Resistance of buildings during an earthquake by Guillaume MELIN

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

BACHELOR OF ARCHITECTURAL TECHNOLOGIES AND CONSTRUCTION MANAGEMENT

7th SEMESTER DISSERTATION

Author :

Guillaume MELIN (177626)

Consultants :

Professor Bixiong Li

Dissertation realized in partnership with Sichuan University in Chengdu, CHINA.

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

Acknowledgement This dissertation has been written in order to achieve the 7th and last semester of my Bachelor of sciences in Architectural Technologies and Construction Management in VIA University College in Denmark. I want to thank particularly the Sichuan University (SCU) in Chengdu, in Sichuan, a province of China, which took me as student for a period of 6 months. Thanks to them, I have been able to pick this subject, very important in China, geologically less important in Denmark. VIA University College gave me the opportunity to come in China for that, and I am appreciative. Thanks to my Danish and Chinese consultants who guided me in the good directions: Mr Laurids Green (head of department of Constructing Architect in VIA UC), Mrs. Xia Wang (lecturer in the architecture dept. of SCU) and Mrs. Bixiong Li (head of department of Civil Engineering in SCU). My work on this dissertation has been mainly done by reading books, to have an overall knowledge of the subject, and then some articles and some universities or specialized centres websites helped me to be more meticulous on certain points.

Number of pages (2400 characters): 30 pages. - Characters : 71886 All rights reserved – no part of this publication may be reproduced without the prior permission of the author. NOTE: This dissertation was completed as part of a Bachelor of Architectural Technology and Construction Management degree course – no responsibility is taken for any advice, instruction or conclusion given within!

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

Abstract The subject that is scrutinized in this dissertation is the seismic design. Since I am in China, I have decided to choose this topic to understand another aspect of design, a more restrictive one, where technology’s needs exceed the onlyaesthetical design to create another type of architecture. My problem statement is the following one: “How can buildings structures resist on a seismic tremor?” In order to reply to this problematic statement, I divided my research in a few others formulations, those research questions, are the following: -

How, structurally, are the buildings made of?

What are the consequences of an earthquake?

Which structural systems could prevent different levels of earthquake to get a building down?

How do engineers size seismic-resisting buildings over the world?

The books I have read helped me to get a general idea on some of the earthquake specificities and on seismic design over the past 40-50 years with the birth of new technologies and the unceasing inspections and studies from the past earthquakes to evolve towards a quake-proof construction world. To get more specific, internet (articles, online courses of some universities, specialized centres on earthquakes, codes …) provided me the rest of information I needed. All resources used are usually listed at the end of each part and at the end of the dissertation - List of references. The illustrations used are also listed - List of illustrations- in the following page.

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

List of Contents ACKNOWLEDGEMENT................................................................................................... 2 ABSTRACT .................................................................................................................... 3 LIST OF CONTENTS........................................................................................................ 4 LIST OF ILLUSTRATIONS ................................................................................................ 7 CHAPTER 1. INTRODUCTION........................................................................................ 8 CHAPTER 2. THE STRUCTURAL DESIGN OF CONSTRUCTIONS ...................................... 10 2.1 THE STRUCTURE AS THE BUILDING’S SKELETON ................................................................. 10 2.1.1 Structural engineers .......................................................................................... 10 2.1.2 The structural engineer/architect relationship ................................................. 11 2.1.3 Static equilibrium .............................................................................................. 12 2.1.4 Codes and standards ......................................................................................... 12 2.2 BUILDING MATERIALS .................................................................................................. 13 2.2.1 Material density ................................................................................................ 13 2.2.2 Internal forces ................................................................................................... 13 2.2.3 Material extension ............................................................................................ 14 2.2.4 Most common structural material .................................................................... 14 2.3 FINANCIAL INFLUENCES................................................................................................ 16 2.3.1 The client ........................................................................................................... 16 2.3.2 Country specificities........................................................................................... 16 2.4 STRUCTURAL DESIGN HISTORY EXAMPLES ........................................................................ 17 2.4.1 Before Antiquity................................................................................................. 17 2.4.2 Romans .............................................................................................................. 17 2.4.3 Traditional Chinese architecture ....................................................................... 18 CHAPTER 3. THE SEISMIC RISKS ................................................................................. 20 3.1 EARTHQUAKE CHARACTERISTICS .................................................................................... 20 3.1.1 Geology.............................................................................................................. 20 3.1.2 Type of waves .................................................................................................... 21 3.1.3 Peak ground acceleration .................................................................................. 22 3.1.4 Forces involved. ................................................................................................. 22 3.1.5 Extra-consequences ........................................................................................... 24 3.2 A YOUNG TECHNOLOGY ............................................................................................... 24 3.2.1 The 20th century................................................................................................. 24 3.2.2 Retrofitting ........................................................................................................ 25

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

3.3 EARTHQUAKE IN L’AQUILA, CENTRAL ITALY (2009) ........................................................... 26 3.3.1 Characteristics ................................................................................................... 26 3.3.2 Geographic area and history ............................................................................. 26 3.3.3 Damage ............................................................................................................. 27 3.4 EARTHQUAKE IN WENCHUAN, SICHUAN PROVINCE, CHINA (2008)...................................... 28 3.4.1 Characteristics ................................................................................................... 28 3.4.2 Geographic area and history ............................................................................. 28 3.4.3 Damage ............................................................................................................. 29 CHAPTER 4. EARTHQUAKE-RESISTANT DESIGN .......................................................... 31 4.1 K.I.S.S. PRINCIPLE ..................................................................................................... 31 4.2 CAPACITY DESIGN ....................................................................................................... 32 4.2.1 Ductility ............................................................................................................. 33 4.2.2 Hierarchy of strength ........................................................................................ 33 4.3 RESISTING EARTHQUAKES’ FORCES ................................................................................. 34 4.3.1 Horizontal planned resistance ........................................................................... 34 4.3.2 Vertical planned resistance ............................................................................... 35 4.4 MAIN STRUCTURAL SYSTEMS ........................................................................................ 35 4.4.1 Shear walls ........................................................................................................ 36 4.4.2 Braced systems .................................................................................................. 36 4.4.3 Moment resisting frames .................................................................................. 37 4.4.4 Mixed systems ................................................................................................... 37 4.5 COMMON ISSUES TO AVOID .......................................................................................... 38 4.5.1 Structural discontinuity and off-set ................................................................... 38 4.5.2 Soft storey ......................................................................................................... 38 4.5.3 Short column ..................................................................................................... 39 4.5.4 Torsion ............................................................................................................... 40 4.5.5 Infill walls........................................................................................................... 40 4.5.6 Buildings pounding ............................................................................................ 40 4.5.7 Re-entrant corners............................................................................................. 41 4.6 PARTICULAR SYSTEMS ................................................................................................. 41 4.6.1 Seismic separation gap...................................................................................... 41 4.6.2 Stairway ............................................................................................................. 41 4.6.3 Bridge between buildings .................................................................................. 42 4.7 NEW TECHNOLOGIES ................................................................................................... 42 4.7.1 Seismic proof constructions ............................................................................... 42 4.7.2 Dampers ............................................................................................................ 42 4.7.3 Carbon fibres ..................................................................................................... 43 4.7.4 Innovative structural configurations ................................................................. 43

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

CHAPTER 5. THE DESIGN PROCESS AND THE BUILDING STANDARDS OVER THE WORLD 45 5.1 PERFORMANCE-BASED DESIGN ...................................................................................... 45 5.1.1 Performance level.............................................................................................. 45 5.1.2 Hazard level ....................................................................................................... 46 5.1.3 Probability of the earthquake ........................................................................... 46 5.2 THE DESIGN PROCESS .................................................................................................. 47 5.2.1 The building ....................................................................................................... 47 5.2.2 Peak ground acceleration.................................................................................. 47 5.2.3 Building response spectra ................................................................................. 48 5.2.4 Seismic force ...................................................................................................... 48 5.3 COMPARISON BETWEEN CODES AND STANDARDS .............................................................. 48 5.3.1 Factors ............................................................................................................... 48 5.3.2 Intensity scale .................................................................................................... 49 5.3.3 Probability of the earthquake ........................................................................... 49 CHAPTER 6. SUMMARY ............................................................................................ 50

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

List of Illustrations FIG. 1 : WILLIS FABER DUMAS HEADQUARTERS BY N. FOSTER AND A. HUNT ........................................ 11 FIG. 2 : FREE-BODY DIAGRAM...................................................................................................... 12 FIG. 3 : INTERNAL FORCES........................................................................................................... 13 FIG. 4 : IMPROVED WOODEN PROFILE ........................................................................................... 15 FIG. 5 : GENERAL IMROVED PROFILES ........................................................................................... 15 FIG. 6 : PIT HOUSE CROSS SECTION ............................................................................................... 17 FIG. 7 : TEEPEE HOUSE STRUCTURE ............................................................................................... 17 FIG. 8 : ROMAN ARCH FORCES PATH ............................................................................................. 18 FIG. 9 : CHINESE WOODEN STRUCTURE.......................................................................................... 19 FIG. 10 : TYPICAL CHINESE TEMPLE CROSS SECTION .......................................................................... 19 FIG. 11 : SEISMIC WAVES ........................................................................................................... 21 FIG. 12: MAGNITUDE, RICHTER SCALE GRAPHIC REPRESENTATION ...................................................... 22 FIG. 13: INTENSITY, AMERICAN SCALE........................................................................................... 23 FIG. 14 : ISOSEISMAL MAP ITALY EARTHQUAKE ............................................................................... 26 FIG. 15 : BUILDING COLLAPSED IN THE CITY OF L’AQUILA ................................................................... 27 FIG. 16 : SAN FRANSISCO CHURCH CLOSE TO L’AQUILA WITH THE ROOF COLLAPSED ............................... 27 FIG. 17 : ISOSEISMAL MAP CHINA EARTHQUAKE .............................................................................. 28 FIG. 18 : APOCALYPTICAL SURROUNDING IN BEICHUN COUNTY AFTER EARTHQUAKE ............................... 29 FIG. 19 : BAILHUZEN MIDDLE SCHOOL AFTER EARTHQUAKE ............................................................... 29 FIG. 20 : UNREINFORCED MASONRY IN SICHUAN PROVINCE. ............................................................. 30 FIG. 21 : STEEL BEHAVIOUR WHILE STRESSED .................................................................................. 33 FIG. 22 : LOCATION OF HINGE JOINTS IN A MOMENT RESISTING FRAME. ............................................... 33 FIG. 23 : BOND BEAMS-CHORD .................................................................................................... 35 FIG. 24 : SIMPLIFIED BUILDING WITH SHEAR WALLS ON EACH SIDE TO RESIST EVERY HORIZONTAL FORCES .... 36 FIG. 25 : SIMPLIFIED STEEL FRAME WITH BRACING ........................................................................... 36 FIG. 26 : SIMPLIFIED MOMENT-RESISTING FRAME BUILDING .............................................................. 37 FIG. 27 : SIMPLIFIED BUILDING WITH BRACING AND SHEAR WALLS ...................................................... 38 FIG. 28 : SOFT STOREY EXAMPLE .................................................................................................. 39 FIG. 29 : SHORT COLUMN EXAMPLES WITH SLOPED GROUND OR WITH A MEZZANINE FLOOR .................... 39 FIG. 30 : EXAMPLES OF ISOLATION SYSTEM. ON THE LEFT, RUBBER PLATES, ON THE RIGHT SLIDING BEARING43 FIG. 31 : THE TOWER TAIPEI 101 - IN YELLOW THE TUNED MASS DAMPER ........................................... 44

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

Chapter 1. Introduction

This dissertation has been written within the context of the 7th semester of the bachelor in Architectural Technologies and Construction Management of VIA University College, in the city of Horsens, Denmark. To fulfil this last semester composed of a dissertation and a final project, I chose to go abroad in exchange with the Sichuan University in Chengdu, which is a relevant location in order to write down those pages with professors that are specialized within the field I had picked, but also people leaving here who feel the tremors regularly and that can share their personal experience with me. The general subject I have been intrigued by is the earthquake in the construction field. Myself simply wondering how could they stand with such a massive force shaking the whole ground. Earthquake is one of the most disastrous events that can happen to a building or a civil construction, its enormous energy coming out of it is able to destroy entire cities, or to create a tsunami taking over everything on its way. Being a student in Denmark is not the best way to get in touch with those things since the seismic risks are negligible over there. My travel in China, in a region of the centre, in the province of Sichuan - next to the highest mountain chain in the world, the Himalaya - known for earthquakes offered me the possibility to study those famous - unfortunately for bad reasons - tremors, coming out of the ground to make us thrill. Before coming to Denmark, I was a student in the University of Strasbourg, 8

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

France, where I studied civil engineering and have been graduated with a Bachelor on the relevant field, the earthquake being relatively low in the area - 2-3 on the Richter scale at most -, me and my classmates haven’t been taught a lot about it. Therefore that was a reason now to get more knowledge on seismic risks and seismic design. My problem statement is: “How can buildings structures resist on a seismic tremor?” In this dissertation we will investigate on how the buildings are made of, without any earthquake considerations, the materials that our civilizations have used and how they do stand in usual circumstances, that is to say the way we have built towers, industrial hangars, stadiums, etc... After this recap, we will turn on the big problem for edifices that has existed since the very beginning of our planet, earth movements and their consequences on our societies. After the problems, come the solutions and the different structural systems that we can meet to prevent the buildings to collapse. Finally, we will take a general look on the design process and give examples on codes differences or similarities around the world. As a student in the Constructing Architect programme, my goals are not to go deep into the engineer calculations, even though it surely is very interesting to dig the possible differences on calculations or diagrams. Nevertheless that will not be relevant for this dissertation. Therefore, and as stipulated by the name of the programme, I will limit myself around the architectural technologies against earthquakes going a bit further on some points, with the codes and standards for example. My resources have been mainly books, to get an outline of the seismic design of the last decades and nowadays. Articles and specialized websites helped me to go deeper in the understanding on specific parts of the dissertation.

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

Chapter 2. The structural design of constructions This chapter 2 will introduce the real content of this dissertation. The structure in architecture is an important thing, it is merely the concept that make the building standing. Therefore we will firstly take a look of this field, on the engineers, on the building industry and on their way to work. Besides, a recap of structural design history will end this chapter relating materials and construction technics.

2.1 The structure as the buildingâ&#x20AC;&#x2122;s skeleton 2.1.1 Structural engineers In a construction process, the client is the first person involved in the project, since he/she or a moral person like an institution decides either to create a new project, and therefore a new building or to renovate a building that needs it for different reasons; could be for another function of the construction and then another architectural form would be more appropriate or to upgrade the buildings to the last building codes released, which is an interesting subject, a normal 10

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

refurbishment, or in case of seismic resistant structures, a retrofitting The architect, the person in charge legally to respond to the client needs will create the outline, and so on. The structural engineers can enter the process at several different moments, theoretically and what is usually represented in schools is the engineer starting to work on the project after the outline being finished and after tender bid accepted. It could also be in a very technical construction the engineer being called by the architect to work on the outline and give some advices. In some civil construction, a structural engineer could be the only person in charge. Unfortunately for the engineers, architects are usually more known than them, there is even a specific term for a widely-known architect, and it is a “starchitect”. The engineer stays almost always in the shadow of the architect. Almost, because some engineers are still, wellknown in the building industry and they are always linked to famous architects as well – e.g. Anthony Hunt that has worked with few of Norman Foster and Richard Rogers’ projects. FIG. 1 : WILLIS FABER DUMAS HEADQUARTERS BY N. FOSTER AND A. HUNT

They are the people sizing buildings and they also get their words to say on the precise components of the construction. They are the people taking care of the respect of the seismic codes acting in the country the project is settled and are directly responsible for any damages any deviances.

2.1.2 The structural engineer/architect relationship Engineers and Architects are two complementary actors of the building industry. Beyond the fact that they might have some contentious relationships, their work as one team is important on several points. First of all, the client could lose his money caused by the time schedule postponed - mistakes on the structural design, etc... -, the building could be less sustainable, or could have bigger issues. This relationship becomes even more important when the project is located in a seismic region, the risks may directly be connected to humans’ lives.

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

2.1.3 Static equilibrium This sub-part of the chapter talks about the exact role of the engineer in the sizing process of a building. The static equilibrium is the key in order to define the actions on each component of the structure and size them. The concept of equilibrium is that the external forces acting on the structure cancel each other; the system is thus in equilibrium. If the system is not static - i.e. in equilibrium - it is called to be a mechanism. A free-body diagram is a 2D view that allows an engineer to simplify and find out the value of the forces acting on the structure. Those forces will be matched with the loads acting on the building; vertical loads - i.e. dead loads and imposed loads - and horizontal loads - i.e. the wind being important for long-spanning structures like bridges or high-raise buildings but also seismic resulting forces in certain regions on earth. The path that forces take through the different components is an important aspect of sizing elements, however some structure can be harder than others to represent schematically. The reason for that is that the force paths is not simple to analyse, too many possibilities are possible. Nevertheless, computer-based FIG. 2 : FREE-BODY DIAGRAM software help engineers with those structures and avoid many complicated calculations that moreover take time. (Mcdonald, 2001, p. 9)

2.1.4 Codes and standards The way to size structural components or the solutions that can be brought within the structure are stipulated in the respective codes of the country the project is built in. Those minimum requirements help the building industry to be at a certain level of efficiency towards the resistance of the construction and safety for the people working in, living in, or simply as users of those superstructures. Each country and government has their own standards that have to be respected by any company. This will be discussed on the dissertationâ&#x20AC;&#x2122;s part 5.

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

2.2 Building materials Each material that are used as a construction material or even that can be found anywhere on this planet for whatever reason have their specific characteristics. They allow the construction, to look different but also to act different predictably enough on a structural point of view thence there are more optional ideas to raise an edifice.

2.2.1 Material density One of the most common characteristic is the density, calculated by the weight or the mass (depending on the units we want to involve in the calculations) divided by the volume. The density of the construction materials are directly linked to the total mass of the building, this mass is a very important factor in the seismic design that we will look deeper into in the following part. Indeed, the mass is linked to inertia, the Newtonâ&#x20AC;&#x2122;s second law of motion establishing that the force equals to the mass times acceleration - F = m x a. We will go through it in the next chapter. (Title 3.1.4, p.18)

2.2.2 Internal forces

FIG. 3 : INTERNAL FORCES

Materials are generally submitted to constraints and very important ones quantitatively if the components are used in the building industry. Their functions have to be completely exploited, to do so, architects and engineers need to know what kind of forces they must resist to.

Several forces are at skate and cause intrinsic stress in the structure, the normal force is acting perpendicularly to the plane cross section, and therefore longitudinally on the component. The latter force is currently called axial force; we can find two sorts of thrust in the materials fibres, the compression and the tension. The shear force on the opposite is acting perpendicularly to the length of

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

an object, or, to be more specific, in the same plane than the component cross section. The bending is a combination of the 3 kinds of forces mentioned above, compression, tension, and shear. The result is a bend created on the longitudinal plan of the component. It creates on one of the extreme fibres a high compression and on the opposite side a high tension. Therefore, to resist a bending, the element has to be as resistant in tension as in compression to be completely effective and not oversized. The last force that can occur is the torsion; this is a particular one caused by the forces mentioned above when the centre of the thrust is not adequate with the inertia centre of the plan they act on.

2.2.3 Material extension The expansion of a material can be generated by natural circumstances or by mechanical constraints. Thermal expansion, known by everyone, is the dilatation of elements provoked by a certain difference of temperature. It can be a structural problem if the system is indeterminate. An indeterminate system is the opposite of the determinate system, its particularity is that the unknown forces are too numerous to use the simple method with the free-body diagram. It brings more difficulty on the calculations but also with extensions of elements; indeed theoretically the forces don’t allow any displacement in the system. In order to work fine, it has to consider potential extension within the joints’ areas. The mechanic strain, on the other hand, depends on the forces that apply on the object. The difference of size is the result of the material elasticity against stress. There are two sorts of behaviour while speaking of strain; the elastic mode is defined by a level of strain proportional to the stress applied; the second one, called plastic mode has a more complicated mathematical definition. The latter increases the resistance of the material before yielding.

2.2.4 Most common structural material Timber is one of the most used materials for construction, especially for lowraise residential building. Its characteristics in compression, tension and bending are good depending on the fibres’ directions; it allows a low-medium span definitely enough for a single family house scale. However, timber has some inconvenient defaults, since wood is a raw natural material, the inner structure of it is not regular, wood fibres can be affected by any parasites or we can also find nodes in it. This will affect the resistance of the material; therefore most of the

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

structures in timber we deal with are made with improved methods in factories. Its density being quite low, it becomes even more efficient. Masonry is as the wood, a very common material for a very FIG. 4 : IMPROVED WOODEN PROFILE long time now. It is defined by assembled blocks - concrete blocks, bricks … - linked together by a mortar, a joint between all the blocks that sticks them together. Although masonry is very suitable in term of compression forces, tension is not resisted enough to be chosen for that perspective. Most of the constructions in the roman period were using masonry, with active-forms like arches, vaults, domes... Nowadays, masonry is used for the vertical structure and it is mixed with another type of material for horizontal structure in order to resist bending of slabs, lintels and so on. Usually reinforced concrete fulfils this role. Steel is either a main structural material from buildings, either a complement to the other ones, and that is because of one reason; steel has the best capacities to resist compression and tension. We can find structural steel structures in industrial buildings or in civil constructions as bridges or skyscrapers. The major default of steel is its density, therefore improved shapes I-shape, H-shape, Hollow structural section… are needed to reduce the volume of steel, which means decreasing the weight and the price, and not reducing the webs’ inertia of the beam resisting the bending. FIG. 5 : GENERAL IMROVED PROFILES

Nowadays, reinforced concrete might be the most common structural material, indeed it is used for almost all kind of buildings, resisting all kinds of forces. It is due to concrete resisting very well in compression, and steel helping concrete to reinforce itself while in tension. Skyscrapers, bridges, dams, single-family houses, multi-storeys residential buildings, etc... Improved techniques are common in precast concrete. In Denmark, hollow-core slabs are more efficient because of the structure has less weight, dead loads are minimized then. Also, different techniques of casting exist, they have different consequences on the structure steel frame as the framework to cast concrete, and thus it brings an additional structural reinforcement. (Mcdonald, 2001, pp. 22-36) 15

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

2.3 Financial influences 2.3.1 The client The client is the one who invest in the building, according to the latterâ&#x20AC;&#x2122;s reasons or goals; he might want to choose other technics that are demanded by architects and engineers; the main argument being the cost of those products. It can be dangerous at first sight, even more when seismic design is at play but fortunately codes are made for that kind of cases. When it is a public client, financial mediums are regulated by the government of the country with its budget. However, public institutions are on one hand, imposing the codes and standards on their territory, on the other hand that would be impossible to not be aware of those ones. Private clients might not be aware of the regulations and codes, nevertheless the rules have to be respected, but the client can still change his mind and bring extra cost to the building construction. Nowadays, saving money on every aspect of the construction has become an international sport; a bad thing would be the construction not being able to fulfill its mission, in other words, not being functional anymore. The worst case, particularly during an earthquake, is the structure collapsing and having a contingent of people in danger or lost.

2.3.2 Country specificities Several differences between countries can be guessed, the location is different, and so there are not the same reliefs, the same resources from the ground. That has an impact on the material cost and therefore on the way to construct and neither the same seismic risk. As a consequence of the lack of resources, industries are missing, the country cannot be developed enough; and then constructions might not be strong enough for cause of budget limit. Finally, codes and standards are usually able to prevent structures to collapse and stay functional; however some countries have not official standards on their own even though those countries form a minority among the international community.

Guillaume M. Melin (177626) VIAUC Campus Horsens – 02.12.2013

2.4 Structural design history examples 2.4.1 Before Antiquity Structural design relatively exist since the humanity does; starting with the hut from the Kung people in Africa around 60000BC -, from the Gravetians in Europe - around 28000 BC - or the Tipis in America. Those housings were done with sticks made of timber and ingeniously linked together to restrain a tensile membrane made of plant, or animal skin. Some were more

FIG. 7 : TEEPEE HOUSE STRUCTURE

sophisticated than others, it also depended on the function of the tent; some people were moving regularly in order to survive, while others used to stay at the same location near their vital needs. On a seismic point of view their houses were perfect; those tents are made of an active-form – defined in FIG. 6 : PIT HOUSE CROSS SECTION the next parts - and work in tension of the membrane, the only rigid component is the wooden skeleton of the tent which is light-weight, earthquakes had very small influences on our ancestors’ homes. (Mcdonald, 2001, p. 2) (Ching, et al., 2011)

2.4.2 Romans The romans are known for their engineering; especially for their remarkable public works. The water supply network built at that period of time can compete, 17

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on the amount of water furnished, against the biggest cities’ networks from our contemporary time, which says how improved it was. The system used to provide the famed Romans thermal baths as well as the improved toilet system and the aristocracy villas with water. In order to do so, they improved a lot of construction technics like the semicircular arch, but didn’t invent so much of those latter. Understanding the assets of it, they have built aqueducts, bridges, temples, houses and thus very spacious constructions could have been realized, for example the domes of some cathedrals still standing nowadays. In order to do so, they had noticed the importance of decreasing the weight of the structure - e.g. use of hollow bricks. Their construction materials were first the hydraulic cement made with volcanic ashes, and then later they have started to build with concrete. This concrete mix had lime and volcanic sand as main components. They have also used masonry bricks, copper and bronze in their architecture, those last two were not, by all means, structurally oriented. Roman concrete was remarkably stable under FIG. 8 : ROMAN ARCH FORCES PATH earthquakes, the non-intentional variations in the density of the concrete allowing disruptions of seismic waves. Their constructions, for example the coliseum in Rome and or the aqueducts in south Europe still exist and show their brilliant engineers’ minds. (Raucci & Jewell, n.d.)

2.4.3 Traditional Chinese architecture

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

Traditional constructions in China are made of timber. they developed the timber construction process, the felling, the transportation and the processing being more and more convenient.

FIG. 10 : TYPICAL CHINESE TEMPLE CROSS SECTION

FIG. 9 : CHINESE WOODEN STRUCTURE

Concerning the structural components of the building, the latter stands thanks to a post and beam structure with bounds, tenons

and mortises. Besides, those links between components have a relatively small freedom in their movement, that plus the fact that wood is a good material on a damping perspective, the building lower the risk of damages if horizontal loads apply. The traditional architecture in China is symmetrical; it uses rectangular shapes, circular, hexagonal or octagonal ones. The components are oversized and therefore stronger. A concrete made of lime and earth is use as foundations. (Zhang, s.d.)

Guillaume M. Melin (177626) VIAUC Campus Horsens â&#x20AC;&#x201C; 02.12.2013

Chapter 3. The seismic risks The Chapter 2 dealed brought up the way engineers are in the building industry and also the way building have been built. The seismic risks were already there obviously and some seismic design considerations can be observed in some historical and traditional architecture. This part of the thesis will introduce the phonemenon that is an earthquake, stopping by the geology of our planet until the seismic waves hitting the building. Then, 2 examples will be scrutinized, an earthquake in the center of Italy in 2009 and one in Sichuan province in 2008.

3.1 Earthquake characteristics 3.1.1 Geology Our planet is made out of solid and liquid core. Even though the centre is a solid core made of heavy metals, The crust, the external part of our globe is composed of light materials as basalts and granites and exists as plates covering 20

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the planetâ&#x20AC;&#x2122;s surface. The outer core is liquid and the mantle has its own internal flow. That is the latter that creates tectonic movement. The phenomena of the earthquake come from an energy liberated by the friction between the tectonic plates. The elastic strain generated by the plates rubbing next to each other, will concentrate a massive amount of energy. The latter being released will cause a fault in one of the plate and at the same time an earthquake. This concept is schematically explained by the lateral force exceeding the friction force of the two plates. Andrew Charleston compares it with our fingers snapping. We apply a normal pressure directed on each other on both fingers, and then we apply at the same time a lateral force that will create the snap. (Lindeburg & Baradar, 2001, pp. 1-8) (Charleson, 2008, pp. 4-11)

3.1.2 Type of waves A quake will generate several kinds of waves. Some will be called surface waves while the others are called underground or body waves. When the fault appears, body waves are transported from that point until the surface of the earth, pushing and pulling the soil particles - P-waves - and moving them side to side - S-waves. The P-waves are the fastest to get to the surface; it will reach the building before any others. Primary - P-waves - and secondary - Swaves - are usually reflected back at the surface and back again towards the earth surface. This fact brings a stronger ground shaking - by up to twice as much than in the ground.

FIG. 11 : SEISMIC WAVES

Surface waves are on the other hand a consequence of the body waves. Body waves will hit the surface of the earth at the epicentre. Then, surface waves are created from this latter position going all around. Surface waves are composed of two different ones. Love waves are going sideways in the horizontal plane, this wave is similar to the S-waves unless there are no actions directed up or down. And Rayleigh waves which create an elliptical movement in the vertical plane.

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Those waves will therefore create movements which will act on the building with its inertia mainly in a horizontal plane. (Lindeburg & Baradar, 2001, pp. 1-8) (Charleson, 2008, pp. 4-11)

3.1.3 Peak ground acceleration The PGA value defines the shaking; it is calculated by seismometers or accelerometers, parts of the seismograph. The unit, used in general all over the world is linked to the unit g, the gravitational acceleration. It can also be measured in gal – cm/s² - or in m/s² or in the imperial unit system - ft/s² or in/s². The acceleration of the ground is related to the tremor’s intensity, the more acceleration, the higher will be the intensity and consequently the more damage. If the designs of constructions are the same, as well as the geographical region, we can observe a relatively clear correlation between the intensity and the PGA. The “reference peak ground acceleration” is used to design the buildings within the codes. National zoning maps with RPGA exist and represent a certain earthquake with a certain occurrence, defined in the codes as well. (Lindeburg & Baradar, 2001, p. 12)

3.1.4 Forces involved. The magnitude of an earthquake is defined by the “size” of the latter. Each quake is attached to a value on the Richter scale - named after the American seismologist Charles Richter – for the low and moderate earthquakes and with the moment magnitude scale for the strong and severe earthquake since the Richter one is mathematically not precised enough on this range. The scale is growing on an exponential range. Indeed a difference of 1 FIG. 12: MAGNITUDE, RICHTER SCALE GRAPHIC unit on the scale equals a 10 times REPRESENTATION difference on the wave’s amplitude and 31 times difference on the energy released. The fault being more or less deep 22

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in the ground, the latter is similar to an insulation layer reducing for us, fortunately, this massive difference. So that, when it reaches the external surface of the earth, the difference will not be as big as it was. Following Newton’s law of motion, we have “F” equalled to the inertia force; “m” which is FIG. 13: INTENSITY, AMERICAN SCALE the mass of the building; and “a”. This part of the equation represents the acceleration generated by the shaking. It is especially expressed in function of “g” - universal gravity acceleration - in seismic design - in a way that 0.5g is equal to 4.90 m/s². 2nd

When a quake has been detected, seismographs can detect the intensity of the quake, there are different scales representing it, each of them are going from I to XII - roman numbers; XII being the most severe. For example, the “Leidu scale” is in use in CHINA and the “European Macroseismic Scale” in Europe. Three characteristics are considered, the animals’ perception of shaking, the humans’ one and the surrounding impact. On the opposite of the magnitude, intensity is different at every location where earthquakes have been felt. This value will depend on the layers of soil, on some probable phreatic table and so on, and particularly on the distance between the fault - or also the epicentre - and the location aiming to be measured. A geographic plan is generally made to see the different area of intensity called “isoseismal map” like a topographic plan, but instead of altitude lines, we see intensity lines. As it is brought up on one of the paragraph above, the inertia force of the building is linked to the building. The weight is linked to the inertia; therefore the weight has a connection with the force applied to the building. The heavier a building is, the stronger it has to be to resist the resulting inertia force. When earthquake’s waves reach the foundations, those latter will follow the shaking randomly in the 3 spatial dimensions - x, y, and z. The building will tend not to move considering its weight, this force resulting to keep the building from not moving is called inertia force. And it is this resulting force that is applied to the building during a tremor. The shaking of the building varies according to the soil. The nature of the soil will have a consequence on the magnitude and on the frequency of the quake. With a soil type made of soft sediments, the magnitude will increase and the duration of the shaking as well. On the contrary, with a soil made of rock, the shake will last a shorter time and the magnitude with be smaller. (Charleson, 2008, pp. 15-24) (Murty, n.d., p. Tip 03) 23

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3.1.5 Extra-consequences Earthquakes are not only tremors which provide structural problems. Indeed a couple of other consequences not related to the superstructure can happen. The quake may partly damage the building services, generating electricity sparks, gas leaks .A major problem in the last decades and centuries was the subsequent fires after earthquakes. In 1906, San Francisco was battling against fires for 3 entire days after the earthquake destroying around 90% of the city or more recently, the Northridge quake in 1994 in California. Ground shaking might if some circumstances are gathered lead to dangerous incidents. If the epicentre is located in the ocean and relatively close to the coast – a few hundreds kilometres – this can create a tsunami with the consequences we all know, floods. Identically, the shaking can destroy civil constructions like dams or levees which will cause floods as well. In some regions, earthquake can generate landslides, it happens when the inertia force from the tremor exceeds the intrinsic strength of the soil, it creates a rupture and a part of the land moves over. Dangerous for the stability of buildings, liquefaction happens when the ground is made of a layer of sand or loose soil, a water table high enough to submerge this layer and a ground shaking putting a sufficient pressure. Although this situation is dramatic, the ground losing its ability at bearing structures, the kinds of soil which are subject to such a characteristic are rare. Indeed both characteristic have to match immediately while the ground shaking is still running. The concept is similar to the sand at the beach, having our feet in the sand, and when the wave goes over the sand; our feet sink in the ground. Those consequences are more severe if specific buildings are touched, hospitals, schools, and in another perspective, nuclear power plants - for example, the earthquake in Fukushima, Japan in March 2011 – or some refineries like in Tomakomai, Japan in 2003. (Charleson, 2008, pp. 113-119) (Michigan Technological University, 2007)

3.2 A young technology 3.2.1 The 20th century The 20th century is known for major changes, mainly in technology, the architecture has changed and become high in altitude but also new shapes 24

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appeared, pleasing architects and residents, bringing challenge to engineers, all that to respond to new society demands. It has in the building industry, been a century of analysis on quake resistance. Seismic technologies are relatively new. It has started in the beginning of the century with an earthquake in San Francisco in 1906 and another one in 1908 in the south of Italy; both of them killing around tens of thousands of people. Those 2 events has sounded the death knell for people to notice that improvements were necessary. Consequently, in response to that, instruments were created in order to calculate the ground shaking. 20th

After two other earthquakes in 1923 in Kanto – 140,000 victims – and in 1925 in Santa Barbara, the building industry at that time decide to evolve towards seismic design. Laboratory tests are running and in late 20’s to early 30’s, the first building codes with seismic design is introduced on the municipal territory of Los Angeles. Until the 1950’s, Engineers and architects developed seismic design around strength and stiffness, but evolving with the time, they came up with another concept, named ductility which responded to what used to be their problem statement: “what happens to a structure if its inertia forces exceed those for which it has been designed?. (Charleson, 2008, p. 35) But it is around 1970 that a group of engineers from New-Zealand introduce a specific design approach to resist tremors. Called Capacity design, this latter is based on specific ductility needs in the building. This approach significantly changes the way to build. Made with this system, a building is judged to be 6 times more resistant than a common one. (Charleson, 2008, pp. 34-35)

3.2.2 Retrofitting To retrofit a building is defined as the renovation of the latter up to seismic codes and standards. Apart maybe of the new developed countries and considering the cities that have built a lot during this century, a major problem appears. Since engineers and architects assume the lifespan of constructions around 50 years depending on its function, a lot of them have been thought and erected before or during those seismic studies. Therefore, a major part of buildings in cities all over the world is not designed to resist earthquakes, or at least severe ones. Then the need to retrofit those constructions is undeniable. Unfortunately, that renovation has a cost; private investors might not consider that risk as far as a quake did not happen yet or they merely can’t afford it.

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Also being able to happen, is a relatively important earthquake in a low-risk seismic region, especially with buildings conceived in the second part of the century where constructions were not as able as today to handle it. (Charleson, 2008, pp. 187-209)

3.3 Earthquake in L’aquila, central Italy (2009) 3.3.1 Characteristics The earthquake happened on the 6th of April in 2009, in the region of L’aquila, name of the biggest city around, and particularly touched by the tremor since the epicentre was a few kilometres from it. Its magnitude was over 6 on the Richter scale, precisely 6.3 so that we can define the earthquake as strong. The depth of the fault was 8.8 km and the biggest intensity detected was of VII. (USGS, 2012)

3.3.2 Geographic area and history The area of L’aquila is located in central Italy on the North-East of Rome and surrounded by a mountain belt called the Apennines. The city of L’aquila (about 75,000 inhabitants) was situated at 5 km from the epicentre, Pizzoli 6km, Tenni 59 km and the capital city Rome at 85 km. The quake has been triggered by a normal fault rupture, this region is seismically active; in 1997, an earthquake of magnitude 6.4 was recorded with a prior period of 2 months recording 8 earthquakes of more than 5.0 on the Richter scale; finally a few years later in 2002, an earthquake of magnitude 5.9 shook central Italy.

FIG. 14 : ISOSEISMAL MAP ITALY EARTHQUAKE

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3.3.3 Damage Concerning the damage of facilities, the region is not urban and has a medieval past accordingly to the buildings still present. The constructions, built a few centuries ago down to the 13th century have suffered from the shaking. In the cities around the epicentre, 25% to 50% of the edifices have been severely damaged. However L’aquila city centre, being the closest to the epicentre, handled the FIG. 15 : BUILDING COLLAPSED IN THE CITY OF L’AQUILA quake relatively well, since rare buildings have been found completely collapsed. Most of the latter are only partly destroyed; it is due to the good material used likely hosted by the ancient aristocracy. Therefore, buildings of good quality has, with no surprise, resisted better. The intensity of the earthquake has been very different within close areas, so that some cities have suffered from a violent shaking. The region has very different sediments layers on the valleys, and that may have caused differences. The peak ground acceleration – i.e. PGA – exceeded 0.35g while the average design level of the constructions was at most 0.25g from the 2003 codes upgrade for this region. That is only for the newest buildings that represent a few percentage of the constructions, before that, the buildings were designed at 0.23g.

FIG. 16 : SAN FRANSISCO CHURCH CLOSE TO L’AQUILA WITH THE ROOF COLLAPSED

Critical buildings have also been damaged, the main hospital of the region had to be evacuated following to some apparent cracks at the top of the column on the ground floor. More dramatically, a whole student dormitory collapsed in L’aquila killing the students inside – the earthquake happened during the night.

The Italian government condemned seven members of a commission responsible of the buildings’ situation in the region, charged for reckless homicides. (EERI, June 2009)

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3.4 Earthquake in Wenchuan, Sichuan province, China (2008) 3.4.1 Characteristics The Wenchuan earthquake was recorded on the 12th of May in 2008. Its magnitude was 7.9 on the Richter scale which can be defined as a major earthquake. It killed 69,000 people, 370,000 injured, 1,5 million Chinese citizens had to be relocated. About the constructions, 216,000 buildings were damaged with 6,900 of them being schools. The intensity of the earthquake was VII in Chengdu, VIII in Mianyiang and IX in Tianpeng area. (USGS, 2013)

3.4.2 Geographic area and history The epicentre was situated close major cities like Mianyang – by 150 km - and the Sichuan province capital city, Chengdu – by 80 km.

The rupture depth was 19 km, and the area covered by the shaking was enormous - i.e. 260,000 m². The reason for that is the length of the fault, which is 270 km long. The earthquake is due to the reverse thrust of the Tibetan plateau - which represents a soft crust - against the Sichuan basin – the strong crust. With its magnitude and intensity, that is strongest and most devastating earthquake from the FIG. 17 : ISOSEISMAL MAP CHINA EARTHQUAKE current 21st century. The Longmen Shan fault is a very active one, earthquake with a 4,0 magnitude are common. It can be explained by the characteristic of the fault – the fact that the Tibetan plateau is going up, those are very active situations. (USGS, 2013)

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3.4.3 Damage The predominant structure in Sichuan is the unreinforced brick masonry, especially in rural areas which are not trustworthy during an earthquake. In the cities the buildings resisted pretty well, the majority of them being pretty young. Most of the ones that collapsed were built with the codes from 1976 and 1989 upgrades. The peak ground accelerations were recorded by 3 stations, the first one was 957 gal – gal is a geodesy unit related to the movement FIG. 18 : APOCALYPTICAL of the ground, 1 gal = 1 cm/s², so that 981 gal = SURROUNDING IN BEICHUN COUNTY AFTER EARTHQUAKE 1g – at a distance of 22 km form the epicentre with 60 seconds of shaking duration. The second one recorded a peak at 802 gal at 88 km from the epicentre but 1 km from the fault with a shaking that lasts 90 seconds. The third one recorded 550 gal at 150 km form the epicentre, 75 km from the fault and lasts 150 seconds. Those peaks define a very brutal earthquake which left no chances for the buildings without seismic design. Therefore, the disaster happened in the rural zones, close to the fault but also where the last seismic codes were not applied. The constructions being already not strong enough, the shaking triggered several landslides where the conditions were gathered and formed quake-lakes. However some buildings avoided those critical regions and resisted structurally well close to the fault, it is the case of Bailuzhen middle school or Tongji middle school done with moment resisting frames – defined in the next part. The shaking had FIG. 19 : BAILHUZEN MIDDLE SCHOOL AFTER consequences on dams and EARTHQUAKE bridges since Sichuan province had an important amount of them. Some bridges’ spans collapsed and some issues with dams happened, for 29

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example, the Zipingpu dam area located at 17 km from the epicentre has recorded some displacements on the parapet wall part around 10cm. As a result, the dam has been entirely drained a few days later in order to solve safety issues detected. Moreover, 30,000 km of pipelines in the region had to be restored; the stations recorded a PGA of 0.1g and a bit more near them. Consequently, although Chengdu hasn’t been touched severely by the quake regarding to the buildings’ structures, the water supply network related to the city FIG. 20 : UNREINFORCED MASONRY IN SICHUAN was damaged. The authorities PROVINCE. even distributed pills all over the city to disinfect the water. Power outage touched all the regions around but Chengdu, the other regions were without electricity for a period of 10 to 20 days depending on the difficult accessibility of the different areas. (EERI, October 2008)

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Chapter 4. Earthquake-resistant design

We have observed in the prior chapter that earthquake needs to be considered by everyone has a big risk for the human lives, bu also for the economy of a country. In this part we will talk about the solutions. Studies and researches have been conducted during the whole 20th century and continue to be at the beginning of the 2000’s. Engineers and architects have successfully invented methods to avoid constructions to collapse and therefore to save humans’ lives, technologies improving all the time, an earthquake can occur nowadays and keep edifices intact, without putting in troubles their functionality. Consequentely we will go through all the problems that a quake generates and how buildings can be designed and built to resist.

4.1 K.I.S.S. Principle This principle is a basis to design constructions in a seismic region. KISS Keep It Simple and Symmetrical - is a way to think in order to create a building 31

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resisting shaking well. On a design perspective, the structure of the building has to be stable; in such a way that floor plans are coherent and homogenous, and the cross sections as well. This philosophy facilitates the design of buildings that can be submitted to complex force patterns due to the random shaking, and thus thwarts those edifices from main structural issues. (Charleson, 2008, p. 126)

4.2 Capacity design Capacity design is a way to conceive a building with an increased aptitude at standing against a quake. It has been developed by a research team from NewZealand and nowadays, all the buildings are created by their concept. The latter rests on the ductility and the damping efficiency of the structure in itself - i.e. the absorption of the shaking within the structure. The deflection of the buildings is therefore controlled. The method deals with the reinforced concrete, the concrete cannot enter in a ductile state, but steel bars can. In order to get so, they have to reach the plastic range of the steel. The characteristic of the steel defines for this metal, contrarily to the concrete, several states under axial forces. The steel has an elastic range – the strain being proportional to the force applied – and after a certain state called the yield point the steel becomes plastic. The particular effect of that plastic range is that the steel will be more flexible within its intrinsic crystal structure – strain hardening - and the most important be still resistant. In the plastic range, the element will strain more than in the elastic one, therefore this strain, will act like a mechanical damper with concrete keeping the whole building component united. Capacity design particularly takes care of the vertical components of the constructions, indeed, in order not to collapse those ones need to keep bearing the loads in the same axis – Dead loads, imposed loads. This technology allowed architects and engineers to build better high-raised buildings within medium and high-risk regions in the last quarter of the 20th century. (Murty, n.d., pp. Tips 09-17)

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4.2.1 Ductility When earthquake’s waves hit the building, it will create a deflection explained by the inertia force in a prior part of the report and that we can analyse in the period of vibration of the building. This deflection need to be controlled, in order not to break, the structure needs to be ductile. Although, only some critical parts need that characteristic, these are the bottom of building on the vertical structural elements – columns, shear walls – and on horizontal FIG. 21 : STEEL BEHAVIOUR WHILE elements close to the junction with the vertical STRESSED ones – i.e. in a cross section, two parts that are ductile on each side on the horizontal element. Those critical parts need to create sort of a hinge joint, and are called structural fuses. Their locations allow the building to be more flexible, and therefore to direct the earthquake power into those hinge joints. (Murty, n.d., pp. Tips 08-09)

FIG. 22 : LOCATION OF HINGE JOINTS IN A MOMENT RESISTING FRAME.

4.2.2 Hierarchy of strength In a practical way, a structural fuse is built by creating a hierarchy of strength. This hierarchy puts on the top of it, the force that will generate the fuse. If another force is too strong and break the component before the fuse is active, then the building will collapse. A major work will be done to counter those other forces. In other words, the fuse must be active before the other forces put the building down. During an earthquake shear force might contribute to a failure in the vertical elements; steel reinforcements have to be replaced by others. Since it is perpendicular reinforcements – called ties – that handle shear force, the diameter of those bars can be increased for a bigger strength, the spacing between them should also be decreased. 33

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The footing of the foundation shouldn’t overturn, if it does, important damage could appear. Even if that issue may bring big issues, it is easy to counter, the footing will be dimensioned according to the final designed bending strength and then, that would not be a problem. Over-reinforced concrete has also to be considered as a risk; by adding bigger diameter and more reinforcements the latter will not yield and therefore will not become ductile. The concrete not being strong enough would break into pieces; the solutions are to increase the depth of the element or to take off some of the steel. (Murty, n.d., pp. Tips 17-18)

4.3 Resisting earthquakes’ forces This part will deal with the force paths coming from the earthquake fault, passing by the inertia force of the building, to the foundations resisting stress. The seismic forces go up-down and side-to-side, the vertical resultant force is insignificant though, first of all because the building is designed to resist dead loads and imposed loads with a safety coefficient, and second of all because the vertical movements don’t provoke any direct risks for the building to collapse – nevertheless an exception occurs for constructions having long distance spanning, which is not the case for usual buildings. During an earthquake, the issue comes from the horizontal plan – inertia force -; it is the one disturbing factor for the stability of constructions by deflecting them. Buildings must resist this force.

4.3.1 Horizontal planned resistance The inertia force is coming on the horizontal plan; theoretically, the point of action will be the centre of inertia of the building. Here, engineers consider a centre of inertia on each storey with a different force; the inertia force may be the same, but the moment of inertia is not, and that is the cause of the deflection – the distance between the foundations and the storey height.

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To get a good structural behaviour, hinge joints should be present on both side of the slab in order to absorb the deformation submitted by the vertical components on the horizontal diaphragm. As a consequence, this latterâ&#x20AC;&#x2122;s design is specifically thought to improve its bending â&#x20AC;&#x201C; horizontal bending. (as shown in Fig. 22.) Similarly to an I-shape beam, the diaphragm is made of chords going around the slab and a web. The web merely is the centre of the slab and the chord is the component which penetrates the wall structure with specific reinforcements to improve the moment of inertia and likewise be FIG. 23 : BOND BEAMS-CHORD stronger. The chord is a bond beam cast in the wall-slab junction in a concrete structure a usual beam in wooden or steel structures. To sum up, horizontal diaphragms need to transmit the inertia force to the vertical structure, however by doing that, internal forces will occur in the horizontal structure, strength and ductility is therefore demanded. (Charleson, 2008, pp. 126-140)

4.3.2 Vertical planned resistance Inertia force acts on the horizontal plan but for the building to stand this horizontal force needs to be resisted by the foundations which is usually the only support of any construction. The force paths must go through the vertical components until the foundations. Those structural parts of the construction are the critical ones. Without them, the slabs cannot be supported; moreover the vertical components are the ones that counter the cantilever effect from the earthquake, the inertia force. Those components are detailed in the next part (part 4.4). (Charleson, 2008, pp. 144-154)

4.4 Main structural systems

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When clients ask to build in an area known to be shaken by the tectonic plates’ movement, architects and engineers have several solutions. This part is about the differences between them.

4.4.1 Shear walls This system is by far the most efficient. As it has been said on the main part about structural design at the beginning of this paper; monolithic walls resist very well to horizontal force applied in their longitudinal direction.

FIG. 24 : SIMPLIFIED BUILDING WITH SHEAR WALLS ON EACH SIDE TO RESIST EVERY HORIZONTAL FORCES

The shaking of the quake generates a random inertia force, but this latter can be projected on two axis – e.g. x and y - on a 2D plan of the construction. If shear walls are built with their directions following the axis, all the forces no

matter which directions will be resisted. Even though It might be the best choice structurally speaking, shear walls systems propose a very closed design, indeed, the shear walls can be highly stressed – especially at the bottom of the construction – openings are not recommended and are avoided. Fortunately, shear walls can separated by coupling beams – i.e. a beam which very high and connect two shear walls – or they also can be built on just a part of the building’s length. (Murty, n.d., p. Tip 23) (Charleson, 2008, pp. 66-75)

4.4.2 Braced systems Also seen in the first main part about structural design, they are used in light industrial buildings particularly and on low-rise buildings. The braced system uses triangle geometry with braces in tension to resist forces coming – as the shear wall – from the perpendicular plane to its length. 36 FIG. 25 : SIMPLIFIED STEEL FRAME WITH BRACING

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Braced-systems are made of steel, as mentioned on prior parts of this paper, ductility, a major piece of capacity design, is brought in reinforced concrete by this specific material. Therefore, we can also create hinge joints in braced systems, in order to so, the bracing bar is placed eccentrically; that would create the structural fuse and absorb the tremors resultant forces. This system stays a light weight construction method, foundations needs to be controlled to be sure they won’t overturn under the seismic cantilever reaction; tension piles can be built to prevent that to happen (Charleson, 2008, pp. 76-80)

4.4.3 Moment resisting frames This is the system used with columns and beams; it has to fulfil 3 criteria. First of all, the columns have to be deep enough to resist significant bending moments, second of all the beams and columns need to have similar depths, even though the column is usually recommended to be stronger and then having a bigger section; and finally a rigid connection between the column and the beam. The beam needs to be smaller at some critical points to create hinge joints in the beam by focusing the weak part of the structure on the horizontal structure’s member. On a design point of view, moment frames have almost no limits on the buildable shapes and opening for natural lights are not an issue. (Charleson, 2008, pp. 83-88) FIG. 26 : SIMPLIFIED MOMENT-RESISTING FRAME BUILDING

4.4.4 Mixed systems This system is defined by the use of 2 or more of the systems mentioned above on the same axis. If engineers and architects mix systems, they will not respect the KISS principle – Keep It Simple and Symmetrical -, the force path will be difficult to understand and computer-based programs will be needed to apprehend all load cases. It is a system that should be avoided, however shear walls and moment frames systems can be complementary on a certain kind of building; the high-rise 37

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buildings. The deflections of the 2 systems are totally different, and a good use of them can improve the seismic design of this kind of construction. In certain case, it can allow the failure of some components on purpose with a sort of backup structural system. The earthquake force will focus on the weaker system like it does with the structural fuses. (Charleson, 2008, p. 90)

FIG. 27 : SIMPLIFIED BUILDING WITH BRACING AND SHEAR WALLS

4.5 Common issues to avoid 4.5.1 Structural discontinuity and off-set The vertical continuity of the structure during an earthquake is fundamental; this concept integrates several things to avoid in order not to face a discontinuous structure, which leads to the buildingâ&#x20AC;&#x2122;s failure. Firstly, the building vertical load system made of different components at each storey needs to be well connected together; the force path will therefore go directly to the foundation and possibly in the ground. A homogenous structure means also a continuity, for example if a shear wall is present on the 2nd floor, this component should be there on all the other floor to form one continuous entity; important issues could happen if this is not respected and we will go through it in the next paragraphs. The fundamental statement is that the vertical pattern of the building must be homogenous from floor to floor and well connected.

4.5.2 Soft storey As it has been adduced above, if the structure is not homogenous, it creates issues. A soft storey is merely a storey that is weaker than the others. The structural fuse is similar to it, the earthquake is going to focus on the weaker part 38

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of the construction, with a structural fuse, everything is done for that to happen and then no risk is generated on the building’s ability to stand. A soft storey will be weak but it will not absorb anything, in that particular case, all the forces will be concentrated on this latter and will cause a failure. A failure caused by a soft storey is disastrous; the whole building is strongly affected and may collapse entirely or partly according to its architectural features. (Murty, n.d., p. Tip 21)

FIG. 28 : SOFT STOREY EXAMPLE

4.5.3 Short column 2 types of short column exists, on one hand there are columns that find themselves shorter than others in a moment resisting frame and on the other hand, the column that don’t fulfil their structural function by being brittle because of external matters.

FIG. 29 : SHORT COLUMN EXAMPLES WITH SLOPED GROUND OR WITH A MEZZANINE FLOOR

The first ones can be placed in a certain way that their length is shorter than the others with still the same function to do. An easy example of that is when an edifice has to be constructing on a slope. The foundations can be on a different level, which brings more complications or can be on the same level. In both case one column will be shorter than the other. The stiffness of the column will not be enough – the stiffness of a column is proportional to its length cubed: L3.

The main problem is the column blocked by the soil; therefore to outwit that, the solution is to build the foundations on the same level with a pile serving as a column with a movement gap that parts the soil and the vertical structure. The structure is therefore equal on both sides. The other ones can be brittle because of masonry. In a moment resisting frame, the gaps between columns and beams can be filled. And similarly to the soil, this can be an issue. If the masonry does not fill the entire wall, then part of the columns are weak and break under the massive shaking. 39

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(Murty, n.d., p. Tip 22)

4.5.4 Torsion The torsion is caused when the centre of resistance of the construction does not match the centre of mass. In other words, if the building is not symmetrical along the inertia force on the x or y axis, the building will be submitted to torsional thrusts. However, the centre of resistance can be further than the centre of mass if the structural frame is capable to support the torsion with a specific sizing.

4.5.5 Infill walls A moment resisting frame is very opened, the columns and beams being literally a skeleton. Masonry walls, here without any structural function helps to infill the gaps. The problem occurs because the blocks prevent the building from deflecting correctly, the walls are weaker than the force coming from the earthquake, infill walls will suffer damage and will cause damage on the structure. If it does happen in the ground floor, it can result as a short storey. A stiffer structure or separation gaps â&#x20AC;&#x201C; on the top and on the sides -would be appropriate to solve those issue. The masonry needs also to get some vertical reinforcements going through it with a short and regular spacing between the components, in view of a strong earthquake the masonry could part horizontally by not following the exact force applied to the building. (Charleson, 2008, p. 159)

4.5.6 Buildings pounding This issue concerns the medium and high rise buildings; those buildings have to respect a certain distance from each other. The vertical cantilever effect cause by the earthquake will deflect the constructions, if they touch each other, the resonance generated will cause important damages. The solution is therefore to create a seismic gap and stiffer the construction to minimize the deflection. A separation gap on the roof level can be around 700mm long. (Charleson, 2008, p. 137) 40

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4.5.7 Re-entrant corners In case of a building with a C-shape or an L-shape, the building will suffer torsion. An irregular re-entrant corner is approximatively declared when B > 0,15 A where A is the façade of the building and B is the re-entrant façade. Consequently, either the formula in the prior sentence should be considered, either a separation gap between the two parts of the building should be created. (Charleson, 2008, p. 132)

4.6 Particular systems 4.6.1 Seismic separation gap Separation gap in constructions are usual, they are required for the dilatation of the material - a characteristic seen in the first part – or to separate different building structure parts. In a seismic region, those specific gaps get some changes. Because of the deflection, and in order to prevent the buildings to pound each other, structure needs to be stiffer, overlapping cantilevers with sliding joints could be used to handle the relatively long spanning gap.

4.6.2 Stairway The stairway needs to stay functional during and after the quake, they are the escape routes, in case of fire after the tremor, their usability will be vital for the residents or users in the construction. Stairways links two different storeys, because of the deflection, the slabs have not the exact same movement; the one above will move on a longer distance. As a result, monolithic stairway should get specific systems to behave well during an earthquake. In order to limit the effects of the shaking on the component, some mechanical translation support can be added. Sliding joints – Teflon strips - at each floor allow drift in any direction, components are pin or casted on the level above and allowed to translate on the inferior level. (Charleson, 2008, p. 168)

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4.6.3 Bridge between buildings A normal fitted bridge between two buildings during a quake will fail very easily and the consequences would be terrible. When a component as a bridge is connected to two different buildings, with as a result a different period of vibration, the latter must be suited to rotate in plan and to slide on elevation. The sliding must be allowed for one connection only. (Charleson, 2008, p. 140)

4.7 New technologies 4.7.1 Seismic proof constructions This technic is also called base-isolation; the goal is to part the foundations with the ground and the superstructure. Hence, the waves coming from the ground have no influence on the building; so that the seismic isolation decreases the peak acceleration of the building. This technic has been used “over 2000 buildings in the world since the late 70’s … 1500 are in Japan”. Critical buildings are subject to it due to their function; for example, hospitals that needs to stand against any earthquake intensity. However, the seismic-proofing has limits. The building shouldn’t be too flexible, that is to say buildings that have more than ten storeys and more than 1.0sec of period of vibration. The soil will be considered as bad if it is soft, for the same reason, soft soil increase the period of vibration of the building. The wide separations gap needed could possibly be a problem. It is the best method to outwit a quake; the building avoids theoretically any contact with the waves. (Charleson, 2008, p. 218) (Lindeburg & Baradar, 2001, p. 169)

4.7.2 Dampers

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Seismic prrof constructions are possible thanks to those components. The dampers are similar to shock absorbers in a car; within the construction FIG. 30 : EXAMPLES OF ISOLATION SYSTEM. ON THE LEFT, RUBBER they are located in the PLATES, ON THE RIGHT SLIDING BEARING moment resisting frame within frames in diagonal; a sort of moment-resisting frame with a bracing-system. But instead of being just that, which would not be good for the structure and particularly its deflection, the bracing is composed on one side of a damper. This side will absorb the shock and reduce the horizontal drifts; slender structures are therefore possible for designers. (Lindeburg & Baradar, 2001, p. 169)

4.7.3 Carbon fibres Nowadays, new construction materials are available on the market like the fibre reinforced composite materials. They are made of carbon fibres, which are around ten times stronger than mild steel â&#x20AC;&#x201C; steel with low percentage. It can be used to reinforce walls or columns by wrapping of bandaging them. They are stuck to the wall with s special synthetic resin. This method is interesting while retrofitting constructions since it does not include any demolition processes. (Lindeburg & Baradar, 2001, p. 173)

4.7.4 Innovative structural configurations Some structural features can decrease the inertia force acting on the building. The concept is to have a part of the building connected to the main structure with dampers. The different parts of the building will not vibrate on the same basis, which is supposed to cancel a certain amount of the resulting force by absorbing one another. They are called tuned mass damper or harmonic absorber, one of the most well-known example is the Taipei 101, located in Taiwan.

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FIG. 31 : THE TOWER TAIPEI 101 - IN YELLOW THE TUNED MASS DAMPER

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Chapter 5. The design process and the building standards over the world The prior part summarized all the possibilities that exist nowadays to counter an earthquake. From a very simple structural choice or by extreme and innovative â&#x20AC;&#x2DC;tricksâ&#x20AC;&#x2122; like the tuned mass damper ine the tower Taipei 101. In this part 5, that is to say the last one, we will deal with the design process of the constructions in a seismic area, obvisouly this process is dfferent. The codes and standards will also be adduced in thispart and especially the difference on certain point between the chinese standards, the american, and the european ones.

5.1 Performance-based design 5.1.1 Performance level Codes and standards focused nowadays on the potential victims of quakes, by trying to decrease them. As it has been mentioned on the second part about the seismic risks, institutions responsible for building codes went slowly into that goal by passing from different phases in the 20th century. Every buildings created today 45

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shouldn’t theoretically collapse entirely under an earthquake, allowing significant damage though – thanks to the generalised application on capacity design on the worldwide building codes. For example, the Eurocodes stipulate 4 performance-levels: -

Operational

Immediate occupancy

Life safety

Near collapse

(Fardis, 2009, pp. 1-36)

5.1.2 Hazard level The hazard level is complementary to the performance level. If we follow the theory, an operational performance should be fulfil for a building returning frequently; respectively, immediate occupancy with an occasional occurrence of the earthquake, life-safety with a rare one and near collapse with a very rare one.

5.1.3 Probability of the earthquake The non-collapse requirement is defined by the value called “design seismic action”. It represents a 10% probability of exceeding during a 50 years span, which represents the usual span the buildings are constructed for. The occurrence of this quake should be 475 years which also means a 0.2% exceeding probability in a single year. Those values represent the performance level of life safety. Of course, the client can decide a better performance for his building to reach, with those performance levels he knows the risks and consequently the designer is also protected by that if something happens to the client’s property. As a result, the client can’t sue the designer or engineers if all the codes have been respected, whatever the outcome of the catastrophe. For other performance requirements, we can note the damage limitations, which keep the building in the elastic range, with an earthquake return period of 95 years and still an occurrence probability of 10% The design peak ground acceleration is calculated by the reference peak ground acceleration – that is found on the hazard zonation maps - times the importance factor of the building Indeed, the buildings are submitted to importance factors, the life-safety being the goal. Therefore more the building is a risk for humans’ losses, more the factor will be important. 46

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A few examples, still with the Eurocodes, an ordinary building will get his factor equalled to 1.0. if the building is aimed to receive an important amount of people indoor, the factor goes up to 1.2; it is the case for schools. For critical buildings, that need to resist for public needs after an earthquake like a hospital, the factor is 1.4. On the opposite, a building with low-importance for humans’ lives has a factor of 0.8. (Fardis, 2009, pp. 2-9) (Lindeburg & Baradar, 2001, pp. 13-14)

5.2 The design process In this sub-part, we will dwell on the design of the building from the beginning of the project until the dimensioning of the constructions.

5.2.1 The building All the buildings and projects are different one another, that is to say each building will behave with different manners while the seismic waves will hit it. The period of vibration of the building is one of its most important characteristic; like any physical object, buildings have an intrinsic vibration frequency and as a result a period as well. Some characteristics of the building, architectural and structural features, have an influence on it. The height is the most influent among them, along with the weight and the main structural system in use. As a result, the building will oscillate on a given rhythm when the waves will reach it. The oscillation is outlined by the modes of vibrations. There are as many modes as storeys in the building, the first ones – usually first three – being considered within the building codes because they are submit to the majority of the dynamic energy. (Charleson, 2008, pp. 18-23)

5.2.2 Peak ground acceleration The reference peak ground acceleration is defined by the history on earthquakes on a specific region, zonation maps with this value are done to know what kind of ground acceleration the earth is able to provide. As it has been said in 47

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the prior sub-part, this reference ground acceleration represents the ground acceleration on rock, the soil if it is softer, would change this acceleration, as well as the importance factor of the building. A peak ground acceleration, in order to design the building will come out. (Lindeburg & Baradar, 2001, pp. 17-19)

5.2.3 Building response spectra Those diagrams were firstly done practically, with shaking table and different buildings, captors were recording the response of the building and were point out to form a curve. Nowadays, with computer-based technologies gathering all kinds of software, the response spectra are mathematically modelled. Therefore we can see the buildingâ&#x20AC;&#x2122;s acceleration in function of the period of vibration. The beginning of the curve starting at 1.0 represents the design ground acceleration. And we observe that if the period of vibration is too low, the buildings acceleration is increased. The phenomenon is called resonance. Those curves are different if the soil changes; the kind of soil adds a factor on the equation of the curve. (Charleson, 2008, pp. 21-22) (Fardis, 2009, p. 10) (Lindeburg & Baradar, 2001, pp. 20-22)

5.2.4 Seismic force With the acceleration of the building we can calculate the inertia resulting from it and size the building. While dimensioning the elements strength will be consider in order to resist the bending moment and the shear force that seismic force generates. As well as stiffness, present with the maximum storey drift requirements. Capacity design has a role in those sizing, and as it has been seen, particularly in the critical areas of the building, plastic hinges.

5.3 Comparison between codes and standards 5.3.1 Factors

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The factors used in general are a bit different or the same. The philosophy stays the same but the methods change. The institutions in charge in their respective countries can have a look on the standards of their neighbor, the only things that separate them is the habits of their engineers and architects working within the country. The different codes and standards arrange themselves to be close to each other – probability of exceeding occurrence which is the same – but has still deep differences.

5.3.2 Intensity scale The intensity scale is a way to quantify the impact the earthquake had over the surroundings and over people. Characterized by the humans feeling and animals feeling during the quake and on the structures and general facilities damage, it is quantified by a 12 grades scale different all over the world. In Europe, the scale is called “European macroseismic scale”, while in China, they call it “Leidu scale”. Small differences can be noticed accordingly to the subjectivity of the institutions making them, but the philosophy is the same. The grades are both noted in roman numbers and the XII grade is the most severe. the intensity is usually correlated pretty well with its distance from the earthquake epicentre. (Fardis, 2009)

5.3.3 Probability of the earthquake As it has been mentioned in the prior paragraph (5.1.3, p.37), the codes assume a certain earthquake to occur. The US codes are not so much different compared to the Eurocodes, both aim towards a life safety statement. The probability of occurrence of the seismic design required is the same, that is to say 10% with 475 years of return period. The US codes are established with a MCE value – maximum considered earthquake – which serves as a base for the different performance level. It represents the usual earthquakes that occurred on the specific region with an increasing factor of 1.5. For building of ordinary risks, MCE will be multiplied by a factor of 2/3. As for the Eurocodes, the other risks are stated as large occupancy buildings and safety critical facilities, respectively multiplied by a factor of 5/6 and 1.0. (Fardis, 2009, pp. 2-8)

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Chapter 6. Summary In the history of architecture and structural engineering, the materials used by humans to build their homes, their cult places like temples or else are the same as today. They are basically materials that we find in nature, wood, rock, earth, water, or metal have been used to build constructions. As civilizations and societies, we have been , we are and we will be able to improve our technics. In a simplified version of the building industry today, a structural engineer and an architect work together to create a project and to build it. This relationship is important in order to complete the latter in the best conditions, those two professions are complementary and must work well together in critical structure – i.e. seismic structure. Earthquakes produce a significant amount of energy aimed to move and shake the ground, buildings on the other hand are aimed to stay stable and not to move. Obviously, with this amount of energy, humans and mechanical systems can’t fight against it, we must outwit the earthquake in order to save lives and goods. A seismic design with particular technics is needed to limit ruination of constructions and avoid any human loss. The systems and the way to design buildings to facilitate the integration of an inertia force in the building have been thought since the second half of the 19th century until nowadays. The two examples of earthquake – in Italy 2009, and in Sichuan, China 2008 - in this dissertation shows that whatever the magnitude of the tremor, damage can appear, it does depend only on the mistakes done during 50

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construction. The future is therefore oriented towards an earthquake proof world with isolation systems. Engineers and architects need still to follow standards and codes. About them, we can observe that they are very similar. That is explained by the fact that institutions all over the world, that is to say, universities, laboratories, centres of researches are helping each other, sharing their information. On the other hand, countries have different landscape and risks, in response factors change without surprises. Earthquake-resistance is definitely a worldwide community working together to avoid those past natural disasters. Although, a lot of safety are imposed on new constructions, old ones have kept their issues with the time and that is one of the important to do in critical regions, retrofit!

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LIST OF REFERENCES Charleson, A., 2008. Seismic Design for architects: outwitting the quake. Charon Tec Ltd éd. Oxford: Elsevier Ltd. Ching, F. D. K., Jarzombek, M. & Prakash, V., 2011. A global history of architecture. 2nd ed. New Jersey: John Wiley and sons, Inc.. EERI, June 2009. Special Earthquake Report - Abruzzo, Italy, Earthquake of April 6, 2009, s.l.: s.n. EERI, October 2008. Special Earthquake Report - Wenchuan, Sichuan Province, China, Earthquake of May 12, 2008, s.l.: s.n. Fardis, M. N., 2009. Seismic Design, Assessment and Retrofitting of Concrete Buildings. Patras: Springer. Lindeburg, M. R. & Baradar, M., 2001. Seismic design of building structure: a professional's introduction to earthquake forces and design details. 8th ed. Belmont, CA: Professional publications, Inc.. Mcdonald, A. J., 2001. s.l.:Reed Educational and Professional Publishing. Michigan Technological University, 2007. What Are Earthquake Hazards?. [Online] Available at: http://www.geo.mtu.edu/UPSeis/hazards.html Murty, C. V., n.d. Earthquake Tip, Kanpur, India: Indian Institute of Technology. Raucci, S. & Jewell, T., n.d. Engineering in Ancient Rome, s.l.: Trustees of Union College. USGS, 2012. Magnitude 6.3 - CENTRAL ITALY. [Online] Available at: http://earthquake.usgs.gov/earthquakes/eqinthenews/2009/us2009fcaf/us2009fcaf.php [Accessed 31 October 2013]. USGS, 2013. Earthquake hazard archive - Magnitude 7.9 - EASTERN SICHUAN, CHINA. [Online] Available at: http://earthquake.usgs.gov/earthquakes/eqinthenews/2008/us2008ryan/ [Accessed 29 October 2013]. Zhang, Z., s.d. Traditionnal Chinese buildings and their performance in earthquake, s.l.: s.n.

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