STRUCTURE magazine December 2019 by structuremag

STRUCTURE DECEMBER 2019

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SOILS & FOUNDATIONS Design and Construction of Tunnels Cities of the Future Hanging a Monumental Stair

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LeMessurier Calls on Tekla Structural Designer for Complex Projects Interoperability and Time Saving Tools

Tekla Structural Designer was developed specifically to maximize collaboration with other project parties, including technicians, fabricators and architects. Its unique functionality enables engineers to integrate the physical design model seamlessly with Tekla Structures or Autodesk Revit, and to round-trip without compromising vital design data. “We’re able to import geometry from Revit, design in Tekla Structural Designer and export that information for import back into Revit. If an architect makes geometry updates or changes a slab edge, we’ll send those changes back into Tekla Structural Designer, rerun the analysis and design, and push updated design information back into Revit.”

Tekla Structural Design at Work: The Hub on Causeway

For over 55 years, LeMessurier has provided structural engineering services to architects, owners, contractors, developers and artists. Led by the example of legendary structural engineer and founder William LeMessurier, LeMessurier provides the expertise for some of the world’s most elegant and sophisticated designs while remaining true to the enduring laws of science and engineering. Known for pushing the envelope of the latest technologies and even inventing new ones, LeMessurier engineers solutions responsive to their clients’ visions and reflective of their experience. An early adopter of technology to improve their designs and workflow, LeMessurier put its own talent to work in the eighties to develop a software solution that did not exist commercially at the time. Their early application adopted the concept of Building Information Modeling (BIM) long before it emerged decades later. While LeMessurier’s proprietary tool had evolved over three decades into a powerhouse of capability, the decision to evaluate commercial structural design tools was predicated on the looming effort required to modernize its software to leverage emerging platforms, support normalized data structure integration and keep up with code changes. After a lengthy and thorough comparison of commercial tools that would “fill the shoes” and stack up to the company’s proprietary tool, LeMessurier chose Tekla Structural Designer for its rich capabilities that addressed all of their workflow needs. According to Derek Barnes, Associate at LeMessurier, ” Tekla Structural Designer offered the most features and the best integration of all the products we tested. They also offered us the ability to work closely with their development group to ensure we were getting the most out of the software.”

One Model for Structural Analysis & Design

From Schematic Design through Construction Documents, Tekla Structural Designer allows LeMessurier engineers to work from one single model for structural analysis and design, improving efficiency, workflow, and ultimately saving time. “Our engineers are working more efficiently because they don’t need to switch between multiple software packages for concrete and steel design. Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product,” said Barnes. LeMessurier engineers use Tekla Structural Designer to create physical, information-rich models that contain the intelligence they need to automate the design of significant portions of their structures and efficiently manage project changes. TRANSFORMING THE WAY THE WORLD WORKS

“Tekla Structural Designer has streamlined our design process,” said Craig Blanchet, P.E., Vice President of LeMessurier. “Because some of our engineers are no longer doubling as software developers, it allows us to focus their talents on leveraging the features of the software to our advantage. Had we not chosen to adopt Tekla Structural Designer, we would have needed to bring on new staff to update and maintain our in-house software. So Tekla Structural Designer is not just saving us time on projects, it is also saving us overhead.

Efficient, Accurate Loading and Analysis

Tekla Structural Designer automatically generates an underlying and highly sophisticated analytical model from the physical model, allowing LeMessurier engineers to focus more on design than on analytical model management. Regardless of a model’s size or complexity, Tekla Structural Designer’s analytical engine accurately computes forces and displacements for use in design and the assessment of building performance.

“Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product.”

Positioning a large scale mixed-use development next to an active arena, a below grade parking garage, and an interstate highway, and bridging it over two active subway tunnels makes planning, phasing and engineering paramount. Currently under construction, The Hub on Causeway Project will be the final piece in the puzzle that is the site of the original Boston Garden. Despite being new to the software, LeMessurier decided to use Tekla Structural Designer for significant portions of the project. “Relying on a new program for such a big project was obviously a risk for us, but with the potential for time savings and other efficiencies, we jumped right in with Tekla Structural Designer. It forced us to get familiar the software very quickly.” “Tekla Structural Designer allowed us to design the bulk of Phase 1 in a single model,” said Barnes. The project incorporates both concrete flat slabs and composite concrete and steel floor framing. “Tekla Structural Designer has the ability to calculate effective widths based on the physical model which is a big time saver,” said Barnes. “On this project, the integration with Revit, along with the composite steel design features enabled us to work more efficiently. Adding the ability to do concrete design in the same model was a bonus because we had both construction types in the same building.” “Tekla Structural Designer helped this project run more efficiently, and in the end it was a positive experience,” said Blanchet.

“Tekla Structural Designer gives us multiple analysis sets to pull from, which gives us lots of control. Most programs don’t have the capability to do FE and grillage chase-down. For the design of beam supported concrete slabs, Tekla Structural Designer allows us to separate the slab stiffness from the beam stiffness, so if we choose to we can design the beams without considering the influence of the slab. In the same model we can use a separate analysis set to review the floor system with the beams and slab engaged,” said Barnes. Barnes also shared similar benefits with concrete column design. “Tekla Structural Designer does grillage take-downs floor-by-floor, finds the reactions and applies them to the next floor. This allows us to view column results both for the 3-dimensional effects of the structure as a whole and from the more traditional floor-by-floor load take-down point of view. Doing both has always required significant manual intervention, but Tekla Structural Designer puts it all in one place.” “We reduce the possibility for human error because with Tekla Structural Designer less user input is required,” said Barnes. “Tekla Structural Designer automatically computes many of the design parameters, such as column unbraced lengths. The assumptions made by the software are typically correct, but we can easily review and override them when necessary.”

“Tekla Structural Designer provided the best fit for our workflow compared to other commercially available software.”

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Erratum

EDITORIAL BOARD

It was brought to our attention that there was an error in the July 2019 Structural Design article, ASCE 7-16 Provisions for Lateral Drift Determination – Part 1. The author apologizes for the error. The text (page 34) should have read: P-delta effects must be considered, per Section 12.8.7 of ASCE 7-16, when the stability coefficient per Equation 12.8-16 and 12.8-17 is more than 0.10 but less than or equal to θmax. If the design story drift is obtained from first order analysis, it should be increased by an incremental factor Ad, or a P–∆ analysis shall be conducted to obtain the second-order drift.

Chair John A. Dal Pino, S.E. FTF Engineering, Inc., San Francisco, CA chair@STRUCTUREmag.org Jeremy L. Achter, S.E., LEED AP ARW Engineers, Ogden, UT Erin Conaway, P.E. AISC, Littleton, CO Linda M. Kaplan, P.E. Pennoni, Pittsburgh, PA Charles “Chuck” F. King, P.E. Urban Engineers of New York, New York, NY Emily B. Lorenz, P.E. Chicago, IL Jessica Mandrick, P.E., S.E., LEED AP Gilsanz Murray Steficek, LLP, New York, NY

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Contents D ECEM BER 2019

SPECIAL SECTION

26 NCSEA EXCELLENCE IN STRUCTURAL ENGINEERING AWARDS The National Council of Structural Engineers Associations (NCSEA) announced the winners of the 2019 Excellence in Structural Engineering Awards in November. Read highlights of each of the award-winning projects in this issue.

Features

Columns and Departments

22 TURNING UNDERGROUND TO BUILD THE CITIES OF THE FUTURE

By Brian Gettinger, P.E., Brad Watson, P.E., and Mike Shiflett, P.E.

In response to breakneck growth, major urban areas are building

By Edward M. DePaola, P.E., SECB

Structural Design Design and Construction of Tunnels By David Ward, P.E.

different kinds of engineering marvels – instead of skyscrapers reaching toward the sky, they are digging deep underground.

Editorial 10 Ways to Spend Ten Bucks

Structural Practices Geo-Structural Challenges for

This move underground presents its own set of structural

Advancing Tunnel Design and Construction

engineering challenges.

By Rouzbeh Vakili, Ph.D., P.E., P.Eng., Alexander Herzog, P.E., and Philip Lund, P.E.

33 NAPA COUNTY HISTORIC COURTHOUSE – PART 1

By Luke Wilson, S.E., Brett Sheilds, P.E., and Kevin Zucco, S.E.

Assessing the damage from the 2014 South Napa Earthquake

By Frank Griggs, Jr., D.Eng., P.E.

Codes and Standards ASCE 7-16 Provisions for Lateral Drift Determination – Part 2

and developing appropriate repair programs for the Napa

By Abdulqader Al-sheikh

County Historic Courthouse was supported by a wholistic 3-D BIM approach that revealed valuable insights.

Historic Structures Desjardins Bridge Disaster of 1857

InSights Automation of Construction Documents and Details By Charles Portelli, AIA, and Nick Mundell

36 HANGING A MONUMENTAL STAIR

By Dan Wray, P.E., and Bryan Starr, P.E., S.E.

CASE Business Practices A Check-Up of Your Firm’s Quality Assurance Plan

Instead of anchored and reinforced at the lower level, this

By Jeff Morrison

30,000-pound stair hangs from a four-pronged structural mast on the underside of the building’s 17th floor.

On the Cover

The ingenuity of structural engineers tasked with

designing the unique gridshell for the roof at Jewel Changi Airport were acknowledged with a 2019 NCSEA Award of Excellence. Read more on page 29.

In Every Issue 4 40 44 46 48

Advertiser Index Resource Guide – Earth Retention NCSEA News SEI Update CASE in Point

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, the Publisher, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions. D E C E M B E R 2 019

A Powerful Software Suite for Detailed Analysis & Design of Reinforced Concrete Structures

EDITORIAL 10 Ways to Spend Ten Bucks By Edward M. DePaola, P.E., SECB, F.SEI, M.ASCE

“C

hoose a job you love, and you will never have to work a day in You may be asking, “What difference can a $10 donation make?” your life,” Confucius (supposedly) once said. If this is true, I Well, to make a difference, you do not have to do something big or have not worked in more than 40 years. Yes, I am one of those unbeliev- costly. You do not have to start by chairing a committee or mentorably lucky people who love what they do. Where I am today is because ing all of the younger engineers in your office. But if you think that of a thousand little things that there is nothing you can do to happened to me along the way, Individual commitment to a group effort – make a difference, think harder. most of them positive, some of Students who have come to the them negative, but all of them that is what makes a team work, a company Structures Congress – often work, a society work, a civilization work. meaningful to me. These expewith the Futures Fund’s help – riences have made me look at have returned year after year, – Vince Lombardi myself and my career from a difand many have advanced in our ferent perspective than I think I would have had they not occurred. professional societies; there is a whole alphabet of them. Yes, you can Of course, it starts with the contributions of my parents, relatives, make a difference, and $10 is all it takes. friends, professors, spouse, children, co-workers, and, well, you get To put all of this in perspective, I offer the following list of 10 ways the idea. You probably have your own list of important people. But to spend ten bucks, and how each one of them relates to everything the biggest thing I have learned in my life is the importance of the else in our lives: words “thank you.” Now, I know you don’t need anyone to tell you 10) Buy a pack of cigarettes. No, wait. That’s a bad idea. how to thank all of those people in your life. So this editorial will 9) Go to a coffee chain and buy a cup of coffee. Better yet, you take a slightly different approach. can get a coffee and a bagel from the guy with the street cart I want to thank my profession as well as all of the people in it. I also on the corner for $1.50. want to share with you how I give thanks, and how you might give 8) Buy a cheap bottle of wine. Not that you would really want thanks too. to drink it. The easiest way is to give back to the profession. For me, that has 7) Buy a cool gadget. Still, you will be bored with it in a few meant being very active in all professional societies, at first by volundays, and it will end up on a shelf collecting dust. teering for committees, then by organizing activities, and eventually by 6) Take a cab down 7th Avenue from 42nd Street to 34th Street. serving in leadership roles. I am proud to tell you that on October 1st Nope, it is faster (and better for you) if you walk. of this year, I became the Chair of the Structural Engineering Institute 5) Buy your spouse a box of chocolates or some flowers. Bad Futures Fund (SEIFF) www.asce.org/SEIFuturesFund. idea – when they find out how cheap you are, you will be in The Fund is about precisely what you think it is about: the future the doghouse. of our profession. All gifts that we receive are used to fund four 4) Buy a lottery ticket because, “Hey, you never know.” But strategic initiatives: you probably have a better chance of getting hit by lightning • Invest in the future of the structural engineering profession (don’t do that, either). • Promote student interest in structural engineering 3) Deposit it in a savings account. At an interest rate of 0.1%, • Support younger-member involvement in SEI in 693 years you’ll have $20. But at least we are moving in • Provide opportunities for professional development the right direction. Every year, we provide scholarships to students and young profes2) Invest it. A share of Ford Motor Company is currently tradsionals to attend the Structures Congress. Last April, we had 20 ing at about $8.50 – and you can still get that coffee and students, and 25 young professionals attend. For next April, we have bagel! Now we are getting somewhere. happily funded up to 65. Also, we are funding expenses for young 1) Invest it in the future of our profession. Contribute $10 to professionals to participate in standards committee meetings. This the SEI Futures Fund. Best idea ever! gets them started with active involvement in SEI, and, as a bonus, Keep in mind that the SEI Futures Fund has partnered with the ASCE we are seeing that many of them continue further, becoming more Foundation to leverage its expertise in managing philanthropic gifts. You active and taking on more leadership roles in SEI. have our word that 100% of your gift goes directly to the SEI Futures When I think of a “fund,” I know that big donors always come to Fund for investment in our profession, free of any administrative burden. mind. Yes, SEIFF has corporate donors, including my firm, but we So please join me and all of my colleagues on the Futures Fund also have private donors, including my wife and me. And for the board in contributing to our efforts to invest in the future of fiscal year 2019, the fund has received over $60,000 from 5 firms our profession. Yes, you can make a difference – thank you!■ and 119 individuals, an average of about $390 each. But here is the kicker: SEI has over 30,000 members, and if each one of us gave just Edward M. DePaola is President and CEO of Severud Associates $10, we would have more than $300,000 to invest in the future of Consulting Engineers PC, in New York City. Ed is deeply involved with our profession. That is almost three times what we have funded for many professional organizations. (edepaola@severud.com) our 2020 initiatives. STRUCTURE magazine

‘‘

D E C E M B E R 2 019

structural DESIGN Design and Construction of Tunnels By David Ward, P.E., LEG

ransportation tunnel design and construction requires accepting the inherent uncertainty associated with sub-

surface conditions that can be characterized but never truly known. This article follows the geotechnical arc of a typical tunnel project from explorations and testing, to engineering analysis, supporting the selection of tunnel excavation methods, characterizing ground behavior, developing parameters for liner design, and estimating surface settlements and effects on adjacent structures. It provides examples from completed projects to highlight the risk and mitigation methods associated with underground construction.

Unlined rock tunnel, subsequently supported with rock bolts and shotcrete to address rockfall risk.

Explorations and Testing

Existing Tunnels

The elements of the geotechnical exploration program will vary based on the design phase, whether it is existing tunnel rehabilitation or new tunnel construction, whether it is a soil, rock, or soil and rock tunnel, and the anticipated complexity of the subsurface conditions. Typical steps for both existing and new tunnels include: 1) reviewing available data and 2) designing and implementing an exploration and testing program. The exploration program should be developed collaboratively with the design team and the owner to obtain both the parameters required for the design and to address identified types and areas of risk and uncertainty.

The goal of the exploration program for existing lined tunnels is to understand the liner parameters and the anticipated loads. The program is designed to answer questions like: Can the existing liner accommodate new loads over or adjacent to the tunnel? Can the existing loads be accommodated if the liner is notched to improve tunnel clearance? How will the ground behave if portions of the tunnel liner are removed? For existing unlined tunnels, the exploration program focus is the required rock mass parameters for design. The program is designed to answer questions like: Will additional support be needed for tunnel enlargement? Can the tunnel accommodate changes in loading? What ground support or liner types are required to address a change in tunnel use? The first exploration step should include reviewing existing geotechnical reports, design drawings, as-built drawings, and construction records or notes. Information on construction problems or the tunnel performance during use could help identify areas requiring additional analysis. The second step is fieldwork to confirm the liner condition and configuration and understand the ground behind the liner. Even when design drawings and construction records are available, some confirmation explorations may be warranted. Where data is not available or there is a reason to suspect that the tunnel was not constructed in accordance with project plans, the field exploration effort will be greater. Liner explorations could include probe holes drilled to estimate the liner thickness, concrete core samples for strength testing and identifying reinforcement, geophysical methods to identify the liner thickness and steel reinforcement, and condition mapping to identify liner distress areas. Some of these same methods can be used to assess conditions behind the liner. Probe holes and geophysical methods can be used to

Specialized rock coring equipment for explorations within an existing tunnel.

D E C E M B E R 2 019

Drilling holes in an existing liner to fill with lightweight grout, for voids identified during the exploration program.

Final cast-in-place liner being installed within an initial, gasketed, pre-cast, concrete segmental liner.

characterize voids and the distance to competent rock. Specialized rock core drilling, such as used on the Cape Creek Tunnel near Florence, Oregon, can be performed from within the tunnel to obtain samples of the liner and soil or rock. The results of these explorations were used to confirm the presence of voids and collect samples that were used and tested to identify appropriate grouting methods, existing liner stability, and new liner design parameters. The frequency and spacing of these explorations will depend on the anticipated liner and ground condition variability. The Ownerâ&#x20AC;&#x2122;s risk tolerance for differing site condition claims during construction can also inform the frequency and spacing. Existing stable, unlined rock tunnels provide an unparallel opportunity to map the geologic structure and characterize the rock mass. Sampling for testing rock strength or characterizing joint infilling may be needed for the final design. Plotting all the data, including construction data, on the same tunnel map can be useful for identifying patterns and correlations. For example, there may be a correlation between identified overbreak or collapse areas during original construction and current liner distress areas. Alternatively, mitigation during original construction may have addressed the area of concern sufficiently such that no additional work will be required.

and improve the designersâ&#x20AC;&#x2122; understanding of the subsurface conditions. A third exploration phase reduced the spacing between borings to about 300 feet and focused on the critical mined station excavation, the landslide hazard areas at the portals, and the Interstate 5 undercrossing. An approximately 150-foot-deep test shaft, telescoping down in diameter from 16 feet to 10 feet at a depth of 100 feet, was constructed during the final design phase to provide additional information on soil properties and ground behavior, and to provide the opportunity to bidders to observe in situ soil conditions. More expensive testing methods, such as aquifer pumping tests and test shafts, are commonly not completed until final design. The exploration and testing quantity and sequencing can also vary based on whether the project will be design-bid-build or design-build. For soil tunnels, sonic core drilling to obtain nearly continuous soil cores is increasing in popularity. Mud rotary or hollow stem auger borings and penetration testing are still useful, particularly for characterizing conditions at portal or shaft locations. Reliance solely on penetration test samples may still be suitable in geographic areas with a history of successful tunnel construction and relatively homogeneous ground. Testing could include soil classification, unit weight, soil strength, deformation parameters, modulus, hydraulic conductivity, abrasion, x-ray diffraction, cobble or boulder strength, corrosion parameters, and combustible or noxious gas. For rock tunnels, the exploration program would likely include vertical and angled rock core drilling, downhole testing to identify discontinuity spacing and orientation, permeability tests, and rock core tests. These tests could include unit weight, rock strength, modulus, durability, petrographic analysis, x-ray diffraction, abrasivity, and drillability parameters. Rock outcrop mapping, for rock mass parameters and to develop subsurface profiles, is also common. Geophysical testing to characterize the depth of soil overburden at portals and shafts may also be appropriate. For the 24-footwide, 3,100-foot-long Wheeler Gulch Tunnel in Colorado, where the geologic conditions could largely be interpreted from outcrop mapping and helicopter access was required, only a single rock core boring was performed. The exploration and test results are used to characterize the ground type distribution; characterize ground behavior; develop engineering

New Tunnels Exploration program guidelines for new tunnels are available, including the AASHTO Manual on Subsurface Investigations published in 1998. The program for new tunnels is designed to answer questions like: What are the spatial distributions of soil and rock types and groundwater conditions? What are the design soil, rock, and groundwater parameters? What is the anticipated ground behavior? Are difficult conditions like abrasive ground, obstructions, or noxious/ combustible gas present? For transportation tunnels, the program is typically phased. For example, on the Beacon Hill Tunnel project in Seattle, Washington, only three explorations were performed along the approximate one-mile-long corridor to help select a preferred route. Once the alignment was selected, four additional explorations and related testing were performed to fill data gaps at the portal locations 10 STRUCTURE magazine

design parameters for structures and liners; and, develop baseline parameters to assist potential contractors with understanding the geotechnical risks.

and ability to take advantage of a non-circular cross-section led to the selection of SEM as the preferred method of construction.

Analysis for Existing Tunnels

The initial and final liner selection will depend on the subsurface conditions and also on the method selected. The shielded soil tunnels are commonly initially supported with precast concrete segmental liners followed by a cast-in-place final liner. SEM initial liners are usually a combination of shotcrete and lattice girders with other SEM toolbox items like grouting or presupport. The final liner is commonly shotcrete. Other methods, such as roadheader, drill-and-blast, or gripper TBM tunnels, could be initially supported with items such as spot or pattern bolts, mesh, and steel sets and wood lagging. The Federal Highway Administration’s Technical Manual for Design and Construction of Road Tunnels – Civil Elements provides design considerations and procedures for the common liner types used for transportation tunnels.

Work on existing tunnels discussed in this article falls into one of three major categories, 1) lining a previously unlined tunnel, 2) partial liner removal, and 3) liner replacement. The parameters used in design and analysis will vary depending on the work being performed.

Liner Design For existing unlined rock tunnels, a common working assumption is that the tunnel is statically stable, and the liner design will primarily need to support future loads. These future loads could include seismic, groundwater, or the development of additional rock load associated with weathering. Shear or weathered zones and loose rock blocks that cannot be safely removed may require additional support. The geotechnical engineer’s role on these projects is usually focused on estimating the potential additional groundwater and rock load and identifying likely reinforcement and liner types. For the Fishhook Tunnel in Idaho, the design incorporated differing permanent ground support requirements to address the differing long-term support requirements at the portals, where shear zones were present, and the remaining portions of the tunnel. For existing liner notching or modification, numerical analysis is commonly required to accommodate the design of the irregular liner geometry resulting from the notching. The numerical analysis will require assumptions regarding the original methods used to excavate and temporarily support the ground to develop an estimate of the current load. The results may indicate that voids behind the liners need to be filled to reestablish the ground-liner interaction. For liner replacement, understanding the ground behavior when the existing liner is removed is critical for determining the need for ground improvement or presupport. For example, on the Cape Creek Tunnel project, a combination of permeation grouting, void filling grouting, and drilled steel reinforcement was used to pre-support the ground during liner removal. The replacement liner design methods are the same as those discussed later for new tunnels.

Analysis for New Tunnels Method Selection The tunneling method selection could be the contractor’s responsibility or could be predetermined by the Owner and Owner’s engineer based on permitting limitations or project-specific requirements. The geotechnical data and analysis performed during design are used to help determine the appropriate excavation method. A partial list of methods that could be considered, depending on the anticipated subsurface conditions, includes: slurry pressure balance tunnel boring machine (TBM), earth pressure balance TBM, gripper TBM, openface TBM, sequential excavation method (SEM), boom-mounted, milling excavator (roadheader), and drill-and-blast. Required crosssectional shape and tunnel length, available work area, local contractor experience and availability, and project schedule can also influence the excavation method selected. The Sound Transit E330 Tunnel in Bellevue, Washington, is an example of where the project schedule

Liner Design

Settlement The tunneling-induced ground movement magnitude can be the result of the Contractor’s selected means and methods. However, some ground movement is almost inevitable as a result of changing the stress in the ground during construction and the elastic response of the ground. In other words, assuming “zero settlement” is not practical. The estimation of settlement can vary from using empirical relationships between an assumed volume loss, soil type, tunnel diameter, and depth to a three-dimensional numerical analysis which accounts for anticipated soil and groundwater parameters, the state of stress in the ground before construction, and all of the subsequent construction steps. The effort is often directly related to the risk of ground-movement-induced damage. Where the tunnel is deep, has a relatively small diameter, and predominately single-story wood-framed structures are present over the alignment, the empirical analysis may be sufficient. The empirical analysis can also be used as a screening tool to identify buildings or structures that could require additional analysis. On the Alaskan Way Viaduct Replacement project in Seattle, Washington, the empirical analysis was used to estimate the settlement. Relationships between settlement, angular distortion, and damage were used to identify buildings and structures for additional analysis. For selected structures, such as the pile-supported Viaduct, numerical analysis was performed to estimate the potential settlement and damage as well as evaluate potential mitigation methods.

Conclusion Geotechnical input for tunnel projects will vary depending on whether the work is being done for an existing tunnel or proposed tunnel, and whether the tunnel is in soil or rock. While relatively standardized approaches are available for analyzing new tunnels and new liners, modifications to existing tunnels can require significantly more analysis to address uncertainty and non-standard liner geometries parametrically. The effort and requirements to estimate settlement-induced damage can also vary widely from a quick empirical-based analysis to screening structures for potential damage to an in-depth soil-structure interaction analysis.■ David Ward is a Senior Associate at Shannon & Wilson, Inc., in Seattle, WA. (dcw@shanwil.com)

D E C E M B E R 2 019

structural PRACTICES

Geo-Structural Challenges for Advancing Tunnel Design and Construction By Rouzbeh Vakili, Ph.D., P.E., P.Eng., Alexander Herzog, P.E., and Philip Lund, P.E.

rban areas are becoming more densely built: consequently, surface space is less available. The United Nations recently projected that

68% of the worldâ&#x20AC;&#x2122;s population would live in urban areas by 2050 (up from 55% at present). Thus, it is expected that cities, counties, and states will increasingly look to underground structures as alternatives to surface infrastructure to address space constraints. The increase in the number and extent of tunneling projects also increases the complexity of urban infrastructure development, necessitating structural and geotechnical or tunneling engineers to collaborate closely to deliver an efficient and practical design while managing impacts on existing structures. This article highlights challenges that may be faced in urban tunneling projects during design and construction phases and provides examples of coordination between disciplines to reduce risk factors such as ground settlement or cost overruns. Some factors requiring enhanced coordination between structural and geotechnical engineers include subsurface investigations for locating and designing underground structures, identifying and minimizing geotechnical risks, and the design of excavation support systems to prevent damage to existing structures and other infrastructure and to manage the impacts of tunneling on adjacent structures.

Figure 1. Pressurized tunnel boring machine.

peak-hour train storage beyond the terminal station, the size was set by ventilation requirements, and the depth was fixed by the elevation of the sump pit for the tunnel dewatering system. Tunnel designers must also consider the effects of tunnel excavation on existing and future structures within the zone of influence. The team of geotechnical and structural engineers must work jointly to produce a constructible design for a tunnel project. That design must both satisfy each disciplineâ&#x20AC;&#x2122;s technical considerations and accommodate all other disciplinesâ&#x20AC;&#x2122; requirements through a process of fine-tuning and refinement. Accommodations can range from shifting a tunnel element, such as an access shaft, a few hundred feet along the alignment in order to avoid poor geologic conditions, to moving the Development of Alignment tunnel alignment horizontally a few feet in order to avoid the deep One of the first steps in any tunneling project is the selection of an pile foundations of an existing building or vertically to avoid ground overall tunnel alignment. The alignment may be constrained horizon- anchors of an abandoned excavation support system. tally or vertically by factors such as geological conditions, proposed Since subsurface conditions are a controlling factor for alignment selecrail station locations, connections to existing infrastructure, existing tion, it is crucial to have an experienced geologist on the design team to underground structures, or available right of way space. Traditionally, review the existing subsurface information and evaluate the soil or rock alignments will follow paths with limited surface obstacles to avoid formations during the initial phases of the project. Based on the location sub-surface property acquisition and to mitigate the risk of settle- and depth of the selected alignment and the subsurface conditions, the ment. However, in dense urban locations, the alignment will often geotechnical and structural designers will select the most appropripass under or close to existing strucate type of excavation method and tunnel tures (e.g., buildings, tunnels, or other structure. Pressurized face Tunnel Boring below-ground structures) or pass under Machines (TBMs), such as slurry and earth an unoccupied area that will be a future pressure balance machines, are often used development site and impose additional for urban tunneling through soil to control load on the tunnel. Therefore, for urban ground deformations, prevent groundwater tunneling, designers must consider the inflow, and minimize the risk of damage to requirements of multiple engineering adjacent buildings and utilities (Figure 1). disciplines including rail or roadway However, other types of tunneling, such operations, ventilation, egress, tunnel as mined drill-and-blast tunneling in rock, safety systems, and architectural goals. As the sequential excavation method (SEM) a practical example, on a recent complex in soft ground, or cut and cover excavaurban underground rail project, the locations, are used in urban tunneling, typically tion of a deep underground shaft was set for non-circular or large diameter openby train operation requirements such as Figure 2. Shear zone in rock tunnel excavation. ings or in the presence of poor subsurface 12 STRUCTURE magazine

Figure 3. SOE system comprised of slurry walls, struts, and tieback anchors.

Figure 4. Urban excavation abutting existing tunnel (left) and other structures.

conditions or obstruction constraints. For shallow vertical alignments, cut and cover could be the preferred method; however, it creates the most community disturbance. Cut and cover excavation design requires considerable interaction between geotechnical and structural engineers for designing the temporary supporting system (e.g., soldier pile, slurry wall, secant pile, sheet pile) and waterproofing design. Cut-and-cover construction also requires coordination with civil engineers to address maintenance of traffic issues.

ground improvement methods. Protection of existing structures often influences the type of excavation support and bracing preloading requirements for cut-and-cover tunnel projects.

Site Investigation (Below and Above Ground) Developing a subsurface investigation program (including field and lab testing) is a critical part of the design process. The depths and locations of the borings must be selected strategically to capture as much variation in the soil and/or rock conditions as possible. Therefore, having an experienced geologist on the design team and having local experience with the in-situ soil and rock types are crucial factors in developing a successful subsurface investigation program. In addition, structural engineer input (e.g., shaft or cavern depths and diameters) is required in selecting borehole locations and depths. It is necessary to have a thorough understanding of the geologic conditions before executing the design because unexpected ground conditions can cause significant delays and complications during construction. As an example, unanticipated highly fractured rock encountered during tunneling can cause issues for tunnel advancement and worker safety. Figure 2 shows a shear zone encountered during tunneling through a dolomitic rock formation, which delayed a project because the TBM grippers were not able to bear on competent rock to push the TBM forward. The figure shows the steel rings and mesh that had to be installed within the shear zone area to stabilize the tunnel heading, introducing extra cost and delay. The structural design of the final tunnel lining had to be revised because the internal diameter changed due to the deformed ground and intrusion of initial support elements. This delay and expense could have been avoided with more up-front costs on subsurface investigation. In addition to subsurface conditions, the condition of existing structures along the tunnel alignment must be investigated. Structural engineers generally collect building and historical information and flag structures sensitive to settlement, which require special consideration, such as landmark or masonry buildings. The pre-construction inspection reports should include the types and depths of the foundations, structural materials and connections, and existing defects. The effect of tunnel construction on the existing structures must be evaluated during the design process, allowable movement thresholds determined, and strengthening or protection methods designed if required. Protection measures may include traditional underpinning or use of

Design Collaboration The design of cut and cover tunnel structures in urban areas requires close, multidisciplinary collaboration. Before excavation can begin, a complex network of buried utilities, such as gas and electric lines, sewers, telecommunications, and various other conduits, must be relocated or supported in place. Information regarding the location and type of utilities may be limited or nonexistent. The Support of Excavation (SOE) system sometimes needs to accommodate utilities that cannot be relocated â&#x20AC;&#x201C; for example, by using jet grout columns as temporary walls in lieu of traditional elements such as sheeting or secant piles to allow a sewer to pass through the excavation. The SOE system itself must be designed to create the required architectural and structural space while minimizing impact to adjacent structures (Figure 3). Property limits can restrict space options and the method of SOE support. For example, when permission cannot be acquired to install temporary tieback anchors below an ownerâ&#x20AC;&#x2122;s property, pipe struts may be used as an alternative means to support the SOE walls, but these restrict the temporary working space. Thus, design collaboration between civil engineers, electrical or telecommunications engineers, geotechnical engineers, and structural engineers is essential to satisfy the requirements of utility companies and other stakeholders and maintain the safety of the public. Rehabilitation and expansion of existing underground rail tunnel structures can require nonstandard support of excavation designs, requiring close coordination between structural and geotechnical engineers. Several projects the authors have been involved with have featured excavations above or adjacent to cut and cover tunnel boxes (Figure 4). Since it is often not feasible to install sheeting or piles on the tunnel roof, concrete button piers placed on the roof, above the tunnel walls, have been used successfully as SOE walls. These concrete button piers, cast-in-place using individual shoring boxes before mass excavation, act as soldier piles while minimizing damage to the existing structure. The button piers transfer additional load to the existing tunnel columns and walls. In some cases, the tunnel box itself becomes part of the SOE system with struts bearing on the exterior wall of the tunnel box and transferring earth and surcharge loads induced by mass excavation to existing tunnel slabs. Some existing tunnel structures pre-date common structural shapes, such as wide flange beams, and require more detailed structural analysis. Historical drawings become critical references for allowable stress checks. The process of designing an appropriate D E C E M B E R 2 019

Figure 5. Numerical modeling of complex urban tunneling.

Figure 6. Numerical modeling evaluating the effect of tunneling through existing structures.

SOE system, which can remain below the allowable stress increase of historical steel and cast-iron elements, requires close coordination between geotechnical and structural engineers. Mined tunneling (e.g., drill-and-blast through rock) also requires an iterative design process between geotechnical and structural engineers. The authors have worked on the development of many rock tunnels and caverns where the ground conditions and corresponding feasible excavation methods strongly influence the proposed geometry of final structures or architectural elements. New underground rail stations built in dense urban areas often require significant excavation beyond public platform areas. Ancillary shafts for ventilation or fire protection, electric substation vaults, passenger entrances, emergency egress tunnels, and cross passages between adjacent rail tunnels are all common elements of design in addition to multiple entrance tunnels, shafts, and connections to existing transit infrastructure. These multiple excavations often intersect or are adjacent to each other, which creates zones of increased stress within the rock mass. Geotechnical engineers perform rock mass stability analyses to determine if the architectural or structural configuration is feasible. During this iterative process, tunnels are sometimes relocated to allow for wider rock pillars to support overburden loads such as rock and soil cover, adjacent building loads, or other infrastructure (Figure 5). Rock mass quality and rock joint geometry will also dictate the type and extent of temporary excavation support. As discussed above, urban tunnel designers should also consider the effect of tunneling on existing structures. A recent project required an assessment of the effect of tunneling through lightly loaded timber piles that support an existing marine bulkhead. Due to the critical nature and complexity of the proposed tunnels and the relative locations to the existing bulkhead supported on timber piles, a threedimensional numerical analysis was performed (Figure 6 ). The results of the analysis were used to estimate the ground surface settlement at different construction stages and to evaluate the impact on the existing vertical and battered piles. Cutting the existing piles would impose additional loading on the adjacent piles. This analysis was performed to evaluate the amount of the load that would be transferred to adjoining, un-cut piles, and evaluate the geotechnical and structural capacity of foundation.

choices can be made during the design process to reduce the risk of damage to adjacent structures, instrumentation monitoring of existing structures is a fundamental part of the construction process to provide a quantitative assessment of the tunneling operation and selected construction technology. The collected field measurements can also be used to refine the design analyses and modify construction procedures, if necessary. Ground deformation monitoring is particularly crucial for shallow urban tunnel construction with a slurry or earth pressure balance shield. Empirical equations and numerical modeling, with analyses informed by precedent projects, are commonly used at the design phase to estimate the ground movement due to tunneling and for determining appropriate TBM face pressures. During construction, collected ground deformation data is reviewed against predicted values. This may result in previously performed analyses being modified and TBM operations parameters being adjusted. Collected ground deformation data can also be beneficial for any tunnel project that might be constructed in the future. On a recent project, field data collected in the mid-20th century was used to calibrate the analyses related to a new subaqueous tunnel at a nearby location.

Instrumentation and Monitoring All underground excavation causes stress redistribution in the ground, which leads to ground deformation. Mitigating the associated risk is an essential factor during the design process. Although various 14 STRUCTURE magazine

Conclusion Underground structures are in direct contact with natural ground materials, and that simple fact makes tunnel design a multidisciplinary problem. The subsurface conditions and the sizes and types of tunnels change from project to project, but one factor is constant: urban tunnel design and construction requires knowledgeable and experienced geotechnical engineers, structural engineers, systems engineers, geologists, and other disciplines to deliver a project successfully. Overcoming each challenge and providing the ideal solution as urban environments densify necessitates seamless communication and collaboration between multiple engineering disciplines. Individuals with different backgrounds and specialties must work together, collaboratively, to develop a project that meets clientsâ&#x20AC;&#x2122; expectations and offers the greatest added benefit to communities and society.â&#x2013; All authors are employed at the Geotechnical and Tunneling Technical Excellence Center of WSP USA. Rouzbeh Vakili is a Senior Geotechnical Engineer. (rouz.vakili@wsp.com) Alexander Herzog is a Senior Geotechnical Engineer. (alex.herzog@wsp.com) Philip Lund is a Senior Structural Engineer. (philip.lund@wsp.com)

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historic STRUCTURES Desjardins Bridge Disaster of 1857 By Frank Griggs, Jr., Dist. M.ASCE, D.Eng., P.E., P.L.S.

quire Whipple built two wooden swing bridges in Ontario, Canada, in 1853 on the Great Western

Railway. One was near Hamilton, Ontario, at Dundas and the other over the Welland Canal at Thorold Station just west of Niagara Falls. The bridge at Hamilton, called the Desjardins Bridge, opened in early November 1853. The turntable was on the bank and not in the center of the bridge as was generally the case. Large masonry abutments had been built to cut down the length of the swing span required. In addition, a wooden structure was built, upon

Failed swing bridge decking at Dundas. Illustrated London News, 1857.

which the swing span rested when it was in an open position. The waterway was 66 feet wide and required a swing span of eighty feet (from the center of turntable to the far abutment) to reach the other side of the channel. The counterweight span was 36 feet (44 feet from the center of the turntable) making the entire span 124 feet, including the 16-footdiameter turntable panel. The truss pattern had tension ties and counters in each panel and no verticals. The truss height was 18 feet with the truss members on the small side for contemporary structures. Whipple wrote in his 1869 appendix to his 1847 book, A Work Bridge Building …for Iron and Wooden Bridges: After being used nearly 3½ years, and borne the passage of, probably, 10,000 trains, and passed through vicissitudes testing its capacities pretty thoroughly, including the swamping of a heavily loaded freight car, which broke loose upon a steep grade and ran on to the bridge (off of the track) with such speed and force as to break 6 or 8 beams; on the 12 th of March, 1857, it met with a disaster involving consequences most lamentably fatal. On that day, the Locomotive Oxford, drawing a passenger train eastward, with a broken axle, ran off the track upon the downgrade approaching the bridge and could not be stopped till it went crashing into the timber of the bridge; probably coming in collision with the light lattice timbers on the right hand side, cutting them off, and letting the engine and train down some 40 feet into the canal. Such was the conclusion arrived at by the Coroners’ Jury, after a long and thorough investigation and the hearing of a vast amount of

Swing bridge at Dundas on the Great Western Railway.

16 STRUCTURE magazine

testimony by experts and others. The Jury also found that the Bridge was “safe and sufficient” for the traffic of the Road, with engines & trains upon the Track; but not sufficient, in case of their running off the track while passing over the bridge. The bridge had never been supposed to be safe for the passage of trains off the track, and it is believed that few bridges in the country would be reliable in such conditions. There was, however, a general concurrence of opinion, in the testimony at the Inquest, that the bridge would have borne, before straining the materials to their utmost capacity, from 3 to 5 times the weight that would ever come upon it in ordinary usage. There was another bridge of the same kind, and built at the same time, for crossing the Welland Canal at the Thorold Station, upon the same road; and the length of time the structure sustained the heavy traffic of that road should be regarded as a demonstration of the adaptability of this Plan of Bridging to railroad purposes. It was still winter in Canada West; the canal was not operating, and the bridge was “spiked” closed and acted like a fixed span rather than a swing bridge. Newspaper accounts of that fateful day of March 12, 1857, told of the disaster, with one giving the following description of the events occurring as the 4:10 P.M. train out of Toronto reached the bridge, We have said that the train had passed the switch apparently all right; in a moment or two, the locomotive enters on the bridge; one sharp, shrill whistle gives the only warning to the passengers that between them and eternity there is left scarcely sufficient time to say, “May the Lord be merciful!” The Oxford sinks through the floor of the bridge, and goes down, with that brave driver BURNFIELD, who perished at his post, in the execution of his duty. Next comes the tender, and then the baggage car, in which there were two or three persons. And then the first passenger car, with its fifty precious souls, comes down the rails and takes the fatal leap, either turning a complete somersault or careening over upside down. It lies across the bed of the canal, the

ice being broken through, and the car is about half filled with water. and appearance I have witnessed since my arrival at the bridge on But there is yet another car-load of mortal beings poised for a moment the 20 th instant, that the immediate cause of the disaster on the 12 th on the top of the wall, and then it, too, plunges into that fearful abyss instant, was the violent collision of some part or parts of the locomotive leaving the hind trucks on the rails above a poor remnant of a whole attached to the ill-fated train with the timber of the ill-fated bridge train, which but a moment before was as perfect as skill could make either directly or through the medium of some interposed body. it, and bore homeward many a manly heart and fondly-beloved father, The inquest continued for another six days with other men testifying. mother, husband, wife, brother, sister and child. A few escaped, and Thomas Keefer, a civil engineer of some renown and later President others perished in the attempt; but not less than fifty-seven or sixty lives of ASCE, was one of the last to testify. He stated, “if I am correct in were, “at one fell swoop,” cut off in the twinkling of an eye, as it were, my belief of the immediate cause of its destruction on the 12th March, and souls were landed on the shores of eternity which had no time to any wooden bridge with the roadway upon the lower chord would reflect of its grandeur or its despair. have shared a similar fate.” The news of the disaster spread rapidly throughout the United On Tuesday, April 8, 1857, the Jury handed down its findings that, States and Canada, and even abroad. In its April 14, 1857 issue, in part, were as follows: the Illustrated London News ran a major article of the disaster entitled “Frightful Railroad Accident in Canada.” It quoted the Rochester Union of March 13, “the bridge appears to be a frail structure, to be thus easily destroyed.” That everyone was not happy with the coming of the railroad was evident in an editorial in the Chatham Western Planet writing about this disaster and the previous wreck that had taken place near their city, which said, Better, infinitely better, that the whistle of the locomotive had never woke the echoes of our forest than that it should have sounded the death knell of so many human beings who have dyed this road with their blood. Use for all types of concrete and grout applications, from slabs-on-grade to The jury for the coroner’s inquest was containment tanks, multi-story post-tension structures to bridge decks. sworn in the day after the accident in the boardroom of the Great Western Railway. During the first three days of the inquest, ADVANTAGES testimony was given by people who had survived the crash or had witnessed the ¡ Maximize joint spacing (up to 300 ft, L/W 3:1) ¡ Enhance compressive and flexural strengths failure. They all said that they had always ¡ Prevent shrinkage cracking and curling ¡ Eliminate pour/delay strips thought the bridge to be safe. Richard Bond reported he had, “been ¡ Thinner slabs and walls viable ¡ Reduce long-term relaxation of P/T tendons and shear wall stresses connected with railways for 20 years; con¡ Reduce reinforcement requirements sider myself able to judge when a bridge ¡ Minimize creep and moment ¡ Improve durability and lower permeability is right or wrong; examined the bridge ¡ Minimize waterstops about a fortnight or three weeks before the ¡ Increase abrasion resistance 30-40% accident; it was all right; always considered the bridge a safe one, and am aware of no report ever having been made that the bridge was not safe.” On March 25, the Jury went to Thorold to inspect Whipple’s other swing bridge. The big day for Whipple came on the ninth day of the Inquest, March 26, when he gave his testimony in the case. He gave a lengthy statement describing his design methods and concluded, From what proceeds, it is abundantly evident to me that the bridge over the by CTS Cement Manufacturing Corp. Desjardin Canal was not broken down by the simple pressure of the traffic passing over it bearing fairly on the track rails; Contact us for more information and project support at 888.414.9043 and it is my decided opinion from the CTScement.com examination I have made, and the facts

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The jurors aforesaid also find that the said bridge over the Desjardin canal was built of wood, and constructed of sufficient strength for the conveyance of the traffic of the line safely and securely over the said bridge, provided that the locomotive and cars remained on the railway track, but that the said bridge was not built of sufficient strength to sustain an engine and train in case they should run off the track while passing over the said bridge. The jurors are of the opinion that the only certain way of providing against a similar catastrophe, at the same place, would be the erection of a permanent bridge, and they would, therefore, strongly urge on the Government to cause the same to be built forthwith, and also that the Toronto and Great Western lines should have separate tracks over said structure, thereby doing away with switches, which are always objectionable in such places. The jurors would further recommend the renewal of the former law, compelling trains to come to a dead stop before passing on this and all similar bridges, believing as they do, that the lamentable accident might have been avoided had this precautionary measure remained in full force. Later view of the site with iron swing span on the same abutments replacing Whipple’s Bridge Whipple was completely exonerated of any wrongdoing and a Whipple Truss on high piers replacing the suspension bridge. or responsibility for the failure. As he said, he had never The jurors aforesaid find that the immediate cause of the accident designed the bridge to handle trains off the track. With the death of was owing to the breaking of the forward axle of the engine-truck 59 people, however, many questioned the ability of engineers close to the wheel on the right, at a point on the road not ascertained, to design safe bridges. The failure was attributed more to an in consequence of which the left forward wheel of the truck left the operational problem than an engineering design problem.■ rail at or near the switch near the bridge, causing the locomotive when entering on the bridge to diverge to the right crushing and Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having tearing away its supports, and precipitating the whole train into the restored many 19 th Century cast and wrought iron bridges. He is now an canal, and resulting in the calamity which forms the subject of this Independent Consulting Engineer. (fgriggsjr@twc.com) melancholy inquiry… ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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CODES and STANDARDS ASCE 7-16 Provisions for Lateral Drift Determination Part 2: Seismic and Wind Drift By Abdulqader Al-sheikh

his article is the second of a two-part article on ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, and its provisions for lateral drift determination. The first article (STRUCTURE, July 2019) discussed main points influencing seismic drift computation. Column base restraint conditions. In this article, the effect of soil flexibility and cracking of reinforced concrete elements on seismic drift computation force-resisting systems have different levels of nonlinear response, of structural systems is addressed. It also discusses the determination and these levels are represented by the values of response reduction of the level of loads for checking wind drift, return periods of wind factors R. The reduced stiffness of these various systems also varies speed maps, and allowable wind drift limits. A brief comparison based on the permitted degree of nonlinear deformation. Special between seismic drift and wind drift, in connection to their nature, moment frames, for example, may have higher stiffness reduction and a determination procedure is covered. factors among a group of seismic force-resisting systems because this system is designed to permit a high level of nonlinear deformation. Selecting reduction factors for different seismic systems in association Effect of Soil Flexibility Modelling with their behavior under seismic forces is not clearly stated in ACI Soil flexibility can have a significant effect on the behavior of a struc- 318-14; however, engineering judgment may be employed to select the ture. ASCE 7-16-12.7.1 (Foundation Modeling) states, “For purposes best value for stiffness reduction based on the type of seismic lateral of determining seismic loads, it is permitted to consider the structure force resisting system and the intended level of nonlinear deformation. to be fixed at the base. Alternatively, where foundation flexibility is considered, it shall be in accordance with Section 12.13.3 or Chapter Wind Load Level 19.” Therefore, structural engineers must decide the most appropriate analytical assumptions for the structure considering its construction Wind design has been brought into strength level design since 2010 details. The Figure illustrates four types of base restraint conditions (ASCE 7-10). As a result, many changes have been incorporated in that may be considered. comparison to older versions. Unlike seismic drift, which is determined Higher flexibility (pinned base) lengthens the period of the build- at the strength load level, wind drift is still a serviceability concern and ing, resulting in a smaller calculated base shear but larger calculated thus calculated at the service load level (low mean recurrence interval, story drifts. MRI). Since 2010, ASCE 7 has considered two wind load levels, which One drawback to the pinned-base condition is that the story drift of are the strength load level (high mean recurrence interval, MRI) maps the frame, especially the story drift in the lowest story, is difficult to with MRI 300, 700, 1700, and 3000 years and the service load level keep within allowable code limits. If the story drift of the structure (low mean recurrence interval, MRI) maps with MRI 10, 25, 75, exceeds acceptable limits, rotational restraint can be increased at the and 100 years. The strength load level is used for checking strength foundation by a variety of methods, as illustrated in the Figure. design requirements while service load level is used for complying with serviceability requirements such as drift and vibration.

Effect of Structural Elements Cracking Seismic design is based on consideration of nonlinear response. It is, therefore, necessary to consider the reduced stiffness of seismic system elements due to cracking. ASCE 7-16 Section 12.7.3 states that, for reinforced concrete structures, stiffness properties of concrete and masonry elements shall consider the effects of cracked sections. ACI 318-14, Building Code Requirements for Structural Concrete, also states in its Commentary R18.2.2 that, for lateral displacement calculations, assuming all structural elements to be fully cracked is likely to lead to a better estimate of the possible drift than using uncracked stiffness for all members. According to ACI 318-14 – R18.2.2, the assumption of I = 0.5Ig for all structural members of the seismic resisting force system shall be permitted. It should be noted that various seismic 20 STRUCTURE magazine

Wind Speed Maps The Appendix C Commentary presents maps of peak gust wind speeds at 33 feet (10 m) above ground in Exposure C conditions for return periods of 10, 25, 50, and 100 years (Figs. CC.2-1 through CC.2-4 of ASCE 7-16). However, the decision of which map to use is not explicitly stated and is left to the discretion of the design engineer. MRI of 10 and 50 years is recommended but, under certain circumstances, the design engineer can use a higher MRI wind speed in consultation with the client. The height of the structure, type of cladding materials, and type of cladding detailing are among the most important reasons that may encourage using wind speed maps with high return periods.

Wind Drift Limit The ASCE 7-16 standard does not suggest an allowable drift limit for wind design as it does with a seismic design but, according to the nonmandatory Appendix CC (Serviceability Considerations) of ASCE 7-16, common usage for building design is on the order of 1/600 to 1/400 of the building or story height without more details. Typical wind drift limits in common usage vary from H/100 to H/600 for total building drift and h/200 to h/600 for interstory drift, depending on building type and the type of cladding or partition materials used. The most widely used values are H (or h)/400 to H (or h)/500 (ASCE Task Committee on Drift Control of Steel Building Structures, 1988). An absolute limit on interstory drift is sometimes imposed by designers in light of evidence that damage to nonstructural partitions, cladding, and glazing may occur if the interstory drift exceeds about 0.4 inches (10 mm).

Effect of Cracking of Structural Elements For wind analysis, the cracking of structural elements has less effect on the structural response of the wind force resisting system. This lesser effect stems from the nonlinear response of a structure, which is not considered in wind analysis. According to ACI 318-14 Commentary, the factors shown in the Table shall be used to consider cracking effects. The factors shown in the Table are calculated by multiplying the moment of inertia factor for strength load level stipulated in Table 6.6.3.1.1 by 1.4 as stated in R.6.6.3.2.2 of ACI 318-14.

Seismic and Wind Drift

Member and Condition

Moment of Inertia

Column Wall

1 Ig Uncracked Cracked

1 Ig 0.5 Ig

Beams

0.5 Ig

Slabs

0.36 Ig

be appropriate because the philosophy of wind design does not allow the nonlinear response. • Allowable drift limits for structures under wind and seismic forces are, to a great extent, different because of the different design philosophies. The allowable drift limits of seismic force-resisting systems are much higher than those permitted for wind force resisting systems. The allowable drift limit for certain seismic systems is about 10 times the drift allowed under wind loading.

Conclusion One of the inherent provisions in most codes and standards is the consideration of building drift under seismic and wind loading. This must be thoroughly addressed because of the high importance of drift control on structural systems and nonstructural elements, such as partitions, glass, and other brittle components. There are many reasons that necessitate limiting drift; the most significant is to address the structural importance of member inelastic strain in the case of seismic design and system stability and to limit damage to non-structural components, which can be a threat to safety, health, and welfare of the public.■ The online version of this article contains references. Please visit www.STRUCTUREmag.org. Abdulqader Al-sheikh is a Structural Design Engineer at AD Engineering Company (AEC) and a Member of the Saudi Council of Engineers (SCE). (abdulqader37@gmail.com)

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Both seismic and wind drift are lateral deflections that take place because of applied lateral design forces, but they have many differences, such as allowable drift limits, nature of drift, and determination procedures. The main differences are summarized as follows: • Seismic drift is recognized as inelastic drift because of the inelastic behavior of the seismic force-resisting system. Thus, a deflection amplification factor, Cd, is used to account for an inelastic drift. On the other hand, wind drift is considered an elastic drift because the wind force resisting system interacts linearly with the design wind forces. Nonlinear response of the wind force resisting system is not permitted. This can be clearly seen from the strict allowable drift limits under wind loads as compared with the relaxed allowable seismic drift. • Seismic drift of structures is determined at the strength-load level (Strength Limit). However, wind drift is still regarded as serviceability limit and is obtained at service load level (service wind speed with return period of 10, 25, 50, 100 years). • ASCE 7-16 and ACI 318-14 have explicitly stated that the effect of reinforced concrete cracking shall be considered for obtaining realistic estimates of seismic drift. Yet, they do not state the same for wind drift. ACI 318-14, instead, states in its Commentary that, for wind design, effective stiffnesses representative of pre-yield behavior may

Table of moment inertia permitted for elastic analysis at service load level.

D E C E M B E R 2 019

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UNDERGROUND TO BUILD THE

CITIES OF THE FUTURE T O D 21 -C ST UNNELING PENS THE OOR TO ENTURY INFRASTRUCTURE DEVELOPMENT

By Brian Gettinger, P.E., Brad Watson, P.E., and Mike Shiflett, P.E.

he twentieth century saw great monuments to structural engineering erected across the United States, from the Empire State Building to the Golden Gate Bridge and from the Hoover Dam to the Sears Tower. These skyscraping and massive steel-and-concrete structures each challenged preconceived notions of what was possible and left a legacy of engineering and architectural excellence. In the twenty-first century, urban areas continue to grow rapidly. Some of the fastest expansion is in Texas, where the Kinder Institute at Rice University projects that the population of the state’s urban centers may double by 2040. Four of the largest urban areas – Austin, San Antonio, Dallas, and Houston – already have accounted for 85% of Texas’ overall population growth since 2010. Unrelenting growth in these regions stresses the existing infrastructure, particularly linear civil infrastructure: transportation, water, sewer, and stormwater assets. Surging population presents a two-pronged challenge for city planners – they face more demand coupled with shrinking Figure 1. Flooding caused by Hurricane Harvey's massive, persistent rain resulted in $125 space for construction. Meanwhile, greenfield sites are gobbled up billion in damages and 65 deaths across the Houston region and Southeast Texas in 2017. for development, and existing transportation corridors and utility Courtesy of ThinkStock. easements are already full. Houston, We Have a Problem In response to this breakneck growth, Texas’ major urban areas are building different kinds of engineering marvels. Instead of skyscrapers Before August 2017, Houston might have been best-known for the reaching toward the sky, they are digging deep underground to install National Aeronautics and Space Administration’s landing a man on new infrastructure. In the same way that the 102-story Empire State the moon, or hosting the largest livestock exhibition and rodeo in Building challenged engineers in the 1930s, this move underground the world. After August 25, 2017, the city also became an exhibit presents its own set of challenges, particularly in Houston, with its for urban flooding in the devastating aftermath of Hurricane Harvey. unique geotechnical conditions. Massive, persistent rains fell on the sprawling urban and suburban landscape, with its flat terrain and clayey and minimally absorbent soils, triggering devastation never before seen in the city (Figure 1). Up to 60 inches of rain fell during Harvey, leading to widespread flooding resulting in $125 billion in damages and 65 deaths across the Houston region and Southeast Texas. Following Hurricane Harvey, Houston’s engineering community got to work brainstorming solutions to mitigate the impacts of future flooding events, but few traditional solutions seemed feasible. These traditional solutions for flood mitigation included building regional and local detention basins and Figure 2. Tunneling facilitates construction of an inverted siphon to move large volumes of stormwater safely underground. 22 STRUCTURE magazine

widening flood conveyance channels, creeks, and bayous so that additional flow could be conveyed downstream to Galveston Bay. Houston’s urban growth, particularly along the waterways, quickly showed that this approach would require extensive property acquisition – an unpopular, time-consuming, and expensive proposition. What if, instead of moving stormwater at the surface, it could be conveyed underground, which would take it through densely populated urban areas with minimal community and environmental impacts? That is precisely what the Harris County Flood Control District (HCFCD) wanted to find out in a study beginning in the summer of 2019. HCFCD, the Figure 3. Structural design of concrete segments used for tunnels continues to improve, making the segments United States Army Corps of Engineers’ non- more durable and corrosion-resistant. Courtesy of Cylonphoto/123rf.com. federal local sponsor for the region, is responsible for approximately 2,500 miles of bayous and tributaries that drain However, clayey and sandy soils, high groundwater, and creeping stormwater from Harris County. HCFCD’s service area encompasses growth faults had dissuaded consideration of tunnels in Houston Houston and some of Texas’ fastest-growing suburbs. Tunneling has – until now. not been a tool in the agency’s arsenals; so, HCFCD leveraged a grant from the United States Economic Development Administration and Geotechnical Meets Structural Engineering local funding from voter-approved bonds to start the process of studying the feasibility of tunneling for stormwater management (Figure 2). Tunneling is a nexus between geotechnical and structural engineerTunneling for stormwater management is not a new idea; the concept ing. For a project to be safely and successfully constructed, existing has been proven in Chicago, Washington, D.C., and even in Texas earth and groundwater pressures must be balanced by the excavation cities such as San Antonio, Austin, and Dallas, which all have or are equipment, and the permanent shafts and tunnel lining systems must constructing large-diameter, inverted-siphon stormwater tunnels. support earth and hydrostatic loading over their design lives. continued on next page ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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are evenly distributed throughout the entire volume of the segment and are present closer to the surface of the concrete than steel reinforcement, the segments are more durable, reducing the risk of damage to the edges and corners due to handling. Steel fiber reinforcement improves the overall quality and reduces the need for rework. Synthetic fibers offer similar benefits as steel fibers plus even greater corrosion protection because they are innately corrosion resistant. Bekaert Maccaferri, a manufacturer of steel fiber for segments, indicates that impact resistance of steel fiberreinforced segments is 20 times higher than for unreinforced concrete. Impact resistance is a benefit of both steel and synthetic fiber-reinforced concrete. Recent laboratory testing of synthetic fiber-reinforced concrete found that strain during impact loading was reduced by as much as 68 percent compared to unreinforced concrete. Figure 4. Tunnels are the skyscrapers of the underground, facilitating needed higher-density The first tunnel solely using synthetic fiber-reinforced seginfrastructure development in urban areas and presenting unique engineering challenges the ments is currently under construction near Kansas City, same way the first skyscrapers did in the 20 th century. Courtesy of Gui Yongnian/123rf.com. Missouri. These advances in segment reinforcement technology provide greater durability, longer service life, and Tunneling in rock, as is common in Dallas, Chicago, and the Upper an improved final product at a comparable or lower cost than the Midwest, often can rely on the excavated rock formation to be self-sup- traditional methods. porting with minimal initial support systems. Excavation in competent limestone often can be opened and left without a permanent support Crossing a Different Kind of Fault system with no geotechnical consequence. In Houston, the clays, sands, and high groundwater require ground support systems to be Improvements in tunneling technology over the last 30 years have installed before excavation or immediately after excavation. These are mitigated many risks associated with tunneling in soft ground with much different and more complex systems. high groundwater tables. But the greatest geotechnical and structural Excavation equipment must use pressurized-face tunneling equipment challenge facing tunneling in Houston may be crossing active growth to control the earth and groundwater pressure that continually acts faults that are slowly but continuously creeping toward the Gulf of on the tunnel boring machine, and also varies along the tunnel align- Mexico. The most-active faults generally cross west and northwest ment as different geologic formations are encountered. Earth pressure Houston in a northeasterly direction, including the Long Point and balance and slurry tunnel boring machines are both pressurized-face Brittmoore Faults, which have been measured to creep up to one-half tunneling equipment that could be used in Houston depending on inch per year. Although they are not at risk of rupture or generating the soil and groundwater conditions that are encountered. seismic events, the consistent creeping of these faults poses a challenge Immediately following excavation, the pressurized-face tunnel boring to any concrete structures crossing them, as evidenced by the frequent machine erects precast concrete segments for both the initial and concrete pavement patches required on Houston’s Interstate 10 above permanent excavation support system for the tunnel. The precast the surface expression of the Long Point Fault. concrete segments are typically produced at a manufacturing facility, Special structural design considerations must be made for these fault cured, and then transported to the project site for erection. The seg- crossings to ensure that the tunnel can withstand the potential fault ments can also be fabricated on or near the site. Factory production displacement over its service life. Prospective mitigation concepts are of precast segments results in higher repeatability and quality across being evaluated as part of the study commissioned by HCFCD and large batches than cast-in-place concrete structures, which can be will be developed in greater detail in the future. challenging in large and deep tunnels. In most tunnel applications, the concrete segments (Figure 3, page 23) Skyscrapers of the Underground form a concentric compression ring as they counteract the earth and groundwater pressure deep underground. In unique circumstances, if Houston’s unique geology, urbanization, and extreme weather events the tunnel’s internal pressure due to the conveyed water, wastewater, or are encouraging innovative thinking about infrastructure. Tunneling, stormwater is expected to exceed the confining earth and groundwater once considered impractical, is now at the forefront of the effort to pressure, post-tensioning may be required in the segments. This is to build resiliency into one of America’s largest urban areas (Figure 4). This ensure that the segments remain in compression. effort, still in the early planning phases, will require innovative Traditional concrete segment structural designs incorporated steel engineering and construction solutions to serve as a backbone reinforcement. As in other concrete structures, this reinforcement for the next century of infrastructure investments.■ is the leading cause of deterioration. As the steel corrodes, the rust Brian Gettinger is the Tunneling Services Leader for Freese and Nichols, increases the volume of the steel, creating tensile cracks that can result Inc. (brian.gettinger@freese.com) in spalling, cracking, and other failures. Concrete segment designs are now using alternatives to steel reinforcement, including steel wire Brad Watson is a Freese and Nichols Principal and Manager of the firm’s mesh and, more recently, steel fiber and synthetic fiber reinforcement Structural Group. (bbw@freese.com) to prolong concrete service life, particularly in corrosive environments. Mike Shiflett is a Senior Geotechnical Consultant in Freese and Nichols’ Steel fibers are noncontinuous and discrete, so there is less potenWater Resources Design Group. (mms@freese.com) tial for propagation of corrosion activity. Also, because the fibers 24 STRUCTURE magazine

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EXCELLENCE STRUCTURAL ENGINEERING IN

AWARDS

he National Council of Structural Engineers Associations (NCSEA) is pleased to publish the winners of the 2019 Excellence in Structural Engineering Awards. The awards were announced on the evening of November 14 at NCSEA’s 27th annual Structural Engineering Summit in Anaheim, California. Given annually since 1998, each year the entries highlight work from the best and brightest in our profession. Awards were given in seven categories, with one project in each category named the Outstanding Project. The categories for 2019 were as follows: • New Buildings under $20 Million • New Buildings $20 Million to $100 Million • New Buildings over $100 Million • New Bridge and Transportation Structures • Forensic | Renovation | Retrofit | Rehabilitation Structures up to $20 Million • Forensic | Renovation | Retrofit | Rehabilitation Structures over $20 Million • Other Structures The 2019 Awards Committee was chaired by Carrie Johnson (Wallace Engineering, Tulsa OK). Ms. Johnson noted: "We had two rounds of judging to allow the judges more time to focus on each individual project. The preliminary round was performed via electronic voting by a group of NCSEA Past Presidents and the final round was done in Austin, Texas, by engineers from the Structural Engineers Association of Texas (SEAoT). The judges had an enormous task of trying to determine winners from an impressive group of submittals. The group of winning projects is outstanding.” Please join NCSEA and STRUCTURE® magazine in congratulating all the winners. More in-depth articles on several of the 2019 winners will appear in the Spotlight section of the magazine over the 2020 editorial year.

Category 1: New Buildings under $20 Million

OUTSTANDING PROJECT Brother James Gaffney, FSC, Student Center Romeoville, IL | Wight & Company

Photos courtesy of Paul Schlossmann Photography

26 STRUCTURE magazine

Brother James Gaffney, FSC, Student Center at Lewis University is designed to be distinctive, projecting an image of a contemporary, forward-thinking university. The focal point is an 80- x 70-foot clear span roof framing which pays tribute to the university’s aeronautics heritage. The roof incorporates custom trusses which uniquely resemble the cable-and-strut construction of vintage airplanes. Trusses extend through the southern curtainwall and frame a large overhang. Sloped columns support the overhang and incorporate CastConnex specialty connections. The entire structure is exposed including floors, roof, decking, walls, columns, and stairs. Architecturally exposed structural steel specifications were strictly adhered to.

Category 2: New Buildings $20 Million to $100 Million

OUTSTANDING PROJECT Rufus 2.0 Spheres

Seattle, WA | Magnusson Klemencic Associates

Courtesy of Bruce Damonte

The centerpieces of Amazon's new corporate headquarters in Seattle, the first-of-their-kind Spheres, feature three intersecting glass-and-steel structures enclosing five freestanding floors. Comprised of organically shaped steel sections, the 65,000-square-foot workspace features treehouse meeting rooms, waterfalls, a paludarium, a four-story living wall, and is packed with more than 40,000 plants from over 50 countries – a collection worthy of a top-tier conservatory. MKA collaborated with the architect, contractor, detailer, and fabricator through all phases of design, fabrication, and erection. Integrating fabrication and erection constraints into the design during earlier stages, the team delivered the architect’s vision within the owner’s challenging schedule requirements and budget. Category 3: New Buildings over $100 Million

OUTSTANDING PROJECT 181 Fremont

San Francisco, CA | Arup

Photos courtesy of Jay Paul Company

181 Fremont is a 56-story mixed-use building featuring luxury residential and commercial space. Its innovative damped mega-brace structural design facilitated a reduction in building stiffness to decrease seismic demands while still satisfying stringent occupant comfort criteria for wind-induced vibration, eliminating the need for a tuned mass damper. Mega-columns designed to uplift slightly in a major earthquake also limit seismic demands in the tower and foundation. The building’s resilience-based design approach earned it a REDi Gold rating, having been designed to remain essentially elastic and achieve immediate reoccupancy after a 475-year earthquake.

Category 4: New Bridges or Transportation Structures

OUTSTANDING PROJECT

41st Street Steel Arch Pedestrian Bridge Chicago, IL | AECOM

Through an international design competition, the City of Chicago constructed a new signature pedestrian bridge at 41st Street connecting the Bronzeville neighborhood with the lakefront while creating an inviting atmosphere with an aesthetically pleasant iconic structure. The new 41st Street Pedestrian Bridge is 1500 feet in length and incorporates twin 240-foot-long inclined arches on graceful sweeping S-curves to span over Lake Shore Drive and Metra Electric/CN Railroads. The structure has very complex and complicated geometry, which created challenges to design, fabricate, and erect over extremely active railroad tracks (263 trains daily) and a major highway carrying 100,000 vehicles daily. D E C E M B E R 2 019

Category 5: Forensic / Renovation / Retrofit / Rehabilitation Structures under $20 Million

OUTSTANDING PROJECT

“Leaning Tower of Granby” Historical Renovations/Savoy Norfolk, VA | Speight Marshall Francis

The 1907 Hotel Savoy was dubbed the “Leaning Tower of Granby” because the nine-story building developed a 21-inch northward tilt. After a decade of annual inspections, it was determined that it had stopped settling for over fifty years. A committed developer resolved to straighten it regardless of cost – a feat that had never been accomplished on a building that tall. An innovative plan to correct the lean on the 3,250-ton building using custommade 200,000-pound hydraulic-powered jacks involved nearly 70 sheets of drawings for each step of the process. Subsequent historic renovations transformed Savoy into modern apartments with ground-floor retail.

Category 6: Forensic / Renovation / Retrofit / Rehabilitation Structures over $20 Million

OUTSTANDING PROJECT

Hudson Commons: Innovative Approaches to Vertical Expansion New York, NY | WSP USA

Hudson Commons is a unique repositioning project on 34th and 9th Ave in New York City that pushes the boundaries of vertical expansion. Designed by KPF with WSP as the engineer of record, the eloquent renovation adds 17 floors of steel to an existing 1960s era concrete structure, bringing the new building’s total rentable area to 700,000 square feet. The complexity of the problem inspired innovative solutions, including shotcrete column jackets, a new reinforced concrete core threaded through the building, shoring free demolition, and a range of foundation retrofit methodologies. The exposed concrete core punctuates the sleek tower addition. Category 7: Other Structures

OUTSTANDING PROJECT Vessel, New York’s Staircase New York, NY | Thornton Tomasetti

Vessel is the centerpiece of Hudson Yards, a 16-building complex on the West Side of Manhattan. The steel structure features a lattice of 154 interconnecting flights of stairs, 80 landings, and nearly 2,500 individual steps. As the structural engineer for the project, Thornton Tomasetti worked closely with the client and project team to develop the design from concept through design development, design-assist, fabrication, and construction. Vessel is a sculpture on an industrial scale, a beautifully refined design that balances form and function and celebrates craftsmanship and attention to detail to create a defining object in New York’s urban fabric. 28 STRUCTURE magazine

AWARD WINNER – CATEGORY 1

Waffle

Culver City, CA | NAST Enterprises Corp.

Waffle is a four-story tower with a gently curving surface sculpted with vertical and horizontal steel fins, home to Vespertine restaurant in Culver City, CA. Four aggressively bending steel pipe columns on the inside provide structural support for the building. The elevator shaft, annexed on the west, provides for circulation in addition to hiding the thermal expansion joints of the steel shell. Two sets of stairs, levitated on the inside and the outside corner, are designed with slotted and slip connections to accommodate the flexibility demands of the building. Corner fins are cantilevered, utilizing the extent of steel capacity.

AWARD WINNER – CATEGORY 1

Rose Park Pool Operations Building Billings, MO | Cushing Terrell

A large, singular curved roof was constructed over two smaller structures to create a new pool house. Cantilevered steel columns fixed on drilled shafts support the high roof curved steel girders. Large glue-laminated beams of naturally durable cedar support a metal deck that follows the roof curve. The high roof construction sequence provided quick shelter for underlying construction during winter conditions. Careful detailing, material selection, and assemblies remove unnecessary finishes and result in a raw structure that is both form and finish and is void of noticeable connections.

AWARD WINNER – CATEGORY 2

Hale Centre Theatre

Sandy, UT | Dunn Associates, Inc.

Hale Centre Theatre is a world-class theater experience that is truly one of a kind. It features a centrally located round stage with seating radiating concentrically outward, each row increasing in diameter. When patrons experience a show at the Hale Centre Theatre’s center stage, the viewing angle is 360-degrees. The theatre is approximately 130,000 square feet with two separate stages – a theater-in-the-round, which seats over 900 patrons, and the smaller Jewel box theater that seats 460. The larger of the two has the functionality to telescope vertically above stage level and then to retract while rotating 360-degrees.

Courtesy of Hines

AWARD WINNER – CATEGORY 2

AWARD WINNER – CATEGORY 3

Greensboro, NC | Stewart

Singapore | BuroHappold Engineering

New York, NY | WSP USA

NCA&T Student Union

Stewart has exceeded North Carolina A&T State University’s expectations, designing a focal point and multicultural hub for the country’s largest historically black public university. Since its opening, enrollment has soared. The entrepreneurial spirit of the school is expressed through complex design. The three-story, cutting-edge facility is LEED silver certified and structurally innovative. It features long cantilevers extending off the building from every angle, a unique exterior wall system, cantilevered floors, and two expansive, minimally supported monumental stairs, which demand attention and invite students into the center of the social energy on campus.

Jewel Changi Airport

The new mixed-use complex at Jewel Changi Airport in Singapore delivers an exceptional experience for the 85 million passengers that pass through it every year. BuroHappold Engineering, as the design engineer for the roof, created an incredibly complicated glassand-steel thin-shell gridshell that encloses the interior forest and commercial spaces. The gridshell roof itself contains more than 5,000 solid steel nodes and 14,000 member elements. At the apex of Jewel’s glass roof is an oculus that showers approximately 10,000 gallons of water per minute, as a 130-foot indoor waterfall, through this spectacular central garden space.

53W53 – MoMA Tower

53W53, the pyramid-shaped luxury-condominium building envisioned by Pritzker Prize-winning Architect Jean Nouvel, is a 728,000 GSF high-rise of 82 stories, reaching 1113 feet in height on a narrow site of 87 feet in width, resulting in a 1:12 slenderness ratio. It is the first time a structure of this magnitude and complexity is done with reinforced cast-in-place concrete diagrids. Adjacent to the Museum of Modern Art, the new building will also provide 65,000 GSF of additional unobstructed gallery spaces, at levels 2, 4 and 5, that will complement the current program of the existing premises of the MoMA Museum. D E C E M B E R 2 019

AWARD WINNER – CATEGORY 4

AWARD WINNER – CATEGORY 5

Kittery, ME and Portsmouth, NH | FIGG/Hardesty & Hanover

Montreal, Canada | TYLI-SLI Joint Venture

Templeton, CA | SSG Structural Engineers, LLP

Sarah Mildred Long Bridge The new Sarah Mildred Long Bridge carries US Route 1 Bypass over the Piscataqua River between Kittery, Maine, and Portsmouth, New Hampshire. The crossing consists of vehicular approach bridges stacked over railroad approach bridges leading to a movable lift span over the navigation channel. The 2,434foot segmental vehicular bridge provides two 12-foot lanes with 5-foot shoulders and bridge railings for cyclists. Span lengths vary from 132 to 320 feet. The 1,437-foot segmental railroad bridge provides a heavy rail line that serves the Portsmouth Naval Shipyard. Span lengths vary from 69 to 160 feet.

Samuel De Champlain Bridge Opened to traffic on July 1, 2019, the Samuel De Champlain Bridge spans the St. Lawrence River in Montreal. The over 2-mile (3.4-km) viaduct with a signature cable-stayed bridge represents the most high-profile infrastructure project in North America. The project faced stringent design and performance criteria, various site constraints, and an aggressive 48-month design-build schedule. Challenges included limits to field construction during severe winter months, wind and seismic hazards, navigational channel closures, imposed no-construction zones, and more. Large-scale precasting, modular construction methods, and creative erection sequencing were incorporated to meet the design-build schedule.

Epoch Tasting Room

Bringing life back to the original York Mountain Winery, condemned after the 2003 San Simeon earthquake (6.6M), the new Tasting Room’s full reconstruction salvaged original materials from selective demolition, taking great care to repair the original stone winery and preserve the “bones” of the building. A delicate procedure due to 100 years of soil pressures, water infiltration, and seismic deformation, the existing building was carefully dismantled, with its usable materials cataloged, stored, and repurposed to save the character of the circa-1907 clay brick, heavy timber framing, and to shore and re-point the un-reinforced stone walls of the original cellar.

Courtesy of Doublespace Photography

AWARD WINNER – CATEGORY 5

AWARD WINNER – CATEGORY 6

Ann Arbor, MI | SmiithGroup

Atlanta, GA | HOK

Ottawa, Ontario, Canada | Fast + Epp

Regents of the University of Michigan

The Clements Library project involved the addition of a state-of-the-art preservation storage space for a priceless collection of American history, while subtly marking its presence in relation to the existing 1923 Italian Renaissance-inspired building. The addition was placed underground in response to site constraints. This solution presented a sophisticated structural challenge involving underpinning a portion of the existing building during the 35-foot deep excavation. The result yielded a barely visible perimeter wall of black granite, peeking slightly above grade, and two glass entrance and access "jewel" boxes – subtle signifiers of the incredible undertaking of this construction. 30 STRUCTURE magazine

Hartsfield-Jackson Atlanta Int'l Airport Modernization As part of the modernization of the world’s busiest airport, HOK delivered a structural and architectural icon. Dual 864-foot-long structural steel canopies clad in ETFE flank the existing terminal and support new pedestrian bridges over the existing roadways. Interdisciplinary design and parametric structural modeling were key in providing a structure that welcomes and shelters passengers at the airport threshold. Surmounting challenges ranging from complex existing conditions to accommodating uninterrupted airport operations were key in exceeding the client’s expectations and enlivening the airport for the 21st century.

National Arts Centre Architectural Rejuvenation The National Arts Centre in Ottawa underwent a significant rejuvenation for Canada’s 150th anniversary. A key project feature is a striking coffered ceiling covering a 60,000-sqaure-foot glass-clad extension, a piece of structural artistry visible from the street. The roof was formed of 28 hybrid wood-steel panels pre-fabricated offsite, with electrical, mechanical, and acoustic integration. The ceiling promotes engineered wood products fabricated in Canada and the ability to span large spaces using a hybrid system where each material is used according to its strengths. The rejuvenated Centre has become a new beacon for arts and culture.

AWARD WINNER – CATEGORY 7

Northeastern University’s Interdisciplinary Science & Engineering Complex – Spiral Stair Boston, MA | Summit Engineering, PLLC

The spiral stair is a five-story, cantilevered, monumental steel plate stringer stair comprised of steel bent plate treads and hollow structural section (HSS) cross-member framing. The stair is located within the main atrium of the building and is supported at each floor level by an A-frame structural steel frame system serving as both the primary support and the stair landing at each floor. The stair employs sloped and curved glass guard rails with stainless steel pipe railings. The stair was completed on schedule and became the focal point of the new Interdisciplinary Science and Engineering Complex at Northeastern University.

AWARD WINNER – CATEGORY 7

Reinventing the Gabion: ROMO Backcountry Estes Park, CO | STRUCTURALIST

Iconic Longs Peak is the tallest mountain in Rocky Mountain National Park and one of the most frequented 14ers (> 14,000 feet) in Colorado. NPS collaborated with Colorado Building Workshop, the design-build program at the University of Colorado Denver, to design and construct new backcountry privies. The new privies success is due to their unconventional hybrid structural system and prefabrication. The solution? A series of prefabricated gabion walls and 1⁄8-inch steel plate moment frames that triangulate lateral loads resisted by the stone ballast. This innovative assembly allowed for rapid on-site construction and architecture that disappears into the surrounding landscape.

2019 PANEL OF JUDGES

The judges for this year’s Excellence in Structural Engineering Awards were: PRELIMINARY ROUND ‒ NCSEA Past Presidents Vicki Arbitrio, P.E. ‒ Gilsanz Murray Steficek Craig Barnes, P.E. ‒ CBI Consulting, LLC Marc Barter, P.E., S.E. ‒ Barter & Associates Bill Bast, P.E., S.E. ‒ Thornton Tomasetti Tom Grogan, P.E. ‒ Haskell Ron Hamburger, S.E. ‒ Simpson Gumpertz & Heger

Carrie Johnson, P.E. ‒ Wallace Engineering John Joyce, P.E. ‒ Engineering Solutions, LLC Jim Malley, S.E., P.E. ‒ Degenkolb Ben Nelson, P.E., S.E. ‒ Martin/Martin Sanjeev Shah, P.E., Esq ‒ Lea + Elliott Mike Tylk, S.E. ‒ TGRWA

FINAL ROUND ‒ Structural Engineers Association of Texas (SEAoT) Davy Beicker, P.E Beicker & Associates

Jim Goes, P.E., S.E ATS Engineers, Inspectors & Surveyors

Jeremy Klahorst, P.E. Datum Engineers

Galen Schroeder, P.E. Datum Engineers

Matt Carlton. P.E. WJE

Tom Grogan, P.E. Haskell

Joe Luke, P.E. LCRA

Kris Swanson, P.E., S.E. DCI Engineers

Brian Caudlee, P.E. Walter P. Moore

Brian Johnson, P.E. Backbeat Structural Design

Ben Nelson, P.E. Martin/Martin

Larry Swayze, P.E. LM Swayze Engineers PLLC

Bobby Chamra, P.E. Building Diagnotics, Inc.

Carrie Johnson, P.E. Wallace Engineering

Gary Pickett, P.E. Pickett Kelm Associates

Bob Tieman, P.E. Page Sutherland Page

Angela Galloway, P.E. Amtech Solutions

Bill Kelm, P.E. Pickett Kelm Associates

Lemar Porter Retired

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Napa County HISTORIC

Courthouse By Luke Wilson, S.E., Brett Shields, P.E., and Kevin Zucco, S.E.

Figure 1. Entry showing damage; taken the morning of the Earthquake.

3:20 AM, August 24, 2014, the Napa County Historic Courthouse was severely damaged in the magnitude 6.0 South Napa Earthquake, which induced nearby ground motion readings indicating spectral accelerations ranging from 0.4g to 1.7g for low period structures. Most obviously, the top of the south east corner at the front of the building (east face), along with its attached dental cornice, collapsed outward to the sidewalk below (Figure 1). The Initial review also revealed a partial collapse of the exterior brick wall on the north elevation. At one corner of the building, the top two to three feet of brick wall fell inward, collapsing the ceiling framing above the jury room (Figure 2). There was significant additional damage throughout the exterior and interior masonry walls and the building frequently appeared in news coverage as a prominent downtown public building affected by the Napa Earthquake. The Courthouse was subsequently red-tagged by the City of Napa (the building was deemed unsafe for occupancy or entry, except as authorized by the local building Authority Having Jurisdiction per Applied Technology Council, ATC 20-1), beginning the long process to assess the damage, repair, and reoccupy the historic structure.

History The Napa County Historic Courthouse was constructed in 1878, with an estimated $51,000 construction cost equivalent to approximately $1.3 million in 2019 dollars. It was added to the National Register of Historic Places in 1992. Before this building was built, two previous courthouse buildings existed on the same site. The original building was a prefabricated structure shipped to the city by barge in 1851. Just five years later, in 1856, the original building was replaced with a site-built courthouse that was ultimately deemed

unsafe due to settlement and wall cracking. In 1874, construction began on the current courthouse, which has been serving the local community for over 140 years. The current courthouse architectural design was provided by the Newsome Brothers, who also designed the Napa Opera House and the William Carson Residence in Eureka, with the assistance of local architect Ira Gilcrest. The Courthouse, along with the Hall of Records and Administrative Annex, occupies a city block bounded by 2nd, 3rd, Coombs, and Brown Streets in downtown Napa. The land for the Courthouse site was donated for use by City founder Nathan Coombs. As part of the original construction, a two-story jail was constructed west of the courthouse with a small access corridor between the two buildings. In 1918, the Hall of Records building was constructed adjacent to the jail on the west end of the block. When the jail was demolished in 1977, a new Administrative Annex was built as infill between the Hall of Records and the Historic Courthouse to create a singleoccupancy space. While the 1977 infill project is seismically separate from the Historic Courthouse, several large openings reinforced with concrete frames were added to the west wall of the Courthouse building, and multiple smaller openings were either added or infilled to accommodate new circulation patterns in the combined space. Additionally, in 1977, a seismic retrofit of the Historic Courthouse was performed with concrete pilasters added in the north and south exterior walls and the interior hallway corridor walls, and out-of-plane wall anchorage hardware was installed throughout. In 2003, a tenant improvement to the court clerkâ&#x20AC;&#x2122;s office added an approximately 25-foot-long concrete infill shear wall between the first and second floors Figure 2. Courtroom view from the attic where brick partially collapsed along the north corridor wall. the ceiling below. continued on next page D E C E M B E R 2 019

Figure 3. Horizontal offset of brick in the attic.

Figure 4. Partially collapse of URM wall due to adjacent Administrative Annex Framing.

The Courthouse is a 15,000-square-foot, two-story, unreinforced brick masonry building with wood-framed floors, ceiling, and roof. The original construction included an octagonal bell tower with an onion dome roof that was damaged in the 1906 San Francisco Earthquake and eventually removed in the early 1930s. The roof framing consists of straight sheathing over 2x rafters and site-built, large rough-sawn timber trusses, while the ceiling framing below consists of conventional 2x framing. Roof and ceiling framing both span between perimeter and corridor bearing walls. The floor consists of assorted finishes over straight sheathing with rough-sawn 3x12 joists. ZFAâ&#x20AC;&#x2122;s longstanding relationship with Napa County and direct involvement with the Historic Courthouse since 2006 offered familiarization and knowledge of the building invaluable to the process after the earthquake and repair solutions beyond.

non-structural damage, including broken sprinkler lines that caused additional water damage. The out-of-plane wall anchorage included failures of both the original government anchors (approximately 8-inch-diameter iron plate on the far face of the brick wall anchorage by a flat plate through the wall to the wood framing beyond) and the 1977 retrofit adhesive anchors. Observed failures included: wythe pullout, retrofit anchor adhesive bond failure, buckling of 2x diagonal braces, net tension rupture of 2x braces, and bolted connection failures in 2x members. With few exceptions, out-of-plane wall anchorage failures were concentrated at the roof/attic level.

Damage from 2014 Earthquake

Documenting the Damage In lieu of traditional damage documentation methods, in which reviews are completed on a room-by-room basis, generally looking at a wall from one side at a time, ZFA employed a wholistic 3-D approach. Detailed and scaled field observations of damage on both sides of walls (cracks, deflections, displacement of wythes, localized collapses, and failures in out-of-plane wall anchorage), as-built conditions differing from the original construction, and 1977 reconfiguration documents were combined with original construction documents to create a 3-D BIM model. All observed wall cracks were modeled with different color and weight 3-D model lines. Blue lines indicated cracks occurring on the north or east faces, while red indicated cracking on the south or west faces of walls. Model line weights were also varied to depict crack size thresholds. Wall profiles were edited to show localized collapsed areas and voids.

ZFA was brought in as part of a team tasked with completing the Courthouse repair after the building was shored under a prior contract with a separate design team. Before beginning repair design and drawings, ZFA completed an extensive damage documentation effort to reveal and illustrate the level of damage to the client and the insurance companyâ&#x20AC;&#x2122;s peer review engineer for confirmation of required repair scope. In addition to obvious partial collapses, the building sustained significant damage at the second-floor level and along the front of the building. The front (east) faĂ§ade, consisting of a series of reentrant corners stepping out horizontally towards the front entrance, experienced significant corner damage throughout. Observed damage included: diagonal cracking of walls leading to in-plane and out-ofplane horizontal wall displacements up to three inches (Figure 3); multiple localized or partial collapses of brick walls (Figure 4); failure of both the original out-of-plane government roof-to-wall anchors and the 1977 retrofit wall anchorage; and significant Figure 5. Site photo of damaged wall and heat map showing offset. 34 STRUCTURE magazine

Figure 6. BIM Model showing entry damage (see Figure 1 for actual photo of area).

Figure 7. Example damage documentation drawings (see Figure 5 for an actual photo of the wall).

A 3-D exterior site scan was completed shortly after the earthquake for use in shoring design, and an internal 3-D scan of each room was completed during the damage documentation phase. The resulting data point cloud was linked into the BIM model to verify dimensional assumptions and aid in building deflection review and assessment. Sections were cut through the walls with the point cloud to illustrate out-of-plane wall displacements and verify wall thicknesses that were otherwise difficult to identify solely through field observations. The 3-D scans were also used to generate “heat maps” (Figure 5) showing relative out-of-plane displacements in a colorized gradation to augment the documentation drawings. Using the damage documentation 3-D BIM model (Figure 6), twostory full-length wall elevations and 3-D views clearly illustrated crack patterns on both sides of walls (Figures 7 and 8). Using this wholebuilding approach to documenting the damage, significant two-story diagonal crack patterns were revealed that extended through wall faces, providing valuable insight into the global building behavior and resulting damage extent from the earthquake.

Assessing the Damage FEMA 306, Evaluation of Earthquake Damaged Concrete and Masonry Wall Buildings, was used to classify observed failure modes and provide an estimated loss of strength for each wall pier along each wall line. Typical failure patterns included wall-pier rocking, in-plane flexural cracking, and out-of-plane flexural cracking. Additionally, significant

corner damage was observed due to the reentrant corner configuration along the eastern front façade, resulting in the partial collapse of two walls at the roof. In addition to the more common damage patterns documented in FEMA 306, weak pier/spandrel joint damage patterns were also observed at exterior corners and reentrant corners. The combination of the field observations, 3-D BIM modeling, and FEMA 306 analysis created a summary of the damage documentation that was used to develop a conceptual repair approach for review and discussion with the insurance company’s peer review engineer. Because of the historic materials and construction techniques, 140 years of use and modification, and the wide range of damage throughout, a single repair option was not appropriate. The repair concept, therefore, used a combination of traditional brick repair methods, repointing, grout injection, and localized areas of brick rebuild along the western portion of the building. However, the more heavily damaged eastern portion and corridor walls required a creative repair approach to save the historic fabric of the building and provide improved structural performance. This repair approach included the use of Fabric-Reinforced Cementitious Matrix (FRCM), one of the first applications in California, and wall reconstruction with specially-detailed CMU construction to replace the walls in the areas of heaviest damage. A more detailed review of the various repair and rehabilitation techniques utilized will appear as a future article in STRUCTURE.■ All authors are with ZFA Structural Engineers in Santa Rosa, California. Luke Wilson is an Associate Principal. (lukew@zfa.com) Brett Shields is an Engineer. (bretts@zfa.com) Kevin Zucco is an Executive Principal. (kevinz@zfa.com)

Project Team

Figure 8. Two-story section through the main hallway showing damage documentation.

Owner: County of Napa Structural Engineer: ZFA Structural Engineers Historic Preservation Architect: TreanorHL Architect: TLCD Architecture Owner’s Rep: AECOM General Contractor: Alten Construction D E C E M B E R 2 019

View from Level 15.

HANGING A MONUMENTAL STAIR

n late 2018, Little, a national architecture and engineering firm, moved its Charlotte, NC, office into a newly constructed building in the heart of the city’s uptown. In addition to being a flexible, adaptable environment pursuing both LEED and WELL certification at a Silver level (targeted to be one of the first WELL spaces in Charlotte), the interior upfit focuses on vibrancy, energy, rawness, complexity, and, most of all, connectivity. Helping to drive all of these factors through the 14th, 15th, and 16th floors occupied by the firm, Little incorporated an open, internal staircase that acts as a focal point – an architectural center of gravity. This monumental centerpiece, however, is not a typical connecting stair. Little used its engineering, design, and architectural expertise to create a structure that not only facilitates impromptu meetings and idea exchange but provides an unexpected visual statement – starting at the top.

Aligning Architectural and Engineering Goals

Instead of being traditionally anchored and reinforced at the lower level, which would disturb existing tenants on the 13th floor below, this 30,000-pound stair hangs from a four-pronged, structural mast on the underside of the building’s 17th floor. Ensuring structural reliability was a challenge for By Dan Wray, P.E., and Bryan Starr, P.E., S.E., the design team. The team knew it did not want to LEED AP BD+C add significant strengthening to the existing building structure. Using the existing building as it was originally designed reduces the carbon footprint of the International WELL Building Institute (IWBI) defines WELL as “Buildings and Communities renovation while also making it more cost-effective. that help people thrive.” Where LEED focuses on the performance of a building, WELL With the stair connecting three of Little’s floors, the focuses on the performance of the occupants. WELL Building Certification was developed team was able to remove the mildly reinforced conthrough a medically proven research matrix that focuses on the health and wellness of crete slab and a 21-inch mildly reinforced concrete building occupants. WELLv1 is based on seven concepts (Air, Water, Nourishment, Light, beam at two levels, totaling 56,000 pounds of conFitness, Comfort, and Mind) with over 100 features (www.wellcertified.com). crete – more weight than the actual stair. The team distributed the load of the stair to the underside of the 17th-floor beams with bolted steel channels to support the stair and designed for the live loads required by code. The team initially evaluated a straight hanger to suspend the stair from the 17th floor. However, this type of hanger pushed almost the entire load to the 17th floor, which would have required strengthening the existing structure. Instead, after several iterations, the team crafted a four-pronged structural mast to transfer some of the load, due to flexibility of the prongs, to the 16th and 15th floors, allowing the existing structure to adequately carry the appropriate load. Approximately 55 percent of the dead and live loads are carried by the 17th floor, while the 16th and 15th floors support the remainder of the load transferred from the inside HSS14x4 stringers (see Figure 1 and further explanation below). Every project attempts to combine architectural intent with engineering design but, in this unique case, the two aligned perfectly. The architects introduced a concept of a flaring mast to symbolize a ‘spark’ (one of Little’s core values is to ‘spark’ a spirit of excitement and discovery). This introduced the structural flexibility needed to better share the stair load between multiple floors. Four 2¼-inch-diameter pins accomplish the mast connection to the 17th floor. This connection is utilized to eliminate the transfer of any moment into the existing structure while Four-pronged hanger at Level 16. complimenting the rawness of the design.

36 STRUCTURE magazine

only be directly in front of the lobby elevators but also between four PT girders to avoid cutting any post-tensioned cables. Also, the placement eliminated requirements for reinforcing the existing slab which was then cantilevered past the nearest girder. The maximum cantilever of the concrete slabs and beams cut to form the stair opening on the 15th and 16th floors was 2 feet, 7 inches from the edge of an existing PT girder. The existing slab was checked for this cantilever and reinforcement was deemed not necessary. One existing 12-inch-wide beam was removed from each slab for a length of 22 feet. The remaining beam on either side of the stair slab opening was checked to verify stresses and deflections were within allowable code requirements. The PT girders were checked with the removal of the concrete slab to verify they were not overstressed and still met ACI serviceability for uncracked members.

Building the Stair Structure Figure 1. Monumental stair section.

Creating the Stair Opening Beyond the mast concept, the design team had to evaluate the location of the staircase carefully. The existing cast-in-place concrete structure of the building consists of 48-inch-wide post-tensioned (PT) girders in both directions between columns supporting mildly reinforced beams at approximately 11-foot centers and a one-way 5-inch-thick mildly reinforced concrete slab. The location of the 17-foot by 22-foot floor opening for the stair was selected to not

The main stair structure was designed to give the impression that the stair is “floating.” Two HSS10x6 outriggers are cantilevered from the steel mast at each level supporting each HSS14x4 stringer, and an HSS14x4 outrigger is cantilevered from the mast to support the landing. The main HSS14x4 stringer runs along the inside edge of the stair directly under the inside railing and is supported by the 15th and 16th floors as well as the HSS10x6 outriggers at the intermediate landings, framed back to the center mast. A secondary HSS6x6 stringer runs along the stair approximately 2 feet, 4 inches from the outside edge of the stair (Figure 2, page 38). The architects requested the edge of the stair treads be exposed steel, which was termed the ‘zipper.’ The 10 gage stair treads cantilever

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Base of the stair.

2 feet, 4 inches past this secondary stringer and support the C4 zipper, which in turn supports the outside railing. The design of the stair also included checking step live loading with live load only on half the stair, as well as only on the landings, to verify stresses and movement. Differential deflections of the 15thand 16th-floor structural members were also checked as altering these movements changed the amount of load supported by the 17th floor.

Executing the Installation Even with diligent planning from both the engineering and design team, the stair execution did not come without its challenges. One obstacle was the limited size of the building freight elevator and getting materials to the 16th floor. The solution – the stair was

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Figure 2. Monumental stair isometric.

constructed at the fabricator’s (CM Steel) shop and then cut into 42 pieces to be delivered and re-connected on site. Once in the space, the stair was pieced back together using full penetration welds. The construction sequence took advantage of the existing floor by installing the hanger framework on the underside of the 17th floor before cutting the new holes in levels 16 and 15. The team also had to pay careful attention to not damaging rebar and PT cables while adding the connections to the 17th floor and attaching the stringers to the 15th and 16th floors (bolted to the PT girders). All reinforcement and PT cables were located by the use of X-ray and ground-penetrating radar (GPR) before drilling. While the structural integrity of the stair was important, so was its architectural design. A winding ribbon of structural steel that creates a finished backbone rendered in white is a stark contrast to the rawness of the steel that it threads together. All exposed steel was left to patina for several months in the field and was later rubbed with a protectant bee’s wax. Bee’s wax was selected to meet the WELL requirements for the space. The apparent hand of the craftsperson was also integral, and the materials used were critical in how the stair would invite users. In this case, the more ‘raw,’ the better. Bolted connections, welds, bends, and cuts became the narrative to the inherent beauty of material, in how they look, feel, and sound. The visible welds were lightly ground smooth to keep with the rawness. The stair railing surrounding the stairway was envisioned by the design team to mimic the shading of architectural sketches. This three-story monumental stair, suspended in the center of Little’s space, is sensible and understandable yet impressive in unexpected ways. Since physical activity is essential to our health, this feature, luring employees away from the elevator and encouraging movement, has been a key feature in supporting the environment’s pursuit of WELL Silver Certification.■ Dan Wray is Little’s Structural Engineering Studio Principal in Charlotte, NC, and an officer with the Charlotte chapter of the SEAoNC. (daniel.wray@littleonline.com) Bryan Starr works remotely for Little from Boise, Idaho, where he is an officer with SEAI. (bryan.starr@littleonline.com)

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INSIGHTS Automation of Construction Documents and Details By Charles Portelli AIA, RIBA, CDT and Nick Mundell

here is no shortage of articles that make claims about how our work can and will be automated in the future. With the recent surge of development happening on the topic of Machine Learning, these claims continue to grow. The reality is, our industry is in a constant state of evolution. When we look 20 years in the past, the early introduction of computers into the AEC space created a significant disruption that brought a level of efficiency by augmenting the traditional document delivery process. Analysis applications helped streamline laborious and time-consuming calculations engineers had been doing manually for decades, while CAD applications significantly increased the efficiency in creating and revising the 2-D documents that our industry heavily relies on. The introduction of the computer did not eliminate architecture or engineering as a profession but rather played an active role in its evolution, allowing us to design more beautiful and seemingly gravity-defying buildings. Digital processes have become widely adopted by the AEC industry and are currently the lifeblood of almost every firm. As the computer automated certain processes of the past, it created new opportunities ripe for exploration. Much automation is developed by homegrown solutions within forward-thinking firms, which focus on the development of in-house applications to assist in streamlining projects, allowing the firm to be more innovative in their engineering solutions. These custom solutions and design workflows help optimize project delivery.

30 Hudson Yards An example of automation is the work done on the recently completed tower, 30 Hudson Yards. Designed by Kohn Pedersen Fox, this building consists of a 73-story, 1,268-foot glass tower above reclaimed land spanning the west side rail yards of Manhattan. Thornton Tomasetti (TT) was commissioned for the engineering and detailing of the steel structure. From the early onset, all project stakeholders set an objective to deliver the project using BIM. The project documentation and models were delivered STRUCTURE magazine

via LOD400 to allow the models to be used for construction and fabrication of the steel structure. At the time, TTâ&#x20AC;&#x2122;s CORE studio developed a common data environment (CDE) called TTX, later rebranded as Konstru, which allowed engineers to move model data between analysis, documentation, and detailing applications seamlessly. Multiple applications were used to analyze the structural systems, from lateral to gravity; each system required its own method of analysis. The results of these models contained valuable information for the detailing and construction documentation process. Konstru integrated directly into the analysis applications, extracting the necessary data and porting it to the documentation applications natively. The data is serialized into a common format that is applicationindependent. Eliminating data silos meant the model data can now easily be supportive in other BIM platforms for further development of the project. The project team did not have to maintain many concurrent instances of models across the variety of applications being used on the project, which is an inefficient and time-consuming process, not to mention could be fraught with human error making coordination grueling. The Konstru platform assisted in keeping up with the rapid pace of project development. Member locations were constantly in motion as program spaces were being updated by the design team to maximize leasable area. Trying to make these updates manually would have made the model obsolete by the time all framing members were updated. The single model set, which contained the geometric information, as well as force data, saved countless hours in coordination. The tower model was exchanged weekly with project stakeholders to keep everybody up to date.

black-box scenarios and, second, by taking active steps in the evolution of traditional processes for project delivery. This led to the development of an application, AutoConnect. AutoConnect utilizes the Tekla application programming interface (API) and leverages force data embedded in the model elements, coupled with structural code and standards to auto-generate appropriately sized connection details. Traditionally, each connection detail would be modeled individually. AutoConnect reads attribute data built into each member to identify the forces exerted on the member. When this data is coupled with design criteria and rule sets, connections can be automatically generated in the 3-D model. To increase transparency into the process, safeguard measures were implemented to log all the generated connections so they may be reviewed in conjunction with the BIM data in the model. A Connection Manager was developed to look at the connections generated and allows engineers to plot out the different connection types to identify which connections can be standardized for ease of fabrication, reducing overall cost.

What a Future Project May Look Like The skills within firms are continually evolving. Designers and engineers are becoming more facile with developing bespoke solutions. Automation allows engineers to focus on creating elegant solutions. As an industry, we should aim to go beyond basic BIM and parametric models and aspire to positively impact the evolution of the AEC industry.â&#x2013; The online version of this article contains references. Please visit www.STRUCTUREmag.org.

One Vanderbilt On One Vanderbilt, engineers within the firm executed the project using the CDE, while CORE studio played a support role. The objective was to empower project engineersâ&#x20AC;&#x2122; level of automaton proficiency. This required a bit of a change in office culture, first gaining the trust of engineers to demystify any

Charles Portelli is the Senior Associate Application Developer at Thornton Tomasetti. (cportelli@thorntontomasetti.com) Nick Mundell is the Director of CORE Modeling at Thornton Tomasetti. (nmundell@thorntontomasetti.com)

D E C E M B E R 2 019

NCSEA

NCSEA News

National Council of Structural Engineers Associations

2019 SEA Grant Program Recipients

The NCSEA Grant Program was developed to assist Member Organizations (SEAs) in growing their Association and promoting the structural engineering profession, in accordance with the NCSEA Mission Statement: NCSEA advances the practice of structural engineering by representing and strengthening its Member Organizations. The 2019 Grant Recipients are: Structural Engineers Association of Central California (SEAOCC) to enhance SEAOCC’s new Structural Engineering, Engagement, and Equity (SE3) Committee Structural Engineers Association of San Diego (SEAOSD) to support the EERI San Diego-Tijuana Regional Earthquake Scenario Study and a Special Wind Region Study

2019 Excellence in Structural Engineering Award Winners The winners of NCSEA's 2019 Excellence in Structural Engineering Awards were announced at the Summit in Anaheim, CA last month. This program annually highlights some of the best examples of structural engineering ingenuity throughout the world. Turn to page 26 to learn about this year's winning projects!

Structural Engineers Association of Illinois (SEAOI) to host a Young Professionals Workshop Structural Engineers Association of Kansas/Missouri (SEAKM) to launch a SEAKM SE3 Committee Panel Discussion & Networking Event, and to assist with STEM classes for local elementary school students Structural Engineers Association of Massachusetts (SEAMass) to launch a SEAMASS SE3 Committee - Interactive Seminar Series Structural Engineers Association of New York (SEAoNY) for a screening of the documentary Leaning Out with panel Structural Engineers Association of Ohio (SEAoO) for a Young Members’ Track at SEAoO’s Annual Conference Oklahoma Structural Engineers Association (OSEA) to assist OSEA’s efforts in the Engineering Fair E-week 2020 Bridge Competition Structural Engineers Association of Texas (SEAoT) to support a local SE3 Speed Mentoring event Structural Engineers Association of Washington (SEAW) to assist with a Joint Oregon and Washington Special Regions Wind Study Congratulations to this year's recipients!

2018 Grant Program Recipient SEAOH Performs Student Outreach The Structural Engineers Association of Hawaii (SEAOH) was awarded $1,000 for Structural Engineering Student Outreach as part of the 2018 Grant Program. SEAOH used the funds to host a series of weekly sessions working with middle school students. These weekly sessions introduced students to bridge engineering concepts and guided them through the design and creation of their own foam bridges. At the completion of the program, they held a ceremony for final presentations and awards; at this ceremony each student took turns load testing their bridges and presenting their design concepts to a panel of judges. The bridges were scored on aesthetics, structural design, and design efficiency, and the students were scored on their presentation skills. The event was deemed a success by all involved! The feedback received was positive and students even stated that they walked away with a better understanding and an interest in the complexity of bridge design. 44 STRUCTURE magazine

News from the National Council of Structural Engineers Associations MO Public Outreach Challenge â&#x20AC;&#x201C; Finalists Announced This year, the NCSEA Communications Committee invited SEAs to participate in the very first Member Organization Public Outreach Challenge to inform and educate other industries, professions, and the general public about Structural Engineering! The purpose of this challenge was to help improve the visibility and recognition of practicing structural engineers with outreach through news articles, videos, blogs, and any other creative content to spread the message about our profession and its critical role in society.

Congratulations to the very first winners of the MO Public Challenge! 1st Place: Structural Engineers Association of California (SEAOC) 2nd Place: Structural Engineers Association of Illinois (SEAOI) 3rd Place: Structural Engineers Association of Oregon (SEAO)

Young Member Group of the Year Announced Each year, NCSEA awards the Young Member Group of the Year award at the Structural Engineering Summit. This award recognizes Young Member Groups that are providing a benefit to their young members, member organization, and communities. The 2019 Finalists were: Minnesota Structural Engineers Association, Structural Engineers Association of Georgia, Structural Engineers Association of Massachusetts, and Structural Engineers Association of Metro-Washington.

Congratulations to the 2019 YMG of the Year Structural Engineers Association of Minnesota

NCSEA Webinars

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Mass Timber Structural Floor and Roof Design Dr. Scott Breneman

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Efficient Design of Long-Span Composite Steel Deck-Slabs Vitaliy Degtyarev, Ph.D, P.E., S.E.

Long-span composite steel deck-slabs are a special type of composite slab, which allows for the design of buildings with large open spaces and requires special design consideration not typical for conventional composite steel deck-slabs. In this session, the ways of obtaining efficient designs of long-span composite slabs by providing supplemental reinforcing bars for extra strength, deflection control, and establishing fire resistance are presented. Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 states. D E C E M B E R 2 019

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SEI Local Leaders Conference

50+ local SEI Chapter and Young Professional Leaders participated in a Leadership Facilitation Skills Bootcamp October 25-26, at ASCE in Reston, VA. The extended program was made possible with support from Computers & Structures, Inc., and the SEI Futures Fund. #SEILocalLeaders19

Iconic Global Structures Conference in Dubai “Attending the Iconic Global Structures Conference in Dubai was a great experience. It was interesting to learn about how structures – from the Singapore Sports Hub stadium to the London Eye to new super tall multi-use buildings – compared to their cohorts in other parts of the world. A major conversation topic was the methodology of building beyond the code – as most of these structures pushed the boundaries of structural engineering. It was really cool to hear how these engineers were able to take core principles and manipulate them to achieve safety and efficiency. It highlighted the dynamism of structural engineering SEI/ASCE leaders at Burj Khalifa and made me all the more excited to join the field!” Genevieve Graham, S.M.ASCE Read more at www.asce.org/SEINews.

Membership

Thank you to 2019 SEI Sustaining Organization Members

Elite Level: Alfred Benesch & Company • Hayward Baker, Inc. Basic Level: Boswell Engineering • Geopier Foundations • Hardesty & Hanover • International Code Council Schnabel Foundation Company • Simpson Gumpertz & Heger, Inc. • Simpson Strong-Tie • Walter P Moore Join SEI as a Sustaining Organization Member to reach more than 30,000 SEI members year-round, and show your support for SEI to advance and serve the structural engineering profession. www.asce.org/SEI

SEI Online

Order Your Bridges 2020 Calendar Today $13.95 | 2 for $20 + shipping/handling www.asce.org/Product.aspx?isbn=9780784415375

SEI on Twitter

Errata 46 STRUCTURE magazine

SEI on Facebook SEI Standards Follow us: @SEIofASCE

Visit www.asce.org/SEIStandards to: View ASCE 7 development cycle

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata. If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.

News of the Structural Engineering Institute of ASCE Students/Young Professionals

International Research – Applications Now Open International Research Experiences in Civil, Construction, and Environmental Engineering through ASU in collaboration with ASCE and NSF. Application open for Spring 2020 semester abroad. https://ireccee.engineering.asu.edu

Congratulations to Recipients of the 2019 O.H. Ammann Research Fellowship in Structural Engineering Sina Basereh, S.M.ASCE – University of Buffalo Ran Cao, S.M.ASCE – The City College of New York Fei Ding, P.E., A.M.ASCE – University of Notre Dame

Mirela Tumbeva, S.M.ASCE – University of Notre Dame Christopher Michael Zaverdas, S.M.ASCE – Rensselaer Polytechnic Institute

Advancing the Profession

SEI President Glenn Bell on Future Vision for Structural Engineering View his address from October 22 at Northeastern University www.youtube.com/watch?v=SRRX4V3j-bE.

THANK YOU

Thank you to SEI Futures Fund Donors who gave through the Ashraf Habibullah/ Computers & Structures Inc. 4:1 Challenge Match this summer! The total amount raised during the challenge through August was $21,875 from 71 donors. Through your combined generosity, more than $60,000 will go to SEI strategic initiatives investing in the future of SE, student and young professional involvement, and professional development. Learn more and give www.asce.org/SEIFuturesFund.

Structural Standards Coordination Council (SSCC) By J. Greg Soules, P.E., F.SEI, F.ASCE, Chair, Structural Standards Coordination Council

There are a lot of moving parts in the development of SEI standards. One of these moving parts is the coordination of SEI standards with codes and standards produced outside of SEI. The Structural Standards Coordination Council (SSCC) is an SEI board-level committee established to do just that. The mission of the SSCC is to provide an organized mechanism for planning and coordinating the development schedules of structural standards developed and maintained by U.S. standards development organizations (SDO) for the benefit of public safety, health, and welfare, as well as for the benefit of structural engineering practice. The SSCC has several overall goals: 1) Promote the code adoption of SSCC member standards 2) Improve communication between SSCC member organizations. 3) Promote coordinated development schedules among SSCC member organizations. 4) Provide a forum for coordinating technical content in SSCC member standards. 5) Provide a forum for enhancing the usability of SSCC member standards. 6) Provide a forum for discussing issues of common concern. 7) Provide a forum for longer-term discussions and coordination for structural standards code development. 8) Provide a forum to discuss educational outreach for standards.

9) Provide a platform for coordination with other SDOs and code bodies in areas of mutual concern. The following organizations are SSCC members: • ASCE/SEI • ACI (American Concrete Institute) • AISC (American Institute of Steel Construction) • AISI (American Iron and Steel Institute) • AWC (American Wood Council) • The Masonry Society (TMS) • NCSEA (National Council of Structural Engineers Associations) With regards to the publication of ASCE/SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, the overall goal is for ASCE 7 requirements to be coordinated with the material standards published by the member organizations shown above and to have ASCE 7 and these material standards adopted into the IBC. On occasion, the ICC changes its process and timetable for the submission of standards to be adopted into the IBC. When these changes occur, the members of the SSCC must work together to produce coordinated standards. Sometimes, the revised ICC schedule cannot be met. When this happens, some of the material standards will not be fully aligned with ASCE/SEI 7 for up to 3 years. For the good of the structural engineering profession, the SSCC tries to prevent this problem from happening. D E C E M B E R 2 019

CASE in Point Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has recently updated its Contract Library, and they have re-issued updated Contracts that have been reviewed by outside legal counsels. Below is a handy guide for firms to know which contract is appropriate to use in certain situations. You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

CASE Contracts – Usage Guide Structural Engineer is Retained

Structural Engineer is Retaining Additional Entity

CASE Agreement #1 – An Agreement for the Provision of Limited Professional Services. This agreement is intended for use for small projects or investigations of limited scope and time duration.

CASE Agreement #9 – An Agreement Between Structural Engineer of Record and Design Professional for Services. This agreement is intended for use when the Structural Engineer of Record, when serving in the role of Prime Design Professional or as a Consultant, retains the services of a sub-consultant or architect.

CASE Agreement #2 – An Agreement Between Client and Structural Engineer of Record for Professional Services. This agreement is intended for use when the client, e.g., owner, contractor developer, etc., wishes to retain the Structural Engineer of Record directly. This agreement may also be used with a client who is an architect when the architect-owner agreement is not an AIA agreement.

CASE Agreement #10 – An Agreement Between Structural Engineer of Record and Geotechnical Engineer of Record. This agreement is intended for use when the Structural Engineer of Record retains geotechnical engineering services. It can also be altered for use as an agreement between an Owner and the Geotechnical Engineer of Record.

CASE Agreement #3 – An Agreement between Owner and Structural Engineer as Prime Design Professional. This agreement is intended for use when the Structural Engineer serves as the Prime Design Professional.

CASE Agreement #11 – An Agreement Between Structural Engineer of Record and Testing Laboratory. This document is intended for use when the Structural Engineer retains testing services.

CASE Agreement #4 – An Agreement between Client and Structural Engineer for Special Inspection Services. This agreement is intended for use when the Structural Engineer is hired directly by the Owner to provide Special Inspection services.

Other Situations

CASE Agreement #5 – An Agreement Between Client and Specialty Structural Engineer for Professional Services. This agreement is intended for use when the structural engineer is hired directly by a contractor or sub-contractor for work to be included in a project where you are not the Structural Engineer of Record. CASE Agreement #6 – An Agreement Between Client and Structural Engineer for a Structural Condition Assessment. This agreement is intended for use when providing a structural condition assessment. CASE Agreement #7 – An Agreement for Structural Peer Review Services. This agreement is intended for use when performing a peer review for an Owner or another entity and includes responsibilities and limitations. CASE Agreement #8 – An Agreement Between Client and Structural Engineer for Forensic Engineering (Expert) Services. This agreement is intended for use when the engineer is engaged as a forensic expert, primarily when the Structural Engineer is engaged as an expert in the resolution of construction disputes. It can be adapted to other circumstances where the Structural Engineer is a qualified expert.

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CASE Agreement #12 – An Agreement Between Structural Engineer of Record (SER) And Contractor for Transfer of Digital Data (Computer Aided Drafting (CAD) or Building Information Model (BIM)) Files. This agreement is intended for use when transferring CAD or BIM files to others. CASE Commentary #A – Agreement for Use with and Commentary on AIA Document C401 “Standard Form of Agreement Between Architect and Consultant,” 2017 Edition. This document is intended for use as a letterform of an agreement that adopts the AIA C401 by reference. This Agreement is intended for use when the ownerarchitect agreement is an AIA B-series. A scope of services is included. The purpose of the commentary is to point out provisions that merit special attention. CASE Commentary #B – Commentary on AIA Document A295 – 2008 “General Conditions of the Contract for Integrated Project Delivery,” 2008 Edition. This document provides commentary on AIA Document A295 Integrated Project Delivery. CASE Commentary #C – Commentary on AIA Document A201 “General Conditions of the Contract for Construction,” 2017 Edition. This document provides Commentary on AIA document A201-2017 sections that merit special attention.

News of the Council of American Structural Engineers Donate to the CASE Scholarship Fund! The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at minimum, in an ABET-accredited engineering program. Since 2009, the CASE Scholarship program has given $32,000 to help engineering students pave their way to a bright future in structural engineering. We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates. One way to enhance the ability of students to pursue their dreams to become professional engineers is to offer incentives in educational support. Your monetary support is vital in helping CASE and ACEC increase scholarships to those students who are the future of our industry. All donations toward the program may be eligible for a tax deduction, and you don’t have to be an ACEC member to donate! Contact Heather Talbert at htalbert@acec.org to donate.

CASE Winter Member Meeting February 27-28, 2020

The 2020 CASE Winter Member Meeting is scheduled for February 27-28, 2020, in New Orleans. The agenda for the meeting includes:

Thursday – February 27 1:30 pm to 5:30 pm CASE ExCom Meeting 6:15 pm to 7:30 pm CASE Project Speaker

Friday – February 28 7:30 am to 8:30 am Shared Breakfast 8:30 am to 10:00 pm CASE Roundtable – Stacy Bartoletti, Moderator 10:00 am to 10:30 am Shared Morning Break 10:30 am to 12:00 pm Technology Panel Discussion – Kevin Peterson, Moderator 12:00 pm to 1:15 pm Shared Lunch 1:30 pm to 5:30 pm CASE Breakout Sessions

Registration can be found at www.acec.org/coalitions/upcoming-coalition-events. Questions? Contact Heather Talbert at htalbert@acec.org.

Manual for New Consulting Engineers An HR Favorite for New Hires

ACEC’s best-seller, “Can I Borrow Your Watch?” A Beginner’s Guide to Succeeding in a Professional Consulting Organization offers new engineers a head start in the business of professional consulting. This essential guide is tailored to the unique needs of engineering firms, and the skills and experiences rookie consultants need to be successful in a large organization, including: • Proposal Preparation • Project Management

• Financial Management • Client Relationships • Staff Management

With over 140 pages of consulting expertise, this resource is the perfect addition to any new staffer’s welcome pack or in-house orientation. It can even be a useful resource for more seasoned engineers looking to refine their skills. To order this book, go to www.acec.org/bookstore. Bulk ordering is available; for more information, contact Maureen Brown (mbrown@acec.org).

Follow ACEC Coalitions on Twitter – @ACECCoalitions. D E C E M B E R 2 019

CASE business practices A Check-Up of Your Firm’s Quality Assurance Plan By Jeff Morrison

any times, the phrases quality assurance and quality control are used interchangeably. Quality assurance (QA) is process-focused, where the processes are put in place to ensure the correct steps are performed. Quality control (QC) is used to verify that deliverables are of acceptable quality and that they are complete and correct. As this relates to structural engineering, one may think of QA as the process we go through during our design and construction document production phases and QC as the review exercise at the end of a project to determine how well the process worked in producing a quality set of construction documents that met the needs of the client, owner, and contractor. It is critical to make sure that all staff understand the important goals of this process, such as increasing the quality of the firm’s work product, decreasing liability, and serving as a learning opportunity. The process should be approached in a context of teamwork and camaraderie; this can go a long way in building a culture of quality and obtaining buy-in from all staff. The goal of the process is not to be burdensome or add another task but rather, in the long term, to increase efficiency and learning.

Recommendations Have a kick-off meeting with the entire project team to get the project off on the right foot in the early Concept/Schematic Design phase. Involve all staff working on the project, including engineers, technicians, and the QA reviewer. At this meeting, share the project background information, client, consultants, scope, and schedule. Review the main elements of the project such as gravity framing systems, lateral-load-resisting systems, foundation system, basic wall sections, serviceability requirements, and any special requirements or unique details that can be identified. This can also serve as a mini design charette to brainstorm ideas and start thinking about some of the more challenging aspects of a project. Review with the technical staff the organization of drawing sheets, building information modeling (BIM) model, as well as BIM execution plan requirements and BIM level of development requirements. After completion of the Design Development phase, a formal QA review should be performed. This is an excellent opportunity to 50 STRUCTURE magazine

review all major structural elements, design criteria, typical wall sections, and details. This check is important to make sure all primary elements are properly accounted for in the most efficient, economical, and constructible way early in the project. It will also serve as a check to verify if the assumptions and decisions made at the project kick-off are still accurate or if any adjustments need to be made. A thorough QA review should be performed near the end of the Construction Document phase; additional reviews at appropriate milestones during this phase should also be performed for larger or more complex projects. These check-in reviews can serve to make sure the project stays on track throughout a lengthy design process. At this point, the review should be focused on the details of the project and coordination with architectural and other consultants. Dimensional and detail coordination is essential at this stage. At this point, an engineering design and construction documents checklist can be used as a valuable tool.

Challenges Several factors can make the QA process for structural engineering firms challenging. Our design process and each project are typically different and unique. Whether it be a fast track schedule, architectural challenges, existing building or site conditions, or complex owner requirements, all of these items make the development and implementation of a quality assurance plan that applies to all project sizes and types a challenge. • Time – We are all busy, and making time for additional review during the design process can be a challenge. However, the QA process needs to be looked at as one that can be a great tool to increase efficiency, improve quality, provide excellent learning opportunities, reduce construction phase issues, and decrease liability exposure. • Scalable – The plan should be nimble and scalable to work for the smallest to largest project. The smallest project may only require a brief review of the final documents at the end of the project. The largest and most complex projects will likely require multiple check-ins during each phase to verify that the project team is staying on track and on schedule.

Resources CASE has several documents and tools available to assist with the QA process. These include: CASE 962-D – A Guideline Addressing Coordination and Completeness of Structural Construction Documents Tool 1-2 – Developing a Culture of Quality Tool 9-1 – A guideline Addressing Coordination and Completeness of Structural Construction Documents (includes a drawing review checklist) Tool 9-2 – Quality Assurance Plan • Accountability – What will be the means to track accountability to make sure the process is being followed? This needs to be a consistent emphasis to all staff during regular staff meetings. Adequate time needs to be built into the production schedule to allow for both the QA review, and analysis and response to the comments. This will build a culture of quality, and, over time, it will become an ingrained part of the project planning process.

Questions to Consider A few questions to consider in the review of your firm’s QA plan: • Who will perform the QA reviews, and how will this be assigned for each project? • What projects require a QA review, and how will this be determined for each project? • When will the QA reviews occur? • Why are we doing this? Buy-in from all staff and understanding the importance of this process is critical to making sure it happens and that the process is given the care and attention it deserves. • How will this fit in our work process schedule?■ Jeff Morrison is Vice President and Senior Project Engineer at Lynch Mykins in Raleigh, NC. He also serves on the CASE Toolkit Committee. (jmorrison@lynchmykins.com)

D E C E M B E R 2 019

ACI 212.3R-10

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Report on Chemical Admixtures for Concrete

Reported by ACI Committee 212

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