February 2024 by structuremag

See how high-strength steel is enabling sustainable design. nucor.com/madeforgood

SEE HOW OUR PARTNERSHIP OPENED NEW POSSIBILITIES FOR SUSTAINABLE ARCHITECTURE. Read the case study at nucor.com/madeforgood/spiral-case-study

See how high-strength steel is enabling sustainable design. nucor.com/madeforgood

SEE HOW OUR PARTNERSHIP OPENED NEW POSSIBILITIES FOR SUSTAINABLE ARCHITECTURE. Read the case study at nucor.com/madeforgood/spiral-case-study

STRUCTURE FEBRUARY 2024

NCSEA | CASE | SEI

STEEL

CONCRETE

INSIDE: D.C.'s Storey Park

Delegated Design in Structural Steel MIT-Kendall Metro Canopy HSS Welds for Steel Shapes

18 39 52

ADVERTISER index

Please support these advertisers

Atlas Tube ........................... Cover Gatefold Bull Moose Industries.............................. 21 Cast Connex. .........................................4 Computers & Structures Inc. ......... Back Cover Enercalc ................................................6 IDEA StatiCa ........................................ 22 JLG Industries ...................Inside Back Cover NCEES................................................ 15

STRUCTURE FEBRUARY 2024

NCSEA | CASE | SEI

STEEL CONCRETE

INSIDE: D.C.'s Story Park

Delegated Design in Structural Steel MIT-Kendall Metro Canopy HSS Welds for Steel Shapes

18 39 52

New Millennium.................................... 17 Nucor ................................................. 11 Nucor ........................................... Onsert Nucor Vulcraft....................................... 25 Simpson Strong-Tie ................................. 23 Strongwell, Inc. ..................................... 29 Williams Form Engineering Corp. ............. 19

STRUCTURE

CIRCULATION

CONNECTING A CITY THROUGH

subscriptions@structuremag.org

EFFICIENT CONNECTIONS

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

February 2024

Linda M. Kaplan, P.E. Pennoni, Pittsburgh, PA

Available Only at

Jessica Mandrick, P.E., S.E., LEED AP Gilsanz Murray Steficek, LLP, New York, NY

Digital Issue

STRUCTUREmag.org

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)

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 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

On the Cover: A final rendering of the Storey Park project in Washington D.C. depicts the residential component and seven-story bridge. Image courtesy of HKS Architects.

In Every Issue 3 Advertiser Index 73 Resource Guide-Steel 74 NCSEA News 76 SEI Update 78 CASE in Point

Managing Editor Shannon Wetzel swetzel@structuremag.org Production production@structuremag.org

MARKETING & ADVERTISING SALES Director for Sales, Marketing & Business Development

When the city of Houston needed a way

Monica Shripka Tel: 773-974-6561 monica.shripka@STRUCTUREmag.org

to get pedestrians across 17 lanes of busy traffic, they turned to Atlas Tube

STRUCTURE magazine (ISSN 1536 4283) is published monthly ®

Jumbo HSS and Shuriken™ connection

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 31, Number 2, © 2024 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.

technology to get the job done. See how we were able to deliver on the city’s need to create a sleek, easier-to-build bridge.

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.

FEBRUARY 2024

A PEDESTRIAN BRIDGE THAT’S ANYTHING BUT PEDESTRIAN. Constructing a 600-foot pedestrian bridge over one of the busiest highways in America was never going to be easy. But early collaboration and innovative thinking brought forward a plan that would simplify the process and ensure a successful outcome — all with as little interruption to traffic as possible. The secret weapon? The dynamic duo

work on-site — leading to a faster build at

of Atlas Tube Jumbo HSS and Shuriken

a reduced cost and less disruption than a

connection technology.

traditional build process. By utilizing this innovative method of assembly, the people

Made and melted in the USA, the thicker-

of Houston are able to enjoy not only

walled Jumbo HSS gave designers the

better access to transportation, but

ability to keep the stringer size at 16x16,

a better looking, easier on-site build.

giving the bridge a slimmer and sleeker look. Featuring three 200-foot sections, no field welding was needed to assemble on-site.

• 600-foot span, built and delivered in three sections • 17 lanes of traffic crossed

That’s because of the game-changing Shuriken connections that allow bolted

• Critical link between community and transit center

HSS splices using standard hardware.

• Galvanized per Texas DOT requirements

Shuriken allowed for more efficient

• 1,216 Shuriken

fabrication in the shop and field, and less

Shuriken splice locations No gusset or stiffener plates

16x16" 5/8" Wall stringer 12x12" Verticals

10x10" Diagonals

Scan the QR code to download this project’s case study or to see all the ways Shuriken can simplify your next build with our Shuriken design book.

atlastube.com/houston

Contents Cover Feature

48 THE STORY OF A TRUSS: STOREY PARK By Seth Rogge, Macenzie Smith, and Chris Woitowicz

Storey Park, located in the heart of the NoMA (North of Massachusetts Ave) neighborhood in Northeast Washington, D.C., was previously the site of an old Greyhound bus depot lot. Situated near the Metro entrance and adjacent to Union Station, this unique city block of land was ready for redevelopment.

FEBRUARY 2024

Features

Columns and Departments

39 BREAKING THE MOLD

7 Editorial ‘I Didn’t Say I Wasn’t Going to Do It, Only That I Didn’t Have the Time.’

By Rebecca Lubrano, Paul Kassabian, and Christian Sjoberg

The new MIT entrance pavilion in Cambridge, Massachusetts, is designed by Simpson Gumpertz & Heger (SGH) using first principle structural concepts, built with carbon and glass fiber, optimized with AI/machine learning, and fabricated by boat builders.

44 ENHANCING PUBLIC SAFETY IN EVENT PLANNING By William Gorlin, PE, SE

While temporary structures encompass a wide range of applications, this article specifically concentrates on their utilization for live events and related entertainment industry uses.

52 KNOW YOUR HSS WELDS By Mike Manor, PE, MLSE, and Cathleen Jacinto, PE, SE

Maximizing the benefits of HSS sections in a design involves understanding the intricacies of various welding types for connections between members.

By Angelina V. Stasulis, P.E., S.E.

8 Structural Influencers Dr. Nehemiah J. Mabry, P.E. 10 Structural Observations To Align or Not to Align By Doug Steimle, P. E.

Structural Design 14 Watch Out for Unevenly Loaded Welds y Duane K. Miller B 18 Strategies for Better Delegated Design in Structural Steel y Michael Stubbs B 24 Rising to the Challenge: Seismic Retrofit of the Fox Plaza Tower y Matt Skokan, Ph.D., S.E., Metin Oguzmert, Ph.D., B S.E., and Saiful Islam, Ph.D., S.E.

28 InSights Bringing Low-Carbon Concrete Forward y Don Davies, P. E. S. E., and Larry Sutter, Ph. D., P. E., F. B ASTM, F. AC

30 Structural Analysis Current Canadian and U.S. Approaches to Snow Drift Loads y Michael O’Rourke, Ph. D., P. E., John E. Cocca, P. E., B and John F. Duntemann, P. E., S. E.

34 Outside the Box Reclaiming the Role of Structural Engineer as a Form Finder y Edmond Saliklis B 56 Technology Robots in Cold-Formed Steel Projects y Venkata Charan Kumar Gajulapalli, PE and Karthik B Ramagiri

58 Structural Forum A Structural Engineer’s Role in Preservation of Existing Buildings y David Cocke, S. E. B 61 Code Updates 2024 IBC Significant Structural Changes y John “Buddy” Showalter, P. E., M. ASCE, M. B NCSEA, and Sandra Hyde P. E., M. ASCE, M. NCSEA

66 InSights Sustainable Steel 101 y Steve Rys, P.E. B 70 Historic Structures Exploring Transition Buildings and Construction y Jeffrey S. Barnes, and Craig E. Barnes, P.E., S.E. B 72 Business Practices Strategies for Navigating the Engineering Talent Drought y Eric Stern and Mandi Spindler B 80 Iconic Structures Birmingham’s Sloss Furnaces y John A. Dal Pino B

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. FEBRUARY 2024

EDITORIAL ‘I Didn’t Say I Wasn’t Going to Do It, Only That I Didn’t Have the Time.’ By Angelina V. Stasulis, P.E., S.E.

y partner sighed as I asserted I was adding yet another thing to my already overflowing plate, then calmly responded that sentiment was exactly why I always felt stressed and unfulfilled. While walking down the buffet of life, it’s easy to say “yes” to revamping the company detail library, committing to a fitness routine, or planning the neighborhood Halloween party. After all, there’s value in doing each of these things. However, taking on too much means that, even if we’re constantly doing something, we’re never actually accomplishing anything. In our world of polished social media images and celebrated Who’s Who lists, the illusion of success prevails. It’s tempting to “lean in,” “say yes,” and “do it all” in an attempt to “follow your passion,” but these buzzword mantras oversimplify the path there. When success doesn’t come quickly, it’s easy to assume those at the top are different from us somehow, superhuman, even. I’ve been on the receiving end of that assumption more than once, and always find it surprising, because I never feel like I have it all together. In fact, Shonda Rhimes’ quote, “Whenever you see me somewhere succeeding in one area of my life, that almost certainly means I am failing in another area of my life” best summarizes my experience. Success is fueled by sacrifice, choosing one thing over others: morning workouts or relaxing, alarm-free mornings, managing project teams or time for personal skill development, exciting travel with colleagues or cozy weekends with family, writing this editorial or curling up with a good book, etc. Recognizing this, I’ve reframed my original adage a bit. It’s not a matter of “not having time,” but knowing where the new activity falls on my priority list and being intentional about what’s going to take a backseat for a bit. Saying “yes” to something means saying “no” to something else. To help limit that impact, any new task needs to come with a clear objective. Without a “definition of done,” projects can quickly turn into endless Sisyphean tasks, expanding to fill whatever time is allocated, much like those standing one-hour calls we all have on our calendars. The field of software development has a

STRUCTURE magazine

concept known as the “minimum viable product” (MVP). Whenever you see me somewhere Rather than trying succeeding in one area of my life, to create the best possible product, that almost certainly means I am failing in developers aim to another area of my life. avoid unnecessary —Shonda Rhimes work on components that might not align with user needs by delivering the bare minimum at interanalysis model and using hand calculations mediate check-in points along the way. This for a simple flexible diaphragm lateral analysis. doesn’t mean delivering an inferior product, The key here is ensuring we have the right but rather limiting their work with a clearly minimum requirements for our MVP, which defined scope to gather insights to refine and requires effective communication. Oftentimes enhance the product in subsequent itera- people ask for one thing, when they really tions. This approach is particularly valuable need another. in dynamic environments where requireBy focusing my efforts, I’ve been able to ments evolve over time, like that of structural condense my work hours into only four days engineering. a week. I use my extra day for routine tasks I’ve leveraged this concept in a few ways like laundry or yoga, but also as contingency throughout my professional life. To create a time for unexpected or one-time events, training program for our firm, I started with like a dentist appointment or home repair. a series of “basic training modules.” These are Consolidating my priorities has provided the 2- to 3-page Google Docs that provide back- flexibility to handle these commitments, and ground on a variety of tasks like retaining wall more satisfaction out of the work I choose to design and link resources from our company commit to. I’m no longer struggling to check drive or the internet. Going forward, I hope shop drawings while listening to a committee to grow this into a full internal wiki to serve conference call, because I’ve trained my staff as a searchable clearinghouse for our firm’s enough to delegate the shop drawings and collective knowledge and experience. For now, have chosen only one committee to actively though, the limited content is already paying participate in. Despite a reduced workload, I dividends for our new hires. feel more fulfilled, anticipating the next chalTo create time for these overhead projects/ lenge, and maintaining the inclination to say investments, I’ve had to revisit how I approach yes, knowing there’s always an opportunity project work. Project profitability is contin- to come back for seconds (or thirds) in the gent on efficient use of time for our majority buffet of life.■ fixed-fee workstyle. Since we design in parallel with our architectural counterparts, we must be careful not to get too far ahead of them, as Angelina V. Stasulis, P.E., S.E. is a Senior Structural Engineer this can result in costly rework when design with Shear Structural in Atlanta, Georgia, and member of changes. By identifying key needs for each NCSEA’s Board of Directors. She enjoys running before given phase, we limit our liability. In practice the sun comes up, competing with the international bronze this might look like: offering a typical bay medalist barbershop show chorus, sketching free body of framing with approximate quantities to diagrams of playground equipment for her two young assist in early pricing rather than generating a children, and is always interested in taking on more things full set of general notes, plans, and details or she “doesn’t have time for.” not bothering to create a full 3D structural

FEBRUARY 2024

structural INFLUENCERS Dr. Nehemiah J. Mabry, P.E. Dr. Nehemiah J. Mabry, P.E., is an engineer, educator, and CEO based in Raleigh, North Carolina. With advanced degrees in Mathematics and Civil/ Structural Engineering, his career spans roles as a NASA Researcher, Bridge Design Engineer, and Professor. Dr. Mabry is also the CEO and Founder of STEMedia, an edtech and digital media company for young professionals in the Science, Technology, Engineering, and Math (STEM) community. He can be reached at nehemiah@stemedia.com. STRUCTURE: “Being good at math” is often cited by engineers as the reason they went into engineering. Being a really good engineer takes more than that. What brought you into engineering? Would you tell us about the intangibles such as inspiration, mentorship, role models and personal drive that played a role in your success and that of your students? Mabry: I believe it takes creative curiosity, a penchant for problem-solving, and dedicated drive; the math and science skills come when you have those. During my junior year of high school, my father encouraged me to explore engineering, which led me to an invaluable NASA internship. This experience solidified my passion, culminating in a BS, MS, and a PhD in engineering (though there were TONS of ups and downs along the way). Both of my parents have been inspirational cornerstones of support in my journey, championing my academic goals and lending their insight where they could. Furthermore, mentors like my former engineering boss, David Simpson of WGI, and my NASA advisor, Dr. Curtis Banks, have been invaluable in shaping my trajectory. STRUCTURE: There is a raging debate as to whether kids can discover and pursue a STEM career starting late in high school or in college, or whether it has to start much earlier than that. As an educator, what are your thoughts on the subject? Mabry: It’s never too early or late to discover and pursue STEM, in my opinion. Though I had related interests early on, I didn't really discover and decide to pursue an engineering career until late in high school. Some people might get introduced as toddlers, 8 STRUCTURE magazine

while I know some who are making career pivots into tech in their 40s and 50s. There's always room to try your hand at STEM, so my thoughts are always, “Go for it!” STRUCTURE: STEM professionals aren’t always known as the best writers and communicators. But obviously you wouldn’t be where you are today if you weren’t great at it. How do we fix that? Mabry: To be honest, I was a very reluctant speaker, and especially a reluctant writer growing up. I couldn't stand it. However, it was my mother who frequently placed me in situations where effective communication was necessary to get what I wanted. College and grad school also crystallized the importance of these skills, particularly as I stepped into leadership roles. I've since come to view effective communication as an indispensable tool in my STEM toolbox, aiding in my pursuits. Now, I strive to learn from every proficient communicator I encounter. I absorb what stands out, incorporate it into my style, and hone it until I master the particular skill myself. Put yourself in positions to exercise that muscle and you will get better. STRUCTURE: Would you tell us a little about your professional engineering career before STEMedia? Mabry: I am still a practicing professional engineer, however before committing fulltime to STEMedia, I spent the bulk of my hours as a bridge design engineer and inspector in North and South Carolina. In the design role, I was responsible for conceptualizing and refining bridge structures, doing everything from load analysis to material selection, and utilizing CAD drawings for design details, while making sure we complied with all relevant standards. As an inspector, I assessed existing bridges to ensure their safety and determined their rating. This involved visually checking for any signs of wear or damage, using specialized tools for in-depth evaluations, and teaming up with preservation and maintenance professionals for any required repairs or strengthening. STRUCTURE: Many engineers decide at some point in their careers to start their own engineering business. But not many have taken the path you chose. Would you tell us what inspired you and what

unexpected obstacles you encountered and overcame? Mabry: I am inspired by inspiring. It may sound like a play on words, but the positive responses to my speeches, videos, and executed projects are what fuel my motivation. While facing challenges like limited financial resources, numerous rejections, and even personal self-doubt, the WINs and the genuine impact made in others' lives outweigh them. I have passion for entrepreneurship in both technical and the creative arenas, so I don’t rule out starting an engineering firm in my future. Regardless, I cherish the process of transforming ideas into reality in a manner that uplifts people. STRUCTURE: Would you tell our readers about your firm STEMedia? Mabry: STEMedia is a media company and education platform dedicated to transforming STEM education and professional development. With a track record spanning over a decade, we are passionately committed to bridging gaps, promoting diversity, and fostering inclusivity in the STEM fields. I encourage anyone reading to visit www. stemedia.com to learn more about our impactful mission and initiatives. STRUCTURE: What is STEMedia working on today that really excites you? Mabry: Currently, STEMedia is actively producing a series of digital TV shows and advanced engineering courses tailored for our platform. What truly excites me is our growing partnerships with like-minded companies and organizations that share our mission to elevate and empower. STRUCTURE: Would you describe for our readers the effort (time and people) that goes into producing say a 30-minute educational program? How do you market and sell educational programs? Is it different from selling engineering services? Mabry: Producing a 30-minute educational program involves extensive pre-production work, including research, scriptwriting, and storyboarding. The actual production requires coordination among camera crews and talent, while post-production focuses on editing, sound, and graphics. This endeavor, spanning weeks to months, demands a collaborative effort from a dedicated team, just like on engineering projects. While

the subject matter knowledge is also similar, marketing educational programs differs from selling engineering services in that we target educational institutions in STEM, but engineering practice emphasizes business-to-business relationships and technical professionals in the industry. STRUCTURE: What do you think STEM will look like in another 15 to 20 years? Mabry: In the future, I think much of STEM education and practice will occur in high-tech simulated environments. Technology will provide us the chance to learn in more realistic scenarios. Additionally, advances will allow us to experience this across different platforms and from various sources. STRUCTURE: What do you hope to be doing in another 15 years? Mabry: I hope to be the creator of that educational content and experiences for the next generation! STRUCTURE: What impact do you think Artificial Intelligence (AI) will have on STEM education and the professional engineering community? Mabry: AI will enable these environments to become more personalized and responsive to individual needs and idiosyncrasies. For example, if someone's current teacher or supervisor isn't ideal, there will be countless other options to curate learning experiences and even contribute to business projects far beyond their locale. STRUCTURE: Let’s assume all future STEM students will read your interview in STRUCTURE. What would you like to tell them? The floor is yours. Mabry: The World is waiting on you. There is a unique blend of talent, passion, and purpose that lies within you, and your STEM career is one opportunity to bring that to life. The ideas and innovations that you now and will one day work on have the potential to change someone else’s world for the better. When things get tough, and you can't seem to figure it all out, STAY WITH IT. You may not know it now, but if you keep giving it your best, then one day it will all come together. I believe that. STRUCTURE: Outside of engineering, what do you like to do? Mabry: I truly cherish the moments with my wife and our three girls. It brings me great joy to plan and host events that nurture their personal, academic and spiritual growth, as well as benefit our community. Crafting creative content that inspires will always be a passion of mine; whether it's how I pay the bills or not. And for some occasional downtime, I enjoy playing my bass guitar. ■ FEBRUARY 2024

structural OBSERVATIONS To Align or Not to Align Walls studs and floor framing in multi-story construction. By Doug Steimle, P. E.

ne of the more hotly debated topics in the multistory wood community is whether wall stud and truss or joist alignment is required over the full height of a building, or whether it is beneficial to allow the framing to be unaligned. Let’s look at the advantages of each at a high level.

Aligned Bearing Wall Studs and Truss or Joist Framing By aligning bearing wall studs with floor and roof trusses, we create a straightforward load path and eliminate the requirement for horizontal load transfer elements through the floor system depth. Alignment of studs also helps with coordination with other trades. Alignment of studs means alignment of spaces between studs, which allows plumbing stacks or conduit to run vertically throughout the height of the building without interfering with wall studs, floor framing, or load transfer elements (Figure 1).

Figure 1. Aligned bearing wall studs and floor framing detail.

Unaligned Bearing Wall Studs and Truss or Joist Framing There are constructability benefits and efficiency gains in the design of the wall studs when allowing wall studs and floor and roof framing to be unaligned. From a constructability standpoint, there is more tolerance and flexibility in the field when alignment isn’t required. An unaligned approach could also allow for optimized stud spacing at any given level to accommodate accumulated loads (Figure 2).

Partially Aligned Bearing Wall Studs and Truss or Joist Framing

Figure 2. Unaligned bearing wall studs and floor framing detail.

When we talk about alignment, there are two alignment elements to consider: • Bearing wall stud alignment from floor to floor. • Floor or roof framing alignment with the wall studs Regardless of whether the bearing wall studs align from floor to floor, the reactions of the floor or roof framing must also have a load path to the bearing wall studs. In most situations, the trusses or joists bear on the double 2× top plates of the wall assembly below. It may be undesirable to align the wall studs with the floor framing. For example, wall studs may want to be at 16 inches o.c. 10 STRUCTURE magazine

to accommodate minimum spacing for exterior finishes, while floor joists may want to be at 24 inches o.c. to maximize floor sheathing capacity (Figure 3).

Which is the Preferred Approach? Before we determine the best approach for our project, we need to know more about it. • How many levels does the building have?

REIMAGINE … EFFICIENCY MATERIAL STRENGTH DESIGN

SUSTAINABILITY

Design with Aeos™ high-strength ASTM A913 structural steel. A high strength-to-weight ratio means fewer tons and a lower carbon footprint for your project. Aeos is also the lowest embodied carbon steel of its kind, made with more than 95% recycled content using a circular steelmaking process with electric arc furnace (EAF) technology.

DISCOVER THE AEOS™ ADVANTAGE nucor.com/aeos

AST M A9 1 3 HIGH-STRENGTH STRUCTURAL STEEL

• What are the magnitudes of the floor framing reactions on the wall? • Are we able to have a full-depth rim board as a load transfer element, and if so, are we limited to solid sawn lumber or can we use an engineered wood product? • Are we able to have a ribbon board at the top of the floor truss as a load transfer element? • Are there other limitations on wall stud spacing or truss spacing? Once we determine the magnitude of the truss or joist reactions, the magnitude of the accumulated wall loads, and the potential span distance of load transfer elements, we can have a conversation with the framer and contractor about the feasibility, detailing, and cost implications of unaligned framing and the various methods at our disposal.

Can Double 2× Top Plates Be Used to Serve as Transfer Elements in an Unaligned Framing Condition in Multi-Story Construction?

Figure 3. Partially aligned bearing wall studs and floor framing detail.

As noted, the double 2× top plates in a conventional wall framing system are often used to transfer loads. If the reactions are small and the spans are short, it may be feasible for the double 2× top plates to resist concentrated loads (truss/joist reactions) as a transfer framing element. This common condition is not specifically addressed in the International Building Code, or the American Wood Council (AWC) National Design Specification, or other codes or referenced standards. But there are things to consider when trying to determine the capacity of double 2× plates: • If using typical double-plate construction, do both plates act together as a composite member? If so, does adequate nailing between the two need to be provided for shear flow? • If the plates are assumed to act independently of each other, what is the load distribution between them? • How does the typical stagger/lap of plate elements impact the internal forces in either a composite or non-composite approach? • Should this analysis assume both plates are simple spans for one stud bay? Should one of the plates be assumed continuous for several adjacent bays, or should both plates be continuous for several bays? There are multiple methods of conducting this analysis that can produce significantly different results. Two of the analysis methods are presented here. Method 1: Assume that the two plates are acting together as one composite 3-inch deep member. Although some engineers use this analysis method, it can be unconservative since nail slip and flexibility in wood connections aren’t easily 12 STRUCTURE magazine

considered in textbook shear flow calculations, which rely upon rigid connections between components (such as built-up steel W-shapes with welded connections). Method 2: The consensus among many engineers is to assume that the two plates act independently, with the load distributed to each plate based on their stiffness. In other words, for common top plate construction, half the load goes to each plate. As far as splice locations and looking at continuous vs. simple span members, there should be some offset in top plate splices to provide continuity in at least one top plate member. A simple span will produce the worst bending effects, and a continuous span will produce the worst shear effects. Enveloping the worst-case approach of both analyses allows for flexibility in wall framing construction. The downside of using double 2× top plates as load transfer elements is that the capacity of the plates is low. A double 2 × 6 SPF (spruce, pine, fir) top plate spanning 16 inches between wall studs can only support a reaction of 1,000 to 1,400 lbs. when using the Method 2 approach. See Figure 4 for top plate testing. The

Figure 4. Washington State University testing of top plate bending. (Photo courtesy of Don Dolan.)

Figure 5. Rim board detail.

AWC Wood Frame Construction Manual (WFCM) prescriptively limits floor joist spans to 26 ft. to specifically address this issue. The WFCM commentary to Section 3.1.3.2a states the following: C3.1.3.2a Framing Member Spans. Framing member spans are limited to 26 feet for floors based on the bending capacity of the double top plates supporting floor framing members. The worst-case assumption is that a floor framing member bears directly between two studs, creating a concentrated load at mid-span of the top plates. Section 3.1.3.3g required band joists, blocking, or other methods to transfer roof, wall, and/or floor loads from upper stories to alleviate the concern of additional loads being transferred through the floor framing members into the top plate.

Higher Capacity Load Transfer Elements in an Unaligned Framing Condition If the double 2× top plate does not have sufficient capacity to support the reaction of the truss/joist, it is possible to support the truss or joist on the full-depth rim board or girder truss first and then distribute loads to the top plates. The cost of the fasteners required to achieve this load path should be considered. Method 1: Use full-depth rim boards. In this scenario, the stud above transfers loads to the bottom plate of the wall, through the floor sheathing, and into the full depth rim board. The rim board can then distribute loads to the top plate below, which then transfers loads to the unaligned studs (Figure 5). Method 1A: Use a full-depth girder truss. Method 2: Use a ribbon board at the top of the floor truss system

Figure 6. Ribbon board detail.

(specific to parallel chord trusses). The ribbon board, often a 2 × 4 or 2 × 6 oriented vertically, flush top with the top of trusses, can be designed to collect loads from the studs above and transfer them directly to the end of the floor trusses to which they are attached. If the double top plate below the floor truss system is structurally adequate to support the reaction of the floor trusses (spanning between misaligned studs below if necessary), then nothing further is necessary. If the double top plate is not structurally adequate to support the truss reactions, then another ribbon board can be placed flush to the bottom of the trusses to redistribute the loads to the plates and studs below (Figure 6).

Conclusions and Recommendations The capacity of conventional double 2× top plates to serve as a load transfer element is often overlooked when designing multi-story light wood frame projects. The design process for bearing walls, as well as the detailing of floor-to-wall intersections, should consider whether higher capacity load transfer elements are required for unaligned construction or if defaulting to the alignment of studs with trusses or joists results in simpler construction.■

Doug Steimle, P. E. is a principal at Schaefer in Cincinnati, Ohio. He’s a licensed structural engineer with 25+ years of experience designing wood structures across the country. He is recognized in the design of tall wood and hybrid-frame buildings both for mixed-use and multi-residential, and is an advocate + thought leader on mass timber construction (doug. steimle@schaefer-inc.com).

FEBRUARY 2024

structural DESIGN Watch Out for Unevenly Loaded Welds Welds with nonuniform loading require special design attention. By Duane K. Miller

any welds are evenly loaded: the applied loads are uniformly transferred through the throat and length of the welds. Such welds are easily designed and routinely perform as expected. Other welds, however, are unevenly loaded. One end of the length of the weld may be more severely loaded than the other, or the loading along the weld throat may be non-uniform. The design of these welded connections is more complicated. Fortunately, design codes dealing with welded connections, such as the American Institute of Steel Construction’s AISC 360, Specification for Structural Steel Buildings, and the American Welding Society’s AWS D1.1, Structural Welding – Steel, provide design rules to accommodate unevenly loaded welds. The unfortunate situation is that engineers and detailers involved with the design of welded connections may be unaware of the need to treat such connections in a special manner. When unevenly loaded welds are treated incorrectly, the performance of the welded connection will likely be compromised. This article summarizes five of the ten most frequently encountered conditions involving unevenly loaded welds. The remaining five conditions will be covered in Part 2 of this article in the March issue of STRUCTURE Magazine.

Condition 1: Unevenly Loaded Because of Bending About the Root of Fillet or PJP Groove Welds Consider two ¼-inch fillet welds on either side of a tee joint, each with a length of 1 inch, as shown in Figure 1. The calculated capacity of this pair of fillet welds is 10.5 kips when loaded in tension or in shear. Next, consider the same amount of fillet weld but placed on one side of the joint (i.e., a single ¼-inch fillet weld that is 2 inches long), as shown in Figure 2. The capacity of the fillet weld shown in Figure 2 is the same as the weld shown in Figure 1 if loaded in shear. However, if loaded in bending, as shown in Figure 3, the weld is unevenly loaded: strains are concentrated in the weld root, and the capacity of the welded connection will be significantly reduced. The same condition occurs with partial joint penetration (PJP) groove welds. The uneven loading

Figure 1. Double-sided fillet welds in tension.

14 STRUCTURE magazine

shown in Figure 3 should be avoided. The AISC Steel Manual (AISC 2023) offers this advice: “When lateral deformation is not otherwise prevented, a severe notch effect can result, as illustrated in Figure 8-10 . The use of a single-sided fillet or PJP groove weld in joints subject to rotation about the toe of the weld is discouraged. Using a weld on each side will eliminate this condition.” One solution, as listed in the manual, is to use double-sided welds, as shown in Figure 1. If double-sided welding is not possible, stiffeners or gussets can be used to preclude rotation about the weld root. The key is to eliminate any joint rotation that will result in tension about the root of the single-sided weld.

Condition 2: Unevenly Loaded Because of a Single Transverse Weld in an End-Loaded Lap Joint End-loaded lap joints are typically detailed with double transverse fillet welds, as shown in Figure 4. While there is some local eccentricity associated with the connection, no special precautions are needed for this condition. Under some conditions, it may be impossible to obtain access for welding on the second side of the joint, resulting in the conditions shown in Figure 5. This is a problematic configuration, prohibited by AISC 360 (AISC 2022) as follows: “Lap joints

Figure 4. End-loaded lap joint with double transverse fillet welds.

Figure 2. Single-sided fillet weld in shear.

Figure 3. Uneven loading on a single-sided fillet weld in bending.

Figure 5. Uneven loading in an end-loaded lap joint with a single transverse fillet weld.

Figure 6. End-loaded lap joint with single transverse fillet weld plus restraint.

Figure 8. Fig. C-J2.2 from AISC 360, used with permission.

Figure 7. End-loaded lap joint with single transverse fillet weld plus plug weld.

joining plates or bars subjected to axial stress that utilizes transverse fillet welds only shall be fillet welded along the end of both lapped parts, except where the deflection of the lapped parts is sufficiently restrained to prevent opening of the joint under maximum loading.” When it is impossible to weld along the ends of both lapped parts, the restraint needed to prohibit the opening of the joint may be achieved through the stiffness of the joined members or a mechanical support, as shown in Figure 6. A plug or slot weld on the accessible side of the joint is another potential solution, illustrated in Figure 7. Longitudinal fillet welds may also be an acceptable alternative.

Condition 3: Unevenly Loaded Because of Short Spacing Between Transverse Welds in End-Loaded Connections The third condition is the same as discussed in Condition 2 but focuses on the overlap dimension as compared to the number of welds. The eccentricities associated with end-loaded lap joints can be typically ignored; however, when the overlap dimension is too small, these eccentricities can result in unevenly loaded welds. AISC 360 requires the following: “In lap joints, the minimum amount of lap shall be five times the thickness of the thinner part

joined, but not less than 1 in. (25 mm).” This concept is illustrated in Figure 8, Part A. The unacceptable condition is shown on the right-hand side of Figure 8. Localized rotation of the connection will concentrate strains at the weld toe, resulting in uneven loading.

Condition 4: Unevenly Loaded Because of Shear Lag Welds may be unevenly loaded because of shear lag. Shear lag is defined in the glossary of AISC 360 as “Nonuniform tensile stress distribution in a member or connecting element in the vicinity of a connection.” The concept of shear lag is illustrated in Figure 9, where an end-loaded lap joint is shown. For this condition, AWS D1.1 Clause 4.9.2 offers a simple solution: “If longitudinal fillet welds are used alone in lap joints of end connections of flat bar or plate members, the length of each fillet weld shall be no less than the perpendicular distance between them.” AISC 360 Table D3.1 provides shear lag factors for other geometric configurations where some, but not all, of the cross-sectional elements are joined by welds or mechanical fasteners. The shear lag factor “U” is a reduction factor, always less than 1.0, that accounts for the unevenly loaded condition. Two primary factors are used to determine “U”: the length of the weld and the distance of the weld from the neutral axis, identified as x-bar. To reduce the shear lag factor (that is, to achieve a value closer to 1.0), the weld length can be increased, or x-bar can be reduced. In most cases, increasing the length of the weld is the simplest solution. This inevitably requires increasing the amount of overlap in the connection.

Condition 5: Unevenly Loaded Because of a Long Weld in an End-Loaded Connection Figure 9. Uneven loading due to the effects of shear lag.

16 STRUCTURE magazine

Condition 5 is the opposite of Condition 4: rather than having a weld that is too short, this condition occurs when the weld in

an end-loaded connection is too long. As shown in Figure 10, for very long welds in end-loaded connections, the majority of the load is transferred through the ends of the long welds, with relatively little load transferred through the center. AISC 360 addresses this condition by providing three relationships that compare the fillet weld leg size “w” to the fillet weld length “l.” When the length “l” is less than 100w, no adjustment is required. In other words, the weld is considered evenly loaded when l < 100w. For weld lengths of 100w – 300w, the effective length is modified by this relationship:

Figure 10. Uneven loading on a very long weld in an end-loaded lap joint.

Preview for Part 2

β = 1.2 – 0.002(l/w) < 1.0 The actual length “l” is multiplied by the β factor, reducing the length permitted to be used for calculation purposes. Finally, for lengths greater than 300w, the effective length is fixed at 180w. These relationships apply to end-loaded connections; they do not apply to longitudinal welds that are used to join webs to flanges on plate girders where the weld is subject to shear due to bending. While the described design rules are valid, these conditions very rarely occur. For example, consider a 5/16-inch fillet weld, where 100w is 31 inches; up to this length, no consideration of the effect of long weld length is needed. For this weld, 300w is 94 inches; this is the point where the effective length becomes fixed at 180w, or 56 inches. Only very large end-loaded connections will be affected by these provisions.

Uneven weld loading due to other common conditions, such as combining transverse welds with longitudinal welds or using welds in combination with bolts or rivets, will be discussed in Part 2 of this article in the March issue of STRUCTURE. More detailed information on how to mitigate these conditions can be found in AISC 360, the AISC Steel Construction Manual, and in AWS D1.1. ■

Duane K. Miller, PE, ScD, is a recognized authority on the design and performance of welded connections. He retired after 44 years of service with The Lincoln Electric Company who retains his services as a consultant.

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

FEBRUARY 2024

structural DESIGN Strategies for Better Delegated Design in Structural Steel Collaboration, clarity, and conciseness are the keys to success. By Michael Stubbs

ver the last two decades, the practice of delegating the design of structural steel connections and structural steel stairs to an engineer retained by the steel fabricator has grown more and more popular. As such, the execution of these projects has seen many different methods to get to a final design and fabricated steel. This article is intended to provide some history into delegated design, a review of the documents that define standard practice, and some strategies and best practices for specifying delegated design.

Standard Practices

Figure 1. This chart illustrates the impact that good concept, schematics, design development, and construction documents have on project cost.

History of Delegated Design Delegated design was originally developed to provide a more efficient project for the owner. By allowing steel fabricators and steel erectors some degree of leeway in the way the steel frame and stairs are constructed, they can leverage their expertise, experience, and particular equipment to maximize both cost and schedule savings. Fabricators are allowed to select connections based on the most efficient fabrication methods for their shop. Erectors are able to influence the construction sequence to minimize constructability issues and provide shorter schedules. The overarching intent is to produce the most economical structure possible with the shortest construction schedule to maximize the project owner’s return. The Engineer of Record (EoR) takes every opportunity to make the project successful for the project owner, and a well-executed delegated design does just that. However, when bad practices are introduced into the project, delegated design can negatively impact both the cost and construction schedule of the steel package.

Importance of Early Decisions One way that delegated design projects can have negative issues is when critical decisions are postponed to construction phase as opposed to being made during design. Figure 1 shows that the impact of good planning and strategies are greater early in the project where the cost of delays and issues is much greater during construction. The EoR can help a project significantly by identifying decisions and coordination items that need to be made during design. 18 STRUCTURE magazine

The American Institute of Steel Construction (AISC) has made many advances in standardizing how delegated design is executed. This can be seen in the most recent edition of the Code of Standard Practice (CoSP). The ANSI/ AISC 303-22 Code of Standard Practice for Steel Buildings and Bridges incorporates changes to better suit projects with delegated design. Chapter 3 of the CoSP outlines the expectations and requirements for Design Documents and Specifications on del-

egated design projects. It is important to note that delegated design is not a requirement. The CoSP has three options for how connections are specified; Section 3.2.3 of the CoSP, titled “Requirements for Connection,” outlines these three methods. The method by which the connections shall be designed will be selected and specified by the owner’s designated representative for design (ODRD) and is typically the EoR. The methods are summarized below: • The complete connection design shall be shown in the structural design documents. • The connections shall be designated in the structural design documents in such a way that an experienced detailer can complete the connection design. An example of a steel connection detail using Method 2 is shown in Figure 2. • The connection shall be designated in the structural design documents or specifications to be designed by a licensed engineer working for the fabricator. This method is often referred to as “delegated design.” Section 3.2.3 (3) of the CoSP outlines the requirements of how delegated design shall be specified and executed. Since, on most projects, the CoSP is referenced as a specification that shall be followed in the structural design documents, most steel fabricators, detailers, and steel erectors expect these provisions to be followed. In the absence of information to the contrary, most fabricators, detailers, and steel erectors will assume that they must follow the CoSP and will bid on the project accordingly. For this reason, any EoR that specifies projects with delegated design should become familiar with the CoSP section 3.2.3 and, when necessary, make sure any deviations from the CoSP are clearly defined in the structural design documents. Listed below is a brief summary of the provisions in the CoSP: • 3.2.3 (3) (a) Requires Substantial connection information. This is an important addition to the 2022 CoSP. Substantiating

since 1922

Figure 2. Detail of a steel connection using Method 2 is shown.

connection information is the documents that are prepared by the engineer designing the connections for the fabricator. The Substantiating Information is the documents that the EoR will review. It can also include plans and detailed diagrams depicting the structural connections. • 3.2.3 (3) (b) The fabricator (and, in turn, the connection engineer) shall submit in a timely manner representative examples of the substantiating connection information. This provision is intended to have all team members agree on the format for the substantiating connection information prior to the preparation of the calculations and design drawings. This step is intended to facilitate a fast and more efficient approval process. • 3.2.3 (3) (c) Requires the connection engineer to review the shop drawings and confirm in writing that the shop drawings properly incorporate the connection design. It is important to note that the CoSP does not require the connection engineer to stamp the shop drawings. In fact, the CoSP commentary discourages the practice. This provision also clearly states that approval of the shop drawings by the connection engineer does not replace the approval process of the EoR. • 3.2.4 States that all member reinforcement, including stiffeners, web doublers, plates, and bearing stiffeners located away from the connection, shall be designed by the EoR and shown in sufficient detail. Section 3.2.3 of the CoSP also has the following requirements for the structural design documents: • The EoR shall provide project-specific details that show the conceptual configuration for the order of magnitude of the forces to be transmitted. This provision is intended to provide the fabricator and the erector sufficient information to accurately bid on the project with the need to retain an engineer during the bidding process. It is unreasonable and counterproductive to expect fabricators to spend money on a connection engineer for projects they have not been awarded. It benefits everyone involved in a project when the structural design documents are complete enough to price a project accurately. • The EoR shall specify any restrictions on the connection in the structural design documents. This can include the type of bolts 20 STRUCTURE magazine

used, connection types that cannot be used, and the grade of connection materials. • The loads, including transfer forces, shall be provided in such a way as to facilitate the connection design. • The EoR shall clearly state whether the loads provided are service or factored loads and whether ASD or LRFD shall be used in the calculations for the connection design. • Substantiating information is required to be included in the approval documents. To help with structural steel projects with stairs, AISC has published Design Guide 34: Steel-Framed Stairway Design. Chapter 9 of Design Guide 34 specifically addresses the Delegated Design of Stairs. This chapter outlines the responsibilities of the EoR, the architect, and the stair design engineer. It clearly states that the stair design engineer is typically not the most knowledgeable about egress and building requirements with respect to stairs. For this reason, the Design Guide recommends that the architect provide sufficient details for the geometry of the stairs to be determined. The Design Guide also recommends that the EoR provide details for how the stair shall attached to the main frame, details, and dimensions for the slab openings. In addition to outlining the various roles of a stair design project, Design Guide 34 provides a designer checklist in Appendix A. This checklist is a great tool for helping in the preparation of both architectural and structural documents associated with delegated stair design. The Council of American Structural Engineers (CASE) has developed many useful documents that assist with the preparation of design documents. Three of the documents that CASE published are listed below: • 962 – National Practice Guidelines for Structural Engineers of Records • 962-B Practice Guidelines for Specialty Structural Engineers • 962-D a Guideline Addressing Coordination and Completeness of Structural Construction Documents 962—National Practice Guidelines for Structural Engineer of Record has multiple provisions. Some that pertain directly to delegated design include maintaining responsibility for the primary structure with the EoR. This means that, ultimately, the EoR is responsible for the structural integrity of the project. This guide also puts the responsibility of communicating the project requirements on the EoR. The submittal review process and responsibilities during that process are also defined in this guideline. 962—National Practice Guidelines for Specialty Structural Engineers. This guideline is intended for all engineers who do delegated design, including Pre-Engineered Metal Building Engineers, Joist Engineers, and Pre-Cast Concrete Engineers. Despite being all-encompassing, it has some good recommendations that can apply to delegated connection and stair design. It requires the connection engineer to meet the qualifications outlined in the structural design documents. The connection engineer is required to request information or clarification when necessary. The connection engineer shall notify the EoR of perceived errors in the structural design documents. Still, the connection engineer is not responsible for conducting a peer review of the structural design documents. 962-D—A Guideline Addressing the Coordination and Completeness of Structural Construction Documents provides industry standards for what should be included in structural design documents. It also provides recommendations for how design criteria shall be communicated.

Strategies for Successful Delegated Design The following are strategies the author has developed to help make delegated design projects run more efficiently. It is important to remember that the ultimate goal of delegated design is to increase the return on investment of the project for the project owner by providing a more economical design and a faster construction process. These strategies are intended to do just that.

Strategy 1: Make the Process Collaborative Delegated design was developed to leverage the expertise of the fabricator, erector, detailer, and specialty engineers to make the project better. EoRs who specify delegated design should go into the process open-minded and willing to let these other members of the team play a part in the design. When all team members contribute, the project is better and runs more smoothly.

Strategy 2: Provide Accurate Reactions Too often, EoRs use widely conservative blanket specifications for how the beam reactions are to be calculated. These include such provisions as 55% of the Uniform Load Tables, the full shear capacity of the beam, or tables with conservative loads. None of these methods are as effective as showing the actual reactions for each beam on the drawings. As stated previously, the ultimate goal of delegated design is to make the project more efficient. That is impossible to do when the connections

are being designed for wildly conservative end reactions. When accurate reactions are provided, the connections are designed to be sufficient for the loads that the beam will see in its lifetime. Also, loads that are shown as dead, live, wind, and seismic separately allow for load reductions to be taken into account per the ASCE 7 load cases. For example, when a connection engineer is designing a beam-to-column connection with a wind transfer force and the dead and live load reactions are provided separately, the connection engineer can reduce the live load per the ASCE 7 load cases. When the reactions are not shown separately, the connection will be designed for the full live load in excess of code requirements.

Strategy 3: Make the Project Easy to Bid CoSP requires the EoR to provide sufficient information in the structural design documents for the fabricator and erector to bid on the project accurately. The structural design documents should provide some depiction of all connections that are appropriate to the magnitude of the forces that are being transmitted. The depiction should also show stiffener and doubler plates that may be required. During the preparation of the approval documents, the fabricator and erector can develop the actual connection geometry in conjunction with the connection engineer. By providing sufficient information to bid on the project, the EoR avoids costly and time-wasting confrontations associated with change orders, and they are helping ensure that the bidding process is as competitive as possible.

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

FEBRUARY 2024

Strategy 4: Use Substantiating Documents The structural design documents should require the submission of substantiating documents. The EoR should also clearly define their expectations for the substantiating documents. Substantiating documents are the best method to clearly communicate the connection and/or stair design to the EoR as well as the detailer. Substantiating documents also avoids a common point of confusion: whether or not the connection/stair engineer should sign and

seal the shop drawings. Since only portions of the shop drawings reflect the connection/ stair engineer’s work, having them sign and seal documents that contain more than just their work can be problematic. First, it causes an insurance issue. There are examples where connection engineers have signed and sealed shop drawings with errors that were not the connection engineer’s fault, yet they were still held liable. It also causes issues with many state engineering acts, which require engineers to only sign and seal documents prepared under their direct supervision.

The practices outlined by the CoSP for the use of substantiating documents and the connection engineer providing written confirmation that they have reviewed the shop drawings alleviate the need for the connection engineer to sign and seal the shop drawings. This avoids multiple legal entanglements.

Strategy 5: Clearly Define Expectations for Submittals The earlier in the project, the fabricator, erector, detailer, and specialty engineer understand the expectations of the EoR for review of the approval documents, the faster and better the approval documents can be prepared. Special requirements, such as the format for substantiating documents, connection design criteria, connections that can or cannot be used, or material specifications, should be clearly specified in the structural design documents.

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

Strategy 6: Manage the Architect’s Expectations This strategy primarily pertains to stairs and architecturally exposed structural steel. It is important during the design process that the EoR ensures the architect’s expectations are achievable. This can be as simple as giving the architect an estimate of stringer size for ornamental stair or bolt counts and weld sizes for exposed connections. Too often, submittals are delayed because the architect’s vision simply isn’t feasible for the project. Clear communication between the EoR and the architect before the project is released for bid can avoid confusion. Delegated design under the right circumstances can benefit all parties involved in the design, fabrication, and erection of structural steel and stairs. The project starts with the EoR. As such, the EoR and how they specify the project can have the greatest impact on how well the project goes. All engineers who specify delegated design should strive to make their structural design documents as concise and easy to use as possible.■

Michael A. Stubbs is the President of Stubbs Engineering, Inc., a full-service structural engineering firm headquartered in Las Cruces, New Mexico. (mstubbs@stubbseng.com)

22 STRUCTURE magazine

structural DESIGN Rising to the Challenge: Seismic Retrofit of the Fox Plaza Tower Unique steel moment frame building seismic retrofit benefits from full-scale testing and nonlinear analysis. By Matt Skokan, Ph.D., S.E., Metin Oguzmert, Ph.D., S.E., and Saiful Islam, Ph.D., S.E.

n response to increasing concerns about seismic threats in the Bay Area, the owners of San Francisco's Fox Plaza Tower tackled the challenge head-on. This 30-story structure, constructed in 1966 on the site of the former Fox Theatre, underwent a significant transformation to enhance its performance during a seismic event. This preemptive move was crucial, considering the U.S. Geological Survey's prediction of a 72% probability of the region experiencing a major earthquake—magnitude 6.7 or higher—within 30 years. This article delves into the remarkable journey of this project. From the comprehensive seismic analyses to the collaborative efforts between industry and academia in full-scale lab testing and the application of an innovative retrofitting scheme, the Fox Plaza Tower has set a precedent in earthquake preparedness for high-rise buildings. The property on San Francisco’s busy Market Street serves commercial and residential purposes, with offices occupying the 4th to the 12th floors and apartments from the 15th to the 30th floors. The tower's footprint is rectangular in shape, measuring 75 ft. by 224 ft. The layout of the lateral force resisting system and moment frame connection types differ notably between the upper and lower halves of the building, influenced by differing story heights and clearances for balconies at the apartments. The existing building employs a unique and unconventional lateral force-resisting system consisting of steel moment-resisting frames with haunched beam assemblies. These assemblies incorporate Pre-Northridge welded moment connections, which attach to the strong and weak axes of built-up columns in a space-frame configuration. Most of the beams at the lower levels have a WT section welded to the bottom, extending for approximately one-fourth of the beam's length from the column face. Notably, there is no lateral bracing at the bottom flange of the beams and WT sections. Reinforced concrete floor slabs are supported by structural steel beams and girders. Figure 1 shows the layout of moment frames at the typical commercial and residential floor levels. Below grade, there is a sizeable two-level parking area that also supports two other buildings. The foundation system utilizes concrete pile caps at the steel columns with 11-in.

Figure 2. Photo of existing moment frame connection with haunched beams.

24 STRUCTURE magazine

Figure 1. Plan view showing moment frame configuration.

diameter step-taper Raymond piles that are 56 ft. long. The analysis of the structure revealed potential damage risks to both the upper and lower sections of the building depending on the nature of seismic events. Therefore, a custom seismic retrofit scheme was developed to counter these threats, which includes strengthening the connections at the upper apartment levels and adding perimeter braced frames with fluid viscous dampers at the lower office levels. The project followed the ASCE 41-13 Seismic Rehabilitation of Existing Buildings standards.

Full-Scale Testing Early in the project, it became apparent that there was no existing research on the behavior of the unique haunched beam-to-column moment connections used in the building. Furthermore, there was minimal data on Pre-Northridge beam-to-column weak axis connections without a haunch. To overcome this, full-scale testing was conducted in the laboratory at the University of California, San Diego, by a group led by Professor ChiaMing Uang. The ASCE 41 standards recognize the use of experimental data to bridge the gap between the standard specifications and unique, often overlooked, details and characteristics in buildings. This allows for a more precise analysis of structures and ensures the accuracy and reliability of computational models. In this case, the goal of the testing was to understand how these specific connections respond to cyclic loading, with a focus on three types of connections: (a) common beam-to-column weak axis, (b) haunched beam-to-column weak axis, and (c) haunched beam-to-column strong axis. Figure 2 presents an image of a moment frame connection from the project site, showing how the haunched beams are framed into columns in both orthogonal directions. The design team created a laboratory testing program specific to the project, which aimed to predict the performance of these unique connections. The test specimens consisted of a column with a beam framing into one side and

Figure 3. Haunched beam-to-column weak axis connection hysteresis results (left) and test specimen (right).

utilized the AISC cyclic quasi-static loading sequence, expressed in terms of the inter-story drift ratio. The initial loading phase encompassed six cycles at 0.375%, 0.5%, and 0.75% drift, respectively. Another set of four cycles followed at 1% drift and then continued with two cycles each at progressive levels of 1.5%, 2%, 3%, 4%, and so on. This sequence was followed until the specimen either failed or the test setup was considered to pose a safety risk. Connections featuring haunched beams generally displayed ductile behavior during tests, without the sudden weld fractures commonly seen in Pre-Northridge connections. However, the introduction of the haunch led to lateral-torsional buckling, resulting in strength and stiffness degradation behavior. This buckling occurred before the full plastic strength of the beam with the WT haunch section could develop. As a result, the vulnerable beam flange welds at the column were safeguarded, but at the cost of reduced strength in the beam-column assembly, which was more than initially expected. The plastic hinging was observed at the tips of the haunches, with no inelastic behavior evident at the face of the columns. Figure 3 depicts the haunched beam-to-column weak axis connection test specimen and the moment versus plastic rotation backbone curve used in the non-linear analysis model.

Seismic Performance Objective and Existing Structure Evaluation To reduce the risk of collapse during a major seismic event, the building owners had voluntarily undertaken the effort to improve the seismic performance of the existing structure to achieve, at a minimum, Collapse Prevention structural seismic performance at the BSE-1N (475-year) seismic hazard level and to minimize the Life Safety hazard. As such, the design team conducted a detailed performance-based seismic assessment of the existing building using non-linear response history analysis procedures, employing the CSI Perform-3D analysis software. URS Corporation, the project geotechnical engineer, performed a site-specific seismic hazard evaluation for the building site, providing 10 pairs of spectrally matched horizontal ground acceleration records corresponding to a BSE-1N (475year) earthquake hazard level. These records were then subjected to a 90-degree rotation during the analysis, leading to twenty unique ground motion simulations. The findings from the analyses showed significant issues in the longitudinal direction of the building due primarily to most of the columns being oriented in the weak axis, with beam-column sub-assemblies exhibiting more severe strength reduction and degradation compared to the transverse direction. In two out of the 20 simulated ground motions, the story drifts in the longitudinal direction exceeded 10%, and the analyses did not converge, suggesting that the building could potentially collapse. Additionally, three other simulated records showed excessive story drift ratios exceeding 4% (beam plastic rotations of 0.03-0.04 radians) in the longitudinal direction. The transverse building direction did not show any issues with potential 26 STRUCTURE magazine

Figure 4. Beam-to-column weak axis moment connection retrofit detail.

collapse as most of the columns were oriented in the strong axis. The frame configuration, moment connection types, and frame member proportioning vary significantly between the upper and lower parts of the structure due to the very different use of the spaces. The non-linear analyses of the existing building revealed that while the upper portion of the structure was vulnerable to certain ground motions, the lower part was susceptible to others. This difference was primarily due to the frequency contents of different ground motions exciting the various higher dynamic modes of the structure. Note that the heavy built-up column splices consisted of welded connections, and hinging of the columns was not observed during the analyses.

Seismic Strengthening Plan The building's vulnerability to seismic activity necessitated a retrofit to reduce the risk of collapse and limit excessive deformations. The difference in the dynamic behavior of the upper and lower portions of the building required a hybrid seismic strengthening solution. The design team developed a retrofit plan for the upper residential floors in the longitudinal direction to improve the ductility of the beam-tocolumn weak axis Pre-Northridge moment connections. This retrofit was specifically applied to the connections from the 17th to the 21st floors. The retrofit detail used two pairs of reinforcing ribs welded to the top and bottom beam flanges that provide an alternate load path for tensile forces, effectively reducing the stresses on the vulnerable flange CJP welds. In addition, fillet welds were added between the beam web and the shear tab to strengthen and stiffen the connection. This retrofit strategy aligns with the detail that K-C Tsai and Egor Popov developed and tested to improve the performance of the column weak-axis moment connections at the University of California Berkeley in the late 1980s. Figure 4 illustrates the retrofit detail used for the beam-to-column weak axis connections. At the lower commercial floors in the longitudinal direction, the excessive drift of the structure resulting from the reduced strength and degradation of the beam-column connections due to lateral torsional buckling could have been addressed by adding lateral bracing to the beams. However, this solution would have been very invasive and could have resulted in issues with the beam flange welds. Although it is not ideal, the lateral torsional buckling behavior of the beam-column assemblies allowed for significant plastic rotation capacity and did not cause substantial strength degradation. As such, two braced frame alternatives were initially considered for retrofit at the lower commercial levels: buckling restrained braces (BRB) and viscous dampers. Although the BRB alternative did result in greater story drift reductions, it also increased the total structural stiffness, consequently leading to higher

Figure 5. Plan and elevation views showing viscous damper frame retrofit.

story shears, which redirected more energy to the upper levels, which increased the story drifts. Furthermore, the BRB option posed substantial challenges for the existing foundation due to the large overturning forces exerted on the columns. The design team selected the viscous damper option for the lower levels because it offered a more structurally elegant solution. It successfully reduced the story drifts at the lower levels to acceptable levels without adversely affecting the foundation or the upper levels since it does not increase the initial elastic stiffness of the structure. The braced frames with viscous dampers were situated along the two longitudinal faces of the building on the first seven floors above ground, with a total of 14 dampers being utilized. Figure 5 illustrates the viscous damper frame retrofit used at the lower levels. Since their first use by the space industry in the 1960s, viscous dampers have found wide application in civil engineering. These compact devices, which turn kinetic energy into heat, help reduce stress and deflection in various structures, not just buildings. Due to the precision required in their design, their properties are closely controlled. Before use, each damper is rigorously tested for reliability. Taylor Devices manufactured and tested the dampers used in the project at their plant in Buffalo, New York. Figure 6 shows the beam-to-column weak axis moment connection retrofit detail and the viscous damper frames during construction. The use of viscous dampers and experimental data for the retrofit design and evaluation necessitated an independent peer review. Accordingly, the design and analysis of the building were peer-reviewed by a team at Simpson Gumpertz & Heger in San Francisco. The design team incorporated the inelastic properties of the improved moment connections at the residential levels and the fluid viscous damper frames into the non-linear analysis model of the building, which was used in the evaluation of the overall response of the improved structure and the design and evaluation of the new and existing structural components. Figure 7 compares the inter-story drifts before and after the proposed retrofit scheme and shows that the proposed seismic retrofit scheme will minimize the potential for collapse and significantly reduce the damage potential of the building for the BSE-1N (475-year) earthquake. In addition, beam plastic rotations are kept below 0.02 radians, which ensures that there will not be any severe strength and stiffness degradation at the moment connections. Plant Construction Company served as the general contractor on the project. The onsite construction spanned approximately ten months and was completed in March 2019. This project's unique attributes are as follows: • The enhancements to the structural system significantly reduce the earthquake-induced story drifts by improving the structure's energydissipating capacity without adding any extra stiffness. Integrating the retrofit solution with the existing building avoided significant additional load on the foundations and other structural elements,

Figure 6. Completed beam-to-column weak axis retrofit connection (left) and viscous damper frame (right).

Figure 7. Comparison of story drifts between the existing building and retrofitted building.

striking a delicate balance in addressing the seismic deficiencies present in the two distinct upper and lower halves of the building. • At the upper residential levels, the beam-to-column weak axis connection strengthening was designed to minimize the impact on the tenants by restricting the construction activity to one floor or even one apartment unit at a time. On average, construction activity within an affected apartment unit lasted only two to three days. The retrofit work impacted less than 5% of the residents at the upper levels at any given time during the construction. • At the lower office levels, the strategic choice of braced frames with viscous dampers enabled the retrofit work to be confined to just one bay on each side of the building perimeter at the lower levels. As a result, most commercial operations remained functional during the construction. • The full-scale testing of the building’s unique moment connections filled a knowledge gap, thereby aiding the structural engineering community in gaining a deeper understanding of historic beam-tocolumn connections in steel moment frames. The project also marked a productive alliance between academia and industry professionals. • The design and development of the seismic retrofit scheme and the associated peer review process highlight the innovative nature of structural engineering, ensuring that standards keep pace with new developments and the diverse characteristics of existing building structures. ■ Matt Skokan, Ph.D., S.E., is Principal at Saiful Bouquet Structural Engineers. (mskokan@ saifulbouquet.com) Metin Oguzmert, Ph.D., S.E., is Project Manager Consultant at Saiful Bouquet Structural Engineers. Saiful Islam, Ph.D., S.E., is Chairman and CEO at Saiful Bouquet Structural Engineers. FEBRUARY 2024

INSIGHTS Bringing Low-Carbon Concrete Forward The movement toward sustainability and the use of low-carbon materials. By Don Davies, P. E. S. E., and Larry Sutter, Ph. D., P. E., F. ASTM, F. AC

hroughout the construction industry, there is a movement toward sustainability that is affecting what and how we build, and a central part of this movement is the use of low-carbon materials. Although all materials are subject to scrutiny, concrete has received specific attention, and rightfully so, concrete is the most widely used material in the world. On a unit mass basis, concrete has one of the lowest carbon footprints of all manufactured materials. Still, the enormous volume used acts as a multiplier that makes concrete worldwide one of the largest single sources of emitted carbon dioxide (CO2). Simultaneously, concrete is the cornerstone of civilization. From transportation systems to water and sanitary systems to building construction for both private and public owners, concrete is essential. So, for concrete, our climate challenge is to Sustainability movements in the construction industry have paid specific attention to reducing the carbon footprint reduce its carbon footprint, knowing its of concrete, among other materials, because of its widespread use. use will continue. Stepping up to this challenge, the Climate Works Foundation has recently launched a Task Force to support our proven SCMs to their full availability. But “additionality,” a series of articles, tests, and concrete project case studies that which includes both lower carbon approaches to making cement help educate and de-risk the use of lower-carbon concrete. This binders and new ASCMs, is also needed. article is the first of a series that will frame this challenge, identify For new approaches to be effectively used and scalable, we need opportunities for impact, educate on important issues around testing and industry standards to establish acceptance criteria. This lower carbon concrete, and then, through shared case studies, includes testing for consistency, strength development with time, show examples of lower carbon concrete in use. pumpability, placement predictability, and long-term durability. Of course, cost also needs to be considered. Currently, national building codes such as ACI 318 recognize Where to Focus ASTM C150 Portland cement, ASTM C595 blended cement, and ASTM C1157 performance-based cement as acceptable, with While Portland cement is often 15% or less of the final concrete SCMs limited to fly ash, slag cement, and silica fume. Concrete mix volume measured by weight, it makes up nearly 90% of its mixtures are also subject to approval by the licensed design profesglobal warming potential (GWP). So, concrete avoidance and sional and local code official based on the use case and specific substitution are keys to the material's carbon reduction strategies. mixture designs. Trial batch testing and mixture performance Avoidance is achieved by reducing the total cement in a mixture data are typically required prior to approving a mixture for use. or optimizing the concrete volume required within a design. Diligence in testing and review should not be compromised. Substitution is accomplished by using an alternative cementi- However, which ASTM and ACI standards should be used for tious material (ACM) to fully replace the Portland cement, a testing new material is not clear and often not consistently applied, supplementary cementitious material (SCM) to replace a portion so the Task Force would help provide guidance. of the Portland cement or both. When a new material has been appropriately validated through Historically, the most common SCMs have been coal fly ash or third-party validation programs such as that being launched by blast furnace slag. Still, with the closure of domestic coal-fired ACI, there is still another significant barrier to its adoption. power plants and the limited supply of blast furnace slag, there is Unless prior batch testing data exists, or the ready-mix supplier a growing list of alternative supplementary cementitious materials is brought on early, and they are given a budget for testing, the (ASCMs) starting to be considered. We should continue to use design/construction/ready-mix supplier team is often reluctant

28 STRUCTURE magazine

to consider new mixtures or materials. There is typically no time to generate new mixture performance data, and there is a disincentive to take any new material or financial risk. But if we are to accelerate cleaner and lower carbon concrete use, we need to de-risk this barrier. Fortunately, there is an opportunity we are yet to leverage fully. Large projects often require hundreds of concrete placements over the course of many months to years. All projects, even small ones, have an opportunity to run parallel studies during the concrete’s construction, finding non-critical locations to test a trial batch of next-generation material and collecting data to inform either later placements on the same project or future projects down the road. Advancing an industry culture of continual testing and evolving improvement is another approach the Task Force will be encouraging.

articles and case studies from the Climate Works Foundation Task Force are a step in that direction. Future articles will include inputs on performance-oriented concrete

specifications, standardized test protocols, in-field placing demonstration standards, and other topics developed as this effort evolves.■

Don Davies is the Co-Founder of Davies-Crooks Associates. A mission driven consulting and adaptive re-use development practice focusing on lowering our climate impacts on the planet. Davies-Crooks Associates supports AEC industry leaders, government, and non-profits for creating impact at scale. Lawrence L. Sutter, Ph. D., P. E. is the Principal Engineer at Sutter Engineering, which specializes in providing technical support to companies bringing new products and technologies to the concrete construction business. The major focus of the firm is to help the cement and concrete industry attain carbon neutrality by the year 2035. He is a Professor Emeritus in the Materials Science & Engineering Department at Michigan Tech.

Who’s Working On This?

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

While far from being the only ones working on this challenge, the material and structural engineering leaders of the Climate Works Foundation Task Force will be supporting a series of placed concrete mixture demonstration projects. This will include evaluating mixtures with a 30% and a more ambitious 60% or greater GWP reduction compared to baseline mixtures from each case study project. The Task Force will target multiple strategies that are regionally appropriate across the country. The goal is to evaluate multiple scalable strategies that meet carbon reduction targets while retaining performance and constructability needs. Where tradeoffs occur, those will be documented as well. Shared case studies will highlight the lessons learned and advance a collective industry understanding. Another notable and related effort, one this Task Force will collaborate with, is coming from the tech sector Green DC Futures Team (see: https://climateaccord.org/news/greenerconcrete-for-data-centers-an-open-letter/).

Where Do We Go From Here? Breaking down the puzzle of lower carbon concrete into smaller and more manageable topics and then working solutions to those topics is something we collectively know how to do if we lean into the challenge and share our knowledge. The forthcoming FEBRUARY 2024

structural ANALYSIS Current Canadian and U.S. Approaches to Snow Drift Loads Snow drift procedures in Canada and the U.S. are compared and evaluated. By Michael O’Rourke, Ph. D., P. E., John E. Cocca, P. E., and John F. Duntemann, P. E., S. E.

istorically, the snow drift provisions in the early editions of ASCE 7 were adopted from Canadian practice. Although there are similarities between practices in the United States and Canada, the current U.S. snow drift provisions differ in significant ways from the current Canadian provisions. The objective of this article is to identify and discuss these differences to provide structural engineers with a better understanding of snow drifting in general and of specific differences for those with projects in Canada. The comparison is restricted to the simple leeward roof step drift load, which is arguably the most important snow load since it results in more snow-related structural performance issues. Specifically, simple geometry refers to nominally flat upper and lower-level roof surfaces, no upwind or downwind parapets on the upper-level roof, and a step size that does not impact the lower-level roof drift size. Also, the roof elevation difference at the step is less than 16.5 ft. (5 meters) since the Canadian code allows a reduction in drift load for larger step sizes.

U.S. Snow Loads In the current U.S. provisions as defined by the American Society of Civil Engineers Standard Minimum Design Loads and Associated Criteria for Buildings and Other Structures, ASCE/SEI 7-22, the drift height is a function of the ground snow load pg (in pounds per square foot), the length of roof upwind of the drift (or “fetch”) lu (in feet) and a winter wind parameter W2 (dimensionless). Specifically, the leeward roof step surcharge drift height, hd (in feet), is given by hd = 1.5

0.74 g

l w c 0.70 u

1.7 2

(1)

where γ is the snow density (lb/ft3) given by γ = 0.13 pg + 14 ≤ 30 lb/ft3

(2)

Equation 1 was developed using multiple regression analysis of numerically simulated maximum annual snow drift loads for 10 upwind fetch distances over a 35-year period at 46 sites across the U.S. A general discussion of each of these variables follows. Ground Snow Load: As one would expect, the drift height is an increasing function of the ground snow load. The larger the ground snow load, the more snow that is available for drift formation. Table 1 presents the drift height in ASCE 7-22 as a function of pg, normalized by the drift height for pg = 40 psf. As 30 STRUCTURE magazine

Table 1. Influence of ground snow load pg upon ASCE 7-22 drift height. Drift height is normalized by that for Pg = 40 psf.

Pg (psf)

γ (pcf)

Normalized hd

15.3

0.67

17.3

0.89

19.2

1.0

21.2

1.07

23.1

1.12

shown, hd is nominally proportional to pg to the 0.3 power, pg0.3. Upwind Fetch: Similar to the influence of pg, the drift heights in the current ASCE 7-22 provisions are an increasing function of upwind fetch distance lu. From Equation 1, hd is proportional to the fetch to the 0.35 power (lu)0.35. Winter Wind Parameter: In ASCE 7-22, W2 is the percent of time the wind speed exceeds 10 miles per hour during October through April; values for the lower 48 states are presented in ASCE7-22 map, Figure 7.6-1. W2 values vary from 0.25 in the Intermountain West to 0.65 in parts of the Midwest. As shown in Equation 1, hd is proportional to W2 to the 0.85 power, W20.85.

Canadian Snow Loads The snow load, S, in the 2020 version of the National Building Code of Canada (NBCC) is as shown in Equation 3. S = Is [Ss (Cb Cw Cs Ca) + Sr]

(3)

where Is is the importance factor for snow loads which equals 1.0 for the roofs of ordinary importance considered herein; Ss is the 50-year ground snow load (in kilopascals), Cb is the basic roof snow load factor; Cw is the wind exposure factor which equals 1.0 for roofs involving snow drift accumulation; Cs is the slope factor which equals 1.0 for the flat roofs considered herein; Ca is an accumulation factor, and Sr is the 50-year associated rain load (in kilopascals). Note that, somewhat surprisingly, the design roof snow load in Canada is not a function of the roof ’s thermal condition. As shown above, the Canadians have chosen to include rain with their snow load. Herein, the rain contribution will be neglected, as the focus of this article is loading due to drifted snow. The Cb factor is the Canadian version of the ASCE groundto-roof conversion factor of 0.7, which is a function of the Cw

factor and a characteristic length of the upper or lower roof, lc. The characteristic roof length lc (m) is defined as

Table 3. Influence of ground snow load Ss upon the NBCC Drift Height. Drift height normalized by that for Ss = 40 psf.

Ss (psf)

γ (pcf)

Normalized (hd)NBCC

(4)

15.3

0.56

where w is the smaller plan dimension for the roof and l is the larger plan dimension. Defining the aspect ratio Ar as l/w, lc becomes

17.3

0.83

19.2

1.0

1 lc = w a 2 - A k

21.2

1.12

23.1

1.21

w2 l

lc = 2 w –

(5)

Table 2. Characteristic roof length normalized by the smaller and larger roof plan dimensions.

Aspect Ratio Ar

lc/w

lc/l

1.0

2.0

1.5

0.75

3.0

1.66

0.55

4.0

1.75

0.43

5.0

1.8

0.36

Theoretical Consideration

The Cb factor is 0.8 for a characteristic roof length lc of 230 ft. (70 m) or less and increases non-linearly to 1.0 for lc of about 1400 ft. (440 m) or more. This factor accounts for the inability of wind to remove snow from very large roofs. Drift Loads: NBCC accounts for drift loads via the accumulation factor Ca. The drift atop the lower-level roof at the step is given by the parameter Cao. For the case considered herein, where the leeward step size does not influence the drift size F Cao = C

(6)

Where for the case considered herein with no parapets, F = 0.35β

clc Ss + C b ≤ 5

(7)

where γ is the snow density (same as the ASCE 7 relation in Equation 2) and β =1.0 for leeward drifts. (β is a factor that is dependent upon the direction of the wind, whether leeward or windward.) Note that the first part of Equation 7 corresponds to the surcharge drift while the second term (i.e., Cb) corresponds to the balanced snow load atop the lower-level roof. Since Cao is multiplied by dimensionless factors (i.e., Is, Cb, Cw, and Cs) and Ss in Equation 3, it is a load with units of psf or kPa. As such, for comparisons with the drift surcharge height hd in ASCE 7-22, one needs to divide by the snow density. Hence, the relation for drift height as per the NBCC, (hd) NBCC, absent the rain load, is (hd)NBCC = Is Ss (Cw Cs) ;0.35

cl c E S s /γ

(8)

l S s cc

(9)

with Is = Cw = Cs = 1.0. That is (hd)NBCC is proportional to

Note that (hd)NBCC is nominally proportional to Ss0.38. In relation to the fetch distance, (hd)NBCC is proportional to the square root of the characteristic roof length lc. However, to facilitate comparisons with ASCE 7-22, it can be shown that for a given Ar ratio, (hd)NBCC is proportional to the traditional upwind fetch lu to the 0.5 power, (lu)0.5.

Hence, in relation to the ground snow load, (h d) NBCC is proportional to the square root of Ss divided by the snow density γ. The influence of the ground snow load upon the NBCC drift height is shown in Table 3, normalized by that for Ss = 40 psf.

Although the hd relations in ASCE 7-22 were based on multiple regression analysis of numerically simulated drift data, one can estimate the expected influence of lu and pg from theoretical considerations. The drift load (half the drift height times drift width times snow density with units of lbs./ft.) is the snow transfer from the upper-level roof times a trapping efficiency. For the simulated drifts, the trapping efficiency was taken to be a constant, 50%. The snow transfer, due mainly to snow saltation (wind-driven snow particles bouncing along the snow surface), is a function of the wind speed to a power and the fetch distance. The snow transport is proportional to the square root of the fetch for lu ≤ 750 ft. For larger fetch distances, the transport rate is constant for a given wind speed. That is, for lu of 750 ft. and larger, we have the “infinite fetch” transport rate. In relation to fetch distances, for the ASCE 7-22 relationship, the simulated drift database had 10 fetch distances ranging from 25 to 1000 ft. There was only one fetch equal to or larger than the transport rate transition fetch distance of 750 ft. That is, 90% of the simulated drift database upon which the ASCE 7-22 drift relation was based had a lu of 750 ft. or less. Based upon this, it will be assumed herein that the upwind fetch distance is 750 ft. or less. Specifically, the transport rate will be assumed proportional to the square root of the fetch distance (lu)0.5. The drift formation process stops when either a) the wind stops blowing, or the speed is less than the drift threshold of 10 MPH, or b) the driftable snow has been blown off the upper-level roof. The theoretical influence of lu and pg upon the drift height is based upon two scenarios. High Wind Scenario: In this scenario, the wind speed is large enough to completely remove driftable snow from the upper-level roof after each snowfall event. For this scenario, the drift load Ld (lbs/ft) on the lower-level roof would be proportional to the upwind fetch and the ground snow load. For example, consider an upper-level roof north of the lower-level roof. If the wind is out of the north 100% of the time, then Ld = lupg. If the wind is out of the North 50% of the time, then Ld = 0.5 lupg. Ld is proportional to lupg For a large roof step, the drift width is typically 4hd (drift slope of 1:4). Hence, the drift height hd is proportional to the square FEBRUARY 2024

root of the drift load Ld divided by the snow density. Using the ASCE 7 relation for snow density. hd is proportional to

Ld 0.5 0.5 c = l u P g / 0.13Pg + 14

Hence, for the High Wind Scenario, the drift height is proportional to the upwind fetch to the 0.5 power, lu0.5. It can be shown that the influence of pg upon the drift height for the high wind scenario is nominally the same as in Table 1. That is, for the high wind scenario, hd is nominally proportional to the ground snow load to the 0.3 power, pg0.3. Low Wind Scenario: In this scenario, the wind speed is low enough that some snow remains atop the upper-level roof and does not contribute to drift accumulation on the lower-level roof. For this scenario, the drift load Ld is proportional to the transport rate, which in turn is proportional to the square root of the fetch. Ld proportional to

As such, the drift height is proportional to the upwind fetch lu to the 0.25 power, (lu).25. hd proportional to

Ld 0.25 c = l u / 0.13Pg + 14

The influence of Pg upon the drift height for the Low Wind Scenario is presented in Table 4. Table 4. Theoretical influence of ground snow load pg for low wind scenario. Drift height normalized by that for pg = 40 psf.

Pg (psf)

γ (pcf)

Normalized hd

15.3

1.12

17.3

1.05

19.2

1.00

21.2

0.95

23.1

0.91

Notice that for the low wind scenario, the ground snow load has little or no influence.

Comparisons to Theory ASCE 7-22: In relation to the ASCE 7-22 provisions, the drift height hd is nominally proportional to the ground snow load pg to the 0.3 power and proportional to the upwind fetch to the 0.35 power. Hence, the influence of both parameters is consistent with the expected behavior in the low and high wind scenarios. That is, for the ground snow load, pg0.3 is within the theoretical range of pg0.0 (low wind) to pg0.3 (high wind). Similarly, for the upwind fetch, lu0.35 is within the theoretical range of lu0.25 (low wind) to lu0.5 (high wind). These comparisons are presented in Table 5. Table 5. Parameter influence for drift height in ASCE 7 provisions and NBCC provisions in comparison to theoretical considerations.

Variable of Interest

ASCE 7 NBCC

Theory Low Wind

Theory High Wind

lu0.35

lu0.5

lu0.25

lu0.5

pg0.3

pg0.38

pg0.0

pg0.30

32 STRUCTURE magazine

Figure 1. Plan view of the two-level roof. U indicates the upper-level roof, and L indicates the lower-level roof.

NBCC: In relation to the NBCC provision, the drift height is nominally proportional to the ground snow load to the 0.38 power and proportional to the traditional fetch lu to the 0.5 power. Hence, the influence of l u is consistent with theory; that is, lu0.5 is within the range of lu0.35 (low wind) to lu0.5 (high wind). However, using the NBCC approach, the influence of ground snow load, pg0.38, is outside the expected range of pg0.0 (low wind) to pg0.3 (high wind). As a result, the NBCC provisions would tend to underpredict drifts for low-ground snow load sites and over-predict for high-ground snow load sites. The NBCC provision does not utilize a winter wind parameter in its drift load procedures. Hence, a US/Canadian comparison of the influence of the W2 parameter in ASCE 7-22 is not possible. Similarly, a US/Canadian comparison of the influence of the NBCC snow source aspect ratio Ar is not possible since ASCE 7-22 does not utilize that parameter. However, it is possible to estimate from theoretical considerations the expected relative influence of the NBCC characteristic length lu and the corresponding aspect ratio A r. As noted above, the NBCC characteristic length cleverly decreases the fetch for upper roof snow sources that are long in the along-wind direction and short in the cross-wind direction lc ≤ lu (l parallel to lu), while increasing the fetch for the reverse (i.e., (lc ≥ lu) for w parallel to lu. If the aspect ratio is 3.0 or 5.0, the NBCC drift height is 74% or 60%, respectively, of that for Ar = 1.0 if w is perpendicular to lu. If w is parallel to lu, the NBCC drift height is 29% or 34% (for an aspect ratio of 3.0 or 5.0, respectively) larger than that for Ar =1.0. In theory, the aspect ratio has two potential influences upon the drift height. As sketched in Figure 1, the first potential influence is the fact that the along wind fetch distance AB is somewhat larger than the traditional fetch lu. However, it can be shown that the resulting expected increase in the drift height at B is effectively negated by the corresponding increase in the new drift surcharge width on the lower-level roof, which would now likely be parallel to AB. The second potential influence of the aspect ratio upon leeward drift size is that a larger or smaller number of wind directions could contribute to significant drift formation atop the lower-level roof. That is, in Figure 1, wind parallel to AB, CB, or DB could contribute to leeward drift formation. Wind along line AE could cause snow transport from the upper-level roof. However, it is unlikely that, that snow will end up atop the lower-level roof. Hence, in Figure 1, wind direction within plus or minus θ of north is assumed herein to potentially contribute to leeward drift

formation. Table 6 presents θ as a function of aspect ratio Ar as well as θ normalized by that for Ar = 1.0. That is, for a roof with l parallel to lu (lc ≤ lu) and an aspect ratio of 5 (l = 5w), the wind directions potentially contributing to drift formation are 21 percent of those for Ar = 1.0. For a roof with w parallel to lu (lc ≥ lu) and Ar = 5, wind directions potentially contributing to drift formation are about 2.6 times that for Ar = 1.0. Table 6. Contributing wind angle θ as a function of aspect ratio Ar, θ normalized by that for Ar = 1.0.

Parameter

influence of the traditional upwind fetch in the NBCC provisions is consistent with that from theoretical considerations. In contrast, the influence of the ground snow load (pg0.38 versus a range of pg0.0 to pg0.3) is close. Table 6 suggests that the influence of aspect ratio in the NBCC provisions has the expected trend but is not consistent with a theory that drift size is proportional to the range of contributing wind direction. As such, it is the authors’ opinion that the ASCE 7-22 provisions for snow drift loading are, in general, more consistent with expected behavior than the NBCC provisions.■

Aspect Ratio Ar lc ≤ lu

lc = lu

lc ≥ lu

5.0

3.0

1.0

3.0

5.0

Θ (deg.)

5.7˚

18.4˚

26.5˚

56.3

68.1

Θ (normalized)

0.21

0.69

1.0

2.12

2.57

hNBCC

0.60

0.74

1.0

1.29

1.34

Table 6 also shows the relative influence of aspect ratio as per the NBCC provisions mentioned above.

Conclusion In summary, Table 5 shows that the influence of lu and pg in the ASCE 7-22 drift load provisions is consistent with that from theoretical considerations. Table 5 also shows that the

Full references are included in the online version of the article at STRUCTUREmag.org.

Michael O’Rourke is a Professor Emeritus of Civil Engineering at Rensselaer. He served as the chair of the ASCE 7 Snow and Rain Load subcommittee from 1997 thru 2017. John F. Duntemann, P. E., S. E. is a Senior Principal at Wiss, Janney, Elstner Associates in Northbrook, Illinois. He is the current Chair of the ASCE 7 Snow and Rain Subcommittee and a Fellow of the Structural Engineering Institute (SEI). John Cocca, P. E., is an Associate Principal at Wiss, Janney, Elstner Associates in New Haven, CT. He is the Vice Chair of the ASCE 7 Snow and Rain Subcommittee and a member of the Structural Engineering Institute (SEI).

ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

Available Online

2023/24

STRUCTURAL ENGINEERING

Structural Engineering Resource Guide

Resource Guide

From Structure Magazine

www.structuremag.org

Annual Publication with Industry Resources for SEs and Profiles from STRUCTURE’s Advertising Partners

Find your next industry partner for anchors, foundations, concrete, software, wood products, steel and cold-formed steel, connections, and more.

302310-SERG-Cover.indd 77

9/24/23 12:23 PM

FEBRUARY 2024

OUTSIDE the BOX Reclaiming the Role of Structural Engineer as a Form Finder Unleashing creativity within constraints. By Edmond Saliklis

n the realm of 3D form creation, the engineer, architect, and sculptor each wield their unique skills and knowledge, drawing inspiration from historical precedents. However, it is within the realm of structural engineering that the greatest constraints and the least unbridled freedoms lie. The structural engineer shoulders the weight of codes and regulations, necessarily ensuring life safety and compliance. Yet, it is precisely within these constraints that the engineer's ability to create groundbreaking designs emerges. Polytechnic training, coupled with aesthetic sensitivity, is the domain of the new structural engineer of the 21st century. Renowned engineer Peter Rice, in his autobiographical work “An Engineer Imagines,” emphasized the engineer's essential role in realizing design ideas that can only be brought to life through their creator’s technical expertise. Rice recognized that aesthetics, as well as engineering mechanics, are intertwined in the process of form-finding. This notion echoes the philosophy of Eduardo Torroja, founder of the International Association for Shell and Spatial Structures (IASS), who believed that rationality and imagination must converge in the design process for optimal results. Torroja meticulously shaped each part of a structure to enhance both its appearance and efficiency. He believed that the creation of truly beautiful structures required an engineer to have a serene and acute artistic sensibility, a fecund creative imagination, and the technical skill to comprehend the behavior of structures.

A Call to Action Today, the structural engineering profession finds itself at a critical juncture, in need of a revival and a reaffirmation of its role as a form finder, i.e., creating “spatial structures” and consequently designing “special structures.” In the late 20th century, too much creative control was ceded to the architect, and in the early 21st century, the specter of design by Artificial Intelligence looms large. A “Call to Action” is proposed here, which reinvents a venerable form-finding tradition guided by the engineer’s unique skill set. This tradition flourishes when form-finding is guided by “Discipline and Play.” Just as a game without rules descends into chaos, a lack of playfulness stifles innovation. Regrettably, engineers have often succumbed to a formulaic and robotic mindset, eroding the vitality of our profession. To overcome this challenge, it is imperative to reimagine and reinvent 19th-century geometric analysis and design tools for the 21st century. At California Polytechnic State University and in a new startup company, Poly Shells LLC, a new set of geometric tools has been created to analyze and design efficient and elegant thin shell structures. While architects are trained to be form finders and spend years honing their design skills, they often do so completely apart from any guidance from engineering faculty in most universities. The master 34 STRUCTURE magazine

Figure 1. Strut pinned only at A.

engineers possess a unique set of technical tools and knowledge that allows for the creation of form in a distinctive and impactful way, a path that is parallel to, but separate from, architectural form-finding. Engineers such as Pier Luigi Nervi, Robert Maillart, and Peter Rice exemplified this capability, as their contributions to the field of structural engineering have left an indelible mark on the history of structures. The brilliant civil engineer Iannis Xenakis is an exemplar of an extraordinarily creative structural engineer. Xenakis, working alongside, but not relying on, the architect Le Corbusier, showcased his exceptional form-finding abilities with the creation of the magnificent 1958 Philips Pavilion. The Philips Pavilion is an ideal springboard for this “Call to Action” as it combines mathematical rigor with architectural wonder.

A New Form-Finding Method With this in mind, a new geometric form-finding technique that empowers engineers to create spatial structures rapidly has been recently created and field-tested with a variety of college students. At its core, this methodology involves the sequential stabilization of points in 3D space, focusing on restraining unstable spherical and circular paths or “mechanisms” experienced by unconstrained structural elements. Through a surprisingly simple and approachable parametric and digital design palette, an infinite array of designs can be swiftly generated, exploring boundless possibilities while simultaneously achieving unconditional stability and minimalism through static determinacy. A single strut AB, as shown in Figure 1, which is pinned at its base and shown as Point A, is unstable, with Point B swinging along a

Figure 2a. Sequential form-finding. Figure 3a. The intersection of three spheres creates a stable point.

Figure 2b. Intersection of two spheres is a circular mechanism. Figure 3b. The intersection of one sphere and one circle creates a stable point.

spherical path at the unrestrained end of the strut. This is a degree of freedom that must be “locked down” or prevented with subsequent judiciously placed members. Consider now a second strut CD, pinned at its own base Point C. If the placement of Points A and C are such that there is an intersection between the two spheres, as shown in Figure 2a, a circular mechanism or degree of freedom remains for the node at the apex of the two struts as shown in Figure 2b. Notice that the designer controls the lengths of strut AB and strut CD. These are the radii of the spheres surrounding base Point A and Point C. Yet a two-member structural form is not stable, as the circular degree of freedom or mechanism still exists at the False Crown, and it must be locked down. This False Crown is an unstable point due to the circular mechanism formed by the two pin-ended struts. However, the intersection of three spheres, as in Figure 3a, or the intersection of one sphere and one circle, as in Figure 3b, is a unique “Final Crown.” There are two such Final Crowns, one above the ground plane and one below. We choose to focus on the Final Crown above the base, and we use that to create an unconditionally stable tripod. New struts can be built off the stable elevated Crown. A new design workflow has been created, wherein points in 3D space are linked through this parametric algorithm. While there is nothing special about the initial arrangement of points in 3D space, through a series of steps guided by a mentor, even inexperienced designers generate many ideas quickly. Mentoring of form-finding includes the use of symmetry, or starting with a simple, planar polygon on the base, the

use of spheres with matching radii on the corners of the polygon can be a nimble starting point, and as previously stated, circles, which are the intersections of the spheres, are the key tools. It is the intersection of these circles that generates the arrangement of points in 3D. The designer ensures that the structure is stable and statically determinate before making any major changes to the parameters of the polygon or the spheres on the corners of the base polygon. Many delightful forms can be created instantly using this technique. The forms are unconditionally stable if new spheres are created from pinned points on the base plane or unconditionally stable points, such as a previously found stable Crown point, elevated at some height off of the base. Figure 4 shows such sequential stacking of elements. Many techniques have been recently tested at California Polytechnic State University in design studios and in General Education classes for non-engineers. One such technique uses symmetry and a regularly assigned pattern of radii to the spheres being used, as shown in Figure 5a. This rapidly generates a series of intersecting circles, shown in Figure 5b. The intersection of the circles is the starting point of playful yet disciplined form-finding, as shown in Figure 5c.

The Double-Skin System This pedagogy and research is part of a larger endeavor to link “form-finding” to “form testing” to “form making.” Curvilinear shell structures are an ideal vehicle for such explorations as they FEBRUARY 2024

Figure 5a. This image reflects a technique using symmetry and a regularly assigned pattern of radii to the spheres being used.

Figure 4. Building off of an elevated stable point.

are economical and can be made of the ultimate renewable material, timber. These pedagogical and research studies allowed Professor Saliklis and Nathan Lundberg to form Poly Shells LLC—a conceptual design company specializing in an innovative timber double-skin thin shell system. This eco-friendly construction method utilizes precisely cut plywood polygons, resulting in lightweight yet remarkably stiff structures. Vertical couplers separate the top and bottom layers of the double-skin. A polygon cap is placed above the top layer, and another cap is placed below the bottom layer. The entire double-skin timber shell can be formed into hyperbolic paraboloid cladding surfaces between the struts of a space frame, or the double-skin can be a standalone funicular shell. The double-skin timber shell is rapidly assembled by hand without the need for screws, clamps, glue, or brackets. The ethos of this workflow embodies the union of high-tech design thinking with low-tech construction methods. Figure 6 shows the double-skin system, while Figure 7 is an example of what can be built in several hours by hand using the double-skin system as a standalone shell. The caps covering the panel on the top layer of the shell are shown in Figure 8 in an exploded view, while Figure 9 shows another rapidly assembled free-standing double-skin Poly Shell. The form-finding of spatial structures can present some insights into the design thinking of past brilliance like that of the engineer/architect Enrico Castiglioni or the architect/professor Anne Ting. This new design methodology of spatial structures is rapid, rigorous, surprising, and infinitely variable, something that Ting and Castiglioni would immediately recognize as both structurally rational and meticulously beautiful. Figure 4 is, in fact, a re-creation of Anne Ting’s unbuilt City Tower design for Philadelphia. This new design approach has been successfully applied in various studio environments and seminars, 36 STRUCTURE magazine

Figure 5b. Stabilized points from intersecting circles.

Figure 5c. Stable elements from unconditionally stable points.

allowing for the creation of efficient, economical, and elegant forms. The new hybrid structural typology, fusing the space truss with hyperbolic shells, will be known as the Xena Form. The Xena Form was so-named as an homage to Xenakis, but this new structural typology finds echoes in Castiglioni’s and Ting’s precedent-setting forms. The Xena Form is a minimal structure insofar as it is a statically determinate spatial structure, with straight or curved struts forming the structural skeleton. The evolution of an Xena Form with curved struts is shown

Figure 6. The Poly Shells double-skin system.

Figure 8. Another view of the double-skin system.

in Figure 10. Poly Shells’ double-skin cladding is formed in hyperbolic paraboloid areas. The struts bound these hyperbolic paraboloids, and some struts can float above or below the shell surfaces as the designer chooses. In 1958, Xenakis cloaked his rather heavy Philips Pavilion with concrete cladding panels, whereas our new Xena Forms are lean and green, light and stiff. Poly Shells LLC has received industry input from experts in timber construction including LEVER Architecture, NOUS Engineering, PCL Construction and Woden Fire Consultants to explore the potential for realizing the first prototype structure at a Universityowned site. The site-specific application will be studied further in 2024. Figures 11a, 11b, and 11c show three different Xena Forms as sunscreens in an arboretum. The forms in Figures 11a and 11b have struts floating above the hyperbolic paraboloid double-skin shells. Still, the form in Figure 11c is a twisted Möbius Strip, which is an abstract mathematical construction, wherein the inner surface is twisted to become the outer surface over the length of the strip. In our Xena Forms, the structural struts bound the hyperbolic paraboloid shell, which acts primarily as cladding. All three forms use lodgepole pine struts. By embracing this new approach, we have the opportunity to redefine the role of the engineer, reigniting the creative spirit within our profession and shaping a future where innovative and elegant structures abound. Through the convergence of playfulness and discipline, we can chart a path toward a more vibrant and impactful structural engineering practice. The collaboration between academia and industry, exemplified by our partnerships with leading design and construction firms, can bridge the gap between theory and practice, ensuring that students

are equipped with the skills and knowledge necessary to navigate the complexities of modern structural engineering. With the incorporation of innovative techniques like the double-skin thin shell system, structural engineering undergraduate programs can expose students to cutting-edge technologies that are reshaping the field.

Figure 9. A manually assembled free standing Poly Shell.

Figure 7. A manually assembled free-standing shell.

Figure 10. Design evolution of Xena Form with curved struts. FEBRUARY 2024

Figure 11a. Xena Form with floating struts.

Figure 11b. Another Xena Form with floating struts.

Figure 11c. Xena Form as Möbius Strip.

The transformation of structural engineering education requires a shift in mindset—an acknowledgment that structural engineers have the potential to be form finders and to shape the built environment in innovative and aesthetically pleasing ways. By embracing a more open-ended and imaginative pedagogy, structural engineering educators can empower students to become form finders themselves, capable of creating efficient, economical, and elegant structures that push the boundaries of what is possible. This evolution in education will contribute to a renaissance in our profession, unlocking the creative potential of structural engineers and fostering collaborations that lead to groundbreaking designs. By embracing creativity within the constraints of codes, regulations, mathematics, and physics, fostering interdisciplinary collaborations, and integrating emerging technologies, structural engineers can unleash their artistic and innovative potential, reshaping the future of form-finding and leaving a lasting impact on the built environment. The structural engineer's role as a form finder will be revitalized, 38 STRUCTURE magazine

ensuring that the discipline continues to thrive as a powerful force in the 21st century. This new empowerment is a re-claiming of a centuries-old idea that the technically trained engineer can control the forces in a structure by manipulating its form.■ Edmond Saliklis is a professor in the Department of Architectural Engineering at California Polytechnic State University San Luis Obispo, where he teaches structural engineering courses and architecture studios. He earned his Ph. D. at the University of Wisconsin-Madison and he is a licensed Civil Engineer in California. He is the author of three recent textbooks Structures: A Geometric Approach (2018) and Structures: A Studio Approach (2020), both published by Springer International and Architectural Structures: Visualizing Load Flow Geometrically (2022), published by Routledge, Taylor & Francis Group. He recently co-authored a chapter titled Thin Shelled Concrete Structures for the latest edition of the Structural Engineering Handbook. He is also CEO of Poly Shells LLC, a conceptual design company.

Breaking the Mold

Designing and constructing the new MIT Kendall Canopy. By Rebecca Lubrano, Paul Kassabian, and Christian Sjoberg

ow do you create a new entrance for a world-famous technology university? It must act as an enduring symbol for those entering the university for the first time as well as for the students and faculty every day. It must represent all that is best of what is technologically possible now in the built environment, be rooted in the history of the institution, and look forward to what will be possible. This is the new MIT entrance pavilion in Cambridge, Massachusetts, designed by Simpson Gumpertz & Heger (SGH) using first principle structural concepts, built with carbon and glass fiber, optimized with AI/machine learning, and fabricated by boat builders (Figure 1).

Initial Concept Design: Principal Stress Patterns Located above the MIT-Kendall Metro station, the new entrance pavilion points towards the main campus and, specifically, the famous infinite corridor of MIT. With the physical constraints of the two metro headhouses and the elevator below, the pavilion had to touch the ground lightly and sparingly. Following the MIT motto of “mens et manus” (“mind and hand”) representing the ideal of education for practical application, the design team used an approach of principal stresses and topological optimization to aid with early form finding. For those familiar, this approach only produces singular optimized forms when the solution parameters are defined clearly. At the same time, many options drive the design process: e.g., focus on self-weight, include lateral forces, maintain a form, define internal reinforcement, allow the form to be defined by the principal

stresses, etc. We explored different options as an iterative discussion point with the design architect and record architect. Examples of a few of these options are shown in Figure 2. In parallel, we discussed materiality to represent technology, including early thoughts such as a stone and titanium structure (to form a 3D reverse-arch structure that would complement and advance the nearby Collier Memorial), metal foams (lightweight and stiff cellular material researched by MIT faculty), and others. Ultimately, the two above design approaches of structural system and materiality fused to produce a 160-ft. long FRP shell structure 26 ft. in the air on multiple slender steel columns. Fiber-reinforced polymer (FRP) uses high-strength fibers in a polymer matrix to form a composite material which, unlike reinforced concrete, can easily have the thin fibers aligned in various directions to align with inherent stresses, essentially a fully designed material. Thus, this project represented the use of one of the newest structural materials (FRP) for a stiff, angular pavilion structure, the inclusion of newer fabrication techniques to the risk-averse construction industry (here from the boat building industry), and the ability to adapt the placement of multiple columns to meet multiple needs of structural optimization, pedestrian access, safety, and visibility. Bridge and boat design approaches for the FRP shell structure used an outer stressed skin with material placed where best for overall bending and with internal stiffening diaphragms to allow the outer panels to be fully utilized while also providing stiff load paths for the local column loads to connect with the overall structure (and avoid shear lag problems seen on the early steel box girder bridges such as Milford Haven/Cleddau and Westgate bridges). FEBRUARY 2024

Figure 2. Preliminary structural form options. (Renderings by SGH Digital Lab)

additionally compounded the size of the parameter space. Note the HSS steel columns act as vertical cantilevers and also the lateral system of the overall structure. Thus, their location and size directly affect overall system stiffness, which, in turn, directs the load to each column. We initially planned a typical brute force analysis approach but realized the solution space was over 200 million model options (more precisely, 11^8), and even at 30 seconds per analysis run using SAP2000, it would take approximately 200 years to run all options! We were also wary of traditional optimization methods, which converged on a single optimum that was potentially not favorable aesthetically. We needed a better way of explorFigure 1. Completed canopy at the MIT MBTA entrance. (Photo by Rebecca Lubrano, SGH) ing more diverse yet still relatively high-performing local optima. In the summer of 2018, the SGH Digital Lab explored a Column Layout Development machine learning-assisted optimization strategy for predicting higherperforming regions of the parameter space and based this on previous Much of the design iteration between engineer and architect moved research by Sjoberg et al. Given the predicted set of diverse yet highfluidly in scale from the overall approach to details of the hidden perim- performing configurations, we could then perform validation analysis eter gutter, etc. This was necessary as the pavilion is a singular object of on the designs and discuss their merit as final design candidates. To structure and architecture. Here, we provide just two examples of many create a predictive model of performance, we produced a training set that are related to the design process. for our supervised neural network model. Each object in the training set consisted of the normalized spacing and size parameters for the columns, as well as the resulting demand capacity ratio (DCR) values for the AI/Machine Learning columns and maximum stress and deflection values for the FRP shell. The entire workflow, including parametric modeling, meshing, propThe Architect (NADAAA) challenged the team to determine which erty/load assignment, and result collection, was automated across column layouts would provide the best distribution and allow for the multiple software programs to run the analyses iteratively and collect thinnest columns. No-go areas compounded the problem for head- data for the training set (Figure 3). The team created a parametric model houses, emergency egress, below-grade structures, and sight lines. The using Rhino and Grasshopper, then wrote custom nodes in C#, which combination of these various constraints prevented the column layout used the API for SAP2000 to insert the meshed geometry into a template from following a typical bilaterally symmetrical and evenly distributed SAP2000 file and assign properties/loads. The script also controlled the pattern. Asymmetrical support of execution of the analysis runs and the FRP shell structure further the logging of result data. Once enforced the need to analyze the this pipeline was completed, we effects of specific column congenerated a random sampling of figurations rigorously. the parameter space and allowed Given the remaining candithe analysis iteration loop to run date areas for the columns in overnight. The resulting dataset the plan, we began to establish of 1300+ input/value pairs gave the parametric layout logic. To us some limited insight into introduce sufficient regularity for the higher-performing regions fabrication purposes, a series of of the solution space. Still, the linear runs of column locations high dimensionality made direct were defined. Each linear run interpolation between input veccould then be subdivided into tors unproductive. The goal was N number of regularly spaced to explore whether the supercolumns. Following this logic, vised machine learning model the 11 resulting runs of colwould be more capable of filling umns could each have between the gaps between the parameter one and (length/min spacing combinations than our previous +1) columns. The range of posinterpolation methods. Figure 3. Diagram showing the iterative workflow used to explore design options. sible cross-section diameters With the training set populated, (Images by Christian Sjoberg, SGH)

40 STRUCTURE magazine

we began the process of selecting the network architecture. We split the already small training set to allow for a validation set between 100-300 pairs. We tested various commonly used parameter encodings and activation functions to avoid over-fitting and select our final architecture. Training was relatively fast with the small dataset, allowing us to make modifications quickly and monitor loss metrics. Ultimately, once we were satisfied with the performance when predicting the validation set, we explored the parameter space with our predictive model. Two methods were used to leverage this new model. The first and simplest was to quickly generate a larger random sample of possible designs and their predicted performance values. Since the machine learning model in milliseconds could now predict each design’s performance, we were able to generate 10,000+ pairs and store them in CSV format quickly. We then filtered out rows with result values outside of the desirable ranges. We sorted the data by the performance metrics to manually identify a diverse yet performative set of potential designs. The second method we explored was to use the trained network as a fitness approximator within a genetic algorithm. Genetic algorithms are a type of solver that works by iteratively creating populations of solutions, evaluating them using a “fitness” function, and creating a new population from some of the characteristics of the top performers plus some random “mutations.” We set up a custom implementation of this solver in Python and reviewed the top solutions it produced. Ultimately, the solutions that this method provided were not a significant improvement over the best results of our large random sampling. Each of the selected designs would then move on to the validation stage, where the model was regenerated from the input parameters, and the actual analysis results could be collected for comparison to the predicted. Of the predicted top performers, we filtered for those whose performance was verified through analysis. We manually selected a small but diverse series (a “top 10”) to review with the architect and project team. Ultimately, this study took two days to set up, two days to run, and one day to check—far less than 200 years. We provided the diverse “top 10” to the Client as 3D online models using an internal tool developed by the SGH Digital Lab. One example is shown in Figure 4 as a static image of the model where we used circles as visual indicators of the vertical loads at the column bases (which is a visual simplification recalling the model is more complex than gravity-only as the columns are the lateral system and hence integral with the overall gravity and lateral stiffness of the system).

sculptures such as Harry Bertoia’s sounding sculptures in Chicago and elsewhere, Skylar Tibbits’ “Resonance” sculpture in the Boston Seaport (structural design by SGH) for human interaction and movement, and even long cantilever traffic lights at wide intersections seen worldwide. We proactively had a meeting with the design team and owner to share expected movements at various wind speeds during the design stage. From experience, setting expectations on non-standard projects while still in design is far better than providing justifications in response to concerns after construction. We used the combined approach of the Beaufort scale to represent various wind speeds as relatable descriptions (e.g., “Inconvenience felt when walking against the wind” for 32-38 mph winds) and produced animations of expected pavilion structural movement seen from a pedestrian at ground level. This produced a helpful discussion related to the multiple shareholders, such as pedestrians, the Boston Metro Department (MBTA), and tenant views from the adjacent buildings. We did similar studies up to 120 mph hurricane winds showing what that is actually like outside (typically not experienced as people are told to “Take refuge …stay away from windows.”). Hence, the larger visible movement of the pavilion would likely be one of the least concerns of people if and when such an event occurs . With these expectations established, the owner accepted a H/120 horizontal deflection for the canopy under the code-level wind loads (where H was the height of the canopy in inches), which is significantly less stringent than the industry standard for occupied structures (H/600 to H/400). This increase in allowable deflection was critical to maintaining the slender columns the architect envisioned for the project but still kept the canopy movement nearly imperceptible during the 32-38 mph winds (deflections smaller than H/1000).

FRP Design Responsibilities

When approaching a standard building project with standard delegated design elements, the team generally knows and understands the roles and responsibilities of each individual party (e.g., with the delegated design of a curtain wall system) to meet the performance criteria, applicable building code requirements, and the standard of care. For the FRP canopy, as structural design progressed, the combination of a unique design and unique procurement through boatbuilders meant this required a clear definition as it made sense that the FRP subcontractor would take on final responsibility for the design of the FRP, being able to select materials and construction methods preferred Column Performance Criteria by their shop, while SGH maintained responsibility for the columns and the connection to the lid structure. As noted previously, the behavior The building code prioritizes stiffness as it relates to the consequences of and, thus, design of the FRP canopy was directly linked to the location movement (e.g., cracks in brittle finishes, etc.). In the case of pavilions, and stiffness of the columns and vice versa. This is analogous to delsculptures, and other touchable/visible structures, there is little guidance. egating the design of the beams in a moment frame while maintaining There are cases of acceptable large movements in outdoor structures/ responsibility for the column design and the overall moment frame. At the request of MITIMCo (the client, the investment arm of MIT), SGH took on design responsibility for the overall pavilion system of the columns and the canopy. Given the integrated nature of the system, this required the FRP subcontractor to account for the column stiffFigure 4. Visual summary of one column layout option produced by ness in their model and for SGH AI process. (Figure by SGH Digital Lab) to review the entire FRP canopy Figure 5. Canopy design responsibility diagram. (Figure by Paul design for proper consideration Kassabian, SGH) FEBRUARY 2024

of the supporting structure, the columns, and the FRP canopy itself, including compliance with all building code requirements. SGH provided the diagram in Figure 5 to all parties during design to explicitly clarify this arrangement. Although the diagram may be simple, it clarified multiple misunderstandings in the team from email/meeting discussions and set the project up for success.

Figure 6. Ansys Analysis Model images. (Model by Lyman Morse/SGH)

FRP Code Applicability and Testing With standard building materials, we have individual codes to follow during design, such as ACI 318 for concrete and the AISC Steel Manual for steel. At this point in time, the IBC does not have an “FRP Manual.” There are general provisions within the IBC for plastics, including some specific to FRP, but these primarily relate to fire and thermal performance. The design strength of the material falls under IBC Figure 7. FRP canopy during installation of bulkheads. (Photos by Paul Kassabian, SGH) 1706.2 “New Materials,” where the design strength and permissible stresses are to be established via tests found to be acceptable by the building official. Analysis Approach For the structural design of the materials in the FRP canopy, the FEA model was based on predefined material properties from the During the latter portion of the design phase, the canopy analysis manufacturer, Vectorply, who then performed destructive physical was done in SAP2000, developing conservative weights and stifftesting on the materials to verify the assumed properties were conser- nesses for the FRP canopy and a preliminary design for the columns. vative and thus acceptable. All the testing information, including fire Once in construction, the FRP subcontractor, with input from SGH, performance testing, was reviewed and accepted by the construction developed an integrated FEA model in Ansys Mechanical. The model team, design team, and building official to confirm the adequacy of included core and skins for the top and bottom canopy panels, canopy the proposed system for the final construction of the FRP canopy. bulkheads (including access panel openings), solid attachment bracket

Figure 8. FRP canopy halves delivery and installation on site. (Photos by Paul Kassabian, SGH)

42 STRUCTURE magazine

Figure 9. Completed canopy installation. (Photos by Rebecca Lubrano and Paul Rosenstrauch, SGH)

locations, stainless steel columns, and composite steel beams of the supporting structure below. The lid structure of the metro station below was mostly a 14-in. thick concrete structure, but five of the columns were located over concrete on a metal deck with composite steel beams. Given how sensitive the structural load paths were to relative stiffness, the less stiff support condition at the composite steel beams needed to be accounted for in the model (Figure 6). The analysis included a residual capacity analysis on the stainless steel columns based on ASCE 7-10 Section 2.5.2.2. This did not consider any impact or other loads imposed by the damaging event. Instead, it focused on the redistribution of loads after one of the loadbearing elements is damaged to the extent it no longer contributes to the structural system. The residual capacity analysis concluded that the columns have sufficient residual capacity to prevent progressive collapse should any one column be unable to support any load following the occurrence of a damaging event.

Mold in Two Halves and Infusion During the FRP design, the project team actively discussed the logistics of transporting the canopy from the boatbuilder’s shop in Maine to the project site in Cambridge. Given the proximity of the final site to the Charles River, the intent was to put the canopy on a barge to travel down the coast and be picked off the boat in Cambridge. Given the 160-foot length, the team decided to build the canopy in two halves that would then be connected onsite in Cambridge. The two halves could fit on the same barge and would be more manageable to erect onsite. The actual fabrication of the FRP canopy was similar to that of a boat. Each top-side half was molded and infused upside down as one piece. Given the rotational symmetry of the canopy, the same mold was used for both halves. The bulkheads were hand-laid onto this top surface in the shop, and the bottom panels were individually molded and infused. The two halves were then packed on the barge and shipped down the coast (Figure 7).

Installation on Columns The stainless steel columns were installed prior to the FRP canopy’s arrival on the site. The columns were temporarily bolted in place with oversized holes to allow for adjustments to the column locations if needed. While the columns and canopy were being constructed, many installation logistics were finalized. The arrival of the FRP canopy had to be timed such that the columns were already installed and to avoid conflicts with multiple events in the area, especially given the need to shut down roadways between the barge on the river approximately half a mile down the road from the project site. The FRP canopy arrived in Cambridge during the day on September 21, 2022, and was picked by crane off the barge on the night of September 23, when the appropriate roads could be closed (Figure 8). Once at the site, the south half was picked first and placed on the previously installed columns, columns plumbed and fully welded in place prior to the canopy being released by the crane. The same process followed with the north half, and the two halves were then bolted together. Over the next several months, the canopy received its finishing touches, including the installation of the metal soffit cladding for the lighting and other elements designed in close coordination with the structure and architecture, prior to the site officially opening to the public in February 2023 (Figure 9). ■ Full references are included in the online version of the article at STRUCTUREmag.org.

Rebecca H. Lubrano, P.E.: Rebecca H. Lubrano is a Senior Project Manager with Simpson Gumpertz & Heger in Waltham, MA (rhlubrano@sgh.com). Paul E. Kassabian, P.E., P.Eng., C.Eng.: Paul E. Kassabian is a Principal with Simpson Gumpertz & Heger in Waltham, MA (pekassabian@sgh.com). Christian H. Sjoberg: Christian H. Sjoberg is a Senior Developer with Simpson Gumpertz & Heger in Waltham, MA (chsjoberg@sgh.com). FEBRUARY 2024

Enhancing Public Safety in Apex Event Planning i

Plaza

A Mass Timber High-Rise By Eric R. Ober, P.E., and Chelci E. Dell, P.E.

44 STRUCTURE magazine

The Revolution in Temporary Structures Regulations by the International Code Council By William Gorlin, P.E., S.E.

n a landmark decision, the International Code Council (ICC) Board of Directors has ushered in a new era of safety, awareness, and operational criteria for temporary structures with its certification of crucial changes to the International Building Code (IBC). The forthcoming 2024 edition of the code will feature provisions that will reshape the landscape of regulations governing temporary structures, particularly those used by the entertainment industry for public assembly and live events. These alterations not only address long-standing gaps in safety standards used by professionals and public officials involved in the design and management of these temporary structures but also signify a strengthened emphasis on ensuring the wellbeing of event attendees. The benefits of these changes extend beyond the implementation of new regulations for structural design and use to also include the newly heightened awareness, improved coordination, and sharing of responsibilities between the International Fire Code (IFC) and the IBC. While temporary structures encompass a wide range of applications, this article specifically concentrates on their utilization for live events and related entertainment industry uses.

A Historical Gap in Temporary Structure Regulations Historically, the IBC lacked a comprehensive set of regulations specifically tailored to temporary structures, exposing a crucial gap in safety and practical use requirements within the entertainment industry. Prior editions of the IBC contained very limited provisions for temporary structures. IBC Sections 108 and 3103 defined “temporary” as less than 180 days and briefly touched on conformance to the code, permit requirements, construction documents, location regarding fire safety, and means of egress. IBC directed users to the IFC for temporary tents, umbrella structures, and other

membrane structures. IBC stated that the building official is authorized to issue a permit for temporary structures, which shall conform to IBC requirements “as necessary to ensure public health, safety, and general welfare.” This meant that any relief in the code-prescribed design loads and use of temporary structures was up to the discretion of the authority having jurisdiction. As a result, the application of design criteria was vastly different from location to location depending on the judgment of the building official, some of whom required strict adherence to the building code, while others allowed the use of more relaxed industry standards or their own opinions about what loads and use conditions would be acceptable. For example, City ‘A’ may require full hurricane wind loading on an event structure erected for three days, while the neighboring City ‘B’ may only require a modest wind load and a weather action plan for the same event. To make matters more challenging, some jurisdictions issue permits for special event structures through building code enforcement, while others issue permits through fire safety enforcement. Such large discrepancies had made it difficult to plan a temporary event, particularly a touring event, and produced vastly inconsistent outcomes in terms of safety. This oversight became increasingly apparent as the popularity of outdoor entertainment events and public assemblies surged. The absence of dedicated regulations for design requirements left event organizers, designers, and engineers grappling with a lack of guidance on critical matters such as structural integrity and stability and usage parameters for temporary structures. The entertainment industry in the U.S. started developing its own technical standards in 1996 when the Technical Standards Program (TSP) was established by the Entertainment Services and Technology Association (ESTA). ESTA is an ANSI-accredited Standards Development Organization and has produced dozens of important technical standards that have been approved as American National Standards.

Festival-goers at the annual Albuquerque, New Mexico, International Balloon Fiesta enjoy a variety of entertainment options housed in temporary structures over the course of nine days, including musical and dance performances, chainsaw carving, fireworks, and lightshows. FEBRUARY 2024

A key standard pertaining to temporary special event structures is ANSI E1.21, which was first approved and published in 2006 as “Entertainment Technology - Temporary Ground-Supported Overhead Structures Used to Cover the Stage Areas and Support Equipment in the Production of Outdoor Entertainment Events.” Updated in 2013 and again in 2020, this standard was expanded to address a broader range of structures and retitled “Entertainment Technology - Temporary Structures Used for Technical Production of Outdoor Entertainment Events.” This standard is notable in that it contains provisions for using reduced environmental loads and for incorporating operations management plans (OMP) into the management of temporary structures to ensure safety, including weatherrelated safety (i.e., action plans). While the scope of the standard excludes structures occupied by the general public, the OMP concept has been utilized for various temporary events at the discretion of event operators and building officials. The entertainment industry received a notable jolt in 2011 with the weather-induced collapse of a stage structure at the Indiana State Fair. In response to this tragedy, a wide variety of industry professionals created the Event Safety Alliance (ESA) to proactively promote “life safety first” for live events, using the British Event Safety Guide (HSG195) as a model. This group rapidly produced a guide tailored for the U.S. market, which introduced many improved practices for the industry. Ultimately, ESTA and ESA worked together to harmonize efforts and develop a series of new ESTA standards. One such standard now referenced in section 3103.8 of the 2024 IBC and in the upcoming revision of E1.21 is ANSI ES1.7 “Event Safety Requirements – Weather Preparedness.” Another complementary standard is ANSI ES1.19, “Safety Requirements for Special Event Structures.” While the entertainment industry was developing all these standards, few had been adopted as law by States or cities, so they remained voluntary. A few places, such as New York City and the Commonwealth of Kentucky, developed their regulations for temporary special event structures inspired by provisions in the ESTA standards. It is important to note that the 2018 IFC included notable changes to Chapter 31 by adding a section on temporary special event structures, which references E1.21. This change set into motion the subsequent revisions in the IBC.

Key Aspects of the New IBC Provisions for Temporary Structures While the lack of in-depth provisions for temporary structures in the IBC has been acknowledged for a long time, in response to the new 2018 IFC provisions for temporary special event structures, the 2021 IBC crossreferenced these temporary structures back to IFC. Then, a notable milestone occurred in 2019 when a group led by several building officials proposed significant changes during ICC hearings to the temporary structures section of IBC. Members of ESTA’s Technical Standards Program attended these ICC meetings in a show of support and engaged in discussions with the authors of the proposed change. While the change was not approved in 2019, it planted the seeds for the 2022 IBC ad hoc committee that developed proposed changes. These changes have been approved for the 2024 IBC and include the modifications to Section 108, “Temporary Structures, Equipment and Systems,” Section 202, “Definitions,” Chapter 16, “Structural Design” Sections 1608, 1609, 1612, 1613, 1614, and 1615, and Section 3103, “Temporary Structures.” Key aspects of the code changes pertain to definitions, loads, and actions. A temporary structure is still defined as a structure erected for a period of 180 days or less; however, the code now includes detailed criteria and provisions for extending the use period beyond 180 days. It is important to note that Section 3103.1 of 2024 IBC indicates that temporary special 46 STRUCTURE magazine

Temporary structures at live outdoor entertainment events that gather an assembly of people, such as a hot dog eating contest on Coney Island (above) and PokemonGo Fest in Chicago (right), would fall under new code changes in the 2024 International Building Code.

event structures, tents, umbrella structures, and other membrane structures erected for less than 180 days shall also comply with the IFC. The code introduces a new, important concept in defining a publicoccupancy temporary structure as any “... building or structure erected for a period of one year or less that serves an assembly occupancy or other public use.” The implication is that temporary structures “serving an assembly occupancy” are required to have the same level of reliability and performance as permanent structures. The reliability of a temporary structure relates to several factors in the 2024 IBC, including: 1. Service life, which allows a reduction in snow and wind loads for temporary structures that have a cumulative service life less than or equal to 10 years since the probability of the maximum environmental load occurring is less than for a permanent structure with a lifespan of 50 years or more. 2. Defined Risk Category, which affects the factor in the table of reduced snow and wind loads. 3. Allowance of weather seasons to be considered in determining environmental loads. 4. Allowance of seismic loads to be reduced by multiplying the calculated value by a factor of 0.75. 5. Actions to mitigate risks to occupants prior to a large environmental load occurring on the site. Public-occupancy temporary structures can take advantage of controlled occupancy procedures, which must follow the operations management plan of ANSI E1.21 and weather preparedness of ANSI ES1.7, as well as the emergency action plan requirement stipulated in the new code Section 3103.8. Each of these factors allows temporary structure design environmental loads to be reduced below those used for permanent structures, based on achieving a similar level of reliability. The code provisions allow snow and ice loads to be neglected if the temporary structure is not used in a cold weather environment. In addition, the code allows the design wind speed to be reduced to a specified value based on

the Risk Category in hurricane-prone locations outside of hurricane season. If controlled occupancy procedures are implemented, then the ground snow load and design wind pressure may be reduced by multiplying each value by a factor of 0.65. The code includes provisions for installation and maintenance inspections of temporary structures assembled using transportable and reusable materials, citing provisions in ANSI E1.21. The code also includes a provision allowing a registered design professional to use live loads that are lower than those prescribed in IBC Table 1607.1 if such reductions are rational and can be warranted. An example of utilizing this provision would be in a purpose-built performance stage, in which certain areas have designated uses that are subject to lower loads than a code-defined stage. Additionally, the new code contains an important distinction that has previously not been addressed. It affirms temporary non-building structures or temporary structures not occupied by the general public (“ancillary structures”), which are in the vicinity of areas accessible by the public or a public-occupancy temporary structure, pose similar hazards to the general public as the public-occupancy temporary structure. As a result, these ancillary structures are required to have the same Risk Category and occupancy load as a public-occupancy temporary structure and, therefore, the same environmental load conditions and limitations. Section 3103.6 of 2024 IBC defines the applicable “vicinity” as “…within a radius equal to 1.5 times the height of the temporary non-building structure” to areas occupied by the general public.

A Vision for Change: Collaborative Efforts The newfound collaboration and alignment between the IFC and the IBC signify a significant shift towards a more comprehensive approach to public safety, particularly in the realm of temporary structures. Members of ESTA's Technical Standard Program, including this article’s author,

played a pivotal role by actively engaging with the ICC ad hoc committee responsible for developing the proposed code changes. Their involvement extended beyond mere participation, as they provided invaluable feedback on drafts and offered recommendations for specific code provisions. This collaboration between industry experts and regulatory authorities fostered an environment of mutual understanding and acceptance of the underlying principles. Furthermore, the ICC ad hoc committee's composition, including both code officials and individuals affiliated with subcommittees of ASCE 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, underscores the commitment to a multidisciplinary approach. These committee members are not only focused on refining code requirements but are also actively planning to contribute to developing a new Appendix to ASCE 7 dedicated to addressing the unique challenges posed by temporary special event structures. This initiative reflects a forwardthinking perspective, recognizing that standards need to evolve in tandem with one another. ESTA's Rigging Working Group, responsible for developing ANSI E1.21, recognized the need to harmonize their standards with the upcoming 2024 IBC provisions. This conscientious effort culminated in an upcoming revision to ANSI E1.21, which includes changes related to determining environmental forces. These changes are designed to align seamlessly with the applicable provisions outlined in Chapter 3103 of the 2024 IBC, effectively bridging the gap between industry-specific standards and regulatory requirements. This collaborative endeavor represents a paradigm shift in the industry's approach to public safety concerning temporary structures. It places a heightened emphasis on proactive measures, risk mitigation strategies, and a comprehensive understanding of the unique challenges posed by various event scenarios. The industry's dedication to a well-coordinated approach ensures that safety is not just a matter of compliance but a shared responsibility that transcends codes and regulations, ultimately enhancing the overall safety, reliability, and predictability of temporary structures in live events and beyond.

Conclusion The certification of meaningful changes to the International Building Code for temporary structures represents a significant milestone for temporary structures used for public assembly and live events. This collaboration amongst the ICC, ASCE committee members, and ESTA’s Rigging Working Group, as well as the alignment between the IFC and IBC, signify a more comprehensive approach to public safety, focusing on proactive measures, risk mitigation, and a deeper understanding of temporary structure challenges across various event scenarios. This synergy enhances safety protocols and cultivates a culture of diligence and preparedness within the industry, benefiting event attendees, organizers, public officials, and professionals. By prioritizing structural reliability, design integrity, and usage parameters, these provisions improve public safety and enhance the quality of event planning and execution. As the industry embraces these changes, attendees and professionals can look forward to more immersive and memorable experiences at outdoor entertainment events. ■ William Gorlin serves as McLaren Engineering Group’s Vice President of the Entertainment Division, is a graduate of Cornell University, and is registered as a Professional Engineer in 26 states. His 37 years of experience include engineering of scenic, entertainment, and amusement structures, staging, rigging, theatrical infrastructure, buildings, show action equipment, scenic elements, theme park attractions, architectural theming, sculptures, and other frameworks, worldwide.

FEBRUARY 2024

A final rendering of Storey Park project depicts the residential component and seven-story bridge. Image courtesy of HKS Architects.

The Story of a Truss: Storey Park By Macenzie Smith, Seth Rogge, and Chris Woitowicz

torey Park, located in the heart of the NoMA (North of Massachusetts Ave) neighborhood in Northeast Washington, D.C., was previously the site of an old Greyhound bus depot lot. Situated near the Metro entrance and adjacent to Union Station, this unique city block of land was ready for redevelopment. With such a large block in a prime location, the development team quickly realized this building would embody the true spirit of mixed-use development (Figure 1). With three levels of below-grade parking, the original design of a U-shaped building could accommodate several uses. Initially programmed for an office building on the west side and apartments on the east side, the building had a natural delineation between these two functions both in façade and floor plate. Needing to look like one building, the ground floor and the roof were at the same elevation; but with office floors requiring more floor-to-floor height, the west office building had two fewer floors than the residential building. Strategically locating columns between the two functions allowed

48 STRUCTURE magazine

the building to be laterally still tied together without the unnecessary burden of an expansion joint (Figure 2). The design was completed, permitted, and ready to construct with one exception: an office tenant. With prime ground-floor retail and a white-hot residential market, the office tenant was the last piece of the puzzle. As time dragged on and the office market continued to suffer, change was inevitable. Revising the office to residential space was, unfortunately, not a viable option. The solution was the next best alternative for the area: a flagship hotel. Given the current square footage, a new hotel program would require more space. With the Washington D.C. height restrictions, going vertical was not an option. The only logical option was to connect the U-shaped building and make it donut-shaped. However, due to parking and planning requirements along with other zoning ordinances, only the top seven floors could be connected together. With a span of over 60 feet x 60 feet, floors 7 through 13 would be connected together. The challenge: the area below the new connection needed to be free and clear of columns and other structural framing.

Large transfer beams could not be added due to aesthetics, height requirements, and the affect on adjacent rooms/ façade. Several options were studied, and eventually, a reverse king post truss with vertical tension rods was determined to be the best option, both structurally and aesthetically (Figure 3).

The Truss To support floors 7 through 13 with the thinnest slab profile possible, columns would be located within the 60-feet × 60-feet connection. These columns would then transfer the load to the underside of the seventh floor through a reverse king post truss (Figure 4). With such an exposed structural element, the aesthetics of the truss also played an important role. The king post truss comprised a W27×258 top chord that spanned front to back, the two 10-inch diameter solid steel tension rod diagonal members, and the W14×109 hanger post. At the apex where the hanger post connects with the diagonal members, 1 ⅝-inch-thick plates sandwich the members together. These plates provided some out-of-plane and torsional restraint for the hanger post. With the high amount of shear in the web members, ½-inch-thick doubler plates were utilized on each end. The concrete columns supporting the floors were strategically located along the top chord members. ½ inch-thick web stiffeners were used due to the high concentration of forces. The vertical end reactions from the reverse king post truss were then transferred back to the supporting building with diagonal tension rods. The tension rods were located at each end of the top chord. With such a large force generated from the floors above, 8-inch-diameter solid rods were used for support. The rods were connected to the truss using 2 ½-inch-thick end plates. Large, specialized clevises were fabricated to connect the ends of the tension rods. The rods continued vertically up, creating a V shape on the front and back of the bridge, and were connected to full building concrete columns at the main roof. With such large tension and shear forces, a W14×257 steel column was embedded into the main building concrete columns at the main roof. These composite columns were connected to the main roof and level 12 diaphragm by an embedded plate with shear studs (Figure 5). To overcome the substantial shear forces, the large embed plates were needed to transfer the load back to the diaphragm. With multiple floors supported on the king post truss, the analysis of the post-tensioned slabs at levels 7 through 12 was crucial to understanding both the relative short- and long-term deflections of each floor along with the stiffness behavior of the slab. As the king post truss and hanger rods displaced, additional stresses within the slab needed to be accounted for in the design. Using 3D finite element analysis, stresses were reviewed in the slab to determine critical areas needing additional top or bottom reinforcement. Post-tensioning effects were manipulated to account for additional internal stresses. Although the design was documented in the drawings, the construction and erection sequence was a critical piece in the process. Temporary shoring installation and removal

Figure 1. The site was originally a Greyhound Bus station, shown here in 2007. Image courtesy of Google Earth.

Figure 2. An early rendering of the Storey Park project included an office component. Image courtesy of HKS Architects.

Figure 3. This 3D view shows the structural elements of the suspended floor system.

FEBRUARY 2024

Figure 4. Elevation of the reverse king post truss is illustrated. Image courtesy of SK&A Structural Engineers.

Figure 5. A W14 steel column was embedded in the composite column at roof to anchor tension rods at roof.

50 STRUCTURE magazine

Figure 6. Tall, heavily loaded shoring of the new addition was one of several challenges to overcome.

Figure 7. Tension rod installation is shown.

below the bridge element was a challenge to the team (Figure 6). This shoring stood 54 ft. from the ground level/second level slabs up to the underside of the seventh floor and supported loads of up to 870 PSF. The shoring needed to support the flat slab above and the complex shape of the king post truss during initial installation, formwork, reinforcement, and placing of concrete. The shoring then needed to be removed in such a way that the four tension rods supporting the truss would be loaded in an identical manner without allowing for any settlement of the slabs or unbalanced loading. Several options were evaluated to allow for the installation and uniform loading of the rods through a progressive lowering of the shoring towers. However, these options became increasingly complex, leading the team to evaluate the design of the rods. The goal was to find a system that allowed for tensioning of the rods after installation but prior to the removal of the shoring below the 7th-floor slab. The tension rods could not have become engaged until the main roof had been poured and gained the required strength. This made the tension rods unusable for any temporary support. After several discussions and iterations, the plan was to form/reinforce/ pour concrete by floor up to the seventh floor. Formwork installation for the seventh floor under the bridge began as the rest of the structure reached the fifth floor so the truss could be set and formed around as the seventh floor structure was placed. The truss was then set, and the seventh floor concrete was placed. The rest of the building above level 7 was placed up to the roof level. The concrete-encased steel columns from the 12th floor to the roof were installed prior to the roof concrete being placed. Formwork was removed between the seventh floor and the roof to allow for the installation of the supporting tension rods. The

tension rods were installed and tensioned (Figure 7). Once inspected, the shoring below the bridge was removed. Throughout this process, surveyors performed monitoring on the concrete structure and king post truss to ensure allowable deflection was not exceeded.

Conclusion Once the initial deflection was determined after the strategic removal of the shoring towers, the façade and waterproofing could then be installed with relative ease. Collaboration between ownership, contractor, architect, structural engineer, and several subcontractors allowed for a creative solution to a unique challenge.■ Seth Rogge is a Principal with SK&A Engineering. He has significant experience with transit-oriented developments, multi-story office and residential buildings, mixed-use/retail developments, and parking garages as well as hospitals, academic facilities, and associated structures. (sethr@skaengineers.com). Macenzie Smith is a Project Manager with SK&A Engineering. She has almost 10 years of experience with a heavy focus on conventional and posttensioned concrete structures. Her portfolio includes projects with challenging lateral systems, unique bridge structures, and complex post-tensioned transfer girders for large residential developments. (macenzies@skaengineers.com). Chris Woitowicz is a Project Executive with John Moriarty & Associates, doing work in their Massachusetts, D.C. and North Carolina offices. Chris has experience on projects of all types including low, mid and high rise residential, life science, office, hospitality and student housing. (woitowicz@jm-a.com).

FEBRUARY 2024

Know Your HSS Welds

Insights into elevating weld designs to achieve successful hollow structural section connections. By Mike Manor, P.E., MLSE, and Cathleen Jacinto, P.E., S.E.

ollow structural section (HSS) steel shapes allow for many geometric framing possibilities in structures with the benefit of aesthetic appeal. Maximizing the benefits of HSS sections in a design involves understanding the intricacies of various welding types for connections between members. Several considerations when designing welded HSS connections include the flexibility of the HSS connecting face, corner radius of square and rectangular HSS, and interior access for welding the tube. This article will explore weld selection and design considerations to ensure successful, cost-efficient HSS structures.

Basic Terminology

Figure 1. HSS connection terminology is illustrated.

the effective throat must be considered per AWS 2020. Two more weld types to be aware of are complete joint penetration welds (CJP) and partial joint penetration welds (PJP). For CJP welds, the weld metal at the root extends all the way through the joint thickness, while for PJP, the weld metal only partially fills the joint. Types of PJP welds for HSS include flare-bevel and flare-V-groove welds. As shown in Figure 4, flare-bevel welds are located between a flat surface, such as a plate or end of the HSS branch, and the curved HSS corner. Flare-V welds occur where two HSS corners are welded together. Usage of these welds as they relate to HSS will be discussed later in this article, but for more in-depth information about weld geometry, strength, installation, and more, see AWS 2020 or AISC Design Guide 21.

Starting with basic terminology, in HSS connections, there is a single chord plus one or more branches, as shown in Figure 1. The chord is the main supporting member. The branch(es) are the supported elements (plates or HSS, for example) attached to the chord that can impose shear, axial, and/or moment loads to the chord. When two HSS members are connected, they can be in a matched or stepped configuration, as shown in Figure 2. In a matched connection, the branch width is the same as the width of the chord member perpendicular to the chord axis. For stepped connections, the branch is narrower than the chord. Additionally, there are terms for common connection types for plate-to-HSS and HSS-to-HSS connections, as shown in Figure 3. Plate connections are either transverse or longitudinal T- or Cross-Connections. HSS-to-HSS connections are typically T-, Y-, Cross- (X-) or K-connections. Uneven Load Distribution Moving on to weld definitions, HSS connections have various weld types to consider. The strength for all In HSS connections, branch members types of welds is based on the effective are often attached to a connecting face throat of the weld, which is the miniof an HSS chord. When the branch mum distance of the weld cross-section is welded transverse to the HSS confrom the weld root to the opposite side. necting face and imposes an axial or The weld root is the point where the out-of-plane load normal to the chord two members being welded are closest wall, the HSS face acts as a plate spantogether. The first weld type for HSS ning in its weak direction between is a fillet weld, which has a triangular the supporting side walls. As demoncross-section (AWS 2020). Fillet welds strated in Figure 5, the connecting face are most commonly used for plates conis stiffer near its sidewall supports and necting to HSS wall faces as well as less stiff at its midspan. This results in for T-, Y-, and K-connections. When nonuniform stress distribution across the angle between branch and chord is Figure 2. Connected HSS members can be in a matched (left) or the width of the connecting face. The other than 90 degrees, adjustments to stepped (right) configuration. stiffer portion resists a higher share 52 STRUCTURE magazine

Figure 4. Shown are types of groove welds in HSS connections. Figure 3. Common HSS connection categories are illustrated here.

of the load, and likewise, the load distribution in the weld will follow the plate stress. To account for this, a reduced effective HSS face width is assumed in connection design per AISC 360-22 Specification Equation K1-1. For additional information, see the Steel Tube Institute article: https://steeltubeinstitute.org/resources/ understanding-local-yielding-due-to-uneven-load-distribution/. For square, rectangular, and round HSS, a reduced effective weld length must be considered during weld design. The effective weld length can be calculated per AISC 360-22 Table K5.1 and K5.2. Note that Table K5.2 is new in the 2022 Specification to address round HSS sections.

HSS Corner Radius The type of weld used for a connection is influenced by the geometry of the HSS sections in combination with the connection configuration. Due to the manufacturing process, square and rectangular HSS members have a radius in each corner with an allowable variation in tolerance. To account for this range, different radius values are used for geometry in calculations. When determining the workable HSS flat width for stepped connections, the radius is assumed as 2.25tnom per Part 1 of the AISC Steel Manual of Construction 16th Edition. However, for flare-bevel welds in matched connections, an average corner radius of 2t is used (where t is the HSS design wall thickness) per the footnote of AISC 360-22 Table J2.2. Understanding which corner radius value to use is crucial when specifying flare-bevel groove partial joint penetration (PJP) welds, as the radius determines the weld’s effective throat size and strength capacity. In turn, the radius is based on the HSS thickness (see Figure 4). Hence, HSS thickness is necessary to consider for reasons beyond just the weight and strength of the member. When using AISC 360-22 Table J2.2 to determine weld strength, note that these are the Figure 5. Uneven load distribution in maximum effective throat sizes for flare transverse HSS connections. groove welds. If no effective weld size

is specified for a flare-bevel groove weld, the fabricator will typically fill the weld region flush, potentially requiring many weld passes. Therefore, specify the required effective throat size (S) for flare-bevel PJP welds to save cost and reduce welding time. Flarebevel and flare-V-groove welds can be underfilled as long as the effective throat is specified in the contract documents, with one exception: some categories of Architecturally Exposed Structural Steel (AESS) require that welds be filled flush. Matched and stepped connection configurations also impact the weld type used. Stepped rectangular HSS-to-HSS and plate-toHSS connections can generally use fillet welds, while matched HSS-to-HSS rectangular connections require both fillet and the more costly flare-bevel groove welds. Therefore, it is recommended to provide stepped connections instead of matched, where possible, to minimize fabrication costs. In a matched connection, the weld geometry at the corner radius and root opening varies depending on HSS member size, as shown in Figure 6. This bears greater impact when joining thin branches to thicker chords. The AISC 16th Edition Steel Manual specifies a nominal weld root opening, R, of zero inches, with permissible tolerances of up to 1/16” as detailed and up to 1/8” for fit-up. Should the gap between the chord corner radius and branch wall surpass these limits, available methods to close the root before the final connection weld include profiling the branch to fit around the chord or using steel or weld material for backing. The branch thickness also provides an upper bound limit to the effective throat in flare groove welds (Figure 4c), so HSS thickness must be considered.

Interior Accessibility for HSS When welding the end of an HSS member to a plate or another HSS, a weld can only be placed on the exterior side of the HSS wall, therefore resulting in a single-sided weld. Some typical locations where this is relevant are in trusses or at column baseplates where FEBRUARY 2024

the branch is under axial loading. Single-sided welds to tensionloaded HSS members are subject to a local eccentricity, are partially unrestrained from rotation, and are prone to local bending about the weld axis, which can lead to increased stress at the weld root (see Figure 7). Research was conducted on the effects of single-sided welds on HSS, leading to revisions in AISC 360-22 that clarified when the use of the weld directional strength increase factor is appropriate. Per AISC 360-22, depending on the load angle relative to the weld axis, fillet weld strength can be increased, as noted in AISC Equation J2-5, by a weld directional strength factor, kds. The minimum factor is 1.0 for a longitudinal load parallel to the weld axis with a maximum of 1.5 for a load perpendicular to the weld axis. However, this Figure 6. Options to close the weld root when connection geometry is beyond the allowed additional strength is only applicable if strain compatibility of the tolerances of prequalified connections. weld is considered. Thus, single-sided welds at tension members are unable to take advantage of the increase, while double-sided welds can. The weld root opening effect is less significant in round HSS we will further explore HSS considerations for various weld types, connections due to the curved weld axis, and thus, the directional including a discussion on alternatives to CJP welds. increase is permitted for round HSS connections. Summarizing, per AISC 360-22 Specification Section J2.4 Commentary and Olson 2020, the weld directional strength increase: HSS Fillet Welds • It is not permitted for rectangular HSS members to base or cap plates in tension (kds=1.0). Fillet welds are the easiest type of weld for both designers and • It is permitted for round HSS to base or cap plates in tension. welders to work with and are great for stepped HSS connections. • It is permitted for HSS members to be connected either to a Access is limited to only one side of HSS walls at connections, and, rigid plate or an HSS chord in compression. as a result, the leg size of the fillet can often exceed the thickness • It is permitted for longitudinal plate-to-HSS connections, assum- of the HSS member being connected. (Note that AISC 360-22 ing welds on both sides in any loading. Section J2.2b, in referring to maximum weld sizes at the end edge • It is permitted for transverse plate-to-HSS connections, assum- of the steel member, does not apply to fillet welds at matched or ing welds on both sides and the use of reduced effective weld stepped connections). length per AISC 360-22 Table K5.1 and K5.2, in any loading. When calculating the strength of fillet weld connections, the Accessibility also impacts the installation of backing material as strength of both the fillet weld material and the base material required for Complete Joint Penetration (CJP) welds. CJP welds must be checked with the lowest value controlling the capacity require backing material to ensure sound weld material throughout of the joint. For HSS, the ASTM A500 material available in the the weld thickness. If the backing material is not continuous, stress current marketplace is either grade C or dual graded for both B risers can occur with the possibility of premature weld failure at the and C. Looking at AISC Design Guide 24 1st Edition Table 2-2, stress concentration. Without the backing material, the back side the calculations demonstrate that the base metal rupture limit of the weld would be exposed to air, which would cause a portion state for ASTM A500 Grade C material governs over yielding. of the weld material to be compromised. It can be difficult and Tables 2-2 and 2-3 of the Design Guide aid in verifying the HSS quite costly to install backing along the full length of a CJP weld wall thickness is adequate to transfer the shear along the weld. in closed sections, especially at the corners of rectangular HSS. The Note that in 2022, there were updates to ASTM A500, revising backing material must be carefully sized and installed for continuity the Fy and Fu values to be 50ksi and 62ksi, respectively, for both around the entire interior HSS perimeter, and then be welded in round and rectangular HSS. Updates for a second edition of AISC place along the inside edge of the tube. This Design Guide 24 are underway, which can be labor-intensive work necessary to include a weld table revision to reflect the prepare for a CJP weld. Once the CJP weld current values but are not yet available as is complete, there is another complication. of this writing. For static and wind loads, the backing mateWhen designing fillet welds, consider the rial can remain in place for the life of the number of weld passes required for installastructure. However, portions of the seismic tion and only specify the weld size required design code require the removal of backing to meet the load demand. The larger the fillet bars in seismic connections after the weld weld, the more welding passes it takes to is complete such as column to baseplate complete the weld. Per AISC 16th Edition connections among others. AISC 341 and Manual Table 8-12, a single pass is adequate AWS D1.8 provide specific information on for welds up to 5/16” with the number of when backing removal is required. For an passes increasing quickly for larger weld HSS member, backing on the inside is not sizes. Factoring the number of weld passes accessible for removal. Challenges related to and the bevel preparation required for backing material are one reason why CJP groove welds, fillet welds up to ¾”are genwelds should not be the default weld for erally more economical than groove welds. HSS connections. In the following sections, Figure 7. Single sided fillet weld under eccentric tension load. 54 STRUCTURE magazine

HSS Partial Joint Penetration (PJP) Welds

weld certifications. As discussed earlier, CJP welds also require backing material, which can be a challenge PJP weld strengths are calculated to install in closed sections. per AISC 360-22 Table J2.5 with Therefore, the recommendation is effective throat (S) per Tables J2.1 to leave CJP as the last option. There for bevel groove welds and J2.2 for are only rare occasions where CJP flare groove welds. These effective welds are required such as specific throats are also graphically shown categories of AESS in high seismic in AISC 16th Edition Manual Table applications. A common miscon8-2. You can see in Figure 8 that ception is that CJP welds are better different welding processes can be than PJPs for a variety of reasons, but used, resulting in the same effective this holds true primarily for fatiguethroat and capacity. Thus, it is recloading conditions. While CJPs fully ommended for PJP welds to call out develop the HSS wall strength in the the required (S) in the weld symbol connection, the reality is that PJP (include the parenthesis to indicate welds can also fully develop the HSS this as the effective throat) to allow wall strength but without all the extra the fabricator to determine the best hassle of backing bars and inspection. welding process for the project. Specifying the required effective weld Designers should also ensure rotasize (PJP weld) rather than leaving tion about the root of PJP welds Figure 8. Effective throat of PJP groove welds. the weld symbol blank (CJP weld) cannot occur. can go a long way toward the achieveOccasionally, a PJP weld may not ment of an economical connection. have sufficient capacity to meet the load requirements. If it is not Also, specifying a fillet-reinforced PJP weld (Figure 9) is a great possible to revise the connection geometry to increase the effec- alternative to a CJP weld. For more information, see the Steel tive throat, then there is still another option to consider before Tube Institute article: https://steeltubeinstitute.org/resources/ specifying a much more costly CJP weld. A reinforcing fillet cjp-hss-welds-be-informed-before-you-specify/. weld can be added to a PJP weld to increase the effective throat (S). Figure 9 shows images demonstrating the effective throat for PJP welds reinforced with fillets which can be compared to Conclusion Figure 8 for a PJP alone. Notice that when reinforcing the PJP, the capacity is NOT equal to the sum of the two weld capacities. The design and selection of welded connections for Hollow Instead, the welding capacity is determined based on the effec- Structural Steel (HSS) sections are crucial for achieving successful tive throat of the combined weld, which is the shortest distance and cost-efficient HSS structures. Understanding key factors such from the joint root to the weld face of the diagrammatic weld as the flexibility of the HSS connecting face, the corner radius minus any applicable groove weld reductions. To specify fillet geometry of square and rectangular HSS, and the challenges of weld reinforcement, see AISC 16th Edition Manual Table 8-2 for the accessibility inside the tube is essential. Additionally, the amount pertinent prequalified PJP weld sketch. of work required to perform the various types of welds will greatly impact the cost of HSS connections. Fillet welds are a cost-effective choice for many HSS connections, but engineers must carefully HSS Complete Joint Penetration (CJP) Welds assess the size and number of weld passes required. PJP welds provide versatility and can be reinforced with fillets if needed. While CJP CJP welds are often specified when a non-bolted option is welds offer a quick easy design, they come with added complexity, desired in connecting two HSS members. However, they are cost, and inspection requirements, making them a last resort. The the highest-cost weld to specify and are often overkill. Besides choice of weld type should align with project requirements while requiring a significant amount of time to complete the weld, CJP minimizing unnecessary expenses. A thoughtful approach to weld welds trigger additional installation and inspection requirements. design, considering HSS geometry, loading conditions, and cost All CJP welds require special inspections both prior to and after efficiency, can lead to the successful realization of aesthetically the welding process and some configurations require additional appealing and structurally sound HSS structures. For additional HSS design guidance and resources, visit the Steel Tube Institute website: https://steeltubeinstitute.org. Full references are included in the online version of the article at STRUCTUREmag.org.

Figure 9. Effective throat of fillet reinforced PJP groove welds.

Mike Manor and Cathleen Jacinto serve as HSS Technical Consultants to the Steel Tube Institute and are practicing Structural Engineers with FORSE Consulting. (hssinfo@steeltubeinstitute.org).

FEBRUARY 2024

TECHNOLOGY Robots in Cold-Formed Steel Projects The integration of automation brings safety, precision, and competitiveness. By Venkata Charan Kumar Gajulapalli, PE and Karthik Ramagiri

raditional construction methods such as stick-framing coldformed steel (CFS) walls have been in use for a long time. Stick framing involves wastage of material and requires skilled field labor, slower build time, and scheduling around favorable weather. With advancements in technology, we are now witnessing the early stages of innovation in the architecture, engineering, and construction (A/E/C) industry using automation. CFS projects have been at the forefront of adopting these emerging technologies, setting a precedent for the A/E/C industry. Robots have become integral to the CFS world, providing CFS framing industries with a competitive edge through automation, increased safety, and enhanced production. Applying these technologies creates efficiencies and introduces new challenges for individuals in the A/E/C industry. In the world of CFS, automation through robots plays a vital role in every step of the manufacturing process by: 1.) Automating the roll forming process to various shapes and sizes of CFS. 2.) Using robotic arms for framing CFS into panels or pods and adding sheathing to the panels. 3.) Automating rasping using CNC machines. 4.) Using robots to spray and sand interior walls. And 5.) Deploying field printer robots to position walls/pods accurately in the field.

Robots in the CFS World Automating the Roll Forming Process The traditional stick-framing process involves preordering and procuring large quantities of material. At the construction site, the studs are cut to the required length and dimensioned on the track per the engineering drawings. Alternatively, using an automated roll former, the studs and tracks are rolled precisely to spec using the digital BIM models, reducing wastage and rework.

A robotic arm welds boxed studs. Photo courtesy of Digital Building Components

Robotic Arms for Framing and Sheathing Resembling a human arm mounted to a base, a robotic arm offers multiple degrees of freedom. Its primary application includes welding sub-assemblies together, such as stud-to-track welds, built-up stud components, and CFS-to-structural steel welds. These robots also weld entire panels by utilizing custom welding torches to reach intricate weld positions in the frame. The weld positions are precisely defined in 3D space with BIM software. The generated CAD files are further used to simulate the robot path. This capability extends to welding joints like lap, flare bevel, and other welds as well as welding materials up to 20–22-gauge galvanized steel. Laser profile scanners attached to the robot's end-effector are used to find deviations in the CFS frame within fabrication tolerances before welding. However, robotic arms are sensitive to material tolerances. For example, AISI 240 allows up to 1/8 inches between stud-to-track gaps for load-bearing walls and ¼ inches for curtain walls using 16-gauge material. Deviations in wall plumbness, straightness, and levelness can affect robot welding quality. Incomplete fusion/burn-through of CFS materials is a potential issue, demanding quality check inspection. Spot welding addresses some of the issues associated with robots doing fillet welds. However, spot welds have lower allowable capacity than fillet welds, necessitating project-specific considerations. Robotic arms are also employed for tasks like screwing studs to tracks and placing heavy sheathing boards. The software can be customized to add screws at specific locations and required spacing.

Automating Rasping Using CNC Machines

An automated roll former reduces wastage and rework. Photo courtesy of Digital Building Components

56 STRUCTURE magazine

Rasping is a technique used to make the surface of EPS/insulation board even and flat before applying finishes. This method produces tiny beads of EPS which when inhaled creates health risks. CNC rasping machines are used as an alternative to hand rasping for finishing

A robotic arm adds welds to CFS panel. Photo courtesy of Digital Building Components

panels. These machines use larger-diameter bits to rasp panels, achieving acceptable roughness and precision efficiently. Next-generation rasping CNCs can be designed with additional degrees of freedom for cutting out reveals and adding slopes in EPS with high precision.

they move on wheels and scan the room to add finishes to the wall. Similarly, spraying and sanding processes can be performed in the prefab shop with fixed robots while panels move on conveyors within a paint booth, thereby increasing productivity and safety.

Spraying and Sanding Robots

Field Printers

These robots have been employed on construction sites for spraying drywall mud, sanding, and applying paint. At construction sites,

Robots are employed to mark layouts of wall panels/pods on slabs, enhancing accuracy and reducing human error. A 2D layout drawing is given as input to the field printer robot including the locations, and the sizes of MEP penetrations. Current field printers require human attention to navigate obstacles and for initial setup. Future iterations of field printers have the potential to use Lidar technology that can be integrated to navigate obstacles and define slab boundaries.

Conclusion The integration of robotic technologies has brought increased efficiency, precision, safety, and competitiveness to every stage of CFS construction. Robots have revolutionized the CFS industry by increasing productivity and efficiency throughout the construction process. However, challenges related to material tolerances and specific project requirements must be carefully considered when implementing robotic technologies in CFS projects.■

A robotic arm prototype sands a panel. Photo courtesy of SurePods

Field printer robot in action marking a layout on a slab. Photo courtesy of Digital Building Components

Venkata Charan Kumar Gajulapalli, P.E is a Structural Engineer at Digital Building Components. He is a member of CFSEI and SEAoA. (charanv@digitalbuilding.com) Karthik Ramagiri is a Controls Engineer at General Motors with previous experience in developing automated equipment for A/E/C industry. (karthikramgiri@gmail.com)

FEBRUARY 2024

structural FORUM A Structural Engineer’s Role in Preservation of Existing Buildings By David Cocke, S. E.

ave you ever received a call from a developer asking you to take a look at an old building? Sometimes, they might ask if you could tell them that it’s not feasible to save it so that they can justify tearing it down. Other times, they might ask you whether it’s possible to restore it. We, as structural engineers, need to understand our role in scenarios like this. Let’s assume that a developer is looking to purchase a rundown building. They ask several questions about the building to make sure their project will be successful. They might ask the architect, “Can it be functional?” They might ask the mechanical engineer, “Can it be energy efficient?” They might ask their realtor, “Will it be marketable?” Finally, they might ask the structural engineer, “Can it be safe?” Often, developers are most nervous about the structure, and because of that, it turns out that the structural engineer can be the “deal breaker.”

Chapman University Villa Park Packing House – before.

Working With Owners/Developers As structural engineers, we need to keep our role as potential deal breakers in mind and appreciate what is being asked of us. The first opportunity to provide an opinion might be when the potential owner/developer asks us to provide a feasibility study about an existing building. The feasibility question can become an interesting dilemma for structural engineers. A common definition of “feasible” is capable of being done with the means at hand and circumstances as they are. When evaluating the feasibility of saving an existing building, the circumstances are the condition and integrity of the structure, while the means at hand is the dollar amount that will be required and the developer’s desire and capacity to spend that amount. In other words, we generally can retrofit or repair any building; it is just a question of the cost and whether the developer is willing to spend that amount. For that reason, structural engineers should hesitate to make a statement about whether the retrofit is actually feasible or not without several qualifications. We can only determine what structural measures are needed and help determine the cost, but we should not decide whether the developer should spend it or not. For example, if Bill Gates were to want to save a particular building, he could likely afford it, regardless of the cost. Therefore, it’s feasible. Of course, it does take some special skills and the right attitude for 58 STRUCTURE magazine

a structural engineer to work well with the developer, architect, and contractor to make an existing building project successful. One needs to understand the nature of existing buildings and how they were constructed, be creative, be able to evaluate multiple approaches, not be stubborn, and be responsive, particularly during construction. Communication is key, too, so that they understand and can make informed choices. Potential buyers can be especially skeptical in earthquake-prone regions. Some common questions are, “How do you know that the building is not adequate to resist earthquakes?” or “Hasn’t the building been through several big earthquakes?” or “What size of earthquake are you proposing to design the retrofit to resist?” Multiple tools are available to help us respond appropriately, including not only our analysis tools and in-situ exploratory and material testing but also other risk analysis tools, especially FEMA P-58-7, A Guide to Stateof-the-Art Tools for Seismic Design and Assessments. A map showing shaking at the site from a historic earthquake is a very powerful image for communicating with your client.

Strategies for Older Buildings There are several points for us to remember about working with older buildings. Sometimes, a developer will want to use the term “unsafe” as an excuse to allow for demolition by neglect—we need to give them objective information about saving it while making a good

judgment about the owner’s true interest in doing the project right. We should also realize that an older building’s seismic performance may be better than first expected. If the owner agrees, we might have to expend extra effort to determine that actual performance—it’s too easy to conclude that an older building doesn’t meet current seismic life-safety standards and, therefore, conclude that the building cannot be saved. Of course, more effort means more fees, so that has to be clear to the client from the start. We also now have the tools to clarify different performance objectives versus the cost to the client so that they can understand and make an informed decision. In earthquake regions, we should also consider that time Sandi Simon Center for Dance, Chapman University. is a factor—the odds of a damaging earthquake in the short term are very low, and a phased retrofit can help The California Historical Building Code is basically a “performanceto save the building in the long term. Also, when the potential cost based” code, meaning that it allows the structural engineer to set a of earthquake repairs is factored in, a life cycle cost analysis may target performance. It is generally much less prescriptive than the show that retrofitting is the smart financial choice. The structural regular building code. It provides “alternate” means to other code engineer needs to be confident enough to offer at least a reasoned requirements to reach the project objectives. ASCE 41, Seismic guess on structural costs based on recent experience—it isn’t Evaluation and Retrofit of Existing Buildings, provides a toolbox always comfortable, but at least a relative comparison between for evaluating older buildings and identifying the deficiencies to be structural schemes might help before a cost consultant is retained addressed. The California Historical Building Code does allow for by the buyer. Even beyond the monetary cost considerations, it partial retrofitting, so the mitigation measures can be prioritized, is fairly certain that saving a building typically saves more energy starting with potential falling hazards such as chimneys, parapets, and carbon than demolition and replacement with a new building. masonry facades, and hollow clay tile. After that, the next critical Looking down from 30,000 feet, there are generally three optional deficiencies can be addressed. Of course, the owner/developer might strategies for seismic retrofitting: 1) Adding strength and rigidity not have the funds immediately available building to retrofit the (brute force method), 2) supplementing the existing structural building completely. Temporary stabilization or “mothballing” can system with new structural elements, or 3) changing the dynamic be an effective strategy to help save a building. Water intrusion and response of a structure using base isolation, passive dampers, or fire protection are critical considerations for immediate stabilization other elements to lessen the seismic force demand. Of course, and might be necessary to prevent further deterioration while the performance objectives, costs, schedule, disruption, and other funding is gathered to proceed. factors also have to be considered when choosing a strategy.

Historic Buildings When working on the preservation of a designated historic building, it is also necessary to follow a few rules to be consistent with the Secretary of Interior’s Standards for the Rehabilitation of Historic Buildings: • Respect the existing structural system • Utilize the existing structure in combination with new strengthening and/or structural elements • Establish the desired performance objective • Understand the importance of the existing architecture/history/ social context • Repair and supplement when possible instead of replacing • Be surgical whenever possible to be “invisible”—it is usually desirable for the added strengthening elements to be hidden • Strive for reversibility, although this is not always easy with structural elements. The standards are based on the concept that later in the structure’s life, the added seismic elements could be removed for future installation of improved systems as well as repair of historic features and materials. California is one of the few states to have a historical building code.

Existing Warehouse – before.

FEBRUARY 2024

New exterior – after.

Case Studies I want to highlight two recent projects; one is a designated historic building, and another is a decades-old nondescript industrial building. The owners of both buildings were interested in changing the use of their building, i.e., “adaptive reuse.” Chapman University in Orange, California, purchased a former citrus packinghouse by the railroad tracks on the outskirts of town. The 1920s building had been out of use for decades, but the northfacing lights in the sawtooth roof were thought to provide a wonderful interior space for promoting student creativity. The building’s seismic strengthening included a steel tension rod roof diaphragm addition at the bottom chord of the roof trusses, strengthening of the roof trusses using post-tensioned steel rods, new plywood sheathing added to the interior face of existing exterior walls, addition of new strong-backs to laterally brace historic hollow-clay tile exterior walls that separated the packing house from the historic cold storage, and reframing the historic canopy extending along the north elevation. A new two-story mezzanine addition, built in a large floor opening, houses a performance space on the lower level and classrooms on the second level. The mezzanine’s masonry walls were used as new interior

shear walls for the existing floor. Creatively placed opening curvatures in the floor opening allow light to extend down to the lower level. The historic wood floor framing was repurposed as architectural wall finishes on the new partition walls throughout the lower level. Although an upgrade was required because of the change in risk category, the California Historical Building Code provided flexibility to the architect and structural engineer in retaining the original materials and supplementing them with new elements. The structural steel was tested for material properties and weldability, and the hollow clay tile was braced in place. The design team worked with the university to develop several options for use, including a student activities center, art museum, dance studio, and others. The new facility, the Sandi Simon Center for Dance, is now open and has won numerous design awards. The university saved money by reusing the existing structure, demonstrated its support for sustainability, and provided a space that is perfect for dance. The second project involves an older concrete building in Culver City, California, with nearly zero curb appeal. Approximately 50,000 square feet in size, the roof structure of arched timber beams and parabolic shapes is beautiful. The private owner fell in love when he entered the building and chose to invest in an adaptive reuse that celebrated the beauty of the structure. A seismic upgrade was required because of the extent of the modifications and included some thickening of the perimeter shear walls, new roof anchors, and cross-ties in the roof diaphragm. To create an open space, a huge box truss consisting of HSS12×12 chord members measuring 8 feet by 8 feet by 150 feet long was installed to allow the removal of a line of eight columns down the central spine of the building. The parking capacity was expanded by constructing a subterranean level below the ground floor slab. The slab-on-grade was removed with careful shoring of the perimeter foundation walls and replaced with a new two-way slab spanning between new concrete columns. With theatrical lighting and a contemporary two-story new structure of concrete and heavy timber framed wood floors in the front, the space is now used for performances, car shows, parties, conferences, and other events. So, the next time the phone rings and a client asks whether you could evaluate a building, consider that you are now in a critical role to do some good: preserve some history, improve your community’s resilience to earthquake damage and dangers, provide your client with a good investment and do some good for the environment. It is actually one of many important roles that we as structural engineers have and gives us the significant opportunity to help our society as we move forward.■

David Cocke, S. E. has been practicing Structural Engineering in California since 1981. He founded Structural Focus in 2001 after 20 years at another firm. He is a registered Structural Engineer in California and several other states, with expertise in seismic evaluation, historic preservation, retrofits and new design. Some of his most notable projects include renovations to the Hotel del Coronado, Glendale Masonic Temple, Wilshire Boulevard Temple, Red Bull North American Headquarters, Walls Annenberg Center for Performing Arts, Netflix Egyptian Theater, Culver City’s new Amazon Studios and numerous projects for all of the major southern California film studios. David has been very professionally involved, has served in leadership positions as past president of EERI and SEI and on numerous board of directors including SEAOC, several preservation organizations and is a current member of the NIST Advisory Committee on Earthquake Hazards Reduction. (dcocke@structuralfocus.com). New interior – after.

60 STRUCTURE magazine

code UPDATES 2024 IBC Significant Structural Changes Steel (IBC Chapter 22)—Part 4.

By John “Buddy” Showalter, P. E., M. ASCE, M. NCSEA, and Sandra Hyde P. E., M. ASCE, M. NCSEA Note: This multi-part series includes a discussion of significant structural changes to the 2024 International Building Code (IBC) by the International Code Council (ICC) (Figure1). Parts 1, 2, and 3 ran in the November 2023, December 2023, and January 2024 issues. Part 4 includes an overview of changes to IBC Chapter 22 on steel.

his article provides an overview of the reorganization of Chapter 22 on steel in the 2024 International Building Code (IBC) and a discussion of new sections for metal building systems, industrial boltless steel shelving, industrial steel work platforms; and stairs, ladders, and guarding serving steel storage racks and industrial steel work platforms. Only a portion of the chapter’s total number of code changes is discussed in this article.

2201.3 Protection. The protection of steel members shall be in accordance with the applicable reference standards within this chapter.

Reorganization of Chapter 22

2201.5 Anchor Rods. Anchor rods shall be set in accordance with the approved construction documents. The protrusion of the threaded ends through the connected material shall fully engage the threads of the nuts, but shall not be greater than the length of the threaded portion of the bolts.

IBC Chapter 22 covers requirements for the quality, design, fabrication, and construction of steel structures, including structural steel, cold-formed steel, steel bar joists, steel cable structures, and steel storage racks. Changes to the 2024 IBC include an editorial reorganization of Chapter 22 for better flow, usability, and clarification of steel provisions in the building code (see the summary table). New provisions for structural stainless steel, cold-formed profiled steel diaphragm panels, steel decks, industrial boltless steel shelving, steel work platforms, and metal building systems have also been added, along with applicable reference standards (Figure 2). References to several existing steel standards, including ANSI/AISC 360-22 Specification for Structural Steel Buildings, are also updated in the 2024 IBC. Not all editorial and renumbering changes are shown here.

2201.4 Connections. The design and installation of steel connections shall be in accordance with the applicable reference standards within this chapter. For special inspection of welding or installation of high-strength bolts, see Section 1705.2.

SECTION 2205 2202 STRUCTURAL STEEL AND COMPOSITE STRUCTURAL STEEL AND CONCRETE 2205.1 2202.1 General. The design, fabrication and erection of structural steel elements and composite structural steel and concrete elements in buildings, structures and portions thereof shall be in accordance with AISC 360.

SECTION 2201 GENERAL 2201.1 Scope. The provisions of this chapter govern the quality, design, fabrication and erection of steel construction. 2201.2 Identification. Identification of steel members shall be in accordance with the applicable referenced standards within this chapter. Other steel furnished for structural load-carrying purposes shall be identified for conformity to the ordered grade in accordance with the specified ASTM standard or other specification and the provisions of this chapter. Where the steel grade is not readily identifiable from marking and test records, the steel shall be tested to verify conformity to such standards.

(Reference to several steel standards, including ANSI/AISC 360-22, ANSI/AISC 341-22 and ANSI/AISC 358-22, are also updated in the 2024 IBC as reflected in IBC Chapter 35. The remainder of IBC Section 2202 includes editorial and renumbering changes and is not shown for brevity.) SECTION 2203 STRUCTURAL STAINLESS STEEL 2203.1 General. The design, manufacture and erection of austenitic and duplex structural stainless steel shall be in accordance with AISC 370. SECTION 2210 2204 COLD-FORMED STEEL

Figure 1. More information on the code changes can be found in the 2024 Significant Changes to the IBC available from the International Code Council.

2210.1 2204.1 General. The design of coldformed carbon and low-alloy steel structural members not covered in Sections 2206 through 2209 shall be in accordance with FEBRUARY 2024

SECTION 2208 STEEL DECK 2210.1.1 2208.1 Steel decks. The design and construction of cold-formed steel floor and roof decks and composite slabs of concrete and steel deck shall be in accordance with this section SDI SD. The design of cold-formed steel diaphragms shall be in accordance with additional provisions of AISI S310, as applicable. Figure 2. Standards ANSI/AISC 370-21, AISI S310-20w/S1-22, and SDI SD-2022 are now referenced in the 2024 IBC

AISI S100. The design of cold-formed stainless-steel structural members shall be in accordance with ASCE 8. Cold-formed steel light-frame construction shall comply with Section 2211. The design of cold-formed steel diaphragms shall be in accordance with additional provisions of AISI S310 as applicable. Where required, the seismic design of cold-formed steel structures shall be in accordance with the additional provisions of Section 2210.2 2204.2. 2210.2 2204.2 Seismic design requirements for cold-formed steel structures. The design and detailing of cold-formed steel seismic force-resisting systems shall be in accordance with Section 2204.2.1 and 2204.2.2 as applicable. Where a response modification coefficient, R, in accordance with ASCE 7, Table 12.2-1, is used for the design of cold-formed steel structures, the structures shall be designed and detailed in accordance with the requirements of AISI S100, ASCE 8, or, for cold-formed steel special-bolted moment frames, AISI S400. 2204.2.1 CFS special bolted moment frames. Where a response modification coefficient, R, in accordance with ASCE 7, Table 12.2-1, is used for the design of cold-formed steel special bolted moment frames, the structures shall be designed and detailed in accordance with the requirements of AISI S400. 2204.2.2 Cold-formed steel seismic force-resisting systems. The response modification coefficient, R, designated in ASCE 7, Table 12.2-1 for "Steel systems not specifically detailed for seismic resistance, excluding cantilever column systems" shall be permitted for systems designed and detailed in accordance with AISI S100. Such systems need not be designed and detailed in accordance with AISI S400. SECTION 2205 COLD-FORMED STAINLESS STEEL 2205.1 General. The design of cold-formed stainless steel structural members shall be in accordance with ASCE 8. SECTION 2211 2206 COLD-FORMED STEEL LIGHT-FRAME CONSTRUCTION 2206.3 Cutting and notching. The cutting and notching of holes in cold-formed steel framing members shall be in accordance with AISI S240 for structural members and AISI S220 for nonstructural members. (Additional changes to this section were renumbering only and are not shown for brevity.) 62 STRUCTURE magazine

(Previous Sections 2210.1.1.1 through 2210.1.1.3 have been deleted without substitution.) CHAPTER 35 REFERENCED STANDARDS American Institute of Steel Construction (AISC) ANSI/AISC 341-22 16 Seismic Provisions for Structural Steel ANSI/AISC 358-22 18 Prequalified Connections for Special and Intermediate Steel Moment Frames for Seismic Applications ANSI/AISC 360-22 16 Specification for Structural Steel Buildings ANSI/AISC 370-21 Specification for Structural Stainless Steel Buildings American Iron and Steel Institute (AISI) AISI S310-20 w/S1-22 North American Standard for the Design of Profiled Steel Diaphragm Panels, 2020 Edition, with Supplement 1, 2022 Edition Steel Deck Institute (SDI) SDI SD-2022 Standard for Steel Deck SDI NC—2017 Standard for Noncomposite Steel Floor Deck SDI RD—2017 Standard for Steel Roof Deck SDI-C—2017 Standard for Composite Steel Floor Deck—Slabs Change Significance: Based on the extensive reformatting of IBC Chapter 22, Table 1 provides a summary of revised section numbers and content. This article will provide additional discussion of new provisions added to IBC Sections 2210, 2211, 2212, and 2213. IBC Section 2203 introduces a new section on structural stainless steel with reference to the new ANSI/AISC 370, Specification for Structural Stainless Steel Buildings. This standard provides a uniform practice for the design of structural stainless steel-framed buildings and other structures. IBC Section 2208 references the new SDI steel deck (SD) standard. The three previous SDI steel deck standards (RD, NC, and C) were combined into a single standard that covers both roof and floor deck applications. This change removes the previously referenced SDI RD (Steel Roof Deck), NC (Non-Composite Steel Floor), and C (Composite Steel Floor) standards and substitutes the new combined steel deck standard. IBC Sections 2204 and 2208 on cold-formed steel members and decks include a new reference to AISI S310 with newly developed Supplement 1. This standard, first published in 2013, includes design provisions for diaphragms consisting of profiled steel decks or panels, which include fluted profiles and cellular deck profiles. The diaphragm may be installed with or without insulation between the panels and supports and may be supported by materials made of steel, wood, or concrete. Supplement 1 clarifies the design requirements for diaphragms filled with structural concrete used in the construction of floors. It also provides direction in calculating the resistance of perimeter

attachment fasteners, including headed studs, welds, screws, and proprietary fasteners. Cold-formed stainless-steel provisions are placed in IBC Section 2205 with reference to existing standard ASCE 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members. The ASCE 8-20 standard was previously referenced in the 2021 IBC Section 2210 on cold-formed steel.

Metal Building Systems (IBC 2210) New IBC Section 2210 includes provisions and an accompanying new definition to clarify design requirements for metal building systems (Figure 3). CHAPTER 2

Table 1. Summary of Revised Section Numbers and Content in 2024 IBC Chapter 22-Steel

2021 IBC Section 2201 General

Content added from other sections

2202 Identification of Steel for Structural Purposes

Content deleted and comparable language added to 2201.2

2203 Protection of Steel for Structural Purposes

Content deleted and comparable language added to 2201.3

2204 Connections

Content deleted and comparable language added to 2201.4

2205 Structural Steel

Renumbered as 2202 and renamed to capture composite structural steel and concrete

DEFINITIONS METAL BUILDING SYSTEM. An integrated set of fabricated components and assemblies that form a complete or partial building shell that is designed by the manufacturer. This system typically includes but is not limited to primary framing composed of built-up structural steel members, secondary members that are cold-formed steel or open-web steel joists, a metal panel roof system and exterior wall cladding. The system is manufactured in a manner that permits plant or field inspection prior to assembly or erection. SECTION 2210 METAL BUILDING SYSTEMS 2210.1 General. The design, fabrication and erection of a metal building system shall be in accordance with the additional provisions of this section. 2210.1.1 Design. The design of metal building systems shall be in accordance with Sections 2210.1.1.1 through 2210.1.1.4, as applicable.

2206 Composite Structural Steel Content deleted and enabling language and Concrete Structures added to Section 2202

New Section 2203 Structural Stainless Steel created New Section 2205 Cold-Formed Stainless Steel created 2207 Steel Joists

No section number change

2208 Steel Cable Structures

Renumbered as 2214

2209 Steel Storage Racks

No section number change

2210 Cold-Formed Steel

Renumbered as 2204; a new Section 2208 Steel Deck was created

2211 Cold-Formed Steel LightFrame Construction

Renumbered as 2206

2210.1.1.1 Structural steel. The design, fabrication and erection of structural steel shall be in accordance with Section 2202. 2210.1.1.2 Cold-formed steel. The design of cold-formed carbon and low-alloy steel structural members shall be in

Summary of Section and Content Changes

New Section 2210 Metal Building Systems created New Section 2211 Industrial Boltless Steel Shelving created New Section 2212 Industrial Steel Work Platforms created New Section 2213 Stairs, Ladders, and Guarding for Steel Storage Racks and Industrial Steel Work Platforms created

Figure 3. The IBC defines a metal building system as an integrated set of fabricated components and assemblies that form a partial building shell that is designed by the manufacturer.

FEBRUARY 2024

accordance with Section 2204. 2210.1.1.3 Steel joists. The design of steel joists shall be in accordance with Section 2207. 2210.1.1.4 Steel cable. The design, fabrication and erection of steel cables, including related connections, shall be in accordance with Section 2214.

Figure 4. Industrial boltless steel shelving is covered by Section 2211 of the IBC. (Photo courtesy of A&A Boltless Rack and Shelving)

2210.2 Seismic design. Where required, the seismic design, fabrication and erection of the structural steel seismic force-resisting system shall be in accordance with Section 2202.2.1 or 2202.2.2, as applicable.

Change Significance: New IBC Section 2211 references standard ANSI/MH 28.2, developed by the Storage Manufacturers Association (SMA) of the Material Handling Institute (MHI) for the design, testing, and utilization of industrial boltless steel shelving with assistance from the Federal Emergency Management Agency (FEMA) Seismic Code Support Committee. This standard applies to boltless shelving placed on mobile carriages; multi-level boltless shelving systems, such as pick modules, catwalks, and deck-overs; and boltless shelving used in conjunction with an automated storage and retrieval

Change Significance: Metal building systems are significantly different from other forms of steel construction, especially regarding the shared design responsibilities between the metal building system manufacturer and registered design professional for the project. Metal building systems often contain assemblies made of a variety of components, such as structural steel, cold-formed steel, and steel cables. These systems are typically highly optimized structures that are heavily dependent on bracing components to function as designed. Some bracing components consist of materials that are not considered to be "structural steel." This addition to IBC Chapter 22 and the accompanying new definition clarifies the design requirements for metal building systems. Many construction documents still use the nonexistent Metal Building Manufacturers Association standards as the governing design requirements. Adding specific referenced standards for the components of a metal building leads to accurate construction documents and improved construction practices. By clarifying the design requirements for different metal building system parts, the special inspection requirements are improved. New IBC Section 1705.2.6 provides special inspection provisions for these systems (see STRUCTURE December 2023 for more information).

system (Figure 4). The structural framing components for these systems are made of cold-formed or hot-rolled steel structural members. ANSI/MH 28.2 does not apply to industrial steel pallet racks (addressed by ANSI/MH 16.1), industrial cantilever racks (addressed by ANSI/MH 16.3), boltless shelving structures not fabricated from steel, industrial steel bin shelving, or shelving systems built with slotted metal angles. Boltless shelving is typically a prefabricated, free-standing, non-building structure that utilizes a designed framing system. It is generally located within an industrial or warehouse environment that does not allow access by the general public. Personnel working within the confines of spaces with boltless shelving structures are presumed to be properly trained, physically able, and appropriately attired for the intended working environment. ANSI/MH 28.2 applies to boltless shelving structures installed within a building and potentially subjected to seismic loads. Loads from other environmental exposures, such as snow, wind, or rain loads, are not addressed in this standard. ANSI/MH 28.2 does not cover any design requirements that need to be addressed for supported equipment that would subject a shelving system to significant dynamic loading or harmonic vibration that has the potential to cause structural damage or metal fatigue. For applications beyond the scope of ANSI/MH 28.2 evaluation by a qualified design professional is required.

Industrial Boltless Steel Shelving (IBC 2211)

Industrial Steel Work Platforms (IBC 2212)

New IBC Section 2211 references standard ANSI/MH 28.2, which applies to industrial boltless steel shelving.

New IBC Section 2212 references standard ANSI/MH 28.3, which applies to industrial steel work platforms.

SECTION 2211 INDUSTRIAL BOLTLESS STEEL SHELVING

SECTION 2212 INDUSTRIAL STEEL WORK PLATFORMS

2211.1 General. The design, testing and utilization of industrial boltless steel shelving shall be in accordance with MHI ANSI/MH 28.2. Where required by ASCE 7, the seismic design of industrial boltless steel shelving shall be in accordance with Chapter 15 of ASCE 7.

2212.1 General. The design, testing and utilization of industrial steel work platforms shall be in accordance with MHI ANSI/MH 28.3. Where required by ASCE 7, the seismic design of industrial steel work platforms shall be in accordance with Chapter 15 of ASCE 7.

CHAPTER 35 REFERENCED STANDARDS

Material Handling Institute (MHI) ANSI/MH 28.2-2022 Design, Testing and Utilization of Industrial Boltless Steel Shelving

Material Handling Institute (MHI) ANSI/MH 28.3-22 Design, Testing and Utilization of Industrial Steel Work Platforms

64 STRUCTURE magazine

Change Significance: New IBC building code provisions that apply Section 2212 references standard to employee access ways serving variANSI/MH 28.3, developed by the ous material handling structures. SMA for the design, testing, and This information has been incorutilization of industrial steel work porated into the newly referenced platforms with assistance from standard ANSI/MH 32.1. This stanthe FEMA Seismic Code Support dard applies to fixed stairways and Committee. ladders along with guards for elevated An industrial steel work platform platforms used in material handling is typically a prefabricated freestructures. The stairways and ladders standing non-building structure are attached to structures such as with an elevated surface that utiracking pick modules, shelving pick lizes a pre-designed framing system modules, decked-over platforms, and and is located within an industrial free-standing work platforms. Newly or similarly restricted environment referenced standard ANSI/MH 28.3 (Figure 5). Flooring may include applies to the design, testing, and other structural or non-structural utilization of industrial steel work elements such as concrete, steel, and platforms (see Section 2212 in this Figure 5. Industrial steel work platforms are covered under Section 2212 of engineered wood products. article for background). the IBC. ANSI/MH 28.3 does not apply to Not all stairs, ladders and guards platforms whose structural framing for steel-framed platforms and work components are not made from steel. This standard is based on areas are governed by ANSI/MH 32.1. Application is limited to the work platform being restricted from general public use, and a very specialized subset of stairs, ladders, and guards for "industhose working on the work platform are properly trained, appro- trial steel work platforms used in material handling structures" priately dressed, and physically able to work on such platforms within its scope. For example, consider a steel-framed platform in an industrial or warehouse environment. used to service HVAC equipment in a factory. The guards on the ANSI/MH 28.3 applies to work platforms potentially subjected HVAC access platform have to comply with IBC Section 1607.9, to seismic loads. Loads from other environmental exposures, such which covers loads on handrails and guards. The stairs or ladder as snow, wind, or rain loads, are not addressed in this standard. used to access the HVAC platform also have to comply with the For applications beyond the scope of ANSI/MH 28.3, evaluation structural and architectural requirements in the IBC, not ANSI/ by a qualified design professional is required. MH 32.1. Extending the stair, ladder, and guard provisions of ANSI/MH 32.1 standard to all "industrial steel work platforms" is not appropriate.

Stairs, Ladders and Guarding for Steel Storage Racks and Industrial Steel Work Platforms (IBC 2213)

New IBC Section 2213 references standard ANSI/MH 32.1, which applies to fixed stairways and ladders along with guards for elevated platforms used in material handling structures. SECTION 2213 STAIRS, LADDERS AND GUARDING FOR STEEL STORAGE RACKS AND INDUSTRIAL STEEL WORK PLATFORMS 2213.1 General. The design and installation of stairs, ladders and guarding serving steel storage racks and industrial steel work platforms shall be in accordance with MHI ANSI/ MH 32.1. CHAPTER 35 REFERENCED STANDARDS Material Handling Institute (MHI) ANSI/MH 32.1-2018 Stairs, Ladders and Open-Edge Guards for Use with Material Handling Structures Change Significance: The Rack Manufacturers Institute (RMI) and SMA, two product groups of MHI, have compared and compiled Occupational Safety and Health Administration (OSHA) and

Conclusion Structural engineers should be aware of significant structural changes in the 2024 IBC Chapter 22 for steel. Changes include an editorial reorganization of the chapter for better flow, usability, and clarification of steel provisions in the building code. New provisions for structural stainless steel, cold-formed profiled steel diaphragm panels, steel decks, industrial boltless steel shelving, steel work platforms, and metal building systems have also been added along with applicable reference standards. Reference to several existing steel standards, including ANSI/AISC 360-22 for structural steel, are updated. New IBC Section 2210 includes provisions and an accompanying definition to clarify design requirements for metal building systems. New IBC Section 2211 references standard ANSI/MH 28.2, which applies to industrial boltless steel shelving. New IBC Section 2212 references standard ANSI/MH 28.3, which applies to industrial steel work platforms. Finally, the new IBC Section 2213 references standard ANSI/MH 32.1, which applies to fixed stairways and ladders along with guards for elevated platforms used in material handling structures.■

John “Buddy” Showalter, P. E., (bshowalter@iccsafe.org) is Senior Staff Engineer of ICC’s Consulting Group. Sandra Hyde, P. E., (shyde@iccsafe.org) is Managing Director of ICC’s Consulting Group. FEBRUARY 2024

INSIGHTS Sustainable Steel 101 An introduction to sustainability for structural engineers using steel. By Steve Rys, P.E.

CAs, EPDs, GWP … many engineers attempt to dive into the world of sustainability and find themselves swimming in a sea of acronyms. While a designer might wish—or need— to incorporate sustainability into projects, the lack of concise background information can leave someone new to sustainability feeling quite intimidated. Rest assured that integrating carbon reduction into designs can be as simple as adding a couple of columns to an existing design spreadsheet. Carbon emissions, such as CO2 and other greenhouse gases, directly associated with the building and construction sectors are responsible for about 40% of the annual emissions generated globally [1]. Digging deeper, the iron and steelmaking industry is responsible for 7-8% of annual carbon emissions produced globally [2]. Roughly 50% of all steel produced serves the construction (building and infrastructure) sector [3], so structural engineers have a direct influence on about 4% of global carbon emissions when using steel. Several state and local governments are exploring such laws as Buy Clean legislation to impose carbon limits on building products during procurement. Carbon criteria have also begun to be included in private development projects. Therefore, it is imperative for structural engineers to become familiar with sustainable design procedures and strive to reduce embodied carbon on their projects.

What Is Embodied Carbon? For structural engineers, the primary focus in the realm of sustainability is on embodied carbon. Embodied carbon is the measure of CO2 and other greenhouse gases emitted into the atmosphere that can be attributed to a building’s materials and its construction. Activities in this category include the extraction of raw materials, refinement and manufacturing of materials into products, transportation of those refined products, construction of the structure, and the deconstruction and disposal of materials

Figure 1. Life Cycle Stages of a Building. Courtesy: Nucor Corp.

66 STRUCTURE magazine

at the end of a structure’s life cycle (Figure 1).

Measuring Embodied Carbon The world of sustainability has many complexities; thankfully, the measurement of embodied carbon is not one of them: Embodied Carbon = Material Quantity x Carbon Factor

This calculation is performed for each building material to be used (wide flange steel, metal decking, concrete, rebar, etc.) and then summed to attain a total embodied carbon value for the design. Engineers determine material quantities for their structures (i.e., cubic yards of concrete or tons of steel) through typical design procedures. The carbon factor, often referred to as carbon intensity or emission intensity, is a multiplier providing the carbon emissions produced per unit of material quantity to measure the potential impact on the environment. Global Warming Potential (GWP) is the metric typically used to measure a product’s carbon footprint. GWP includes CO2 as well as any other gas emitted that has the potential to increase the earth’s mean temperature. The quantity and impact of gases are scaled relative to CO2, combined, and quantified in units of kilograms of CO2 equivalent (kg CO2e).

Environmental Product Declarations GWP values are found in Environmental Product Declarations (EPDs). EPDs are third-party verified reports that transparently document the environmental impact of a building product over its life cycle. The EPDs must conform to a set of accounting rules known as Product Category Rules (PCR) and ISO standards. Care should be taken to ensure proper ISO standards are met, and PCRs are matching when comparing materials or steel from other countries outside of the United States to have an equal EPD comparison. EPDs typically cover impacts from the product stage (A1-A3 per Figure 1), which are referred to as “cradle-to-gate.” It is important to understand the boundaries or stages covered by each EPD as it can vary, such as the difference between EPDs for “fabricated” versus “unfabricated” steel products. An EPD also documents various environmental impact categories, such as Ozone Depletion Potential (ODP) or Acidification Potential (AP), as well as energy consumption and waste. In simpler terms, the EPD is analogous to a nutrition label and documents the product’s impact on the environment instead of food to one’s body. The material quantity parallels the serving size, and the GWP corresponds to calories per serving. The additional impact categories can be thought of as the various macronutrients. The two primary types of EPDs are industry-wide EPDs (IW-EPDs) and manufacturer EPDs. Manufacturer EPDs are also sometimes referred to as product EPDs. Industry-wide EPDs are produced by industry organizations such as AISC or SDI that provide GWP values representing a weighted average over a representative sample of suppliers for a given type of product. Manufacturer EPDs are produced by an individual supplier for a specific product. If a supplier produces the same product type at multiple facilities, the supplier may elect to establish separate sitespecific EPDs or list the GWP for each facility under one EPD.

Sustainable Structural Design Considerations Initial Design and Material Selection The first step of integrating carbon reduction into structural design is to choose the most appropriate material or product (ex., wide flange v. HSS) for the given project or application. There is no single material or product considered to be the most sustainable. Thus, multiple options and hybrid solutions should be considered. Engineers should calculate embodied carbon using each respective material quantity and associated GWP for the designs being considered. It is recommended that engineers use

industry-wide GWP values in the early phases of design since product sources will not be known until a project’s procurement team purchases the material. Keep in mind that the design with the lowest material quantity or the product with the lowest GWP may not always be the most sustainable option. Always consider embodied carbon when designing for sustainability, not just material quantity or GWP!

Steel Sourcing Considerations Steel manufacturing can be subdivided into two principal processes: “extractive” blast furnace-basic oxygen furnace (BF-BOF) production and “circular” electric arc furnace (EAF) production. Powered by coal and fossil fuels, BOF mills extract and use raw materials (primarily iron ore, coal, and limestone) and up to 30% recycled scrap metal to produce steel. By contrast, EAF mills use electricity to melt down and recycle scrap metal (up to 99%+ recycled content) and direct-reduced iron (DRI) to repurpose old metals and make new steel.

Figure 2. According to the 2021 EAF & BOF Steel Production Metrics, steel produced in a BOF facility emits 3.5 times as much carbon into the atmosphere on average than a facility producing EAF steel. Courtesy: The World Steel Association.

EAF steel is a much more efficient and sustainable means of producing steel. Per the World Steel Association, every ton of steel produced in a BOF facility emits an average of 3.5 times more carbon into the atmosphere compared to an EAF facility (Figure 2). Therefore, any project striving to be more sustainable must prioritize the use of EAF-produced steel. It is important to note that different steel products have different ranges of GWP values (Figure 3). This is closely related to the percentage of recycled content of which the product is composed. Thinner steels, such as sheet mill products, including hollow structural sections and metal decking, require more

KEEP IN MIND The design with the lowest material quantity or the product with the lowest GWP may not always be the most sustainable option. Always consider embodied carbon when designing for sustainability, not just material quantity or GWP. Figure 3. GWP (mt CO2eq / mt) for Steel Products, per IW-EPDs. Courtesy: SDI, AISC, CRSI. FEBRUARY 2024

virgin iron and alloys relative to scrap metal in order to maintain surface quality during the roll-forming process. Products with reduced surface quality concerns, such as rebar, can have nearly 100% recycled content.

should provide the specific facility GWP values from which the product is to be procured. Step 2: Establish maximum GWP limits as material procurement criteria for each product. Products exceeding GWP thresholds may not be procured. Design Optimization • It is recommended to allow for some percentage (5-15%) of nonconformCarbon reductions can be further ing material to provide purchasing DID YOU KNOW? All wide flange achieved through conventional flexibility in these early stages of sussections produced in the United States design refinement, the exploration tainability documentation. of new technologies, and the use of • Many specifications currently are made in EAF mills, and only five W high-strength materials. There are indicate minimum recycled content sections in the AISC steel manual are established and emerging structural percentages. These requirements are not currently domestically produced? technologies in the marketplace that generally quite conservative and Contact your steel suppliers to can facilitate a sustainable project. not overly impactful. Specification understand section availability. Reach out to suppliers directly to writers may require the submisinquire about availability and applision of recycled content letters but cability to your project. should ultimately allow the GWP Here are a few easy-to-implement requirements to govern procuresolutions: ment decisions. • Grade 65 Wide Flange Sections (ASTM A913): Grade 65 struc• If any new materials or proprietary technologies are used in the tural steel should be the new normal for strength-controlled design, they must be added to the specifications, and General elements such as columns and truss elements. A 10-25% tonnage Notes drawings as allowable or required materials. reduction can be expected for only a slight cost premium from the mill and no difference in lead time compared to traditional A992. In addition, there are enhanced weldability characteristics inherent Sustainable Design Example to ASTM A913 Grade 65 steel. Also, inquire about the availability of Grades 70 and 80, as they are emerging in the market. The graphic on the opposite page shows a short design case study that • Grade 80 Rebar (ASTM A615 or A706): Explore replacing typical illustrates how easily embodied carbon reductions can be achieved. Grade 60 rebar in vertical or direct tension elements (columns, shear walls, etc.) for larger diameter bars (typically #10-#11). In addition to tonnage reductions of over 5-15%, horizontal Life Cycle Assessments tie requirements and congestion can be reduced from those of Grade 60. Life cycle assessments (LCAs) take simple embodied carbon calculations • Design with a yield strength of 50 ksi and specify ASTM A500 to the next level. Embodied carbon calculations consider the GWP for Grade C for all HSS sections (rectangular/square and round). Do products used in the construction of a structure. LCAs, on the other hand, not specify pipe (ASTM A53, Fy = 35 ksi) sections unless required. consider the GWP for the manufacturing of the products, construction • Design with a yield strength of 50 ksi for all angles and channels. methods, maintenance and service life factors, and end-of-life scenarios. Thankfully, there are many tools at an engineer’s disposal to measure the Specification Updates and Selective Procurement impacts of building materials on the environment. Many of these tools are free, plug into software that engineers use daily, and provide detailed Selectively procuring more sustainable materials is another layer of impact breakdowns of buildings—all at the click of a button. Whole embodied carbon refinement. Designers can implement the use of Building LCAs are the 201 level to the 101 embodied carbon calculations. sustainable materials on a project by updating and expanding project By assessing a building over its entire life cycle, more design considerations specifications. Below are recommended changes to project specifica- and time-dependent scheme comparisons can be unlocked. tions that can increase the sustainable qualities of a project: Step 1: Require the submission of manufacturer EPDs for each product (wide flange, HSS, plate, decking, rebar, etc.) as formal submittals Conclusion to be reviewed by the design team. This Structural engineers have the opportunity to make a positive, dramatic will allow designimpact on the greenhouse gases emitted across the globe. In the not-soers to refine their distant future, embodied carbon will be considered equally important embodied carbon to cost and constructability to align with new legislation and project calculations beyond requirements. Armed with baseline sustainability knowledge and the industry-wide GWP aforementioned recommendations, structural engineers can effectively assumptions and reduce the carbon footprint of their structures. ■ allow the procurement teams to review Steve Rys, PE, has over a decade of structural design and construction experience GWP information if and now serves the New York City AEC community as a mill representative for required per Step 2. Designers can update and expand specifications to increase Nucor, North America’s largest steel producer. (steve.rys@nucor.com) The EPDs submitted the use of sustainable materials. Courtesy: Nucor Corp. 68 STRUCTURE magazine

Sustainable Design Example A 16% material quantity reduction was realized by optimizing the original design using A913 Grade 65 wide flange sections. An 18% GWP reduction versus the AISC industry average value was achieved by selectively procuring material from a facility in Arkansas with published EPDs. The product of those respective reductions resulted in a 31% embodied carbon savings.

Courtesy: Nucor Corp.

FEBRUARY 2024

historic STRUCTURES Exploring Transition Buildings and Construction

‘Transition buildings’ was a term coined by post-transition historians that described the period when steel began to be utilized. By Jeffrey S. Barnes, and Craig E. Barnes, P.E., S.E.

he term “transition building” was coined by post-transition historians. It derives from builders of that period utilizing steel to create greater building heights and floor spans. The word “skyscraper” had its origin in either New York or Chicago, but who holds bragging rights for the oldest skyscraper? Research suggests the oldest skyscraper is the 150-foot-tall Temple Court Building located in Manhattan and completed in 1883, while others declare Chicago had the first skyscraper—the Home Insurance Building, at 138 feet high, completed in 1885. Regardless of whether it is New York or Chicago for the home of the earliest skyscraper, one thing is certain—skyscrapers addressed a country that started booming socioeconomically and provided the need for more space. Buildings transitioned from low-rise to high-rise seemingly overnight, and the technology had to keep up with the demand. Low-rise masonry structures grew to high-rise masonry structures that were load-bearing and could only be erected as quickly as the floors were constructed. In other words, the pace of building construction was, at times, directly linked to the pace of wall and floor construction. Thus, in colder climates such as Chicago, New York, and Boston, a high-rise could take 2 to 3 years to complete due to winter conditions. This was a costly and time-consuming process. Imagine being the owner of a 10-story masonry building and watching it take shape laboriously over a period of 2 to 3 years, maybe close to 4 years, before you can realize any return on investment. But it was not just labor costs and time costs that drove technological advances: logistics also became a factor—material production (casting iron, firing bricks), fabrication (cutting steel to length, punching rivet holes), shipping, hauling (materials from railroad terminal to site) and placement (steel erection and bricklaying). In Boston, cement had to be brought in by train, which would add to the cost. Brick and stone required a large amount of space to fire and store prior to being used. As a result, that high-rise with a small footprint might have required 10 times its footprint of laydown space to be able to erect that high-rise, which added to the cost. As demand for more space increased, the demand for the high-rise also increased, meaning more loads were introduced to the structure. As a result, masonry walls could no longer bear any more loads past a certain height. This is another reason why the steel frame came into play. By the 1890s, three types of building technologies were available: bearing walls, cage frames (see sidebar), and skeleton frames. The bearing-wall building has thicker walls at the bottom and thinner walls at the higher sections of the building. This framing required interior masonry walls to support the floors above. The cage-framed building was a hybrid where the masonry walls provided lateral resistance to wind, and the steel frame directed gravity loads to the ground. The masonry walls in cage-framed buildings were slightly thinner than in 70 STRUCTURE magazine

Photo by Epicgenius - Own work, CC BY-SA 4.0, https://commons.wikimedia.org/w/index.php?curid=110106849

One of the earliest skyscrapers, the Temple Court Building, built in 1883 at 5 Beekman Street in New York, today is interconnected with a 51-story condominium tower. By the 1890s, steel frames began to be used in high-rise construction.

bearing-wall buildings. Some interior masonry walls required for cage framing were replaced with cast iron columns. The skeleton-framed building had connecting steel beams and columns that supported the gravity and lateral loads; the masonry walls did not contribute much structurally and remained at a constant thickness from top to bottom. This concept is the precursor to today’s high-rise buildings. Donald Friedman, president of Old Structures Engineering and a structural consultant in historic and old buildings, created a model to illustrate typical costs of the three types of building technologies available in the late 1890s. The models are 13 stories and do not include interior finishes, stairs, windows, or elevators; they are just the basic structure. Based on data for construction at that time, a FEBRUARY 2024

Cage Frame Defined

Cage frame was a term created to be able to illustrate the two types of building systems known at that time that combined – cage and skeleton. The term “cage” is not used to describe buildings today. “Structural Systems of Early Skyscrapers: The Case for New York,” Friedman, Donald. April 24, 2012, should be consulted. bearing-wall building may contain 4,900 tons of brick and 180 tons of steel for a bare structural cost of $122,000. A cage-framed building may require 2,600 tons of brick, 82 tons of cast iron, and 250 tons of steel for a bare structural cost of $80,000. A skeleton-framed building may need 1,800 tons of brick and 520 tons of steel for a bare structural cost of $74,000. As a building owner, opting for the more cost-efficient technology is a no-brainer and allows for a building with greater capacity than a bearing wall structure. As the economy transitioned and began to grow, high-rise structures did, as well. These thinner buildings with smaller footprints allowed for a great number to be erected in a shorter amount of time and to greater heights as the country underwent rapid expansion, doubling the total cost of construction from $31.5 million in 1860 to $76.0 million in 1890. Builders were also transitioning in different ways as the building industry began to understand the benefits of rolled steel wide-flange sections, with a greater variety of shapes being provided.

Metals Used in Construction Often confused but often not the same are wrought iron, cast iron, and steel. Wrought iron, with less than 0.1% carbon (mild steel) and less than 0.36% carbon (hard steel), is a commercial form of iron that is tough, malleable, and relatively soft for easy tooling. This form of wrought iron is often used in non-structural items such as tables and chairs. Cast iron, with a chemical content ranging between 2% -4% carbon and 1% -3% silica, is iron that has been melted, poured into a mold, and allowed to solidify. Cast iron, partially because of its poor tensile strength, is found more often in transitional structures as structural columns and cast as pintles. Pintles are frequently found in heavy timber construction, where they are used to transfer timber column loads at floor/ column joints. In addition, cast iron is very brittle, which makes it hard to shape or machine. Structural steel is an alloy or a combination of iron and other alloys such as carbon, manganese, STRUCTURE magazine

nickel, copper, etc. To make the desired grade of steel, typically less than 2% carbon is added to iron, and the rest is made up of many other elements. When onsite, if you have any doubts about which material is present, strike the metals with a grinder, and the change in spark quantity will be the giveaway—the more spark, the higher the carbon content. All iron ore-based products are produced through the process of smelting. The smelting process requires iron ore (of which there are different grades found in nature), coke, and limestone heated to melt the metal out of the ore. Young engineers should become familiar with the process. A familiar product that results from the smelting process is slag. Although largely a waste product, slag, which contains metal oxides and silicon, is improving its image as it is being used in a variety of highway products, including additives to concrete. Production smelting that engineers are familiar with originated in two- or more-story high vertical blast smelters that were shaped like an open-topped milk bottle and lined with refractory material (that resists high heat). The more efficient Bessemer converter superseded the vertical blast furnace, a larger, more rounded metal container lined with refractory material. This was replaced by the more efficient electric arc furnace, first utilized in France in 1907. In the late 1800s and early 1900s, there were many small steel fabricators producing structural steel sections. These fabricators produced their own catalogs of available structural sections (Figure 1). Examples of these early fabricator catalogs are Wrought Iron & Steel 1884, Pencoyd Iron Works 1891, Carnegie, Phipps & Co.-1892, Bethlehem Steel pamphlet 1907, and the Bethlehem Steel blue book 1911. Another interesting tidbit is that steel at that time could have a 50 kips per square inch (ksi) ultimate tension strength and an Elastic Modulus of 30 ksi. Uniform safety factors were not universally accepted. One of the catalogs recommended railroad bridge members be designed to 1/5 of the ultimate steel value, highway bridges to 1/4 of the ultimate strength value, and roof trusses to 1/2 of the ultimate steel value. In conclusion, this article barely scratches the surface of what transition buildings are and hopefully has interested the reader to continue their discovery. ■ Jeffrey S. Barnes is the Principal of Barnes Consulting Group LLC and may be contacted at jeff@barnes-consulting. com. Craig E. Barnes, P.E., S.E., is also a Principal of Barnes Consulting Group LLC. (craig@barnes-consulting.com) Figure 1. Fabricators from the late 1800s and early 1900s provided catalogs of available structural sections, which included (a) zees, (b) channels, (c) I-beams, (d) tees, (e) angles, and (f) individual pieces to act as composites for metal deck. Individual fabricators of the day also offered non-standard pieces that builders could commission for the particular project. A question mark (g) suggested those items were manufactured in particular shapes and sizes and made to a particular specification. FEBRUARY 2024

business PRACTICES

Strategies for Navigating the Engineering Talent Drought

Discover cutting-edge strategies, from revolutionizing recruitment to redefining retention and upskilling for success, as we navigate a landscape where innovation is not just a goal but the key to a brighter, more innovative future in the architecture, engineering, and construction industry. By Eric Stern and Mandi Spindler

echnology is the driving force behind innovation in today’s evolving architecture, engineering, and construction industry, and securing top talent is crucial in the talent drought. To stay ahead in this competitive market, engineering firms must explore innovative approaches to talent acquisition and retention, like cultivating strong employer brands that resonate with prospective candidates and investing in professional development to meet industry demands. The engineering labor shortage is not an industry concern but a societal one. The success of critical infrastructure development, our energy future, and economic growth depend on attracting and developing engineers. Below are strategies for organizations to not only endure the labor shortage but thrive by recruiting, retaining, and developing top talent.

Recruitment Revolution Recruiting top engineering talent beyond traditional methods is the initial step in addressing the talent shortage. Engineering firms in today’s competitive job market must establish compelling reputations as employers of choice that resonate with prospective employees beyond attractive compensation and benefits packages. Successful employers are shifting their focus toward employee wellbeing and flexibility in response to the evolving workforce's demands and turnover. Employees now expect firms to provide benefits such as technology for flexible work arrangements and access to mental health resources. Empowering employees to balance work and personal life is key to avoiding burnout and prioritizing employees as people. 72 STRUCTURE magazine

Allowing employees to manage their work arrangements makes people feel valued, builds trust, and encourages free thinking. When employees have the freedom to experiment with different work styles, they can explore creative solutions to challenges. A culture of innovation provides employees the time and resources to think creatively. Organizations can create a culture where employees advance, and the business prospers by placing trust at the core of flexible work strategies.

Retention Re-Defined Retaining engineering top talent must be an ongoing strategy, not a one-and-done effort. Skilled engineers are in high demand, so retention is as critical as recruiting. Organizations must balance attracting new talent, providing professional growth and advancement opportunities, and keeping team members engaged to maintain a competitive edge today. Effective retention begins with a purpose-driven workplace culture, including a clear purpose statement and values that inspire and guide employees to align their contributions with the firm’s greater objectives. In doing so, organizations can create a compelling work environment that retains talent, empowers ethical decision-making, drives innovation, enhances reputation, and ensures long-term sustainability. Nurturing a connected workplace where employees feel heard and valued is critical for engagement and retention. Companies can focus actions on employee feedback by providing platforms for connection, input, and inclusion. Employee resource groups (ERGs), supportive leadership, and two-way communication are pivotal in fostering belonging. Employees are more likely to bring their whole selves to work, actively contribute,

and feel a sense of belonging in companies that value diversity and inclusion—increasing employee engagement, retention, and satisfaction. Engineers shine in innovative environments. Successful firms encourage and support engineers to explore ground-breaking projects and technologies. Adopting emerging technologies leverages innovation to solve industry-wide challenges, streamlines daily workloads, and empowers engineers to prioritize better — helping organizations stay competitive and agile in an ever-changing world.

Up-Skilling for Success Upskilling, or teaching employees skills beyond their present role, is vital for employees and employers as the industry and technology evolve. Engineering firms can address skill gaps and foster ongoing growth by investing in professional development. This commitment to personal growth benefits individual employees and the organization, ensuring it remains adaptable in the face of change. One innovative approach to upskilling is encouraging software skills development for existing employees who could apply these skills in their current roles. Individuals with diverse skill sets and experience levels become a powerful force for internal innovation and growth. Seasoned software and technology-focused employees can mentor and train others, instill an understanding of automation, and identify future opportunities for advancement and innovation. Training current automation-skilled engineers in basic software development can yield fantastic results, both from a return-on-investment perspective as well as when looking to drive innovation. Organizations that strategically invest in training and development to bridge this gap

will find success and long-term retention by creating a scalable approach to innovation and technology adoption. Engineers are not only responsible for designing and implementing innovative solutions but also for leading teams, making strategic decisions, and effectively managing resources. Comprehensive training in leadership and business management is crucial for engineers to thrive as technical experts and effective leaders and drive success in their organizations. A solid understanding of business management is vital for engineers to grow in their roles and ensure projects are executed properly, within budget, and aligned with organizational goals. In a landscape characterized by fierce competition, engineering firms must think beyond traditional strategies and instead invest in purpose and innovation to resonate with prospective candidates and retain top talent. The path ahead is clear: organizations must adapt and prioritize the well-being and professional growth of their engineers. By doing so, they are not only addressing a talent shortage but also advancing technology. In this industry where the pace of change is relentless, those who invest in their talent will thrive. Together, we embark on a journey to bridge the gap between talent and opportunity, and in doing so, we pave the way for a brighter, more innovative future.■ Eric Stern serves as Technology Innovation Director at Ulteig Engineering in Denver, Colorado. He leads innovation and emerging technology applications across the firm’s Power, Renewables, Transportation and Water engineering practices (eric. stern@ulteig.com) Mandi Spindler serves as the Talent Director for Ulteig Engineers in St. Paul, Minnesota. In her role, she leads the talent acquisition, talent management, DE&I, and Culture functions (mandi.spindler@ulteig.com).

STEEL guide NCSEA

Phone: 312-649-4600 Email: ncsea@ncsea.com Web: www.ncsea.com/education/sereview Product: SE Refresher and Exam Review Course Description: NCSEA's SE Review Course is completely on-demand and allows you to study at your pace but with instant access to the material and instructors. Vertical and lateral courses available cover several topics, including structural steel. Review course materials and watch the recordings when it is convenient for YOU for an entire year.

RISA Technologies

Phone:949-951-5815 Email: info@risa.com Web: risa.com Product: RISA 3-D Description: Whether designing custom hot rolled steel shapes or cold-formed steel wall panels, RISA-3D has you covered. With steel databases and design codes from all over the world, you'll never have to look anywhere else.

ENERCALC, LLC

Phone: 800-424-2252 Email: info@enercalc.com Web: https://enercalc.com Product: ENERCALC SEL / ENERCALC 3D Description: Save hours on every steel design with ENERCALC—now with FEM capabilities. Beams, columns, two dimensional frames, force distribution in bolt groups and more. The clear, simple user interface makes it fast & easy to setup, confirm & “what-if” your designs. Member optimization improves your efficiency and saves time!

Not listed? Monthly 2024 Resource Guide forms are available on our website. STRUCTUREmag.org

FEBRUARY 2024

NCSEA News Diversity in Structural Engineering Scholarship Program Open for 2024 Applications

he Diversity in Structural Engineering Scholarship Program was established by the NCSEA Foundation to award funds to structural engineering students who identify as historically underrecognized (including but not limited to Black/African Americans, Native/Indigenous Americans, Hispanics/Latinos or Spanish, Asian, Native Hawaiian or Pacific Islander, and other people of color, those with disabilities, veterans and LGBTQIA+).Scholarships are presented annually to junior college students, undergraduate students, and/or graduate students pursuing degrees in structural engineering. The Diversity in Structural Engineering Scholarship Program has awarded more than $60,000 to scholarship recipients since the program’s inception in 2021. In 2023, the NCSEA Foundation granted ten scholarships, which comprised four newly established named scholarships within the program: The Applied Technology Council Diversity in Structural Engineering Scholarship, The Degenkolb Engineers Diversity in Structural Engineering Scholarship, The Martin/Martin Skyrise Scholarship, and The Steven B. Tipping Memorial Scholarship for Innovation and Excellence in Structural Engineering, along with the SEA-MW Diversity in Structural Engineering Scholarship. As we begin the 2024 scholarship season, five groups have generously provided funds to create even more opportunities for aspiring structural engineers in the form of named scholarships. This year, the program will be awarding more than $60,000 to scholarship recipients. Applications are due on March 15, 2024. More information, including the application form and each partner scholarship, can be found here: http://www.ncsea.com/about/foundation/diversityscholarship/. Partner scholarships in addition to those awarded by the NCSEA Foundation, are as follows:

Applied Technology Council Diversity in Structural Engineering Scholarship—$3,000

Degenkolb Engineers Diversity in Structural Engineering Scholarship—$5,000 ($2,500 per semester)

SEA-MW (Metropolitan Washington) Diversity in Structural Engineering Scholarship—$3,000

SEAONC (Northern California) Diversity in Structural Engineering Scholarship—$3,000

Forell | Elsesser Engineers Scholarship—$3,000 SEAoNY (New York) Diversity in Structural Engineering Scholarship—$3,000 Holmes Diversity in Structural Engineering Scholarships—two at $5,000

Wood Solutions Scholarships—four at $5,000 Martin/Martin Skyrise Scholarship—$5,000

Steven B. Tipping Memorial Scholarship for Innovation and Excellence in Structural Engineering—$3,500

follow @NCSEA on social media for the latest news & events! 74 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

Fuel the Future: Speakers Wanted for the 2024 Structural Engineering Summit!

eize the opportunity to contribute to the 2024 Structural Engineering Summit in Las Vegas, Nevada, at the MGM Grand Hotel from November 5-8. We’re actively seeking speakers and session ideas—your chance to share insights with both seasoned and emerging engineers. The NCSEA Structural Engineering Summit, the only national event in 2024 dedicated solely to practicing structural engineers, hosted nearly 1,000 attendees in 2023 and stood as a testament to the vibrancy of the profession. Every year, the Summit proves to be a hub of enthusiasm and knowledge exchange, hosting a series of dynamic events that captivate practicing structural engineers, executives, students, trailblazers, educators, and young engineers alike. Our sessions are dedicated to empowering structural engineers with advanced insights, encompassing topics such as artificial intelligence, optimal design practices for enhanced profitability, interpretation of new codes and standards, recent project case studies, and a myriad of subjects pertinent to practicing structural engineers. If you have an impactful presentation idea that can benefit practicing structural engineers in the present or future, we want you! Presentations typically span from 45 minutes to 2 hours. Join us in shaping the future of structural engineering! For details and speaker submissions, visit https://bit.ly/2024SummitSpeakers or scan the QR code. Don’t miss this chance to inspire, connect, and thrive!

NCSEA Webinars

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

Secure access to the highest-quality webinars at an incredible value with an NCSEA Webinar Subscription! Subscribers receive: • At least 25 live webinars a year featuring high-quality speakers and relevant subjects. • All technical webinars are Diamond Review-approved in all 50 states. As an ICC Preferred provider, NCSEA webinars meet the renewal requirements of ICC Certifications. • Unlimited 24/7/365 access to NCSEA’s Recorded Webinar Library—more than 170 recorded webinars available at your fingertips. • Unlimited free continuing education certificates for each webinar so multiple viewers at the same location can receive credit for every live webinar. • Available anywhere! NCSEA’s webinar subscription can be used wherever you are. Visit www.ncsea.com/education for the latest news on upcoming webinars and other virtual events.

Recommendations for Performing Structural Engineering Quality Assurance Reviews

FEBRUARY 2024

SEI Update Education

SEI Futures Fund Thank you to Computers and Structures, Inc., and our many generous donors for maximizing the CSI - SEI Futures Fund 3 to 1 to $250,000 match offer in 2023! The campaign raised over $330,000 for the SEI Futures Fund to benefit the structural engineering profession! Learn more at www.asce.org/SEIFuturesFund

SEICon24 Register and book your room for SEICon24 at NASCC: The Steel Conference! We look forward to seeing you in San Antonio March 20 – 22 as we celebrate the structural engineering profession alongside our colleagues at The Steel Conference. SEICon24 includes much of what you enjoy from Structures Congress – outstanding keynote speakers, expert technical sessions, fun receptions, PDHs, and more! Join us in San Antonio to learn, engage, and network. www.SEICon24.org.

Advancing the Profession

NIST Grant Contractor Report on Advancements in Computational Wind Engineering Now available from workshop held May 2023 at ASCE. Wind engineering practitioners and researchers for buildings developed the workshop and subsequent roadmap to standardize and apply computational wind engineering (CWE) The impetus for the project was the extensive casualties and property losses that have occurred over the last several decades because of damaging hurricanes, tornadoes and other wind events affecting the United States. NIST has continued to research and provide leadership in the advancement of knowledge of these hazards and to the development of standards that will lead to more resilient communities across the nation. The workshop process included a review of the literature that identified current issues in the areas of Computational Fluid Dynamics Design Tools, Verification and Validation Benchmark Testing, System Reliability and Risk, Storm Type and Generation, and Structural Engineering Applications. This was followed by an extensive workshop preparation process, a two-day workshop to obtain input from the top experts in these areas, and report preparation and review. The workshop identified a broad range of research and development activities to advance the use of CWE in practice with the goal of reducing the impacts of these severe wind events. This report includes discussion and specific recommendations on the following nine topics: 1. 2. 3. 4. 5. 6. 7. 8. 9.

Development of guidelines/minimum requirements for the application of CWE, including quality assurance/quality control protocols; Development of consensus-based validation case studies using reliable wind tunnel data; Full-scale observation and instrumentation with CWE integration; Enhancing existing and developing new databases appropriate for verification and validation (V&V) of CWE; Community vulnerability through physical testing for component fragility (residential scale); Verification and Validation (V&V) virtual wind tunnel (with potential interactive design tools); Integration of mesoscale simulations with urban scale models; Sensitivity analysis and uncertainty quantification in Computational Fluid Dynamics (CFD); and Leverage CWE to improve understanding between wind characteristics and effects.

Access at https://doi.org/10.6028/NIST.GCR.23-047. 76 STRUCTURE magazine

News of the Structural Engineering Institute of ASCE

Honoring the Legacy of Dr. Dennis Mertz Dr. Mertz was a professor at the University of Delaware, visionary leader in bridge engineering, founder of the ASCE Journal of Bridge Engineering, and member of the SEI Board of Governors who passed away in 2016. An ASCE award in bridge engineering is being established in his memory, to be given annually for the best paper in the ASCE Journal of Bridge Engineering. If you would like to donate to establish the award endowment, please give at https://swipesimple.com/links/lnk_ccfd747f, or make check payable to ASCE, and mail to: ASCE Accounting – designated for Dennis Mertz Award, 1801 Alexander Bell Dr, Reston, VA, 20191.

Congratulations to 2024 SEI Fellows: Ossama Ali Ahmed, Ph.D., P.E., PMP, F.SEI, F.ASCE Bradley Byrom, P.E., S.E., F.SEI, M.ASCE Vijaya Gopu, Ph.D., P.E., F.SEI, F.ASCE Sriram Kalaga, Ph.D., P.E., F.SEI, F.ASCE Linda M. Kaplan, P.E., F.SEI, M.ASCE David Lattanzi, Ph.D., P.E., F.SEI, M.ASCE

Saurabh Mittal, P.E., S.E., F.SEI, M.ASCE Spencer Quiel, Ph.D., P.E., F.SEI, M.ASCE Dorothy Reed, Ph.D., P.E., F.SEI, F.ASCE Girum Urgessa, Ph.D, P.E., F.SEI, F.ASCE Brian Uy, Ph.D., P.E., F.SEI, F.ASCE Ioannis Zisis, Ph.D., F.SEI, M.ASCE

The SEI Fellow grade of membership recognizes accomplished SEI members as leaders and mentors in the structural engineering profession. Reach out to congratulate your peers, and encourage your SEI member colleagues to apply to advance to SEI Fellow. Prospective SEI Fellows must be current SEI, actively involved, licensed P.E./S.E., 10 years responsible charge (typically post P.E.). Complete application package by November 1 at www.asce.org/SEIMembership

Now Available at www.asce.org Load and Resistance Factor Design (LRFD) for Pultruded Fiber Reinforced Polymer (FRP) Structures, ASCE/SEI 74-23, is intended for use in the design of new buildings and other structures constructed of pultruded fiber reinforced polymer (FRP) composite structural shapes, connections, and prefabricated building products.

Follow SEI on Social Media: FEBRUARY 2024

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 Committee… 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 Basisof-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/

Now more than ever we need to support the upcoming generation of the workforce. Give to the CASE Scholarship today! 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 78 STRUCTURE magazine

News of the Coalition of American Structural Engineers Upcoming Events 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

Small Firm Workshop: Leading Your Firm Into the Future February 25-26, 2024 New Orleans, LA

Presented by William Siegel and Kevin McMahon of McMahon Siegel Group, and David Whitmire of Thinc Strategy, attendees will discover the strategies for thriving in today’s dynamic business landscape, navigate change with resilience, and learn the art of successful succession planning. Don’t miss out on this opportunity to elevate your business to new heights. Held in New Orleans, LA, the workshop precedes Coalitions Winter Meeting: Innovation Forward, where ACEC’s coalitions gather and dive into the future of business and technology. Register for the Small Firm Workshop and a single event, or combine with the Coalitions Winter meeting and make the most of these events.

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

Innovation Forward: Advancing your Business with Tomorrow’s Technology Are you ready to steer your firm toward success in the ever-evolving landscape of today’s marketplace? Discover the strategies that are propelling companies to new heights, including the game-changing forces of AI and the digital marketplace. Join us at ACEC’s Winter Coalitions Meeting, in New Orleans, where you’ll have the unique opportunity to learn from your peers about their plans for tomorrow to ensure they remain at the forefront of competition in today’s dynamic business world. Don’t miss this opportunity to dive deep into the future of business and technology. Secure your spot at ACEC’s Coalitions Winter Meeting today and set your course for success in the competitive marketplace of tomorrow. https://program.acec.org/2024-winter-coalitions-meeting FEBRUARY 2024

iconic STRUCTURES Birmingham’s Sloss Furnaces A look into the history of steelmaking. By John A. Dal Pino

Figure 1. The furnace complex for the Sloss Furnace Company is the oldest remaining blast furnace site in the Birmingham, Alabama, Iron and Steel District.

he story of the Sloss Furnace Company is more than a story about a historic manufacturing plant. Like most structures, the iron-producing furnaces were built to generate income for the owners by filling a product need produced at a competitive price that utilized locally available raw materials and human capital. The Sloss Furnace Company also fits into the larger economic development framework of the post-Civil War South within a timeline that runs from before the Civil War through the early 1920s when poorer Southerners, including many formerly enslaved people or their descendants, moved North in search of better opportunities, through the 1950s. The furnace complex is the oldest remaining blast furnace site in the

80 STRUCTURE magazine

Birmingham Iron and Steel District (Figure 1). The furnaces are prime examples of post-war efforts to rebuild and industrialize the largely agrarian South after the Civil War, during which history has shown that the more industrialized North had a distinct manufacturing advantage. It is also a story of the City of Birmingham, Alabama, which was founded in 1871, taking its name from Birmingham, England, one of England’s major industrial cities at the time. Due to a unique combination of available raw materials within a 30-mile radius, this part of central Alabama grew into a major industrial center focusing on the iron and steel industry. Birmingham also became a major railroad crossroads, allowing for the export of finished products to distant markets. The iron and steel industry

in the Birmingham District became the leading iron and steel-making center in the South (overtaking Tennessee), competing with its chief Northern rivals, principally the mills located in and around Pittsburgh, Pennsylvania. Besides supplying domestic markets, the Birmingham plants also exported large amounts of pig iron to foreign markets starting in the 1890s, competing with Middlesbrough, England, and Glasgow, Scotland.

Overview of Southern Iron and Steel Production Iron production in the South dates to the 1790s. At that time, the production was on a small, local basis, with blacksmiths and mechanics operating foundries and forges in a number of locations Figure 2-3. A cross section of a blast furnace rebuilt in 1940 is shown (left) along with a photo of the base of a Sloss furnace. throughout Georgia, Tennessee, and the Carolinas. The main product was wrought iron (a form of steel) that could be turned into tools and other metal implements. The region, furnace went into operation (“blew in” in industry parlance) in 1882, however, made little progress compared to the North in building larger-scale and the second in 1884. The ovens of the original Sloss furnaces were production prior to the Civil War. In 1860, the South was producing only designed by the English inventor Thomas Whitwell, who first introduced about 25% percent of the nation's bar, sheet, and railroad iron and less than his “stoves” into the U.S. in the 1870s. Hargreaves, an acquaintance of 10% of its pig iron. The war spurred a boom in Southern iron production, as Whitwell, brought them to use at Sloss. Pig iron (an intermediate product much-needed gun forging works and arsenals were erected throughout the in the production of steel with a high carbon content of 5% to 6%) was region to supply the war effort. the main product of the Sloss Furnaces. In its first year of operation, the The Red Mountain Iron Company built its famous Oxmoor plant near Sloss Furnaces produced 24,000 tons of pig iron, producing the product Birmingham in 1862. The plant, located along Shades Creek in the Oxmoor at a cost of about $8 per ton less relative to a total cost between $15 and Valley of the Birmingham Industrial District, was the first to make use of $20 a ton at the Northern plants. Alabama’s red hematite ore in significant quantities and helped establish the The iron ore for the Sloss Furnaces was extracted from company-owned pig iron industry in the area. The iron ore was mined from Red Mountain, mines, Sloss #1 and Sloss #2, located on 40 acres of land on the south side of a southwest-to-northeast ridge located to the south of Birmingham. The Red Mountain, roughly 10 miles southwest of the furnaces. Red Mountain Oxmoor plant, as well as the Irondale and Tannehill furnaces, were destroyed is a southwest-to-northeast trending ridge and the southernmost tail of by Federal troops near the war’s end as the Northern Army marched south- the Appalachian Mountains. The remaining mines and mine buildings, ward. After the war, southerners revived and rebuilt the iron industry, and including some equipment, are preserved in Red Mountain Park. Coal/ it eventually became an important part of the industrialized "New South." coke, limestone/dolomite, and other ingredients were supplied from other non-company mines located throughout the Birmingham area.

Sloss Furnace Company The Sloss Furnace Company was the creation of James Withers Sloss. Sloss was born on April 7, 1820, of Scotch-Irish descent in Mooresville, Alabama, located roughly 80 miles north of Birmingham. He was a second-generation American, his father having emigrated from County Derry, Ireland, in 1803. Sloss appears to have received little formal education, but at the age of 15 was able to land a job as an apprentice bookkeeper for the local butcher. After the traditional seven years of apprenticeship, he was able to purchase his own store in Athens, Alabama. He also ran his own plantation and served as a Colonel in the Confederate Army. After the war, he became involved in the railroad industry and started working alongside the noted Alabama industrialist Daniel Pratt (17991873) at the rebuilt Red Mountain Iron Company’s Oxmoor Furnaces. Following success there, Sloss joined Truman Aldrich and Henry Fairchild DeBardeleben in establishing the Pratt Coal and Coke Company located in Birmingham, suppliers of coal and coke to the local furnaces. Sloss started his own firm, the Sloss Furnace Company (hereafter referred to as Sloss Furnaces or Sloss), in 1881 after leaving his partnership with DeBardeleben and Aldrich. He engaged Harry Hargreaves, a Swiss-English engineer, to supervise the construction of two blast furnaces. The first

The Basics of Pig Iron Production The raw materials needed for pig iron production are iron ore, coke (processed coal), and limestone/dolomite. Pure iron does exist naturally in the Earth’s crust, but is bound up in iron oxide-containing rock. Coke is the fuel that is made by heating coal until impurities are burnt away and it becomes nearly pure carbon. Limestone/dolomite serves as a flux, helps remove impurities, and is a necessary source of calcium carbonate in the production process. It takes roughly three tons of red hematite ore and 2,500 pounds of coke to make one ton of pig iron. A blast furnace is essentially a tall hollow steel tower (perhaps better described as a tank, vessel, or caldron) that serves as a container for the smelting of iron. The original furnaces at Sloss were approximately 60 feet tall and 18 feet in diameter, lined with heat-resistant, refractory brick. See Figure 2 for a cross-section and Figure 3 for the base of a Sloss furnace. At Sloss, the raw materials were lifted by elevator from supply bins located on the ground to raised filling platforms and then fed by hand into the top of the furnace. Simultaneously, very hot air (1,600 F) is created in tall cylindrical boilers (see Figure 4), dried, and then injected under pressure through nozzles at the base into the raw material mixture. FEBRUARY 2024

The coke burns when the injected oxygen approximately 600 African-American workers. combines with the carbon in the coke, formPig iron is later processed by melting and expoing carbon monoxide. The carbon monoxide sure to blasting of air to oxidize and remove reacts with the iron oxides to form iron and the impurities (the extra carbon), and then carbon dioxide as a byproduct. The “pure” iron made into steel products. The furnaces in the sinks to the bottom of the furnace (due to its Birmingham area were hampered by the high greater density), while the lighter byproducts phosphorus and silica content of the locally of the process, a material called slag, floats to available iron ore. To combat this disadvantage, the top. Both the molten iron and the slag are Sloss employed the latest industry developments then drawn off independently. Then, more and retained the services of expert metallurgists fuel is added, and the process continues. such as Edward Uehling and James Pickering At Sloss, there were two furnaces, and the Dovel. Despite these efforts, the economical “hot metal” from each was transported into production of steel from the local pig iron large adjoining casting sheds where the liquid remained beyond his grasp. Therefore, plants iron was funneled through channels on the like Sloss focused on the production of pig iron, ground into sand-lined forms to create “pigs,” although later, some plants, but not Sloss, also each weighing around 110 lbs. Once cool, the produced cast-iron pipe and pipe fittings. Other pigs were removed from the forms by hand and Figure 4. Tall cylindrical boilers produced very hot air that common uses of cast iron included such prodthen hand-carried by workers to the nearby was then dried and injected at the base of the furnace into ucts as cooking skillets, kitchen stove plates, train platforms of the Louisville and Nashville the raw material mixture. and automobile engine blocks. Railroad and the Alabama Great Southern Railroad for shipping. The slag, which is a waste product that was historically of no value, is used today as a The Later Years replacement for Portland cement in concrete. The local ore generated twice as much waste slag as the Northern plants, representing a cost Like any commodity produced to a widely accepted standard, price is disadvantage. To the extent possible, the exhaust gases were collected almost always a prime factor in purchasing decisions, and competition in tanks and fed back into the process. can be fierce. Distinguished scholar C. Vann Woodward has noted that As one might expect in these pre-OSHA days, the work was quite danger- "during the depression of 1884 and 1885, Southern iron made its first ous since, in addition to the high temperatures and exposed conditions, successful invasion ... into the Northeastern market. This precipitated a the workers on the filling platforms were inadvertently breathing exhaust hard-fought struggle for sectional dominance in the iron industry that gases rising up and escaping through the throats of the furnaces. was almost as much discussed as that between Northern and Southern The pig iron-making process is a continuous one that runs day and cotton mills." night until the furnace needs to be shut down for lack of raw materials or James Sloss retired in 1886 at the age of 66 and sold the company maintenance, which may include the re-building of the insulating refrac- to local businessmen. With help from Wall Street financiers, the firm tory brick. During the early years of operation, and actually well into the was reorganized and re-named the Sloss Iron and Steel Company. Sloss 20th century, daily production at the Sloss was around 100 tons of iron died in 1890. Following the depression of 1893 and the boom in 1898 per day. This production was much lower than other mills, particularly resulting from the Spanish-American War, the company expanded and those in the Northern states, and was largely the result of a labor-intensive was once again re-organized, this time as the Sloss-Sheffield Steel and operation designed around the use of local, largely untrained, African- Iron Company. American workers rather than a highly mechanized one. In the late 1920s, the owners were forced to modernize and re-design As noted above, Sloss had a major cost advantage over its Northern the 1890s-era technology of the plant as a result of several factors, competitors due to the local labor market conditions. But Sloss, who had including competition from the North, “unfair” government-controlled no formal experience or training in running an efficient manufacturing transportation pricing system for Southern iron, and an exodus of operation, found it challenging to run his furnaces and to maintain his African American workers to the North in search of a better life. The workforce. He found the local white population disdainful of manual plant was eventually sold in 1952 to the United States Pipe and Foundry labor and “imported” white workers who flocked to the Birmingham Company, which continued operations until it was closed for good in area unruly and undependable. In the segregated South, it was also 1970. difficult to mix the white and black workers. He, therefore, depended None of the original furnaces remain today. Yet what does remain on a plantation model employing unskilled dates from the early part of the 20th century formerly enslaved people. He apparently paid and stands today as a fine example of the iron a good wage, $1.50 a day, although Sloss soon and steel technology from a bygone era; it discovered that good wages alone were not is preserved in the Sloss Furnaces National enough to retain workers and to have them Historic Landmark, dedicated in 1981, located report for work on Monday mornings. His just east of downtown Birmingham.■ solution was to construct company tenement housing (which remained until the 1950s) and company stores to attract what he believed John A. Dal Pino is a Principal with Claremont were more reliable married workers. After Engineers, Inc. in Oakland, California. He serves paying for housing and food, Sloss figured that as the Chair of the STRUCTURE Editorial Board nearly half of wages ($15 to $20 per month) Some boilers still stand at the site of the Sloss Furnace (jdalpino@claremontengineers.com). could be saved. In its heyday, Sloss employed Company complex today. 82 STRUCTURE magazine