The Bridge and Structural Engineer

B&SE_Volume 47_Number 1_March 2017

The Bridge & Structural Engineer JOURNAL OF THE INDIAN NATIONAL GROUP OF THE INTERNATIONAL ASSOCIATION FOR BRIDGE & STRUCTURAL ENGINEERING

Bridge Engineering

The Bridge & Structural Engineer

Indian National Group of the International Association for Bridge and Structural Engineering ING - IABSE

Contents :

Volume 47, Number 1 : March 2017

Editorial ●

From the Desk of Chairman, Editorial Board : Mr. Alok Bhowmick

iv

●

From the Desk of Guest Editor : Dr. Subramanian Narayanan

v

1.

Conceptual Bridge Design Beyond Signature Structures Walter Kaufmann, Beat Meier

1

2.

Aesthetics in Bridge Structures: The Role of Engineers, Architects and Builders Roumen V. Mladjov, S.E.

9

3.

Bridge Foundation Systems : Variants & Expedients V.N. Heggade

16

4.

Seismic Response Control of Reinforced Concrete Bridges with Soil-Structure Interaction Abdul Matin, Said Elias, Vasant Matsagar

34

5.

A Novel Lightweight Composite Deck System for Long-Span Bridges Xudong Shao, Lu Deng, Anil Agrawal

42

6.

Fatigue Design of Steel Bridges Sougata Roy

47

7.

Construction Methodology & Geometry Control During Construction of Cable Stayed Bridge Over Bardhaman (Burdwan) Station Yard, Eastern Railway, India Anirban Sengupta, Yogesh Waingankar

8.

Calibration of IRC:112 Provisions with Life-365 Model Ajoy Mullick

72

9.

Collapse of Kolkata Flyover - Practitioner’s Perspective Prabhakar Narasingarao, Dr. Subramanian Narayanan

79

10.

Learning for Civil Engineers from recent Bridge Collapses Vivek Abhyankar

85

11.

Accelerated Bridge Construction with Folded Steel Plate Girders Dr. Subramanian Narayanan

94

12.

Some Hydrologic and Hydraulic Aspects of Planning and Design of Road Bridges Prof. S.K. Mazumder

103

13.

Movable Bridges : Case Study of Port-Said Bascule Bridge Prof. Hussein H. Abbas, Prof. Mazhar Mohamed Saleh, Dr. Samir Soliman Marzouk

112

14.

New Technology Bridges the Gap Between the Analytical and Physical Model Alex Mabrich

119

58

Contents

Special Topic : Bridge Engineering

Panorma ●

Highlights of the ING-IABSE Workshop on “Inspection, Investigation and Repair/Rehabilitation of Bridges & Flyovers” held at Bangalore on 20th and 21st January, 2017

122

●

Office Bearers and Managing Committee - 2016

124

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

i

The Bridge & Structural Engineer ING - IABSE

Journal of the Indian National Group of the International Association For Bridge & Structural Engineering

June 2017 Issue of the Journal will be a Special Issue with Focus on Urban Transport Structures Salient Topics to be covered are : 1. Urban Transport Planning 2. Sustainability Issues 3.

Mass Rapid Transport Systems

4.

Accelerated Bridge Construction Techniques

5.

Traffic Management Issues in Urban Areas

6. Use of ITS in Urban Planning 7. Case Studies

The Bridge & Structural Engineer ING - IABSE

Journal of the Indian National Group of the International Association For Bridge & Structural Engineering

September 2017 Issue of the Journal will be a Special Issue with Focus on Structural Engineers for Sustainable Development Salient Topics to be covered are : 1. Structural engineers role in safety & sustainability 2. Sustainable approach to the structural design of buildings & bridges 3. Sustainability & cultural heritage in structural design 4. Measuring sustainability & Life Cycle Assessment (LCA) 5. Aspects of Sustainability related to road and rail infrastructure 6. Aspects of Sustainability related to buildings 7. Case Studies Those interested to contribute Technical Papers on above themes shall submit the abstract by 30th June, 2017 and full paper latest by 30th July, 2017 in a prescribed format, at e-mail id : ingiabse@bol.net.in;ingiabse@hotmail.com

ii

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

March 2017

B&SE: The Bridge and Structural Engineer, is a Quarterly journal published by ING-IABSE. It is one of the oldest and the foremost structural engineering Journal of its kind and repute in India. It was founded way back in 1957 and since then the journal is relentlessly disseminating latest technological progress in the spheres of structural engineering and bridging the gap between professionals and academics. Articles in this journal are written by practicing engineers as well as academia from around the world.

All material published in this B&SE journal undergoes peer review to ensure fair balance, objectivity, independence and relevance. The Contents of this journal are however contributions of individual authors and reflect their independent opinions. Neither the members of the editorial board, nor its publishers will be liable for any direct, indirect, consequential, special, exemplary, or other damages arising from any misrepresentation in the papers.

The advertisers & the advertisement in this Journal have no influence on editorial content or presentation. The posting of particular advertisement in this Journal does not imply endorsement of the product or the company selling them by ING-IABSE, the B&SE Journal or its Editors.

Front Cover : Top Left : Typical Example of Retrofitting for fatigue in Steel Bridges by connection stiffening, where out of plane bending is excessive and even holes drilled at crack tips fails to arrest crack growth.

Editorial Board Chair:

Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt. Ltd., Noida

Members:

D.O. Tawade, Chairman ING-IABSE & Member (Technical), NHAI Mahesh Tandon, Managing Director, Tandon Consultants Pvt. Ltd., New Delhi A.K. Banerjee, Former Member (Tech) NHAI, New Delhi Harshavardhan Subbarao, Chairman & MD, Construma Consultancy Pvt. Ltd., Mumbai Nirmalya Bandyopadhyay, Director, STUP Consultants Pvt. Ltd., New Delhi Jose Kurian, Former Chief Engineer, DTTDC Ltd., New Delhi S.C. Mehrotra, Chief Executive, Mehro Consultants, New Delhi

Advisors:

A.D. Narain, Former DG (RD) & Additional Secretary to the GOI N.K. Sinha, Former DG (RD) & Special Secretary to the GOI G. Sharan, Former DG (RD) & Special Secretary to the GOI A.V. Sinha, Former DG (RD) & Special Secretary to the GOI S.K. Puri, Former DG (RD) & Special Secretary to the GOI R.P. Indoria, Former DG (RD) & Special Secretary to the GOI S.S. Chakraborty, Former Chairman, CES (I) Pvt. Ltd., New Delhi B.C. Roy, Advisor Transportation AECOM & Chief Executive RUPL, New Delhi Published: Quarterly: March, June, September and December

Top Right : An example of the Inverset™ system developed by M/s Amcrete Products, Inc., in which the superstructure and the decking surface, is cast upside-down suspended from wide flange steel girders, causing a prestressing effect in the steel girders, and when the section is turned upright for placement, the deck will be in a compressive state.

Publisher:

Bottom Left : Typical View of the rotating leaf in the open position of a Movable Bridge.

All editorial communications should be addressed to Chairman, Editorial Board of Indian National Group of the IABSE, IDA Building, Ground Floor, Jamnagar House, Shahjahan Road, New Delhi-110011.

Bottom Right : An example of the Acrow Panel Bridging System, known as the 700XS® System, which is a light bridge composed of large orthotropic deck units and tall truss systems.

ING-IABSE C/o Secretary, Indian National Group of the IABSE IDA Building, Ground Floor (Room Nos. 11 and 12) Jamnagar House, Shahjahan Road New Delhi-110011, India Phone: 91+011+23388132 and 91+011+23386724 E-mail: ingiabse@bol.net.in, ingiabse@hotmail.com,

secy.ingiabse@bol.net.in

The Bridge & Structural Engineer, March 2017

Disclaimer :

Submission of Papers:

Advertising: All enquiries and correspondence in connection with advertising and the Equipments/Materals and Industry News Sections, should be addressed to Shri R.K. Pandey, Secretary, Indian National Group of the IABSE, IDA Building, Ground Floor, Jamnagar House, Shahjahan Road, New Delhi-110011.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

Journal of the Indian National Group of the International Association for Bridge & Structural Engineering

The Bridge and Structural Engineer

iii

From the Desk of Chairman, Editorial Board

This issue of the Journal “The Bridge & Structural Engineer” is dedicated to the theme of “Bridge Engineering”. In this issue, we have tried to collect papers from bridge experts to cover various aspects of the art and science of bridge engineering, covering about accelerated bridge construction, bridge aesthetics, concept designs, hydrologic and hydraulic aspects, new technologies, durability, seismic response, soil-structure interaction, materials, design, fabrication, construction, inspection, evaluation, safety, and case studies on bridge collapses.

For this special issue, we are privileged to have Dr N. Subramanian as our Guest Editor. Dr Subramanian is a renowned structural engineer of international fame with an illustrious academic background. He is also a renowned practicing structural engineer, who devotes many waking hours (may be even some dreaming hours) for the social cause and in disseminating knowledge to the young engineers through many avenues. He is an active member of SEFI forum. He has authored more than 25 books in various topics …some of them are first of its kind. He has also published 240 technical papers and several discussions in National/International Journals.

This Journal “The Bridge & Structural Engineer” has experienced substantial growth since September 2013, when it received a face lift. The journal has received many accolades from members as well as non-members of ING-IABSE, who subscribe to this journal. The sale of this publication from the stalls in various seminars and workshops have gone up significantly in the recent past which has influenced our secretariat to print additional number of copies with a change in page budget.

While this is an encouraging development and shows the growing popularity of the magazine, there

iv

Volume 47 │ Number 1 │ March 2017

are some recent trends that is a cause for concern.

The recent statistics for the journal indicates that turnaround time for printing of this publication is getting delayed by about 2.5 to 3 months, despite sincere effort from the editorial board to set it right.

The primary cause for the increased turnaround time is mostly delays in getting the committed papers on time, delay in the review process by referees. Some of the blame can be attributed also to those of us, who are co-ordinating the entire process.

It is up to us as members of this association, to make a concerted effort to maintain or upgrade the quality of this journal, keep our tradition of excellence and continue to attract the best papers in the field for this journal. There is a surge in the number of journals (including the emergence of web-based publications) over the past decade and this has thrown new set of challenges to the editorial board and to the ING-IABSE executive committee.

I, therefore, extend this earnest plea to all readers, expert authors, reviewers, who may be called upon by the ING-IABSE secretariat to write a paper or to conduct a review for B&SE journal in the near future. An important responsibility is being thrust upon your shoulders and I hope, you will give this task the importance and value that it deserves and deliver in a timely manner, so that as a reader you receive your copy of the journal in time.

Happy Reading !

(Alok Bhowmick)

The Bridge and Structural Engineer

From the Desk of Guest Editor

When Er. Alok Bhowmick, Chairman of the editorial board, invited me to guest edit the March 2017 issue I was a bit perplexed to choose the theme for this issue, as many important themes were covered in the past. Finally, we chose the theme as Bridge Engineering, in order to cover a number of research & developments that are going on around the world. We chose the following eleven sub-themes: 1.

2.

3.

4.

5.

6.

Conceptual design of bridges Aesthetics in bridge construction Bridge foundation engineering Composite deck system Fatigue & fracture critical bridge inspection Service life prediction for reinforced concrete bridges 7. Accelerated bridge construction to rehabilitate aging highway structures 8. Efficient methods for upgrading or reinforcing existing bridges 9. Hydrology & hydraulics in bridge design 10. Floating bridges, and 11. Future bridge designs Later the following three themes were added: 12. Failure analysis of bridges 13. Construction techniques 14. Soil-structure interaction It is gratifying that we received papers on most of the sub-themes except on Fatigue & fracture critical bridge inspection, Efficient methods for upgrading or reinforcing existing bridges and Floating bridges. This may be due to the fact that there are no floating bridges in several parts of

The Bridge and Structural Engineer

the world- five of the world’s nineteen floating highway bridges (four that are among the largest) are located in Washington State, USA. The SR 520 Albert D. Rosellini Evergreen Point Floating Bridge, completed in 2016 is the World’s longest and widest floating bridge. Other notable floating bridges are Sunset Lake Floating Bridge, Vermont, Eastbank Esplanade, Oregon; Nordhordland bridge and Bergsoysund Bridge in Norway; Demerara Harbour Bridge, Guyana; Yumemai Bridge, Osaka, Japan (World’s first floating swing bridge); William R. Bennet Bridge, Kelowna, British Columbia; Queen Emma Bridge, Curaçao; Floating bridge in Dubai, and Hobart Bridge in Tasmania. Three papers have been reprinted with permission from original sources.

In any country, bridges serve as the critical links of the transportation network and hence should be maintained to remain safe and functional during their service lives to enable personal mobility and transport of goods to support the economy and ensure high quality of life. In the first paper, Professors Kaufmann and Meier of Germany demonstrate with several examples the conceptual design of bridges. They show that in any bridge project, a collaboration of interdisciplinary bridge design teams is necessary and if the construction process is considered as an integral part of the design concept, aesthetic and economic demands can be simultaneously satisfied. Bridges, unlike buildings, are in the realm of structural engineers, who also cater to their aesthetic appeal (for example, Robert Maillart, Christian Menn, Othmar Hermann Amman and Fritz Leonhardt designed several aesthetic bridges). Large bridges may even become the icon of the city in which they are constructed

Volume 47 │ Number 1 │ March 2017

v

(e.g., Golden gate bridge as icon for San Francisco).

Er. Mladjov of USA in his paper presents the basic principles of good structural design and explains how an engineer, with or without the assistance of an architect, can produce an elegant and appealing bridge structures. Designing aesthetic bridges requires holistic approach, and a team effort.

The design and construction of bridge foundations involve four specialized disciplines of civil engineering, viz., Geotechnical, Geological, Hydrological and Structural, and hence quite complex compared to land based foundations.

Er. Heggade in his interesting paper explains with numerous practical examples, different deep foundation techniques that warrant special construction expedients. He observes that in many cases, the bridge foundations are to be modified during construction due to geotechnical surprises, and does not fit into standard theoretical solutions and also are not well documented. Er. Matin, Er. Elias, and Prof. Matsagar analytically investigated the effect of soil-structure interaction on the responses of three span continuous reinforced concrete bridge, installed with tuned mass damper(s). The significant effects of the soil surrounding the pier on the response of the controlled bridges have been observed. It is also found that the seismic responses of bridge with TMDs are considerably altered.

Profs. Xudong and Lu of China and Prof. Anil of USA discuss about the new lightweight composite deck system developed by them. This composite deck system consists of a conventional orthotropic steel deck topped by a thin, compactly reinforced, ultra-high performance concrete layer.

The experimental investigations conducted by them revealed the excellent static and fatigue performance of this system. The system has great potential for application in long-span bridges, and already been adopted in China. Dr. Sougata Roy of USA provides an overview of the fatigue design and detailing requirements for steel bridges.

The fatigue design guidelines for S-N curves and selecting details from fatigue resistant categories of AASHTO and Eurocode 3 codes are discussed and compared, noting that the current IRC:24 incorporates Eurocode 3 provisions. Distortion induced fatigue is well explained with solutions to rectify and reduce its effects.

vi

Volume 47 │ Number 1 │ March 2017

Though software packages may be used to analyse and complicated bridge geometry, several problems may occur during the construction of cable stayed bridges. Sengupta and Waingankar of STUP Consultants explain the construction methodology and geometry control adopted by them, while constructing a cable stayed bridge over Bardhaman (Burdwan) station yard, Eastern Railway, India.

Dr. Ajoy Mullick examines the rationale of provisions in IRC: 112 to guard against chloride corrosion and carbonation corrosion. He argues that a proper choice of w/c ratio and cover depth will minimize depth of carbonation, provided the concrete is well compacted and cured. Though the provisions are adequate for guarding against chloride ingress also, additional protection measures may be required, when the concentration of acid soluble chloride ions on the surface exceeds 1.0 percent by weight of cement.

Failure of any crucial bridge not only results in precious loss of lives, injury and huge property loss, but also affects the economy of the region. Er. Prabhakar and I have shown clearly that the recent collapse of the Kolkata Flyover, in India is due mainly to the unconventional connection details adopted at the junction of the vertical Pier and the cantilever girder of the flyover. The importance of proof checking, certification, and continuing education are emphasized, in order to eliminate such failures in future. Er. Vivek of Afcons briefly discusses about seventeen bridge failure cases occurred in recent years, and stresses the importance of learning from these failures.

The urgent need to adopt accelerated bridge construction practices, to replace dilapidated bridges, and to avoid economic loss and reduce the hardship to the existing users is stressed in my paper.

To solve this problem, the Short Span Steel Bridge Alliance (SSSBA), USA organized the Modular Steel Bridge Task Group and several solutions emerged from this group. These solutions are outlines and more details about the two innovative systems of Folded Plate Girder (FSPG) and the Press-Brake-Formed Tub Girders are provided.

Both are similar composite construction techniques and offer several advantages, including sustainable solution, better life-cycle costs and durability.

The Bridge and Structural Engineer

Foundation scouring has been reckoned as the leading reason for bridge failures in USA. Half of the 500 collapses that happened between 1989 and 2000 were caused by it! Prof. Mazumder of Delhi College of Engineering discusses the various hydrologic and hydraulic aspects of planning and design of road bridges. He has reviewed the existing methods of computing scour under bridge piers and abutments. The limitation of IRC method of scour computation (which is based on Lacey’s theory) and the need for employing mathematical models for scour computation are outlined. It is well known that if detected early enough, foundation-scouring is easy to fix by dumping riprap, into the water around the bridge piers. Several innovative methods have been proposed recently to detect scouring, including the one by Dr Prendergast of Delft University, in which accelerometers (similar to those used in smart phones) are used to monitor changes in the patterns of vibration due to foundation erosion, and forewarn problems.

Movable bridges can be constructed in different ways, namely: vertical lift, bascule and swing types.

The Bridge and Structural Engineer

Profs. Abbas, Saleh and Marzouk of Egypt explain the details of the of a movable bridge, recently built in the city of Port-Said, Egypt, founded on one of the Suez Canal branches. This bridge consists of six bays with an intermediate bascule movable bay having an open angle of 70 degrees. In the final paper Er. Mabrich explains the advancements taking place in 3D design software, that are making it possible for bridge designers and engineers to close workflow gaps from model to analysis.

I wish to thank all the authors who readily contributed papers and also IABSE and the STRUCTURE magazine to reprint papers already published by them. I also wish to thank Er Alok Bhowmick and ING-IABSE for giving me this opportunity to be a guest editor of this special issue of the journal. Hope that the papers selected by me will be useful to the practicing engineers to learn about the various facets of bridge engineering.

(Dr. N. Subramanian)

Volume 47 │ Number 1 │ March 2017

vii

Brief Profile of Dr. N. Subramanian Dr. N. Subramanian is an award winning author, consulting engineer and mentor now living in Maryland, USA , and former Chief Executive of Computer Design Consultants, Chennai. A Ph.D. and Post Doctoral Fellow of the Indian Institute of Technology, Madras, he has more than 40 years of professional experience which include teaching, research, and consultancy. He also worked in the then West Germany as Alexander von Humboldt Fellow with Prof. J. Lindner and Prof. Ch. Peterson. He has served as a consultant to several leading organizations in India and designed several noteworthy structures involving RCC, structural steel, and cold-formed steel. More than 600 microwave/transmission towers were designed by him, one of which earned certificate of merit from the Consultancy Development Centre, DSIR, Govt. of India, 1992.

Dr. N. Subramanian also developed several special-purpose software packages for analysis and design of different types of structures. Being a member/fellow of several professional bodies, he took very active part in these organizations, and is the past All India Vice president of Association of Consulting Engineers (India) and also the Indian Concrete Institute.

Dr Subramanian has authored more than 25 wide selling books and more than 240 technical papers, published in National and International journals/conferences. He is also the author of Steel Structures: Design and Practice (OUP), Design of Reinforced Concrete Structures (OUP), Access to Computer Education (OUP) and Space Structures: Principles and Practice (Multi-Science, U.K.). Three of these books earned the coveted ACCE-Nagadi Award and are prescribed as text books.

Dr Subramanian is in the editorial/review boards of several prestigious International journals and earned several awards, including the Tamilnadu Scientist Award and the Life-Time Achievement Award given jointly by the Indian Concrete Institute and L & T. He is active as mentor to young engineers and is one of the main contributors to Structural Engineering Forum of India, which has more than 21,500 members. He served as the President of Rotary Club of Madras Metro.

D. P. Jaiin & Co. Infrastructu ure Pvt Ltd., Nagpur, believes in positive thinking g, yen for learning, lateral l thinking and d receptivity to new ideas. The T company is tea am player to all its clients right from concepttion to commissionin ng. It has been activeely involved in – H Highway Constructio on ( NH & SH) C Construction of build dings & industrial civvil works IIrrigation projects likke water reservoirs, construction c of c canals, canal structu ures and lift irrigation n projects.

Designing and constrruction of Runways, Aprons & Aircraft D H Hangers.

M Mining & heavy eartthwork projects.

U/6 6, HIMALAYA ACCOR RD APARTMENT, OPP P. LAW COLEGE, AMR RAVATI ROAD, NAGP PUR – 440010.

TEL.: 91-712-2535357 Fax: 91-712-2560839 Email: dpjain.company@dpjaingr d roup.com Website: www.d dpjaingroup.com

viii

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Conceptual Bridge Design beyond Signature Structures

Walter Kaufmann Professor, ETH Zurich Zurich, Switzerland kaufmann@ibk.baug.ethz.ch

Beat Meier CEO, dsp Ingenieure & Planer AG Greifensee, Switzerland meier@dsp.ch

Walter Kaufmann graduated from ETH in 1992 (doctorate 1998). He then worked as a structural engineer in Spain and Switzerland before joining ETH Zurich in 2014 as Chair of Concrete Structures and Bridge Design.

Beat Meier graduated from ETH Zurich in 1991. He joined dsp Ingenieure & Planer in 1998 and took over as CEO in 2014. During the past 25 years, he successfully contributed to numerous projects and bridge design competitions.

Abstract The present paper outlines important aspects of the collaboration in interdisciplinary bridge design teams, using some of the authors’ recent successful projects as illustrative examples. It is emphasized that design competitions have significantly contributed to the fact that in Switzerland – contrary to international tendencies – hardly any sculptural, inefficient and excessively expensive bridges were built over the past decades, while aesthetic criteria and innovative approaches are appreciated even in the design of minor bridges and civil engineering structures.

Keywords: Bridges, bridge design, conceptual design, design competitions, signature structures.

1.

promoted it as an ideal alternative to the countless anonymous, often ugly utility structures based on purely technical and economic criteria.

Fig. 1 : Salginatobelbrücke, R. Maillart, 1930 [2]

Swiss Tradition of Bridge Design

Conceptual bridge design has a long-lasting tradition in Switzerland. Engineers like Robert Maillart, Alexandre Sarrasin and Christian Menn (Figs. 1 to 3) significantly contributed to the evolution of today’s international state-of-art in bridge design and technology and to the recognition of the cultural value of modern engineering structures.

More than 30 years ago, Billington [1] introduced the terms efficiency, economy and elegance as common denominator of their approach to bridge design and The Bridge and Structural Engineer

Fig. 2 : Pont du Gueuroz, A. Sarrasin, 1934 [2]

Volume 47 │ Number 1 │ March 2017

1

architects without much knowledge nor concern for construction methods, see for example Fig. 5.

Fig. 3 : Felsenaubrücke, Bern; Ch. Menn, 1974 [2]

This type of holistic, integral approach to conceptual design, adding sustainability and innovation as additional design goals, is what the authors, presumably like many other structural engineers, are striving for when conceiving a new bridge.

2.

International Trends Bridge Design

in

Conceptual

However, instead of following Billington’s approach, international bridge design unfortunately appears to have evolved in two different, opposite directions over the past decades.

On one hand, a trend to so-called signature bridges, conceived with the primary goal of producing new landmarks, is observed. Some of these bridges are indeed beautiful, efficient and reasonably economic structures, see for example Fig. 4. Note that this bridge has been conceived by an eminent structural engineer, not an architect.

Fig. 5 : Sheikh Zayed Bridge, Abu Dhabi; Z. Hadid, 2010 [4]

While these bridges may achieve the goal of attracting attention by looking beautiful or original to the general public – which, being a matter of taste, is at least debatable in many cases, see for example Fig. 6, their execution often requires enormous efforts, resulting in disproportionate costs and carbon footprints.

On the other hand, less important structures are ever more designed for minimum cost, completely neglecting their aesthetic and cultural importance – in spite of the fact that, considering their envisaged lifespan, these structures will inevitably leave a significant imprint on their environment.

Fig. 6 : Ponte do Milenio, Ourense A. Varela/Pondio Ingenieros, 2001

3.

Fig. 4 : Puente Del Tercer Milenio; Zaragoza J.J. Arenas, 2008 [3]

Many of these bridges, however, are essentially giant sculptures with complicated and inefficient structural configurations, typically sketched by renowned

2

Volume 47 │ Number 1 │ March 2017

Contemporary Swiss Bridge Design

Fortunately, these trends have had little impact in Switzerland. Hardly any sculptural, inefficient and excessively expensive bridges have been built here, and aesthetic criteria and innovative approaches are appreciated even in the design of minor bridges and civil engineering structures. Doubtlessly, this

The Bridge and Structural Engineer

awareness is fostered by design competitions, which are regularly organized by public clients. In these competitions, the project entries are judged on a holistic set of criteria, and usually, structural engineers join forces with architects and other experts to form design teams.

4.2 Conceptual Design Process

4.

While aesthetical criteria may dominate the structural concept of an urban small span footbridge, structural engineering aspects, including construction methods and economic considerations, must dictate the concept of large-span bridges. Otherwise, as mentioned above, disproportionate costs and carbon footprints will be the consequence.

Bridge Design Competitions

In the following, the authors are sharing their personal thoughts with respect to conceptual bridge design, based on their experience, particularly in design competitions. These ideas are certainly debatable and lack scientific rigour. Still, the authors hope to animate their colleagues to think critically about their role in conceptual bridge design, and stimulate the discussion.

4.1 Interdisciplinary Teams Modern day bridge design encompasses classical engineering issues such as structural analysis, construction processes and economy, but many other aspects as well. Among these additional facets, there are many topics, such as geotechnics, hydrology, environmental protection, traffic engineering and, of course, aesthetics. The latter comprises the conformity with the surroundings, urbanistic aspects, shapes and proportions as well as other facets crucial for a good structural concept.

Structural engineers are not specifically trained for many of these aspects, and only few geniuses are capable of mastering all of them and design a splendid bridge as shown in Fig. 4. Therefore, collaborations of structural engineers with architects (and specialized consultants for additional topics) have become common. While some years ago, many Swiss structural engineers felt embarrassed if clients asked for design teams including an architect to participate in design competitions, it has become normal today. Based on positive experiences with collaborations in competitions, many engineers are consulting nowadays with architects not only in design competitions, but also in normally tendered, challenging projects.

It is crucial for a successful collaboration that all the team members share a high level of professional skills, mutual respect and open-mindedness, particularly regarding ideas of other team members. In addition, they must have a true interest and basic competences in the other disciplines involved, such that all the team members can share ideas using a common vocabulary.

The Bridge and Structural Engineer

Conceptual bridge design is far from being a linear process. Rather, it is interdisciplinary, highly interactive and iterative. To foster creativity, the collaboration should be non-hierarchical, except for organizing the team (deadlines, deliverables).

Independently of the importance of structural or aesthetical criteria, engineers and architects should develop the structural concept, integrating all relevant criteria, in a dialogue. Neither should the engineer develop the concept and the architect refine it, nor the architect rely on the engineer just to check the feasibility of his idea. Just as it is impossible to obtain a good cake if the ingredients are baked separately, it is unlikely to obtain a good structural concept if the team members develop their ideas separately and assemble them a posteriori. Instead, the design team should take on the role of pre-modern master-builders, reuniting the tasks of architect and engineer.

The authors are convinced that a good structural concept – as a result of an integral, holistic design approach – is characterized by the fact that neither the concept as a whole, nor parts of it, can be credited to one of the team members, just like it is impossible to attribute the taste of a perfect soup to one of its ingredients.

4.3 Role of Structural Engineers Mastering the technical challenges in bridge design and construction has become ordinary today, and the age of great structural engineering protagonists and bridge designers may be over. However, designing a bridge in order to satisfy all technical requirements such as safety, serviceability and durability still requires a high level of education, professional skills and experience, as well as a lot of hard work. Moreover, structural engineers carry most of the responsibility in bridge projects and assume the risks that come with construction. Therefore, independently of the significance of the different aspects in the design, a structural engineer – or a structural engineering firm – should generally adopt the role of the author of the project. Efforts are required in order to achieve that

Volume 47 │ Number 1 │ March 2017

3

the public recognize the leading role of structural engineers in bridge design, rather than talking about the “architect” of the bridge.

There are of course bad examples of collaborations, where either the engineer made a sculptural or ornamental concept of the architect feasible, or the architect decorated an ill-conceived structure.

However, these cases must not be taken as reasons to discredit collaborations in interdisciplinary design teams in general. Rather, it is up to us engineers to take on an active role in the design process and discuss on a par with the architects and other team members.

4.4 Competition Juries The jury members take on a key role in design competitions, as they evaluate the entries and select the best project. To a certain extent, the winning project will always reflect the attitude of the jury. It seems evident that the cumulated competences of all jury members should be at least equal to those of the participants and cover all the aspects involved in a project, even if the jury may be assisted by experts pre-examining the entries. Unfortunately, this is not always the case.

It does of course make a huge difference if the jury is dominated by politicians or sponsors with the idée fixe of building a landmark bridge, or rather by highly skilled and experienced engineers and architects as well as far-sighted representatives of the client, conscious of spending tax money. Fortunately, the latter applies to most bridge design competitions in Switzerland, which is why bridge design here has profited a lot from them.

5.

The structural concept of the winning project is the result of an intensive evaluation of the topographical, environmental and geological issues at the project site. The new bridge is visible from many locations and has a great impact on its surroundings. Therefore, the design team opted for a prominent and elegant structure, without overpowering the spectacular alpine landscape. Rather than an arched bridge (that would have fitted well geometrically, although its bases would have been hidden by the forest), a concept minimizing horizontal foundation forces was favoured because the solid rock suitable for bridge foundations is found only at considerable depth, and above it, the left valley slope is unstable. Finally, due to the steep valley slopes complicating site access, the structural concept aimed at building this bridge with the least amount of access points.

Based on these considerations, a conventional, yet carefully designed and detailed concrete box girder bridge was proposed. The variable depth girder has only two piers (other girder bridges proposed in the competition had three or more), corresponding to the site accesses, and is constructed using the balanced cantilever method. The superstructure is monolithically connected to the piers and has expansion joints and longitudinally movable bearings at the abutments, such that the required pier stiffness during construction can also be used in the final structure. Therefore, the abutments, located in the steep, partly unstable valley slopes, merely have to resist horizontal forces caused by friction of the bearings.

Examples

In the following, three of the authors’ recent successful projects, all located in alpine surroundings, are presented, focusing on the development of their quite contrasting structural concepts.

5.1 Inn Bridge Vulpera The Inn Bridge Vulpera [5], opened to the traffic in 2010, connects the villages Scuol and Tarasp (Canton Grisons). It spans the Inn gorge at a height of about 70 m with a slope of 7.5%. Anticipating that the design and the construction of the bridge were going to be both technically as well as aesthetically challenging, the client decided to organize a design competition in 2005, stating clearly that an economical solution was sought.

4

Volume 47 │ Number 1 │ March 2017

Fig. 7 : Inn Bridge Vulpera, 2010

The Bridge and Structural Engineer

The pier locations were determined taking into consideration the structural and aesthetic constraints.

Criteria included: a well balanced span layout, similar pier heights and no additional pier on the Tarasp side.

The total length of the bridge is 236 m, with spans of 59 + 104 + 73 m.

The shape of the superstructure and the piers, as well as the envisaged construction sequence, were developed foremost by considering the structural performance and optimized with regard to their appearance and the construction process (polyhedral, relatively simple geometries allowing for a repeated use of the same formwork with minimal adjustments).

In this way, an elegant structure could be built without negative impact on construction cost. For example, while the bottom slab of the box girder has a constant width, its side faces become visible due to the inclined webs of the variable depth girder, showing the continually increasing depth of the bottom slab and emphasizing the flow of the forces without complicating the formwork. The dimensions of the pier sections correspond with the bending moments in the longitudinal and transverse directions, with a waist located in the area of minimal forces.

The shaft and the movable rings were built top-down such that they could already be used for stabilizing the excavation during construction.

5.2 New Versam Gorge Bridge The new Versam Gorge Bridge [6] near Versam (Canton Grisons) is located close to the existing bridge built in 1897, one of the very few arched steeltruss bridges remaining in Switzerland. In addition, it is located in a dramatic landscape close proximity to the Rhine Gorge, which is in the federal inventory of national importance. Hence, the design was established in agreement with the Swiss nature conservation and heritage agencies.

Due to the relatively good rock quality, the site was attractive for statically efficient arch-type solutions.

In order to avoid ingratiating similarities with the existing slender, almost transparent bridge, the design team opted for a large, generous and forceful looking strut frame structure, with a span of 80 m between the strut supports.

Fig 9 : New Versam Gorge Bridge, 2012

Fig 8 : Inn Bridge Vulpera, 2010

Piers and abutments are founded on caissons in the stable rock. For the pier on the unstable left valley slope, an excavation of 18 m depth was required, using the bottom 4.7 m – the part lying in the solid rock – as caisson foundation with 10 m diameter. On top of this caisson foundation there is a cylindrical, hollow shaft including movable rings, separated by compressible joint material, which can adjust to the slow horizontal deformations of the deep unstable layers for an anticipated period without intervention of at least 100 years. Above the movable rings, a stiff hollow shaft of approximately 8 m height resists the pressure from the faster creeping slope on the surface.

The Bridge and Structural Engineer

The shapes of the superstructure and the piers were developed in view of the internal force flow and optimized with respect to the appearance and construction process. The cross-sections are variable, with the greatest dimensions in the transition zone between the superstructure and the substructure. The pier shape tapering towards the bottom, resulting in a compact pier base, was chosen to fit in with the topographic conditions (dip of the slope strongly skewed to the bridge axis) as well as geotechnical considerations.

The rough terrain with steep valley slopes and the limited site access (falsework could only be delivered in small pieces due to the narrow access roads including low-profile tunnels) demanded a clear assessment of

Volume 47 │ Number 1 │ March 2017

5

the construction process in order to guarantee a costeffective solution.

The design team decided to opt for an unpretentious bridge fitting harmoniously into the dominant surrounding landscape, being as light as possible, but without affecting its robustness and durability. The proposed design, winning the design competition in 2007, is a multi-span continuous prestressed concrete bridge with a constant, lightweight open cross-section.

The total length of the bridge is 441.0 m, with spans of 21.0 + 14·28.5 + 21.0 m. In order to achieve a high durability and minimum maintenance, all piers are monolithically connected to the bridge deck, with only two expansion joints, located at the abutments (as compared to 6 expansion joints and 20 hinged connections in the existing bridge).

Fig. 10 : New Versam Gorge Bridge, 2012

In order to minimize horizontal reaction forces in the steep slopes, the 112 m long bridge rests on pot bearings moving that are movable longitudinally at both abutments. However, in order to achieve a high durability in the forested and shady, moist environment, no expansion joints were provided.

The small pavement cracks that must be expected in this semi-integral solution with the given movement length were accepted by the client.

5.3 Replacement of Steinbach Viaduct The Steinbach Viaduct [7] crosses the Sihlsee Reservoir (Canton Schwyz) with a length of about 440 m. The old, narrow bridge was no longer adequate to cope with today’s requirements and involved high maintenance costs, particularly due to excessive settlements and the high number of expansion joints. Therefore, a two-stage design competition was launched in 2006 in order to find the best possible solution for its replacement.

The task of replacing the existing bridge proved to be very challenging. The choice of the structural concept was primarily influenced by the fact that the bridge had to be built without influencing the level of the reservoir, with water level fluctuations of up to 8 m, while temporary dams in the lake were inadmissible for environmental protection reasons. Furthermore, in view of the low load bearing capacity of the subsoil (soft lacustrine sediments up to 100 m depth) and its sensitivity to settlements, the choice of the foundation system was essential. Finally, the largely visible crossing over the lake, with the new, 13 m wide deck only just above the waterline at high reservoir levels, and its location in a recreational area posed high demands on the appearance.

6

Volume 47 │ Number 1 │ March 2017

Fig. 11 : Steinbach Viaduct, 2014

The length of the normal span as well as the shapes of the deck and the piers were developed from the static requirements and optimised with regard to aesthetic considerations as well as the construction procedure.

For example, in order to facilitate an optimal use of a launching-girder without intermediate supports, the cross-section of the deck is constant, without diaphragms nor transverse frames, the pier geometry allows for a direct support of the launching girder, and the normal span is relatively short. The latter also enables a very slender superstructure, to give the impression of an unpretentious, elegant strip over the water even at high water levels.

The height of the piers above the lake bottom varies between 8 and 12 m, and their appearance changes greatly with the changing water level. Hence, finding a pier geometry with proportions appropriate with full as well as empty reservoir – and satisfying the constraints of the envisaged erection procedure – was particularly challenging. In their uppermost part, the external geometry of all piers is identical, enabling the use of the same formwork. However, in The Bridge and Structural Engineer

order to ensure a reference of the piers to the water level, the horizontal connection joining the pier arms is located at the same elevation on each pier, which was achieved using a variable horizontal stop-end shuttering on the inside of the pier formwork. The abutments are designed to always remain above the water line, facilitating the ecological connectivity above the water level even when the reservoir is full.

The choice of the foundation concept, particularly regarding its construction, was crucial for the cost-

effectiveness of the project. Piers and abutments are founded on prefabricated spun concrete piles Ø45 cm up to 36 m long, dimensioned based on the results of in-situ static load tests carried out one year before construction of the bridge. The 16 piles of each pier are connected by a massive pile cap, which ensures the load transfer and is only slightly embedded in the ground in order to minimise the impact on the lake bottom.

Fig. 12 : Steinbach Viaduct, 2014

The foundations and the piers were built entirely using pontoons. First, the piles were driven. Then, a sheetpile caisson was installed, followed by underwater excavation and the pouring of an underwater steel fibre reinforced concrete base. Next, the sheet-pile caisson was pumped out, using the piles and the underwater concrete in order to ensure safety against uplift. Now, the pile cap and the pier could be constructed in dry conditions and finally, the sheet-pile caisson was flooded and the sheet-piles removed. These works were carried out simultaneously on several piers in the sense of a line construction site, in order to achieve the required construction speed corresponding to the superstructure.

The latter was built span by span in a 3-weekcycle in two construction seasons, starting each year from the abutments. During the winter break in between, the launching-girder was moved from the middle of the lake to the second abutment such that, before pouring the closure span, the two halfbridges could be pressed apart. In this manner, the creep and shrinkage deformations of the 441 m long The Bridge and Structural Engineer

deck and the corresponding imposed deformations of the monolithically connected piers could be partly compensated.

6.

Conclusions

As already stated, the view of the authors regarding the collaboration in interdisciplinary design teams may lack generality and scientific rigour, and the same applies to the examples presented above. Still, some conclusions can be drawn.

First of all, the projects presented, all of them with structural concepts developed by interdisciplinary design teams following the proposed holistic approach, were successful. They convinced the clients and the competition juries, respectively, and – more importantly – all of them passed the litmus test of construction without negative surprises, neither technically or aesthetically, nor economically. Hence, they demonstrate that the proposed holistic approach is particularly suited for technically and aesthetically challenging projects.

Volume 47 │ Number 1 │ March 2017

7

Furthermore, the examples demonstrate that if the construction process is considered as integrating part of the design concept, aesthetic and economic demands can be simultaneously satisfied.

Finally, it is evident that – at least if economic criteria are relevant – structural engineers must take on an active role in conceptual bridge design, in spite of the fact that nowadays, almost any structure (even giant sculptures) can be calculated and built.

7.

Acknowledgements

The authors are grateful to all other members of the design teams involved in the projects presented in this contribution: E. Imhof Architekt, ACS-Partner AG and Dr. Vollenweider Geotechnik AG (Inn Bridge Vulpera); Prof. A. Deplazes (New Versam Gorge Bridge), and Feddersen & Klostermann, Spataro Petoud Partner, F. Preisig and Fellmann Geotechnik (Steinbach Viaduct).

This paper was originally presented in the IABSE Conference-Structural Engineering: Providing solutions to Global Challenges, Sept. 23-25, 2015, Geneva, Switzerland and published in the proceedings.

It is reprinted here with permission from IABSE Secretariat.

8

Volume 47 │ Number 1 │ March 2017

References 1.

Billington, D. The Tower and the Bridge. Basic Books, New York, 1983.

2.

Marti, P., Monsch, O., Schilling, B. IngenieurBetonbau. vdf an der ETH, Zürich, 2005.

3.

Arenas, J.J., Capellán, G., Beade, H., and Martínez, J. El puente del tercer milenio: retos en el diseño de puentes desde la perspectiva de la ingeniería creativa. IV ACHE Conference on Bridges and Structures, 2008.

4.

Lengweiler, R., Constantoulakis, C.,Shebl, A., and Lüthi, M. Verzerrte Bögen auf massiven Auflagerkraken. TEC21, 2011, pp. 28-33 & 49-50.

5.

Kaufmann, W., Müller, O., and Vogt, R. Inn Bridge Vulpera. Structural Concrete in Switzerland. fib Congress Washington, Swiss Group of fib. 2010, pp.81-86.

6.

Meier, B. and Müller, O. New Versam Gorge Bridge. Structural Concrete in Switzerland. fib Congress Mumbai, Swiss Group of fib. 2014, pp.113-117.

7.

Kaufmann, W. Replacement of Steinbach Viaduct – new bridge over the Sihlsee.

Structural Concrete in Switzerland. fib Congress Mumbai, Swiss Group of fib. 2014, pp. 107-112.

The Bridge and Structural Engineer

AESTHETICS in BRIDGE STRUCTURES: the Role of Engineers, Architects and Builders Roumen V. MLADJOV, S.E.

Roumen Mladjov, born 1937, received his structural engineering degree from the University of Sofia, Bulgaria.

He worked for Metalproect and Metalstroj, Sofia, Bulgaria, later as an associate with Middlebrook + Louie, USA, for 25 years. His main interests are structures, seismic safety and efficiency in structures.

Structural & Civil Engineer Independent Consultant San Francisco, CA, USA rmladjov@gmail.com

Summary

2.

This paper discusses the aesthetics in bridge design and the respective roles of engineers, architects, and builders in bridge construction. How to make a bridge structure elegant and pleasant to the eye? Do we need architects for bridge design? The paper presents the basic principles for good structural design and how an engineer, with or without the assistance of an architect, can deliver an elegant and appealing bridge structure. To be elegant a bridge does not need to be expensive or extravagant; the simplest bridge with an “honest” structure is often best.

The Caravan Bridge, a single arch stone structure in Turkey built around 850 BC is considered the oldest still functioning bridge. In his Histories, Herodotus reports a bridge on stone piers built in Babylon over a channel of the Euphrates River around 550 BC [1]. He also describes temporary military pontoon bridges built by the Persian armies: one on the Ister River (Danube) and two long ones used for crossing the straits of Hellespont (Dardanelles) during their invasion of ancient Greece around 500 BC and 480 BC, respectively. This is about 2,500 years before Bosphorus Bridge I, the first permanent bridge between two continents, was completed in 1973.

Keywords: aesthetics, bridge designs, engineers, builders, structural elegance, cooperation.

1.

Introduction

Bridges have been built since the beginning of civilization and are among the oldest structures used by mankind. From meeting purely utilitarian necessities, bridges have evolved with time to become symbols of human progress, of cities and entire countries. Among the thousands of bridges around us are the bridges that we all admire, the bridges that are the symbols of the eternal human aspiration for building longer and taller, stronger and faster.

When discussing bridges, important issues to consider are aesthetics and the respective roles of engineers, architects and builders in designing a bridge. What makes a bridge structure elegant and appealing? Do we need to involve architects in the bridge design?

A brief review of bridge design and construction development will help with the answers.

1

Engineers, Architects and Builders

For centuries up to the early 19th Century there were no structural-bridge engineers and architects1. These professions and “titles” simply did not exist at that time. Writers and scholars often refer to architects when describing ancient constructions; however, these “chief builders” practiced the combined tasks of present-day engineers, architects, artists, and craftsmen. The tasks of these modern professions were performed by a single practitioner, who was learning these skills by apprenticeship, following the experience of their predecessors and the “trial-anderror” method. Later, during the Middle Ages, with the building of Gothic cathedrals, the leaders of larger projects were called “master builders”.

Master builders produced remarkable structures over the centuries, before the first engineers and architects started receiving a formal education. Later, during the Industrial Revolution, with the need for

Architect derives from Greek arkhitekton (arkhe “leading” + tekton “builder”), meaning chief builder.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

9

more construction (bridges and buildings) and the development of engineering knowledge, the functions and duties of a single master builder were separated among architects, engineers and builders. It was only during the 19th Century that civil engineering and architecture took off as technical professions.

During the “Heroic Age” of bridge engineering, Thomas Telford, Isambard K. Brunel, Robert Stevenson, John, Washington and Emily Roebling2, and Alexander Gustave Eiffel built astonishing bridge structures. To this day, we still admire and consider these bridges as part of the highest achievements in engineering. Among these bridge builders, only Washington Roebling and Eiffel had formal engineering education (Eiffel being a chemical engineer). The others were self-taught without a formal education in engineering; the bridge-engineering genius John Roebling had completed his academic education, but did not take the final exam. At that time, being able to provide a safe and efficient bridge was valued more, than having formal degrees.

The general term “engineer” is used in this article for all structural, bridge, civil or self-educated engineers and builders. All remarkable bridges of the past were designed and built by engineers. There are no reports of architectural involvement (in the current meaning of the term) in their design. Only a few bridges in the past required the skills and knowledge of current day architects. For example, the Venetian Rialto Bridge, the Ponte Vecchio in Florence, and the London Tower Bridge used architects for the building structures on these bridges; others like Pont Alexandre III in Paris used architects due to their rich ornamentation.

Today many engineers working on bridges believe that, due to their education, experience and skills, they are able to work alone and do not require the involvement of architects in bridge designs, except for secondary elements like vehicle/pedestrian barriers and light poles [2]. Other engineers do work with architects, or at least consult an architect for their bridge design. Since their structures are mostly exposed, bridges may be considered as the most “sincere” constructions. Therefore, even if an architect is not involved, the bridge designer must consider aesthetics; he/she is the one that best knows how to resolve the challenge of balancing the contradicting requirements of robustness and slenderness, and to 2

10

obtain safety and elegance at the same time. So what is the answer to the initial question “Do we need an architect to design a bridge?”

To respond to this question, it is necessary to look at the specifics of bridge aesthetics. Most bridge professionals agree on the basic principle that a bridge has to be robust (strong, stiff and resilient), functional, efficient and economic, but also that it should be elegant – slender with simple forms and well proportioned. It has to be in harmony with its surrounding environment, and if possible, to embellish its natural site (Figs. 1 & 2). Even as early as the Roman period, Vitruvius had formulated three important structural qualities – firmitas, utilitas, venustas – meaning that a structure should be solid/robust, useful and beautiful [3]. In modern times, David Billington has set the basic principles of good structural design as: efficiency (of materials), economy (of cost and time) and elegance (slenderness, elegance and good proportions) [4]. Since then, some of the most prominent bridge engineers, Fritz Leonhardt, Michel Virlogeux, and Christian Menn, have expressed the same understanding of the essential qualities of bridge design [5-7].

Fig. 1 : Golden Gate Bridge, San Francisco, 1937, Recognized as One of the Greatest Achievements in Bridge Engineering; (a) Overall view, (b) South Tower, a View from the Bridge Deck

Fig. 2 : Sunniberg Bridge, Switzerland, 2006, Designed by Christian Menn with Longest Span of 140 m- an Example for a Bridge, Built in Harmony with its Environment.

The Brooklyn Bridge is John Roebling’s (the father) most famous structure; Washington (the son) then continued the work after the death of his father. When Washington became ill with “caisson disease”, acquired during the construction of the bridge, and could not leave his room, the management continued with the decisive help from his wife Emily.

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Regarding the robustness and functionality of bridges, all bridges shall be designed and built to satisfy their main function – to transfer pedestrians, vehicles and/ or trains from one side of an obstacle to the other side.

This means that all bridges must have the necessary strength and stiffness to safely carry the prescribed loads, as specified in bridge codes. For this reason, the robustness is a must for all bridges and is not discussed further in this article.

The engineer’s role is to provide an efficient and economic structure, while also trying to make it elegant. The engineer’s task is to select the most appropriate bridge type and the correct parameters for a specific project; the builder can also provide significant inputs in this process, if involved from the beginning. The architect, while advising the engineer on aesthetics, should avoid recommendations that may significantly increase the cost. While aesthetics is more or less subjective, efficiency and economy can be measured objectively by the cost, main structural materials, and construction time, with respect to the bridge spans lengths [8].

3.

Designing Strong and Elegant Bridges

How can an engineer, with or without help from an architect, deliver an elegant and appealing bridge structure?

There is a consensus among professionals that a welldesigned bridge in conformance with the structural “basic principles” usually results in an elegant, wellproportioned and appealing structure without the need for additional ornamentation. A bridge design should also take into consideration the visual exposure and appearance of the structure in relation to its site environment. Based on their exposure and location, bridges are: ● ● ● ● ●

Non-visible structures, usually short span bridges on roads without underpasses, not requiring specific attention to aesthetics; Common short or medium span bridges, requiring regular attention to aesthetics; Long-span bridges or such with significant exposure, requiring special attention to aesthetics; Complex bridges with exposure and long approaches, requiring special attention to aesthetics; Some bridges span a river, strait, gorge or ravine directly, often without any approaches. Such locations increase the bridge’s visibility exposure and are more aesthetically demanding.

The Bridge and Structural Engineer

The most important part of bridge design is the overall concept for the structure and its elements – the selection of the appropriate structural system for the bridge considering its specific function, site location, and required spans. The concept is always the most important, challenging, and creative part of engineering. Economy depends mainly on the efficient design concept. Good design concepts minimize future difficulties both in the design office and on the construction site. While experienced engineers can deliver excellent projects even without an architect, it would be preferable for engineers to work in collaboration with an architect with knowledge and understanding of bridge design and aesthetics. Few engineers have the advantage of both engineering and architectural training, with skills like Santiago Calatrava. Therefore, perhaps it is a good idea for engineers to work with an architect on their next bridge project. It is also important for engineers and architects to work closely with the builders, from the early phases of design.

An ideal solution for most bridge professionals would be a simple straight-line structure with constant depth and without any supports within the span. However, such a solution is possible only for relatively short spans, as it is limited by the strength of the structural material. The required depth for simple or continuous supported girders even at medium spans will make the depth-to-span proportion either inefficient or unattractive; therefore, requiring the use of other static systems with slender superstructures.

Renowned bridge engineers have provided the following recommendations for designing more attractive bridges [2], [5-7]: 1. The bridge should be appealing by itself, in harmony with its environment, scale and character of the site. When complementing its site environment, a bridge is regarded as a high achievement (Figs. 1 & 2); 2. A bridge system should be selected considering its surrounding – over a river, over a sea, over deep canyon, within urban areas, etc.; 3. Transition from approaches to main spans should be smooth with appropriate relation between neighboring spans; 4. Simple forms expressing the flow of forces should be used, maintaining clear order and unity for the entire structure; 5. Slender bridge elements and minimum types of elements should be used; 6. Fewer and lighter pier-supports should be used for more transparency of the substructure; Volume 47 │ Number 1 │ March 2017

11

Light and shadow effects may be used to visually enforce the slenderness of the bridge components; 8. The design should permit appropriate maintenance during the lifetime of the bridge.

Useful recommendations for bridge design are published in Bridge Aesthetics Sourcebook- Practical Ideas for Short and Medium Span Bridges [9].

harp, etc.), general aesthetic advice for the best visual design in cable-stayed bridges;

7.

4.

Pedestrian bridges, where the relatively shorter spans, combined with higher visibility and lighter loads, provide more opportunities for design creativity;

5.

Bridges with above-deck structures, regardless of whether they are arch, suspension, or cablestayed structures, have significant visual effect on everyone on the bridge deck and therefore, such bridges could benefit from close collaboration between the engineer and the architect.

6.

Secondary elements like vehicle barriers, pedestrian railings, and light poles. These are usually considered the architect’s domain; however, even the best-designed secondary elements cannot save a mediocre bridge design.

5.

Long-Span Suspension and Cable-Stayed Bridges

The Role of Architects in Bridge Design

An architect can provide essential assistance to engineers in the following aspects of a bridge project: 1.

Selection of the most appropriate structural system and the overall concept for the project;

2.

Improvement in the proportions of main bridge components – ratio of depth to span lengths, ratio of central to side spans, providing good order and proportions;

3.

4.

Form shaping of the main components – piers and superstructure elements; design of the towers and pylons for suspension and cablestayed bridges – their composition (single, two, three or more columns at one support), shape of the frames, configuration of cable-stays (fan,

The long-span bridge design is mainly governed by structural efficiency. These bridges benefit from the natural elegance of their structural systems; as stated

by Michel Virlogeux in [6], “The scale of longspan bridges alone gives them an inherent majesty.”

(Fig. 3).

Fig. 3 : From Left to Right: (a) Hardanger Bridge, Norway, 2013 with 1310 m Central Span;( b) SF- Oakland Bay Bridge, 1936, Two Central Spans each of 704 m

In suspension bridges the towers with their imposing size and shape, combined with the natural elegance of the catenary main cables, have predominance on the projected image. This “natural” inherited quality of the suspension structures is usually enhanced with appropriate articulation of the tower legs and cross-girder ties. Here again the role of an architect is helpful for achieving maximum aesthetic effect.

Well-designed towers provide a feeling of elegance and strength at the same time.

12

Volume 47 │ Number 1 │ March 2017

Architect Irving Morrow provided one of the best known aesthetic improvements to the Golden Gate Bridge in San Francisco. The Art Deco style of the bridge towers and the selected “International Orange” color significantly contributed to the fame of this amazing 1937 structure, considered even today as one of the greatest bridges ever built (Fig. 1). The Golden Gate Bridge is always on the list of the “Greatest” and “Most Famous” bridges in the world. It is one

The Bridge and Structural Engineer

of the few structures that have enhanced their site environment.

The towers of several bridges, considered as some of the best, are shown in Figs. 4 and 5; while the architects involved in their design have not received similar credit as Irving Morrow, it is obvious that the shape of these majestic and elegant towers have

benefitted from the contribution of architectural expertise. For example, the suspension towers of the San Francisco–Oakland Bay Bridge, 1936 (Fig. 5d), with stylization by architect Timothy Pflueger, have inspired the designers of the towers of Akashi-Kaikyo Bridge (Fig. 5e) the longest span in the world.

Fig. 4A : From Left to Right: Towers of Cable-Stayed Bridges, from Left to Right: (a) Normandy Bridge, France, 1995; (b) Russky Island Bridge, Russia, 2014; (c) Oresund Bridge, Denmark to Sweden, 2000.

Fig. 4B : From Left to Right: Towers of Cable-Stayed Bridges, from Left to Right: (a) Tatara Bridge, Japan, 1999; (b) Millau Viaduct, France, 2004; (c) Rion–Antirion, Greece, 2004.

Fig. 5 : From Left to Right: Towers of Suspension Bridges, from Left to Right: (a) Brooklyn Bridge, New York, 1883; (b) George Washington Bridge, New York, 1931; (c) Verrazano-Narrows Bridge, New York, 1964, (d) San Francisco- Oakland Bay Bridge, 1936; (e) Repeated Tower Shape Similar to 5d for Akashi-Kaikyo, Japan, with 1991 m Central Span, 1998; (f) Great Belt East Bridge, Denmark, 1998.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

13

For cable-stayed bridges, the towers also have a major significance in creating an attractive overall image. The relatively shorter spans for “middle long-range” structures allow more “free” treatment of the tower’s design; therefore, more creativity and originality – more possibilities for innovative work between engineers and architects. With all this said, regarding the multitude of options in suspension and cable-stayed bridges, a bridge does not need to be extravagant. Indeed, as mentioned earlier, bridges are the most “honest” constructions as their structure is exposed. Ludwig Mies van der Rohe’s principle “Less is More” is valid as much for bridges as for buildings.

6.

The Role of Builders in Bridges

In creating a successful bridge project the role of the builder is much larger than just providing the construction. It is important to involve an experienced builder3 in the team as early as possible in the selection of the bridge system. The builder can supplement the effort with his knowledge of efficient construction methods, and can prevent the designers from selecting a system that may lead to significant problems during design and construction. In addition to organizing and managing the construction, the builder often has to design detailed phases of the structural assembling, with all necessary

temporary structures. This process requires close collaboration with the (design) engineer. Once the project construction starts, the builder’s main task is to provide high quality and to deliver the project on time and within budget.

7.

Aesthetics, Efficiency and Economy

It is important to keep a good balance between the aesthetics and the efficiency and economy in bridge design. Any deviation to extremes in either direction has negative effects. For example, one of the most talented bridge designers, Santiago Calatrava, has created Alamillo Bridge at Seville, Spain for Expo’1992 (Fig. 6a). It is a beautiful but controversial bridge design, due to its deviation from basic cablestayed systems, resulting in high inefficiency. The unusual omission of back span cable-stays creates a dramatic view and contributes to the attractiveness of the bridge, but such a concept should be discouraged for any bridge that is not built as a monument.

Similar comments are valid for the Erasmus Bridge in Rotterdam, Netherlands (Fig. 6b), 1996 credited to Ben van Berkel, the architect of the bridge. While the Erasmus Bridge became the symbol of Rotterdam, no one can pretend that its structure complies with the basic rules of statics, another deviation resulting in a much higher cost.

Fig. 6 : From Left to Right: a) Alamillo Bridge, Seville, Spain, 1992, Santiago Calatrava, main Span 200 m; b) Erasmus Bridge, Rotterdam, Netherlands, 1996, Ben Van Berkel, main Span 284 m.

Some pedestrian bridges with spans 100 – 150 m (330 – 490 feet), designed with an eye to the extraordinary, have significantly higher costs per unit area than most suspension and cable-stayed

bridges with much longer spans of 500 – 1000 m (1650 – 3300 feet) [10].

According to Christian Menn and Michel Virlogeux, the art of engineering with its optimization of

In the terms of the article, builder is also an engineer, but on the construction or contractor’s side

14

Volume 47 │ Number 1 │ March 2017

1

The Bridge and Structural Engineer

economy and elegance requires creativity and fantasy, and engineers should avoid multiplied repetitive structures and illogical shapes [6-7].

Creativity in good design is essential, but “excessive originality” should only be found in justified exceptions.

A bridge does not need to be expensive or extravagant – the simplest bridge with sincere structure is often the best [11]. The elegance of bridge structures as discussed in this article can be obtained following a few basic rules, using simple forms and proportions in compliance with the static of structures. The right balance between the leading role of engineers and an important contribution of architects and builders is essential for creating a successful bridge project.

8.

Acknowledgements

This paper was originally published in the October 2016 issue of Structure Magazine (pp.8-11), a publication of the National Council of Structural Engineers Associations (NCSEA), USA. It is reprinted with the permission from both Structure Magazine and NCSEA.

References 1.

Herodotus, The Histories, University of Chicago Press, 1988, 710 pp.

2.

Walter, R. “Engineers, Architects and Bridge Design”, Structural Engineering International (SEI), Vol.6, No.2, May 1996.

The Bridge and Structural Engineer

3.

Vitruvius, The Ten Books on Architecture, Dover Publications, Mineola, N.Y. , 1960.

4. Billington, D., The Tower and the Bridge, Basic Books, Inc, New York, 1983.

5. Leonhardt, F., “The Significance of Aesthetics in Structures”, Structural Engineering International, Vol. 6, No. 2, May 1996, pp. 74-76.

6. Virlogeux, M., “Structural and Architectural Design of Bridges”, Structural Engineering International, Vol. 6, No. 2, May 1996, pp. 80-83.

7. Menn, C., “The Place of Aesthetics in Bridge Design”, Structural Engineering International, Vol.6, No.2, May 1996, pp. 93-95.

8. Mladjov, R., “Efficiency and Economy in Bridge and Building Structures, Parts I and II”, STRUCTURE Magazine, ASCE, No. 5, May 2016, pp. 20-22 and No. 6, June 2016, pp. 63-70.

9. Bridge Aesthetics Sourcebook- Practical Ideas for Short and Medium Span Bridges, Subcommittee on Bridge Aesthetics of the Transportation Research Board, Draft, March 2009, www.bridgeaesthetics.org 10. Sobrino, J. “A Bridge is More Than a Bridge: Aesthetics, Cost and Ethics in Bridge Design”, Structural Engineering International, Vol. 23, No. 3, Aug. 2013, pp. 340-345.

11. Mladjov, R., “Long Span Bridges and the Art of American Bridge Engineering”, SEAOC Convention, San Diego, CA, USA, 2009.

Volume 47 │ Number 1 │ March 2017

15

BRIDGE FOUNDATION SYSTEMS : VARIANTS & EXPEDIENTS V.N. Heggade Sr. Vice President, Gammon India Ltd., Mumbai Venkat.heggade@gammonindia.com

Summary Bridge foundations are very distinct from land based foundations for other structures like buildings and Industrial structures. Invariably, they are deep foundations in rivers and creeks warranting special construction expedients. As it happens in deep foundations, many a times the foundations are to be corrected during course of construction due to geotechnical surprises that are not necessarily classically theoretical as such are not well documented.

In the paper, an attempt has been made to document such variants & expedients.

Key words: Foundations, Investigations, Shallow foundations, Deep foundations, Caissons, Cofferdams, Hybrid foundations.

1.

Introduction

Any structure, including bridges, normally is made up of two parts, the upper being supported by lower part. The upper part is called the superstructure and lower part is called the substructure. Further, particularly in bridges, the substructure consists of foundations, shafts, columns, piers and pier caps, bearings etc. Hence, “foundation can be defined as that part of structure which is generally below the soil or water surface and distributes the loads to the earth underneath”.

16

Volume 47 │ Number 1 │ March 2017

V.N. Heggade is presently the Senior Vice President & Member, Board of Management of Gammon India Limited. He has more than 32 years of experience of designing and constructing bridges. He has more than 105 papers to his credit and is a member of various IRC (Indian Roads Congress) and BIS (Bureau of Indian Standards) committees.

He is a recipient of IABSE-Prize, in addition to a regular contributor to the ING-IABSE journal. He has passion for tall and long structures and is an ardent advocate of sustainability and aesthetics in construction. He is a Fellow of INAE, apart from receiving awards and recognition from almost all civil engineering bodies of the country.

Unlike the foundations of the land based structures, the bridge foundations are in deep waters, marine creeks, mountainous terrains with peculiar geotechnical, geological, and hydrological formations.

Hence the design and construction of bridge foundations involve four specialised civil engineering disciplines, viz., geotechnical engineering, geological engineering, hydrological engineering and structural engineering.

The art and science of foundation design and construction are not in same pace as that of superstructure for obvious reasons that the superstructures are visible, more prone for criticism and hence more attention, whereas foundation being below the visible surface, less prone to criticism and hence less attention. Perhaps, this is the reason why Terzaghi once averred, “There is no glory in the design and construction of foundations”. However, superstructure can be same wherever built while the same cannot be said about foundations. Normally, the failure of the foundations is not due to structural performance, but due to the yielding of soil supporting the foundation.

Improper soil investigation, scour estimation and consequent conception of foundation cause hazardous ramification in terms of economy and time. There are instances where artesian conditions were not taken into cognizance while conceiving the bridge The Bridge and Structural Engineer

foundation, which had led to the disappearance of the well foundation during construction. There is abundant scope of mitigating hazards and durability problems by addressing constructability issues during design solution stage. Inappropriate choice of foundations that are not construction friendly not only warrants huge time and expenditure but also prone to mishaps, the instances are: ●

Adopting open foundations in river or creek which warrant cofferdam on undulating exposed rocky strata where controlling water seepage is almost impossible.

●

Adopting well foundations in a busy area for flyovers where there is always a danger of adjacent buildings and roads sinking due to sand blowing in the wells.

●

Adopting well foundations on a sloping rock when deep foundations are envisaged and later change the entire design concept as the well sinking cannot be controlled on a sloping rock.

Further the depth of foundations is influenced by the various environmental factors such as: ●

Seasonal, expansive and shrinkage nature of soils leading to movements of foundations.

●

Soil erosion by water and wind actions.

●

The location in relation to existing structures or anticipated structures, which may be damaged by construction operations or soil vibrations.

●

Vulnerability of the structures to settlement effects.

predictions of substrata. There are also instances of over reaction in terms of conservatism leading to huge financial drain on the country’s exchequer.

The subsoil stratification is unpredictable, even to an experienced geologist. There are many examples where rocky strata are overlaying on clayey strata and the boulders in between sandy or clayey soil, misleading the strata to be hardpan. Therefore soil investigation shall be carried upto sufficient depth and the procedure of investigation shall give the complete information as to the character of the soil, water condition and state of compaction, leaving no scope for doubts. The onus of responsibility pertaining to soil investigation shall fully lie with owners lest there is absolutely no scope for the contractor to gamble about the local conditions. The kind of approach such as “Introduction is given in good faith, bidders are free to visit site and carry out his own investigation, etc.” will only leave scope for misinterpretation and consequent wrong conception of foundation.

There is now a greater awareness of the necessity of proper soil investigation and the pre-tender investigations are done in some details, but far from realistic requirements. Further, the contracts call for confirmatory borings, which needs once again to be load tested to ascertain the properties of the soil or to establish that the assumed properties of the soil are in order. However, there are still some occasions where owners do not take responsibilities for soil investigation, obliging the contractors to gamble, consequently leading to disputes and overruns in time and cost.

In the following paragraphs, Bridge foundations are discussed in details with the emphasis on aspects associated with expedients and construction.

2.2.1 Bearing medium

2.

●

Rock sound/weathered, conglomerate.

●

Coarse sand and gravel.

●

Fine sand.

●

Silt and clay.

●

Combination of any of the above.

Sub-Strata Investigations

2.1 Geo-technical Investigations1&2 In the selection of the foundation types and depths, the exploration of the geotechnical condition of stratum is most imperative which will enable the designer to decide the most economical foundation type and sites, and the constructor to decide the construction methodology and equipments to achieve economy and speed.

There are numerous occasions where the improper subsoil exploration has led to the faulty conceptualisation and construction. Quite often the cost and time overruns are due to the unrealistic The Bridge and Structural Engineer

The bearing medium may be broadly classified into five categories.

Whenever, the rocky strata is available at reasonable depth, it is quite convenient and safe to rest the foundation on top of rocky strata as the crushing strength of rock is normally higher than foundation material. The rocky or hardpan founding medium offers unyielding strata, avoiding the complication of the effects of differential settlements on the structure.

When deep foundations are embedded into rocky

Volume 47 │ Number 1 │ March 2017

17

strata, apart from the very high end bearing capacity, the side friction derived for bearing is quite enormous compared to other bearing mediums. In case of rocky strata, it is essential to establish whether the strata is sloping and if so to what extent or degree, before taking up the execution of bridge foundations. There are instances where well foundations are designed on steeply sloping rocky stratum which had to be abandoned due to excessive tilts and shifts.

Gravely and coarse sandy strata are vulnerable to scouring conditions, though this is next best suited as bearing medium, to that of rock and hardpan.

The characteristic of fine sand as bearing medium varies according to the water content in it. In some conditions, it is quite stable whereas under some other condition such as exposed to vibrating foundations, thorough investigation needs to be done.

The extremely fine-grained strata, i.e. soft and clay are considered to be poor bearing medium as they are prone to pronounced settlement. The consolidation settlement in clay due to vertical compression occurs over a long period of time. The clay being a highly impermeable material, the squeezing out of water entrapped within the fine grains (dissipation of pore pressure) is a time consuming phenomenon. However, normally the vertical compression does not lead to the elastic deformation of silt or clay particle itself resulting in crushing.

The bearing capacity of the soil is related to shear failure in the ground. For the foundations in clays undrained shear strength is the criteria as for the clays with low permeability, the construction generally takes place under undrained condition. After the end of construction, along with the consolidation, the clay gains more strength with time. However, in case of granular soil, owing to very high permeable nature, due to the speedy dissipation of pore pressure, the drained condition has been obtained faster. Thus, for foundations on clays either the shear failure or settlement may govern the design whereas for the granular soil, choice of the bearing pressure is virtually controlled by the settlement criteria for large areas of foundations.

It is vital to distinguish between gross and net foundation pressures.

IRC:78-2000 clearly distinguishes the gross and net bearing pressures.

‘Gross bearing pressure is stated to be the total pressure at the base of the foundation on soil due to possible combination of loads and the weight of the earth fill, whereas Net pressure intensity is defined to 18

Volume 47 │ Number 1 │ March 2017

be the difference in intensities of the gross pressure and the original overburden pressure.

There are many variations of soil bearing capacities provided in National Codes and soil investigation reports. The internationally accepted soil bearing pressures are explained below.

The failure of the foundation strata is defined only in case of shear. Therefore the ultimate bearing pressure is the value at which the ground fails in shear.

The maximum safe bearing pressure is the intensity of applied pressure that the soil will safely support without risk of shear failure irrespective of the settlement which may develop, with a factor of safety.

The allowable bearing pressure is the allowed intensity of applied pressure, taking into account maximum safe bearing pressure as well as settlements.

The change in the natural structure of the soil, depth and width of the foundation also influence the bearing pressures to a large extent. In the undisturbed state and also due to the large failure surface area in case of deep and wide foundations, the bearing capacities can be much larger than calculated by various bearing capacity theories.

2.2 Hydrological Investigation4 Indian subcontinent is full of rivers which have to cater for extreme rainfall that are very intense but infrequent and spatially highly variable. Water depths in some rivers may vary from zero to 15 – 20 m in a few days or even 24 hours. Velocities may attain 6 m/s and flow directions may change considerably within a channel during a flood, particularly near river bends.

This warrants proper hydrological investigation in terms of : ●

Hydrological data upon which to base estimates of the magnitude of floods for design purposes.

●

Hydraulics of flow in alluvial rivers and flow through bridge waterways and around bridge piers.

●

Reliable methods for estimating scour at bridge piers.

●

Prediction of the occurrence of impact and accumulation of debris against the bridge structure.

The ignorance of the above may result either in underestimation leading to catastrophe or overestimation leading to scouring of national exchequer. In this connection, it is demonstrated The Bridge and Structural Engineer

below by a case study that “bigger and better codes of practice” would not help in the design and construction of flood-resistant structures because every case is unique. A better solution could be to improve the training of engineers in hydrology and the hydraulics of waterways.

In one of the states of North East a 3.0 km bridge with multiple spans of 120 by cast in-situ balanced cantilever method is under construction (Fig. 1).

The new bridge is 80 m down stream of an existing bridge with the same span arrangement having piers parallel to the existing bridge. The earlier bridge was provided with a guide bund one side as the other side was having non erodible hilly terrain. For calculations of the scour depth for the earlier bridge regime width was considered as linear waterway, as the non-erodible bank was not available on one side to cater for the meandering river. Since the new bridge which is 80 m downstream is flanked on either side by non-erodible banks due to already constructed guide bund for the existing bridge, the linear waterway to be considered for calculation of scour depth becomes the distance between the non-erodible banks minus summation of obstructions provided by piers instead of regime width empirical expression given in IRC codes that has a constant ‘c’ varying from 4.8 to 6.3. If this empirical expression is insisted upon for linear water way calculations, the difference in the foundation depth works out to be more than 10 m.

bridge engineers are somewhat reluctant to use this data. However, in the absence of correct silt factor for boulderly bed there is a problem wherein the selection of this important parameter is left to the judgement, discretion and experience of the designer.

Also, the results obtained by the above formulae for bouldery bed are erratic and impracticable. In many of the projects, the silt size used for calculating the silt factor to be used in Lacey’s formula are of the size of a fully grown man as could be seen from the Fig. 2. As the scour depth and founding levels are based on this formula, the foundations cannot be humanly installed up to founding levels.

Fig. 2 : Scour Depth & Silt Factor for Bouldery Strata

2.3 Geological Investigations

Fig. 1 : under Construction Bridge 80 m D/S from Existing Bridge

In sub-Himalayan terrain of Himachal Pradesh, Jammu & Kashmir, and North East states, the strata is not exactly rocky but the boulders cemented by soil matrix forming some kind of conglomerate. From the surface, to a large depth the stratum is uniform.

Silt factor plays a significant role in finalising the scour depth and also the founding levels of the bridge structures. Due to various uncertainties associated, the bridge engineers are confronted with the difficult job of choosing an appropriate value of silt factor.

The IRC Codes cater for a maximum silt factor of up to 2.42 (Applicable for heavy sand) only. Though IS 7784 (Pt-I) gives a increase beyond the range of 2.42 in discrete jumps of 4.75, 9, 12, 15 & 24, The Bridge and Structural Engineer

Fig. 3 : Top: Final GAD of Anjikhad Bridge. Bottom: Approach Foundations Completed Prior to Stop the Work

In the Himalayan hilly terrain, in addition to geotechnical investigation, Project Geomorphology and Geology needs to be studied. Normally the terrain is an alternative bands of shale and sandstone or boulders studded in soils. In addition, the presence of hot springs and artesian conditions needs to investigated. The detailed seismic studies including Volume 47 │ Number 1 │ March 2017

19

the active seismic shear faults should be carried out. The absence of the these studies have led to the abandoning of projects half way through in many instances, as has happened in the case of Anjikhad bridge (Fig. 3) due to treacherous geological conditions encountered during execution.

3.

Shallow Foundations

The foundations having the depth from the surface, lesser than twice the width may be generally defined as shallow foundations, though the same may not be a mandatory requirement. Shallow foundations are seldom called open foundations as the soil or rock upto founding level is openly excavated. In case of spread foundations, the masonry of structure is made wide at the bottom giving suitable steps whereas RCC footing is nothing but an RCC slab spreading load over a larger area. The RCC raft is a much wider common slab, supporting more than one structural element or columns.

The shallow foundations are relatively economical and speedily constructible, but do not necessarily provide rigid and unyielding support, when they are resting on soil stratum.

Previously, it was a general practice to make shallow foundations with plain concrete or solid masonry.

Because of the low bending strength, the depth of the foundation required used to be very high. Gradually, the plain concrete or masonry foundations were replaced by steel grillage foundations and RCC foundations.

In bridges where open foundations5 are adopted under water, cofferdams are (Fig. 4) constructed to retain the soil from collapsing into the foundation pit.

The cofferdam is a temporary structure enabling the exclusion of water in a given area with a reasonable amount of pumping, providing sufficient working space to build the foundation. The cofferdams are usually constructed in place and normally depend upon the natural bottom strength. The walls are provided with bracings in braced cofferdams (Fig. 5) to resist the lateral pressure. In some cases, precast bottom is extended by steel follower. The walls of the cofferdam must be tight and the soil at the bottom should be impervious to get water-tight enclosure. If the soil at the bottom is not impervious, a layer of concrete can be put at the bottom and the pumping water can resume immediately after the hardening of concrete. This method can be used up to the depth of 5.0 m in water.

20

Volume 47 │ Number 1 │ March 2017

Fig. 4 : Cofferdams for Open Foundations

Fig. 5 : Top Left: Braced Cofferdam, Top Right: Precast Concrete with Steel Follower, Bot: Transportation & Placement of Precast Conc Cofferdam at Location

When the greater depths are encountered, the steelcaissons fabricated in the yard are floated into position (Fig. 4). After proper positioning, through the water ballast, the caisson is lowered and penetrated into the subsoil. Then the inside of the cofferdam is dried up by de-watering pumps and the construction of open foundation is carried out in the conventional manner.

Quite often the cofferdams are to be rested on the undulating rock surface below the water. For shallower water depths, concrete cofferdams are adopted (Fig. 5). Since the cofferdams are resting on the rock mass, it is quite a challenging task to control the seepage of water inside the cofferdams, by various methods including the concreting of space between cofferdam resting edge and rock surface. Puddle marine clay bags (Fig. 6) packed along the periphery of resting edge of cofferdam could be used to enable the exclusion of water from foundation area.

Fig. 6 : Shoring & Waterproofing Arrangement Inside

The Bridge and Structural Engineer

The cofferdam shall be designed in such a way that the aggregate cost of construction, maintenance, and pumping is minimum. It is not imperative to obtain completely watertight cofferdam, as the cost of the same is extremely prohibitive. It is impossible to completely stop the water running between the rock and the bottom of the cofferdam, when the cofferdams rest on the rock. However, if the foundation is on clay, sheet piles driven sufficiently down below should stop the leakage.

When deep excavations and removal of excavated materials are required to be carried out for shallow foundations, the grabbing cranes, winches and buckets are generally used. The side slopes are protected by strutting, shoring, and sheet piling depending upon the depth of excavation.

the ground level. The foundations are 23 m diameter having an edge thickness of 2.5 m and tapering to pier diameter of 5.5 m in a height of 2 m, thus making the total depth of the foundation to 4.5 m.

For each foundation 28 m square × 18 m deep sheet pile cofferdam (Fig. 8) was constructed by driving AU 25 sheet piles by vibro-hammer. As the excavation progressed in stages through sandy and clayey strata, till the rock level at around 19 m depth was reached, the cofferdam was stabilised by struts and whalers at four designed levels. Since the sheet piles could not get sufficient embedment in to the rock, the same were pinned by Toe pins made up of 230 mm diameter pipes having ISMB 200 sections inserted and pressure grouted.

For the de-watering in shallow excavation, the normal pumps with 75 to 150 mm delivery pipes are adequate. Deeper excavation warrants submersible pumps. These pumps are submerged in water during de-watering and have an integrated electric motor and pump fitted with foot valve. Being devoid of separate suction pipe and foot valve, and lesser in weight compared to conventional pumps, they can be easily handled.

Boats, barges, pontoons, launching tugs, anchors, and floating crafts, etc., are some of the plants and equipments, which are invariably, used in the construction of shallow foundations in the waterbound bridges.

Fig. 8 : Sheet Pile Cofferdam for P19 Foundations

While water table at the location was reduced by well point (Fig. 9) with a system consisting of deep bores and five HP pumps outside the cofferdam, open dewatering was carried out using 35 HP pumps inside the cofferdam. After having brick wall as shuttering, 1652 cubic metre of nonstop concreting was completed in 40 hours. However fixing of 280 t of reinforcement took 40 days.

Fig. 7 : Signature Bridge P19 Foundations

The signature bridge which is under construction on Yamuna river at Delhi has two massive independent open foundations P19 to support peculiar pylon legs (Fig. 7) that are resting on rock at around 20 m below The Bridge and Structural Engineer

Fig. 9 : Dewatering & Concreting of P19 Foundation

Volume 47 │ Number 1 │ March 2017

21

4.

Deep Foundations

In the shallow foundations, the soil support is applied near the usable portion of the structure whereas in deep foundations6; the soil supports are applied far deeper from the usable part of the structure.

Invariably, deep foundations are adopted wherever good bearing medium is not available at shallower depths. Therefore “Deep foundations can be defined as a type of foundation which transfers building loads to the earth farther down from the surface than a shallow foundation does, to a subsurface layer or a range of depths”.

Fig. 10 : Deep Foundation Variants

Deep foundations are generally classified into three categories: ● Well/caisson foundations ● Pile foundations ● Hybrid foundations (Combination of the above two)

The well foundations are structural elements, which are wholly or partially constructed at higher levels at the location of its position itself and sunk to the desired level by various construction expedients, whereas caissons are structural elements which are wholly or partially constructed at higher levels in a fabrication yard, floated to the final position and sunk to the desired level by various construction expedients.

Thus, though there is a subtle difference in the definition of caisson and well foundations, in the succeeding paragraphs, both will be termed as caisson/ well foundations.

The well foundations are very popular for bridges in India, compared to its counterpart pile-foundations, because of its obviously apparent robustness. On the other hand, piles are preferred option in Western and European countries. Wells are termed as monoliths in European parlance, whereas the same are called cylinders or drilled shaft in American practice.

The structural element of caisson/well foundations consist of: ●

Cutting edge

●

Kerb

●

Steining

4.1 Well Foundations & Caisson Foundations

●

Bottom plug

Well/caisson foundations are very popular in Indian subcontinent due to its sturdiness and robustness.

Over the period, the design and construction practices are streamlined and codified. Apart from the bridges, transmission tower foundations and intake wells, these specialised foundations are sporadically exploited even for flyovers and water tanks (Fig. 11).

As the spans for the bridges increase, the caisson/well sizes also increase. In India, for the Second Hooghly Bridge, 24.0 m dia wells have been adopted and till now the deepest well/caisson constructed is up to 70.0 m below water level ((Fig. 14).

●

Top plug/Intermediate plug

●

Caisson/Well cap

Fig. 11 : Application of well Foundations, Right Bottom: 24 m Dia Second Hoogly Bridge Caisson

22

Volume 47 │ Number 1 │ March 2017

The wells/caissons have strong kerb and cutting edge to facilitate sinking. The inside sloping face of the kerb and cutting edge enables reverse arch action to the bottom plug. The provision of steel strakes in the cutting edge and kerb is essential to avoid cracking when the caissons pass through hard strata and requires blasting for sinking. Normally the caissons are circular in shape to facilitate streamline flow barring in some cases like the large lateral thrust due to earth pressure has to be resisted in case of abutments. In these cases, single or multiple cell rectangular or double ‘D’ shaped caissons are provided. When the caissons are founded in alluvial and clayey strata, the stability of the well is achieved with a minimum grip length of 1/3rd the total height, while caisson founded on hard rock are normally embedded into rock by 150 mm, and 33% tension is permitted at the founding base after the redistribution. The dredge holes of the caissons are filled up with tremie concrete or pre-packed grouted concrete, which is known as colcrete.

The Bridge and Structural Engineer

Well/caisson foundations are further classified into four categories by virtue of its construction expediency. ●

Conventional construction on land

●

Sand islands

●

Floating caissons

●

Pneumatic sinking

In the open type of foundations, the cutting edge fabricated of mild steel is laid on the ground level and curb sufficiently reinforced to suit the strata is concreted. The material inside is gradually scooped out to facilitate the sinking under its own weight. As the sinking proceeds, the steining is built up in lifts, normally of around 2.5 to 3.5 m to further the sinking due to increase in weight. Sometimes Kentledge load is used in order to facilitate sinking. If the foundation interior is divided into a number of compartments, the drifting, i.e., shift and the tilting, can be controlled by dredging the appropriate compartment. Normally, the tilt of 1 in 80 and shift of 150 mm is considered to be tolerance limits and the design caters for the same.

Cranes and grabs (Fig. 12) are the standard equipments deployed for well sinking. In the absence of cranes, the builder hoists, with apt capacity of double drum winches, are also employed. The essential requirement of the grab is that it should bite into the soil and can be closed when operated from top. The length and breadth in both opened and closed condition shall be as small as possible, yielding maximum quantum of earth carrying capacity. The grabs can work satisfactorily under the condition of soft sand and soft clay. However, when encountered with soft rock, stiff clay and kankars, chisels are employed for loosening the strata.

Fig. 12 : Well Sinking by Mucking, Kentledge & Steining Building

When well foundations are chosen in perennial river, where the velocity and depth of the water is low, sand islands or artificial peninsulas (Fig. 13) are constructed, providing sand bags to support the sand in the location of pitching of cutting edge.

The sand island or construction of cofferdam bund is an ingenuous application, which is in practice in India for many years. Here, the island is made to bring the ground level above water level and wells The Bridge and Structural Engineer

are constructed on the level surface. The rest of the methodology of construction of well is similar as explained earlier.

Fig. 13 : Islands or Artificial Peninsulas for Kerb Casting

4.1.1 Floating caisson method At depths more than 5.0 m, the bottom section of steel caisson (cofferdam) is normally constructed in a fabrication yard (Fig. 14), brought to the edge of the river on a rail track and slipped into river via a judiciously designed tilting slipway and floated to the location. The floating caissons are designed to achieve the desired stability and the draught requirement during floatation. In some cases, the floating gantry mounted on barges is used for handling and transporting these floating caissons.

Fig. 14 : Fabrication Transportation, Pitching & Concreting of Floating Caissons

Once in position, the caisson is lowered by filling up the annular space between the double wall with water to sink and embed to the riverbed. Further the space is filled up with concrete due to which the sinking progresses. As the caisson sinks due to concreting of steining in lifts, the further formwork sections are added and the process continues. In some cases where concreting requirement is very high, floating batching plants, with placer boom arrangement, are employed.

In some cases like in Bogibeel bridge at Dibrugarh, a temporary platform/cofferdam (Fig. 15) at the edge of the river is used for floating the caissons. Once the caissons are assembled on temporary platform or

Volume 47 │ Number 1 │ March 2017

23

cofferdam, the platform is broken so that caisson is floated in the water and then towed to the location by tugs and held in position by tethering arrangement for concreting.

used for treatment of workers suffering from caisson sickness. It is always ensured that the pumped in air is fit for human breathing by provision of filters for purification of air.

4.1.3 Jack down method of sinking Well sinking is normally done by gravity method.

IRC:78-2014 gives an empirical expression, where steining thickness is calculated on the basis of the kind of soil strata encountered which overcomes the frictional resistance offered by surrounding soil, and the well sinks by gravity.

By adopting jack down method (Fig. 17) of sinking, it is possible to reduce the thickness of the steining.

Fig. 15 : In Bogibeel Bridge, Caissons Assembled on Temp Platform, Platform Broken and Towed by Tugs

4.1.2 Pneumatic sinking method

Fig. 16 : Well Sinking by Pneumatic Pressures

Quite often, open sinking is not a feasible option, whenever grabbing at higher depths needs to be carried out under water, or when boulder or rocky strata is encountered during sinking. To overcome these difficulties, it is necessary to have people working inside the caissons/well, which warrants exclusion of water in the working place. The pneumatic or compressed air caisson (Fig. 16) is closed at the top by a steel or concrete air deck and the inside water is pushed out by pumping compressed air at the designed pressure, with the maximum limit of 3.5 atmospheres, beyond which human beings are vulnerable to fatal caisson sickness. The pneumatic sinking arrangement consists of air locks for entrance of men and materials, medical lock, compressors, generators and tools for sinking. A compressed air chamber is provided for the workers to get used to the working pressures. This chamber consists of two interiors where the first interior is pressurised from atmospheric pressure to working pressure after the entry of workers and the second interior is already pressurised to the working pressure. This pneumatic chamber is normally tested for 1.5 times the maximum working pressure and is accompanied with a medical lock chamber at the platform level. This chamber is 24

Volume 47 │ Number 1 │ March 2017

Fig. 17 : Typical Jackdown System

In this system, a downward pressure is applied on the well by centrally held jacks that are connected to ground anchors through gripper rods. Jack down method of sinking has many advantages over the conventional gravity sinking. ●

Faster sinking compared to conventional method.

●

Controlled sinking and prevention of tilt and shift of well.

●

Sinking of non-uniform thickness of well steining.

●

Easy sinking in hard stiff clay strata in deeper depth.

●

Easy rectification of tilt and shift.

●

Prevention from jumping of well.

●

No kentledge requirement.

●

Minimal influence zone during sinking and ascertain less chances of blow.

●

Nominal crane working hours and no additional crane required in big size well.

The Bridge and Structural Engineer

4.1.4 Tilt and shift rectification of wells10

●

Pulling by winches/prestressing, etc.

Sinking of wells for foundations demands experience and warrants special expertise. Some of the problems associated with sinking of the well foundations are:

●

Excavation on opposite side of tilts

●

By the application of pneumatic pressures (Fig. 20)

●

Sinking through clay, soft rock & boulder strata.

●

Cracks in steining & expensive corrective method.

●

Jumping of well, consequent damages

●

Dewatering/blasting for sinking, and consequent kerb cracks.

●

Hazards of pneumatic sinking, casualties

Well foundations are designed for the tolerance limits of 1 in 80 tilt and shift of 150 mm at the founding level. Among all the problems associated with well foundations, excessive tilts (> 1:80) and shifts (> 150 mm) lead to rejection of wells, redesign of well caps and spans as such warrant rehabilitation/ correction, though time consuming.

There are instances where the originally envisaged spans had to be doubled due to abandoning of wells (Fig. 18) because of the excessive tilts and shifts which could not be rectified as in one of the bridges near Mumbai and for a ropeway project in Guwahati.

Fig. 18 : Abandoned Wells due to Excessive Tilt & Shift

Over the period contractors have evolved numerous tilt and shift rectification methodologies that can be categorized as (See Fig. 19):

Fig. 20 : Rectification by Pneumatic Pressure

In one of the railway bridges in Western part of India, a novel solution was used reversing the principle of pneumatic sinking. The well was tilted to the inclination of almost 1:2. All the other measures of rectification, such as Kentledging, pulling by prestressing, and excavating within and outside the wells, failed. The contractor was almost on the verge of abandoning the well, which meant huge time overrun and cost overrun, including redesign of the spans, as at the same position foundation could not be located.

As shown in the Fig. 20, the top of the well was sealed with a combination ISMB girders and steel plates, making the chamber inside the well airtight. The provision was made in the lid to let the compressed air into the chamber. Structural steel towers were erected on top of the lid for winches which were attached to anchor blocks.

When the pneumatic pressure was created inside the well, by letting compressed air inside pushing the water outside, and the well pulled simultaneously by winches, at around 2.5 kg/cm2 pressure the well suddenly got popped up and started floating, as shown in Fig. 20.

4.2 Pile Foundation6

Fig. 19 : Kentledging, Pulling, Judicious Excavation

●

By eccentric kentledging.

The Bridge and Structural Engineer

Pile foundations are the most ancient discovery of deep foundations, and are very popular in Western and European countries. They may be defined as column type structural support, which may be Volume 47 │ Number 1 │ March 2017

25

pre-fabricated or cast-in-situ. Piles can be classified into four categories on the basis of its construction. ●

Pre-fabricated driven piles

●

Driven and cast-in-situ piles

●

Bored piles

●

Bored pre-cast piles.

suitable segments, and lengthened by on site welding.

The process continued, till the founding level was reached.

The first two types are called displacement piles while the bored piles are known as replacement piles.

Normally, the ground conditions, location and type of structure influence the choice of the pile type and the installation technique. For example, pre-fabricated driven piles are chosen for loose water-bearing sands and gravels through which they can be driven easily. The same is not suitable for bouldery and hard strata where the driving would be hardly possible.

The tubular piles having low ground displacement are adopted where deep penetration is required in dense sand and gravels. In stiff clay, very dense sand, and gravels and also where piles are to be socketed into the rock, bored cast-in-situ piles are invariably preferred.

In India, for bridges invariably bored cast in-situ piles are used, rarely bored precast piles are adopted.

The size of the piles adopted for bridges varies from 0.80 m dia to 2.5 m diameter, and the depths could be as high as 70 m. Piles are connected to the pile caps to ensure necessary rigidity against transverse deflections. Pile caps are usually laid with the soffit at low water level to facilitate the ease of construction, thereby minimising the cost.

4.2.1 Small replacement piles Tabular RCC or PSC precast piles, steel piles and also sections like ‘H’ or ‘box’ are also termed as small displacement piles. Tubular piles are preferred for marine structures where they can be fabricated and driven in large diameters to resist lateral forces.

The Dolphin Jetties at Chitgong, Bangladesh (Fig. 21) steel tabular piles of BS43A grade were used. The approach jetties have vertical piles of 0.60 m diameter with varying depths of up to 25.0 m, whereas in breasting and mooring dolphins, the horizontal loads are resisted by raker piles with the maximum rake upto 1:5.

The piles for the breasting dolphins are of 1.0 m in diameter with maximum lengths of 35.0 m, penetrated into medium to dense sandy soil. These piles are driven open ended by vibratory hammers in 26

Volume 47 │ Number 1 │ March 2017

Fig. 21 : Piling for Dolphin Jetties at Chittagong

Prefabricated structural steel pipes were brought to the unloading point by trailers, and using hydro/ crane this pipe segment was unloaded to the material handling pontoon. This was lifted up by floating barge-mounted crane, and through the guides fixed to temporary platform on temporary piles; the piles are pitched to initial position. Using vibro hammer, which is fixed at the top of the pile, the pile segment was driven to the level of 2nd guide. Vibro hammers, consists of pairs of exciters rotating in opposite direction, which can be mounted on pile. Their weight combined with vibrating energy sinks the pile into soil. Vibratory hammers are very effective for driving small displacement piles into large or medium sand.

Ideally a pile should be vibrated near the natural frequency for the maximum efficiency.

The energy absorbing properties, size range and high resistance to lateral loading and buckling of these piles, combined with modern corrosion protection techniques like cathodic protection make them particularly suitable for oil and gas production platforms, jetties, piers, and mooring dolphins. The large diameter piles can achieve very high bearing capacities.

However, there appears to be various interpretations with regards to the bearing capacity calculations of small displacement piles.

Normally, the method of installation of displacement piles tends to compact granular surrounding, and hence the bearing capacity coefficient ‘K’ increases.

A critical component in the evaluation of unit skin friction is the coefficient lateral pressure ‘K’ of the soil. The different techniques used for the pile The Bridge and Structural Engineer

installation may have a profound effect on the value of ‘K’. Recommended values of ‘K’ to be used for the evaluation of pile skin friction capacity vary from Kop to Kp where Ko is soil pressure coefficient at rest and Kp is passive pressure coefficient.

After extensive experimentation, Levancher and Sieffert (1984) proposed that the ‘K’ value may be estimated using k = ko Kmo, where kmo is the modifying coefficient depending upon the installation method of piles. It was concluded that Kmo varies between 2 to 3.

On the other hand, in clayey strata, the soil in the immediate vicinity of the driven pile is displaced and usually remoulded a certain distance from the pile side. In this zone, the pore pressure caused by pile driving normally dissipates quickly, and the soil may regain much of its original shear strength after consolidation. Again, this may not happen in stiff consolidated clays. In addition, in clayey strata, during driving, the soil structure is disturbed thereby reducing the shear strength.

Though the ‘Nordlund’s method”, while calculating the coefficient of earth pressure, takes into account the volume of soil displacement by a unit length of penetration, till the introduction of ‘IC’ method, there was no clear guidelines for working out the friction capacity for open ended driven piles.

Fig. 22 : Jamuna Bridge at Bangladesh

The ‘IC’ method was specially developed for the Jamuna River Bridge in Bangladesh (Fig. 22), by the Imperial College, London. The method is sound and based both theoretical and practical considerations and particularly takes into account the effects of soil displacement, and dilation. Different equations are given for working out the frictional resistance of soil for open ended driving and closed ended driving, accounting for the volume displacement of soil. The The Bridge and Structural Engineer

external shaft resistance for tension loading on openended piles is considered approximately 10% lower than compression loading. This is based on CPT values. However, in the absence of CPT results, it is recommended that CPT values may be computed using the relationship with SPT, available in literature.

4.2.2 Large replacement piles The replacement piles, i.e. bored cast in-situ or precast piles have large capacities and vary in size in India from 0.80 m to 2.50 m diameter. The bores are usually formed by percussion or rotary driving methods.

Though the percussion method is considered to be a primitive method, in certain circumstances where working place is not available, it is an effective and economical construction expediency for replacement piles.

In this method, bores are formed by tripod rigs or A frames, and operating the cutting tools like bailer and chisel through single drum winch (Fig. 23). Bailer is a heavy tube having flat valve at the bottom and the cutting edge is used for advancing the bore in the soil, while the valve helps in mucking out activity. The chisel is used for boring into the rock and normally the weight of the chisel in tonnes is double the size of the bore in metre.

Fig. 23 : Conventional Percussion Method

In the case of Thane Creek Bridge near Mumbai (Old Thane Creek Bridge), 2.50 m diameter tubes were bored into rock using Hoschtrasser system of piling. Hollow cylindrical RCC pre-cast piles were transported through crane mounted barge (Fig. 24) and pitched into position. The piles were anchored into the rock by anchor rods and grout. The tubes were subsequently withdrawn without disturbing the grouted piles.

Volume 47 │ Number 1 │ March 2017

27

rotary head. The casing is rotated and pushed down as the boring continues. The soil boring tools have to be changed depending upon the strata encountered. Once the boring is completed, the bore is cleaned using cleaning bucket, the reinforcement cage is lowered, and the concreting is done by the tremie method.

Fig. 24 : Pitching of Precast Pile (Hollow) of 2.5 m Dia

Due to the availability of heavy duty augurs, the mechanical or hydraulically operated rotary rigs are extensively used these days for large diameter boredcast-in-situ piles. The mechanical rigs, which are generally crane mounted, can bore through soft rock, having strength up to 1,000 t/m2.

On the other hand, state of the art hydraulic rigs can bore into the hard rock, having strength up to 5,000 t/m2. However, recently it has been reported that these rigs were used to bore in rocks having strength up to 14,000 t/m2. The hydraulically operated rigs (Fig. 25) are generally crawler mounted, and can bore upto 60.0 m depth and 3.0 m diameter. Unlike, mechanical rotary rigs, these rigs can also install casings during the boring operation.

For maintaining the stability of the drilled bores, temporary casings are generally used up to 8.0 m depth, and in some cases, the casings are used to the full depth. In highly collapsible strata, the permanent liners are insisted upon as the extraction of the liner, after setting of concrete, may damage the concrete shaft itself.

When the bentonite suspension with 6% weight is used to protect the sides, especially in sandy soil like in Delhi, where many flyovers are in progress, normally a positive head of 1.5 m above the water table needs to be maintained. Generally, for underwater concreting, the tremie method is quite popular. In this method, the concrete, having a minimum slump of 150 mm, is poured through tremie pipes having diameters of 150 to 200 mm. The essential condition is that the pipe is kept always well below the concrete and water interface, and the green concrete continuously displaces water or bentonite suspension from below.

4.2.3 Large replacement piles in water bodies When the water depth is shallow or if it is possible to close the river or creek even at deeper depths, like in the case of Godavari bridge (Fig. 26) at Rajmundry, artificial peninsulas are created to mobilise and place the rigs for carrying out the piling.

Fig. 25 : Modern Rotary Rings for Piling

The essential boring tools of these rigs are soil or rock augur, bucket and core barrel, and cleaning bucket.

The rotary rig fitted with suitable tool is moved into the location of the piling. After establishing the reference point, verticality of the mast, etc., the boring is commenced. Once the bore depth reaches around 2.0 m, the casing or liner with the internal diameter, generally around 50 mm higher than the pile diameter, is installed using special attachment to 28

Volume 47 │ Number 1 │ March 2017

Fig. 26 : Artificial Peninsulas for Piling in River

Marine piling is normally done by either end on gantry (Fig. 27) method or spudded barge technology. In the end on gantry method, the gantry moves on the wheels of the well box, which is fitted on top of already driven piles. If the distance between the piles in the direction of the piling is large, temporary piles are driven to The Bridge and Structural Engineer

support the gantry on the wheels. The front end of the gantry cantilevers enough to support the piling rig, which is ready for continuing with the diving of liners.

Either percussion method or rotary boring could be employed in this arrangement. However, driving of casing or liner upto refusal is very essential, as the concreting has to be done under water, and Bentonite suspension for side collapse cannot be used.

Fig. 27 : Left: spudded Barge, Right: End on Gantry

In spudded barge method of piling (Fig. 27), the condition is that the pipe is always well below framed decks supported on pontoons and spudded into the riverbed. The pontoons are filled with water to attain higher stability before the piling. The frame is transferred after dewatering the pontoon, following the completion of the piling, in each location.

5.

Steep sloping rocky strata are encountered at founding level.

●

For enhancing foundations.

the

capacity

of

existing

5.1 When All Tilt and Shift Rectification Measures Have Failed In one of the foundations in Western India for 5.5 m diameter well, the resultant positional shift was 993 mm (Fig. 28). The well foundations are designed for the shift of 150 mm and tilt of 1 in 80. In this case, the tilt was 1 in 420 which was well with in the limits of permissible 1 in 80. For the resultant eccentric pier position, the stresses in the well steining as well as bearing pressures were well within permissible limits.

However as the shift was very high and it was not possible to pull back the well with in shift limits, hybrid solution by way of connecting the cap to 3 numbers of 1.2 m diameter bored cast in situ piles were adopted.

Hybrid Foundations10

The selection of the foundation type is very imperative for deep foundations, which is normally dictated by the constructability and construction feasibility.

Quite often in a river or creek, where the bridge foundations are to be laid on the rocky strata, caissons or open foundations are selected depending on the degree of shallowness of the founding strata. This solution is extremely inappropriate where the rocky strata is undulating abruptly, so much that within the foundation area, the level difference of the rock could be to the tune of 5.0 to 10.0 m. In such cases, it is almost impossible to stop the seepage for exclusion of water, within the cofferdam, where open foundations are adopted, and caissons where bottom plugging has to be done. Under such circumstances, pile foundations are most appropriate as the base area of the individual piles is comparatively much smaller.

The combination of pile foundation with well or open foundation is known as Hybrid foundation.

In majority of the cases hybrid foundations are necessitated because of the inappropriate selection of foundation and unforeseen circumstances realised during execution rather than a design choice.

There are three distinct circumstances where this technology is used for bridge foundations: ●

●

All the tilt rectification measures have failed.

The Bridge and Structural Engineer

Fig. 28 : Piles Provided to Arrest Further Tilt after Plug

Fig. 29 : Pentagonal Cap for Hybrid Foundation

In another case in Eastern India for a tapered well of bottom diameter of 7.5 m (Fig. 29), the tilts and shifts were beyond permissible limits. However, the stresses in the steining as well as bearing pressures were well within the limits. Well P4 was tilled from 1 in 155 to 1 in 76 in A2 direction and 1 in 171 to 1 in 32 in U/S direction during sinking of well from RL -28.142 (i.e, 25.744 m from GL) to RL -33.28. Well was also shifted due to tilting from 918 mm in A2 direction and 1389 mm in U/S direction. Tilt and shift rectification were tried by Kentledge method as well Volume 47 │ Number 1 │ March 2017

29

as pulling well by prestressing cables. When both methods could not yield the results, the well had to be strengthened by additional piles of 1.5 m diameter, surrounding the well, and integrated by a common pentagonal cap.

5.2 When Steep Sloping Rocky Encountered at Founding Level

Strata

are

There are many instances where well foundations are conceived on steeply sloping rock, which culminated either in deserting of foundation or in some the project itself. In the Joghigopa Bridge, Assam and the Signature Bridge, Delhi, which is currently under construction, novel solutions in the form of hybrid foundations were designed and constructed, when encountered with a similar situation.

At the Jogighopa Bridge7, on river Brahmaputra in Assam, out of the 19 intermediate well foundations of the 2,285 m long bridge, 17 foundations were 11 m x 17 m double ‘D’ shaped wells. These wells were sunk up to 67 m depth in the river bed through predominantly sandy/silty strata. At locations 17 and 18 however, hard rock strata existed at a depth of 40 m and that too in a steeply sloping condition (Fig. 30).

The foundations were on exposed rock with transverse force of about 5000 t.

● ● ● ●

Remove the sand in dredge-hole by grabbing and air-lifting so as to clean the entire area including that below the well curb.

Construct the concrete bottom plug.

Construct the balance six piles to complete anchoring of the foundation.

Construct RC plug over the bottom plug in dry condition after dewatering the well.

Jet grouting (Fig. 31) utilizes energized fluid jets, usually water surrounded by compressed air. This jet erodes the surrounding soil and fine particles of loosened soil are removed by airlift pressure.

The remaining soil is simultaneously mixed with cement grout to form a column of material which has properties similar to that of a low strength concrete, that is, up to about M20 grade.

Fig. 31 : Jet Grout Shaft, Nozzle & Column

Fig. 30 : Pile Integrated Wells for Joghigopa

Hybrid foundations were designed and the following construction sequence was adopted: ●

Sink the well up to about one metre above the top of rock strata.

●

Stabilize the soil around the well curb above the rock strata by forming a grout barrier.

●

Support the steining by constructing six out of the twelve 1.5 m diameter RC piles through 1.65 m diameter holes kept in the well steining, with about 10 m anchor length in the rock strata.

30

Volume 47 │ Number 1 │ March 2017

The plugging and anchoring of the well foundation was carried out after jet grouting activities were completed. The work of jet grouting and subsequent plugging was carried out at such a large depth, and hence any visual inspection was not possible. In order to ensure the safety of the foundation, the owners decided to strengthen the same by providing 10 nos. external piles of 1.5 m diameter having an anchorage in the rock of about 10 m. These external piles were integrated with the well cap so that in any eventuality the load could be shared by these external piles (Fig. 32).

Fig. 32 : Left: Internal Piles in Steining, Right: External Piles Connected to Well Cap

The Bridge and Structural Engineer

In Zuari railway bridge of Konkan Railways, another variation of hybrid deep foundations was successfully adopted. For some of the caisson foundations, the depth exceeded 30.0 m and pneumatic sinking was required to proceed with the sinking. However, beyond 3.0 atmospheric pressures, the working hours would have been greatly reduced, causing enormous time over run. To overcome these problems, some of the caissons were made to rest on 8 numbers of 1.5 m diameter bored cast in-situ pile foundations (Fig. 33, left).

Fig. 33 : Hybrid Foundations for Zuari & Kaliabhomara Bridges

In Kaliabhomara Bridge near Tezpur (Fig. 33 right) constructed in the Eighties, as per preliminary subsoil investigation report the strata for founding the P2 well appeared to have dense sand. However, after sinking 25 m deep, up to RL 35.6 m, against the design founding level of RL 5.0 m, the work slowed down as it encountered boulder strata. For two seasons the work of removal of boulders continued and after removing 100 boulders the well reached a level of RL 32.08 m. The stability calculations established that if the scour could be controlled at RL 45.0 m, the well would be safe. However, under seismic loads there was a tendency to develop tension. To arrest scour at RL 45.0 m, a boulder apron around the pier was provided. This was feasible since the well was near the bank. The well was protected by providing 60.0 m diameter, 3.0 m thick crated boulder garland packed under dry conditions, prevailing at the time of construction of the bridge, at the location of well under pier P2.

foundation, as the same has to neutralize uplift of 6000 t under critical load condition (Fig. 34). Originally, the foundation was designed as 2 independent wells with maximum diameter of 17.5 m each connected by a common well cap of thickness 4.5 m. The wells were designed on the basis of single bore holes at the CGs of the wells and was terminated at around half meter above rock line, assuming horizontal rock line.

Fig. 34 : Sloping Rock & Hybrid Foundation

As in the river Yamuna, there were instances of sloping rock; geotechnical investigations were carried out around the periphery of both the wells, before taking up the actual construction of foundation. To the surprise of all rocky strata was varying drastically within the diameter of wells. It was as high as 9 m from one edge to the deepest point within the well diameter. This warranted a change in the design.

In the revised design, 16 numbers of 1.2 m diameter was provided in each well through the steining and anchoring into rock by 6 m. Wells were sunk by Jack down method of sinking and as the steining was built up with 1.3 m diameter voids left in the steining, so that 1.2 m diameter piles can be done later through these voids. Bored cast in-situ piles were done using specially imported RCD rigs. The sequence of construction is shown in Fig. 35.

To cater for the tension under seismic conditions, four numbers of bored cast in situ piles of 1.5 m diameter on the outer periphery outside the well and 1 No. similar pile in the middle though the dredge hole were provided. Apart from this a combined well-pile cap, square in size was provided so as to act as a composite unit.

In the Signature bridge, Delhi, which is currently under construction, the P23 foundation is a very critical The Bridge and Structural Engineer

Fig. 35 : Sequence of Construction of Pile Integrated Wells

Volume 47 │ Number 1 │ March 2017

31

5.3 For Enhancing the Capacity of Existing Foundations

to occur, especially since the spread footings were supported on a layer of shale.

Micropiles as a part of the hybrid foundation for rehabilitation and enhancing the capacity of existing foundations has to yet catch up in India. In Western countries micropiles are extensively used for enhancing capacities especially for: ●

Seismic retrofit

●

Arresting structural settlement

●

Resisting uplift/dynamic loads

●

Underpinning

The 430 m U.S. Highway 70 Bridges8 over Lake Hamilton (Fig. 36), USA had 6 intermediate piers supporting 7 spans of nine steel girders each constructed between 1982 and 1984 (Sullivan, et al.

2007) The foundations were 1.8 m deep footing and 9 m deep seal on sandstone/shale rocky strata. During the routine inspection in 1991, cracks were observed in footings and seal in piers 2, 3, 6 and 7. In the preliminary engineering stages three rehabilitation solutions were proposed; micropiles, cofferdam supported concrete jackets, and pile supported concrete jackets. Finally, foundations were strengthened by 24 numbers of 180 mm diameter micropiles through footing and seal, having capacity of 100 t, and an embedment of 5 m into rock.

Fig. 37 : Hybrid Footings with RCC Piles for Minto Bridge Rehabilitation

The rehabilitation solution was a deep foundation system and consisted of twelve 750 mm diameter, 9.6 m long, reinforced cast-in-place concrete piles (Monnier, et al., 2015). The upper 1.5 m was supported by 914 mm steel jacketed caisson. Two new pier footings 2.8 m thick were located above existing spread footings. Tension and compression reinforcing dowels into existing footings were used to provide full load transfer between existing and new structures.

Being inspired by these case studies above, for one of the bridges in Himalayan terrain, the enhancement of foundation capacity is proposed by way of hybrid drilled shaft with Micropiles10 In the alternative one, the 62 numbers of inclined 300 diameter micropiles were to be taken through the shaft (Fig. 38, top) of 8 m diameter and 6 m deep M40 grade of concrete.

It involved cutting of 6 layers of 32 diameter reinforcement within the concrete.

Fig. 36 : Hybrid Footings with Micropiles

The bridge on PTH 10 over the Souris River9 was constructed in 1979 as a two lane, 137.5 m long, three-span continuous bridge, situated in a deep valley north of Minto in Canada. The substructure consists of two cast-in-place concrete abutments founded on driven steel H-piles, and two cast-in-place concrete piers supported on shallow spread footings founded on shale. During the 2011 Flood, scouring of the river bottom led to concerns for the potential of undermining of river piers. Due to the height of the existing piers (approximately 12.0 m high), the authorities were concerned with risks associated with the stability of the structure if any undermining were 32

Volume 47 │ Number 1 │ March 2017

Fig. 38 : Proposed Hybrid Drilled shall with Micropiles

As such, it was decided to avoid making so many cores including cutting of reinforcement within the The Bridge and Structural Engineer

concrete shaft and have micropiles outside the shaft (Fig. 38, bottom) being integrated to existing pier through a pile cap by a proper shear and moment transferring mechanism.

6.

Conclusions

The design and construction of bridge foundations involve four specialised disciplines of civil engineering, viz., Geotechnical, Geological, Hydrological and Structural, and hence quite complex compared to land based foundations. The importance of substrata exploration in time, for the selection of type of foundations, has been highlighted in each and every forum by experts-but still not much attention is given till date. It is necessary to evaluate the pros and cons of choosing a particular type of foundation at the project approval stage itself. Invariably, the selection of a wrong type of foundation for a particular location, increases time and cost overrun, causes distress during construction, and in some cases imposes problems to realise technical requirements.

Another major concern, where the projects of 2 to 3 years are being prolonged to even 15 years in some cases, is the estimation of scour depths for bouldery strata in sub-Himalayan terrain. Presently, the codes specify the methodology on the basis of Lacey’s Regime Theory, which was developed for uniform alluvial channels. Though the same can be reasonably applied for alluvial rivers, where the weighted mean diameter of the silt varies upto 1.50 mm, presuming to apply the same to bouldery strata considering 1 to 2 m diameter boulder as a silt is too far fetched.

With the advancement in technology and sophistication in soil investigation methods in recent years, logically ‘factor of safeties’ should have been reduced gradually over the period as FOS normally express the degree of uncertainty. Paradoxically, it is observed that for bridge foundations, the depth and size of the foundations, for the same loadings at the same parallel locations, are gradually on ascendancy.

This is a phenomenon observed generally in Civil Engineering in India.

The Bridge and Structural Engineer

References 1.

Indian Geo Technical Journal, Vol. 25, No.1, January 1995.

2. Nayak, N.V., Foundation Design Manual, 4th edition, Dhanpat Rai Publications, New Delhi, 1996.

3. Reddi, S.A., “Unique features of foundation nos 17 &18 of Jogighopa bridge”, Foundations of Major Bridges: Design & Construction, IABSE Colloquium, New Delhi, 1999.

4. Nayak, N.V., and V.N. Heggade, ‘The innovations in Foundation Practices’, Key note paper no K08, Indian Geotechnical Conference 2006, December 14-16, 2006, IIT Madras.

5. Heggade, V.N., ‘Investigations, Design and Construction of Foundations’, New Building Materials & Construction World, February 2006, Vol.11, Issue-8.

6. Heggade, V.N., ‘The Innovative methods in Design and Construction of Bridge Foundations’, Key note paper in Work shop on Deep Foundations (Well/Piles) For Bridgesoptimal solutions, Central Roads Research Institute, New Delhi, November 29-30,2006.

7. Ponnuswamy, S., and V.C. Sharma, Bridging River Brahmaputra, NFRailways, Maligaon Guwahati,1988.

8. Sullivan, R.P., Jian Huang, J., and Klevens, G.A., “Rehabilitation of U.S. 70 bridge over lake Hamilton”, The Magazine of Deep Foundation Institute, Summer 2007.

9. Monnier, D., Yathon, K., Eden, R., Fingas, R., and Shehata, E., “Challenges and Innovative solutions for bridge foundation repairs”, 2015 Conference of the Transportation Association of Canada, Charlottetown, PEI, 20pp. [http://conf. tac-atc.ca/english/annualconference/tac2015/ s20/monnier.pdf].

10. V.N. Heggade, “Hybrid technology for rehabilitating deep bridge foundations”, INGIABSE Workshop on“Inspection, Investigation and Repair/ Rehabilitation of Bridges and Flyovers” , Bengaluru, 20th and 21st January 2017.

Volume 47 │ Number 1 │ March 2017

33

Seismic Response Control of Reinforced Concrete Bridges with Soil-Structure Interaction

Abdul MATIN

Said ELIAS

Vasant MATSAGAR

Structural Engineer Graduated Student, Indian Institute of Technology (IIT) Delhi, India matin994@gmail.com

PhD Research Scholar Indian Institute of Technology (IIT) Delhi, India eliasrahimi959@gmail.com

Associate Professor Indian Institute of Technology (IIT) Delhi, India matsagar@civil.iitd.ac.in

Abdul Matin, born 1984, received his civil engineering degree from the Herat Univ.

Said Elias, born 1986, received his civil engineering degree from the Herat Univ.

Vasant Matsagar, born 1976, received his civil engineering degree from the Government Engineering College Aurangabad.

Summary The effect of Soil-Structure Interaction (SSI) on the responses of three span continuous Reinforced Concrete (RC) bridge installed with tuned mass damper(s) (TMD/s) is investigated. The TMDs are placed at the mid-span of the bridge and each tuned with a modal frequency, while controlling up to first few modes as desirable. The importance is placed on determining the physical parameters that affect the response of the system and identifying the circumstances under which it is necessary to include the SSI effects in the design of seismically controlled bridges. The soil surrounding the foundation of pier is modeled by frequency independent coefficients and the complete dynamic analysis is carried out in time domain using direct integration method. It is observed that the soil surrounding the pier has significant effects on the response of the controlled bridges. In addition, it is also seen that the seismic responses of bridge with TMDs are considerably altered.

Keywords: Bridge; earthquake; reinforced concrete (RC); SSI; tuned mass damper; TMD.

1.

Introduction

Reinforced Concrete (RC) bridges are the most important links for surface transportation networks.

34

Volume 47 │ Number 1 │ March 2017

They are the most important lifeline structures and their failure, as a result of any seismic event, seriously obstructs relief and rehabilitation work. A number of bridges have collapsed during past earthquakes, all over the world. Bridges are especially vulnerable to damage and can easily collapse as a result of earthquake ground motions. The main reasons for their failure are their structural simplicity (lesser redundancy), and their fundamental time period coinciding with the earthquake. The fundamental time period of vibration of most of the bridges is found to be in the range of 0.2 to 1 sec. From the past earthquakes, it is noted that the predominant time periods had been in this range, thereby it has caused the seismic response of bridges to amplify. The effectiveness of tuned mass dampers (TMDs) for vibration control of long span bridges and tall buildings, due to wind and earthquake excitations, have been extensively studied. Optimal linear vibration absorber for linear damped primary system was determined by Randall et al. [1] using graphical solution. It was also reported that a small offset in tuning of the frequency could result in decreased efficiency of a single TMD. Use of multiple tuned mass dampers (MTMDs) has also been studied extensively, showing that MTMDs are more The Bridge and Structural Engineer

effective than single TMD (STMD). Luu et al. [2] have shown the effectiveness of MTMDs to control the vibration of bridges, caused due to trains moving at high speeds. Recently, Matin et al. [3] reported that TMDs are the best solution to control the bi-direction response of concrete bridges subjected to earthquake ground excitations. Pisal and Jangid [4] reported that placing all multiple tuned mass friction dampers (MTMFDs) at mid-span showed better performance, as compared to the randomly placed devices along the span of the bridge. However, the above mentioned studies ignored the effect of soil-structure interaction (SSI). Tongaonkar and Jangid [5] reported that the soil surrounding the pier has significant effects on the response of isolated bridges and under certain circumstances; the bearing displacements at abutment locations may be underestimated if the SSI effects are not considered in the response analysis of the system. Their investigation shows that consideration of SSI in the analysis will result in enhancement of safety and reduction in design costs. However, no study has been reported wherein the TMDs have been installed in bridges with consideration of SSI. Hence, the objectives of this study include (1) the placement of TMDs at mid-span of RC bridges, and (2) tuning of the TMDs to higher modal frequencies for seismic response mitigation of a concrete bridge with considering of SSI.

2.

Structural Model of Bridge without/with TMDs

In this study a concrete continuous span bridge is considered. The assumptions made in this study are as given below: 1.

Uncontrolled bridge, and controlled with TMD systems are assumed to remain in elastic range.

2.

The bridges with/without TMDs are modeled as a finite element models divided into a number of small discrete segments and a node connecting the two adjacent segments together. Two degrees of freedom are considered at each node and the masses of each segment is assumed to be distributed-between two adjacent nodes.

3.

Mass contribution of non-structural elements, such as parapet walls, kerbs, and wearing coat, are also considered, because they also produce inertial forces; however, their stiffness is neglected.

The Bridge and Structural Engineer

4.

The horizontal and vertical components of an earthquake are generally uncorrelated. The two horizontal components of ground motion on the bridges with/without TMDs are considered, while the effect of vertical component is not considered.

5.

At least ninety nine percent of total mass is included in the controlled modes.

6.

The soil supporting the pier foundation is modeled as spring and damper acting in the horizontal and rotational directions. Viscous damping is used to simulate the radiation damping in the soil, which is developed through the loss of energy emanating from the foundation in the semi-infinite soil medium (Tongaonkar and Jangid, [5]).

7.

The foundation is represented for all motions using a spring-dashpot-mass model with frequency-independent coefficients.

The modeling of the foundation on deformable soil is performed in the same way as that of the structure and is coupled to perform a dynamic SSI analysis (Tongaonkar and Jangid, [5]).

Fig 1(a) shows the general elevation of three span continuous RC bridge. Fig. 1(c) shows a finite element model of the concrete bridge installed with TMDs, duly considering the flexibilities of both the bridge deck, and piers. The bridge bearings are supported on reinforced concrete piers and rigid abutments. In Fig.1(b), kd-x, kd-y, cd-x and cd-y are the stiffness and damping of a TMD in longitudinal (E-W) and transverse (N-S) directions. The mass of TMDs is mi which are all installed at mid-span of the RC bridge.

The system has additional degrees-of-freedom at the base of pier due to flexibility of foundation or SSI effects (refer degrees-of-freedom xn and yn in Fig. 1(c)). The above assumptions will lead to the mathematical model of the bridge as shown in Fig. 1(c).

A sufficiently accurate consideration of soil behavior can be obtained, if the soil stiffness and damping coefficients of a circular mass less foundation on soil strata are evaluated by the frequency independent expressions, Spyrakos [6]. The stiffness and damping coefficients of soil medium are expressed by

Volume 47 │ Number 1 │ March 2017

... (1)

35

Fig. 1 : (a) General Elevation of Three Span Continuous Concrete Bridge, (b) Schematics of the TMD, (c) Finite Element Model of Three Span Continuous Bridge with SSI.

... (2)

... (3)

... (4)

where stiffness of the swaying and rocking springs are represented as Ks, and Kr and the damping corresponding dashpots are indicated as Cs and Cr, respectively. G is the soil shear modulus; Vs is the shear wave velocity for soil; a is the radius of circular footing, υ is Poisson’s ratio for the soil and H is the depth of the soil stratum overlying a rigid bedrock, . The above expressions are also valid and for the limiting case of a large soil stratum, in which case the term H diminishes, (Tongaonkar and Jangid, [5]). The equations of motion of the bridge installed with TMDs, under two horizontal components of an earthquake ground motion, can be expressed in matrix format as, 36

Volume 47 │ Number 1 │ March 2017

... (5) where [Ms], [Cs], and [Ks] are the mass, damping, and stiffness matrices of the bridge, respectively of order (2N + 2n + 2) × (2N + 2n + 2); N and n indicate the degrees of freedom of the bridge and the TMDs, respectively; [Q] = {X1, X2 ,...Xn, x1, xn, xs, xθ, Y1, Y2, are the ...Yn, y1 ...yn + ys, yθ] T, displacement, velocity, and acceleration vectors, is the earthquake ground acceleration respectively; as the earthquake ground vector, including acceleration in the longitudinal: N-S and transverse: E-W directions, respectively; and {r} is the vector of are the influence coefficients. Further, displacements of the ith node of the bridge in the N-S and E-W directions, respectively.

3.

Modeling of the STMD and TMDs

Earthquake excitation often dominate multi-mode of the bridges. Due to this effect the bridge can be damaged and is unsafe for riding. Therefore, the first few modal frequencies are controlled in both The Bridge and Structural Engineer

directions simultaneously and frequency of each TMD is calculated from the following:

... (6-a)

... (6-b)

where the tuning frequency ratios are: fi–x = 1 and fi–y = 1 respectively for the N-S and E-W directions. respectively are the frequencies of Here, the TMDs and first few natural frequencies of the bridge in the longitudinal (N-S) direction. The frequencies of the TMDs and first few natural frequencies of the bridge in the transverse (E-W) direction respectively are and . The effectiveness of the TMDs can be improved by suitably designing parameters of the TMDs, using the following: ... (7-a)

... (7-b)

Then, total mass of all the TMDs (mt) is calculated from the known Mt and assumed total mass ratio μ. Subsequently, the masses of each TMD unit are calculated as:

... (8-a)

... (8-b)

The damping ratios of the TMDs are kept same and the damping (cn, i) of the TMDs are calculated as:

... (9-a)

... (9-b)

4.

Numerical Study

Seismic response of a three span continuous bridge with piers and a box girder is investigated under various real earthquake ground motions. The earthquake ground motions used for the investigation include: Imperial Valley, 1940 earthquake (recorded at the El Centro station); Loma Prieta, 1989 earthquake (recorded at the Los Gatos Presentation Center); and Kobe, 1995 earthquake (recorded at the Japan Meteorological Agency - JMA station). The peak ground acceleration (PGA) of the Imperial Valley, Loma Prieta, and Kobe earthquake ground motions applied on the bridge are: 0.35g, 0.57g, and 0.86g in the longitudinal direction and 0.21g, 0.61g, and 0.82g in the transverse direction, respectively; where g denotes the gravitational acceleration. The material properties and dynamic properties of this bridge are given in Table 1. The properties of this bridge are taken from the bridge studied by Tongaonkar and Jangid [5]. The Fundamental periods of the bridge in both directions are 0.53 sec. the other frequencies of the structure in both longitudinal and transverse directions are (0.49, 0.25 and 0.15 sec) and (0.47, 0.24 and 0.16 sec), respectively. The TMDs are designed as coupled to control the modal responses in both directions (longitudinal and transverse) and the TMDs may have two springs and dash-pots, which are attached to the single mass.

Table 1 : Section and Material Properties of Concrete Bridge Member Properties Area (m2) Moment of inertia in N-S direction (m4) Moment of inertia in E-W direction (m4) Young’s modulus of elasticity (KN/m2) Mass per unit volume (KN/m3) Length/height (m) Fundamental time period in N-S direction (sec) Fundamental time period in E-W direction (sec) Damping ratio Shape

The Bridge and Structural Engineer

Deck 3.57 2.08 2.08 3.6 × 107 23.536 3@30 = 90 0.53 0.53 5% Rectangular

Piers 1.767 0.902 0.902 3.6 × 107 23.536 10 0.15 0.15 5% Circular

Volume 47 │ Number 1 │ March 2017

37

4.1 Effectiveness of the TMDs In this section, the performance of the TMD schemes in response control of RC bridge is investigated.

The TMDs are designed for the bridge with rigid foundation, and their design parameters are given in Table 2. The TMDs are designed with the same damping ratio of 5%, as that of the main structure. The mass ratios for the different TMD schemes remained the same for comparison and the value is 1% of the

total mass of the bridge. Thereafter, the SSI effect is included to insure the performance of the TMDs. Three types of soil properties are considered, which are hard soil, medium soil, and soft soil. The shear wave velocity (Vs), shear modulus (G), and other dynamic properties of different types of soil are given in Table 3, which are taken from the study reported

by Tongaonkar and Jangid [5]. Also, Poison’s ratio (υ) is taken 0.4, as reported by Tongaonkar and Jangid [5].

Table 2 : Design Parameters for the STMD and TMDs in the RC Bridge Schemes STMD 2TMDs 3TMDs

TMDs TMD-1 TMD-1 TMD-2 TMD-1 TMD-2 TMD-3

Frequency, ωi (rad/sec) N-S E-W 11.81 11.81 12.57 11.81 12.57 25.26

11.81 11.81 13.13 11.81 13.13 25.7

Mass, mi (ton) N-S E-W 92.79 (1% of Mt) 49.27 51.3 43.53 41.5 44 46 39 37 9.65 9.7

The response quantities of the bridge in longitudinal as well as transverse direction are plotted for the uncontrolled, controlled with STMD, controlled with 2TMDs placed at mid-span, and controlled with 3TMDs placed at mid-span. The variation of

Stiffness, ki (kN/m) N-S E-W 12948 6874 6874 6189 6159 6159

12948 7157 7157 6409 6409 6409

Damping, ci (kN-sec/m) N-S E-W 109.6 58 54.7 52 49 24

109.6 60.6 54.5 54 49 25

normalized pier base shear, deck displacement, and deck acceleration with varying soil types, in the longitudinal and as well transverse directions, under different real earthquakes are plotted in Figs. 2 and 3.

Table 3 : Dynamic Properties of Different Types of Soil Properties of Soil Shear Modulus, G (MPa) Shear Wave Velocity, Vs (m/s) Translational stiffness of soil medium, Ks (108N/m) Rocking stiffness of soil medium, Kr (108N/m) Translational damping coefficient, Cs (107N s/m) Rocking damping coefficient, Cr (107N s/m)

38

Volume 47 │ Number 1 │ March 2017

Hard 3.57 394 4.29 1.8 1.04 0.967

Type of Soil Medium 0.357 134 0.429 0.18 0.307 0.285

Soft 0.179 99.6 0.214 0.09 0.206 0.191

The Bridge and Structural Engineer

Fig. 2: Variation of the responses (pier base shear, deck displacement and deck acceleration) of the RC bridge from with different soil type under different earthquakes ground motion (Imperial Valley, 1940, Loma Prieta, 1989, and Kobe, 1995) in longitudinal direction.

It is observed that nTMDs are effective and responses are considerably reduced as compared to that of uncontrolled bridge and STMD. It implies that all the TMDs schemes are effective in response reduction of RC bridges under the earthquake forces. In addition, it is observed that the nTMDs schemes are much effective for bi-direction earthquake excitations as

The Bridge and Structural Engineer

compared to the STMD. It is noticed that by increasing the flexibility of the pier foundation the normalized peak pier base shear and deck acceleration are reduced significantly. However, it is observed that the deck displacement is increased drastically by increasing the flexibility of the pier foundation.

Volume 47 │ Number 1 │ March 2017

39

Fig. 3 : Variation of the responses (pier base shear, deck displacement and deck acceleration) of the RC bridge from with different soil type under different earthquakes ground motion (Imperial Valley, 1940, Loma Prieta, 1989, and Kobe, 1995) in transverse direction.

It is observed that in some cases deck displacement in rigid foundation (80 mm) increased in case of foundation with soft soil (500 mm), it shows > 80% increase in the response. Also, the performance of the STMD is found to be insignificant especially under Loma Prieta earthquake ground motion. It is concluded that the displacement at the center of the deck increased significantly in the RC bridge by considering SSI. In addition, significant reduction in deck displacement is achieved by installing the TMDs. Further, base shear and deck acceleration reduced significantly in both directions after installation of the

40

Volume 47 │ Number 1 │ March 2017

STMD and TMDs. Also, the soil type greatly affects the design parameters of the STMD and nTMDs schemes and seismic responses of the bridge with flexible foundation, and the nTMDs are more robust as compared to the STMD.

5.

Conclusions

Multi-mode control of reinforced concrete (RC) bridge including soil-structure interaction (SSI) under earthquake ground motions is presented. Tuned mass dampers (TMDs) are installed for multi-mode control of the RC bridge including SSI. Comparison of

The Bridge and Structural Engineer

seismic responses is made for the bridge installed with single tuned mass damper (STMD), two tuned mass dampers (2TMDs), and three tuned mass dampers (3TMDs), under different real earthquake excitations.

The following conclusions are drawn from the results of the numerical study presented here: 1.

The displacement at the center of the deck increased significantly in the RC bridge by considering SSI.

2.

Significant reduction in deck displacement is achieved by installing the TMDs.

3.

Base shear and deck acceleration reduced significantly in both directions after installation of the STMD and TMDs.

4.

The soil type greatly affects the design parameters of the STMD and nTMDs schemes and seismic responses of the bridge with flexible foundation, and the nTMDs are more robust as compared to the STMD.

2.

3.

4.

5.

References 1.

RANDALL S.E. HALSTED D.M. and TAYLOR, D.L.“Optimum Vibration Absorbers for Linear Damped Systems,”, Mechanical Design, Vol. 103, No. 12, 1981, pp. 908-913.

The Bridge and Structural Engineer

6

LUU, M., ZABEL, V. and KÖNKE, C. “An Optimization Method of Multi-Resonant Response of High-Speed Train Bridges using TMDs”, Finite Elements in Analysis and Design, Vol. 53, 2012, pp. 13-23.

MATIN, A., ELIAS, S., and MATSAGAR, V. “Seismic Control of Continuous Span Concrete Bridges with Multiple Tuned Mass Dampers”, Proceeding of Second European Conference on Earthquake Engineering and Seismology (2ECEES), Istanbul, Turkey, 2014.

PISAL, A.Y., and JANGID, R.S. “Vibration Control of Bridge Subjected to Multi-Axle Vehicle Using Multiple Tuned Mass Friction Dampers”, International Journal of Advanced Structural Engineering, Vol. 8, No. 2, (2016), pp. 1-15.

TONGAONKAR, N.P. and JANGID, R.S. “Seismic Response of Isolated Bridges with Soil–Structure Interaction”, Soil Dynamics and Earthquake Engineering, Vol. 23, 2003, pp. 287302.

SPYRAKOS, C.C. “Assessment of SSI on the Longitudinal Seismic Response of Short Span Bridges”, Construction and Building Materials, Vol. 4, No. 4, 1990, pp. 170-175.

Volume 47 │ Number 1 │ March 2017

41

A Novel Lightweight Composite Deck System for Long-span Bridges

Xudong SHAO Professor Hunan University Changsha, Hunan, CHINA shaoxd@hnu.edu.cn

Lu DENG Professor Hunan University Changsha, Hunan, CHINA denglu@hnu.edu.cn

Anil AGRAWAL Professor City College of New York New York, USA agrawal@ccny.cuny.edu

Xudong Shao, born in 1961, received his Ph.D. degree in Bridge Engineering from Hunan University. He is a professor in Civil Engineering at Hunan University. His main area of research is related to large-span and new type of bridge structures.

Lu Deng, born in 1984, received his Ph.D. Degree in Civil Engineering from Louisiana State University. He is a professor at Hunan University.

His main area of research is related to bridge-vehicle vibration and vehicle loads.

Anil Agrawal, born in1965, received his Ph.D. Degree in Civil Engineering from University of California, Irvine. He is a professor at the City College of New York.

His main area of research is related tohazard mitigation of bridges.

Summary Conventional orthotropic steel decks are susceptible to fatigue damage under cyclic heavy vehicle loads.

In the past, a number of countermeasures have been proposed to deal with this problem, but none proved to be very effective. To address this problem, a new lightweight composite deck system was proposed. This composite deck system consists of a thin, compactly reinforced, ultra-high performance concrete layer on top of the steel deck. The stiffness of the bridge deck is increased significantly with this composite deck system. Consequently, stress levels at fatigue crackprone details are reduced significantly. The proposed deck system has shown excellent static and fatigue performance, and great potential for application in long-span bridges. The relevant studies and results are presented in this paper.

Keywords: orthotropic steel deck; ultra-high performance concrete; lightweight composite deck; experimental test; application.

1.

Introduction

Orthotropic steel decks (OSDs) have been used commonly in long-span bridges to reduce the self42

Volume 47 │ Number 1 │ March 2017

weight, and therefore improve the spanning ability of these bridges. The OSDs are usually covered with a 2 to 3-inch-thick asphalt wearing course. Under cyclic heavy traffic loads, these steel decks are susceptible to fatigue cracks, while asphalt overlays also suffer from cracking and shoving problems, which compromise the serviceability and durability of the bridge deck.

To address these problems, a number of countermeasures have been proposed in the past, including increasing the thickness of deck plates, refining the configuration of fatigue-prone details, enhancing the welding quality, etc. However, none of these approaches have proved to be very effective, since none of them provide much benefit for increasing the stiffness of the deck plate. Recently, Buitelaar et al. [1], Murakoshi et al. [2], and Dieng et al. [3] have proposed to use a reinforced high performance concrete (RHPC), a steel-fiber-reinforced concrete (SFRC) overlay and a fiber-reinforced UHPC (UHPFRC) layer, respectively, to strengthen the stiffness of the steel deck. However, these attempts did not achieve satisfactory results.

Cracks developed in the RHPC and SFRC, while sliding occurred between the steel deck and UHPFRC layer. The reason was either because the concrete did not have sufficient cracking strength or the concrete The Bridge and Structural Engineer

layers did not develop sufficient composite action with the steel deck.

To systematically address the aforementioned issues, a novel Lightweight Composite Deck (LWCD) system was proposed. Substantial amount of research effort has been made during the past six years to explore the basic behaviors of the LWCD, including the material property of the UHPC, shear performance of the shear studs, static and fatigue performance of the LWCD, etc. Some of these studies are briefly introduced in this paper.

2.

LWCD System

The LWCD is composed of a conventional OSD covered by a 1.38- to 2.36-inch-thick (35-60 mm) UHPC layer (Figure 1). The OSD and UHPC are connected through headed studs to ensure that the desired bonding performance of full composite action could be achieved between the two structural components. In the LWCD, the UHPC layer functions as a structural component and is designed to have the same service life as that of the OSD. In order to ensure the desired cracking strength and fatigue performance, the UHPC is compactly reinforced with a steel mesh, as shown in Fig. 1.

Design of Highway Bridges and Culverts in China [4] was investigated based on the FE analysis using the ANSYS program. The performance of a normal OSD without UPHC layer was also studied for the purpose of comparison. The main dimensions of the cross section are as follows: t=0.47 inch, b=0.31 inch, h=10.31 inches, s=12.05 inches, l= 12.36 inches (t=12 mm, b=8 mm, h=262 mm, s=306 mm, l= 314 mm) (refer to Figure 1). The UHPC layer was 1.77-inch (45-mm) thick. Steel rebars with 0.39-inch (10-mm) diameter were arranged in both directions with centerto-center spacing of 1.48 inches (37.5 mm). The stress levels at the six typical fatigue-prone details in the steel deck were examined and compared. The analysis results are shown in Fig. 2.

Fig. 1 : Schematic of the LWCD

3.

Behaviors of LWCD

The Humen Bridge, a suspension bridge that has a main span of 2,913 feet (888 m) and was opened to traffic in 1997 in Guangdong, China, was selected as the test bed for evaluating the performance of the proposed LWCD. Bridge deck segments and longitudinal deck strips were fabricated and tested in the laboratory. Finite element (FE) analysis was also performed to develop the field testing plan.

3.1 Static Performance The performance of the LWCD under the design vehicle loads specified in the General Code for The Bridge and Structural Engineer

Fig. 2 : Comparison of Stress Ranges in Fatigue-Prone Details

As shown in Fig. 2, with the addition of UHPC on the steel deck, the stress ranges in all six details of the OSD have been reduced significantly, especially in the rib-to-deck welds where the stresses are reduced by 82% and 51% in the deck plate and rib, respectively; and are below the corresponding constant-amplitude fatigue limits (CAFLs) specified in the bridge Volume 47 │ Number 1 │ March 2017

43

design codes [5], indicating that these details would theoretically not have fatigue problems during their service life.

3.2 Performance of Headed Studs The headed studs used in the LWCD have a height of 1.38 inches (35 mm) and diameter of 0.51 inch (13 mm), resulting in a height-to-diameter ratio of 2.7.

Push-out tests were performed to study the behavior of the short headed studs embedded in the UHPC. The test results show that when the load was increased to a certain value, the headed studs were sheared off from the steel plates while the UHPC layer was intact with no observable cracks developed, indicating that even with a low height-to-diameter ratio of 2.7, the studs could still develop full shear strength in the LWCD. 3.3 Performance at Negative Bending Moment Zone When exposed to traffic loads, tensile stresses develop at the negative bending moment zones on the UHPC layer, e.g. at the diaphragm sections. To reveal the behavior of the UHPC layer under such negative bending moments, a static load test was performed on a steel-UHPC composite beam specimen (Fig. 3), which consisted of an OSD strip and a 1.77-inch-thick UHPC layer. In the test, the load was incrementally increased until the specimen failed.

Fig.3: Set-up of the Static Load Test

The test results show that when the bottom flange of the OSD began to yield due to excessive compression, no visible cracks were observed on the UHPC surface.

When local buckling developed at the bottom flange of the OSD at the peak load, cracks with a maximum width of 0.01 inch (0.3 mm) were observed. These 44

Volume 47 │ Number 1 │ March 2017

observations clearly indicate that the OSD failed prior to the UHPC layer.

3.4 Fatigue Performance Fatigue tests were also performed on the LWCD specimen. With the compact reinforcement inside, the cracking strength of the UHPC used in this study can reach 6.19 ksi (42.7 MPa) [6], as compared to 1.16-1.45 ksi (8-10 MPa) without reinforcement. The fatigue load was set to produce a stress range of 3.09 ksi (21.3 MPa), which is half of the cracking strength at the most critical location of the UHPC layer. The test results showed that the UHPC layer developed no fatigue cracks after 3.1 million cycles at this stress level. Based on FE analysis, the design load only causes a maximum stress range of about 1.45 ksi (10 MPa) in the UHPC layer, indicating that the UHPC layer can meet the design requirements in terms of fatigue safety.

4.

Application to Field Bridges

To date, the LWCD has been applied to fifteen bridges in China, among which the first pilot project was the Mafang Bridge constructed in 1984. This bridge consists of fourteen 210-foot-long (64-m) simply-supported spans. Due to the heavy traffic, the pavement suffered from serious deterioration, and cracks were also observed in the OSD. In 2011, a major retrofit was undertaken for the asphalt overlay [8], five different retrofitting schemes adopting different wearing courses were adopted for different spans, including a 3.15-inch-thick (8-cm) stone asphalt concrete layer, 3.15-inch-thick (8-cm) epoxy asphalt layer, 3.15-inch-thick (8-cm) sandwich plate, 2.76-inch-thick (7-cm) polymer asphalt concrete layer, and the proposed compactly reinforced UHPC layer (on the 11th span). In order to examine crack development in the UHPC layer, the first 177 feet (54 m) of the 11th span was covered by a 1.97-inchthick (5-cm) UHPC layer with a 1.18-inch-thick (30mm) asphalt overlay on top, while the remaining 33 feet (10 m) was covered by a 3.15-inch-thick (8-cm) UHPC layer without an asphalt overlay.

Three routine checks have been performed during the past four years and no fatigue degradation has been observed in the LWCD. No further crack propagation on the OSD and obvious deterioration in the asphalt pavement were observed. No cracks were observed

The Bridge and Structural Engineer

on the top surface of the 3.15-inch-thick (8-cm) UHPC layer. On the other hand, crack propagations have been observed on the steel decks and serious degradation of the pavement has been observed in decks retrofitted using four other retrofitting schemes approximately 4 years after the installation of decks (see Fig. 4). Figure 4(e) is the deck using LWCD, which is completely damage-free after 4 years. It should be noted that all 5 decks shown in Figure 4 have been subjected to the same traffic loading during the last 4 years.

5.

1)

The UHPC layer improves the stiffness of the bridge deck significantly, leading to significantly reduced vehicle-induced stresses in the steel deck and therefore a pronounced extension of the fatigue life of the steel deck;

2)

The UHPC layer needs no major retrofits or replacement during the service life of the bridge. Therefore, although the LWCD scheme has a slightly higher initial cost compared to conventional schemes that adopt an epoxy asphalt overlay on top of the OSD, its life-long total cost, including costs related to the maintenance and retrofitting of the asphalt overlay, is much (estimated at 85%) lower since the cost of the asphalt overlay in the LWCD scheme is much lower;

3)

The weight of the LWCD is comparable to that of the conventional “OSD + asphalt overlay” system. In addition, field applications have demonstrated that it is convenient and feasible to construct the LWCD on either a newly-built bridge or an older bridge, making it a very promising deck system for long-span bridges.

6.

Conclusions

Advantages and Potential Use of the LWCD

Field verification of LWCD, compared to other retrofit schemes of the deck shown in Fig. 4, indicates excellent potential use of the LWCD. In summary, the LWCD has the following advantages over the conventional “OSD + asphalt overlay” system:

In conclusion, the LWCD has shown excellent static and fatigue performance and significant potential for application in long-span bridges. High cracking strength and low permeability of the UHPC layer along with excellent bonding between UHPC layer and steel deck are the keys to ensure desired performance and durability of the LWCD. Further research should focus on the effects of the following parameters: (1) the ingredients and material ratios, (2) type, shape and volume ratio of the steel fibers, (3) reinforcement ratio of the UHPC, (4) layout of the shear studs, and (5) thickness and size effect of the LWCD specimen on the performance of the LWCD. In addition, structural optimization should be pursued to further reduce the cost and to ensure that the stress range levels of the key details are below their CAFLs.

7.

Fig.4: Service State of Five Retrofitting Schemes on the Mafang Bridge after Nearly 4 Years of Service (Photos Taken in Sep. 2015). (a) Stone Asphalt Concrete; (b) Epoxy Asphalt; (c) Sandwich Plate System; (d) Polymer Modified Asphalt Concrete; (e) Proposed LWCD.

The Bridge and Structural Engineer

Acknowledgements

The authors acknowledge the following funders for their support of this research: the National Natural Science Foundation of China (grant number 51178177), and the Transportation Science and Technology Major Project sponsored by the Ministry of Transport of China (grant number 2011318494160).

Volume 47 │ Number 1 │ March 2017

45

References 1.

Buitelaar, P., Braam, R., and Kaptijn, N., “Reinforced High Performance concrete overlay system for Rehabilitation and Strengthening of or tho tropic steel bridge Decks”, Proc., 1st International or tho tropic Bridge Conference, Sacramento, CA, U.S.A, 2004, pp. 384-401.

2.

Murakoshi, J., Yanadori, N., and Ishii, H., “Research on Steel Fiber Reinforced Concrete Pavement for Ortho Tropic Steel Deck as a Countermeasure for Fatigue”, Proc., 23th U.S. Japan Bridge Engineering Workshop, Tsukuba, Japan, 2007.

3.

Dieng, L., Marchand, P., Gomes, F., Tessier, C., and Toutlemonde, F., “Use of Uhpfrc Overlay To Reduce Stresses in Orthotropic Steel Decks”, Journal Of Constructional Steel Research, Vol. 89, No. 0, 2013, pp. 30-41.

46

Volume 47 │ Number 1 │ March 2017

4.

5.

MTC, General Code for Design of Highway Bridges and Culverts (JTG D60-2004), China Communications Press, Beijing, China, 2004.

(In Chinese).

ECS,Eurocode3 (EN1993-1-9): Design of Steel Structures, Part 1-9: Fatigue, European Committee for Standardization, 2005, pp. 14-15.

6.

Shao X.D., Yi D.T., Huang Z.Y., Zhao H., Chen B., and Liu M.L., “Basic Performance of the Composite Deck System Composed of Orthotropic Steel Deck and Ultra-Thin Rpc Layer”, Asce Journal of Bridge Engineering, Vol. 18, No. 5, 2013, pp. 417-428.

7.

Cao, J.H., Shao, X.D., Zhang, Z., and Zhao, H., “Retrofit of Anortho Tropic Steel Deck with Compact Reinforced Reactive Powder Concrete”, Structure and Infrastructure Engineering, Vol. 12, No. 3, 2016, pp. 411-429.

The Bridge and Structural Engineer

Fatigue Design of Steel Bridges Sougata ROY Principal Sougata Roy, LLC Bethlehem, PA, USA sougata.roy@sougataroyllc.com

Summary This paper presents an introductory overview of the fatigue design and detailing requirements for steel bridges. Historical performance of welded steel bridges around the world shows that fatigue damage under variable live load spectrum controls their design. Fatigue cracks mostly precipitate at the toes of the welded attachments, where stress concentration at the weld toe notch, welding residual stresses, and micro-discontinuities at the heterogeneous fusion zone facilitate fatigue crack growth. While robust fatigue design recommendations for a target service life, based on nominal in-plane stress range and connection detail classification, exist in specifications around the world, majority of the fatigue cracks in service are distortion induced, mitigation of which require careful detailing practices. Fatigue cracking, if undetected, can lead to unstable fracture and progressive collapse. The fatigue design guidelines in the AASHTO and Eurocode 3 specifications are discussed and compared, noting that the current IRC:24 incorporates Eurocode 3.

Keywords: steel bridges; fatigue design; fatigue fracture; welded connections; detailing; fabrication; distortion-induced-fatigue; AASHTO; Eurocode 3; IRC:24.

1.

Introduction

1.1 Need for Fatigue Design Fatigue cracking leading to fracture is one of the primary considerations for service performance of steel bridges carrying vehicular loads. Historical performance of welded steel bridges around the world shows that fatigue damage under variable The Bridge and Structural Engineer

Sougata Roy, born 1966, received bachelor degree in civil engineering from the University of Calcutta, India, and masters and doctoral degrees in civil engineering from Lehigh University, USA. He has more than 16 years’ experience in academia and another 13 years’ experience in structural engineering consultancy and research. His expertise is in fatigue and fracture of steel bridges and structures.

live load spectrum can diminish their functionality and require significant repair and retrofit. Fatigue cracks have mostly precipitated at the toes of the welded attachments, and occasionally have grown from the weld root. Uninhibited fatigue crack growth can lead to cross-sectional fracture and potentially collapse of non-redundant structures. Most fatigue fracture problems have occurred due to lack of adequate attention to detailing, which is the most important aspect of fatigue fracture design. Fatigue fracture limit state must be considered in design and detailing of steel bridges for life-cycle cost-effective performance.

1.2 Phenomenon of Fatigue Fracture Fatigue is a progressive, localized, and permanent structural damage that occurs in a material subjected to repeated or fluctuating tensile strains at nominal stresses often much less than the static yield strength of the material (although, at the location of fatigue damage, the local strain exceeds yield).

Fatigue damage culminates into crack initiation and progressive stable propagation under tensile stress range that (if not intervened) may result in unstable fracture after a sufficient number of fluctuations, when the fatigue crack reaches a critical size. Note that contrary to fatigue crack growth that is driven by the change in stress or stress range, unstable fracture occurs when the crack size or a flaw becomes critical for the total applied stress.

Fatigue cracking in a component is attributed to stress concentrations due to geometric (macro) and/or material related (micro) structural discontinuities. In steel bridges, fatigue cracking particularly precipitates at the welded connections due to several conditions

Volume 47 │ Number 1 │ March 2017

47

inherent to welding. First, the stress concentration at a connection is locally intensified by rapid sectional transition at the weld bead. Second, crack like flaws (discontinuities) such as trapped slag or lack of fusion exist at the weld toe and the weld root that promote the fatigue crack growth. And finally, the presence of high tensile welding residual stress fields, in the order of the material yield strength, at the weld toe and the weld root facilitate fatigue crack propagation.

Because of the high tensile residual stresses, the welded connections experience fatigue cracking even when subjected to nominal compressive stresses. Moreover, most of the fatigue life of a welded connection is expended in growing micro-

(a)

discontinuities through the tensile residual stress field.

Accordingly, the total applied stress range including stress reversals must be considered for assessing the fatigue performance of welded connections. Fig. 1 shows fatigue crack growth from the weld toe at a stiffener-to-flange connection, starting from a trapped slag or slag inclusion at the weld-to-base metal fusion boundary. Fig. 2 shows the exposed fatigue fracture surface in a plate girder, where the fatigue crack grew from the unfused root of the longitudinal webto-flange weld, in a plane normal to the weld axis.

Note that these discontinuities are normal occurrence in any welded fabrication that meets the quality requirements.

(b)

Fig. 1 : Fatigue Cracking at the Toe of a Stiffener-to-Flange Weld: (a) Macro-Etched Section; (b) Rectangle Inset Under Optical Microscope

1.3.1 Total life (safe life) approach

Fig. 2 : Exposed Fatigue Fracture Surface

1.3 Approaches to Fatigue Design Primarily there are two approaches to assessing fatigue performance, the total life (or safe life) approach, and the damage tolerant (or fail safe) approach. Depending on the different service operation conditions, different fatigue assessment or design strategies are followed.

48

Volume 47 │ Number 1 │ March 2017

The total life or safe life approach is the classical approach that assumes that no fatigue crack exists in the structure during service; fatigue failure is defined by fatigue cracking. The fatigue design life is characterized by the total life endured without significant cracking at the applied stress range.

In this approach, the fatigue design is performed using curves that are obtained experimentally under controlled cyclic stress for different structural details.

The safe life method generally provides an acceptable level of reliability that a structure will perform satisfactorily during its design life without the need for regular in-service inspection for fatigue cracking.

As such, the safe life method is appropriate for nonredundant structures, where local formation of cracks in one component could rapidly lead to failure of the component or the structure. A few variations of the total life approach exist depending on whether nominal stresses or local stresses are considered or whether the fatigue design aims for a finite or infinite life.

The Bridge and Structural Engineer

1.3.2 Damage tolerant (fail safe) approach In contrast to safe life approach, the basic premise of the damage tolerant or fail safe approach is that all engineering components are inherently flawed; and existence of macro cracks in a structural component does not define failure. The fatigue life is defined by the number of cycles required to propagate the initial crack (such as micro-discontinuities at the weld toe or an existing fatigue crack) to some critical dimension for unstable fracture. This propagation life or design (useful) fatigue life is estimated using Fracture Mechanics. Thus, the damage tolerant approach may be applied to redundant structures, where in the event of fatigue fracture load redistribution between components or structural elements can occur. A structure designed by this approach will perform satisfactorily during its design life with an acceptable level of reliability, provided that a prescribed inspection and maintenance regime for detecting and correcting fatigue damage is implemented throughout the life of the structure. Depending on whether fatigue crack propagation in service is accepted or inhibited, this approach can be implemented for finite and infinite life design.

2.

Traditional Fatigue Design

2.1 S-N Curves for Constant Amplitude Loading Fatigue performance of various welded and bolted connection details for steel bridges has been experimentally studied for decades by many investigators [1-13]. Typically, small-scale specimen tests result in longer apparent fatigue lives, indicating a component size effect on their fatigue resistance.

Therefore, fatigue performance of full scale structural components was evaluated. Full-scale testing of welded members also indicated that the fatigue resistance is primarily controlled by the stress range or the live-load stress, and the effect of mean stress such as the dead load stress is not significant due to the high tensile welding residual stresses at the site of fatigue cracking. Similar fabrication induced high tensile residual stresses exist at the root of bolt threads. The research findings were included as design guidelines against fatigue fracture limit state in the bridge design specification around the world [14-17], wherein the fatigue performance of the connection details are defined as a function of the detail type, and the endurance for an applied nominal stress range.

The connection details exhibiting similar fatigue performance are grouped into detail categories, which capture their notch severity (the local stress The Bridge and Structural Engineer

concentration including the connection geometry and weld effect) or, the propensity to fatigue cracking.

Each category of details exhibit an exponential relationship between the endured life (N) at a given stress range (S), such as:

N = A · S–m

... (1)

which is traditionally presented on a log10-log10 base, with N as the abscissa and S as the ordinate.

The constant A for each detail category somewhat represents its notch severity. Note that N is the dependent variable, and S is the independent variable.

When tested under constant amplitude (CA) loading, the S-N data for welded steel connections exhibit a threshold stress range, below which the connection exhibits practically infinite life, i.e., noticeable fatigue cracking does not occur. Above this threshold, the S-N relationship is liner in the log-log base. The design S-N curve, defined by the 95% confidence to the 95% probability of survival life of the test data, or the lower bound fatigue life, usually exhibit a slope of m = -3 for conventional steel, up to the threshold stress range, and is obtained by linear regression of log10 N against log10 S. Note the variability of the data is on N for a given S. For a large data set the design S-N curve is approximately equal to the mean curve minus two standard deviations of the data. Beyond the threshold stress range, the S-N curve is defined by a horizontal line (parallel to the N-axis or abscissa) through the threshold stress range, often referred to as the constant amplitude fatigue limit (CAFL) or the constant amplitude fatigue threshold (CAFT). The intersection of this bilinear S-N curve is called the “knee”. Fig. 3 shows an example of the fatigue design curves in the US highway [14] and railway [15] bridge specifications. Seven S-N curves are defined for detail categories A, B, B´, C´, C, D, E, and E´ in order of decreasing fatigue strength.

Fig. 3 : AASHTO Fatigue Design Curve

Volume 47 │ Number 1 │ March 2017

49

2.2 Design Requirement Traditionally, the design against premature fatigue cracking requires the fitness of a connection detail to be matched with imposed in-service stress levels and its endurance requirement. The imposed stress range is the nominal stress rather than the local concentrated stress at the connection details.

The nominal stress is calculated using mechanics of material equations for axial and flexural actions, such as P/A (where, P is the axial force and A is the cross sectional area) and/or, M/Z (where, M is the bending moment and Z is the section modulus), and does not include the local effect of stress concentrations of welds and attachments. The local effects and the inherent welding residual stresses are implicitly included in the design S-N curves, which are primarily generated from simple specimens containing different connection details, fatigue tested under bending or pure axial stresses and are presented in terms of the tested nominal stresses and the fatigue life endured. This design methodology is limited to simple connection details, primarily subjected to in-plane stresses in the component (or load-induced fatigue) that have been classified by fatigue testing.

2.3 Treatment of Variable Amplitude Loading While the service load histories for bridges consist of stress ranges of varying amplitudes, hereafter called variable-amplitude (VA) loading, most of the design S-N curves were generated based on fatigue testing performed under CA loading, primarily due to convenience and limitation of resources. A relationship between CA and VA loading is necessary for fatigue design of bridges using the available S-N curves. This relationship is most simply derived by applying Palmgren-Miner’s [18] linear fatigue damage accumulation rule starting from a stress range (or live load) histogram (or spectrum), which concisely represents the variable loading on bridges by capturing its characteristics in terms of at least the stress (or load) range magnitude and the number of occurrences (or frequency) of each stress (or load) range level. Accordingly, an equivalent or effective CA stress range (Sre) producing the same damage or fatigue crack growth as the VA stress range is given by:

... (2)

where, Sri is the ith stress range in the histogram; fi is the frequency of occurrence of Sri; and m is the slope 50

Volume 47 │ Number 1 │ March 2017

of the S-N curves. Experimental studies [3, 7, 12, 19] show that when the fatigue threshold is sufficiently exceeded by the VA stress spectrum, the fatigue resistance continues to reduce with the number of cycles. The rate of change in fatigue resistance below the CAFL depends on the frequency of exceedance. A CAFL exists for a variable amplitude stress spectrum with less than 1 in 10,000 exceedance.

Thus, designing against variable amplitude load spectrum requires determination of the stress range spectrum experienced by a particular detail, analytically or experimentally. Alternatively, an equivalent fatigue design vehicle may be estimated analogous to the effective stress range that represents the entire traffic spectrum crossing the bridge.

... (3)

where, Wi is the gross vehicle weight (GVW) of the ith vehicle in the traffic histogram (spectrum); fi is the frequency of occurrence of Wi, and m is the slope of the S-N curves. Analysis based on this design vehicle is then expected to produce the effective CA stress range, which can be used directly with the design S-N curves. Two assumptions are underlying to this approach: (1) the stress range induced at a detail by a vehicle crossing the bridge is proportional to its GVW; and (2) all vehicles crossing the bridge produce the same fatigue damage as is done by an equal number of design vehicles.

A key point to note is that in determining both the effective stress range or the design vehicle, typical lighter vehicles or their effects i.e., smaller stress ranges towards the lower end of the spectrum or the histogram are truncated, since they do no contribute much to the fatigue damage and their inclusion produce unduly conservative results.

3.

Fatigue Design Specifications

3.1 AASHTO Specifications 3.1.1 Overview The AASHTO [14] fatigue design specifications, developed based on comprehensive experimental research [1-4, 6-10, 12, 20], pioneered the modern fatigue design specifications around the world.

The AREMA [15] specifications are similar to the AASHTO specifications, except for the load model representing railway vehicles. As mentioned earlier, the AASHTO specifications adopt a safe life design The Bridge and Structural Engineer

approach following the traditional nominal stress based design, where the fatigue resistance of connection details are classified based on their notch severity.

Seven detail categories are defined. The CAFL occurs at an increasing number of cycles for lower fatigue categories or classes. Sometimes, different details, which share a common S-N curve (or category) in the finite-life regime, have different CAFL. The fatigue design stress is determined using a design truck, having a GVW as 75% of the HL-93 truck with a fixed rear axle spacing of 9 m. This vehicle represents typical 4 and 5 axle trucks that are prevalent in the US truck traffic spectrum, and are responsible for most of the fatigue damage. The 75% or 0.75 value is not a true load factor in the sense of Load and Resistance Factor Design (LRFD); however, it is included in the specifications as a load factor to the HL-93 truck.

Only one vehicle is to be placed in the most critical disposition for determining the fatigue design stresses.

Recognizing that most bridges carry a large number of truck traffic during their life time, AASHTO requires infinite life fatigue design except for small Average Daily Truck Traffic (ADTT) experienced by low volume rural roads. Field measurements show that the fatigue limit state stress range corresponding to an exceedance of 1 in 10,000 is typically about twice the effective stress range. Accordingly, infinite life design is ensured by limiting the stresses due to twice the fatigue design truck to the CAFL of a detail. This is implemented in the specifications for most common bridge details as a load factor of 1.5 for infinite life design, twice of that for the finite life design. The AASHTO specifications define the infinite and finite life design limit states respectively as Fatigue I and Fatigue II load combinations.

3.1.2 Detail classification

Fig. 4 : AASHTO Fatigue Detail Categories

The Bridge and Structural Engineer

The categorization of several standard connection details are tabulated, including three dimensional representations (sketches) showing the primary loading directions and potential crack locations, the CAFL, and the constant A for finite life assessment.

Similar representation of detail categories is common in the fatigue design specifications around the world, including the Eurocode 3 (and its incorporation into the IRC 24 [21]). The following is a brief simplified overview of the categorization of the connection details in the AASHTO specifications. The Eurocode 3 and other specifications are also based on similar rationale for classification. Examples of detail categories as per the AASHTO specifications are shown in Fig. 4. Base metals are classified as Category A, the highest possible fatigue category, with the potential for fatigue crack growth from rolled inclusion and other fabrication defects. Due to stress concentration, small unfilled holes are classified as Category D. Accordingly, riveted and mechanically fasted joints other than preloaded high strength bolted joints are classified as Category D. Properly tensioned highstrength bolted joints loaded in shear are classified as Category B, where the slipping is prevented. Pin plates and eye-bars are designated as Category E details, in terms of the stress on the net section.

All welded joints are classified between categories B and E´. Welded joints are described as longitudinal or transverse depending on whether the weld axis is parallel or perpendicular to the primary stress range.

Continuous longitudinal welds exhibiting fatigue crack growth from the weld root normal to the weld axis are classified as Category B and B´. Rest of the welded connections are common attachments that typically exhibit fatigue crack growth from the transverse weld toe, except for fatigue crack growth from a large volumetric defect in a transverse groove weld or from the root and through the throat of an under-designed transverse fillet weld. These attachments are detailed and classified based on whether they are load carrying or non-load carrying, and further based on their length and thickness parallel to the primary stress. Note that longer and thicker attachments parallel to the primary stress direction tend to attract more stress (or result in greater stress concentrations) and as such their fatigue behaviour is akin to load-carrying, even if they are not directly loaded. With increasing stress concentration the attachments are classified as lower or more severe categories.

Depending on their length parallel to the primary stress direction, the attachments that are not specifically Volume 47 │ Number 1 │ March 2017

51

designed to carry significant load or the so called nonload carrying attachments are classified as: Category C if less than 51 mm; Category D if between 51 mm and 101 mm; and Category E if greater than 101 mm. Note that a transverse stiffener, welded to the web and/or to the flange of a girder is a special case of short attachment and is designed as Category C with respect to the stress in the flange against fatigue crack growth at the transverse weld toe. Welded cover plates are examples of long attachment and are classified as Category E and Category E´, when the flange thickness exceeds 19 mm.

In most cases, load carrying attachments are transversely loaded, such as a gusset plate connecting the lateral load system to the main girder. These attachments can also be characterised as so called non-load carrying with respect to the stresses in the main member and therefore are subjected to an interaction effect. In practice, each of these stress ranges is checked separately. To avoid, any possibility of fatigue crack growth from the weld root, the load carrying attachments are often recommended to be full penetration groove welded and non-destructively inspected to ensure no volumetric defects exist. If a fillet or partial joint penetration weld is used to connect the load carrying attachment or a discontinuous load carrying member, such as a cruciform joint, the fatigue strength is reduced as a function of the weld leg size, the member thickness and the unfused root width [22].

The fatigue resistance of attachments also depend on the transition radius and the weld finish at the end. For example, with a 51 mm transition radius or less, the attachment is classified as Category E. The attachment can be classified as Category C with 152 mm end transition radius, and as Category B when the weld reinforcement is ground smooth with a tangential transition on to the main member, eliminating the stress concentration and the end transverse weld and thereby the associated mode of fatigue crack growth from the weld toe.

3.2 Eurocode The Eurocode [16] allows fatigue assessment using either damage tolerant method or safe life method.

Detailed guidelines are provided for safe life design following the traditional nominal stress based design using detail classification. Additional guidelines are provided for safe life design based on geometrical (hot spot) stress, usually estimated by advanced Finite Element Analysis (FEA) that includes the local stress 52

Volume 47 │ Number 1 │ March 2017

concentration due to the connection geometry, but excludes the weld notch effect, which is implicitly considered by the empirical fatigue design curves.

Two sets of design S-N curves are provided in log10-log10 basis for normal (direct) and shear stress ranges. The fatigue resistances for normal stress ranges are defined by a series of 14 S-N curves corresponding to standard detail categories. These curves are shown in Fig. 5 against the AASHTO fatigue design curves for comparison. All design curves are parallel to each other having a slope of -3 up to 5×106 cycles, changing to a slope of -5 up to 108 cycles, and then becoming horizontal parallel to the N-axis or the abscissa. The stress range corresponding to the horizontal line is called the cut-off level, also referred to in the literature as the variable amplitude fatigue limit (VAFL), below which no cumulative fatigue damage occurs under variable amplitude loading. The stress range corresponding to the 5×106 cycles is termed the CAFL.

Fig. 5 : Eurocode 3 Fatigue Design Specifications, Compared with AASHTO Specifications

Two bilinear S-N curves are specified for shear stress ranges having a slope of -5 up to 108 cycles and then becoming horizontal (defined as the cut-off level).

The S-N curve for each detail category is designated by a number, which represents in MPa the fatigue strength at 2×106 cycles. When test data were used, the value of the reference fatigue strength was calculated for a 75% confidence level of 95% probability of survival for log10N, taking into account the standard deviation of the test data, the sample size and residual stress effects. At least 10 data points were included in the statistical analysis. It appears that not all the curves were developed based on experimental data.

The specification allows for modification of fatigue strength for non-welded or stress-relieved welded The Bridge and Structural Engineer

details in compression. It also allows for modification of fatigue strength for size effect due to thickness or other dimensions.

The specification considers partial safety factor for fatigue strength along with the partial factors for load actions such as to produce an acceptable reliability of performance in terms of the consequences of failure.

For a safe life methodology the partial fatigue strength safety factors are defined as 1.15 and 1.35, respectively for low and high consequence of failures.

Five fatigue load models (FLM) are specified in the Eurocode [23] incorporating varying degrees of accuracy and for application with finite and infinite life design considerations. The FLM 1, 2, and 3 are used to determine the maximum (tensile) and minimum (compressive) stresses at the connection details due to all possible disposition of these loads on the bridge, and consider their algebraic difference (constant amplitude stress range) for design with the S-N curves. The FLM 4 and 5 are used to determine the stress range spectrums (variable amplitude stress range histogram) at the connection details, resulting from the passage of lorries on the bridge.

The fatigue design is performed using the stress range spectrums along with the S-N curves, assuming linear cumulative fatigue damage. The difference is that the FLM4 uses idealised vehicle spectrum and the stress range spectrum is determined analytically, whereas, the FLM5 uses actual measured data. The FLM1 is similar to the Frequent Load Model LM1 used for strength design limit states, consisting of uniformly distributed and concentrated loads, but their magnitudes scaled respectively to 70% and 30% to avoid undue conservatism. The uniformly distributed load is allowed to be neglected. The FLM 2 consists of a set (or distribution) of five different “frequent” or idealised lorries of different axle spacing (including both tandem and tridem axle groups), axle load, and wheel configurations. The FLM3 consists of a single vehicle of four axles and two identical wheels. Similar to the FLM2, the FLM4 consists of a set of five different “standard” idealised lorries with different distribution for long distance, medium distance and local routes. The FLM 1 and 2 are typical of heavy traffic on European main roads and motorways, and to be used for infinite life design. The FLM 3, 4, and 5 are to be used for finite life design. The FLM1 includes multiple presence effects. The FLM2 is more accurate than the FLM1, when simultaneous presence of several lorries can be ignored. The FLM3 is used for simplified design verification. The FLM4 is more The Bridge and Structural Engineer

accurate than FLM3, when simultaneous presence of several lorries on the bridge can be ignored. The FLM5, based on actual measured traffic data is the most accurate.

3.3 Comparison of the Design Specifications Although both the AASHTO and the Eurocode specifications incorporate the traditional nominal stress based fatigue design methodologies, several key differences exist between these design provisions.

First, the AASHTO specifications do not include any design provisions for shear stress range, since fatigue performance of welded connections are primarily controlled by normal stress ranges. Second, the detail classifications are more refined in the Eurocode.

Over the finite life regime with slope -3, the highest detail class of the Eurocode, Category 160, matches the highest detail Category A of the AASHTO specifications; however, the lowest detail Category 36 of the Eurocode is lower than lowest detail Category E´ of the AASHTO specifications. The Eurocode detail categories 125, 100, 90, 71, 56 and 40, respectively match the AASHTO detail categories B, B´, C & C´, D, E, and E´ respectively. It may be noted that the lower bound or the design S-N curves in the AASHTO specifications, generally match the upper bound of the test data for the next lower design category [8]. Considering the significant scatter in the fatigue test results, such refinement of the design S-N curves in the Eurocode are questionable.

The major difference between the AASHTO and the Eurocode fatigue design provisions lies in the infinite life design approach. The Eurocode assumes the CAFL for all detail categories at 5 million cycles, whereas the full scale test data that forms the basis for the AASHTO design S-N curves significantly differ [8]. The VAFL is defined at 100 million cycles for all detail categories. The corresponding fatigue resistance is 54.9% of the Eurocode specified CAFL. An infinite life is ensured if the CA stress range due to FLM1 or FLM2 is less than this CAFL. According to AASHTO, the infinite life for a detail category is ensured by limiting twice the stress range due to the design truck to the AASHTO specified CAFL. Interestingly, the ratio of the Eurocode cut-off level to the AASHTO CAFL increases from 0.39 to 0.87 in decreasing order of the matching categories (i.e., Category 160 to 40 or Category A to E´), bracketed between 0.46 and 0.60 for most of the categories. This suggests that that both specifications provide similar fatigue resistance for most details; however, Eurocode is un-conservative for the lower category details. It is Volume 47 │ Number 1 │ March 2017

53

implicit in the AASHTO specifications that when the VA stress range spectrum exceeds the CAFL of a detail category more than 1 in 10,000, the cumulative fatigue damage is estimated using the finite life S-N curve extended below the CAFL with a slope of -3. For VA stress range spectrum, the Eurocode uses the bilinear curve, with slopes of -3 up to the CAFL, and -5 until the cut-off level for estimating the cumulative fatigue damage. Debate exists between these two infinite life design approaches, particularly with the use of a curve of -5 slope, which depends on the frequency of exceedance of the CAFL by the VA stress range spectrum. Considering the lack of sufficient VA test data, particularly in the long life regime, including tests subjected to VA loading with different exceedance, and significant scatter that is often observed near the fatigue threshold, the AASHTO approach is certainly conservative, and is supported by the available test data [3, 19, 7, 12].

Evidently, the Eurocode fatigue load model is significantly elaborate compared to a simple AASHTO fatigue load model.

4.

Distortion-Induced Fatigue

The fatigue design guidelines presented so-far were developed for load-induced fatigue under primary inplane stresses; however, most fatigue cracking in steel bridges are displacement-induce, caused by distortion of cross-sections and out-of-plane deformation of elements that induced high localized bending stresses [24]. Out-of-plane distortion occurs in girder webs, mainly at attachment plates for diaphragms, transverse stiffeners and floor beams, as schematically illustrated in Fig. 6, due to incompatibility of displacements between connected components during passage of vehicles across a bridge. Assuming a fixed ended beam subjected to relative support displacement, the stresses responsible for the cracking at the weld toes at the end of the web gap are inversely proportional to the square of the gap length (L) and directly proportional to the relative displacement (∆). Very high stresses and stress reversals develop normal to the welds at the gap ends (point of fixity) that are usually small, even for small out-of-plane displacements, leading to fatigue cracking within a short period of service. These unintended secondary stresses are often overlooked during design. The solution to such problems, where the connection geometry (such as the gap dimension, the web thickness etc.) affects the magnitude of stresses under an imposed displacement, lies in proper detailing that eliminates the secondary 54

Volume 47 │ Number 1 │ March 2017

stresses either by stiffening or softening of the connection, and needs to be addressed on a caseby-case basis. Advanced FEA are often necessary to develop effective retrofit strategies, which must be verified by field measurements on prototype.

Significant knowledgebase has been generated over the years that can provide guidance for new design and retrofit detailing.

M = 6El∆/L2 Fig. 6 : Schematic of Web Gap Distortion

In older bridges, transverse stiffeners and attachment plates were not welded to the tension flange of welded I-girders and box girders for the fear that a fatigue crack initiating in the flange would lead to a brittle fracture. This well-intended but outdated practice originated in Europe in the 1930’s from unexpected brittle fractures in early welded bridges, which was attributed to the welded details, but was primarily due to the poor quality of the steel. This practice has been the root of the distortion-induced cracking, which can be prevented by welding stiffeners to the web and the flanges. Fig. 7 shows a crack that formed along the fillet welds at the diaphragm connection plate-to-web connection of a plate girder, where the connection plate was not welded to the flange. The fatigue cracking in these gaps typically occurs in a longitudinal direction along the fillet weld toe of the longitudinal web-to-flange joint, or at the termination of the vertical fillet weld of the connection plate-toweb joint, or at both locations, as shown in Fig. 7. In many cases, the displacement causing this distortion is limited, and the cracking arrests as the compliance of the gap increases (end fixity reduces by crack growth) and the stresses are reduced. Because most of the cracks are oriented longitudinally, there is typically no reason to be concerned about fracture of the girder, unless due to the shear in the web the cracks turn and propagate across the web normal to the in-plane principal stresses. It is typically a mistake The Bridge and Structural Engineer

to weld repair such cracks, as this restores the high stresses, which originally caused the cracking and will certainly reinitiate the cracking at the weld repair.

In some cases with limited distortion, holes may be drilled or cored at the crack tips to arrest propagation by blunting the crack tip. However, cracks can reinitiate if the out-of-plane bending is excessive, as seen in Fig. 8 where the holes drilled at crack tips failed to arrest crack growth.

allowing the small fatigue crack to remain after drilling or coring holes at the crack tips. Another way to increase the flexibility of the joint is to remove part of the stiffener or connection plate and increase the length of the gap, as shown in Fig. 10. If the same displacement occurs over a greater length gap, the stresses are significantly reduced.

Fig. 9 : Retrofitting by Connection Stiffening Fig. 7 : Distortion Induced Fatigue Cracking in the Web Gap

In most cases, problems with web-gap-cracking can be solved by rigidly connecting the attachment plate to the tension flange, or stiffening of the connection, which introduces alternate load path from the cross member to the girder, and eliminates the out-of-plane distortion. To retrofit existing bridges, a rigid tee or angle may be connected to the attachment plate and the tension flange using high-strength bolts. Holes must also be drilled at the ends of short cracks to prevent any possibility of crack propagation into the web under in-plane stresses. An example of retrofitting by connection stiffening is shown in Fig. 9.

Fig. 8 : Retrofitting Fatigue Cracks by Drilling Holes

Alternatively, the stresses can be reduced by increasing the flexibility of the connection or, softening the connection. The flexibility may be increased by The Bridge and Structural Engineer

Fig. 10 : Retrofitting by Connection Softening

Another example, where the best details are more flexible, is the connection angles for simply supported beams. Despite common assumptions, such simple connections can transmit up to 40 % of the theoretical fixed-end moment, even though they are designed to transmit only shear forces. This unintentional end moment may crack the connection angles and/or the beam web cope. For a given load, the moment in the connection decreases significantly as the rotational stiffness of the connection decreases. The increased flexibility of connection angles allows the limited amount of end rotation to take place with reduced angle bending stresses. A criterion to provide sufficient flexibility requires the angle thickness t be limited as [25]:

Volume 47 │ Number 1 │ March 2017

... (4)

55

where g is the gauge length of the outstanding leg (connected to the supporting element) and L is the span length of the simply supported beam.

Attention should be given to fabrication practices, such that defects such as nicks, cuts, notch, undercuts and volumetric defects are avoided.

In cases where the section distortion is not displacement induced, such as a load being transferred between two relatively rigid components across a thin element causing load induced bending, hole drilling or increasing the flexibility of a connection will not be effective. In these cases, and in many displacement limited cases, the best solution to distortion-induced cracking is to increase the rigidity of the connection.

In new construction, the bridge specifications now recommend that connection plates should be rigidly connected to both the flanges and the web.

6.

5.

1.

Fisher, J.W., Frank, K.H., Hirt, M.A., and Mcnamee, B.M., ‘Effect of Weldments on the Fatigue Strength of Steel Beams”, NCHRP Report 102, Highway Research Board, Washington, D.C., 1970.

2.

Fisher, J.W., Albrecht, P.A., Yen, B.T., Klingerman, D.J., and Mcnamee, B.M., “Fatigue Strength of Steel Beams with Transverse Stiffeners and Attachments”, NCHRP Report 147, Transportation Research Board, Washington, D.C., 1974.

3.

Schilling, C.G., Klippstein, K.H., Barsom, J.M., and Blake, G.T., “Fatigue of Welded Steel Bridge Members under Variable Amplitude Loading”, NCHRP Report 188, Transportation Research Board, Washington, D.C., 1978.

4.

Fisher, J.W., Hausammann, H., Sullivan, M.D., and Pense, A.W., “Detection and Repair of Fatigue Damage in Welded Highway Bridges”, NCHRP Report 206, Transportation Research Board, Washington, D.C., 1979.

5.

Gurney, T.R., Fatigue of Welded Structures, Cambridge University Press, Cambridge,1979.

6.

Fisher, J.W., Barthelemy, B.M., Mertz, D.R., and Edinger, J.A., “Fatigue Behavior of FullScale Welded Bridge Attachments”, NCHRP Report 227, Transportation Research Board, Washington, D.C., 1980.

7.

Fisher, J.W., Mertz, D.R., and Zhong, A., “Steel Bridge Members under Variable Amplitude Long Life Fatigue Loading”, NCHRP Report 267, Transportation Research Board, Washington, D.C., 1983.

Conclusions

Requirements for fatigue design of steel bridges were presented. Fatigue of steel bridges under traffic loading is the most significant issue affecting their service performance, and must be considered for life-cycle cost-effective design. Research and case studies of in-service fatigue cracking of steel bridges over the past 40 years have helped in formulating design guidelines and improved detailing practices, implementation of which have prevented fatigue cracking in new construction and ensured durability.

However, the risk of fatigue fracture of many steel bridges that are not designed accordingly and the economic impact of retrofitting these bridges to keep them functional remain high.

Historically most of the fatigue cracking of the welded steel bridges occurred at attachment details under in plane loading, as well as at the web gaps from distortion. Careful detailing is the most important aspect of fatigue fracture design. The attachment connections are the most severe of the fatigue critical details, which are characterized by crack growth at the weld toe. These connections can be designed by traditional classification approach using nominal stress-based S-N curves, and selecting details from fatigue resistant categories. The distortion induced fatigue cracking can be addressed by detailing practices that eliminate the secondary stresses driving these cracks. In most cases, the web-gap-cracking can be prevented by rigidly connecting the attachment plates to the tension flange. Where the distortion is displacement controlled, the stresses can be reduced by increasing the flexibility of the connection. If distortion is limited, holes may be drilled or cored at the crack tips to temporarily arrest propagation.

56

Volume 47 │ Number 1 │ March 2017

Acknowledgements

The author is grateful to his mentor and colleague Dr. John W. Fisher for sharing the vast knowledge on fatigue fracture of steel bridges. The author is also indebted to his students and research assistants for their contributions to his research over the years.

Finally the author is thankful to the ING-IABSE for the opportunity to publish this manuscript.

References

The Bridge and Structural Engineer

8.

9.

Keating, P., and Fisher, J.W. “Evaluation of Fatigue Tests and Design Criteria on Welded Details”, NCHRP Report 286, Transportation Research Board, Washington, D.C., 1986.

Miki, C., Toyoda, Y., Mori, T., and Enokido, N. “Fatigue of Large-Scale Welded Girders under Simulated Highway Loadings.” JSCE Structural Engineering/Earthquake Engineering, Vol. 7, No. 2, 1990, pp. 283s-291s.

10. Fisher, J.W., Jin, J., Wagner, D.C., and Yen, B.T., “Distortion-induced Fatigue Cracking in Steel Bridges”, NCHRP Report 336, Transportation Research Board, Washington D.C., 1990.

11. Maddox, S.J., Fatigue Strength of Welded Structures, Abington Publishing, Cambridge, 1991.

12. Fisher, J.W., Nussbaumer, A., Keating, P.B., and Yen, B.T. “Resistance of Welded Details under Variable Amplitude Long-Life Fatigue Loading”, NCHRP Report 354, Transportation Research Board. Washington, D.C., 1993.

13. Roy, S., and Fisher, J.W., “Modified Aashto Design S-N Curves for Post-weld Treated Welded Details”, Journal of Bridge Structures - Assessment, Design and Construction, Vol. 2, No. 4,2006, pp. 207-222.

14. Aashto Lrfd Bridge Design Specifications, 7thEdition, American Association of State Highway and Transportation Officials, Washington, D.C., 2015.

15. Manual of Railway Engineering, American Railway Engineering and Maintenance of Way Association. Washington, D.C., 2016.

16. Eurocode 3: Design of Steel Structures. Part 1.9: Fatigue, Comité Européen de Normalisation, Brussels, 2005.

The Bridge and Structural Engineer

17. Specification for Highway Bridges: Part II Steel Bridges, Japan Road Association, Tokyo, 2002.

18. MINER, M.A., “Cumulative Damage in Fatigue”, Journal of Applied Mechanics, Vol. 67, 1945, pp.A159-A164.

19. Tilly, G.P., and Nunn, D.E., “Variable Amplitude Fatigue in Relation To Highway Bridges”, Proceedings of the Institution of Mechanical Engineers (London),Vol. 194, 1980, pp. 259267.

20. Moses, F., Schilling, C.G., and Raju, K.S., “Fatigue evaluation procedures for steel bridges”, NCHRP Report 299, Transportation Research Board, Washington, D.C., 1987.

21. IRC:24 Standard Specifications and Code of Practice for Road Bridges – Section V : Steel Road Bridges (Limit State Method), 3rd Revision, Indian Road Congress, New Delhi, 2010.

22. Frank K.H., and Fisher J.W., “Fatigue Strength of Fillet Welded Cruciform Joints”, ASCE Journal of the Structural Division, Vol.105, No.ST9, 1979, pp.1727-1740/ 23. Eurocode 1: Actions on Structures - Part 2: Traffic Loads on Bridges, Comité Européen de Normalisation, Brussels, 2003.

24. Fisher, J.W., Fatigue and Fracture in Steel Bridges: Case Studies, John Wiley, 1984.

25. Yen, B.T., Zhou, Y., Wang, D., and Fisher, J.W. “Fatigue Behavior of Stringer-Floor beam Connections”, Proc. 8th Annual Int. Bridge Conf., Paper IBC-91-19, Engineers’ Society of Western Pennsylvania, Pittsburgh, PA, June 1012, 1991, pp. 149-155.

Volume 47 │ Number 1 │ March 2017

57

Construction Methodology & Geometry control during construction of cable stayed bridge over Bardhaman (Burdwan) station yard, Eastern Railway, India

Anirban Sengupta Jt. Vice President & Chief Technical Officer, STUP Consultants Pvt. Ltd. anirban.sengupta@stupmail.com

Yogesh Waingankar Senior Manager, STUP Consultants Pvt. Ltd.,

Graduated from Jadavpur University, West Bengal, India in the field of Civil Engineering in the year 1984. Worked in the field of Bridge & Industrial Plant (Steel, Power) engineering at STUP for 26 years and at Larsen & Toubro Ltd, for 6 years. Worked in engineering for PSC (Precast segmental, in-situ construction), steel composite, steel construction, suspension, extradosed, cable stayed bridges PSC Silos of large diameter, structures having large diameter pile. Published papers in FIB symposium & IABSE conference and other national & international conferences. Worked with various international & national standards like Euro, British standard, ASHTOO, Malaysian, Indian, DIN. Worked at various countries, Malaysia, Sri Lanka, Lao PDR, Bhutan, Marshall Island, Italy. Having member of Institute of Engineers (India), Indian Geotechnical Society, Indian Society of Earthquake Technology.

Graduated from the Government College of Engineering, Karad, Maharastra, India in the year 2003 and Masters in Structural Engineering from Visveswarya National Institute of Technology, Nagpur, Maharastra, India in the year 2005. Worked in the field of Bridge engineering at Stup since 2005 till date. Involved in design of long flyovers, river bridges with PSC structure (Precast & in-situ type, segmental box girder construction), steel composite bridges, extradosed & cable stayed bridges.

Summary

The method of construction and geometry control for a cable stayed bridge over Bardhaman Railway Yard and the predicted profile from a FEM construction stage analysis model have been discussed. Results obtained at site are compared with the predicted values and the actions initiated for complying with the design profile, deliberated in the paper.

Key Words: Stay Cable, Pylon, Creep, LUSAS, Stressing, Deflection, Deck Erection Crane, Back Span.

1.

Introduction

Eastern Railway decided to construct a Road over Bridge (ROB) over 9 nos. broad gauge electrified

58

Volume 47 │ Number 1 │ March 2017

track at Bardhaman station yard without provision of any foundation structure in between the lines over the ROW of about more than 100 m.

Considering the requirement of large clear span, over 9 (nine) existing electrified track with 25 kV AC traction of more than 98m width and limited space for approach to merge with existing road, the challenge was to construct a long span bridge. The AC traction line catenary wire clearance from the formation level of deck is very limited, the shortest being 1918 mm.

The depth of the girder & overall deck structure needed to be restricted. The type of long span bridges to suit such demand are Cantilever construction bridges like PSC balanced cantilever; Segmental bridge; Extradosed cable bridge; Cable stayed bridge; The Bridge and Structural Engineer

Suspension bridge. But with the control of depth required for a span of 120m, the Cable stayed bridge is found to be the best solution.

Moreover the erection and construction of such a bridge, with a clearance from contact wire maximum of 2.37 m and minimum of 0.788 m is a greater challenge.

2.

Strcutural Arrangement

The salient features of the bridge is as follows (Figs. 1, 2, 3 & 4):

Total Length of the Bridge : 188.431 m Carriageway width in each lane : 7.5 m Number of Lanes in each Direction : 2 CP1 to P1 (Steel composite deck) : 124.163 m P1 to CP2 (RCC Deck) : 64.265 m Width of bridge deck for main span : 27.7 m Width of bridge deck for back span : 28.2 m Footpath : 1.5 m on both sides Cross Slope : 2 %

Fig. 1 : Elevation and Plan of the Bridge

Fig. 3 : Cross-Section at main Span

Fig. 2 : Typical Cross Section of the Bridge

The Bridge and Structural Engineer

Fig. 4 : Plan of the Bridge

Volume 47 │ Number 1 │ March 2017

59

It is a cable stayed bridge with harp system.

3.2 Stage – 1 – Erection of 1st (First) Panel

The main span over railway station yard is having steel box type beams with RCC deck. The RCC deck consists of 130 mm precast plank and 120 mm cast-in-situ slab. The back span is of RCC construction monolithic with pylon P1 and pier CP2.

A. Materials for the first panel are fed by tractor and trailer.

The pylon is of RCC construction upto the deck level and of steel construction above deck level.

All steel component are connected to the concrete part with anchor bolts.

The steel girders over pier CP1 rest on bearing.

The main span has been divided into ten (10) segments of 12 m supported by nine (9) cables. The back span is also having nine cables at a spacing of 6.881 m.

3.

Constuction Method

Ascertaining the stresses in various components during the erection of superstructure for a cable stayed bridge is equally important, as at service condition.

Construction condition may be more critical in some cases than service condition and depends on sequence of construction/erection adopted, span, structural arrangement, stressing sequence, long term effects like creep in concrete, strength of material at the time of erection, temperature effect during construction across the section. Considering the main girders of steel construction, some of the effect like creep are not critical in this case. In this bridge the RCC back span remained supported on temporary trestles and shuttering system till the end of erection of all cables and completion of 1st stage stressing of all the cables. The back span of 64.265m is having more stiffness compared to the stiffness of pylon and main span. Hence stressing calculation at construction stage had to be carried out keeping in mind the above fact.

3.1 Prior Work Done A. Concreting of entire rear/ back span between P1 and CP2 completed and allowed to attain full strength. Concrete supports not removed or disturbed.

B. Four (4 m) meter long stubs (0 – 4 m) of the three longitudinal steel girders to be embedded in the pier concrete using long holding down bolts. Operation done with great control, supporting the girders and holding them in the right position to the proper line and level till the concrete is poured and sets.

60

Volume 47 │ Number 1 │ March 2017

B.

Mobile crane of adequate capacity is used for erection.

C.

The outer girder on the Howrah side (4m – 14m) is erected first.

D. Thereafter central girder and outer girder on the Asansol side are erected one after other.

E.

While erecting each girder above, the bolting of the joint with the previous stub should be completed in all respects before the slings are released.

F.

1) Temporary platform for workers and for inspection, erected at main joint

2) Safety net fixed under the steelwork with nylon ropes and hooks/Tensile fabric cloth.

3) Profile alignment and levels of steelwork erected thus far, inspected and checked.

4) Bolts of all joints now fully tightened.

Same procedures followed in all construction stages.

G. Cross girders between the main girders are erected.

H. First the cross girder nearest the pylon is erected; then the middle girder of the panel and lastly the cross girder farthest from the pylon are erected one after the other. They are securely bolted to the cross girder stubs attached to the main box girder.

I.

Stay Cables - The first set of stay cables on the steel side and the corresponding cables on the concrete side (viz. 2 nos. 6009, 2 nos. 6010, i.e. cable with outer girder & 1 no. 7009, 1 no.

7010, cable with central girder) are erected by the specialist agency and stressing done to the required level.If necessary, temporary wire rope supports for the box may be used.

J

Erection of Precast Panel – Precast concrete panels are erected between the cross girders covering the full width from the central to the outer box girder on each side.

K. In-Situ Concreting - After fixing shuttering on the sides of the outer beams, deck reinforcement placed in situ concreting of deck carried out.

L.

A minimum of 3 days of curing done before any work on the concrete.

The Bridge and Structural Engineer

Fig. - Stage - 1

3.3 Stage -2 (Erection of Deck Erection Crane (Dec) A. Panels 2 onwards are erected by the custom-built Deck Erection Crane (DEC) which is assembled on the deck.

3.4 Stage -3 3.4.1 Erection of second panel (14 m to 26 m) In the DEC, the cross frame projects out of the longitudinal gantry by about 8m, while lifting outer longitudinal girder. It is necessary to prevent the cross frame from unbalanced loads. This is done by lifting one outer girder by an outer winch while the winch at the opposite outer end is used to hold the other outer girder as counterweight, this being an additional precaution.

The Bridge and Structural Engineer

Fig. - Stage - 2

Volume 47 │ Number 1 │ March 2017

61

A. Move the DEC to correct position for erecting girders of the second Panel (14 m – 26 m) B. Move cross girder frame to correct position to pick up central box girder.

C. Pick up central girder and move it to its location.

D

Erect the central girder in position and secure by drifts and bolts with Panel 1, taking care to release the slings only after the box girder is secured and all drifts are replaced by bolts.

Fig. - Stage - 3

E.

Move cross frame back to correct position to pick up outer box girder.

F. Feed both outer girders simultaneously.

G. Cross frame to move forward carrying outer girders to position.

H. Erect both outer girders & taking care to loosen the slings only after both girders are secured with bolts and drifts.

B.

Move cross girders for the Panel at the deck level through central track.

C

Adjust winch positions for cross girder erection and move cross head back for picking up cross girders.

D. Winch on the cross head pick up the first cross girder on the Howrah side.

E.

Cross head moves forward to correct position for erecting cross girders.

F.

Cross girder is swung on the hook and erected in position.

G. The first cross girder on the Asansol side is erected exactly as above.

H. The other cross girders two on the Howrah side and two on the Asansol side are erected as above, the cross frame being moved to pick up the girders and moving to position to erect the girder in its position.

3.5 Stage - 4 (Erection of Second Set of Cables

3.4.2 Erection of cross girder In panel 14 M – 26 M

Cable two nos. each 6008 and 6011 on outer girder and one no. each 7008 & 7011 on central girder erected in position. Temporary wire rope supports to be used if necessary.

A. Cross girders are erected one by one and both halves of the girder also being erected one by one.

Tension as necessary imparted to cables checking profile simultaneously, making necessary adjustments.

Fig. - Stage - 3

62

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

3.6 Stage - 5 (Erection of Temporary Platform and Safety Nets and Profile Check) A. Temporary platform for workers and for inspection, erected at main joint.

B. Safety net fixed under the steel work with nylon ropes and hooks/tensile fabric cloth.

C. Profile adjustment and levels of steelwork erected thus far, inspected and checked.

D. Bolts of all joints now fully tightened.

3.7 Stage -6 (Erection of Precast Conrete Slabs and Pouring of Cast-In-Situ Concrete) A. Erect Precast concrete slabs (across the cross girders) closely, for a length of 12m along the span erect any side shuttering as required. Fix reinforcement as per drawing.

B. Pour in-situ M 60 grade of concrete for a length of 12 m segment above.

C. Check deck levels and tower verticality and make necessary adjustments, if required.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

63

3.7.1 Special note For safety, power blocks of OH traction line is

absolutely necessary and arranged during all stages of the deck erection system.

3.8 Stage -7 (Erection of Panels From 26 M To 110 M) The next seven panels (26 m to 110 m) are erected in the identical manner as Panel 14 m – 26 m, the

crane tracks for DEC and for material movement are progressively laid on the deck concrete and DEC moved as required from Panel to Panel.

3.9 Stage -8 (Erection of Panels from 110 M To 120 M)

D. Second/Final stage stressing is done for achieving profile.

Deck Erection Crane is dismantled and erection done with the use of mobile crane operating at ground level.

Even as the construction needs to be carried out over running rail track having electric traction no of safety issues and precaution measure has been taken.

3.10 Stage -9 (Final Completion of Panels from 26 M To 110 M)

1.

Trial assembly of all fabricated item has been carried out at fabrication yard prior to dispatch so that there will not be any mismatch of connection faced during erection.

2.

Operation of Deck erection crane has been tested at fabrication yard/shop prior to dispatch and erection over back span.

3.

During erection of girders the deck erection crane shall be anchored to the deck.

3.10.1 Erection of miscellaneous items - barriers, handrails, parapets, laying of wearing coat & final stage of stressing A. Crash barrier and handrails erected.

B.

Parapets erected/cast.

C.

Deck wearing coat laid.

64

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

4.

Deck erection crane and other mechanical equipment has been designed with 25% additional incidental load.

5.

Sizing of lifting rope has been carried out with a factor of safety of 5.

6.

Sizing of all drives has been carried out with 25% margin on power requirement.

7.

All drives are reversible type to allow both direction movement i.e. lifting & lowering.

8.

All brakes has been provided with Fail safe device.

9.

Earthing of steel girders has been provided for protection from 25 kV AC traction.

10. Safety net has been provided below each segment under erection.

11. Temporary access platform has been fixed from the erected segment outer & central girder, Cross girder for tightening the HSFG bolts, checking and inspection.

Based on the above the construction stage analysis to control the geometry has been carried out.

4.

Modelling

Geometry of deck and pylon has been modelled using beam elements in LUSAS. Deck has been modelled as grillage of longitudinal and transverse members.

Longitudinally deck is modelled as three main beams representing deck slab. Transverse beams (cross girders) have been modelled with steel composite properties. For modelling solid deck part of anchor span, slab is divided into no of beams. Spacing of beams in longitudinal and transverse direction has been kept to maintain the ratio of spacing near to unity. Main beams have been modelled as separate entity. Spacing of transverse beams for the main span has been kept same as that of the cross girders of each panel. Accordingly contribution of the deck slab has been considered for working out the properties.

Fig. 5 : Modelling Node Points on Outer, Central & Cross Girder

At pylon location and at the end of anchor span, deck has been integrated with substructure. So the substructure has also been modelled as part of grillage.

The wall has been divided in to longitudinal (vertical) and transverse members. The pile cap has been modelled along with spring supports with the stiffness mentioned in original service stage design report.

Pylon has been modelled with line elements along the centre line of members viz. pylon legs, cross ties, and anchor points in pylon head. Cables have been connected to pylon at height of their intersection with the centre line of pylon obtained from the drawings in longitudinal elevation.

Cables have been modelled as bar elements between pylon and deck without sag. Cables have been The Bridge and Structural Engineer

connected to longitudinal beams of deck at their point of intersection with beam centre line. Cross beams have been located at each intersection points.

Effect of sag has been worked out separately and accounted in the overall analysis by superimposition.

However alternatively sag of cables can be incorporated using beam elements for modelling the cables.

Longitudinal vertical profile of the deck has been followed precisely while modelling the deck. Also the vertical position of the longitudinal elements (relative to each other in transverse direction) has been modelled such that the transverse slope of deck is precisely modelled. Figs. 6A & 6B shows the complete LUSAS model below.

Volume 47 │ Number 1 │ March 2017

65

Fig. 6A : LUSAS Model of the Bridge

Fig. 6B : LUSAS Model of the Bridge

4.1 Section Properties of Various Members The section properties of members considered in the analysis model are as per the dimensions of girders provided in the drawing.

4.2 Material Properties Concrete properties: The widely accepted CEB-FIP Model 1990, which is used to represent the concrete properties with age effect is used. Variable Creep and shrinkage effects are considered in this model. The Compressive Strength of concrete varies with time is represented as-

Other concrete properties: Poison ratio = 0.15

Comp. strength = 40000 kN/m2.

Density = 2.548 t/m3 Relative humidity = 70% Coefficient of thermal expansion = 0.000012 Steel properties: Young’s modulus = Density = 7.85 t/m3.

200 E+6 kN/m2.

Poison ratio = 0.3 Coefficient of thermal expansion= 0.000012 4.4 Support Condition

s = 0.25 (For normal cement concrete) Concrete strength required is 50 MPa (cube) = 40 MPa (cylindrical) 66

Volume 47 │ Number 1 │ March 2017

At pier CP1, pin support has been assumed at bearing level. So only vertical and transverse translations have been restrained and other displacements have The Bridge and Structural Engineer

been allowed to be free. Pylon P1 and Pier CP2 are modelled as pilecap with spring supports of stiffness mentioned in original service stage design report which is based on soil structure interaction.

Temporary supports with their corresponding stiffness’s have been assigned to rear concrete deck.

These supports are modelled as “Compression only” spring supports and will be ineffective when the deck lifts off after cable stressing. The stiffness’s of these supports are calculated according to the actual section provided.

4.5 Loading Self weight of deck has been applied as body force

to longitudinal members, and the weight of cross girder has been applied as UDL on corresponding member. The weight of steel stiffeners, diaphragms have been precisely considered and their respective loading locations have been shown below. To account for the weight of evenly distributed stiffeners/studs, material density is modified appropriately.

4.5.1 DL considerations for central girder – (Fig. 7) The following table and figure shows a typical 12 m segment of main girder along with the DL considered.

Fig. 7 : DL Considerations with their Respective Locations (MG2)

4.5.2 DL considerations for outer girders – (Fig. 8) The following table and figure shows a typical

12 m segment of main girder along with the DL considered.

Fig. 8 : DL Considerations with their Respective Locations (MG1)

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

67

4.5.3 DL considerations for pylon The weights of pylon segments are also considered according to the detail drawings by appropriately modifying material density.

Weight of Pylon outer one – 3115 kN Weight of Pylon central one – 5070 kN 4.5.4 DL considerations for deck erection crane (DEC) – (Fig. 9) The self weight of DEC considered for analysis is

applied as four point loads. To account for the weight of rails etc., the weight of DEC has been increased accordingly (7.6% for front and rear support and 25.4 % for CW trolley). Hence the total weight of DEC considered is 1806 kN. The position of DEC at the time of panel erection is decided according to erection position. For ease in casting of deck, the DEC is allowed to move backwards by 8m from its previous position.

Fig. 9 : DL Considerations for DEC and CW Trolley

4.5.5 DL considerations for cable installation equipment and material trolley rails

–

For crash barrier: 1.946 kN/m2 (Applied to total width of 25.7 m)

To account for the weight of cable installation equipment, 1 MT load has been considered at each splice point location of main girders. For each of the six material trolley rails, 1 kN/m load has been assumed.

–

For wearing coat: 1.430 kN/m2 (Applied to total carriageway width)

5.

Panel Erection Time Construction Stages

4.5.6 SIDL considerations SIDL considerations has been based on the service stage design report as follows:

68

Volume 47 │ Number 1 │ March 2017

Cycle

and

The typical 27 day time cycle for complete erection of a panel is as given below.

The Bridge and Structural Engineer

Table 1 : Panel Erection Time Cycle Sr. No. 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

6.

ITEM Erection of MGs and CGs Fix handrails and access platforms Stay cable installation and stressing for main span Checking of cable forces & report submission Joint survey of working points Placement of precast slab and in situ concreting Concrete curing for 85% strength Joint survey of working points & report submission Geometry control and approval by DDC Re-stressing/stress adjustment if recommended DEC track extension Trolley track extension Stay cable installation and stressing for back span cable of next panel Move DEC forward Slack/cushion considered for planning

Methodology for Geometry Control

6.1 Analysis and Output The target is to achieve the required geometry of the bridge deck after completion of the bridge work construction i.e. open to traffic. The required geometry achieved 1) by adjusting the stay cable forces and 2) by providing pre-camber to the bridge deck. In many bridges, combinations of the two methods are adopted, but in the present case, it has not been feasible to provide the pre-camber to the pre-fabricated steel girder units and hence the geometry control is being exercised through adjusting the cable forces.

Cable forces for the each panel have been adjusted such that the upward deflection of the panel due to stressing of corresponding cable will counteract the net total downward deflections due to erection of subsequent panels and due to laying of SIDL.

●

DAYS 5d 2d 2d 1d 1d 11 d 7d 1d 3d 1d 2d 2d 4d 1d 2d

After concreting of the main span unit.

Also the cable forces for each of the above stages have given.

6.2 Monitoring of Structure To ascertain the behaviour of structure, it has been necessary to monitor the structure at site. In each panel cycle, deck and pylon has been surveyed at following stages: ● ●

● ● ●

After stressing of back span cables.

After movement of DEC to next position (Only front panel two points of each outer & central girders).

After erection of steel panel.

After stressing of main span cable.

One day after casting of slab

The cable stressing forces and expected deck deflections for all the construction stages have been calculated. For each panel, the deflections have been given for the following stages.

Panel tip point and anchor point for all the erected panels has been monitored along with top of pylon legs and cable anchor point of the pylon legs have been monitored at the end of each of the above stages.

●

After stressing of back span cable.

6.3 Corrective Measures

●

After Movement of Deck Erection Crane (DEC).

●

After erection of steel panel.

●

After stressing of main span cable.

After each stage, the survey report for deck and pylon deflections have been sent to the designer. After studying the difference in the deck RLs and pylon deflections, any deviations from the expected values, the corrective measures have been proposed.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

69

●

The pylon deflections can be corrected effectively with back span cable stressing.

●

The geometry control of the deck can be done with front cable forces.

●

Cable re-stressing is proposed if required.

For any proposal, the stress checks for all main girders, pylon and cables have been performed. Intermediate re-stressing for adjustment of profile has been carried only after completion of any segment i.e. the deflected shape was always within the controllable limit with respect to the predicted result.

The above procedure has been followed till the 9th panel erected and stressed the last cables 6001 for outer girder & 7001 for central girder & 7001. The 10th panel has been erected using DEC and the girders has been rested on packing with jacking arrangement at CP1.

The final stage of stressing data has been provided after carrying out the construction of Crash barrier and wearing coat, getting the survey data and analysed activating the load for crash barrier and wearing coat.

The profiles as predicted based on CSA and as obtained from the survey has been compared for every stage and stress in cables to be applied has been provided to site.

7.

Conclusion

After final stage re-stressing based on the survey data received from site and after completion of construction of crash barrier & wearing coat, the profile of deck has been achieved as follows and matching with that of calculated result from stage stress analysis having very minor variation within the perceptible limit (Figs. 10, 11, 12).

Fig. 10 : Deflected Shape Howrah Side Girder

Fig. 11 : Deflected Shape Central Girder

Fig. 12 : Deflected Shape Asansol Side Girder

70

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Fig. 13 : On the Verge of Completion – After Completion of Panel – 9

References

5.

1.

Manual for FEM package LUSAS for construction stage analysis provided by the developer.

2.

Acceptance of Stay Cable systems using prestressing steel – fib (CEB-FIP) publication 2005, fib bulletin 30.

3.

CEB-FIP (fib) model code 1990 published by Thomas Telford.

7.

4.

Recommendations for Stay cable Design, Testing and Installation by Post Tensioning Institute, 6th Edition, 2012.

8.

The Bridge and Structural Engineer

6.

Data sheet for Cable Anchorages of M/s Soletanche Freysinet.

Construction stage analysis of cable stayed bridge by Marko Justus Grabow, Thesis submitted to the faculty of the Technical University of Hamburg in partial fulfilment of the requirement for the degree of DiplomaIngenieur in Civil & Environmental Engineering, 2004, Germany.

Construction and Design of Cable Stayed Bridge

by Walter Podolony & John B. Scalzi, published

by John Willey& Sons, 1986.

Cable Supported Bridges by Niels Gimsing, published by John Wiley& Sons, 1983.

Volume 47 │ Number 1 │ March 2017

71

CALIBRATION OF IRC:112 PROVISIONS WITH Life-365 MODEL Ajoy Mullick Consultant, Cement and Concrete Technology, New Delhi, India ajoy_mullick@rediffmail.com

Dr. Mullick received degree in civil engineering from Patna University, India and MSc and PhD from University of Calgary, Canada. He is former Director General, National Council for Cement and Building Materials, New Delhi.

Summary

1.

Durability provisions in IRC:112 are aimed to provide service life of 100 years to concrete bridges. Models for predicting the time to non-compliance with a specified durability criterion for concrete structures are used as tools for service life design and prediction.

Among the various deteriorating mechanisms, the kinetics of corrosion of steel reinforcement due to chloride ions and carbonation have been described by mathematical relationships. Chloride corrosion processes find application of service-life prediction software like Life-365, which is described. The design approach for other mechanisms of deterioration is to aim to avoid these by ‘design out’ strategy, which does not require use of models.

Among the significant features of Concrete Bridge Code IRC:112–2011 [1] are the new scheme of classification of service environments, and recommendations for water-binder ratio and depth of cover suitable for each. These are reproduced in Tables 1 and 2 respectively. The provisions in Table 2 are essentially for chlorideand carbonation induced corrosion of steel. For other mechanisms of deterioration e.g. sulphate attack, alkali –silica reaction, frost attack and abrasion etc., cover depth is not important. For these, limit of waterbinder ratio, choice of cement type and chemical admixture should ensure durability; a ‘design out’ strategy [2].

The provisions of water-binder ratio and depth of cover in IRC:112 for different service environments are compared with predictions by Life-365 software.

Comments are offered on adequacy of the Codal recommendations.

Models for predicting service life requires estimation of time of non-compliance with a specified durability criterion related to corrosion of steel are used in the modelling. Softwares like Life-365 are available for prediction of service life, taking into account the various environmental and mix parameters [3]. Use of Life-365 in checking the provisions of IRC:112 are described in the paper.

Keywords: Bridges, carbonation, Code, concrete, corrosion, diffusion coefficient, durability, Fick’s law, life estimation, modelling.

Introduction

Table 1 : Classification of Service Environment (Table 14.1, IRC: 112) Sl. No.

Environment

i)

Moderate

ii)

Severe

Wet, rarely dry, humid (relative humidity > 70 percent), completely submerged in sea water below mid-tide level, concrete exposed to coastal environment,

iii)

Very severe

Moderate humidity (relative humidity 50 to 70 percent), concrete exposed to air-borne chloride in marine environment, freezing conditions while wet,

iv)

Extreme

Cyclic wet and dry, concrete exposed to tidal, splash and spray zones in sea, concrete in direct contact with aggressive sub-soil/ground water, concrete in contact with aggressive chemicals.

72

Exposure conditions Concrete dry or permanently wet, concrete continuously under water.

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Table 2 : Durability Recommendations for Service Life of at least 100 years (20 mm Aggregate) (Table 14.2, IRC:112) Exposure Condition Moderate Severe Very Severe Extreme

2.

Concrete Mix Properties Maximum Water/ Minimum Cement Minimum Grade of Cement Ratio Content, kg/m3 Concrete 0.45 340 M25 0.45 360 M30 0.40 380 M40 0.35 400 M45

Fluid Transport in Concrete

IRC:112-2011 [1] emphasises importance of fluid transport on durability. It states ‘one of the main characteristics influencing the durability of concrete is its permeability to the ingress of water, oxygen, carbon dioxide, chloride, sulphate and other potentially deleterious substances. Degree of permeability is governed by the constituents, the mix proportions and workmanship used in making concrete. A suitably low permeability can be achieved by having adequate cement content, low water cement ratio, use of blended cements, ensuring complete compaction of the concrete and adequate curing’ (clause 14.1).

Most of the deterioration processes e.g. corrosion, carbonation, alkali-silica reaction, sulphate attack, frost attack require presence of water or moisture in concrete. The various processes of transport of fluids – both liquid and gases – into concrete can be summarised as follows; ●

●

Flow of water due to application of a hydrostatic head, characterised by water permeability coefficient.

Water absorption and uptake of water resulting from capillary forces, characterised by a sorptivity coefficient.

●

Ion diffusion: movement of ions as a result of concentration gradient, characterised by ion diffusion coefficient.

●

Other variants are possible; like gas diffusion, water vapour diffusion, pressure induced gas flow etc.

In view of so many possible modes, one should really be concerned with a notion of collective ‘penetrability’ of fluids; nevertheless, the commonly accepted term is ‘permeability’, which is mostly adopted to describe transport of fluids through concrete [4].

Fluid transport depends mainly upon the structure of The Bridge and Structural Engineer

Minimum Cover, mm 40 45 50 75

hydrated cement paste. The microstructure that forms upon hydration of cement consist of solids having pores of various sizes in addition to the spaces originally occupied by the water. The porosity depends upon the age, the degree of hydration, the water/ cement ratio and the type of binder [5].

2.1 Kinetics of Chloride Ingress The rate of ingress of chloride ions from the outside surface to the cover concrete has, most often,been expressed following Fick’s Second law of diffusion, given by;

... (1)

Where, CS = surface chloride level, X = depth from surface, CX = chloride level at depth, X, t = exposure time, D = chloride diffusion coefficient, and erf = error function, given by;

erf (x) =

2 π

∫

x

0

2

e − t dt

... (2)

The phenomenon is depicted schematically in Fig. 1 [5]. The trend indicates that the amount of chloride ion diffused inside concrete decreases with the depth, X from the surface, increases with exposure time, t and is lower, the higher the chloride diffusion coefficient, D. To indicate ‘non-compliance’, Cx should equal or exceed the permissible limit of chloride content specified in the Code, and X is the depth of cover. It is to be noted that any ‘background’ chloride, i.e. from the ingredients of concrete, has been kept out of both sides of equation 1. Nevertheless, such chloride ions gets added in reaching threshold Cx. Volume 47 │ Number 1 │ March 2017

73

Fig. 1 : Depiction of Chloride Ingress in Concrete by Fick’s Law [5].

Two other parameters above require quantification.

The first is the chloride diffusion coefficient, D; and the other is the of concentration of chlorides on the surface, Cs. Value of D is lower, the lower is the water/binder ratio and given by the relation;

D28 = 1 x 10 (-12.06 + 2.40 x w/b) m2/s

... (3)

D is lower when mineral admixtures are added, in the manner indicated in Figs. 2 and 3, reproduced from Ref. 3; and decreases with age.

Fig. 3 : Influence of Silica Fume on Chloride Diffusion Coefficient [3]

The values of D obtained from Figs. 2 and 3 are at 28 days. Further decay occurs with time. Value at time t, D(t) with respect to Dref at time tref is given by;

D(t) = Dref. {tref/t}m

... (4)

Where, m is the diffusion decay index, a constant. Use of slag or fly ash influences decay in the value of D beyond 28 days and up to 25 years by the relation;

Fig. 2 : Dependence of Chloride Diffusion Coefficient on Water-Binder Ratio [3]

74

Volume 47 │ Number 1 │ March 2017

m = 0.2 + 0.4 (% FA/50 + % SG/70)

... (5)

Cs - The other variable, amount of chloride ions (Cs) at the surface of concrete depends upon the location

The Bridge and Structural Engineer

i.e. sea front, marine environment, coastal or inland.

The structural parts in contact with sea water will have high value of Cs, because four Cations – Na+, Mg2+, Ca2+ and K+, and two Anions – Cl- and SO42make up more than 99 percent of dissolved materials in the seawater. Among these, Na+ and Cl- ions are most abundant. 78 percent of the dissolved solids are NaCl and 15 percent MgCl2 and MgSO4. In coastal environments, amount of airborne chlorides depend upon the distance from the sea shore, wind velocity and many other factors. In inland constructions, chloride ions dissolved in soil and sub-soil water rise due to capillary action. Measurements of ambient air quality by environmental agencies do not record chloride ions present. So, the values have to be obtained by in-situ measurements or historical data.

Survey conducted in the Arabian Gulf indicated chloride profile with distance from the sea as shown in Fig. 4 [6]. Measurements of 20 – 25 years old RCC structures in Mumbai damaged due to corrosion of reinforcement, located 12 km from Thane Creek (Thane), near sea coast (Kandivali) and close to the sea (Ulwe), indicated concentration of total acid soluble chlorides as follows [7]; Ulwe – 0.13 % by wt. of concrete i.e. 1.0 % by weight of cement,

does not react with the hydrated cement. The highest rate of carbonation is expected to occur at a relative humidity of 50 to 70 percent [4, 5].

Life – 365 model, described below does not account for carbonation [3]. The only way to check the Codal provision is to ensure that carbonation front does not reach the reinforcement over the design service life.

The rate of carbonation in concrete is expressed in a simplified form [2, 8];

D = K. √t

... (6)

Where, D is the depth of carbonation, mm t is the time in years, and K is the carbonation rate coefficient, which is an environment and concrete related constant.

The value of K in concrete of low water/cement ratio, well compacted and cured has been estimated to be between 2 and 6, i.e. 2 < K < 6 [8]. For these values, the depth of carbonation over 100 years (t = 100) will be between 20 mm and 60 mm. For an average value of K = 4, the estimated depth of carbonation will be 40mm. fib-53 stipulates such a value for ‘high grade’ concrete as shown in Fig. 5 [2].

Kandivali – 0.053 % by wt. of concrete i.e. 0.4 % by weight of cement.

The values at Thane were lower.

These values have been used in the case studies described later.

Fig. 5 : Increase of Carbonation Depth with Time – Influence of Concrete Quality [2]

Fig. 4 : Chloride Profile – Distance from the Sea in Gulf Region [6]

Rate of carbonation IRC:112 stipulates that carbonation is not significant when the pores of concrete are saturated, because of slow rate of diffusion of CO2 in water compared to that in air. On the other hand, if there is insufficient water in the pores, CO2 remains in gaseous form and The Bridge and Structural Engineer

For the Arabian Gulf, cover of 60 – 70 mm, w/c = 0.4 and 28 days concrete strength 35 – 40 MPa has been recommended in coastal areas where carbonation is high due to relative humidity between 55 – 75 % [6]. Taking all these into consideration, stipulation of maximum water-binder ratio 0.40 and minimum cover depth 50 mm in IRC:112, when RH is between 50 to 70 percent (‘very severe’ category in Table 2), are expected to safeguard against carbonation destroying the alkaline passivity around the reinforcement.

Volume 47 │ Number 1 │ March 2017

75

3.

Application of Life-365 Model

The features of Life Estimation model Life-365 and application software are described in Ref.3, hence are not repeated here. Suffice it to say that it is used for predicting the service life of reinforced concrete exposed to chlorides. It uses a simplified approach based on Fickian diffusion that requires only simple inputs. The input parameters are the geographical location and temperature, type and size of structure, nature of exposure, chloride concentration at the surface and permissible chloride level in concrete, mix composition, cover depth, and any protection to concrete surface or the reinforcement etc. The chloride diffusion coefficient is based on water-binder ratio and mineral admixtures used, as explained before [3].

The concept of service life is explained in Fig. 6. When corrosion is caused by chloride ingress,

the service life is usually assumed to be equal to the initiation time, i.e. time taken for the critical threshold concentration of chlorides to reach the depth of cover. The period of propagation, which may be of short duration, is traditionally not taken into account, because of uncertainty with regard to the consequences of localised corrosion [2]. In IRC:112-2011 for concrete bridges, 100 years’ service life is the stage when limiting chloride ions reach the level of reinforcement, so that corrosion can be initiated. This is clearly the ‘initiation’ phase in Figure 6. The total service life including ‘propagation’ phase could be 10 – 20 percent more. However, Life365 Service Life Prediction Model [3] adds 6 years as ‘propagation phase’ to the ‘initiation time’ and the total is the ‘service life’. In this paper, ‘Service life’ has been taken as the ‘Initiation period’ as in IRC:112.

Fig. 6 : Models for Propagation of Corrosion of Steel in Concrete [2]

76

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

3.2 Calculation of Service Life A typical RCC circular bridge pier, 1500 mm dia. X 20 m high, exposed to airborne chlorides of different magnitudes was examined. Provisions of IRC:112 for w/c ratio and depth of cover for the given exposure condition were adopted (Table 2). Permissible level of acid soluble chloride inside concrete was taken as 0.2 percent by weight of cement, as per Cl. 14.3.2.3 of IRC:112 [1]. Using Life -365 model, the time of inset of corrosion was calculated. Allowing another 6 years for the propagation of cracks to unacceptable level, the total service life was computed [3].

Case I – Bridge in coastal environment Hot, airborne chlorides, concentration of acid soluble chloride ions on surface, Cs = 0.40 % by weight of cement.

‘SEVERE’ service environment, maximum w/c ratio is 0.45, minimum cover depth = 45 mm (Table 2).

RCPT value of 1500 Coulombs has to be obtained (Cl. 18.6.7 of IRC:112), which is not possible by use of OPC alone and mineral admixtures have to be added. Consider incorporation of 30 % fly ash in the total cementitious material.

Permissible acid soluble chloride content, Cx = 0.20 % by weight of cement.

Using these parameters, the calculated service life is > 100 years.

Case II – Bridge near the sea front Hot, relative humidity – 50 to 70 percent, concentration of acid soluble chloride ions on surface, Cs = 1.00 % by weight of cement.

‘VERY SEVERE’ service environment, maximum w/c ratio is 0.40, minimum cover depth = 50 mm (Table 2).

RCPT value of 1200 Coulombs has to be obtained.

Consider incorporation of 30 % fly ash and 7 % silica fume in the total cementitious material.

Permissible chloride content, Cx = 0.20 % by weight of cement, as above.

Since high performance concrete is used, the cover depth can be reduced by 5 mm (Note 4 under Table The Bridge and Structural Engineer

14.2 of IRC:112). However, for computation, 50 mm cover is used.

Using these parameters, the calculated service life is < 100 years. Hence, either the water/binder ratio has to be lower than 0.40, or, additional protection measures have to be adopted.

The additional protections can be in the form of use of galvanised reinforcement, use of surface coatings to the concrete etc. (Cl.14.4.1 of IRC: 112). Galvanised reinforcement is expected to have a greater tolerance to the level of chloride before onset of corrosion, i.e. higher value of threshold Cx. In Life-365 model, however, use of galvanised steel reinforcement has only the effect of enhancing propagation time after initiation of corrosion from 6 years to 20 years.

IRC:112 stipulates that use of surface coating inhibits penetration of chlorides and CO2 [1]. In Life-365 model, waterproofing membranes or sealants applied to the exposed surface of the concrete are expected to impact the rate of chloride build- up on the surface during its life time (20 years and 25 years respectively) and no change in effective chloride diffusion coefficient [3].Thus, benefits of use of these protective measures as envisaged in IRC:112 are not reflected in the same manner in Life- 365 model.

Using either of the above protection strategy, adequate service life is expected.

4.

Conclusions

The rationale of provisions in IRC:112 to guard against chloride corrosion and carbonation corrosion are examined. Chances of carbonation front reaching the reinforcement bars under adverse atmospheric conditions (RH between 50 to 70 percent) are minimised by the choice of w/c ratio and cover depth prescribed, provided the concrete is well compacted and well cured. The provisions are adequate for guarding against chloride ingress; however, additional protection measures may be required, when the concentration of acid soluble chloride ions on the surface exceeds 1.0 percent by weight of cement.

References 1.

Indian Roads Congress, Code of Practice for Concrete Road Bridges, IRC:112-2011, 280 p.

Volume 47 │ Number 1 │ March 2017

77

2.

fib (CEB – FIP), Structural Concrete, Textbook on behaviour, design and performance, Bulletin 53, Volume 3, Second Edition, December 2009.

376 p.

3.

LIFE-365TM Consortium III, Life-365 Service Life Prediction Model, Version 2.2, August 2013, 86 p.

4.

Neville, A. M., Properties of Concrete, 4th ed., 2000, Pearson Education Asia, 844 p.

5.

Mullick, A.K., ‘Durability Provisions in Concrete Bridge Code IRC:112’, The Bridge and Structural Engineer, Vol. 44, No. 2, June 2014, pp. 101 – 109.

78

Volume 47 │ Number 1 │ March 2017

6.

Haque, M.N., Al-Khait, H. And B. John, ‘Proposals for a Draft Code for Designing Durable Concrete Structures in the Arabian Gulf’, The Arabian Journal for Science and Engineering, Vol. 31, No. 1C, June 2006, pp.

205 – 214.

7.

Pattanaik, Suresh C., Gopalakrishnan, E., and Patro, Sanjay K., ‘A Study on Deterioration of Reinforced Cement Concrete Structures in Mumbai’, The Indian Concrete Journal, Vol. 89, Issue 5, May 2015, pp. 51 – 58.

8.

Bertolini Luca, Elsener Bernhard, Pedeferri Pietro, Polder Rob, Corrosion of Steel in Concrete – Prevention, Diagnosis, Repair, WILEY-VCH, 2004, 392 p.

The Bridge and Structural Engineer

COLLAPSE OF KOLKATA FLYOVER - PRACTITIONER’S PERSPECTIVE

Prabhakar NARASINGARAO Consulting Engineer Mumbai, India nrprabhakar@gmail.com

Dr. Subramanian NARAYANAN Consulting Engineer Gaithersburg, MD 20878, USA drnsmani@gmail.com

Er. N. Prabhakar, BE, CEng(I), MIStructE (UK) has over 55 years of professional experience with 13 years in UK. He has designed numerous structures such as chimneys, hyperbolic natural draught cooling towers, large water tanks, and TV towers. He authored many software packages and 25 technical papers, and served as codal committee member. He is a member of many professional institutions such as IE (India), ISWE, and ICI. His cartoons appeared in ICJ for the past 19 years and a collection was published by Ambuja Cements.

Dr. N. Subramanian, a doctorate from IITM, has worked in Germany as Alexander von Humboldt Fellow during 1980-82 and 1984. He has 40 years of professional experience which includes teaching, research, and consultancy in India and abroad. He has authored 25 books and 240 technical papers and has won the Tamil Nadu Scientist Award, the Lifetime Achievement Award from the Indian Concrete Institute (ICI) and the ACCE(I)-Nagadi best book award for three of his books. He also served as the Vice-President of ICI and ACCE(I). He is in the editorial/review committee of several journals.

Abstract The recent collapse of Kolkata Flyover has put several question marks on the engineering practices adopted in the construction of flyovers. It has eroded the confidence of the common man on engineers and construction companies responsible for their construction, and certainly blotted their professional reputation. Many questions were raised in the news media on the basis of the award of the contract, with a political tone. The usual suspicions of using substandard materials, lack of supervision, etc. were also raised, but very little discussion was there in the media on technical matters like under-design of structural elements and inadequate connections between the elements for the collapse. The IIT report submitted to the Govt. of West Bengal also found fault with all the aspects of this collapse-from quality of materials, faulty approval of design to improper project execution. This paper focuses its attention on the design and detailing aspects only as it may be the main reason for collapse of the Kolkata Flyover.

The Bridge and Structural Engineer

The importance of proof checking, certification, and continuing education are emphasized, as these alone can eliminate such failures in future.

Keywords: Collapse connection detailing.

1.

analysis:

Steel

flyover;

Introduction

Bridges and flyovers are critical links in any transportation network. Failure of any crucial bridge/ flyover not only results in precious loss of lives, injury and huge property loss, but also affects the economy of the region. For example, it was found that the collapse of I-35W Mississippi River Bridge (which was used by more than 140,000 vehicles per day) resulted in huge economic loss to Minnesota, USA –about $17 million in 2007 and $43 million in 2008. Each failure should be analysed and the causes should be reported widely, so that other engineers who are involved in similar projects will not repeat the same mistake and can learn from the mistake of others. Hence in this Volume 47 │ Number 1 │ March 2017

79

paper, the causes of the recent failure of flyover in Kolkata are reported.

On 31st March 2016, a segment of flyover in Kolkata which was under construction collapsed suddenly causing casualty of 26 people in a short period, and injuring more than 80 people severely (see Fig. 1). The first reaction of the construction company was that it was an “Act of God”, as such a collapse had never happened in their 27 years of experience of constructing bridges [Ref. 1 and 2].

This collapse, in addition to the periodic reporting of collapses occurring all over India, has eroded the confidence of the common man in engineers and construction companies responsible for their construction, and certainly blotted their professional reputation.

Fig. 1 : Scene after the collapse of a Span of Kolkata Flyover

Following the collapse, several questions were raised in many national newspapers and TV channels about the basis of the award of the contract, selection of sub-contractors and material suppliers, etc., with a political tone due to state elections that followed few weeks later. “The entire system is at fault” said many.

The usual suspicions of using sub-standard materials, lack of supervision, etc., were also raised, but very little discussion was there in the media on technical matters like under-design of structural elements and inadequate connections between the elements for the collapse. These matters were discussed in detail in the website Sefindia.org of Structural Engineering Forum of India (SEFI), which has more than 20,000 Structural Engineers as members. Many senior SEFI members participated in the discussion forum as lot of information emerged from the site visit of collapsed structure, and many detail photographs from collapsed site were also made navailable later.

80

Volume 47 │ Number 1 │ March 2017

From the discussion of an experienced Bridge Engineer and SEFI member, had with a visiting professor of IIT, Kharagpur, who had visited the accident site, it was known that this flyover collapse was triggered by the failure of a pier cap. This professor had also mentioned that the structure had given sufficient warning 6 to 7 hours before the actual failure. It was revealed that when concreting of the deck slab was half way, the bolts in the cross girder of the steel girders, placed on cantilever pier cap, sheared off. This apparently happened because of excessive deflection of the pier cap, over which these longitudinal girders were placed. The failure occurred when concreting of deck slab above the steel girders, on one carriageway, had just been finished.

The concrete was still in the green state when the failure took place. Some close-up photographs gave much needed clues of the behaviour and failure of the structural elements, particularly the connection details between the structural elements. In addition, as per the Hindu dated 12th Aug. 2016, the committee set up

by West Bengal Chief Minister Mamata Banerjee to probe the collapse, found defects in multiple aspects of the flyover construction. It found the design of the flyover to be faulty, and also pointed out inconsistency in construction material, faulty approval of design, lack of quality check and improper project execution on part of the Kolkata Metropolitan Development Authority.

2.

Brief Details of The Flyover

The long-delayed 2.5-km Vivekanda flyover under the Jawaharlal Nehru National Urban Renewal Mission was expected to tackle congestion in Burrabazar area - the location of one of the largest wholesale markets in Asia - up to the Howrah station, the gateway to the city. This flyover consists of two carriageways made of composite construction, i.e., reinforced concrete deck slab over steel plate girders which are supported on steel piers at intervals along the length of the flyover (see Fig. 2). The project’s foundation was laid in 2008 and work on the Rs.164-crore project began on February 24, 2009. It was scheduled to be completed in 2012 but land acquisition issues delayed its completion. The implementing agency also ran into financial troubles.

The Bridge and Structural Engineer

Fig. 2 : Part plan of Kolkata Flyover at the Location of Collapse

3.

Failure Analysis

In the absence of detail drawings about actual dimensions of the flyover, sizes of structural members and their connection details, to analyse the cause of collapse, one has to rely heavily on the photographs shown below that got published in the SEFI website (see Figs. 3 to 6). Moreover, close look of these photographs will show that the main cause of failure is the peculiar joint detail adopted at the cantilevered beam at pier 40(C). The first photograph shows the twisting of steel plate girders placed on top of cantilever girders, which indicated that the failure could have been due to lateral torsional buckling of the girders, as there may be inadequate bracing to their top flanges[Ref.3].

Fig. 4 : Complete collapse of the Two Steel Cantilever Girders Over the Central Pier

Fig. 5 : Close-up view of the Pier Cap Fig. 3 : Twisting of Steel Plate Girders Placed on Top of Cantilever Girders

Fig. 4 Shows the Complete Collapse of the Two Steel Cantilever Girders of About 7.5 m Length Over the Central Pier 40(C) which Supported the Carriageways.

The close-up views of the pier cap, as given in Figs. 5 and 6, and provided by Er. Subhajit Chaudhuri in the SEFI website has revealed that the collapse was indeed due to the peculiar joint detail adopted by the designer.

The Bridge and Structural Engineer

Fig. 6 : Close-up view at the Top of Pier Cap

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

81

From Figs 5 and 6 it is clearly seen that the box section of the cantilever girders was not connected to the vertical face of the Pier 40(C) by either bolting or welding. The design strength of the cantilever girders was provided only by the top plate of the box section (see Fig. 5) and 4 nos. small sized beams below it.

The posting by Er. Tridibesh Indu on 8th April 2016, at SEFI website states as below: “Kindly note that the Pier 40(C) which had collapsed on 31st March 2016 was supporting two simply supported spans. On one side of the pier, deck slab on both the carriageways were already cast. On the other side, concreting for

deck slab was done for one of the carriageways while the other carriageway slab was not cast prior to the collapse.”[ Ref. 3].

Hence, it is seen that Cantilever Girder no. 1 (see Fig. 7) carried the full dead load from the deck slab on both sides, whereas Cantilever Girder No. 2 carried the dead load from the deck slab on one side only. This is also clear from the photograph shown in Fig. 8. It has to be noted from Fig. 8 that the concrete debris had fallen on one side whereas on the other side the bare plate girders have fallen down, without having any concrete over them.

Fig. 7 : Collapse of Cantilever Girders at Pier 4º(C) with Plate Girders and Concrete Deck Slab

Fig. 8 : At Failure, Concrete Deck Slab had been Laid on one Cantilever Side only

The cantilever girders did not collapse when the concrete had been laid on one side of the girders only.

When the new concrete was laid on the other side which was supported by Cantilever Girder No. 1, this girder collapsed first due to flexure and shear failure, and collapse of Cantilever Girder No. 2 took place following it because of a common beam supporting them. This means that the joint at the cantilever girder was not designed even for full dead load condition of having concrete deck slab on either side 82

Volume 47 │ Number 1 │ March 2017

of the girders, apart from the deficiency they had with deflection. When the loads were applied, the bottom edge of the cantilever beam was pushed into the hollow steel column (see Fig. 5), making it to dent.

Due to this there was heavy tension in the top plate of the hollow cantilever (see Fig. 7b), which teared off, when it exceeded the ultimate tensile force, as shown in Fig. 6. This initiated shear failure of the side plates of the of the hollow cantilever beam, thus resulting in total collapse. In addition, as seen in Fig. 4 and 6, the cantilever girder 1, had a large opening near the supporting end (near the pier) and also a spliced connection. It is not clear how it had affected the shear capacity of the beam. In any case such an opening near the support is undesirable and will surely reduce the shear capacity of the girder. Another major point of weakness was the inadequate number of bolts in the splices, where 16 mm diameter bolts were used in many important locations.

It has to be noted that other spans of the flyover did not fail. It is because in other spans, there are two piers supporting the hollow beams, in which case it will be in simply supported condition, and hence there will not be any problem. Since, in this section The Bridge and Structural Engineer

(40(C)), only one column is provided, the cantilevered beam resulted in a failure, as explained above.

In addition, it is apparent that the cantilever girders were not at all designed to carry any super-imposed (vehicular) loads that would be there on the flyover when it will be put into service. Had the cantilevered beam survived somehow this dead load condition, it would have failed in service, resulting in more serious collapse.

This collapse could have been averted, had the designer allowed the hollow cantilever to go over the column and supported it on the hollow column, using a cap plate connection, in which case, there will not be any reduction in the size of the cantilever beam at the support (where the bending moment will be maximum), which initiated the failure.

Considering the seriousness of this collapse, the structural design and quality of construction for the whole length of the flyover that has been built already, is to be thoroughly checked, and even load tested as per IRC procedures, for structural safety and stability of the whole flyover before it is put into service.

4.

Proof Checking and Certification

In countries like USA and Europe, the designs of important buildings and bridges will be proof checked by some competent authorities, who will independently check the analysis, design and detailing of the structures. Such a procedure eliminates the percentage of failures, and any mistake made by the original designer, is found and corrected at the design stage itself. Moreover, the contractor who builds the structure is also well qualified and certified, and hence even if there is a constructability problem, which is missed even by the proof checker will be identified by him/her and will be rectified before construction.

With the proliferation of engineering colleges in our country coupled with the non-availability of dedicated and qualified teachers, the quality of education is poor. Moreover, inexperienced engineers think that if a computer analysis is done and if the computer results are followed verbatim, the structure will be safe. Unfortunately, it is not a correct assumption, as proved again in this case. Fresh engineers are not able to draw even simple bending moment or shear force diagrams or deflected shapes of simple structures [Subramanian, 2011]. A good engineer is one who is in a position to check the results using a simplified, The Bridge and Structural Engineer

‘back on the envelope’ calculations. It is high time engineers coming out of colleges are certified as it is done in countries like USA, through rigorous testing.

In addition, their certification should be extended after the initial period by interviews and additional continuing education courses. It is because, there is an exponential growth of research and development that is going on in several establishments, all over the world, and hence it is impossible for the practicing engineer to know these developments and adopt them correctly in their day-to-day practice. Professional organisations like the American Society of Civil Engineers (ASCE) are debating to consider masters degree in Engineering as the minimum qualification for doing practice.

5.

Conclusions

The close-up photographs of the collapsed cantilever girders clearly show some unconventional connection details of the cantilever girders to the vertical Pier 40(C) supporting them. The continuity of the cantilever girders which are made of box section, were provided only through the top flange of the girders and 4 nos. small sized beams placed below it. Otherwise, there are no connections between the pier and the girders at the vertical faces by way of seating or web cleats at the face of the pier to support the girders and to resist vertical shear from the girders. These are certainly a bizarre way of doing the connection details which make us wonder whether there was really any involvement of a qualified structural engineer on this job. If a third-party proof-checking had been made on the design and drawings of this supporting structure, prior to construction, it would have saved the collapse and 26 lives. In view of this collapse, the whole length of the flyover that has been built already is to be thoroughly checked for structural safety and stability before it is put into service. It is also important to introduce compulsory certifications and continuing education, in order that such failures are minimized in future.

6.

Acknowledgement

Many photographs cited in this article are taken from the Structural Engineering Forum of India, Sefindia. org website [Ref.3]. The authors wish to thank and acknowledge the numerous Structural Engineers who participated in the discussions and provided the photographs.

Volume 47 │ Number 1 │ March 2017

83

References 1.

http://www.thehindu.com/news/national/allare-at-fault-says-iit-report-on-kolkata-flyovercollapse/article8975296.ece

2.

http://www.theweek.in/news/india/whatcaused-kolkata-flyover-collapse-experts-speak. html

84

Volume 47 │ Number 1 │ March 2017

3.

http://www.sefindia.org/forum/viewtopic. php?t=17673

4

Subramanian, N., “Are Our Structural Engineers Geared up for the Challenges of the Profession?”, The Indian Concrete Journal, Vol. 85, No.1, Jan.2011, pp.20-26.

The Bridge and Structural Engineer

Learning for Civil Engineers from recent Bridge Collapses Vivek ABHYANKAR Structural Engineer AGM-Design, Afcons Mumbai, India abhy_vivek@hotmail.com

Summary India experiences heavy rain fall in the month of June – July – August. During this season many rivers experience vigorous foods. Small villages situated on the banks of big rivers like Ganga, Yamuna, Godavari etc. get media coverage only when flood water enters in habitable areas. In heavy rains often the roads get waterlogged and the connectivity of remote villages gets lost. In extreme events the bridges and roads get washed away with the river water.

In recent past Industry came across many such bridge failures during rainy season and also during construction stages. But most of these failures were attributed to carelessness and could have been avoided with proper precautions. All these collapses killed several lives and resulted into loss of money and time and left sorrow behind! What did we (Engineers) learnt from these failures, is a matter of discussion. In the present paper author has tried to present various dimensions of this topic and the various modes of failures observed and the aftereffects of a few selected cases. The aim is not to point fingers towards anyone but to bring the issue to forum for improvement. Hope after reading this paper the unity amongst the field professionals will improve for the good cause!

Keywords: HFL; geometric torsion; instability; uplift bearings; seismic arrestor; dilapidation; design-life; structural health monitoring; retrofitting.

1.

General

Bridges are the backbones of surface transportation sector. Bridges establish connectivity over an obstacle like river, gorges, existing roads, railways etc. to render a smooth, shortest riding surface for vehicles,

The Bridge and Structural Engineer

Born 1977, Started career as site engineer, did PG in Structural engineering, Gold Medallist. Worked with HCC, TCE, BV; visiting professor, author of papers, coauthor in two books. Working with Afcons as AGM-Design. Area of interest design, value engineering and coordination in Bridges, Underground metros.

railways, waterways, pedestrians etc. Bridges experience problems due to negligence at various steps like - during planning and design process; during soil investigation; during actual construction and during maintenance. Many roads which are constructed a few decades back or more, are rarely inspected for modified vehicle loadings (especially bridges laying on busy routes). Negligence at any one or more stages in bridge engineering may prove to be detrimental, leading to problems of various types like vibrations, cracking, corrosion, instability and (catastrophe) local / global collapse.

In India the theoretical knowledge of bridge engineering (planning, design, detailing) is available upto sufficient extent, and various eminent consulting firms are able to rigorously plan and design the bridges; whereas the construction, monitoring, testing, maintenance and repairs are not known to sufficient extent due to lack of research and lack of funding / time. Availability of skilled man power is another fact which adds to the severity; the young generation finds lot of interest in social net-working but show hardly any patience towards engineering research work (lab-testing). Lack of budget to perform lab tests is equally responsible for restrained growth of engineering profession in India. Even there are limited numbers of accredited material testing labs available in land.

Another practice in India is ‘lowest bidder’ wins the tender. But often at lowest rate it may be difficult for any consultancy or contractor to render highest quality. So to meet the project’s basic requirements (without reducing the desired profit) the agencies have to cut the corners or turn into ‘arbitration / litigations’ demanding for increase in cost after winning works; or Volume 47 │ Number 1 │ March 2017

85

sometimes leave the work in between. In today’s time most of the eminent Indian construction companies and consultancies are facing financial crises in transportation projects because of delay in receiving payments from clients; delay in land acquisition, getting reduced payment due to changes in scope after the award of work; money spent behind arbitration and litigation and so on ! because of such scenario, the remuneration paid to civil engineers is discouraging and also leading to brain drain. Bureaucracy, biased media and hidden political agendas stay at the apex of such disappointing status. Favouring specific agencies for specific works also colludes the professional scenario and kills healthy competition.

The bridge failures in recent years substantiate the above points. Repeated occurrence of such collapses indicate that – ‘we Civil Engineers in India are unable to gear up and move forward towards a change’ (especially the engineers placed at client side); it is very much painful fact! After certain failures

instantaneous changes are seen; but they remain for a small time and lose steam very soon!

In this paper author has briefly discussed about seventeen bridge failure cases occurred in recent years, learning from these failures and the way forward.

2.

List of recent Bridge Collapses (in Road, Metro, Railway, foot overs)

Bridges failures can be classified into two main types :- (i) Serviceability failures and (ii) Stability/strength or overall failure (a catastrophe). The serviceability failure could be like – cracking, vibrations, potholes in wearing coat, worn-out expansion joints etc. whereas the stability or strength failure is catastrophic in nature due to the enormous loss of material, lives, adjoining properties and time. It is the serviceability failure, which if neglected then can slowly blow up and convert into a catastrophe. Table. 1 below shows a short list of such selected catastrophes occurred in recent past which were churned by media for some time.

Table 1 : Showing Summary a Few Recent Bridge Collapses Yr. Failure

Age

Type

Status

Location

Type of Super Structure

Reason of Collapse

Loss of Life/ damage

Road Bridge Collapse 2013

5 Yrs

Road

2014

--

Road

2016

--

Road

2016

100+ Yrs

Road

2016

44 Yrs

Road

2015

--

Road

2016

--

Road

2016

--

Road

86

Instability Kolkata – Curved Steel Major damage no caused during Ultadanga area composite life loss vehicle hit During Curved RCC Poor detailing in Surat Huge loss Construction box of bearings Poor detailing Kolkata Steel During (near Ganesh of connection Sever loss Construction composite Talkies) in piers Mumbai – Goa Slab type, Highway on brick Old Operating Sever loss river Saraswati masonry dilapidated near Mahaad piers Himachal Slab type Heavy Rains Huge loss Operating Pradesh Steel truss Operating Shimla (HP) Instability Major damage type (Bailey) Sillod – RCC slab Kannad Old Minor damage, with stone Operating Road near dilapidated connectivity lost piers Aurangabad One truck got During Kolkata, PSC I-Girder Instability crushed under construction Howrah girder Operating

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Yr. Failure

Age

Type

Status

Location

Type of Super Structure

Reason of Collapse

Loss of Life/ damage

2016

--

Road

Operating

Odisha

Concrete slab

Central pier

Time loss

Metro Segmental Bridge Collapse 2007

0

Metro

During Construction

Delhi

Box Segmental

2009

0

Metro

During Construction

Hydrabad

Box Segmental

2012

0

Metro

During Construction

Mumbai

Box Segmental

2013

0

Metro

During Construction

Chennai

Box Segmental

Bridge All caused erection gantry severe damage to (launching structure and life girder) gave loss away

Railway Bridge Collapses 2002

--

Rail

Operating

Rafigunj

Steel Plate Girder

Old dilapidated

130+

1981

--

Rail

Operating

Bihar

Steel Truss

Derailment

300 or more

--

Rail over

Operating

Pokhra

Steel Plate Girder

Old

Foot over Bridge 2010

--

Foot over

During construction

New Delhi

2.1 Ultadanga Flyover, Kolkata A flyover at Ultadanga area of Kolkata city connects two important parts of the city; it was in fully operational state till 2013. In March 2013 one of the curved steel spans collapsed down in early morning (late midnight) when a truck hit crash barrier; full span came down and fell in nala below along with

Steel Arch suspended concrete deck

Brand new

Loss of time and life

the truck. This is a classic example of bridge failure at ‘minimum load condition’. It is said that probably the ‘uplift/tension’ bearing and seismic arrestor could have arrested the dislodgement of span and hence the collapse. The structure was later restored. But two other recent collapses in Kolkata make us to think, why so repeated bridge collapses and what did we learn!.

Fig.1: (Left) Bird’s Eye View of Ultadanga Bridge Collapse; (Right) Collapses Curved Span

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

87

2.2 Surat Bridge Surat bridge collapse is another example of collapse of a simply supported curved bridge span, which was engineered carelessly. It killed innocent workers and put the careers of many innocent professionals at stake. The superstructure had acute curved concrete super-structure (90º turned in plan leading to severe geometric torsion), with inappropriate support mechanism. The complete episode tarnished the image of profession in front of the society. Such collapses lead to wastage of national resources – men, material and money. Often the failures are reported in such a way that the effectiveness of finding is questionable! i.e. the facts are turned and twisted and blame game goes between various parties involved. In the race of ‘timely completion’, many minute details are not reviewed, rigorously.

got brutally killed, many cars, etc. got trapped below debris.

In EPC type projects the entire responsibility is assigned to the contractors. But while awarding the contract the clients should verify ‘design capability’ and previous experience of the contracting agency (and the agency to which the design is out sourced by the contractor/s).

Fig. 3 : Flyover Collapsed in Kolkata near Ganesh Talkies

It is said ‘No Lunch is Free in this world’; but in Civil Engineering this fact is still not imbibed by the clients as a result of which the clients want the contractor to build ‘Taj’ at the cost of ‘hut’. The recent bridge collapses are the classic examples of this tendency.

Fig. 2 : Flyover Collapsed in Surat City

2.3 Kolkata Flyover Near Talkies This is a second failure of bridge occurred in Kolkata in last three years. More than hundred innocent lives

2.4 Bridge on River Saraswati Savitri river is wide vigorous perennial river which flows through Konkan region of western Maharashtra through town named Mahad. About a century back a road bridge was constructed on Savitri river using locally available masonry and the technology at that time. The bridge was an engineering achievement in those days and it was laying on the busy route from Mumbai to Goa; but was not maintained adequately.

Fig. 4 : (Left) Arial view of Washed Span of Bridge on Savitri River (Right) Closer look on Masonry Pier

88

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Still surprisingly just before the collapse the bridge was inspected (?) and certified to be Safe! during heavy rain the bridge collapsed at night time. Many of the vehicles did not notice this and unfortunately simply fell down in vigorous river flow from the broken span.

Post collapse the military search operation was carried out with full speed, it faced lot of hindrances because vigorous water flow and reptiles in water; heavy capacity cranes and magnets were utilised to trace the vehicles but it was all in vain till water resided!

What did we, ‘Professionals’ learnt from this collapse? did we ever questioned it or the news vanished like other media news!! Did we question – how to link the ‘design life’ of a bridge to the quality of construction and degree of maintenance? Did we link the ridding level to foreseen high flood level (HFL)? Many question, many views – but no codal provisions / guidelines for practitioners!!

2.5 Bridge in Himachal Pradesh (Pathankot)

members kept mum on it. The bridge failures are attributed to three reasons – (i) incorrect design (ii) bad construction (iii) inadequate maintenance.

2.6 Steel Bridge Collapse near Manali, H.P. Yet another river bridge collapse! This was a steel Bailey bridge connecting North Portal access of a Rohtang Tunnel in Himachal Pradesh. Spanning across river Bias (distributary) has quitea strategic location, near village Sisu. It collapsed when a loaded truck was passing over the bridge, and resulted into complete cut-off for some time. Himachal Pradesh and J&K states have many locations which are inaccessible for heavy vehicles, sophisticated construction equipment. Heavy snow falls, unpredictable weather, hilly terrane, local social beliefs often cause hindrance in the development works (especially construction and maintenance activities). Irrespective of all these hurdles the BRO and local contractors play appreciable role in the national development.

But often it is found that the fund is sanctioned with restrains and contractors have to work with lot of restrictions in challenging situations. Manali bridge collapse makes us to think about these other aspects (other than technical) as well.

Fig. 5 : Another River Bridge Collapsed in Pathankot

During 2016 rains, when at one place the Savitri river bridge was collapsing at other place ‘Pathankot’ near Himachal other bridge collapsed due to similar reason; flood. This bridge collapse was equally important as it brings our attention to ‘dilapidated’ bridges which get often washed out during vigorous rains. But this bridge received limited media coverage, and the fraternity

Fig. 6 : Steel Bridge Collapse in Rohtang,Manali

2.7 Sillod – Kannad Road Near Aurangabad

Fig. 7 : Silod Bridge (Left) Close view of Long Crack in Mid Span (Right) Span Collapsed Like a Biscuit!

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

89

This was a minor concrete slab type bridge which collapsed during rainy season. It was under operating stage; but was not very old bridge. It collapsed because of basic poor quality of construction. There are so many minor bridges and culverts built in India by local contractors where the quality of construction is questionable. Honey combed concrete, reduced cover, corroded reinforcement are the common observations; due to untrained workers/engineers. Often contractors hide/patch-up such work using local plastering. The irony is when such bridges collapse, many people blame the designer!

2.8 Minor Road Bridge, Odisha One more River bridge failure due to sinking of central pier; reminds us about faulty construction practices, inadeqaute geotechnical/hydrological studies and inadequate maintenance.

Fig.8: Minor Bridge Collapsed in Odisha Due to Sinking of Central Pier

Till now major/minor road bridges collapsed in rivers and busy city areas were discussed. In further section a few Metro and Raiwlay bridges are discussed.

2.9 Metro Bridge Construction

Spans

Collapsed

During

Fig. 9 : Elevated Metro Span Collapse, at Two Different Locations Delhi due to De-Stabilized LG

Following photographs show the catastropic failures of elevated segmental bridge viaducts during construtcion in major cities like New Delhi, Chennai, Mumbai, Hyderabad and other places. These failures took place due to construction-safety lapses. All of these failures were catastrophic, resulted into loss in

time, money and life. It is pertinent to note that almost all of them took place after cosidrable safe construction was done; this indicates that safety can not be neglected at any point of time; Safety should become a Habbit – whould be exercised from start till end of the project.

Fig. 10 : Elevated Segmental Spans Collapsed in Mumbai (Left) and Hydrabad (Right) at Two Different Locations Due to De-Stabilized Erection Gantries/Supporting Staging

90

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Fig. 11 : Elevated Segmental Spans Collapsed in Busy Area of Chennai City

From above photos a common observation that can be drawn is – ‘importance of Bridge Erection Gantries’.

All most all the engineering colleges in India teach high end subjects like – Earthquake, FEM, PlateShells etc. but forget to teach the ‘Planning and Design of Temporary Works’ like formwork and launching/erection gantries, which leads to most

of the construction stage failures. Irony is after learning above ‘high-end’ subjects the students leave nation and go abroad for better jobs/education; those who stay inland, hardly get an opportunity to use these subjects in day-to-day practice. We need a ‘Applied Education’ in real sense! (at least at graduate level).

Fig. 12 : Elevated Segmental Spans Collapsed in Sone River Bridge Due to De-Stabilized Erection Gantry

2.10 Rafigunj Rail Bridge Collapse

Fig. 13 : (Left) Rafigunj Rail Bridge Collapse (Right) Baghmati Train Collapse, Yr. 1981

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

91

9th September, 2002 Rafigunj area of Bihar State, in midnight a Rajdhani train got derailed from a wrecked steel bridge. About 120 passengers were reported to be dead. The incedence raised many questions on safety of railway brdiges and AC coaches which caused considerable hurdles in rescue operations in pourng rains. It also pointed out towards the ‘structural health-monitoring’/safety audits of old bridges. In year 1981 more fatal accident had happened in Bihar where a passenger train had collapsed in the Baghmati river, smashing the adjoining steel truss bridge, where more than 300 people were died. Although Indian railways has strict procedure of review and checking , there were many sever rail bridge incidences which raised questions on – conceptual planning; structural design; construction and maintenance of rail bridges (because of space restrictions only two cases were covered here).

At a few stretches of railways, a speed restriction was imposed near certain railway bridges. But is this a correct solution? The fatigue specifications were also debated and kept ever improving. The ‘derailment case’ is now a days added in design basis note. But are these actions really working? We Civil Engineers need to study more!

After the road bridges, metro viaducts and rail bridge collapse cases one last case study of a pedestrian bridge is presented below.

2.11 Delhi Footover Bridge Near Stadium

gives different face to the mishap and facts and figures get disappeared. One regional language movie was developed on this incidence by some organisation (but facts are spiced up in films).

3.

Discussion, Conclusions and Acknowledgements

Till now various case studies in road, rail, metro, footover bridges in concrete and steel were covered.

The data and reasons presented were best known and published data in media. Readers may do more study on each collapse case, independently. Here more than just the facts or reasons of failures what is more important is future course of action. A few of these cases were designed properly but were not executed properly; whereas a few were designed and constructed properly but were not maintained properly; whereas a few had severe design/conceptual lacunae. A few (like one on river Saraswati) were too old and were probably unfit for modern highway loads apart from dilapidation. But all bridges had one common feature – which is ‘we do not learn from bridge failure unless it repeats!!’ it is really heart wrenching fact.

Apart from the technical aspects in bridge collapses the Bureaucracy, media, local social customs add to the complexity. One another notable fact amongst the Civil Engineering profession is ‘hiding the facts/data’. Many organizations spend crores of rupees on activities like - document management system; knowledge management; technical trainings; management reviews etc. but often the mishaps put a question mark on the effectiveness of these tools.

Apart from above one more notable reality is technical staff give-up in front of the political pressures from top and are forced to scarify the safety procedures in the race of ‘timely completion’ of projects.

Fig. 14 : Arch Suspended Steel Foot Over Bridge, Delhi

This foot over bridge collapsed during the haste of common wealth games held in Delhi in year 2010.

All the contractors had a stretched target of timely construction before starting of the games. The said bridge had steel arch on top and the concrete deck suspended using steel rods. But a few rods got snapped off and entire deck fell down on the live operational road below; blocking the complete road which connects two main marts of the city. Bureaucracy 92

Volume 47 │ Number 1 │ March 2017

Scarcity of competent technical staff for design – execution – monitoring is another issue which is linked to the remuneration and quality of engineering education that is imparted in India. The list of facts is really endless. This paper was composed purely for awakening the professionals and for the benefit of young practicing engineers to be aware of the pitfalls in the current practices and remain alert in their designs, constructions and has no intension to point out mistakes of any person, group or organisation/s.

Lack of ‘experimentation’ and lab-based research by young working professionals is one reason of slow progress which contributes erroneous construction works. Also the practice of awarding contracts to The Bridge and Structural Engineer

lowest cost tenderkills the potential of research in engineering works.

After all these, we as a Civil Engineering Professionals are on the mission of ‘Nation-Development’. The bridge collapses not just leave behind the loss of property, time or life but a deep sorrow, grief and curses. Let’s all join our hands together and give India a better, accident free infrastructure!

Author would like to acknowledge the Editors and IABSE office to permit me to express my views in this special issue on Bridge Engineering on

The Bridge and Structural Engineer

such a sensitive topic. Author would also like to thank various websites on Internet from where the information was confirmed / referred (it is practically difficult to mention all of them). The contents are best reviewed at multiple stages; still in case of any omission, misprint, please ignore the same keeping in mind the intended purpose of this paper – i.e. Quality Improvement in Engineering !

References ● ●

List of Indian railway collapse.

Various Internet websites and new sites.

Volume 47 │ Number 1 │ March 2017

93

ACCELERATED BRIDGE CONSTRUCTION WITH FOLDED STEEL PLATE GIRDERS Dr. Subramanian NARAYANAN Consulting Engineer Gaithersburg, MD 20878 USA drnsmani@gmail.com

Summary The recent infrastructure report card of the American Society of Civil Engineers showed that several bridges have reached their useful service life and have to be replaced. This scenario is true in all parts of the world, as most of the bridges, built 50 or more number of years ago, are under severe distress, and require extensive repair/rehabilitation or even replacement.

As these bridges are under service, there is an urgent need to adopt accelerated bridge construction practices, to avoid economic loss and also reduce the hardship to those who are using these bridges.

To solve this problem, the Short Span Steel Bridge Alliance (SSSBA), USA organized the Modular Steel Bridge Task Group and several solutions emerged from this group. Out of these, two innovative systems, namely, the Folded Plate Girder (FSPG) System and the Press-Brake-Formed Tub Girders System are discussed. Both are similar composite construction techniques and offer several advantages. They are also sustainable and have better life-cycle costs and durability, as welding is minimized. As they are fabricated completely in the factory and erected at site in a few hours, they are best suited for accelerated bridge construction. They provide an efficient solution to the problem of replacing the ageing bridges all over the world.

94

Volume 47 │ Number 1 │ March 2017

Dr. N. Subramanian earned his PhD from IIT, Madras in 1978. He worked in Germany as Alexander von Humboldt Fellow during 1980-82 and 1984. He has 40 years of professional experience which includes teaching, research, and consultancy in India and abroad. Dr. Subramanian has authored 25 books and more than 240 technical papers, published in International and Indian journals and conferences. He has won the Lifetime Achievement Award from the Indian Concrete Institute (ICI), the Tamil Nadu Scientist Award, and the ACCE(I)-Nagadi best book award for three of his books. He served as the past National Vice-President of ICI and ACCE(I). He is also in the editorial/review committee of several journals.

Key words: Accelerated Bridge Construction; Folded Plate Girder (FSPG) System; Press-brake-formed tub girder; Composite girder, innovative design; Sustainability; Life cycle cost.

1.

Introduction

According to the 2017 Infrastructure Report Card of the American Society of Civil Engineers, bridges get a C+ grade, roads receive a D, and America’s overall infrastructure rating is a D+, the same grade given in 2013. As per this report card, the U.S. has 614,387 bridges, and four in 10 are 50 years or older.

In addition, about 56,007, i.e., 9.1% of these bridges, were structurally deficient in 2016, and on an average there were 188 million trips across a structurally deficient bridge each day. While the number of structurally deficient bridges is decreasing in USA due to the replacement of deteriorating bridges, the average age of America’s bridges keeps going up and many of them are approaching the end of their design life (the average age increased marginally, from 42 to 43 years). The backlog of bridge rehabilitation in USA alone is estimated at $123 billion. These facts outline the importance of repairing and replacing aging bridges all over the world. Though such a report is not available in India, the situation in India may be similar or even worse, due to several factors, such The Bridge and Structural Engineer

as poor maintenance and non-adherence of quality in construction practices. In 2009, the Federal Highway Administration (FHWA) challenged the North American steel industry to develop alternatives that are economical, have long service life with minimal maintenance, and meet the needs of the industry, including Accelerated Bridge Construction (ABC).

To develop ideas to meet this challenge, the Short Span Steel Bridge Alliance (SSSBA) organized the Modular Steel Bridge Task Group – consisting of 30 organizations representing the SSSBA, Steel Market Development Institute, National Steel Bridge Alliance, National Association of County Engineers, steel bridge fabricators, University faculty members, steel manufacturers, government organizations, and bridge owners. This group developed several solutions based on sound research and development and extensive testing conducted in universities. Some of these solutions are as given below [see Ref. 7]:

The Simple for Dead Load and Continuous for Live Load system (which is a special bridge construction process rather than an application of special bridge elements), again developed by Prof. Azizinamini and associates (Azizinamini, 2014). It involves placing simple span steel members across the piers initially but adding the required concrete diaphragm later in construction to create a continuous structural system.

An example of the simple for dead load and continuous for live load system is provided in Fig. 2.

The Inverset™ system developed by M/s Amcrete Products, Inc., in which the superstructure and the decking surface, is cast upside-down suspended from wide flange steel girders, causing a prestressing effect in the steel girders, and when the section is turned upright for placement, the deck will be in a compressive state. An example of an Inverset™ Bridge system is provided in Fig. 1.

Fig. 2 : The Simple for Dead Load and Continuous for Live Load System (Source: Ref. 7)

Fig. 1 : The Inverset™ System Developed by M/s Amcrete Products, Inc. (Source: Ref. 7)

The Folded Plate Girder (FSPG) System, developed

by Prof. Azizinamini and his associates when he was with the University of Nebraska-Lincoln (see Section 2.0 for more details).

The Press-Brake-Formed Tub Girders System (also known as folded plate girder or folded plate), developed by Prof. Karl Barth and associates of West Virginia University (see Section 3.0 for more details).

The Bridge and Structural Engineer

The Pre-topped Girder Section- This prefabricated bridge system includes combinations of superstructure elements and decks fabricated together before transporting them to the job-site. This system, results in faster construction due to the on-site bolted connections and the lack of field welding.

The details of this system as well as the assembly of the components at an actual bridge are provided in Fig. 3.

The modular steel girder/cast-in-place deck system developed by M/s SDR Engineering Consultants.

This system is similar to the pre-topped girder system except that the deck is cast at the bridge site. Cold formed steel plates are attached to the steel girders, which act as formwork for the bridge deck. Wire mesh is welded to the cold formed plates to provide reinforcement for the concrete deck that is poured on site. The details of this bridge system are shown in Fig. 4.

Volume 47 │ Number 1 │ March 2017

95

Fig. 3 : Pretopped Girder System (Source: Ref. 7)

Fig. 4 : The Modular Steel Girder/Cast-in-Place Deck System (Source: Ref. 7)

Acrow Panel Bridging System, also known as the 700XS® System, is a light bridge composed of large orthotropic deck units and tall truss systems.

The trusses of this type of bridge are 50% taller than alternate panel bridges and provide 50% greater bending strength and 20% greater shear strength.

An example of an Acrow Panel Bridge is shown in Fig. 5.

for wider bridges. An example of bridge using this system is shown in Fig. 6.

Fig. 6 : Side View of the Railroad Flatcar Bridge (Source: Ref. 7)

Fig. 5 : The Acrow Panel Bridging System (Source: Ref. 7)

The Railroad Flatcar System, which uses decommissioned railroad flatcars as the superstructure of the bridge. This system is basically useful to short span, low volume county roads. For a single lane road one flatcar can provide the entire superstructure, whereas multiple flatcars have to be placed adjacently 96

Volume 47 │ Number 1 │ March 2017

It was decided in October of 2011 that the shallow steel press-brake-formed tub girder system provides the best solution and meets the FHWA goals of economics, innovation, and ABC practices (Michaelson and Barth, 2017). ABC is not just about accelerating the schedule at the job site but rather accelerating the entire process of design, manufacturing and construction. Hence, only the details of this as well as a similar Folded Steel Plate Girder (FSPG) System, are discussed further.

2.

The Folded Steel Plate Girder (FSPG) System

The Folded Steel Plate Girder (FSPG) System consists of inverted tub sections made by cold-bending flat plates using a break press (see Fig. 7a) and has many advantages for both steel fabricators (e.g., lighter and The Bridge and Structural Engineer

more stable configuration, accelerated schedule, and competitive construction costs) and bridge owners (e.g., versatility, reduced maintenance and lifecycle cost, and proven technology). The maximum span length for this system is currently limited to about 18 m, reflecting the longest press breaks that are available in the industry. The depth of the girder

ranges between 425 mm and 890 mm. Design tables have been developed for the preliminary girder sizing (see Macey, 2015). The FSPG system is considered to have “the potential to revolutionize the short-span bridge market” (Azizinamini, 2009) and even received AISC Special Achievement award in 2011.

Fig. 7 : Details of the Folded Steel Plate Girder (FSPG) System

Fabrication of the folded steel plate girder at the factory involves the following five simple steps: Cold bending the steel plates: Cold bending of 10 to

12.5 mm thick plates (A709 Grade 50 steel) using break press (Fig. 8). This operation takes approximately 2-3 hours.

Fig. 8 : Cold Bending of Plates to the Required Shape Using a Break Press (Source: Macey, 2015)

Installation of necessary hardware: Welding of bearing stiffeners, sole plates and bolting of flange separators, which takes approximately 1-2 days (Fig. 9).

The Bridge and Structural Engineer

Installation of shear studs: welding shear studs may take about a day (Fig. 10).

Volume 47 │ Number 1 │ March 2017

97

Fig. 9 : Installation of Necessary Hardware (Source: Macey, 2015)

Fig. 10 : Installation of Shear Studs (Source: Macey, 2015)

Corrosion protection of the girders: Corrosion protection may be provided either by hot-dip galvanizing, metalizing, or painting (Fig. 11).

The galvanizing has a guarantee of 25 years and reduces maintenance over that time frame-to a level equivalent to or better than that of concrete. It is better

to add removable, galvanized bird screens to the open bottom of the FSPG girders, which will allow easy inspection of the girders while still keeping birds and other critters from nesting inside the bottom flanges.

Instead of such measures, weathering steel could also be used to prevent corrosion.

Fig. 11 : Corrosion Protection by Hot-Dip Galvanizing (Source: Macey, 2015)

Pre-casting the deck, end diaphragms, and barriers: This operation could be completed in about one week (Fig. 12). It has to be noted that the concrete may be precast off-site or on-site. The neutral axis of the composite girder is located in top flange or deck.

Concrete end diaphragms accommodate thermal movement and end rotation. The details of end diaphragm can be varied based on girder depth and skew.

All the above mentioned steps could be completed in the factory in about 4-6 weeks time and the completed units may be transported to the site by means of trucks, as shown in Fig. 13.

98

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

Fig. 12 : Precasting of the Deck, End Diaphragms, and Barriers (Source: Macey, 2015)

The Bridge and Structural Engineer

Fig. 13 : Transportation of the Composite Unit (Source: Macey, 2015)

Fig. 15 : Closure Pours of Side-by-Side Placed FSPG Sections (Source: Macey, 2015)

Typically, a 12 m long folded plate girder with precast deck will weigh about 10,900 kg. Hence, a relatively lightweight crane can be used at the construction site to lift and place them in position, on elastomeric bearing pads, placed over existing abutments, as shown in Figs. 7(b), 7(c), and 14. Each composite girder could be erected at site within an hour.

The FSPG bridge system has the following advantages in its design and construction (Azizinamini, 2009 and Stockhausen, 2017):

Fig. 14 : Erection of Composite Units Over Existing Abutments (Source: Macey, 2015)

A typical two-lane county type bridge will require three such folded girder sections placed side-byside and connected longitudinally, as shown in Figs. 14 and 15. These side-by-side placed FSPG sections are connected using cast in place closure pours between the concrete slabs as shown in Fig. 15.

Completion of closure pours take approximately 2 hours per joint (Macey, 2015).

The folds in the plates are kept uniform while the thickness (10 mm or 12.5 mm) and their crosssectional dimensions are varied depending on the span of the bridge. Different top and bottom flange widths or web depth are obtained just by changing the bend locations. Hence it is possible for the fabricators to fabricate the girders for any span from just two plate thicknesses.

The Bridge and Structural Engineer

1.

The inverted tub shape is a stable bridge girder configuration during fabrication, shipping, and erection. It also does not require internal or external cross frames for either local or global stability. Flange separators provided at regular intervals, brace the bottom flanges and transmit lateral loads. Elimination of cross frames reduces the cost considerably. As welding is minimized, fatigue and fracture observed in older steel bridges, is not a concern. In addition, only conventional equipment and practices are used for the casting of concrete.

2.

The weight of the girders are approximately 1/3 the weight of concrete beams. The system uses less steel by weight compared with typical ABC steel stringer construction. Shipping weight and length reduce hauling permits.

3.

The top flange of this system is wide enough (about 625 to 890 mm) to serve as a work platform, and help to reduce construction hazards associated with workers walking on girders during construction.

4.

The system requires only smaller work force and uses smaller capacity cranes. Hence the overall cost is competitive with other ABC construction types.

5.

Box girder bridges are very efficient systems and used already in spans longer than 90 m. The box sections used in such long span bridges are deep and hence facilitate internal inspection. However, in short-span bridges, the required depth of the box is so small that it prohibits crawling inside the box for inspection. This is one of the reasons for not using box girder bridges for short-span Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

99

bridges. For this reason, in this system, the box is made open at the bottom, allowing for easy inspection.

6.

7.

As discussed earlier, the fabrication at the factory takes relatively short time. Fully assembled composite units arrive at site and no field welding or bolting is required. Hence, this system is amenable for accelerated bridge construction and the completed bridge may be opened to traffic within 24 hours of erection.

As the folded plate girder has no joints, minimized fatigue prone details, and provided with corrosion protection at the factory itself, it results in a sustainable design and expected to have a service life of 100 years or more, with reduced maintenance costs.

Extensive research and development work on this system was carried out by Prof. Azizinamini and his associates at the University of Nebraska-Lincoln,

for seven years. Design evaluations were performed as per AASHTO LRFD specifications to assess the feasibility of the system. To verify the performance and capacity of the system, extensive testing as well as FEM modeling were also carried out by them.

Two companies – CDR Bridge Systems, LLC, Pittsburgh and High Bridge Solutions (HBS), LLC, Trenton, NJ - have been granted exclusive distribution rights to construct FSPG bridge systems. The following four bridges have already been constructed using this system: 1.

The first bridge was built during 2011 in Uxbridge, Massachusetts, at a cost 20% less than the estimated cost of $2 million. It has a span of 14 m to centerline of abutments and a width of 10.74 m. Four folded plate girders were fabricated with an attached precast concrete slab (See Fig. 16 for the typical section of this bridge),

Fig. 16 : Typical Section of the Uxbridge, Massachusetts Bridge

2.

A 17.68 m span Primrose East Bridge, in Boone County, Nebraska was constructed in 2014.

Four pre-topped folded steel plate girders were erected in just two hours. Closure pours of ultrahigh performance concrete (UHPC) were used,

3.

A two-lane, 15.2 m bridge near Bradford, Pennsylvania was erected in October 2016 in less than three hours. Closure pours of this bridge took only a few days, which enabled the bridge to be reopened in 30 calendar days, five days ahead of the accelerated, five-week schedule for the project detour. This is a full month faster than it would have taken for a conventional castin-situ construction (Stockhausen, 2017), and

4.

The bridge in Lewiston, Pennsylvania was opened to traffic just two weeks later in 2016, and it took only 2½ hours for the erection. CDR

100

Volume 47 │ Number 1 │ March 2017

Bridge Systems was also able to reduce the total production time for the Pennsylvania bridges to as little as 14 weeks, including steel procurement (Stockhausen, 2017).

University of Massachusetts Amherst (UMass) instrumented and monitored the performance of the bridge at Uxbridge since construction for twenty months, with load testing and also did analysis to present an independent evaluation. Field data obtained were compared with 2D hand calculations as well as the results obtained from SAP- 2000 and ANSYS finite element models with 3D effects and nonlinear soil-structure interaction (Civjan, et al., 2014). Stresses in concrete and steel components were found to be within expected ranges of values during the service-level loads. Long term data collection and truck load testing indicated a conservative design of The Bridge and Structural Engineer

this innovative structure. Through the first two years of service, no signs of distress were noted and readings were within the expected range of bridge behavior.

Based on this study, the FSPG system was found to be an effective system for accelerated construction of short span bridges (Civjan, et al., 2014).

3.

The Press-Brake-Formed Tub Girders System

The Press-brake-formed tub girder system is similar to the FSPG system, except that the steel trapezoidal section is placed in the reverse direction as compared to the FSPG system, as shown in Fig. 17. Similar

to the FSPG system, either hot-dipped galvanized or weathering steel could be used. Once the shape has been formed using press brakes, shear studs are welded to the top flanges. A reinforced concrete deck is then cast on the girder in the fabrication shop and allowed to cure, and the composite modular unit is transported to the site. These standard modules are longitudinally joined at the site using ultra-high performance concrete (Michaelson and Barth, 2017).

Unlike FSPG system, it is a closed system, since the girder is closed at the bottom and hence has increased torsional stiffness.

Fig. 17 : The Press-Brake-Formed Tub Girder System (Source: Ref. 7)

The system offers several advantages and they are also similar to the FSPG system: (1) The girder itself is simple to fabricate, requiring minimal welding, (2) Because of the systemâ&#x20AC;&#x2122;s modular composite design, there is a reduced need for additional details such as stiffeners or cross-frames, and (3) The composite unit can be easily shipped to the bridge site, allowing for accelerated construction and reduced traffic interruptions.

While modular precast concrete decks are recommended, multiple deck options are available.

Examples include the use of full-depth/partial-depth precast concrete deck panels, cast-in-place concrete decks, or more advanced composite decks such as the Sandwich Plate System (a structural composite material comprising two metal plates bonded with a polyurethane elastomer core). Extensive research and development work on this system was carried out at West Virginia University. Based on this study, for each standard plate width and thickness, an optimum depth of girder was obtained (Michaelson and Barth, 2017). To verify the performance and capacity of the system, three years of testing (on eight press-brake tub girder specimens donated by Nucor Corporation, SSAB Americas, United States Steel Corporation, and The Bridge and Structural Engineer

EVRAZ North America) as well as computer analysis using FEM modeling were carried out.

Unlike the FSPG system, the press-brake-formed tub girder system is an open source. The Short Span Steel Bridge Alliance has developed standardized plans using standard plate sizes (widths of 1.50, 1.82, 2.13, 2.44, 2.74, and 3.00 m). The designs were developed to achieve maximum structural capacity. Press-brakeformed tub girders are versatile for multiple-deck options. They can be used for both tangent and skewed configurations, as well as simple and continuous spans. They are recommended for single spans up to 18 m. Spans may be spliced together to form a multispan bridge greater than 18 m.

The Amish Sawmill Bridge located in Fairbank, Iowa is the first Press-brake-formed tub girder steel bridge in the USA and was opened to traffic on 8th January 2016 (see Fig. 18). Researchers from West Virginia University, Marshall University, and the University of Wyoming monitored and studied the field performance of this bridge. Two more press-brake-formed tub girder bridges are scheduled for construction in West Virginia (Michaelson and Barth, 2017). This system offers significant cost savings, ease in shipment and fabrication, accelerated construction and design Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

101

versatility, and has the potential to replace numerous short span bridges in future.

Fig. 18 : The Amish Sawmill Bridge, Buchanan County, Iowa (Source: Ref. 7)

4.

Azizinamini, A. (2009). “A New Era for ShortSpan Bridges.” Modern Steel Construction, American Institute of Steel Construction (AISC), Sept., 2 pp. www.aisc.org/globalassets/modernsteel/archives/2009/09/2009v09_new_era.pdf

2.

Azizinamini, A. (2014). “Simple for Dead Load--Continuous for Live Load Steel Bridge Systems,” Engineering Journal, AISC, Vol. 51, 2nd Quarter, pp. 59-82.

3.

Civjan, S.A., Sit, M.H., and Breña, S.F. (2014) “Field and Analytical Studies of the First Folded Plate Girder Bridge”, Transportation Research Board 2014 Annual Meeting on Celebrating Our Legacy, Anticipating Our Future, Washington, D.C., Jan.12-16, 19 pp.

4.

Macey, M.J. (2015), “Folded Steel Plate Girder System-Applications in Accelerated Bridge Construction”, AASHTO SCOBS Technical Committee for Construction (T-4) Presentation, Annual Meeting, Apr. 21. http://bridges. transportation.org/Documents/2015%20 SCOBS%20presentations/Technical%20 C o m m i t t e e / T- 4 % 2 0 P r e s e n t a t i o n % 2 0 7 _ Matthew%20Macey_FSPG%20-%20CDR%20 Bridge%20Systems.pdf

5.

Michaelson, G.K., and Barth, K.E. ( 2017), “A New Shape for Short Span steel Bridges”, Structure Magazine, NCSEA/CASE/SEI of ASCE, Mar. 2017, pp.26-29.

6.

Stockhausen, T., “A New Take on Plate Girders”, Modern Steel Construction, AISC, Vol. 57, No.1, Jan. 2017, pp. 56-59.

7.

http://www.shortspansteelbridges.org/steelsolutions/systems.aspx

Acknowledgements

The photos used in this paper were adopted from the references given below.

102

1.

Conclusions

The Modular Steel Bridge Task Group formed by the Short Span Steel Bridge Alliance has developed several alternative bridge systems to meet the needs of the industry, including Accelerated Bridge Construction (ABC). Out of these solutions, the technology of folded steel plate girders is appealing because it offers significant cost savings, ease of shipment and fabrication, accelerated construction, design versatility, and sustainability. In addition to their reduction in weight, their life-cycle costs are also competitive. As steel is the world’s most recycled material, at the end of useful life of these bridges, they may be transformed into other steel products. With so many benefits, this technology provides the potential for accelerated bridge construction, not only in USA but also throughout the world, and may solve the challenge of replacing aging critical infrastructure.

Naturally, several state Departments of Transportation in USA have already expressed interest in building such steel tub girder bridges in their jurisdictions.

5.

References

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

SOME HYDROLOGIC AND HYDRAULIC ASPECTS OF PLANNING AND DESIGN OF ROAD BRIDGES S.K. Mazumder Former AICTE Em. Professor Delhi College of Engineering/Delhi Technology University Somendrak64@gmail.com

Summary In depth knowledge in hydraulics and hydrology is needed while planning and designing a bridge and its foundations. Several data, e.g., topography, stream flow, rainfall, soil and sub-soil, have to be collected and analysed. It is essential to conduct morphologic study e.g. plan form, bed form, meandering processes, river behavior etc., for selection of site and safety of bridge. Computational procedure for design flood, HFL, waterway requirement in different valley settings, afflux etc. have been explained. Existing method of computing scour under bridge piers and abutments has been examined. Limitations of IRC method of scour computation based on Lacey’s theory and need for employing mathematical models for scour computation have been outlined.

Born in 1938, Prof. S.K. Mazumder, a Ph.D. in Civil Engg. (IIT,KGP), has 57 years of teaching, research & consultancy experience in hydraulic and water resources engineering. Further details of Prof.

Mazumder is available in his website: www.profskmazumder.com

under the bridge has to be made very scientifically for safety as well as economy. Underestimation of waterway and scour may result in the failure of a bridge, loss of properties and outflanking of bridge.

Due to inadequate waterway provided under Bagmati bridge on NH-57 there was severe meandering of river Bagmati upstream and downstream resulting in damage to the road, the village and the agricultural lands as shown in Fig. 1. Overestimation of waterway, on the other hand, will not only increase the cost of the bridge, it will also provide an opportunity to the river to play in its meandering belt under the bridge causing non-uniformity of flow distribution, which may result in high scour under some of the bridge spans and silting in some others.

Key Words: River morphology, design flood, waterway, afflux, scour depth

1.

Introduction

Large numbers of bridges are being constructed all over India by the railways and roads authorities for better and faster communication and connectivity to the different parts of the country. Some of the roads and road bridges are new; but a large numbers of existing bridges are being widened from 2-lanes to 4-lanes. For the safe design of a bridge, knowledge of structural and foundation engineering is essential. Hydrologic and hydraulic aspects of planning and design of a bridge is equally important in deciding its location, waterway, afflux, scour, hydraulic forces, river training measures, etc.,[1]. Computation of waterway The Bridge and Structural Engineer

Fig. 1 : Meandering of River Bagmati U/S & D/S of Bagmati Bridge on NH-57

Most of the rivers, especially those in the North and North-East of India [2] pass through a variety of Volume 47 │ Number 1 │ March 2017

103

terrains, e.g., hilly and mountainous, sub-hilly and trough, braided and meandering zones with wide flood plains, deltaic and tidal reaches, etc. Fixing waterway for a bridge under different terrains requires an intimate knowledge of river morphology, river-mechanics and alluvial stream processes [3], hydrology [4], hydraulics [5], etc. Any arbitrary decision regarding waterway under a bridge without considering its past history and behavior of the river, in the near and far field, may create unforeseen problems in future during the life span of the bridge [6,7]. Costly river training measures may be necessary to prevent outflanking and damage to bridge and adjoining structures.

Primary objective of writing this paper is to focus on some of the important hydrologic and hydraulic aspects of planning and design of a bridge and its foundations.

2.

Investigations/Data Collection

A number of routine investigations are to be carried out for the safe design of a bridge. These are briefly described below.

2.1 Topographic Data Topographic sheets are used for determining the catchment area, terrain slope, river course and its tortuously, land use, soil and cover conditions, etc. When they are not readily available due to the classified nature of certain catchments (restricted areas), satellite imageries obtained from Google Earth can be used. Use of digital terrain maps by GPS/GIS are very useful to obtain topographic information and in visualizing terrain conditions and the river behavior.

2.2 Hydrologic Data: Hydrologic investigations e.g. rainfall, stream flow, flood history, dominant flow, stream forms and their tributaries, sediment characteristics, debris flow etc., are vitally needed for determining the location, waterway, scouring etc., and to avoid future problems of failure and excessive maintenance cost.

2.2.1 Peak Flood and High Flood Level (HFL) Data Peak flood discharge and corresponding flood levels are necessary for finding design flood, design HFL,

104

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

waterway, and the deck level of a bridge. In the case of major bridges on large rivers where gauging (stagedischarge) data are readily available, design peak flood for a return period of 100 years is computed

by Gumbel, Log Pearson type-III method or other similar methods of frequency analysis. At least 15 to 20 consecutive years of annual peaks and the corresponding high flood levels are required.

2.2.2 Rainfall Data Where flood data is not readily available, rainfall data in the catchment is of vital importance to determine the design flood. Depending upon the return period of peak flood, maximum probable rainfall of 100 year return period is found from frequency analysis of rainfall data. Since continuous recording type rain gauges are now installed in many parts of the country, it is desirable to use such rainfall records of different storm durations for the estimation of design peak flood.

2.3 Stream Data Stream survey data e.g. contour plan, L-section, cross-sections, High Flood Level (HFL), etc., are vitally required for fixing the location of the bridge and waterway required for the bridge[8]. IRC Pocket book for bridge engineers gives the details of river survey data to be collected at a bridge site.

2.4 Morphologic Data Morphologic investigations should be carried out in regard to the history of river behavior in the vicinity of the bridge site, change in flow pattern in the past, etc. Morphologic behavior of a river is governed by the flow of water and sediments in the river. Many a river engineers [9-12] have developed procedures for determining stability and regime characteristics of rivers as briefly discussed below.

2.4.1 River Plan-form Based on flow of water (Q), sediment (Qs), bed slope (S0), and stream power (Q.S0), Schum [11] developed criteria to decide plan-form of streams depending on water and sediment flow as illustrated in Fig. 2(a).

The Bridge and Structural Engineer

Fig. 2(a) : Different Plan Forms of a Stream like, Straight, Meandering and Braided (Masse)

Fig. 2(b) : Lane’s Criteria for Finding river Regimes (Lane)

2.4.2 River Regime and Meandering Lane [13] and Garde [12] developed similar criteria for finding stream stability and meandering process by developing following laws ●

QSe α Qsd50 … by Lane

... (1)

●

Q6/7 Se7/5 α QS d50 3/4 …. by Garde

... (2)

Knowing mean annual flow in a river and bed slope, different regimes of a river can be predicted as illustrated in Fig. 2(b).

2.4.3 Meandering Process River meanders migrate both laterally (faster rate) and longitudinally (slower rate). Understanding the migration process (Fig. 3) and the effect of bridge on the change on meander pattern is necessary for deciding waterway and designing river training measures[14]. Many times, fixing waterway without proper morphological study and meandering process has resulted in washout of the bridge, excessive cost of maintenance, and other problems related to the safety of the bridge and the approach embankments.

3.

Computation of Design Flood

IRC:5[8] & IRC:SP-13[15] recommend peak flood of 100 year return period for safe design of bridges and appurtenant works like guide bunds, approach embankments, etc. Additional discharge varying from 10% to 30% (depending on the catchment area) is to be added with peak design flood for design of pier and abutment foundations [17]. For freeboard under a major bridge, AASTHO [18] recommends a peak flood of 500 year return period and corresponding HFL for safety of the bridge. Computations of peak flood and corresponding HFL are briefly discussed underneath.

The Bridge and Structural Engineer

Fig. 3 : Illustrating Lateral Migration of River

3.1 For Gauged Catchments Most reliable method of peak flood estimation is to collect past annual flood data for 15 to 20 consecutive years depending upon the return period of design flood. Frequency analysis is performed by tabulating the recorded peak floods and the high flood levels according to their magnitudes with the highest value on top and lowest at the bottom. There are several methods of computing design flood of any given frequency/return period e.g., Gumbel method, Log normal method, Log Pearson type-III etc. All of them are essentially probabilistic methods of best curve fitting of extreme value distributions. Details of these methods are given in standard hydrology text books [4,19].

3.2 For Ungauged Catchments Peak flood/HFL data are not available in many catchments, especially for streams in remote and inaccessible areas. However, rainfall data collected

by Indian Meteorological Department (IMD) are usually available for years. Depending on the size of catchment areas, a number of reliable methods of Volume 47 │ Number 1 │ March 2017

105

flood estimation, based on observed rainfall data are briefly discussed below.

3.2.1 Rational Method The rational method is appropriate for estimating peak discharges for small catchments up to about 25 sq. km. Rational Method presupposes an uniform critical rainfall intensity continuing indefinitely and uniformly all over the catchment. The runoff at the outlet of a catchment will increase until the Time of Concentration TC, when the whole catchment is contributing flows to the outlet. The peak runoff is given by the following expression:

Q = 0.028 P f A I where,

... (3)

Q = Maximum runoff in Cumecs, A= Catchment area in hectares, I = Design Rainfall intensity in cm/hr for the selected frequency and duration equal to the time of concentration, P = Coefficient of run-off for the given catchment, f = Spread factor for converting point rainfall into areal mean rainfall. Further details of computation of peak flood by Rational method may be found in IRC:SP:42[20] and IRC:SP:13 [15].

3.2.2 SCS Method (Run-off Curve Number Method) SCS (Soil Conservation Services) method or Runoff Curve Number (CN) method of estimating direct runoff from storm rainfall is developed by U.S Soil Conservation Services. Relation between rainfall, runoff, initial abstraction, and potential maximum retention can be expressed as;

Qr = (P-Ia) / [(P-Ia) +S] 2

... (4)

where, Qr = storm runoff depth in mm, P = storm rainfall in mm, Ia = initial abstraction in mm = 0.2S, S = potential maximum retention in mm = (25,400/CN) – 254 Further details of computation of peak flood by SCS method, by using SCS curve numbers are available in IRC:SP:42 [20].

3.2.3 Unit Hydrograph Method Unit hydrograph can be prepared synthetically by using physiographic data like area of catchments, length of stream, longitudinal bed slope, soil and cover conditions, etc. Daily rainfall corresponding to design flood return period is found from iso-hyetal curves for the catchment. Hourly distribution of rainfall and rainfall excess-values corresponding to design storm are found. By using the unit hydrograph and rainfall 106

Volume 47 │ Number 1 │ March 2017

excesses, flood hydrograph is prepared and the peak flood is determined. Details of computing peak flood by using synthetic unit hydrograph method are available in Flood Estimation Reports [21] prepared jointly by CWC, RDSO, IMD & MORTH, Govt. of India.

3.2.4 Use of Hydraulic Structures Peak floods can be estimated by recording HFL upstream and downstream of existing hydraulic structures in the river, e.g., dams and barrages, bridges, culverts and other cross-drainage structures. General equation for measuring flow past such hydraulic structures may be written as

Q = Cd LeffH3/2

... (5)

where, Q is flow rate in cumec, Leff is the effective waterway in m, H is the head above crest in m, and Cd is the coefficient of discharge in m1/2/s. Cd-value varies from structure to structure depending upon whether the flow is free or submerged, geometry of the structure, etc. Cd-values for dam/ spillways under free and submerged conditions may be obtained from USBR [22], IRC:SP:13[15], and Mazumder and Joshi[23].

3.2.5 Using Manning’s Equation When stream cross-section is available, Manning’s equation can be used to determine stream flow

Q = (1/n) (AR2/3.S1/2)

... (6)

where, n is Manning’s roughness coefficient, R is hydraulic mean depth in m given by R= A/P, and S is the energy slope, A is area of cross-section normal to flow in m2, P is wetted perimeter in m and Q is flow rate in m3/s.

Manning’s n-values can be obtained from standard textbook of hydraulics by Chow[5]. Assuming different stages (water levels), Q-values corresponding to the different stages can be found from Manning’s equation for the given stream section. Stage-discharge curve can be obtained by plotting discharges against corresponding stages/water levels. Design peak flood can be obtained from the stage – discharge curve corresponding to measured HFL or vice versa.

4.

Estimation of Design Hfl

Design HFL corresponding to design peak flood can be found from stage-discharge curve where flow records are available. Stage – discharge curve can also be prepared by Manning’s equation discussed The Bridge and Structural Engineer

under section 3.2.5. These are normal HFL assuming that the low water bed level (usually surveyed during lean flow period) remains unaltered during flood.

Actually, there is always some change in bed level due to scouring of bed during passage of high floods.

River bed usually undergoes retrogression (especially downstream of hydraulic structures like dams and barrages) resulting in lowering of HFL. Aggradations occur in rivers where heavy sediment load comes from landslides. Photograph no.1

illustrates heavy sediment deposition due to landslides in Vishnuprayag-HEP Barrage on Alaknanda River, Uttarakhand in June-2013. River bed level rose around 17 m due to landslides and flooding debris. Obviously such aggradations will cause rise in normal HFL. HFL can also be estimated by using software such as HEC-RAS, MIKE-11, and Mike-21. HFL upstream of structures can be found by adding afflux with normal HFL downstream [24].

Photo.1 : Aggradations of Alaknanda River Bed at Vishnuprayag HEP Barrage due to Landslides June, 2013 (Photos Show Bed Levels Before and After Flood).

[Courtsey: S.D.Sharma, GMR Group of CO. New Delhi]

5.

Estimation of Waterway

When a new bridge is to be constructed, a designer has all the freedom to provide waterway as required.

As per IRC-5[8], waterway (W) should be equal to Lacey’s regime waterway (P) given by equation:

P =W= 4.8Q1/2

... (7)

where, Q = Design flood discharge in m3/sec, P = Wetted perimeter in meter, and W = Linear waterway in metre. The code also stipulates that the waterway so found should also be compared with linear waterway at HFL corresponding to design flood discharge, and the minimum of the two should be adopted as the clear waterway under the bridge. The methodology for determining waterway under different situations is discussed briefly below.

5.1 In a Hilly Terrain In a hilly or mountainous terrain where the river flows in gorges with steep bed slope, the flow is usually in supercritical state when depth (y) is small and velocity of flow (V) is very high. Lacey’s waterway in such terrain is very high compared to linear waterway at The Bridge and Structural Engineer

HFL. Thus the minimum waterway under the bridge will be determined by the linear waterway at HFL. Any restriction of normal waterway under a bridge in supercritical flow will result in the formation of hydraulic jump upstream which is not desirable.

Moreover, restriction of normal waterway will affect free movement of gravels and boulders which move along the river bed during flood season.

5.2 In a sub-hilly/Trough Terrain In a sub-hilly/trough region, slope of river bed and stream power per unit width per unit weight (QSo) reduce drastically, resulting in deposition of the sediments brought from the mountainous stretch.

In this stretch, the river is found to be unstable and changing its course periodically. As a result, a fan shaped delta type formation occurs. It is better to avoid construction of any bridge since there is always a risk of outflanking of the bridge due to its shifting course [7]. In such stretches, Lacey’s waterway is only a guideline but the actual waterway to be provided may be more, depending on width of the fan shaped braided area-which may be several times more than Lacey’s waterway. Too much restriction of flood plain should be avoided to ensure free flow of water and Volume 47 │ Number 1 │ March 2017

107

sediments. Physical and mathematical model study should be carried out to fix up waterway, alignment, location of the bridge, and protective works.

IRC:5-2015[8] is applicable only in straight rivers without any flood plain.

5.3 In a Meandering Flood Plain

where,

In this region, the river bed and bank consists of fine alluvial soil which can be as easily eroded as deposited. Due to an inherent instability, the river erodes its outer bank and the eroded materials get deposited on the inner bank opposite to the eroded one [25]. Guide bund and approach embankments with pitching are to be provided where the wide flood plain is restricted. Excessive restrictions of meandering flood plain of a river create high afflux and many unforeseen problems [26]. Elliptical type guide bunds as per design, proposed by Lagasse, et al.

[27], should be provided for the safety of the bridge.

5.4 In a Deltaic Region In the deltaic stretch, longitudinal bed slope and stream power are so low that even fine silts and clays deposit in the channel beds and banks. River divides and starts flowing in multiple channels forming deltas. Many of the rivers in their deltaic stretch are also subject to backflow during high tides. Thus, determination of waterway in deltaic channels is a very difficult task due to unsteady varying flow over time, unless river is trained with flood embankments to follow a steady course. Submergence of area in between the embankments occurs due to storage of incoming flood water during tidal lockage period. When an all weather road is be constructed in such tidal stretch, waterway under the bridge across the river should be sufficient enough to avoid undue afflux above normal high flood level. It is prudent to carry physical and mathematical model study for a final decision.

6.

Computation of Afflux

Afflux is the rise in water level upstream before and after the construction of a bridge. IRC:5-2015[8] stipulates a maximum afflux of 150 mm. High afflux due to excessive constriction of normal waterway should be avoided, as it may result in hydraulic jump, and consequent scour downstream, increase in overall cost of construction, and many other unforeseen problems, viz., outflanking, silting, damage to properties, river instability , and costly protective measures.

Afflux computed by the following empirical Molesworth equation [28] prescribed in 108

Volume 47 │ Number 1 │ March 2017

h1 * = [V2 /17.88 + 0.015] [(A/A0) 2 –1 ]

... (8 )

h1 * is the afflux in m, V is the mean velocity of flow in the river prior to bridge construction, i.e., corresponding to normal HFL in m/s, A and A0 are the areas of flow section at normal HFL in the approach river section and under the bridge respectively in m2.

Molesworth equation (8) is not applicable for rivers with wide flood plains and non-uniform approach flow. In such a situation, Bradley [29] suggested the following equation for determination of afflux.

h1 * = 3( 1- M) Vn2/2 g

... (9)

where, M = A0/A, Vn is the mean velocity of flow under the bridge with water level same as under normal flow condition.

7.

Bridge Scour

Determination of scour around bridge piers is important while deciding the foundation level of piers and abutments. It is a universal practice to find total scour depth as sum of general scour, contraction scour, and local scour, except in India where the total scour depth in piers is arbitrarily determined as 2R below HFL or R below mean scoured bed level as given in IRC-5 [8] and IRC-78[17]. Hydraulic radius or mean scoured depth (R = dsm) in a river is computed by Lacey’s theory [30]. The multiplying factor 2 is based on observed scour depths in 17 major railway bridges [31] given in a annual report (Tech.) by C.W.P.R.S., Pune [32]. All the piers investigated were founded on very fine and uniformly graded soil (d50-varying from 0.17 to 0.39 mm). Yet, the same factor 2 is adopted for computing scour in piers founded even on coarse and graded soils (e.g., gravely and bouldery soil) having 2 mm < d50 < 300 mm and σg >1.3) without any verification from the field. σg is the geometric standard deviation given by the expression.

σg = (d84/d16) 0.5

... (10)

Scour around pier below river bed is governed by many parameters such as type of pier, pier thickness, shape of pier nose, flow obliquity, flow conditions, and sediment characteristics-which are not considered in the IRC formula. Based on these parameters, several mathematical models have been developed in India and abroad, and proposed by Kothyari, et al. [33], The Bridge and Structural Engineer

Melville and Coleman [6], Breusers & Raudkivi [34], and Richardson and Davis [35] for predicting maximum local scour depth to be measured below river bed level.

Mazumder and Kumar [36] computed total scour depths in six bridge piers founded on cohesion less uniform fine bed materials (d50 < 2 mm, σg < 1.3) and compared them with those found by IRC method based on Lacey’s theory. It was found that the IRC method overestimates scour in all the cases and the error was found to vary from 5% to 275%. Holnbeck [37] observed local scour depths in fine soil in river

Maine in USA, and compared the observed scour values with predicted ones by using HEC-18 Model.

It is noticed that the predicted scour depths are highly conservative as compared with observed ones.

Mazumder and Dhiman [38] made exhaustive study on local scour in bridge piers in Missisipi river on coarse graded soil d50 > 2 mm, σg > 1.3 and compared them with the observed ones, and those obtained by IRC method. It is found that in all the cases, IRC method overestimates the local scour depth. Fig. 4 illustrates the effect of size and gradation of bed materials on local scour depth in bridge piers.

Fig. 4 : Showing Variation of Local Scour Depth in Bridge Piers with Size and Gradation of River Bed Materials

8.

Other Factors

Apart from the various hydraulic and hydrologic considerations discussed under sections 1 to 7 above, other factors such as hydrostatic force, buoyant force, drag and lift forces, wave forces, and the effect of debris on these forces should be taken into account for the safe design of bridge structures. These are available in the publication by US Department of transportation, Federal Highway Administration [39].

Problems, New Age Int. Pub. Pvt. Ltd., New Delhi, 3rd Edition, 2000, 686 pp.

4.

Raghunath, H.M., Hydrology: Principles, Analysis and Design, Wiley Eastern Ltd. Delhi, 1985.

5.

Chow, V.T., Open Channel Hydraulics, McGraw-Hill Book Co., New York, 1970.

6.

Melville, B.W. and Coleman, S.E., Bridge Scour, Water Resources Publications, LLC, Vol. I and II, 2000.

7.

Mazumder, S.K., “River Behaviour Upstream and downstream of Hydraulic Structures”, Proc. Int. Conf. On Hydraulic Engineering Research and Practice (ICON-HERP-2004) in honour of Prof. K.G.Rangaraju, Dept. Of Civil Engineering, IIT, Roorkee, Oct 26-28, 2004.

8.

IRC:5, Standard Specifications and Code of Practice for Road Bridges-Section-I, Indian Roads Congress, New Delhi, 2015.

9.

Lacey, G., “Stable Channels in Alluvium” Paper 4736, Proc. of Institution of Civil Engineers,

References 1.

Mazumder, S.K., ”Morphology and Training of Rivers Near Bridges”, Indian Highways, Vol. 44, No.7, July 2016, pp.25-35.

2.

Mazumder, S.K., “Determination of Waterway under a Bridge in Himalayan Region - Some Case Studies”, Paper presented at 70th IRC congress held at Patna on 11-14 Nov., 2009, and Published in the Journal of IRC, Vol.70-2, July-Sept., 2009.

3.

Garde, R.J. and Ranga Raju, K.G., Mechanics of Sediment Transport and Alluvial Stream

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

109

Vol. 229, William Clowes & Sons Ltd., London, U.K. ,1930, pp. 259-292.

10. Lane, E. W. ,“A Study of the Slope of Channels Formed by Natural Stream flowing in “Erodible Material” U.S. Army Core of Engineers, Missouri River Division, Omaha, Sediment series 9, 1957.

11. Schumm, S.A. ,“Plan form of Alluvial Rivers”, Proc of the International Workshop on Alluvial River Problems, Univ. of Roorkee, March 18-20, 1981.

12. Garde, R.J., River Morphology, New Age International(P) Ltd., New Delhi, 2006.

13. Lane, E.W., “Stable Channels in Alluvium”, Trans. of ASCE, Vol.120, 1955.

14. Chitale, S.V. , “ Shape and Mobility of River Meanders”, Proc. XIX Congress of IAHR, Vol. 2, pp. 281-286, New Delhi,1981.

15. IRC:SP:13, Guidelines for the Design of Small Bridges and Culverts, Indian Roads Congress, New Delhi, 16. IRC:SP:42, Guidelines for Road Drainage, Indian Roads Congress, New Delhi, 2013.

17. IRC:78, Standard Specifications and Code of Practice for Road Bridges, Section-VII, Indian Roads Congress, New Delhi-110 011, 2000.

18. AASHTO, Izzard, C.F. ‘Section 14 -Drainage’ in Hand Book of Highway Engineering, 1994.

19. Subramanya, K. , Engineering Hydrology, McGraw Hill Book Co., New York, 2013.

20. IRC:SP:42, Guidelines for Road Drainage, Indian Roads Congress, New Delhi, 2014.

21. “Flood Estimation Reports” Prepared Jointly

by CWC, IMD, RDSO and MORTH, Govt. of India, and Published by Hydrology Division of Central Water Commission, New Delhi. [www. cwc.nic.in/main/webpages/Flood_estimation_ reports.html].

22. USBR, ‘Chapter-F- Hydraulics of SpillwaysCulvert Hydraulics’ in Design of Small Dams, (Int. Edition), Oxford & IBH Pub. Co Ltd., New Delhi, 1968, pp. 326-33.

23. Mazumder, S.K., and L.M. Joshi, “Studies on Critical Submergence of Flowmeters”, Journal of Irrigation and Power, CBIP, Vol.32, No.2, April, 1981.

110

Volume 47 │ Number 1 │ March 2017

24. Mazumder, S.K. and Dhiman, R., “Computation of Afflux in Bridges with Particular Reference to a National Highway”, Proc.,HYDRO-2003, CWPRS, Pune, Dec.26-27, 2003.

25. Mazumder, S.K., “Stability of River Downstream of Hydraulic Structures”, Proc. of VIII APDIAHR Congress, Vol II, CWPRS, Pune, Oct. 20-23, 1993, pp 273-282.

26. Mazumder, S.K., “River Behaviour near Bridges with Restricted Waterway and Afflux- Some Case Study”, Indian Highways, Vo.37, No.10, Oct. 2009.

27. Lagasse, P.F., Schall, P.F., Johnson, F., Richardson, E.V. and Chang. F., Stream Stability at Highway Structures, 2nd Edition, Department of Transportation, Federal Highway Administration, Hydraulic Engineering Circular No.20, Washington, D.C., 1995, 200 pp.

28. Molesworth, G.L., Pocket Book of Engineering Formulae for Civil and Mechanical Engineers, E & F N Spon, London, 1871.

29. Bradley, J. N., Hydraulics of Bridge Waterways: Hydraulic Design, 2nd Edition, Federal Highway Administration, Washington, D.C., 1978.

30. Lacey, G., “Stable Channel in Alluvium”, Journal of Institution of Civil Engineers, U.K., Paper no. 4736,Vol. 229, 1930, pp. 259-292.

31. CBIP, River Behaviour, Management and Training, Vol. I, Central Board of Irrigation and Power, New Delhi, 1989.

32. CWPRS, Annual Report (Tech.), Central Water & Power Research Station, Pune.

33. Kothyari, U.C., Garde, R.J., and Ranga Raju, K.G., “Temporal Variation of Scour Around Circular Bridge Piers”, Journal of Hydraulic Engineering, ASCE, Vol. 118, No.8, 1992, pp.

1091-1106.

34. Breusers, H.N.C., and A.J. Raudkivi,” Chapter-5 Scour at Bridge Piers”, in Scouring, A.A. Balkema Pub., IAHR Hydraulic Structures Design Manual, 1991.

35. Richardson, E.V. and Davis, S.R., Evaluating Scour at Bridges, Report No. FHWAIP-90-017, 1995.

36. Mazumder, S.K., and Kumar,Y.K, “Estimation of Scour in Bridge Piers on Alluvial Non-Cohesive The Bridge and Structural Engineer

Soil by Different Methods”, IRC Highway Research Bulletin. Oct. 2006, Presented in the 67th IRC Congress at Panchkula, Haryana, 1721 Nov 2006.

37. Holnbeck, S.R., Investigation of Pier Scour in Coarse-Bed Streams in Montana, 2001 through 2007- Scientific Investigations Report 2011–5107, by U.S. Department of the Interior ,U.S. Geological Survey, U.S.A., 2011.

The Bridge and Structural Engineer

38. Mazumder, S.K., and R.K. Dhiman , “Local Scour in Bridge Piers on Coarse bed MaterialObserved and Predicted by Different Methods”, Journal of The Indian Roads Congress,Vol.74-4, April-June 2016 , pp.126-135.

39. Zevenbergen, L.W., L.A. Arneson, J.H. Hunt, and A.C. Miller , Hydraulic Design of Safe Bridges, Publication Number FHWA-HIF-12018, Federal Highway Administration, U.S. Department of Transportation, April 2012, 280 pp.

Volume 47 │ Number 1 │ March 2017

111

MOVABLE BRIDGES : CASE STUDY OF PORT-SAID BASCULE BRIDGE

Prof. Hussein H. Abbas Chairman of EHAF Consulting Engineers. habbas@ehafconsulting.org

He received his Ph.D. in 1971 from France. He is the founder and the president of EHAF Consulting Engineers founded in 1971, and operates in Egypt and in eight Arab gulf countries. He served a dean of Engineering – Al Azhar University.

He authored and co-authored more than 150 scientific referred papers published in journals and presented in international conferences.

Prof. Mazhar Mohamed Saleh

Dr. Eng. Samir Soliman Marzouk

Former Head of Structural Department of the Faculty of Engineering, Cairo Univ. mazhar1793@hotmail.com

Head Deputy of Structural Department at EHAF Consulting Engineers samir.soliman@ehafconsulting.org

He received his bachelor degree in 1978, and his Ph.D. in 1989.

He is teaching structures and steel bridges in the Faculty. He joined EHAF Consulting Engineers Group in 1980, and he acts presently the head of the structural department. He leads design teams of engineers in tunnel, railway, roadway, and pedestrian bridge projects. He participated to different conferences, sharing several papers in Engineering journals.

He received his bachelor degree in 1985, and Ph.D. in 2004. He joined EHAF Consulting Engineers in 2004. He is registered as chartered consultant Engineer for design and review of different types of steel and concrete structures, in the Egyptian Syndicate of Engineers.

He is known for his skills in repairing and strengthening of structures. He also shares technical papers in different conferences.

Summary Movable bridges can be constructed in different ways namely: lift, bascule and swing types. Movable bridges can change position to allow for passage of boats below. The main advantage of this type of bridges is the lack of high piers and long approaches. The use of steel is in many cases mandatory to optimize the design within a reasonable budget. The moving parts i.e. electromechanical and control systems need to be designed considering the duty condition, environmental exposure and loads due to dead, wind and seismic conditions.

A case study will be presented for a movable bridge recently built in the city of Port-Said, and the basic information will be provided. Studies were conducted and concluded that a movable bridge is preferred over other bridge options. The bridge that will be presented in this paper, as a case study, is a new born roadway single-leaf bascule bridge, operated on 2015, founded 112

Volume 47 │ Number 1 │ March 2017

on one of the Suez Canal branches. The bridge consists of six bays with an intermediate bascule movable bay having an open angle of 70 degrees.

Keywords : Movable, Suez Canal, bascule, fulcrum, bridge balance, lifting system.

1.

Introduction

Movable bridges are in some sites required to enable navigation across waterways, providing a suitable vertical clearance above the highest water level (H.W.L.). This clearance can be just few meters but may go up to 70 m to allow for the giant cruise ships, large vessels and container carriers to sail safely underneath. The three basic design types of movable bridges are: vertical lift, bascule and swing. Movable bridges are more expensive to operate and maintain than fixed stationary ones. A few ancient drawbridges were built 4000 years ago in Egypt and 2600 years in Chaldean Kingdom of the Middle East. But in the fifteenth century, Leonardo da Vinci did not only The Bridge and Structural Engineer

design and succeed to build up a bascule bridge, but also draw plans and construct swing retractable scale bridge models. The first medieval movable bridges were short wooden bascules, hinged at end and raised by a system of ropes and pulleys.

The most spectacular lift bridge in the world is the Bacalan-Bastide Bridge, which features four slender towers that can raise the deck nearly up to 61 m above the river and is thus considered one of the highest vertical clearances of any lift bridge in the world. The bridge is designed to carry in addition, the expanded light rail system of Bordeaux city, Fig. 1. London iconic Bridge crossing Thames River is a bascule historic bridge build in 1886–1894. Its total length is 244 m and its longest span amounts to 82 m. The clearance of the bridge above water level is 8.6 m when closed and 42.5 m while open. Egypt, has the longest swing span worldwide as Al Ferdan Railway Bridge across the Suez Canal consists of 2 side spans of 150 m and a central one of 340 m. The bridge towers are 80 m high. The central spans of this asymmetrical movable bridge swings (150 + 170) m around 2 tables one at each side of the Canal. To avoid unbalance moment due to dead load on the slewing bearing, the shorter span length is loaded with additional counterweight. Today, as the Suez Canal Bridge has been enlarged at this location near the city of Ismailia, there is a need for a bridge extension to connect the 2 enlarged ends of the Canal.

Fig. 1 : The Lift Bacalan-Bastide Bridge in an open Position

Different mechanism systems were introduced for operating the movable bays. The main concerns in the mechanical system selection are the weight of the moving bay as its equilibrium has to be maintained during the open and close stages. This reflects on the importance of the best selection for the decking system to reduce the energy requested to control the bridge movements.

2.

Bridge Description

The most recent movable bascule type bridge built in Egypt was completed in 2015. This bascule bridge is located in Port Said and takes the name of the city.

The total length of the bridge is 126 m and consists of 6 spans of varying lengths of 20 and 22 m. Five among the 6 spans are fixed while one is movable, Fig. 2.

Fig. 2 : New Bridge Elevation

The approach bridge deck is designed as a multiple steel girder system acting in a composite fashion with a 25 cm reinforced concrete slab above. The movable span is a bascule single-leaf orthotropic steel deck selected to reduce the weight which needs to be raised, Fig. 3, while the steel lifting tower is about 12 m tall, Fig. 4. The bridge width is about 15 m to accommodate 4 traffic lanes two in every direction. In the opening position, the leaf is rotating to an angle of 70 degrees with the horizontal.

The Bridge and Structural Engineer

The new Port-Said roadway bridge B, is constructed on one branch of the Suez Canal nearby an old bridge A, as shown in Fig. 5, to link Port-Said with Port-Fouad cities. This old bridge had been in operation for many years, but it was too narrow and heavily corroded. The Administration decided to build a new bridge, better adapted to the present needs and the increasing traffic volume.

Volume 47 │ Number 1 │ March 2017

113

Fig. 3 : Intermediate Section of the Orthotropic Bridge Deck

bascule intermediate span, providing a navigation channel with infinite vertical clearance. The fixed bays are designed as composite multi-girder steel bridge with reinforced concrete slabs, while the movable bay has been designed as plate girder with orthotropic steel deck and a light pavement above.

The movable bay dimensions has a length and width of 19.2 m and 14.8 m respectively. The spacing between balance weight supporting columns is 22 m, allowing to a maximum open angle of 70 degrees.

3.

Fig. 4 : Section of Bridge at the Cylinder Support

Fig. 5 : Location of Port Said Movable Bridge the old Narrow (A) and the New Bridge (B)

A two-way bridge can play an important role to accommodate the forecasted traffic from and to the harbor and the city. Comparative traffic studies have been carried out for the different possible alternatives of fixed, elevated and movable bridge designs. The comparative studies on various design features and selection method have been examined and then concluded that the movable bridge option is preferred over other systems, from technical and economic points of view.

The new bridge design consists of six bays each of about 22 m span and width of 14.8 m, with movable 114

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

Bridge Deck

The movable bridge deck has been constructed as an orthotropic steel deck with a light pavement, that for two main reasons. Firstly, it has lighter weight with respect to other decking systems, and thus requires less power to rotate the leaf with smaller ballast counterweights. Secondly, they deliver the entire floor load, when the deck is rotated, it transfers its load directly from the deck plate to the edge girders, i.e. reducing the internal forces in the upper and lower trunnion pins. The orthotropic deck, 15.74 m wide is supported by 5 main girders 2.90 m apart and transverse beams spaced every 2.0 m. The pavement consists of a layer of tar mixed with cement of thickness 80 mm The orthotropic panel detail was shaped as the trapezoidal closed-rib system for its higher flexural and torsional rigidities. The high torsional rigidity contributes to a better distribution of concentrated transverse loads and consequently, to a reduction in the deck plating stresses.

4.

Bridge Mechanism

Port-Said movable bay is a single leaf bridge. The main components of this bascule bridge are: the movable steel deck, two tie rods, a balance frame including an overhead counterweight of 178 tons made of blocks of cast iron, a portal support H-frame and an operatorâ&#x20AC;&#x2122;s service building. The service room is located in one side of the high tower. The H-tower provides support to the main counterweight boom and to the pivot of the main operating struts.

The movement of the bridge is controlled by a The Bridge and Structural Engineer

computerized system to follow all the necessary safety steps in each operation.

The bridge opens like a lever on fulcrum. The fulcrum fits into the girder of the bridge and is made of a trunnion shaft attached to the leaf girder via a hub, and supported on bearings to permit rotation of the leaf.

The trunnion hub-girder assembly form the pivotal element of the bascule mechanism to open and close the girder of the bascule bridge, power is supplied to the assembly by means of hydraulic cylinder system electrically equipped to control the gradually bridge opening and closing. The bridge has a counterbalance weight to assist in the lifting operation of the deck, Fig. 6. The opening and closing operations are performed by two servo-hydraulically controlled pistons. In the event of failure of either piston operation using only one piston is possible but at a reduced speed.

Fig. 6 : View of the Rotating Leaf in the Open Position

The operating system includes the gearbox, driving unit, shafts and brakes. The stroke length for the hydraulic cylinder reaches 2.4 m. It is found that the forces exerted on the hydraulic cylinder changed its magnitude with change of the open angle. The wind load has a great impact on the force direction and magnitudes. The bridge completes its open within 5 minutes. The maintaining of cylinders open, constant speed had been achieved by using hydraulic cylinders with variable speed to accommodate the acceleration and deceleration at the ends of the stroke.

The total bridge decking dead load is 187.5 tons counter balanced with 178 tons as shown in Fig. 7. The balance arm is supported by two columns connected with puller to the steel deck of the main girders. The puller is provided nearly to the end of the balance arm. Its function is to balance the steel deck loads during deck rotation on the trunnion pins. The mechanical studies for the operating system included both the kinematic study for the bridge, the analysis of the forces exerted on the bridge components, bridge stability at lock, balance arm trunnion pins and the bearing system, Fig. 8.

The design of movable bridge lifting mechanism has to account for the probable source of misalignment of the used structural elements. Rolled sections and plates have some degree of lack of straightness, twist and camber and substantial residual stresses due to welding. The abnormal wear and fatigue, shift of foundations due to barge collision and impact can cause disorders in the bridge system. Also, the long term mechanical action and excessive vibration of machine and structure elements can lead to loss of rigidity.

Fig. 7 : Plan showing the Bridge Components

The Bridge and Structural Engineer

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

115

Fig. 8 : Elevation showing the Bascule Mechanism

During operation, nearly the entire dead load is transferred to the bearings. The bearings have to possess also friction to mitigate the contribution of mechanism load at different opening angles. Additional forces, also arise on the push-pull (hydraulic cylinder) due to the extra displacement in the vertical direction, Fig. 9. A computer system is necessary to provide both the operator interface as well as a monitoring system for the hydraulic power supplies and the control systems.

A bascule bridge may be simply represented as a rotating mass with the entire weight of the bridge acting at its center of gravity. The balance of the bridge is thus dependent upon the offset of the bridge center of gravity from the point of rotation.

Fig. 9 : Hydraulic Cylinder Extra Displacement.

5.

Loading Combinations

The bridge has been designed to meet 90 tons traffic loading. In addition to the loading combinations stipulated by the relevant Egyptian design code for bridges. The following cases have been studied to

116

Volume 47 â&#x201D;&#x201A; Number 1 â&#x201D;&#x201A; March 2017

include the bridge behaviors under different operating positions of open and close positions during the bridge rotation. Four cases are envisaged in this study: a)

Bridge opened with no wind.

b)

Bridge opened while wind acts in the same direction.

c)

Bridge opened while wind acts in opposite direction.

d)

Bridge in closed position and the pressure in the hydraulic cylinder is zero.

In cases a, b & c the calculations are checked at different angles to obtain the upper and lower limits for the calculated parameters. The considered angles from the start till the full opening are 0, 10, 20, 30, 40, 50, 60, 65, 70, 71 & 72 degrees. It is found that the critical case for bridge trunnion pin is the case (d). The frictional forces during operation can be reduced through a lubrication system. The interested readers may refer to the AASHTO movable bridge specifications that include the design guidance for the mechanical and electrical system.

6.

Parametric Study of Wind Load Effect

The wind load direction affects the forces exerted on both the rotating and supporting system. Parametric study has been carried out to monitor such effect on the movable system at different opening angles. The study referred to the start open state of the bridge as comparative base line. The wind load has been The Bridge and Structural Engineer

considered in three cases, positively when the wind load was applied with the raising direction of bay and negatively in the opposite direction.

Figs. 10 & 11 show the variation of hydraulic cylinder forces and pressure, respectively versus the bridge angles at different directions of wind loads. The system stability was maintained by avoiding the negative force resulted from the action of force reversing. The angles varied from close to open state; i.e., from 0 to 70 degrees. The pressure envelope ranges from 20 to 95 tons per side. The minimum positive 20 tons force adds safety against sudden high speed of wind.

The selected hydraulic cylinder in accordance has a limit range from 30 to 150 bars, as recommended by the AASHTO. The percentage of wind load on the hydraulic cylinder at the open state with respect to the starting open state for both the positive and negative wind directions ranges from a reduction of 76.6% to an increase of 2.22%, respectively. While the effect on the hydraulic pressure varied from a reduction of 72.8% to an increase of 6.67%, respectively.

from 25 to 140 mt. The percentage of wind load effects on the driving torques and column base moment at the open state with respect to the starting open state for both the positive and negative wind directions range varied from a reductions of 82.14% and 14.28%, respectively.

It is worthy to note that the maximum design forces on the hydraulic cylinder is reached at the complete open (θ =70 degrees), while the maximum torques on driving units and moment on the supporting columns is reached at the start of open angles (θ= 0 degree), with the opposite wind direction. These results are dictated from the exigency of maximum lifting arm at the start open angle.

Fig. 12 : Hydraulic Cylinder Pressure vs. Bridge Angle at Different Wind Loads.

Fig. 10 : Driving Torque vs. Bridge Angle at Different Wind Loads

Fig. 13 : Hydraulic Base Moment vs. Bridge at Different Wind Loads

7.

Fig. 11 : Hydraulic Cylinder Force per side (2 Cylinders) vs.

Bridge Angle at Different Wind Loads

Figs. 12 & 13 show the variation of driving torque and column base moment, respectively versus the bridge angles at the different directions of wind loads. Under the different loading combinations, the envelope of driving torques and column base moment ranges The Bridge and Structural Engineer

Testing

The movable bridge bay has been subjected to different stages of inspection and tests. The inspection included the deck fabrications, erection, and electromechanical hydraulic devices installation. Tests included the bridge load carried out on the bridge bays and the cold test for the bay open. The cold test visualized the performance and balances between the bascule and lift span at the different open angles till to complete opening, Fig. 14. Comparison of Volume 47 │ Number 1 │ March 2017

117

the static and dynamic simulation of mechanical behavior of the hydraulic system and the actual measured behavior was necessary. The cold test included also the inspection for the performance of the provided alternative operating system, represented by a complete hydraulic set equivalent to the original one but manually pumped, in case of drop in electricity.

in a movable bridge project. These constrains were considered starting from the concept design stage and extended up to the detailed drawing stage. The project required detailed inspection of material condition and structural configuration. The motor of the bridge lifting system has to be strong enough to operate the bridge against normal and higher wind pressures.

Movable bridge balance is an essential consideration when constructing a new movable bridge. Bridge balance should be monitored throughout the construction phase, utilizing spreadsheets that have been compiled based on the detailed calculations to ensure that a severe imbalance does not develop.

References 1.

AASHTO 2007, “LRFD Movable Highway Bridges Design Specification”, 2nd. Edition, American Association of State Highway Bridges and Transport Officials, Washington, DC.

2.

Chris Peacock, “10 Amazing Bridges”, Andrews UK limited, 2011.

3.

Johnson. J & Curran. P, “Gateshead Millenium Bridge- an Eye for Engineering”, Civil Engineering, No. 156, Feb., 2003, pp14-25.

4.

Sean A., Bluni, P.E., Partner, Hardesty & Hannover, LLP “Comparison of Movable Bridge Design in Domestic and Foreign Market”, 12th Biennial Symposium, Nov. 2008, Orlando, USA.

5.

Kolgin,T., “Movable Bridge Engineering”, Wiley, 2003.

6.

Lim, T. C., et al, “ Moveable Bridges – Selection and Design”, 3rd International Conference on New Dimensions in Bridges , Flyovers, Overpasses & Elevated Structures, 9–10 April 2003, Malaysia.

7.

Giernacky, R. G., et al., “Movable Bridge Balance and Counterweight Design Considerations for Designers and Constructors”, Proceedings of Heavy Movable Structures, Inc., Thirteenth Biennial Symposium, Florida, 2010.

Fig. 14 : Movable Bridge Bay Simulation Model

In order for the bascule leaf to be properly aligned, key checks have been performed during and after erection. The principal causes of misalignment are derived from errors in machinery, inaccurate layout during construction and improper installation of racks, bearings, shafts, pinion gears and tread plates.

At last, steel bascule bridges are economical, compact and useful transportation structures if they are well designed and built, properly maintained and suitably aligned.

8.

Concluding Remarks

The basic engineering aspects of Port-Said movable bridge have been presented. From the precedings the prominent aspects of the project were: The project is a good example of how functional and limited budget constrains can be taken into account

118

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

New Technology Bridges the Gap Between the Analytical and Physical Model Alex MABRICH

Mr. Mabrich, Senior Consultant, provides consulting, training and support services for multidisciplinary design projects at Bentley Systems.

His bridge, roadway, site, drainage, and geotechnical expertise ranges from resurfacing roadway projects thru multilevel complex interchanges globally. Registered PE in Florida, he graduated from Universidad Ricardo Palma in Lima, Peru and obtained his Master of Science in Civil Engineering degree from Florida International University.

Senior Applications Engineer Bentley Systems, Inc. Exton, PA, USA Alex.Mabrich@Bentley.com

Summary Bridge designers and engineers have been working in a disconnected workflow for years. The design team builds a physical model, while the engineer designs the analytical model. The gap between the two leads to increased risk of errors, delays, and cost.

Now, advancements in 3D design software are making it possible for bridge designers and engineers to close workflow gaps from model to analysis.

Keywords: BIM, Bridge Design, 3D Design Software, Open Bridge Modeler, Accelerated Bridge Construction.

1.

Introduction

An architect models a building with ornate columns and decorative friezes. It is beautifully designed and an accurate representation of what the building will look like when it is constructed. But when the structural engineer gets his hands on it, that beautifully designed building turns into a model of sticks and nodes. This is what is known in the design and engineering field as a gap between the analytical model used for design calculation and the physical model, which is the real representation of the building.

This same scenario applies to bridge engineering where a designer might model a bridge structure, for example, with decorative guardrails, barriers, supports and attractive lighting along the piers.

Typically, this model is presented to the client as an actual representation of the bridge, which the client will use during public hearings to get the goThe Bridge and Structural Engineer

ahead on the project. Again, the bridge engineer sees something different when he takes this bridge model and calculates the loads and stresses. He only sees the analytical model and not the physical representation of the bridge. In these two examples, the work is being done twice – the design team builds the physical model, while the engineer designs the analytical model – with no connection with the physical model.

Bridge designers and engineers have been working in this disconnected workflow for years, and the consequences are increased risk of project errors, delays and added costs. The challenge for bridge designers is to make the vast information generated by their structure useful for the other project team members, further downstream in the bridge life-cycle.

Another challenge is to use information generated from surveyors before the project starts, and information from other transportation projects that surround them.

The current advancements in 3D design software are now making it easier to bridge the gap between the model and design and analysis, providing all the necessary details for an accurate representation of the structure in the real world.

2.

Establishing Environment

a

Physical

Model

In a physical model environment, the goal is to have one model that starts from conceptualization and as the design progresses, other disciplines add their data to the model. For example, the road engineer adds the highway alignment, profile and sections, the structural engineer adds the beams and steel specifications, and the contractor applies the construction specifications Volume 47 │ Number 1 │ March 2017

119

and so on. Bridge information Modeling (BrIM) has evolved from being a catch-phrase to reality in which the stakeholders are demanding more intelligent models.

The 3D model of the structure that is the cornerstone of BrIM starts as a CADD model, and is enriched with information. The relevant properties of the model are no longer levels or layers, colour or thickness of lines but are instead viewed as columns, pile caps, beams as well as the enabled properties including concrete strength, steel grade, plate thickness, and tendon types.

Designers, engineers, and contractors have begun to realize that 3D modeling brings great potential beyond the design stage- for example, during construction and later at the inspection and maintenance stages. Indeed, new technology is now available that gives designers and engineers the ability to verify clearances, generate geometric reports, and add information that pertains to construction practices – material specifications, construction dates, installation reports, quality control logs, and other information the contractor thinks is necessary to store in the model.

With Open Bridge Modeler, Bentley is Helping Designers and Engineers Bridge the Gap in Workflow from Model to Analysis and the Gap Between Disciplines.

In this BrIM environment, changes are easily made, the model is updated with new information and the work continues without any duplication of work. The result is an information-rich federated model. While the model includes all the design data, the specific applications that each project participant uses allows access to only the parts of the project they need to see. This seamless and integrated workflow prevents project team members from having to convert, translate, or import data, which could result in an increase in design time, and lost information and data integrity.

120

Volume 47 │ Number 1 │ March 2017

3.

Building an Information-Rich Model

When properties are added to the model using any CADD software, your computer draws lines up to a certain colour, layer or level, leaving the project team member to determine what it means. For example, a level 10 colour yellow, thickness five identifies the bridge deck and little else. In a BrIM environment, however, the engineers can click on an element in the model and see the bridge deck with steel reinforcement, with the designer’s name and who inspected it, and when. As more information is added from design, construction, and inspection, the model becomes The Bridge and Structural Engineer

more and more intelligent – a complete 3D model.

With advancements in infrastructure technology, we can move away from the old way of doing things with spreadsheets, and have the information at our fingertips when we need it.

Increasingly, BrIM technology is being used in both the public and private sectors, particularly in public private partnerships and joint ventures where project teams need to work collaboratively. In these situations, all project participants are involved from the beginning to the end of the project, working together on the same venture. The process provides huge benefits because any conflicts that are found can be corrected on the spot, which increases productivity, reduces costs, and maintains the project schedule. This is a far more integrated process than traditional methods where the designer bids the plans of a bridge design to the transportation department and then the contractor needs to decipher the project on his/her own.

4.

Open Bridge Modeler

A BrIM methodology brings all of the components together in a 3D model with easy access to the parts of the design that you need. Open Bridge Modeler, Bentley Systems’ latest addition to its comprehensive bridge application portfolio,

The Bridge and Structural Engineer

allows users to easily manage design changes with built-in, user-defined relationships among bridge components throughout the project life-cycle. The software enables bridge designers and engineers to close workflow gaps from model to analysis, and among disciplines.

Advancements in 3D Design Software are now Making it Easier to Bridge the Gap Between the Model and Design and Analysis, Providing all the Necessary Details for an Accurate Representation of the Structure in the Real World.

Moreover, Open Bridge Modeler grants access to the roadway geometry without having to perform translations or import operations. Users can generate complete bridge geometry reports, including civil and bridge element reports, deck and beam seat elevations, quantities, and cost estimates.

Volume 47 │ Number 1 │ March 2017

121

HIGHLIGHTS OFof THE ING-IABSEEditorial WORKSHOP ON From the Desk Chairman, Board

“INSPECTION, INVESTIGATION AND REPAIR/REHABILITATION OF BRIDGES & FLYOVERS” HELD AT BANGALORE ON 20TH AND 21ST JANUARY, 2017 The Indian National Group of the IABSE in co-operation with Govt of Karnataka, Public Works, Ports and Inland Water Transport Department successfully organised a two day Workshop on “Inspection, Investigation and Repair/Rehabilitation of Bridges & Flyovers” at Bangalore on 20th and 21st January, 2017. The Workshop was well attended by more than 200 delegates from various Govt.

Departments as well as other private and public organizations.

The aim of the workshop was to provide an insight into the probable causes of distress through detailed inspection, investigation and testing and discuss the available methods and techniques of their repair and rehabilitation to enhance their serviceability during the design life.

During this two days Workshop, several eminent engineers in the field of Bridge & Structural Engineering made presentations on the different aspects of bridge repair and rehabilitation and present some interesting case studies for benefit of the participants from Karnataka Public Works & Inland Water Transport Department and other state PWDs and Consultants associated with the topic.

Participation of delegates in floor intervention and discussions was very encouraging.

The Workshop was inaugurated by Shri Laskshminarayana, IAS, Principal Secretary, Public Works & Inland Water Transport Department, Govt. of Karnataka by lighting the traditional lamp in the presence of Shri D.O. Tawade, Chairman, INGIABSE, Shri Siddgangappa, Secretary, Public Works & Inland Water Transport Department, Govt of Karnataka, Shri N Lakshman Rao Peshve, Chief Engineer (NH), Govt of Karnataka, Shri AK Banerjee, Chairman, Scientific Committee and Dr. Harshavardhan Subbarao, Vice President & Member Technical Committee, IABSE Parent Body, Zurich as well as other dignitaries.

Shri Laskshminarayana, IAS, Principal Secretary, Public Works & Inland Water Transport Department, Govt of Karnataka extended warm welcome to the participants of the Workshop. Shri D.O. Tawade, delivered his address during the Inauguration. Shri K. Srinath, Suptd Engineer (NH) proposed Vote of Thanks.

The Workshop on “Inspection, Investigation and Repair/Rehabilitation of Bridges & Flyovers” was addressed by the following eminent experts covering the following themes:

Friday, January 20, 2017 Session 1 – Inspection, Investigation, Testing and Bridge Management System 1.

Mr. A.K.Banerjee

–

Key Note Address

2.

Dr. Lakshmy Parameswaran

–

Condition Survey and Bridge Management System

3.

Mr. R.K. Jaigopal

–

Detailed Investigation and Testing

4.

Dr. Harshavardhan Subbarao

–

Forensics in Bridge Engineering

Session 2 – Repair and Rehabilitation 5.

Dr. Paul Jackson

–

Strengthening of Hammersmith Flyover in London

6.

Mr. Vinay Gupta

–

Repair Materials and Techniques

7.

Mr. P.Y. Manjure –

Rehabilitation of Bridges & Other Structures – The Challenging Discipline

8.

Mr. Samir Surlaker –

Concrete Bridge Repair – State of the Art Materials & Methods

122

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

Saturday, January 21, 2017 Session 3 – Case Studies 9.

Mr. Alok Bhowmick & Mr. P.Y. anjure

–

Rehabilitation of Chopan Bridge over river Sone in U.P

10.

Prof. Ananth Ramaswamy

–

Repair and Retrofitting of Masonry Arch Bridges

11.

Mr. V.N. Heggade –

Hybrid Technology for Rehabilitating Deep Bridge Foundations

12.

Mr. Paolo Franchetti

–

13.

Dr. Sanjeev Kumar Garg

–

14.

Mr. Sudarshan S Iyengar/ – Mr. N. Lakshmana Rao Peshve

Restoration of Flood Affected Bridges - Case Studies Gareke Kandy Bridge –

15.

Mr. Umesh Rajeshirke

–

Rehabilitation of Durgabati Bridge on NH 2 in Varanasi

16.

Mr. Repaka Srinivasa Rao/ Mr Ashok Kumar

–

Condition Survey of Bridges in Karnataka State

Repair & Rehabilitation of Steel Bridges

The concluding remarks of the Workshop were presented by Shri A.K. Banerjee, Chairman, Scientific Committee on 21st January, 2017. He expressed the hope that the outcome of the Workshop would have enriched the delegates. The delegates who attended the Workshop mentioned that the subject matter of the Workshop is very timely. Shri Lakshman Rao Peshve, Chief Engineer (NH) and Chairman, OC proposed Vote of Thanks.

A light music with dinner was organized in the evening of 20th January 2017 for the participants who rejoiced the evening.

The Workshop was a great success.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

123

Indian National Group of the IABSE Office Bearers and Managing Committee - 2016 Chairman 1.

Shri D.O. Tawade, Chairman, ING-IABSE & Member (Technical), National Highways Authority of India

Vice-Chairmen 2.

Shri B.N. Singh, Additional Director General, (I/C) Ministry of Road Transport and Highways

3.

Shri Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt. Ltd.,

4.

Shri A.K.S. Chauhan, C.O.O., GR Infraprojects Ltd.,

Honorary Treasurer 5.

The Director General (Road Development) & Special Secretary to the Government of India, Ministry of Road Transport and Highways

Honorary Members 6.

Shri Ninan Koshi, Former Director General (Road Development) & Addl. Secretary

7.

Prof. S.S. Chakraborty, Past Vice-President, IABSE

Persons represented ING on the Executive Committee and Technical Committee of the IABSE 8.

Dr. Harshavardhan Subbarao, Vice President & Member, Technical Committee of IABSE & Chairman and Managing Director, Construma Consultancy Pvt. Ltd., Past Member of the Executive Committee and Technical Committee of IABSE

Honorary Secretary 11. Shri R.K. Pandey, Member (Projects), National Highways Authority of India

Members of the Executive Committee 12. Shri A.D. Narain, Former Director General (Road Development) & Addl. Secretary 13. Shri A.K. Banerjee, Former Member (Technical), NHAI 14. Shri A.V. Sinha, Former Director General (Road Development) & Special Secretary 15. Shri G. Sharan, Former Director General (Road Development) & Special Secretary 16. Shri R.P. Indoria, Former Director General (Road Development) & Special Secretary 17. Dr. Lakshmi Parameswaran, Chief Scientist, Bridges & Structures Div., CSIR-Central Road Research Institute 18. Shri Ashwinikumar B. Thakur, Group Engineer, Atkins India 19. Shri Sarvagya Kumar Srivastava, Engineer-inChief (Projects), Govt of Delhi 20. Dr. Mahesh Kumar, Engineer Member, Delhi Development Authority

Secretariat 21. Shri R.K. Pandey, Member (Projects), National Highways Authority of India

Prof. S.S. Chakraborty, Past Vice-President, IABSE

22. Shri Ashish Asati, General Manager, National Highways Authority of India

10. Dr. B.C. Roy, Past Vice President & Member, Technical Committee, IABSE

23. Shri K.B. Sharma, Under Secretary, Indian National Group of the IABSE

9.

124

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer

MEMBERS OF THE MANAGING COMMITTEE – 2016 Rule-9 (a): A representative of the Union Ministry of Road Transport and Highways 1.

Shri D.O. Tawade, Chairman, ING-IABSE & Member (Technical), National Highways Authority of India

Rule-9 (b): A representative each of the Union Ministries/Central Government Departments making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 2.

CPWD - nomination awaited

3.

NHAI - nomination awaited

4.

Ministry of Railways - nomination awaited

Rule-9 (c): A representative each of the State Public Works Departments/Union Territories making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 5.

Govt of Andhra Pradesh – nomination awaited

6.

Govt of Arunachal Pradesh – nomination awaited

7.

Shri Ajoy Chandra Bordoloi, Commissioner & Special Secretary to the Govt of Assam

8.

Govt of Bihar – nomination awaited

9.

Govt of Chattisgarh – nomination awaited

10. Shri Sarvagya Kumar Srivastava, Engineer-inChief (Projects), Govt of Delhi 11. Govt of Goa – nomination awaited 12. Govt of Gujarat – nomination awaited

19. Govt of Madhya Pradesh – nomination awaited 20. Dr. D.T. Thube, Chief Engineer, Govt of Maharashtra 21. Shri O. Nabakishore Singh, Additional Chief Secretary (Works), Govt of Manipur 22. Govt of Meghalaya – nomination awaited 23. Shri Lalmuankima Henry, Chief Engineer (Buildings), Govt of Mizoram 24. Govt of Nagaland – nomination awaited 25. Govt of Orissa – nomination awaited 26. Govt of Punjab – nomination awaited 27. Govt of Sikkim – nomination awaited 28. Govt of Tamil Nadu – nomination awaited 29. Govt of Tripura – nomination awaited 30. Govt of Uttar Pradesh – nomination awaited 31. Govt of Uttarakhand – nomination awaited 32. Govt of West Bengal – nomination awaited 33. Shri Mukesh Anand, Chief Engineer, Union Territory Chandigarh Rule-9 (d): A representative each of the Collective Members making annual contribution towards the funds of the Indian National Group of IABSE as determined by the Executive Committee from time to time 34. Major V.C. Verma, Director (Mktg), Oriental Structural Engineers Pvt. Rule-9 (e): Ten representatives of Individual and Collective Members

13. Govt of Haryana – nomination awaited

35. Shri G. Sharan, Former DG (RD) & Special Secretary

14. Govt of Himachal Pradesh – nomination awaited

36. Shri A.K. Banerjee, Former Member (Technical), NHAI

15. Govt of Jammu & Kashmir – nomination awaited 16. Govt of Jharkhand – nomination awaited

37. Shri AV Sinha, Former DG (RD) & Special Secretary

17. Govt of Karnataka – nomination awaited

38. Shri R.P. Indoria, Former DG (RD) & Special Secretary

18. Shri K.P. Prabhakaran, Chief Engineer, Govt of Kerala

39. Shri. Atul D. Bhobe, Managing Director, S.N. Bhobe & Associates Pvt. Ltd.

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

125

40. Shri N.K. Sinha, Former DG (RD) & Special Secretary

53. Shri Bageshwar Prasad, CEO (Delhi Region), Construma Consultancy Pvt. Ltd.

41. Dr. Lakshmi Parameswaran, Chief Scientist, Bridges & Structures Div, CSIR-Central Road Research Institute

54. Shri Alok Bhowmick, Managing Director, B&S Engineering Consultants Pvt. Ltd.

42. Shri Rakesh Kapoor, General Manager, Holtech Consulting Pvt. Ltd.,

Rule-9 (i): Honorary Treasurer of the Indian National Group of IABSE

43. Shri Inderjit Ghai, CEO, Consulting Engineers Associates

55. The Director General (Road Development) & Special Secretary to the Govt of India

44. Shri Ashwinikumar B. Thakur, Group Engineer, Atkins India Rule-9 (f): Four representatives of Bridge and Structural Engineering Firms

Rule-9 (j): Past-Chairman of the Society, for a period of three years, after they vacate their Chairmanship

45. Shri M.V. Jatkar, Executive Director (Technical), Gammon India Ltd.

Rule-9 (k): Secretary of the Indian National Group of IABSE

46. The Managing Director, UP State Bridge Corporation Ltd.

56. Shri R.K. Pandey

47. Shri T. Srinivasan, Vice President & Head Ports, Tunnels & Special Bridges, Larsen & Toubro Ltd.

Rule-9 (l): Persons who have been awarded Honorary Membership of the Parent Body

48. Vacant Rule-9 (g): Two representatives of the Engineering Colleges / Technical Institutes / Universities / Research Institutes 49. Prof. A.K. Goel, Director, Indian Railways, Pune 50. Shri V.L. Patankar, Director, Indian Academy of Highway Engineers Rule-9 (h): Four representatives Engineering Firms

of

Consulting

51. Shri A.D. Narain, President, ICT Pvt. Ltd. 52. Shri Krishna Sandepudi, Vice President, Aarvee Associates Architects Engineers & Consultants Pvt. Ltd.

126

Volume 47 │ Number 1 │ March 2017

57. Shri Ninan Koshi 58. Prof. S.S. Chakraborty Rule-9 (m): Persons represented ING on the Executive Committee and Technical Committee of the IABSE 59. Dr. Harshavardhan Subbarao Rule-9 (n): Past Members of the Executive Committee and Technical Committee of the IABSE 60. Prof. S.S. Chakraborty 61. Dr. B.C. Roy

The Bridge and Structural Engineer

K & J PROJECTS PVT. LTD. K & J is a leading Civil Engineering Infrastructure Consultancy firm in Central India which is incorporated in 2004 as a Private Limited Company under the Companies Act, 1956. K&J has dedicated team of over 350 professional staff with state-of-the-art knowledge of international standards. With its Head Office at Nagpur, Maharashtra, K&J has on-going projects in the State of Andhra Pradesh, Arunachal Pradesh, Bihar, Chhattisgarh, Gujarat, Maharashtra, Madhya Pradesh, Mizoram, Tamilnadu, Telangana, Uttar Pradesh and West Bengal.

Services Offered : Highways ¾ Feasibility Studies ¾ Detailed Project Report ¾ Project Management Consultancy / Construction Supervision & Quality Control ¾ Authority Engineer Services ¾ Independent Engineer Services ¾ Pre-bid & Detailed Design Services to Concessionaire Railways ¾ Preliminary Location Surveys ¾ Final Location Surveys ¾ Structural Designs for ROB/RUB/Bridges Major Clients ¾ NHAI ¾ MORT&H ¾ PWD / R&B ¾ Railways

Registered Office : SHIVAM, 16 NIT Layout, Ravinagar, Nagpur – 440 033 Tel. : 0712-2556402, Fax : 0712-2556403, Email : contact@knjprojects.com Website : www.knjprojects.com

The Bridge and Structural Engineer

Volume 47 │ Number 1 │ March 2017

127

FORTHCOMING EVENTS OF THE ING-IABSE

1.

Indian National Group of the International Association for Bridge and Structural Engineering (ING-IABSE) is organising an International Seminar on “Innovations and Aesthetics in Design and Construction of Bridges & Tunnels” at Nagpur on 8th and 9th July, 2017.

2.

ING-IABSE is also organising a two day Workshop at Guwahati on the topic of “Project Preparation and Repair/Rehabilitation of Bridges and Flyovers” in association with National Highways and Infrastructure Development Corporation Limited (NHIDCL) sometimes in the month of October, 2017.

For any enquiry about the above Seminar/Workshop, please address to the following: Shri I.K. Pandey Secretary Indian National Group of the IABSE IDA Building (Ground Floor) Jamnagar House, Shahjahan Road New Delhi-110011 Tel: 011-23388132, 23386724 E-mail: ingiabse@hotmail.com; ingiabse@bol.net.in

128

Volume 47 │ Number 1 │ March 2017

The Bridge and Structural Engineer