January STRUCTURE 2025 by structuremag

STRUCTURE JANUARY 2025

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Renovating the Egyptian Theater

INSIDE: 3D-Printed Homes in Texas

Fire Qualifications for Connections Parking at Oyster Point The Building Code Highway

20 24 38

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EDITORIAL BOARD Chair John A. Dal Pino, S.E. Claremont Engineers Inc., Oakland, CA chair@STRUCTUREmag.org Marshall Carman, PE, SE Schaefer, Cincinnati, Ohio Erin Conaway, PE AISC, Littleton, CO

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Sarah Evans, PE Walter P Moore, Houston, TX Linda M. Kaplan, PE Pennoni, Pittsburgh, PA Nicholas Lang, PE Vice President Engineering & Advocacy, Masonry Concrete Masonry and Hardscapes Association (CMHA) Jessica Mandrick, PE, SE, LEED AP Gilsanz Murray Steficek, LLP, New York, NY Brian W. Miller Cast Connex Corporation, Davis, CA Evans Mountzouris, PE Retired, Milford, CT Kenneth Ogorzalek, PE, SE KPFF Consulting Engineers, San Francisco, CA (WI) John “Buddy” Showalter, PE International Code Council, Washington, DC Eytan Solomon, PE, LEED AP Silman, New York, NY

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On the Cover: The seismic retrofit of the historic Egyptian Theatre in Los Angeles, which hosted the world’s first red carpet premiere, serves as a model for balancing modern engineering demands with the preservation of architectural heritage.

In Every Issue 3 Advertiser Index 43 SE News 48 CASE in Point 50 NCSEA News 52 SEI Update

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 32, Number 1, © 2025 by The National Council of Structural Engineers Associations, all rights reserved. Subscription services, back issues and subscription information tel: 312-649-4600, or write to STRUCTURE magazine Circulation, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.The publication is distributed to members of The National Council of Structural Engineers Associations through a resolution to its bylaws, and to members of CASE and SEI paid by each organization as nominal price subscription for its members as a benefit of their membership. Yearly Subscription in USA $75; $40 For Students; Canada $90; $60 for Canadian Students; Foreign $135, $90 for foreign students. Editorial Office: Send editorial mail to: STRUCTURE magazine, Attn: Editorial, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. POSTMASTER: Send Address changes to STRUCTURE magazine, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. STRUCTURE is a registered trademark of the National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

Image courtesy of Mike Hume at historictheatrephotos.com.

JANUARY 2025

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Contents Cover Feature

32 UNVEILING SECRETS By Jonathan Lehmer, SE

The seismic retrofit of the Egyptian Theatre in Los Angeles serves as a model for balancing modern engineering demands with the preservation of architectural heritage.

JA N UA RY 2025

Features 24 REDEFINING STRUCTURAL EFFICIENCY & RESILIENCE By Mei Kuen Liu, SE, and Chris Petteys, SE

A 10-story parking structure in South San Francisco achieves efficiency and resilience beyond the code-level life-safety performance objective with a concrete precast hybrid moment frame system.

28 FROM NOZZLE TO NEIGHBORHOOD By David P. Langefeld, PE, and Sam Covey, PE

A 100-home 3D-printed community in Austin, Texas, showcases structural innovations in home building.

Columns and Departments 7 Editorial Where Are All the Young Engineers? By Michelle Ryland, SE, RA

8 Structural Influencers Tanya de Hoog 12 Structural Design Innovations in Timber Concrete Composite Structures By Kirby Beegles, PE, SE

14 Optimizing Beam Hanger Placement in Mass Timber Structures y Dong Han, Ph.D, Lori Koch, MS, PE, B Max Closen, MASc

18 Grouted Mechanical Splices in Reinforced Concrete y Kayla Hanson, PE B

InSights 20 Trends for Fire Qualification, Design of Post-Installed Reinforcing Bars & Anchors y Kenton McBride, Ph.D, PE B 22 Bringing Lower Carbon Concrete Usage to Buildings y Don Davies, PE, SE B 38 Codes and Standards The Building Code Highway y David Sparks, SE, PE B 41 FAQ on SEI Standards y Jennifer Goupil, PE B 56 InSights Tech-Driven Monitoring Protecting America’s Infrastructure y Kelsey Kidd, Worldsensing B 57 Historic Structures 19th Century Mississippi River Bridges y Dr. Frank Griggs, Dist. M. ASCE 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. JANUARY 2025

EDITORIAL Where Are All the Young Engineers? By Michelle Ryland, SE, RA

hen I was just beginning my career, anyone to join me. How can I get the young 4. Different a colleague invited me to an SEAOI engineers in my office to see the value in priorities. dinner meeting at the famed Cliff Dwellers engagement if I can’t get them to an event in Another colClub on South Michigan Avenue overlook- the first place? league admitted ing Grant Park in Chicago. Having nothing I left the session with few definitive answers his extracurricuscheduled for that evening, I said yes. While and still felt a piece was missing. Back in lar passions, like I don’t remember the presentation topic, I Chicago, I went straight to the source physical activities, do remember meeting several people, whom and—using this editorial assignment as an were more fulfilling than networking and preI met again the next month, who introduced excuse—directly asked my younger colleagues sentations. I appreciated his honesty. me to more people, and eventually I felt com- why, despite all my efforts, encouragement, 5. Taking the first step is hard. While fortable attending events on my own. and invitations, nobody comes to the events some recognized the value of professional I soon found the young member group (in with me? Do you not have time? Do you communities, they struggled to prioritize Illinois, we call it YEC—Young Engineers not see the value in attending these types of attendance. Competing commitments, like Committee) and served on its board for events? Or is it like Taylor Swift sings “It’s me, pursuing a PhD, and preferences for daytime over six years. From there, I became SEAOI’s hi, I’m the problem, it’s me”? Fortunately, they over evening events were also factors. Delegate to NCSEA, joined and eventu- all assured me that was not the case! 6. Networking can be intimidating. Social ally co-chaired and chaired anxiety, shyness, and insecurity the NCSEA Communications were common concerns. One Committee, joined SEAOI’s feared senior engineers would ask Board of Directors in 2019 and difficult questions, while another the Executive Board in 2023, and doubted his conversational abilimost recently joined NCSEA’s Engagement requires empathy, addressing ties due to a language barrier – an Board in 2024. eye-opening admission considerThat first dinner meeting was misconceptions, and creating welcoming ing his daily interactions in the more than a decade ago and while office. I doubt my story is unique, the opportunities that fit their needs. None of this felt earth-shattering, reality is my path to professional but these conversations offered organization leadership, and the a fresh perspective on engaging personal friendships gained along junior staff. To succeed, I need the way, truly began with accepting to rethink how I communicate a simple invitation. I’ve tried to pay it forward I spoke to five colleagues, representing over and support their involvement. Engagement by inviting young engineers in my office to half of the staff junior to me. Three were in requires empathy, addressing misconceptions, similar events but with limited success. their mid-20s and had graduated within the and creating welcoming opportunities that In a quest for enlightenment, I attended last two years, two were in their mid-30s, one fit their needs. the “How to Engage Young Members at a was pursuing a PhD, three were men, two were Thirteen years ago, my career trajectory Local and National Level Successfully” ses- married or engaged, and one was a non-native shifted because someone extended an invitasion at the NCSEA Summit last November. English speaker. Here’s what I learned: tion. Today, I remain committed to doing the This panel discussion by the Young Member 1. Nobody reads emails. While a broad same—not just inviting, but inspiring young Support Committee shared ideas of different generalization, it was surprising how often engineers to find the sense of belonging and events and activities that would be appealing they claimed ignorance despite my email opportunity that shaped my journey. ■ to young engineers, but didn’t sufficiently receipts. address the “successfully” part of the session 2. Professional organizations are mistitle, in my opinion. During the Q&A, I posed understood. Some assumed these groups my dilemma to the panel: were for senior professionals, not young SEAOI hosts a variety of events—educa- engineers. One even likened them to exclutional, social, volunteer, outreach, and young sive, unwelcoming student organizations he’d member focused; my company encourages encountered in the past. Michelle Ryland is a dual licensed structural engineer involvement in professional organizations, 3. Burnout from past experiences. One and architect with Klein and Hoffman in Chicago. She pays for membership and supports the time colleague avoided involvement due to prior also serves on the NCSEA Board of Directors. and cost of event attendance; I invite people negative experiences with technical committo come with me to events, and I rarely have tees at a previous organization.

STRUCTURE magazine

JANUARY 2025

structural INFLUENCERS Tanya de Hoog Tanya de Hoog, 2024 President of the Institution of Structural Engineers and Chief Engineer, Eminence and Innovation Officer at Aurecon, is a structural engineer with nearly 30 years of experience working on a diverse range of award winning and landmark projects globally. Based in Sydney, Australia, de Hoog leads efforts to empower engineers and designers at international design, engineering and advisory company Aurecon to consistently deliver technical excellence in a changing industry and attain eminence as leaders in their respective fields. In 2006 she founded Thornton Tomasetti's London office and until recently was a senior leader in their New York HQ. She also holds positions at Imperial College, London Department of Civil and Environmental Engineering Industry Advisory Board, and University of Cambridge, Department of Architecture, MDes Advisory Board. de Hoog is committed to raising awareness of the vital role structural engineers play in driving climate action, ensuring public safety, and making a positive impact on people's lives through their work. STRUCTURE: You have been described as full of energy, intensity and optimism, along with displaying technical excellence and great attention to detail. Would you tell us a little about your life and upbringing that made you the person you are? de Hoog: I grew up in Canberra, which as the seat of government in Australia attracts people from all over the world. My dad was an applied research mathematician, while my mom was a creative individual who also shared a curiosity for new cultures and new experiences, and even worked in Indonesia in her 20s. This blend of scientific rigor and artistic creativity profoundly shaped my perspective. My mom’s stories and treasures from her time in Indonesia, along with my dad’s global scientific community, instilled in me a love for different cultures, the idea of travelling, and a passion for learning. Born in Los Angeles during my dad’s first post-doctorate position at UCLA, I was exposed early on to diverse experiences and ideas that cultivated my creativity, energy, and optimism. Playing sports and spending time outdoors in Australia’s hot climate, I developed a deep connection to nature and a strong sense of community. My mom’s passion for design, influenced by her family of dressmakers, sparked my appreciation for creating beautiful, functional things. These experiences taught me the value of integrating creativity with technical excellence and the importance of environmental stewardship. The multicultural and intellectually stimulating environment of my upbringing has deeply influenced my commitment to sustainability, always searching for new, exciting, interesting ways to progress, as well as community impact. STRUCTURE: How do you define an 'influencer' in the field of structural engineering, particularly in terms of leadership, evolving the field, and impacting society and the environment? de Hoog: It’s exciting and encouraging to see the explosion of an Influencing era in terms of the opportunity it creates for many more 8 STRUCTURE magazine

structural engineers to raise the profile of the profession, increase public understanding of it, and to share knowledge. We need more structural engineers in the world, and influencing using social media is one tool that has the potential to be a powerful force. The bulk of my career has been pre-influencer era and with nearly 30 years of experience, my aim is to be one of those influencers in the structural engineering field who actively change, evolve, and lead our profession to deliver better, more efficient, and positive outcomes for society and the environment. People in leadership positions have the ability to influence people at scale and enact greater change with their work and message, ideally extending beyond the structural engineering profession to the entire built environment industry. STRUCTURE: What sets you apart as an influencer in the field of structural engineering? de Hoog: Because of my background, my contribution to the field lies in my holistic approach which integrates technical excellence with a deep sense of social and environmental responsibility. Rather than focus solely on technical advancements, I emphasize the broader impact of engineering on society and the environment. My influence extends beyond building better structures to building a better world through

thoughtful, purpose-driven engineering. Although not every project will allow for STRUCTURE: What does "engineering significant carbon reductions, engineers with purpose" mean for you personally and can still make incremental improvements what do you hope it will mean for others? by sharing knowledge and best practices de Hoog: The concept of finding a vocaglobally. This is why I am driven to ensure tion that has meaning and purpose is not equal access to knowledge and resources for new or specific to engineering. However, engineers worldwide, enabling them to make from a structural engineering perspective, informed and impactful decisions that are the profession was originally formed with locally relevant and powered by the depth the mandate to uphold public safety, which of knowledge from a global profession. in my view is ‘Engineering with Purpose.' As my own career evolved, I started to recognize STRUCTURE: What do you do to learn that it was the impact and outcomes of the new things? Any suggestions for others? projects I was working on, and the people I de Hoog: A solid foundation is the most was working with that was what motivated powerful ingredient for successful change me. In the last decade, this has become even from my experience. In engineering terms, more specific in terms of accelerating positive mentors helped me understand that manimpact towards people in our communities aging change comes from having a robust and protecting the planet. The world needs understanding of the fundamentals of engimore engineers, especially given the current neering and a strong collaborative team of and future challenges of climate change, and Tanya de Hoog speaks at the Australasian Structural engineers who have requisite experience. we need to accelerate and scale our impact. Engineering Conference 2024. Delivering new ideas is a team endeavor. This For this reason, I want people to recognize is why I believe so strongly in the power of their potential for impact early in their careers mentoring and being mentored, as it provides in order to maximize it. valuable insights and different perspectives. Staying curious, embracIn my new role as Chief Engineer at Aurecon, I am able to lead and ing new technologies, actively seeking opportunities for growth and influence thousands of engineers, encouraging them to identify their being comfortable outside our comfort zones, both personally and core values and apply them in their work. This approach aims to professionally, are essential. address the most pressing issues of our time by creating solutions for our client's complex problems that are innovative and meaningful. STRUCTURE: Awe-inspiring structures are all around us. Ultimately it's about engineering that serves society and the envi- But in reality, many of those structures are intended for and ronment, creating a better future for people and the planet. used by a limited audience. How should SEs design structures to touch more people and create more equity? STRUCTURE: For lack of a better term, “first world” engineers de Hoog: There is much need for more attainable housing, can reduce carbon emissions by choosing material A versus mate- increased sustainability, resilience, safety, and community amongst rial B, designing with structural different forms, etc. But most many other societal challenges. Therefore, I believe we will reconof the world lacks those options, building some form of stacked sider in the future what awe-inspiring structures mean; might they concrete box with lots of walls and very few windows. What can be a combination of scale, beauty, functionality and fairness? To SEs do to help those with limited material choices, particularly design for equity, structural engineers must consider the broader in areas of the world experiencing most of the population growth? social impact of their projects. This means designing structures that de Hoog: Structural engineers play a crucial role in addressing are accessible, affordable, and beneficial to a wide range of people. climate change and its impacts. While reducing carbon emissions Equity in design is about ensuring that everyone has access to safe, is important, it's just one aspect of our responsibility. Ensuring that healthy, and inspiring spaces, and that these spaces are designed to structures are resilient, adaptable, and sustainable is equally crucial. ensure everyone can thrive in and around them. This includes designing buildings that can withstand environmental changes, can be reused, and contribute to a circular economy. STRUCTURE: How can SEs improve their contribution to the design process and make the resulting structure even better than the architect’s original vision? Would you cite an example of when your contributions impacted the design for the better? de Hoog: An example from my experience is the design of Shirley The world needs more engineers, especially Chisholm Recreation Center in Brooklyn. By working closely with given the current and future challenges of the like-minded, purpose driven Architects like Studio Gang and climate change, and we need to accelerate the whole design and build team led by Lend Lease, we were able to incorporate innovative structural solutions that not only met and scale our impact. For this reason, I the aesthetic goals but also improved the building’s performance want people to recognize their potential and sustainability. This kind of synergy between disciplines leads for impact early in their careers in order to better outcomes.

to maximize it.

STRUCTURE: Your career has taken you to jobs around the globe. How did you do it, and from your experience, what are the more risk-averse engineers missing out on? JANUARY 2025

de Hoog: I have always been fortunate to work with and for people who have supported me. The confidence I had to make the leap from one thing to the next came from having built up a solid base and a supportive community. It is why I am so driven to increase awareness of the power of the structural engineering community we have access to through our institutions. Having the ability to work in different countries has been one of the most rewarding aspects of my professional journey, and I believe that other engineers can benefit immensely from similar experiences. By being open to new challenges, and always having a curious mindset and desire for continuous improvement, they can develop a more wellrounded skill set, build a global network, and bring fresh insights to their work. STRUCTURE: Engineers appear to be under increasing pressure for profit (low fees) and from demanding schedules (fast-tracking), negatively impacting their design creativity, thoroughness, and learning during construction. How have you tackled those dilemmas in your own career? de Hoog: Balancing profit pressures and demanding schedules is challenging, yet it’s possible to take some control of the business of engineering with the right approach, the right team, and most importantly the right clients. People who are aligned in their goals and have a depth of experience or access to it are essential for success. In my career, I’ve focused on fostering a culture of creativity, efficiency, and innovation. This means streamlining processes, leveraging technology appropriately, and encouraging creative problem-solving. By far the most successful approach after technical excellence is cultivating clients who understand the value and importance of our work, and see us as an essential partner, beyond the traditional role of the engineer that sizes the structure. By maintaining high standards and prioritizing long-term relationships over short-term gains, we can achieve a balance that benefits both our clients and our teams. STRUCTURE: What lessons did you learn from opening an office in London that would be helpful to others starting a new endeavor? de Hoog: When I co-founded the London office of Thornton Tomasetti, the goal was to deliver creative, innovative engineering solutions alongside some of the world’s best Architects from UK and Europe. The joy of engineering for me and for our team came from the process of being involved in these types of projects. The vision for the office never changed the strategy, and the approach evolved with the changes to the environment around us. Opening an office in London taught me the importance of adaptability and local knowledge. Understanding the market, building a strong team, and establishing a clear vision were key factors in our success. For others starting a new endeavor, I would recommend thorough research, building a strong support and client network, and being prepared to pivot as needed. More importantly though, establish your ‘Why’—the purpose behind your engineering that you can go back to you whenever the road gets bumpy … which it will! STRUCTURE: Mentoring is important, whether it is one 10 STRUCTURE magazine

on one or more evenly spread to many people. What have you found to work best and what can other engineers learn from your experiences? de Hoog: Effective mentoring is about building trust and providing tailored guidance. One-on-one mentoring allows for personalized support, while group programs can create a sense of community and shared learning. In my experience, combining both approaches works best. For example, the senior women’s mentoring at The Institution of Structural Engineers International I recently ran provided small group mentorship while also unintentionally creating an impactful network among participants. Engineers can learn from this by being proactive in seeking and offering mentorship, being open to feedback, and creating environments where knowledge sharing is encouraged. My recent experience is that many engineers are very generous with their own knowledge and willingness to share it and mentor others. It is more challenging to find equally as many engineers more experienced in their career who put their hand up and ask for mentoring. Our careers become exponentially more impactful as we get older and gain more knowledge and my view is that the impact of support through mentoring as our careers progress allows us to leverage and multiply the impact of our work by gaining different knowledge, perspective, and advice from others. STRUCTURE: In terms of reducing our harmful impacts on the planet, what should be doing differently? de Hoog: Reducing our harmful impacts requires a collective effort across all sectors of society. For engineers, it means prioritizing sustainable practices in our designs and operations. This needs to be balanced against risk and future adaptability. In order to do this, we must appreciate and then embrace a culture of lifelong learning and continuously develop our professional capabilities to address these challenges. The good news is that in our professional lives, we have the ability to have up to 100 times more impact than in our personal lives. So this means, the most powerful way for engineers to positively impact the planet is to practice engineering, and do it in a way that improves what was done yesterday. The most available example of this today is in reducing embodied carbon in design by reducing the amount of material required through repair, reuse, reduction in volume of material, use of lower carbon materials, and several other means. Of course, there are a vast number of other options available now and these are increasing. ■

structural DESIGN

Innovations in Timber Concrete Composite Structures

Timber-concrete composite, or TCC, framing systems provide a unique opportunity to leverage components that are inherent to mass timber buildings to create a strong, stiff floor system that could not be achieved using timber alone. By Kirby Beegles, PE, SE

ost engineers were trained to use certain structural systems for a building archetype based on their mentor’s experience. The passage of this institutional knowledge is invaluable, however the prevalence of sustainable design practices in contemporary architecture means we must challenge those preconceptions. Structural engineers’ ability to influence the amount of embodied carbon in buildings gives them unique opportunities to think outside the box and encourage clients to use more environmentally friendly materials in construction. While steel, concrete, and masonry will continue to serve an important role in the built environment, there are certainly applications where replacing them with timber will result in a more sustainable structure. Historically, timber-concrete composite (TCC) construction was used as a rehabilitation mechanism for bridges and buildings. Some of the earliest cited examples of TCC systems are found in Europe when a lack of available steel stimulated the development of alternative methods of construction after WWI and WWII. Following these developments, the use of TCC designs spread to the refurbishment of historic buildings in Europe. Its use has since evolved, now gaining traction as a method for achieving long spans in new buildings. Despite its use in Europe, Canada, and other regions, few projects in the U.S. have implemented TCC systems. In fact, the Woodworks Innovations Network (a database that catalogs wood projects across the U.S. and Canada) lists only 11 instances of building designs that used TCC systems in these countries. This may be a result of the lack of guidance and design provisions that pertain to this construction. This contrasts with steel-concrete composite (SCC) systems which have well-developed provisions and design equations that can be implemented by designers in the same way that non-composite elements are designed. However, interest in TCC systems is beginning to gain traction, with the completion of some recent high-profile projects such as Limberlost Place in Toronto and the KF Aerospace

stair in Kelowna, British Columbia. These projects have achieved impressive results; the composite CLT girder slab-band reaches a clear-span of about 30 feet using a 7-ply CLT panel at Limberlost Place, eliminating the need for deep glulam beams and keeping the floor structure remarkably thin. The KF Aerospace stair spans 70 feet between floors over the length of its spiral form.

Applications The primary motivation for using a TCC floor system lies in the fact that floors in mass timber buildings often contain a cementitious topping. This topping is most often required for acoustic treatments or fire protection but is not typically taken advantage of for its potential contribution to the structural capacity of the floor. The topping can be gypcrete, but using concrete has certain advantages acting as an exposed topping slab, to route conduit, or as a diaphragm. By connecting timber framing to the topping slab to transmit internal shear forces, designers can create strong and stiff members while utilizing less timber or achieving longer spans than a non-composite system. While it can be challenging to achieve the same strength and stiffness as a concrete or steel-concrete composite frame, using timber beams produces a structural system that is more sustainable than these other materials. Where sustainability is a high priority outcome, several case studies have shown that replacing carbonintensive materials with timber provides a considerable decrease in the embodied carbon of the structural system. Aesthetics and biophilic design are also reasons to use exposed timber in a structural system. Implemented as a refurbishment of buildings for adaptive reuse, TCC systems can extend a building’s life-span for future generations.

Codes and Standards The International Building Code (IBC), through reference to the National Design Specification (NDS) for Wood Construction, does not explicitly acknowledge timber-concrete composite systems, however this does not preclude designers from using these systems. Through use of the IBC alternative methods provisions (IBC 2024 section 104.2.3), a timber-concrete composite frame could be designed, detailed, and constructed through collaboration with the

12 STRUCTURE magazine

Authority Having Jurisdiction for the project. There is some precedence for TCC systems to become incorporated in U.S. building codes. Eurocode 5 includes design provisions and equations for mechanically jointed beams, which have been applied to composite member analysis. Further, some countries, such as Canada, have created a design guide which details a process of analyzing the system for use in buildings.

Materials and Construction The typical materials used in timber-concrete composite systems are the concrete slab, a timber member (often a wood beam or plank), and the interface shear connector that ties these two materials together (synonymous with the headed shear studs typically used in SCC systems). The topping slab needs to be thick enough to carry the internally induced compressive forces and act as a proper base material for the anchorage of the interface shear connectors. A slab thickness of 3-to-4 inches is a good starting point. A thicker slab will induce unnecessary weight to the system, and a thinner slab might not be able to provide adequate anchorage for the shear connectors. The timber component can be either a beam or a plank. Solid-sawn elements or engineered wood products (EWP) are acceptable, but it is not ideal to use a built-up beam (for example one that is a multi-ply Laminated Veneer Lumber beam) due to the complexity of providing a shear transfer mechanism that equally engages all the individual plies. The wood member should be a species recognized by the NDS or similar governing standard for material design that provides design stress ratings for bending, shear, tension, and compression based on the grade and species of wood. The shear interface connectors that transfer horizontal stresses between the timber and concrete components are a critical element to define and evaluate as part of the analysis process. Typical mechanical shear connectors are self-tapping wood screws, truss plates, or lag screws. The timber member can also be notched, allowing the concrete to interlock with the wood. The shear connector must have a known strength, typically gleaned through laboratory testing using the same base materials as the composite system (timber and concrete). In addition to establishing the yield and ultimate strength levels of the fastener, it is critical to establish the stiffness of the fastener as this property plays a major role in the stiffness of the overall composite section.

Mechanical Behavior and Design Calculations FP Innovation’s Design Guide for TCC Structures in Canada (hereafter referred to as the “Design Guide”) suggests the use of the Gamma Method to determine capacity. This method is recognized in Eurocode 5 as a closed-form solution for calculating the stiffness of a partially composite beam. The procedure may not be intuitive to designers who are accustomed to SCC systems. Some of the unique aspects of TCC evaluation are: • The need to consider both immediate and long-term loading conditions for both strength and serviceability, since longterm creep of the timber and concrete change the internal stress distribution and thus the strength of the composite section. • The need to consider the fastener stiffness at both service and ultimate levels, since the slip of the interface fastener changes the internal stress distribution to the concrete and timber elements. • If fire-resistant construction is required, the assembly would require either testing per ASTM E119/UL 263, or an evaluation of the effect of charring on the capacity of the timber components and interface fasteners. A critical parameter for determining the stiffness of the system is the failure mode of the interface fasteners. The Design Guide notes that when the fasteners fail in a brittle manner, or are not allowed to yield, the moment capacity is determined by the effective bending

capacity of either the timber component or concrete component, using the Gamma Method:

Mr = min (Mr,y,t; Mr,y,c) Where

Mr,y,t =

(EI) eff Tr,t Mr,t c t (EA) t at Mr,t + (EI) t Tr,t

Mr,y,c = 0.9z c f lc Sc Refer to the Design Guide for symbol definitions. Brittle fasteners must be strong enough to preclude failure prior to the failure of the timber or concrete component. The strength of the fasteners is evaluated as part of the shear analysis for the beam. A limitation of the Gamma Method is that it assumes each component exhibits linear-elastic behavior. This may overestimate the capacity of the composite section if the fasteners yield prior to failure of the timber or concrete components. In the case of ductile fasteners, a third limit-state is introduced: the Elasto-Plastic bending moment resistance (Mr,EP) The bending resistance then becomes:

Mr = min (Mr,y,t; Mr,y,c; Mr,EP) Where

2 bh 2 hc,eff l + v b,c c 6c,eff + v b,t bt6ht Mr,EP = N b ht + t + hc 2 2

The shear capacity of the composite section is also influenced by the failure mode of the fasteners. With brittle fasteners, the shear capacity of the entire composite section is likely to be controlled by the fastener strength alone, with the fasteners at each critical section being evaluated for the horizontal shear stress at that location.

Vr = min (Vr, c,conn; Vr,c,t; V c,c) r,

Varying the spacing of the fasteners along the span of the beam can provide an economic design:

For ductile fasteners, when the highest-stressed fasteners near the end of the beam yield, the internal forces redistribute to the timber or concrete components until the weaker material fails, similar to the behavior of steel-concrete composite framing. In this scenario, designers can take advantage of the behavior to gain higher shear strength by considering the Elasto-Plastic shear capacity of the composite system:

Vr = min (Vr,c,t; Vr,c,c; Vr,EP,t; Vr,EP,c) Note that the connection strength is not directly evaluated. Instead, it is a component of the equations used to determine the shear strength JANUARY 2025

Fig. 1. As shown in this illustration of a cross-section of a TCC beam, the CLT plank cannot be continuous because the concrete must pour down to bear on the glulam beam for a proper connection of the shear fasteners.

Fig. 2. This proposed construction sequencing of the TCC floor framing has similar erection efficiencies as standard mass-timber construction techniques.

of the timber or concrete components, evaluated using the Gamma Method or Elasto-Plastic model as indicated above. The Elasto-Plastic shear capacities are given by:

Vr,EP,t = cVr,t -

mVr,conn (ht + t) (EI) 0 mVr,conn m + L r m Lm2 (EI) c

Vr,EP,c = cVr,c -

mVr,conn (2hc - hc,eff + t) (EI) 0 mVr,conn m + L r m Lm2 (EI) c

The shear capacity is also checked against the fastener’s strength under service-level loads to preclude the risk of fatigue failure. While the specifics of evaluating the bending and shear capacity are complex, the take-away for the designer is to recognize the critical role the interface connector plays in the capacity of the composite section. Those accustomed to the provisions relevant to SCC composite systems should be prepared to account for aspects of the interface shear connector that do not typically drive the design of SCC systems. The type of failure (brittle versus ductile), stiffness of the fastener, and long-term creep effects must all be accounted for in the strength equations. In conclusion, the design process for TCC beams is a bit more complex than other types of framing. The influence of each component on the strength and stiffness results in more equations to be calculated, and each step of the analysis must be evaluated twice: once for ultimate strength and a second time for long-term effects. Fortunately, because the equations offer a closed-form solution, the process can be automated through the use of spreadsheets or other analysis tools. More information on determining the capacity of TCC framing systems can be found in the Design Guide.

Advantages of TCC Systems Utilizing the analysis procedure presented in the Design Guide, the author conducted a study of a structural framing bay for a typical 14 STRUCTURE magazine

office building. The study evaluated a 30 feet x 30 feet column grid framed with three different systems: steel-concrete composite members, non-composite timber members with cross-laminated timber (CLT) decking, and TCC members. The proposed TCC system consisted of the concrete topping, glulam framing, and CLT planks spanning between purlins, acting as the forming surface for the concrete topping and gravity support for loads between the purlins. Because the concrete must pour down to bear on the glulam beam for a proper connection of the shear fasteners, the CLT plank cannot be continuous (Fig. 1). The author proposes that while this design is unorthodox, it is viable by simply re-orienting the boards of the CLT so that the outer plies (the strong direction) are along the short dimension of the panel rather than the long dimension. This would permit CLT panels to be placed with the long dimension parallel to the supporting joists rather than perpendicular, allowing for similar erection efficiencies experienced using standard mass-timber construction techniques (Fig. 2). At the time of publication, at least one North American CLT manufacturer has published it is capable of providing a transverse layup in their CLT panels. Other layout techniques may be possible but were not considered as part of this study. The goal of the study was to determine whether it was possible to create a floor framed using a TCC system that has a similar depth to an SCC floor for a given bay size, since the depth of the framing is often a critical design constraint and can cause design teams to steer away from timber systems. The system was designed to be constructed without the need for shoring prior to the concrete curing ; an important consideration as shoring can lengthen the construction schedule and add cost. The study concluded the following : 1. The overall floor depth of the TCC floor system was only slightly deeper than the comparable SCC floor system (Fig. 3). 2. The timber concrete composite floor system resulted in a timber utilization intensity (TUI) of 0.51 cubic-feet per square-foot. This compared favorably to an all-timber floor system which resulted in a TUI of 0.56 for an optimized design (with no limitations placed on the member depth) or 0.67 for a floor limited to the minimum depth possible.

Fig.3. The SCC and TCC structural systems used in the case study are compared.

(The TUI in this case only considers the floor framing members and CLT). The environmental impact of the timber-concrete composite floor showed that the timber system has less Global Warming Potential (GWP) than the SCC system. It did have a higher impact than an all-timber system. This was primarily due to the additional concrete required to create a topping slab thick enough to make the TCC system viable. While initially this appears to be a disadvantage in the TCC system, it is important to keep in mind that the primary goal of this study was finding a viable alternative to steel, with the secondary benefit being a shallow floor depth compared to an all-timber system (Fig. 4).

expect from these other structural systems. In order to facilitate adoption, the author proposes: • Full-scale testing of TCC floor members, particularly those with beams as the timber component. • Creation of a design guide similar to the Design Guide for TCC Floors in Canada, but correlated to United States design standards and units of measurement. • When successful testing of a system has been completed, incorporation of these provisions in the NDS would make the methodology more accessible to a broad design audience. This will help enable designers to make use of timber framing in more applications, providing an alternative to more carbon-intensive materials. • Creation of additional design examples, including examples where glulam or other EWP beams are used as the timber component, owing to the fact that the procedure for determining strength and stiffness will be slightly different than with planks.

Conclusions Some design and construction firms have already begun to study the positive benefits of TCC Systems. With better familiarity, resources, and testing of this system, more implementation among designers could occur. ■

Future Development and Testing TCC systems currently in use and those evaluated in the design guide example problems are generally limited to sections created using concrete and timber planks rather than beams. This limits the practical applications of the system and does not acknowledge the benefits that can be achieved by creating sections that incorporate a beam element. These systems can be used in place of more carbonintensive materials such as steel and concrete, while still providing the long spans and minimal depths building owners have come to

Kirby Beegles is a structural engineer with Martin/Martin. He is a leader on their Wood and Mass Timber Subject Matter Expertise team and heads the Advocacy and Education components of their SE2050 committee. This article draws from a case study he completed as part of his graduate research at the University of Colorado at Denver.

Fig. 4. Shown is the GWP Comparison for the floor systems evaluated. The environmental impact is a measurement of the GWP per square-foot. This was evaluated using a simple LCA considering the industry-average GWP for stages A1-A3 and not considering the benefit of biogenic carbon.

JANUARY 2025

structural DESIGN

Optimizing Beam Hanger Placement in Mass Timber Structures Designers must balance the sometimes conflicting needs of gravity, fire safety, and seismic drift. By Dong Han, PhD, Lori Koch, MS, PE, Max Closen, MASc

ass timber buildings have gained significant traction in recent years, with building code updates making it easier than ever for designers to create taller timber structures meeting prescriptive code requirements. Their popularity is further boosted by their sustainability, visual allure, and abbreviated construction timelines compared to traditional building methods. A factor contributing to these expedited schedules is the prefabricated nature of mass timber components, including cross-laminated timber (CLT) wall, floor, and roof panels, as well as glued-laminated timber (glulam) beams and columns. These elements can be efficiently assembled on-site using either custom steel or pre-engineered connectors. Beam hangers, a type of pre-engineered connector, are often used to connect beams to columns in mass timber structures. Their placement on the beam end is influenced by three main factors: applied gravity loads, connection fire protection requirements, and seismic lateral drift limits. While wind loads do cause lateral drift, there are currently no codified limits for drift due to wind. Seismic drift limitations are established in ASCE 7, and in some cases, also subject to local building code regulations. The ideal placement of the beam hanger with respect to the neutral axis of the beam varies among these factors, requiring designers to accommodate sometimes conflicting needs. Figure 1 illustrates optimal positions for gravity demands (below the neutral axis), fire protection (above or around the neutral axis), and lateral drift accommodation (on the neutral axis).

Applied Gravity Loads To reduce wood splitting risk under gravity loading, designers should place the beam hanger at the lowest practical point on the beam end while complying with geometry requirements, including prescribed end and edge distances (Fig. 1A). This positioning helps reduce stress concentrations at the lowermost fastener, which can induce cross-grain tension and lead to splitting. Reinforcement, such as self-tapping screws, can be used to minimize splitting but requires careful detailing and coordinated installation. Reinforcement is typically unnecessary if the ratio between the distance from the top fastener to the loaded edge and the beam depth exceeds 0.70, as shown in Figure 1A. This guideline is known as the “70% rule.”

Fig. 2. Effects of seismic lateral drift at a beam-to-column joint.

16 STRUCTURE magazine

Fig. 1. Optimal beam hanger positions accounting for (A) applied gravity loads, (B) fire protection, and (C) seismic lateral drift.

Connection Fire Protection Designing fire protection for mass timber connections requires understanding the demands on connections during a fire and the options for complying with fire and building code requirements. In exposed mass timber construction, where fire resistance is provided by the mass timber itself, beam hangers are often placed higher in the beam end to provide adequate wood cover at beam-to-column joints. If this placement contradicts the "70% rule," reinforcing screws can be added at the potential origins of stress development. These screws can be generally countersunk into the timber to isolate them from fire. This process involves drilling holes prior to installation and then sealing them with wood plugs (Fig. 1B). Research, such as that conducted by Létourneau-Gagnon et al. (Applied Sciences 11.8 (2021): 3579) in Canada, is currently underway to further examine the performance of self-tapping screws when exposed to fire.

Seismic Lateral Drift When accommodating lateral drift, the ideal beam hanger placement is on the neutral axis of the section. The top and bottom edges of the beam experience the highest levels of stress and deformation. By placing the beam hanger closer to the neutral axis, the associated stresses and deformation are reduced. A comprehensive approach is essential to achieve optimal connection performance under seismic loading, taking into account factors like gravity loading and tensile stresses perpendicular to the length of the column. If deviations from the "70% rule" occur, reinforcing screws can be installed perpendicular to the grain to mitigate adverse effects. Seismic drift introduces tension and stress in the fasteners in the column, heightening splitting risk. To counter this effect and maintain a continuous load path, additional perpendicular-to-grain reinforcing screws may be required in the column (Fig. 1C). Furthermore, if fire safety presents a concern, the previously mentioned methods to protect reinforcing screws should also be applied.

Fig. 3. Methods for reducing stresses while ensuring fire safety.

Fig. 4. Methods for reducing joint stresses in cases without stringent fire protection requirements.

Drift-induced Load Demand Increase

When fire protection regulations permit an exposed connector or do not require a bonded joint, increasing the gap between the beam and the column can effectively delay the onset of a direct load transfer between them, thereby reducing stresses caused by imposed drift (Fig. 4A). This isolating approach can improve joint performance in a drift scenario and thus should be considered during design. In situations with an exposed connector as shown in Figure 4, the gap size matches the connector thickness (assuming no housing or cut-end detailing), which may not fully prevent bearing of the CLT panel end against the column under significant drift conditions. In cases where a smaller gap is required to meet connection fire protection requirements, bearing may take place even at lower levels of drift. In either scenario, the behavior will align with earlier discussions when drift intensifies sufficiently to close the gap and initiate bearing at the interface. Until the gap at the interface closes, the connector’s position has minimal effects on joint stiffness or stress development; instead, it determines the joint rotation where bearing initiates. Under this condition, the connector independently supports gravity load and rotational demands, resembling a soft hinge. Reinforcement must be applied as needed to minimize the risk of wood splitting. Situations with elevated seismic risks and specific design considerations may necessitate a larger gap between the CLT panel and the column, as illustrated in Figure 4B.

In practical applications, beams usually support CLT panel floors. These panels are attached to the underlying beams using screws, resulting in added stiffness and allowing for composite behavior between the beam and the panel system. One effective measure to manage seismic-induced deformations at these connections involves placing the beam hanger on the neutral axis of the assembly. This positioning considers the influence of the CLT panel above, which shifts the center of rotation. Failing to account for this impact can significantly increase the prying forces exerted on the connector, potentially compromising its structural integrity during seismic events. For simplicity’s sake, a model comprising a CLT panel attached to a beam connected to a column is used to illustrate the seismic impact, as shown in Figure 2A. In this model, the connector is positioned on the neutral axis of the beam, instead of the entire assembly. During a seismic event, as the column undergoes racking to the left, the CLT panel imposes minimal force on the connection because there is no direct load path (Fig. 2B). In contrast, when the column racks to the right, the CLT panel bears against it, establishing a direct load path that elongates the lever arm. This scenario exacerbates the prying effect at the connection (Fig. 2C), resulting in increased tension within the assembly.

Reducing Joint Stresses While Ensuring Fire Safety Two effective strategies exist to mitigate the stresses at beam-to-column connections during earthquakes, both of which involve isolating or eliminating the load path for these stresses. One approach is to create a greater distance between the narrow edge of the CLT panel and the column face. This wider gap reduces bearing at the interface, thereby diminishing the panel’s impact on joint stiffness (Fig. 3A). Proper sizing of the gap or machining of the panel end is essential to fully isolate the panel under maximum drift condition. Another approach aligns the column flush with the top face of the beam, enabling the CLT panel to extend over the column (Fig. 3B). To prevent direct load transfer between these components, the panel should be notched at the bottom, directly above the column. This design can improve the panel’s isolation from the column in response to seismic movements.

Reducing Joint Stresses in Cases Without Obligatory Connection Fire Protection Requirements For buildings or sections where connection fire protection requirements are not mandatory, an alternative approach can be used to address drift-induced stresses at beam-to-column connections.

Conclusion Optimizing the placement of beam hangers in mass timber structures represents a critical aspect of ensuring their structural integrity and safety. This process requires a holistic approach that addresses gravity loading—a constant concern—alongside fire safety, seismic drift, or both. There are multiple approaches to mitigate seismic drift-induced stresses at beam-tocolumn connections, and the selection among these methods depends on the prioritization of fire protection in the overall design considerations. ■ Dong Han, Ph.D, is a Senior Scientific Writer with MTC Solutions. He studied advanced materials for environmental, energy, and biomedical applications and has been involved in scientific publishing for over a decade. Lori Koch, MS, PE, is a Senior Product Engineer with MTC Solutions. She is a board member for SEAVa, serves on the NCSEA Continuing Education Committee, and is a licensed Professional Engineer in Virginia (support@mtcsolutions.com). Max Closen, MASc, is President of MTC Solutions. After completing a timber engineering degree at the University of Applied Sciences in Rosenheim, he pursued a master’s degree at the University of British Columbia, where his research on mass timber connections contributed to their first commercial application. JANUARY 2025

structural DESIGN

Grouted Mechanical Splices in Reinforced Concrete

Efficient load transfer, reduced costs, and simple installation make grouted mechanical splices a good option for resilient concrete structures. By Kayla Hanson, PE

ffective load transfer is crucial in reinforced concrete structural design but can become convoluted with increasing structure size and complexity. Design procedures typically presume continuous materials; however, casting an entire concrete structure monolithically is impractical. Most structures are composed of multiple segments due to jobsite space limitations, transportation weight and size restrictions, equipment capacities, and overall constructability. Historically, effective load transfer and proper segment connection has been achieved using reinforcement bar lap splices or welded splices. Another option to provide the connection vital to a structure’s performance and resilience is mechanical splicing. Mechanical splices use a coupler or sleeve to join two pieces of reinforcement and create a continuous connection that transfers forces and is ultimately stronger than the reinforcement bars it joins. Mechanical couplers may be grout-filled, steel-filled, threaded, use shear bolts, use swaging (a cold-forming compressive process), or incorporate a combination of these characteristics to hold the adjoining rebar in place, develop strength, and create continuity. Engineers specify grout-filled mechanical splices for optimization and resilience. Grout-filled mechanical splices eliminate the need for lapping and welding rebar—saving material, time, and labor—and as a result, can reduce congestion. Rebar used in fully grout-filled mechanical splices does not require end preparation or use of threading equipment. Even greater efficiency and cost reduction is realized when the splices are installed ahead of time by the rebar fabricator. In precast and tiltup applications, grout-filled splice systems tend to have an equivalent material cost to other types of mechanical splices; grout-filled splice systems for cast-in-place applications can be notably more economical than other mechanical splice options.

Grout-Filled Mechanical Splices Grout-filled mechanical splices typically use a cylindrical-shaped coupler or sleeve that is open-ended and varies in length, wall thickness, and diameter. The interior of the grouted portion of the sleeve is deformed to aid bonding between the grout and the sleeve and to increase slip-resistance. Some fully grouted sleeves come equipped with set screws to aid sleeve positioning during rebar assembly. Half-grouted couplers typically do not use set screws.

Grout-filled mechanical splices are widely used in applications including horizontal and vertical connections in cast-in-place, tilt-up, and precast concrete construction. These systems are common in connections between concrete beams, walls, columns, piers, and foundations. Most options available today are designed to meet ACI 318 requirements for mechanical splices. Most grouted mechanical couplers can be used with Grade 60 rebar that complies with ASTM A615, A706, A775, and A767. When using Grade 80 or Grade 100 rebar, the sleeves typically have the same geometry, but are cast with a stronger metal and are filled with stronger grout. Grouted couplers also can be epoxy-coated or galvanized.

Sleeve Size vs. Rebar Size Some grouted mechanical couplers are designed to accommodate different rebar sizes, typically corresponding to the sleeve size or two bar sizes smaller, depending on the supplier. For example, a #8-size sleeve may be designed to accommodate #8, #7, and #6 rebar. This interchangeability reduces the need to stock a different sleeve size for each rebar size. In some precast applications, sleeves are intentionally up-sized to provide greater tolerance in the field. For example, a #9- or #10-size sleeve can be used with #8 rebar Shown is a fully-grouted mechanical coupler system. to provide more annular space around the #8 rebar than a #8-size sleeve, affording more room to align and insert the field-set rebar into the sleeve during installation. Depending on the supplier, some fully grouted couplers can be used as transition couplers to join two different-diameter bars. The variation in bar sizes that can be joined is typically limited to a difference of two sizes. For example, a #10 and a #8 bar could be joined in some fully grouted couplers.

Rebar Embedment This illustration shows a half-grouted, half-threaded coupler.

18 STRUCTURE magazine

Adequate rebar embedment within mechanical couplers is imperative to fully develop the splice’s strength. Rebar embedment length typically

follows the sleeve size rather than the rebar size. Grouted couplers usually require rebar embedment equal to 6 bar diameters with respect to the sleeve size, not the bar size. For example, a #8 bar used in a #10 sleeve requires an embedment length of 7.62 inches (6 x 1.27 inches = 7.62 inches, where 1.27 inches is the diameter of a #10 bar, which corresponds to the #10 sleeve size).

Advantages of Grouted Mechanical Couplers Mechanical couplers offer benefits over traditional lap splices or welded splices, including reduced labor by eliminating tying or welding rebars, reduced steel consumption, and less rebar congestion by eliminating laps. While numerous styles of mechanical couplers are on the market, a one-size-fits-all solution does not exist. Each type offers advantages in different applications. Fully grouted couplers do not require steel overlap or special rebar end preparation like threading or saw-cutting. These grouted sleeve characteristics save contractors time and labor, contribute to jobsite safety, and eliminate the need for special rebar equipment.

Grouting While grouted couplers have many advantages over other mechanical couplers, they do require a grouting step, which is not needed with other mechanical coupler types. This step may seem intimidating to those unfamiliar with the installation of non-grouted mechanical couplers. Fortunately, clear and easily repeated processes exist for grouting sleeves. Grout is typically engineered specifically for this application. Only specify grout recommended by the sleeve supplier. The grout is generally high early strength, high ultimate strength, non-shrink, and highly flowable. Most grouts intended for use in mechanical splices are designed to achieve 4,000 psi compressive strength within 24 hours of casting and usually have design strengths over 10,000 psi. No proportioning is required when mixing grout or grouting the sleeves, and only simple equipment is needed –a drill, mixer blade, and steel mixing bucket. Simply measure the water into the bucket, add the entire bag of dry grout mix to the water, and mix. The process only takes a few minutes and can be done while the previous grout batch is injected into sleeves using a handy pump. The time and labor savings afforded by having one person mixing grout and one person injecting grout typically offset the cost of assigning two personnel to this task.

Three Rules for Optimal Grouted Mechanical Splice Performance: 1. 2. 3.

Ensure proper rebar embedment within the sleeve or coupler. Follow grout preparation best practices. Ensure the grout fully fills the sleeve or coupler.

Specification and Drawing Considerations Grout-filled mechanical coupler suppliers can provide engineers

This is a typical coupler orientation above the joint in a wall or column at a foundation.

with generic language or tailored specification language upon request. At a minimum, ensure specifications call out the items listed below: 1. Coupler product name, company name, and compliance required with the applicable sections of ACI 318. 2. Product name of grout required to be used in the system. 3. Requirement for couplers to be installed at the locations shown in the drawings and in accordance with supplier’s instructions. Additionally, at a minimum, ensure engineering drawings include: 1. Coupler product name and coupler size. 2. Reinforcing bar size. 3. Product name of grout to be used in the system.

Embracing Advanced Splicing Techniques Grouted mechanical splices are a crucial historical innovation in the field of reinforced concrete construction, offering significant advantages over traditional lap and welded splices. By providing efficient load transfer, reducing labor and material costs, and simplifying installation processes, these splices enhance both the performance and resilience of concrete structures. Understanding the anatomy, application, and installation of grouted mechanical splices is essential for engineers and contractors seeking to optimize their construction practices. As the construction industry continues to evolve, embracing advanced splicing techniques like grouted mechanical couplers will be key to achieving stronger, more durable, and more cost-effective infrastructure. ■

Kayla Hanson, PE, is Splice Sleeve North America’s Business Development Manager. Hanson graduated from Purdue University in 2013, and is active in various ACI, ASTM, and CRSI committees.

A fully grouted mechanical coupler system is depicted here. JANUARY 2025

inSIGHTS

Trends for Fire Qualification, Design of Post-Installed Reinforcing Bars & Anchors

Recent and upcoming advances in qualification and design of adhesives will provide clarity and confidence for designers. By Kenton McBride, Ph.D, PE

ire design of structural connections is vaguely addressed and loosely understood. Structural engineers are given prescriptive code requirements to address structural capacity during fire events, leaving unanswered questions, including the applicability of these requirements to adhesives used in structural connections. In some cases, engineers and jurisdictions have steered away from such adhesives due to lack of clarity on this issue. Chapter 6, Types of Construction, of the 2024 International Building Code (IBC) provides basic fire-resistance rating requirements for structural components of buildings. While no specific language is provided in Chapter 6 that directly addresses the fire-rating requirements of connections, it can be inferred that connections should be able to carry at least the lower of the fire-resistance rating of the two connected members, including both cast-in and post-installed solutions. Where fire ratings are not required, e.g., for nonstructural elements, it is assumed that sprinkler systems and other active fire protection measures are sufficient to address fire events. IBC 2024 Chapter 7, Fire and Smoke Protection Features, expands on the requirements with the following statements that address connections: • Section 704.2, Protection of the primary structural frame, requires that where protection is needed to address fire-resistance rating within the primary structural frame, encasement shall be provided on all sides, “including connections to other structural members.” • Section 704.5, Attachments to structural members, allows that “The edges of lugs, brackets, rivets and bolt heads attached to structural members shall be permitted to extend to within 1 inch (25 mm) of the surface of the fire protection.” • Section 704.5.1, Secondary attachments to structural members, expands the 704.5 requirement to require that any connections to steel members requiring fire protection be protected with the “same fire-resistive material and thickness as required for the structural member” with additional prescriptive requirements for the length of protection along the connected non-rated member. Minimum cover requirements for reinforced concrete and minimum thickness of insulating materials are provided for reinforced concrete and steel members in Section 721, Prescriptive fire resistance. Reinforced concrete cover requirements align with the principles outlined in ACI 216.1-14(19), Code Requirements for Determining Fire Resistance of Concrete and Masonry Construction Assemblies, which is directly referenced by IBC Section 722, Calculated fire resistance. These prescriptive cover requirements incorporate various reinforced concrete failure modes without further discussion of connections. Existing design of structural connections often implicitly accounts for the fire design scenario through the prescriptive requirements of IBC Chapter 7. However, some aspects of fire resistance of connections may not be fully addressed and may merit additional design checks, including the use of structural adhesives. Adhesives used in both steel-to-concrete and concrete-to-concrete fire-rated connections often include polymers that

Even in cases where the minimum cover and protection requirements of the IBC are met, the temperatures incurred by the adhesive layer in these connections could result in reduced adhesive capacities that should be checked against the design structural loads corresponding to the fire event.” are sensitive to elevated temperatures. Even in cases where the minimum cover and protection requirements of the IBC are met, the temperatures incurred by the adhesive layer in these connections could result in reduced adhesive capacities that should be checked against the design structural loads corresponding to the fire event. To address the issue of adhesive strength under fire exposure, multiple assessment and design procedures are underway or have recently been completed. Post-installed reinforcing bar products are evaluated in accordance with the International Code Counsel Evaluation Service (ICC-ES) Acceptance Criteria for Post-installed Adhesive Anchors and Reinforcing Bars in Concrete Elements (AC308), which serves as a basis for Evaluation Service Reports (ESRs) and a forthcoming ACI 355 document titled Qualification of Post-Installed Reinforcing Bar systems in Concrete. In AC308, criteria for evaluating the capacity of post-installed reinforcing bars at various temperatures are provided. The AC308 assessment of temperature resistance provides the necessary input parameters for design using the “Resistance Integration Method,” where the adhesive embedment is broken into contributing segments. Similarly, the resistance integration method can be applied to adhesive anchors connecting steel to concrete. Equation 1 and Figure 1 demonstrate the basic concept of design using this methodology (ignoring other applicable design factors), which can be summarized as follows: 1. A number of segments is chosen along the embedment. The greater the number of segments, the more accurate and less conservative is the calculated capacity. 2. For each segment, the highest temperature of the segment is identified and assigned to the entire length of the segment. 3. The temperature of each segment is mapped to the corresponding bond capacity of the adhesive at that temperature. 4. The bond capacities of all segments are summed to produce the adhesive capacity of the connection. n

Nba,i = / r : d : x ii : n i=1

20 STRUCTURE magazine

hef

where τθi is the product-specific bond stress corresponding to the greatest temperature in Segment i. This Resistance Integration Method will be formalized into future American design documents and guides. While other anchoring failure modes may also be affected by fire conditions, the extent of the effect and the necessity to address them explicitly remains unclear, as does the fire capacity for other shear fracture mechanisms in concrete, including sectional shear, punching shear, and interface shear, none of which are explicitly accounted for in design. In European design, highly conservative prescriptive capacities are provided for anchorage failure modes, but these requirements do not account for reduced temperatures due to typical fire protection on anchored connections. IBC Section 704.5 currently provides a prescriptive stand-in to address these failure modes. Further study should be made to verify the broad applicability of IBC Section 704.5 to anchorage and additional study should be made to determine the fire resistance of other shear failure mechanisms in reinforced concrete. These recent and upcoming advances in qualification and design of adhesives for post-installed reinforcing bars and anchors will provide clarity for designers, paving the way for confident fire-rated design of connections in combination with the existing prescriptive requirements of the IBC. ■ Kenton McBride, Ph.D, PE, is director of codes and standards at Hilti North America. Fig. 1. Illustration of the Resistance Integration Method for an adhesive anchor

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JANUARY 2025

inSIGHTS

Bringing Lower Carbon Concrete Usage to Buildings

This article focuses on the new roles and responsibilities of structural engineers to lower embodied carbon, and how to collectively advance lower carbon concrete usage responsibly. By Don Davies, PE, SE; Anne Ellis, PE; and Thomas Van Dam, PhD, PE

mbodied carbon (EC) emissions, measured through Global Warming Potential (GWP) benchmarks and thresholds, are increasingly prominent in owners’ project expectations and in construction policies, certifications, and specifications. Great strides are being made to reduce the industry’s net carbon output and to find new materials and methods other than the carbon-intensive portland cement for producing lower carbon concrete. Prioritizing, evaluating and validating lower EC concrete necessitates new roles, responsibilities and changes to our structural engineering practice. For decades, byproducts of industrial processes including fly ash and ground granulated blast furnace slag (i.e., slag cement) have been utilized as sustainable and cost-effective supplements and replacements for portland cement. A submitted and accepted concrete mixture utilizing these materials, or other supplementary cementitious materials (SCMs) like ground glass or natural pozzolans, may have earned project LEED points in prior projects. Increasingly prominent in owners’ project expectations and in construction policies, certifications, and specifications, though, is the requirement to quantify the EC of products used in constructing a building project. For concrete, it’s no longer enough just to specify the use of SCMs; reporting concrete mixtures’ carbon footprint through Environmental Product Declarations (EPDs) is becoming the norm. Programs asking for EPDs and the estimation of a project’s EC impacts within their standards include the United States Green Building Council (USGBC) LEED v4.1, the International Living Futures Institute (ILFI) Net Zero Carbon Certification, and the forthcoming American Society of Heating Refrigeration and Air Conditioning Engineers (ASHRAE)/International Code Council (ICC) Proposed Standard 240P. Similarly, federal and state governmental policies and actions are emerging, with states like California, Colorado, Oregon, Minnesota, Maryland, New York, and New Jersey leading the way. Requirements to quantify the EC of products may be reflected in material intensity policies, building intensity policies, Whole Building Life Cycle Assessment requirements, EPD requirements, tax incentives and/or cement reduction requirements. While harmonization in approaches and criteria is lacking, the underlying math is straight forward: A project's EC = estimated material quantities times the estimated material’s EC. This is the underlying basis to help guide next steps.

Who Should Do What? Ideally, the owner, at times aided by a sustainability consultant, should 22 STRUCTURE magazine

establish EC goals in the programming phase of a building project as defined within the Owner Project Requirements (OPR) documents, as well as the design team’s Basis of Design (BOD) documents. These goals then flow down to the project sustainability specifications (typically section 01 81 13). An advance summary report should define the total project GWP threshold for each major building component. This summary report typically requires advanced estimates of material quantities for the different building applications. While concrete mixture GWP benchmarks may be established, only the project level GWP threshold, which is the summation of each specific concrete mixture quantity estimate times its GWP benchmark, should typically become a contractual obligation. When establishing concrete GWP benchmarks and thresholds, soliciting early input from potential ready-mix suppliers on their capabilities is encouraged. As the responsible party for the development of project-specific structural concrete requirements, the structural engineer is responsible for aggregating this information within the project structural contract documents including the structurally relevant specifications and general notes, with the concrete GWP benchmarks set at the mixture level, and overall concrete GWP thresholds set at the project level. The National Council of Structural Engineers Association (NCSEA) Sustainable Design Committee has developed and recently published consensus-based recommended structural engineering general notes for this purpose. Further, the architect may add finish and tolerance requirements, which may or may not impact the concrete mixture design. The construction teams may also specify construction-related performance requirements including but not limited to set time, pumpability, and finishing. This contractor information often will be identified within the concrete bid documents, constituting means and methods of construction, and appropriately should not become part of the structural contract documents. Once the full concrete mixture requirements are complete, the construction team can proceed with material procurement. The concrete producer submits the proposed concrete mixtures including test data, for review and approval, demonstrating that the collective of the proposed mixtures meet or exceed the performance criteria and the established GWP project threshold set for concrete. The owner’s inspection and testing agency conducts responsible acceptance testing of the concrete delivered to the jobsite, in keeping with the project specifications. The construction team should be responsible for tracking and reporting as-built material volumes, the concrete GWP, and all other required material and product GWP thresholds. This information

can then be checked against earlier estimates. Collectively, this allows for a thoughtful tracking and management of both the concrete and the other GWP impacts accumulated through construction.

only the EC ambitions and new responsibilities that come with it, but also the augmentations to the contract drawings and project specifications facilitates success. In doing so, the use of lower carbon concrete will yield substantial opportunities to reduce the EC associated with building construction. ■

But What About Lower Carbon Concrete? Don Davies, PE, SE, is Principal at Davies-Crooks Associates in Seattle. (don@davies-crooks.com) Anne Ellis, PE, Hon. M. ACE, F. ASCE, NAC, is a Principal with Ellis Global. (anne@ anneellis.com) Thomas Van Dam, PhD, PE, is a Principal with Wis Janney Elstner - Transportation Research Group. (TVanDam@wje.com)

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There are numerous levers to pull to achieve lower carbon concrete starting with reducing the amount of portland cement in the mix. This can be achieved through improved quality control, better graded aggregates, and the use of SCM and alternative cements, including ASTM C595 blended cements. In limited markets, lower carbon hydraulic cements that satisfy ASTM C1157 may be available. Changing from “business-as-usual” concrete is often not as simple as a drop-in replacement. Short-term and long-term performance characteristics will likely change and some of these materials will require pilot test trials and mock-ups prior to their use. When using any new concrete materials in a building project, including lower carbon, it is incumbent upon all parties to use project-ready concrete for all specified components of the final, non-temporary construction. Projectready lower carbon concrete: • Uses standard-compliant materials and mixtures supported with appropriate testing data validating use for the intended application. • Achieves approval for use through the rules established within a project’s contract documents. • Has sufficient material availability to meet the project demand and schedule needs. Advancing innovation via pilot test trials of lower carbon concrete is an important step in a material’s transition toward becoming projectready. Piloting concrete should be restricted to controlled conditions where tear-out can be tolerated, such as non-structural applications like sidewalks, or temporary blinding slabs. Concrete placement in trials should be under the supervision of the responsible parties for ultimately signing off on that final use condition. Additionally, novel materials are coming into the market that don’t yet meet criteria of Standards Development Organization (SDO) such as ASTM or others, and/or project contract document approvals. These materials are only appropriate for testing in research and development efforts. Given their lack of standards, they typically should not be used as part of the final project. Individual and team experience and sophistication with establishing, measuring, and validating EC will vary greatly now and in the future. Successful outcomes are improved by timely and effective communications with all impacted parties including the owner, architect, construction team, and the ready-mixed concrete producer. Informing the entire team of not

JANUARY 2025

Image Credit: Jason O'Rear

Redefining Structural Efficiency & Resilience A 10-story parking structure in South San Francisco achieves efficiency and resilience beyond the code-level life-safety performance objective with a precast concrete hybrid moment frame system.

By Mei Kuen Liu, SE, and Chris Petteys, SE

ith the recent boom in the biotechnology industry, the Oyster Point neighborhood of South San Francisco, California, evokes large, busy life science campuses more so than its namesake bivalve mollusks. Serving the workers on 865,000 square feet over three buildings of laboratory space on the new life science campus, the Kilroy Oyster Point Phase Two Parking Structure provides 1,961 parking spaces on a 260 feet square footprint over ten stories. As large as the parking structure appears on the approach, drivers may be surprised to realize they have just entered on Level Three, as it is uniquely carved into a hillside to minimize its towering height. With vehicle entrances and exits distributed on three sides and on two different levels, aided by the use of reversible lanes with dynamic LED digital signage guiding traffic, the time it takes to enter and exit the parking structure is minimized. The parking structure’s four-bay configuration has two central park-on ramps and pedestrian vertical circulation points that include three stair towers and five elevators. The elevators are arranged in a dual-bank that are strategically located to minimize the pedestrian travel time from the parking space to their final destination.

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As the first impression to those entering the campus, the parking structure serves as the gateway to other buildings. The parking structure’s aesthetic picks up design cues from the buildings it serves. The exposed prefabricated precast concrete is color-integrated and accentuated with a perforated metal screen that undulates along its elevations with form, color and light.

System Description The structural concrete elements are precast under controlled conditions at Clark Pacific’s Woodland manufacturing plant outside of Sacramento, California. The gravity system is comprised of precast double tees, 63 feet to 68 feet in span, that are supported by precast girders, which are in turn supported on the corbels of gravity columns. A thin, 3 ½-inch thick cast-in-place concrete topping slab over the double tees ties the precast gravity elements together and acts as the seismic diaphragm. For its lateral system, the parking structure layout is a perfect candidate for precast hybrid moment frames (PHMF). With almost a

Image Credit: Kilroy

Image Credit: Forell Elsesser Engineers

A structural Revit model of the parking structure in the Oyster Point neighborhood of San Francisco is shown here.

The parking structure is shown in red here on the overall campus plan.

square plan, Forell | Elsesser Engineers are able to efficiently locate the moment frames along the four exterior elevations of the building. The ramps are strategically located in the two interior bays of this four-bay parking structure, meaning each level of the PHMFs is completely flat and not complicated by the ramp geometry. This translates to much greater fabrication and erection efficiency. Seismic joints are provided at mid-height of each ramp such that they do not interconnect the floors and inhibit inter-story drift and therefore don’t negatively impact the performance of the moment frames. The moment frame elements are designed to all have the same dimensions to streamline precast production. The Moment Frame Columns (MFCs) are 30 inches by 48 inches and Moment Frame Beams (MFBs) are 24 inches wide by 50 inches deep. At first glance, the MFBs appear to be very deep: in fact, the beam depth is carefully coordinated so these perimeter beams can be upturned and double as vehicle barriers and guardrails. Minimum code required guardrail height is the primary driver to the MFB depths. Compared to the conventional cast-in-place moment frame system, the PHMF dissipates seismic energy through a similar mechanism but in well-defined locations. The precast MFCs are delivered to the site as multi-story elements, and each precast MFB is single bay, spanning between adjacent columns. The only elements that go through the beam and column joints are the unbonded post-tensioning tendons, located at the mid-depth of the MFBs, and special rebar that is descriptively termed “energy dissipating rebar,” typically located at the top and the bottom of the MFBs. At the concrete plant, the MFCs and MFBs are cast with sleeves for these jointing elements. Once the moment frames are erected at the project site, the post-tensioning tendons are stressed to a pre-determined tension, similar to bridge PT construction, and then locked, cut, and grouted along with the conduits for the energydissipating rebar. Since the tendons are sheathed, they are unbonded between end anchor points. The energy dissipating rebar is installed in corrugated metal sleeves through the column and into the connecting beams. The most important feature of the PHMF system is that a portion of the energy-dissipating rebar is de-bonded before the grout is injected into the full length of the sleeves. This de-bonded region of the energy-dissipating rebar serves as the designated yielding element in the lateral system. During an earthquake, the moment frame beam-to-column joint opens and closes, which puts these energy-dissipating rebar to work by axial yielding. After the seismic event, the post-tensioning tendons, which

are designed to remain elastic, will restore the frame to its original position with no residual drifts. As such, the only yielding locations of the PHMFs are limited to the beam and column joints, and near the base of the MFCs to a lesser extent.

Validation Through Testing Clark Pacific Director of Operations Randy Clark was instrumental in leading the design, fabrication, and installation of the precast structural components utilized in the Precast Seismic Structural Systems (PRESSS) research program that constructed a five-story, 60 percent scale shake table test performed at the University of California San Diego over 20 years ago. This research significantly advanced the understanding of the seismic-resistant design of PRESSS, of which the PHMF was one of the lateral systems being tested. "As a young engineer, I didn't fully grasp the profound implications of the work we were undertaking,” Clark said. “It seemed like just another project at the time. However, in retrospect, understanding the wealth of knowledge gleaned from those tests and the transformative impact this has had on the field of building science, I feel a deep sense of pride for being part of such a seminal project." During testing, the PHMF was subjected to high seismic story drifts and performed with flying colors. At the early stages of loading, the researchers observed low levels of damage, limited to inelastic action at the beam-to-column connections, as intended. They gradually increased the seismic loading and loaded the specimen to a drift level of 4.5 percent, which is 225 percent of the code-allowed design drift level of 2 percent and observed no significant strength loss in the structure.

Design and Analysis The analysis of this system is similar to a conventional cast-in-place special concrete moment frame system, apart from an important difference in the establishment of the effective sections to use for the frame elements, and more accurately, the beam and column joint. Many analytical investigations were conducted in the preceding two decades, and the methodology developed by Forell | Elsesser Engineers draws on those earlier studies, as well as practical limitations of model run time and project schedule. JANUARY 2025

Image Credit: Jason O'Rear

The parking structure layout, which is an almost square plan, is an ideal candidate for precast hybrid moment frames. Forell | Elsesser Engineers located the moment frames along the four exterior elevations of the building.

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Image Credit: Clark Pacific

To study the effects of joint opening at the beam-column interface, conventionally reinforced (not post-tensioned), hence ACI 318 recForell | Elsesser Engineers examined results from a detailed, component- ommended section modifiers are appropriate. level, nonlinear analytical study in CSI SAP2000 by Computers & The design of PHMF elements is governed by the ACI Standard 550.3 Structures, Inc., using solid elements. As noted previously, only two Design Specification for Unbonded Post-Tensioned Precast Concrete Special types of elements go through the joint. The energy-dissipating rebar Moment Frames. While it is not the intention of this article to be a primer is modelled as nonlinear axial hinges to capture potential yielding and to this ACI Standard, it is worth noting that the frame element design the corresponding energy dissipation. The post-tensioned tendons are is governed by seismic drifts and rotation of the beam-column joints, designed to remain linear and are modelled as linearly elastic link ele- and that it is an iterative process to satisfy multiple competing criteria. ments. In a conventional cast-in-place concrete moment frame structure, the beam section modifier is used to account for the effective section after cracking. In the case of a PHMF structure, however, the postConstruction Advantages tensioned beams are designed to remain elastic (no cracking), and this modifier is used to capture the effects on beam-column joint opening. A prime benefit of precast construction is that most of the time- and A part of the topping slab can be included to the MFB section (similar labor-intensive work has already been done at the plant. Since the project to T-beams), but since the slabs are located near the middle of the MFB is developed as a campus with multiple buildings, having a staging area depth, the slab contribution was deemed not significant in the analyses. Jointing the topping slab around the MFC is not recommended due to maintenance concerns. By calibrating the two models, Forell | Elsesser Engineers were able to distill the nonlinear behavior from the solid model as a single number to use in the linear frame model as a beam modifier. Compared to the ACI 318 recommended value for an effective beam section of 0.35, they found that a lower modifier of 0.30 is more appropriate. It is worth emphasizing that while the modifier in ACI is a result of cracking, the modifier used here is to account for the opening of the beam-to-column joints; the individual precast elements remain uncracked. While a lower value modifier means a softer structure with larger seismic drifts, this softening also lengthens the structure’s fundamental period and hence reduces the associated spectral acceleration. This results in a reduced seismic PHMF components in the parking structure include fiber reinforced grout, post tensioned strands, multi-strand anchor, and mild base shear for the structure. The MFCs are steel reinforcement.

Image Credit: Clark Pacific

PHMF joint rotations are illustrated. The yielding locations of the PHMFs are limited to the beam and column joints and near the base of the moment frame columns.

is sometimes not possible. The precast parking structure team was able to schedule truck shipment of precast elements on a just-in-time basis, thus eliminating the need for a large lay-down area. As opposed to a typical average crew size of 53 workers for a cast-inplace project, you would typically only see a team of 24 workers at the project site for precast construction. While the total labor between the two construction types is comparable, the precast system is able to shift about 75 percent of the total labor offsite. This reduction in field labor translates to a reduction in field construction duration from about 40 weeks to just 22 weeks—a significant reduction by any measure.

USRC Platinum Rating With a mission to educate, advocate, and promote resilience-based design that considers the impacts of natural disasters as an essential component of long-term sustainability, the US Resiliency Council (USRC), a 510(c)(3) non-profit organization, was established in 2011 by cofounders Evan Reis and Ron Mayes as a way to educate building stakeholders and the public about the importance of resilient design to the community and the inseparable link between resiliency and sustainability. Its earthquake building performance rating system is being used by public and private owners and communities and is forming the basis of economic and financial incentives being developed by lenders and insurers to reward high-performing buildings. Using the PHMF system, the parking structure received the prestigious Platinum Verified Earthquake Rating from the USRC. This is the highest rating awarded by the USRC and represents the pinnacle in structural earthquake performance. In order to achieve a Platinum rating, a building must meet strict performance thresholds in three categories: Safety, Damage, and Recovery. Platinum-rated buildings suffer negligible damage, with repair costs less than 5% of the building’s replacement cost. They are also expected to allow for functional recovery from immediately to within a few days of a major seismic event. Injury and blocking of building egress paths is also unlikely. The evaluation to determine building performance is based on performance-based design philosophies and the methodology outlined in FEMA P-58 “Seismic Performance Assessment of Buildings.” The methodology is based on a probabilistic approach to risk assessment. Instead of evaluating the building performance based solely on a single maximum considered earthquake, the probabilistic approach considers a range of possible earthquake scenarios and their likelihood of occurrence, as well as the variability in building response and potential consequences of different damage states. To evaluate the performance of key components in the building, FEMA P-58 uses fragility curves

The construction crew erects framing for the parking garage.

that represent the probability of certain levels of damage that may occur to a building component (structural, architectural, mechanical, electrical, etc.) given a specified degree of ground motion intensity. Each curve is component-specific and is developed and built from intensive empirical testing data. Rooted in the decade-long PRESSS research program, the PHMF system is backed by testing and aided by advanced analysis. In the third decade of implementation in the San Francisco Bay Area, PHMF structures continue to provide owners efficient structures with savings inherent with precast construction, in addition to enhanced seismic performance. With the USRC ratings program, we can now quantify the seismic performance beyond the minimum code-level life-safety performance in the face of high seismic demands and deliver a structure that is efficient, resilient, and sustainable. ■

Mei Kuen Liu, SE, is a Senior Associate with Forell | Elsesser Structural Engineers, San Francisco, California. Chris Petteys, SE, is President and CEO of Forell | Elsesser Structural Engineers, San Francisco, California.

JANUARY 2025

A neighborhood of houses in Texas was built with walls printed layer by layer onsite and highlights material and construction innovations in structural design.

From Nozzle to Neighborhood A 100-home, 3D-printed community in Texas showcases structural innovations in homebuilding. By David P. Langefeld, PE, and Sam Covey, PE

dditive construction, also called 3D printing, is a rapidly advancing technology and an innovative method for the construction of wall systems and other structural and nonstructural elements, that departs from the traditional technique of formed-and-poured concrete used for cast-in-place and precast concrete. In the case of Wolf Ranch, a master-planned community located north of Austin, TX, in the vibrant, growing city of Georgetown, 100 3D-printed homes meant a departure from the traditional technique of stick-framed and concrete construction. As a leader in construction-scale 3D printing robotics, software, materials, and construction technology, ICON partnered with nationwide homebuilder, Lennar, internationally renowned design architect, BIGBjarke Ingels Group, and Austin-based structural engineering firm, Fort Structures, to build the largest community of 3D-printed single-family residences to date. The 3D-printed homes, offered in eight floorplans ranging from about 1,600 to 2,100 square feet, blend contemporary Texas ranch style aesthetics and energyefficient designs that highlight the benefits of resiliency and sustainability with the digital possibilities of additive construction. This article overviews the 3D-printed wall system used in the project.

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The 3D-printed ranch homes were offered in eight floor plans ranging from 1,600-2,100 square feet.

Wall System Testing and Approval by AHJ 3D printing utilizes a material batching and delivery system and a robotically controlled process of extruding mortar or concrete layerby-layer along a designated print path. Section R104.11 of the 2021 International Residential Code (IRC) allows for alternative materials and methods, such as 3D-printed structures. The Authority Having Jurisdiction (AHJ) reviews and approves the alternative if it’s acceptable and equivalent to systems and products already incorporated in the Code. The acceptability and equivalency of the 3D-printed wall system was supported by a suite of tests conducted by accredited third-party laboratories. These tests assessed the material properties and the wall system’s structural capacity. The material testing consisted of measuring the compressive strength, interlayer tensile bond strength, flexural bond strength, shrinkage, and freeze-thaw resistance. The structural testing consisted of measuring the axial capacity, out-of-plane flexural capacity, and in-plane shear capacity of over two dozen full-scale wall specimens following the general guidance of ASTM E72, “Standard Test Methods of Conducting Strength Tests of Panels for Building Construction.” Material and structural test data and reports consistent with the requirements of Section R104.11 of the 2021 IRC were made available, and the 3D printing and structural design processes were discussed prior to the AHJ’s plan review of the structural engineer’s signed and sealed structural drawings. Furthermore, test results, design procedures, and equivalency checks as described in the ICC reference document AC509, “Acceptance Criteria for 3D Automated Construction Technology for 3D Concrete Walls” were formally summarized. Therefore, in addition to review by the AHJ, the 3D-printed wall system was reviewed by the International

Code Council Evaluation Services (ICC-ES). Based on their review, ICC-ES issued ESR 4652, “ICON 3-Bead Wall System,” which describes the wall system in more detail. The ESR, grounded in material and full-scale structural tests, provides basic information about this new type of wall system for structural engineers and demonstrates to AHJs that this wall system meets or exceeds the performance of existing wall systems already specified in the IRC. Structural engineers can use this general information, as well as more detailed information provided by ICON, to develop project specific details and designs. Regular collaboration and communication with the structural engineer and the AHJ combined with the extensive testing regime and third party evaluation and validation of the 3D-printed wall system enabled a smooth and timely plan review process, building permit issuance, and construction.

Structural Design Overview Each of the 100 homes consists of a metal roof and prefabricated wood roof trusses sitting atop the 3D-printed wall system. Each home is supported on a post-tensioned concrete slab-on-ground foundation. Though the 3D-printed wall system was innovative and new, the foundation was typical for central Texas in terms of slab thickness, grade beam depth and spacing, post-tensioning, and the 28-day design concrete compressive strength of 3,000 psi. The 3D printed wall system utilized a three-bead wall system, as the gravity force- and lateral force-resisting systems. The wall system consists of several parts, namely, beads, shells, and cores. A bead refers to a single, nominally 3/4-inch tall by 2-1/2-inch-wide extrusion of 3D printing material, called Lavacrete, added layer-by-layer by a 3D printer, called JANUARY 2025

The 3D-printed wall system underwent a series of tests by third-party laboratories to assess the material properties and structural capacity.

Vulcan, following a designated print path. A shell refers to a stack of multiple layers of one or more beads, and a core refers to a bounded vertical space printed integrally with the shell that is later reinforced and filled with Lavacrete. This 3D-printed wall system contains both an exterior and interior shell and is named the three-bead wall system because it contains three beads at any given elevation—a single bead on the exterior shell and a double bead on the interior shell. The exterior shell protects the structural wall from impacts and transfers the out-of-plane loads to the interior shell via the cross ties. The double-bead interior shell is the structural portion which spans horizontally between the cores. The cores span between the foundation and the top of wall as simply supported elements and provide the resistance for axial, bending, and shear. The exterior and interior shells are separated by a cavity which is filled with insulation for an overall nominal wall thickness of 12 inches. These two shells are connected via C- or Z-shaped cross ties consisting

of 3/16-inch diameter plain wire spaced 18 inches on-center in both directions. At least one cross tie is included within each core. Each shell contains horizontal reinforcement consisting of #2 or #3 bars located in the center of the shells and spaced either 8 or 12 inches on center vertically. The horizontal reinforcement is designed to resist volume changes from temperature and shrinkage and, in the case of the interior shell, to span between cores. Due to the additive construction process, both the cross ties and horizontal reinforcement are placed incrementally during the print process. The vertical cores are the main structural elements, resisting gravity, out-of-plane, and in-plane loads. Each core consists of a grouted element vertically reinforced with one #5 reinforcing bar. The vertical reinforcement in the cores is placed at the conclusion of the print process, and the cores are subsequently infilled with Lavacrete. Vertical cores are located at the edge of each opening, at locations of high gravity load, and not more than 6 feet on center.

The structural details of a 3D-printed wall produced for homes in the Wolf Ranch community are diagrammed here.

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A 3D printer using a material batching and delivery system and a robotically-controlled process extrudes mortar or concrete layer by layer at the construction site.

At the top of the walls, a wood top plate and wood rim beam are bolted to the 3D-printed wall with a 3/4-inch diameter threaded rod that is coupled to the #5 vertical reinforcing bar. The continuous wood beam spans over windows and between the cores. The design team decided to use a wood rim beam to eliminate the need to shore and grout bond beams at all the openings which resulted in quicker construction. The roof trusses bear on the wood rim beam atop the exterior side walls and are connected to the top plate with hold down hardware and fasteners.

Testing and Quality Control During Construction The structural engineer specified the requirements for inspections and testing for the construction of the 3D-printed wall system. During printing, the Lavacrete was routinely sampled from the nozzle to measure its fresh properties, including temperature, slump, density, and air content. In addition to measuring the fresh properties, sets of 3-inch by 6-inch cylinders were molded and submitted to a third party laboratory for compressive strength testing. At a minimum, fresh material samples were obtained and cylinders were molded once after the first 2,500 linear feet of printing and every 10,000 linear feet thereafter. For a single home, this sampling frequency typically yielded approximately 15-20 quality control samples and compressive strength tests. The Lavacrete average 28-day compressive strength for the project was greater than 4,000 psi, well more than the specified design strength of 2,000 psi. While the structural engineer performed periodic observations, the construction documents required ICON to submit material test data, photographic evidence documenting the placement of reinforcement, print progress updates in terms of layers per day, and evidence of the application of a bond agent if interlayer print times exceeded 120

minutes. Reports were submitted to the structural engineer using traditional construction management software. In addition to the project-specific requirement, the ESR for the threebead wall system was contingent on ICC-ES reviewing, approving, and inspecting quality documentation and procedures. ICON is subject to semi-annual follow-up inspections from ICC-ES.

Conclusion Construction-scale 3D printing is the latest, state-of-the-art building technology producing attractive, resilient, and energy-efficient homes. This method is one solution to the growing skilled labor shortage and the increasing costs of construction. U.S.’s first community of 3D-printed homes offers a glimpse into a promising path toward delivering resilient, beautiful, technology-driven homes that meet rising demand. ■

David P. Langefeld, PE, has a background in structural modeling and analysis, condition assessments, and testing supporting complex forensic engineering investigations. At ICON, he leads the evaluation and ongoing compliance of ICON’s structural building systems at the local, state, and federal levels, and he sits on multiple consensus-based committees for 3D printed concrete that are working to develop standards, tests, and guidelines for the construction industry. Sam Covey, PE, is the founding principal of Fort Structures, overseeing multiple 3D-printed concrete projects as the Engineer of Record. At Fort Structures, Covey leads the construction technology division, specializing in structural engineering for innovative building systems along with engineering a wide range of specialty commercial structures and high-end custom residences.

JANUARY 2025

Image courtesy of Mike Hume at historictheatrephotos.com.

UNVEILING SECRETS

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The seismic retrofit of the Egyptian Theatre in Los Angeles serves as a model for balancing modern engineering demands with the preservation of architectural heritage. By Jonathan Lehmer, SE and Melineh Zomorrodian, SE

he historic Egyptian Theatre by Sid Grauman was designed as one of the great movie palaces in a Revival-Egyptian style in 1921 and opened to the public in late 1922 by hosting the world’s first red carpet movie premiere. The building's original structure consisted of reinforced concrete frames with hollow clay tile infills at all interior and exterior walls. Prior earthquakes have resulted in the collapse of similar building types. The City of Los Angeles passed an ordinance requiring the demolition or analysis and possible retrofit of concrete frame buildings designed prior to January 1977. The design team, led by architects Studio 440 and structural engineers Structural Focus eagerly got to work to retrofit this landmark. Work on the Egyptian Theatre, which is listed on the National Register of Historic Places, was performed according to the City’s ordinance and the California Historical Building Code. During construction, unforeseen conditions required the design and construction teams to collaborate towards flexible solutions to accomplish an effective retrofit and rehabilitation. The Theatre re-opened to the public in late 2023. The Theatre is a mostly single-story building with an attached threestory portion at the southwest corner. The building is approximately 200 feet long, 130 feet wide, and 64 feet tall. The concrete roof structure is multi-tiered and supported by a series of concrete trusses to create an unobstructed interior with minimal interior columns. Concrete beam and column frames form the perimeter walls and all columns are generally supported on spread footings (Fig. 1). The Theatre received several renovations and changes over the years, including modification of the proscenium (the dividing line between the stage and auditorium) structure to increase the viewable stage width for a wide projection screen and a renovation and voluntary seismic retrofit in 1997/1998 following significant damage from the 1994 Northridge earthquake.

PROJECT TEAM Owner: Netflix, Los Angeles, CA Structural Engineer of Record: Structural Focus, Gardena, CA Architect (General): Studio 440, Hollywood, CA Preservation Architect: Historic Resources Group, Pasadena, CA Mechanical, Electrical, Plumbing Engineer: Syska Hennessy, Los Angeles, CA Geotechnical Engineer: Geotechnologies, Inc., Glendale, CA General Contractor: Whiting-Turner, Irvine, CA Construction Manager: Lincoln Property Company, El Segundo, CA

Fig. 1. The Egyptian Theater has a sloping mid-level roof over an exterior portico entry, a low flat roof over the interior lobby area, a mid-level flat roof over the auditorium, and a high flat roof over the stage.

JANUARY 2025

City of Los Angeles Retrofit Ordinance Strong earthquakes have caused severe damage to concrete buildings designed and constructed without ductile detailing requirements implemented in the 1976 Los Angeles City Building Code, or similar codes from that period. The City of Los Angeles implemented an ordinance that required the compliance, retrofit, or demolition of concrete buildings permitted before January 1977 within city limits, with exceptions for detached single-family dwellings or detached duplexes. The prior 1997/1998 voluntary seismic retrofit of the Theatre was determined to be inadequate for the requirements of this ordinance. The team proceeded with an ASCE 41 approach to meet the ordinance requirements.

Material Testing and Condition Assessment The design team located detailed original architectural and structural drawings of the Theatre and the structural drawings of the previous voluntary retrofit which were invaluable. However, despite having the detailed drawings, a major challenge faced at the project's start was the lack of material properties in the original documents. A material testing program was prepared based on the ASCE 41-17 Comprehensive Data Collection Requirements and according to the LADBS Information Bulletin P/BC 2020-153 to obtain the concrete and reinforcing strength of the original construction. As part of the material testing, the height and spacing of the deformations on the reinforcing bars were recorded to calculate the rebar development and splice lengths using guidance by the Concrete Reinforcing Steel Institute. A total of 72 concrete cores and 15 rebar samples were tested to determine the expected strength, lower bound strength, and applicable knowledge factor based on the coefficient of variation for each member type according to ASCE 41-17. These final values were used in the analysis and design. Foundation testing was delayed until construction due to inaccessibility and an assumed foundation material strength from ASCE 41-17 based on the building's age was used during design and confirmed by testing during construction. ASCE 41-17 and the LADBS ordinance required a Visual Condition Assessment of the building when performing a Tier 3 Risk Category III evaluation because the original and previous voluntary retrofit drawings of the Theatre contain detailed information about the member sizes, configurations, and reinforcing. To comply with the required Visual Condition Assessment quantities, all locations of material testing and an additional 40 original beams and 10 original columns were specified to be visually inspected for damage, degradation, and plumbness. The condition assessment verified the accuracy of the construction documents and found limited concrete damage and cracking which was not unexpected for a 100-year-old structure.

Project Retrofit Scope Structural Focus’ experience with seismic retrofits of similar non-ductile concrete buildings informed the decision to avoid using any of the existing non-ductile concrete frame system as a vertical lateral force resisting system. A proposed 34 STRUCTURE magazine

Fig. 2. As shown in this Egyptian Theatre ETABS Model, original concrete roof slabs are in gray; new or previous retrofit concrete shear walls are in red; original non-ductile concrete frame columns are in light blue; original non-ductile concrete frame beams are in dark blue; and new collectors are green.

seismic retrofit system of new concrete shear walls was developed, and the behavior of the building with the proposed seismic retrofit upgrades was analyzed in ETABS using a linear dynamic response-spectrum-based modal analysis procedure (Fig. 2). To confirm a linear dynamic analysis was acceptable, ASCE 41-17 section 7.3.1.1 was followed and required the new lateral force resisting system did not have in-plane and out-ofplane discontinuities. Section 7.3.1.1 allows a linear dynamic analysis with prescribed limitations if a weak-story or torsional strength irregularity is present. The layout of the new lateral force resisting system avoided a weak-story irregularity, but a torsional strength irregularity was confirmed to exist in the east-west direction; however, the DCRs in that direction were confirmed to be below the prescribed limitations defined that allows Linear Dynamic Analysis to be utilized. To create diaphragm continuity across the various roof elevations, new concrete diaphragm transfer walls were designed as deformation-controlled elements to avoid adding interior shear walls that would interrupt the auditorium area. Utilizing the analysis results from the linear dynamic model, the shear and flexural strengths of the existing concrete diaphragms were analyzed per ASCE 41 as deformation-controlled actions. While the flexural strengths were sufficient, the existing shear strength of the lightly reinforced 3-inchthick diaphragms was deficient adjacent to various shear wall lines. Fiber-reinforced polymer (FRP) diaphragm shear strengthening demands were determined per ASCE 41 as deformationcontrolled actions and specified on the structural drawings for deferred submittal design (Fig. 3). The diaphragm connections to the shear walls were then analyzed per ASCE 41 as force-controlled actions and found to be deficient. To mitigate the deficiencies, new collectors were designed to increase the shear transfer length with the addition of FRP between the diaphragm and collectors to increase the shear transfer strength (Fig. 2). The new collectors and FRP shear transfer strengthening were both designed as force-controlled actions. At limited areas, the compressive strength of the existing concrete beams was adequate for the collector compression demands but due to inadequate reinforcing Fig. 3. Early coordination between Structural Focus and quantity and development the collector tension various FRP suppliers was required to ensure the diaphragm demands required adding FRP tension collector FRP strengthening design requirements specified on the structural drawings were feasible. strengthening on the existing beams. The FRP

Fig. 4. During shear wall reinforcing, the installation of rebar dowels required frequent coordination despite the design considerations because the existing reinforcing proved challenging to avoid. Filling abandoned drilled holes as the drilling proceeded was important to avoid losing excessive capacity of the existing concrete members.

tension collector and shear transfer demands were analyzed and specified on the structural drawings as force-controlled actions for deferred submittal design after coordination with various FRP suppliers to confirm feasibility of design requirements prior to bidding. The retrofit design continued with the analysis of the new and previous retrofit concrete shear walls for in-plane shear and flexure as deformation-controlled elements per ASCE 41. Each individual wall pier (bounded by existing beams and columns) was analyzed and designed/ strengthened with appropriate m-factors (~1.75 to 2.25 for BSE-1E and ~2.25 to 3.25 for BSE-2E). The existing concrete walls required shear strengthening, which was feasible using FRP. For walls requiring FRP strengthening, the required equivalent horizontal reinforcing with 60 ksi yield strength was specified in the structural drawings along with the existing material strengths and the classification of the member action for deferred submittal design. The new and existing concrete shear walls were analyzed for out-ofplane shear and flexure as force-controlled elements per ASCE 41. Infills were determined to span either horizontally between columns or vertically between beams based on the available continuity of the

reinforcing and the out-of-plane aspect ratios of the wall infills. The existing columns and beams were analyzed for the appropriate out-ofplane loads where required and confirmed to be adequate without any added strengthening. Continuity of the shear walls for in-plane and out-of-plane demands through the existing concrete beams and columns was achieved by doweling rebar through the existing concrete members (Fig. 4). The strength and shear friction transfer of these bars was critical for the intended shear wall behavior to act as single large wall panels per wall elevation. The design utilized large diameter bars for these dowels to reduce the number of drilled holes and the likelihood of hitting/damaging existing reinforcing bars in the existing concrete members. At shear wall special boundary zones which continued through existing beams, it was selected to demolish the length of the beams where the boundary zone occurred while keeping the existing beam reinforcing intact. This enabled the boundary zone reinforcing to be placed continuously through the beam and the portion of beam demolished was then replaced by new concrete. Temporary shoring of the existing beam was required while the ends were demolished and was accomplished with small steel pipe members that were eventually embedded in the new concrete walls. The preservation and protection of the historic plaster finishes on the interior faces of the new exterior shear walls required careful consideration during the construction of the shotcrete walls. Avoiding damage to the historic plaster was of paramount importance for the project. The contractor, in coordination with the design team, elected to use small light gauge steel shaft studs to hold the backing for the shotcrete wall along with the waterproofing membrane (Fig. 5). The shaft studs were buried inside the shotcrete wall without interrupting the continuity of the reinforcing. All new and existing shear walls extended down to be supported on new concrete foundations. The new foundations were designed to carry the full seismic load, while the original building foundations under the existing concrete columns remained unchanged to support gravity loads. The new foundations were epoxy doweled into existing foundations for continuity and load transfers, utilizing the gravity loads from the existing columns to resist the overturning seismic loads on the new foundation. The existing building foundations are at different elevations throughout the site, and at some locations, they are +/- 15 feet below grade. To avoid surcharging or undermining the existing foundations, the bottoms of the

City of LA Ordinance Requirements: • Buildings must meet 75% of current LA Building Code (LABC) seismic demands for strength with gravity members having deformation compatibility for 100% LABC drifts. • OR Buildings must meet ASCE 41-17 Building Performance Objective for Existing Buildings (BPOE) using a tier 3 procedure. For a Risk Category III structure: Performance Level

Hazard Level

Damage Control

BSE-1E: Not less than 75% BSE-1N

Limited Safety

BSE-2E: Not less than 75% BSE-2N

• Qualified Historic Buildings shall comply with the 2019 California Historical Building Code (CHBC), which permits maximum seismic forces to be limited to 40% of the building or component mass for Risk Category III buildings.

Fig. 5. A shotcrete test panel with shear wall infill confirmed that proper protection of the plaster and concrete consolidation around the stud flanges was consistently achievable.

JANUARY 2025

Fig. 6. Foundation construction required further coordination with the existing field conditions on a case-by-case basis; some steps that were previously assumed to be required were eliminated, and step heights were adjusted to reduce the number of steps as much as possible.

Fig. 7. After the portico columns were strengthened, they were analyzed to verify the plastic rotational capacity was adequate for the end rotations due to the inelastic building drift.

new and existing foundations were aligned in the structural drawings. This proved challenging during construction due to the required depth of excavations, adjacent property lines, adjacent building surcharges, and steps needed in the new foundations to accommodate elevation changes. Due to the project schedule, rebar shop drawings were produced based on the original drawings and fabrication began prior to excavation. Adjustments were made in the field as existing foundations were exposed and found to deviate from the original drawings, causing rebar congestion and step layout issues (Fig. 6). The independent gravity columns of the Theatre that were not integrated into new concrete shear walls were analyzed as deformation controlled secondary members according to ASCE 41 for seismic drift compatibility. The analysis revealed that the independent columns had inadequate shear strength to develop the full expected plastic flexural capacity of the columns and were classified as shear-critical. To strengthen the columns to achieve a flexure-critical condition, the required FRP shear strengthening to develop the full expected plastic flexural capacity was specified on the structural drawings as equivalent grade 60 shear reinforcing along with the existing material strengths and the classification of the member action for deferred submittal design (Fig. 7). The existing elaborate historic plaster ceiling system was another critical element to brace without compromising the historic fabric of the Theatre. The 1-inch-thick ceiling was supported only by vertical wires and created a potential falling hazard during a seismic event. Utilizing the option of meeting the California Historical Building Code (CHBC) as permitted by the ordinance, a new lateral bracing system was designed with light gauge steel framing.

Project Renovations Scope Major renovations to the Theatre were included in the project with the goal of enhancing the audio and visual capabilities and user experience to achieve a state-of-the-art movie experience similar to other modern theaters. Additionally, in collaboration with the historical 36 STRUCTURE magazine

preservation architect Historic Resources Group, the design team was driven to further reveal and highlight the original architecture of this grand movie palace. Although the original Theatre design did not include a lobby area, the new architectural layout required one that would also host a control booth. A new steel framed platform for these functions was designed to visually and acoustically separate the non-enclosed entry from the auditorium. Below the lobby platform a new concrete partial basement provides the necessary area for an emergency generator and mechanical equipment. A new light gauge steel framed projection booth on the low roof is accessed by elevator from the lobby and is capable of projecting modern movie formats and nitrate film formats (one of only five nitrate film projection theaters in the United States). Various stage improvements utilizing conventional steel and light gauge steel framing provide the infrastructure for presentations and for the large projection screen. Steel tension cable supported speakers in the auditorium provide high quality Dolby Atmos audio and a new auditorium seating platform constructed of tiered raised access floor systems leads from the lobby platform down to the stage. Finally, a new steel stair and catwalk provides access to the fly loft and original organ loft from inside the three-story structure at the southwest corner of the Theatre. A defining feature of both the 1997/1998 renovations and the new renovations is the design for “reversibility.” Reversibility is characterized as features designed for the possibility of future removal without negatively impacting the original features and structural stability of a building. Its applicability extends to architecture, mechanical, electrical, plumbing, structural, and all other renovation features of a building. This design feature is especially important for historic structures to avoid unnecessarily impacting historic fabric and to allow the structure to undergo future renovations without significantly altering the historic structure. The design team collaborated so the new renovations are designed to be either standalone without tying into the core and shell structure, or removable without negatively impacting the structural strength of the core and shell structure like the cantilever partition walls around the new lobby, or the tension cables supporting the speakers.

Fig. 8. Concrete frame repair at southwest portion consisted of shoring where required to remove and repair poorly placed concrete, mortar or epoxy injecting cracks, cleaning, and repairing corroded reinforcing, and placing repair mortar at spalls and to achieve proper clear cover on reinforcing bars.

Fig. 9. To adequately transfer the new girder loads into the columns and to enlarge the column concrete area, an interior reinforced concrete column enlargement was designed to be epoxy doweled to the full height of the original column and to achieve direct bearing under the ends of the new proscenium girder.

Hidden Conditions and Damage During construction, despite the initial findings of the visual condition assessment during design, after the removal of all the exterior plaster finishes widespread poor-quality concrete and damage was found. Poor quality concrete consisted of inadequate consolidation, rock pockets, honeycombing, and inadequate reinforcement clear cover. Damaged concrete consisted of cracking, reinforcement corrosion, spalling, and delamination. The cracked concrete is likely partially due to damage during the Northridge earthquake and other regional earthquakes. A repair program was enacted to rectify these structural issues encountered since the full design strength of many of the original elements was necessary for the retrofit (Fig. 8). During the early phases of the design, it was noted that the original proscenium columns on each side of the stage did not exist, and no documentation of this modification could be found. The original proscenium was designed with a narrow view of the screen, and it is likely that as movie projecting technology progressed, the desire for a wider screen resulted in the widening of the proscenium. As construction commenced and finishes were removed, it was alarmingly discovered that the columns had been demolished without adequately modifying the structure of the roof to increase the proscenium girder span from the original approximately 42 feet to the modified approximately 70 feet. Considering the tributary roof areas loading the proscenium girder line consisted of the high roof over the stage, the organ loft roof, and the organ loft floor slab, large shoring towers to support the deficient proscenium girder were installed. A new large concrete girder was designed to support the tributary roof and organ loft loads and span approximately 70 feet between the perimeter columns. The original perimeter columns that now supported the new girder were analyzed for the increased load and found to be deficient for confinement and concrete area. Since the columns could only be enlarged towards the interior to avoid changing the exterior historic features of the Theatre, the requirements for added confinement were designed as additional FRP strengthening and the equivalent grade 60 confinement reinforcing was specified on the structural drawings for deferred submittal (Fig. 9). During construction, it was also discovered that an undocumented sloping

concrete topping slab had been previously added on the original low roof of the Theatre over the new lobby area. In some areas the topping slab was up to 12 inches thick and significantly increased the weight of the low roof compared to the original 3-inch slab. Since the weight of the topping slab was not considered in the retrofit design already under construction, as well as the requirements for installing FRP directly on the structural roof diaphragm slab, the topping slab was selected to be removed. Removal was time consuming and required care to be taken to avoid damaging the structural roof diaphragm slab because the topping slab was found to consist of hard rock concrete that was well bonded to the 3-inch slab.

Conclusion The comprehensive retrofit and rehabilitation of the historic Egyptian Theatre in Hollywood stands as a remarkable example of how engineering innovation can merge with historical preservation and modern renovations to achieve seismic resilience and the successful re-birth of a landmark historic building. By adhering to the rigorous requirements of the City of Los Angeles’ mandatory retrofit ordinance, the project successfully navigated the complexities of retrofitting a non-ductile concrete (NDC) structure without compromising its historical integrity. Through the strategic use of fiber-reinforced polymer (FRP) and traditional strengthening techniques, the Theatre was fortified to meet modern seismic demands while preserving its iconic architectural elements. Moreover, the careful bracing of historic elements like the historical plaster ceiling and the reinforcement of the proscenium girder illustrated the commitment to maintaining the Theatre's historical character while upgrading its structural performance. ■ Jonathan Lehmer, SE, is a project engineer at Structural Focus in Gardena, CA. His project experience ranges from new high rise to retrofit and rehabilitation and historic preservation. Melineh Zomorrodian, SE, joined Structural Focus in 2008 as an intern, and remained with the firm following her Masters graduation from UCLA. She specializes in structural analysis, design, and construction administration. JANUARY 2025

codes & STANDARDS

The Building Code Highway

The Post-Tension Institute (PTI) DC10.5 is one of many referenced standards in the building code, and it stands alone from the requirements of the originating codes, as well as the ACI. By David Sparks, SE, PE Reprinted with the permission of the Post-Tensioning Institute (PTI).

ngineers-in-training coming out of school have little idea of how to apply building code to what they are taught in school. On top of that, many senior engineers, those that would even call themselves “experts,” also do not understand the building code. With this lack of understanding comes a corresponding lack of knowledge regarding how to apply the numerous other standards referenced in the building code. The PostTension Institute (PTI) slab-on-ground standard (DC10.5) is just one of those many referenced standards that exist in the main building code. To best understand the building code, specifically the International Building and Residential Codes (IBC and IRC aka “I Codes”), think of the codes like a highway. The highway itself is marked with exit signs that branch off in paths to other standards (these are referenced in Chapters 35 and 44 for the IBC and IRC respectively)—and even some loops that lead back from one code or one standard to another. Sometimes municipalities will unconditionally adopt the I Codes by reference. Other times, the municipalities will amend the I Codes as necessary to achieve the path through the codes that make the most sense for them. The same may also be true for entire states, like California for example, where the state may adopt and amend the I Codes as a statewide code. Amendments to the I Codes are much like road closures or detours. In those cases, exits from the code to other standards are closed, and/or specific paths through the code are eliminated or changed. Researching amendments, whether municipal or state, is the important first step before beginning your journey into and through the I Codes. This first step provides a route with the closures and detours in mind so that we do not get lost along our journey. The flow chart in Figure 1 helps illustrate the route through the I Codes. While both the IBC and IRC have references to other standards, this

Fig. 1. This flow chart illustrates the route through the I Codes.

38 STRUCTURE magazine

flow chart includes only those pertinent to the discussion for establishing a path to the PTI and American Concrete Institute (ACI) standards. The arrows in the flow chart indicate a one-way direction on the path through the code. To arrive at the IRC, the municipal or state codes delegate the scope directly or as typically done in Chapter 1 of the IBC. The exit ramp to the IRC is necessary in Chapter 1 of the IBC so that the scope of any IBC references does not take place before arriving in the IRC. Once you arrive in the IRC, it gives some alternate reference standards, but in IRC Chapter 3, direction is given that two triggers exist to leave the IRC other than the alternate reference standards: 1. Engineered rational analysis, which requires the use of the IBC, is acceptable instead of the IRC provisions. 2. Portions of the structure that are non-compliant with the IRC shall use the IBC. As of 2024, both the IBC and the IRC contain exit ramps to the PTI DC10.5 document. Before the 2024 edition of the I Codes, the only way to access the PTI DC10.5 standard was to go through the IBC. With the return path from the IRC to the IBC for portions of the structure that are non-compliant with the IRC, this does not appear to be an issue at first. However, the IBC exit ramp to the DC10.5 document occurs in Chapter 18 under subsection 1808.6.2 for slab-on-ground foundations within section 1808.6, “Design for Expansive Soils.” The remainder of IBC Chapter 18 does not address slab-on-ground foundations on stable soils. While the IBC has not and still does not mention stable soils, the 2024 IRC Chapter 5 Section R506.2 references the proper title of the DC10.5 document, “Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive and Stable Soils.” Before the release of DC10.5-19, the title of the DC10.5-12 reads “PTI DC10.5-12: Standard Requirements for Design and Analysis of Shallow Post-Tensioned Concrete Foundations on Expansive Soils.” The DC10.5-12 predecessor, DC10.1-08, reads “PTI DC10.1-08: Design of Post-Tensioned Slabs-on-Ground.” Both the DC10.5-19 and DC10.5-12 documents make the following statement regarding their predecessor, “This standard is based on PTI DC10.1-08. Refer to this document and the commentary on this standard for background and interpretational information

that clarifies its application.” Chapter 2 of PTI DC10.1-08 deals with stable soils. As such, practicing engineers have used the PTI standards for many years on stable soils as the standard of practice to design PT Slabs on stable soils, but the IBC technically does not explicitly state it. The resulting inconsistency regarding stable soils that persists today in the IBC leaves an opening for assertions that the IBC does not reference the PTI for stable soils. While this interpretation disagrees with the document that the IBC references, it is something that needs to be corrected.

Exiting on a One-Way Road— Do We Need to Go Back? Once we exit the IBC or IRC and enter into the destination of the PTI DC10.5 standard, there is no reason to go back or to access other standards, unless specifically instructed to do so within PTI. In the case that the PTI does not address specific structural criteria, the standard should indicate a path for the Fig. 2. Durability destinations within the I Codes are summarized with the original highway map shown here. designer to return to the originating standard (IBC/IRC) for those criteria. So using the highway analogy, we made it to destination. There are no secondary exits, roundabouts, or U-turns leading DC10.5 town, but they may not have all the things necessary to stay to other standards or back to the IBC and IRC. there and may indicate a return to the last exit. In the case of the PT slabWhile the PTI DC10.5 is demonstrably independent of the other on-ground, concrete strength and durability are two of the requirements standards, it is interesting to note that Chapter 19 of the IBC directly needed. So the question is – does the PTI standard have that information? references ACI 318. Specifically, under IBC section 1904 “Durability,” Within DC10.5-19 in Chapter 10.0 “Materials,” the standard requires the first sub-section, 1904.1, lists ACI 318 durability requirements as the that concrete shall have a minimum compressive strength of 2500 psi (17 standard for the IBC and ACI 201 by reference. However, before leavMPa) at 28 days. Moving on to Section 10.4 “Durability,” the required ing 1904.1 an exception exists about Group R-2 and R-3 occupancies. compressive strength increases to 3000 psi (21 MPa) for concrete exposed These building types include apartments, condominiums, townhomes, to freezing and thawing or to deicing chemicals. and one-and two-family dwellings - residential and light-commercial The standard continues to guide the designer on necessary measures applications. The only durability requirement under the exception is that for the PT slab-on-ground when the site has various levels of soil sulfates the minimum concrete compressive strength shall be 3000 psi (21 MPa) and soil chlorides. These measures include changing the cement type and with no further action given including the w/cm ratio. minimum compressive strength for sulfates and protecting the tendons The 2024 IRC indicates that concrete slab-on-ground floors adhere and conventional steel from chlorides. Therefore, at high sulfate levels, to ACI 332 or the provisions of R506—which includes DC10.5 under like freezing and thawing, the minimum 28-day compressive strength R506.2 specifically addressing PT slab-on-ground. Therefore, the final increases to 3000 psi (21 MPa). There are no requirements about the destination from the 2024 IRC is DC10.5 independent of ACI 332. water-to-cement ratio (w/cm) of the concrete. While the reference out of the IRC may seem insignificant to this disThe PTI DC10 requirements are consistent with the minimum strength cussion, it is extremely important. Neither the IBC nor the IRC refer requirements contained in ACI 332-14, but not with ACI 332-20. The out to ACI about PT slab-on-ground. If they did, then there would be commentary to ACI 332-14 states that the w/cm ratio has a limit in a conflict between ACI 318 and ACI 332. This is further justification place by controlling the minimum required compressive strength of the that DC10.5 is a standalone document. concrete. While ACI 332-20 does now contain limits for w/cm ratio, All the various, independent, durability destinations within the I Codes the PTI DC10.5 document does not. can be summarized by the adaptation of the original highway map shown From PTI DC10.5, there are no references to ACI about durability, in Figure 2. nor are there references back to the IBC since the PTI receives scope as a standalone document. In fact, in the commentary of DC10.5 Chapter 1, it states (underlines added by the author), Do Any Road Signs Confirm the “Post-tensioned concrete foundations designed by this standard generDestination in DC10.5? ally meet the requirements for plain concrete specified in Chapter 14 of ACI 318-14. These foundations will typically contain less reinforcement In July 2017, the PTI Technical Advisory Board (TAB) published – prestressed and non-prestressed – than the ACI318 requirements Technical Note 21 (PTI Technical Note 21, Issue 21—July 2017, “TN21”) for reinforced concrete. This standard is intended to be a standalone further clarifying the independence of the PTI DC10.5 document. TN21 document uniquely developed for the design of post-tensioned concrete gives exit signs for those that arrived in the ACI 318, and by reference foundations on expansive and stable soils and is supported by the perfor- ACI 201, for the design of PT slab-on-ground foundations. mance of many thousands of existing conformant foundations. As such, While TN21 uses ACI 318-14 for its facts, improved and clarified it is intended that this standard be independent of ACI 318 and the verbiage exists within ACI318-19. According to TN21, ACI 318-14 conflicting parts of the general building code into which this standard Section 1.4.7 states that “This Code (ACI 318-14) does not apply to design is incorporated.” and construction of slabs-on-ground, unless the slab transmits vertical loads The PTI DC10.5 document lets the designer know that it is the end or lateral forces from other portions of the structure to the soil.” At first glance JANUARY 2025

Fig. 3. All rodes designing a post-tensioned slab-on-ground lead to the PTI DC10.5 standard.

this infers that ACI 318-14 might apply. However, the commentary R1.4.7, clearly states (bold font added by the author), “Detailed recommendations for design and construction of slabs-on-ground and floors that do not transmit vertical loads or lateral forces from other portions of the structure to the soil, and residential post-tensioned slabs-on-ground, are given in the following publications: • ACI360R. • PTI DC10.5-12”. (Emphasis added by the author) In the most current ACI at the time of this article (ACI318-19), Section 1.4.6 states, “For one- and two-family dwellings, multiple single-family dwellings, townhouses, and accessory structures to these types of dwellings, the design and construction of cast-in-place footings, foundation walls, and slabson-ground in accordance with ACI 332 shall be permitted.” However, the commentary for section 1.4.6 then indicates an exit route to DC10.5-12 for expansive soils. What do we do about stable soils? The commentary goes on to refer to section 1.4.8 for stable soils which ultimately lands at the ACI 360R standard. Entering Chapter 10 of ACI 360R-10 for the design of post-tensioned slabs-on-ground, we find a reference to the PTI as an applicable design procedure under section 10.2.4. Once again, all roads and all exits have ended within the PTI DC10.5 document. As simply and clearly stated by TN21 (underline added by the author): “This Technical Note is to clarify that the post-tensioned residential slabs-onground were never intended to be governed by the ACI 318 Building Code.” Finally, the PTI published an article titled “Code Requirements for Sulfate Durability in Residential Concrete” in February 2008 of the PTI Journal in Volume 6 Issue 1 authored by Mr. Ken Bondy. It is important to note that a review of this article occurred under the PTI Journal publication policies. The article discusses that in historical practice the w/ cm ratio did not apply to residential concrete design and construction. Typical compressive strengths used were between 2000 and 3000 psi, which equated to w/cm ratios of 0.8 and 0.6, respectively. Therefore, litigation ensued because the w/cm exceeded the requirements for sulfates (0.45 or 0.5 depending on conditions) of the ACI tables. At the time of the journal article, ACI 332 had not yet included limitations in the form of w/cm ratio. ACI 332 now does include limitations, but recall that ACI 318, 201, and 332 do not have scope on the DC10.5 document. Some of the early engineers in the PT slab-on-ground field did not have the luxury of the information presented in this article. As 40 STRUCTURE magazine

a result, those engineers reacted to the many lawsuits regarding sulfate durability allegations by changing their plan specifications and, according to the journal article, this “… has resulted in a significant increase in the cost of many new homes in California, Nevada and Arizona, with no related benefit.” The journal article also addresses the question: Why different durability requirements for residential concrete? The journal article suggests that factors of safety in the range of 30 are present under light residential loads. In addition, it makes the point that reducing the w/cm ratio from 0.6 to 0.45 effectively changes the service life of the concrete from 150 to 200 years, while the materials in the structure on top of the concrete only have a service life of roughly 75 to 100 years. Finally, after going through the various durability requirements for PT slab-on-ground within the PTI, which do not (then or now) include limitations on w/cm ratio, the journal article summarizes as follows (bold font added by the author): “…They (the codes) will include a requirement for sulfate-resistant cements, but they will not require direct limitations on w/cm, which are difficult to control in fresh concrete and impossible to evaluate precisely in hardened concrete. This will clarify and refute the erroneous allegation that the mere reference to a sulfate-resistant cement by a licensed design professional somehow also triggers a requirement for a limitation in w/ cm. In this model code, w/cm ratios will be indirectly controlled, when necessary, by specifying a minimum concrete compressive strength. These new code criteria are consistent with long-standing successful practices for sulfate durability in residential slabs and foundations. They should help in reducing opportunistic lawsuits which have resulted in increased costs to homeowners with no related benefit, and are based simply on a lack of clarity in code wording rather than a real deficiency in performance.”

Conclusion Understanding the scope and code flow within the building code is a necessity. Without a proper map of the route through the code for a posttensioned slab-on-ground, an engineer, designer or lawyer might get lost along the way. The result is a potential increase in lawsuits regarding the applicability of standards within the code. Should the engineer, designer, or lawyer get lost in the ACI because of a lack of understanding, it is important to note that all those roads in the ACI regarding post-tensioned slab-on-ground still lead back to the PTI (Fig. 3). Paths exist to the PTI DC10.5 document as the standard for the design and construction of PT slab-on-ground foundations on expansive and stable soil types. There are areas within the I Codes and ACI with references to the PTI DC10.5 document that still need additional wording reflecting the correct application of the document to stable as well as expansive soils. Efforts are underway by the author and others to make the path as clear as possible to engineers, designers, and lawyers. Even so, the DC10.5 standard is a standalone from the originating codes as well as the ACI. Very simply stated, DC10.5 is “… independent of ACI 318 and the conflicting parts of the general building code into which this standard is incorporated.” ■

Full references are included in the online version of the article at STRUCTUREmag.org. David Sparks, PE, SE, is vice president of research and development for Felten Group, which is an architectural and structural engineering consulting firm specializing in residential construction.

codes andSTANDARDS

FAQ on SEI Standards Questions you always wanted to ask. By Jennifer Goupil, PE

his quarterly article addresses some of the questions received about structural standards developed by the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE). Questions from engineers, building officials, and other design professionals are often considered to develop future editions. These topics and more are discussed on the ASCE Peer-to-Peer Standards Exchange Forum. ASCE/SEI members can ask and answer questions in the forum. Visit https://collaborate.asce.org/standardsexchange/home to learn more and read about other topics.

ASCE 7-22 Errata What are errata items for ASCE 7-22, or any standard for that matter, and how do I get them? The definition of errata items for the purposes of SEI Standards are items that were balloted correctly by the Standards committee but, unfortunately, were not printed correctly in the final published version of the Standard. Errata can happen for several reasons, but the most common is that the language that was balloted was not included in the final file for publication. The process of balloting standards provisions is iterative and the final revisions—which can often include editorial changes, typos, or coordination of section numbers—are the most common type of errata. These items occur last in the balloting process and can be erroneously overlooked when creating the final files for publication. However, since these items were balloted and adjudicated correctly by the committee, they are considered part of the Standard and are identified as errata and published. All of the errata for ASCE standards that are developed by SEI are included on a webpage titled “ASCE 7 & SEI Standards” in a drop-down list at the bottom of the page. Errata are available as a free download from the ASCE Library and are also accessed via the Standard’s own page in the ASCE Library. Errata are published periodically as the items are identified and confirmed by the Standard’s chair. The errata for ASCE 7-22 and Supplement 1 has been published on July 24, 2024, and is available as a free download.

Important ASCE 7-22 Errata What am I supposed to do with the Errata that has just been published for ASCE 7-22? And what are the most important items for me to be aware of? When the errata is published, owners of the Standard should annotate the sections that have been identified in the errata. This will ensure that designers are using the Standard as it was intended to be published and will also aid in clearing up any confusion that may result from typos or incorrect pointers. Several important errata have been included in the July 2024 publication, including the following: • Correction of Equation 12.10-8 to remove the 0.8 coefficient for the lower bound of Cpi. The lower bound of Cpi = Cpo. Refer to the July 2023 edition of Structure Magazine for a more detailed explanation of this errata item. • Note 7 in Figures 25.5-1B, -1C, -1D for the Basic Wind Speeds

• •

for Risk Category II, II, and IV (respectively) are corrected for approximate probability of exceedance in 50 years to be 7%, 3%, and 1.6% respectively. Note these errata were corrected prior to map adoption into the 2024 International Building Code (IBC) and are shown correctly in Figures 1609.3(2), (3), and (4), respectively in the IBC. Parapet wind loads in Figure 30.6-1 is corrected for the leeward parapet for Load Case B. This should be for Zones 4 or 5, which aligns with the windward parapet pressures. Corrections of referenced standards in Supplement 1 including the reference to the 2020 edition of Seismic Design of LiquidContaining Concrete Structures and Commentary, published by the American Concrete Institute. This correction includes a corresponding edit to remove an exception from Section 15.7.7.3 and adds clarifying language for sloshing forces in the commentary.

Heavy Live Loads and Permitted Reductions* I have a technical question about provision 4.7.3 in ASCE 7-22. Suppose there is a 1st floor column of a 3-story building that supports 2 floors (not including roof), which have a live load = 125 psf, and I want to find the axial live load in the 1st floor column. I have seen some sources state that the 2nd floor live load cannot be reduced because the 2nd floor column only supports one floor, but the 1st floor column can be reduced since the 1st floor column supports more than one floor. Other sources have the live load on both floors reduced since the 1st floor column supports two or more floors, and that is what the axial load is being computed for. What is the correct methodology to apply this provision? The first interpretation is correct. Each story-length of column is treated as a unique element. In this case, the first story column supports two floors, and the second story does not. *Reprinted from ASCE Peer-to-Peer Standards Exchange Forum. Log-in to ask your Qs online! This article’s information is provided for general informational purposes only and is not intended in any fashion to be a substitute for professional consultation. Information provided does not constitute a formal interpretation of the standard. Under no circumstances does ASCE/SEI, its affiliates, officers, directors, employees, or volunteers warrant the completeness, accuracy, or relevancy of any information or advice provided herein or its usefulness for any particular purpose. ASCE/SEI, its affiliates, officers, directors, employees, and volunteers expressly disclaim any and all responsibility for any liability, loss, or damage that you may cause or incur in reliance on any information or advice provided herein. If you have a question you want to be considered in a future issue, please send it to sei@asce.org with FAQ in the subject line. Visit asce.org/sei to learn more about ASCE/SEI Standards. ■ Jennifer Goupil, PE, F.SEI, F.ASCE, is the Managing Director of the Structural Engineering Institute and the Chief Resilience Officer for the American Society of Civil Engineers.

JANUARY 2025

Structural Engineering Executive Retreat 2025 Gain Insights. Build Your Network. Elevate Your Vision.

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Claim Management: Share lessons to prevent and handle claims and near-misses effectively. Next-Gen Talent: Discover strategies to attract and retain young engineers. Future-Proofing: Learn to shift from current projects to future opportunities.

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SE NEWS

The Kreher Preserve and Nature Center (KPNC) cut the ribbons on its new Environmental Education Building on Dec. 6, 2024. Engineered from cross-laminated timber (CLT), the building serves as a meeting space, educational classroom, and place of learning for everyone to enjoy the natural world. Photo courtesy Auburn University.

Auburn’s new environmental education building features CLT-construction A

uburn University unveiled its newest venue for environmental education at the Kreher Preserve and Nature Center (KPNC) during the official dedication and ribbon cutting of its Environmental Education Building held Friday, Dec. 6. Nestled at the entrance of the 120-acre tract of the Kreher Preserve and Nature Center located on North College Street in Auburn, Alabama, the structure will be a dynamic focal point of the KPNC that will welcome tens of thousands of visitors and program participants annually. Housing the KPNC’s Woodland Wonders Classroom and the multi-purpose City of Auburn Community Classroom, the new state-of-the-art building

provides climate-controlled space for educational offerings and special events and gatherings. The initial concept and design of the structure was completed by Boston’s Leers Weinzapfel Associates, led by Tom S. Chung, FAIA, LEED AP BD+C as principal architect and Su Poon, AIA, LEED AP. With funding from the Alabama Legislature, the building was constructed using cross-laminated timber, or CLT, a mass timber product engineered using southern yellow pine produced in Dothan, Alabama. As such, the building showcases Alabama’s forest industry and the advancement of CLT as a sustainable alternative

to traditional building materials. Further enhancing the CLTconstructed design, the team incorporated exposed natural wood ceilings, walls, floors and abundant windows to blur the boundaries between indoor and outdoor spaces, promoting a sense of wonder and exploration of the natural world. The building sits above the land on footings, and the landscape design incorporates natural features like the “Johnny Lawrence Memorial Rain Garden,” which minimizes environmental impacts and maintains natural water and runoff systems. Among its primary uses, the building will be the home to Kreher’s Woodland Wonders Nature Preschool, an emergent,

nature-based curriculum that invites children to learn through the experience of nature. “This Environmental Education Building is a testimony to that mission,” said Auburn President Christopher B. Roberts. “It embodies the united partnership between the state, university, city and community members who have joined forces to create this dynamic space for our youth and community to gather, learn, and appreciate the bounty of the natural world.” “Aesthetically, this building is a showpiece of the beauty of CLT construction, but within its walls, it will accomplish something much more powerful,” said Auburn President Christopher B. Roberts. ■ JANUARY 2025

Paris’ Racine Tower renovation complete I

n November 2024, Maud Caubet Architectes completed the Racine project in Paris's 12th arrondissement for Alderan (SCI EWOK), the project owner. The project involved the renovation of the former headquarters of the Office National des Forêts (ONF), a 35-meter-high tower (115 feet) built in 1970 into new offices for the Local Business Skills Operator (OPCO EP). The extensions and the new crown represent a spectacular transformation. Project cost was €24 million, or about $25.2 million. The Racine project is situated in a predominantly residential area, and the tower stands as a testament to an era when urban development relied heavily on slab construction, with largely artificialized ground levels. The reclamation of fertile soil thus became a foundational constraint The top floor of the Racine Tower includes a greenhouse and café. Photo credit: MAUD CAUBET ARCHITECTES

for the project. In this context, the initially envisioned graduated garden has been reinterpreted as terraced landings. A 175-square meter (1,884 square-foot) productive green roof allows for the renaturation of the entire site, despite the densification brought about by the project. Two parking levels, out of five underground levels, were restored to living spaces with natural light. A large, curved patio, 12 meters (39 feet) deep, redefines the new spaces opening onto the terraced garden. Finally, the project repurposed the volume of the parking ramp to create an unusual space with direct access to the new patio. The planned adaptability of the structure to easily change its purpose (offices, housing, activities, etc.) was anticipated through a design that minimizes, in advance, the scale and cost of future modifications. The restructured tower is inherently flexible, allowing for future transformations: the building's repetitive

Friendship bridge connects Brazil and Paraguay T

he completion of the Ponte da Integração BrasilParaguai, the friendship bridge linking Brazil and Paraguay, is a long-awaited second connection between the two countries with a main span of 470 meters, making it the longest in Latin America. A project of this size required suitably large expansion joints, which were delivered by the structural protection specialists at MAURER. The Ponte da Integração 44 STRUCTURE magazine

Brasil-Paraguai is an asymmetric cable-stayed bridge with a total length of 760 meters. The pylons reach up 190.17 meters in Brazil and 185.04 meters on the Paraguayan side. The bridge crosses the Paranà river and connects the Paraguayan city of Presidente Franco with Foz do Iguaçu in Brazil. The bridge has an average width of 17.8 meters (58.4 feet), with lanes measuring 3.6 meters (11.8 feet) wide in each direction. It also

has a 3-meter-wide (9.8 feet) hard shoulder and a pavement measuring 1.7 meters (5.6 feet) in width. It will relieve the strain on the friendship bridge inaugurated in 1965 between Foz do Iguaçu and Ciudad del Este. This older bridge will then be closed to heavy goods vehicles, which will only be allowed to use the new bridge. The record span required the largest expansion joint constructions ever built in either country. Fitted at the abutments at both

ends of the bridge, these flexible elements compensate for movements that the bridge makes due to traffic, wind, and temperature fluctuations in relation to the mainland. Expansion joints also ensure that vehicles can drive across this juncture unimpeded, regardless of the transition construction’s displacement. The joints are installed perpendicular to the direction of travel. Bridges in both countries are usually built with simple rubber

grid of slab posts and the strategic placement of vertical circulation elements ensure complete reversibility of uses. In fact, all or part of the tower can accommodate hotel services, student housing, or co-living spaces, in addition to the service and sports facilities on site. Double-height extensions and a bioclimatic greenhouse enhance the range of uses. This way, the tower can be fully 'lived in,' offering the possibility to live, work, study, have fun, grow a garden, love, and age—all at the same time. The former attic on the 10th floor has been replaced with a timber frame, creating space for a greenhouse integrated into the building's structure. Reaching skyward, this glass crown houses an educational urban agriculture unit and a café, both accessible to all building users. This new treelined, transparent, and luminous space offers residents a unique view of Paris and transforms into a distinctive lantern for the local community at night. ■ A large curved patio opens onto a terrace garden. Photo credit: MAUD CAUBET ARCHITECTES

The friendship bridge between Brazil and Paraguay was due to be inaugurated in 2024. Photo: Consórcio Ponte Foz

profiles and narrow rubber expansion joints. But this was not sufficient for the friendship bridge. The project called for

sophisticated, low-maintenance swivel joist expansion joints. What makes the swivel joist expansion joints special is the

way they allow for movements in all directions without damage or significant resistance. They permit lateral, longitudinal, and

vertical movements in relation to the direction of travel, as well as any rotations. The profiles rest on top of the parallel swivel joists, which, with the exception of those at the edges, run at a slight angle to the direction of the road. This spreads the bridge’s tensile and shear movements evenly and without fatigue across the gaps between the profiles. As a result, the expansion joints are set to do their job for at least 50 years. Construction of the bridge began in 2019. The main bridge section was completed in August 2023, with work on the ancillary structures still ongoing. The bridge was scheduled to be opened to traffic by the end of 2024. ■ JANUARY 2025

PTFE tensile membrane amphitheater unveiled at Doral Central Park in Florida A

new PTFE tensile membrane amphitheater structure by Birdair Inc., a specialty contractor for custom tensile membrane structures, was unveiled at Doral Central Park in Doral, FL, in December. The amphitheater—a core element of the multi-phase $168 million Doral Central Park project—features a cutting-edge 10,600 square foot PTFE tensile membrane roof and will serve as a prime destination for concerts, shows, cultural events, and more in the Doral area. Birdair's scope of work included the design, fabrication, and installation of the PTFE-coated fiberglass membrane, all the supporting structural steel system inclusive of the sweeping front arched truss and all perimeter

columns, structural cables and end fittings, and membrane clamping hardware. Fellow members of the project team included Bermello Ajamil, a global architecture and engineering firm based in Coral Gables, FL; Bliss & Nyitray, Inc., a structural engineering firm that offers threshold inspections, feasibility studies, planning and more; and Kaufman Lynn Construction, a full-service commercial construction company that operates in Florida and North Carolina. PTFE-coated fiberglass tensile membrane is a versatile material that can withstand climates ranging from the arctic to the desert heat with an expected project life exceeding 30 years. Its combination of inertness, thermal stability, and surface

The amphitheater is part of a multi-phase project that will span over 80 acres.

properties make it ideal for projects requiring superior weather and fire resistance. The principal element that differentiates PTFE fiberglass membrane from conventional glazing is

its advantageous shading coefficient. In very warm climates, even low lighting levels make PTFE fiberglass membrane an energy saver versus conventional systems. ■

COWI completes infrastructure upgrade for the Blue Ridge Parkway's I-26 crossing C

OWI, a leading international engineering consulting group, in partnership with Structural Technologies, has successfully completed its role as Construction Engineer for a crucial bridge replacement along North Carolina's iconic Blue Ridge Parkway. Owned by National Park Service, which also served as the Engineer of Record, and designed by the Federal Highway Administration and the Eastern Federal Lands Highway Division, the new bridge replaces an existing structure to accommodate the widening of I-26 from two to four lanes in each direction, significantly reducing congestion 46 STRUCTURE magazine

along this critical route. The construction of the new bridge faced notable challenges, including restricted site access and storage, phased traffic, environmental sensitivities, and complex engineering demands. COWI addressed these challenges by redesigning the foundations to provide sufficient stability of the structure during erection, engineering a specialized lifting and launching operation for superstructure segments utilizing a custom lifting frame (by VSL International), and designing a temporary stability tower and end span dunnage. COWI also conducted detailed construction stage analysis, controlled casting and

erection geometry, and produced shop drawings to ensure structural integrity and a successful project execution. By parameterizing engineering processes, COWI substantially reduced the time spent on structural analysis, geometry control, and development of 3D models of all precast structural elements. Early-stage reinforcement clash detection enabled the team to quickly modify the structural detailing and increased the efficiency of precasting operations. The unique construction sequence which utilized a single precast segment delivery location at each of the bridge pier tables, ground-based crane, and the

lifting frame, helped minimize environmental disruption by heavy equipment. COWI, in collaboration with Structural Technologies designed temporary structures made from existing precast piles and repurposed steel materials to lower the project's carbon footprint. “This was a complex project in a challenging environment, made all the more significant by the bridge forming part of the iconic Blue Ridge Parkway,” said Jan Zitny, senior bridge engineer at COWI. “Facing and overcoming numerous challenges alongside Structural Technologies helped us forge a strong partnership, which was instrumental to the project's success.” ■

IN BRIEF AISC releases Fourth Edition Seismic Manual The American Institute of Steel Construction has published the fourth edition AISC Seismic Design Manual, which expands upon the guidance in the third edition and incorporates the 2022 AISC Seismic Provisions for Structural Steel Buildings, the 2022 AISC Specification for Structural Buildings, and recent design guides. This revision adds design examples for multi-tiered buckling-restrained braced frames, concentrically braced frame column bases, and connection

design at the intersection of braces in a concentrically braced frame. Another updated example considers partial-jointpenetration groove welds for a column splice in a special moment frame. New tables summarize applicable requirements of the AISC Seismic Provisions as well as second-order amplifier values for use with approximate second-order analysis. An updated discussion of diaphragms considers load path, challenges in analysis, and common assumptions, along with guidance on diaphragm modeling. The new Seismic Manual is available at aisc.org/ publications.

McAdams expands southeast presence with acquisition of Florida-based AVID Group McAdams, a multidisciplinary design and engineering firm headquartered in Raleigh, NC, specializing in civil/site design, planning and design, transportation, water resources, and geomatics, has announced the acquisition of AVID Group, LLC, a Central Florida (Orlando/ Tampa) civil engineering firm known for its expertise in commercial and retail projects throughout Florida. This strategic acquisition will strengthen McAdams' position in the southeast and enhance its ability to serve both existing and new clients.

Exo’s digital innovation recognized for “advancing the world’s infrastructure” E

xo, a leader in structural engineering and asset management for utilities, was both an award finalist and an award recipient at Bentley’s Year in Infrastructure 2024, Going Digital Awards, an event celebrating organizations that advance global infrastructure through innovation and visionary projects. Exo was honored as a finalist for its pioneering use of iTwin Capture and Power Line Systems in the Ohio Falls Hydro Station project, demonstrating digital advancements in life extension and sustainability for transmission and distribution infrastructure. The Ohio Falls project, led by Exo’s VP of Engineering Services, Michael Miller, PE, tackled the challenge of two corroded transmission towers, built in the 1920s, whose lines span the Ohio River. With inadequate technical drawings available, Miller’s team utilized digital

twin technology to create precise models of the aging towers, analyzing the feasibility of extending their service life rather than replacing them. The innovative remediation is projected to save the utility company an estimated 10 years of permitting delays and approximately $80 million by avoiding unnecessary tower replacements. In addition to being named a finalist, Exo received the esteemed "Founders’ Honor" for its work in stabilizing a critical 161kV overhead transmission tower, which supplied power to several hospitals. By creating a digital twin and engineering a stabilization solution, Exo ensured the transmission line remained in service, avoiding extended outages. The project saved four to six weeks of surveying time thanks to the use of Bentley’s digital applications, ensuring uninterrupted power to the hospitals at full capacity. ■

The Ohio Falls Hydro Station includes two towers built in the 1920s supporting a transmission line crossing the river. Exo used an integrated modeling solution to model the existing structures and analyze their reusability.

A critical 161kV overhead transmission tower had been damaged during a recent flood. Exo selected Bentley’s iTwin Capture to accurately survey the deflected shape of the structure. JANUARY 2025

CASE in Point

ACEC Winter Coalition Meeting to highlight sustainability efforts

he 2025 Coalitions Winter Meeting brings together ACEC’s coalition members to discuss strategic initiatives and industry challenges for the year ahead. This year’s Winter Meeting will be held February 25-26 at the Hyatt Regency Phoenix in Arizona and will highlight sustainability efforts and the industry and is a chance for the CASE Executive Committee (ExCom) and its subcommittees to work on this year’s publication and education initiatives for CASE members. This gathering provides a valuable opportunity for coalition leaders and attendees to collaborate on solutions impacting the engineering profession. The ACEC Winter Coalition Meeting is open to everyone. To register for this event scan the QR code or visit www.acec.org/education-events/events/coalitions-winter-meeting/.

Blueprint for Success: Developing the Next Generation of Firm Leaders

ith the engineering industry in the recruitment trenches fighting for talent in a scarce market, any organization that can turn its high-potential employees into future leaders—and its good managers into great ones—will enjoy a tremendous competitive advantage in the workforce wars. In every business, you’ll find them: those superstar employees whose drive, dedication, and sheer talent make every project better for their being part of it. They are an invaluable asset, so what can managers do to create a path forward—and upward—for these high potential employees? Conversely, how can these high performing employees prepare to pick up the mantle of leadership? ACEC’s Small Firm Workshop, “Blueprint for Success,” to be held February 23-24 in Phoeniz, AZ, will help firms answer some of those questions. Conceived for firm leaders and high-potential

employees alike—and with unique content tailored for both—this comprehensive, handson workshop will focus on cultivating the next generation of leadership. This day-and-a-half long session will offer principals actionable strategies to help top performers grow into confident leaders. At the same time, those top performers will take part in concurrent sessions focused on skills—both hard and soft—that one must bring to leadership roles. Join us in Phoenix, where you and your top performers will gain valuable insight into how managers become mentors—and how exceptional employees can evolve into visionary leaders. Attendees will earn 8 PDHs.

ACEC’s Coalition of Professional Surveyors and USIBD to present session on “Navigating BIM and Digital Twins”

n January 14 from 1:30-2:30 p.m. Eastern Time, an online education session on BIM will be hosted by the ACEC COPS Coalition and USIBD. It will focus on standardizing and optimizing building documentation practices across project stages, from design to completion. The session will address developing best practices, ensuring high-quality documentation, maintaining ethical standards, and supporting the collaboration between professionals like architects, engineers, and surveyors. The session is a must for professionals seeking to enhance documentation quality, efficiency, and cross-team coordination. Attendees will earn 1 PDH. Register at www.acec.org/events. For more on USIBD’s mission, visit https://usibd.org/about-us/.

48 STRUCTURE magazine

News of the Coalition of American Structural Engineers Explore CASE’s bestsellers of the year

xplore CASE’s top publications that inspire and inform professionals like you. From cutting-edge research to actionable insights, this year’s bestsellers are not to be missed. Plus, if you’re not a CASE member, don’t forget to use your discount code NCSEASEI2022 at checkout for exclusive savings.

Tool 1-4: Creating a Culture of Recruitment and Retention This tool offers valuable insights into the unique challenges and opportunities associated with recruiting and retaining Millennial (born 1981–1996) and Generation Z (born 1996–2012) employees. As these generations rapidly become the majority in the workforce, understanding their professional values, goals, and definitions of satisfaction is essential for engineering firms aiming to thrive now and in the future. With priorities and benefits that often differ significantly from those of earlier generations, Millennials and Gen Zers are reshaping workplace expectations. Firms that embrace these shifts and adapt accordingly will be better positioned for long-term success.

Tool 2-3: Employee Evaluations Enhance your employee performance evaluations with this comprehensive publication, designed specifically for structural engineering offices. This customizable tool not only streamlines the evaluation process but also elevates it, offering fresh ideas, innovative techniques, and thought-provoking questions to refine and improve your current practices. Whether your evaluations are formal and scheduled or more informal and flexible, this resource is tailored to fit your firm’s unique needs while strengthening your risk management program. Equip your firm with the tools to foster growth, accountability, and excellence.

Tool 3-6: Career Path Planning Unlock the potential of your team with our Career Path Planning Tool. This invaluable resource provides a ready-to-use template and practical guidance on the importance of mapping career paths within your firm. By identifying individual strengths, weaknesses, and seniority, as well as

assessing your firm’s overall talent landscape, you’ll gain critical insights to shape your recruitment strategies and strengthen your workforce. Perfect for preparing performance reviews, this tool equips leaders with meaningful discussion points, actionable feedback, and a way to share growth opportunities that inspire and retain top talent. Pair it with CASE Tool 2-3 for impactful evaluations and CASE Tool 1-4 to foster a culture of growth and retention.

White Paper: Teaming Agreements Streamline your joint projects with clear, comprehensive teaming agreements! Essential for successful collaborations among contractors, design professionals, and structural engineers, these agreements define roles and responsibilities upfront—especially valuable in design-build projects. This commentary offers insights into standard teaming agreements from top organizations, including: • AIA: Contract C102-2015 • EJCDC: Contracts D-580 and E-580 • DBIA: Contract No. 580 • ConsensusDocs: Contracts 296 and 498 Explore these agreements and other indispensable CASE publications at CASE Resources. Got ideas for tools that could enhance your business? We’d love to hear them! Reach out to us at coalitions@acec.org and help shape the future of structural engineering resources.

Support the future of structural engineering with CASE Scholarship Fund

he CASE Scholarship Fund is more than just financial support; it’s an investment in the next generation of structural engineers who will shape the built environment and drive innovation forward. Each year, CASE provides scholarships to dedicated students working toward a career in engineering, empowering them to pursue their education and make meaningful contributions to the industry. Your donation, no matter what the amount, can have a lasting impact. It helps cover tuition, supports students’ access to critical resources, and ultimately brings talented, enthusiastic new professionals into the

field. Let’s ensure that financial barriers don’t stand in the way of their potential. Join us in building a stronger, more resilient future by contributing to the CASE Scholarship Fund today. Together, we can make a difference! Below is a QR code for you to make your donation today. Please remember to select the CASE Scholarship Fund when contributing. Thank you for your support! JANUARY 2025

NCSEA News A transformative era: survey highlights AI’s growing role in structural engineering and the built environment

recent survey conducted by the National Council of Structural Engineers Associations underscores the transformative potential of artificial intelligence (AI) within structural engineering and the broader built community. The findings showcase both the current state of AI adoption and the profession’s readiness to embrace its exciting future.

Key Highlights From the Survey: Expanding AI use: Almost 30 percent of respondents report using AI tools weekly or daily, reflecting early momentum in leveraging AI for tasks such as internal administration, design optimization,

and sustainability enhancements. Growing interest: About 67 percent of respondents expressed interest in adopting AI if equipped with the right tools and training, signaling willingness to innovate. Organizational awareness: With only about 26 percent of respondents reporting that their organizations have formal AI policies, there is an opportunity for AI leadership across the profession. Shared concerns: Transparency, accountability, and ethics emerged as top considerations, with 85 percent of respondents identifying transparency as a key area to address. These concerns underscore the importance of collaborative approaches to ethical AI integration.

A Profession Poised for Transformation

Aditya Kaushik, Walter P. Moore, Structural Engineers Association of Colorado

“We are at the threshold of an exciting transformation for structural engineering and the built environment,” says Tricia Ruby, president of the NCSEA Foundation Board of Directors. “While challenges remain, the enthusiasm and curiosity shown in our survey and town hall conversations reflect a profession eager to lead the way in unlocking AI’s potential.

Dave Martin, Degenkolb, Structural Engineers Association of California

Insights From the NCSEA AI Town Hall

AI Grant Team Project Manager: John-Michael Wong, KPFF, Structural Engineers Association of California

Ayush Singhania, Simpson Gumpertz & Heger, Structural Engineers Association of California Andrew Sundal, HGA, Minnesota Structural Engineers Association Emre Toprak, Arup, Structural Engineers Association of Metropolitan Washington Sheng Zheng, Martin/Martin, Structural Engineers Association of Colorado

AI Advisory Board KP Reddy, Founder & CEO, Shadow Ventures Robert Otani, Senior Principal & Chief Technology Officer, Thornton Tomasetti Zak Kostura, Associate Principal, Advanced Digital Engineering, Arup Kimon Onuma, Founder & President, Onuma, Inc. Roark Redwood, Sr. Vice President of Technical and Government Solutions, National Institute of Building Sciences KiSeok Jeon, VP of Digital Advisory, STV Farahnaz Soleimani, Assistant Professor, Oregon State University .

50 STRUCTURE magazine

The survey findings align with insights from an NCSEA-hosted town hall, which drew about 750 participants. Live polling during the event revealed that about 65 percent of attendees believe AI will significantly transform the structural engineering profession. The NCSEA AI town hall was hosted by Emily Guglielmo, P.E., C.E., F-SEI, NCSEA Foundation past president, and featured NCSEA’s AI Grant Team. While the town hall polling revealed enthusiasm for AI, it also confirmed the need for resources and guidance, as about two thirds of participants indicated the need for support to integrate AI effectively into their work. “Structural engineers have always been at the forefront of innovation in the built environment,” says Alfred T. Spada, NCSEA executive director. “As AI continues to evolve, NCSEA is committed to equipping the profession with the tools, knowledge, and ethical frameworks needed to shape the future responsibly and effectively.”

Building a Collaborative Roadmap for AI Adoption To meet this demand for AI resources, the NCSEA Foundation is advancing efforts through its AI Grant Team and AI Advisory Board. These initiatives aim to create a comprehensive vision and roadmap for how AI can empower structural engineers to drive innovation, address complex challenges, and build resilient, sustainable communities.

News from the National Council of Structural Engineers Associations NCSEA Foundation drives support with $30,000 fundraising campaign

he National Council of Structural Engineers Associations wrapped up its December 2024 fundraising campaign in celebration of 30 years of advancing the structural engineering profession. The campaign aimed to raise $30,000 to support the NCSEA Foundation’s mission of building a stronger, more resilient future for the profession and the communities it serves. Through widespread support from engineers, SEAs, and allies of the profession, the campaign helped generate crucial funding for scholarships, mentorship programs, and grants to local Structural Engineering Associations (SEAs). These efforts drive innovation by advancing AI, sustainability, and cutting-edge technology; build leadership by funding scholarships and mentorships to prepare the next generation of engineers; and raise awareness by promoting the critical role of structural engineers and empowering SEAs to create meaningful change in their communities. As a 501(c)3 organization, contributions to the NCSEA Foundation are tax-deductible. To learn more about the NCSEA Foundation and how you can contribute to its initiatives, visit www.ncsea.com/foundation.

Leadership Week 2025 dates announced

CSEA has announced that Leadership Week 2025 will take place May 5-8 at the Hilton Garden Inn O’Hare in Des Plaines, Illinois. This annual event brings together the NCSEA Board of Directors, committee chairs, and selected SEA leaders. NCSEA Leadership Collaboration (May 6-7) is an invite-only event for Board members and committee leaders to align initiatives, strengthen engagement, and advance NCSEA’s strategic goals. SEA Leadership Retreat (May 7-8) brings together SEA leaders selected by their associations to connect with NCSEA committees, share best practices, and explore strategies for strengthening their local associations. Leadership Week demonstrates NCSEA’s commitment to empowering leaders and fostering innovation in the structural engineering profession. Stay tuned for more details on this impactful event.

NCSEA Webinars

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

January 9

Eccentrically Braced Frames

January 21

Snow Drift Loads in the U.S. and Canada—Similarities and Differences

Purchase an NCSEA webinar subscription and get access to all the educational content you’ll ever need! Subscribers receive access to a full year’s worth of live NCSEA education webinars (25+) and a recorded library of past webinars (170+) – all developed by leading experts; available whenever, wherever you need them! Recommendations for Performing Structural Engineering Quality Assurance Reviews JANUARY 2025

SEI Update Building tomorrow—supporting the future of structural engineering with the SEI Futures Fund

inda Kaplan has taken on the role of Chair for the SEI Futures Fund, bringing with her extensive experience and a deep commitment to advancing structural engineering. Having first connected with the Futures Fund as a scholarship recipient, Kaplan experienced firsthand how the fund can transform careers and empower the next generation of leaders. Several members of her scholarship class, including Kaplan herself, have since ascended to board-level positions within SEI, underscoring the program’s profound and lasting impact. Under Kaplan’s leadership, the Futures Fund is expanding its reach, thanks to successful partnerships like the significant matching donation from CSI and Ashraf Habibullah. Kaplan is now championing additional opportunities, encouraging both individuals and companies to contribute. Smaller personal or corporate matches, she emphasizes, can create a ripple effect, multiplying contributions and enabling the fund to provide more scholarships and support new initiatives. “Every dollar donated helps build the foundation for the future of our industry,” she notes, pointing to the direct benefits of funding educational opportunities and outreach programs.

For aspiring engineers, applying for the Futures Fund offers unparalleled benefits. Scholarships provide financial support, professional growth opportunities, and a pathway to engage with SEI’s robust network of industry leaders and peers. For donors, contributions to the Futures Fund do more than help individual scholars—they bolster the profession as a whole by fostering innovation, public awareness, and a stronger pipeline of future engineers. Kaplan also highlights the long-term societal impact: “Supporting the Futures Fund isn’t just about creating engineers; it’s about educating the public on the importance of infrastructure and ensuring that we have the leadership to meet tomorrow’s challenges.” Now is the time to invest in the future of structural engineering. Whether through a personal donation, corporate match, or spreading awareness, every contribution makes a difference. Visit the SEI Futures Fund website— go.asce.org/seifuturesfund —to learn more about how to apply or donate and join Kaplan and her team in shaping the future of engineering. Together, we can ensure that the profession thrives for generations to come.

Congratulations to the 2025 SEI Fellows

he 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. The 2025 SEI Fellows are: Faheem Ahmad, P.E., S.E., F.SEI, M.ASCE Alice Alipour, Ph.D., P.E., F.SEI, M.ASCE Kevin Aswegan, P.E., S.E., F.SEI, M.ASCE Preetam Biswas, P.E., F.SEI, M.ASCE Michael Constantinou, Ph.D., F.SEI, F.ASCE C. Kennan Crane, Ph.D., P.E., F.SEI, M.ASCE Evan Hammel, P.E., F.SEI, M.ASCE Kenneth Herrle, P.E., F.SEI, M.ASCE Peter Lee, P.E., S.E., F.SEI, M.ASCE Brian McDonald, Ph.D., P.E., S.E., F.SEI, F.ASCE Kevin Moore, P.E., S.E., F.SEI, M.ASCE Duane Otter, Ph.D., P.E., F.SEI, M.ASCE Zulma Stella Pardo Vargas, Ing., F.SEI, M.ASCE Andrea K. Reynolds, P.E., S.E., F.SEI, M.ASCE Josh Sebolt, P.E., F.SEI, M.ASCE Rohit Soni, P.E., F.SEI, M.ASCE Donald Taylor II, P.E., F.SEI, M.ASCE 52 STRUCTURE magazine

SEI’s first ever sustainability symposium

ark your calendars for the first ever Structural Engineering Sustainability Symposium in North America: 25 Years to 2050, What can you do? This free, virtual, half-day event brings together structural engineers and related industries to discuss practical steps to meaningfully reduce the climate impact of the industry. The conversation will go beyond sustainability 101 and feature lessons learned, best practices, and case studies including: discussing LCA results with key stakeholders, procuring better materials, circular construction successes, and more. Join us and learn practical strategies that can be implemented today wherever you are in your climate journey. Register here: www.eventbrite.com/e/seis-north-americanstructural-engineering-sustainability-symposium-tickets1089164844369?aff=oddtdtcreator.

News of the Structural Engineering Institute of ASCE ASCE eLearning: Delivering client value for the structural and civil engineer

oin us for the first of a three part ASCE eLearning webinar series by the SEI Business Practice Committee: Client value for structural/civil engineers | ASCE on Tuesday, February 4, designed to equip students, young and mid-level structural/civil engineering professionals with essential skills for success. The Client Value session delves into the art of building strong, lasting client relationships and consistently delivering exceptional results. Learn how to drive sustainable business growth by mastering

often-overlooked skills that can propel your career to new heights. Don’t miss this opportunity to enhance your career with valuable knowledge and practical strategies. Register now and prepare for upcoming sessions on Productivity and Entrepreneurship. ASCE eLearning webinars are free for members. Encourage nonmember colleagues to join at www.asce.org/SEIMembership, then sign up for the webinar. Membership for students and the first year after graduation are free.

Structures Congress 2025 is heading to Phoenix this April

oin SEI for Structures Congress 2025 from April 9 – 11, 2025 in Phoenix, Arizona. The technical program will provide 11.5 PDHs and is packed with sessions on blast, bridges, buildings, business/ professional practice, career/leadership, climate change/sustainability/ lifecycle assessment, codes and standards, education, forensic, natural disasters, non-building/special structures, non-structural, research and technology. Check out a preview of the technical program in this issue and start planning your Structures Congress experience.

Check out Local SEI Chapter and Graduate Student Chapters and get involved. www.asce.orgSEILocal

SEI Webinars January 22

ASCE/SEI 41-23 Tier 1 and Tier 2 Updates - NEW

February 03

Designing Nonbuilding Structures Using ASCE/SEI 722

February 06

Structural-Condition Assessment of Existing Structures

JANUARY 2025

Structures Congress 2025 Technical Program at a Glance Phoenix, Arizona | April 9– 11, 2025 | www.structurescongress.org

9:30– 10:30 AM

Towards Zero Carbon: A Roadmap for the Structural Engineering Profession

Enhancement of Structural and Glazing Elements with Elastomeric Polymers

ASCE 7-28 Wind: Past, Present & Future

11:00– 12:30 PM

Designing Tall Buildings: What I Wasn’t Taught in School... Until Now

Leveraging Structural Steel in the Pursuit of Net-Zero

Beginning the Discussion: Lessons from the Baltimore Key Bridge Collapse

Structural Condition Assessment of Existing Buildings: Standards & Guidance

Steel-Timber Floor Systems: Research & Practical Applications

A Compendium of Sustainable Materials and Strategies

Structural Collapse Investigations - Case Studies

Incorporating Future Conditions in ASCE 7 to Support Engineers & their Clients

ASCE 7-28 Seismic: A Mid Cycle Update

3:30– 5:00 PM

(BU-7) Saving Structures: Assessment and Strengthening Techniques

1:30 – 3:00 PM

THURSDAY, APRIL 10, 2025

Update on NIST’s Investigation of the Partial Collapse of Champlain Towers South in Surfside, Florida [Special Session]

8:15– 9:15 AM

EPA’s Implementation of IRA Programs for Sustainable Construction Procurement

Numerical Methods for Cold Form Steel Modeling

9:30– 10:30 AM

The Art of Office to Residential Conversions in New York City

The State-of-Practice of Embodied Carbon in Structural Engineering

Impact Modeling and Response ASCE at Work in for Falling Debris and Intentional Washington: Collaboration Events to Advance National Codes & Standards

(BU-4) Building the Future Now: Top to Bottom and In-Between

1:30 – 3:00 PM

Structural CFS Building Applications for EOR: Floor, Roof, and Bearing Wall

11:00– 12:30 PM

FRIDAY, APRIL 11, 2025

(BU-5) Evolving Concrete: Code Changes, Implementation, and Research

A Panel on Embodied Carbon

Structural Blast Design: Case Studies and Applications

New Requirements for ASCE 24-24 and ASCE 7-22 for Design in Flood Prone Area Part I

New Methods and Strategies Towards Net-Zero Carbon Structures

Defects Investigation -Testing and Inspections

New Requirements for ASCE 24-24 and ASCE 7-22 for Design in Flood Prone Area Part II

View full program at structurescongress.org

KEY:

BLAST

BRIDGES

BUILDINGS

BUSINESS/ PROFESSIONAL PRACTICE

CAREER/LEADERSHIP

CLIMATE CHANGE/ SUSTAINABILITY/ LIFE CYCLE ASSESSMENT

CODES AND STANDARDS

On the Verge of Collapse: Risk Management for Structural Engineers

The Dam Life of a Structural Engineer

Bridge Asset Maintenance / UHPC

Make the black box transparent: XAI Application in Structural Engineering

Innovative Mass-Timber Design: From Research To Construction

What the Practicing Structural Engineer May Want to Know

Guidelines and Specifications on Structural Design of Modules

Geo-Structural Aspects Impacting Bridge Load Rating

Integrating AI and Structural Engineering

Advances in Blast Modeling and Testing

Meet the Future of Structural Engineering

Building Inclusive Futures Part 2: Equity in Natural Hazards Research

Innovative Bridge & Tunnel Case Studies

Research on Damage and Repair

Advancing Structural Designs with State-of-the-Art Structural Optimization

BIM Execution Planning Musical Chairs

Curtain Wall and Nonstructural CFS Building Applications for the EOR

Life-Cycle Assessment based on Damage Detection & Maintenance Strategies

Sharing the Story of a Real Structural Engineer’s Claim

Emergency Response Engineering – A Blend of Technical and Soft Skills

Seismic Response/Assessment

Latest Trends in the Mechanics of Construction Materials and Structures

The Subject of Ethical Structural Engineering

NIST/SEI Tornado Workshop Results - Research Needs, ASCE 7 & Beyond

Fatigue / Bridge Analysis

Intelligent Infrastructure for the Future

AI and MI in Structural Engineering

New Methodologies for Seismic Retrofit, Cordoning, and Repair

Prestressed Concrete Bridges

Recent Advances in CLT and Wind Engineering

EDUCATION

FORENSIC

NATURAL DISASTERS

NON-BUILDING/SPECIAL STRUCTURES

NON-STRUCTURAL

RESEARCH

TECHNOLOGY

inSIGHTS

Tech-Driven Monitoring Protecting America’s Infrastructure IoT technologies can help engineers identify deficiencies in real time, before failures happen. By Kelsey Kidd, Worldsensing

xtreme heat, wildfires, flooding from torrential downpours, and violent hurricanes and storm systems test the stability and resiliency of the nation’s bridges, highways, and dams. In the U.S., organizations like the Cybersecurity and Infrastructure Security Agency (CISA) are tasked with protecting critical infrastructure against severe weather threats. But even before considering the potential impacts of violent weather, the general state of the U.S. civil infrastructure is concerning. About 1 in 3 bridges either need repairs or need to be replaced, according to a 2024 American Road and Transportation Builders Association report. U.S. highways currently have $1 trillion in needed repairs. And more than 4,000 dams are in poor or unsatisfactory condition. As a result, the pervasive view is that the U.S. infrastructure is not adequately prepared to withstand severe weather and other meteorological challenges, putting the arteries we rely on to connect with one another at greater risk of failure and collapse. The continuous monitoring of structures for vibrations, changes in tilting, and gauging the impact of temperature on the integrity of a structure will harden and protect civil infrastructure amid environmental threats and avert catastrophes that can cost both lives and billions of dollars. Relying on human structural engineers to conduct this level of monitoring on a continuous basis is simply not feasible and would be incredibly costly, especially considering the number of critical structures at risk across the country. There is also the challenge of determining who among the many companies involved in design, construction, and maintenance assumes responsibility for ongoing structural monitoring. This is why structural engineers, safety consultants, and any organization responsible for civil infrastructure safety must more readily embrace and deploy technologies that can heighten resilience and reduce the risk of failures. Today, advancements in IoT (Internet of Things) technology makes this possible. Using a series of connected sensors and devices that can capture and transmit data via software systems and applications, engineers can receive continuous data flows and monitor the stability of structures from afar, receiving alerts of any dramatic or worrisome changes and deploying teams to where repairs are needed most. For example, a tiltmeter can be deployed to conduct 3-axis inclination monitoring of bridges, highways, or skyscrapers susceptible to strong winds, measuring the relative changes in inclination on the ground or in the structures themselves. When combined with existing geospatial monitoring, the collected data can offer detailed information on even the highest fixed structures, ground movements, and differential settlements in slopes or infrastructure. Flood monitoring systems, which combine water level sensors and weather stations with a central connectivity hub, can be used to accurately monitor water level parameters, which can prevent travel routes from being impacted by surface-level floods. Accessible through a visualization platform, the continuous flow of data allows engineers to set alerts, receive notifications when thresholds are reached, and

56 STRUCTURE magazine

A tiltmeter is installed for three-axis inclination monitoring.

provide timely and detailed information to local communities under threat when unexpected events occur. And any vibrational effects to buildings can be measured by using a wireless vibration sensor featuring a 3-axis MEMS accelerometer, capable of recording data up to 1000 Hz from a 4k Hz signal. The sensor’s configurable settings allow it to meet different state regulatory standards, helping to support safety protocols and enabling timely evacuations in emergency situations. Historically, infrastructure maintenance has been reactive in nature. Crews are typically deployed in the field only after an urgent situation arise or, sadly, after a failure has occurred. As weather events continue to stress the nation’s aging infrastructure, those urgent situations are likely to increase in volume. IoT technologies can help engineers and safety consultant identify deficiencies in real time, before failures happen. ■

Kelsey Kidd is North American Sales Director for Worldsensing, a provider of IoT solutions for monitoring critical infrastructure.

historic STRUCTURES

A vintage postcard shows the Fort Snelling Bridge, with Fort Snelling on the right.

19th Century Mississippi River Bridges Fort Snelling Bridge, Minnesota 1880. By Dr. Frank Griggs, Dist. M. ASCE

he Fort Snelling Road bridge was a joint venture between the federal government and Ramsey County of Minnesota. Minnesota had passed laws in 1876, 1878, and 1879 pertaining to a bridge at this site. The 1878 law stated, “That said bridge shall be constructed so as to offer reasonable and proper means for the passage of vessels under the same, and width at least one hundred (100) feet between piers therefor.” On March 29, 1878, the county voted $100,000 to build the bridge, and the Congress in the same year approved “The Sundry Civil appropriation bill ... (which) contained an item appropriating $65,000 for aiding the construction of a free wagon bridge at the military reservation at Fort Snelling.” This amount “being one-half of the cost as estimated by the U. S. Engineers.” The Bill stated, “the height of said bridge shall be at least 68 feet above high-water mark, and that a span of at least 200 feet in the clear be provided from the right or Fort Snelling bank of said river toward the left bank thereof.” On April 4, 1878, Joseph S. Sewall, then an engineer in St. Paul, was chosen to make surveys by a committee appointed to oversee the design and construction, and on the 14th of the same month he was elected engineer of the work.” Sewall completed the plans and specifications for a cantilever bridge and sent them “to the Sec’y of War for approval, and by him referred to a commission composed of Gen’l Alfred H. Terry and G. K. Warren, by whom they were approved.” In 1878 Warren had prepared a report entitled “Bridging the Mississippi River between St. Paul and St. Louis” for the Chief of Engineers, U. S. It covered the 16 bridges that were in existence up to 1876. The river valley at the site “had a width of about 800 feet, with steep

rocky bluffs 80 to 90 feet in height, on each side. The low water channel is only about one-half this distance in width.” Steamboat traffic, while not as heavy as farther downstream, was still to be guaranteed free passage at all times. Sewall prepared a specification on July 5, 1878, that was becoming standard practice and requested designs and bids for an iron bridge at this site in August 1878. The specifications stated that river traffic would have to be maintained and that the “superstructure be so proportioned and constructed that with a load of 1,800 pounds per lineal foot upon any part of the structure, there should be no tensile strain upon any part of the structure exceeding 12,000 lbs. per square inch, and no compressive strain exceeding one-fourth the breaking load of the part as calculated by Gordon’s formula.” The specification also prescribed the design be based upon a wind pressure of 50 lbs. per square foot ... and iron in tension to have an elastic limit exceeding 25,000 lbs. per square inch. Bids were be submitted by September 30, 1878. A remarkable 31 bids based upon the designs of the different bridge companies were submitted. The bids ranged from $39,444 to $69,000 but Sewall selected the design of Horace E. Horton & Company of Rochester, Minnesota, who had designed an unusual bridge consisting of five short truss spans on iron towers and a cantilever span with two, trusses serving as anchor spans. His bid, which was accepted on September 30, 1878, was for $44,328. Masonry bids had been received in August and the contract awarded to Michael O’Brien of St. Paul for $63,397. It was finished on June 19, 1879. Horton was to eventually receive $63,397 for the superstructure. The bridge would JANUARY 2025

Fig.1. The Fort Snelling Bridge connected St. Paul on the right and Fort Snelling on the left (note, drawing lacks a member on the left end of the suspended span).

have an 18-foot-wide roadway and a five-foot wide sidewalk. Engineering News reported on the bids in its January 4, 1879, issue noting, “There were 17 bidders, and 31 plans offered … The engineer, J. S. Sewall recommended that the bid of H. H. E. Horton & Co. of St. Paul, Minn., on their "Plan C" be accepted and the proper steps taken to secure the approval of the Secretary of War. The following are the companies that submitted bids, with several having options. The italicized firms have previously built spans across the Mississippi. H. A. Streeter, Globe Iron Works, Chicago Delaware Bridge Co., New York Morrison, Field & Co., Buffalo, N. Y. C. Shaler Smith, St. Louis Cunningham & Keepers, Milwaukee L. Soulerin, Chicago Wrought-Iron Bridge Co., Canton, O.H. Phoenixville Bridge Works, Pennsylvania Detroit Bridge & Iron Works Louisville Bridge & Iron Co. (Ky.) Passaic Rolling Mill Co.. New York New York Bridge Co. F. E. Cauda, Chicago H. E. Horton & Co., St. Paul Keystone Bridge Co.. Pittsburgh, Pa. Clinton Bridge Co., Iowa American Bridge Co., Chicago Horton’s Plan C involved one of the first cantilever spans in the United States. James Eads had used cantilever methods in erecting arches for his St. Louis Bridge in 1874 and C. Shaler Smith had erected his Kentucky High Bridge using cantilever methods in 1876. This bridge was built as a continuous three span bridge, but after erection the top chord was cut resulting in the end spans being supported by a brackets off the middle span that served as the anchor span. Smith would later build a cantilever over the Mississippi at St. Paul, Minnesota, in 1780 to be described in a future article. Horton was the first to build what became a true cantilever and the first to have the suspended span lifted into place and pinned to cantilever arms. It is not known what the other bridge companies had submitted. Horton’s bid of $44,325.00 was not the lowest. In fact, five reputable firms had under bid him. Despite this Sewall and Allen probably chose Horton based upon his proposed erection technique. As can be seen in Figure 1, Sewall and Horton started on the Fort Snelling side (the south or west side) of the river and built a masonry anchorage pier with the end truss tied down to the anchorage. They would then build on falsework the first 135-foot anchor span. A one panel bracket was cantilevered off the anchor span and over the first main stone pier. Starting from the St. Paul side of the river, they had five simple trusses of 72.5-foot span resting on four iron towers of 18 feet width. The next span, built on falsework as well, and setting on stone piers, served as an anchor span for another one panel long bracket. This span was 180 feet long, discounting the bracket. The suspended span was slightly over 200 feet in length. The total length of the bridge was 1,020 feet. Since the trusses was designed for much lighter loads (carriage and foot traffic) its weight was not great and 58 STRUCTURE magazine

Horton decided to fabricate the suspended span off site, float it to the site on barges and lift it into place from the barge. This span had top chords of riveted channels and eye bar bottom chords and diagonals. It, like all the long span trusses, had intermediate posts (verticals) to support the deck at shorter intervals. No record is known to exist of the lift and the equipment used in the lift. Captain Charles J. ALLEN was assigned to oversee the work, and he wrote: “The contractors for the superstructure made their own bargains with the Keystone Bridge Company, of Pittsburgh, Pa., for the iron. Considering it as necessary to have the shop tests superintended by some person in whose report’s confidence could be placed, I informed the Board of Bridge Commissioners that such should be provided for. They requested me to nominate a person, to be paid by themselves. Colonel Merrill, to whom I applied, kindly named Mr. Charles Davis. He was accordingly employed by the Board of Bridge Commissioners … The contract between Horton & Co. and the Bridge Commissioners provided for the superstructure to be finished by September 1, 1879, and requiring a forfeiture of $25 per day for each and every day after that date that the completion of the superstructure was delayed. Allen further wrote, “I required a test live load to be placed upon the bridge of 1,500 pounds per linear foot. Ice was found to be the most convenient material with which to load the bridge … It was agreed between the bridge engineer (Mr. Sewall) and myself that the deflection should not exceed the 1/1000 part of the length of span. The Fort Snelling Bridge passed its load test successfully and Ramsey County and the residents of St. Paul could now cross the Mississippi on a new, and somewhat novel, bridge.” The total cost for the bridge was $134,925 and opened March 15, 1880, or just short of a year after the contract for the superstructure was awarded. Allen concluded his report with, “The bridge is probably the strongest wagon bridge in this vicinity, although without merit as to architectural effect. It will withstand all, but such extraordinary gales or hurricanes as defy engineering skill, and which occasionally visit the Mississippi Valley. The final test was a severe one.” A St. Paul newspaper closed its article on the opening of the bridge with, “It is for this and future generations to profit by a pioneer project realized in solid stone and tenacious iron—graceful but strong, beautiful but durable.” Another newspaper wrote, “St. Paul can boast of a bridge, which in beauty, strength and durability, is unsurpassed by any other which spans the rolling waters of the Mississippi, from the Falls of St. Anthony to the Jetties, excepting the massive structure at St. Louis.” The bridge was replaced in 1909 by a double steel arch bridge to accommodate heavier loads. It in turn was replaced in 1961 by a continuous steel girder bridge. ■

Dr. Frank Griggs, Dist.M. ASCE, specializes in the restoration of historic bridges, having restored many 19th Century cast and wrought iron bridges. He is now an Independent Consulting Engineer (fgriggsjr@verizon.net).

It is a profession that promotes progress and prosperity.

You know what, Syed?

Ashraf

One of the reasons I really love structural engineering is because it is a profession that reflects the positive side of humanity.

WHY I LOV E S T RUC T URAL ENGINEERING And above all, it is a profession that reflects happiness!!

Ashraf

It is a profession that creates jobs and vitalizes the economy.