STRUCTURE magazine | December 2023

STRUCTURE DECEMBER 2023

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STRUCTURE DECEMBER 2023

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EDITORIAL BOARD Chair John A. Dal Pino, S.E. Claremont Engineers Inc., Oakland, CA chair@STRUCTUREmag.org Jeremy L. Achter, S.E., LEED AP ARW Engineers, Ogden, UT Erin Conaway, P.E. AISC, Littleton, CO Linda M. Kaplan, P.E. Pennoni, Pittsburgh, PA Charles “Chuck” F. King, P.E. Urban Engineers of New York, New York, NY Nicholas Lang, P.E. Vice President Engineering & Advocacy, Masonry Concrete Masonry and Hardscapes Association (CMHA) Jessica Mandrick, P.E., S.E., LEED AP Gilsanz Murray Steficek, LLP, New York, NY Jason McCool, P.E. Robbins Engineering Consultants, Little Rock, AR Brian W. Miller Cast Connex Corporation, Davis, CA Evans Mountzouris, P.E. Retired, Milford, CT Kenneth Ogorzalek, P.E., S.E. KPFF Consulting Engineers, San Francisco, CA (WI) John “Buddy” Showalter, P.E. International Code Council, Washington, DC

On the Cover: 2023 Structure of the Year Winner, Children’s Museum of Eau Claire, Wisconsin.

Photo courtesy of KPFF Consulting Engineers & ERA Structural Engineering

Eytan Solomon, P.E., LEED AP Silman, New York, NY Jeannette M. Torrents, P.E., S.E., LEED AP JVA, Inc., Boulder, CO

EDITORIAL STAFF Executive Editor Alfred Spada aspada@ncsea.com

Production production@structuremag.org

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MARKETING & ADVERTISING SALES Director for Sales, Marketing & Business Development Monica Shripka Tel: 773-974-6561 monica.shripka@STRUCTUREmag.org STRUCTURE magazine (ISSN 1536 4283) is published monthly ®

by The National Council of Structural Engineers Associations (a nonprofit Association), 20 N. Wacker Drive, Suite 750, Chicago, IL 60606 312.649.4600. Periodical postage paid at Chicago, Il, and at additional mailing offices. STRUCTURE magazine, Volume 30, Number 12, © 2023

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, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.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, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. POSTMASTER: Send Address changes to STRUCTURE magazine, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.

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.

D ECE M B ER 2023

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Contents D ECEM BER 2023 NCSEA SEE Awards

26 2023 STRUCTURAL ENGINEERING EXCELLENCE AWARDS Congratulations to the 2023 SEE Award Winners! Structure of the Year goes to Children’s Museum of Eau Claire by KPFF Consulting Engineers & ERA Structural Engineering.

Columns and Departments 7 Editorial Celebrating the 10th Anniversary of the SEI Futures Fund!

20 Structural Design Determination of Blast Loads on Buildings

By Abdulqader Mohammed and Abdulrahman Salah

46 Codes and Standards Solar Arrays Designed Incorrectly for Wind Uplift Loads

By Joseph Burns, P. E., S. E., FAIA, F. SEI, F. ASCE,

By Joe Maffei, S. E., Ph.D., Gwenyth Searer, S. E.,

F. IStructE, F. IABSE

Rob Ward, S. E., and Rafael Sabelli, S. E.

8 Letter to Editor Addressing Retention Is Good for Profession

25 Structural Forum Structural Pilgrimage to East Coldenham Elementary School

By Jim D’Aloisio P. E., LEED AP

48 Code Updates 2024 IBC Significant Structural Changes

By John “Buddy” Showalter, P. E., M. ASCE, M. NCSEA, and Sandra Hyde P. E., M. ASCE, M. NCSEA

10 Structural Influencers Emily Guglielmo, S. E., P. E., F. SEI

38 Structural Forces Building Settlement

By Dilip Khatri, Ph. D., S. E.

12 Structural Observations The Sky’s The Limit, But At What Cost ?

By Roumen V. Mladjov, S. E., P. E.

By Neil Wexler

42 Historical Structures 19th Century Mississippi River Bridges #10

By Dr. Frank Griggs, Jr.

16 Structural Design The Nγ factor in Soil Bearing Capacity Calculations

By Dr. N. Subramanian, Ph. D., F. ASCE

58 Structural Verse What’s an Engineer Doing

In Every Issue 3 Advertiser Index 45 Earth Retention Guide

44 Business Practices The Art of Delegated Steel Design

By Michael A. Stubbs P. E., S. E. and Adam Sanchez P. E.

52 NCSEA News 54 SEI Update 56 CASE in Point

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

STRUCTURE magazine is not a peer-reviewed publication. Readers are encouraged to do their due diligence through personal research on topics.

D ECE M B ER 2023

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EDITORIAL Celebrating the 10th Anniversary of the SEI Futures Fund!

What? Yes, the SEI Futures Fund is a decade old this month!

By Joseph Burns, P. E., S. E., FAIA, F. SEI, F. ASCE, F. IStructE, F. IABSE

T

he Structural Engineering Institute of ASCE created the SEI Futures Fund (SEIFF) in December 2013 to support the SEI Vision, and so this month is an excellent time to celebrate what we all have accomplished. It is also a time to look forward to what we will be supporting in 2024.

I will end with a request to consider the SEIFF in your year-end giving, especially since your gift now will have quadruple the impact on our ability to fund our future strategic initiatives through the CSI 3 to 1 gift match.

What Is the SEI Vision?

In 2013, SEI published A Vision for the Future of Structural Engineering and Structural Engineers: A case for change. The SEI Vision lays out an inspiring view of what the structural engineering profession could be by the year 2033, and it makes recommendations for SEI Board of Governors’ action to lead us to that vision. The SEI Vision is that the Structural Engineering Profession will be: • A unique, fully engaged profession with a strong identity; • Recognized for the contribution the profession makes to º the creation of inspiring structures, º public safety and risk management, and º economic and sustainable use of resources,the use of innovative technologies; • Stewards of the built environment; and • Attractive career path to the best and brightest.

What Is the SEI Futures Fund?

Recognizing the pressing need for professional volunteerism and financial resources necessary to bring about the SEI Vision, SEI established the SEI Futures Fund (SEIFF). Every dollar contributed goes directly to profession-building initiatives identified and approved by the SEIFF Board. The SEI Futures Fund operates in collaboration with the ASCE Foundation for administrative and professional fundraising expertise, so that 100% of your gifts are utilized to support our grants.

The goals of the SEI Futures Fund are unique: STRUCTURE magazine

to advance the art, It is very exciting to work on the ASCE 7 Flood science, and practice Loads Committee. We are working on the first of structural engimajor upgrades to this Chapter in many cycles. neering for a brighter As the Young Professional representative on the future for our profescommittee, I am helping to shepherd the group’s sion. It does this by thoughts and concepts into ballots for voting. funding four strategic - Jessica Mandrick P.E., S.E., LEED AP; initiatives outside the Gilsanz Murray Steficek LLP ASCE 7-22 Flood normal bounds of the SEI operating budget.

Loads Subcommittee Secretary-Historian-Balloteer It does not support scientific research. The four strategic priorities for funding are to: What Can You Do • Invest in the future of the profession, • Promote student interest in structural to Support the SEIFF? engineering, • Support younger members involvement in An Incredible Match Gift Opportunity!

SEI, and • Provide opportunities for professional In 2023, Computer and Structures, Inc. development. is matching donations to the SEI Futures Donors may be individuals, companies, or orga- Fund three-to-one up to $250,000! This nizations. The Board’s strategy for individuals has very generous challenge grant expires on been top-down, ensuring we have the support January 1, so please consider a year-end and commitment of the SEI Board, the SEIFF contribution to the SEI Futures Fund, as it Board, and the many SEI committees, chapters, will quadruple your donation’s impact on and members. Total grants from the SEIFF to our ability to fund our strategic priorities SEI initiatives over the past five years have been and future grants. We have one month left $1,027,000! to meet this challenge, and we need your support in fully achieving the remarkable matching gift. Many thanks to Ashraf What Will the SEIFF Habibullah and CSI for this challenge!

Support in 2024?

This year’s SEI Futures Fund Board members are Joe Burns, Linda Kaplan, Jim For 2024 the SEI Futures Fund Board has Harris, Dan Linzell, and Otto Lynch. Please committed more than $200,000 for these reach out to any of us if you have questions. strategic programs: We also want to thank Jon Magnusson for • Student and Young Professional Scholarship his excellent stewardship of the SEI Futures to engage at in-person SEI conferences. Fund as its Chair for 2022-2023, and parApply by January 3 for scholarship to ticularly for his role in securing the very SEICon24 March 19-22 at NASCC in San generous challenge grant from Computers Antonio – www.SEICon24.org/scholarships! and Structures, Inc. • Young Professional Travel Support for SEI Learn more and give at www.asce.org/ Standards committees.

SEIFuturesFund. Thank you!■ • SEI Carbon Impacts Workshop • Publish SEI Prestandard for Calculation Joseph Burns P. E., S. E., FAIA, F. SEI, F. ASCE, Methodology for Structural Systems in F. IStructE, F. IABSE, is a Managing Principal at Whole-Building Life Cycle Assessment Thornton Tomasetti in Chicago, and Chair of the • Advocacy for the Flood Loads Supplement SEI Futures Fund Board. #2 to ASCE 7-22 D ECE M B ER 2023

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LETTER to EDITOR

Addressing Retention Is Good for Profession

T

he article “Improving Engagement and Career Longevity in Structural Engineering” struck a chord with me. The author's candidness was perfect. The points were relevant and will help bring to light actual barriers to retainment in structural engineering. As a 20 something, female structural engineer, this article made my day. About a month ago, I almost left the industry to pursue a stress analyst role in aerospace. I ended up deciding to stick it out in the building industry a little longer. I have worked so hard to get where I am, and it is hard to let all of that effort go. To save money, I completed my master's degree in structural engineering in a year. To keep up with increasing levels of responsibility, I took the SE exams a year after I started my first job. For me, structural engineering sounded like a great career, as it combined lifelong technical learning and architecture. We play a vital role in the built environment, especially with regard to public safety.

However, lingering hardships can make this profession frustrating for female structural engineers. Sometimes it is just hard to deal with the "boys club" mentality of our profession, especially when it comes to CA work, and the poor maternity and paternity leave benefits in many small A|E firms make it feel like I am penalized as a female engineer for starting a family. I have seen female engineers get overlooked for promotions after they have a child, and this is really discouraging. There is a lot of lingering 1950s mentality left among firm leaders, unfortunately, but it is encouraging to see people advocate for good improvements in our profession. I have also witnessed

a lot of turnover among structural engineers in the building industry, and it has made me question my own reasons for going into the profession.

Addressing retention is good for the profession as a whole because when young structural engineers like me start questioning if we made a mistake in getting into this industry, we jeopardize losing the next generation.

Name withheld upon request

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structural INFLUENCERS Emily Guglielmo, S. E., P. E., F. SEI Emily Guglielmo, is a Principal at Martin/Martin, Inc. and leader of the San Francisco Bay Area office. She is Chair of the ASCE 7-28 Seismic Committee and is a voting member of the Wind and Main Committees. Emily is the chair of the National Council of Structural Engineers Associations (NCSEA) Code Advisory Committee. She has presented lectures on structural building code provisions nationally and internationally and was recognized with the Susan M. Frey NCSEA Educator Award. Emily currently serves on the boards of the Charles Pankow Foundation, the NCSEA Foundation, and the AISC Education Foundation. She is the President of the Structural Engineers Association of California (SEAOC) and served as the President of NCSEA in 2020-2021.

How did you become so involved in the development of codes and standards?

My journey into the world of code development can be traced back to a pivotal moment in my childhood. At the age of 8, I was a competitive gymnast, training for hours each day. One October evening, I was running down the vault runway when I was knocked off my feet. Sitting on the mat, I watched the lights sway, ultimately learning the San Andreas Fault had ruptured near the Loma Prieta peak. Witnessing the aftermath of collapsed roadways, bridges, and buildings, along with the realization of the significant loss of life, left a profound impact on my career choices.

As I pursued this career, I developed a deep passion for earthquake engineering, recognizing the critical role of robust seismic codes. However, as with many colleagues, the complexity of the codes frustrated me. When should an engineer apply the Ωo provisions, and why is Chapter 12 of ASCE 7 so convoluted? I do not believe in complaints without constructive participation.

However, a path to contribution was elusive for me.

Several years later, my journey took a significant turn at a Structures Congress, for which I was a Young Professional scholarship recipient. During an early, chilly morning run around Lake Michigan, I had the privilege of meeting John Hooper, who held the position of Chair of ASCE 7 Seismic.

From that day forward, John served as my mentor and advocate, facilitating my engagement in national code committee work.

While such work can be an exercise in patience, my participation has provided me with a deep understanding of technical topics, a mechanism to contribute to the improvement of our profession and communities and numerous lifelong friendships.

What do you think is the biggest misconception engineers have about the development of codes and standards?

One of the most common misconceptions among engineers is the belief that "codes are written by a bunch of academics who are just looking to get their research published." In reality, the process of developing codes and standards requires numerous technical experts from diverse backgrounds, including practicing engineers, producers, academics, government representatives, and building officials.

10 STRUCTURE magazine

Another prevalent misconception is that "the people who write codes just like to change things for the sake of change."

I know today this belief couldn't be further from the truth.

Those professionals who volunteer their time to update codes are genuinely committed to improving our profession.

While we are cautious about unnecessary change, we do carry the responsibility to advance our profession by consistently improving our codes. When compared to other industries, like technology and medicine, their embrace of innovation and change is integral to success. If our aim is to attain higher compensation or command greater respect, we must similarly embrace change, even if it entails mastering new aspects of our codes every six years.

I personally invite anyone who holds strong reservations about our codes to consider participating in a code committee meeting. You will find countless hard-working engineers giving their time and energy to improving our profession. Join us.

What advice would you give to young engineers?

My advice to young engineers is simple: Say Yes! In today's world, where many advocate to "say no," I believe that young engineers should seize every chance. There are countless opportunities specifically reserved for young engineers, from scholarships to designated committee positions. Take advantage of these prospects; step through each door and make connections.

Throughout my career, I've found that saying yes, especially when faced with challenging or intimidating situations, has resulted in new opportunities, meaningful relationships, and increased career satisfaction. Embrace the unknown, and don't be afraid to stretch beyond your comfort zone.

How do you balance your volunteer engagement with your professional responsibilities and personal life?

Balancing volunteer engagement, professional responsibilities, and personal life is a continuous journey, and my approach has evolved. In the early days, I sought a strict work-life balance, often leaving me overwhelmed and perpetually unsuccessful. Instead, I’ve shifted this mindset. Rather than seeking balance, I focus on being content and present with my choices. I may choose to leave work early to coach a soccer game, but recognizing such a choice might result in an evening work session. I might miss a school concert, opting to attend an out-of-town committee meeting. At the end of the day, if I am satisfied with my choices and present in my activities, then I’ve found my balance.

What do you think is the biggest challenge facing our profession in the next ten years?

I believe that one of the most significant challenges (and opportunities) facing our profession in the next decade is the rapid Emily accepting the 2023 NCSEA Service Award. advancement of Artificial Intelligence (AI). We stand at an inflection point and must make a choice avenues for value creation. If we harness AI's capabilities to to resist or strategically embrace AI's disruptive potential. handle monotonous tasks, it will liberate engineers to focus In the next five years, AI is likely to replace many of the tasks on innovation and creative problem-solving. we currently perform. Engineers who are unprepared to answer the question "what's next" run the risk of being left behind. Who inspires you?

To ensure a meaningful future, we must actively explore new My three incredible kids. They are a constant source of inspiration, reminding me to be brave, to ask thought-provoking questions, and to approach each day with boundless energy.

Their curiosity, resilience, and unwavering enthusiasm inspire me to be a better person.

Family Vacation in Banff, Canada.

What do you think you will be remembered for in terms of your personal legacy, and/or what are you most proud of?

I’m not preoccupied with a personal legacy, but I do hold a deep aspiration for the structural engineering profession that I will leave behind. I hope my contributions will contribute to a profession that is markedly more diverse and adaptable and seizes new opportunities.

Throughout my career, I've had the privilege of learning from others benefiting from the wisdom, energy, and innovations of those who came before me. I hope my efforts serve as a launching pad for future generations of engineers, propelling them to take structural engineering to new heights. My pride is in the enduring impact of my work on the future evolution of our profession.■ D ECE M B ER 2023

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structural OBSERVATIONS The Sky’s the Limit, But at What Cost?

Extreme structure development and the future.

By Roumen V. Mladjov, S. E., P. E.

S

ince ancient times, building greater, stronger, and faster has been part of humanity’s aspiration for greater achievement.

The desire to surpass prior achievement is strongly embedded in the human mind and essential for development and progress. Since their projects can be easily compared, measured, and assessed for efficiency, engineers and builders have historically been on the frontline of this effort.

Exceptionally tall buildings and long-span bridges have a powerful impact and often become landmarks and symbols of a society’s financial, technological, and sociopolitical prowess. Earliest in this respect was the ziggurat temples of ancient Mesopotamia, most famously that at Babylon (the historical prototype of the Tower of Babel in the Bible), of which Nebuchadnezzar II (605–562 BC) boasted to have “raised its top to heaven” while just 107 ft (60m) tall. Not surprising as the first modern “skyscrapers” in 1884–1888 were not taller than 131–164 ft (40–50m). Similarly, Egyptian pharaohs strove to dazzle with the height and size of their pyramid tombs, most famously those at Giza. However, Khufu’s tomb (shortly after 2600 BC), whose original height was 481 feet (146.6m), remained the tallest building until the fourteenth century.

The Lighthouse of Alexandria, a building with multiple floors with fire on top, was built by Sostratos of Knidos (282–246 BC) at the entrance of Alexandria’s harbor. It stood probably some 328 feet

(100m) tall. Damaged by earthquakes, the structure collapsed completely in the 14th century.

Medieval European cities likewise competed with the size and height of their cathedrals. The Lincoln in England exceeded the height of Khufu’s pyramid with its spire completion in 1311, reaching 525 feet (161.5 m). In modern times, the Eiffel Tower, erected in Paris in 1889, became the tallest man-made structure (until 1930) at 986 feet (300.5m), later increased to 1,083 feet (330m) with an antenna.

This was the first monument built not for a higher power but to demonstrate technical advancement and ability. It also became a symbol of its city, country, and humankind’s everlasting strive toward new heights and achievements.

Modern skyscrapers originated in Chicago and New York City in the 1880s, with the Home Insurance Building in Chicago (1885), ten stories rising 138 feet (42.1m), later amplified to twelve stories (1891) and 180 feet (54.9m). The total height and number of stories became the subject of fierce competition in erecting these buildings.

An example would be the Chrysler Building in New York City (1930), 1046 feet (318.8m), designed to exceed the height of both a skyscraper completed just before it and of the Eiffel Tower. Walter Chrysler specifically directed his architect, William Van Allen, to “build it taller than the Eiffel Tower!” However, as soon as the next year, the record for the tallest building passed to the Empire State Building (1931),

Figure 1 The Tower of Babel by P. Bruegel the Elder (left), 19 th century Eiffel Tower (center), 21st century One World Trade Center, NY (right).

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Figure 2 (left to right) Chrysler Building, Empire State Building, John Hancock, Chicago 1969, World Trade Center 1972 – 1973, NY, and Sears (Willis) Tower, 1973 Chicago.

with 1250 feet (381m), which was also the fastest-built skyscraper.

The United States continued to lead in tallest buildings with the famous Chrysler Building, Empire State Building, New York City; World Trade Center, New York City, 1973, and Sears (now Willis) Tower, Chicago, 1973, until the completion of Petronas Towers in Kuala Lumpur in 1998.

The construction industry keeps track of and ranks accomplishments in the tallest buildings and structures, largest covered arenas, and longest bridge spans. Each new structural record has outcompeted prior top achievements in some respect, calling for exceptional designers and builders, innovative ideas and technology, efficient structural systems, stronger materials, excellent organization, and persistent efforts. This renders the competition for improvement essential for structural progress.

The inspiration to build greater, stronger, and faster is admirable, but is there a limit, or should there be one? Engineers can build a 9,800 feet (3,000m)-span bridge, but would the same be advisable for a 6,600 feet (2,000m)-tall tower or a 3,300 (1,000m)-wide building? No doubt they can achieve such extreme tasks, but are they really needed and functionally justified? Will they be useful and efficient once built?

Engineers and builders continue building taller and larger as such demands arise; however, in many cases, these needs have already been satisfied, if not surpassed. There is no genuine or pressing need for more skyscrapers taller than 2,500 feet (800m). A 3,300 feet (1,000m)-tall building cannot be more efficient to build and maintain than two 1,600 feet (500m) ones with equal floor area. Perhaps, by the 2020s, the era of super skyscrapers may be over. Satellites circling the earth provide better service than excessively tall TV and radio towers.

This leaves long-span bridges in specific rare cases where such are needed due to natural, socioeconomic, or geopolitical factors. Extreme structures have driven builders to use their best knowledge and experience and inspired further development in structural engineering.

Unfortunately, plenty of extreme structures had to be abandoned or even demolished soon after completion, becoming superfluous or unsustainable, including large sports arenas built for unique events.

Remarkable examples of this are the NY State World Fair Pavilion; Houston Astrodome, Texas; Olympic Stadium, Athens; the 105-story hotel in Pyongyang, North Korea, etc.

Despite such considerations, the urge to build taller and greater to demonstrate prosperity and superiority remains. Enterprising engineers and builders, commissioned and encouraged by powerful, wealthy, and ambitious states, corporations, or individuals, continue trying to surpass previous achievements. Since overcoming the challenges posed by such projects have contributed to the further development of structural and technological knowledge, instead of suppressing the natural desire for greater achievement, it might be better to redirect such competitiveness toward more efficient and economic structures.

What is more beneficial for society – to build a single record-breaking tall building or multiple tall structures at 80-85% of the cost and materials?

It may be necessary to consider most new projects as a balance between meeting the justified requirements and achieving a higher level of efficiency to achieve this shift in values. Such undertakings would require the persistent and combined effort of professional, structural, and architectural associations, universities, and journals.

In this time of heightened ecological consciousness, changing energy and climate conditions, and aim for sustainability in all aspects of life, it makes sense to put the needs of the community and the Earth above self-purpose and pride. Therefore, build as tall, long, and strong as necessary but efficiently, combining science and reason: build more with less!

There are already buildings taller than 800m, stadiums larger than 300m, and bridge spans longer than 2,000m, with current record achievements as follows: D ECE M B ER 2023

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Figure 3 Sydney Harbor bridge, Australia, 1932, L=1,650 ft (503 m) (left); Golden Gate Bridge, San Francisco, USA, 1937, L = 4,200 ft (1,280 m) (center); Akashi Kaikyo Bridge, Japan, 1998, L = 6,532 ft (1,991 m) (right).

The tallest building is Burj Khalifa, Dubai, UAE, 2010; the tallest building in the US is One World Trade Center, New York, 2014, 94 stories, 1,777 feet (541 m) tall, replacing the two World Trade Center towers, destroyed by terrorist attacks on September 11, 2001. Among the tallest buildings, the most efficient are: • Sears Tower (now Willis Tower), Chicago, 1973: 1,451 feet (442m), 108 stories • John Hancock Center, Chicago, 1969: 1,128 feet (344m), 100 stories • Burj Khalifa, Dubai, UAE, 2010: 2,717 feet (828m), 163 stories

The tallest free-standing tower is Skytree Tower, Tokyo, Japan, 2012, a steel structure with a roof at 1,624 feet (495m) and an antenna at 2,080 feet (634m). The tallest guyed mast is the KVLY-TV mast, Blanchard, North Dakota, 2,064 ft (629 m). The world’s tallest nonbuilding structure ever built was the Warsaw Radio Mast, 2,121 feet (646m), built in 1974 and collapsed in 1991.

By that time, the height of skyscrapers had been surpassed by the Ostankino TV Tower in Moscow (1967) at 1,762 feet (540m) and the CN Tower in Toronto (1976) at 1815 feet (553m).

The development of the longest bridge spans is no less fascinating than the development of tall structures, starting from the ancient still-in-use bridges (spanning a few meters) to the George Washington Bridge, 1931, New York, the first bridge surpassing 3,500 feet (1067 m), up to todays’ bridges with about 6,600 feet (2,000 m) spans. Moreover, a newer system – hybrid suspension, a combination between classic suspension and cable-stayed was recently introduced with promising more efficient structure for long spans (still in theoretic concept).

All these are great achievements, enlarging the boundaries of structural possibilities. But, of course, the longest span is not the only element that counts. There are multiple ratings for the ten Greatest bridges; some are always part of these lists: Sydney Harbor, Golden Gate, and Akashi.

Needs of a Growing World Population The world population was 8 billion in September 2022 and is estimated to reach 9 billion in 2042. The world population continues to grow, and people satisfy their needs by building more houses, factories, larger cities, and infrastructure. How can we, as engineers, help make this growth less damaging to the planet? The simple response is to design and build more efficiently with fewer materials – concrete, steel, masonry, and wood. Saving even 10 or 15% in materials will reduce the carbon footprint by the same percentage. It would be most beneficial to redirect the “competition” in construction from building extreme or record-setting structures to building more efficiently.

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Structural Efficiency and How to Measure It The simplest way to determine the most efficient structural system for different types of structures is to compare their highest achievements. But this does not help estimate a specific project’s efficiency in terms of cost or construction materials. The main goal of efficient and economic design is to build a specific structure for the lowest possible cost, with fewer construction materials, while providing a high level of functionality and safety based on the design criteria and code requirements. The amount of construction cost measures the efficiency and economy of a structure, the quantity of materials used, and the total structural weight, all of them per unit area (ft2 or m2) – for example, $/ft2, concrete in ft3/ft2 or steel in pounds/ft2. For example, among the tallest buildings, the most efficient are: • Sears Tower (now Willis Tower): 0.36 kg/m3 • John Hancock Center: 0.43 kg/m3 • Burj Khalifa: 0.50 kg/m3

While the Burj Khalifa, currently the tallest skyscraper in the world, is among the most efficient, it is worth noting that it (and others built recently) is significantly less efficient than skyscrapers built in the 1960s and 1970s.

Comparing cost, materials, or weight per unit area works for comparable spans L or heights H. Still, it does not work for significantly different dimensions, as larger structures have more demand than smaller structures.

Structures with different dimensions can be compared using efficiency/ economy (E/E) coefficients, which are structural cost, materials, or weight divided by the product of the area times the structural span L (or the height H for tall buildings), where L is the single-bay span or the average span for structures of two or more spans. The E/E cost coefficient is $/ (m2 × L); for structural steel, it is kg/ (m2 × L); for concrete, it is m3/ (m2 × L). The smaller the E/E coefficient, the higher the structure’s efficiency in cost or materials.

For example, engineers can probably use bridge efficiency as the best option for estimating structural system efficiency because the “bridge” constitutes 95% or more of the total project. Steel is the material used for most extreme structures. Lately, composite steel-concrete is gaining a place, mostly for structural elements resisting high compression forces (as concrete-filled steel sections or tubes (CFST/S).

The most efficient bridge systems are: Table 1 Bridge Efficiency by System

System

Average Span Length ft (m)

E/E steel (kg/m3)

E/E cost ($/m3)

Suspension

3,967 (1,209)

0.98

16.79

Cable-stayed

1,683 (513)

2.46

35.27

Steel continuous girders

673 (205)

3.00

14.20

Steel arch

722 (220)

4.50

39.83

Similarly, just as extreme structures inspire further development, more efficient structures would stimulate excelling in the structural economy. Engineers should be encouraged to design and build more with less. It is time for competitions like “best projects of the year” to recognize the importance of efficiency and economy in their considerations. Building more with less is essential for efficiency in construction and a necessity for our future.■ Roumen Mladjov, S. E., P. E., Roumen’s main interests are structural and bridge development, structural performance, seismic resistance, efficiency, and economy. (rmladjov@gmail.com).

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Currently, the most efficient bridge system is the suspension, as shown in Table 1. The longest spans utilizing this system at present are Çanakkale Bridge, Dardanelles, Turkey (2022): 6,637 feet (2,023m) and Akashi-Kaikyo, Japan (1998): 6,532 feet (1,991m).

There were projects for even longer suspension steel bridge spans like at Messina Strait with a span of 10,827 feet (3,300m) and T.Y. Lin’s at Gibraltar Strait with a span of 16,400 feet (5,000m), but these were never completed.

Considering the E/E coefficients above, one can compare systems with close average lengths. For larger differences in span length, the significant increase in demand for longer spans should be considered.

It is important to avoid extreme minimalism, as the main requirement for any structure is to be strong, resilient, and safe, based on structural code requirements and the judgment of the responsible structural engineer.

Assembling a database for different types of structures with information for project parameters, efficiency and economy data will help engineers and their clients select the most appropriate and efficient system from the early design stages. Such a database could be developed with help from academia, professional organizations and publications, and engineering, architectural, and building companies. Academia should teach students efficiency and economy from the earliest stages of their professional training. Reducing structural materials quantity in construction is the most efficient way to limit its carbon footprint and preserve the environment. Moreover, competing for more efficient structures would involve a larger base of engineers than the very few who have had the chance to participate in the design of extreme projects. Involving more engineers in such efforts would increase the chances of innovative solutions and greater efficiency.

Conclusion The competition for more extreme structures is far from over. The continuing development of design, technology, and construction materials allows for building even more extreme and efficient structures: “The Sky is the limit.” This urge for greater achievement has contributed to today’s high accomplishments and is a powerful tool for further structural progress. Unfortunately, when an elite team is working on a new record-setting structure, the question of efficiency and economy recedes to second place. Therefore, when developers, engineers, and builders are “reaching for the Sky,” they should remember that achieving more efficient and economical projects is equally important to save resources and the environment. While competing for record achievements advances structural progress, we should remember that efficiency and economy are at least equally important.

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structural DESIGN The Nγ factor in Soil Bearing Capacity Calculations Learn about different philosophies used in the determination of soil bearing capacity.

By Dr. N. Subramanian, Ph. D., F. ASCE

O

ne of the first steps required in any structural design is determining the safe bearing capacity of the soil, qa, at the site. Terzaghi (1943) was the first to present a comprehensive theory for evaluating the ultimate soil-bearing capacity of shallow foundations.

Terzaghi suggested that for a continuous or strip foundation (i.e., one whose width-to-length ratio approaches zero), the failure surface in the Figure 1 General shear failure surface as assumed by Terzaghi for a strip footing (after Terzaghi, 1943). soil at ultimate load may be assumed as a logarithmic spiral surface (see Figure 1). (Note Terzaghi’s bearing capacity equations have now been modified to that this is the case of general shear failure.) consider the effects of the foundation shape, depth of embedment, and The ultimate soil-bearing capacity equations contain three factors: load inclination. The details of these equations may be found in any Nc, Nq, and Nγ as shown in Eqn.1. Although there is not much dis- standard textbooks on Soil Mechanics [e.g., Bowles (1996), Coduto pute in the calculation of Nc and Nq, there is disagreement regarding (2015)]. It must be noted that many design engineers still use the using the proper value of Nγ. The differences in the reported Nγ Terzaghi equation, which is considered to provide fairly good results. values are substantial compared with those proposed initially by Terzaghi. Hence, one must be careful while selecting the Nγ value in their calculations.

Bearing Capacity Factors Using an equilibrium analysis, Terzaghi expressed the ultimate bearing capacity of continuous or strip foundation as: Although there is no dispute in the calculation of Nc and Nq, there is more disagreement regarding the proper value of Nγ . Some of Q qu = = cNc + γ0 Df Nq + 0.5γ1BNγ (1) the proposed equations for these factors by different researchers are A discussed below.

Where A = area of footing in ft2, B = width of footing in ft, c = cohesion in psf, Df = depth of surcharge in ft (See Figure 2), Equations Suggested by Terzaghi (1943) Q = ultimate bearing capacity, lb, qu = ultimate bearing capacity in psf, γ0, γ1 = unit weight of soil in lbs/ft3, Nc, Nq, Nγ = bearing The bearing capacity factors were defined by Terzaghi (1943) as capacity factors that are non-dimensional and are functions only below: of the soil friction angle ϕ. This equation (1) is known as BuismanTerzaghi equation.

Nc = ^Nq - 1h cotz (4) The net ultimate bearing capacity and safe bearing capacities will be 3r z as follows: (Deducting γ0Df from qu to get qnet) e 2c 4 - 2 mtanz Nq = (5) z qnet = cNc + γ0Df (Nq − 1) + 0.5γ1BNγ (2) 2cos 2 c 45 + 2 m qsafe = qnet /FS + γ0Df

(3)

It must be noted that no safety factor is applied to the surcharge.

16 STRUCTURE magazine

Nγ = 0.5 c

K pc - 1 m tanz cos 2 z

(6)

Figure 2 Depth of Surcharge: (a) ordinary footing; (b) footing in the basement; (c) raft foundation

Where Kp γ is the passive earth pressure coefficient =

Nc = 2 + π = 5.14

1 + sinz 1 - sinz

for ϕ = 0

(9a)

Nγ = 2(Nq + 1)tanϕ

For foundations that exhibit the local shear failure mode in soils, Terzaghi suggested the following modifications to Equation (1): Q 2 qu = A = 3 cNc´ + γ0Df Nq´ + 0.5γ1BNγ´ (Strip foundation)

(7)

Nc´, Nq´, and Nγ´ are the modified bearing capacity factors and

2 calculated by replacing ϕ by ϕ´= tan−1 a 3 tanz k The variation of Nc´, Nq´, and Nγ´ with the soil friction angle ϕ is given in Table 2.

Guidance for selecting general and local shear failure is given in Table 3.

(10a)

Vesic (1975) considered the possibility of local shear failure in his bearing capacity equations. Terzaghi (1943), on the other hand, only considered general shear failure. Local shear failure is a type of bearing capacity failure that occurs when the soil beneath the footing is not strong enough to support the load.

The soil shears and yields, but the footing does not tilt. This type of failure is common in loose sands and soft clays. General shear failure is a type of bearing capacity failure that occurs when a large portion of the soil beneath the footing shears. This type of failure is common in dense sands and stiff clays. Vesic’s bearing capacity equations are more complex than Terzaghi’s equations because they account for the possibility of local shear

Equations Suggested by Vesic (1975) Vesic (1975) used the following formulas to compute the bearing capacity factors:

z Nq = e rtanz tan 2 c 45 + 2 m Nc = (Nq − 1)cotϕ

for ϕ > 0

ϕ

Nc’

Nq’

Nγ’

0

5.7

1.0

0

5

6.74

1.39

0.074

(8)

10

8.02

1.94

0.24

(9a)

15

9.67

2.73

0.57

20

11.85

3.88

1.12

ϕ

Nc

Nq

Nγ

25

14.80

5.60

2.25

0

5.70

1.00

0.0

30

18.99

8.31

4.39

5

7.34

1.64

0.5

35

25.18

12.75

8.35

10

9.61

2.69

1.2

40

34.87

20.50

17.22

15

12.86

4.45

2.5

45

51.17

35.11

36.00

50

81.31

65.60

85.75

20

17.69

7.44

5.0

25

25.13

12.72

9.7

30

37.16

22.46

19.7

35

57.75

41.44

42.4

Void ratio, e

Soil condition

Recommended method

40

95.66

81.27

100.4

>0.75

Loose

Local shear

45

172.28

173.28

297.5

<0.55

Dense

General Shear

Medium

Interpolate between 1 & 2 (mixed shear)

50

347.50

415.14

1153.2

Table 1 Terzaghi’s modified bearing capacity factors Nc, Nq, and Nγ (Source: Bowles,1996)

Table 2 Terzaghi’s modified bearing capacity factors Nc’, Nq’, and Nγ’

0.55 to 0.75

Table 3 Guidance for the selection of general and local shear failure (IS 6403:1981)

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ϕ

Nc

Nq

Nγ (Vesic)

Nγ (Kumbhojkar)

Nγ (Hansen)

Nγ (Mayerhof)

Equation (13)

Nγ (German code)

0

5.14

1.00

0.0

0.0

0.0

0

0

0.00

5

6.49

1.57

0.45

0.14

0.08

0.07

0.466

0.05

10

8.35

2.47

1.22

0.56

0.39

0.37

1.31

0.26

15

10.98

3.94

2.65

1.52

1.18

1.13

2.94

0.79

20

14.83

6.40

5.39

3.64

2.95

2.87

6.20

1.97

25

20.72

10.66

10.88

8.34

6.76

6.76

12.96

4.51

30

30.14

18.40

22.40

19.13

15.07

15.66

27.66

10.05

35

46.13

33.30

48.03

45.41

33.93

37.16

61.48

22.62

40

75.31

64.20

109.41

115.31

79.55

93.70

145.20

53.04

45

133.88

134.88

271.76

325.34

200.82

262.75

374.04

133.90

50

266.89

319.97

762.89

1072.80

570.20

876.36

1092.58

380.21

Table 4 Bearing capacity factors Nc, Nq, and Nγ by Vesic and other researchers

failure. Vesic’s equations also include shape factors and depth factors that account for the effects of footing shape and depth on bearing capacity (shape and depth factors are not discussed here).

The German code DIN 4017 uses different values of Nc and Nq than the Terzaghi bearing capacity formula. These values are based on more recent research and were found to be more accurate for German soil conditions. The German code DIN 4017 uses the following expression for Nγ . Note that in this equation (Nq − 1) is used instead of (Nq + 1): Nγ = (Nq − 1)tanϕ

(10b)

It has to be noted that Reissner (1924) was the first to formulate the soil bearing capacity problem, and using some simplifying assumptions, solved it by the methods of theory of plasticity (Vesic, 1975).

For weightless soil (γ = 0), he derived the equations for Nq and Nc as given in Equations (8) and 9(a) (Vesic, 1975). It is important to note that both Reissner’s and Terzaghi’s equations for Nq are approximations of the actual bearing capacity of a footing. The most accurate way to determine the bearing capacity of a footing is to perform a load test.

Values of Nγ Suggested by Other Researchers

Nγ = 1.5(Nq − 1)tanϕ

(11)

Mayerhof (1963) proposed the following equation: Nγ = (Nq − 1)tan (1.4ϕ)

(12)

Following Terzaghi’s assumptions, Kumbhojkar (1993) developed a numerical solution procedure to determine Nγ and found that the results agreed with Terzaghi’s calculations. The Nγ expressions suggested by different researchers are compared in Table 4. Note that the values of Nc and Nq remain the same for all the researchers.

Chen and McCarron (1991) suggested the following equation for Nγ: z Nγ = 2(Nq + 1)tanϕ tan c 45 + 5 m

(13)

The above equation results in a value that is 1.036 times the value of Vesic for ϕ = 5°, 1.235 times the value of Vesic for ϕ = 30°, and 1.376 times the value of Vesic for ϕ = 45°.

Han et al., 2016 Rough footing

λ (as per Han et al., 2016, Smooth footing)

ϕ

Brinch Hansen (1970) provided a lower bound expression to experimentally determined values. His expression is of the form:

Bolton and Lau (1993)

Smith (2005)

Kumar (2009)

0

1

10

100

104

104

5

0.085

0.210

0.242

0.246

0.248

0.495

0.09

–

0.087

10

0.281

0.619

0.707

0.721

0.723

1.446

0.29

–

0.282

15

0.699

1.411

1.605

1.637

1.641

3.282

0.71

0.70

0.699

20

1.579

2.968

3.375

3.445

3.452

6.904

1.60

1.58

1.577

25

3.461

6.137

6.995

7.145

7.163

14.32

3.51

3.46

3.457

30

7.653

12.92

14.80

15.16

15.19

30.38

7.74

7.65

7.644

35

17.58

28.47

32.88

33.76

33.86

67.73

17.8

17.6

17.55

40

43.19

67.50

78.96

81.44

81.74

163.4

44

43.2

43.08

45

117.6

178.1

212.1

220.4

221.3

442.7

120

118

117.1

50

372.0

547.6

667.8

701.7

706.2

1412.6

389

372

370.0

Table 5 The factor Nγ of strip footings using the method of characteristics (Han et al., 2016)

18 STRUCTURE magazine

Smooth footing

ratio λ = 0, the computed values by them matched the values of other researchers. However, when λ = 104, the computed values were almost double that of λ = 0. The comparisons in Table 5 show that the value of Nγ increases with the increase in λ for a certain friction angle. For smooth and rough footings, when λ = 104, the value of Nγ approaches Chen’s theoretical upper bound value (1975).

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TAYLOR DAMPED D MOMENT FRAME E™ SIMPLIFIED

D ECE M B ER 2023

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The cohesion term (cNc) predominates in cohesive soils, whereas the depth term (γ0 Df Nq) predominates in cohesionless soils; only a small increase in Df will increase qu substantially. The base width term (0.5γ1BNγ) increases bearing capacity for both cohesive and cohesionless soils. According to Coduto (2015), there is much more disagreement regarding using the proper value of Nγ. This is because relatively small changes in the geometry of the failure Summary and Conclusions surface below the footing can create significant differences in Nγ, especially in soils with high friction angles.

The safe bearing capacity (SBC) of the soil, qa, at the site is It is seen from Table 4, that for angles larger than 35°, the bear- required to design any structure founded on shallow foundaing capacity factors change rapidly and by significant amounts. tions. Although the settlement analysis has to be performed to As the estimated bearing capacity qu is further reduced to the determine SBC, equations proposed by Terzaghi and later by allowable safe bearing capacity, qsafe, by using a safety factor, the others to calculate the ultimate bearing capacity have been used probability of qsafe being safe is very high. Vesic (1975) recom- and adopted in codes with a safety factor. The equations involve mends that the depth factors (not discussed here) should not be three bearing capacity factors: Nc , Nq, and Nγ . Although the values used for shallow foundations (D/B ≤ 1) due to the uncertainties of Nc and Nq have been accepted by researchers, the value of Nγ in the quality of overburden. Vesic (1975) commented that the alone is not accepted and alternate equations have been proposed widely used Terzaghi factors shown in Table 1, though not sub- by many researchers. A few of these alternate equations have been stantially different numerically, are being gradually abandoned, presented and compared. It is seen from Table 4, that for angles as they are based on obviously incorrect failure patterns. He also larger than 35°, the bearing capacity factors change rapidly and by mentioned that an increasing trend among practicing engineers significant amounts. Hence, one has to be careful while selecting and researchers is to use the Nγ values given by Vesic (1975), the Nγ value in their calculations. It is seen that the Nγ equation which are based on the theory by Reissner and later researched suggested by the German code, DIN 4017, is not only simple in

by Caquot-Kerisel, and given in Table 4. format but also seems to give reasonable values.■ Some researchers have found that the value of Nγ is related to the soil friction angle ϕ and other parameters, such as q, c, γ, Full references are included in the online version of the article and B. For example, Xiao et al. (1998) calculated the bearing at STRUCTUREmag.org. capacity using the method of characteristics (MOC) and found that Nγ is affected by all q, c, γ, and B and that Nγ is affected by ϕ and γB/(c + qtanϕ) only when the load is vertical. Michalowski Dr. N. Subramanian, Ph. D., F. ASCE, FNAE is a consulting engineer living (1997) and Silvestri (2003) studied the influence of c/γB and q/γB in Gaithersburg, MD. He has a doctorate from IITM, India, and he also on Nγ using the limit analysis method and the limit equilibrium worked in Germany for 2 years as an Alexander von Humboldt Fellow. He method and have shown that for a given value of ϕ, the value has over 45 years of professional experience in consultancy, research, and of Nγ significantly changes with c/γB or q/γB. Zhu et al. (2003) teaching. He can be reached at (drnsmani@gmail.com). showed that Nγ is related to the friction angle ϕ and also to the surcharge ratio λ [λ = (q + ccotϕ)/γB].

Han et al. (2016) used a finite difference analysis program. They found that the devices inc. value of Nγ is influenced not only by the friction angle ϕ but also by the surcharge ratio λ (they note that this conclusion is only valid for Mohr–Coulomb soil). They also studied the relationship of Nγ between smooth and rough foundations (footingsoil interface friction) and found that the value of Nγ for a smooth footing is only DAMPER R DESIGN about half that for a rough footing. Thus, implying that the roughness of the footing base has a large impact on Nγ. Instead, small values of the interface friction δ (10° No N o Peer Review wR Required equ uired to 15°) are required to obtain rough footNo Tim Time-History me History A Analysis na ing effects [Chen and McCarron (1991)].

Table 5 shows the results from their studies Quick Design Times for various values of λ for smooth footing, a particular value of λ for rough footing, and also the results obtained by Bolton and Lau (1993), Smith (2005), and Kumar (716) 694-0800 | www.taylordevices.com ESR-4769 (2009). It is seen that when the surcharge

Photo credit: John Doogan/WSP

Discussions on the Computed Values of Bearing Capacity and the Factor Nγ

19

structural DESIGN Determination of Blast Loads on Buildings Designing for blast loading.

By Abdulqader Mohammed and Abdulrahman Salah

T

his article focuses on the structural aspects of designing for blast resistance.

This involves quantifying blast overpressures from accidental explosions and establishing design blast loads from these overpressures. This article provides a guide for design engineers and others to design new blast-resistant buildings and evaluate existing buildings for blast resistance. It provides the basic considerations and principles involved in determining blast loads on buildings.

Blast Wave Parameters

Figure 1 Characteristic shapes of blast waves & impulse. Courtesy of Buildings and Infrastructure Protection Series/Federal Emergency Management Agency’s BIPS 06/FEMA 426: Reference Manual to Mitigate Potential Terrorist Attacks against Buildings, 2nd Edition.

For blast-resistant design, the most significant feature of an explosion is the sudden release of energy to the atmosphere, which results in a pressure transient, or blast wave. This has a sudden, almost instantaneous rise in pressure above ambient atmospheric conditions to a peak free field (side-on or incident) overpressure. The peak side-on overpressure gradually returns to ambient with some highly damped pressure oscillations. This results in a negative pressure wave following the positive phase of the blast wave. Side-on pressure refers to the rise in pressure above ambient produced by a shock wave sweeping unimpeded across any surface (walls or roof ) not facing the blast source.

The integrated area under the pressure verse time function is known as the impulse: I = ∫P(t)dt I = impulse (pounds per square inch-milliseconds (psi-ms) or megapascals-milliseconds (MPa-ms) P = Pressure (psi or MPa) T = time (ms)

-

Pso td Io Pso− td− Io− Pr qo U Lw

Peak side-on positive overpressure Positive phase duration Corresponding positive impulse Peak side-on negative pressure (suction) Negative phase duration The associated negative impulse Peak reflected pressure Peak dynamic (blast wind) pressure Shock front velocity Blast wavelength

Peak Reflected Pressure, Pr Upon encountering a surface, the free field blast wave generated by an explosion undergoes reflection. This phenomenon causes the surface to experience significantly greater pressure than the incident side-on value. Typically, the reflected pressure magnitude is determined by an amplifying ratio of the incident pressure:

Pr = Cr Pso , Impulse is a measure of the energy from an explosion imparted to a building. Both the negative and positive phases of the pressure-time waveform contribute to impulse. Figure 1 shows how impulse and pressure vary over time from a typical explosive detonation.

Cr = Reflection coefficient

For peak overpressures up to 20 psi (138 kPa), the expected range for most accidental vapor cloud explosions, Newmark provides a simple formula for the blast wave reflection coefficient at normal direction to the surface as follows:

Blast Wave Parameters for Blast Loading The principal parameters of the blast wave required to define the blast loading for the building’s components are: 20 STRUCTURE magazine

Cr = Pr / Pso ≈ (2 + 0.05 Pso) Cr = Pr / Pso ≈ (2 + 0.0073 Pso)

(Pso in psi) (Pso in kPa)

Dynamic (Blast Wind) Pressure, qo This blast effect is due to air movement as the blast wave propagates through the atmosphere. In the low overpressure range with normal atmospheric conditions, the peak dynamic pressure, qo, can be calculated using the following empirical formula from Newmark: qo = 2.5 Pso / (7 Po + Pso) ≈ 0.022 Pso2 qo = 2.5 Pso / (7 Po + Pso) ≈ 0.0032 Pso2

(psi) (kPa)

where, Po = ambient atmospheric pressure. The net dynamic pressure on a structure is the product of the peak dynamic pressure, qo, and a drag coefficient, Cd. The drag coefficient depends on the shape and orientation of the obstructing surface. For a rectangular building, the drag coefficient may be taken as +1.0 for the front wall and −0.4 for the side and rear walls and roof. Please note, the net dynamic pressure, in turn, will be added to the blast pressure to get the total pressure on walls; please refer to the sections later in this article for front, side and rear walls.

Shock Front Velocity, U In the free field, the blast wave from an explosion travels at or above the acoustic speed of the propagating medium. In the low-pressure range and for normal atmospheric conditions, the shock/pressure front velocity in the air can be approximated using the following relationship from Newmark: U ≈ 1130 (1 + 0. 058 Pso)0.5 U ≈ 345 (1 + 0.0083 Pso)0.5

(ft/s) (m/s)

Figure 2 Idealized shock and pressure loads. Courtesy of Design of Blast-Resistant Buildings in Petrochemical Facilities, Second Edition.

The actual design overpressures may be stated to the design engineer in two ways: - The simplest is a set blanket statement such as; “All buildings shall be designed for a peak reflected overpressure of X psi (kPa), a peak side-on overpressure of Y psi (kPa), and duration of Z milliseconds.”

- Specifying overpressures and durations based on the distance between the structure and a potential source. The building engineer would then determine design loads based on the appropriate distance.

Commonly used criteria include SG-22 (withdrawn) and CIA (being revised). Both documents specify at least two blast overpressures for buildings spaced 100 feet (30 meters) from a vapor cloud explosion hazard as follows: - High pressure, short duration, triangular shock loading: Side-on overpressure of 10 psi (69 kPa) with a duration of 20 milliseconds.

- Low pressure, long duration, triangular loading: Side-on overpressure of 3 psi (21 Pa) with a duration of 100 milliseconds.

Building Blast Loading Blast Wave Length, Lw The propagating blast wave at any instant in time extends over a limited radial distance from the explosion source. The pressure is largest at the front and trails off to ambient over a distance, Lw, the blast wavelength. Values of Lw for high-energy explosives can be obtained from UFC 3-340-02. In the low-pressure range, the length of the blast wave can be approximated by:

To design a blast-resistant building, the design engineer first needs to determine the loads on the building as a whole and on each structural component such as a wall, roof, frame, etc. Depending on its distance and orientation relative to the blast source, the building, and its components, will experience various combinations of blast effects (reflected overpressure, side-on overpressure, dynamic pressure, and negative pressure). The design engineer can determine the blast loads

Lw ≈ U td

Idealized Blast Wave Parameters To simplify the blast-resistant design procedure, the generalized blast wave profiles shown in Figure 1 normally are idealized or linearized, as illustrated in Figure 2 for a shock wave and pressure wave.

Determination of Vapor Cloud Design Overpressures Because there are no codes or industry standards for determining what blast overpressures should be used, the design blast loads normally are supplied by the facility owner. The facility owner can perform a blast risk assessment of the targeted buildings to determine the blast pressure. Blast risk assessment of buildings can be done by a specialized consultant.

Figure 3 Blast loading general arrangement for a rectangular building. Adapted from Forbes 1998, courtesy of American Concrete Institute.

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Figure 4 Front wall loading. Courtesy of Design of Blast-Resistant Buildings in Petrochemical Facilities, Second Edition.

Figure 5 Roof and side wall loading. Courtesy of Design of Blast-Resistant Buildings in Petrochemical Facilities, Second Edition.

for the various building components, as illustrated below.

As Figure 3 indicates, triangular blast loading on the roof starts with a high pressure (P) and short duration (t) at the edge of roof facing the explosion source and gradually changes to lower pressures and longer durations from the middle to the far end of the roof.

An example of blast loads calculation is provided at the end of the article for a better understanding and demonstration of blast loads computations.

computed by equating the impulse for each load shape and using the same peak pressure, Pr. The impulse, Iw, under the bilinear pressure-time curve, is:

Front Wall Loading

The duration, te, of the equivalent triangle is determined from the following equation: te = 2 Iw / Pr = (td − tc) Ps / Pr + tc

The walls facing the explosion source will experience a reflected overpressure. The magnitude of the reflected pressure, Pr, is determined as an amplifying ratio of the incident pressure:

Pr = Cr Pso where, Cr = reflection coefficient The reflected overpressure decays to the stagnation pressure, Ps, in the clearing time, tc, as defined below and illustrated in Figure 4.

Ps = Pso + Cd qo tc = 3 S / U < td Where, S = clearing distance, the smaller of BH, or BW/2 BH = building height BW = building width As indicated in the tc equation, the duration of the reflected overpressure effect, tc, should not exceed that of the free field positive overpressure, td.

In order to use the dynamic response charts based on a triangularshaped load, the bilinear pressure-time curve shown in Figure 4 can be simplified to an equivalent triangle. This equivalent load is

Figure 6 Effective overpressure values. Courtesy of UFC 3-340-02.

22 STRUCTURE magazine

Iw = 0.5 (Pr − Ps) tc + 0.5 Ps td

Side Wall and Roof Loading The side walls are defined relative to the explosion source, as shown in Figure 3. These walls will experience less blast loading than the front wall due to a lack of overpressure reflection and attenuation of the blast wave with distance from the explosion source. The general form of roof and side wall blast loading is shown in Figure 5. tr: Rise Time Duration td: Positive Phase Duration to: Total Positive Phase Duration As a blast wave travels along the length of a structural element, the peak side-on overpressure will not be applied uniformly. It varies with both time and distance. A reduction factor, Ce, accounts for this effect in the design.

Values for Ce (refer to Figure 6) depend on the length of the structural element, L, in the direction of the traveling blast wave, Lw. If the blast wave is traveling perpendicular to the span, then L should equal a nominal unit width of the element. The equation for side walls is as follows:

Pa = Ce Pso + Cd qo

where Pa = effective side-on overpressure

Figure 7 Rear wall loading. Courtesy of Design of Blast-Resistant Buildings in Petrochemical Facilities, Second Edition.

Rear Wall Loading

Reflected Overpressure

Rear wall loading normally is used only to determine the net overall frame loading. Because the rear wall load is opposite to the front wall load, its inclusion tends to reduce the overall lateral blast force. For buildings where a blast load occurs from any direction, rear wall effects normally are neglected conservatively. The shape of the rear wall loading is similar to that of side and roof loads.

The effective peak overpressure is similar to that for side walls and is calculated using the equation shown below (Pb normally is used to designate the rear wall peak overpressure instead of Pa). The equation for rear walls is as follows:

Pr = [2 + 0.05 (Pso)] Pso = [2 + 0.05 (6 psi)] (6 psi) = 13.8 psi (95 kPa)

Clearing distance S = minimum of BH or BW/2 = 15 ft (4.5 m)

Reflected Overpressure Clearing Time Pb = Ce Pso + Cd qo

where Pb = Rear Wall Peak Overpressure

Blast loading Calculations Example This example illustrates the calculation of blast loading on the components of a building subjected to a shock wave traveling horizontally.

The building dimensions are as follows: Width, BW = 93 feet (ft) (28.4 meters (m)) Length, BL = 67 ft (20.4 m) Height, BH = 15 ft (4.5 m)

tc = [3 (S / U) < td = 3 (15 ft) / (1,312 ft/s) < 0.05 s = 0.034 s Drag coefficient, Cd = 1.0

Stagnation Pressure Ps = Pso + Cd (qo) = (6 psi) + (1.0) (0.8 psi) = 6.8 psi (47 kPa)

Front Wall Impulse Iw = 0.5 (Pr − Ps)tc + 0.5 Ps td = 0.5 [(13.8 psi) - (6.8 psi)] (0.034 s) + 0.5 (6.8 psi) (0.05 s) = 0.289 psi-s (2 kPa-s)

Effective Duration Blast Loading Parameters Peak side-on overpressure, Pso = 6 psi (41 kPa) duration, td = 0.05 s

te = 2 Iw / Pr = 2 (0.289 psi-s) / (13.8 psi) = 0.042 s

Shock Front Velocity

Side Wall Loading

U = 1130 (1 + 0.058 Pso)0.5 = 1130 [1 + 0.058 (6 psi)]0.5 = 1,312 ft/s (400 m/s)

This calculation will be for a wall segment, L1, 1 foot wide (0.3 m).

Drag coefficient, Cd = −0.4

Equivalent Load Coefficient Length of Pressure Wave Lw = U (td) = (1,312 ft/s) (0.05 s) = 66 ft (20.1 m)

Lw/L1 = (66 ft) / (1 ft) = 66 Ce = essentially 1.0

Equivalent Peak Overpressure Peak Dynamic Wind Pressure qo = 0.022 (Pso)2 = 0.022 (6 psi)2 = 0.8 psi (6 kilo pascals (kPa))

Pa = Ce Pso + Cd qo = (1.0) (6 psi) + (−0.4) (0.8 psi) = 5.7 psi (39 kPa)

Rise Time

Front Wall Loading The front wall is assumed to span vertically from the foundation to the roof.

The design will be for a typical wall segment one foot wide.

tr = L1 / U = (1 ft) / (1,312 ft/s) = essentially 0.0 s duration, td = 0.05 s

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Roof Loading

Total Positive Phase Duration

The roof is a slab spanning between roof beams of 8 feet length.

For the design of the roof, a section 1 foot wide by 8 feet long will be used.

L1 = 8.0 ft (2.4 m) Drag coefficient, Cd = −0.4

to = tr + td = (0.006 s) + (0.05 s) = 0.056 s

Conclusion

Equivalent Peak Overpressure Lw/ L1 = (66 ft) / (8 ft) = 8.25 therefore Ce = 0.9

Equivalent Peak Overpressure Pa = Ce Pso + Cd qo = (0.9) (6 psi) + (−0.4) (0.8 psi) = 5.1 psi (35 kPa)

Rise Time tr = L1 / U = (8 ft) / (1,312 ft/s) = 0.006 s

Obtaining a basic knowledge of blast loading and their respective forces is an essential step in the design of blast-resistant buildings. Blast pressures are computed as uniform pressures on building front walls, side walls, roof, and rear walls. These blast pressures can be resisted by the building’s lateral resistance systems, i.e., by shear walls, framing systems or both. The pressure on walls and roofs can be transferred to the lateral resistance system by either the tributary or dynamic reaction methods.■ Full references are included in the online version of the article at STRUCTUREmag.org.

Abdulqader Mohammed, Master of Engineering from KFUPM. A senior Civil/Structural Engineer at Alkhorayef Water and Power Technologies (AWPT). He is also a Member of Saudi Council of Engineers (SCE) (abdulqader37@gmail.com).

Abdulrahman Salah, a structural engineer, currently working as a research assistant at the University of Houston while pursuing his Ph. D. (engbinsalah@gmail.com)

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structural FORUM Structural Pilgrimage to East Coldenham Elementary School Recommitting to safety of the public.

By Jim D’Aloisio P. E., LEED AP

E

arlier this summer, while traveling in New York’s Hudson Valley, I stopped at East Coldenham Elementary School.

It was a quiet summer day. People came and went while I spent a few moments sitting on a bench in a small memorial garden near the front entry of the building. I consider my visit to be a structural pilgrimage. On November 16, 1989, a strong straight-line wind, sometimes called a microburst or derecho, hit the building’s south face during lunchtime.

The central portion of the cafetorium wall collapsed, or blew in, on top of dozens of children. In the catastrophe and its aftermath, a total of 10 children lost their lives, and 18 others were injured.

The events of that horrible day are probably incomprehensible to many people. Some might consider it to have been an “act of God.” Some may think of it as a testament to the unbridled and unpredictable power of severe weather events. But to structural engineers, it represents much more. We should not turn away from the lessons of the East Coldenham disaster.

The collapsed wall was the non-load-bearing gable end of a long section of building with a shallow double-pitched roof. At the center of the wall was a rectangle of masonry roughly 20 feet wide and 13 feet high, bearing on a foundation wall and surrounded on both sides and across the top by window glazing and aluminum mullions. There were no wind columns, wind girts, or lateral braces. Apparently, when the building was constructed in 1959, neither the structural engineer nor the architect designed any specific mechanism to resist strong wind loading on the wall.

Structural engineering practice and building codes have greatly evolved over the past 60-plus years. Certainly, today, such a design omission would be considered an act of malpractice. Still, out of respect for the children, families, and community of East Coldenham, engineers might consider the following takeaways: • Engineers should design holistically, collaborating with the rest of the design team, rather than defaulting to “siloing” or designing only the parts and pieces for which we are 100% responsible. This leads to more coordinated projects and reduces the possibility of gaps in the design.

While wind loading is certainly the purview of the structural engineer, other design aspects such as conditions of structural thermal bridging should be assessed, to prevent not only energy loss but to reduce the potential for condensation, which could lead (and has led) to corrosion and compromise of structural elements.

• When performing structural condition reviews of existing buildings, engineers must realize that there are buildings that may contain stillundetected structural flaws. As buildings continue to age and, in some cases, deteriorate, and the likelihood of experiencing large-magnitude sporadic snow, wind, or seismic loads increases, structural deficiencies will make themselves apparent. The East Coldenham building held its

dangerous secret for 30 years. Let’s try to identify these flaws before another catastrophe hits.

• The public’s implicit trust in the safety of buildings and other structures that they live, work, and learn in every day lies in the hands of the structural engineering community. Let’s do everything we can to honor that trust.

• Such important, tragic, and instructive stories are not shared within the engineering community nearly enough.

Lest East Coldenham be dismissed as an anachronistic anomaly, in 2009, I came upon another flawed cafetorium wall in an elementary school. Built in 1993, it measured 80 feet long and 25 feet high and was constructed of eight-inch hollow, unreinforced CMU and an independent exterior brick wythe. Competence check: Does that make you raise your eyebrows? The wind resistance of this wall was well below code requirements, yet there it was – built and occupied – with children. Fortunately, we were able to call attention to this problem and reinforce this deficient wall before a too-strong wind occurred. It seems like we, as a profession, still haven’t learned the lesson well enough.

I returned from my pilgrimage to East Coldenham and recommitted to our profession’s fundamental canon as engineers: to hold paramount the safety, health, and welfare of the public. There are precious lives at stake.

We have to do better.■ Jim D’Aloisio P. E., LEED AP, is a Principal with Klepper, Hahn & Hyatt of East Syracuse, NY. He has been a practicing structural engineer for over 30 years, specializing in building condition reviews, structural forensic engineering, and implementation of strategies to reduce embodied and operational carbon in buildings. (jad@khhpc.com)

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2023 STRUCTURAL ENGINEERING EXCELLENCE AWARDS T

he National Council of Structural Engineers Associations (NCSEA) is pleased to publish the 2023 Excellence in Structural Engineering (SEE) Awards winners. The awards were announced during NCSEA’s Structural Engineering Summit, held

November 7-10, 2023, at the Disneyland Hotel in Anaheim, California. A video of the awards presentation can be found on

the NCSEA website. The winning entries recognize the most innovative and groundbreaking work of our profession’s top talents.

The 2023 Awards Committee was chaired by Carrie Johnson (Wallace Design Collective, PC, Tulsa, OK). Ms. Johnson noted: “The judging was conducted in two rounds. The first round was performed by a group of NCSEA Past Presidents and the second round was completed in Portland by members of the Structural Engineers Association of Oregon (SEAO). The judges were truly impressed by the quality, innovation, and creativity of the award submissions.”

Learn More About This Year’s Winners

The judges selected the winning projects based on the following criteria: • Complexity of criteria or unique problems • Creativity of structural design • Innovative application of new or existing materials or techniques • Commitment to sustainability through efficient use of materials, resilience, circular economy principles, or embodied carbon reduction • Ingenuity of design for constructability challenges and efficient use of labor

Awards were given in eight categories, with an Outstanding Project awarded in each of the following categories: • New Buildings under $30 Million • New Buildings $30 Million to $80 Million • New Buildings $80 Million to $200 Million • New Buildings over $200 Million • New Bridges and Transportation Structures

• Forensic | Renovation | Retrofit | Rehabilitation Structures under $20 Million • Forensic | Renovation | Retrofit | Rehabilitation Structures over $20 Million • Other Structures

After the judging was complete in each category, an overall 2023 Structure of the Year award was selected from the eight Outstanding Projects. This resulted in a lively discussion among the judges who compared each project based on the complexities and ingenuity required.

Congratulations to the 2023 SEE Award winners! Keep an eye out for in-depth articles on several of the 2023 winners in future issues of STRUCTURE.

THE JUDGING WAS CONDUCTED IN TWO ROUNDS. THE JUDGES WERE TRULY IMPRESSED BY THE QUALITY, INNOVATION, AND CREATIVITY OF THE AWARD SUBMISSIONS. Carrie Johnson, P. E., S. E., Chair, 2023 Awards Committee 26 STRUCTURE magazine

2023 PANEL OF JUDGES ROUND 1: PAST PRESIDENTS Vicki Arbitrio, P. E., Gilsanz Murray Steficek Barry Arnold, ARW (retired) Craig Barnes, Socotec Marc Barter, Barter & Associates Bill Bast, S. E., Socotec Jim Cagley, Cagley & Associates Tom DiBlasi, DiBlasi & Associates

Tom Grogan, Haskell/Retired Emily Guglielmo, Martin/Martin Carrie Johnson, P. E., Wallace Design Collective Jim Malley, Degenkolb Ben Nelson, P. E., Martin/Martin Ed Quesenberry, Equilibrium Sanjeev Shah, SSI Consulting

ROUND 2: STRUCTURAL ENGINEERING ASSOCIATION OF OREGON Damian Andreani, Catena Mike Astrella, Catena Peder Golberg, James G. Pierson, Inc. Tom Grogan, Haskell (retired) Kylean Gunhus, Miller

Carrie Johnson, Wallace Design Collective Amit Kumar, City of Portland Nisarg Mehta, DCI Ben Nelson, Martin/Martin Ed Quesenberry, Equilibrium

CATEGORY 1: NEW BUILDINGS UNDER $30 MILLION AND STRUCTURE OF THE YEAR

Structure of the Year

CHILDREN’S MUSEUM OF EAU CLAIRE EAU CLAIRE, WI

Structural Design Firm: KPFF Consulting Engineers & ERA Structural Engineering Architect: Steinberg Hart | Holzman Moss Bottino General Contractor: Market & Johnson

Outstanding Project

SRT Design Offers Resilient and Cost-Effective Solution Creating a 26,000-square-foot biophilic structure using structural round timber (SRT) required creativity. Using non-milled whole ash and sugar maple trees, the childrens’ museum design featured two stories: the bottom story of SRT supported a 5.5-inch thick concrete slab, topped by the second story of SRT, using the trees’ branches to hold up the building’s roof trusses.

The design, by KPFF Consulting Engineers & ERA Structural Engineering, is organic and created a few challenges—namely, shrinkage of the SRT. The wood materials, from certified forests, reduce embodied carbon, support circular economy principles, and promote resilient forest practices by helping to slow the propagation of infestations. “While many projects begin with a clear understanding of what a cost-effective, constructable solution will look like, this project’s unique architectural vision called on us to develop a solution from scratch,” according to the project submission. “We achieved this through a highly iterative design process that included the entire design and construction teams. We studied column bay spacing to optimize truss layout, we studied more than a dozen truss Ben Nelson of Martin/Martin representing the 2023 Panel of Judges configurations and geometries, and we iterated on connection details to presents the 2023 SEE Structure of the Year Award to Erik Lofthus of KPFF develop the most cost-effective solutions.” Overall, this project proved to Consulting Engineers at the Awards Celebration event on November 9th during the NCSEA Structural Engineering Summit in Anaheim, CA. be an architectural, cost-effective, collaborative feat.

NORTH HOUSTON SPORTS COMPLEX WARNER ROBINS, GA | PES STRUCTURAL ENGINEERS, INC.

Finalist

Approximately 100 miles south of Atlanta, Ga., the new North Houston Sports Complex in Warner Robins is home to the city’s recreation department. It fills a gap in recreational facilities for the 10th largest municipality in the Atlanta metro region. The 77,000-square-foot Claude Lewis Recreation Center features an exposed structural steel frame, used to create large, open spaces. The design included long-span special joists to clear span the recreation courts, intentional truss design to maximize openness for the translucent Kalwall panel cladding, and a carefully coordinated lateral system that balances the desired architecture with construction cost.

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CATEGORY 2: NEW BUILDINGS $30 MILLION TO $80 MILLION

University of California, San Diego, Epstein Family Amphitheater SAN DIEGO, CA

Photos courtesy of Thornton Tomasetti, and Philipp Scholtz Ritterman

Structural Design Firm: Thornton Tomasetti Architect: Safdie Rabines Architects General Contractor: BN Builders

Outstanding Project

Warm and Accommodating Welcome to Campus As the first stage of creating a “front door” to the UC San Diego campus, the Epstein Family Amphitheater was designed to fit within the landscape and offer the backdrop for events large and small—from as small as 50 to as many as nearly 2,500 people. Using an in-house software system, the design by Thornton Tomasetti included various geometric components and materials, including a concrete back wall, 40-foot tall ethylene tetrafluoroethylene (ETFE)-clad diagrid steel shell, and rounded roof slab with a vegetative roof.

The amphitheater includes a 3,000-square-foot subterranean concrete restroom facility, 400-square-foot ticketing and vending building, and the ability to support complex rigging for local theater to larger traveling performances. It features two bowl sections and a sloping lawn—all of which contributes to a design that is meant to be versatile and flexible for various types of daily and special events. From informal study sessions to exhibits and performances, this theater was designed to accommodate myriad experiences.

Orange County Museum of Art COSTA MESA, CA | JOHN A. MARTIN & ASSOCIATES, INC.

West Hollywood Recreation and Aquatics Center

Photo courtesy of Jasmine Park

WEST HOLLYWOOD, CA | LPA DESIGN STUDIOS

Finalist

Finalist

OCMA’s structure enables a uniquely complex architecture while meeting criti- The West Hollywood Aquatics and Recreation Center is made of three cal safety and cost thresholds. Creative engineering and a collaborative process separate structures, each with its own set of unique structural engineering facilitated highly visible elements, such as large, column-free spaces, that sup- challenges. Two rooftop pools are supported by a two-way trussed space frame port heavy plaza loads, non-orthogonal architectural elements that cantilever above 100-feet wide by 160-feet long column-free multi-court gym. Adding in places more than 30 feet off the primary structure, and a cantilever-trussed to the design challenge, the structure also cantilevers 30 feet over a public classroom wing that frames a public space. Less visible but no less important roadway, providing connectivity to the new Community Center, which is are the structure’s flexibility to withstand California’s significant seismic forces, a itself supported by an existing parking structure below. Finally, a two-level high bearing strength that sustainably and economically accommodates reduced 70-foot long grand stair structure relies on a horizontally cantilevered steel column sections for thinner profiles and less material usage. truss to link the new buildings to West Hollywood Park.

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CATEGORY 3: NEW BUILDINGS $80 MILLION TO $200 MILLION

Ascent MILWAUKEE, WI

Structural Design Firm: Thornton Tomasetti Architect: Korb + Associates Architects General Contractor: C.D. Smith

Photos courtesy of KAA Design Group/Nairn Olker

Outstanding Project

Timber! World Record-Breaking Glulam Building The 25-story, 493,000-square-foot Ascent building is the tallest timber building in the world—and it’s helping to expand this market in the United States. The building is mixed-use, hosting 259 apartments, a 6th-floor pool, and top-floor amenities room.

The first six stories, made of concrete, create a base for the 19 stories of European-sourced glulam timber. And, in total, it has optimized both financial and aesthetic considerations.

“At 284 feet tall, such a building would have been unthinkable just a few decades ago. Thanks to groundbreaking research and dedication, mass timber construction has begun to gain traction, and Ascent is the embodiment of that reality,” according to the submission. New test data, a state-of-the-art cross-laminated timber (CLT) diaphragm design, prefabrication of structural components, and timber material reduction all contributed to achieving this world record-breaking feat.

Limberlost Place at George Brown College

(W)rapper LOS ANGELES, CA | ARUP

Photo courtesy of Salina Kassam

TORONTO, ONTARIO, CANADA | FAST + EPP

Finalist

Finalist

The 10-story Limberlost Place is located on George Brown College’s Toronto (W)rapper, located near the Newport-Inglewood fault in Los Angeles, waterfront campus and holds the distinction of being Ontario’s first tall timber is a 17-story office building in Los Angeles that features a striking steel institutional building. It showcases a harmonious blend of sustainable design, exoskeleton made of 5,000 feet of curvilinear, built-up plate boxes. In structural innovation, and an advanced prefabricated envelope system. Fast + collaboration with EOMArchitects, Arup structural and civil engineers Epp developed a groundbreaking large span beamless CLT timber-concrete leveraged digital tools to design a base isolated, unclassified framing composite “slab band” system, accompanied by perpendicular CLT infill panels, system in a highly seismic region resulting in one of the safest and most supported on glulam columns. The unique “slab-banded” system enables resilient office buildings in the world. At 180,500 square feet and 17 architectural flexibility and unobstructed mechanical distribution. Limberlost stories tall, this office building is the first of its kind in the city, adding Place has become a beacon for the mass timber movement. aesthetic appeal and a column-free interior.

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CATEGORY 4: NEW BUILDINGS OVER $200 MILLION

Google Bay View MOUNTAIN VIEW, CA

Structural Design Firm: Thornton Tomasetti Architect: BIG, Heatherwick Studio, Adamson Associates General Contractor: Whiting-Turner

Photos courtesy of Iwan Baan

Outstanding Project

Human-Centric, Sustainable Solution Fast-Tracked for Google Strategy and sequencing kept this fast-tracked new corporate campus for Google on schedule. The campus, called Google Bay View, is made of three separate buildings, each with roofs made of canopies that look similar to inverted shells. They’re beautiful—and they were challenging.

The 400,000- to 600,000-square-foot buildings cover only two floors to help keep the buildings on the same plane.

To create the “sweeping” canopies, 160 feet above grade at the peak, the team used compressive structural typologies with tensile net structures. They achieved longer spans, 500 by 500 feet at the widest part. The results? An eggshell-thick sustainable solution that used fewer materials and offered less weight than comparable structures. Those qualities also required planning for the roof and facade. The team spent two years designing and planning to ensure the sequencing would meet this fast-tracked schedule and meet seismic standards.

“For decades, Google has been committed to providing the healthiest human-centric environments possible,” according to the submission. “Google Bay View has the largest geothermal installation in North America and a water-positive campus that shows what’s possible. These are some of the largest buildings in the world to pursue LEED Platinum v4 certification and Living Building Challenge certification.”

Long Beach Civic Center LONG BEACH, CA | NABIH YOUSSEF ASSOC. AND SKIDMORE, OWNINGS & MERRILL

Finalist

New York University—John A. Paulson Center NEW YORK, NY | SEVERUD ASSOCIATES CONSULTING ENGINEERS, PC

Finalist

The Long Beach Civic Center is a state-of-the-art facility that provides the NYU’s John A. Paulson Center combines performing arts spaces, housing, and City and Port of Long Beach, Calif., with performance-based infrastructure athletic facilities within one building. The five-story, steel-framed podium feacommensurate with its size and status as a world-class shipping and logistics tures a 350-seat proscenium theater and other venues acoustically isolated from hub. The center consists of an eleven-story City Hall tower, an eleven-story noise and vibration. Steel trusses span over two below-grade levels and provide Port Headquarters tower, and a single-story elliptical-shaped City Hall Council space for basketball courts and a swimming pool. The cellars were designed as Chambers constructed over a two-story subterranean parking garage. The City a waterproofed “bathtub” and are supported by drilled caissons. An 18-story Hall and Port Headquarters towers are LEED Gold. The center is a pioneer faculty housing tower and 13-story dormitory block are framed with steel and in the use of resilience-based earthquake design and meets the Resilience-based the GIRDER-SLAB system, which reduced erection time, dead load, and Earthquake Design Initiative (REDiTM) Gold performance objective. structural depth. The building is expected to achieve LEED Gold certification.

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CATEGORY 5: NEW BRIDGE AND TRANSPORTATION STRUCTURES

Sixth Street Viaduct Replacement Project LOS ANGELES, CA

Structural Design Firm: HNTB Corporation Architect: HNTB Corporation General Contractor: Skanska-Stacy and Witbeck

Outstanding Project

LED Lights, Camera, Action! New Viaduct Showcases Several Firsts The new Sixth Street Viaduct opened in July 2022 and features 10 pairs of unbraced, canted arch ribs between 30 and 60 feet high in homage to the original steel double arched viaduct. The 3,060-foot-long structure crosses 18 active railroad tracks, U.S. Highway 101, and the Los Angeles River. It features programmable LED lights, five sets of stairways at the intersection of the double archways, pedestrian/bicycle ramps, and a 9-degree outward cant to offer views of downtown LA or the San Gabriel Mountains.

Behind the scenes, the team used a new seismic isolation design methodology setting a worldwide precedent using 32 isolators placed mid-height on the 18 Y-bents. According to the submission, this is the first time this method has been used in the United States and it is the longest use on a concrete network tied arch bridge in the world.

This structure showcases several other firsts, too, such as being the first California Bridge to use Grade 80 concrete reinforcement and the first U.S. bridge to use DYWIDAG multistrand post-tensioned couplers.

Port Lands Bridges

Nancy Pauw Bridge

TORONTO, ONTARIO, CANADA | SCHLAICH BERGERMANN PARTNER

BANFF, ALBERTA, CANADA | STRUCTURECRAFT

WITH ENTUITIVE

Finalist

Finalist

This family of three bridge types (each paired for six total) are key infrastructure The glacial Bow River traces its way through Canada’s Rocky components of Toronto’s waterfront revitalization. The aesthetically unified Mountains and the town of Banff in one of the world’s first national bridges provide the new Port Lands neighborhood with light-rail, vehicular, parks. Spanning the Bow, in the heart of the community, is the new cycle, and pedestrian connections, creating a critical link. Designed as hybrid Nancy Pauw footbridge, fulfilling a 108-year-long dream. Responding shell-arch bridge structures, they are essentially curved tied arches with a planar to the wish for natural materials, StructureCraft designed and built deck connected by hangers. Innovative fabrication techniques combined with an unusually slender 80-meter clear span out of sustainable timber, state-of-the-art engineering allowed for maximized material efficiency and a shallow high-thrust arch that appears effortless in this beautiful reduction of costs. With four of the six now in place, the bridges have become setting. Its rise to span is 1:20, its span to depth ratio is more than emblematic of Toronto’s evolution, with unique and compelling structural forms. 65, and the two bridge sections were erected in a day.

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CATEGORY 6: FORENSIC | RENOVATION | RETROFIT | REHABILITATION STRUCTURES UP TO $20 MILLION

Sandi Simon Center for Dance at Chapman University ORANGE, CA

Structural Design Firm: Structural Focus Architect: Lorcan O’Herlihy Architects (LOHA) General Contractor: R.D. Olson Construction

Outstanding Project

Second Chance at Showtime for Historic Building

Photo courtesy of Connor Steinkamp Photography

Now home to Chapman University’s Sandi Simon Center for Dance, the 1920s industrial building originally housed a citrus packing house.

To get it ready for showtime, it required historic preservation and expansions, including seismic strengthening of perimeter walls, restoration of the original sawtooth roof and clerestory windows, and aesthetic upgrades. An unconventional solution of strengthening of hollow-clay tile block wall included helical anchors, which required in situ tensile capacity testing. The roof required strengthening, too: seismic and gravity, the latter due to new acoustic ceilings and mechanical, electrical, and plumbing (MEP) distribution systems.

Despite having the same footprint as the original structure, the center now also includes a two-story mezzanine, creating light using floor opening curvatures. The original wood flooring was repurposed as wall finish. One unexpected reuse of the original structure was the canopy sag. According to the submission, “The canopy was re-supported with new steel strongbacks tied to the new steel tension-rod diaphragm and the second-floor framing. The restoration intentionally maintained the canopy sag, resulting from years of deferred maintenance, as a sign of the building’s various lifetime stages.” The center’s overall result is a balance of historic features and modern upgrades.

Downers Grove North High School Learning Commons Roof DOWNERS GROVE, IL | WIGHT & COMPANY

Finalist

A previously unused courtyard at the Downers Grove High School in Downers Grove, Illinois, became a center for collaboration with the introduction of a new roof enclosure. Eight custom-framed king post trusses form an X shape and are framed by a steel perimeter, filling the 65-foot wide area and creating the look of a floating roof.

Tubular steel purlins top the trusses to form a support for skylights and acoustical steel roof decking. Reinforcement of existing columns and the use of the courtyard as opposed to a new structure create a sustainable angle to this geometric project.

Interested in learning more about the NCSEA SEE Awards program? Visit www.ncsea.com/awards/ Consider submitting a project in 2024; the call for submissions occurs in the spring.

32 STRUCTURE magazine

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LYNCH MYKINS Humanity Isn’t Optional: Successful SE Companies are People-First

Y

ou can have state-of-theart software, the latest tools and gadgets, top-notch resources and suppliers, and the excitement of big and complex projects. But, if you don’t have happy engineers who are excited about relating to them, creating with them, communicating with them, and capable of presenting the value of their ideas to clients with confidence, does state-of-the-art even matter? It’s a means to an end.

You already know the structural engineering industry is Anna Lynch, P. E., CEO challenged with bringing fresh Lynch Mykins Structural Engineers talent into it and keeping them.

Over the past 7 years, under the leadership of CEO, Anna Lynch, Lynch Mykins Structural Engineers has been focused on creating and nurturing a people-first culture that attracts fresh talent and has reduced attrition rates lower than the industry average.

Today, engineers in the U.S. have incredible opportunities to leave their mark with heart, fun memories, joy, and pride in the places they’re building. We’re not talking about the future of structural engineering. We’re talking about today. Now. We’re talking about proven facts like humans are wired for connection, laughing reduces stress, and social interaction is a basic human need like food and water.

Knowing this and believing that a traditionally stressful industry can be flipped on its side, Anna deconstructed the journey of a structural engineer’s role, growth, and development. Then, she redesigned it and reinvented her 49-year-old firm, with a keen eye on the goal of thriving people and thriving business for decades to come.

It didn’t happen overnight, but it did happen quickly and intentionally. With care, research, communication, and heart-led leadership, the company has created one of the most unique external brands, with an internal people-first culture that fully supports it!

How Lynch Mykins employees experience a people-first culture:

Email us at culturecoaching@lynchmykins.com to get our FREE CULTURE IDEAS for SE firms to get you started!

Lynch Mykins’ “people-before-profit” commitment is demonstrated by their investment in the individual employees to help them become the best person, and engineer they can be. When people feel supported, happy, and connected, profits always result - now, and ten years from now.

In our hard skills world, the left brain is no longer enough. Successful engineers are learning strategies to practice using the whole mind to be their best with clients and each other. At Lynch Mykins, human connection and creativity upstage perfection every day of the week!

If you’re not interested in building a culture from scratch, hello! Consider joining ours!

Scan this QR code and see if we have the perfect job for you.

“Just because we’re engineers doesn’t mean we need to perpetuate the stereotype of engineers. It’s refreshing to work with a CEO who’s in it for the right reasons, encourages us to be ourselves, and provides us with the support and personal development to do it with confidence. I moved across the country for this and I love laughing, playing music, dancing, high-fiving coworkers in a place where work is fun.”

“In a people-first company, diverse thinking leads naturally to accepting diversity in every way - people, ideas, dress, projects, conversation. I’m no longer held back by who I am. I’m respected, appreciated and I’m growing without limits now. When people are allowed to be real, clients get real ideas, real conversation, real connection, and creativity. Not some distant, overly processed, uncomfortable attempt at trying to be something they’re not.”

“This industry can be stressful, so Lynch Mykins has figured out a ton of ways to reduce the stress and make it fun. We have Happy Hours, a nap room, ping-pong and pool tables. No one dreads Mondays because in our Monday morning meetings, we lift each other up, shout-out each others’ victories, share upcoming events and lessons learned. It really does feel like a family that truly wants the best for everyone!”

lynchmykins.com

Chris Cobb, Senior Project Manager, Raleigh, NC

Nicole Zechman, Design Director, Charlotte, NC

Victor Torres, Project Engineer, Richmond, VA

CATEGORY 7: FORENSIC | RENOVATION | RETROFIT | REHABILITATION STRUCTURES OVER $20 MILLION

TSX Broadway and Palace Theater Redevelopment NEW YORK, NY

Structural Design Firm: Severud Associates Consulting Engineers, PC Architect: Mancini Duffy General Contractor: Pavarini McGovern (Construction Manager)

Outstanding Project

To New Heights: Historic Theater Raised 31 Feet in Times Square Befitting the Times Square location, the TSX Broadway and Palace Theater Redevelopment included both historic components and modern advancements. That meant lifting up the 100+-year-old 8-story steel framed theater (with the 36-story concrete hotel tower that was built on top in the 1990s) 31 feet into the air to make room for storefronts below. According to the submission, “TSX Broadway expands the limits of how existing structures can be effectively repositioned and augmented to create revitalized buildings with improved functionality.”

The work that was done in the ’90s wouldn’t suffice for the new lift of the building, 25 million pounds per column, so the updated design included post-tensioning girders, encasing steel columns with concrete reinforced with rebar, demolishing old columns, bonding or doweling new concrete to existing slabs, and drilling caissons below the cellar into the bedrock.

“TSX Broadway makes a significant contribution to sustainability as a large-scale reuse of an existing facility,” the submission continues. “Complete demolition would have been wasteful while retaining 25 percent of the floor construction reduced the demand for new materials. All concrete substituted slag for up to 40% of standard cement; reinforcement and structural steel framing contain over 90% recycled material.”

Colorado Convention Center Expansion

UCSD York Hall Historic Seismic Retrofit

DENVER, CO | MARTIN/MARTIN, INC.

SAN DIEGO, CA | LPA DESIGN STUDIOS

Finalist

Finalist

The Colorado Convention Center embarked on one of the largest overbuild University of California, San Diego’s York Hall, built in 1966, is a expansion projects in the United States, adding 200,000 square feet of usable 122,000-square-foot historic mid-century building. Immediately recognizable space above its existing facility. To overcome the challenges of this massive, perched atop its 300-foot long fluted column arcade, the lift-slab buildings overbuild expansion, the design-build team employed creative design and are ringed with more than 800 quarter-ton, precast concrete fins. A seismic construction methods. The team collaborated closely to work within the retrofit meeting the University of California’s seismic performance standard limitations of the existing structure with minimal disruptions to the conven- preserved the historically significant architecture and kept the building fully tion center’s operations. The expansion’s unique features include cantilevered operational during construction. The seismic retrofit included the complete floor spaces, innovative use of existing columns, and multi-function floor replacement of multiple historic fluted concrete columns, new shear walls, trusses engineered to support a 500-ton crawler crane during construction. and strengthening of three-quarters of the precast concrete fins.

34 STRUCTURE magazine

314-568-3827

CATEGORY 8: OTHER STRUCTURES

KF Aerospace Spiral Stair KELOWNA, BC, CANADA

Structural Design Firm: StructureCraft Architect: Meiklejohn Architects General Contractor: Sawchuk Developments

Outstanding Project

Free-Standing Stairway to Space: CLT Used to Ascend

Blue Owl Signature Stair

Albright Knox Gallery (Common Sky)

NEW YORK, NY | CRAFT | ENGINEERING STUDIO

BUFFALO, NY | THORNTON TOMASETTI

Finalist

Finalist

CRAFT | Engineering Studio developed a structurally superior Common Sky is a work of art and functional canopy enclosing the Albrightcurved stair design alternative without requiring specialty fabrica- Knox Gallery’s Town Square. Angled facets of glass and mirrors transform tion equipment. They used a planar mesh of triangular flat plates the square with kaleidoscopic light and shadow. The canopy reaches down to create a curved surface, maximizing the staircase’s cross-sectional to the ground at a single point, maintaining the space’s asymmetry. The properties. Each plate was laser cut from sheet steel and assembled canopy is composed of steel hollow sections, arranged in two layers. They in a single shop, minimizing errors. Customized stair sections are supported at the perimeter on spherical slide bearings installed atop were fabricated as welded assembly modules, providing flexibility existing building columns and within the courtyard by a new trunk column in module sizes based on delivery and site access. The innovative and footing. Due to the arched geometry, the roof is of variable height with approach improved efficiency and accuracy. a maximum overall elevation of 23 feet.

36 STRUCTURE magazine

Photo courtesy of Studio Other Spaces

Befitting the aerospace client, StructureCraft designed a free-standing cross laminated timber (CLT) 70-foot spiral staircase connecting the first and second floors. The solution needed to be able “to both bend and warp and create forces in combination of short- and weak-axis bending as well as torsion,” according to the submission, and without any cut-offs. This may be the first time CLT has been manufactured in this way.

Another unique aspect was the use of thin boards glued to the edge on the top and bottom layers and a cross layer of boards on the flat side.

The team used models to consider solutions to challenges, such as predicting spring back and understanding the flow of the timber/concrete composite sections. Aesthetics were also a concern, especially at the base of the stairs where the connection was. The team came up with a solution to create a sculptural concrete bullnose, located under the stairs. On the second story, the team tied the concrete topping into the floor slab.

Installation proved challenging as well, since the second floor wouldn’t allow for a crane to lift the lower half in place. Instead, the team used a rigging system, hooked up to the existing beam structure, and lifted manually with adjustable chain hoists. Bottom, middle, and top connections lined up perfectly on site; the concrete was poured; and the unsupported spiral stairway was complete.

since 1922

structural FORCES Building Settlement

Considering the effects of ground movement on internal structural forces.

By Dilip Khatri, Ph. D., S. E.

M

any structural failures share one common theme: uneven ground settlement. As structural engineers are trained at a university, they assume that the "fixed base" of our buildings is FIXED. However the effect of settlement on the aboveground structure can be dramatic. As a building sinks unevenly, moments and shears are redistributed to stiffer adjacent elements (columns, beams, walls, etc.) which add to their total load demand. Like a human being injured in one leg, weight is redistributed to the other leg, and a person's center of gravity shifts and the body can become unstable, leading to their fall.

Consequently, a building will redistribute weight because of stiffness variation from the sinking columns, which adversely affects the other columns. Analysis of a simple moment frame confirmed that the bending moments would increase 20% to 35% for a 1-inch deflection, and deflections will dramatically increase to 3 to 4 inches at a single column line.

Let us start with a statement found in almost all of our college structural analysis textbooks: "All structures are stable and fixed/pinned at their base, with the foundation never moving."

Figure 1 illustrates a simple 3-story moment frame building with fixed base connections. Points A, B, and C are assumed to be level and never displace vertically or horizontally (small deflection theory). Our education in structural analysis, design, retrofit, codes, and basic theory is formulated on this critical assumption: the ground never moves, or if it does, the movement is too small to be of any significance.

Reality Check: The Ground Does Move Over time, the support of a structure will change due to varying soil conditions, moisture levels, mild earthquakes, and possible landslide conditions. These factors are not part of our conventional wisdom as structural engineers because we are never taught that this may happen. It is completely outside of our envelope of expectation.

Examining our building codes, structural textbooks, Figure 1 Typical multi-bay moment frame with vertical loads. and research 38 STRUCTURE magazine

efforts for the past 80 years shows that "we" [structural engineers] work from the premise that the building will not move. We establish our design practice using sophisticated analysis methods such as finite element analysis, dynamic analysis, and nonlinear analysis, all founded on stable foundations.

Certainly, for many buildings and bridges, this has proven to be a good working methodology. Iconic structures like the Figure 2a Multi-bay moment frame with flexible Golden Gate Bridge foundation reactions. (almost 90 years of service), the Empire State Building (90+ years of service), and the Taj Mahal (over 400 years) have lasted well beyond their expected lifespan and defied collapse through numerous natural events.

But recently, there have been structural failures that are baffling investigators and give a reason for the re-examination of our standard of practice stemming from ground movement. A recent landslide in Rancho Palos Verdes, California is a prime example of ground movement which no conventional structure could have survived. Many structural engineers would lay the blame on the geotechnical engineers and geologists and claim this is "not our fault." Not true. Structural engineering is not just the "structure above ground" but includes the long-term stability of the foundation. We should be looking holistically at the entire system, not just "our part" above ground. We should remember that geotechnical engineers and engineering geologists provide information and data on soil conditions with recommendations to structural engineers for our design. They are not building design professionals; structural engineers are. Just like architects who perform their scope of design dealing with form, function, aesthetics, lighting, colors, shading, and the ethos of the structure, structural engineers have to take responsibility for our expertise and provide cautionary recommendations as necessary during the design phase.

Therefore, Figure 1 from our structural textbooks is not reality but instead is more like Figure 2A. Each base support in twodimensional space has three degrees of freedom (DOF) and can displace in two dimensions, plus rotate in-plane. Each DOF

Figure 2b Base reactions with six degrees of freedom.

a simple beam. As the vertical stiffness of Point B degrades, see Figure 4B, the load reaction at Point A will increase. The vertical loads will shift to the remaining support point as the stiffness degrades at the other reaction point.

Equilibrium is the fundamental equation that keeps every structure standing.

The balance of the forces and moments is formulated from Newton's Law. When one support loses its capacity to accept the vertical force demand, then that force (mass multiplied by the acceleration due to gravity) has to be transferred to other supports. It must go somewhere, and so the redistribution of the loads (and stresses) in the structure is automatic and follows the basic laws of physics. If we revisit our textbooks and recall the classical method of Moment Distribution (developed by Hardy Cross), this analysis method illustrates the re-distribution of moment to balance at the frame joints.

Practical Analysis Geotechnical reports will give an estimated long-term settlement, often in the range of ½-inch to 1-inch for firm soils, over the life of the structure. This is an estimation based on the soil conditions at the time of completion of the construction of Figures 3a Single bay moment frame with one Figures 3b Single bay moment frame with one the building. Therein lies the basic fallacy: degree of freedom. degree of freedom deflection plot. soil conditions can change over time. During has a stiffness coefficient. Figure 2B illustrates three dimensions design, we assume that the moisture content and bearing capacity where the structure has six DOFs: three linear elastic springs and will not change over time, but they can. For example, seepage three rotational springs. The concept of multi-degree of freedom from a leaking water main or in-ground swimming pool will systems is usually part of graduate school structural engineering certainly affect the soil parameters. Over-watering from irrigation degree programs, but these principles are still usually introduced will affect the soil capacity. Dewatering on adjacent construction only for the understanding of the superstructure in conventional sites can lower the water table. These factors are not included in structural analysis, but not foundation movement. a soils report because the geotechnical engineer is not expected For simplicity, we look at Figure 3A and examine the behavior to forecast them, but these factors do occur in reality. as Ky degrades. When Ky degrades, see Figure 3B, the frame These factors suggest the need for structural monitoring and will deflect capacity analysis over time, at least for structures where deteriofrom Point ration leading to failure would have consequences for the public.

B to B’, and We cannot the reactions assume that will shift to “everything” Point A as the will remain frame redis- static over Figures 4a Simple beam with fixed pinned supports. tributes the 50 to 100+ vertical loads years. For to the stiffer example, column.

Florida has This basic instituted a structural timeline for redistribustructural tion of loads monitoring is observed in and recerti- Figure 5 Schematic view of a real structure with foundation Figures 4b Simple beam with one degree of freedom base reaction. Figure 4A for fication of elements and deflection plot.

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soil stiffness results in shifting load and moments in the superstructure. Let us take this principle and apply it to a basic building frame system, as shown in Figure 5.

The dimensions were taken from the plans of a recently collapsed building and are for a one-story version of a garage structure. As Point D deflects downward, the deformed structure above shows the elastic curve and movement of Point B, with the assumption that Points A and C are stable for simplicity. In reality, we do not know if Point C is stable, but for this analysis, we will assume it is.

If we take this concept and extend it to a multi-story, multi-bay structure (see Figure 6), the analysis becomes more complex because of the variability in the ground movement.

Figure 6 Multi-bay tall building with varying ground settlement reactions.

Figure 7 Single Bay moment frame with fixed base and then adjusted with three degrees of freedom.

Figure 8 Structural analysis using RISA-2D for varying stiffness at right reaction.

building occupancy on a 10-year schedule. New York City recently experienced a sudden collapse of a Manhattan parking structure and has instituted a similar law. California (and all other states) are considering similar measures, like California’s Balcony Law (Senate Bill 326), which requires structural examination every 6 to 9 years for wood balconies.

Portal Frame Analysis For simplicity, this paper examines a single-bay portal frame. A portal frame is a snapshot of a larger multi-story moment frame system and illustrates these concepts with a basic analysis where one support sinks. Modeling this with a portal frame allows an engineer to analytically determine reaction results when ground movement occurs at one side of the frame. Degrading 40 STRUCTURE magazine

Case Study Of Single Bay Moment Frame

Let us look at a single-bay frame taken from a recent collapse (see Figure 7), which has Cases A and B. Case A: Conventional portal frame analysis with stable foundation support Case B: Portal Frame analysis with degrading foundation stiffness at Point D Case A provides a symmetrical moment diagram and is in many textbooks, so it is not presented here. Case B is shown with actual vertical loads from the building calculation but with no lateral loads.

The moment redistribution and shifting of the reaction load is evident with a modest 1.28-inch deflection, see Figure 8. This affects and magnifies the moment values higher at Point A [N1] as Point D [N3] displaces further, and the moment at Joint B [N2] is increased. In principle, the structural theory is proven here that ground displacement will affect the moments, shears, and axial loads in the frame structure above. Similar conclusions will apply to other building types (i.e., shear wall structures, braced steel frames, concrete frames, wood frames, etc.).

Examples Of Structural Damage From the textbook to the real world, we are now faced with physical evidence of structural cracking in columns and beams that may threaten the superstructure.

Figure 9 is such a case for a three-story complex with subterranean parking that has extensive cracking in column locations and is currently under citation.

Figure 10 is a 22-story steel high-rise that has areas of water damage in the subterranean parking area, as shown in

Figure 9 Subterranean parking structure with structural cracking indicates potential ground settlement and/or lateral movement.

Figure 10 High rise building resting on 4 levels of subterranean parking.

Figure 11 Corrosion effects on steel frame moment connection in parking structure.

Figure 12 Corrosion damage to steel moment bolted connection. Deflection checks are important to determine if amplified moments could cause cracking.

Figure 13 Subterranean parking column cracking with no seismic lateral connection on the beams.

Figure 11. Examination of the structural connections is one part of the investigation. Structural engineers are also investigating whether ground displacement has led to asymmetric moment distributions and amplified stresses in the moment frame joints.

Figure 12 shows a beam connection with deterioration that passed testing, but this is not sufficient to conclude if the structure has displaced vertically. A physical survey should be part of the investigation process.

Figure 13 illustrates a potential column overstress that may be due to settlement and/or water damage.

Our profession should take a proactive stance on this issue and call for research focused on this topic to upgrade and address our codes and standards. Our industry has an unfortunate "slow response" time to institute code changes. Still, we, as practicing structural engineers, should be examining our design practice based on practical analysis, not standards formulated by researchers and theoreticians.■

Conclusions The structural theory of stable foundations ought to be questioned based on recent and historical examples of settlement that have affected the structural distribution of loads. Structural engineers need to recognize these phenomena as potentially damaging, assess root causation, and address these concepts in standards, guidelines, and code provisions.

DISCLAIMER: The author, Dr. Khatri, is not part of any investigation team or research group funded by any entity. The examples cited here are for discussion only and do not suggest that these are established/proven conclusions for open cases. Dr. Khatri is a structural engineer with over 40 years of academic and professional experience and is not purporting to represent any structural opinions on open failure investigations, their designers, or causes of failure.

Dr. Khatri, Ph. D., S. E., is a consulting engineer and owner of Khatri International Inc., based in Las Vegas, NV and Arcadia, California. He has been a structural engineer for 41 years and is licensed across the USA, Canada, and Australia. He may be reached at (dkhatri@aol.com).

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historical STRUCTURES 19th Century Mississippi River Bridges #10 Government (arsenal) bridge 1872

By Dr. Frank Griggs, Jr.

B

y 1870, the wooden Rock Island Bridge (Structure October 2022) needed replacement. The Federal Government decided to replace it with an iron structure crossing the land just south of the earlier bridge on the island's west end. In May 1870, Major W. H. Benyaurd, the Army engineer placed in charge of the bridge, prepared a plan of the bridge site and located the position of the swing span close to Rock Island with an extensive set of specifications to be sent to prospective bidders. He wrote, “These specifications were general in their nature, and drawn up so that the bidders would have equal opporView from the Island looking westerly towards Davenport with the 100’ span in the foreground. tunities to compete for the bridge, no particular form of truss, nor any particular patent being specified, that would debar some firms as the low bidder and Kellogg, Clarke & Company as the highest from proposing, while at the same time, the Government would have bidder, with the contract going to Smith’s company. Benyaurd wrote, the benefit of a greater number of plans to choose from, and be better enabled to secure a proper structure.” The specification was for an The plans, &c., having been carefully considered, the award all-wrought iron structure and included the following: was made to the Baltimore Bridge Company, and a contract entered into on the 13th of October, 1870, for the draw-span, The superstructure will consist of two spans of 260 feet, three one span of 260 feet and one span of 220 feet, the amount of spans of 220 feet, fixed in a permanent position, and one draw available funds on hand being only sufficient for these three span, with two equal openings of 160 feet and a total length of spans, and current expenses, the remainder of the appropria366 feet. Each span will consist of two trusses, 33 feet between tion having reverted into the Treasury. The contract for the centers of top and bottom chords, with wagon-road on bottom remaining spans over the main river, and the approach spans chords, and single-track railroad intermediate, allowing 12 at each end, was not entered into until August 1871, after feet headway for wagon-road. The trusses are to be placed 18 the money which had reverted into the Treasury had been feet apart in the clear, with two sidewalks, each 5 feet wide, re-appropriated. outside, on level with wagon-road and floor.

The bridge was not mentioned in the 1866 Federal Law governing In other words, it would be a double-deck bridge with the road- bridges across the Mississippi and Missouri Rivers, but the requireway on the lower level and the railroad on the upper level. He sent ments of that law were followed. The layout of the bridge starting the plans and specifications to many firms and got proposals from on the Iowa or Davenport side and running easterly was as follows, Kellogg, Clarke & Co., Phoenixville, Pennsylvania; Baltimore Bridge Company, Baltimore, Maryland; L. B. Boomer & Co., Chicago, a span of 196 feet, for railroad only, spans Second and Front Illinois; Keystone Bridge Company, Pittsburgh, Pennsylvania; and streets, Davenport, these streets intersecting at this point. Then Detroit Bridge Company. Kellogg & Clarke, with the Detroit Bridge follow the double-deck spans across the river, in the following Company, had built the Quincy Bridge and Boomer, the first Rock order: One of 260 feet; three of 220 feet each; one of 260 feet; Island Bridge; Keystone had also built the Keokuk and Hamilton draw span, 368 feet; finally, a railroad span of 100 feet, over Bridge. The only firm that had not built a bridge across the Mississippi the wagon road on the island, leading to the bridge, making was the Baltimore Bridge Company under C. Shaler Smith. Bids were a total length of bridge of 1,815 feet. This got the bridge to opened in late October 1870 with Detroit Bridge & Iron Company the Island.

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Engraving of the bridge from Rock Island, on the right, to Davenport, Iowa.

Smith adopted Pratt Trusses for his single-level spans at the ends and Whipple double intersection trusses for his doubledeck spans. For the double deck spans, “The trusses are 33 1/2 feet high between pin-centers, and placed 19 feet apart, allowing about 17 1/2 feet clear space. There are two floors: the upper, for the railroad, placed a little below the center of the trusses, and the wagon floor on the bottom chords. On each side of the bridge, outside of trusses, on a level with wagon road floor, are foot walks 6 feet wide, protected by substantial railing. About 12 feet clear headroom is allowed for wagons.” The verticals were built up with Phoenix sections fabricated with 4, 6, or 8 elements, with the smaller sections near mid-span and the large sections near the ends of the spans. The lower chords were wrought iron links, and the two-panel diagonals of iron bars were similar to but smaller than the lower chord members. “The top chord is 20 inches deep and from 22 inches to 24 inches wide, depending upon the panel's position. Each segment is formed of four plates, 20 inches by 1 inch to 1 1/4 inches thick, connected together at top and bottom by 6-inch channel bars, filling-strips being also introduced between the channels and plates, in certain of the segments.”

Benyaurd wrote of the swing span, “The draw-span is a double Whipple, reversed-pin connections throughout. Each of the seven panels of the top chord, from the ends, is formed of two eye bars, each 9 inches by 1 1/8, to which eyes from 15 inches to 18 inches in diameter are welded; these eye bars form the outsides of the chord and are connected together by horizontal plates and diagonal strips, making each segment stiff, and from 20 inches to 24 inches wide, depending upon the position of the panel. The next four panels are made up merely of eye bars, from 9 inches by 1 1/4 to 9 inches by 1 inch, with eyes 18 1/2 inches, having respectively 4, 6, 8, and 10 bars in a panel. The remaining panel is a short one and used as a connecting link between the two arms; it is formed of four pieces, 18 1/2 inches by 1 inch, 5 feet long.” The swing span was built on the swing span island and was completed in mid-February 1872. The approach spans were then erected, and the main channel bridge was finished on May 1, 1872. Benyaurd had gotten the additional funding for the approach spans over the

main channel. They were completed and tested on May 8, 1872, under a load of six flat cars loaded with sand weighing a total of 68,000#, and the deflections were never much over an inch.

Benyaurd finished his report with, Too much credit cannot be given to the Baltimore Bridge Company for the manner in which they performed their part of the contract and for the excellent workmanship and material that they put upon the bridge. And it is to be regretted that they were not so fortunate in a pecuniary point of view as they were in erecting the strongest and most perfectly finished bridge in America.

It was the longest, heaviest, and only double-deck swing span in the world at the time. The total weight of iron in the main channel spans was 6,675,000#, with a weight of iron and wood of 1,565,465# in the swing span alone. To move this great weight required a new mechanism that Smith developed to be powered by a steam engine mounted at the mid-span of the swing.

Bids for the spans over the easterly channel were asked for in November 1872 for “a through truss bridge 721 feet long in five equal spans; to have a roadway 20 feet in the clear and with two sidewalks each five feet in the clear; the roadway to be floored with oak plank 3 inches by 6 inches, and the sidewalks with pine plank, 3 inches by 6 inches, the latter to have a suitable hand-rail properly braced, the floor beams of wrought-iron, the floor joists to be of white pine.” Bids were opened in November 1872. The contract for these spans was awarded to Clarke, Reeves & Company, the successor firm to Kellogg and Clarke & Co., for $60,250. It was completed on July 1, 1873.

This bridge would survive until 1896 when Ralph Modjeski built the existing bridge on the original but reinforced piers.■ Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19th Century cast and wrought iron bridges. He is now an Independent Consulting Engineer (fgriggsjr@verizon.net).

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business PRACTICES The Art of Delegated Steel Design Strategies for successful delegated design in structural steel.

By Michael A. Stubbs P. E., S. E. and Adam Sanchez P. E.

F

or decades, delegating the design of steel connections and stairs to engineers under the steel fabricator’s contract has become increasingly popular. Projects with delegated design have had mixed results. Very often, the outcome of delegated design is significantly impacted by the way delegated design is specifi ed in the construction documents. This article provides Engineers of Record (EoR) strategies to improve efficiency and economic benefi t to the project owner when they specify the delegated design of steel components.

It is important to understand that delegated design was originally developed to allow steel fabricators and steel erectors the flexibility to complete the design in a manner that saves both Typical connection that will require accurate reactions. time and money in the construction of the projects. Under the ideal delegated design of Standards typically run smoother and more efficiently. AISC model, fabricators can design connections and stair configura- has also created the use of Substantiating Documents, which tions that maximize their fabrication equipment and procedures’ makes the communication between the EoR and Specialty best and fastest methods. Delegated design allows erectors to Engineer much simpler. It is highly recommended that EoRs influence the design to minimize construction obstacles and familiarize themselves with Section 3 of the Code of Standard leverage their experience to provide more efficient construc- Practice and specify delegated design accordingly. tion sequences. EoRs should ensure that the specifications they The Coalition of American Structural Engineers (CASE) have are writing maximize the ability of fabricators and erectors to three documents that can assist EoRs with making their coninfluence the project. struction documents efficient. These are: When an EoR specifies delegated design, they invite the fab• 962 National Practice Guidelines for Structural Engineers ricator, erector, detailer, and specialty engineer (the engineer • 962-B National Practice Guidelines for Specialty Structural responsible for designing the connections and the stairs) to Engineers the design process. The design process, by its very nature, is • 962-D A Guideline Addressing Coordination and collaborative. The EoR should ensure the project is set up to Completeness of Structural Construction Documents make the relationship as collaborative as possible.

AISC has made considerable progress in defining the methods EoRs should also concentrate on some straightforward stratthat EoRs should specify delegated design. Section 3.2.3 (3) egies to help make their projects with delegated design run of ANSI/AISC 303-22 “Code of Standard Practice for Steel more smoothly.

Building and Bridges” has been updated to facilitate delegated 1. Make sure the use of ASD vs. LRFD is clearly defined. connection design. Projects that follow this section of the Code 2. The construction documents should provide accurate

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reactions where possible. When universal methods of calculating reactions are provided, such as 55% of the Uniform Load Tables, the project can be designed for overly conservative reaction forces. This can cause inefficient conditions. Short small beams used can often have specified reactions that will exceed the actual reaction by as much as 300% 3. When connections are designed to resist wind and seismic forces, the reactions should be broken up into dead, live, wind, and seismic forces. This allows the Specialty Engineer to take advantage of reductions in forces allowed by ASCE 7 load cases.

4. Clearly define connection types that are not acceptable.

5. Provide sufficient information for the fabricator and erector to bid on the project without the need to design connections during the bidding process.

6. Provide specific instructions on what is expected regarding the Substantiating Documents. The instruction can include drawing and details required, calculation format and requirements for correlation to the shop drawings. This will avoid the need for resubmittals and project delays.

EoRs should also consider the methods used to specify a delegated design for stairs. First, the EoR should take an active part in assisting the architect in properly defining the stairs.

This is especially true with monumental stairs. The architect often needs help determining the size and configuration of stair members along with the location of supports. The EoR

must clearly define where the stairs are expected to attach to the primary structure. “AISC Design Guide 34 – Steel-Framed Stairway Design” provides an excellent resource for assisting with the design of stairs and how to specify the delegated design of stairs correctly.

How documents that are prepared under the contract of the steel fabricator are signed and sealed can also be a point of contention on projects with delegated design. Signing and sealing shop drawings can be problematic for Specialty Engineers.

This can present the Specialty Engineer with excessive liability and uninsurable conditions. Therefore, specifying that the Specialty Engineer stamp shop drawing is not recommended.

Substantiating Documents allows Specialty Engineers to sign and seal their work while not creating liability conflicts. As mentioned above, the EoR should be explicit regarding the expectations for the substantiating documents in the construction documents.

Mr. Stubbs will present strategies EoRs can employ to make delegated design projects more efficient at the 2024 North American Steel Construction Conference. This presentation will provide a more detailed discussion on improving the delegated design.■ Michael A. Stubbs P. E., S. E., President, Stubbs Engineering, Inc. (mstubbs@stubbseng.com) Adam Sanchez P. E., Senior Structural Engineer, Stubbs Engineering, Inc. (asanchez@stubbseng.com)

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STRUCTUREmag.org

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CODES and STANDARDS Solar Arrays Designed Incorrectly for Wind Uplift Loads Statement on required load combinations for wind uplift.

By Joe Maffei, S. E., Ph.D., Gwenyth Searer, S. E., Rob Ward, S. E., Rafael Sabelli, S. E.

L

ate last year, the Structural Engineers Association of California (SEAOC) learned that some engineers designing ballasted rooftop solar arrays using Allowable Stress Design (ASD) have not been applying the appropriate load combination for wind uplift (i.e., Load Combination No. 7a from ASCE 7 Section 2.4.1), either ignoring it or modifying it based on an incorrect understanding of its basis. In response, the organization issued the statement shown in the inset on the next page.

The statement was authored by the SEAOC Wind Committee and approved by the SEAOC Board of Directors in November 2022. (The statement is also posted at https://tinyurl.com/SEAOC-Statement ) In the cases that prompted SEAOC’s statement, the engineers ignored ASD Load Combination No. 7a (i.e., 0.6D + 0.6W) from Section 2.4.1 of ASCE 7 and instead considered only ASD Load Combination No. 5a (i.e., 1.0D + 0.6W). For Risk Category II, ignoring the load combination intended to govern uplift (i.e., Load Combination No. 7a) essentially means that the ballast is designed for wind speeds with a return period of approximately 40 years instead of the intended and required 700-year return period.

Thus, a ballasted solar array designed this way would have an annual probability of uplifting from the roof approximately 17 times higher than a code-compliant design. (Over a 30-year service life, the probability of uplift of a solar array would be about 50% rather than the intended 4%.) This is a substantial reduction in the stability of the solar array, and, as emphasized in the SEAOC statement, it is not appropriate to use the full dead load (even if it is known with complete certainty) to counteract the ASD-level wind load.

Basis of ASD Load Combinations ASD is intended to achieve reliability by providing a factor of safety between ASD-level load effects (informally called “service level”) and nominal resistance. In Load Combination 7a, 0.6W is the ASD-level load effect, and the dead load D provides the nominal resistance. The 0.6 factor applied to D represents the factor of safety against uplift, equal to 1/0.6=1.67. Using 1.0D instead of 0.6D would eliminate the factor of safety, which is an integral part of ASD. Just because the dead load is well known does not mean that a coefficient higher than 0.6 can be used for dead load.

Although the basis for the 0.6 factor on dead load may be unclear to some, the requirement to use it in Load Combination 7a is unambiguous. Section 2.4.1 clearly states that whichever load combination “produces the most unfavorable effect … shall be considered.” Thus, all potentially governing load combinations must be considered; none can be ignored.

46 STRUCTURE magazine

Solar array on the roof of a reservoir. Photo courtesy of Noelle Yuen, Maffei Structural Engineering.

Getting the Word Out The authors have recently found cases in which this erroneous design practice is still taking place; thus, we are working to get the word out to engineers and agencies that work with solar energy structures and building officials. Please help by forwarding this article or the link to the SEAOC statement to anyone you know who might be interested.

Because of the high probability of uplift for such designs, any solar arrays that may have been designed with incorrect load combinations should be reviewed and, if necessary to meet code requirements, be redesigned and retrofitted. Retrofitting would typically require additional ballast weight to hold down the array, with further modification or measures needed if the array configuration, the racking system, or the capacity of the roof structure cannot accommodate the additional ballast.■ Joe Maffei is the Founding Principal at Maffei Structural Engineering in San Francisco, CA. He is a member of the SEAOC Wind Committee. (joe@ maffei-structure.com) Gwenyth Searer is a Principal at Wiss, Janney, Elstner Associates, Inc. She is the current Chair of the SEAOC Wind Committee and a member of the ASCE Minimum Design Loads on Buildings and Other Structures Standards Committee. (gsearer@wje.com) Rob Ward is a Senior Structural Engineer at Maffei Structural Engineering in San Francisco, CA. He is a member of the SEAOC Wind Committee. (rob@maffei-structure.com) Rafael Sabelli is a Senior Principal at Walter P Moore and a member of the ASCE Minimum Design Loads on Buildings and Other Structures Standards Committee. (rsabelli@walterpmoore.com)

Statement by the SEAOC Wind Committee on Wind Uplift and Allowable-Strength Design Load Combinations for Ballasted Solar Array Designs, November 11, 2022 (Authored by the SEAOC Wind Committee, and reproduced here with permission from SEAOC) The Structural Engineers Association of California (SEAOC) Wind Committee has recently learned that some engineers designing ballasted rooftop solar arrays for wind uplift have been using the Allowable Stress Design (ASD) load combinations of the ASCE 7 standard, the International Building Code (IBC), and the California Building Code (CBC) incorrectly.

These engineers have proposed ignoring the required Load Combination No. 7 in Section 2.4.1 of ASCE 7-16, (0.6D + 0.6W), which leaves Load Combination 5, (D + 0.6W), governing the design for uplift1.

In the above equations, D is the dead load of the ballasted panel system, and 0.6W is the allowable-stress design-level design wind uplift force.

The justification that SEAOC has seen for this practice assumes that the 0.6 factor on dead load is intended only to represent uncertainty in the dead load. This assumption is incorrect. Rather, the factor was derived so that ASD load combinations would give results similar to designs that use Load and Resistance Factor Design (LRFD).

The degree of certainty that one may have in the dead load is not a justification to change this factor.

It is SEAOC’s opinion that a design that ignores or modifies the 0.6D + 0.6W allowable stress load combination does not comply with ASCE 7, the IBC, or the CBC, and is in direct conflict with code provisions. Further, such an approach cannot be justified by any code provisions that allow alternate design approaches. Failing to use the required 0.6D + 0.6W load combination in the allowable stress design procedure is unconservative and will lead to designs with a substantially higher probability of wind uplift failure than a code-compliant design.

The 2009 paper “Counteracting Structural Loads: Treatment in ASCE Standard 7-05” by Ellingwood and Li2 directly addressed the issue of ASD Load Combination No. 7. The paper states, “the factor applied to dead load in situations where its effect is counteracted by the effects of other lateral or uplift forces should not be increased above 0.6.”

In summary, ignoring or modifying the 0.6D + 0.6W allowable stress load combination violates the building code, and use of 100 percent of the dead load to resist allowable-stress design-level wind uplift will result in markedly unconservative designs and, consequently, a higher probability of solar array failures.

1 ASCE 7-16, the 2018 IBC, and the 2019 CBC all have identical ASD load combinations.

2 Ellingwood, Bruce R and Li, Yue, “Counteracting Structural Loads: Treatment in ASCE Standard 7-05”, Journal of Structural Engineering, January 2009, pp. 94-97.

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code UPDATES 2024 IBC Significant Structural Changes Special inspection & soils/foundations (IBC Chapters 17 & 18)—Part 2.

By John “Buddy” Showalter, P. E., M. ASCE, M. NCSEA, and Sandra Hyde P. E., M. ASCE, M. NCSEA

T

his multi-part series discusses significant structural changes to the 2024 International Building Code (IBC) by the International Code Council (ICC). Part 2 includes an overview of changes to IBC Chapter 17 on special inspection and IBC Chapter 18 on soils and foundations. Only a portion of these chapters’ total number of code changes are discussed in this article. More information on the code changes can be found in the 2024 Significant Changes to the International Building Code, available from ICC (Figure 1).

Metal Building Systems Metal building systems (Figure 2) are significantly different from other forms of steel construction, especially regarding the shared design responsibilities between the metal building system manufacturer Figure 2 Metal building system.

and registered design professional for the project. Adding new metal building system provisions, a definition, and material requirements clarifies the design requirements for the systems (a future article in the series will provide more details). New IBC Section 1705.2.6 provides special inspection provisions for metal building systems.

1705.2.6 Metal building systems. Special inspections of metal building systems shall be performed in accordance with Sections 1705.2.1, 1705.2.3, 1705.2.4 and 1705.2.5 and Table 1705.2.6.

The approved agency shall perform inspections of the erected metal building system to verify compliance with the approved construction documents.

Figure 1 2024 Significant changes to the IBC.

48 STRUCTURE magazine

Type

Periodic Special Inspection

1. Installation of rafter/beam flange braces and column flange braces.

X

2. Installation of purlins and girts, including specified lapping.

X

3. Purlin and girt restraint/ bridging/bracing.

X

4. Installation of X-bracing, tightened to remove any sag.

X

Table 1705.2.6 Special Inspections of Metal Building Systems.

Change Significance: With clarification of the design requirements for different metal building system components, special inspection requirements become clearer. Today, many construction documents list nonexistent Metal Building Manufacturer Association standards as the governing design requirements. By placing material and special inspection requirements into the IBC, designers can create more accurate construction documents, and building departments can more easily inspect the construction.

New special inspection requirements complement the new provisions for metal building systems in IBC Section 2210. Metal building systems are typically highly optimized structures heavily dependent on bracing components to function per the design. Some bracing components consist of materials that are not considered to be "structural steel." As these components didn’t previously have special inspections listed in IBC Section 1705.2, an inspection of the completed installation of these critical components was often overlooked.

Metal building systems often contain assemblies made of various components, such as structural steel, cold-formed steel, and steel cables. While the individual components are often covered by fabricator special inspections and tests found in the subsections of IBC Section 1705.2, the “systems” used in metal building systems are often unique and have not been specifically identified in the steel section.

The new language adds explicit requirements for special inspection of commonly used systems not previously identified elsewhere.

Type

2. Reinforcing bar welding: a.

Verify weldability of reinforcing bars other than ASTM A706.

—

X

AWS D1.4 ACI 318: 26.6.4 26.13.1.4

Inspect welding of reinforcement for special moment frames, boundary elements of special structural walls, and coupling beams.

X

—

AWS D1.4 ACI 318: 26.13.3

Inspect welded reinforcement splices.

X

—

—

Inspect welding of primary tension reinforcement in corbels.

X

—

—

e.b. Inspect singlepass fillet welds, maximum 5/16''.

—

X

AWS D1.4 ACI 318: 26.13.3

X—

X

AWS D1.4 ACI 318: 26.13.3

b.

c.

Special Inspection Of Reinforcing Bar Welding IBC Section 1705.3.1 references AWS D1.4 Structural Welding Code – Reinforcing Steel which contains requirements for rebar inspection.

The construction documents must indicate the type and location of welded splices and any other welding. Reinforcement must be of a material quality that forms a strong weld where welding is required. Reinforcing steel per ASTM A706 Standard Specification for Deformed and Plain Low-Alloy Steel Bars for Concrete Reinforcement is typically specified. Other rebar must be tested before welding begins to determine whether welding is an option or if mechanical splices are required for the reinforcement. Ideally, rebar weldability is determined before construction begins. Changes to IBC Table 1705.3 coordinate the special inspection provisions for welding of reinforcing steel (Figure 3) with the provisions in Section 26.13.3 of ACI 318-19 Building Code Requirements for Structural Concrete.

Continuous Periodic Referenced Special Special Standarda Inspection Inspection

d.

f.c.

Inspect all other welds.

Table 1705.3 (excerpt) Required Special Inspections and Tests of Concrete Construction.

Figure 3 Reinforcing bar welding.

Change Significance: IBC Table 1705.3 Item 2(b) adds a new requirement for continuous special inspection of reinforcement welding in special moment frames, boundary elements of special structural walls, and coupling beams as required by ACI 318 Section 26.13.3.2(d). Because of their critical nature, new Item 2(c) was added to require continuous special inspection of all welded reinforcement splices. New item 2(d) for continuous special inspection for welding primary tension reinforcement in corbels highlights another difficult area to weld.

Previous Item 2(b) for periodic inspection of single pass fillet welds is renumbered as Item 2(e). Former Item 2(c) for special inspection of all other welds is renumbered as Item 2(f ) and revised to permit these welds to be performed as a periodic special inspection since the critical welds covered by new Items 2(b), 2(c), and 2(d) have been introduced into the table as separate continuous special inspections.

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a permit.

IBC Section 1807.2.5.2 height requirement provisions reference IBC Section 1015.3, which mandates a 42-inch-high guard measured vertically from the adjacent walking surface. IBC Section 1807.2.5.3 opening limitation requirements reference IBC Section 1015.4, which restricts openings to the passage of a 4-inch-diameter sphere for the required guard height.

Grade Beams Figure 4 Guards on retaining walls.

Guards On Retaining Walls Guards are systems comprising posts, handrails (where required), and balusters or panels. Guards are required to prevent a person from falling to a lower elevation that might cause injury. The code was silent on requirements for guards on top of retaining walls. These conditions commonly occur in public places such as parks and schools. New IBC Section 1807.2.5 includes provisions for guards on retaining walls (Figure 4) consistent with guard requirements elsewhere in the code. The exception exempts conditions where a retaining wall is not accessible to the public.

In high seismic regions, grade beams (Figure 5) must be designed as ductile per ACI 318 unless the beam is strong enough to resist the anticipated maximum earthquake force as determined by the seismic load combination with an overstrength factor from ASCE 7-22 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, Section 2.3.6 or 2.4.5. In other words, grade beams must be designed for overstrength or ductility. Accordingly, the exception in IBC Section 1810.3.12 for grade beams in deep foundation systems was modified so that only the ductile detailing provisions in ACI 318 Section 18.13.3.1 are exempt when grade beams are designed for the overstrength factor.

Section 1809 Shallow Foundations

1807.2.5 Guards. Guards shall be provided at retaining walls in accordance with Sections 1807.2.5.1 through 1807.2.5.3.

Exception: Guards are not required at retaining walls not accessible to the public.

1807.2.5.1 Where required. At retaining walls located within 36 inches (914 mm) of walking surfaces, a guard shall be required between the walking surface and the open side of the retaining wall where the walking surface is located more than 30 inches (762 mm) measured vertically to the surface or grade below at any point within 36 inches (914 mm) horizontally to the edge of the open side. Guards shall comply with Section 1607.9.

1807.2.5.2 Height. Required guards at retaining walls shall comply with the height requirements of Section 1015.3.

1807.2.5.3 Opening limitations. Required guards shall comply with the opening limitations of Section 1015.4.

1809.14 Grade beams. Grade beams shall comply with the provisions of ACI 318.

Exception: Grade Beams not subject to differential settlement exceeding one-fourth of the thresholds specified in ASCE 7 Table 12.13-3 and designed to resist the seismic load effects including overstrength factor in accordance with Section 2.3.6 or 2.4.5 of ASCE 7 need not comply with ACI 318 Section 18.13.3.1.

Section 1810 Deep Foundations 1810.3.12 Grade beams. Grade beams shall comply with the provisions of ACI 318.

Exception: Grade beams not subject to differential settlement exceeding one-fourth of the thresholds specified in ASCE 7 Table 12.13-3 and designed to resist the seismic load effects including overstrength factor in accordance with Section 2.3.6 or 2.4.5 of ASCE 7 need not comply with ACI 318 Section 18.13.3.1.

Change Significance: IBC Section 1807.2.5.1 parameters for proximity to a walking surface and vertical distance to the surface or grade below are consistent with IBC Section 1015.2 for guards in the interior of a building and IBC Section 105.2 for work exempt from permits. IBC Section 105.2 Item 4 exempts retaining walls less than 4 feet tall from a permit; however, that distance is measured from the bottom of the footing making the grade difference approximately 30 inches.

IBC Section 105.2 Item 6 indicates a sidewalk or driveway with more than Figure 5 Single-span grade beam profile (ACI Detailing Manual MNL-66(20) Figure FND-3: Grade Beam Profile & a 30-inch grade change would require Schedule).

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Change Significance: This clarification for shallow and deep foundations is needed since ASCE 7 Section 12.13.9.3.1 permits the downdrag of pile design based on the significant differential settlement during an earthquake.

Differential settlement exceeding one-fourth of the threshold may impose moments and shears in the grade beam that exceed those computed with seismic load effects, including the overstrength factor, in which case the ductile detailing requirements for grade beams in ACI Section 18.13.3.1 would be required for structures assigned to Seismic Design Category D, E, or F. According to the ACI 318 Commentary for this section, “Grade beams resisting flexural stresses from column moments should have reinforcement details similar to the beams of the frame above the foundation.”

ASCE 7 Section 12.13.9 exempts foundation elements likely to be affected by earthquake-induced settlement from complying with ductility requirements when their deformation is less than limits which vary based on risk category.

Therefore, provisions for grade beams in foundation systems in the IBC were modified so that while grade beams designed with an overstrength factor need not meet the ductile detailing provisions in ACI 318, the exception is only permissible when differential settlements are less than one-fourth of the thresholds in ASCE 7 Table 12.13-3. All other provisions of ACI 318 for strength, durability, reinforcing steel cover, etc., are still applicable.

IBC Section 1809.14 is a new section that adds the grade beam provisions in the deep foundation section to the shallow foundation section. The grade beam must either be ductile or strengthened to resist forces from a maximum considered earthquake by including overstrength in its design.

Deep Foundation Elements In those sections of IBC Chapter 18 that specifically deal with the types of elements most used in the construction of deep foundations, there are limitations placed on the stresses that can be used in the deep foundation element design. In most cases, the allowable stresses are stated as a percentage of the strength of the element’s material. For example, in the case of piles made of steel, the allowable stresses are prescribed as a percentage of the yield strength of the steel. The allowable design stresses designated in IBC Chapter 18 for each deep foundation type are intended to provide a safety factor against the dynamic forces that may cause damage, for example, from driving a deep foundation element, and to avoid overstress in the element under the design loads and loads induced by subsoil conditions.

Material Type and Condition

Maximum Allowable Stressa

3. Steel in compression Pipes, tubes or H-piles, where justified in accordance with Section 1810.3.2.8

0.5 Fy ≤ 32,000 psi

Other pipes, tubes or H-piles

0.35 Fy ≤ 24,000 psi

5. Steel in tension Pipes, tubes or H-piles, where justified in accordance with Section 1810.3.2.8

0.5 Fy ≤ 32,000 psi

Other pipes, tubes or H-piles

0.35 Fy ≤ 24,000 psi

Helical piles

0.6 Fy ≤ 0.5 Fu

Table 1810.3.2.6 (excerpt) Allowable Stresses for Materials Used in Deep Foundation Elements.

1810.3.2 Materials. The materials used in deep foundation elements shall satisfy the requirements of Sections 1810.3.2.1 through 1810.3.2.8, as applicable.

Sections 1810.3.2.1 through 1810.3.2.7 are unchanged and not shown for brevity.

1810.3.2.8 Justification of higher allowable stresses. Use of allowable stresses greater than those specified in Section in Table 1810.3.2.6 that must be justified in accordance with this section shall be permitted where supporting data justifying such higher stresses is filed with submitted to and approved by the building official. Such substantiating data shall include the following: 1. A geotechnical investigation in accordance with Section 1803.

2. Load tests in accordance with Section 1810.3.3.1.2, regardless of the load supported by the element.

The design and installation of the deep foundation elements shall be under the direct supervision of a registered design professional knowledgeable in the field of soil mechanics and deep foundations who shall submit a report to the building official stating that the elements as installed satisfy the design criteria.

Change Significance: As previously written, this section could have allowed the override of the allowable stresses in IBC Table 1810.3.2.6 when a pile passed a load test – which is not the intent. Several foundation types in IBC Table 1810.3.2.6 have multiple allowable stresses for the same material type. For example, IBC Table 1810.3.2.6 provides for a higher allowable compressive yield stress of 0.5Fy for steel piles when justified per IBC Section 1810.3.2.8, which requires a geotechnical investigation and a load test. Otherwise, steel piles are limited to 0.35Fy. In other words, the pile test is to justify the higher values in IBC Table 1810.3.2.6, not to allow even higher values than those already tabulated.

Conclusion Structural engineers should be aware of significant structural changes in the 2024 IBC for special inspections and soils/foundations. New special inspection provisions for metal building systems have been incorporated. Changes to IBC Table 1705.3 coordinate the special inspection provisions for welding of reinforcing steel with the provisions in ACI 318-19. New provisions for guards on retaining walls are added consistent with guard requirements elsewhere in the code. Provisions for grade beams in deep foundation systems were modified so that only the ductile detailing provisions in ACI 318 are exempt when grade beams are designed with an overstrength factor. Finally, new provisions allow tabulated higher allowable stresses for deep foundation elements when evidence supporting the higher capacity is submitted and approved by the building official.■

Look for more of the series in previous and upcoming issues of STRUCTURE.

John “Buddy” Showalter, P. E. Senior Staff Engineer of ICC’s Consulting Group. (bshowalter@iccsafe.org) Sandra Hyde, P. E. Managing Director of ICC’s Consulting Group. (shyde@iccsafe.org)

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NCSEA News Congratulations to This Year’s NCSEA Grant Program Recipients!

The NCSEA Grant Program awards SEAs funding for projects that advance their SEA and the structural engineering profession in accordance with the NCSEA Mission Statement. Supported by the NCSEA Foundation, the SEA Grant Program has delivered more than $80,000 in grants since its inception!

Here are the 2023 Winners and their projects:

Structural Engineers Association of Colorado (SEAC) Colorado Special Wind Region Wind Study SEAC plans to commission a state-wide Colorado wind climate study with the following objectives: (1) to establish meaningful and accurate boundaries for the Colorado SWR for incorporation into ASCE 7, (2) to determine reliability-based design wind speeds, utilizing the most current data and methodologies, spanning the entire Colorado SWR, and (3) to determine what winter windiness parameter(s) should be utilized for snow drifting design within the Colorado SWR using the provisions of ASCE 7-22 and later.

Structural Engineers Association of Illinois (SEAOI) Professional Networking and Headshots Event SEAOI will host a gathering that aims to provide group networking opportunities and offer attendees the chance to have professional headshots taken. The idea originated from a discussion within the SEAOI WiSE group, highlighting the difficulty many face in accessing professional headshots. While the headshots and networking event will be free, recipients are requested to create a post or video promoting SEAOI and/or NCSEA events on their LinkedIn or other social media, which will also be shared on SEAOI's social channels.

Structural Engineers Association of Kentucky (SEAoK) Young Members Group (YMG) SEAoK will launch a YMG in the state of Kentucky, with the goal of cultivating future SEAoK and NCSEA leadership, broadening membership, doing college outreach, primary school outreach, study sessions for PE/SE exams, and for a community of other structural engineers to socialize and engage with.

Structural Engineers Association of Kansas & Missouri (SEAKM) Young Members Group (YMG)

SEAKM is establishing a YMG under SEAKM Kansas City Chapter to attract more young engineers to join SEAKM. To do this, they plan to host events targeted at a younger audience than the typical SEAKM monthly meetings to encourage broader engagement. Once established, the goals of the YMG would include helping young engineers enhance their knowledge and skills, acquire licenses, advance their careers, foster future leadership within SEAKM, and promote structural engineering in general.

Structural Engineers Association - Metropolitan Washington (SEA-MW) SE3 GrowthGuidance Mentorship Program The SEA-MW SE3 GrowthGuidance Mentorship program will connect entry-level engineers with experienced mid-level and seasoned professionals. It will foster open dialogue, clarify career goals, address obstacles, and promote personal growth, all while discussing the unique challenges of the structural engineering profession. Additionally, it will aim to provide a platform for current and future leaders to share practical insights and enhance their leadership skills.

Structural Engineers Association of Northern California (SEAONC) Definition of Special Wind Regions in California SEAONC will attempt to delineate the boundaries of the Special Wind Regions in California, as well as the appropriate Basic Wind Speeds within these Special Wind Regions. Once SEAONC completes the wind studies, the results will be communicated to the various jurisdictions and the engineering community in California via webinars and white papers. SEAONC’s plan is to submit the studies, the associated white papers, and proposed changes to ASCE 7 in time to see the changes adopted in ASCE 7-28, and in time to be adopted into the 2030 International Building Code.

52 STRUCTURE magazine

News from the National Council of Structural Engineers Associations Structural Engineers Association of New Mexico (SEANM) Young Members Group (YMG) - Seminars

SEANM YMG will establish a series of specialized seminars tailored to the young members, held periodically throughout the year. These seminars will explore unconventional subject matter, such as addressing burnout and mental health challenges within the industry, imparting financial literacy skills, and enhancing time management capabilities. In addition to these contemporary topics, the aim is to provide insights into more conventional subjects such as Quality Assurance/Quality Control (QA/QC) practices and common oversights made by newer engineers.

Structural Engineers Association of New York (SEAoNY) Diversity Committee Capacity-Building Program & Learning and Development Series The SEAoNY Diversity Committee is collaborating with Shani Dellimore Barrax of the Aurora Change Agency to address declining participation across committees by revitalizing the community through a needs assessment and three proposed programs. A Self-Identification Campaign initiated with an anonymous survey, aims to collect demographic data from SEAoNY members. The obtained demographic information will guide future programming to encourage diversity, equity, and inclusion within SEAoNY and its member firms.

Structural Engineers Association of Ohio (SEAoO) Embodied Carbon Calculation and SE 2050 Workshops SEAoO will host a half-day workshop in Columbus, Cincinnati, and Cleveland to outline sustainable design practices to local firms, educate them on SE 2050, and guide them through the process of joining SE 2050.

Structural Engineers Association of Oregon (SEAO) Structural Engineering Engagement and Equity SEAO will host a two-part event. Part one is to provide the less experienced structural engineer the opportunity to network with their more knowledgeable counterparts in senior leadership roles. Part two is a guided panel discussion featuring three to five individuals in senior leadership positions. The panelists will be selected in consideration of their work credentials and background diversity to provide an eclectic knowledge base to draw from. During the guided discussion portion, the panelists will be asked questions sourced from a community survey presented in the local SEA newsletter.

NCSEA Webinars

Visit www.ncsea.com/education for the latest news on upcoming webinars and other virtual events.

December 14, 2023

An Introduction to the New ASCE Solar PV Structures Manual of Practice

January 25, 2024

Boston University Data Science Center

February 6, 2024

Elevator 101

Purchase an NCSEA webinar subscription and get access to all the educational content you’ll ever need! Subscribers receive access to a full year’s worth of live NCSEA education webinars (25+) and a recorded library of past webinars (170+) – all developed by leading experts and available whenever, wherever you need them!

Courses award 1.0 -1.5 hours of Diamond Review-approved continuing education after completing a quiz.

Recommendations for Performing Structural Engineering Quality Assurance Reviews

Recommendations for Performing Structural Engineering Quality Assurance Reviews

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SEI Update Happy Holidays – All the best to you and yours this holiday season!

Education

Bridge to Building a Stronger SEI The SEI Bylaws were recently revised as an important milestone to advance the SEI vision, and to achieve the new SEI strategic board structure and the evolution of the committee structure into the Technical and Professional Communities. The SEI reorganization also includes identification and acceleration of Focused Initiatives, and the creation of an Advisory Council. Learn more at www.asce.org/SEI

ASCE/SEI Substation Structure Design Guide-MOP 113 Program January 25 Provides structural design guidance and function as a comprehensive resource for outdoor electrical substation structures and foundations.

Join us for dialogue with industry experts, a technical presentation, and Q&A. Register at www.asce.org/SEIEvents

SEICon24 March 20-22 at NASCC: The Steel Conference Join us in San Antonio to learn, engage, and network with the structural engineering community while taking advantage of the broader opportunities at The Steel Conference. SEICon24 is the SEI annual conference in 2024.

Students & Young Professionals (35 or younger): Apply by January 3 for an SEI Futures Fund scholarship to participate. www.SEICon24.org Plans are also underway for Structures Congress, April 9-11, 2025 in Phoenix.

Advancing the Profession

SE2050: Committing to New Zero

Sign up to receive the SE2050 Monthly Newsletter and learn how you can contribute to reaching net zero embodied carbon structural systems by 2050. www.SE2050.org

Renew your SEI/ASCE Membership Remember to renew for 2024 to continue your benefits and member rates on programs. www.asce.org/SEIMembership

Follow SEI on Social Media: 54 STRUCTURE magazine

Leverage the CSI 3 to 1 Match and Celebrate 10 Years of the SEI Futures Fund Make the most of year-end giving and invest in the future of Structural Engineering at www.asce.org/SEIFuturesFund.

CASE in Point Tools To Help Your Business Grow...

CASE has committees that work together to produce specific resources available to members, from contract documents to whitepapers, to help your business succeed.

If you are a member of CASE, all CASE publications are free to you. NCSEA and SEI members receive a discount on publications. Use discount code - NCSEASEI2022 when you check out.

Check out some of the brand new CASE Publications developed by the Guidelines CommitteeÉ CASE 976-C: A Review and Commentary on the American Institute of Steel Construction 2022 Code of Standard Practice for Steel Buildings and Bridges The importance of the AISC Code of Standard Practice (AISC 303-22), referred to herein as the Code or COSP) to the construction community is manifested in its almost 100 years of use and development.

This Code establishes the trade practices for the steel industry. Generally, this involves the acceptable practices and responsibilities of the Fabricator and Erector and the responsibilities of others such as the Owner’s Designated Representative for Design (ODRD) – (usually the Structural Engineer of Record), the Owner and the Owner’s Designated Representative for Construction (ODRC) – (usually the General Contractor or Construction Manager or similar authority at the jobsite) as they relate to the work of the Fabricator and Erector. The 2022 COSP addresses many recent changes in the practice of designing, purchasing, fabricating, and erecting structural steel and is therefore a continuation of the trend of past improvements and developments of this standard.

CASE White Paper Beyond the Code: Shrinkage Cracking

CASE recognizes that the International Building Code or other governing codes do not address all aspects of structural engineering and design. Often, the most common issues where the owners, or the contractor or the design team are not aligned deal with what is not clearly addressed by the various codes or design guidelines. This is the second in a series of “Beyond the Code” white papers that will attempt to collate design considerations that need to be discussed with the owners at the beginning of a project to establish a clear Basis-of-Design for the project. By proactively bringing up the design consideration in front of the owners, the Structural Engineer can set up realistic expectations and discuss the cost impact of alternative designs.

This white paper in the “Beyond the Code” series discusses shrinkage cracking in concrete with an explanation of why it occurs, common locations they occur, and strategies to mitigate them becoming a risk in your project.

You can purchase these and other Risk Management Tools at You can also browse all of the CASE publications at https://www.acec.org/member-center/get-involved/coalitions/case/resources/ Is there something missing for your business practice? CASE is committed to publishing the right tools for you.

Have an idea? We’d love to hear from you!

Follow ACEC Coalitions on LinkedIn: www.linkedin.com/in/acec-coalitions 56 STRUCTURE magazine

News of the Coalition of American Structural Engineers Upcoming Events Risk Management Industry Update December 7, 2023 1:30–2:30 pm ET Online Hosted by ACEC Coalitions and presented by Chad Wilson, General Counsel, Psomas.

The discussion will include:

Review the current trends in Professional Liability Claims (PLC) Restrictive Covenants: Non-Competes, Non-Solicits Challenges to PLS statute(s) Generative Artificial Intelligence (AI) https://www.acec.org/event/risk-management-industry-update/

Joint Town Hall Event with CASE, NCSEA, and SEI February 21, 2024 2:00–3:30 pm ET Online Leadership from CASE, NCSEA, and SEI will host a virtual joint town hall event to discuss how the three organizations are progressing to fulfill the Vision for the Future of Structural Engineering (adopted April 2019), highlighting initiatives to advance the profession and enhance member engagement.

The town hall is an opportunity to catch up on things you might have missed and gain insight into what the three organizations are doing moving forward.

This complimentary event is open to all CASE, NCSEA and SEI members. https://program.acec.org/ joint-town-hall-event-case-ncsea-and-sei

Coalitions Winter Meeting February 26–27, 2024 New Orleans, LA

Now more than ever we need to support the upcoming generation of the workforce.

Give to the CASE Scholarship today!

Save the date! The Coalitions Winter Meeting will be in New Orleans, LA. The interactive roundtables, educational content, and technology demonstrations will be all about Innovation Forward: Advancing your Business with Tomorrow’s Technology.

More information coming soon.

D ECE M B ER 2023

57

structural VERSE What’s an Engineer Doing

By Neil Wexler To the office you keep going For the task that keeps on growing.

What are you doing every day At the office … you don’t play.

So an engineer’s required, Mostly to do Design.

Buildings, bridges, client brings, You will detail all those things.

What about Managing To make things happening?

And the things the others do, You’re responsible for them too.

Final task is Marketing, Engineers do targeting.

You’ll oversee that too, But …you will let the others do.

Engineering is hard task, It requires lots of math.

But important very much Is the talking to the mass.

So important understand Public Safety is the brand, Public Safety is Number One, For the task that you just planned.

Otherwise, you will be thinking, What’s this engineer bringing ?

Market, Manage and Design This is what he is assigned.

Whatever task may be required, At the office you’re admired, You are so much in the know, Others …aspire for the show.

And remember Number One! For the job that just begun, Never deviate from zee, At the office… let it be!

Neil Wexler, Ph. D., P. E. NWexler@nwexler.com

58 STRUCTURE magazine

A CARBON NEUTRAL FUTURE

STARTS TODAY.

PCA’S ROADMAP TO CARBON NEUTRALITY. THE BUILT ENVIRONMENT IS CHANGING FOR GOOD. Learn how to get involved: cementprogress.com

© Computers and Structures, Inc. 2023