STRUCTURE magazine - December 2018

A Joint Publication of NCSEA | CASE | SEI

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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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December 2018

Evans Mountzouris, P.E. The DiSalvo Engineering Group, Ridgefield, CT Greg Schindler, P.E., S.E. Sammamish, WA Stephen P. Schneider, Ph.D., P.E., S.E. BergerABAM, Vancouver, WA John “Buddy” Showalter, P.E. American Wood Council, Leesburg, VA

STRUCTURE® magazine (ISSN 1536 4283) is published monthly by The National Council of Structural Engineers Associations (a nonprofit Association), 645 N. Michigan Ave, Suite 540, Chicago, IL 60611 312.649.4600. Application to Mail at Periodicals Postage Prices is Pending at Chicago, IL and additional mailing offices. STRUCTURE magazine, Volume 25, Number 12, C 2018 by The National Council of Structural Engineers Associations, all rights reserved. Subscription services, back issues and subscription information tel: 312-649-4600, or write to STRUCTURE magazine Circulation, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611. The publication is distributed to members of The National Council of Structural Engineers Associations through a resolution to its bylaws, and to members of CASE and SEI paid by each organization as nominal price subscription for its members as a benefit of their membership. Yearly Subscription in USA $75; $40 For Students; Canada $90; $60 for Canadian Students; Foreign $135, $90 for foreign students. Editorial Office: Send editorial mail to: STRUCTURE magazine, Attn: Editorial, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611. POSTMASTER: Send Address changes to STRUCTURE magazine, 645 N. Michigan Avenue, Suite 540, Chicago, IL 60611.

STRUCTURE is a registered trademark of the National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

CONTENTS

Features 19 A BRIDGE IN THE FUTURE

Columns and Departments EDITORIAL

7 SEI Futures Fund

By Glenn R. Bell, P.E., S.E., SECB

By Bob Niccoli, P.E. S.E., Sean-Philip H. Bolduc, P.E., and Peter Chou, P.E. Unique challenges for the Town Center Parkway Rail Support Structure led to the development of an unconventional solution, a structure that later would be unearthed to become a bridge. The structure will begin its life as a slab on grade but will become a two-span rigid frame bridge at an undefined time in the future.

INFOCUS

8 The Northridge Earthquake

By John Dal Pino, S.E. STRUCTURAL PERFORMANCE

10 Geoseismic Design Challenges in Mexico City

By Sissy Nikolaou, Ph.D., P.E., George Gazetas, Ph.D., Evangelia Garini, Ph.D., Guillermo Diaz-Fanas, P.E., and Olga-Joan Ktenidou, Ph.D. STRUCTURAL FORENSICS

14 Structural Engineers in Fire Investigations

By Dan Eschenasy, P.E., SECB

22 EXCELLENCE IN STRUCTURAL ENGINEERING AWARDS

ENGINEER’S NOTEBOOK

28 Design of Tall Walls in Wood Structures

By John “Buddy” Showalter, P.E., Bradford Douglas, P.E.,

The National Council of Structural Engineers Associations

and David Low, P.E.

(NCSEA) announced the winners of the 2018 Excellence in Structural Engineering Awards in October. Read highlights on

STRUCTURAL DESIGN

31 Variable Modulus of Subgrade Reaction – Part 2

each of the award-winning projects in this issue.

By Apurba Tribedi CASE BUSINESS PRACTICES

34 Developing the Next Generation of Structural Engineers

By Michael A. Stubbs, P.E., S.E., and David V. Jáuregui, Ph.D., P.E. ON THE COVER The Lincoln Avenue Pedestrian Bridge, Lone Tree, CO, was an Award winning project in the New Bridges or Transportation Structures category of this year’s Excellence in Structural Engineering Awards program.

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.

STRUCTURE magazine

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December 2018

IN EVERY ISSUE 4 Advertiser Index 37 Resource Guide – Earth Retention 38 NCSEA News 40 SEI Update 42 CASE in Point

Editorial SEI Futures Fund Pursuing Our Vision for Structural Engineering

By Glenn R. Bell, P.E., S.E., SECB, F.SEI, F.ASCE, 2019 Chair, SEI Futures Fund Board of Directors

I

n 2013, the Structural Engineering Institute published A Vision for Examples of activities that SEIFF has supported in the last few the Future of Structural Engineering and Structural Engineers: A case years include: for change (the SEI Vision – www.asce.org/SEI). The SEI Vision lays • Scholarships for students and young professionals to attend out an inspiring view of what the structural engineering profession Structures Congress could be by the year 2033, and it • Support to launch the new SEI makes a number of recommendaGlobal Activities Division tions for SEI Board of Governors’ • Beta testing to live stream sesaction to lead us to that vision. sions at Structures Congress The recommendations include • Creation of an SEI Global bold initiatives involving (1) Practices Guideline document education for innovation and (to be published early 2019) leadership and (2) professional • Research in support of SE practice for innovation and leadlicensure ership. Recognizing the massive • Support for ASCE Continuing need for professional volunEducation Webinars for SEI teerism and financial resources Chapters and SEI Graduate necessary to bring about the SEI Student Chapters Vision, SEI established, in 2013, 2018 SEI Futures Fund Student and Young Professional Scholarship Recipients. • Stakeholder workshops for the SEI Futures Fund (SEIFF).

Continuing Education Every dollar contributed to the SEI Futures Fund goes directly to Initiatives that the SEIFF Board has funded for 2019 include more profession-building initiatives identified and approved by the SEIFF than $100,000 for: Board. The SEI Futures Fund operates in collaboration with the ASCE • Investigating the Future of SE Licensure: This will provide a comFoundation for administrative and professional fundraising expertise. prehensive view of SE Licensure and the various options for a The goals of the SEI Futures Fund are unique: to advance the art, path to 2033. science, and practice of structural engineering for a brighter future • SEI Standards lecture for SEI Chapters: This initiative seeks a for our profession. It does this by funding strategic initiatives outside win-win by increasing knowledge of SEI standards and providthe normal bounds of the SEI operating budget. It does not support ing resources for increasing attendance at SEI Chapter meetings. scientific research. The four strategic priorities for funding are to: • SE 2050 Sustainability Commitment Initiative Workshop: This • Invest in the future of the profession, will support a planning workshop for the SE 2050 Initiative • Promote student interest in structural engineering, for structural engineers to meet embodied carbon benchmarks.

• Support younger members involvement in SEI, and • SEI Codes & Standards Young Professional Program: This is an • Provide opportunities for professional development. extension of a highly effective initiative to involve young profesFundraising is driven primarily by the Futures Fund Board members. sionals in various SEI codes and standards activities.

Donors may be individuals, companies, or organizations. The Board’s • SEI Student & Young Professional Scholarships to Structures Congress: strategy for individuals has been top-down, ensuring we have the supThese scholarships are intended to draw more students and young port and commitment of the SEI Board, the SEIFF Board, and the professionals into SEI activities by supporting scholarships to many SEI committees, chapters, and members. Progress along this attend Structures Congress. route has been excellent. Presently, about 65% of all donations come • SEI Local Leadership Conference Facilitation Training: This from individuals, 20% from corporations, and 15% from ticket sales follows the SEI Vision goal of leadership development for at the annual gala held at Structures Congress (more on this below.) structural engineers.

However, we have far more to go. Only a small fraction of SEI’s more So, how can you help? First, if you are not already a regular donor, than 30,000 members contribute. Imagine what we could do for please consider becoming one. This is our future at stake! If you are the profession if everyone was engaged! a donor and are comfortable recruiting others, let us know. We need A tremendous supporter of SEI and the Futures Fund (and the to leverage our voice. And, finally, if you have an idea for a proposed structural engineering profession in general) has been Ashraf SEI Futures Fund activity to advance the profession, let us know, Habibullah, founder and CEO of Computers and Structures Inc. regardless of whether you or someone you know are interested in Since 2016, Ashraf has hosted a gala at Structures Congress cel- advancing the idea. ebrating the structural engineering profession and promoting its This year’s SEI Futures Fund Board is Ed DePaola, Anne Ellis, Jon future. The pinnacle social event at Congress, it is always great fun Magnusson, John Tawresey, and myself. We would be pleased and profoundly inspiring. We invite you to join us next April at to talk with you about getting involved.

Structures Congress in Orlando.

Learn more and give at www.asce.org/SEIFuturesFund.■ STRUCTURE magazine

7

December 2018

InFocus The Northridge Earthquake 25 Years On

By John A. Dal Pino, S.E.

I

n the early morning of January 17, 1994, the ground shook hard in the northern San Fernando Valley area of Los Angeles.

The structural engineering profession was shaken hard too. That was 25 years ago, and how have things changed!

The death toll was thankfully small (less than 75 people). Monetary damage, however, was considerable (estimated at between $15 and $40 billion). However, Los Angeles is a large place; after the earthquake, if you had just dropped into town and had not heard the news on the television or radio, you could have driven around much of the city and outlying areas and not have noticed that an earthquake had occurred. But, if you had a keen eye as any good structural engineer does, you would have noticed the telltale signs of an earthquake here and there (the usual toppled CMU walls, broken glass, brick parapets lying on the sidewalks, etc.). But you would have also stumbled upon a few severely damaged buildings nestled amongst otherwise unscathed structures. Occasionally you would have discovered a building tightly wrapped in yellow caution tape showing no apparent damage and asked yourself, “Hmm, what happened there?”

Looking back on the Northridge earthquake today, it was a truly once-in-a-lifetime event and, taken as a whole, the damage, both observable and hidden, changed the course of structural engineering. It changed the course of structural engineering for the second time, but more on that later. The earthquake occurred in a relatively young, western city full of mostly modern buildings. This made it the perfect laboratory for structural engineers to learn, albeit at society’s expense. Earthquakes occur all over the world every year but, for the most part, few yield data applicable to U.S. practice because the buildings and construction techniques are too different. Now, twenty-five years later, it is important to reflect on what happened in Northridge, what the profession learned, and how life changed for a large segment of society, not just structural engineers.

Before Northridge, there was another Los Angeles earthquake that also “changed

engineering,” the 1971 San Fernando earthquake, located roughly in the same area as Northridge. For many engineers, this earthquake produced the first visual evidence of what a damaged but life-safe building looked like. The resulting damage led to significant changes in the building code (1976 UBC vs. the 1973 UBC) as engineers came to appreciate that design base shear levels were too low and detailing provisions, particularly for concrete tilt-up warehouses and concrete frames (buildings and highway structures), needed significant improvements. The earthquake also led to the creation of California’s Office of Statewide Health Planning and Development (OSPHD) as a result of the near-collapse of the new Olive View Hospital in Sylmar.

Back to the Northridge earthquake. The most noticeable and significant damage consisted of wood apartment buildings “pancaked” onto cars, massive highway structures collapsed, toppled parking garages, and non-ductile concrete frames cracked and sometimes collapsed. The occasional heavily damaged building in a largely undamaged area signaled ground motion focusing in the bowl that is the San Fernando Valley.

Yellow tape was drawn around buildings that sustained some unique damage, unexpected by a vast number of engineer practitioners.

Behind the tape, stiff diagonal braces had torn apart like aluminum beer cans, welded steel moment resisting connections had cracked even in lightweight buildings with wood floors, the roofs of tilt-up buildings had collapsed (again), and one hospital designed to high seismic standards was damaged and out of commission.

In retrospect, it appeared that the profession was caught largely unaware by the 1994 Northridge earthquake and did not expect the kinds of damage that occurred, particularly in the newer buildings, since the code had been modified significantly just 20 years before, along with other improvements in the intervening years. Since the types of damage were generally unexpected, the structural engineering profession was again shaken from its status quo, and things changed again. The pressures of schedule and fee just do not allow

STRUCTURE magazine

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December 2018

engineers too much time for theorizing about how every aspect of a building will perform in an extreme event. The code also provided a life-safety banner that could be used as a protective shield.

Historians argue that, rather than smooth transitions, it is unexpected jolts to the system that change society. So it goes with earthquakes too. As the details about the damage emerged, it became apparent that many aspects of the building code were based on other than historical experience and test data, and that buildings designed to minimum code levels experienced a lot more damage than society was willing to accept. The results of a few tests on a small number of prototypes had been extrapolated to permit construction of large buildings that bore little resemblance to the original concept.

A limited set of data had become codified, and buildings were built with little knowledge about their probable performance. To be fair, there are always a few visionaries who anticipate everything and knew this was going to happen someday. The author has had the pleasure to work for and with many such people but, in general, these are rare individuals.

For the 25th anniversary year of the Northridge Earthquake, STRUCTURE will publish a series of articles on what was learned from the Northridge Earthquake and how it changed structural engineering. The authors were carefully chosen from the most knowledgeable people in our profession. The articles will describe how the profession designed buildings before and how things changed.

For younger engineers, the articles can serve as a history lesson and, for older engineers, a way to reminisce about the past and hopefully start a conversation with their staffs about engineering into the future.

Please share your thoughts about these articles with us as the year goes on.■ John A. Dal Pino is a Principal with FTF Engineering located in San Francisco, California. He serves as a member of the STRUCTURE Editorial Board. (jdalpino@ftfengineering.com)

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exico City’s geoseismic design challenges arise from its unique local geology combined with high tectonic activity. This article discusses the challenges that continue to grow, as the center of the ruins of the Aztec capital has turned into the most prominent modern metropolis of Latin America, with economic growth that demands taller buildings. The response of Figure 1. Evolution of the lake system at Mexico City Valley the regional soils has repeatedly and (Ovando-Shelley et al., 2013). destructively materialized in the form of soil basin amplification phenomena (one- and original water area of 700 km2 (270 mi2) except two-dimensional) that manifested as seismic waves for a small lake near Xochimilco. propagating through the natural valley’s topograNew human settlements between the 16th and phy. This topography is filled with 18th centuries by the conquering Spaniards caused soft, high-plasticity elastic clays the biggest impacts on the built environment due that were a part of the drained to further deforestation, agriculture, pasture lands, lake that once existed below the construction, and land reclamation. The Spaniards City. A vivid recent reminder was tried to control the water by replacing the Aztec the 2017 Mw7.1 Puebla-Morelos system of dikes and canals with streets and squares, Earthquake, a déjà-vu precisely on draining the lakes and removing forestland. These the 32nd anniversary of the September 19th, 1985, actions resulted in severe floods, some of which Part 1: A 32-year Déjà-vu Michoacán Ms8.0 Earthquake, with an epicenter drowned the city for months and even years on end.

400 kilometers (km) (~250 miles) away from the Moreover, the Valley of Mexico is mainly

By Sissy Nikolaou, Ph.D., P.E., DGE, city center. The 1985 toll of more than 39,000 formed by volcanic materials, while the surface George Gazetas, Ph.D., deaths and nearly 10,000 building collapses was layers consist of alluvial deposits, mostly lacusEvangelia Garini, Ph.D., attributed to a multi-resonance of seismic incident trine clays. A large part of Mexico City is built Guillermo Diaz-Fanas, P.E., waves, soil deposits, and structures. In 2017, the on top of highly plastic soft clay sediments interand Olga-Joan Ktenidou, Ph.D. soil amplification repeated from a different type layered with thin silt and sand layers. The clayey of earthquake that was 120 km (~78 mi) away, deposits of Mexico City are fairly unique, conwith only slightly different recorded motions than taining volcanic ash, and are characterized by an those in 1985. This article provides an evalua- unusually high plasticity index (PI ≈ 200-300), a tion of selected recorded motions within the City natural water content (wn ≈ 200 - 600), and low that correlates with observed differences in the shear wave velocity, Vs, within the 40 to 90 m/s level and distribution of damage between the two (~130 - 300 ft/s) range. This saturated deposit events that occurred 32 years apart. The evaluation is extremely compressible and has caused settleis a combined result of experiences and studies ments of 9 m (~30 ft) since the beginning of the performed over more than 2 decades by the co- 20th century (Tapia et al., 2000) which, in turn, authors – designers at WSP and researchers at have induced damaging differential settlements in NTUA, and the recent findings many monuments, structures, and infrastructure by the ATC reconnaissance team built in the former lake area (Figure 2). that traveled to Mexico City, In the past few decades, the City has grown from which is gratefully recognized about 78 km2 (30 mi2) to a metropolis about 100 for contributions to this article. times larger, as shown in Figure 3. The lakebed This is Part One of two articles continues to sink by up to 30 cm/year (~12 in/ that focus on the geotechnical year), as groundwater is extracted to support its and structural design challenges more than 20 million inhabitants. Development of Mexico City. of tall buildings with deeper basements over the past two decades has increased exponentially to accommodate the rapid growth.

Geology and The problematic soil conditions, continuous Tectonics water extraction, and frequent strong seismic Mexico City is mostly built on a activity have presented unique challenges in basin formerly occupied by the the design and construction of Mexico City. ancient Lake Texcoco that has The Spanish settlers identified these difficulties evolved since the Aztec arrival since the mid-16th century after observing their in the 13th century, as shown buildings sinking. In 1550, Cervantes de Salazar, in Figure 1. Currently, the lake author and rector in Mexico, concerned about Figure 2. Continuing sinking of the Mexico City lake bed causing visible differential settlements in buildings. is completely drained from its the continuous settlements and seismic activity,

PERFORMANCE

Geoseismic Design Challenges in Mexico City

STRUCTURE magazine

10 December 2018

Figure 3. Mexico’s Colonia Federal harmonic and radiant geometry from above. Courtesy of Digital Globe, 2015.

introduced a height limit of two stories (Cervantes de Salazar, 1978). Per Rosenblueth & Ovando (1991), this was probably the first rule for earthquake-resistant design in the Americas.

Currently, a Performance-Based Design (PBD) approach has been followed in the seismic design of new tall buildings. PBD also presents challenges, many of which are a result of the behavior of the unique soft soils that seem to have similar behavior over past strong earthquakes.

A 32-year Earthquake Déjà-vu A vivid and destructive manifestation of such amplification was observed in Mexico City during both the 1985 Ms8.0 Michoacán and 2017 Mw7.1 Puebla-Morelos earthquakes.

The 1985 event occurred in the subduction zone offshore of western Mexico, nearly 400 km (~250 mi) from the city. Yet, it caused enormous damage with tens of thousands of deaths mostly concentrated in the northwestern part of the Mexico City lake region.

It became abundantly clear that the soft clay layers were the main culprit of the disaster, having amplified the ±2 seconds period of the motion of the incoming waves. As shown in Figure 4, the 2017 event occurred as the result of normal faulting in Central Mexico at a depth of approximately 50 km (~30 mi), approximately 120 km (75 mi) away from Mexico City. The damage was significant in the City, with 40 building collapses and at least 400 deaths, but lower than those in 1985 thanks to mainly the improved local seismic code. Although the two events had different seismological characteristics of magnitude, distance from the epicenter, and orientation, they generated quite similar soil amplification effects.

Figure 4. Mechanisms of 1985 Michoacán and 2017 Puebla-Morelos earthquakes (USGS, 2017).

The incident waves that constitute the seismic motion, propagating from the bedrock through the soil, undergo changes in both their amplitude and frequency characteristics.

Since the amplitude usually increases, the term “soil amplification” is used to describe the phenomenon, as discussed in a previous article (STRUCTURE, Nikolaou, February 2008). The effects of soil amplification on a structure depend on its location with respect to the lake area, as shown in Figure 5 (Nikolaou et al., 2018), that is largely described by the three zones defined in the Mexican Building Code (OGDF, 2017): Hilly (I), Transition (II), and Lake (III). More specifically, a typical geotechnical profile within the lakebed varies in thickness H and, as shown in Figure 6, consists of an upper fill deposit, a regional soft plastic clay, and dense layers of silty sand and dense clay that reach a stiff deposit referred to as “bedrock.”

However, soil amplification in the lakebed is controlled primarily by the thickness of the upper soft clay that varies within the lakebed, as shown in Figure 5. Past events have documented striking soil amplification effects (Romo & Auvinet, 1992) attributed to the behavior of the soft clay deposit and particularly its high plasticity, PI, that controls the non-linear effects in soils. Vucetic & Dobry (1991) showed that the Mexico City clay behaves practically within the linear elastic Zone I

Zone II

range even for high shear strain amplitudes, γ, on the order of 10-3. This is accompanied by low damping, which means that the straining of the soil does not absorb the seismic energy and that it keeps being amplified as the waves propagate upwards within this layer.

Taking advantage of this unique characteristic offers the opportunity to understand the cause and the selective spatial distribution of damage by examining the soil amplification using simplified engineering approximations.

Mexico City profiles may be generalized simply as a soil column of soft clay with Vs = 80 m/s (260 ft/s) and PI = 200 overlying the assumed bedrock at a depth H from the ground surface. Extensive analytical studies have validated this simplified approach, by Gazetas (1988) and the ATC reconnaissance geotechnical study for the Puebla-Morelos earthquake (Gilsanz & Nikolaou, 2018). The fundamental period of vibration for this soil column, Ts, equals 4 H / Vs. Using contour maps of the estimated Mexico City Ts offered in the local building code from micro-zonation (Ordaz & Perez-Rocha, 1992), engineers can estimate H and the primary vibration characteristics of the soil.

For example, the SCT site (Figure 5) is within the area of concentrated severe damage in 1985 and can be used to compare recordings and analytical predictions from the 2017 earthquake. The site has an H of 40 m (~130 ft) Zone III

Figure 5. East-West section and code-specified seismic zones: Hilly (I), Transition (II), and Lake (III).

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Figure 7. Concentrated damage in buildings highlighted in red in the 1985 disaster, due strong motions affected by soil resonance at T = 2 seconds and rich excitation between 1.5 and 2 seconds. Green-shaded buildings did not suffer significant damage.

Figure 6. Acceleration response spectra from recordings at the rock (UNAM) and surface (SCT) stations in the 2017 (teal) and 1985 (pink) events.

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with an actual soil layering shown in Figure 6.

However, this model is simplified with a uniform soft clay layer excited by motions recorded at a rock outcrop at the UNAM (National Autonomous University of Mexico) recording station for both the 1985 and the 2017 events. The relative similarity of the UNAM rock spectra, with a distinct highperiod content, could be attributed to the roles of the large-scale geometry of the wider (≈ 10 km or 6 mi) and deeper (≈ 500 m or 1600 ft) lake basin (Singh et al., 1993) and the dense built environment of Mexico City that may cause building-to-building effects and elongation of the recorded motions.

To estimate the peak Spectral Acceleration (SA) at the surface, the known UNAM rock

peak ground acceleration, PGA, is multiplied by an amplification factor, A. The fundamental soil period is Ts = 4 H / Vs = 2 seconds, and an approximate theoretical resonance amplification at that same period is A ≈ 11, assuming soil damping of 4%. The predicted peak SA at the SCT station site for this period would be SASCT(1995) ≈ A × SAUNAM(1995) ≈ 11 × 0.08 = 0.9 g, a value close to the maximum of the average spectrum from the two horizontal records of the 1985 earthquake (Figure 6 ).

The agreement between observations and simple analysis is excellent, confirming the claim that it is the resonance of the soil deposit at T ≈ 2 seconds that is subjected to a high-period excitation and is rich at the period range 1.5 < T < 2 seconds. This produced

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the strong destructive ground motions of the 1985 disaster which targeted only buildings at this period range, as shown in Figure 5.

In 2017, the upper clay soil was probably slightly stiffer as the continuing consolidation increased its density and stiffness, validating the assumption that a smaller soil period of Ts ≈1.8 seconds is a reasonable approximation for this event. This produced the same amplification at the resonance of A ≈ 11 and a peak SA at the resonance at the SCT site, equal to SASCT(2017) ≈ A × SAUNAM(2017) ≈ 11 × 0.05 = 0.55 g, which is, again, in excellent agreement with the record. The response of the simplified soft clay profile with H = 40 m to the excitation of the 2017 UNAM record is also shown in Figure 6. While the soil amplification effects at the resonance period of the soil column of Ts ≈ 1.7 seconds are evident, the overall shape of the response spectrum deviates from the 2017 SCT record. This can be attributed to the limitations of the onedimensional approach to a problem that is clearly three-dimensional due to the geometry of the lake. Overall, the records at the SCT site from both events have similar shapes, with the peaks shifted to lower periods and amplitudes in the 2017 event, which is to be expected due to the smaller amplitude and stiffer soil properties.

The analytical predictions support the 2017 observation of minor damage at sites with a clay thickness of around 40 m (~130 ft), as shown in the buildings map of Figure 8 developed by the Geotechnical Extreme Events Reconnaissance (GEER) Association. This map shows collapsed buildings for the 1985 event in blue and the 2017 event in red, with the soil

zoning of the Mexico City Building Code (OGDF, 2004) in the background. This demonstrates that the higher period and higher level of damage in 1985 was within Zone (III) of the city center, shown in Figure 5, concentrated at the northwestern part with H = 20 to 40 m (65 to 130 ft). The 2017 collapses were also within Zone III, but at the western part with H = 10 to 35 m (33 to 115 ft). The estimated peak SA design value for the post-1985 buildings was 0.4 g, almost double the 1985 design values of 0.25 g.

This contributed to the improved structural behavior in 2017, with practically no collapses of post-1985 buildings.

Observations and Conclusions A very similar resonance phenomenon between the ground shaking and certain buildings occurred in both the 1985 Michoacán and 2017 Puebla-Morelos Mexico earthquakes. This was driven by soil amplification of the seismic waves due to the soft clay conditions, even though the two events had fundamental differences in magnitude, epicentral distance, and genesis mechanisms.

However, the level of damage of the 1985 disaster was concentrated in the lakebed SCT recording station region with a fundamental vibration period of about 2 seconds, affecting buildings of periods between 1.5 and 2 seconds. The damage was substantially less in 2017, which can be explained by the smaller recorded accelerations that

Figure 8. Collapsed buildings in 1985 (blue symbols) and 2017 (red symbols) on the contour map of soil zoning and vibration period (GEER, Mayoral, et al., 2017).

peaked in relatively shorter soil periods due to densification during the 32 years since 1985. Distinct high-period content of the rock motions could be due to the lake basin geometry and the dense construction that may have caused building-to-building effects. Both the lake and the density of construction may also have contributed to a longer duration of the seismic motions.

Overall, the lower ground motions, combined with the updates and enforcement of higher seismic design standards since 1985, played a significant role in the decrease of damage and the lack of collapses of post-1985

Lexicon ATC – Applied Technology Council cm – centimeter g – acceleration at the Earth's surface due to gravity, equal to 9.81 m/s2 ft – feet; ft/s – feet per second km – kilometers; km2 – square kilometers m – meters; m/s – meters per second mi – miles; mi2 – square miles M – magnitude is a number that characterizes the relative size of an earthquake and is based on measurements of the maximum motion recorded by instruments.

In addition to the well-known Richter scale, which measures local magnitude, two commonly used scales are (i) surface-wave magnitude (Ms) that reflects the amplitude of surface seismic waves and (ii) moment magnitude (Mw) that reflects the energy released. The two scales yield similar values for the same event.

NTUA – National Technical University of Athens in – inches OGDF – Órgano del Gobierno del Distrito Federal SCT – Ministry of Communications and Transportation. One of the strong motion accelerometers is installed at the ground surface of the parking lot of the SCT building wn – natural water or moisture content, equal to the ratio of the weight of water to the weight of the solids within the mass of soil

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buildings in the 2017 event. The second part of this article will present: (i) geotechnical and structural design and building code evolution of Mexico City towards performance and resilient-based seismic designs of highrise buildings over the last 20 years, and (ii) overcoming the challenges of going higher and digging deeper in this unique environment (Rahimian, 2017).■ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

Sissy Nikolaou, Ph.D., P.E., DGE, is an Assistant Vice President and Principal Engineer of WSP USA’s Geotechnical & Tunneling Technical Excellence. (sissy.nikolaou@wsp.com) George Gazetas, Ph.D., is a Professor of Geotechnical Engineering and Soil Dynamics at the Civil Engineering Department of the National Technical University of Athens, Greece. (gazetas@central.ntua.gr) Evangelia Garini, Ph.D., is a Researcher of Geotechnical and Earthquake Engineering at the Civil Engineering Department of the National Technical University of Athens, Greece. (geocvemp@yahoo.gr) Guillermo Diaz-Fanas, P.E., is a Senior Engineer at the Geotechnical & Tunneling Technical Excellence of WSP USA. (guillermo.diazfanas@wsp.com) Olga-Joan Ktenidou, Ph.D., is an Associate Researcher at the Institute of Geodynamics of the National Observatory of Athens, Greece. (olga.ktenidou@oq.gr)

structural

FORENSICS

T

he usual investigation of a fire incident consists of an effort to establish the origin and causes of the fire. More detailed investigations may be expanded to estimate the fuel quantity, the heat developed, and its duration. Building re-occupancy may take place only after a structural engineering assessment of the effect of the fire on the existing structural system.

Even in cases where a significant structural collapse occurred during the fire incident, the investigation remains focused on issues of evaluating potential arson, negligence, or fire code compliance. Typically, the fire is deemed the primary cause of the collapse and, as a result, the scope of the investigation remains limited to fire issues and does not include the totality of the incident.

Structural Engineers in Fire Investigations

By Dan Eschenasy, P.E., F.SEI, SECB

Dan Eschenasy is the New York City Buildings Department Chief Structural Engineer. He is an Honorary Member of SEAoNY and a member of the SEI Structural Design for Fire Conditions Standard Committee.

Fire has a relatively limited effect on masonry and concrete structures but can dramatically change the capacity and stability of steel structures and can consume wood structures.

Published cases of collapse incidents involving steel buildings have described an assessment of the condition of code prescribed fire protection of the steel. For several World Trade Center (WTC) buildings, structural engineers partnered with fire experts for the evaluation of how changes in the stability or capacity of steel members led to the structure’s collapse.

Collapses of Wood Floors or Roofs For wood-framed buildings, because of the capacity of the fire to spread quickly and consume wood, a collapse is considered a likely outcome and structural engineers may not be called to participate in the evaluation of the incident.

Buildings with bearing masonry walls and wood floors resist fires somewhat better – combustible floors might collapse, but this collapse is not expected to engage the walls.

Since the mid-1800s, successive New York City (NYC) building codes have required the ends of wood joists be “fire cut” to allow the burning floor to collapse without inducing torsional effects in the supporting masonry walls, thus preventing the walls’ collapse.

Since fire investigators can Figure 1. Debris field – roof charred joists cover collapsed first floor. establish the presence of “fire STRUCTURE magazine

14 December 2018

cuts,” structural engineers may not be called to join the investigation.

Structural engineers can bring valuable input as fire might not have been the only trigger of the collapse. For example, they can determine the building loads that existed at the location of the collapse. Even if the fire caused the collapse, the structural engineer’s participation can bring additional technical context and information, and lead to a more complete understanding of the incident.

There may be cases where the fire is the immediate cause of the incident but not the primary or essential cause of the collapse. One could argue that the National Fire Protection Association’s NFPA 921 – Guide for Fire and Explosion Investigations also should recommend expanding forensic investigations to include hypotheses beyond fire. An investigation in The Bronx illustrates such a case where the collaboration between fire and structural specialists led to the determination that the cause of the collapse was due to structural defects rather than fire.

Bronx Fire and Floor Collapse The author was called to participate in the New York City Fire Department’s (FDNY) investigation of a fire incident that occurred at a “99 Cents” store in the Bronx. The fire completely destroyed the wood roof framing of this one-story plus basement structure (Figure 1). During the firefighting operations, the ground floor collapsed, taking the life of two firemen that were fighting in the basement. Several years prior, the store had suffered another roof fire. Subsequently, the roof had been rebuilt. The ground floor, including the supporting wood columns, were built as mill construction (slow-burning construction) using sturdy elements. Along the perimeter, the ground floor was supported by masonry bearing basement walls. Typical of mill construction, the center-heavy timber girders were supported by bolsters set on top of 8-inch by 8-inch timber posts. Almost all the roof joists burned and were lying on top of the collapsed first floor.

As the piece-by-piece removal and examination of the debris progressed, the probability of the fire causing the ground floor collapse decreased – the ground floor supporting structure only had a few joists that displayed charring. The areas of wood breakage were away from the char. One seriously considered hypothesis of causation was the load that existed on the floor – the store was full of merchandise with very narrow space between shelves. The original carrying capacity of the floor was 100 pounds per square foot (psf ).

Figure 3. Rotten base of timber post.

All the debris content of the first floor was trucked away and weighed; merchandise and floor load amounted to about 70 psf.

The debris field indicated that the collapse had occurred by means of the failure of connections of some timber girders to posts (Figure 2). There was evidence of prior and repeated tinkering with the support of the ground floor.

Ultimately, the cause of the collapse was deemed to be rot existing at the base of the timber columns (Figure 3). When erected, these posts had their bottoms extended about 12 inches into the ground where they rested on natural marble outcrops. Over the years, the rot had led to a slowly progressing settlement of the center columns (the perimeter masonry bearing brick walls had not settled). The center columns had lost much of their bearing function.

As the settlement had progressed slowly, the owner was able to use shims, wood cribbing, and other types of shoring to mitigate the settlement’s effect. These “fixes” did not include the repair of broken beam-to-post connections. During the fire, the sudden additional load of water (from fire hoses) and firefighting personnel led to the abrupt settlement of one of the supporting columns. The conclusion was that the floor failure was due to the disturbance imposed on the system by the large, sudden movement caused by the abrupt shortening of the column.

Only the collaboration between fire and structural engineering professionals made possible the determination that the fire was the event that started a chain, but the underlying cause of the collapse was the presence of significant rot at the base of the column.

One should note that, by the time this store was erected (1928), there were numerous publications and handbooks

warning about the dangers of embedding wood columns (posts) in soil. As a result, the established practice (which was not adhered to in this case) was to set the bottom of wood posts on pedestals raised 6 inches above the finished floor.

Wood Bowstring Roof Trusses The 1984 collapse of the roof of a Waldbaum’s store and the resulting death of seven firefighters is deeply embedded in

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Figure 2. Failure of connection and collapse of timber girder supporting first floor.

the lore of the FDNY. The roof framing consisted of a system of wood bowstring trusses. Bowstring trusses consist of an arched top chord (the bow) joined at each end by a straight bottom chord (the string).

From a structural engineering perspective, bowstring trusses differ from other types of trusses in that the bottom chord essentially has the same tension force over its entire length. One available analysis of this collapse describes it in terms of the difficulty of fighting fires in tall attic spaces. This fatal event is mentioned as a warning in many books dedicated to firefighting. The primary recommendation is the use of defensive strategies for fighting fires in buildings with roofs supported by wood bowstring trusses.

The typical firefighter’s concerns with wood trusses include the high probability that the failure of one single element might lead to the collapse of an entire frame.

In 1988, five firefighters lost their lives during a fire as a result of the collapse of a bowstring truss roof in Hackensack, NJ. A fire investigator observed that the collapse might have been structural, as the section of wood breakage did not show char.

Between 1930 and 1960, in many areas of the country, wood bowstring roof trusses were a common solution for hangars, warehouses,

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Seattle

San Francisco

St. Louis

Tacoma

Los Angeles

Chicago

Lacey

Long Beach

Louisville

Portland

Irvine

New York

Eugene

San Diego

Sacramento

Boise

Engineering of Wood Bowstring Trusses By 1996, the paper, “Investigating and Repairing Wood Bowstring Trusses” by Kristie and Johnson in the Practice Periodical for Structural Design and Construction was drawing attention to the structural design deficiencies of wood bowstring trusses.

Kristie and Johnson indicated that, due to heavy snows in 1979, there had been a

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Photo by Dan Howell

and movie houses. They were sold and marketed via catalogs and manufactured by approved fabricators. Due to the lack of a national system for reporting and issuing advisories about structural failures, we lack a clear count of how many bowstring collapses have occurred. Fatalities of volunteer and professional firefighters are counted by the National Institute for Occupational Safety and Health (NIOSH). Including the Waldbaum’s incident, there have been at least 15 firefighter fatalities in fires of wood bowstring truss roofs.

Severud Associates

Celebrates 90 Years of Engineering Excellence Award-Winning Structural Engineers Continue to Make Lasting Mark on Skyline and Profession Severud Associates, founded in 1928 by Fred Severud, established itself by engineering modest projects using simple methods. Nine decades later, its projects are more ambitious and the tools used more sophisticated. But regardless of a project’s complexity or size, the staff’s expertise, experience, and client engagement have ensured the firm’s ongoing success.

Engineers are the primary asset at Severud Associates. Pioneers Fred Severud, Eivind Elstad, and Max Krueger laid the foundation of the firm’s reputation for innovation and creativity while also training successive generations of talented structural designers. The current principals maintain that commitment to education and service to the engineering profession.

And throughout its 90-year history, Severud Associates has been fortunate to work with many prominent architects, developers, corporations, government agencies, and other institutions. Most are longstanding and repeat clients who can attest to the quality of the firm’s service. All are appreciated for their contributions to the firm’s list of more than 16,000 successful projects. www.severud.com

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December 2018

good number of collapses in the Chicago area. These authors found that the main deficiency was an overestimation of the wood tensile capacity in the first half of the 20th century when the allowable tensile stress was considered equal to the tensile stress in bending. Depending on the type of wood, this overestimation might have reached 60%. Additionally, the older design practice overestimated the capacity of bolted splice connections by close to 30%. Around 2008, several local chapters of Structural Engineers Associations (Texas, Washington) republished an article by Gilham and McKee of Western Wood Structures that described how bowstring trusses failed to meet present day (2008) codes.

The report notes that, under heavy snows, there are breaks in truss elements but “the breaks seldom lead to a total roof collapse.”

A review of a drawing of the Bowstring Roof Truss in the 1944 Timber Engineering Company’s (TECO) Typical Design reference manual reveals, with some approximation, the level of noncompliance to the NYC code.

TECO analyzed two loading combinations (Figure 4): (a) a total 40 psf dead and live load over the entire span, and (b) a 40 psf dead and live load for one half of the span and a 10 psf dead load for the balance of the span. As expected, the entire length of the bottom truss chord had to be designed for the same tension force. The TECO notes refer only to a minimum 1200 pounds per square inch (psi) extreme fiber bending stress. As an example, if the bottom truss chord used allowable stresses, say 50% of what we accept today, it would mean that, per code, the truss could carry a total load of about 20 psf. After subtracting the dead load, the remaining 10 psf represent the weight of about 5 to 6 inches of wet snow. The fact that many trusses still stand might be explained by the high factor of safety used for wood strength; since the 1920s, the factor of safety for wood has varied somewhere between 6 and 4, mainly because of the large variability in the strength of natural materials.

Inspection Program Engineers at the NYC Buildings Department (DOB) became aware of the structural design flaws in 2013 when the FDNY Safety Chief flagged a series of articles, including the NIOSH Report

Figure 4. Reproduction of loading combinations from TECO catalog (circa 1944).

F2012-08, Volunteer Lieutenant Killed and Two Fire Fighters Injured Following Bowstring Roof Collapse at Theatre Fire – Wisconsin, (November 11, 2012). The report detailed a wood bowstring truss fire that had resulted in one firefighter fatality and two injuries.

The Report mentioned that “Bowstring truss roof systems may suffer from a little-known phenomenon related to inaccuracies in early industry-accepted truss design.” A review of NYC accident records revealed two prior cases of bowstring truss roof collapses. Both incidents had occurred in vacant buildings and did not involve injuries (Figures 5 and 6 ).

The engineering reports observed that the roofs were covered with several inches of snow. One of the collapsed bowstring trusses was of the “Belfast“ type (which used a lattice system for webbing) and might have been produced by several different manufacturers (Figure 5). One should note that, in many instances, design errors are latent; that is, they are not evident in the absence of an analysis of the original design and additional material testing.

After receiving the FDNY information, a number of facilities with wood bowstring truss roofs were inspected to form an opinion on their condition. In several cases, it was discovered that the trusses had pushed the supporting masonry piers outward – a clear sign of weakening or excessive relaxation of the bottom chords under tension. Other deficiencies noted were related to wet conditions; decay was most common, especially at the areas of the truss end where water could have penetrated through the walls or leaks around the parapet flashing.

An additional incident involving a bowstring truss collapse under snow load occurred while the FDNY and DOB exploratory investigation was underway. Consequently, all owners of buildings with bowstring roofs were ordered to engage a structural engineering consultant to perform condition assessments and structural analyses. The

Figure 5. Collapsed “Belfast” bowstring truss. Wood failed at bottom chord mid-span. Note relatively small snow cover.

department also issued a bulletin clarifying what editions of national standards were permitted to be used for the assessment of wood members. They identified close to 150 existing buildings with bowstring roof trusses. Only about 10% of the buildings had trusses that did not require repairs. The DOB’s collaboration with the FDNY led to repairs and an increase in safety for occupants, and also identified the structures where Figure 6. Bowstring truss failed at connection. firefighting would need to use special tactics. had a structural cause. One can only wonder Black’s Law Dictionary defines a defect as if, in some of the incidents, that might have an imperfection or shortcoming, especially in a been the case. Even if there had been no direct part that is essential to the operation or safety correlation between the collapse in fire and the of a product. Before the 1960s, the testing structural capacity, the original design defects procedure for the tensile capacity of wood compromised the bowstring truss system’s members was inaccurate and led to a design structural reliability and, therefore, reduced defect that caused several collapses under the time available for firefighting from inside snow loads. One can infer that this defect such burning buildings. might have also been a contributing factor This article illustrates the benefits that can to the collapse of wood bowstring trusses be brought about by the participation of in fire conditions. For instance, the F2012 structural engineers in fire investigations to NIOSH report notes an incident where the determine a structural root cause of a colroof was covered with snow. A picture in the lapse (The Bronx case) or to place in evidence report displays about the same snow cover structural contributory factors to a collapse as in Figure 5. A new tile ceiling had been (bowstring trusses). Structural engineers installed, hanging from the truss and cover- can help identify and differentiate classes ing the old ceiling. From the description, one of buildings that require special firefighting can assume that the roof might not have been tactics (e.g., wood bowstring trusses vs. other very far from its design capacity. In these truss types). The collaboration of fire and conditions, a relatively small disturbance structural specialists also requires that strucof the structural system might have led to a tural engineers gain a better understanding loss of stability. of firefighting operations and the effect of fire on specific building assemblies.

For their work on bowstring trusses, the Summary DOB forensic team (Jill Hrubecky, Timothy It seems, in the aftermath, there was no spe- Lynch, and Yegal Shamash), in collaboracific structural analysis nor report regarding tion with Simon Ressner of FDNY, received wood bowstring trusses that had collapsed in an SEAoNY Special Award For fire conditions. As a result, no consideration Outstanding Contribution to was given to hypothesize that the collapses Public Safety.▪

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A BRIDGE IN THE

FUTURE

By Bob Niccoli, P.E. S.E., Sean-Philip H. Bolduc, P.E., and Peter Chou, P.E.

Figure 2. Completed structure in the interim phase.

H

ave you ever heard the phrase, “don’t judge a book by its cover”? The same quote can apply to the Town Center Parkway Rail Support Structure in Reston, Virginia. At first look, the hidden structure appears to be a reinforced barrier along a track. However, just like an iceberg, the true intricacies lie beneath.

Dulles Corridor Metrorail Project – Phase 2 Capital Rail Constructors, a Joint-Venture between Clark Construction Group, LLC and Kiewit Infrastructure South, won the Phase 2 Dulles Corridor Metrorail Project in May of 2013. Parsons serves as Lead Engineer for the Joint Venture. Phase 2 of the project consists of the remaining six stations, 11.4 miles of track and systems to connect Dulles International Airport to Washington D.C. Like so many other projects before, there were many keys to the success of this project: addressing the diverse needs of multiple stakeholders, developing a design approach that provided a robust but economical structure necessary for both current and future needs, and, finally, incorporating the construction of the structure into the total project without a significant delay to the overall schedule.

Origin of the Town Center Parkway Rail Support Structure

WSP), determined that a traditional overpass structure was not feasible.

Constructing an underpass beneath an active Metrorail line would be difficult to permit after the Metrorail line was operational. In 2013, a resolution was approved between the MWAA, WMATA, VDOT, and FCDOT that a structure carrying the rail lines would need to be built during Phase 2 Metrorail Project work. The proposed schedule was to eliminate or at least minimize the disruption of the rail service in the future construction of the Town Center Parkway extension.

To move this feasibility study forward without impacting Phase 2, FCDOT developed preliminary engineering plans for the structure, then MWAA and WMATA developed a cost for final design and construction with the help of Capital Rail Constructors. Once all parties reviewed the cost estimate, Fairfax County determined the change order to incorporate the rail structure was a betterment to the area and recommended this project be incorporated into Phase 2.

The successful early collaboration between all stakeholders was necessary to include this new project into the Metrorail Project schedule.

The Town Center Parkway Rail Support Structure was added into Phase Final Design and Analysis 2 of the Dulles Corridor Metrorail Project after contract award. At this location, the proposed Metrorail alignment runs within the median of The new structure presented unique challenges including stipulations the International Airport Access Highway (DIAAH). In early 2012, from each stakeholder for the project to move forward. Stipulations a study, led by the Fairfax County Department of Transportation included minimizing long-term maintenance costs, a maintenance(FCDOT) in collaboration with MWAA, free interim phase (prior to excavation for WMATA, and VDOT, assessed the feasibility the Town Center Parkway), and flexibility of extending the Town Center Parkway south for the future road alignment. This led to the to Sunrise Valley Drive, passing either under or development of an unconventional solution, over the Dulles Toll Road (DTR), DIAAH, and a structure that later would be unearthed to the future Dulles Metrorail corridors. become a bridge, with an excavation depth The origin of the Town Center Parkway extension not yet decisively defined. The structure will project began after the planning and preliminary begin its life as a slab on grade supported by design of the Metrorail Project. The anticipated three secant pile walls (“interim condition”) start of construction of the Town Center Parkway but will become a two-span rigid frame bridge extension is not until sometime after the comwhen the Town Center Parkway extension is pletion of the Metrorail Project. FCDOT, with constructed at an undefined time in the future Figure 1. Example 3-D FEM model. the help of Parsons-Brinckerhoff (now a part of (“final condition”).

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continued on next page

The test for the design team was to detail a structure based on a concept for a road whose layout would not be finalized until after the structure was completed while meeting the durability and resilience requirements in the interim and final phases. The complexity of the design was in providing a flexible solution that would not restrict the future Town Center Parkway Extension alignment.

The strategy was to define an envelope for the future road that all stakeholders would approve, then analyze and design the structure around the envelope.

The two-stage process in determining the envelope was first to outline the physical restraints involving the minimum and maximum excavation depths and roadway widths. The second was to define a series of excavation approaches within which the future construction team could work.

By embracing design techniques and innovative thinking more commonly seen in tunnel construction, the design and construction teams were able to deliver a structure that met all stakeholder requirements and provided flexibility for the future excavation of the structure.

Figure 3a. Elevation view of the structure in the interim condition.

Figure 3b. Elevation view of the structure in final condition.

Structural Modeling and Design Loading

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Capturing the geometry and construction impacts required the use of complex modeling techniques and post-processing solutions. The approach utilized Soil Structure Interaction (SSI) Modeling to determine the soil responses and 3-Dimensional Finite Element Modeling (3-D FEM) to evaluate the concurrent loading created from the SSI models with loads such as track loading and other transient loading.

Because future excavation will be performed within and below the structure, the actual ground forces and construction sequences on the structure drive the system’s structural behavior (stiffness).

For this structure, the SSI design approach was the appropriate analysis rather than a traditional pre-determined force (strength limit) equilibrium design approach. The SSI approach captured the stiffness of both structure and soils, as well as the interaction

STRUCTURE magazine

between the surrounding soil excavation. The SSI model simulated the non-linear, elastoplastic soil deformation behavior to provide an accurate structure deformation prediction.

Multiple excavation sequences were considered to determine the worst case scenario force effects acting on the framed structure. This information was used to capture how the future contractor may perform the unearthing of the structure. Examples of these variables are initial construction activities including “over-excavation” to lay down a subbase for the future Town Center Parkway and whether the future contractor would unearth the structure one span at a time or in parallel. These different boundary conditions, including the long-term soil effects, were combined to determine the controlling concurrent loading at critical locations within the structure.

The results of the SSI models provided soil pressures and structure deformations necessary to derive non-linear soil springs for the 3-D FEM. The future at-rest pressures were also evaluated by combining a semi-SSI approach with the SSI derived non-linear soil springs. The derived structural deflections and soil loading from the SSI models created the basis for the structural models.

The secant pile walls and deck slab elements were then analyzed using 3-D FEM. The walls and track slabs were detailed as rigid frames to minimize soil deflections in the walls and track deflections in the slabs in the future conditions. Significant moments and torsional loads at the interface between the walls and track slabs had to be considered due to the skews of the walls.

The surface loads, including the at-rest soil pressure, pore pressure acting on the faces of the walls, and the deck slab loads from the ballast and tracks were

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develop a construction procedure including a reinforcement arrangement that would maintain the necessary spacing to ensure proper concrete placement. This included innovative construction techniques, such as the Contractor’s use of a custom in-field reinforcement bending machine capable of bending bundled #14 secant pile rebars that provided the necessary development for the moment connection between the secant pile walls and the track slab (Figure 4).

Conclusion

Figure 4. Field bending of bundled #14 reinforcement.

applied as area loads along the surfaces of the solid members in the 3-D FEM. The Project Directed Rail live loads were modeled as moving loads along alignments to capture the influence of the Inbound and Outbound track loads. Examples of other loads within the 3-D FEM included train derailment force modeled as a modified vehicle, temperature loads, wind loads, rolling forces, seismic forces, concrete creep and shrinkage, longitudinal train braking forces, and a future vehicle collision force against the center wall from an impact along the future Town Center Parkway. The output from the 3-D FEM models was combined through postprocessing techniques developed by Parsons, and the capacity of the structure was determined using AREMA Reinforced Concrete Specifications. Figure 1 (page 19) shows an example 3-D FEM model of the structure with soil loads applied based on one of the excavation conditions.

Construction Planning and Implementation

Bob Niccoli, P.E., S.E., is a Senior Structural Engineer at Parsons Corporation in Boston, MA. (robert.niccoli@parsons.com) Sean-Philip H. Bolduc, P.E., is a Senior Project Engineer at Parsons Corporation in New York, NY. (sean-philip.bolduc@parsons.com) Peter Chou, P.E., is an Engineering Manager at Parsons Corporation in Oakland, CA. (peter.chou@parsons.com)

Comprehensive information on the design of foundation members.

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Construction planning of this project started early in the design process. How the structure was built now would dictate to a large degree how the future excavation for the Town Center Parkway extension would be accomplished. The structure was constructed by first excavating to the bottom slab depth; then the secant pile walls were installed. Once the walls were finished, the slab and barriers were completed and the ballast placed, covering up the entire structure except for the barriers. Figure 2 (page 19) is a photo of the completed structure in the interim condition. Figure 3a shows an elevation view of the structure in the interim condition and Figure 3b an elevation view of the structure in final condition.

The walls required heavy reinforcement to provide excavation flexibility.

The design and construction teams worked closely throughout the final design and construction phases to

The Town Center Parkway Rail Support Structure presented many challenges that required unconventional solutions. The ability to incorporate changes in the design-build delivery method allowed for increased coordination between team members and paved the way for successful construction. By utilizing state-of-the-art soil interaction and structural engineering analysis tools, techniques, innovative construction procedures, and open collaboration between all parties including Fairfax County, WMATA, MWAA, and Capital Rail Contractors, a design was envisioned and implemented that could meet the needs for the local community for years to come.■

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STRUCTURE magazine

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December 2018

21 ANNUAL ST

EXCELLENCE

STRUCTURAL ENGINEERING IN

AWARDS

Courtesy of Hariri Pontarini Architects

T

he National Council of Structural Engineers Associations (NCSEA) is pleased to announce the winners of the 2018 Excellence in Structural Engineering Awards. The awards were announced on the evening of October 26 at NCSEA’s 26th Annual Structural Engineering Summit in Chicago, Illinois. The awards have been given annually since 1998 and each year highlight work from the best and brightest in our profession.

Awards were given in seven categories, with one project in each category named an Outstanding Project. The categories for 2018 were: • 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 2018 Awards Committee was chaired by Carrie Johnson (Wallace Engineering Structural Consultants, Inc., Tulsa, OK). Ms. Johnson noted: “This year, we moved to an electronic form of submitting and reviewing the awards. We had a 20% increase in the number of entries and revised the judging process to include two rounds of judging. The preliminary round was performed via electronic voting by a group of NCSEA Past Presidents, and the final round was done in person by individuals from the Structural Engineers Association of Northern California. They had an enormous task of trying to determine winners from an outstanding group of submittals. The group of winning projects is truly impressive.”

Please join NCSEA and STRUCTURE® magazine in congratulating all of the winners. More in-depth articles on several of the 2018 winners will appear in the Spotlight section of the magazine throughout the 2019 editorial year.

Category 1: New Buildings under $20 Million

OUTSTANDING PROJECT

Ocosta Elementary School and Tsunami Evacuation Tower Westport, WA | Degenkolb Engineers

More than 100,000 people are at risk from a tsunami on the Pacific Northwest coastline. Tsunami waves are expected to hit the coast within 30 minutes of an earthquake on the Cascadia Subduction Zone. For locations like the Westport peninsula (Washington), this is insufficient time for evacuation. Degenkolb Engineers designed the structural system for Ocosta Elementary School and its 1,000-person capacity Tsunami Vertical Evacuation Refuge, which is the first tsunami evacuation structure in the United States. These structures represent a milestone in improving tsunami safety for the Ocosta School District and the neighboring community, and forge a path for others.

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

OUTSTANDING PROJECT

Bahá’í Temple of South America Santiago, Chile | Simpson Gumpertz & Heger

Photos courtesy of Hariri Pontarini Architects

High in the foothills of the Andes Mountains outside of Santiago, Chile, the Bahá’í Temple of South America is the stunning vision of architect Siamak Hariri, of Hariri Pontarini Architects, who wanted to create a temple of light for spiritual inspiration. The temple’s nine-leaf motif celebrates the faith’s spiritual beliefs and evokes oneness with nature. Despite significant challenges – including a high-seismic zone, untested materials in thin structural applications, and high durability and reliability requirements – a collaborative team of designers, fabricators, and builders from three continents came together over fourteen years to successfully bring the architect’s vision to life.

Category 3: New Buildings over $100 Million

OUTSTANDING PROJECT

University of Texas Engineering Education and Research Center Austin, TX | Datum Engineers/Datum Gojer Engineers

The University of Texas Engineering Education and Research Center (EERC) is the showcase for structural engineering for the UT Cockrell School of Engineering, which perennially ranks in the top 10 schools in the country. Structural engineering creativity, ingenuity, and innovation are on display throughout the 430,000-square-foot facility.

While the whole building is itself an achievement of engineering, the numerous individual pieces display the beauty of creative and innovative design solutions in various functional elements such as bridges, shade structures, stairs, and roofs.

Category 4: New Bridges or Transportation Structures

OUTSTANDING PROJECT Nigliq Bridge

Colville River, AK | PND Engineers Inc.

CD5 is the first commercial oil development on Alaska Native lands within the boundaries of the National Petroleum Reserve Alaska (NPR-A). Ten years in the making, this project was a significant milestone in oil development in Arctic Alaska, and crossing the Nigliq Channel of the Colville River was one of the project’s greatest challenges. The Nigliq Bridge, completed in 2015, is 1,421 feet long and consists of eight spans up to 200 feet in length. It provides access to heavy oil field service vehicles weighing up to 175 tons and supports six pipelines while surviving massive ice loads.

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

OUTSTANDING PROJECT

Preservation and Seismic Strengthening of Congregation Sherith Israel San Francisco, CA | Wiss, Janney, Elstner Associates, Inc.

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

OUTSTANDING PROJECT

University of Connecticut Downtown Hartford Campus Hartford, CT | Silman

Congregation Sherith Israel is a historic, unreinforced masonry building with a naturally lighted dome that rises over 100 feet above the floor of its ornately finished vaulted sanctuary.

Though damaged only modestly by the 1906 earthquake, it was subject to a local seismic upgrade ordinance. Various innovative surgically-installed strengthening techniques – including the first known use in North America of super-elastic nitinol for seismic resistance and an extensive network of horizontal and vertical center cores – were implemented to supplement the structure’s inherent strengths. The design was developed to permit all historically significant features to remain virtually undisturbed by the work.

At the University of Connecticut’s new Hartford Campus, retaining an essential piece of Connecticut history, the existing Hartford Times Building (Donn Barber, 1920) façade presented unique structural challenges. The façade was repurposed as the framework for a new stateof-the-art facility built around a public courtyard. Retaining the historic façade required Silman to employ modern techniques to problem-solve, which resulted in innovative solutions like detaching the north and south walls and designing a reinforcing system for the west wall. The 5-story building includes space for classrooms and offices for academic programs that were previously located at UCONN’s West Hartford campus.

Category 7: Other Structures

OUTSTANDING PROJECT Halo Board at Mercedes-Benz Stadium Atlanta, GA | HOK

The Video Halo Board at Mercedes-Benz Stadium is a unique structure designed and built using innovative technology at unprecedented speed. At 56 feet tall and 1,100 feet in circumference, it is the largest video scoreboard in the United States. The support structure contains as much steel as a 150,000-squarefoot office building and is supported by a long span roof structure that expands and contracts by several inches under service load conditions. Through design and construction, the structural team developed project specific computational design tools to successfully deliver Mercedes-Benz Stadium’s centerpiece on an accelerated schedule.

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AWARD WINNER – CATEGORY 1

AWARD WINNER – CATEGORY 1

AWARD WINNER – CATEGORY 2

Kansas City, MO | Wallace Engineering – Structural Consultants, Inc.

New York, NY | Silman

Mountain View, CA | Holmes Structures

A. Zahner Company

St. Luke’s School Expansion

In 2015, Zahner commissioned a radical new expansion to their Kansas City campus. Consisting of 35-foot-tall vertical “pods” resembling truncated cones, the engineering office is intended to act as a showpiece of Zahner’s creativity and manufacturing capabilities. Wallace designed custom-shaped aluminum fins consisting of CNC waterjet-cut aluminum with extruded flanges at each edge to provide strength. Assembled using stainless steel self-tapping screws, Zahner fabricated these fins at their existing factory on site. Built by and for a manufacturer of high-end architectural metalwork, the office’s exposed aluminum structure and detailing inspires both Zahner’s clients and employees.

To avoid a costly seismic retrofit and to reduce the impact of construction on the existing structure below, the addition to the 1955 two-story St. Luke’s School building was constructed as a completely independent structure perched above the existing one, supported on eight 40-foot-tall super columns strategically threaded through the existing structure to new, independent foundations. This allows for seamless integration between the new and old spaces all while providing necessary seismic isolation. The 19,000-square-foot addition, which provides additional classrooms as well as a larger gymnasium, was erected without requiring any lost school days or displacing any students.

Intuit

Holmes Structures engineered a new four-story office building with one level of below-grade parking and a stand-alone five-story parking lot. The post-tensioned concrete office building is unique in its trapezoidal shape (due to site constraints and client goals). To meet the client’s desire for an open all-hands meeting forum without structural interruptions, Holmes Structures implemented a 90-foot, concreteencased steel truss that supports the upper two stories of the building and allows for a columnfree space in the atrium. The desired solution overcame three significant site constraints: limited lot lines and trapezoidal cantilevers, high water table, and anticipated uneven settlement.

Courtesy of Steve Proehl AWARD WINNER – CATEGORY 2

AWARD WINNER – CATEGORY 3

Austin Central Library Austin, TX | Datum Engineers

The Austin New Central Library is a wonder of engineering, architectural, and construction achievement, not to mention civic commitment. Countless dramatic, breathtaking, unique spaces and features are packed inside the building, situated on a former brownfield site on the north shore of Lady Bird Lake along Austin’s beloved hike and bike trail at the south edge of downtown. Structural engineering is on display throughout, in both obvious and subtle ways, including steel and glass skybridges, steel and wood framed reading porches, zig-zagging steel stairs, an exposed steel beam roof structure, a folded atrium roof and 10-foot wide continuous skylight, and an immense mat foundation.

King Abdulaziz International Airport Jeddah, Saudia Arabia | Arup

King Abdulaziz International Airport embarked on a comprehensive redevelopment program in 2006. The project included a new 1.4-kilometer-long (0.87-mile), 670,000-square-meter (7.2-million-square-foot) international passenger terminal, designed to handle over 30 million passengers per year with 46 domestic and international departure gates and 94 boarding bridges, an international departures hub, internal automated people mover, and the world’s tallest air traffic control tower. Arup helped deliver a structural design that met architectural expectations and a stringent timeline, while significantly reducing overall concrete and steel quantities in the building in comparison to the original design.

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AWARD WINNER – CATEGORY 3

Salesforce Tower

San Francisco, CA | Magnusson Klemencic Associates

Transforming San Francisco’s skyline, Salesforce Tower is a slender, tapering, 61-story, 1.4-million-square-foot performance-based seismic design office tower, and the tallest building west of the Mississippi (based on the highest occupied floor). The developer’s goals include wide-open office space with column-free leasing bays and corners, without structural encumbrances. In other words, no outriggers, belt trusses, buckling-restrained seismic braces, or dampers of any kind. Meeting these goals in a 1,070-foot-tall tower sitting on complex soil strata, touching the adjacent Transit Center, and approximately 8 miles from the San Andreas Fault, required creative, complex, never-done-before innovations and structural engineering “firsts.”

AWARD WINNER – CATEGORY 4

AWARD WINNER – CATEGORY 4

Lincoln Avenue Pedestrian Bridge

Gut Bridge Replacement South Bristol, ME | Hardesty & Hanover

AWARD WINNER – CATEGORY 5

Hotel Nikko Pre-Northridge PJP Column Splice Repair

The new Lincoln Avenue Pedestrian Bridge enables walkers, joggers, and bicyclists to cross a busy roadway safely. The cable-stayed structure, spanning 170 feet, features a large, leaf-shaped mast on its south end that rises 78 feet. Six pairs of cables extend from the leaf to support the bridge. The bridge sizing was a balancing act between the strength, stiffness, economy, and close attention to aesthetic vision. The Thornton Tomasetti project team worked closely with the fabricator and developed local finite element models of connections to optimize the configuration and sizing of the pylon and to facilitate an accelerated procurement process.

The 80-year-old Gut Bridge, a bobtail swing bridge, had undergone multiple mechanical failures. The bridge provides the only route from the mainland to Rutherford Island crossing the narrow “Gut,” and requires over 8000 openings per year. Working with the community and Maine DOT, the firm designed an aesthetically pleasing, innovative bascule bridge under budget.

The requirement of a small-scaled structure with operational reliability and quick openings was successfully delivered with a state-of-the-art structure. The new superstructure utilized a combination of details that formed a new type of bascule span which includes cable-stays, steel orthographic deck, and welded steel box girders.

AWARD WINNER – CATEGORY 5

AWARD WINNER – CATEGORY 6

AWARD WINNER – CATEGORY 6

Sacramento, CA | Buehler & Buehler Structural Engineers, Inc.

Laie, HI | J.M. Williams & Associates, Inc.

Philadelphia, PA | The Harman Group

Lone Tree, CO | Thornton Tomasetti

E. Claire Raley Studios for the Performing Arts

Polynesian Cultural Center Renovation

The historic Fremont School is a two-story concrete frame with unreinforced masonry perimeter walls. The Sacramento Ballet’s performance vision required 50-foot clear spaces in the west wing.

So, all twelve existing interior 2-story concrete columns were eliminated and the floor load redistributed to 4 steel columns. The Buehler and Buehler team developed a genuinely innovative structural solution that utilized 36 post-tensioning strands squeezed between the existing 1923 corridor edge beams that are exposed to view from the first floor studios. It was believed that this approach is the first of its kind in the adaptive reuse of a historic building.

Hawaii’s number one attraction is the Polynesian Cultural Center located in Laie, on the Island of Oahu. The center was originally constructed in 1963. The north shore of the island can be subject to hurricane winds, torrential rains, and termites. Although the center has been well maintained, it was still in need of major repairs and renovation work. This project includes converting a 1989 IMAX theater into a themed show attraction with the illusion of a volcano, repairing and renovating the Gateway Building into a fine dining hall, repairing village huts, building a new wind-resistant Chief ’s Hut, and ultimately replacing the marketplace with new restaurants and shops.

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San Francisco, CA | Degenkolb Engineers

Degenkolb Engineers evaluated Hotel Nikko, a high-rise pre-Northridge moment frame structure, using non-linear time-history procedures of ASCE 41-13 augmented with changes proposed for the 2017 update. Column splice weld demands had consistently exceeded their fracture stress. Using emerging research in fracture mechanics, a retrofit scheme was developed that avoided the use of intrusive supplemental damping or braces. The project reaffirmed this empirical formulation through finite element modeling. Applying the process to a variety of column sizes and weld details establishes a procedure to reveal susceptibility in similar building construction types.

Aramark Headquarters Renovation and Overbuild Adaptive re-use of buildings is commonplace.

What was uncommon was the challenge to the Harman Group and the Varenhorst/Gensler architectural design team to adapt a turn of the century occupied five-story concrete, automobile warehouse building with diagonal column grids and 40-inch-diameter columns into functional office/retail space. Additionally, while occupied, complete a 6-story steel framed vertical expansion, cut out the existing concrete core, rebuild two lateral concrete cores supported on hundreds of micro-piles, and insert a 6,000-square-foot atrium and new full building-length promenade for an incredible experience on the urban waterfront at 2400 Market Street.

AWARD WINNER – CATEGORY 6

AWARD WINNER – CATEGORY 7

AWARD WINNER – CATEGORY 7

Babb, MT | JVA, Inc.

Vancouver, WA | Martin/Martin Consulting Engineers

Columbus, IN | Pierce Engineers, Inc.

Many Glacier Hotel

The Grant Street Pier

The Great Northern Railway built the iconic Many Glacier Hotel in the 1910s. Less than a century later, significant deficiencies in the building systems put the hotel at risk of closure. The structural design focused on strengthening the snow and seismic load resisting systems while maintaining the building’s historic character. The phased rehabilitation included the installation of shear walls supported on micropiles, internal reinforcing of the stone masonry chimneys, and integration of the 4-story log columns into the lateral system. Retrofitting elements are mostly concealed, and visitors are rewarded by experiencing the building’s original charm.

The centerpiece for Vancouver, Washington’s new waterfront district is the Larry Kirklanddesigned Grant Street Pier. The triangular deck, cantilevered 100 feet over the river, is supported by a lone, cable-stayed mast creating a signature landmark for the new development. Voids in the concrete and tapered edges make the pier light and elegant, while tuned mass dampers and custom connections ensure its structural integrity. Floating columnless above the water, the eye-catching pier preserves undisturbed native fish migration. The final design exceeded the expectations of the City of Vancouver; they are overjoyed with the outcome.

Wiikiaami

Wiikiaami is a 50-foot-tall artistic structure located in Columbus, Indiana. Wiikiaami translates to “wigwam” in the native language. The conical, wire-frame structure, inspired by the homes of the Miyaamia people indigenous to Indiana, consists of A706 reinforcement, welded at the connections, with sizes ranging from #4 to #9 bars. Wiikiaami’s curved geometry, tapering from 30 feet in diameter at the base to 5 feet at the peak, was fabricated using a CNC cutwood framing system and rests on helical soil anchors. Full 3D analysis was used to minimize reinforcement that could be easily field bent to accommodate the desired curvature.

2018 Panel of Judges

The judges for this year’s Excellence in Structural Engineering Awards were: FINAL ROUND Structural Engineers Association of Northern California (SEANC)

PRELIMINARY ROUND NCSEA Past Presidents Craig Barnes, P.E. CBI Consuling, LLC Sanjeev Shah, P.E., Esq. Lea + Elliott John Joyce, P.E. Engineering Solutions, LLC Ron Hamburger, S.E. Simpson Gumpertz & Heger Vicki Arbitrio, P.E. Gilsanz Murray Steficek Bill Bast, S.E. Thornton Tomasetti Ben Nelson, P.E. Martin/Martin Carrie Johnson, P.E. Wallace Engineering Tom Grogan, P.E. Haskell

Angie Sommer, S.E. ZFA Structural Engineers

Kevin Moore, S.E. Simpson Gumpertz & Heger

Darrick Hom, S.E. Estructure

Marc Steyer, S.E. Tipping Structural Engineers

Emily Guglielmo, S.E. Martin/Martin

Marko Schotaunus, S.E. Rutherford + Chekene

Emily Setoudeh, EIT Buehler&Buehler

Nick Sherrow-Groves, P.E. Arup

Jack Moehle, P.E., Ph.D. UC Berkeley

Peter Lee, S.E. Skidmore, Owings & Merrill

Jim Malley, S.E. Degenkolb

Richard Dreyer, S.E. Holmes Structures

Johnny Thibau, S.E. GFDS Engineers

Tim Hart, S.E., Ph.D. Lawrence Berkeley National Laboratory

Jonathan Buckalew, S.E. Nabih Youssef

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engineer's

NOTEBOOK

W

ood studs designed to resist wind loads in either loadbearing or nonloadbearing tall wall applications are good examples of resilient design. Tall walls can be defined as those exceeding the International Residential Code (IRC) prescriptive limit of 10 feet for loadbearing walls. Proper design of wood structures to resist such wind loads also requires correct use of wind load provisions. Minimum design loads must be in accordance with the governing building code or, where applicable, other recognized minimum design load standards such as American Society of Civil Engineers’ ASCE 7 Minimum Design Loads for Buildings and Other Structures. Wind load provisions have been developed for design of major structural elements using “main wind-force resisting system” (MWFRS) loads and secondary cladding elements using “component and cladding” (C&C) loads. Elements and subassemblies receiving loads both directly and as part of the MWFRS – such as wall studs – must be checked for both the MWFRS loads and C&C loads, independently.

Studs should be designed using MWFRS pressures when considering the combined interactions of axial and bending stresses; and designed using C&C pressures when considering axial or bending stresses, individually. This interpretation was developed because only MWFRS pressures provide loads which have been temporally and spatially averaged for different surfaces (MWFRS loads are considered to be time-dependent). Since C&C loads attempt to address a “worst case” loading on a particular element during the wind event, these loads are not intended for use when considering the interaction of loads from multiple surfaces (C&C loads are not considered to be time-dependent).

This is not unusual. In most cases, it can be considered the controlling limit in wind design of loadbearing and non-loadbearing exterior studs.

However, until sufficient boundary conditions are placed on this simplification, both MWFRS and C&C load cases should be considered.

Design of Tall Walls in Wood Structures

By John “Buddy” Showalter, P.E., Bradford Douglas, P.E., and David Low, P.E. John “Buddy” Showalter is Vice President of Technology Transfer and Bradford Douglas is Vice President of Engineering for the American Wood Council. David K. Low is President of DK Low & Associates, a consulting firm in Charlottesville, Virginia. Contact Mr. Showalter with questions at bshowalter@awc.org.

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Resources The American Wood Council (AWC) has developed several code-referenced design standards for wood construction, for a variety of building types, to aid structural engineers in addressing the challenges associated with high wind.

The National Design Specification for Wood Construction (NDS®) includes necessary design procedures and design value adjustment factors for wood products. The design values for wood studs and the beam and column buckling formulas used to design studs for axial and lateral loads are incorporated in the NDS. The Special Design Provisions for Wind and Seismic (SDPWS) provides specific design procedures for wood members, fasteners, and assemblies to resist wind and seismic forces. In addition to shear wall and diaphragm design, SDPWS offers design criteria for members and connections subject to out-of-plane wind loads. One very specific provision pertinent to tall wall design is the wall stud bending strength and stiffness design value increase, where reference bending and bending stiffness values are permitted to be increased based on the presence of exterior wood structural panel sheathing and interior gypsum wallboard with specific attachment requirements.

The Wood Frame Construction Manual (WFCM) is another AWC standard integral to wood design, providing engineered and prescriptive design requirements for one- and two-family dwellings. However, it also serves as a useful tool in the design of non-residential buildings in Risk Category I or II that fit within the WFCM scope of building size and assigned loads.

Examples include buildings where the bottom floor is used for stores, offices, and restaurants.

For buildings within its scope, WFCM contains both engineered and prescriptive solutions for wind, seismic, and gravity loads. The engineered provisions in WFCM Chapter 2 offer, for example, tabulated wind loads and gravity loads based on assumptions from ASCE 7 provisions.

December 2018

Prescriptive wood solutions in Chapter 3 tabulate both loadbearing and non-loadbearing stud lengths for common lumber species resisting wind loads for various deflection limits and sheathing types. For the prescriptive solutions in WFCM Chapter 3, loadbearing walls are limited to a maximum of 10 feet, while non-loadbearing walls can be 20 feet tall.

In Chapter 2, WFCM permits loadbearing studs up to 20 feet tall. For C&C wind pressures, the localized bending stresses are computed independent of axial stresses. For MWFRS pressures, bending stresses in combination with axial stresses from wind and gravity loads are analyzed. For buildings limited to the conditions in WFCM, the C&C loads control the stud design. A comprehensive WFCM Commentary provides all the background assumptions and example solutions for the tabulated values.

Design Example The following loadbearing stud wall design example demonstrates standard design checks for limit states of strength and deflection based on methods outlined in AWC’s 2015 NDS, 2015 SDPWS, and 2015 WFCM Workbook, along with ASCE 7-10 (see end note).

The objective is to design a 19-foot tall loadbearing wall stud in a two-story building with a 25-foot mean roof height, 32-foot roof span, and 2-foot overhangs. Wind loads are 160 mph Exposure B. Additionally, the following gravity loads are assumed for the roof: • dead load = 10 psf • live load = 20 psf • ground snow load = 30 psf These loads are assumed for the attic and ceiling: • dead load = 15 psf • attic live load = 30 psf The approach is to analyze wall framing as part of the MWFRS exposed to in-plane and out-of-plane load combinations specified by ASCE 7-10. Studs are then analyzed with out-of-plane C&C wind pressures only. The analysis involves an iterative approach. Initial values are selected for member properties (depth, number of members, species, and grade), and analyses completed. Then, stresses and deflections are determined and compared to allowable values. At this point, the member properties are varied, with analyses repeated until stress and deflection criteria are satisfied.

Southern pine No. 2 grade 2x8s are analyzed assuming 16-inch-on-center (o.c.) spacing, wood structural panel exterior sheathing, and ½-inch gypsum wallboard interior sheathing with the following reference design values from the 2015 NDS Supplement Table 4B: • Fb = 925 psi • Fc = 1,350 psi

• E = 1,400,000 psi • Emin = 510,000 psi The wall stud bending strength and stiffness design value adjustment factor from SDPWS Table 3.1.1.1 for a 2x8 is equal to 1.25. Load duration factors (CD) apply to the bending and compression design values, but not modulus of elasticity. CD also varies depending on the shortest load duration in the load combination, for load combinations including: • wind, CD = 1.6 • roof live loads, CD = 1.25 • snow loads, CD = 1.15 Allowable Stress Design (ASD) load combinations per ASCE 7-10 are evaluated for this example (Figure 1). Both balanced and unbalanced snow loads are analyzed. For this example, an unbalanced snow load of 360 plf provides the highest snow loads on the studs.

MWFRS wind pressures are calculated using the “envelope procedure” contained in ASCE 7-10 Chapter 28. The velocity pressure exposure coefficient for a building located in Exposure B with a 25-foot MRH is 0.70 per ASCE 7-10 Table 28.3-1, and a factor of 0.6 adjusts the pressures associated with a 700-year mean return period wind to allowable stress design. The velocity pressure calculates to 23.4 psf.

ASCE 7-10 Figure 28.4-1 shows the external pressure coefficients for interior and end zones for two cases – winds generally perpendicular to the ridge and winds generally parallel to the ridge. Wind perpendicular to the ridge produces the highest external wall pressure coefficients. Reactions at the top of the bearing wall are determined by summing overturning moments about the top of the leeward wall for both load cases and determining the controlling reaction to use in the design. Horizontal projections are used in the analysis. The out-ofplane MWFRS pressure on the wall at interior zones is calculated as 17.3 psf.

Load Combinations 1, 2, 3, and 4 model gravity-only loads (dead load, live load, and/or snow load). Load Combinations 5, 6a, and 7 include MWFRS loads. Load Combination 6a controls for the load combinations that include wind loads. The bearing walls must resist distributed loads from the attic floor and roof and out-of-plane MWFRS loads proportional to the width of their tributary areas. Using NDS column, beam, and combined bending and axial load provisions, the interaction value calculated per NDS Equation 3.9-3 is 0.46.

ASCE 7-10 provisions for calculating C&C loads are used assuming a minimum effective wind area of (L)2⁄3. By observation, negative external pressure coefficients are greater than positive external pressure coefficients.

Thus, negative external pressures and positive internal pressures (windward) create the greatest C&C pressures. A C&C pressure

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1.

2.

3.

4.

5.

6a.

D D+L D + (Lr or S or R) D + 0.75L + 0.75(Lr or S or R) D + (0.6W or 0.7E) D + 0.75L + 0.75(0.6W) + 0.75(Lr or S or R) 6b. D + 0.75L + 0.75(0.7E) + 0.75S 7. 0.6D + 0.6W where: D = dead load L = live load Lr = roof live load W = wind load (note the 0.6 load factor will be included in the velocity pressure calculations) S = snow load Figure 1. Evaluation of ASD load combinations.

of -25.5 psf is calculated for this example.

Applying the C&C pressure as a bending load on the studs leads to calculation of a bending stress to bending strength ratio of 0.76 – even larger than the combined bending and axial interaction calculated with MWFRS loads.

Therefore, No. 2 grade southern pine 2x8 studs work from a strength standpoint.

A deflection check using C&C loads reveals an H/Δ of 273 for No. 2 grade southern pine 2x8 studs. Assuming either a flexible finish or gypsum-type finish, code deflection limits are typically H/180 and H/240, respectively.

Therefore, the 2x8 studs are adequate for deflection unless a brittle finish requiring a tighter deflection limit is used.

Conclusion Major structural elements should be designed for MWFRS loads, and secondary cladding elements should be designed for C&C loads.

Components and assemblies receiving loads both directly and as part of the MWFRS should be checked for MWFRS and C&C loads independently. In cases where components and assemblies must be designed for lateral wind loads, the controlling design case will often be wind acting alone. However, each load combination should be considered thoroughly before being dismissed.▪ This article’s example is based on a webinar, Design of Loadbearing Tall wood Studs for Wind and Gravity Loads (DES230), available for free at www.awc.org. Due to space constraints, only highlights of the example are presented here, but full details can be found in the webinar materials.

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n Part 1 (STRUCTURE, November 2018) of this 2-part series, the definition of the modulus of subgrade reaction was presented and the current state of design with regard to its use was discussed. That article further described some potential shortcomings of the simplified theory of subgrade reaction. This article continues to describe the settlement profile convergence method and how it can be implemented into a new design.

Distribution of Ks As much as the value of Ks is important, distribution of Ks and its effect on foundation design is even more important. It is a complex subject and often not used for routine foundation design because of the non-availability of any simple mechanism. For large scale projects, engineers often collaborate and use sophisticated software for soil-structure-interaction.

From the theory of subgrade reaction, we know that the settlement profile of a uniformly loaded flexible foundation takes the shape of a bowl or trough. Does this hold for a supporting soil medium having uniform Ks? We can begin with a simple case study using a standard commercial software package.

Modeling Mat geometry: Square Mat 12 x 12 x 0.5 feet Soil Bearing Capacity: 4 kip/ft² Loading: 1 kip/ft² Solution Ks can be estimated as, Iq 3 × 4⁄144 = 0.0833 kip/in2/in Ks = a = δ 1 where, I = Safety factor = 3.0, qa (assumed) is the allowable bearing capacity (given 4.0 kip/ft²), δ is the allowable soil settlement = 1 inch (assumed) As discussed earlier, the displacement profile is expected to take the shape of a bowl or trough. However, the foundation settled uniformly (Figure 5 ), which did not match our expected behavior.

It is hardly a surprise. We know, for a uniformly loaded mat, subgrade reaction increases from the center towards the edge and, as a result, the foundation displacement profile takes the shape of a bowl. For this solution, a constant Ks value was used (which is often the practice for its simplicity) and, as a result, the program computed erroneous physical behavior.

Extending the Current Practice Fortunately, it is not too difficult to manipulate a computer program to get close to the expected physical behavior. We can extend the model to create varying soil medium from the center

of the foundation towards the edge. In other words, we can use a variable Ks to depict the desired settlement profile of the supporting soil medium.

Solution As shown in Figure 6, divide the supporting soil medium into several bands (five for this example).

Also, assume subgrade reaction increases as much as 100% from the center towards the edge.

Ks1 = 0.0833 kip/in2/in Ks2 = 0.09996 kip/in2/in Ks3 = 0.119952 kip/in2/in Ks4 = 0.143942 kip/in2/in Ks5 = 0.172731 kip/in2/in After running the analysis, the deflection profile takes the shape of a bowl or trough and matches the expected soil settlement profile (Figure 7 , page 32). Also, it is interesting to notice the soil pressure contour. Unlike the case for a uniform Ks, the base pressure contour now shows varying pressure from the center towards the edge (Figure 8, page 32).

So, it is natural to conclude that a flexible mat foundation should always be analyzed using variable moduli of subgrade reaction. It predicts more accurate physical behavior and, hence, the results should be more accurate.

NCNB Corporate Center Variable Ks was used for the second NCNB (Horvilleur and Patel) study, which varied from the lowest value (181 psi/in) at the centroid to the highest value (548 psi/in) at the edge. As expected, the researchers observed a dishing phenomenon.

The two analyses were compared (uniform Ks at 290 psi/in and a variable Ks). Variation in soil pressure was around 11%, which may not be that significant. However, the differences in moment was significant, varying as much as 120%.

Iterative Method It is evident that the use of a variable Ks is better than using a uniform Ks. However, the value of Ks depends on many factors including rigidity of the foundation, soil stiffness, and settlement.

As an example, initial estimated values of Ks for a given zone were derived based on a predicted settlement. After finite element analysis, calculated displacement for a given zone (more precisely for a given node) may not match with the predicted displacement. So, a new Ks value should be calculated and the analysis rerun.

This iterative process should continue until the solution converges.

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structural

DESIGN

Variable Modulus of Subgrade Reaction Part 2: Settlement Profile Convergence Method

By Apurba Tribedi Apurba Tribedi is a Senior Director at Bentley Systems Inc. in Anaheim, CA office. (apurba.tribedi@bentley.com)

Figure 5. Vertical deflection diagram of the uniformly loaded mat foundation.

Figure 6: Banded Ks distribution from lowest at the center to the highest towards the edge. continued on next page

December 2018

A New Approach Figure 7: Displacement profile of the mat foundation when using distributed Ks.

Figure 8: Base pressure contour of the mat foundation when using distributed Ks.

Discrete Area Method The Discrete Area Method (Ulrich) is an iterative method to achieve deflection compatibility between mat deflection and soil settlement. The steps are described as: 1) Create a finite element model for a mat foundation and analyze it using a geotechnical engineer’s best estimated uniform Ks value.

2) Using base pressure from step 1, the geotechnical engineer calculates soil settlement at each node of the FEA model and a new set of Ks values at corresponding nodes.

3) Input a new set of subgrade moduli in the structural finite element model and obtain a new pressure distribution and settlement.

4) Using the pressure profile from step 3, the geotechnical engineer calculates settlement at each node and a corresponding Ks at each node.

Repeat step 3 and step 4 until convergence is achieved. This happens when the displacements predicted by the structural engineer’s finite element analysis match the settlements predicted by the geotechnical engineer. The resulting coefficient of subgrade modulus may differ significantly from the initial estimated values and, as a result, will significantly influence mat design. The convergence may require multiple iterations and, as the process cannot be automated, use of this method demands close collaboration between the structural and geotechnical engineers.

We can compare the example from the earlier section, Distribution of Ks, to the Settlement Profile Convergence Method, studying the new method’s effectiveness. As expected, the displacement profile from both the methods takes the shape of a bowl (Figure 9). However, the shape produced by the Settlement Profile Convergence Method is smooth. It is even more apparent from the resulting base pressure diagrams from each analysis. As discussed earlier, Ks is expected to vary radially from the center towards the corner. The resulting contour diagram (Figure 10) correctly depicts that expected radial distribution. It is a significant improvement over any existing method.

Base pressure, settlement, stiffness, and modulus of subgrade reaction are all intertwined. Dependency among these parameters adds to the complexity and makes it difficult to achieve a quick, definitive solution.

A new computer-based approach has been developed and proposed by the author, the Settlement Profile Convergence Method. It is an iterative solution to converge estimates of foundation displacement and soil settlement. The most critical aspects of the solution are the initial estimation of soil settlement and normalization of the settlement profile. This minimizes the effect of Conclusion various factors used in different proposed equations for soil stress calculation.

A computer program can be developed to use The steps are described as: the Settlement Profile Convergence Method 1) Generate a mesh to create a finite ele- effectively. It is an iterative solution; the ment analytical model of the foundation total solution time will extend marginally. slab.

However, as it depicts the physical behavior 2) Use the standard Boussinesq’s equation, more closely, the final results are expected or any other method, to calculate soil to be more accurate. Current practice is not stress under each meshed node for a necessarily conservative. It is over-simplified given loading. and can be erroneous. Soil-structure inter3) Normalize settlement values between 1 action is a complex subject, and this new and the ratio between maximum over method is an additional tool at the engineer’s minimum settlement. disposal for automating a foundation analy4) Calculate Ks for each node from allow- sis. It does not replace engineering judgment able bearing capacity and the calculated or experience. Close collaboration among soil settlement from step 3. structural and geotechnical engi5) Calculate nodal spring constants by mul- neers is highly recommended for tiplying Ks with the nodal tributary area. the best possible outcome.■ Assign spring constants as compressiononly springs.

The online version of this article 6) Run the finite element analysis. contains references. Please visit 7) Extract nodal displacement values from www.STRUCTUREmag.org. the analysis.

8) Repeat step 3 through step 7.

9) Compare the new displacement profile with the displacement profile from the last analysis.

Figure 9: Revised displacement diagram from 10) Repeat steps 8 and 9 until two consecu- Settlement Profile Convergence Method. tive analyses converge or fall within a reasonable tolerance limit.

Effectiveness of the New Method The new method is significant for several reasons.

• It focuses on the shape and relative displacements rather than the absolute displacements, which minimizes the effect of initially estimated soil settlement.

• The process is automated and complementary to the current FEA based mat foundation analysis.

Figure 10: Revised base pressure contour for 0.5• It considers structural rigidity. foot thick mat foundation.

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December 2018

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CASE BuSinESS PrACtiCES Developing the Next Generation of Structural Engineers

By Michael A. Stubbs, P.E., S.E., DBIA, and David V. Jáuregui, Ph.D., P.E.

T

he world of Structural Engineering is in one of the most significant transitions we have seen in recent history. As we exit the more recent Great Recession and establish new normal economic conditions, we are also dealing with rapidly changing technology, the effects of a more global economy, and ever-evolving contract structures and delivery methods. The growth of the economy in the last few years has provided new opportunities and filled the backlog of most firms, but we still see pricing pressures. BIM, finite element modeling, and the growing sophistication of computer programs are changing workflows.

Where we used to compete against a small group of local firms that we knew, we are now finding ourselves competing with firms across the country and even internationally.

When you combine this with the retirement of Baby Boomers, happening at a record rate, it is easy to see why structural engineering firms are so desperate for new talent. Finding young, ambitious engineers is just the start.

You have to recruit them and then enhance their education with experience to make them productive as quickly as possible. Recruiting, refining, and retaining new engineers is one of the biggest challenges facing firm leaders today.

Young engineers are experiencing new and bigger pressures than their predecessors. Because of the shrinking Baby Boomer workforce and the fact that Generation X is one of the smallest generations in recent history, Millennials are expected to fill the ranks of firm leadership earlier in their career.

They also must become proficient in building codes, design procedures, contract structures, risk management, and technology in a more complicated environment than we saw even ten years ago. Staffing a firm with new leaders is difficult for both the firm principals and the young engineers.

We, as a profession, can help develop the next generation of structural engineers by getting involved in their collegiate education as advisors, mentors, and teachers. Many colleges and universities throughout the U.S. welcome input and assistance from professionals. By getting involved at the collegiate level, professionals give student engineers a real-world perspective that builds upon the material covered in their classes. Also, it gives

firm leaders a chance to vet potential new hires in an environment where you get a better picture of their work ethic over a prolonged period, rather than in a short interview.

Guest lecturing a class is an easy way to get involved. Whether you lecture on a technical topic or soft skill, the use of examples from current projects will serve as a valuable lesson.

Giving students an opportunity to see how the skills they are learning in college are applied to real projects provides a perspective that can make learning more exciting, thoughtprovoking, and easier to connect practical application to abstract concepts.

Professionals can also get involved in the student chapters of professional organizations.

ASCE’s concrete canoe and AISC’s steel bridge competition are examples of activities where professionals can spend time with students in a fun and educational environment. These programs give professionals a means to share design techniques while assisting students in competitions against other universities, which indirectly ties to today’s competitive market for engineering services.

Co-ops and internships are also good ways for firms to get involved in the education of students. These opportunities can be invaluable to a young engineer, and to the firm. These soon-to-be engineers gain valuable experience, and the firm can “test drive” a potential new hire. This is a great way to recruit young engineers before the competition even knows they are entering the job market.

Once you hire a new engineer, the task of giving them the experience and mentoring they need can be overwhelming for both the senior engineer and the junior engineer.

One approach that works is to give the young engineer a cycle of assignments with steep learning curves that plateau periodically. This can be done with a combination of projects that expand the young engineer’s skills with projects that are familiar. Giving a new engineer a project that challenges them followed by a project very similar helps the engineer gain confidence as they perform tasks on their own. It also gives the senior engineer a chance to get out of the constant training mode and progress with their daily work and responsibilities.

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December 2018

The senior engineer does have to provide the young engineer opportunities for as broad a skill base as possible. The experience and skill set of the young engineer should be continuously reviewed, and projects and tasks assigned that will continue their growth. As they become proficient at technical skills, it will be time to make them responsible for more project management tasks like budgeting, scheduling, and communicating with clients and team members.

It is also essential to give the young engineer feedback and insights as to why certain things are done. Routine trips to job sites should be common, an excellent opportunity for senior engineers to teach the “hows” and “whys” behind the things we do. Senior engineers should also take the time to debrief young engineers after meetings. Continually teaching young engineers the thought process that produces desired outcomes is important to growing new firm leaders.

Ultimately, one goal should be to have the young engineer become licensed. Licensure allows up-and-coming engineers to expand their job role and their responsibility in the firm. This starts by teaching them the importance of licensure while they are still in college. It should be continued when they enter the workforce. Senior engineers can help by providing experiences that will aid in passing exams. Young engineers should also be trained in the responsibilities associated with licensure and ethics involved with being a professional engineer.

CASE has prepared a toolkit that assists in training new engineers. Tool 5-2: Milestone Checklist for Young Engineers can be downloaded from CASE’s website by members and is for sale for non-members. This tool provides a guide for the skills young engineers should develop before becoming licensed.■ Michael A. Stubbs is the President of Stubbs Engineering, Inc., a full-service structural engineering firm Headquartered in Las Cruces, NM. (mstubbs@stubbseng.com) David V. Jáuregui is the Department Head and a Professor in the Department of Civil Engineering at New Mexico State University in Las Cruces, NM. (jauregui@nmsu.edu)

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NCSEA News

News from the National Council of Structural Engineers Associations

2018 Structural Engineering Summit Draws Record Attendance The 2018 Structural Engineering Summit attracted more than 600 attendees from across the country to Chicago, IL, to celebrate the profession. The event featured over 45 education sessions for the practicing structural engineer, social and networking events, and a trade show with 68 exhibitors.

NCSEA was assisted by the local member organization, the Structural Engineers Association of Illinois (SEAOI), to host a one-of-a-kind river cruise.

The Structural Engineering River Cruise was held on the new Odyssey that hit Chicago’s shore line at the end of September. The glass-enclosed boat allowed views of the skyline and provided a unique venue for SEAOI representatives to present about select structures on the river.

The 2018 Summit featured a keynote by Ron Klemencic, P.E., S.E., Hon. AIA, who described engineering as an ever-evolving discipline. Klemencic reviewed how some of the most impactful innovations in recent years were developed, and what areas are ripe for the next wave of advancements. NCSEA also invited local mentor and influencer, Stacy Hanke, to inspire attendees to build trust, project confidence, and stay accountable. The Summit held a Friday luncheon with another keynote from Ashraf Habibullah, S.E., who emphasized the importance that education plays for young engineers to enter the professional world “to lead, influence, and inspire.”

The NCSEA Awards Banquet at the Summit featured the presentation of NCSEA Special Awards, including: • NCSEA Service Award to Barry Arnold, P.E., S.E., for work on behalf of NCSEA that is beyond the norm of volunteerism • James M. Delahay Award to Jonathan (Jon) C. Siu, P.E., S.E., for outstanding contributions towards the development of building codes and standards.

• Robert Cornforth Award to Ryan A. Kersting, S.E., for exceptional dedication and exemplary service to an NCSEA Member Organization and to the profession.

• Susan M. Frey NCSEA Educator Award to Ronald O. Hamburger, S.E., for his genuine interest in, and extraordinary talent for, effective instruction to practicing structural engineers.

In addition, the Excellence in Structural Engineering Awards were presented for exceptional structural engineering projects in: • New Buildings Under $20 Million • New Buildings $20 Million to $100Million • New Buildings over $100 Million • New Bridges/Transportation Structures • Forensic/Renovation/Retrofit/Rehabilitation Structures up to $20 Million • Forensic/Renovation/Retrofit/Rehabilitation Structures over $20 Million • Other Structures The Summit also marks the end for the 2017-2018 NCSEA Board of Directors, and ushers in new members for 2018-2019. The Board welcomes new directors Richard C. Boggs and Paul J. Rielly. Williston “Bill” Warren IV shifted from President to Past President as former Vice President, Jon Schmidt entered the President Position.

The board said goodbye to Past President, Tom Grogan, and Director, Chun Lau. The complete 2018-2019 Board of Directors includes: Jon Schmidt, P.E., SECB, President; Susan Jorgensen, P.E., SECB, LEED, Vice President; Emily Guglielmo, S.E., P.E., Secretary; Ed Quesenberry, S.E., Treasurer; Richard C. Boggs, P.E., SECB, LEEP AP, Director; David Horos, Director; Paul J. Rielly, P.E., S.E., SECB, Director; and Stephanie Young, Director.

Next year’s Structural Engineering Summit will be held November 12–15 at the Disneyland Hotel in Anaheim, CA. Information on attending and exhibiting will be available soon at www.ncsea.com.

STRUCTURE magazine

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December 2018

Last year, NCSEA’s Structural Engineering Emergency Response (SEER) Committee and the International Code Council signed an agreement to join forces to improve SEER’s 2nd Responder Roster by creating a single database of volunteers between the two organizations.

As of October 2018, the two organizations have joined forces to create the Disaster Response Alliance (DRA) to help communities get up and running as quickly as possible after a major disaster. The DRA maintains a single, national database of skilled volunteers willing to assist with response and recovery activities. These activities include post-disaster safety assessments, rapid safety assessments, detailed safety assessments, other building damage assessments, inspections and other code-related functions in the aftermath of a disaster. The DRA’s national database of volunteers is available to local and state jurisdictions as well as federal government agencies for pre- and post-disaster assistance. To add your name to the database, visit www.disasterresponse.org.

SEAMASS Uses NCSEA Grant Funding to Enhance Mentor Program

NCSEA Webinars

Register by visiting www.ncsea.com.

December 11, 2018 Understanding the AISC Direct Analysis Method of Design Donald White, Ph.D. This webinar lays out the key fundamental concepts associated with the Direct Analysis Method; various aspects of the method’s practical application are discussed.

January 15, 2019 Making Money: Design Services, Budgeting, and Scope Management Howard Birnberg This webinar discusses what considerations need to be assessed before signing a design services contract, as well as the design fee budget and why projects run over their design budgets.

January 22, 2019 Resilient Design & Risk Assessment Using the Quantitative & Building-Specific FEMA P-58 Analysis Method Curt B. Haselton, Ph.D., P.E. This course will cover how the FEMA P-58 analysis method is now being used in practice for both resilient design of new buildings and risk assessment of existing buildings for earthquake hazards.

January 29, 2019 Ethics in the Practice of Engineering Robert Kirkman, Ph.D. This webinar will consider the function of engineers in their societal role as professionals, and the habits or dispositions of character appropriate to that role.

Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 States.

STRUCTURE magazine

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December 2018

News from the National Council of Structural Engineers Associations

Over the past year, the Structural Engineers Association of Massachusetts Young Member Group (SEAMASS YMG) has made an effort to follow NCSEA’s mission to “advance the practice of structural engineering by representing and strengthening its Member Organizations” through their involvement in the Greater Boston ACE Mentoring Program. Their continued involvement as mentors has provided students with the opportunity to gain valuable exposure to the Structural Engineering profession, increasing their understanding of the occupation and inspiring them to pursue a career in the field.

This year’s program started in mid-October, and those involved with the program are eager to make use of the new teaching supplies. The addition of computers and structural design software to the ACE program will introduce the students to structural design software early on and provide a stepping-stone to the profession’s modern workflow. The SEAMASS YMG acquired six lightly used computers from a local engineering firm, and used the grant money to refurbish the computers as necessary. With the grant funding SEAMASS purchased new batteries, power cords, and Microsoft office licenses (Word, Excel, and PowerPoint) for the computers. The group was able to acquire free student versions of the structural design software to stretch their budget further. With the additional funds, SEAMASS YMG purchased a K’Nex set and shake tables for the ACE program, which will give the students a more hands on experience with structural engineering and help them understand the fundamentals of building frames and lateral design.

The group looks forward to using this equipment to encourage future students to pursue their passions through a career in structural engineering.

NCSEA News

ICC and NCSEA SEER Create National Database

Learning / Networking

SEI Update

The News of the Structural Engineering Institute of ASCE

YOU ARE INVITED On behalf of the Structural Engineering Institute of ASCE, we invite you to Structures Congress, April 24-27, 2019, in Orlando, Florida. Join more than a thousand of your peers from the U.S. and abroad for a great program of technical and professional learning, networking, and fun social activities.

We provide the perfect blend of interactive presentations on cutting-edge knowledge and practical application to advance your career and the profession. The program includes presentations on Blast, Bridges, Buildings, Business & Professional, Career Development, Codes & Standards, Education, Forensics, Materials, Natural Disasters, Non-Building, Non-Structural, Research, and more. You will learn from the experts including those that develop ASCE/SEI standards, hear from stimulating speakers and leaders, and pick up new tools and resources to exceed in every stage of your career. View the full program and register at www.structurescongress.org.

Be inspired to greater creativity and innovation by keynote speakers: Anthony Atala, M.D., on Regenerative Medicine: Building Tissues and Organs; Ashraf Habibullah, P.E., M.ASCE, on Structural Engineering: Indispensable to Civilization; and Scott Mallwitz with Walt Disney Imagineering.

Special Sessions are provided to strengthen skills on Communication, Conceptual Design, and a report on the Grenfell Tower Fire. The program also includes the Women in

Structural Engineering (WISE) Reception, new vendor sessions, and the Civil Engineering Podcast with Anthony Fasano. And, of course, there is the overall fun of being in Orlando, so we have secured discounts for golf and Disney to round out your trip.

Earn PDHs and enjoy fun social events like the Friday Special Evening Reception Celebrating the Future of SE, hosted by CSI. Make sure you purchase this additional ticket! 100% of ticket proceeds fund SEI strategic initiatives through the SEI Futures Fund in partnership with the ASCE Foundation.

Come early on Wednesday to check out an SEI committee. Invest in your future and experience all SEI/ASCE offers to prepare you to lead and innovate in structural engineering.

Encourage a student or young professional to apply for an SEI Futures Fund scholarship. Join us – and Laura Champion David Cocke bring your colleagues.

P.E., M.ASCE, S.E., F.SEI, F.ASCE, We look forward to Director, SEI of ASCE SEI President 2019, Structural Focus seeing you in Orlando!

NEW – Structural Fire Engineering Structural Fire Engineering provides best practices for performance-based structural fire engineering design, the calculated design of a structure to withstand the thermal load effects of fire, which have the potential to alter the integrity of a structure based on specific performance criteria.

When structural systems are heated by fire, they experience thermal effects that are not contemplated by conventional structural engineering design.

Traditionally, structural fire protection is prescribed for structures after they have been optimized for ambient design loads, such as gravity, wind, and seismic, among others. This centuryold prescriptive framework endeavors to reduce the heating of individual structural components with the intent of mitigating the risk of structural failure under fire exposure. Accordingly,

the vulnerability of buildings to structural failure from uncontrolled fire varies across jurisdictions – which have differing structural design requirements for ambient loads – and as a function of the building system and component configuration. As an alternative approach, ASCE/SEI 7-16 permits the application of performance-based structural fire design (also termed structural fire engineering design) to evaluate the performance of structural systems explicitly under fire exposure in a similar manner as other design loads are treated in structural engineering practice.

Manual of Practice (MOP) 138 addresses the current practice, thermal and structural analysis methods, and available information to support structural fire engineering design, and is a valuable resource for structural engineers, architects, building officials, and academics concerned with performance-based design for structural fire safety.

Membership

Join or Renew SEI/ASCE

For innovative solutions and learning, to connect with leaders and colleagues, and to enjoy member benefits such as SEI Member Update monthly e-news opportunities and resources – visit www.asce.org/myprofile or call ASCE Customer Service at 800-548-ASCE (2723).

Errata

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.

STRUCTURE magazine

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December 2018

SEI Futures Fund in Collaboration with the ASCE Foundation Give a year-end gift to:

• Invest in the future of structural engineering • Support student and young professional involvement in SEI • Provide opportunities for professional development Learn more and give at www.asce.org/SEIFuturesFund.

Students and Young Professionals (35 and younger): Invest in Your Future

SEI Update

Advancing the Profession

Join SEI/ASCE for technical, professional, and leadership experience to advance your career and the profession. Apply by January 4 for an SEI Futures Fund scholarship to participate and get involved in SEI at Structures Congress. www.asce.org/SEI

At the recent SEI Electrical Transmission and Substation (ETS) Structures Conference in Atlanta, Archie D. Pugh, P.E., PMP, M.ASCE, was presented with the 2018 Gene Wilhoite Award. Archie is a highly regarded leader in transmission engineering at AEP, with more than twelve years serving in a critical role in the siting, permitting, design, and construction of the Wyoming-Jackson Ferry, 765kV Transmission Line Project, the first 6 conductor bundle 765kV line in North

America. Archie is recognized with the Wilhoite Award for his leadership skills, willingness to share his knowledge and experience, and continuous education of his staff and peers in the electric utility industry. These attributes were critical to the success of the Wyoming-Jackson Ferry project and continue to serve him well in his position overseeing operations of AEP transmission systems in 7 states. Archie has also served on the SEI ETS Conference planning committee since 2006 (Chair in 2012), is a past member of MOP 113, and an alum of Virginia Tech with a B.S. and M.S. in Civil Engineering.

SEI Local Leaders Conference Forty-five local SEI Chapter leaders from across the U.S., including several SEI Graduate Student Chapter leaders, participated in the SEI Local Leaders Conference October 5-6, 2018, at ASCE in Reston, VA. The meeting included an update on SEI initiatives, exchange on best practices to serve local members, presentations from SEI President David Cocke on Vision initiatives and from ASCE staff on tools and resources for recruiting and retaining

members, K-12 outreach, engaging student and young professionals, etc. The program also included a tour of the U.S. Capitol and visit to the House floor with former Congressman Swett from NH, as well as ASCE Government Relations University and training on Personality as it relates to Leadership. Connect with your local Chapter or start one at www.asce.org/SEILocal. The next SEI Local Leaders Conference will be October 24-26, 2019, with a particular focus on Leadership/Facilitating Consensus training, made possible by the SEI Futures Fund in collaboration with the ASCE Foundation.

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SEI Online

Follow SEI on Twitter @ASCE_SEI

SEI Standards

SEI News

Visit www.asce.org/SEIStandards to: • View ASCE 7-22 Committee Meeting schedule and archive • Submit proposals to revise ASCE 7 STRUCTURE magazine

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Check out the latest news items at www.asce.org/SEI.

December 2018

The News of the Structural Engineering Institute of ASCE

Congratulations to the 2018 Gene Wilhoite Innovations in Transmission Line Engineering Awardee

Did you know?

CASE in Point

The News of the Council of American Structural Engineers

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 your young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several tools available for firms to use to enhance their internal policies and procedures – from office policy guides to employee reviews.

Tool 1-3

Sample Policy Guide

Tool 4-3

Sample Correspondence Guidelines

Tool 2-2

Interview Guide and Template

Tool 5-3

Tool 2-3

Employee Evaluation Templates

Managing the Use of Computers and Software

Tool 2-5

Insurance Management

Tool 5-5

Project Management Training

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

CASE 962 Practice Guidelines for Structural Engineer of Record UPDATED: The purpose of this document is to give firms and their employees a guide for establishing Consulting Structural Engineering Services and to provide a basis for dealing with Clients generally and negotiating Contracts in particular. Since the Structural Engineer of Record (SER) is usually a member of a multi-discipline design team, this document describes the relationships that customarily exist between the SER and the other team members, especially the team leader. Further, this Guideline promotes an enhanced Quality of Professional Consulting Structural Engineering

Services while also providing a basis for negotiating a fair and reasonable compensation. Additionally, it provides a basis for Clients to better understand and determine the Scope of Services that the Structural Engineer of Record should be retained to provide.

The committee did an all-inclusive update to this document plus added two new sections on the responsibilities for drawing releases for steel mill orders and Guaranteed Maximum Price releases.

You can purchase this and the other Risk Management Tools at www.acec.org/bookstore.

CASE Winter Planning Meeting

The 2019 CASE Winter Planning Meeting is scheduled for February 7-8, 2019, in Tampa, FL. The agenda includes: Thursday – February 7 1:00 pm – 5:30 pm CASE Executive Committee Meeting 6:00 pm – 8:00 pm CASE Roundtable Speakers: NCSEA SE3 Committee Members

Friday – February 8 7:30 am – 8:30 am Shared Breakfast 8:00 am – 12:00 pm CASE General / Toolkit Committee Meeting CASE Contracts Committee Meeting CASE Guidelines Committee Meeting CASE Programs & Communications Committee Meeting 12:00 pm – 1:00 pm Lunch 1:00 pm – 4:30 pm CASE General / Toolkit Committee Meeting CASE Contracts Committee Meeting CASE Guidelines Committee Meeting CASE Programs & Communications Committee Meeting 4:30 pm – 5:00 pm Committee Wrap-up Session

If you are interested in attending the meeting or have any suggested topics/ideas from a firm perspective for the committees to pursue, please contact Heather Talbert at htalbert@acec.org.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

STRUCTURE magazine

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December 2018

CASE 976-A – Commentary on Value-Based Compensation for Structural Engineers The importance of receiving adequate fees for structural services is vital for the engineering practice to thrive. If fees are not adequate, the structural engineering professional becomes a commodity; libraries are not maintained, computer software and equipment become outdated, and the quality of our product declines significantly.

Value-Based Compensation is founded on the concept that there are specific services, which may vary from project to project, that provide valuable information to the client and whose impact on the success of the project is far more than the prevailing hourly rates. Value-Based Compensation is based on the increased value or savings these innovative structural services will contribute to the project. As a result, the primary beneficiary of an innovative design or a concept is the owner, but the innovative engineer is adequately compensated for his knowledge and expertise in lieu of his time.

This document discusses the list of changes published in the preface of the 2010 Edition and provides some commentary to these changes. This document also addresses areas of the COSP that may not be well understood by some SERs but will likely have an impact on the structural engineer’s practice of designing and specifying structural steel.

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 • Financial Management • Client Relationships • Project Management • 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).

Fresh EJCDC Contracts to Meet Modern Market Demands EJCDC’s newly released 2018 Construction (C-Series) Documents are a significant modernization, revision, and expansion of the 2013 C-series and now the state-of-the-art in construction contract documents.

The updated edition comprises 25 integrated documents, including: • Fundamental contract documents such as the Standard General Conditions, the Small Project Agreement, and Supplementary Conditions • Forms for gathering information needed to draft bidding documents • Instructions for bidders and a standard bid form STRUCTURE magazine

• Bonds including bid, performance, warranty (new for 2018), and payment bonds • Administrative forms, such as change orders and a certificate of substantial completion EJCDC C-700, Standard General Conditions of the Construction Contract, has been extensively refreshed and updated, too.

The new EJCDC 2018 C-Series also includes expanded and updated “Notes to Users” and “Guidelines for Use” to provide more specific instructions, and it eliminates the need for notary and corporate seals.

To purchase these and other EJCDC documents, go to www.acec.org/bookstore.

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December 2018

CASE is a part of the American Council of Engineering Companies

CASE 976-D – Commentary on 2010 & 2015 Code of Standard Practice for Steel Joists and Joist Girders The specification of joists and Joist Girders can provide an economical structural solution, but there are very specific requirements that must be understood by all parties. The updated 2010 SJI COSP provides a more practical approach to specifying joists, introduces new design terms for use by the structural engineer, and identifies and clarifies topics that may have been subject to varying interpretation in the past. The more recently released 2015 SJI COSP provides additional clarifications and minor revisions.

CASE 976-C – Commentary on Code of Standard This Commentary provides observations and analysis of the Practice for Steel Buildings and Bridges revisions and additions in both documents and discusses The 2010 COSP addresses many recent changes in the practice specific aspects of the COSP that have a direct impact on of designing, purchasing, fabricating, and erecting structural the structural engineer’s practice of specifying steel joists. steel and is, therefore, a continuation of the trend of past Familiarity and understanding of the entire SJI COSP are improvements and developments of this standard. It is impor- necessary to ensure the proper design and documentation tant to note the Structural Engineer can change any of the of steel joists and Joist Girders. However, the Commentary’s requirements of the Code of Standard Practice by specifying an discussion highlights sections of interest to the specifying alternative in the Contract Documents. structural engineer.

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

CASE in Point

CASE Practice Guidelines Currently Available

Project: Center for Naval Aviation Technical Training Complex | Engineer: SMR-ISD Consulting Structural Engineers Photo by Pablo Mason, courtesy of Harper Construction Company

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