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WIND/ SEISMIC INSIDE: Sideyard Seismic Strengthening Frequently Asked Seismic Questions Airport Drive-Through Canopy
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STRUCTURE ® magazine (ISSN 1536 4283) is published monthly by The National Council of Structural Engineers Associations (a nonprofit Association), 20 N. Wacker Drive, Suite 750, Chicago, IL 60606 312.649.4600. Application to Mail at Periodicals Postage Prices is Pending at Chicago, IL and additional mailing offices. STRUCTURE magazine, Volume 28, Number 3, © 2021 by The National Council of Structural Engineers Associations, all rights reserved. Subscription services, back issues and subscription information tel: 312-649-4600, or write to STRUCTURE magazine Circulation, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.The publication is distributed to members of The National Council of Structural Engineers Associations through a resolution to its bylaws, and to members of CASE and SEI paid by each organization as nominal price subscription for its members as a benefit of their membership. Yearly Subscription in USA $75; $40 For Students; Canada $90; $60 for Canadian Students; Foreign $135, $90 for foreign students. Editorial Office: Send editorial mail to: STRUCTURE magazine, Attn: Editorial, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. POSTMASTER: Send Address changes to STRUCTURE magazine, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. STRUCTURE is a registered trademark of the National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.
Contents M ARCH 2021
By Jason Thompson, P.E., S.E.
Sideyard is a five-story-tall and slightly wedge-shaped structure showcasing Oregon-sourced mass timber construction. It serves as a case study of how the structural engineering community can navigate the design and documentation of such projects in the future.
ROCKY MOUNTAIN METROPOLITAN AIRPORT DRIVE-THROUGH CANOPY By Todd Robbins, P.E., SECB, and Jason McCool, P.E.
Designing a drive-through canopy may not sound very exciting to most engineers. Finding out it is a drive-through at a high-altitude airport with hurricane-force wind speeds can quickly change that first impression. The clear-span arched trusses supporting the 40-foot-high aircraft canopy were the most prominent part of this unique project.
Columns and Departments 7 Editorial Learning and Leading into the Future
30 Technology Communicating in a BIM World
By Joseph G. DiPompeo, P.E.
8 Structural Economics The Economics of Seismic
Strengthening 12 Just
By Dr. Kristopher Dane and Mike Bolduc, P.E.
32 CASE Business Practices Do You Have a Force
Majeure Clause in Your Engineering Services Contract?
By Terrence Paret et al.
the FAQs Frequently Asked Seismic Questions
By Emily Guglielmo, P.E., C.E.
By Bruce Burt, P.E.
42 Business Practices Opening a Branch Office
16 Structural Forensics Investigative Peer Reviews By Cole Graveen, P.E., S.E.
25 Construction Issues Building Faster and Better By Ashley P. Thrall, Ph.D., and Angelene Dascanio, MEng Sc, MS
28 Structural Practices The Basics of the 5% Rule By Kirk Wagner, S.E.
March 2021 Bonus Content Feature Clemson’s Mass Timber Outdoor Education Center Spotlight Resiliency for Affordable Housing
By Stephen Lehigh, P.E.
In Every Issue Advertiser Index Resource Guide – Software Updates NCSEA News SEI Update CASE in Point
Additional Content Available Only at – STRUCTUREmag.org By David Impson, P.E., S.E., Andrew Ruffin, P.E., S.E., and Brian Haygood, P.E.
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. M A R C H 2 0 21
EDITORIAL Learning and Leading into the Future By Joseph G. DiPompeo, P.E., F.SEI, F.ASCE
ow! COVID-19 and the challenges of 2020 have come out for an encore at the start of 2021. (I am writing this in January for publication in March.) Hopefully, this will be the only encore, and we will soon realize widespread vaccination against COVID-19 for movement toward a new normal for our businesses and livelihoods. Throughout this last year, we have seen stunning examples of leadership, both good and bad, including our leaders at all levels of government – local, state, national – as they struggled with the crises of 2020. We have seen countless problems related to the pandemic, economy, civil unrest, business disruption, and people unable to work. So, where am I going with this in a Structural Engineering magazine? Remember all those math and science problems, all those classes you took that did not have much to do with anything, but you spent hours solving math, science, and logic problems? After 4-7 years of that and practicing as a structural engineer, I will assume that almost everyone reading this is an experienced and trained problem solver. That is what we do. We build things, we fix things, we solve problems. While this is a challenging time for all of us, it is a great time to be a problem solver and a Civil/Structural Engineer. We have already shined during the COVID crisis. The widely used and referenced COVID-19 map produced by Johns Hopkins University Department of Civil and Systems Engineering has played an invaluable role in tracking the COVID-19 pandemic. The world is awash in problems that need solving. I encourage all of you to brush off your leadership and problem-solving skills, get out of your cubicles, and help! Be leaders at your jobs and in your careers. Get involved in your community. Join local boards and commissions where you can put your talents to use. Volunteer. Let the world know you are a structural engineer, and show your community what you can do. Developing structural engineers as leaders is a key strategic vision initiative and one of the goals of my year as SEI President. While many things have not gone as planned for my term, this goal remains. SEI is actively working on several initiatives to develop leadership and soft skills among structural engineers. While the announcement and rollout of some of these programs has been delayed by the past year’s events, we are working to bring them to you as soon as possible. In the meantime, I urge you to use the skills you already have to do something greater than yourself or your career and show the world what we can do! In addition to leadership development, SEI is also reviewing our organizational structure to optimize it to best serve and advance structural engineering. SEI is 25 years old, and we are still operating under the system set up at our founding. With the growth of the past 25 years and STRUCTURE magazine
the significant changes and challenges of the past year, many trends that were already happening were tremendously accelerated by the pandemic and the shift to virtual. We live and work in a vastly different world and in vastly different ways than 25 years ago and are exploring how best to position SEI for the next 25 years and beyond. Civil and Structural Engineers also have a tremendous opportunity to contribute to fixing and rebuilding our infrastructure. While we are good at solving all kinds of problems, infrastructure is literally what we do. With a changing political climate on infrastructure spending and climate issues, it is looking like we will be in great demand. Let’s get out there While this is a and build and fix things and let the world know we are challenging time the ones doing it! The events of 2020 also for all of us, it is a sparked a discussion on diversity within our progreat time to be a fession. I heard stories of things that I did not think problem solver and happened anymore from our members. We have a a Civil/ Structural new Board-level Committee Engineer. on Diversity, Equity, and Inclusion with a passionate leader and team. We also have a newly formed Boardlevel committee on Resilience. This is also an energized group, and I am confident both will make great strides. Many thanks to SEI’s past president Glenn Bell and other leaders for their work on Confidential Reporting on Structural Safety – CROSS-US. This is the U.S. version of a program that was started in the UK and has grown to Australasia, Southern Africa, and Germany (coming soon). CROSS-US is a confidential database sharing failures, near misses, and incidents to learn from and not repeat them, improving structural safety. Submit and review reports at www.cross-us.org. We can all learn from each other and make a better, safer future. I invite you to share what we can do to make your membership more valuable to you. Please email me or SEI Managing Director, Laura Champion at email@example.com with any thoughts or ideas you have. Lastly, I also encourage you to give to the SEI Futures Fund to support programs that invest in the future of structural engineering www.asce.org/SEIFuturesFund. I am honored to be SEI President this year. While my term will look very different than expected, we have much to do to solve problems and learn and lead into the future. I will do my best to keep moving SEI forward through the aftermath of 2020 and a hopefully much brighter and better 2021!■ Joe DiPompeo is current SEI President and President of Structural Workshop LLC in Mountain Lakes, NJ.
M A R C H 2 0 21
structural ECONOMICS The Economics of Seismic Strengthening Reconsidering Costs in Areas of Lower Seismicity
By Terrence Paret, Gwenyth Searer, P.E., S.E., Kari Klaboe, P.E., S.E., and Hayley Proctor, P.E.
he economics of reducing seismic risk has generally received less attention in regions of lower seismicity than in higherrisk regions of the country. Though less heralded, the subject is important: what investment in risk reduction is appropriate to the extant risk? Three relevant subtopics are explored in this article. First, the code methodology for calculating design seismic forces results in geographically variable "effective return periods" across the U.S., causing design forces in lower seismicity areas to be based on rarer events than in regions with greater seismicity. Second, FEMA's annualized economic loss estimates demonstrate that community-wide seismic improvement generally does not “pay for itself.” Third, empirical data from the 2011 Mineral, Virginia, earthquake – which subjected a considerable inventory of precode and non-compliant buildings to design-level shaking – demonstrates the seismic adequacy of the “as-is” building inventory in low and low-moderate seismicity areas.
Figure 1. Example derivation of effective return period for Charleston, South Carolina.
Effective Return Period Before the International Building Code (IBC) and ASCE 7-98, Minimum Design Loads for Buildings and Other Structures, the design earthquake across the U.S. was based on a mapped probability of exceedance of 10 percent in 50 years. Newer codes and standards instead use ground motions having a probability of exceedance of 2 percent in 50 years, multiplied by a uniform scale factor of 2⁄3 to derive design accelerations (the DBE). Though the scale factor may have maintained parity for California’s seismic design forces before and after the change, the consequences stemming from the introduction of this scale factor are far-reaching. In short, the codified seismic design requirements for major cities across the U.S. now yield haphazard outcomes for the “effective” return period of the design earthquake. Therefore, while the current code articulates seismic hazard for design as a uniform return period across the U.S., the codified design process instead achieves strikingly nonuniform results in terms of effective DBE hazard. Any decision about whether to improve, or to require improvement, of a building or inventory of buildings, and about the appropriate target for that improvement, should account for this. The authors define an “effective return period” as an effective probability of exceedance over a specific duration of time. To illustrate the concept, effective return periods for nine locations in the U.S., representing areas of low, moderate, and high seismicity, were derived. These effective return periods were derived using hazard curves obtained from the USGS Unified Hazard Tool for each location, adjusted to obtain maximum response values. The procedure for deriving the effective return period is graphical. As illustrated in Figure 1, the hazard curve was plotted, the 2,475-year return period values (i.e., MCE event) were multiplied by 2⁄3 as required by U.S. codes, and the hazard curve was re-entered to obtain the effective return period associated with this reduced value. This operation was applied to the hazard curves for nine cities, as tabulated in Table 1. Values for both the MCE and the MCER STRUCTURE magazine
are instructive because U.S. codes evolved from reliance on the MCE to a “risk-adjusted” MCE, referred to as the MCER, beginning with the adoption of ASCE 7-10. City-to-city variability in Table 1 is striking, demonstrating that designs in lower seismicity areas have longer effective return periods than designs in higher seismicity areas. This means that areas for which seismicity ought to be of less concern are required to design for less frequent, rarer events than areas with more substantial seismic hazards. The data also illustrates that the evolution from the MCE to the MCER did not rectify this discrepancy. The fifth column of Table 1 provides the effective return periods and probabilities of exceedance values for one-half the MCER, a level consistent with the reduced demands of the International Existing Building Code (IEBC) for certain existing structures undergoing alteration or repairs. Again, the values vary widely due to the use of a scalar multiplier on probabilistic events.
Annualized Earthquake Loss The U.S. Federal Emergency Management Agency (FEMA) studied the economic risks posed by earthquakes quantified via two risk indicators: • Annualized Earthquake Loss (AEL): The estimated longterm cost of earthquake damage to the inventory of existing buildings in a specific geographic area (e.g., state or metropolitan area) on a per-year basis (i.e., annualized). • Annualized Earthquake Loss Ratio (AELR): The AEL divided by the replacement value of the building inventory and expressed as a ratio of dollars of damage to dollars of inventory. The top portion of Figure 2 is a FEMA-generated map of the U.S., color-coded to reflect the AELR for each state/territory. The colorcoded scale used is not linear and appears biased toward smaller AELRs, which obscures the relative hazard posed in each area. To better visualize the relative risk, the authors revised FEMA’s map to
Table 1. Effective design return periods and probabilities of exceedance in 50 years.
⁄3 × MCE1
600 (8% in 50yrs)
990 (5% in 50yrs)
380 (12% in 50yrs)
210 (21% in 50yrs)
⁄3 × MCER2
⁄2 × MCER
1200 (4% in 50yrs)
1880 (3% in 50yrs)
730 (7% in 50yrs)
400 (12% in 50yrs)
New Madrid, MO
1600 (3% in 50yrs)
1330 (4% in 50yrs)
730 (7% in 50yrs)
560 (9% in 50yrs)
Salt Lake City, UT
1100 (4% in 50rs)
1790 (3% in 50yrs)
930 (5% in 50yrs)
640 (8% in 50yrs)
1880 (3% in 50yrs)
960 (5% in 50yrs)
650 (7% in 50yrs)
1550 (3% in 50yrs)
1920 (3% in 50yrs)
1070 (5% in 50yrs)
750 (6% in 50yrs)
Las Vegas, NV
2080 (2% in 50yrs)
1120 (4% in 50yrs)
730 (7% in 50yrs)
New York City, NY
2240 (2% in 50yrs)
1270 (4% in 50yrs)
860 (6% in 50yrs)
2200 (2% in 50yrs)
1100 (4% in 50yrs)
680 (7% in 50yrs)
Based on ASCE 7-98 and from Searer et al.; Based on ASCE 7-16; 3Deterministically capped.
reflect AELRs based on a linear color scale with five uniform increments, as shown in the bottom portion of the figure, with the lowest increment of $0 to $220/$1,000,000 representing negligible overall economic risk. The resulting map demonstrates that the vast majority of the geography of the U.S. has a negligible overall economic risk of seismic damage, with only eight states and one territory having an AELR greater than $220/$1,000,000.
AELR and Individual Buildings
Case Study The above AELR data is notably at odds with the common understanding that most older buildings in areas of lower seismicity do not fare well when measured against either the seismic design requirements in the IBC or ASCE 41. However, the 2011 Mineral Earthquake (Virginia) provides a substantial data set with which to resolve the disparity. That event exposed an inventory of buildings presumably numbering in the hundreds of thousands to ground shaking that exceeded design or MCE levels. Thus, it provides ground-truth empirical data to assess the economic reasonableness of pursuing inventory-wide seismic strengthening of existing buildings in areas of lower seismicity. Much of the building inventory that was shaken most strongly was older pre-code, or at least pre-modern-code, unreinforced brick masonry construction. The building
Although AELR values have certain limitations, the hazards posed by earthquakes can also be visualized on a per-building basis. Using AELR values for a given metropolitan area, the authors computed the potential economic exposure for an “average” building, say an office building, with a replacement value of $20 million in the nine cities. It was assumed that an “average” existing property might have about 20 years remaining in its life until it is either demolished or undergoes a significant renovation. A cumulative loss estimate for that period was also computed by multiplying the average annualized loss by this 20-year exposure period, as shown in Table 2, page 10. In Oakland, the cumulative 20-year risk for this “average” office building is $575,000. Since strengthening this hypothetical building could easily cost several million dollars, it likely makes little economic sense to do so unless the building is very likely to perform significantly worse than an average building. In areas of moderate seismicity like Seattle or Salt Lake City, with an average cumulative economic risk of only about $260,000 for that same $20 million office building, it would be challenging to justify seismic strengthening from a purely ‘cost of physical damage’ perspective when an upgrade is likely to cost an order of magnitude more than the cumulative loss. In these areas, perhaps focusing on strengthening only the most vulnerable parts of the most vulnerable buildings makes sense. In areas of low seismicity like New York City and Washington D.C., with a 20-year cumulative exposure of $12,000 or less for the average $20 million office building, it appears unreasonable for seismic strengthening of such buildings to be in the discussion since most seismic strengthening activities – except perhaps strengthening only the most vulnerable parts of the most vulnerable buildings – will never come close to paying for itself. Figure 2. Annualized earthquake loss ratios by state, by FEMA (top) and recolored linearly (bottom). M A R C H 2 0 21
major damage. Two buildings collapsed in the town of Mineral, with minor damage to several other buildings. AELR Average Annualized Average 20-year Location No lives were lost in any of these buildings. While the [$ per Million] Loss Estimate Loss Estimate significance of this damage for the individuals whose Oakland, CA $1,437 $28,740 $574,800 property was affected should not be downplayed, Charleston, SC $977 $19,540 $390,800 the post-earthquake survey results demonstrate that buildings in the community-at-large performed well Seattle, WA $704 $14,080 $281,600 within the intent of the code for new construction. Salt Lake City, UT $633 $12,660 $253,200 This empirical evidence demonstrates the dubiNew Madrid, MO $631 $12,620 $252,400 ous premise of promoting seismic improvement on a community-wide basis in the low-seismicity Memphis, TN $434 $8,680 $173,600 Washington D.C. area – at least if the economLas Vegas, NV $182 $3,640 $72,800 ics of preventing building collapse and loss of life New York, NY $29 $580 $11,600 are the driving considerations. Moreover, given the excellent performance of the inventory of older buildWashington D.C. $8 $160 $3,200 ings shaken by MCE-level motions during Mineral inventory was also representative of construction along the eastern (buildings for which seismic forces were never explicitly considered seaboard for which the USGS postulates a design shaking intensity during design), it is unreasonable to conclude that similar inventosimilar to what was experienced in many areas shaken by Mineral. ries of buildings in similar seismic exposures would be seismically For example, Figure 3 depicts measured and calculated ground inadequate or pose a substantial threat to occupants. If seismic motion spectra from various locations in and around Washington strengthening has merit in these areas, it is primarily with respect D.C. during Mineral, as well as the USGS MCER spectrum for to unique heritage structures. For more typical buildings, the focus the National Mall in Washington, roughly 130 kilometers from should be on mitigating demonstrable risks such as unreinforced the Mineral epicenter. It indicates that low rise buildings – which masonry chimneys, ornaments, and veneer rather than wholesale would include the vast majority of buildings in the Washington D.C. upgrading. Of course, likely collapse in an MCE or lesser event that metropolitan area – likely experienced ground shaking exceeding is identified during the assessment of an individual building should the DBE, in some cases by significant margins. While significant, be addressed. The economic arguments presented by the authors potentially life-endangering damage occurred primarily to façade in this article apply to building inventories such as those described ornaments and appendages of a very limited number of unique pre- that have demonstrated an ability to withstand a design event or code, heritage masonry structures in Washington, D.C., little or no greater with minor damage. damage occurred to the tens of thousands of older masonry bearing wall residential and commercial buildings. Where damage was noted, Conclusion it was typically limited to sporadic damage to unreinforced masonry chimneys, veneer, and parapets rather than to primary structural Geographically-variable effective return periods that result from codecomponents. No lives were lost as a result of the earthquake, and the required scaling of the 2,475-year earthquake counterintuitively require dollar value of the earthquake-caused damage has been estimated design for much rarer earthquakes in lower seismicity areas than in areas to total only about $200 to $300 million. of higher seismicity. Return periods should be chosen that make sense for Even in the epicentral region, post-earthquake damage survey the project rather than using a defined percentage of the requirement for results found that heavy damage was sporadic, with up to seven new construction. Both economics and risk are necessary considerations houses destroyed and major damage to 120 houses. Beyond 1.8 miles in strengthening decisions. Understanding that FEMA’s AELRs apply to (3 km) from the epicenter, the damage distribution had moderated large inventories of buildings rather than to individual ones, the AELRs such that, while many residences had some damage, few residences had nevertheless indicate that earthquakes pose little to no risk of costly earthquake damage across most of the U.S. Evidence from the 2011 Mineral Earthquake supports this thesis, as relatively minor economic impacts and no loss of life were observed from DBE- and MCE-level shaking across a broad region. Engineers should focus on mitigating the collapse mechanisms in buildings for which partial or complete collapse is likely because that is where the economics of intervention makes the most sense.■ Table 2. Economic risk posed by earthquakes for nine selected cities.
References are included in the PDF version of the article at STRUCTUREmag.org. All authors are with Wiss, Janney, Elstner Associates, Inc. Terrence Paret is a Senior Principal. (firstname.lastname@example.org) Gwenyth Searer is a Principal. (email@example.com) Kari Klaboe is a Senior Associate. (firstname.lastname@example.org) Figure 3. USGS MCER for Washington, D.C., versus recorded/computed motion. Adapted from Wells et al.
Hayley Proctor is an Associate II. (email@example.com)
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just the FAQs Frequently Asked Seismic Questions By Emily Guglielmo, P.E., C.E., F.SEI
Note: Answers are based on ASCE 7-16, Minimum Design Loads for Buildings and Other Structures.
For diaphragm design, when do I apply overstrength (Ωo ) load combinations and when do I apply the 25% increase on my diaphragm forces? In general, the requirement to use overstrength load combinations is triggered when designing: • Elements supporting the discontinuous wall or frames with an out-of-plane offset (horizontal irregularity Type 4) or an in-plane discontinuity (vertical irregularity Type 4). [Applies to structures in SDC B, C, D, E, and F] • Elements contributing to the overturning resistance of cantilevered columns. [Applies to structures in SDC B, C, D, E, and F] • Collector elements, splices, and their connections. [Applies to structures in Seismic SDC C, D, E, and F] The requirement in Section 22.214.171.124 to increase the diaphragm forces by 25% is triggered when designing diaphragms Modified Response Spectrum for Site Class D and E sites with S1 ≥ 0.2 Exception. in structures in SDC D, E, F with one or more of the following: • Torsional irregularities (horizontal irregularity type 1a/1b) • Reentrant corner irregularities (horizontal irregularity type 2) near 1.0s for the site of interest. However, the two-point spectrum • Diaphragm discontinuities (horizontal irregularity type 3) is potentially non-conservative when the peak MCER response • Out-of-plane offset irregularities (horizontal irregularity type 4) spectral velocity occurs at periods greater than 1.0s, particularly • In-plane discontinuity (vertical irregularity type 4) for structures on softer soil sites and where the seismic hazard is The 25% increase in seismic forces is to be applied to: dominated by large-magnitude events. • Diaphragm connections to vertical elements of the seismic During the ASCE 7-16 cycle, there was insufficient time to adeforce-resisting system (SFRS) quately define a full spectrum capturing the required modifications. • Diaphragm connections to collectors As a result, an interim solution was provided: a geotechnical engineer • Collector elements is to provide a site-specific response spectrum for certain sites. The • Collector element connections to vertical SFRS requirement for a site-specific analysis is triggered when assigning a However, the exception in Section 126.96.36.199 states that load effects site coefficient Fa or Fv. Specifically, for softer soils at high seismic that include the overstrength factor need not be increased by 25%. regions, a requirement to see Section 11.4.8 is noted. Section 11.4.8 Therefore, for a structure in SDC C-F, the collector elements and requires a site-specific analysis. their connections only need to be designed for the load combinations, However, there are some relevant exceptions to this requirement. including the overstrength factor. Structures in SDC D-F that have The most often used exception is for structures on Site Class D and a horizontal irregularity Type 1, 2, 3, or 4 or a vertical irregularity E sites with S1 ≥ 0.2, provided Cs is determined by Eq. 12.8-2 for Type 4 shall be designed for the following forces: T ≤ 1.5Ts and taken as 1.5 times value computed by Eq. 12.8-3 for • Diaphragm connections to vertical SFRS: 25% Increase TL ≥ T > 1.5Ts or Eq. 12.8-4 for T > TL. This requirement forces • Diaphragm connections to collectors: 25% Increase the use of the constant acceleration equation, Cs = SDS/(R/Ie), for • Collector elements: Increase by Overstrength Factor structures with T < 1.5Ts and the amplification by 1.5 of the constant • Collector element connections to vertical SFRS: Increase by velocity, Cs = SD1/T(R/Ie), and constant displacement, Cs = SD1TL/ Overstrength Factor T2(R/Ie), equations. Finally, the code does not require the 25% increase or overstrength Both ASCE 7-16 Supplement 1 and forthcoming Supplement 3 proload combinations to design the diaphragm shears and moments. vide necessary clarifications to the footnotes of the Fa and Fv tables. These updates clarify that it is acceptable to use Fa and Fv values to Why does ASCE 7-16 require a site-specific response calculate Ts and for the calculation of SD1 when the exceptions of spectrum analysis? Is there any way to avoid it? Section 11.4.8 are used. Research has shown that the use of only two response periods ASCE 7-22 will introduce a multi-period response spectrum, elimi(0.2s and 1.0s) to define the Equivalent Lateral Force (and Modal nating the need to perform a site-specific ground motion hazard Response Spectrum Analysis) design forces is reasonably accurate analysis. This 22-point response spectrum will be publicly available when the peak MCER response spectral acceleration occurs at or via a web-based tool to provide acceleration parameters, including near 0.2s and peak MCER response spectral velocity occurs at or SMS, SM1, SDS, and SD1. STRUCTURE magazine
Can you clarify the difference between the two methods available to calculate redundancy, ρ, for my building?
Does ASCE 7 require foundations to be designed for the overstrength factor?
Historically, many structures were engineered utilizing moment connections at all beam-column joints. Subsequent increases in labor cost and the availability of members with large section properties led to engineers concentrating SRFS in a few large elements. Damage from the 1994 Northridge earthquake was concentrated in these buildings with low redundancy. The code was then modified to encourage increased redundancy for structures in Seismic Design Categories D, E, and F. For structures with low inherent redundancy, the required design forces are amplified by 30% to increase strength and resistance to damage. There are several conditions for which the redundancy factor, ρ, is permitted to be taken as 1.0, including for structures in SDC B and C, for drift calculations, for non-structural component forces, collectors, overstrength load combinations, and diaphragms. For structures assigned to SDC D and having extreme torsional irregularity (Type 1b), ρ must be taken as 1.3. For other structures assigned to SDC D and for all structures assigned to SDC E or F, ρ must be taken as 1.3 unless one of the following two conditions is met. If one of the conditions below is met, ρ is permitted to be taken as 1.0. • Method 1: Removing an individual element (brace, beamto-column connection, shear wall or wall pier, and cantilever column) of the SFRS does not decrease the story strength by more than 33% and does not trigger an extreme torsional irregularity. This check only is required to be done at the stories resisting more than 35% of the base shear. • Method 2: The structure must have two bays of SFRS perimeter framing on each side of the structure in each orthogonal direction. This method is only permitted for structures with no horizontal irregularities and only must be checked for stories resisting more than 35% of the base shear.
Typically, ASCE 7 does not require overstrength to be used for foundation design. When designing elements supporting discontinuous walls or frames, overstrength is typically provided for the design of the connections to the foundation but not taken into the foundations. One notable exception is in ASCE 7 Section 188.8.131.52, Cantilever Column Systems, in SDC B, C, D, E, and F. When designing a cantilever column system, the foundations used to provide overturning resistance at the base of cantilever column elements must be designed for overstrength load combinations. A second exception of overstrength required in foundation design is in ASCE 7 Section 184.108.40.206 for pile anchorage requirements for SDC D, E, F. For piles required to resist uplift forces or provide rotational restraint, anchorage into the pile cap must be designed to resist the axial tension force resulting from the seismic load effects including overstrength. Please note that the California Building Code modifies ASCE 7 Section 12.13.1 for certain structures (schools, community colleges, and hospitals) to require the foundation to have the strength to resist the lesser of the following seismic loads the lesser of: • The strength of the superstructure elements • The maximum forces that can be delivered to the foundation in a fully yielded structural system • Forces from the Load Combinations with overstrength factor
Why do the provisions in Chapter 12 of ASCE 7 not mention SDC A? To simplify the provisions, particularly for engineers designing structures in SDC A, there is no need to use Chapter 12. Instead, these
Recent studies of building collapse performance, such as those of the Applied Technology Council’s ATC-63, ATC-76, and ATC-84, show that designs based on the ELF procedure generally result in better collapse performance than those based on MRSA with the 15% reduction in base shear included. Also, many of the designs using scaled MRSA did not achieve the targeted 10% probability of collapse given MCE ground shaking. While scaling to 100% of the ELF base shear does not necessarily achieve the intended collapse performance, it does result in performance that is closer to the stated goals of ASCE 7.
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When doing modal response spectrum analysis (MRSA), why does ASCE 7-16 require me to scale my base shear to 100% of equivalent lateral force (ELF) procedures? Previously, where the combined response for the MRSA base shear was less than the ELF base shear, I only had to scale my forces to 85% of ELF.
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structures must be designed to Section 11.7. This section points the engineer to Section 1.4, General Integrity, which has basic requirements for load-path connections, lateral forces, connections to supports, and anchorage to walls.
Why is there a seismic importance factor, Ie = 1.25 in Table 1.5-2, but only importance factors of 1.0 and 1.5 in Chapter 13? The importance factor for seismic (Ie) is based upon Risk Category and the associated Life Safety, Hazard, and Essential nature of the structure. For building design, Ie = 1.0, 1.25, or 1.5, but for non-structural components (Chapter 13), Ip = 1.0 or 1.5, depending on Risk Category, SDC, component function, weight, and location. It is important to note that Ip might not equal Ie, and, in some instances, Ip may be less than Ie.
I have a building with special reinforced concrete shear walls, and I would like to classify it as a building frame (R=6) rather than a bearing wall (R=5). What is the difference between a building frame and a bearing wall? ASCE 7 defines each system as: • Bearing Wall System: A structural system with bearing walls providing support for all or major portions of the vertical loads. Shear walls or braced frames provide seismic force resistance. • Building Frame System: A structural system with an essentially complete space frame providing support for vertical loads. Seismic force resistance is provided by shear walls or braced frames. Several sources have attempted to clarify the distinction of what qualifies as “major portions of the vertical load.” The Structural Engineers Association of California (SEAOC) Blue Book describes a method for detailing integral beams and columns within shear walls. The integral beams and columns must be capable of carrying the gravity loads of the portions of the wall damaged in a seismic event. This approach can be used to justify a building frame system with an enhanced R-value. The National Earthquake Hazards Reduction Program (NEHRP) Provisions note that a building frame is a system where the gravity loads are carried primarily on columns, not walls, while allowing minor portions of the gravity load to be carried on bearing walls, but not more than a few percent of the building area.
When must my foundations be interconnected with ties? Can I use lateral soil pressure on my pile cap to provide the required restraint? In SDC C, D, E, and F, structures utilizing a deep foundation system, individual pile caps, drilled piers, or caissons must be interconnected by ties per sections 220.127.116.11 and 18.104.22.168. All ties must be able to resist (in tension and compression) a force equal to 0.1SDSD, where D is the dead load of the larger pile cap or column dead-plus-live load. Per section 22.214.171.124.1.1, for liquifiable sites and SDC E and F, individual footings shall be interconnected by ties in accordance with Section 126.96.36.199. These requirements highlight the importance of the foundation system, acting as an integral unit, not permitting one column or wall to move independently from the rest of the structure. ASCE 7 requires that pile caps (and footings in SDC E, F) be tied together to attain this performance. This requirement is especially important where the use of deep foundations is driven by the existence of soft surface soils. ASCE 7 does permit the required restraint to be provided by a slabon-grade or confinement by competent rock, hard cohesive soils, very dense granular soils, or other approved means. However, relying on lateral soil pressure on pile caps is not recommended as ground STRUCTURE magazine
motions are highly dynamic and may vary between structure support points during a design-level seismic event.
Can you clarify when I should be using Chapter 13 versus Chapter 15? Section 15.3 represents a clear delineation between Chapter 13 and Chapter 15, where a nonbuilding structure is supported by another structure. When the supported nonbuilding structure’s weight is less than 25% of the combined effective seismic weights of the nonbuilding structure and supporting structure, the design seismic forces of the supported nonbuilding structure are determined according to Chapter 13. The supporting structure is designed to the requirements of Chapter 12 (if a building) or Section 15.5 (if a nonbuilding structure), with the weight of the supported nonbuilding structure considered in determining the effective seismic weight, W. Even with the 25% threshold described in Section 15.3, there are non-structural components and nonbuilding structures common to both chapters. Some examples include billboards and signs, bins, chimneys, conveyors, cooling towers, stacks, tanks, towers, and vessels. The recommended reference for determining whether to use Chapter 13 or Chapter 15 is Nonstructural Component or Nonbuilding Structure? (Bachman and Dowty, 2008). That article suggests three ways to differentiate between non-structural components and nonbuilding structures: • Size: Non-structural components are typically small, usually less than 10 feet in height. • Construction: Non-structural components are typically shop fabricated. • Function: Non-structural components are primarily designed for functionality, while nonbuilding structures are primarily designed to maintain structural stability.
I heard the requirement to consider accidental torsion is gone in the ASCE 7-16 Standard. Is this true? Why? ASCE 7 has historically required a minimum eccentricity of 5% of the width of a structure perpendicular to the direction being considered to any static eccentricity computed using idealized locations of the centers of mass and rigidity. This requirement is because the locations of the centers of mass and rigidity for a given ﬂoor or roof typically cannot be established with a high degree of accuracy because of mass and stiffness uncertainty and deviations in design, construction, and loading from the idealized case. However, ASCE 7-16 provides a new exception in Section 188.8.131.52 that provides relief from the accidental torsion requirements for buildings that are relatively insensitive to torsion. This provision permits the exclusion of accidental torsion moments when determining the seismic forces for the design of the structure, except for the following cases: • Structures assigned to SDC B with extreme torsional irregularity: type 1b. • Structures assigned to SDC C, D, E, and F with a torsional or extreme torsional irregularity: type 1a or 1b. This relief is supported by research that shows that the inclusion of accidental torsion has little effect on collapse probability for low SDC structures without Type 1b horizontal structural irregularities and for high SDC structures without Type 1a or 1b irregularities. Reference included in the PDF version of the article at STRUCTUREmag.org. Emily Guglielmo is currently Vice-Chair of the ASCE 7-22 Seismic Subcommittee and chair of the TC-2 Task Committee on General Provisions.
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Investigative Peer Reviews What are they, really, and what do they entail? By Cole Graveen, P.E., S.E.
tructural engineering consulting firms are occasionally hired to review a design performed by another engineering firm. The review is frequently a traditional pre-construction structural peer review performed to achieve a better project outcome. The practice of having a traditional peer review performed is becoming more commonplace for Risk Category III and IV buildings, which include tall buildings, buildings with large occupant loads, and essential structures. Peer review is also commonplace for structures designed using performance-based procedures or with new or innovative framing systems. Traditional structural peer reviews may be performed at the request of the owner or developer, to expedite a building department review, or because it is required by the building code or performancebased design guide. These reviews generally occur in a cooperative environment. Other types of reviews can also occur. A less traditional type of structural peer review is an investigative peer review. This type of review occurs when something has gone wrong and originates either during construction or after the structure has been completed. The party requesting the investigative review is interested in whether or not the structural design has caused or contributed to problems that have occurred. An investigative peer review evaluates the structural design and may identify errors or omissions. These reviews are often made more complicated compared to typical reviews as the exchange of documents and information may be delayed or restricted depending upon the relationships between the parties involved and the circumstances initiating the review. The author has been involved in several investiThe party requesting gative peer reviews where the parties have become the investigative review adversarial, and the initial design information prois interested in whether vided for review consists or not the structural of only the drawings and a computer model input design has caused or file. This article addresses investigative peer reviews contributed to problems performed under similar circumstances. Fundamental that have occurred. engineering review tasks are presented with a focus on tasks required to review the structural analysis and design performed using electronic calculations and computer analysis models.
Background Several organizations have produced guidelines or rules addressing engineering peer reviews. A list of references known to the author is provided with the online version of this article. Many of these documents were developed for individual states or cities. However, in 2013, the Council of American Structural Engineers (CASE) published a national practice guideline, Guideline 962-G: Guidelines for Performing Project Specific Peer Reviews for Structural Projects. This STRUCTURE magazine
Guideline is both comprehensive and in-depth, providing information on many aspects of peer review while including specific details on engineering tasks performed in a structural review. While Guideline 962-G is written to address traditional pre-construction structural peer reviews, much of the information contained within can be applied to investigative peer reviews.
Initiation The phone rings, and on the line is an attorney whose contractor client is being blamed for a localized failure that occurred in a recently constructed building. The stakes are high as the building owner is suffering a loss of use and wants the structure fixed now. The contractor claims they built what the engineer put on the drawings, and it is not their fault. The attorney wants to know if the engineer’s design caused or contributed to the failure. The contractor’s records include the drawings and specifications, and the engineer’s calculations will be available soon.
Scope Review This fictional but realistic scenario sets the stage for an investigative peer review. Just like a traditional peer review, the process does not begin until the scope of the review has been established with the client. The client is often focused on the specific portion of the building with the performance problem; however, a broader review approach is almost always necessary. The structural peer reviewer needs to have at least a general understanding of the overall building design. It should also be made clear to the client that any initially agreed upon scope may need to be expanded. As the investigation proceeds and both the structural design and the details surrounding the project are unveiled, the need to review certain aspects of the design in more depth may become apparent. Also, in most investigative peer reviews, including this example, not all of the project information is immediately available.
Documents Regardless of what information is initially provided, the peer reviewer needs to clearly communicate to their client what documents are needed for the review. Merely stating the “design documents” or the “drawings and calculations” will likely result in receiving the bare minimum of documentation, making it difficult to review
General Review Tasks The specific review tasks and sequence for an investigative peer review will depend upon the nature of the structural failure or performance problem and the agreed-upon scope. As previously stated, however, the reviewer will need to have at least a general understanding of the overall building design. Certain review tasks apply to any investigative review. These tasks include: • Understanding the design criteria and building code requirements. The review should independently verify the appliable edition of the building code and material design standards and compare the design criteria contained within the code and standards to the project requirements. State or local amendments to the locally adopted national model building code may change design requirements and need to be understood. • Identifying portions of the structure where the design was delegated to others and evaluating if the
submittal documents and design calculations for those delegated items apply to the review scope. If a review of delegated designs is necessary, both the delegated design itself and the compatibility with the primary structure require review. • Identifying the gravity and lateral load resisting systems and diaphragm types in the structure. Having a global understanding of the structural design will aid in the review of both the overall building behavior and the analysis and design of specific building members. • Performing an initial review of the provided printed calculations, electronic files, and design summary or project narrative to identify the subject matter of each document. It is essential to understand early on if the analysis and design of the entire structure are
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the design or even possibly not getting information for portions of the structure. The list of requested items should include: the project drawings (at a minimum the structural and architectural drawings), the project specifications, design summaries or narratives, engineering reports, structural calculations (hand-written and computer output), copies of the electronic files for computer output calculations and for structural analysis and design performed using computer models, RFI’s, addenda/Supplementary Instructions from the engineer, and documents produced by specialty structural designers involved in the project. The latest versions of these documents should be obtained so that the changes that occurred during the design and construction process are included. Record drawing sets are not always produced but, if available, will incorporate such changes. Otherwise, reviewing RFI’s, engineers' field sketches and directives, and addenda may be necessary to understand the final design. If a design summary or project narrative is available, it can significantly assist with understanding the design intent and criteria. It will likely describe the gravity and lateral load systems and building code design criteria. It should also include project-specific design criteria such as floor deflection and vibration limits, building drift limits, and design loads determined from site-specific studies such as wind tunnel testing, seismic site class testing, and seismic ground motion hazard analysis. The summary may also indicate how software programs were used in the analysis and design of the building (more on this later).
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contained in the received documents. If the analysis or design of portions of the structure is not addressed therein, requests for additional information can be made if necessary. Once these general tasks have been completed, more focused and in-depth peer review tasks can be performed – the specific aspects of the design to be reviewed, and to what degree, will be dictated by the review scope, the complexity of the structure, and the detail provided in the calculations. Independent analysis and design calculations are typically performed on a limited basis to check the results in the provided calculations.
Digital Reviews Electronically-created structural calculations and structural analysis and design performed using computer models present unique challenges to an investigative peer review. Merely reviewing printed output and summary reports from a program provides a limited amount of information. Even the built-in default reports available in many programs do not typically provide enough detail regarding the selected setting options and input or model definition to adequately describe what the software is solving and how it is solving it. Opening the software and reviewing the electronic files is the best way to understand what has been done. The investigative review tasks presented below were developed with a focus on software where the structure or portion of the structure is modeled and then analyzed. However, many of the tasks are also applicable to more straightforward engineering calculation software. A comprehensive review consists of the following areas of the structural model. Software Review: Determine the software version used to create the electronic files and, if at all possible, use the same version to open and review the contents. Opening older files in newer versions of the software can sometimes result in settings being reset to defaults. A newer version may have revised input menus or additional input options, which may reset when the newer version opens the older file. Different software settings will change results, which may then mislead the peer reviewer. When it is not possible to use the same version, the changes between versions should be researched. Model Purpose: The analysis and design of tall or complex structures may be accomplished using multiple software programs or using multiple models created from the same software package. There could be a model simply used for the analysis of the structure, to apply the design loads to the structure, distribute them to the individual members, and determine the member forces, with separate programs for designing the members using those forces. There could also be separate models for gravity load analysis, lateral load analysis, and serviceability checks. If a design narrative or summary report does not describe the purpose of multiple models, the investigative reviewer will need to review each electronic file and software package to evaluate its purpose. General Model Definition: Compare the model to the project drawings and specifications to evaluate if the model is representative of the intended construction. This includes the overall vertical and plan dimensions, the individual member locations, member sections, base of column support conditions, and the connections between members, including member offsets, rigid zones, and end releases. Material Definitions: Software typically has default settings for concrete, steel, masonry, and timber material properties. These default settings usually need to be changed, or copied and modified, to account for multiple steel grades, concrete and masonry strengths, or
to account for increased or reduced stiffness properties. A single material may require multiple material property definitions. Models using advanced analysis types may require nonlinear material properties. Property Modifiers: Adjusting the default member stiffness is commonly performed in structural analysis. For example, concrete beam-column joints are modeled with larger stiffness, while reduced member stiffnesses may be used for serviceability analysis. This can be accomplished in multiple ways, such as by modifying material properties as mentioned above, by applying property modifiers to section definitions or individual members, or by using user-defined members. Load Cases and Combinations: Compare the design loads to the loads contained in the model and review the load combinations for compliance with building code criteria. Note that if advanced analyses are being used, such as response spectrum, time-history, or nonlinear analysis, the means of combining the design loads may be different than when a straightforward linear static analysis is used. Design Modules: Member design is performed based on the results of the analysis. This may be performed in a separate stand-alone program or within the same program. If the program is separate, the transfer of member forces from the analysis program to the design program should be reviewed. In any design program, the design settings will need to be reviewed for agreement with the type of analysis performed, proper edition of the material design standard, and both global and individual member settings, which include items such as unbraced lengths and prescriptive minimum and maximum limits. Model Integrity: Verify that the model runs without errors or warnings. If errors or warnings appear in the log or output file, they need to be investigated to determine if they significantly affect the results. Model Results: Review deformed shape plots and member force plots to evaluate if the behavior of the structure makes sense. These plots illustrate the load paths and member behavior and are the easiest way to find unintended results. Compare the individual load case sum-of-reactions to the intended applied loads. This is a simple way to verify that the design loads were entered correctly into the program.
Summary An investigative peer review evaluates the structural design after something has gone wrong. Unlike a traditional pre-construction peer review, the peer reviewer and the designers are usually not able to interact. This restricts the exchange of information and places greater importance on obtaining the design documents, calculations, and related information. As such, an essential characteristic of an investigative peer reviewer is the ability to communicate to their client which documents are needed and why. The review of electronic calculations and analysis models is typically a part of an investigative peer review. These files need to be opened within the software to understand and evaluate the structural analysis and design performed with the software.■ References are included in the PDF version of the article at STRUCTUREmag.org. Cole Graveen is a Senior Engineer at Raths, Raths & Johnson, Inc., Willowbrook, IL. He currently serves as a voting member of the ASCE 7-22 Main Committee and Chair of the ASCE 7-22 Subcommittee on Dead and Live Loads.
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SIDEYARD Figure 1. Sideyard makes stunning use of the leftover site.
Transforming a Neighborhood with Mass Timber Construction By Jason Thompson, P.E., S.E., LEED AP
burgeoning Inner East District of Portland, Oregon, has experienced a transformation within the past ten years and, with it, a surge of new multi-family and office construction. Within this activity lay a small 9,000-square-foot bermed site, an artifact of a newly completed roadway realignment to connect westbound traffic to the Burnside Bridge – a primary link over the Willamette River into downtown. While useful as a staging and lay-down area to facilitate new building construction around it, project partners Key Development, Andersen Construction, and Skylab Architecture saw potential in the small hemmed-in site and recognized its prominent location and importance as a gateway to the newly created neighborhood. Through 17 months of planning and design and another 14 months of construction, the result of that vision is Sideyard – a nearly 25,000-square-foot office and retail building that serves as a showcase of Oregon-sourced mass timber construction (Figure 1). Sideyard is five stories tall and slightly wedge-shaped in plan, measuring approximately 150 feet long in the north-south direction and a maximum of 56 feet wide in the east-west direction. The foundation consists of simple reinforced concrete shallow spread, strip- and mat-slab footings. The bottom-most level is a daylit basement facing Third Avenue to the west, incorporating a non-structural reinforced concrete slab on grade. Reinforced concrete basement walls restrain a single level of retained soil to the east and south. Reinforced concrete walls also surround two interior stair cores of identical size to resist wind and seismic lateral forces. Figure 3. Exposed timber structure prior At the building roof to cladding installation. and floors, the primary
structural system consists of 5-ply cross-laminated timber (CLT) panels, up to 10 feet Figure 2. Stairway constructed of mass plywood. wide and 40 feet long. The panels span to periodic glued-laminated timber beams varied in size, up to 8.75 inches wide and 27 inches deep, and spaced at approximately 15 feet on-center. Except where framing into concrete walls, the beams span to and are, in turn, supported by glued-laminated timber columns measuring 10.75 inches by 13.5 inches in cross-section. A 4-inch-thick reinforced concrete topping slab at Level 2 provides sufficient diaphragm capacity to facilitate the transfer of lateral loads from the central core walls to the longer and stiffer basement walls. This topping slab also allowed for a polished concrete floor finish at Level 2 – something desired by the project partners. The remaining floors are topped with a non-structural assembly incorporating gypcrete and a sound insulation mat to provide the requisite acoustic characteristics. The columns and underside of all floor and roof structures remain exposed to view, maximizing the natural wood finish’s beauty. All CLT panels and gluedlaminated timber members are of Douglas Fir-Larch species and were furnished by D.R. Johnson of Riddle, Oregon. Construction within the stair cores makes almost exclusive use of yet another mass timber product – mass plywood panels (MPP) by Freres Lumber Company, Inc. of Lyons, Oregon. Three-inch-thick mass plywood treads span between the inside face of concrete core walls to a central, full-height, 7-inch-thick MPP spine wall. The 7-inch-thick mass plywood panels are also used for the primary and intermediate stair landings. All mass plywood remains exposed within the stair cores (Figure 2). Sideyard is one of the first projects to employ mass plywood in a structural application, presenting a unique challenge, as the authority having jurisdiction was unfamiliar with the material. When designing with exposed mass timber, it is essential to understand construction types early as fire protection requirements may dictate member sizes, panel thicknesses, connection concealment requirements, and, therefore, cost. Sideyard is Type IIIA construction, and structural members and connections are required to maintain their required load capacity after a 1-hour fire event. Accordingly,
structural calculations completed in accordance with orthogonal direction, allow the CLT panels to behave the American Wood Council’s Technical Report No. as a monolithic diaphragm in resisting lateral wind 10, Calculating the Fire Resistance of Exposed Wood and seismic loads. CLT panel edges were further Members, consider a 1.8-inch-thick sacrificial char fabricated to allow for their connection along adjalayer on all exposed mass timber member faces and cent concrete walls. a protective 1.5-inch-thick char layer around mass The horizontal leg of a steel ledger angle is recessed timber member connections. Beam and column into the underside of CLT panels by 1.5 inches, and member sizes and CLT panel thicknesses were, in some screws between the ledger angle and panels facilicases, driven by these fire resistance considerations. tate the transfer of seismic loads between the CLT The exterior façade is a mixture of CMU veneer, large diaphragms and core walls. Tight-fitting wood filler Figure 4. Glulam beam ends and storefront windows, and curtainwall, and extends high blocks are used to fully conceal the ledger angles column faces were both furnished above the roofline with braced parapets. Except for with premanufactured Ricon-series and protect them from a possible fire event. Where their self-weight, exterior walls are non-load-bearing. connectors to facilitate tight-fitting necessary, the transfer of seismic loads between CLT Resistance against out-of-plane wind and seismic forces and concealed field connections. diaphragms and concrete shear walls is supplemented is provided by a light wood-framed stud wall gapped at by horizontally oriented premanufactured hold-down the underside of CLT panels above to accommodate vertical deflections hardware by Simpson Strong-Tie. and interstory drifts. As required in Type III construction, the light wood The structural design of Sideyard complies with the applicable proviframing within the exterior wall assembly is fire-retardant treated. The sions of the 2014 Oregon Structural Specialty Code (based on the 2012 CMU veneer predominantly rests atop steel ledger relief angles secured International Building Code) and its reference documents, except that to the edge of CLT panels at each floor and the roof. At Level 3, large the use of the 2015 ANSI/AWC National Design Specification (NDS®) floor openings along the east and west sides led to the introduction of for Wood Construction was employed with the approval of the authority hollow structural steel beams to provide vertical and lateral restraint of having jurisdiction. CLT was initially incorporated in the 2015 NDS. the exterior wall assembly at those locations. Along the east side, these Floor vibrations were considered in accordance with the U.S. Edition steel beams are also integral to a large steel-framed canopy, the front edge of the 2013 Cross-Laminated Timber Manual by FP Innovations. CLT of which is further supported by diagonal tie rods secured to the edge of diaphragms at Levels 3 and above were designed in accordance with the CLT panels above at Level 4. the 2015 white paper titled, Cross Laminated Timber – Horizontal A significant advantage of mass timber is the ability to shop-fabricate Diaphragm Design Example, by Spickler, et al. RISA-3D was used for pieces for ease and speed of erection. This was particularly important at analytical modeling of the glued-laminated timber framing and Level the Sideyard site. Despite having served as a staging area for previous 3 canopy, and ETABS for the analysis of the lateral force-resisting nearby projects, it had very little remaining available space to stage its own system. Revit was used for design coordination and drawing creation. construction materials (Figure 3; A timelapse video of Sideyard’s construcThe use of mass timber in building construction is, of course, nothing tion is found at https://vimeo.com/304267049). Structural members new. Its recent renaissance is mostly due to a focus on sustainability were shop-fabricated to the maximum extent possible by CutMyTimber, and a desire to reinvigorate the lumber industry. This, in turn, has Inc. The ends of glued-laminated beams and faces of columns were led to the fast-moving introduction of panel products and a host of furnished with premanufactured Ricon-series connectors (metal plates premanufactured connectors. However, this speed of technological that connect in a dovetail-like manner) by Knapp AG, of Graz, Austria, advancement is, in some cases, outpacing our understanding of related to facilitate tight-fitting and concealed field connections between them structural performance and the development of corresponding U.S.(Figure 4). Where higher load demands dictate, these connectors are based standards. For example, the development and implementation doubled in a staggered configuration. Custom shop-fabricated concealed of standardized testing of premanufactured timber connectors are steel bearing seats connect timber beams to concrete walls to allow for warranted. Despite a lack of approval from a nationally-recognized construction tolerances. Both ends of columns were further fitted with evaluation service like ICC or IAPMO, the Ricon-series timber concustom-designed steel hardware for field connection to footings below or nectors were approved for conditional use on the Sideyard project additional column sections above. Except at locations where surrounding only by leveraging testing from a previous Portland-based project to floor openings mandate the use of taller two-story members, columns demonstrate continued gravity load-carrying capacity when subjected are fabricated in single-story pieces. At floors, the faces of columns are to the maximum expected earthquake-induced rotation demands. routed to allow adjacent CLT panels to extend into the column section The renewed prevalence of mass timber buildings may also change by 1.5 inches. This detail is not used for bearing resistance of the CLT, but how the structural engineering industry models and documents rather as a means to mitigate their designs. For example, given the hardware congestion typical at thermal, sound, smoke, and the intersections of mass timber beams and columns, BIM Level of fire transmission between Detail (LOD) 400 connection modeling, complete with screws and floors at column locations corresponding isometric views within the construction documents, (Figure 5). may prove useful in mitigating potential constructability conflicts. Recesses were routed into Sideyard has helped transform a neighborhood. At the same time, the long sides of interior it serves as a case study of what is possible in mass timber construcCLT panels to facilitate tion and how the structural engineering community may continuous 1-inch-thick navigate the design and documentation of such projects plywood spline connec- in the future.■ tions between panels. At Levels 3 and above, All photos courtesy of Stephen A. Miller. these splines, along with Jason Thompson is a Principal at catena consulting engineers. strategically-located steel Figure 5. Interior shell awaiting tenancy. (firstname.lastname@example.org) strap connections in the M A R C H 2 0 21
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Rocky Mountain Metropolitan
Airport Drive-Through Canopy By Todd Robbins, P.E., SECB, and Jason McCool, P.E.
Sheltair’s state-of-the-art facility welcomes pilots and passengers at Rocky Mountain Metropolitan Airport. Photo by Bob Beresh, courtesy of Sheltair.
esigning a drive-through canopy may not sound very exciting to most engineers. Finding out it is a drive-through for business jets at a high-altitude airport with hurricane-force wind speeds can quickly change that first impression. The 132-foot clear span arched trusses supporting the 40-foot-high aircraft canopy were the most prominent part of this unique project but far from the only design challenge.
Project Scope When Sheltair decided to expand its aviation network footprint with its first Fixed Base Operator (FBO) facility west of the Mississippi River, they partnered with Tectonic Management Group and Robbins Engineering Consultants as their architect and structural engineer of record, respectively. The Rocky Mountain Metropolitan Airport project in Broomfield, Colorado, included a new 35,000-square-foot hangar, a new 10,400-square-foot FBO building, and arched steel drive-through canopies – one for cars at the street entrance and the other for jets on the aircraft side. The FBO provides a passenger lounge, dedicated office space, conference rooms, restrooms, and
Figure 1. RAM Elements model of the high canopy for aircraft.
dedicated areas for the operation of staff and line service. At 10,088 square feet, the aircraft canopy was almost as large as the building it served. At 15 feet taller than the FBO, both factors made the canopy readily visible to incoming pilots.
Design Challenges The first challenge was due to the winds coming down the slopes of the nearby Front Range mountains. The 155-mph design wind speed (3-second gust) was comparable to the sustained wind speeds of a Category 4 hurricane. For comparison, that exceeds the design wind speeds for Risk Category II buildings in the hurricane-susceptible areas along the Texas Gulf Coast and much of the Atlantic coast. Despite being an open structure for planes to pull under for passenger loading and unloading, the canopy on the building’s airfield side still represented a large surface area for wind. The design team investigated several options before deciding on arched trusses supported by builtup laced columns (Figure 1). Twin W24x192 columns, 4 feet apart with round bar X-bracing and W8 struts between them, formed each of the 4 built-up columns. The arched trusses were 6 feet deep, using WT12x88 chords and double angle webs. The 132-foot clear span had one field-bolted splice point near midspan. Each end of each truss was attached to the W24 column’s inside flange via bolted endplate moment connections (Figure 2). 10- and 20-foot cantilever truss sections were similarly attached to the outside flanges of the columns. Vertical X-bracing installed between each pair of trusses, as well as angle lacing in the planes of both top and bottom chords, formed a space truss at each set of built-up columns. This became the moment frame in the long direction. Each pair of laced columns function as cantilevered vertical trusses in the orthogonal axis. Concrete tie beams joined the columns in the long direction to resist the arches’ thrust under gravity loads. The FBO building had various discontinuities in the diaphragm that required careful cross-checking of design software results to ensure the complete roof-to-ground load path was checked. Similarly, the arched
open structure of the canopies required extensive hand calculations to determine appropriate wind loads to be applied to the analytical models of the canopies. While there are wind external pressure coefficients in ASCE 7-16, Minimum Design Loads for Buildings and Other Structures, for enclosed structures with arched roofs (ASCE Fig. 27.3-3) and open structures with gabled roofs (ASCE Fig. 27.3-5), there are not yet any published for open structures with arched roofs. Therefore, the double-pitched open roof condition with slopes set to the arch slopes at the supports was used to model the actual configuration. The natural frequency was determined by modal analysis to verify the structure was not dynamically sensitive so that the assumption of a rigid structure under wind loading Figure 2. WT truss chord connected to the W24 column via endplates analyzed in Idea Statica was appropriate. Connection. Cantilever to left of column, backspan to the right. Extensive coordination was needed early to avoid rework on the trusses while accounting for shipping 5,673 feet above sea level, that equated to an 18% reduction in restrictions, crane capacities to lift them in place, and the optimum wind pressure for this project. While this reduction is new to the quantity of segments. After discussions with the architect and body of the ASCE 7 standard, it has been in the Commentary for many years and is derived from fundamental physics, so erector, one shipping splice near the midspan was adequate. Each there is no reason not to take advantage of it. pair of trusses was then assembled on the ground with bracing installed and set in place as a single unit with two cranes (Figure 2) Care must be taken in laying out joists and deck on curved support framing. There are limits to how much corrugated deck 3). Splice joints were designed as slip-critical connections with can be bent in the strong axis; not accounting for that can add oversized holes to allow for easier fit-up of the unwieldy truss secsignificant cost and time delays for special third-party crimping tions. Exposed structure at both arched canopies and in the FBO of the deck. As a result, several design iterations were required atrium was considered Architecturally Exposed Structural Steel to balance the joist spacing, deck gage, and panel spacing of the (AESS) Category 3 (feature elements in close view). Establishing large structural steel trusses supporting the joists to be compatispecific expectations for the AESS based on the American Institute ble with field-bending the deck to the truss radius. Also, because of Steel Construction’s (AISC) recommendations in their Code the rolled W-beams of the smaller car canopy were a different of Standard Practice (AISC 303-16, section 10) helped minimize radius, the joist spacing and deck gage were different at that problems in that area. canopy to accommodate field bending the deck. Due to the presence of over 20 feet of expansive fill materials at the site underlain by expansive claystone prevalent in that area, the FBO 3) Be aware of jurisdictional differences in building code adoption. In this case, the airport’s mailing address is on Airport Way in building sits over a crawlspace with the elevated steel-framed ground Broomfield, CO, which is in Broomfield County. At the time floor supported on drilled piers extending a minimum of 18 feet into of the project, both the city and county were still using IBC unweathered bedrock. 2015, which is based on ASCE 7-10. However, the county line cuts through the airport property, with the airport mostly in Lessons Learned Jefferson County, which had already adopted IBC 2018 at that point. This, in turn, changed the applicable wind provisions 1) The reduction in wind pressures due to elevation can be signifiused from ASCE 7-10 to 7-16. cant and should not be overlooked. With a field elevation of
Conclusion In the end, the client made a bold entrance into a new regional market. The high arched canopy provides an unmistakable landmark for pilots while also shielding customers from this high-altitude mountain airport’s harsh weather. And careful structural design and coordination ensured a smooth construction process and a realized architectural vision – a win-win for all parties.■ Todd Robbins is a Principal at Robbins Engineering Consultants in Little Rock, AR. (email@example.com)
Figure 3. Two cranes were used to set the canopy trusses in place.
Jason McCool is a Project Engineer at Robbins Engineering Consultants. (firstname.lastname@example.org)
M A R C H 2 0 21
Building Faster and Better By Ashley P. Thrall, Ph.D., and Angelene Dascanio, MEng Sc, MS
odular, rapidly erectable, and deployable structures are critical for disaster relief, military operations, and the accelerated construction of bridges and buildings. Through various innovative means and methods, permanent or temporary structures can be rapidly constructed in challenging environments, such as rural and austere regions where access to construction equipment is difficult or in heavily congested urban locations where construction time is severely limited. The design of these structures poses different challenges than conventional construction. They have unique requirements such as low self-weight and small packaged volume for transportability. Attention to connection detailing is critical for both performance during deployment and in service. Often there are also limitations on available construction equipment. Despite such challenges, these structures offer innovative solutions to improve lives and reduce costs and time spent in construction. This article highlights recent projects and research in modular, rapidly erectable, and deployable infrastructure to inform the growing interest from industry, academia, and national laboratories in advancing this type of structural design.
Deployable Sheltering Rapidly deployable shelters provide essential infrastructure on military bases and housing for displaced families following disasters. Design criteria include low self-weight, erection without heavy equipment, a small, packaged size, and energy efficiency. To achieve these goals, the Lever Shelter Module (Figure 1), developed in the Kinetic Structures Laboratory at the University of Notre Dame, uses the art of origami to form a folding, rigid-wall deployable structure. It features a novel erection strategy: a lever arm attaches to the back wall, human force is applied as a counterweight, wing walls unfold from the back wall, and the roof unfolds to rest on the wing walls. This creates a self-supporting module that can be connected to other modules to
Figure 1. Origami-inspired Deployable Shelter. Top row (left to right): packaged shelter, lifting via the lever arm, and deployed module. Bottom row (left to right): two models erected, multiple modules interfaced with Tricon container, and full-scale deployed prototype.*
achieve a variety of configurations and sizes. Envisioned uses range from housing to office space. Additionally, the modules can interface with a commonly used Tricon container, as depicted in Figure 1. Tricon containers are used for many military housing components (e.g., latrines, kitchens). Thus, this shelter can employ existing infrastructure. The lever arm eliminates the need for heavy equipment, making it deployable in remote environments or disaster regions. As the rigid walls fold into a small volume, it can be transported on a 463L pallet, making it readily transportable by air, rail, ship, and truck. The rigid walls are sandwich panels composed of a lightweight foam core and fiberglass faces, providing a high strength-to-weight ratio and thermal efficiency. For the military, reducing fuel consumption for heating and cooling the shelter is a priority due to the high cost of transporting fuel in military zones. More importantly, thousands of lives have been lost in missions to transport fuel to sites. Thus, from both a life-loss perspective and a cost perspective, it is critical to reduce fuel consumption. Thermal insulation also becomes important for disaster relief sheltering, especially as it can take months or years to rebuild housing. Research on this novel origami-inspired structure included experimental testing on a half-scale prototype, demonstration of the deployment of a full-scale prototype (shown in Figure 1), numerical modeling, and structural optimization.
Figure 2. 3D-printed Barracks Hut. Photos courtesy of the U.S. Army Corps of Engineers. Rendering courtesy of Skidmore, Owings & Merrill.
When people mention 3-D printing, desktop machines printing plastic generally comes to mind. Additive Construction (AC) takes this to a new scale through the printing of entire structures (buildings and bridges). Over the last decade, this technology has grown from laboratory experiments to full-scale structures, driven by its many advantages. It can bring a focus to the struggle for codes and standards to be more agile to accommodate changing technologies (STRUCTURE, Building Code Compliance: 3-D Printed Concrete Walls, September 2020).
continued on next page M A R C H 2 0 21
Since 2015, the U.S. Army Engineer Research and Development Center (ERDC), Construction Engineering Research Laboratory (CERL), has researched AC and printed multiple full-scale structures. In 2018, ERDC-CERL demonstrated an optimized Barracks Hut (B-Hut) with the primary goals to use: unique geometries, feasible construction practices, and continuous printing operations (Figure 2, page 25). ERDC-CERL worked with structural engineers from Skidmore, Owings, and Merrill (SOM) to optimize the building structure following best practices and current code requirements for concrete structures, while ERDC-CERL provided expertise on AC, performed validation testing, and led the building construction. The B-hut utilizes a unique “chevron” design consisting of a triangular-wave base pattern that morphs to a straight pattern near the roof. This self-stable design allows for different lengths and heights with minimal changes to reinforcement. The walls were printed on a conventional cast-in-place reinforced concrete slab with turned down perimeter footings and preinstalled anchors. The walls were anchored through reinforced grouted cores connected to the precast reinforced concrete L-Beam roof with interconnected beams and slabs. Using AC, it is possible to reduce costs by 40% and utilize unique geometries that can improve structural performance that is generally considered cost-prohibitive. Testing confirmed the “chevron” wall exceeds design loads and outperforms a straight wall by 2.5 times for out-of-plane lateral loading for 3D-printed concrete walls. This effort demonstrated feasibility to complete a small building in 24 hours: 14 hours of print time (printer run time) and a 40 hour elapsed time (the total time to complete). Additive construction shows potential for modernization of military construction and the general construction industry as well.
Figure 3. citizenM Washington DC NoMa. Courtesy of Gensler; building artwork “Circulations,” 2020, by Hannah Whitaker.
walls throughout the building’s height. Lateral loads are transferred to the concrete shear walls via horizontal diaphragms composed of steel beams and X-bracing within the roof of each module. This type of modular design’s main challenge is integrating the separate structural systems of the modules that are prefabricated off-site with the on-site construction.
Accelerated Bridge Construction
Accelerated Bridge Construction (ABC) using prefabricated modular components has become much more commonplace in the last two decades, particularly in urban environments, given high traffic volumes Modular Building and poor detour options. Conventional staged construction can disrupt The citizenM Washington DC NoMa project (Figure 3) is pushing traffic for months, negatively impacting safety for both construction the boundaries as one of the first modular hospitality projects in personnel and the traveling public. Entire bridge replacement over a Washington, DC. The building, designed by Gensler, consists of two weekend is the goal of many ABC bridge projects. The replacement of levels of concrete framing, eight levels of prefabricated steel-framed State Route 30 over Bessemer Avenue in East Pittsburgh, designed by modules, and a steel-framed penthouse. HNTB Corporation, is a prime example (Figure 4). Polcom Group in Poland manufactures the 8- x 50-foot shipping The existing bridge carried daily traffic of over 20,000 vehicles container-sized modules, which typically include two hotel rooms and was in such poor condition that the design of the replacement and a corridor. Before shipping, each module is equipped off-site with scheme was accelerated. The demolition of the existing bridge began mechanical, electrical, and plumbing (MEP) systems and interior at 11:15 p.m. Friday. The entire span’s replacement, including abutfinishes and furniture. Building with prefabricated modules expedites ment seats and approach slabs, was completed and reopened to the project timeline by having a large portion of construction and traffic by 6:00 a.m. Monday for the morning commute. Prefabricated MEP work completed when the modules components included abutment seats, arrive on site. This method requires early, steel beam/concrete deck sections, definitive design coordination between the even integrating sidewalk and traffic architect and MEP engineers. barriers, together with approach and The modules have their own steel framsleeper slabs. Connections between ing, designed by Polcom and ARUP. The components were made with rapid set module columns are designed to carry Ultra-High Performance Concrete with combined gravity loads from the stacked cure times on the order of 12 hours. modules and the penthouse to the postConstruction logistics were key, and tensioned concrete transfer slab at Level the use of lightweight concrete for 2. The transfer slab forms a podium deck, barrier, and sidewalk sections that distributes the loads to the concrete were required to limit pick sizes given column grid below. The concrete podium the constrained site and crane access enables an optimized column grid for restrictions. The roadway underneath hotel facilities and a lobby at the base of the bridge was also milled and repaved, the building, differing from the column and the clearances improved from 13.75 layout of the modules above. feet to over 17 feet to reduce the risk of The building’s lateral load resistover-height vehicle impact. ing system, designed by Thornton Figure 4. Accelerated Construction of U.S. Route. 30 over Bessemer Limiting in-situ work to a 57-hour Tomasetti, consists of concrete shear Avenue in East Pittsburgh, PA. Courtesy of HNTB Corporation. weekend closure required extensive STRUCTURE magazine
pre-planning, design coordination, and prefabrication between designer and contractor. As part of the design, and subsequently validated by the contractor, a micro-schedule of all in-situ work activities was developed down to 15-minute intervals. The contractor’s diligence in high-quality prefabrication and strategic trial assembly over the 6 months between notice of award and in-situ construction were keys to the project’s success.
Conclusion This article summarized four projects spanning the building and bridge industries, from common practices that have been used for decades, such as accelerated bridge construction, to emerging frontiers, such as origami and 3-D printing. The field of modular, rapidly erectable, and deployable structures is broad, ranging from retractable roofs and panelized bridges to deployable space structures. These technologies have developed through both civilian and military efforts. A key to adopting these methods is outreach – cultivating and disseminating knowledge about best practices and opportunities for improvements. This learn-deploy-modify ecosystem, as well as cross-pollination from adjacent industries, offers significant opportunities for the advancement of a new paradigm in design: building faster and better. This article is a product of the new Modular, Rapidly Erectable, and Deployable Structures Committee of the Structural Engineering Institute (SEI). Those interested in joining this committee should contact the authors.
Reference is included in the PDF version of the article at STRUCTUREmag.org. Ashley P. Thrall is the Myron and Rosemary Noble Associate Professor of Structural Engineering in the Department of Civil & Environmental Engineering & Earth Sciences at the University of Notre Dame, where she directs the Kinetic Structures Laboratory. She is the inaugural Chair of the Modular, Rapidly Erectable, and Deployable Structures Committee of SEI. (email@example.com) Angelene Dascanio is a Senior Engineer at Thornton Tomasetti, Inc. and is the Secretary of the Modular, Rapidly Erectable, and Deployable Structures Committee of SEI. (firstname.lastname@example.org)
Acknowledgments Contributors to sections of the article include Eric L. Kreiger and Benton Johnson, Additive Construction, and Theodore P. Zoli, Accelerated Bridge Construction. The origami-inspired deployable structure material is based upon work supported by the U.S. Army Natick Soldier Research, Development, and Engineering Center (NSRDEC) under Contract W911QY-12-C-0128. The members of the Modular, Rapidly Erectable, and Deployable Structures Committee of SEI are gratefully acknowledged for their contributions to the article.■ Note: *Renderings reprinted from Energy and Buildings, 82, C.P. Quaglia, N. Yu, A.P. Thrall, S. Paolucci. “Balancing energy efficiency and structural performance through multi-objective shape optimization: Case study of a rapidly deployable origamiinspired shelter,” 733-745, Copyright (2014), with permission from Elsevier. Photograph reprinted from Ballard et al. (2016) ©ASCE.
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M A R C H 2 0 21
structural PRACTICES The Basics of the 5% Rule By Kirk Wagner, S.E.
recent webinar on the “10% Rule” included in the 2018 International Existing Building Code (IEBC), attended by the author, discussed an exception in the code’s Section 502.5 that allows existing structural elements to remain unaltered if an addition to that structure results in a less than 10% increase in the element’s demandto-capacity ratio. After the webinar, there remained the question, where was the companion webinar, the “5% Rule?” The 5% rule, otherwise known as the Prescriptive Compliance Method, is more straightforward and widely used. It is contained in IEBC Section 502.4 and states the following: Any existing gravity load-carrying structural element for which an addition and its related alterations cause an increase in design dead, live or snow load, including snow drift effects, of more than 5 percent shall be replaced or altered as needed to carry the gravity loads required by the International Building Code for new structures. It is important to note that this exists in the Additions section of the code, but its language and intent are echoed in section 503.3 for Alterations. The Alterations section also includes an additional exception for roofing, which allows for a second layer of roof covering weighing 3 pounds-per-square-foot or less to be applied over an existing roof. This particular example is discussed in more detail later in this article. The Prescriptive Compliance Method intends to allow for some flexibility for contractors and building professionals when doing simple additions, alterations, or maintenance work. It is unreasonable to require an engineer to evaluate an existing structure’s adequacy every time something minor arises. It can also be a daunting task when there is a lack of original plans and calculations, despite the addition being as simple as a new waterproofing material being applied or installing new ductwork. It is also an effort to provide an upper boundary for what can be done before triggering more strenuous requirements, as this is sometimes a grey area. The question of who is qualified to determine the original weight and whether the addition is within 5% remains, but that could be discussed to no end and the line has to be drawn somewhere. This article is an effort to clarify the rule and warn about using it. These portions of the IEBC are very interesting and, more importantly, dangerous to the general public if there are engineering, design, and/or construction individuals utilizing them without a complete understanding of the underlying principles. The author has encountered some situations where this rule was used improperly. The following can recommend ways to proceed should any readers come across similar situations. It is best to start with simple examples and then delve into some more advanced situations.
Example 1 An HVAC unit on the roof of a building needs to be replaced. The existing unit weighs 500 pounds, while the replacement unit weighs 525 pounds. If an engineer is trying to determine if the supporting roof beams are structurally adequate, and assuming size and anchorage locations/requirements are equal for both units, then the beams would fall within the 5% rule. The unit could be replaced without further structural analysis and without analyzing the beams or providing designs to bring them up to current building code requirements (assuming that excessive decay, corrosion, or other adverse effects are not present in the roof beam from the time of original construction). STRUCTURE magazine
Roof beam illustrations for corresponding example situations.
Example 2 Assuming the same building, roof beams, and existing HVAC unit, the replacement unit will now weigh 550 pounds. This would add a design gravity load greater than 5%. Thus, per IEBC Section 502.4, it would require the supporting roof beams to be analyzed and redesigned as required to carry the full new load under the current governing code provisions. Stopping right there would leave readers with a very clear and basic picture of what the 5% Rule means. However, a few more things need to be discussed concerning this rule because they are essential to public safety in structures. What would happen if there were a different situation where the unit has not changed weight, but the new unit’s location was shifted a little along the length of the supporting beam? Has the gravity load to the beam increased? It has not, so the change should still be acceptable under section 502.4, right? Wrong. The following example illustrates this.
Example 3 Assume that the original unit was located at the ¼ point along the beam’s length, and the new unit is to be placed at mid-span. Both units weigh 500 pounds. The design gravity load has not increased above 5%, but the moment demand on the beam has increased by 33%. In this situation, depending on the original moment demand-tocapacity ratio of the beam, it is likely that the beam is overstressed based on the original design building code used. Unfortunately, this is a very common mistake that some people make when thinking they are following the 5% rule. Engineers know that moving a load will have this effect, but sometimes there are people who think they are following the building code, citing the 5% rule, and move on. In this particular case and those like it, the situation would result in an unsafe condition and a possible collapse at worse.
The code section for this 5% Rule applies to all “gravity load-carrying structural elements.” This encompasses not only beams and other spanning members but also the connections. If the HVAC unit’s location moves, then the load carried by the beam connection at the end that the unit shifted toward has now increased, as shown in Example 3 above. Whether or not that increase is greater than 5% is a function of the weight change and the location shift.
The Dangers of Compounding Another improper use of this rule is when it is applied more than once over the life of a structure. In certain industries, it is common practice to add to existing structures repeatedly. One example is when new roofing material is stacked on top of the existing roof rather than stripping the old material. As previously discussed, there is an exception in the code that allows for this to happen once “over an existing single layer of roof covering.” Roofing may not weigh that much, but if this occurs multiple times, it could become a danger. There have been instances where the roof starts to visibly sag in warning or collapse altogether when wind or rain loads push the members over their capacity. There is typically a lack of existing drawings for the structures and even fewer structural calculation packages. Sometimes it is difficult to determine if a portion of the structure was a future addition or part of the original design. This leads many industry professionals to want to rely on the 5% Rule for simplicity due to a lack of existing information. The problem is that using the 5% Rule more than once on the same structure or structural element is in violation of the rule and can have a compounding effect. Say one project added 5% weight ten years after original construction, then another project adds another 5% ten years after that. The original structure has now had not just 10% of the original weight added (which would violate the Rule on its own), but actually, 10.25% weight was added (5% plus 5% of 105%.) Unfortunately, this can be a common occurrence and should make any engineer uneasy if noticed on an existing structure in the field. The author has seen numerous examples of this in the oil and gas industry, where pipes are continually being added and rerouted on existing structures. Suppose a full structural support evaluation is to be avoided. In that case, the engineers involved in these projects must determine if the addition or alteration is within 5% if there have already been additions and their effects. This task is also compounded by the fact that piping loads in this industry occur not just
Roofing layers stacked on top of each other, causing structural members to sag under the weight. (photo source: https://bit.ly/3aLYxIc)
Pipe rack at an oil and gas facility with multiple additions after initial design and construction.
from gravity, live, wind, and seismic forces but also from thermal and pressure expansion/contraction of the piping and its contents.
Additional Considerations Consider the case where the 5% Rule was used correctly in accordance with the code language and intent. The structure is most likely going to be OK, considering that most engineers do not push their demand-to-capacity ratios to 95% from the start. Additionally, statistical factors are applied to loads, assuming Load and Resistance Factor Design (LRFD), that could help offset a 5% gravity load increase. However, how would the structure fare if made of steel, located near the ocean, and showing signs of significant corrosion? Sure, it may be within the code to blindly apply new loads, but this could lead to failure due to the assumption that the current strength of a structure is equal to the original design strength. When evaluating existing structures, engineers know to take these things into account, not make blind assumptions.
Conclusion Engineers have the education and the skills to quantify and investigate existing structures accurately. Rather than rely on vague code language, they should use that knowledge to carry out projects correctly and safely. Of course, the argument against this is that it will require more work like a site visit, extra calculations, etc. This ultimately means it will be more expensive and, for a seemingly minor modification, it could be deemed not worth the expense. A response to that is: what is more expensive, the initial investigation or a collapse of the structure leading to a lawsuit? The risks and outcomes of projects need to be weighed similarly to how the LRFD method weighs loading and member strengths. The monetary and mental cost of stamping and signing off on a structure that collapses needs to be factored in. Yes, the 5% Rule is used by some outside of the engineering profession, and some may suggest that the language be removed from the code entirely to prevent it from being used by those who do not fully understand it. There is a common phrase, “I know just enough to be dangerous.” In situations such as these, this phrase is entirely accurate and not at all a joke. Someone who knows about Section 502.4 in the International Existing Building Code but fails to understand the implications of utilizing it can very easily create an unsafe situation in an existing structure. Everyone, engineers and others alike, should do the job right and not rely on this rule.■ Kirk Wagner is a Senior Structural Engineer with Axis Construction Consulting, Inc., who provides forensic structural engineering and design services. (email@example.com) M A R C H 2 0 21
TECHNOLOGY Communicating in a BIM World By Dr. Kristopher Dane and Mike Bolduc, P.E.
early as 1962, Douglas Englehart presaged BIM as “an evermore-detailed, interlinked structure, which represents the maturing thought behind the actual design.” Since this early vision, building designers have wholeheartedly adopted building information modeling (BIM), software-based design, and 3-D modeling. While BIM adoption has been praised for bringing efficiency to the construction process and benefits in coordination, it has also placed a new set of pressures on the structural designer. Effective communication between architects and engineers has always been critical to developing design concepts into construction drawings. Traditionally, printed drawings were exchanged at key milestones where they could be reviewed by design partners to inform their own work. Drawing coordination was a laborious process that required overlays via physical lightboxes, so a heavy emphasis was placed on communication “outside the drawings.”
Integrated BIM Falls Short Pressure on the AEC industry to innovate and improve efficiency has forced changes for the past 20+ years. Some people may challenge the comparison between traditional manufacturing and AEC as unfair because every structure is unique and customized. However, there is no debate that the AEC market demands shorter schedules and requires structural engineers to produce earlier design packages to jumpstart construction. In response to this schedule pressure, software vendors have touted BIM as the panacea to produce designs with less effort and reduced design conflicts. While it is true that some of the tedium of document management has been reduced (e.g., manually coordinating wall elevations with plans) and coordination has improved, the reality of BIM still does not match the integrated vision that we have been promised. The development of sophisticated “truly integrated” design tools has been limited to high-profile design projects, whereas the more widely adopted tools have not lived up to the hype.
Shortened Design Cycle Uncertainty in the design process is changing (see Figure). In the past, the basic design of a structure “locked-in” during the Design Development (DD) phase and the Construction Documentation (CD) phase was then spent coordinating and detailing the design (blue line). Wholesale changes were rare after the DD phase. Now, shorter schedules require early progress with more frequent iterations. Designs are developed with less information, which carries uncertainty further into the design process (red line). The resulting design iteration adds pressure on the designers to rapidly respond to significant changes and highlights the challenges with our BIM-based workflows. Simply using BIM is not enough to address the schedule pressures and pace of design changes.
BIM is not the Design Platform While this push to show progress earlier affects all disciplines in the design process, structural designers are placed at a distinct disadvantage because of how they use BIM. The analytical design model is often a simplified version of the coordinated BIM, containing just what is needed to understand the structural behavior. The BIM platform is STRUCTURE magazine
Uncertainty is now carried late into design.
typically separate from the multiple required structural analysis tools, which requires designers to develop processes to move information from BIM to analysis and back again. This challenge was acknowledged in the McGraw Hill’s Business Value of BIM survey (2012), “Structural analysis rates among the most difficult activities…indicating a critical need for the industry to address ways to make it easier.” Since 2012, however, few tools have been developed to help couple BIM with structural analysis compared to the number of tools aimed at improving contractor workflows. Even with interoperability tools, such as Konstru, the engineer still has to “leave” BIM to do their engineering work.
The Model is not the Deliverable Compounding the issue above is that the design model is rarely the actual deliverable. Project contracts often include language limiting the reliance on the model while emphasizing that the contract deliverable is still a set of 2-D drawings. This poses a challenge in maintaining the responsibility to deliver a complete structural construction document set. The potential to miss critical details that are not modeled is introduced as daily work processes become more model-driven, and less time is spent looking at the drawings.
The Model does not Capture Design Status In the past, early concept sketches were clearly identifiable as conceptual and preliminary. The team members understood that the ideas presented were subject to change. Now, however, models can appear to be very complete even at an early stage of design. It is harder to convey the preliminary and fluid nature of the design within BIM elements. Software is available to help track changes, either with 2-D PDF overlays or with 3-D model comparisons. These tools help designers identify what has changed, but they do not convey designers’ intent or priority for the change. The architectural intent and priority will direct how a structural designer responds and addresses a potentially long list of changes.
The Model does not Talk Too often, statements such as “it’s in the model” are made. Rather than communicating design intent or priorities “outside the model,” engineers and other consultants are asked to both find and react to
changes in real-time, without the context for “why” the change happened. The result is that tracking and reacting to changes has nearly become a full-time job for consultants. The challenges of changetracking are getting worse as it becomes easier to exchange models. Model transfers have increased in frequency from monthly to weekly, to daily, to live models.
Decide How to Communicate Many cloud-based communication and change tracking tools are targeted at contractors who are engaged when the design is less subject to significant changes. During design, BIM may be hindering communication if the design teams let the models communicate intentions that used to be communicated verbally. Every project team should decide how they want to communicate design changes and ensure that the tools selected serve the project's needs and best fit the schedule. The tools should not dictate the communication process.
Summary Technology is creating and providing tools for designers and engineers to live and work in a hyper-fast design environment. As technology advances, the personal and human nature of design must not be forgotten. Understanding the design intent and the priorities of design elements is a critical piece of coordination between trades. If models are simply shared without context and without communicating, then designers are at the mercy of the software they manipulate. The software cannot be allowed to control how individual design processes are approached. The human element of the SE profession is still paramount to creating coordinated and meaningful designs.
As structural engineers, we can adopt the following strategies to respond to these challenges: • Explain to architectural clients that the structural design process happens outside BIM and show that “frozen background” deadlines are critical to minimizing uncertainty and creating efficient designs. • Lean on software vendors and look internally to find efficiency so that structural analysis is no longer an outlier in BIM integration. • Proactively open lines of communication with the design team when sharing models and reviewing changes. • Set communication guidelines early in the design process to engender a collaborative environment. • Avoid letting the model to speak for us and adopt the right tools at each phase of design. • Understand that communication is a crucial component of successful collaboration in a BIM world. Adopting these strategies will help designers minimize the rework penalty, stay efficient in a slowing economy, and prepare for a future where BIM is the deliverable.■ References are included in the PDF version of the article at STRUCTUREmag.org. Dr. Kristopher Dane is Vice President and Director of Digital Design with Thornton Tomasetti, Inc. and member of the Joint SEI-CASE Committee on Digital Design (formerly BIM). (firstname.lastname@example.org) Mike Bolduc is a Senior Project Manager with Simpson Gumpertz & Heger Inc. and Co-Chair of the Joint SEI-CASE Committee on Digital Design (formerly BIM). (email@example.com)
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CASE business practices Do You Have a Force Majeure Clause in Your Engineering Services Contract? By Bruce Burt, P.E.
has been a turbulent and unpredict- efforts were made to limit the effects of the able past twelve months. Still, it appears triggering event. engineering firms were in a better position to Take the example of a data loss due to server respond to the interruptions resulting from malfunction. This could certainly cripple a COVID-19 than many other companies. With the necessary network infrastructure in place, A force majeure many engineering firms effectively performed clause is intended to design/analysis and small team collaboration in a limit a firm’s liability remote environment. At least early in response to if it cannot fulfill its the pandemic, many surveys showed utilization contractual obligations rates actually increasdue to events prescribed ing within engineering firms. While many firms in the clause. dodged a disruption bullet, not all managed to do so. Valuable lessons were learned about a firm’s resilience, lack thereof, and prepared- firm for an extended period. However, there ness for disruptive events. is a question as to whether a data loss would One lesson to be learned is the importance qualify if it can be argued that most comparaof having protective language in a firm’s bly sized firms have backup systems. Another contracts for events that are beyond a firm’s example is a data breach. Even firms with suitcontrol. Though many companies weathered able security systems in place could fall victim the effects of the coronavirus reasonably well, to ransomware. The specific conditions of a other potential events are lurking that could data loss or breach could determine whether result in costly project delays – delays that a the force majeure clause is applied. firm may be held liable for despite its lack of Most engineering firms’ force majeure control over them. clauses address delays. Although not usual in A force majeure clause is intended to limit a professional service firms, companies in other firm’s liability if it cannot fulfill its contractual industries have included non- or under-perobligations due to events prescribed in the formance in their force majeure clause. Firms clause. The events in a force majeure clause should exercise caution in expanding a force would be considered outside a firm’s reason- majeure clause. An overly broad clause or the able control and usually include “Acts of God” inclusion of additional provisions may negate (fires, floods, hurricanes, earthquakes, etc.), more common and generally accepted force governmental or other regulatory action or majeure contract language. Also, firms must inaction, and other listed events such as war, use care in identifying the kinds of occurterrorist acts, strikes, and labor disputes. A rences that are included in a clause. Having pandemic would certainly be considered an too many specific events may cause a force “Act of God” force majeure event. majeure clause to be strictly interpreted to Two other conditions usually need to be met only the events listed. Using less specific lanto trigger a force majeure clause. First, the guage, such as “epidemics and pandemics,” is event must negatively affect a firm’s ability to more likely to be enforceable than including perform its contractual obligations. Second, reference to a specific condition like COVIDthe firm must demonstrate that good-faith 19. And including “government action” that STRUCTURE magazine
may result from a pandemic or other event should also be considered since many parts of the country suffered significant disruption due to government-mandated closures. An example “Delays” clause might be written as follows: The Structural Engineer (SE) shall not be responsible for delays caused by factors beyond SE’s reasonable control, including but not limited to delays because of strikes, lockouts, work slowdowns or stoppages, government-ordered industry shutdowns, power or server outages, acts of nature, widespread infectious disease outbreaks (including, but not limited to epidemics and pandemics), failure of any governmental or other regulatory authority to act in a timely manner, failure of the Client to furnish timely information or approve or disapprove of SE’s services or work product, or delays caused by faulty performance by the Client or by contractors of any level. When such delays beyond SE’s reasonable control occur, the Client agrees that SE shall not be responsible for damages, nor shall SE be deemed in default of this Agreement. As always, when adding or modifying contract terms, seek the advice of legal counsel. For firms who purchased contracts published by ACEC’s Council of American Structural Engineers (CASE), a force majeure clause template is available for free download at www.acec.org/case/news/publications. Future versions of all twelve CASE contracts will include a “Delays” clause.■ Bruce Burt is Vice President of Engineering with Ruby+Associates, Inc., a constructability-focused structural engineering firm located in Bingham Farms, Michigan. He is a member of the CASE Contracts Committee. (firstname.lastname@example.org)
M A R C H 2 0 21
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Report on Chemical Admixtures for Concrete
Reported by ACI Committee 212
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National Council of Structural Engineers Associations NCSEA Corporate Members
SEAs Honored for Outreach Efforts
The NCSEA Communications Committee challenged its Member Organizations (the state SEAs) to share their most effective program for achieving NCSEA’s goal that “practicing structural engineers are recognized by clients, media, policymakers, educators, students, and the public for the value of their contributions to society.” SEAs were asked to submit up to one entry for each of the following categories: Public Outreach, Political Advocacy, Students and Educators, and Other Outreach Programs Related to Current NCSEA Initiatives. Outstanding Efforts in each category were recognized at the NCSEA Annual Business Meeting, held on January 13, 2021:
SEAKM (Kansas-Missouri): Students and Educators Winner
At the beginning of 2020, SEAKM received a grant from NCSEA to help a local elementary school with supplies for a weekly student STEM learning activity period, and chose to provide Mola or MOLA magnetic structural model kits. This idea blossomed from helping one school A teacher using a Mola Structural Model kit with to an entire school district, and SEAKM’s program inspired a larger collective effort of NCSEA students during SEAKM's presentation. to partner with Mola Models, developing lesson plans, activities, and exercises for structural engineers to partner with schools across the country.
SEAW (Washington): Public Outreach Winner
SEAW’s entry highlighted their lecture series, which presents structural engineering topics to a wider audience. This year, they rose to the challenges presented by COVID and expanded their reach to multiple continents via a well-designed virtual forum. Their execution of collaborating with cooperating organizations (from university engineering and architecture departments to other SEAs and materials organizations), publication in various news outlets, and self-advertising on a widely followed social media platforms was impressive, and demonstrated the ability of structural engineers to capture the interest and attention of the masses, even when faced with incredible constraints and limitations.
SEAW (Washington): Political Advocacy Winner
A screenshot from SEAW's lecture series that awarded them the Public Outreach category.
SEAW presented a comprehensive look at their integration with legislative boards dealing with building codes, licensure, and disaster response. Through their involvement in multiple committees and councils, SEAW is a great example of how structural engineers can be recognized as resources for governing agencies.
SEAOC (California): Other Winner
SEAOC’s submission demonstrated a clear response to NCSEA’s Call to Action, published earlier this year. The SEA has a flourishing and active committee (SE3) dedicated to supporting DEI initiatives through successful mentorship programs, roundtable discussions, events, forums, and presentations. They have promoted and created opportunities with ally organizations to empower local leaders and community members with an understanding of how structural engineers support their community. And they offered substantial documentation of efforts of their funding commitment to scholarships for a diverse group of students, as well as sponsorship and publication of scientific studies, papers, and other reports of special importance and significance to structural engineers. We hope the programs from the winning SEAs can become prototypes for other organizations’ programming. NCSEA will be featuring them in upcoming communications. Additionally, category winners will also be receiving several MOLA Structural kits to help them further their outreach activities.
Call for 2021 Structural Engineering Summit Abstracts
NCSEA is seeking abstracts for the 2021 Structural Engineering Summit. The goal is Potential submission topics include: to host the event both in-person in New York, New York, on October 13-15, 2021, and • Best-design practices virtually throughout the month of October. • New codes and standards Sessions will be 45-60 minutes total, including time for Q&A, and should deliver • Recent project case studies pertinent and useful information that is specific to the practicing structural engineer. These • Advanced analysis techniques parameters apply to both technical and non-technical tracks. Presentations are sought for • Management and business practices topics that would appeal to seasoned engineers or younger engineers new to their careers (or • Diversity and inclusion both). The goal of Summit is to provide structural engineers with tools, techniques, and tips • Resilience that will help them and their firms operate more efficiently and effectively. • Sustainability Submit your abstract March 26, 2021. Visit bit.ly/2021SESabstracts for more details. STRUCTURE magazine
News from the National Council of Structural Engineers Associations NCSEA Foundation Awards SEAs Grants For the fifth year, NCSEA has awarded grants to its Member Organizations through the NCSEA Grant Program. The program was developed in 2014 to assist NCSEA Member Organizations in growing and promoting their organizations and the structural engineering field; the first five grants were presented the following year. Over $70,000 in grants have been awarded since 2015. The 2020 Grant Program was the first program supported by the NCSEA Foundation. The Foundation was established in early 2020 to support the non-profit activities of NCSEA and its Member Organizations that advance the structural engineering profession through technical development, education, and outreach. The Purpose of the Foundation is to fund qualifying initiatives and activities that are intended to aid in the advancement of the science and practice of Structural Engineering as well as promote engagement within NCSEA’s Member Organizations. The Foundation itself is supported by specialty ticketed events throughout the year as well as individual and corporate donations. NCSEA is proud to announce the 2020 Grant Recipients. Congratulations to: SEAC (Colorado) awarded funding to launch a local SE3 Committee.
OSEA, 2019 Grant Recipient, used their fund to volunteer in Oklahoma's E-week 2020 Bridge Competition.
SEAOI (Illinois) awarded funding to create a cohesive library of STEM videos and funding to enhance the association's Remote Site Visits. SEAU (Utah) awarded funding for a Wasatch Front Seismic Study to understand the cost implications of designing buildings for a Collapse Prevention risk target for the 84th percentile Wasatch fault response accelerations, rather than the current Ss and S1 code values. SEAONC (Northern California) awarded funding for SE3 DEI Firm Leader Cohorts to provide a forum in which firm leaders can discuss their commitments to and the challenges in implementing strategies for racial DEI in their firms and the industry. SEAoO (Ohio) awarded funding to further support their students by establishing a new $1,000 award. SEAKM (Kansas/Missouri) awarded funding to enhance their Mola Model Initiative and Student Outreach. Thank you to the SEAs that applied for a grant and continue to work for the betterment of their association and the profession! For more information about the Grant Program and to apply for a 2021 Grant, visit www.ncsea.com.
2021 Excellence in Structural Engineering Awards
NCSEA's Excellence in Structural Engineering Awards annually highlights some of the best examples of structural engineering ingenuity throughout the world. Projects are judged on innovative design, engineering achievement, and creativity. Multiple winners are presented in eight categories with an outstanding winner chosen from each category. The winners will be honored at NCSEA's Structural Engineering Summit this October. Entries are due on July 13, 2021. Structural engineers and structural engineering firms are encouraged to enter. More information about the awards along with submission instructions can be found on www.ncsea.com.
Register by visiting www.ncsea.com.
March 4, 2021
March 23, 2021
April 6, 2021
Overview of Changes and Additions in ACI 318-19
Practitioner's Guide to Designing Buildings in Flood Zones
The State of the Art in Existing Masonry Structural Testing
Royce Floyd, Ph.D., S.E.
Kevin Chamberlain, P.E.
Donald Harvey, P.E.
This webinar will discuss technical changes made to design provisions in the 2019 edition of the ACI 318 Building Code Requirements for Structural Concrete and provide an overview of the reorganization for the 2014 edition that carried over in the 2019 edition.
Most structural engineers will design a building or structure in or near a body of water. It's important to understand the requirements and limitations placed on buildings in flood zones. This webinar will present an experienced practitioner's take on what a structural engineer needs to know in their practice.
Fortunately, there are many tools available for the evaluation of existing masonry and existing masonry distress. This webinar will discuss the latest and greatest in both destructive and nondestructive methods for existing masonry structural evaluation.
Courses award 1.5 hours of Diamond Review-approved continuing education after the completion of a quiz. M A R C H 2 0 21
SEI Update Learning / Networking
SEI Virtual Events
www.asce.org/SEI/virtual-events • SEI Standards Series Join live, virtual sessions for exclusive interaction with expert ASCE/SEI Standard developers on state-of-the-market updates. Participants will learn about technical revisions and review a design example. Attendees are encouraged to join/participate in Live Q&A. Each session is LIVE and only available 1:00 - 2:30 pm US ET. MARCH 18 - ASCE/SEI 43 Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities Registration closes March 16 at 10:00 pm US ET Guests: Andrew Whittaker, Ph.D., P.E., S.E., F.ASCE, F.SEI; Chair of ASCE/SEI Nuclear Standards Committee F George Abatt, Ph.D., P.E., F.ASCE; Vice-Chair of ASCE/SEI Nuclear Standards Committee The 2019 edition of ASCE/SEI 43, Seismic Design Criteria for Structures, Systems, and Components in Nuclear Facilities, ensures that nuclear facilities are properly designed to withstand the effects of earthquake ground shaking while retaining the desired functionally, expressed as target performance goals. Because nuclear facilities process, store, or handle radioactive materials in a form and quantity that pose a potential hazard to the workers, the public, or the environment, it is desirable that nuclear facilities have a lower probability of structural damage caused by earthquake shaking than commercial buildings. While this standard is intended for use in the design of new facilities, it can also be used for existing facilities. Individual session: Member $49, Nonmember $99. Student member: Free registration. REGISTER NOW at https://cutt.ly/9hQDTEo • Panel discussion on The Future Impact of COVID-19 on the Commercial Development Market Tuesday, March 30, Noon - 1:30 pm US ET – Free Registration The event will focus on the current and future impact of the COVID-19 pandemic on the structural engineering profession/construction industry at both local and global levels. Global leaders from the real estate development/construction industry and different geographies will share their thoughts of the post-pandemic world and the industry trends they foresee developing. Event made possible by SEI Global Activities and the SEI Futures Fund. • Structures Virtual June 2-4, 2021 Expanding our reach to advance structural engineering practice! We look forward to sharing the latest advances and new interactive technical/professional learning. Learn more and register at www.structuresvirtual.org #StructuresVirtual21 To sponsor, contact Sean Scully email@example.com
View the program at the website, including: • Expert Technical Sessions: Wildfires, Case Studies/Projects, Design/Analysis, Lattice Towers, Managing Aging Infrastructure, Substations, Loading, Foundations, Special Design Considerations, SEI/ASCE Overhead Line Loading Standard • New SEI Futures Fund Scholarship for Students to Participate • Sponsor/Exhibit – Reserve your space with Sean Scully firstname.lastname@example.org www.etsconference.org #ETSC21
Errata STRUCTURE magazine
SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata. If you would like to submit errata, contact Kelly Dooley at email@example.com.
News of the Structural Engineering Institute of ASCE Advancing the Profession
Congratulations to 2021 SEI Fellows: Sigrid Adriaennsens, Ph.D., F.SEI, A.M.ASCE Mohammad AlHamaydeh, Ph.D., P.E., F.SEI, M.ASCE Mark Bendok, P.E., S.E., F.SEI, M.ASCE Paolo Bocchini, Ph.D., F.SEI , M.ASCE Laura E. Champion, P.E., F.SEI, F.ASCE John Cleary, Ph.D., P.E., F.SEI, M.ASCE Ross Corotis, Ph.D., P.E., S.E., F.EMI, F.SEI, Dist.M.ASCE, NAE John Duntemann, P.E., S.E., F.SEI, M.ASCE
Michel Ghosn, Ph.D., F.SEI, M.ASCE John Lobo, P.E., P.Eng, F.SEI, M.ASCE Peter Marxhausen, P.E., F.SEI, M.ASCE Eric Sammarco, Ph.D., P.E., F.SEI, M.ASCE Ayman Shama, Ph.D., P.E., F.SEI, F.ASCE, LEED AP+C, ENV-SP Jeannette Torrents, P.E., S.E., LEED AP, F.SEI, M.ASCE Cheng Yu, Ph.D., P.E., M.ASCE
The SEI Fellow grade of membership recognizes accomplished SEI members as leaders and mentors in the structural engineering profession. Applicants must be current SEI, actively involved, licensed P.E./S.E., ten years responsible charge (typically post P.E.). View/connect with SEI Fellows and complete an application package by November 1 via www.asce.org/SEIFellows.
SEI SE2050 Commitment Program Visit SE2050.org to view the growing list of firms committed to the SE2050 program and resources for reducing embodied carbon.
Bridges Inspire ASCE is looking for striking photographs of bridges, taken in the United States or abroad, that highlight design and engineering achievements of civil engineers. ascebridgephotos.org
Celebrating 25 years of SEI – Advancing and serving structural engineering
SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle M A R C H 2 0 21
CASE in Point 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. 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 internal policies and procedures – from office policy guides to employee reviews: Tool 1-3 Tool 2-2 Tool 2-3 Tool 2-5 Tool 4-3 Tool 5-3 Tool 5-5
Sample Policy Guide Interview Guide and Template Employee Evaluation Templates Insurance Management Sample Correspondence Guidelines Managing the Use of Computers/Software Project Management Training
You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.
CASE Practice Guidelines Currently Available CASE 962-H – National Practice Guideline on Project and Business Risk Management This guideline is intended to assist structural engineering companies in the management of risk associated with projects and to provide commentary regarding the management of risk associated with business practices. The guideline is organized into two sections that correspond with these two areas of risk, Project Risk Management and Business Practices Risk Management. The goal of the guideline is to educate and inform structural engineers about risk issues so that the risks they face in their practices can be effectively mitigated, thus making structural engineering firms more successful. Structural Engineer’s Guide to Fire Protection This publication is intended for structural engineers with no prior experience or training in fire protection engineering. It is a comprehensive and concise treatment of prescriptive and performance-based methods for designing structural fire protection systems in an easy-to-understand format. CASE 504 – Proposal Preparation Spreadsheet The CASE Proposal Preparation Spreadsheet was developed to assist project managers and administrators in developing cost proposals for a project. The spreadsheet may be easily customized for any organization or project type. It also may be used as a checklist to see that all phases of a project are adequately staffed. CASE 976-A – Commentary on Value-Based Compensation for Structural Engineers Value Based Compensation is a means to step out of the ordinary and establish your value to the team. Value Based Compensation is based on the concept that there are specific services, which may vary from project to project, that provide valuable information to the client and whose impact on the success of the project is far in excess of the prevailing hourly rates. Value Based Compensation is based on the increased value or savings these innovative structural services will contribute to the project. As a result, the primary beneficiary of an innovative design or a concept is the owner, but the innovative engineer is adequately compensated for his knowledge and expertise in lieu of his time. You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.
Follow ACEC Coalitions on Twitter – @ACECCoalitions. STRUCTURE magazine
News of the Coalition of American Structural Engineers Save on CASE Membership!
Can you ever really be too successful? Keep your business thriving – no matter what your competition or the economy is doing – and say YES to membership in ACEC’s Coalition of American Structural Engineers (CASE). An “Association within an Association” that complements your ACEC National benefits. CASE, the oldest of ACEC’s four discipline-specific Coalitions, is a professional community for, of, and by structural engineers who want relevant, useful information – on BIM, international building codes, risk management, and more – to run their businesses better. Join CASE today and you’ll qualify for: • Education: CASE offers a track of 3 dedicated education sessions at both the ACEC Fall Conference and Annual Spring Convention to keep members current with best practices and trends in structural engineering. As a member, you’ll also receive a discounted rate to ACEC webinars focused on structural engineering issues. CASE also provides education sessions at the AISC Steel Conference and the ASCE-SEI Structures Congress. • Resources: Coalition members get free access to over 145 contracts, tools, and publications (a total value of over $5,000!). CASE developed over 70 documents geared toward structural engineering firms. • Advocacy: Your voice matters! Coalition members are often the first ones contacted to share their expertise with Congress and government agencies in response to current legislation and relevant regulatory agendas. Save $75 off your first year’s dues through June 30, 2022! Join CASE by March 31, 2021 and get 15 months for the price of 12!
Interested? Contact Marie Ternieden at 202-682-4323 or firstname.lastname@example.org.
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 $34,500 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 in pursuing 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 tax deduction and you don’t have to be an ACEC member to donate! Contact Marie Ternieden at email@example.com to donate.
Manual for New Consulting Engineers An HR Favorite for New Hires
ACEC’s best-seller, Can I Borrow Your Watch?” A Beginner’s Guide to Succeeding in a Professional Consulting Organization offers new engineers a head start in the business of professional consulting. This essential guide is tailored to the unique needs of engineering firms, and the skills and experiences rookie consultants need to be successful in a large organization, including: • Proposal Preparation • Financial Management • Client Relationships • Project Management • Staff Management With over 140 pages of consulting expertise, this resource is the perfect addition to any new staffer’s welcome pack or in-house orientation. It can even be a useful resource for more seasoned engineers looking to refine their skills. To order this book, go to www.acec.org/bookstore. Bulk ordering is available, for more information contact Maureen Brown (firstname.lastname@example.org). M A R C H 2 0 21
business PRACTICES Opening a Branch Office By Stephen Lehigh, P.E.
Editor’s Note: Today, many firms have tackled remote work in response to COVID, and learned several lessons along the way. Although this article is about issues that arose well before the current pandemic, it is interesting to realize that today’s firms may very well be better positioned to create solutions like those presented here.
he departure of a valuable employee from a company due to a spouse’s relocation can be difficult for a company and the employee. What options does the company have to retain this employee? Is it beneficial for the company to have the employee work remotely, and what are the challenges? I found myself in this position three years ago when my wife was transferred for her job, and we had to decide to move 1,900 miles away. It was a challenging time, to say the least. I enjoyed the company that I worked for and my advancement within the company could be lost, but I had no choice, or at least that is what I thought. I decided to have a private meeting with the president of the company to break the bad news. Walking into the meeting was very nerve-wracking, and the outcome was uncertain. In my mind, there was only one option on the table – for us to part ways. What other option was there? To my surprise, shortly after the words left my mouth, my boss made an offer I could not refuse. He offered me the ability to work as a remote employee. I was surprised and greatly appreciated the offer. It made sense as he did not want to lose a valuable employee, nor did I want to leave. After further discussions, working remotely started to become a reality and we knew that there would be challenges to overcome. However, with today’s technology, the ability of Virtual Private Networks (VPN), internet phone services, and screen-sharing software, we had a positive outlook. This situation was new for both the firm and me, but we had to work together to succeed. During the first few months, the plan was for me to work at home, settle in as a remote employee, work out the kinks, and find out if this option would benefit us both. Most importantly, for this to work, I must be happy
with working at home exclusively. This was far different than being in an office environment each day. There was no more interaction with other employees, watercooler talk, or simply being at an office. It took time to adapt, but
seemed to be an excellent opportunity for us in the market, we decided to open a satellite office and hire an entry-level engineer from the area. We mainly focused on marketing and ways to get our name in the community. We needed the community and potential clients to know we were here and ready to help them with their structural needs. One tool that seemed to pay off was making our new location known on Google. This allowed people to find us quickly when searching within the area for a structural engineer. Although this helped, more needed to be done. Since opening the office, we have focused on researching local businesses to determine how we could benefit them. We have emailed firms to try to set up meetings with their principals and have sent mailings of pertinent and useful structural code information to all of our contacts. Recently, we donated our time to an Eagle Scout troop with a project at a local church. Business is increasing, and we have a positive outlook for the future. From my personal experience, this could not have worked out better for me. From the day I sat down with my company President until now, it has been a journey that I was happy to take. I have since become a shareholder in the company, lead a satellite office, and am now on the Board of Directors. As an employee, the future seemed uncertain when I had to decide to relocate. Working for a company that is dedicated to its employees made this an easy process. Although this path may not work for every firm, it shows that there are options to retain an employee faced with a relocation dilemma. Company alterations come with challenges; the benefits certainly outweighed those challenges for us.■
“There are options to retain an employee faced with a relocation dilemma.”
once a home office was set up, I became more focused and determined to make it work. This was a great opportunity, and I found myself happy with the situation. One major obstacle for me was my role as a Project Manager without having direct contact with engineers working on my projects. Having the ability to sit at a desk with an engineer to work through a project was a thing of the past. This was a big hurdle to overcome. In the beginning, we used screen sharing and video chat software. This process worked at the time, but since then, the use of Zoom and Microsoft Teams has dramatically improved the experience. These platforms now allow me to have face to face interactions at any time with coworkers. To further aid the situation, I traveled three to four times back to the company office. This put me back in front of our engineers a few times a year and gave me the ability to visit with clients. A benefit to having a remote employee is the option to expand the company. After the initial year, we started looking at the bigger picture. Since construction in the area where I relocated was on a steady incline, and there
Stephen Lehigh is a Project Manager for the DiSalvo Engineering Group and heads the Castle Rock, CO branch office. (email@example.com)
M A R C H 2 0 21
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STRUCTURE M A RCH 2021 |
MASS TIMBER Outdoor Education Center Figure 1. Main entry to the Andy Quattlebaum Outdoor Education Center.
By David Impson, P.E., S.E., Andrew Ruffin, P.E., S.E., and Brian Haygood, P.E.
ituated on the shores of Lake Hartwell, the Andy Quattlebaum Outdoor Education Center serves as the new 16,500-squarefoot home of the Clemson Outdoor Recreation and Education program (Figure 1). The building is the first facility east of the Mississippi River and second in the country to use cross-laminated timber (CLT) made from southern pine (SP). It is part of the growing wave of interest in mass timber construction. Clemson University’s history with forestry and effective land use date back to its founding as a land grant university in 1889, but it received a significant push in 2014 with the establishment of the Wood Utilization + Design Institute (WU+D). The Institute is a multi-disciplinary group of members focused on forestry, architecture, construction, and engineering with the goal of better utilizing existing timberland throughout the state and developing creative ways to use timber in construction projects. This group has reached a significant milestone in completing the first mass timber building on campus in recent history. With the university’s history as a land grant institution and the WU+D Institute pushing for advancement in timber construction, it was only fitting that Clemson build a structure to bring students closer to local forests and lakes using the same trees that surround them. The new facility's focal point is a two-story building with an open atrium at the main lobby (Figure 2). The atrium incorporates expanses of glazing along with clerestory windows throughout to maximize natural light. This part of the facility includes classrooms, office space, equipment rental, a ground-level covered patio, and a second-floor terrace overlooking the lake that can be used for events or outdoor classrooms. Behind this building is an open-air boathouse with a patterned steel and timber slat wall for rain screening. The desire to showcase an exposed timber structure required a more cooperative approach with the architectural team at Cooper Carry than a typical project to ensure the structural design and detailing met the architectural intent. Every exposed element and connection was thoughtfully considered to maintain a consistent appearance throughout the facility.
Structural Issues The signature roof structure consists of a single-slope pitched roof with double glulam beams spaced at 16½ feet, spanning 60 feet between steel pipe columns. The 27½-inch-deep glulams cantilever beyond the building on each side with a 14-foot cantilever at the roof over the second-floor patio. The double glulam beams sandwich structural steel pipe columns at the two-story building and wide flange columns at the boathouse. The three-ply, 4⅛-inch-thick CLT panels cantilever out an additional 2½ feet beyond the end of the glulams on the panels’ weak axis to create a strikingly thin profile. The CLT STRUCTURE magazine
Figure 2. Interior lobby with exposed CLT elevator shaft and glulam beams.
panels cantilever as much as 9 feet on their strong axis at the ends of the roof to further this design feature. The second-floor framing consists of 27½-inch-deep double glulams spanning 30 feet between steel columns. The floor construction consists of a 2-inch concrete topping slab on 6⅞-inch-thick, five-ply CLT. These beams cantilever 10 feet to create the exterior terrace with the CLT extending an additional 18 inches to create a unique edge profile. One of the benefits to southern pine, aside from its abundance in the Southeast, is its material property strength. Because of its higher strength relative to other materials, southern pine CLT allows for longer spans without construction shoring that could be required for a comparable span using a composite metal deck. A more challenging aspect of this project design was creating a lateral system that worked with the architectural desire for an open structure. At the two-story building, tension-only rod braces were used in one direction and were able to be accommodated within the enclosed twostory area. For the perpendicular direction, the covered patio provided no opportunities for bracing or wall elements. After considering various options, the design team elected to use a hybrid steel column-glulam beam moment frame approach. A 3-D model was created to accurately determine moment frame stiffnesses and the building’s torsional response due to a relatively stiff rod brace at the north end and five sets of moment frames at the south end. This model was used to determine the required connection forces. As is frequently the case with timber construction, the real opportunity for creativity lies in the connection design. Because wood’s bearing capacity exceeds its dowel-action shear capacity, custom steel brackets were detailed with an L-shaped bearing plate for vertical support of the glulam beams. Bolts were used to transfer moment into these steel brackets welded to the columns. To accommodate the maximum bolt
Figure 3. Steel/glulam moment connection.
Figure 4. Detail of steel/glulam moment connection.
spacing of 5 inches for fasteners connecting wood to rigid elements (National Design Specification (NDS®) for Wood Construction 2015, Section 184.108.40.206), vertical slots were used at the top bolts to allow for wood shrinkage relative to the steel but still transfer moment between the beam and column (Figures 3 and 4 ). The building’s layout features a large open floor plan on the first floor with the elevator in a central location, meaning the shaft is a prominent feature. While not required for load-bearing support, the elevator shaft was erected with 5-ply CLT panels. CLT erection of the elevator shaft was completed in one day, saving between two and three weeks of erection time compared to the labor required for traditional CMU block walls. Because the building was designed for Type V-A construction, the shaft was required to have a one-hour fire-resistance rating. To achieve this with mass timber, the effective char depths table for CLT per NDS Chapter 16 was used to design for the required fire-resistance rating.
Trade and Mechanical Coordination Due to the integration of structural steel supports for the mass timber framing, one of the challenges was coordinating between two trades with different subcontractors during the shop drawing review process. Because of the exposed nature of the building structure, the architecture team was involved in this coordination as well. As the EOR, the author’s firm’s responsibility was to help bridge the gap between the two trades and help guide the team towards a solution that met the architecture team’s visual expectations. The design team did not determine the CLT panel layout, but coordinating mechanical duct drops and general openings during shop drawing review was critical to the design. Because of the layout and the few duct-drops through the floor, duct openings were dictated to be centered in panels to avoid transferring forces across panel joints. This allowed for a slim deck profile without adding framed glulam beams around the openings. Panel capacities were analyzed by taking a moment and shear reduction of the section based on the percentage of panel width removed for an opening. These capacities were then compared to the design moment and shear along the panel length.
Costs While many interested parties had a strong desire for this building to utilize mass timber, the design and construction team still needed to justify mass timber from a cost standpoint. Overcoming the perception
that mass timber would be a significant cost premium proved a challenge and required significant persuasion to include mass timber in the first round of pricing. In the early phases, the design team included a mass timber roof as the base concept but chose to use a traditional steel and composite deck for the 2nd floor. As an alternate, a mass timber floor structure was incorporated into the narrative as a final effort to have it included. While the steel's installed cost was less expensive than the installed cost of the mass timber, the mass timber floor and roof eliminated additional costs for architectural soffits and ceiling finishes. With the mass timber serving as an architectural finish, the timber floor structure presented overall cost savings and became the base design moving forward. As CLT becomes more prevalent as a construction material and additional manufacturers come online, it is anticipated that mass timber will continue to become a cost-effective option for new construction.
Conclusion When beginning any project where the exposed structure will be a focal point of a building’s architecture, it is critical for the structural and architectural teams to understand the desired appearance early in the process. From there, the structural engineer can guide the architect on the most cost-effective combinations of CLT floor panel thickness and beam and column spacings. To create a costcompetitive wood structure, the maximum allowable span of the CLT panels must be used for beam spacing to reduce excessive cost in the CLT and reduce the number of beams and beam end connections. After the layout is set, the focus can shift to various exposed timber connections to ensure that structural and architectural needs are met to achieve a consistent look throughout the building. With this process in mind, the design and construction team created a building that serves as a natural bridge between classroom learning and the lakes and forests surrounding it, inspiring students to explore the world around them.■ David Impson is a Principal with Britt, Peters & Associates, Inc. in Greenville, SC, and has been an advocate for mass timber construction in the Southeast. (firstname.lastname@example.org) Andrew Ruffin is a Project Manager with Britt, Peters & Associates, Inc. in Greenville, SC. (email@example.com) Brian Haygood is a Project Engineer with Britt, Peters & Associates, Inc. in Greenville, SC. (firstname.lastname@example.org)
M A R C H 2 0 21 B O N U S C O N T E N T
SPOTLIGHT Resiliency for Affordable Housing
asa Adelante is a seismically resilient nine-story affordable housing project for low-income seniors, with 25% of the units reserved for formerly homeless seniors. The specially “tuned” reinforced concrete building uses self-centering walls on a rocking mat foundation. Lead extrusion dampers within the foundation control the seismic response. The building has been evaluated to have zero days of downtime for repair after a major earthquake, and the project received a Gold Rating from the US Resilience Council. Mar Structural design immediately moved to make Casa Adelante structurally special when learning that the building would be 100% affordable, serving low-income seniors with 25% of the units set aside for formerly homeless seniors. The designers were compelled to make the nine-story reinforced-concrete apartment building seismically resilient to protect an economically vulnerable population who likely would have very limited resources. There were no illusions that there would be “extra” money from the non-profit developers for improved seismic performance. The reality is that more housing is significantly more important than better seismically performing housing in San Francisco. The region suffers an acute shortage of affordable housing. Mar Structural challenged itself to create affordable seismic resilience with this as context and constraint. Conventional code-conforming buildings can sustain significant structural damage in the process of protecting the safety of the occupants during a major earthquake. However, damage can lead to loss-of-use and, in the case of the Casa Adelante residents, the potential for homelessness. However, some structural forms have intrinsic capacities that are untapped. The approach for Casa Adelante was to utilize a cantilevered wall structure on a rocking mat slab foundation and “tune” it using nonlinear response-history analyses (NLRHA) so that it would be strong, stiff, and capable of self-centering in a major earthquake. To create foundation rocking, designers balanced the strength within elements to get the building to re-align after an earthquake. Specifically, the resilient scheme employed the following steps: The formerly liquefiable site was strengthened with deep soil mixing, providing extra strengthening at the ends of walls to preclude soil failures and ensure rocking. The mat slab STRUCTURE magazine
and the post-tensioned concrete floor plates were made relatively strong for positive bending and relatively week for negative bending. These arrangements allowed re-centering and just enough energy absorption. Two of the transverse walls needed to be located with one end adjacent to a property line. Four lead extrusion dampers were installed within the foundation and coupled to tension piers for these walls. The dampers activate during rocking and make up for the loss of slab yielding. The cumulative effect of these moves is a re-centering structure that avoids extensive residual drifts, expensive structural repairs, and potential loss-of-use during repair, which provides the opportunity for resilience. Two schemes were developed to avoid a cost premium for the high-performance design – a conventional design and a high-performance design. This resulted in an internal competition. The conventional design established the baseline cost that the high-performance scheme could not exceed to be considered viable. The general contractor priced both schemes and found the high-performance scheme cost neutral. With evidence of the greater value of the high-performance scheme, Mar Structural presented the findings to the developers (CCDC and MEDA) and the San Francisco Mayor’s Office of Housing and Community Development. The team green-lighted the high-performance design. Professor Geoff Rogers, from the University of Canterbury, was enlisted to develop the foundation dampers and apply his state-ofthe-art technology to a real project and a great cause. He led the efforts to prototype, test, and produce the dampers while staying within tight cost controls. He also donated all his time to the project. The San Francisco building department requires an expert peer review when using innovative technologies. Professor Greg Deierlein, from Stanford, was the project reviewer. His critical work was also provided pro bono. Once Casa Adelante was designed and performing well, Mar Structural reached out to the US Resilience Council (USRC) and got them excited about the project. The USRC waived their fees and, after a detailed evaluation, they awarded the project a Gold Rating for resilience. Casa Adelante is the first multi-unit housing project rated by the USRC. We believe it is one of the first high-performance designs for
Mar Structural Design was an Outstanding Award Winner for the Casa Adelante project in the 2020 Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings $30 Million to 80 Million.
under-resourced occupants constructed in the U.S. The San Francisco Office of Resilience and Recovery promotes resilient design and targets 40 days of repair and loss-of-use for buildings after a major earthquake. Casa Adelante is rated to have zero days of loss-of-use. The high-performance and conventional schemes were evaluated for economic losses, using FEMA P-58, Development of NextGeneration Performance-Based Seismic Design Procedures for New and Existing Buildings, and the SP3 software (Haselton Baker Risk Group, LLC) that comprehensively assesses seismic damage, loss, and building repair time. The high-performance scheme was more valuable, yielding $500,000 (net present value) savings from earthquake losses. Finally, the contractor performed a postconstruction cost evaluation of the as-built structure against the conventionally designed benchmark. The results were an additional cost of $100,000 for the high-performance design for the $42 million project. The highperformance cost premium was near zero, with only 0.24% extra cost. Although the project slightly missed the cost-neutral target, the team was pleased with how close it was. The result is the resilient performance for affordable senior housing at a great value.■ M A R C H 2 0 21 B O N U S C O N T E N T