STRUCTURE magazine | December 2021

STRUCTURE DECEMBER 2021

NCSEA | CASE | SEI

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INSIDE: Slover Library

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Avoiding Sinkholes Structural Movement Calculating Rain Loads

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Contents

Cover Feature

22 SLOVER LIBRARY

D ECEM BER 2021

By David Mykins, P.E.

The challenge for this project was renovating a 115-year-old historic building, a mid-century commercial building, and connecting the two with a new, modern addition. A tight urban site, unsuitable soils for foundations, a high water table, construction concerns about vibrations, and historic requirements all combined for some unique solutions.

Features 26 BYPASSING EXPANSIVE SOILS WITH HOLLOW CORE PLANKS OVER A CRAWLSPACE

By Eric Homburg, P.E.

The Oscar Johnson Jr. Community Center will utilize mass timber construction. Perhaps the most daunting issue was expansive soils. The solution is building the foundation using a suspended hollow core plank slab system supported by elevated concrete beams constructed on top of belled piers.

Columns and Departments 7

8

Automation and the Future of Structural Engineering

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.

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

44

10

Construction Issues

Avoiding Sinkholes during Mechanized Closed-Face Tunneling

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

47

Structural Monitoring

Repair, Defer, or Do Nothing

By The Structural Engineering Licensure Coalition

48

and Giuliana A. Zelada, P.E.

Engineer's Notebook The Hidden Cost of Copy and Paste – Part 1

By John A. Dal Pino, S.E.

51

Codes and Standards

2021 IBC Significant Structural Changes – Part 2

36

58

CASE Business Practices

By Sandra Hyde, P.E., and

Coordination and Completeness of Structural Construction Documents

John “Buddy” Showalter, P.E.

By Jeff Morrison

Structural Design Pour Strips

By Gordon H. Reigstad Ph.D., P.E., S.E., Jason G. Reigstad, and Jared M. Reigstad, P.E.

39

Spotlight

A New Link in Calgary’s Covered Walkway System

By Jason McCool, P.E.

18

Talking Points

The Case for Science-Based Public Policy

By Steven F. Keppel, P.E., Scott J. DiFiore, P.E.,

17

Professional Issues A Vision for Structural Engineering Licensure

By Hee Yang Ng, C.Eng, P.E.

14

Historic Structures Quebec Bridge Failure #2,1916

By Eytan Solomon, P.E.

By D. Matthew Stuart, P.E., S.E., P.Eng, SECB

Part 4 of this four-part series continues the discussion of the structural investigations, including the Gray columns, new floor openings, and demolition of the second-floor mezzanine to allow for a new second-floor loft. Challenging and interesting, the adaptive reuse of the building was a sustainability achievement and a historical success.

InFocus

Code Updates

Calculating Rain Loads per 2021 IBC

By Anne M. Ellis, P.E.

By Sanja Buncic, C.Eng, and Matthew Smith, M.Eng, P.Eng

32 ADAPTIVE REUSE OF THE HISTORIC WITHERSPOON BUILDING – PART 4

40

Invest in Your Success and the Future of Structural Engineering

28 FIRE ENGINEERING EXPOSED TIMBER AT PIG PATCH HOUSE This single-family custom home was built in England from mass timber panels, suspended insulated precast concrete floor planks at ground floor level, and RC ground beams on mini concrete piles used as foundations. This project demonstrated that mass timber could be successfully applied to satisfy architectural and sustainability goals while meeting life safety requirements.

Editorial

InSights

Adaptation Advantage

By Stephanie Slocum, P.E.

In Every Issue

4 Advertiser Index 50 Resource Guide – Earth Retention 52 NCSEA News 54 SEI Update 56 CASE in Point

D E C E M B E R 2 0 21

5

EDITORIAL Invest in Your Success and the Future of Structural Engineering

By Anne M. Ellis, P.E., F.ASCE

H

ow much are you willing to invest in your success? Structural engineers spend $10s, if not $100s of thousands preparing to launch their careers technically. And that is usually sufficient for the first five years of a career. Beyond this, what are you personally investing to enhance your career and your practice? For our practices, firms, and companies to grow and thrive requires competent people, 21st-century solutions to address 21st-century problems, business savvy, and more. Ensuring a strong pipeline of people, solutions, and business aids necessitates collective investment. No one firm can accomplish this alone. This is where the SEI Futures Fund comes in.

The SEI Futures Fund is your vehicle for impact investing through collective action to improve our profession and careers.

Donations from one individual or organization to the SEI Futures Fund are combined with donations from others providing for significant investments in programs that otherwise may not happen. Since 2014, the SEI Futures Fund has awarded grants totaling over $800,000 to: • Promote student interest in structural engineering • Support younger member involvement in SEI • Provide opportunities for professional development “With a grant from the Futures Fund, the SEI Global Activities Division hosted the Future Impact of Covid-19 on the Commercial Development Market virtual event, kicking off informative and timely presentations on the impact the pandemic and other global events are expected to have on our profession,” said Derek Skolnik, Chair of SEI Global Activities Division.

These SEI Futures Fund investments generate social, environmental, and economic benefits to donors and our profession. The programs considered for investment are evaluated through an impact lens.

Consider these SEI Futures Fund-generated impacts: • Social: Was your firm able to grow through new hires of students and/or young professionals that received one of the hundred+ SEI Futures Fund scholarships to engage at an in-person Structures Congress?

• Environmental: Are you considering structural embodied carbon in your design, capitalizing on the industry benchmarks and targets generated by the SE 2050 database developed thanks to the $25,000 investment from the SEI Futures Fund?

• Economic: Are you better able to manage your practice risks thanks to knowledge gained from the Agreement Basics for Engineers guide included with this magazine thanks to the SEI Futures Fund $13,500 investment to publish and distribute it to 30,000 practicing structural engineers?

“The SEI Futures Fund has allowed us to take a major step forward in advancing the goals of net-zero embodied carbon structures. The grant provided us with the resources needed to develop our beta database of structural embodied carbon that will allow us to establish

national trends of embodied carbon in the structural systems we design. Published targets for structural embodied carbon reduction will be developed as a result. We’ve seen an immediate impact on industry momentum around SE 2050 and its goals. Without the SEI Futures Fund grant, we might not have built the momentum we have today. The SEI Futures Fund grant was the catalyst we needed, and it couldn’t have come at a better time,” said Michael Gryniuk, Chair of SEI SE 2050 Committee.

There are many more programs making an impact and generating returns for the savvy, future-forward structural engineers investing in the SEI Futures Fund.

Consider: • Global Practice Guide to prepare and empower those with the passion and ambition to work globally with the technical, professional, and business requisites for success • Structural Fire Engineering Curriculum to help develop future engineers that can work in innovative environments and address the 21st-century demands of clients • Facilitation and Leadership Training Workshop to equip local chapter leaders with usable tools on facilitation to improve their leadership skills to boost their volunteer and professional endeavors I encourage you to contribute to the SEI Futures Fund and track your individual impact return on investment (IROI):

Your career can

be your very best investment in a

changing economy.

STRUCTURE magazine

IROI = IV-IIA × 100% IIA

where: IV = Impact value IIA = Initial investment amount You will quickly see your investment is delivering outsized returns.

“A Futures Fund scholarship allowed me to attend my first Structures Congress. That opportunity led me to 7 more Congresses, 3 national committee chair positions, 2 task committees, 2 job offers, a position on the Futures Fund Board, and many lifelong friends and mentors.

There’s no doubt my career would look different if I hadn’t received that scholarship,” said Linda Kaplan, SEI Futures Fund Board member.

Your career is the most important investment you will ever make.

It is an alternative and diversification to your traditional investment portfolio. Your career can be your very best investment in a changing economy. As you invest in your career, invest in the SEI Futures Fund with a tax-deductible, year-end gift at https://bit.ly/3osX0O5. Thank you for investing in the future of structural engineering!■ Anne Ellis is the Executive Director of the Charles Pankow Foundation and serves as an outside director on the boards of Alpha Corporation and GEI Consultants. In addition, she is chair of the SEI Futures Fund Board, serves on the ASCE Industry Leaders Council, and is chair of the Board of Directors of the National Institute of Building Sciences.

D E C E M B E R 2 0 21

7

INFOCUS Automation and the Future of Structural Engineering

By Eytan Solomon, P.E., LEED AP

In July 2021, I discussed the “hot” topic of automation and the future for structural engineers with two of the industry’s leading experts in digital design: Rob Otani is a Senior Principal and the Chief Technology Officer at Thornton Tomasetti, and Zak Kostura is an Associate Principal and the Americas Region leader of Advanced Digital Engineering at Arup. Below are highlights from our discussion.

There are two things that we call “code.” First, building codes, meaning the rules for structural design – including the jurisdictional codes and model codes (like the International Building Code [IBC]) and code-referenced standards (like the American Institute of Steel Construction’s AISC 360). And then computer code, meaning the software programs we use for structural analysis and design. A frequent question is – what is stopping ALL of the written building code from becoming written computer code?

self-perpetuating. If you simplify the process of designing, you give yourself the opportunity to iterate that design more and focus on things you would not otherwise be able to. Plus, there are advances in culture and technology.

When you are talking about designing a design process, it sounds like systems engineering.

Kostura: At Arup, before someone gets to the point where they Otani: Nothing is stopping us. It’s just hard to implement. It’s hard to automate anything on a project, in theory, they are supposed to create an app that is going to redirect you to all those other things… A put a workflow together that documents the design process. The lot of engineering is actually not just designing a beam. It is deciding workflow diagram allows you to understand the linear parts of your what beam to design, what material to use, etc. process versus the iterative. It helps you understand the dependenKostura: Yeah, I would say that it may be shocking to a structural cies between the two. For example, if you look at three commercial engineer, but I think building code is typically more forgiving high-rise buildings, you might say that no one piece of software will than computer code. A lot of discretion and judgment goes into design all three of these entirely. But when you map the workflow for engineering that you only appreciate when you try to automate these three projects, you see that certain steps are common. Those it. One of the things that I really had to learn a lot about when I common steps can be a tool or a set of tools. The more you can got into coding software is the concept of user-experience design. channel that and think about it in terms of process, the more clarYou really have to plan what you’re doing and the group of users ity you have about how far you go with the scope of any software. you are trying to help. In our line of work, there is a fundamental question about who uses the software. There’s one line of thinking Can people be taught to think that way before they have really that you can make software for engineers to assist in completing mastered the job? the design. Alternatively, the end-user could be someone else, such as the architect or owner with less understanding of the internal Otani: Well, there are a couple of questions there. The end-user is not process. So it’s important to think going to see a lot of that. I think Zak about the user we are building that was referring to the person who is software for. It defines what the mapping this out. I think you know “A lot of engineering is actually software will do. the machine learning apps that we Otani: So, tools like RAM, SAP have created over the years. We need not just designing a beam. It is 2000, or ETABS already have codeindependent little physics-based based automation in their systems. checks along the way because, you deciding what beam to design, By the way, they didn’t always. And know, engineers hate a black box. what material to use, etc.”

I would argue that is what makes They love Excel because all the forthem reliable tools for structural mulas are very clear. The engineers analysis and design. can write their own little checks along the way. What Zak was talking about, in general terms, is called robotic proIs it only a matter of time before cess automation. They use that in the all design is automated? automotive industry, where a very Kostura: I don’t think so. clear road map is needed to identify Otani: I don’t think it is going to when and what to check along the happen in our lifetimes. way because the engineers just by Kostura: Is Apple ever going to be nature don’t trust anything. done designing the iPhone? No, Kostura: Digital practitioners talk because design is something that is about user journeys. What’s the

Rob Otani

8 STRUCTURE magazine

process that the user goes through? What is the user’s goal, and what is the process to accomplish that goal? It is amazing to me how many project engineers are unable to sit down and articulate the process.

It isn’t a technique we prioritize, so most engineers are not getting better at it.

When the author was in engineering school, in the early aughts, there was a requirement to take one coding class. Should aspiring structural engineering students be taking more coding classes?

training every year to keep current. This has affected the “shelf life” of our project managers: their shelf life used to be their entire careers. Like, someone who was 50 in 1990 could tell someone who was 21 exactly how to do the job – with tracing paper, the green (AISC) book, whatever it was. And today, the senior folks who know their stuff inside and out have a hard time being a mentor to the engineer who is churning away (with all the new software).

And I’m not even talking about Revit, just the engineering tools.

However, most would assume that Otani: I would say yes. I mean, must be and has been the case for everyone is not going to be a condecades in things like aerospace sultant software developer, but you We know that our profit where you clearly have needed kind of need to know what’s posmore advanced computing for the margins on conventional design sible. Like to Zak’s point, if you analysis, and yet they had their recognize something that is a real gray-haired engineers. will not get bigger in the future. pain point, and you have a little bit of software development knowlKostura: The difference is that edge, you can piece things together nobody is expected to make a profit to automate the process. So I think on the first prototype in aerospace it’s awareness. and automotive. In our industry, that Kostura: We are in a time-based is precisely what we are expected to business, right? And we know that do, so we have a lot less rigor than our profit margins on conventional they do. design will not get bigger in the Otani: Aerospace has ridiculous QA/ future. They are probably going to QC. There are checks upon checks continue to get smaller, so I think upon checks. For example, you put we must equip as many engineers as a new pillow in an airplane, and it possible with the ability to readily has to be verified. identify repetitive tasks and perform Kostura: You know, it’s another them more efficiently. That has been the role of technology for 40 delineation between us and aerospace and automotive that you do or 50 years, and I think to Rob’s point, a new way is needed with not have this awkward transfer of risk and responsibility midway competence and literacy in design thinking and programming. through the project, right? From design to construction. You have And every organization is going to face a dilemma around what a team that is incentivized to collaborate and work together. If you task an Engineer to know and what you leave in the hands of someone misses something and the model’s not correct initially, it another professional digital practitioner. A lot of us in engineering is everybody’s problem, not just one person’s problem. And the way are experimenting with Cloud services now, right? You can run up we have carved our industry up makes it harder to undo. So that is an AWS [Amazon Web Services] instance and run your FE [Finite a big issue for all of us.

Element] model there, which is faster. And if you were to use Do you think that will lead to more design-build?

Microsoft, you might choose Azure services over AWS. You might get a price break today, but next year AWS might be cheaper. And if Otani: Yes. I think it will. I think it is going to go in two directions. you do the analysis in the middle of the night, it is even cheaper than I think contractors will start to get more involved in design because during the day. You can save money by analyzing when computing they can have more influence during that time. For example, do not resources are less in demand, like electricity. Cloud economics is use that facade material because we cannot get it for two years. As well an example of a field of expertise that will ultimately come into our as the engineers going further and producing shop drawings. And analytically-heavy industry. Can you expect the engineers to increas- having the contractor we know who will build it fill in the rest because ingly take that on? It seems like most organizations are ultimately we have been staring at this project for two years, right? going to come to a point where they have to make a clear decision Kostura: We are all looking for a way to hedge our profits, right? about what digital skills should be left in the hands of people who Because the conventional things we do are getting harder to make practice structural engineering every day or in the hands of another money doing. So, what else can you do? What added services can you field of conventional building design. They will also need to decide offer? There are many great ways to extend what you’re already doing. how they cover the other areas of digital knowledge, like cloud Otani: In the startup world, they talk about making the process vertical.■ economics and many others.

Otani: Yes, but take that one step further. Recently, I did someThe author would like to thank both Rob and Zak for their thing with a technology-in-architecture practice group. I showed a insights into these critical topics. There is much food for thought graph of the software I had when I first started: Risa 2D and SAP as structural engineers consider how the profession integrates 90. That’s it – those two things. And by the way, there were only with new technologies and innovations. three computers in the entire office. So today, there is probably at least 10 times that amount of software, right? So, the engineer Eytan Solomon is a Senior Associate at Silman and a member of coming out of college needs to know so much more than I knew.

STRUCTURE’s Editorial Board. (solomon@silman.com) And now, the practicing engineer needs to have significantly more

Zak Kostura

D E C E M B E R 2 0 21

9

construction ISSUES Avoiding Sinkholes during Mechanized Closed-Face Tunneling

By Hee Yang Ng, MIStructE, C.Eng, P.E.

R

apid transportation by metro has become an indispensable development for many modern cities. Due to the depth and length of the tunnels, these urban subway lines are often built using circular tunnel boring machines (TBM). Two common types of TBMs are slurry TBMs and earth pressure balance (EPB) TBMs. While the tunnel shield and completed segmental tunnel linings (usually discrete precast concrete panels assembled together or precast rings) are relatively safe, the TBM excavation process can be potentially dangerous. This article looks at some measures and good practices to avoid sinkholes (Figure 1) during closed-face tunneling. The intricacies of tunneling and TBMs are much broader than the overviews included here.

Key Principles of TBM Construction A TBM typically consists of a cutterhead, excavation chamber, and a shield in very simplistic terms. The strong cylindrical steel shield provides a safe environment for removing spoils from the excavation chamber and erecting the permanent linings. At the front of the TBM, a cutterhead is equipped and configured strategically with disc cutters to break down rock and appropriately sized openings to allow excavated materials to be removed from the face. Face pressure is regulated via slurry flow and plastic paste discharge for slurry and EPB TBMs, respectively. It is important to note that the cutterhead needs to overcut a slightly larger diameter to allow the shield to advance by jacking against completed linings. Similarly, the linings being erected within the shield means that there is a “tail-void” or

Figure 1. Sinkhole caused by TBM tunneling work.

annulus which needs to be grouted to minimize risks of ground and lining movement.

EPB TBMs are more suited for cohesive soils (clay/soft ground), while slurry TBMs are for cohesionless soils (sand/rock/mixed ground of soil and rock) with additives/conditioning agents and bentonite slurry added, respectively, to condition the excavated spoils for easy removal. Slurry TBMs are more elaborate in setup as the slurry needs to be recycled, requiring a separation plant on the ground. The better control of face pressure for slurry TBMs allows such TBMs to be used in more difficult (e.g., high groundwater and permeable) and heterogeneous ground conditions. Slurry TBMs are also sometimes equipped with powerful crushers and grizzly bars to deal with boulders (or rock debris) and prevent jamming the TBM. Choosing the correct TBM type with adequate and suitable specifications to deal with the site-specific ground conditions is a critical first step in ensuring safe tunneling (Figure 2).

Ground Settlement

Figure 2. Schematic view of EPB TBM and Slurry TBM.

10 STRUCTURE magazine

A sinkhole is a case of a sudden (and usually substantial) void formed at the ground surface. The usual small magnitude ground surface settlement caused by TBM tunneling can be estimated using a Gaussian (normal) distribution curve. This settlement is due to the volume loss in the ground in the process of tunneling. Volume loss can include losses at the TBM face, the shield, and the tail void. The Gaussian distribution assumes that the area encompassed by the curve is equal to the volume loss. Two key parameters are required to define the curve: the volume loss, Vl, and i, the distance from the tunnel centerline to the inflection point. The volume loss is usually expressed as a percentage of the tunnel face area, say 0.5% to 3%. i is a fraction of the tunnel depth, which can typically be 0.25H for sand and 0.5H for clay, where H is the depth to the tunnel centerline. For clay, this value can

range from 0.4H to 0.7H for stiff clay and soft clay, respectively. Although obvious, to minimize impact to property and structures, surface settlement needs to be minimized, and therefore, volume loss needs to be as small as possible, preferably below 0.5%.

Figure 3 shows a comparison of ground surface settlement for TBM in clay and sand. The tunnel diameter is 19.7 feet (6m) and located at 65.6 feet (20m) below ground, using a volume loss of 2%. Ground settlement in stiff clay and sand will show a narrower trough width. Therefore, the maximum settlement would be higher than soft clay to maintain the same area enclosed by the curve, assuming the same volume loss.

A simplified finite element simulation of volume loss in sand and clay using volume contraction is shown in Figures 4 and 5 (page 12). It can be seen that the Figure 3. Gaussian curve for ground surface settlement due to TBM tunneling. settlement trough in sand is very narrow, with much settlement occurring on top of the tunnel and the soil movement wedge surface propagating upwards at an angle of 60° During TBM excavation, the TBM face has to balance two primary (measured from horizontal) to almost vertically (like a chimney-type sources of loadings, namely from soil and water. In terms of total mechanism). In clay, the settlement trough is much wider, and some stress for undrained loading, where water is not considered explicdesigners assume the soil movement wedge to be at 45° measured from itly, the face pressure required to prevent collapse can be calculated the horizontal tunnel centerline (springline). This is due to the plastic from overburden pressure less Nccu, where Nc is the stability number flow of soft clay, where there is substantial movement horizontally (equal to 9 for deep tunnels with soil cover greater than 3 times from the sides of the tunnel due to the generally higher horizontal the tunnel diameter), and cu is the soil undrained shear strength. stress compared to sand. Whereas for sand, the frictional nature of Notice that Nccu is analogous to the ultimate bearing capacity of the material means that vertical stress is usually higher than horizontal 9cu in piles. Surcharge (if extensive in area) and pressure variation, stress, resulting in a greater tendency of downward soil displacement typically 1.45 to 4.35 psi (0.1 bar to 0.3 bar or 10 to 30 kPa), can above the tunnel. In addition, the failure surface of sand tends to be included if necessary. To control settlement, a more stringent propagate upwards in a path of least resistance, which is vertically Nc is used. For example, to control settlement below 2%, Nc could upwards because the frictional strength decreases most rapidly in the be 4.5 (half of 9). vertical direction with decreasing confining stress.

Similarly, for effective stress, the face pressure required to prevent collapse can be calculated from the sum of water pressure and effective soil balance pressure (submerged soil unit weight times tunnel TBM Face Stability diameter) less soil cohesion, with some coefficients applied (e.g., 0.2 The tunneling process is an unloading process as soil support is and 2 respectively for effective soil balance pressure and soil cohesion). removed when the ground is excavated. Therefore, the control of face The water pressure term is going to be the dominant factor when pressure during TBM operation is critical. Inadequate face pressure the groundwater table is high. Again, to control settlement, a higher could result in the face lacking support and the soil becoming unstable, pressure is necessary to balance the soil pressure (use a higher coefcausing ground loss and possibly a sinkhole. Conversely, the soil might ficient, e.g., 0.6). The maximum face pressure should not be greater be forced upwards if the face pressure is excessive, causing a blow-out than the total vertical overburden pressure to prevent ground heave. or ground heave. Slurry might also be leaked out.

Occasionally, operatives may need to enter the excavation chamber to inspect, repair, clear obstructions, or change worn-out tools (also known as cutterhead interventions). In such an instance, compressed air is required to balance the water pressure to prevent water ingress. As soil pressure cannot be balanced, it may be necessary to ensure such stoppage and interventions are carried out in stable ground, for example, docking within a jet grout block. In addition, dewatering and grouting might be required. Ensuring that the membrane filter cake (an impermeable layer on the soil surface formed by small particles clogged or wedged together under pressure) is maintained and effective is crucial for compressed air support, especially during prolonged stoppages.

Controlling Over-Excavation Figure 4. Ground settlement due to TBM in sand.

Another critical aspect in preventing sinkholes is in the control of over-excavation during TBM tunneling. The theoretical volume of soil to be taken out D E C E M B E R 2 0 21

11

High-Risk Activities

Figure 5. Ground settlement due to TBM in clay.

can be determined by the tunnel geometry, i.e., the product of the face area and length of the tunnel. If the actual excavated amount exceeds this theoretical amount, it means there is over-excavation.

Over-excavation can be potentially dangerous because when more soil is taken out from the ground, underground voids are formed, and these could culminate in a sinkhole being formed at the ground surface. In addition, the formation of a sinkhole can be sudden with few tell-tale signs. Therefore, closely monitoring over-excavation and setting an over-excavation limit is crucial in preventing sinkholes.

Determining over-excavation is not a straightforward task because the soil volume in situ is obviously different from the volume of loose soil after excavation. Some designers estimate the expanded volume or swell of the soil after excavation by applying a “bulking factor” of 1.2 to 1.3 for soil and even higher for rock. However, additives or slurry are added for closed-face tunneling, either EPB or slurry, and need to be accounted for. Furthermore, there is a possibility of water intrusion and additive or slurry loss into the ground. Ground make-up is also likely to vary along the TBM alignment. Therefore, designers have to be aware of the limitations and significance of overexcavation monitoring.

While soil volume may be different before and after excavation, the weight or mass of soil must remain the same. For a slurry TBM, the measurement of flow and fluid density at the input and output is used to monitor over-excavation. As an illustration, to find out the volume of soil excavated, the mass excavated can be found by first multiplying density with flow and time and taking the difference at the input and output pipes. Then, volume can be estimated by dividing mass by density. EPB measurement methods include physical muck skip counts and direct weight measurements.

Designers for TBM tunneling should always specify the limit of over-excavation allowed. Typically, this can be a value of 15% to 20% deviation in volume excavated for a single ring or a rolling average of 5 to 10 rings. When such a limit is breached, the tunneling team needs to know what immediate actions are required for reporting, verification, review, and remedial work. The response required might vary according to the risk to the public; for example, depending on whether tunneling is over a greenfield site (minor consequence) or in close proximity to existing buildings or critical infrastructures (severe consequence). When the over-excavation limit is breached, it is essential not to excavate further until verification and review are done to confirm that it is safe.

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Designers need to be aware of several high-risk activities during TBM tunneling. These include break-in/break-out from shafts, stoppage and interventions, mixed ground (soil and rock) conditions, worn-out cutting tools, and TBM flushing for a slurry TBM. Flushing is a process to circulate slurry in an attempt to revive a choked or jammed TBM. Without shield advancement, the slurry discharge could easily result in over-excavation. Therefore, such a procedure must only be carried out under authorization and close supervision with adequate control and precautionary measures in place. The tunneling team must follow strict procedures when encountering unexpected events, such as a jammed TBM. This is to avoid allowing the TBM operator free rein in carrying out repeated and extended flushing and discharging excessive materials. It is always tempting to repeatedly run the slurry flow in the excavation chamber to revive a jammed TBM. A mixed-face condition occurs where the TBM face has both soil and rock. This is particularly challenging for TBM because of the vastly different stiffness of soil and rock. As the rock at the bottom of the TBM face resists the cutterhead’s advance, the soil at the top of the TBM is continuously drawn into the excavation chamber. Therefore, overexcavation can quickly occur. In addition, broken rock fragments that are too large to enter the cutterhead openings are trapped in front of the TBM and rotated. This overbreak also contributes to over-excavation due to the drawing in of more material compared to TBM advancement.

Precautionary and Remedial Measures In built-up areas, it may be helpful to visually annotate the TBM alignment on the ground surface. In addition, surface surveillance by eye helps detect visible ground settlement, especially when ground settlement readings are not yet taken or made available. In critical areas, it is also helpful to have a drilling rig ready and on standby so that any backfilling of voids by grouting can be carried out immediately.

Ground improvement by grouting is a commonly employed technique in TBM operations. In difficult ground conditions such as mixed-face tunneling, grouting the weaker zones helps to provide a more stable excavation process and minimize the risk of overexcavation. Grouting may also be used in other higher-risk areas, such as break-in/break-out locations.

Conclusion The avoidance of sinkholes is a crucial aspect of safe TBM construction.

Starting with a good understanding of the ground conditions, a suitable TBM tunneling method and machine with appropriate capabilities can be selected. Maintaining adequate face pressure and working within allowable over-excavation limits are the two main control measures preventing sinkholes. When over-excavation occurs, a common error is to rely on grouting a sinkhole as an afterthought in an attempt to rush through tunneling with no regard for over-excavation and excessive slurry discharge. The better way is to suspend excavation and restart after verification and review to confirm it is safe to continue.■ Hee Yang Ng is a Principal Engineer with a building control agency in the Asia-Pacific region.

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structural MONITORING

Repair, Defer or Do Nothing

Structure Movement Monitoring for Efficient Planning and Decision Making

By Steven F. Keppel, P.E., Scott J. DiFiore, P.E., and Giuliana A. Zelada, P.E.

S

ymptoms of building movement can develop obviously and suddenly, or more subtly over years or decades. Common symptoms include cracks in walls or floors, racked windows and door frames, and sloping or uneven floors. While each of these symptoms causes concern, not all are equal. Some symptoms could be a sign of structural deterioration in need of immediate repair. In contrast, other symptoms may be a remnant of previous movement that occurred years ago and has since stabilized. In between, a range of conditions exist. Given the range of potential repair actions and related costs, it is essential to identify the cause and an appropriate scope of repair.

Severe symptoms require immediate investigation to determine whether emergency actions are required. However, suppose the engineer and building owner determine that immediate actions are not warranted. In that case, a thoughtful structure movement-monitoring program serves as a valuable tool to identify the appropriate type, timing, and extent of repairs. Collecting the data and understanding the symptoms provide significant value to the owner, the building, and its tenants, as repairs can focus on what is essential.

This article highlights types of structure movement monitoring (referred to herein as monitoring or the monitoring program) that can be implemented to help the owner isolate the cause of damage and plan for effective repairs, along with several examples that illustrate varying conditions and subsequent actions.

Instrument Types and Considerations Monitoring may employ a variety of instruments depending on the conditions and type of structure. Several instruments commonly used with considerations for selection and installation are listed below. Costs can vary widely depending upon the sophistication of the instruments.

Crack in masonry unit block wall.

• Instruments: Digital or optical levels, total station, 3-D laser scans, deformation monitoring points (DMPs).

• Considerations: Determine whether long-term repeatability is necessary. Identify appropriate location, quantity, and type of permanent or temporary DMPs and reference benchmarks (BMs). Evaluate the level of accuracy needed and sophistication of the equipment

Below-Grade Measurements

• Purpose: Measure soil movements or groundwater elevations.

• Instruments: Groundwater observation wells, extensometers, Crack and Joint Measurements and inclinometers.

• Purpose: Monitor displacement at joints or cracks.

• Considerations: Installation requires test borings which can be • Instruments: Crack monitors and disruptive to building operations and displacement gauges (visual/manual occupancy. Interior access is often more or electromechanical readout; 1-D, challenging than exterior access.

2-D, or 3-D); tiltmeters; hand-held Additional considerations are common levels; and many others. to all instruments. Business considerations • Considerations: Choose based on include the building’s value and expected the configuration, access, and anticiuseful life, time-horizon for repairs, potenpated movement (type, magnitude, tial repair costs, owner budget, and safety. and direction). Sometimes the best Technical considerations include accuracy solution is the least sophisticated requirements, frequency and duration of (e.g., pen marks or pins mounted on data collection, repeatability, manual or either side of a crack and measured automated readings, power source, and with a tape measure or calipers). cell service for remote data transmission.

Logistical considerations include access, Deformation Survey durability, protection from the weather, • Purpose: Measure displacement construction, vandalism, impacts to Cracks in masonry block wall repaired with supplemental of structural components, ground tenant use of the space (e.g., trip hazards carbon reinforcement. The monitoring program verified the surface, utilities, etc. or aesthetics), and possible placement near performance of these repairs.

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sensitive equipment affected by vibrations or levelness (e.g., manufacturing, medical, or testing).

Developing Monitoring Programs

remnants of long-ago movements, and no safety risk exists. This condition is a somewhat best-case scenario, where proactive instrumentation, monitoring, and patience save the owner repair cost and disruption.

In some cases, No Action may result from several rounds (or years) of monitoring and a thorough investigation and assessment.

Developing an effective monitoring program requires investigation to identify Example of No Action the program objectives. The investigation includes a visual assessment, a desktop An owner was concerned that its building study, and sometimes destructive or was settling. Multiple tenants reported invasive activity that includes building cracked wall finishes, gaps between openings or subsurface exploration. wood trim and floors, and sloped floors • Visual Assessment: A visual assessin a 100-year-old, multi-story residential Temporary repair involving underslab void filling. Coring, ment identifies the type of distress building. The structure consisted of wood filling voids with grout, and patching can be performed over symptoms, magnitudes, locations, framing and masonry-bearing walls supa few days compared to a longer construction period for a trends, construction materials, and ported on concrete and stone foundations. permanent repair. age of the symptom.

After performing a visual assessment of • Desktop Study: A review of availthe structure and symptoms, the engineer able data, including construction implemented a monitoring program that drawings, repair records, construcincluded crack gauges, a structure defortion permits, historical photos, or mation survey, and floor levelness readings prior subsurface investigations gives every six to twelve months over two-plus a head start in understanding buildyears. Results showed that the symptoms ing construction, foundation types, were not caused by foundation settleand potential load paths. ment but rather a combination of creep • Invasive Investigations: When of the wood framing and localized framing conditions permit or when historical reconfiguration and repairs over the life of data is absent, an invasive investigathe building. Seasonal dimensional change tion consisting of building openings of wood trim and flooring finishes caused (e.g., at framing connections, gaps to be more apparent in the winter column to foundation interfaces) months, and the foundation was stable. and/or subsurface investigations No repairs were required. The instrumenPermanent repair involving slab replacement is usually more (e.g., soil test borings or test pits) tation remained on the structure to allow disruptive to the client’s space compared to temporary repairs. provides critical information to future readings if new symptoms develop. diagnose a problem.

An immediate visual assessment of severe distress symptoms coupled Repairs (with Verification) with a brief desktop study can quickly identify the potential need for immediate emergency stabilization. In many cases, after ruling In many cases, the need for repairs is evident based on visual inspection. out immediate safety issues, the engineer can develop correlations However, desktop study, instrumentation, and monitoring help define the between building construction (or renovations), foundation type, appropriate type of repair. In some cases, instrumentation and monitorand subsurface conditions, identify possible contributors to dis- ing can verify the adequate performance of a more cost-effective repair. tress symptoms, and identify what to measure. Then, armed with Example of Repairs with Verification knowledge, the engineer can develop a targeted instrumentation plan with the type, location, and quantity of instruments and the An owner reported cracks in the shear walls for a multi-story masonry frequency of data collection to help establish whether movement apartment building. The engineer completed an investigation and has stabilized or is worsening. monitoring program to evaluate the cause. The monitoring program The owner and its engineer must work together to balance priorities. consisted of deformation surveys and crack gauges and showed that For example, what areas are most important to diagnose and repair? the foundation was no longer moving. The engineer developed repairs What areas must be maintained without disruption in the short for the shear wall, supplemented with a monitoring program for a few term? Does the owner’s time-horizon permit monitoring beyond a years after repairs to confirm performance, saving the client the costs year? The owner’s specific needs and input influence the program. and disruption of more significant and conservative foundation repairs.

The monitoring program informs decisions for a range of outcomes, from no action needed, to implementation of repairs, to deferred Repairs (Temporary, with Monitoring) action where repairs are prioritized over time.

Depending on the project, permanent repair may not be the best option for the client. For example, permanent settlement mitigation No Action repairs such as soil improvement grouting, foundation underpinThe engineer’s recommendation may be No Action if the monitoring data ning, or replacement of a slab-on-grade with a structural slab are shows that movement is no longer ongoing, the distress symptoms are often disruptive to the client’s operations, even if repairs are phased. continued on next page D E C E M B E R 2 0 21

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Test pit excavation to observe buried wood pile foundations and extract specimens.

The monitoring program informed the locations for the test pit investigation.

Example of a disruptive foundation underpinning repair requiring capital planning.

Foundation underpinning repairs consist of needle beams supported on micropile foundations.

Depending on the structure, rate of measured movement, cause of settlement, and the client’s use of the space, a temporary repair can “buy time” and serve as an effective means to limit disruptions to operations. While a permanent repair could be the most robust solution, the temporary repair supplemented with monitoring may better suit the client’s needs.

The structure deformation survey was critical for repair planning.

High accuracy and repeatability of the survey data are essential to understanding behavior. In this case, structure deformation monitoring techniques involved a high precision digital level and an invar survey rod, collecting multiple rounds of data, and performing a least-squares adjustment of the elevation data to obtain an appropriate accuracy not achievable with traditional survey methods. This was particularly important since the survey traversed multiple interior building spaces.

For this high-value building, the owner agreed to engage the engineer to evaluate and estimate the remaining service life of the timber piles supporting the structure. Combined with the groundwater and structure movement monitoring program, the engineer identified trends and established a low to high-risk priority matrix for repairs within the structure. This priority matrix helped the owner develop a capital plan for long-term repairs. The owner continues to use the monitoring program to update its capital plan as needed based on measurements as time progresses.

Example of Temporary Repairs with Monitoring An owner reported settlement of a slab-on-grade at a cleanroom medical manufacturing facility. The building’s structural framing was supported on deep foundations and exhibited little to no movement.

The slab-on-grade settlement significantly impacted serviceability and facility operations due to racked door frames, damaged utilities, and reduced equipment usage.

A subsurface investigation identified the presence of voids beneath the slab-on-grade, in addition to compressible subgrade. The engineer implemented a monitoring program combined with temporary repairs in targeted areas to mitigate slab settlement by filling the under-slab voids with a lightweight grout. The monitoring program consisted of frequent structure deformation surveys and annual ground penetrating radar surveys to detect under-slab voids and identify areas at risk of sudden settlement. The monitoring program allowed the owner to slow damage to the floor slab and equipment, manage risk of additional settlement, and maintain facility operations.

Repairs (Deferred and/or Prioritized) In many instances, repairs are warranted but not required immediately.

The owner and engineer can work together to plan for what can often be costly and disruptive foundation repairs. Since repairs are deferred and implemented over time, monitoring is a critical tool to help plan and prioritize repairs.

Example of Deferring and Prioritizing Repairs A historic and iconic masonry building previously experienced settlement due to timber pile deterioration resulting from lowered groundwater levels. The monitoring program consisted of groundwater observation wells, borehole extensometers, tiltmeters, various types of crack monitors, and an extensive network of structure deformation monitoring points on walls and foundations.

16 STRUCTURE magazine

Conclusion There are a variety of instruments available for monitoring the behavior of structures and their foundations. Some tools can be used in day-to-day work; others are more complex and require planning, investigation, and some level of disruption to the tenants.

When initial investigations determine that structural safety is not a concern, time for thoughtful and targeted monitoring typically can save the owner unnecessary disruption and repair costs. An effective monitoring program helps owners understand the source of distress, serves as a tool to document symptoms over time, and ultimately helps the owner and its engineer make decisions about the extent and timeliness of needed repairs, if any. Ultimately, the successful monitoring program allows owners to control priorities and budgets for repairs and provides time to plan for disruptions to occupancy and facility operations.■ All authors are with Simpson, Gumpertz & Heger, Inc. in Waltham, MA. Steven F. Keppel is a Senior Consulting Engineer. (sfkeppel@sgh.com) Scott J. DiFiore is a Principal. (sjdifiore@sgh.com) Giuliana A. Zelada is a Senior Project Manager. (gazelada@sgh.com)

engineer's NOTEBOOK The Hidden Cost of Copy and Paste Part 1

By Jason McCool, P.E.

D

elegated design of steel connections and stairs for various steel fabricators has occupied most of my last ten years at the structural engineering firm I represent. I have seen contract documents from structural engineers and architects from all over the country, from sole proprietors working out of their houses to some of the most well-known design firms with a worldwide presence. With that said, I am often asked by exasperated fabricators or freelance detailers about contract documents that just do not make any sense to them.

Sometimes, I am even offered work because the contract documents are so confusing that the fabricator mistakenly thinks design is being delegated to them. Thus, I sometimes serve as an interpreter first before serving as an engineer, followed by a stint as an unofficial defense attorney of our profession when the cause of the confusion becomes apparent.

What is that cause? Typical details and boilerplate specifications.

Of course, reinventing the wheel on each new job is a good way to lose money, so it is beneficial to reuse as many details and notes as possible. Once you invest time getting a good detail drawn and annotated, you naturally will want to use it as a standard. That often requires making it generic enough that it can be applied to a variety of similar conditions. Unfortunately, those conditions sometimes are not as similar as we would like to imagine.

With that in mind, here are the first three of ten obstacles I have encountered using other engineers’ design documents while working on behalf of steel fabricators over the last ten years: Inapplicable details. Do not add details that are not applicable anywhere on your project, especially if you have not added the details that actually do apply. For instance, is your client’s new project in Bismarck, North Dakota, very similar to their previous one in Hayward, California? Unless you intend to give local fabricators heart attacks, do not copy connection details from a building in Seismic Design Category (SDC) E to a project in SDC A. Another instance of this issue is where copied details reference delegated design when the design is not being delegated on the current project. The result has often been a frantic email from a fabricator asking for a quote on connection design for a project bidding on the day they came across a delegated design reference buried in a detail somewhere. Good housekeeping benefits everyone.

Contradictory specifications. Some cases are simply amusing, like a steel stair specification requiring the stairs on a project in Arkansas to be stamped by a California-licensed structural engineer. Others can raise serious questions and cause unnecessary delays. Do not assume that a caveat like “in the event of discrepancies between drawings and specs, the drawings shall govern” clears up the confusion caused by not reading through your own specs thoroughly. This is especially true if you made the Inapplicable Details mistake above. The specs are still binding on the fabricator, so if you copy a spec referencing Architecturally Exposed Structural Steel (AESS) Category 4 to your typical egress stair, the fabricator can rightly say you specified “showcase elements” and price accordingly. I have received plenty of specs that looked like nobody on the design team bothered reading them.

Specs should not be an afterthought.

STRUCTURE magazine

Outdated references. Unfortunately, drawings are often like houses: they accumulate junk and need a good spring cleaning every year.

In the last few years, I have personally seen general notes referencing the 1979 welding code, the 1997 building code, and the 7th and 8th editions of the American Institute of Steel Construction’s (AISC) Steel Construction Manual (that is the 1970 and 1980 manuals for the younger engineers reading this). My first thought when I see references like that in a set of drawings is, “If something so obviously wrong that would take a few seconds to fix was left that way, what else was overlooked?” Now, in case you think those are just obvious gaffes to pass over, one engineer friend called me asking for advice as he was being ordered by an Engineer of Record (EOR) to redesign and resubmit his steel joist calcs based on the AISC’s 9th edition steel manual (from 1989) as shown on the EOR’s drawings. AISC 360-10 was the steel design standard adopted by reference by that state’s building code at the time. Some design programs are not set up to switch back and forth between different editions of standards. And newer editions of references tend to have more research and testing behind them, allowing capacities to increase in later editions or new limit states to be imposed as deficiencies are discovered.

The flip side is that if you specify “latest edition” of some code or standard just to avoid having to update references in your drawings but have not familiarized yourself with it yet, be prepared for RFIs or back charges from those who have read it more closely. A prime example is AISC’s Code of Standard Practice (COSP, 303-16). The 2016 edition added requirements that the EOR shall provide a bidding quantity for reinforcement of framing at connections (e.g., column web doublers, etc.) if the EOR is not fully designing that reinforcement and is delegating the connection design to the fabricator. See COSP section 3.1.2(2)(b) and its Commentary. I was involved in a project where the structural drawings referred to the “latest edition” of the COSP (2016) but simply had the note “web plates/stiffeners etc. as req’d by design.”

In this case, the EOR, by his own reference to the latest edition, should have provided an approximate tonnage of reinforcing for competing fabricators to bid fairly but did not. References matter.

These might seem relatively minor compared to errors like undersizing a beam, but they can still cause questions and delays.

In the next installment, I will look at 4 more issues to watch out for.■ Jason McCool is a Project Engineer with Robbins Engineering Consultants in Little Rock, Arkansas, and a member of STRUCTURE’s Editorial Board. (jmccool@robbins-engineering.com) D E C E M B E R 2 0 21

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CODES and STANDARDS 2021 IBC Significant Structural Changes Part 2: Foundations (Chapter 18)

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

T

his five-part series (Part 1, STRUCTURE, November 2021) includes discussion of significant structural changes to the 2021 International Building Code (IBC) by the International Code Council (ICC). This installment includes an overview of changes to Chapter 18 on foundations and soils. Only a portion of the total number of code changes to this chapter are discussed in this article. More information on the code changes discussed here can be found in the 2021 Significant Changes to the International Building Code, available from ICC. IBC Chapter 18 provides criteria for geotechnical and structural considerations in selecting, designing, and installing foundation systems to support the loads imposed by the structure above. Basic requirements for all foundation types are provided, including requirements specific to shallow and deep foundations. The following modifications were approved for the 2021 IBC. Code text for each section is included with new text identified by underlining, followed by a brief description of the change significance.

Frost Protection at Required Exits Frost protection for egress doors has been added to the foundation requirements.

1809.5.1 Frost Protection at Required Exits. Frost protection shall be provided at exterior landings for all required exits with outward swinging doors. Frost protection shall only be required to the extent necessary to ensure the unobstructed opening of the required exit doors.

Change Significance: Frost protection must now be provided for exterior landings at all required means-of-egress doors. In addition, where frost protection is required, landing areas immediately adjacent to egress doors must be provided with the same frost protection systems as the building being served by the exit. This protection is designed to prevent concrete landings from heaving, thereby compromising the normal operation of required egress doors.

Such heaving actions can render an egress door entirely unusable.

Section 1809.5.1 is intended to provide heave protection only for the area of a landing immediately adjacent to exit doors and only for the area required to allow a door to swing open at least 90 degrees from a closed position. The remaining portions of a larger patio or sidewalk need not be provided with Figure 2. Helical piles.

18 STRUCTURE magazine

Figure 1. Frost protection is required in front of a swinging door only.

frost protection. Doors that do not swing – for example, a revolving door at a lobby entrance – do not require frost protection (Figure 1).

Helical Piles Calculation of the allowable axial design load for helical piles has been clarified.

1810.3.3.1.9 Helical piles. The allowable axial design load, Pa, of helical piles shall be determined as follows: Pa = 0.5Pu (Equation 18-4) where Pu is the least value of: 1) Base capacity plus shaft resistance of the helical pile. The base capacity is equal to the sum Sum of the areas of the helical bearing plates times the ultimate bearing capacity of the soil or rock comprising the bearing stratum. The shaft resistance is equal to the area of the shaft above the uppermost helical bearing plate times the ultimate skin resistance.

2) Ultimate capacity determined from well-documented correlations with installation torque.

3) Ultimate capacity determined from load tests when required by Section 1810.3.3.1.2.

4) Ultimate axial capacity of pile shaft.

5) Ultimate axial capacity of pile shaft couplings.

6) Sum of the ultimate axial capacity of helical bearing plates affixed to pile.

Change Significance: Larger helical pile elements are now common, and shaft friction can play an important role for larger shaft diameters (Figure 2).

Pa , the base capacity plus the shaft resistance, may now be used when determining the allowable axial load.

Shaft resistance is the shaft area multiplied by the shaft’s ultimate skin resistance for the length above the shallowest bearing plate. Base capacity is the soil or rock ultimate bearing capacity in the bearing layer multiplied by the total number of bearing plates multiplied by the plate area for all plates in that layer of soil or rock.

The term shaft resistance is used to be consistent with Section 1810.3.3.1.4 addressing allowable shaft resistance.

Determination of ultimate capacity Figure 3. Installation of H-piles. by load testing has never been intended to be a requirement for all piles. Adding the reference to Section 1810.3.3.1.2 for load tests clarifies when a load test for ultimate capacity must be made available. Details for the load test are covered in the load test provisions.

Structural Steel H-Piles

Because piles can be subjected to tension caused by overturning moment during an earthquake, mechanical means to transfer the tension to the pile cap must be designed for the required tension force and not less than ten percent of the pile compression capacity. This requirement focuses on the attachment of a pile to the pile cap with enough strength in the connection that the pile steel will not pull out of the pile cap. See Section 1810.3.11 for more information on changes to the pile cap requirements.

Piles located in site class E or F soils – poor, liquifiable, and expansive soils – must satisfy the requirements for moderately ductile members per AISC 341.

Deep Foundation Element Splicing Deep foundation element splices for buildings in Seismic Design Category A and B regions designed by general engineering practices do not have to meet the 50 percent tension and bending capacity requirements.

1810.3.6 Splices. Splices shall be constructed so as to provide and maintain true alignment and position of the component parts of the deep foundation element during installation and subsequent thereto and shall be designed to resist the axial and shear forces and moments occurring at the location of the splice during driving and for design load combinations. Where deep foundation elements of the same type are being spliced, splices shall develop not less than 50 percent of the bending strength of the weaker section. Where deep foundation elements of different materials or different types are being spliced, splices shall develop the full compressive strength and not less than 50 percent of the tension and bending strength of the weaker section. Where structural steel cores are to be spliced, the ends shall be milled or ground to provide full contact and shall be full-depth welded.

Exception: For buildings assigned to Seismic Design Category A or B, splices need not comply with the 50 percent tension and bending strength requirements where justified by supporting data. {remainder unchanged}

The design and detailing of H-piles must now conform with requirements of AISC 341, Seismic Provisions for Structural Steel Buildings, for a structure assigned to Seismic Design Category D, E, or F. 1810.3.5.3.1 Structural steel H-piles. Sections of structural steel H-piles shall comply with the requirements for HP shapes in ASTM A6, or the following: 1) The flange projections shall not exceed 14 times the minimum thickness of metal in either the flange or the web and the flange widths shall be not less than 80 percent of the depth of the section.

2) The nominal depth in the direction of the web shall be not less than 8 inches.

3) Flanges and web shall have a minimum nominal thickness of 3⁄8 inch.

For structures assigned to Seismic Design Category D, E, or F, design and detailing of H-piles shall also conform to the requirements of AISC 341.

Change Significance: Steel H-piles (Figure 3) used in higher seismic design categories are expected to yield just under the pile cap or foundation from a combined bending and axial load. Design and detailing requirements for H-piles in AISC 341 are intended to produce stable plastic hinge formation in the piles.

A plastic hinge is the area along the pile length that yields or stretches with permanent deformation during an earthquake.

Figure 4. Steel pile splice.

continued on next page

D E C E M B E R 2 0 21

19

All other requirements of Section 1810.3.6 continue to apply for pile splices in areas assigned as Seismic Design Categories A and B.

Precast Concrete Piles

Figure 5. Precast concrete piles.

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Change Significance: Steel pile splices (Figure 4) must be designed to resist axial and shear forces as well as moments occurring at a splice location. Conformance with this requirement ensures the structural integrity of the splice. Section 1810.3.6.1 contains restrictive splice requirements for structures assigned to Seismic Design Categories C through F. For low seismic areas, commonly available splices are acceptable in many design situations, such as a splice located deep enough that significant tension or bending demands are not expected or possible. Load requirements at the splice diminish due to soil resistance above the splice when the splice is located at depth.

Splices in low seismic design categories are exempt from having to be designed to fifty percent of the tension and bending strength of the pile material. For example, if friction piles are driven to 240 feet, the splice between the two 120-foot sections is 120 feet below grade.

These piles do not need to be checked for a capacity of fifty percent of the pile tension and bending capacity. The pile is braced at the splice by surrounding soil.

Precast concrete piles are now to be designed following ACI 318, Building Code Requirements for Structural Concrete, rather than IBC provisions.

1810.3.8 Precast concrete piles.

Precast concrete piles shall be designed and detailed in accordance with Sections 1810.3.8.1 through 1810.3.8.3 ACI 318.

Exceptions: 1) For precast prestressed piles in Seismic Design Category C, the minimum volumetric ratio of spirals or circular hoops required by Section 18.13.5.10.4 of ACI 318 shall not apply in cases where the design includes full consideration of load combinations specified in ASCE 7 Section 2.3.6 or Section 2.4.5 and the applicable overstrength factor, Ω0. In such cases, minimum transverse reinforcement shall be as specified in Section 13.4.5.6 of ACI 318.

2) For precast prestressed piles in Seismic Design Categories D through F, the minimum volumetric ratio of spirals or circular hoops required by Section 18.13.5.10.5(c) of ACI 318 shall not apply in cases where the design includes full consideration of load combinations specified in ASCE 7 Section 2.3.6 or Section 2.4.5 and the applicable overstrength factor, Ω0. In such cases, minimum transverse reinforcement shall be as specified in Section 13.4.5.6 of ACI 318. {Sections 1810.3.8.1 Reinforcement through 1810.3.8.3.4 Axial load limit in SDC D-F deleted without replacement} Change Significance: Sections 1810.3.8.1 through 1810.3.8.3.4 of the IBC have been deleted as similar provisions are included in Chapter 18 of the 2019 edition of ACI 318. Two exceptions for precast prestressed piles (Figure 5) are retained in the 2021 IBC. Exceptions 1 and 2 recognize that the volumetric ratio of spiral reinforcement need not be greater than that required for driving and handling stresses when a pile foundation system is designed for load combinations including overstrength. Increased axial forces, shear forces, and bending moments provide a significant factor of safety against nonlinear pile behavior when the design includes overstrength effects.

Pile Caps Pile cap requirements have been updated to align with the 2019 edition of ACI 318.

1810.3.11.2 Seismic Design Categories D through F. For structures assigned to Seismic Design Category D, E, or F, deep foundation element resistance to uplift forces or rotational restraint shall be provided by anchorage into the pile cap, designed considering the combined effect of axial forces

due to uplift and bending moments due to fixity to the pile cap.

Anchorage shall develop not less than 25 percent of the strength of the element in tension. Anchorage into the pile cap shall comply with the following: {No changes to Items 1 and 2} 3) The connection between the pile cap and the steel H-piles or unfilled steel pipe piles in structures assigned to Seismic Design Category D, E, or F shall be designed for a tensile force of not less than 10-percent of the pile compression capacity.

Exceptions: 1) Connection tensile capacity need not exceed the strength required to resist seismic load effects including overstrength of ASCE 7 Section 12.4.3 or 12.14.3.2.

2) Connections need not be provided where the foundation or Figure 6. Pile cap connecting two foundation piles. supported structure does not rely on the tensile capacity of the piles for stability under the design seismic force. concrete piles are now to be designed following ACI 318 {remainder unchanged} rather than IBC provisions. Pile cap requirements have also Change Significance: Steel piles used in higher seismic been updated to align with the 2019 edition of ACI 318.■ design categories are expected to yield just under the pile cap Sandra Hyde (shyde@iccsafe.org) is Managing Director, and John “Buddy” (Figure 6 ) or foundation because of combined bending and axial load.

Showalter (bshowalter@iccsafe.org) is Senior Staff Engineer, both with Design and detailing requirements of AISC 341 for H-piles are intended ICC’s Product Development Group. to produce stable plastic hinge formation in steel piles. Because piles can be subjected to tension caused by overturning moment, mechanical means to transfer such tension must be designed for INTRODUCING the required tension force, but not less than 10-percent of pile compression capacity.

NEW VERSION

Structural engineers should be aware of significant structural changes that have occurred in the 2021 IBC. Frost protection for egress doors has been added to the foundation requirements. Calculation of the allowable axial design load for helical piles has been clarified. The design and detailing of H-piles must now conform with AISC 341 requirements for a structure assigned to Seismic Design Category D, E, or F. Deep foundation element splices for buildings in Seismic Design Category A and B regions designed by general engineering practices do not have to meet the 50 percent tension and bending capacity requirements. Precast

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21

SloVer Connecting Old and New LibraRY

By David Mykins, P.E.

The exterior of Slover Library with Seaboard Building on the left.

I

n response to growth in the region, the vision for Slover Library in Norfolk, Virginia, was a state-of-the-art public space that would serve the 21st century needs of the community and learners of all ages. The challenge: renovating a 115-year-old historic building, renovating a mid-century commercial building, and connecting the two with a new, modern addition. The result is the 135,000-square-foot Slover Library complex that redefines how a public library is used.

Slover is certified LEED Gold. It also won a 2015 AIA/ALA Building Award from the American Institute of Architects and the American Library Association, recognizing excellence in innovative design as leading to the library’s resurgence as a community hub. Slover is also a DBIA (design-build) award winner.

22 STRUCTURE magazine

Slover Library is a portal to global information, serving the public through technology built on robust data and infrastructure. This is accomplished through engaging wall-sized touch-panel displays, custom interactive software, computing and media labs, training spaces, video production, and high-tech meeting spaces. It is a worldclass “village green” space for community engagement and thoughtful conversations on issues important to the region and beyond. Slover’s Sergeant Memorial Collection is the storehouse for the region’s history, artifacts, and genealogy dating back to the settlement of the new world.

No longer is a library defined as just a repository for books. Instead, it is a community anchor for civic engagement and access to nextgeneration thinking, innovation, digital content, research, and creativity. What better way to achieve this world-class placemaking vision than to design a beautiful and engaging “past to present” experience for the community – a deliberate, physical connecting of the old and the new.

Tight Urban Site Since the 1990s, the city of Norfolk has aggressively engaged in rebuilding its downtown with new infrastructure, vibrant mixeduse developments, and a new light rail system. Slover Library was envisioned as the anchor for this new downtown district adjacent to the MacArthur rail station – a vital location to serve its leading-edge purpose in the community. This location, however, presented some challenges. The site chosen for the Slover library is surrounded on three sides by existing structures, two of which, The Seaboard Building and Selden Arcade, would need to connect to the new facility directly.

The historic Seaboard Building is located to the east of the new site.

Built in 1900 as a post office and federal courthouse, the city took over the property in 1934 as Norfolk’s City Hall. It remained so until the mid-60s when the City Hall moved to a new facility. The Seaboard Building housed Norfolk’s Social Services until the early 1970s. After that, it was privately-owned until the City of Norfolk re-purchased it in 2007. It was listed on the National Register of Historic Places in 1981. To the west is the Selden Arcade, a 1930s two-story retail and office space used by retailers and financiers into the 1980s, but many storefronts were vacant by 1995. The City of Norfolk purchased the Arcade in 2003 and recently converted it to serve as a studio space for the more than 40 artists of the d’Art Center. To the south is an eight-story city-owned parking garage.

Because of unsuitable soils that extended to an elevation of about 50 feet below the existing grade, it was determined that the new building would need to be supported on deep foundations. Several options were considered, including driven piles and auger cast piles. One of the factors significantly influencing this decision was that the new building would be constructed immediately adjacent to the existing historic structures. With driven piles, there was great concern that the resulting ground vibrations might cause damage to these buildings.

So, the decision was made to use 14-inch-diameter auger cast piles.

But since they would need to extend to tip elevations of 100 feet below grade, removing the spoils associated with traditional auger cast pile installation would be a considerable cost. The team chose to use drilled soil displacement piles, a type of auger cast pile to mitigate this. Instead of removing the soil material, the soil is displaced laterally into the surrounding soil resulting in a similarly profiled regular shaft pile but with minimal spoils.

These piles had the additional advantage of being able to be installed very close to the existing buildings. This minimized the length that the grade beams needed to cantilever to support the new columns set close to the existing buildings.

Another consideration of tight urban sites is the lack of convenient material laydown areas. This was one of the factors that influenced the decision that the majority of the new structure would consist of cast-in-place concrete over structural steel. The amount of space required to store reinforcing bars is much less than that required for structural steel, and the erection can often be done just as quickly.

High Water Table In Norfolk, the water table is relatively high; most basements in this area extend below the water table. Not surprisingly, the basement in the 115-year-old Seaboard building had a history of leaking and even flooding during significant rain events. So, the first challenge was to provide a reliable waterproofing system to the existing building. The waterproofing system the team employed for the Seaboard building consisted of a network of pipes that could remove a large amount of water from the soil, combined with a positive water barrier on the outside of the basement walls. Next, within the existing basement, a new waterproof membrane was installed on top of the existing slab on grade and extended up the inside of the basement walls. Finally, a new 4-inch topping slab was installed on top of a 4-inch sand bed to finish the basement floor and protect the waterproof membrane.

The new Slover Library needed to connect to that basement to allow elevator access to the Seaboard. But a slightly different waterproofing system was designed for the new structure. The new

Slover main entrance.

system consisted of waterproofing on the outside of the basement walls with an exterior foundation drainage system. Still, inside, there is a structural slab over a vapor barrier designed for the hydrostatic uplift should the water table rise despite the exterior drainage system.

Connecting Old and New As with many historic preservation projects, the team was faced with several unique challenges throughout this project. The most significant structural challenge was the lack of documentation of the Seaboard building’s structure. Destructive investigation in advance of the design was not possible because the building was in use until just before the construction was scheduled to begin. So, the design team was forced to make many assumptions based on limited field investigation. Existing beams were encased in concrete for fireproofing, as was typical for that era. So, the sizes and depths of beams were estimated.

When construction began and crews started to remove some of the finishes, the engineers could verify their assumptions. In addition to determining actual member sizes, it was also necessary to verify the material properties. To do this, engineers obtained samples of the structural steel to confirm that it was consistent with A-36 steel and the concrete to determine the compressive strengths. continued on next page D E C E M B E R 2 0 21

23

Through the verification process, it was discovered that one area, which would be the future home for the library’s precious historic collections, was not able to support this proposed loading. The first thought was that these areas would need to be removed and replaced. But upon further investigation, it was determined that the existing 5½-inch concrete slab could be removed and new beams installed between the existing beams.

A lighter slab on steel deck system was designed on this new steel grillage, thus preserving much of the existing structure.

Because the Seaboard Building is listed on the National Historic Register, the team was required to preserve and protect as much of the original building fabric as possible.

So, connections to the new structure had to be carefully planned and approved by the Commonwealth’s Slover interior.

Department of Historic Resources.

The floor plans were designed to connect the buildings above the first floor with a series of pedestrian bridges over an ample open atrium space. These were coordinated with the locations of existing

windows in the Seaboard Building so that only a small portion of the wall below the windowsill needed to be removed.

Project Success Design and construction occurred between 2009 and 2015. Slover is certified LEED Gold. It also won a 2015 AIA/ALA Building Award from the American Institute of Architects and the American Library Association, recognizing excellence in innovative design as leading to the library’s resurgence as a community hub. Slover is also a DBIA (design-build) award winner.

A combined public and private endeavor, the $65 million building was made possible by a $40 million gift from Frank and Jane Batten and a $21 million contribution from the City of Norfolk. In addition, the project led to the creation of the Slover Foundation, which contributed an additional $4 million to the initial construction.

The library is named in honor of Colonel Samuel L. Slover, a former Mayor of Norfolk and uncle of Frank Batten, Sr.■ David Mykins is the President of Lynch Mykins Structural Engineers PC and a former Chair of CASE. (dmykins@lynchmykins.com)

Project Team Structural Engineer: Lynch Mykins Architects: Newman Architects Ornament: Kent Bloomer Studio Urban Designer: Urban Design Associates Geotechnical Engineer: Schnabel Engineering General Contractor: Turner Construction Lighting Designer: Patrick Quigley Associates Civil Engineer: Timmons Group Landscape Architect: Michael Vergason Landscape Architects Slover Library lobby.

24 STRUCTURE magazine

Bypassing Expansive Soils with Hollow Core Planks Over a Crawlspace

By Eric Homburg, P.E.

Rendering of Oscar Johnson Community Center. Courtesy of Burditt Consultants.

T

he Oscar Johnson Jr. Community Center in Conroe, Texas, will begin construction in 2022. The project’s architecture firm is Burditt Consultants and the structural engineering firm is DUDLEY. The building will utilize mass timber construction and is intended to be an architectural statement for the community. Its 120,000 square feet of public space strives to affect countless lives for more than one hundred years. To fulfill that goal, however, the building needs to stand the test of time. There are many factors and environmental loads at play, but perhaps the most daunting is expansive soils. DUDLEY’s solution is building the Community Center’s foundation using a suspended hollow core plank slab system supported by elevated concrete beams constructed on top of belled piers. The system is designed to decouple as much of the structure as possible from expansive soil movements while saving overall foundation cost and construction time. It also allows for ease of future modification/repair for plumbing and mechanical systems, given that there is direct access to the crawl space.

in approximately 6-inch lifts and is usually required to be done to 95% of the maximum dry density. Compaction efforts also become increasingly difficult as both the building footprint and the excavation depth grow due to increased complexity in logistics/quality control and increased exposure to rain delays (which frequently causes damage to previously completed work). This work must be completed before constructing any concrete foundation elements, adding months to the overall project construction timeline on large projects. Perhaps adding salt to the wound, removing and replacing expansive soil merely mitigates damage due to foundation movement. There will still always be a risk for structural, functional, or cosmetic distress (the latter being most common).

Site Considerations

The Community Center will inhabit a site consisting of an average plasticity index of 50, a very high value. Therefore, initial geotechnical recommendations were that the top 6 feet of native topsoil should be Expansive Soils removed and replaced with soils having a plasticity index of between Expansive soils contain near-surface clay and are thus prone to volu- 10 and 20. This is not an unexpected requirement. The native soils metric changes as moisture content fluctuates. Seasonal moisture present in the site are not of good quality, and their intense shrink/ variations can cause expansive soils to shrink and swell differen- swell potential would likely cause structural damage if left in place. tially and induce swell pressures over 4,000 pounds per square foot. Therefore, soil removal was necessary and could not be worked around.

Engineers conventionally design stiffened slab-on-grade foundations But what if the soil fill operations that follow could be eliminated? to resist the effects of expansive soil movements.

The fill operations are particularly undesirable for this project, with a In regions that contend with highly expansive clay soils, it is foundation footprint of about 65,000 square feet. That would equate common for 2 to 6 feet of existing soil to a little over 14,400 cubic yards of to be removed over the entirety of the compacted fill soil that would take addibuilding footprint and replaced with tional months (depending on weather compacted select fill soils consisting of conditions) to execute in the field cor“What if there was an alternative to low expansive clay soils with low permerectly. Realizing this, DUDLEY decided having to fill the holes we spend so much ability. This cut-and-replace procedure to explore a suspended foundation as time and money digging?” is both costly and time-consuming. The an option. This approach comes with process of compacting the select fill is many benefits, primarily that it removes especially a burden. It must be done the foundation slab from direct contact 26 STRUCTURE magazine

with soil, decoupling it from expansive soil behavior (piers are still in contact with the soil). Additionally, it also precludes the need to purchase and painstakingly place select fill. In aggregate, this decision should prove to save time, save money, and improve quality, a trifecta rarely achieved.

Design and Construction Since this building is being built using a suspended foundation, the excavation only needs to be as deep as the overall depth of the slab framing system plus an air gap for maintenance crew maneuverability. Hollow core planks were selected as the slab system for the project because of their speed of installation given the Community Center’s footprint. Hollow core planks are a precast concrete element that, with the combined power of prestressing and cast-in voids, can span long lengths at light weights for concrete construction. The author’s firm selected a maximum hollow core plank span length of 30 feet based on building layout and then determined that the typical hollow core plank depth should be 10 inches with an additional 2 inches of topping slab for a total depth of 12 inches. Correspondingly, concrete beams supporting Typical section for suspended hollow core planks and beams. the slab were limited to a maximum span length of 30 feet, yielding a 3-foot beam depth. Concrete piers that extend 12 feet below the elements. Once both tasks are complete, the topping slab is poured, existing grade support these concrete beams. Superstructure columns and the foundation is finished. were also located on top of beam-pier connections. Regarding the aforementioned air gap, the entire design team determined it would need Applicability to Other Projects to be 2 feet deep for this project, though this is ultimately a judgment call. Therefore, given the selected slab depth, beam depth, and air gap, DUDLEY believes that the suspended slab system described was the excavation depth for this building will be 6 feet. the best foundation system for this project. The company’s research There are some nuances to consider when designing this system: indicates that this and similar systems are incredibly underutilized • Slab drops can be accommodated by selecting thinner hollow in regions with expansive soil. The author encourages other design core planks and modifying the topping slab thickness. Still, the professionals to explore the possibilities of this foundation type. dropped region should be framed out with beams below, and The viability of these suspended slabs scales with building footprint the maximum slab span may need to be reduced. For example, size due to the interplay of increased select fill costs and economy of the gymnasium space was constructed over 8-inch planks due scale benefits gained with precast concrete elements. Therefore, we to the entire room needing to be dropped 2 inches, but the do not recommend this system for buildings with small footprints. plank span was reduced to 24 feet.

This project used a combination of cast-in-place beams with precast • Soil must be retained at the building perimeter to preserve the hollow core plank slabs. Given the scale of the project and the good accescrawlspace under the slab. This was accomplished by deepening sibility of precast elements, precast concrete is generally more economical. the perimeter concrete beams to extend at least 1 foot below the While precast beams were an option for this project, they were ultimately final grade of the crawl space to act as retaining walls. Tops of the not used due to an irregular column/pier grid. With numerous concrete exterior concrete piers were also lowered to match the bottom of beam-to-girder connections required throughout the project, cast-in-place these beams. A system of soil retainers and cardboard void forms provided a more natural solution. Another benefit of using cast-in-place have been provided to preserve the suspended nature of these beams is the added versatility when it comes to beam penetrations. exterior beams (as is present at all interior beams).

Considering lead time for precast element fabrication is essential.

• It may be tempting to vary the interior beam depths throughout Time saved in avoiding the use of select fill does not matter if the same the structure for economy. However, since the concrete piers amount of time is lost in waiting for precast elements to arrive at the often support multiple beams and the top of the concrete beam job site. In pre-construction meetings, it may be helpful to must align with the bottom of the hollow core plank, detailing remind general contractors to hire a precast manufacturer as and constructing this condition at the piers would prove diffisoon as possible.■ cult. Therefore, this complexity was not worth it for this project, Eric Homburg is a Project Manager at DUDLEY in College Station, Texas. given the number of conditions that could develop.

Mr. Homburg serves as the Structural Engineer of Record for this project.

As for construction, the 6-foot excavation of the building footprint (ehomburg@dudleyeng.com) will happen first. In conventional construction for this region, this is where select fill would be brought to the site. However, that step Project Team will be skipped entirely for this project. Next, the contractor can begin drilling and casting piers. On top of these piers, the contractor Owner: The City of Conroe, Texas will then form up and pour the concrete beams used to support the Structural Engineer: DUDLEY hollow core planks. Once the beams have fully cured, options open Architect: Burditt Consultants up for the contractor to install the building’s columns and hollow core Mechanical Engineer: Cleary Zimmermann Engineers planks simultaneously. This phase should be a relatively quick process Structural Software: ETABS and ENERCALC since each of these tasks only involve the installation of prefabricated D E C E M B E R 2 0 21

27

Fire Engineering Exposed Timber at

Pig Patch House

By Sanja Buncic, C.Eng, MIStructE, and Matthew Smith, M.A.Sc., M.Eng, P.Eng

Example of panel connections.

28 STRUCTURE magazine

P

ig Patch House is a single-family custom house located in England. It is built from mass timber panels used in the walls, upper floor, and roofs, suspended insulated precast concrete floor planks at ground floor level, and RC ground beams on mini concrete piles used as foundations.

There are two types of dowel laminated timber products that have been used in this project. The wall panels are made of timber planks aligned in three directions, a diagonal layer sandwiched between two (or four) orthogonal layers. The floor and roof panels are made of traditional dowel laminated timber, where planks are all installed in one direction.

The mass timber panels used in the walls are made entirely from wood without any resin. In producing these panels, boards are laid on a mounting table in layers in three directions: longitudinal, diagonal, and transverse. Blind holes are drilled, and the solid wood threaded dowel is screwed in. The threaded dowels are made of well-dried and compressed beech. As they are inserted into softwood (spruce), they absorb the ambient humidity of the surrounding spruce board layers, swell up, and connect solidly with the surrounding wood.

The mass timber panels used in floors and roofs are made of timber planks/boards oriented in the same direction and connected with beech threaded dowels.

Pig Patch House is not a square house; it has an irregular shape at both levels. The ground floor is an open plan providing the dining and kitchen area directly over more than a half of the footprint, with some smaller rooms and upper floor stair access occupying the other half. On the upper floor, there are two bedrooms and a large roof terrace. Very few walls align at the two levels, which means that some areas have an upper floor structure spanning over 20 feet (6 meters).

The external walls have large door and window openings up to 10 feet (3 meters) wide. All these factors presented design challenges.

The house is built in a rural area, surrounded by large mature trees and shrubs. The soil in this area is high plasticity clay, prone to shrinkage and heaving due to moisture variations caused by nearby trees. To address this, the foundation system is designed as reinforced concrete ground beams 24 inches wide (600 millimeters) by 18 inches deep (450 millimeters), spanning over 12-inch-diameter (300-millimeter) mini continuous flight auger piles installed at proximately 10 feet (3 meters) on-center.

The ground beams support precast insulated prestressed concrete units, which are laid on top. Due to the presence of prestressing tendons, there are limited areas where resin anchors can penetrate the panels. As a result, the layout of the ground floor units was developed to suit the position of internal and external walls to allow fasteners for the timber wall cleats (plates) to be in this safe zone.

The stability of the building is derived from the external panels, and the horizontal forces are transferred into the walls at the ground floor level by diaphragm action of the upper-floor structure. The shear-center of the ground floor wall is positioned closer to the area with smaller rooms. This was addressed in the distribution of the forces Typical connection detail. that would need to be resisted by the walls.

It is assumed that only external walls contribute to the stability of the building to allow for potential future adaptations.

The upper-floor structure is made of 4-foot-wide (1.2-meter) by max 36-foot-long (11-meter) dowel laminated timber panels. These panels transfer the load and are designed to span as one-way panels. Since the panels act independently, the connection between two adjacent panels was achieved with 11⁄32-inch-diameter (9-millimeter) screws installed at an angle of 45 degrees and a spacing of 8 inches (200 millimeters) along the junction. These screws allow the floor to act as a deep beam for the horizontal loads, transferring the horizontal loads into stability walls.

A similar approach was adopted for the roof diaphragm.

To span over large door and window openings, panels above the upper floor have been designed as deep beams. As noted earlier, the wall panels are constructed from timber planks in lamellas that span in three directions (horizontal, vertical, and diagonal). The diagonal layer is required for the resistance of horizontal (racking) forces. Therefore, only the outer lamellas that run parallel to the opening could be used to bridge over the opening and transfer the load from the upper-floor structure onto the walls. It should be noted that the planks in the lamellas are only connected to other planks by beech dowels. Although the deep beam panel would have worked similarly to a traditional CLT deep beam, the designers assumed that each individual plank in the external lamellas would carry a portion of the total load applied on the deep beam panel.

This load was estimated based on the stiffness of each of the planks.

This was a conservative approach because beech dowels will transfer the load from each of the planks in the external lamellas onto other lamellas and create a mass timber panel of similar properties to a traditional cross-laminated timber panel.

The deep beam over some of the windows also acts as a balustrade for the roof terrace. To achieve this, the cleat plate at the bottom had to be intermittent to allow direct transfer of moments and shear forces into the upper-floor structure through pairs of 11⁄32-inch-diameter (9-millimeter) screws installed at an angle of 45 degrees.

As mentioned above, the upper-floor structure had to span over 20 feet (6 meters) while carrying the load from the upper-floor walls supporting the roof and the upper-floor roof terrace. Because the upper-floor structure is only 6 inches thick (150 millimeters), a single glulam beam was installed over the upper floor and hidden within one of the walls to help transfer the loads. The glulam beam is connected to the floor panel with screws and load-shared between the two.

Fire design was an integral part of the project because most timber panels were left exposed as walls or soffits. The exposed timber, left untreated, did not satisfy prescriptive requirements from Approved Document B of BS 9991:2015, Fire Safety in the Design, Management

and Use of Residential Buildings – Code of Practice, for flame-spread to satisfy the Building Regulations 2010 (UK). The Approved Documents mentioned above are prescriptive documents for fire safety design, which are analogous to the role building codes play in the U.S. They demonstrate to designers how to meet the minimum requirements for fire safety as mandated in the Building Regulations.

Approved Document B Fire Safety – Volume 1: Dwellings, 2019 edition, provide designers with an efficient means of meeting fire safety requirements for common building situations. However, they are not the only option available to design teams. The design team can deviate from the prescriptive guidance if they demonstrate that the building is still safe and meets the requirements of the Building Regulations. In the case of Pig Patch House, there was a desire to leave the timber panels untreated and fully exposed.

This was achieved using a performance-based approach.

Understanding that the untreated timber has an increased rate of flame spread relative to a guidance-compliant solution and that this impacts the early stage of fire development, a mitigation strategy was developed which similarly addressed the early stage of the fire and improved occupant evacuation. Detection was increased throughout the house to alert occupants to a potential fire much sooner relative to a guidance-compliant solution with only a single detector in corridors.

The effectiveness of this mitigation strategy was quantified using fire modeling to determine how long it takes for conditions to become unsafe, evacuation calculations to determine an overall range of evacuation times for occupants, and an event tree analysis to quantify the risk of each scenario. This type of analysis can demonstrate that the holistic fire strategy achieved the required level of performance despite aspects not meeting prescriptive guidance.

This custom house has demonstrated that, by understanding the structural and fire safety nuances of mass timber construction, the material can be successfully applied to satisfy architectural and sustainability goals while meeting life safety requirements. The two-way nature of the Nur-Holz panel allowed for efficient structural framing, while the thoughtful introduction of glulam elements allowed for load transfers between the floors.

It was also shown that a holistic fire safety strategy was able to quantify the performance of the mass timber building while creating design opportunities beyond prescriptive guidance.■ Sanja Buncic is an Associate at Entuitive and leads several major ongoing projects in the UK. (sanja.buncic@entuitive.com) Matthew Smith is an Associate at Entuitive, where he leads the company’s fire engineering service. (matt.smith@entuitive.com) D E C E M B E R 2 0 21

29

NCSEA

National Council of Structural Engineers Associations

NEW YORK: FEBRUARY 14-17, 2022 VIRTUAL: JANUARY 31-FEBRUARY 24, 2022

CALLING ALL STRUCTURAL ENGINEERS FOR THE NCSEA SUMMIT

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Network with the best in the industry, and celebrate structural engineering excellence.

31 EDUCATIONAL SESSIONS INCLUDED IN ALL-ACCESS REGISTRATION Member registration rate breaks down to $32/PDH. Check out the full educational slate online, and register to learn from industry experts on a diverse range of subjects.

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INSPIRATIONAL KEYNOTE SESSIONS 20 Years After 9/11: Lessons Learned & More to Research Dan Eschenasy, P.E., SECB, F.SEI; James W. Feuerborn Jr., P.E.; Therese McAllister, F.SEI; Moderated by Vicki Arbitrio, P.E., SECB, F.SE

Lighten Up & Lead: Leverage Levity to Boost Client Confidence & Employee Engagement

Tami Evans, Employee Engagement & Levity Leadership Expert

NETWORKING OPPORTUNITIES ABOUND

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Interact with exhibitors and enjoy lunches, breaks, and a cocktail reception all on the trade show floor, as well as virtual networking and educational opportunities.

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Structural Investigations Gray Columns The configuration of the fabricated steel Gray column crosssections involved four sets of vertical Carnegie Steel double angles located 90° from each other around the circumference of the section. These were connected to the adjacent angles via articulated, riveted gusset plates uniformly spaced vertically for the full height of the column. A total of four plates were located at each connection level (Figure 16 ).

Because of this unusual configuration, a finite element analysis (FEA) determined that the loads imposed on any one set of double angles by a beam reaction at any level would not be shared with any of the other three pairs of angles over the full height of the column. This condition occurred because of the inability of the riveted gussets to adequately transfer the vertical load of any one of the double angles to the other adjacent double angles. This situation, along with the findings of a weldability analysis conducted for the Gray columns, influenced the development and design of the connection of the new loft beams to these unusual existing columns.

Weldability As indicated previously, it was assumed that the components of the Gray columns were rolled by Carnegie Steel; therefore, a chemical lab analysis of a sample from a steel beam obtained from the 9th floor was used to conduct a weldability analysis. From the findings of the lab test, a calculation of the carbon equivalency (CE) based on the Dearden-O’Neill Equation (from the American Welding Society (AWS) Guide for Strengthening and Repairing Existing Structures, D1.7)

Adaptive Reuse of the Historic Witherspoon Building Part 4: Structural Investigations

By D. Matthew Stuart, P.E., S.E., P.Eng, F.ASCE, F.SEI, A.NAFE, SECB

T

his four-part series discusses the adaptive reuse of the Witherspoon Building in Philadelphia, PA (Part 1, STRUCTURE, September 2021, Part 2, October 2021, Part 3, November 2021). Part 4 continues the discussion of the structural investigations, including the Gray columns, new floor openings, and demolition of the second-floor mezzanine to allow for a new second-floor loft. Numbered photos are provided in the print version of the articles; lettered photos are provided only within the online versions of the articles.

32 STRUCTURE magazine

Figure 16. Riveted gusset plate connectors at the Gray column vertical double angles.

resulted in a CE value of 0.23% for steel with a carbon content greater than 0.12%. This value was significantly less than 0.35% (which would require that special care be taken when welding) and well within the recommended CE range provided in Table 6-1 of The Procedure Handbook of Arc Welding by the Lincoln Arc Welding Foundation. Therefore, the CE value for the beam sample indicated good weldability.

Based on the results of the above analysis, it was determined that the welding procedures, including any required preheating, could comply with the requirements of Section 4.5 of AWS D1.7.

However, although the results of the weldability analysis indicated the Gray columns could be field welded, the results of the FEA concerning the inability of a Gray column to distribute the imposed vertical loads uniformly across the section influenced the design of the new loft beam connections. In addition, there was a real potential for material strength reduction at any one of the vertical double angles due to the heat generated by the welding process, which could weaken the angles and cause a localized failure unless the column was shored the full height of the building. As a result, all new beam connections were designed to connect only to the internal gusset plates.

In addition, an in-situ weld test, recommended by AISC Design Guide 21, Welded Connections – A Primer for Engineers, was also conducted at one of the top gussets at a Gray column that extended into the original mechanical penthouse and was therefore not supporting any appreciable vertical load. The test confirmed that the proposed welds associated with the new loft beam to Gray column connections did not damage the existing section.

Typical 1st-floor loft beam connections to existing Gray column gusset plate details are shown in Figure P, online. Figures 17 and 18 show examples of the erected condition of the same beam-to-Graycolumn connections.

Figure 17. 1st - floor loft beam connection to existing Gray column.

New Floor Openings As previously noted, large trash chute and mechanical chase openings were required full height of the building above the 1st floor. An investigation was conducted to locate the existing concealed beam framing and individual tiles in the area of the building impacted by the openings to minimize the disruption and re-support requirements of the affected clay tile arch framing. The beams were located using handheld ground penetrating radar (GPR) as previously described. The individual tiles were located after the beam locations were established by removing strips of the plaster ceiling parallel to the beam span and perpendicular to the arch span.

Removing the plaster revealed the location and direction of the tile scoring and joints, which confirmed the assumed direction of the arch span and established the location of the joints between adjacent rows of tiles. Once the beam and tile row joints were determined, the locations of the new openings were established to minimize the need to reinforce the remaining tiles after the tiles within the footprint of the openings were removed. This was accomplished by locating the edges of the new openings at a joint between adjacent tile rows and at the end of an arch span corresponding to a supporting floor beam.

This approach avoided interrupting the arching action of any one row of tiles between the beam supports. Unfortunately, the contractor could not control the demolition at the openings well enough to avoid damaging the remaining row of tiles next to the opening. As a result, it was necessary to form and pour cast-in-place reinforced concrete beams between the supporting steel beams to provide adequate

Figure 18. 1st - floor loft beam connection to existing Gray column.

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Carnegie Steel beams, which in turn were also spaced at approximately 5 feet on-center. A lab test of one of the rods removed from a new opening indicated a yield strength of approximately 50 ksi, which equates to a factor of safety of approximately 2.75 for the recommended maximum allowable stress of 18 ksi provided in the Principals of Tile Engineering Handbook of Design for the design of flat arch, hollow clay tile framing.

Utilizing the recommended formula and allowable stress for determining the load capacity of a hollow clay tile arch from the same reference resulted in a uniform load carrying capacity of approximately 280 psf. Deducting the existing topping weight and the tile and plaster ceiling resulted in a reserve load carrying capacity of approximately 170 psf, almost twice the reserve load-carrying capacity of 100 psf determined for the floor beams.

Additional Structural Renovations In addition to previously described structural renovations, strengthening was required for several other areas of the building during the construction phase of the project.

9th Floor Beam Damage Figure 19. New cast-in-place concrete beam at new openings in the arch floor to replace damaged tile.

While drilling new holes for plumbing waste lines, a subcontractor cored through the entire cross-section of an existing, 12-inch-deep, 9th-floor beam at the approximate midspan of the member. Although the beam did not deflect or exhibit any other indication of structural duress, the member was re-supported using two new steel channels that were underslung and straddled the damaged beam.

The new channels were connected by uniformly spaced stiffener plates attached to the bottom flange of the original floor beam to support the damaged section. The channels, in turn, spanned between the supporting girders via hanger connections. The fact that the clay tile arch supported by the damaged beam did not deflect, even though the span of the arch was essentially doubled, is a testimony to the resilience and extraordinary load-carrying capacity of this type of vintage framing system.

support of the remaining damaged tile arch sections at the edge of the openings (Figure 19).

Unfortunately, the investigation could not identify any of the anticipated tie rods typically located at the exterior arch span, which is where the new openings were to be located, next to an exterior wall. Typically, tie rods for flat-arched clay tile framing are not visible because they are generally located at least 3 inches from the bottom of the tile. As a result, it is challenging to locate the steel rods using a ferroscanning device such as a Profometer.

In addition, it is also difficult to locate the rods using GPR due to the significant number of internal cavities associated with hollow clay tiles. The best method of locating tie rods is via the use of X-Rays. In-situ radiography can determine both the spacing and diameter of the rods without damaging the tiles. However, this method of non-destructive testing was not feasible for the project.

Absent tie rod location information, it was necessary to design supplemental ties, which consisted of steel angles located in the remaining arch span next to the openings that were welded to the bottom flange of the steel beams located at each end of the same arch span.

These supplemental ties were required to resist the horizontal arch thrust force at the beam adjacent to the new openings without knowing the actual number of tie rods that would be damaged by the demolition of the tiles within the new openings. As a result, the new ties had to be installed before the demolition of the openings began (Figure 20 ).

During the demolition process, the presence of tie rods was confirmed (Figure Q, online).

The ¾-inch-diameter steel rods were spaced at approximately 5 feet on-center and located near mid-depth of the existing 12-inch-deep Figure 20. New steel angles installed first at the arch span adjacent to the new chute and chase openings.

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Demolished Existing 2nd Floor Mezzanine After the existing 2nd-floor mezzanine was demolished for the proposed new 2nd-floor loft, which was eventually eliminated from the adaptive reuse project, it was necessary to strengthen the existing remaining two-story steel wide flange columns that remained and extended up to support the 1960s 3rd-floor infill framing above. In addition, several of the existing Gray columns in the same area of the building were also strengthened because it appeared that the same members at one time had been braced by framing that had existed within the Witherspoon Hall space but had been removed when the 2nd-floor mezzanine was constructed in the 1960s.

Horizontal, diagonal bracing was installed between the wide flange columns checked for the approximate two-story unbraced length and the existing Gray columns. Some of the wide flange columns had not been erected as a continuous vertical member at two 2nd floor Figure 21. Reinforced discontinuous column at 2 nd - floor mezzanine cantilevered beam. mezzanine beam cantilevers. Instead, they had been interrupted by the same beams (Figure R, online). Reinforcing transfer girder, a splice in the beam near the midspan of the section plates and stiffeners were installed to ensure the remaining stacked was determined to be deficient for the new imposed loads (Figure 22). columns would behave as a continuous vertical member (Figure 21). A new HSS steel column was installed between a new footing at the sub-basement slab and the bottom of the girder at the splice to Existing 2nd Floor Roof Top Unit (RTU) Dunnage reduce the span of the member.

Due to lack of space on the original mechanical penthouse roof for all of the new required RTUs, one of the units was placed Conclusions on an existing exterior steel dunnage frame at the 2nd floor that had been previously used to support Liebert units for the old The structural investigation, analysis, and design associated with the office spaces on the west side of the building at the courtyard adaptive reuse of the historic Witherspoon Building were all chalarea. The analysis and strengthening of lenging and interesting. In the absence the existing dunnage were performed of existing drawings, it was fascinating by another structural engineer hired to discover the concealed aspects of the directly by the mechanical contractor. structure as the building revealed itself As the SEOR, the analysis, design, during the construction phase. From and calculations were peer-reviewed the author’s perspective, the most interby Pennoni. In addition, the responesting aspects of the structure included sibility for ensuring that the existing the Gray columns and the 4th-floor building structure was capable of transfer trusses. supporting the new RTU load and The adaptive reuse of the buildmodified dunnage was assumed by ing was also a sustainability success the SEOR. because, as Architect Carl Elefante Assessment results associated with the stated in a 2007 National Trust for existing supporting structure involved Historic Preservation Journal article, a considerable amount of investiga“…the greenest building is one that tion and exploratory demolition due to is already built…” The restoration of the complexity of the existing building the building was also a historical sucbelow the 2nd floor in the vicinity of the cess, which benefited the developer dunnage in question. The investigathrough the available tax tion results indicated that the existing credits associated with this structure was adequate, including steel type of project.■ beams and a transfer girder over the mechanical sub-basement area that D. Matthew Stuart is a Senior Structural Engineer at Pennoni Associates Inc. in indirectly supported a portion of the Philadelphia, PA. (mstuart@pennoni.com) dunnage above. However, at the exist- Figure 22. Existing 20-inch-deep, Carnegie Steel B3 transfer ing 20-inch-deep Carnegie Steel B3 girder splice.

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structural DESIGN

Pour Strips

Often Overlooked and Misunderstood

By Gordon H. Reigstad Ph.D., P.E., S.E., Jason G. Reigstad, and Jared M. Reigstad, P.E.

F

or over 40 years, the traditional pour strip in concrete construction has been an issue of contention between the engineer of record (EOR) and the contractor, and this challenge continues today. The EOR desires a high-quality slab, which requires more pour strips that are left open longer. The contractor wants faster construction, which requires fewer pour strips and pouring them back sooner.

Shrinkage and restraint-to-shortening (RTS) are at the core of this age-old dilemma, and EORs should not have to sacrifice quality for cost and schedule.

Cost, quality, and schedule are significant factors, and most people in construction say you can only get two out of three on a project.

However, for traditional pour strip construction, you may only get one if higher quality is chosen. Particularly common in post-tensioned (PT) concrete construction, pour strip leave outs cause either significant delays or poor quality, and they are always a safety concern because of the large gaping hole through floor slabs.

The pour strip or leave out (Figure 1) is a 3-foot to 8-foot gap in a floor slab, which is often left open three to four levels below the floor being placed. Pour strips have a long successful history of being used in reinforced concrete (RC) structures before the popularity of PT began in the 1960s. PT pour strips have even more impact from RTS due to the additional volume change from elastic shortening and creep caused by post-tensioning compression forces restrained by structural elements (i.e., shear walls and other stiff vertical members). Temperature change during construction can also significantly contribute to slab cracking, especially during winter when the slab is heated for curing but removed after a few days. All volume change components contribute to cracking and poor slab performance if not dealt with appropriately.

The industry solution used by EORs to allow for this shortening has been a traditional pour strip, which relieves the strain in the slab and is wide enough for a reinforcement lap splice and PT tendon jacking. The biggest complaint from contractors of the traditional pour strip is that it delays construction. Forms of gapless pour strip or strain-relief-joint (SRJ) solutions can resolve the age-old pour strip dilemma and remove contention between the EOR and contractor.

Volume Change and RTS Volume change and RTS are challenging aspects of concrete construction and are often misunderstood. They are even more challenging when PT is involved. The issue is the tensile stress created when slab shortening is restrained by stiff structural elements. A properly designed pour strip temporarily interrupts slab continuity between the stiff elements, lowers RTS, and allows for volume change. Since creep and shrinkage strains increase with time, cracking can be reduced by keeping the pour strip open longer. It is common for EORs to require a pour strip to remain open for 28 or 56 days and sometimes even longer for better performance. Serviceability is the primary design concern, and PT slabs are not designed to crack. Excessive cracking leads to deflection, vibration, and corrosion (parking garages) problems, leading to architectural complications.

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Figure 1. Pour strips minimize restraint and allow for volume change but create scheduling issues and safety concerns. Furthermore, delayed pour-backs have a trickle-down effect for other trades.

Pour Strip Locations For years, the typical pour strip has been part of the Post-Tensioning Institute’s (PTI) Post-Tensioning Manual. When poured back, the basic structural function of a pour strip is to create reinforcement continuity from one side of the leave-out to the other. This provides load transfer for both vertical (such as slab moment and shear) and horizontal loads (like diaphragm, horizontal shear, and drag forces).

Many EORs designing either RC or PT slabs believe that forming and shoring is a means and methods responsibility of the contractor. This is not entirely true because where EORs place pour strips, how they design them, and when they can be poured back dictates how the contractor must build them. Definitions of reshoring and backshoring can be found in ACI 347, Guide for Shoring/Reshoring of Concrete Multistory Buildings. Where slabs are self-supporting, less expensive reshoring can be used, and slabs that are not self-supporting require more expensive backshoring. PT slabs can be designed as selfsupporting, which is not practical for RC slabs, and always require backshoring. A gapless pour strip needs to provide for slab shortening and all the structural functionality of a traditional pour strip. It must create ductile reinforcement continuation for both vertical and horizontal load transfer.

In the case of PT with a traditional pour strip, a mid-span location (between columns) with self-supporting slabs is the most desirable for construction because it only requires reshoring. A slight increase in PT at the bays near the pour strip allows for self-supporting cantilevered slabs without costly backshoring. A mid-span location in RC requires backshoring on both sides of the pour strip. Therefore, most pour strips in RC are placed at a one-fifth-span location, which requires only the long, four-fifths-span to be backshored.

When a concrete contractor is not directly involved in the design process, EORs often select a one-fifth-span location for the pour strip.

This is because, at this location, the flexural moment demand is near zero; it is a point of contraflexure where minimal slab reinforcement is required. Unfortunately, this typically creates a four-fifths-span that

cannot be self-supporting, which requires expensive backshoring.

Contractors who understand backshoring know that the associated costs and schedule delays far exceed any savings in reinforcing steel and PT. Furthermore, backshoring must stay in place until the concrete structure is complete.

Often Overlooked EORs often overlook construction issues created in their designs.

Pour strip locations, how they are designed, and when they can be poured back drives the formwork and shoring requirements. Pour strip construction is not just a means and methods responsibility of the contractor. The pour strip design has a trickle-down effect that delays every other trade and results in longer, more expensive construction projects. The concrete structure is on a project’s critical path, and the pour strip is on the critical path of the concrete structure.

Beyond the apparent cost, quality, and schedule issues traditional pour strips present, they also create a safety hazard. The number one concern on a construction site is safety. Having a large gap in the floor is a significant obstacle that must be protected and bridged to prevent injuries.

Gapless Pour Strips A gapless pour strip that eliminates the large leave-out but still allows for volume change and structural continuity would take the pour strip off the critical path and be the best of both worlds. This type of pour strip would allow EORs to achieve higher quality slabs without increasing cost or lengthening schedules.

When a traditional pour strip is poured back, it achieves two structural functions (transfer of vertical and horizontal loads) utilizing an ACI 318-permitted lap splice providing ductile reinforcing steel continuation (ACI 318, Building Code Requirements for Structural Concrete and Commentary). For a gapless pour strip to be substituted for a traditional pour strip, it must have the same structural functionality.

A gapless pour strip lowers costs, improves quality, and shortens schedules. The length of time a gap is left open is no longer an issue; thus, quality is improved without expense.

Additionally, there is no trickle-down effect that delays other trades, and safety concerns are eliminated. But how can a gapless pour strip be achieved?

turns soft/plastic around 150°F. Therefore, EORs need to be careful using the lockable dowel when fire ratings are required.

Although the lockable dowel creates a gapless pour strip, it falls short and does not provide the same structural functions as a traditional pour strip. Structural engineers must be careful when evaluating the lockable dowel because it cannot transfer the ductile reinforcement needed to withstand all vertical and horizontal loads, including diaphragm chord continuation. In other words, the lockable dowel is only a partial substitute for a traditional pour strip.

Mechanical Splices Steel reinforcement mechanical splice systems (known as rebar couplers) have a long, proven history of providing for the continuation of ductile reinforcing steel, dating back to the 1960s. Tens of millions have been successfully installed worldwide, including in high seismic zones. Since normal reinforcing bar lengths are far short of typical building dimensions, four ACI 318 code-permitted methods of splicing rebar provide reinforcement continuation: lap splices (used in traditional pour strips), mechanical splices (rebar couplers), butt-welded splices, and end-bearing splices.

Mechanical splices were first used in high seismic regions. They are now defined by ACI 318 as Type 1 (non-seismic) capable of developing 125% Fy (yield strength) and Type 2 (seismic) capable of developing 100% Fu (ultimate strength); this is also included in the required ICC-ES Acceptance Criteria for Mechanical Connector Systems for Steel Reinforcing Bars (AC133).

Mechanical couplers are made of steel or a combination of steel and concrete. Fire ratings for mechanical reinforcement splices installed in concrete are prescriptive (and the same as for rebar) according to the International Building Code (IBC) and ICC-ES (defined in Table 721.1 in the latest IBC).

A mechanical coupler is simply another ACI 318-permitted method of splicing reinforcing steel, much like a lap splice utilized by

Lockable Dowels Some have used lockable dowels to create a gapless pour strip. They are typically located at a one-fifth-span since they do not provide reinforcing continuation and instead act as a shear dowel. The lockable dowel can support a selfsupporting four-fifths-span, which allows for less-costly reshoring versus expensive backshoring. However, this is only part of the story as it does not provide structural continuity like the traditional pour strip.

Since this product is a dowel (with limited non-ductile pull-out much like an expansion bolt), it must be used when ductile reinforcing steel continuation is not a concern.

Most structural engineers call this an expansion joint where diaphragm chord steel is interrupted. This does not replace all of the vertical or horizontal loading functions provided by a traditional pour strip.

Fire ratings are typically required in concrete slabs. Beyond its inability to transfer loads like a traditional pour strip, the lockable dowel uses an epoxy grout, which is combustible and

Figure 2. a) PS=Ø Mechanical Reinforcement Splice System provides ductile reinforcement continuation and a gapless pour strip; b) First pour installation of PS=Ø at the AdventHealth hospital in Overland Park, KS. D E C E M B E R 2 0 21

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traditional pour strips. Could a unique mechanical rebar coupler that provides the same structural functionality as a traditional pour strip but allows for volume change be the solution to a gapless pour strip? Yes.

A New Solution The PS=Ø® Mechanical Reinforcement Splice System (Figure 2, page 37) eliminates traditional pour strip leave-outs and maintains reinforcing continuity while allowing for volume change. It features a tapered thread on one end and a grout-filled sleeve on the other. The system meets ACI 318 Type 1 and Type 2 mechanical splice requirements and is ICC-ES approved (ICC-ESR 4213). PS=Ø stands for Pour Strip Zero and allows for free movement, longitudinal and transverse, to the SRJ until the coupler is grouted with a high-strength concrete grout (Figure 3). PS=Ø allows for the original EOR design of a traditional pour strip to be maintained, and since it is a simple substitution, no delegated design is required.

PS=Ø has the same structural functionality as a traditional pour strip but does not require a leave-out. It provides a load transfer of both vertical (such as slab moment and shear) and horizontal loads (like diaphragm, horizontal shear, and drag forces) by connecting ductile reinforcing from one side of the slab to the other using a code-permitted method of splicing rebar.

Applications Both applications apply to one-fifth-span and mid-span locations.

1) The standard application (Figure 4) is where the second slab is poured directly against the first slab. Stressing can be done from either end of the first slab but only from the outside of the second slab.

2) When a temporary leave out or stressing strip is needed, they can easily be added to the system between slabs or at a wall and adjacent slab, then poured back immediately after stressing. This can also be utilized in construction sequencing where a delayed, low-rise building is attached to an adjacent high-rise building.

Figure 3. The PS=Ø mechanical coupler allows for a gapless pour strip and is grouted with a high-strength, non-shrink grout after the EOR specified time; the red arrows indicate the open joint that will be filled with the same non-shrink grout.

Conclusion The traditional pour strip has been the industry solution used by EORs to allow for volume change and relieve RTS in concrete slabs for decades. Although a good solution, it has been an issue of contention between EORs and contractors because of the construction problems it brings. Cost, quality, and schedule are essential aspects of building projects and pour strips have forced EORs to sacrifice quality for cost and schedule.

Real, lasting innovation in construction requires solutions that make designs better and construction more productive. Structural engineers developed PS=Ø to create a gapless pour strip or strain-relief-joint (SRJ) that provides high-quality slabs without increasing cost or lengthening schedules. With PS=Ø, overall projects costs and schedules are significantly reduced and safety is improved, making this rebar coupler an excellent innovation for concrete construction. Lower cost, higher quality, accelerated schedules, and improved safety are achieved. PS=Ø provides the same structural functionality as the traditional pour strip, making it an easy substitution for a gapless pour strip.■ References are included in the PDF version of the article at STRUCTUREmag.org.

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Figure 4. Standard PS=Ø application where the second slab is poured directly against the first slab; all MEP and curtain wall trades have worked without delay across the bottom of this SRJ at the Even Hotel in Rochester, MN.

All authors are with Reigstad Engineers, Inc., developers of the PS=Ø ® Mechanical Reinforcement Splice System.

Gordon H. Reigstad is President. (greigstad@reigstad.com) Jason G. Reigstad is Vice President. (jgreigstad@reigstad.com) Jared M. Reigstad is Vice President. (jmreigstad@reigstad.com)

INSIGHTS

Adaptation Advantage

The Top Non-Technical Skill Structural Engineers Need in an Evolving AEC Industry

By Stephanie Slocum, P.E.

I

n 1996, a bespectacled nerdy high school student leaped out of her car with a movie rental. She slid in the rain, dashing to the Blockbuster drop-off a mere minute before closing. Her singular focus: to return the movie and avoid the late fees that cost more than the initial rental. That teenager was me.

At the time, Blockbuster had a virtual monopoly on movie rentals, despite lackluster service and grumbling by customers about the late fees that were much more than the initial rental cost. Then, one year after my rainy-day dash, Reed Hastings founded Netflix, in part due to frustration with the $40 fine he acquired at Blockbuster. In its early stages, Netflix had no late fees and sent DVDs straight to your house for a flat monthly rate.

At first, the new business model appeared to be small potatoes to the video rental giant. Blockbuster peaked in 2004, with 9000 stores globally and $5.9 billion in revenue that year. However, by 2005 the company lost half its market value while Netflix took off. Today, after 16 years of decline, there is only one Blockbuster store remaining.

Adapt or Fail Business experts have noted multiple points at which Blockbuster could have changed its trajectory, a decline that was both foreseeable and preventable. Yet, in analyzing what happened, a pattern emerges of contentment with the status-quo and blinders when it comes to industry shifts. For example, business plans were driven based on past experience in product design to business development, and new innovations were overlooked.

That same culture permeates many structural engineering firms, creating a general resistance to adaptation and a preference to uphold the status quo. For many organizations, the business model, client acquisition strategies, and the transactional nature of work relationships are not considered potential innovation areas. One need look no further than the industry’s struggles with the shift to work-fromhome to see significant change resistance and, in some cases, a rigidity incompatible with adaptation, let alone innovation.

Blockbuster failed because when the whirlwinds of change swept through their industry, they did not adapt. The same can happen to you and your firm without keeping your pulse on what is going on far beyond your firm in the global business environment driving the broader AEC industry. The key to successful adaptation is your ability to understand and effectively communicate your value proposition to everyone you work with: peers, managers, and clients.

Value Propositions Many structural engineers have little knowledge of the business aspects of the industry in the Figure. Those blinders create a disadvantage when it comes to understanding your value. No one wants a stagnant career or to work in a firm in a Blockbuster-style slow-and-steady demise; however, that is EXACTLY what happens when you lack business knowledge.

You are in the business of structural engineering. Without knowing your business value, you are flying blind when it comes to growth paths. That value determines if you get hired, get a raise or promotion, and the amount of influence you have at work. It determines if you work on high- or low-profile projects. Shifts in what is most valued STRUCTURE magazine

Use the questions in each of these three categories to better understand where your value is in your organization.

can also be triggered by global industry changes like those we have experienced over the last 18 months.

Determining Your Value Value is not determined through self-reflection. Instead, it is in the eye of the beholder. Like feedback, your value is determined through input from others. Particularly when market shifts accelerate, soliciting input directly from clients, peers, and managers ensures you are on the same page about your specific value proposition. Those insights show you how to adapt to leverage and learn the skills and strategies that keep you relevant.

Try this exercise to start the conversation about your value to your firm: 1) Create a list of your weekly work activities.

2) Highlight the 3-5 activities that most contribute to your role and/or success of your organization.

3) Tell your manager(s) you are working on “maximizing your productivity and value to your organization by increasing your highest-value contributions.” Show them the list and ask for input. Do they agree? What do they see as your most valuable attributes?

4) Repeat this exercise with coworkers or others you work with, considering patterns in the data.

Articulating Your Value Asking directly for input gives you immediate data on your value and sometimes even exact words you can share with others about your effectiveness in your role. It eliminates false assumptions and replaces them with actual data about where your true value lies.

Knowing your value is the first step in articulating it in the various work contexts to different stakeholders. Once the data is gathered, the next step is to learn the skillsets necessary to articulate it within the business context of your organization and the AEC industry at large. This skill set is not just necessary to have an adaptation advantage; it sets the stage to position you as a leader in structural engineering and beyond.■ Stephanie Slocum is the Founder and CEO of Engineers Rising LLC and is a career and business strategist for engineers. She is the chair of the Structural Engineering Institute’s (SEI’s) Business Practices Committee and has also been elected to the Board of Governors. (stephanie@engineersrising.com) D E C E M B E R 2 0 21

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code UPDATES Calculating Rain Loads per 2021 IBC

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

S

econdary drainage system rain loads have been updated in the 2021 International Building Code® (IBC) to be consistent with ASCE 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. The following text shows the specific change in the 2021 IBC. Strike-through indicates deleted text and underlined denotes new text.

1611.1 Design rain loads. Each portion of a roof shall be designed to sustain the load of rainwater that will accumulate on it if the primary drainage system for that portion is blocked plus the uniform load caused by water that rises above the inlet of the secondary drainage system at its design flow. as per the requirements of Chapter 8 of ASCE 7. The design rainfall shall be based on the 100-year hourly rainfall rate indicated in Figure 1611.1 15-minute duration event, or on other rainfall rates determined from approved local weather data. Alternatively, a design rainfall of twice the 100-year hourly rainfall rate indicated in Figures 1611.1(1) through 1611.1(5) shall be permitted.

R = 5.2(ds + dh) (IBC Equation 16-19) where: dh = Additional depth of water on the undeflected roof above the inlet of secondary drainage system at its design flow (in other words, the hydraulic head), in inches. ds = Depth of water on the undeflected roof up to the inlet of secondary drainage system when the primary drainage system is blocked (in other words, the static head), in inches.

R = Rain load on the undeflected roof, in psf. Where the phrase “undeflected roof ” is used, deflections from loads

Figure 1. Secondary drainage system outflow.

(including dead loads) shall not be considered when determining the amount of rain on the roof.

1611.2 Ponding instability. Susceptible bays of roofs shall be evaluated for ponding instability in accordance with Section 8.4 Chapters 7 and 8 of ASCE 7.

Change Significance

Figure 2. Design for ponding instability is based on secondary drainage system rain loads.

40 STRUCTURE magazine

Secondary (overflow) system design (Figure 1) has been harmonized with roof rain load provisions for a structure to provide realistic expectations of the roof drainage system and potential roof loading by rainfall (Figure 2). The IBC is now consistent with ASCE 7 provisions. Calculations for the design mean recurrence interval and duration for determining the hydraulic head, dh, are available in both ASCE 7 and the IBC. Legacy ASCE load standards’ design rainfall durations for plumbing systems were between 15- and 60-minutes for a 100-year mean recurrence interval. The 1995 Standard Plumbing Code used the 100-year/60-minute duration for primary drainage system design and a 100-year/15-minute duration storm for the secondary drainage system. In the 2018 IBC and International Plumbing Code (IPC), a 100-year/60-minute duration is used for both the primary and secondary systems. Note that using twice the 60-minute duration is close to the 15-minute duration rainfall rate for

many regions. Also, note that 2021 IPC and 2021 IBC rainfall maps (Figures 1106.1 and 1611.1, respectively) both include a 60-minute duration rather than the 15-minute storm duration. However, the 2021 IBC, by giving two options – the 15-minute duration or twice the 60-minute duration – results in values similar to ASCE 7 when drainage pipe or scupper flow rates are calculated. Note that the 2021 International Plumbing Code has not yet been updated to reflect the 100-year/15-minute (or twice the 100-year hourly) duration rainfall event design requirement for Figure 3. Secondary drainage design assumptions. secondary drainage systems; the structural engineer would be advised to coordinate with the plumbing reasonable. Engineers should be aware of different secondary drain engineer to ensure that the secondary drainage systems are designed options available to plumbers and clearly communicate how changes, for the higher rainfall rate. especially to secondary drain geometry, can impact design rain loads The best source for rainfall data is the National Oceanic and on a building. Important parameters to communicate include assumed Atmospheric Administration (NOAA) National Weather Service static head, hydraulic head associated with secondary drain or scupper Precipitation Frequency Data Server – Hydrometeorological Design size and geometry, rain load, and rainfall rate.

Studies Center (hdsc.nws.noaa.gov/hdsc/pfds/index.html) for precipitation intensity (inches per hour) based on the 100-year mean 2018 IBC Design Example: recurrence interval. NOAA’s data lists both 15-minute and 60-minute duration data.

60-minute rainfall total for primary and secondary systems.

Design rainfall: 3.30 inches for 100-year mean recurrence, Cedar Rapids Station No. 1.

Details of Load and Drain Size Calculations Calculate primary and secondary drain size and the resulting rain To understand why this makes a difference, the following examples load, R. show how to determine rain load, R, assuming rainfall for the city of Primary Drain Cedar Rapids, Iowa. The first example uses 2018 IBC requirements, and the second uses 2021 IBC requirements.

Depth of water (in 1 hr): 3.30 inches (NOAA) Secondary drain size and geometry affect the structural engineer’s Tributary area (primary drain): 100-ft by 50-ft = 5,000 ft2 determination of a maximum height of water above the roof surface Flow rate (volume) to maintain roof drainage: using variables ds and dh for static and hydraulic head (Figure 3).

Q = 0.0104 × A × i (ASCE 7-16 Equation C8.3-1*) Secondary drains should be specified, if possible, to keep rain loads where (page 42): Table 1. Storm drain pipe sizing.

PIPE SIZE (inches)

CAPACITY (gpm) VERTICAL DRAIN

SLOPE OF HORIZONTAL DRAIN ⁄16 inch per foot

1

⁄8 inch per foot

1

⁄4 inch per foot

1

⁄2 inch per foot

1

2

34

15

22

31

44

3

87

39

55

79

111

4

180

81

115

163

231

5

311

117

165

234

331

6

538

243

344

487

689

8

1,117

505

714

1,010

1,429

10

2,050

927

1,311

1,855

2,623

12

3,272

1,480

2,093

2,960

4,187

15

5,543

2,508

3,546

5,016

7,093

[2021 International Plumbing Code Table 1106.2 – courtesy of the International Code Council]

D E C E M B E R 2 0 21

41

Figure 4. Secondary drainage using scuppers.

Figure 5. Secondary drainage using pipe with visible outlet.

Q – flow rate to maintain drainage rate equal to rainfall rate A – tributary area i – water depth in inches per hour * The 0.0104 factor in the equation converts area, in square feet, and rainfall rate, in inches per hour, to gallons per minute.

A = 50 ft × 100 ft = 5,000 ft2 i = 3.30 inches/hr Q = 0.0104 × 5,000 ft2 × 3.30 inches/hr = 172 gal/min From Table 1 (page 41), the minimum drain size is a 4-inch vertical pipe or 6-inch horizontal drain with a minimum slope of 1⁄16-inch per foot

Secondary Drain and Rain Load (R) Calculation Given: ds = specified distance from the roof surface to the bottom of scupper (Figure 4) or top of secondary drain pipe (Figure 5) dh = calculated (tabulated) height of water above the base of scupper or secondary drain based on drain geometry Flow rate (volume) to maintain roof drainage through a scupper or secondary drain is calculated for the secondary drain since the primary drain is assumed to be completely blocked. The secondary system’s design rainfall is 3.30 inches per hour per the 2018 IBC. The flow rate, Q, will be 172 gal/min, identical to the primary drain system. This rain load must leave the roof at the same rate it is falling or faster.

42 STRUCTURE magazine

Typically, once the structural engineer determines the rainfall, static head, hydraulic head, and rain load on a roof, a plumber can size the secondary system pipes or scuppers to a flow rate of 172 gal/ min or greater. An engineer does this check as an iterative process to keep rain loads on the roof rational by limiting the hydraulic head to a reasonable value. Engineers should be aware of different secondary drain options available to plumbers and clearly communicate how changes, especially to secondary drain geometry, can impact design rain loads on a building.

To calculate R: dh (hydraulic head) = 5 inches for a 6-inch-wide, 6-inch-high, closed-top scupper which corresponds to a flow rate of 194 gpm (Table 2) which is sufficient for the calculated flow rate of 172 gpm. ds (static head) = 6 inches (specified distance from the roof surface to the bottom of the scupper) R = 5.2(ds + dh) = 5.2(6 in. + 5 in.) = 57.2 psf (IBC Eq 16-19) As a second iteration, a 24-inch-wide scupper (open- or closed-top of any height) handles 200 gpm with a corresponding 2-inch hydraulic head.

This would reduce the rain load to a more reasonable 41.6 psf.

Note: The 5.2 value in the equation converts depth of water, which is in inches, to pressure in pounds/square foot (psf ) using the density of water of 62.5 pounds per cubic foot (pcf ) and the conversion inches to feet (12 inches per foot); therefore, (62.5pcf )/(12in./ft) = 5.2 psf per inch of water depth.

2021 IBC Design Example: 60-minute rainfall duration for the primary system, 15-minute rainfall duration for the secondary system.

Design rainfall for Cedar Rapids Station No. 1: 3.30 inches for 100-year mean recurrence (60-minute) 1.72 inches for 100-year mean recurrence (15-minute) Calculate primary and secondary drain size and resulting rain load, R.

Primary Drain Using the parameters from the 2018 IBC Example, calculation of the primary drain diameter per Table 1 requires a minimum 4-inch vertical pipe or 6-inch horizontal drain (minimum slope of 1⁄16-inch per foot).

Secondary Drain and Rain Load (R) Calculation Flow rate (volume) to maintain roof drainage through scupper or secondary drain is the flow rate calculated for the secondary drain.

The primary drain is assumed to be completely blocked.

Flow rate (volume) to maintain roof drainage rate equal to rainfall rate: Q = 0.0104 × A × i (ASCE 7-16 Equation C8.3-1) where: Q – flow rate to maintain drainage rate equal to rainfall rate A – tributary area i – water depth in inches per hour (must convert 15 min interval to equivalent hour interval)

Table 2. Flow rate of various roof drains at various water depths at drain inlets.

FLOW RATE (gpm) DRAINAGE SYSTEM

Depth of water above drain inlet (hydraulic head) (inches) 1

2

3

4

5

4-inch-diameter drain

80

170

6-inch-diameter drain

100

190

380

8-inch-diameter drain

125

230

560

1,100

6-inch-wide, open-top scupper

18

50

90

140

194

24-inch-wide, open-top scupper

72

200

360

560

776

6-inch-wide, 4-inch-high, closed-top scupper

18

50

90

140

177

24-inch-wide, 4-inch-high, closed-top scupper

72

200

360

560

708

6-inch-wide, 6-inch-high, closed-top scupper

18

50

90

140

194

24-inch-wide, 6-inch-high, closed-top scupper

72

200

360

560

776

[2018 IBC Commentary Figure 1611.1(2) - courtesy of the International Code Council]

A = 50 ft × 100 ft = 5,000 ft2 i = 1.72 inches/ 15 minutes × 60 minutes/ hour = 6.88 inches/hr Q = 0.0104 (5,000 ft2) (6.88 inches/hr) = 358 gal/min Note that a 15-minute design rainfall has approximately 2 times the flow rate of the 2018 IBC 60-minute design rainfall. This is true for much of the United States but check each location to make sure it is true in that region rather than assuming a fixed 100-percent increase in flow rate.

2021 IBC flow rate (secondary drainage system) 358 gal/min = = 2.08 2018 IBC flow rate (secondary drainage system) 171 gal/min

Secondary drainage system rain loads have been updated in the 2021 IBC to be consistent with ASCE 7-16. For plumbers to be aware of the change to a 15-minute per 100-year rainfall duration, engineers should clearly communicate how changes, especially to secondary drain geometry, can impact design rain loads on a roof.■ Sandra Hyde (shyde@iccsafe.org) is Managing Director and John “Buddy” Showalter (bshowalter@iccsafe.org) is Senior Staff Engineer, both with ICC’s Product Development Group.

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To calculate R: dh (hydraulic head) = 3 inches for a 24-inch-wide, open- or closed-top scupper corresponding to a flow rate of 360 gpm (Table 2), sufficient for the calculated flow rate of 358 gpm. ds (static head) = 6 inches (specified distance from the roof surface to the bottom of the scupper) R = 5.2(ds + dh) = 5.2(6 in. + 3 in.) = 46.8 psf (IBC Eq 16-19) Secondary drain size impacts flow rate but drain size and geometry impact hydraulic head levels. Therefore, to keep the rain load to reasonable design levels, larger secondary drains and geometries should be explored to minimize hydraulic head levels. The examples show that flow rates in the 2021 IBC are approximately double for secondary drain design. However, for comparably sized roofs and secondary drains (24-inch scuppers), the difference in the hydraulic head is 1 inch, which only increases the rain load by about 5 psf.

Conclusion

D E C E M B E R 2 0 21

43

historic STRUCTURES Quebec Bridge Failure #2, 1916

By Frank Griggs, Jr., Dist. M.ASCE, D.Eng, P.E., P.L.S.

A

fter the collapse of the Quebec Bridge in August 1907, an Engineering Committee was formed under the supervision of the Transcontinental Railway Commission to undertake a complete redesign of the bridge. The Board consisted of one Canadian, H. E. Vautelet, one American, Ralph Modjeski, and one British Engineer, Maurice Fitzmaurice. Engineering News wrote of the Board, “The task of the engineering committee is a very great one. Difficult as it would be under any circumstances, the progress and results would be certain[ly] beyond question to engage the most careful attention of the engineering world.

As matters actually stand, this attention will be multiplied by the stimulus of the intense interest awakened by the collapse of the first bridge. There Suspended span collapsing into the St. Lawrence River. will be a more anxious, more rigid scrutinizing of the conclusions and the designs produced by the committee. specifications shall not, in any way, relieve the contractor from This fact renders their work correspondingly more delicate and such responsibility.” of greater responsibility and therefore more difficult. Judgment In other words, the contractor was solely responsible, even though of fine metal will be needed. May it not be corroded by the they had to submit their plans and calculations to the Board for steam rising from the ever-boiling approval. Besides this and the pot of Canadian politics! May the previous bridge, the main differcommittee accomplish its task sucence is that the government now cessfully and worthily.” had a fully funded organization After a design competition, a to oversee design and construccontract was formally awarded on tion work. Nothing would be April 4, 1911, to the St. Lawrence done until checked thoroughly Bridge Company consisting of the by engineers from the Board and Dominion Bridge Company and the the Bridge Company. No assumpCanadian Bridge Company, with a tion would be allowed to stand clause placed in the contract stating, unless agreed to by both parties, “The contractor must satisfy and large-scale tests of members himself as to the sufficiency and would be required if there was suitability of the design, plans, any question. The Board, in fact, and specifications upon which the made its own design independent bridge is to be built, as the contracof the design of the St. Lawrence tor will be required to guarantee the Bridge Company and then comsatisfactory erection and complepared its design with theirs. Any tion of the bridge, and it is expressly discrepancy was adjusted after a understood that he undertakes the complete discussion. The Board entire responsibility, not only for tested quarter-size compression the materials and construction of members at the Phoenix Iron the bridge, but also for the design, Works to verify their designs. In calculations, plans, and specificaaddition, the Company decided to tions and for the sufficiency of erect the anchor and cantilever arms the bridge for the loads therein in a similar manner to the original specified. And the enforcement of Phoenix Bridge plan but to erect any part or all of all parts of the Lifting plan. the suspended span off-site, float

44 STRUCTURE magazine

Lifting details.

and they were being lowered for another lift when, at 10:50 am, a sharp report was heard, and the span was seen to slide off its end supports into the river.

Unlike the first failure, where there were few eyewitnesses to the collapse, this time, the press, photographers, government officials, and the Board were on hand to witness it. Engineering News reported that “many prominent engineers from the United States and Canada were on the suspended span when the lifting operations began. At the intermission in the jacking operations, they came ashore. That saved their lives...”

Thirteen men were killed this time, with fourteen injured. Once again, an intensive investigation into the cause of the accident was launched. It was clear to all that the truss had fallen off of the southwest supporting girder, where the designers used cruciform steel castings at each corner to provide for rotation about two perpendicular axes.

Engineering News reported, “A steel casting, by which the weight of the south upstream corner of the suspended span was transferred to the lifting girder, broke in such a manner that the girder kicked back from under it. This corner of the span dropped into the water, starting transversal rotation of the whole south end of the span… But experience teaches again that disaster may come, even to the most careful.

In the light of what has happened at Quebec, engineers in every rank of the profession must realize anew that there is a lurking

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it in, and lift it intact into place connecting with the ends of the two cantilever arms.

The anchor spans were 515 feet, the cantilever arms were 580 feet, and the suspended span was 650 feet. As the result of the thorough planning and checking of every detail of the design, the fabrication and erection work generally went smoothly. The field inspection staff for the government consisted of 33 men over the erection period.

The bridge company had over 60 men working as engineers, draftsmen, calculators, and checkers on the job. The Dominion Bridge Company and the Canadian Bridge Company devoted their entire fabricating facilities to the bridge throughout its construction.

The suspended span was finished in July of 1916. The span weighed over 5,000 tons and was floated into place and connected to the lifting jacks on September 11. After a short lift, the span slipped off its supports and collapsed into the river. The official report of the Board of Engineers described, in part, the series of events on that fateful day as follows, “Preparations for floating were completed about September 1st, 1916, but the range of tides, at this date, was not suitable…The next series of high tides occurred on September 11th, and, weather conditions being favourable, the span was floated at 3:40 am, and by 4:40 am, it was being towed out into the river…As the tide was running strong, the tugs had little to do but guide the span on its trip up the river. At 6:35 am, the span reached the bridge site, and at 7:40 am, the lifting hangers at all four corners had been connected. At 8:50 am, the jacks began lifting, and during the third lift of two feet, the scows floated clear, leaving the span suspended about 20 feet above the water…Up to this point, the entire operation of floating the span and connecting it to the lifting hangers had worked exactly according to schedule.

Nothing occurred that had not been foreseen and provided for. There was no wind, and every condition was favourable.

As the work remaining to be done was simply a repetition of mechanical operations which had already been successfully performed, it was felt that the most difficult part of the work had been satisfactorily accomplished.

At 10:30 am, jacking operations were resumed, and one more lift was made. The pins had been inserted connecting the lifting links to the fixed jacking girders, thus transferring the load directly to the cantilever trusses. The load on the jacks had been released,

Tipping of lifting beam.

D E C E M B E R 2 0 21

45

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possibility of failure in every task that the engineer performs and that, when such failures come, those who suffer by it deserve the broad and generous sympathy of their brethren. The final success of any great engineering work depends on a multitude of details and on eternal vigilance to see that none are neglected. It is the engineer’s duty to study disasters to engineering works because of the useful lessons they may teach; but let him study them with all sympathy for his brother engineer Quebec Bridge who has borne the responsibility, remembering that he himself might have been in a like situation.” crushed and tipped the remainder of the rocker at the same time The Engineering Record had similar thoughts, writing, kicking out the swinging girder and allowing the corner to fall.

“Engineers, who must constantly face unknown factors in break- With the supports at this corner gone, the long, heavy span could ing new paths, will not lose faith in their brethren at Quebec, not maintain its equilibrium, and it dropped into the river…The though the layman, seeing only the workingmen who were on the susoutstanding fact of a second serious pended span when it fell were thrown loss at the site, may be a doubting into the water. Of these, a number Thomas…Fortunately, sufficient eviwere saved by the small boats, but dence was preserved on the girder some 10 or 11 have not since been The final success of any great to show exactly what happened. accounted for.”

This evidence, combined with the On September 13, 1916, the St. engineering work depends on detailed drawings, leaves it to every Lawrence Bridge Company accepted engineer to draw his own conclufull responsibility for the failure and sions. Some will believe, as does this took “immediate steps to replace a multitude of details and on journal, that greater factors of safety the span.” The Board and the Bridge should have been used in the interCompany decided that the entire lifteternal vigilance to see that mediate castings and the suspension ing apparatus would have to be rebuilt details.” due to excessive deformations of the none are neglected The Railroad Gazette wrote, lifting links occasioned by the fall.

“From examinations of the hangers The investigating team submitted since the accident, it is evident that its report on October 19, 1916. The the intermediate or roller casting most significant finding was that the under the southwest hanger gave disaster was due solely to a failure of way. The fracture of the front lower the casting. pin bracket of this rocker threw the lower pin out of service, and Work on the new suspended span got underway on June 4, 1917, the concentration of the 1,200-ton load on the remaining pieces with the span being completed on August 27. It was floated into place on September 17, or just over one year after the failure. This time, the lift went as planned. The Engineer, London wrote, “one of the greatest, if not the greatest, feat of bridge engineering the world has ever seen was brought to a successful conclusion, on Smart-phone, 4” screen Thursday, September 20th, 1917 at 4:01 pm, when the 10-inch Android or Apple iPhone will run the pins connecting the two sections of the Quebec Bridge to the ends of the cantilever arms were driven.”

As the engineering journals of the time wrote, “The final success of reinforced concrete cross-section analysis app any great engineering work depends on a multitude of details and Details at xsecweb.com on eternal vigilance to see that none are neglected” and all also on Windows, MacOS and iPad engineers must “realize anew that there is a lurking posUSD 20.00 from the various app stores sibility of failure in every task that the engineer performs.”■

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46 STRUCTURE magazine

Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19 t h Century cast and wrought iron bridges. He is now an Independent Consulting Engineer. (fgriggsjr@twc.com)

professional ISSUES A Vision for Structural Engineering Licensure Structural Engineering Licensure Coalition

T

he Structural Engineering Licensure engineers’ work and the complexity of codes. engineering profession would have over its Coalition (SELC), an organization In addition, the profession is challenged to own regulation. Plus, profession-run certififormed by the Council of American Structural acquire an ever-increasing essential formal cation would fill a void should licensure be Engineers (CASE), the National Council of education while many colleges are being diminished in some fashion.

Structural Engineers Associations (NCSEA), forced to reduce classroom hours. These cirWhile there is growing justification for the Structural Engineering Institute (SEI), cumstances call for rigorous credentialing specialized structural engineering licensure, and the Structural Engineering Certification programs that effectively and adequately iden- threats continue to challenge the regulation Board (SECB), has been advocating since tify engineers qualified to design structures. of professional engineers and structural engi2012 for structural engineering neers in particular. For example, in (S.E.) licensure as a post-professional the past several years, some U.S. engineer (P.E.) credential in all U.S. jurisdictions have raised the posjurisdictions. As part of that effort, sibility of reducing regulation of Communication regarding the SELC recently published its Vision professions, including engineering, positive and negative aspects of S.E. for the Future of S.E. Licensure (the with the potential for licensure to Vision), which builds on vision be eliminated. Also, there is some licensure should be more robust documents published by its parent resistance advanced by the strucorganizations. tural engineering community itself, within the engineering community.

Professional licensure has been funprincipally from two fronts: 1) the damental in the U.S. since Wyoming status quo within the profession, first instituted a licensing requirewhich contends that what we have ment for professional engineers in done in the past is good enough for 1907. Civil engineers at that time tended Th e process to attain licensure has the future, and 2) the position held by many to practice across a breadth of disciplines improved over the years. Strict requirements that holding a P.E. license and conform(structural, geotechnical, water resources, for education, experience, and examina- ing to the relevant codes of ethics suffice. transportation, etc.). Over the decades since tion have evolved under the guidance of While both points had merit in the past and licensing first appeared, the profession has the National Council for Examination probably still do somewhat in the present, become compartmentalized, with engineers of Engineers and Surveyors (NCEES). they do not speak well to the future and the now tending to practice mainly in a single Education accredited by Accreditation for profession’s evolution. Structural engineering discipline. Taking responsible charge in more Engineering and Technology, Inc. (ABET) licensure aims to protect public safety by than one specialty is much less common now is typically required. Verification of appli- restricting structural engineering to those than it might have been decades ago. cants’ experience for licensure generally who demonstrate that they are best qualiThe SELC Vision argues that the trend toward is mandated through affidavits from col- fied to practice by passing an objective and specialization, and the increasing complexity leagues or supervisors. The signers of the rigorous examination. Legislative support of engineering, should dictate specialization affidavits attest that the candidate’s experi- for this goal, through licensure, provides a in the licensing process. Jurisdictions in the ence is progressive, requires the application strong base for long-term continuity of the U.S. recognized this early, with Illinois creat- of engineering knowledge, shows self- merits of the profession and its influence on ing a specialty license in structural engineering improvement, teaches the candidate to our built environment. in 1915, less than 10 years after engineering design, and puts the candidate in a position Ultimately, it is on the shoulders of licensure was first introduced. While the expan- of responsibility for the design product. In the structural engineering community sion of S.E. licensure has been relatively slow addition, nearly all U.S. jurisdictions now to advance S.E. licensure in the U.S. since then, now at least twelve states and two recognize that meaningful continuing edu- Communication regarding the positive and territories restrict either structural engineer- cation is essential for the engineer to stay negative aspects of S.E. licensure should be ing practice specifically or the use of the title abreast of new technologies and changes in more robust within the engineering commu“Structural Engineer.” standards and codes. nity. In addition, improving outreach and Trends in the profession are strengthening Recently, certification of structural engi- discussion within the community is necesthe argument that specialized licensure is neers by the profession has been discussed sary to rally support to defeat legislative needed. For example, technology is chang- as a companion to S.E. licensure. The goal of initiatives that would harm licensure. Please ing how engineering is performed, new certification is to raise the bar of structural read SELC’s Vision (www.selicensure.org) sophisticated analyses and design tools allow engineering practice in the eyes of the public if you wish to learn more about marginally qualified engineers to practice and validate additional outcomes necessary the history and importance of S.E. outside their specialty areas, and globaliza- to be in responsible charge, especially as licensure in the U.S.■ tion engages stakeholders in remote and continuing education is mandatory and SELC has been formed to champion the cause potentially unfamiliar markets. Moreover, given that not all states have S.E. licensure. of structural engineering licensure and to build a the growing breadth and extent of informa- In addition, certification by the profession consensus among all stakeholders. tion engineers need to assimilate complicates is advantaged by the control the structural STRUCTURE magazine

D E C E M B E R 2 0 21

47

talking POINTS The Case for Science-Based Public Policy

By John A. Dal Pino, S.E.

T

he scientific method is best described as a series of sequential steps: Question, Research, Hypothesis, Experiment, Observation, and Conclusion. If the hypothesis is “proven” through experiment and observation, the Conclusion becomes a Theory generally supported and accepted by other subject-matter experts. This process takes time and effort and cannot be rushed.

However, in today’s fast-paced, 24-hour news cycle, rarely is enough time allotted for all the steps to be taken, particularly the experiment part. We all want answers fast and show little patience for delays that are warranted. The interpretation of data, or more often the observation of patterns in the data, is now referred to in the public sphere as “the science” and is used as the basis for making public policy on the fly.

“From where I stand, the earth looks flat, so it must be” is a good example of observational data masquerading as science. Unfortunately, this process can result in un-tested hypotheses becoming public policy.

One could argue that if society is trying to make advancements quickly or address critical challenges (whether it is health policy or building codes), relying on observational data is better than waiting for the science to become “settled.” Since most policymakers are, by nature, cautious and risk-averse, it may seem safer to over-estimate the risk while demonstrating an eagerness to act, thereby showing a degree of caring rather than waiting for the scientific process to run its course. But this approach can have unintended consequences. Either the policy will bounce around and change as new information is learned, or the policy will be overly conservative and wasteful, maybe even detrimental. No one wants to be caught looking unprepared and indifferent to the public interest, but the cost to the public can be high in both situations and erode public confidence.

The next time policymakers need to reach the public with an important message, who knows what the response will be. After the balcony collapse that killed several students in Berkeley, California, several years ago, there was a desire by many to take immediate action in the form of stricter building code provisions. Cooler heads prevailed, and the result was the implementation of new inspection requirements. In hindsight, this was probably the right thing to do.

Let’s use a small part of earthquake engineering to demonstrate how the scientific 48 STRUCTURE magazine

method was used successfully to develop improved seismic design procedures and form public policy.

All the steps were taken, and science led the way, whichever road it took.

The field of earthquake engineering in the U.S. as we know it today started in the 1950s thanks to a small group of practicing engineers and academics located mainly in California. This group had an excellent understanding of structural dynamics. They used their knowledge to craft the early seismic code provisions, the basis of which is Northridge Meadows Apartments after the 1994 Earthquake. described in the Structural Engineers Association of California (SEAOC) Blue Book: Buildings, A Cooperative Effort with the Design Seismic Design Recommendations, first published Professions, Building Code Interests and the in 1959. At the outset, the leaders, including Research Community (Applied Technology Henry Degenkolb and his cohorts, and later, Council, 1978) – proposed an outline for university researchers including Professors Egor the building codes used today.

Popov and Vitelmo Bertero at the University of ATC-14 Evaluating the Seismic Resistance California, Berkeley, questioned the status quo of Existing Buildings (1987) and FEMA and decided to do something about it. They 273 NEHRP Guidelines for the Seismic developed questions about seismic performance Rehabilitation of Buildings (Federal Emergency (Question and Research), developed theories on Management Agency, 1997) – established prohow buildings should perform (Hypothesis), cedures for evaluating and retrofitting existing and then went to the sites of a steady stream of buildings. strong earthquakes in regions with similar buildVision 2000 Performance-Based Seismic ings (Experiment) to see (Observation) whether Engineering of Buildings (Structural Engineers they were correct and to gather more data. This Association of California, 1995) – proposed process led to new building code provisions a framework for performance-based, rather (Conclusion and Theory) adopted after public than prescriptive, design. notice, debate, and approval by elected officials.

The field of earthquake engineering was used The 1971 San Fernando and 1994 Northridge here to showcase the scientific method and earthquakes (see STRUCTURE’s 2019 article how it can and ought to form the underpinseries) provided data for new lines of scientific ning of public policy. It has been established inquiry and research and caused many engi- that the scientific method produced significant neers to question the established theory. After advancements as used by the pioneers in this a few years of reflection, discussion, and study, field. Without the efforts of these engineers and new conclusions and theories were reached in researchers, structures might still be designed each case. using a process akin to trial and error. This While building codes (public policy) were example should also serve as a reminder of revised and expanded, there were parallel why experts should be at the head of the table scientific method efforts by many dedicated when public policy decisions are made, ensurstructural engineers, researchers, and govern- ing the basis of those policies will not ment officials to “push the envelope” and be observational data masquerading advance the state of knowledge in earthquake as science, but the real thing.■ engineering. Their contributions and conJohn A. Dal Pino is a Principal with FTF clusions are presented in these important, Engineering located in San Francisco, California. groundbreaking documents: He serves as the Chair of the STRUCTURE ATC-3 Tentative Provisions for the Editorial Board. (jdalpino@ftfengineering.com) Development of Seismic Regulations for D E C E M B E R 2 0 21

EARTH RETENTION guide ENERCALC, Inc.

ENERCALC

Phone: 800-424-2252 Email: info@enercalc.com Web: https://enercalc.com Product: ENERCALC - The Structural Engineering Library Description: NEW! RetainPro 7's wall analysis/design modules are now integrated into the ENERCALC Structural Engineering Library (SEL). SEL's earth retention structure modules provide analysis/design tools for nearly any retaining wall: concrete, masonry, or combinations. Supports cantilevered, restrained, soldier pile, gravity, tapered, gabion, and segmental walls (NCMA 3rd edition) with/ without geogrids (MSE).

Gripple Inc.

Phone: 630-406-0600 Email: grippleinc@gripple.com Web: www.gripple.com Product: Terra-Lock Description: Unique plate and anchor kit to secure a variety of TRM and HPTRM erosion control and slope stabilization matting. Proprietary design, easy to install, and open plate promotes vegetation regrowth.

Listings are provided as a courtesy, STRUCTURE is not responsible for errors.

IES, Inc.

Phone: 800-707-0816 Email: info@iesweb.com Web: www.iesweb.com Product: IES VisualFoundation Description: You need rock solid foundation design. IES offers easy structural software, like VisualFoundation. Quickly model mat slabs, pile caps, and grade beams. Get stability checks, handle punching shear, and make smart codecompliant designs. Try it free, watch videos, find product benefits and pricing at website.

Product: IES QuickRWall Description: Design retaining walls? Try QuickRWall and generate 16 pages of hand calculations and sketches of your design. You simply specify the wall, soil conditions, and other loads. Get instant feedback on pass/fail status, see details of all checks. Start making money on your designs with QuickRWall from IES.

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STRUCTUREmag.org

RISA Tech, Inc.

Phone: 949-951-5815 Email: benf@risa.com Web: risa.com Product: RISAFoundation Description: The ultimate tool for analysis and design of a variety of different foundation types. Featuring an open modeling environment, finite element analysis, and full integration with superstructure analysis programs. You won't find a better choice for retaining walls, spread footings, combined footings, mat slabs, or pile cap design.

Williams Form Engineering Corp.

Phone: 616-866-0815 Email: williams@williamsform.com Web: www.williamsform.com Product: Anchor Systems Description: Williams Form Engineering Corporation has been providing threaded steel bars and accessories for rock anchors, soil anchors, high capacity concrete anchors, micropiles, tie rods, tiebacks, strand anchors, hollow bar anchors, post tensioning systems, and concrete forming hardware systems in the construction industry for over 95 years.

How is your 2022 marketing planning progressing?

Do your goals include: • Reach structural engineers with your marketing messages.

• Build brand recognition and trust amidst the leading structural design professionals.

• Generate leads to grow the sales pipeline.

• Be recognized as the go-to resource for product and industry information.

If you answered yes to any of these, then STRUCTURE can help you succeed!

Check out the 2022 STRUCTURE Media Kit online at STRUCTUREmag.org for Sales, Marketing, & Business Development Monica Shripka Director monica.shripka@STRUCTUREmag.org | Office: 773-974-6561 50 STRUCTURE magazine

SPOTLIGHT A New Link in Calgary’s Covered Walkway System

707

Fifth – Manulife Place camber before installing the cladding to utilized to accommodate thermal movements Pedestrian Bridge is an essen- minimize impacts on the cladding due to in service while improving the durability of the tial link in Calgary’s elevated network of movement. The sensitivity of the structure supports. Additionally, as the bridge straddles pedestrian pathways. The bridge improves to temperature differentials required a careful an active service alleyway, the bridge bents are connectivity within the downtown core, calibration of forces between the stainless- secured by post-tensioned anchor rods to reinadding a new connection from 707 Fifth steel cables and carbon steel frames to attain forced concrete pedestals designed to sustain a – Manulife Place to Calgary’s +15 covered a reasonable camber profile. potential vehicular impact. walkway system, creating a Due to the limited areas in unique architectural expresthe downtown core that could sion in downtown Calgary. It serve as a staging area, the bridge was a challenge that required components were fabricated innovative solutions: providoff-site, then assembled in the ing a seamless connection to adjacent parking lot and lifted adjacent buildings while maninto place on the weekend, aging subgrade conditions, requiring a road closure of only installation sequences, and two weekend days. The presence non-structural coordination. of various utilities in the alleyWith a total length of 270 feet way as well as the proximity of 10 inches, the bridge features an adjacent buildings – including elegant structural system known a landmarked theater – necesas a suspended lenticular truss, sitated a different configuration spanning 96 feet 5 inches. for each above-grade support The bridge consists of 5 spans: and foundation element for the two end cantilever spans of bridge, in some cases accom27 feet 10inches and 28 feet modating for future structures Skidmore, Owings & Merrill was an Award Winner for the 707 2 inches, two side Vierendeel from proposed developments.

Fifth – Manulife Place Pedestrian Bridge project in the 2020 Annual spans of 55 feet 9 inches and 62 Due to the lack of secondary feet 8 inches, and a central sig- Excellence in Structural Engineering Awards Program in the Category mullions, all connections are nature span of 96 feet 5 inches. exposed and therefore require – New Bridges or Transportation Structures.

The structure is supported by close coordination between the pot bearings attached to steel bents roughly The central “suspended lenticular truss” span structural engineers, architects, steel fabrica18 feet tall above grade that straddle the adja- includes tension cables and compression arches tors, and contractors. Given that most major cent service alleyway. The simple and clean whose geometries follow the moment diagram structural elements of the bridge are located architectural enclosure allows the structure under self-weight. The cables have been pre- at eye level with the pedestrian user, special to become a prominent visual feature of tensioned to minimize loads on the arch and attention was paid to the appearances of welds the design. provide camber for the system, resulting in during the fabrication and assembly processes.

The challenge of providing a seamless con- smaller member sizes. An internal stressing In collaboration with cable supplier Pfeifer, nection to adjacent buildings while managing method of cable tensioning was undertaken as the cable fittings and clamps were customsubgrade conditions, installation sequences, part of a careful erection sequence where the arch ized to fit the design aesthetic of the bridge. and non-structural coordination, required members are installed after tensioning but prior The vertical struts feature a double-cruciform innovative structural solutions to successfully to the concreting of the floor slab. The top and shape that provides adequate stiffness while deliver the bridge. bottom chords are designed to be continuous creating visual interest.

The central signature span is an elegant struc- to control higher mode walking vibrations. The By agreeing to construct the bridge, the tural system known as a “suspended lenticular architectural cladding systems were designed to client obtained additional leeway through truss.” Cable tensioning was undertaken as attach directly to the structure and eliminate the Federal Acquisition Regulations (FAR) part of a careful sequence where the arch secondary mullions. And, to minimize impacts to provide higher floors in the main office members are installed after tensioning but on the cladding due to movement, the bridge tower of the project, offering clear views of prior to the concreting of the floor slab. floor was pre-loaded with a weight equivalent to the Canadian Rockies to the west. The bridge The architectural cladding systems were the floor finishes to take out additional camber is also an essential element in connecting designed to attach directly to the structure before installing the cladding. the development to existing as well as future and eliminate secondary mullions, requirInstallation during sub-freezing temperatures developments, providing benefits to both the ing all steel to be fabricated to stringent could result in a higher camber than when public and building tenants. In particular, the Architecturally Exposed Structural Steel installed during warm weather due to the bridge integrates a previously iso(AESS) 4 finishing specifications. The bridge different expansion coefficients of the cables lated portion of the +15 network to floor was pre-loaded to take out additional versus steel frames. Pot bearings were also the rest of the functioning network.■ STRUCTURE magazine

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NCSEA

NCSEA News

National Council of Structural Engineers Associations

Structural Engineering Summit – Feel the Love this February

The NCSEA Structural Engineering Summit will be in New York City February 14-17 and online January 31-Feburary 24. The Summit offers unrivaled educational opportunities, an industry-leading trade show, and unique and fun networking opportunities.

Register for the conference, learn more, and book your hotel room ($219/night at the Hilton Midtown!) www.ncsea.com/events/annualconference Seek inspiration from keynote speakers.

20 Years After 9/11: Lessons Learned and More to Research • Panelists: Dan Eschenasy, P.E., SECB, F.SEI; James W. Feuerborn Jr., P.E.; Therese McAllister, F.SEI • Moderator: Vicki Arbitrio, P.E., SECB, F.SEI • 2021 marked the 20th anniversary of 9/11 and this keynote will feature panelists who were involved in the initial recovery efforts and with the subsequent building performance study. They will discuss the following topics: response of the engineering community, building performance, structural design and fire protection; emergency response legal issues and proposed legislation; and building code changes and recommendations.

Lighten Up & Lead: Leverage Levity to Boost Client Confidence & Employee Engagement • Speaker: Tami Evans • A happy workplace is imperative to success, but how do you remain positive AND professional? Laugh and learn key characteristics to create levity in the worker, workforce, and workplace with tools that help you reach your personal and professional potential through the power of fun. Connection creates culture, and culture creates employee engagement – and the fastest way to connect is through levity. Engagement empowers employees to collaborate through challenges, create innovative solutions, and communicate confidently – not to mention bump your bottom line!

Learn from industry experts on a diverse range of topics, all focused on helping structural engineers succeed. Below is just a sample of the top-notch presentations (31 sessions in total!). The full educational slate is at www.ncsea.com/events/annualconference.

• Wind Loads: Frequently Asked Questions Speakers: Emily Guglielmo, S.E., and Donald Scott, P.E., S.E. • Ask Me Anything – Roundtable with Structural Engineering Principals Moderated by: Francesca Ferrero, P.E. • SE2050: Net Zero Embodied Carbon in Structures Speakers: Megan Stringer, P.E., and Michael Gryniuk, P.E. • Flexible or Rigid? Rethinking Workplace Flexibility Moderated by: Marcus Freeman, P.E.

• Ductile Coupled Reinforced Concrete Shear Walls as Distinct Seismic Force-Resisting Systems S.K. Ghosh, Ph.D. • Leveraging BIM & Technology for Structural Design Mary Shinners, P.E., S.E., and Sarah Scarborough, P.E., S.E. • Historic Materials and Archaic Structural Systems in Renovations Donald Friedman, P.E.

And last, but certainly not least, network with the best in the industry.

• Share your love for structural engineering at the Welcome Party in the Trade Show on Tuesday evening. Come for the food and drinks, and stay for the networking, fun times, and interactive activities!

• Walk the Trade Show floor to connect with industry suppliers; gain valuable information on materials, products, and software innovations; and grab some swag.

• Enjoy lunches and coffee breaks in the Trade Show, and catch up with colleagues, new friends, and industry veterans.

• Celebrate innovation and creativity with the Excellence in Structural Engineering Award winners, and applaud outstanding service and commitment with the Special Award honorees – all at the Awards Celebration event on Wednesday evening.

• Take your networking experience virtual, and enjoy online interactive sessions and a virtual Trade Show.

Reconnecting with your peers, learning from industry experts, networking with product experts at the trade show – what’s not to love! Mark your calendar now and don’t miss the leading event in the structural engineering profession. Visit www.ncsea.com/events/annualconference for more info and to register.

52 STRUCTURE magazine

THANK YOU TO OUR SPONSORS

News from the National Council of Structural Engineers Associations

Excavation Shoring Design Guide and Recorded Webinar Bundle The Perfect Holiday Gift for your Engineering Colleagues

Reviewing a pertinent worked example bolsters any structural engineer’s confidence. The new Excavation Shoring Design Guide is comprised solely of such detailed work examples and provides the real construction details that are necessary to round out complete designs including site characterization and earth pressure diagrams, soil/grout bond strengths, and deflection curves.

The corresponding recorded webinar series (3 PDH’s available) addresses the design and construction aspects of temporary and permanent earth retention systems. Attendees will review the different elements of each earth retention system and when each method is appropriate. Besides sizing the shoring system’s principal members, the series will provide basic construction details necessary to round out complete designs.

Purchase at www.ncsea.com/education. The recorded webinar series and a copy of the digital publication is just $295 (members) / $500 (nonmembers). The digital publication on its own is $119 (members) / $219 (nonmembers) or order the paperback for $169 (members) / $269 (nonmembers).

NCSEA Webinars

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

Now is the time to consider an NCSEA webinar subscription!

First time subscribers receive $100 off their subscription price with discount code SAVE100. Offer expires January 7.

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; and at an affordable price of $1,195 (member) / $1,695 (nonmembers), less than $40/webinar for members!

NCSEA Committee Volunteers Needed – and Appreciated Are you interested in serving on an NCSEA committee to give back to your profession and better the industry? The success of NCSEA and of the structural engineering industry depends on its members to get involved and help advance the mission. Our volunteers help educate on codes and standards, develop courses and publications, advocate for safe structures and post-disaster recovery, promote the structural engineering profession, and more. If you are a volunteer interested in serving on an NCSEA committee, please visit www.ncsea.com/committees to learn more about the various committees and complete the volunteer application. Most committees admit new members on a rolling basis while others add members once a year.

And on that note, from all of us here at NCSEA in this season of gratitude – thank you! NCSEA is fortunate and grateful to have such wonderful committee and board volunteers. Thank you to the entire structural engineering community for your engagement, volunteer hours, and financial contributions that help NCSEA, and the structural engineering industry overall, remain strong and successful. Season’s greetings and happy New Year!

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

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SEI Update Advancing the Profession

What You Need to Know about Supplements 1 and 3 to ASCE 7-16

The 2016 edition of ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures (ASCE 7-16), has three published supplements. Supplement 1 was published on December 11, 2018, and was adopted into the 2021 International Building Code. Supplements 2 and Supplement 3 have been recently published, and all three are available in the ASCE Library as free downloads.

Supplements 1 and 3 work together to address shortcomings in the 2016 standard and should be understood and used by engineers and building officials alike.

Technical Details – Research from USGS and the Applied Technology Council has shown that the use of only two response periods (0.2s and 1.0s) to define the Equivalent Lateral Force (and Modal Response Spectrum Analysis) design forces is reasonably accurate when the peak MCER response spectral acceleration occurs at or near 0.2s and peak MCER response spectral velocity occurs at or near 1.0s for the site of interest. However, the two-point spectrum is potentially non-conservative when the peak MCER response spectral velocity occurs at periods greater than 1.0s, particularly for structures on softer soil sites where large magnitude events dominate the seismic hazard.

As a result, for high seismic hazard locations located on Site Class D and E sites, ASCE 7-16 requires a site-specific ground motion hazard analysis be performed to determine the design response spectrum, including spectral acceleration parameters (SDS and SD1). ASCE 7-16 provides exceptions to requiring a ground motion hazard analysis but lacks sufficient direction to be clearly implemented. Supplement 1 was developed to clarify the requirement; however, the following two issues were uncovered subsequent to the approval of Supplement 1: (1) For Site Class D sites, Exception 2 does not clearly identify the design spectrum that should be used for a modal response spectrum analysis. The intent of the exception is to use a spectrum that includes a 1.5 multiplier on SD1, but the language does not explicitly require this. This oversight can result in long period structures, including tanks, to be under-designed by a factor of up to 1.5. This oversight also leads to underestimating impulsive and convective forces in liquid storage tanks located on Site Class D sites covered by Chapter 15.

(2) For Site Class E sites, to properly utilize Exception 3, a value of Fa for locations where SS is greater than or equal to 1.0 is required.

However, the language in the exception does not provide a value for Fa, nor does Table 11.4-1. This oversight leaves it up to the engineer to select a value without appropriate direction. This oversight also leads to underestimating impulsive and convective forces in liquid storage tanks located on Site Class E sites covered by Chapter 15.

Supplement 3 was developed to correct these issues. Also, Supplement 3 reorganizes the exceptions to simplify the requirements.

Supplements are developed by the ASCE 7 Standard Committee to address important issues in between cycles of development. Supplements 1 and 3 and all other Supplements and Errata to ASCE 7-16 and earlier editions are available at www.asce.org/seiasce7.

John Hooper, P.E., S.E., F.SEI, Dist.M.ASCE, is the chair and Emily Guglielmo, P.E., S.E., F.SEI, M.ASCE, is the vice-chair of the ASCE 7-22 Seismic Subcommittee.

Remember to Support the Future of Structural Engineering with your Year-End Giving Learn more and give www.asce.org/SEIFuturesFund.

Futures Fund Investing in the Future of Our Profession

Follow SEI on Social Media: 54 STRUCTURE magazine

News of the Structural Engineering Institute of ASCE Learning / Networking

Join Us at SEI Events www.asce.org/SEIEvents

The new 2022 SEI Standards Series will preview ASCE 7-22 as a 5-part series that reviews the changes from ASCE 7-16. This unique program includes a dialogue between the leaders and experts who develop ASCE 7 and a detailed technical presentation on the specific changes and three main hazards – Seismic, Wind and Tornado, and Snow/Rain. In addition, information will be provided on the ASCE 7 Digital Products/Hazard Tool. Attendees are encouraged to join the discussion for the extensive live Q&A portion of the session.

• February 10, 2022: ASCE 7-22 Overview & Changes (FREE) • May 12, 2022: ASCE 7-22 Seismic • June 9, 2022: ASCE 7-22 Wind & Tornado • July 14, 2022: ASCE 7-22 Snow/Rain • September 8, 2022: How & Why to Use ASCE 7-22 in Your Practice Learn more at: https://collaborate.asce.org/integratedstructures/sei-standards

Electrical Transmission and Substation Structures Conference October 2-6, 2022, in Orlando

Apply for a student scholarship to participate www.etsconference.org

GET PAID WHAT YOU’RE

WORTH www.asce.org/salaries

Trust the ASCE Member Insurance Program.

ASCEPLANS.COM/SURVEY www.asce.org/freepdh *A Professional Development Hour (PDH) is one contact hour of instruction or presentation. More than 75 percent of U.S. registration boards require continuing education for P.E. license renewal. Visit each registration board’s website to confirm its continuing education requirements. You are required to pass an exam on the webinar’s content to receive a PDH.

Errata

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI. To submit errata, contact sei@asce.org.

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CASE in Point CASE Tools and Resources Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. So whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

CASE has several tools available for firms to use to enhance their own internal policies and procedures – from office policy guides to employee reviews. All Tools are free to Coalition Members!

Tool 1-1 Tool 1-2 Tool 1-3 Tool 1-4

Create a Culture for Managing Risks and Preventing Claims Developing a Culture of Quality Sample Policy Guide Creating a Culture of Recruitment and Retention NEW!

Tool 2-2 Tool 2-3 Tool 2-4 Tool 2-5 Tool 2-6 Tool 3-2 Tool 3-5 Tool 3-6

Interview Guide and Template Employee Evaluation Templates The Risk Management Plan Insurance Management Structural Engineering Job Descriptions Staffing and Revenue Projection Staffing Schedule Suite Carrier Path Planning NEW!

Tool 4-3 Sample Correspondence Guidelines Tool 5-2 Tool 5-3 Tool 5-5 Tool 5-6

Milestone Checklist for Young Engineers Managing the Use of Computers and Software Project Management Training Lessons Learned You can purchase these at www.acec.org/bookstore.

DONATE to the CASE Scholarship Fund!

The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at minimum, in an ABET-accredited engineering program. Since 2009, the CASE Scholarship program has given $37,000 to help engineering students pave their way to a bright future in structural engineering.

We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates. One way to enhance the ability of students to pursue their dreams to become professional engineers is to offer incentives in educational support.

Your monetary support is vital in helping CASE and ACEC increase scholarships to those students who are the future of our industry. All donations toward the program may be eligible for a tax deduction, and you don’t have to be an ACEC member to donate! Contact Michelle Kroeger at mkroeger@acec.org to donate.

56 STRUCTURE magazine

News of the Coalition of American Structural Engineers NEW – 2022 ACEC Winter Meeting San Diego, CA, February 10-11, 2022

The Winter Meeting is open to all CASE members. The agenda for the meeting is found below:

Thursday – February 10 1:30 pm – 3:00 pm

CASE ExCom Meeting

3:30 pm – 5:00 pm

CASE and CAMEE Roundtable, Moderated by Brent White and John Burns

5:00 pm – 6:00 pm

Coalitions Reception

Friday – February 11 8:30 am – 12:00 pm

Educations Sessions – (PDH’s offered) • Remote Monitoring using today’s technologies – How new technologies are changing the way engineers work.

• Navigating the challenges of distance working – Managing legal, financial, and human resource activities for out-of-state employees.

12:00 pm – 1:15 pm

Lunch

1:30 pm – 5:00 pm

Small Firm Coalition Workshop ($$ Paid Event) – (PDH’s offered) • Small Firms and Human Resources – The workforce for the future.

(This workshop continues Saturday morning, February 12, 2021.)

Registration opens at the beginning of December.

To register, go to the 2022 Coalitions Winter Meeting website: http://bit.do/winter2022 Questions? Contact Michelle Kroeger at mkroeger@acec.org.

Structural Engineers Weigh-in!

The ACEC Fall Conference was held from October 28-30, and CASE hosted a structural engineering roundtable that convened people from various firms around the country. Participants discussed trending issues, including workforce challenges, supply chain issues, and licensure.

In addition to the roundtable, CASE hosted three widely attended education sessions, Emerging A&E Risks and Market Realities (speaker Dan Buelow, Willis A&E); Risk Management Challenges with a Virtual Workforce (presented

by Karen Erger and Kevin Holland, Lockton Companies); and Risky Business?

Building Assessments (presented by Mark Baum, Greyling Insurance Brokerage & Risk Consulting).

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

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CASE business practices Coordination and Completeness of Structural Construction Documents

By Jeff Morrison

A

s structural engineers, at the end of the design phase, our work product consists of the construction documents (drawings and specifications), which detail the requirements to construct a given structure to serve an intended purpose. The degree to which the project will be considered a success is directly related to the quality of the construction documents. The documents must be complete, coordinated, clear, concise, and constructible. Poor quality construction documents lead to significantly more time, frustration, and potential liability when issues need to be corrected during shop drawings or, worse, during or even after construction. Building structures today are more complicated than ever, and a successful project involves proactive management of both the technical and administrative sides of the project.

crucial to get every project off on the right foot. Major decisions such as determining the appropriate structural system, lateral load resisting system, typical details and wall sections, foundation system, and coordination with major architectural or MEP systems are critical. If these items wait until too late in the project design phase, we are often left with less-than-optimal structural options.

Therefore, we should provide valuable input early in the project when it can be most useful.

In addition to discussing and reviewing these items with the external project team, having regular meetings with our internal design team, including the engineering staff, BIM staff, and QA reviewers, ensures that all members of the team have the same information and can offer valuable input early in the project and at regular intervals.

The intent was…

Details matter…

In life and structural engineering, communication is essential in making sure we are understood, and we understand others.

Without good communication amongst the design team and between the design team, owner, and contractor, you will not have a successful project. The construction documents are the primary means of communicating with the contractor. It is essential always to keep this in mind and make sure the drawings provide the information the contractor will need to successfully construct the building as we intended.

We must also be proactive in communicating with other design team members the information we need to produce a quality set of structural documents within the project schedule. Requesting this information in writing keeps the project organized and tracks outstanding issues or items awaiting resolution. In today’s fast-track projects with early structural packages, this becomes even more critical. In addition, it is important to have a clear understanding of the intended use of any early structural submittal packages to verify that our drawings are adequately developed and complete for their intended use.

This is the hard part. After the initial excitement of the new project has worn off, the structural details need to be developed, optimized, and coordinated with the architectural and other consultants’ scope of work. The everyday task of advancing the project’s design and details must remain in step with the entire design team. Developing details that are as simple as possible and constructible, without issues, will often prove to be better than designing every unique condition. For example, how many unique column base plate details should I have, can I have the same detail for supporting cladding at every level, can the edge of slab dimensions be simplified. A quality assurance/ quality control (QA/QC) plan consistently implemented on all projects is necessary for any firm. Utilizing a checklist to review the level of coordination and completeness at each major milestone submittal is strongly recommended.

Off on the right foot…

It is well-known that the early stage of a project is the most effective time to influence project cost beneficially. As such, it is 58 STRUCTURE magazine

Back to the drawing board…

Changes invariably occur on most projects throughout the design phase and often even during construction. Projects that require re-design due to budget, owner changes, or other reasons often put time pressure on coordinating these revisions and making sure the ripple effect of these changes is appropriately accounted for throughout the documents.

These changes must be carefully documented in the same way as the original drawings were produced. Oftentimes, unforeseen issues arise if changes are made hastily and without thorough coordination.

It looked good on paper…

Now we have to build it. The construction phase provides valuable insight into the actual quality of the construction documents.

Some examples are the number of Requests for Information (RFI’s), the extent of questions a detailer has on the shop drawings, and how well the structure integrates within the architectural finishes and other trades.

Take note of any issues during construction and use these as lessons learned to apply to future projects.

Summary Producing a quality set of structural construction documents is as much of an art as a science. It requires the structural engineer to have a good understanding of structural engineering issues such as load path, structural analysis, design, and detailing. It also results in an awareness of how buildings are constructed with consideration of construction methods, tolerances, and how to develop the most efficient details. This is a learned skill that takes time to develop, and the process dramatically benefits from involving senior-level engineers who can share their past experiences.

Sharing experiences and regularly discussing the importance of producing highquality construction documents with all staff will prove valuable.■

Resources The following resources are valuable tools for more in-depth learning and development: CASE 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction Documents (Updated 2020) CASE Tool 9-1: Coordination and Completeness of Structural Construction Documents (Updated 2021) CASE Tool 9-2 Quality Assurance Plan Jeff Morrison is Vice President and Senior Project Manager with Lynch Mykins in Raleigh, NC, and a CASE Toolkit Committee member. (jmorrison@lynchmykins.com) D E C E M B E R 2 0 21

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