STRUCTURE magazine - July 2021

STRUCTURE JULY 2021

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WIND/ SEISMIC

INSIDE: Tsubaki Tower

30

8 Multi-Hazard Design Provisions for Wind and Seismic 12 Statue of Liberty 35

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by The National Council of Structural Engineers Associations (a nonprofit Association), 20 N. Wacker Drive, Suite 750, Chicago, IL 60606 312.649.4600. Periodical postage paid at Chicago, Il, and at additional mailing offices. STRUCTURE magazine, Volume 28, Number 7, © 2021

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Contents JU LY 2021

Cover Feature

30 TSUBAKI TOWER

By Steven M. Baldridge, P.E., S.E., Mark Hirschi, S.E., and Yuriy Mikhaylov, S.E.

For the design of the Tsubaki Tower in Guam, spectral accelerations were similar in intensity to Los Angeles and the island nation is situated in “Typhoon Alley.”

With the highest frequency of tropical cyclones on earth, wind loads exceeded design seismic loads by far.

Features 26 UP FROM THE ASHES – PART 2

35 LADY LIBERTY GETS A PRECAST CONCRETE NEIGHBOR

By Ian Glaser, P.E., Jeffrey Schalk, P.E., S.E., and Michael Schuller, P.E.

The

Sperry

Chalet

burned

By Monica Schultes and PCI Staff, Tom Bagsarian, and Becky King

completely as a result of the

The Statue of Liberty Museum’s 83 precast concrete pieces were

Sprague Fire. Part 2 of this

manufactured offsite and shipped and assembled at the northern

series describes the design

tip of Liberty Island. The

and construction of the new

precast concrete panels

Chalet, as designers analyze

had to be designed to

the remaining stone and masonry, and rebuild the structure from

provide lateral stability

the ground up.

for the building structure.

Columns and Departments 7

Editorial

22

The Value of Your Engineering License

Structural Performance Trends in Engineering

Playing Tetris in a Hurricane

Codes and Standards

2021 Special Design Provisions for Wind and Seismic

20

Structural Systems

Structural Analysis

41

Structural Design

An Overview of Slope Stability

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

44

Building Blocks

Shotcrete Today – Not Your Father’s Gunite

By Jeffrey D. Viano, S.E., P.E., et al.

By Charles Hanskat, P.E.

Intersection of AEC and Artificial Intelligence

By Meghana Joshi

47

Structural Carbon

Structural Engineers and the Climate Crisis

By Chris Horiuchi, S.E., et al.

Structural Forum

Ethics Instruction: Are We Covering What We Need To?

By Brandt Saxey, S.E.

Storm Shelter Design

Insights

58

BRBF Global Stability

By Philip Line, P.E., et al.

16

By Ryan Curtis, P.E.

and Christopher Cerino, P.E., SECB

38

Business Practices

Post-Pandemic Career Realignment

By Breanna Gribble, Robert Fields, P.E.,

By Anastasia Athanasiou, Ph.D.

12

50

®

By Brent L. White, P.E., S.E.

8

Structural Practices

By Scott Civjan, Ph.D., P.E.

In Every Issue

4 34 52 54 56

Advertiser Index Resource Guide – Concrete Products NCSEA News SEI Structural Columns CASE in Point

July 2021 Bonus Content Available Only at – STRUCTUREmag.org

Spotlight

Mighty Mac’s New Coat of Paint

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.

J U L Y 2 0 21

5

EDITORIAL The Value of Your Engineering License

By Brent L. White, P.E., S.E.

C

ASE, NCSEA, and SEI, the three structural engineering associa- is respected because it involves a high level of education and experitions that support this magazine, have various shared interests. ence. The difference between an occupation and a profession is key.

Perhaps the most important shared interest is the promotion and Licensure of professionals has a history in the United States that dates support of licensure for structural engineers – the S.E. License. A to the late 1800s. The United States Supreme Court upheld a West Vision for the Future of Structural Engineering Licensure is a detailed Virginia Law to license physicians in Dent v. West Virginia in 1889. paper outlining this common position.

The Court recognized that every This paper, developed by the Structural individual has the right to pursue Engineering Licensure Coalition, has Professional engineering licensure a lawful occupation and that legispreviously been discussed in these latures cannot deprive someone of pages and is something practicing is essential for the safety and that opportunity. However, the Court structural engineers are encouraged concluded that, in this case, licensing welfare of the public. was a way to protect public health and to be familiar with.

The purpose of this article is not to safety. The Dent decision has subsereview the finer points associated quently led to the licensure of scores with S.E. Licensure but to share some of occupations under the guise that thoughts related to the licensure of strucpublic health, safety, and welfare are tural engineers, professional engineers, being protected. The proliferation of and professional licensure in general. licensed occupations since the midLikely, practicing structural engineers 1900s is outstanding. As a licensed recognize the need for some differenprofessional, I find it amazing that tiation relative to structural engineering practice beyond engineering ballroom dance instructors, fortune tellers, hair braiders, and cat practice in general. We recognize that to adequately navigate the groomers are licensed in many states. It is hard to understand how complexities of structural engineering analysis, design, complex code a bad haircut compares to a failed structure relative to the public’s requirements, etc., more than a basic understanding of engineering health, safety, and welfare. The state I reside in has 66 primary occupamechanics and analysis is necessary. This does not downplay the prac- tion categories, many with multiple licenses. This was outstanding to tice of any field of engineering but recognizes that there is a difference. me until I read that my state ranks in the top 10 for fewest licenses.

There are various challenges to this premise from those that oppose Understandably there is a feeling of licensure overload. additional licensing requirements for structural engineers and other Several years ago, I was appointed to serve on an occupational and engineering professions that practice in specific highly technical and professional licensure review committee for the state legislature. This complex fields. The ethical cannon that a professional engineer does committee consisted of individuals with licenses regulated by the not practice beyond their area of expertise is an example. Creating a state, legislators, and the general public. The responsibility of the “barrier to entry” is an economic argument that is also used in protest. committee was to review proposed new or pending legislation related However, implications to the health and safety of the public due to to occupational licensing. My participation provided an interesting improperly performed structural engineering analysis and design opportunity to observe how others viewed the importance of licensing. outweigh these arguments, in my opinion.

I was amazed that the opinions ranged from ‘if it is a job, license it Reaching the goals outlined in the Vision document will take time – as and regulate it’ to ‘there should be no licensure – period.’ I sat next to all change does. Recently, there have been some advances with several a practicing dentist whose opinion was the latter. I asked him several states adopting either title or practice acts related to S.E. Licensure times if anyone should be able to practice dentistry. His opinion was or concerted efforts to advance such legislation. Every positive step that any regulation was unconstitutional and that Caveat Emptor was is an advancement. the standard. I asked him how a person knows how to beware of an As the profession pushes forward with S.E. Licensure objectives, it unqualified dentist, to which he replied that, eventually, that dentist is equally important to recognize the value of professional engineer- would have no patients. Personally, I do not want to be experimented ing licensure in general and understand the value in maintaining on until the market provides a correction. continued support of P.E. Licensure. Recently, several states have Full disclosure, I am a less-is-more person regarding regulation genmade significant efforts to either eliminate professional engineering erally. I think too many professions are licensed, which diminishes licensure or reduce competency and experience requirements. One the importance of professions that truly impact the public’s health, primary argument used is the same as stated above regarding S.E. welfare, and safety. Professional engineering licensure is essential for Licensure – all licensure is an economic barrier to an occupation and the safety and welfare of the public, and S.E. Licensure enhances that opportunity for employment. To practicing P.E.s in any field, this likely safety and welfare. As a profession, while we are pursuing the makes little sense. We all understand and recognize that an unquali- lengthy process of S.E. Licensure in all jurisdictions, we need fied person should not practice engineering, just as an unqualified to be wary of efforts to remove or diminish P.E. Licensure.■ engineer should not practice structural engineering. The practice of Brent L. White is President ARW Engineers, Past President, Structural engineering is not just an occupation – but a profession – a type of Engineers Association of Utah, and the Current Chair, CASE. work that needs special training or a particular skill, often one that STRUCTURE magazine

J U L Y 2 0 21

7

structural PERFORMANCE Trends in Engineering

Why Is Everyone Talking About Performance-Based Multi-Hazard Design?

By Anastasia Athanasiou, Ph.D.

T

he design for multi-hazard mitigation is a new subprinciple of structural engineering, aiming to protect structures from hazards (earthquakes, winds, tsunamis, snowfalls, floods, wild-fire, etc.). This is done by anticipating damage, minimizing consequence losses, and targeting fast recovery in the event’s aftermath. Recurring hazards may be independent or interrelated (concurrent or successive). Earthquakes and winds are typical examples of independent actions, while heavy rain and high winds, main earthquake shocks, and aftershocks are examples of concurrent and successive events, respectively.

Following major international codes, generations of practitioners have been designing buildings to sustain the maximum load expected during their lifetime. This widespread worst-case scenario approach provided society with strong structures of increased initial cost and unmeasured reliability. Reliability is the probability of the structure to meet its performance target under the operating conditions encountered during the intended period of use. Earthquake engineering Figure 1. Simplified performance-based multi-hazard design flowchart. was the first discipline to introduce Performance-Based Design (PBD), targeting desired system performance at various levels parameters are input to mathematical functions that model limit of excitation. The design earthquakes range from service to near col- state probabilities. Damage limit states describe performance goals lapse events and are associated with a certain probability of exceedance and are usually associated with inter-story drift and floor acceleration in the life of the structure. thresholds. Decision parameters quantify the structural performance The mapping of performance objectives to expected excitation levels of components and facilities in meaningful metrics to stakeholders, for enabled the cost-effective seismic design of structures. PBD uses capac- example, repair costs and downtime. Optimized performance-based ity design principles to proportion lateral actions in well-detailed, design minimizes the total expected life-cycle costs, balancing initial ductile elements, such as braces, beams, and columns. Following a and expected failure costs. The risk is defined as the probability of strength hierarchy, deformation-controlled (ductile) elements dissipate exceeding threshold values of the decision variables. Risk assessment input seismic energy through yielding, while force-controlled elements is conditioned on the site and selected design (D). Due to uncertain(beams and columns supporting gravity loads) remain elastic. PBD ties relative to the hazard, the recorded data, and the modeling of the procedures aim to ensure habitat comfort and continuous service system, each of the parameters in Equation 1 is considered within a under minor events and collapse prevention and life safety under probabilistic context and described by a probability density function significant events. (pdf ), p, where p[X|D] is the pdf of parameter X conditioned on the Today, PBD has evolved to the point where state-of-the-art software knowledge of D. Moreover, g[X|D] is the occurrence frequency of is used to perform nonlinear collapse simulations. In contrast, proba- X given D. bilistic methodologies relate seismic performance factors to system Similar advancements are expected to occur in wind engineering, performance capabilities, quantify damage and estimate potential nevertheless at a slower pace owing to the complex nature of wind losses through life-cycle cost analysis. Equation 1 is the benchmark loads and the significant computational effort required to perform relation for seismic loss evaluation (Porter, 2003): wind history simulations. Two key elements for the advancement g[DV|D]=∫∫∫p[DV|DM,D] p[DM|EDP,D] p[EDP|IM,D] of performance-based wind engineering are the refined estimation g[IM|D] dIM dEDP dDM (Eqn. 1) of loads through wind tunnel testing and the implementation of This relation shows how risk assessment can be disaggregated in (i) nonlinear procedures to predict wind-induced demands. Similar to hazard characterization for the definition of the intensity measure seismic design, models employed in wind design account for member (IM), (ii) structural analysis for the estimation of the engineering overstrength and allow for yielding of well-detailed members, designed demand parameters of interest (EDP), such as drifts, floor accel- to resist extreme events without significant loss of strength and stifferations, and stresses, (iii) fragility assessment for the prediction of ness. Equation 1 can be easily adapted in performance-based wind damage measures (DM), and (iv) loss analysis for the estimation of design, where the wind intensity is considered as the mean wind speed decision variables (DV). In fragility analysis, the selected demand at a reference height of approximately 32.8 feet (10m).

8 STRUCTURE magazine

Figure 2. a) Typical floor plan, and b) N-S elevation associated with the numerical model of ¼ floor area.

Current wind practice following major international standards conforms to prescriptive acceptance criteria accounting for stiffness and strength, delivering systems of unquantifiable reliability. The strict wind criteria compromise the benefits of inelastic seismic design for structures designed to resist both winds and earthquakes. The stiffening of the lateral force-resisting system, often required to satisfy stringent wind service criteria, increases initial and potential failure costs, may lead to reduced energy dissipation, trigger unfavorable distribution of forces on force-controlled members, and put life safety at risk under extreme events.

motion, optimal life-cycle costs, minimal disruption on environment and resilience, etc. Costs over the life of the structure include damage and repair costs, relocation costs, indirect costs, costs caused by injuries, and fatalities (Wen and Kang, 2001).

Whereas the dominant hazard controls the optimal design, the less intense hazard may contribute significantly to the overall damage and life-cycle costs and should not be ignored. This implies that a performance-based multi-hazard framework targets optimal design and does not require uniform reliability against different hazards (Wen and Kang, 2001).

Multi-hazard Design Concept

Case Study

Developing PBD approaches for multiple hazards is a significant step towards building a resilient and sustainable civil infrastructure. Figure 1 illustrates the concept of multi-hazard approaches developed by various experts in the field. The design starts with the probabilistic definition of the hazards, expressed in intensities of various mean recurrence intervals (return periods). The use of return periods enables the comparative assessment of induced risks since various hazards are expressed in incompatible units; for instance, winds are described by wind speeds and earthquakes by ground accelerations. Furthermore, if the considered hazards have negligible probability of concurrence, they can be considered independent, and analyses under the single hazards can be run separately.

The development of an accurate structural model, accounting for P-delta effects, material nonlinearity, and fatigue under cyclic loading, is salient for accurate performance evaluation. The peak response (demand parameters) may be assessed through deterministic nonlinear response history simulations or probabilistic methods.

Deterministic response history analyses provide insight into the transient response of dynamic systems under case-specific scenarios.

However, probabilistic methods have the advantage of simulating physical, artificial events and accounting for aleatory and epistemic uncertainties in the data.

The designer, in accordance with the owner and stakeholders, shall set the performance objectives. These objectives are usually expressed in damage limit states and consider occupant comfort under frequent wind loads, life safety under extreme ground

A 15-story office building in Montreal’s downtown is designed for typical seismic and wind loads following the 2015 National Building Code of Canada (NBCC) provisions. Figure 2 shows the typical floor plan and associated N-S elevation for the building. Eight Concentrically Braced Frames (CBFs) with tension-compression braces provide resistance under lateral actions in the two orthogonal directions. The example focuses on the N-S direction.

Initial Design Following the equivalent lateral force procedure (clause 4.1.8.11, NBCC 2015), the minimum seismic base shear is Vmin = 665 kips (2958 kN). The base shear is distributed via the inverted triangular distribution approach along the building height to provide the input forces for the initial section design. A three-dimensional model of the structure is developed in ETABS (2016) to assess the dynamic distribution of the story shear and the associated drifts. With a fundamental frequency as low as 0.29Hz (T1 = 3.41s), the building is classified as dynamically sensitive under wind load, clause 4.1.7 in NBCC (2015). The net wind floor loads, Wi (i = 1,2,..15), are evaluated as the algebraic difference of wind- and lee-ward pressures, multiplied by the corresponding tributary area. The estimated factored wind shear is 1.4W = 1210 kips (5384 kN), i.e., lower than the elastic seismic demand, Rd RoV = 2593 kips (11,536 kN), where Rd = 3, Ro = 1.3 are the ductility and overstrength related factors.

The tension-compression braces were initially designed so that the factored axial force under seismic loads, Cf, is less than or equal J U L Y 2 0 21

9

b) c) to the member resistance, Cr. a) The corresponding resistant factors Cf /Cr, under the factored wind loads (1.4W), take values that are greater in the lower stories where wind is prevalent. This slightly excessive wind demand is expected to be accommodated by the members’ overstrength.

Hence, the brace sections are not further increased, resulting in a small material gain of 882lb (400kg) (the taller the structure, the higher the gain). The ETABs model Figure 3. Peak seismic response at the design level, in terms of a) inter-story drifts, b) residual drifts, and c) absolute floor verifies that the service level accelerations.

(0.75W) inter-story drifts lie Input Motions below the limit value of 1/400 (NBCC, 2015).

Due to a lack of historical data for Eastern Canada, a set of Performance Goals seven ground motions for Montreal, soil C, is created via The performance of the building at the service, strength, and near col- www.seismotoolbox.ca. The motions are scaled in amplitude to lapse level is assessed through independent wind and seismic response match the NBCC 2015 design spectrum in the period range [0.2, history simulations of a sophisticated building model developed in 2] T1. The motion duration is 18s, and the time step equals dt = OpenSees (2015). The model accounts for material nonlinearity, 0.002s, whereas 10s of free vibration upon cessation of motion are fatigue, and second-order effects. Performance is linked to peak inter- included in the analysis. story drifts (δmax) and floor accelerations (αmax). Multi-hazard design An ensemble of 500 wind load sets is created via Monte Carlo is finalized once the following goals are achieved: simulations in MATLAB (2018) based on local pressure data retrieved 1) Under service winds (0.75W, or 1-in-10 years), the building from the wind tunnel testing of a geometrically similar building at is habitable and there is no damage in the cladding (αmax < Tokyo Polytechnic University (https://bit.ly/34L5maj). The data 20 mg and δmax <1/400), where g = 32.174ft/s2 (9.807 m/s2) are scaled in time, and the time step is halved to 0.0665 seconds stands for the standard acceleration due to gravity. to ensure numerical convergence of the algorithm. The total wind 2) For less frequent winds (1.0W, or 1-in-50 years), the duration is 4,365 seconds or approximately 1.2 hours. Due to comresponse is elastic and mild damage is allowed in the putational limitations, only the five statistically most significant cladding (δmax < 1/220). winds are considered.

3) Under design-level earthquakes (1.0E, or 1-in-2,475 years), Response at the Design Level damage to cladding and structural components may occur (δmax < 2.50%); however, such damaged members are repairFigure 3 shows the peak system response under the design level earthable/replaceable (δres < 0.50%), and life safety is ensured. quakes. The mean peak drift is lower than the 2.50% threshold, and 4) For near-collapse winds (identified through incremental analythe mean peak residual drift is less than the 0.50% limit, deeming ses), damage to cladding and structural components is allowed the building repairable after the earthquake. Nonlinearity in the to occur, whereas there is a moderate risk to life safety. upper stories is responsible for the almost constant distribution of More information on the selected thresholds can be found in Isymov accelerations along the building height. Under service winds, Figures (1993) and Griffis and Charney (2016).

4a and 4b, there is no damage in the cladding (δmax < 1/400), and a)

b)

c)

d)

Figure 4. a, b) Peak inter-story drifts and floor accelerations under service winds, c) peak drifts under design level winds, and d) peak drifts under near-collapse winds.

10 STRUCTURE magazine

occupant comfort is ensured (αmax < 20mg). For less frequent winds (1-in-50 years), Figure 4c, cladding remains operational (δmax < 1/220).

a)

Response beyond the Design Level Figure 5a shows the Incremental Dynamic Analyses curves (IDA) relating the peak inter-story drift at the seismic intensity level Sa (T1,5%). The casespecific fragility curve is given in Figure 5b. The median collapse capacity is Sc = 0.20g and, once divided by the design intensity Sa = 0.04g, yields the collapse margin ratio CMR = 5. CMR is significantly larger for systems in which the inelastic response is more evenly distributed throughout the system (FEMA P695, 2009). Figures 5c and 5d show the wind IDA curves (IDWA) that relate the peak inter-story drift to the reference 32.8-foot-height (10m) pressure for the suite of five wind realizations and the corresponding collapse fragility curve. Near collapse, at qnc = 0.087psi (0.60kPa) or 2.4 times above the design wind level qexp = 0.036psi (0.25kPa), braces at the lower stories yield and damage occurs; nevertheless, such damage is repairable (δmax <1/140). The median collapse pressure is qc = 0.094psi (0.65kPa). Contrary to seismic failure, wind failure occurs soon after the first significant yielding of the system. At the design level, the seismic response is highly nonlinear, while wind response is purely elastic. This is because design winds (1.0W, 1-in-50 years) and factored design winds (1.4W, 1-in-500 years) are more frequent than design earthquakes (1-in-2,475 years). However, under severe windstorms, ductility should be exploited.

Since all performance goals are satisfied, the design is considered acceptable. Note that studies in the field of multi-hazard design are currently under development; hence, further issues such as across-wind actions, directionality effects, and life cycle cost analyses are also currently under development.

While this example refers to NBCC (2015), the proposed multi-hazard procedure can be easily implemented under any code/standard. Note that the major international codes/standards use a common theoretical framework for modeling dynamic load effects, with differences lying essentially in the definition of the wind field characteristics (Kwon and Kareem, 2013). The procedure starts with designing the LFRS for the prescribed seismic loads. Following this, the design team should verify that the LFRS has adequate strength and stiffness to resist code-prescribed wind loads. For taller buildings, the wind demands at bottom floors tend to exceed the members’ factored resistance. The initial section design is confirmed if such exceedance lies within a reasonable range, i.e., 10%-15%.

The fundamental concept of the procedure is implementing a multi-hazard design that is both effective and economical, allows the members to use their overstrength to accommodate slightly excessive wind loads at the design level, and exploits the dissipation capacity of well-detailed members under rare events. Once the initial section design is decided, the design team and project stakeholders select performance objectives consistent with the code’s requirements.

Typical performance metrics are acceleration and drift thresholds linked to serviceability and survivability limit states. ASCE/SEI 7-16 (2016), the prestandard for performance-based wind design (ASCE, 2019), the manual for the design of tall buildings under wind (ASCE, 2020), and the papers listed herein provide thorough descriptions of performance objectives and acceptance criteria suitable for earthquake and wind design practice. The initial design and its conformability to the selected acceptance criteria should be assessed using nonlinear response history simulations (at the serviceability level, linear wind simulations are permitted). The building design and respective structural model are finalized once the acceptance criteria are satisfied.■

b)

c)

d)

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

Anastasia Athanasiou is a Post-doctoral Fellow at Concordia University, Canada, working on the multi-hazard assessment of steel structures of different occupancies.

Figure 5. a, c) Incremental dynamic analyses results showing the distribution of collapse statistics for the office building under recurring earthquakes and winds, and b, d) corresponding collapse fragility curves.

J U L Y 2 0 21

11

CODES and STANDARDS 2021 Special Design Provisions for Wind and Seismic

By Philip Line, P.E., Brad Douglas, P.E., Jason Smart, P.E., and Peter Mazikins, P.Eng

T

he 2021 Edition of Special Design Provisions • Added equations for calculating the deflection for Wind and Seismic (SDPWS) was approved of cantilevered diaphragms; as an American National Standard on July 22, • Added provisions for and reference to ASTM 2020, with the designation ANSI/AWC SDPWSD7989 Standard Practice for Demonstrating 2021 (Figure 1). The 2021 SDPWS was developed Equivalent In-Plane Lateral Seismic by the American Wood Council’s (AWC’s) Wood Performance to Wood-Frame Shear Walls Design Standards Committee (WDSC) and conSheathed with Wood Structural Panels; tains provisions for the design of wood members, • Added an 8% shear strength reduction for fasteners, and assemblies to resist wind and seismic wood-frame shear walls nailed with 10d forces. Notable revisions are summarized below common nails and using hold-downs installed (also see Table 1 online for a summary of changes on the inside face of end posts;

by Chapter): • Revised equation for calculation of the • Revised Chapter 3 tables of nominal uniperforated shear wall shear capacity adjustment form load capacities for resistance to out of factor, Co; and, plane wind loads; • Added provisions for the design of CLT Figure 1. 2021 SDPWS is referenced in • Revised organization of requirements in diaphragms and CLT shear walls. the 2021 International Building Code.

Chapter 4 to differentiate between sheathed wood-frame systems and new cross-laminated timber Out-of-Plane Wind Load Resistance (CLT) systems; • Added language to clarify reference conditions (framing Revised tables in Chapter 3 expand the tabulation of nominal uniform materials and nail type and size) for applicability of design load capacities for wall sheathing and roof sheathing resisting out of value tables; plane wind loads. Table 3.2.1A (for wall sheathing) and Table 3.2.2 (for • Revised format of diaphragm and shear wall nominal roof sheathing) now include separate nominal uniform load capacities unit shear capacity tables to a single nominal value for for OSB and plywood resisting out-of-plane wind loads, as well as each configuration (in contrast to tables in prior editions capacities for W24 wall sheathing and Structural I sheathing panels. that tabulated separate nominal values for wind design and seismic design), coupled with revised ASD reduction Revised Organization factors and LRFD resistance factors to work with the revised format; Organization of SDPWS Chapter 4 is revised to better differenti• Added provisions for the vertical distribution of seismic ate between general requirements (i.e., those that are generally force-resisting system (SFRS) strength for structures applicable to all systems addressed by the standard) and those assigned to Seismic Design Category D, E, or F; requirements specific to either sheathed wood-frame systems or new CLT systems. Chapter 4, Lateral Force-Resisting Systems, is now organized as follows: 4.1 General 4.2 Sheathed Wood-Frame Diaphragms 4.3 Sheathed Wood-Frame Shear Walls 4.4 Wood Structural Panels Designed to Resist Combined Shear and Uplift from Wind 4.5 Cross-Laminated Timber (CLT) Diaphragms 4.6 Cross-Laminated Timber (CLT) Shear Walls

Reference Conditions for Framing and Fasteners

Figure 2. Adjoining panel edge locations, sheathed wood-frame diaphragm (select cases).

12 STRUCTURE magazine

Reference framing materials for wood structural panel diaphragms and shear walls are sawn lumber or structural glued-laminated timber (Section 4.1.2.1). Use of other framing materials in diaphragm and shear wall construction (such as Structural Composite Lumber (SCL)) is required to be per the manufacturer’s approved instructions or an

approved evaluation report. This clarification was added to account for product-specific nail size and spacing requirements to limit the potential for splitting in products that may exhibit more splitting than reference framing materials.

Similarly, reference fastener types and dimensions used for sheathing attachment in diaphragms and shear walls and associated with the tabulated nominal unit shear capacity values (such as those provided in Table 4.2A for diaphragms and Table 4.3A for shear walls) are now prescribed in the tables. They are located side-by-side with tabulated nominal unit shear capacities. Revisions to prescribe nail dimensions in nominal unit shear capacity tables and replacement of the term “fastener penetration” with “fastener bearing length” in table column headings were to clarify the full-length nail basis of the tabulated nominal unit shear capacities. Nails of different types or dimensions are considered alternatives to the specified nails.

For diaphragms, Section 4.2.8.1.1 was revised, and a new Figure 4B was added to clarify differences in framing requirements (i.e., either 2x nominal minimum or 3x nominal minimum width of nailed face) for framing at adjoining panel edges that are not continuous and for framing at continuous adjoining panel edges (Figure 2). In addition, for consistency with the full-length nail basis of tabulated unit shear capacities for diaphragms, revisions to 4.2.8.1.1(b) also removed the minimum 1½-inch penetration criterion for closely spaced 10d common nails.

Nominal Unit Shear Capacity – Single Value Format

shear capacity for both wind and seismic better represents minimum strength expectations for wood-frame diaphragms and wood-frame shear wall systems in SDPWS. Also, it simplifies design requirements tied to nominal unit shear capacity.

Vertical Distribution of Story Lateral Strength To reduce the potential for a degraded seismic response due to the presence of a weaker lower story as predicted from FEMA P695 numerical studies of wood-frame wood structural panel shear wall building models, new provisions in Section 4.1.8 prohibit designs in which the seismic force-resisting system (SFRS) lateral design strength for a lower story in wood buildings is less than the SFRS lateral design strength of the story above in seismic design categories D, E, and F. An exception to this prohibition allows the lower story SFRS lateral design strength, Vr(i), to be less than the SFRS lateral design strength of the story above, Vr(i+1), if the lower story SFRS lateral design strength exceeds the lateral design load for that story by the ratio of Vr(i+1)/Vr(i). This criterion is more stringent than weak-story irregularity limits in ASCE 7-16 Minimum Design Loads and Associated Criteria for Buildings and Other Structures, which allow a lower story SFRS to be as much as 35% less than the upper story SFRS in Seismic Design Category D and as much as 20% less than the upperstory SFRS in Seismic Design Categories E and F without requiring strengthening of the weaker lower story to exceed the lateral design load.

Cantilevered Diaphragms

Nominal unit shear capacity tables for wood-frame diaphragms and Table 4.2.3 provides two new equations for calculating diaphragm wood-frame shear walls tabulate a single nominal design value that is deflection at the end of a cantilevered diaphragm from i) a uniformly applicable for both wind and seismic design for a given combination of distributed load and ii) a concentrated load at the end. The equations sheathing, fastening schedule, and framing (in prior editions, separate nominal values were tabulated for wind design and seismic design). Coupled with this new tabulation of a single nominal unit shear capacity for wind and seismic are new ASD reduction factors and LRFD resistance factors for wind and seismic design.

With more than 990 pages and 140 workedFor wind design, there is no change in out examples, the Design Guide on the ACI either ASD or LRFD design strengths 318 Building Code Requirements for Strucfrom prior editions. For the seismic design tural Concrete is an indispensable resource of wood-frame diaphragms (in SDPWS in the proper application of the provisions Tables 4.2A, 4.2B, 4.2C, and 4.2D) and for cast-in-place concrete buildings with wood-frame shear walls (in SDPWS nonprestressed reinforcement.

Tables 4.3A, 4.3B, and 4.3D), there is no change in ASD design strengths.

However, for LRFD design strengths, Featuring... there is a reduction of approximately » A simplified roadmap that can 11% from the previous edition. For the be used to navigate through the seismic design of wood-frame shear walls updated ACI 318 requirements using nominal unit shear capacities from Save 10% – Use Discount Code » Step-by-step design procedures Table 4.3C for gypsum board, gypsum STRUCTURE-2021 at Check-Out and design aids that make designlath and plaster, and Portland cement ing and detailing reinforced conRegular price: $199.95 non-member/ $149.95 CRSI member. plaster, ASD design strength is approxicrete buildings simpler and faster mately 70% of that obtained from prior editions. The reduction in design strength Visit CRSI at www.crsi.org results from the application of a consistent to Purchase Publications and Get CRSI’s FREE Rebar Reference App! ASD reduction factor of 2.8 (a factor of reinforcing steel data • hook details Download FREE Resources! www.crsi.org 2.0 was used in prior editions for SDPWS field inspection data • bar markings identifier* Table 4.3C sheathing systems) across all development lengths calculator* • and more! shear wall systems in the standard. The *premium module, in-app purchase required revised format using a single nominal unit

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rely on the use of apparent shear stiffness, Ga, used in the standard for calculating shear deformations.

full range of opening area ratios. The Shear Capacity Adjustment Factor Table (Table 4.3.5.6) is updated accordingly.

Seismic Equivalency to Wood-Frame Wood Structural Panel Shear Walls

Cross-Laminated Timber Diaphragms

Recognizing the variety of products used in wood-frame wood structural panel shear wall systems (e.g., framing, sheathing, and fastening) and that alternative bracing systems are common and often evaluated for equivalence, the reference to ASTM D7989 (Section 4.3.7.1.1) is made to provide for consistent evaluations of an alternative system’s “seismic-equivalence” to the wood-frame wood structural panel shear walls addressed in the standard.

Shear Wall Strength Reduction

New Section 4.5 CLT Diaphragms adds provisions for the design of CLT diaphragms using principles of engineering mechanics and values of wood member and connection strength in accordance with the National Design Specification® (NDS) for Wood Construction. Requirements include diaphragm shear strength to be based on dowel-type fasteners exhibiting yield modes Mode IIIs or Mode IV per the NDS and use of design force increase factors for the design of wood elements, steel parts, and wood or steel chord splice connections (factors ranging from 1.0 for wind design to 2.0 for seismic).

Combining these requirements is intended to ensure the development of a minimum level of diaphragm overstrength consistent with that provided by nailed wood-frame wood structural panel diaphragms.

Footnote 10 of Table 4.3A requires the Figure 3. Wood-frame shear wall notations application of a 0.92 factor to the nominal (hold-down on the inside face of post shown). unit shear capacity of wood-structural panel sheathed wood-frame shear walls having sheathing attached with 10d common nails and with a hold-down mounted to the inside Cross-Laminated Timber Shear Walls face of the shear wall end post. The factor accounts for reduced strength observed in standardized cyclic shear wall testing that New Section 4.6 CLT Shear Walls adds provisions for the design of employs eccentric hold-downs attached to the inside face (e.g., CLT shear walls (Figure 4), including prescriptive requirements for within the wall cavity, Figure 3). The reduced strength (not observed fasteners, connectors, and individual CLT panel aspect ratios per with other sheathing nail sizes) is believed to be associated with Appendix B. Two CLT shear wall systems are defined: i) CLT shear reduced effectiveness of the eccentric hold-down leading to prying wall, and ii) CLT shear wall with shear resistance provided by high and sheathing nail damage at shear wall corners. However, no such aspect ratio panels only. Associated seismic design coefficients (i.e., R strength reductions have been observed in testing performed on walls = 3 or 4, respectively) are included in the 2020 NEHRP Recommended with different end post details, such as hold-downs mounted on the Seismic Provisions for New Buildings and Other Structures and have been outside face or both faces of end proposed for inclusion in ASCE 7-22. CLT shear walls not conforming posts and rod hold-down systems. to requirements of Appendix B are subject to approval as an alternative method of construction, with default use limited to Seismic Design A and B and seismic design coefficients limited to R = 1.5, Perforated Shear Wall Categories Cd = 1.5, and Ωo = 2.5 unless other values are approved.

Revisions clarify that sheathed areas around openings are to have Conclusion the same nailing (i.e., nail size and spacing) associated with the design The 2021 SDPWS is available in a free view-only electronic format shear capacity of the full-height and for purchase at www.awc.org. Additional information on SDPWS perforated shear wall segments or provisions is available in the SDPWS Commentary. The 2021 SDPWS be included in the area of open- Commentary is scheduled to be available in June 2021. The 2021 ings (Section 4.3.2.3(9). The same SDPWS represents the state-of-the-art design of wood members nailing of sheathed areas above and and connections to resist wind and seismic loads. Reference to the below openings has been used in 2021 SDPWS is included in the 2021 International Building Code.■ perforated shear wall testing that forms the basis of empirical openTable 1 is included in the PDF version of ing adjustment factors. Revised the article at STRUCTUREmag.org. equations for calculating the Shear Philip Line is Director of Structural Engineering, Brad Douglas Capacity Adjustment Factor, Co, (bdouglas@awc.org) is Vice President of Engineering, Jason Smart is simplify the presentation and are Manager of Engineering Technology, and Peter Mazikins is Senior more consistent with the underlyFigure 4. Cross-laminated timber Manager, Engineering Standards with the American Wood Council. shear wall, multi-panel configuration. ing empirical equations over the 14 STRUCTURE magazine

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structural SYSTEMS Storm Shelter Design

By Jeffrey D. Viano, S.E., P.E., Connor J. Bruns, S.E., Matthew H. Johnson, P.E., and Andrea M. La Greca

S

TRUCTURE magazine published Structural Design and Coordination of ICC 500 Tornado Shelters in July 2020, summarizing when a storm shelter is required, design criteria, and lessons learned. The authors of the July 2020 article provide guidance on design criteria, including the significantly increased basic wind speed, the increased internal pressure coefficient which presumes a breach in the building envelope, the increased directionality factor, and increased minimum roof live load, among other design criteria. To supplement the information in the previous article and illustrate the implications of storm shelter design criteria, the Table summarizes and compares wind load design parameters for a fictitious building used conventionally or as a storm shelter.

One of the most important aspects of storm shelter structural design is providing a robust load path for extreme wind loads. Loads need to be transferred from the roof, into the walls, and down into the foundation; the designer must ensure that each structural component along this path can withstand the load demands. This article focuses on typical structural systems and details used for storm shelter design and best practices for delegated design to complement the July 2020 article. This article references the 2014 ICC 500/NSSA Standard for the Design and Construction of Storm Shelters (ICC 500), referenced by the 2015 and 2018 International Building Code (IBC). ICC/ NSSA recently published the 2020 ICC 500, which is referenced in the 2021 IBC.

and missile impact forces. Roofs of storm shelters are often constructed with structural steel, precast prestressed concrete, cast-in-place concrete, or a combination to accommodate the increased loads.

Structural steel-framed roofs typically consist of wide-flange beams and girders, open-web steel joists, customfabricated steel trusses, or a combination. These primary framing elements are subjected to significant wind uplift pressures, often resulting in compression in bottom flanges or joist and truss bottom Figure 1. Exterior wall section at double-tee roof. chords. Consideration of unbraced lengths and required bracing and/or bridging is critically important for a successful design to resist wind uplift pressure.

Roof Systems Concrete slabs on steel framing are typically placed on a composite Roof framing systems in a tornado storm shelter need to be more steel deck. Adequate strength and missile impact resistance can robust than conventional framing systems to resist increased roof live ordinarily be achieved with a concrete-on-steel deck, as long as load, wind uplift pressures, and diaphragm forces in addition to debris the concrete over the upper flutes of the deck exceeds the minimum established by debris impact testing.

Additionally, concrete slabs on steel deck 1 K-12 School Gymnasium in Springfield, IL require substantial reinforcement to resist Design Parameter Typical Building ICC 500 Tornado Shelter Design uplift loads, diaphragm shear, and chord and Basic Wind Speed, V 114 mph 250 mph collector forces.

Reinforced concrete framed roofs may be castDirectionality Factor, Kd 0.85 1.0 in-place or precast prestressed. Cast-in-place Internal Pressure Coefficient, GCpi ±0.18 ±0.55 roofs are often an economical choice for unique Velocity Pressure, qh 29 psf 166 psf geometries or shorter spans. Precast prestressed concrete roofs (typically double-tees) are genMWFRS2 – Wall Pressure3 33 psf 184 psf erally the economical choice for rectangular 2 4 MWFRS – Roof Pressure -28 psf -219 psf buildings with low-sloped roofs and where C&C5 – Wall Pressure6 -38 psf -275 psf longer spans, such as gymnasiums, preclude C&C5 – Roof Pressure7 -73 psf -474 psf efficient cast-in-place concrete options.

Precast concrete elements often require a Roof Live Load 20 psf 100 psf topping and significant reinforcement to resist Risk Category III; Exposure Category C; Topographic factor, K = 1.0; Mean roof height, h = 40 feet; uplift loads, diaphragm shear, and chord and L = B = 80 feet; Flat roof; rigid structure.

MWFRS calculated in accordance with ASCE 7-16 Chapter 27 – Directional Procedure. collector forces. The topping slab can also Combination of leeward and windward wall pressures for MWFRS design. serve to develop the missile impact resistance Maximum roof uplift at horizontal distance of 0 to h/2 from windward edge.

Components & Cladding (C&C) pressures calculated in accordance with ASCE 7-16 Chapter 30, of the roof. Figure 1 depicts a topping slab Part 1 for component with effective wind area = 10 ft . on a roof constructed with precast double Wall pressure at Zone 4 (field of wall) per ASCE 7-16 Figure 30.3-1.

Roof uplift pressure at Zone 2 per ASCE 7-16 Figure 30.3-2A. tees. Precast concrete elements are usually a 1

zt

2 3 4 5

2

6 7

16 STRUCTURE magazine

delegated design, aspects of which are discussed later in this article.

All roof systems must include a robust connection of the framing elements to the wall systems below.

Wall Systems

Foundation Systems

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ICC 500 Section 308 contains provisions for the connection of storm shelters to foundations. In cases where the weight of the structure, cladding, and other permanent dead loads is less than the wind uplift, foundation elements need to be designed for uplift. In many cases, spread footings buried underneath a sufficient depth

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Exterior walls of storm shelters are often used to support roof structures due to their inherent robustness to resist debris and missile impacts and their ability to transfer uplift loads to foundation elements efficiently. The exterior load-bearing walls of a community storm shelter can Figure 2. Plan view – pipe penetration in exterior wall (left); Elevation view A-A (right). also serve a dual purpose as both gravity-bearing walls and lateral load-resisting walls (shear walls). of soil are adequate, but deep foundations may need to be considered Due to high shear demands, large openings in shear walls should be in extreme situations. Uplift capacities of deep foundation elements avoided. In addition to in-plane shear, exterior shear walls need to should be determined with the aid of a geotechnical engineer. be designed for concurrent out-of-plane wind loads. Where roofs In addition to uplift loads, storm shelter foundations experience transfer wind uplift loads into shear walls, the effects of uplift and elevated lateral loads, which are imparted to the underlying soils. overturning must be combined when designing wall boundary Deep and shallow foundation elements should be designed to resist elements and hold-downs. these lateral loads. Shallow spread footings are particularly susceptible Typically, exterior walls for community storm shelters are con- to combined uplift and lateral loads since wind uplift reduces the structed of either reinforced concrete masonry units (CMU) or sliding resistance due to friction. concrete. Exterior load-bearing CMU walls for storm shelters are Community storm shelters with below-grade spaces need to be usually solid-grouted to resist missile impacts. Due to the signifi- designed for buoyancy forces and hydrostatic loads. Per ICC 500 cantly increased out-of-plane wind pressures, the walls may need Section 303.3, underground portions of storm shelters require designing to be doubly-reinforced, with one bar on each side of the CMU to loads “assuming the groundwater level is at the surface of the ground cell. For constructability, the authors recommend using 10-inch or at the entrance to the storm shelter.” An exception to this increased load 12-inch-thick CMU if using two layers of reinforcement. While a is allowed by the ICC 500 if “adequate drainage is available to justify typical CMU wall for a gymnasium structure might consist of 12-inch designing for a lower groundwater level.” This may include dry wells partially-grouted CMU reinforced with a single #5 bar spaced on or storm sewers but is ultimately based on recommendations from the 32-inch centers, a typical wall for a storm shelter might consist of geotechnical engineer.

12-inch fully-grouted CMU reinforced with two #5 bars spaced on 8-inch centers.

Structural Coordination with Other Trades Concrete walls for storm shelters can be cast-in-place, but precast tilt-up panels are cost-effective solutions. Wall panels can be solid or Although unavoidable, penetrations through roof and wall systems insulated sandwich panels as long as they comply with an approved of storm shelters should be limited. Where piping, conduit, ducts, list of missile-tested assemblies. or similar penetrations are required, ICC 500 Section 309.1 stipuBoth concrete and CMU walls require special detailing at the bottom lates that rectangular penetrations greater than 3½ square inches in and top of the wall to resist uplift. Additional steel reinforcement area or circular penetrations greater than 21⁄16 inch in diameter be dowels are often required at the bottom of the wall to transmit uplift and shear loads into foundation elements. Congestion of large diameter bars in CMU cells can be eliminated using mechanical couplers in lieu of lap splices. A cast-in-place concrete ring beam is often cast at the top of CMU walls to transfer diaphragm shear, wind uplift loads, and support of locally high bearing loads from long-span elements. Figure 1 depicts a typical concrete ring beam at the top of a CMU wall supporting precast double tees.

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protected. Similar to those depicted in Figure 2 (page17), protective steel plates can be installed to achieve this protection at wall openings for mechanical, electrical, or plumbing lines. ICC 500 Section 306.8 also requires protecting wall joints between adjacent tilt-up or precast concrete panels unless they are 3⁄8 inch or less in width. Similarly, at openings in roofs where roof vents and duct shafts are required, the structural engineer of record (SER) should provide details for steel plates or other framing to protect the penetration.

Larger openings in walls and roofs should be minimized. As these openings typically support a manufactured component, such as a window, door, or louver, the SER should ensure the structural elements around the opening are designed and detailed sufficiently to receive the minimum anchorage used during the manufacturer’s missile impact test reports for each component. If the wall or roof system differs from that used in the manufacturer’s test reports, increased strength of the surrounding structure is likely required.

Equipment for “critical support systems” on the storm shelter roof is required to be storm rated or housed in rooftop structures that will allow the equipment to remain functional in the event of a tornado as described in ICC 500 Section 701.1.

Delegated Design of Shelter Components Storm shelter design frequently relies on delegating structural and nonstructural component design to the contractor. Delegated structural components commonly include a combination of precast concrete roof or wall elements, open-web steel joists, and structural steel connections. Non-structural components include pre-manufactured windows, doors, louvers, and associated hardware. While these delegated items are typical for most building types, the magnitude of the loads is unique in community storm shelter design. Therefore, the SER should be diligent in clearly detailing the loads and design requirements in the drawings. Similarly, the specifier of other nonstructural components in the storm shelter should do the same.

Design delegation is typically achieved through performance-based specifications authored by various design professionals. The SER’s specifications should require the contractor to engage a specialty structural engineer (SSE) for structural components. For non-structural components, the architect’s specifications should require pre-manufactured components tested for compliance with ICC 500 impact requirements.

The interrelation between the project’s design professionals and the contractor’s SSEs and component manufacturers is complex. The 2018 IBC is vague on the requirements for communication and responsibilities of these parties. Section 107.3.4 refers to the delegated design process as Deferred Submittals. This section requires the design professional in responsible charge to identify deferred submittals within the construction documents and review deferred submittal documents for general conformance to the design of the building. However, several industry-developed documents, such as the Coalition of American Structural Engineers (CASE) National Practice Guidelines for Specialty Structural Engineers, provide guidelines to enhance communication during the design and construction process.

The following is a list to consider for storm shelter delegated design: • Require manufacturer and SSE qualification submittals to demonstrate experience with storm shelter design and construction.

• Include a delegated design matrix that delineates design responsibilities between the SER and SSE.

18 STRUCTURE magazine

• Define ICC 500 roof live load, rain load, wind loads (MWFRS and C&C), and other debris and missile hazard loads for use by the contractor’s SSE. Where a topping slab is anticipated to resist diaphragm shears or other loads, define these loads.

• Perform preliminary designs of delegated components to validate critical dimensional parameters such as concrete wall panel thickness, double tee profile, and steel open-web joist depth, spacing, and minimum bridging.

• Construction documents should include minimum thickness and reinforcement required in precast concrete elements to meet the missile impact criteria of ICC 500 Section 305.1.

• Define foundation design assumptions such as continuity between adjacent concrete wall panels that may impact panel overturning moments or require panel interconnectivity.

Provide design loads at foundation embed plates and design supplemental reinforcement within foundation elements to resist concrete anchorage limit states.

• Provide details for supplemental steel at precast concrete openings that comply with ICC 500 Section 306.8 or 309.1, similar to Figure 2. The authors do not recommend deferring supplemental steel design to the precast manufacturer’s SSE. • Where it is unavoidable to create a structurally independent building, provide capacity-based connection forces at storm shelter interface with non-shelter components such as the host building, canopies, or other appurtenances.

• Review and coordinate performance-based specifications with those of other subconsultants for the building envelope.

In general, the authors do not recommend blanket statements that the SSE’s design shall comply with ICC 500.

Summary The successful design and construction of a storm shelter require active coordination between all parties from the very early conceptual stage to the final construction. More so than traditional buildings, all the systems and components of the storm shelter rely on the others regardless of the traditional lines of scope demarcation between design consultants and/or designers and constructors. Therefore, it is paramount that the design consultants clearly articulate loads and design requirements to be used by the SSE for their design of delegated design items.■ All authors are with Simpson Gumpertz & Heger.

Jeffrey D. Viano is a Senior Consulting Engineer. (jdviano@sgh.com) Connor J. Bruns is a Senior Consulting Engineer. (cjbruns@sgh.com) Matthew H. Johnson is a Principal. (mhjohnson@sgh.com) Andrea M. La Greca is a Project Consultant. (amlagreca@sgh.com)

Previous articles on ICC 500: Tornado Shelters in Schools. Harris, STRUCTURE, September 2016 Hurricane-Driven Building Code Enhancements. Knezevich et al., STRUCTURE, July 2017 Tornado Debris and Impact Testing. Throop et al., STRUCTURE, May 2018 Structural Design and Coordination of ICC 500 Tornado Shelters.

Simon and Dziak, STRUCTURE, July 2020

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INSIGHTS

Intersection of AEC and Artificial Intelligence

By Meghana Joshi

M

edia portrayal of artificial intelligence has been all about a dystopian future and robots. Artificial intelligence was once deemed a figment of scientific imagination, only seen in the characters of futuristic fiction movies. But, as time passed, theoretical and technological developments in computation have (Rachel, Blade normalized artificial intelligence, whether it is Alexa playing your favorite podcasts, Nest fine-tuning your optimal comfort zone, or Siri responding to your questions. Workplace advancements in the AEC profession saw their beginnings with BIM. Still, we have come to realize that revolutionizing the way we draw barely scratches the potential of artificial intelligence in our industry.

The AEC industry is ramping up efforts to incorporate artificial intelligence and information technology to create efficient and impactful building experiences. While business intelligence focuses on producing comprehensible data and in-depth analysis, conceptual understanding of design data through artificial intelligence will help create highly efficient building designs within technical constraints such as site requirements and building codes. Adhering to the set of parameters defined by data, artificial intelligence will gradually phase out pencil and paper renderings and progress towards visually realistic images.

Parametric Design bridges the gap between machine learning and artificial intelligence, and software will continue to offer new and expansive design capabilities through multiple possible iterations within provided constraints. For the uninitiated, artificial intelligence helps us create systems that simulate human thinking and behaviors. In contrast, machine learning capabilities will help these AI systems continually learn and improvise from generated data. Parametric Design will generate solutions that will put the client and community’s best interests forward in creating cities of the future.

The origins of modern parametric design began in the 1970s when Italian architect Luigi Moretti described architecture’s goal as “defining relationships between dimensions dependent on various parameters.”

The 1980s and 90s saw a rise in mathematical models and evolving structures based on parametric criteria and computational design.

Gehry and Partners launched Gehry Technologies in 2002, intending to provide a collaborative design platform and visualization tools for architects. This improved access to adaptive software and computational solutions, paving the way to non-linear imaginations that could be implemented with parametric design solutions. Zaha Hadid and Patrik Schumacher developed advanced parametric design systems and innovative adaptations for architecture and urbanism. In 2008, Patrik Schumacher coined the term “Parametricism” to define the architectural style based on computer technology and algorithms, resulting in fluid and seamless design. As cloud-based technology removed

barriers to access, it is no longer a luxury afforded only by Stararchitects/engineers. Social Justice-driven architects and innovators are creating modular and affordable housing designs with parametric frameworks to find optimal, safe, durable, and sustainable solutions to build strong communities.

Architecture paved the way for conRunner, 1982) struction to seek optimization from artificial intelligence and machine learning. In the past decade, pattern recognition and critical thinking abilities have led to innovations beyond project management and construction administration. Artificial Intelligence quickly overhauled business intelligence through solutions focused on project delivery methods to control schedule and budget. The generative method of design is a unique way of integrating artificial intelligence to develop high-performance building designs based on goal setting and navigating tradeoffs in traditional design. Generative technology coupled with programs such as BIM360 can efficiently manage clash detection within building systems such as mechanical, electrical, plumbing, and fire suppression. Construction safety was enhanced when a Boston-based general contractor developed an algorithm to analyze job safety through photographs. Post-construction intelligent processes were created to operate buildings and facilities efficiently.

Looking at a building through multiple lenses of design, construction, and management perspectives promotes a programming and planning process with long-range preparation for risk mitigation.

The digital shift in the AEC industry breaks down barriers within adjacent markets, helping create impactful ecosystems in the coming years. Facilities Management and Sustainability will rely heavily on technological evolutions based on artificial intelligence to manage their environmental footprint with micro- and macro-changes. The ability to evolve MEP systems based on historical data and revise and regenerate to adapt to site-specific conditions will contribute significantly towards mitigating the effects of building energy usage on climate change. Aesthetics and personalized comfort will be the center of smart building planning; everything from lighting to insulation can be analyzed and optimized. Reduced energy conservation will lead to reduced utility operation costs and minimize the various aspects of the building’s “footprint.”

Another example of an adjacent industry application would be Facebook’s Artificial Intelligence team. Facebook has partnered with Carnegie Mellon on the “Open Catalyst Project” to create efficient and scalable means of storing and using renewable energy.

Artificial intelligence-powered utility planning and management will create uniform access to renewable energy. General Electric’s “Predix” utilizes artificial intelligence to make predictions about the energy infrastructure’s machine health by performing in-depth

I THINK, SEBASTIAN; THEREFORE I AM.

20 STRUCTURE magazine

sensor analysis. Building materials are continually evolving through monitored manufacturing processes to reduce carbon emissions and environmental degradation. A cradle-to-cradle approach will begin with biodiversity conservation for raw materials leading to smart manufacturing and impactful recycling.

Of course, for some, this can create a worry of artificial intelligence replacing traditional building design. Is AI the revolutionary road towards automated building design and construction?

Automation might render some human labor obsolete, but architecture is more than an aesthetic presentation of an enclosed space, just as engineering is more than a collection of beams, columns, pumps, fans, and wires. Repetitive tasks can be delegated to machines, but the empathy and emotional intelligence guiding the design and our duty to the public cannot be replicated. Beyond the standard definition of building design, AEC is about interwoven and interconnected social, environmental, and economic characteristics that define a community. Technological innovations powered with artificial intelligence and machine learning will change built environment

interactions, experiences, and services but the algorithms also come with their own biases. Value judgment and diverse-thought leadership through human control will create fair systems leading to an equitable and inclusive future.

As people become more accepting of innovation and technology and address social injustices in IS AI THE design while also achieving comREVOLUTIONARY mercial successes, the limits are boundless. Thinking beyond the ROAD TOWARDS realm of design, architecture, and AUTOMATED engineering will open inter-professional dialogues and collaborative BUILDING digital innovations to take the AEC DESIGN AND industry forward. By blurring traditional industry boundaries, we CONSTRUCTION? can positively impact how we build and experience the built environment. Architect Michael Sorkin said, “There is a big danger in working in a single medium. The logjam you don’t even know you’re stuck in will be broken by a shift in representation.”

For our generation, that shift will be brought about by artificial intelligence. Just ask Rachel!■ Meghana Joshi is a Senior Project Manager with Little Diversified Architectural Consulting in Newport Beach, CA. (meghana.joshi@littleonline.com)

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structural PRACTICES Playing Tetris® in a Hurricane Ways to Ensure Deployed Flood Barriers Work

By Breanna Gribble, CHMM, ENV SP, WEDG, Robert Fields, P.E., LEED AP BD+C, and A. Christopher Cerino, P.E., SECB, F.SEI, DBIA

I

magine spending millions of dollars on flood protection but, when a storm approaches, the deployable flood barriers cannot be installed in time. Learning from experience and adding some common sense helps reduce the risk of improperly deploying flood barriers. Storage, maintenance, and deployment execution are the primary considerations for barrier selection and must be vetted fully during preliminary and final design to reduce operational challenges. Labeling and storing components in a readily accessible position and having deployment plans that are well thought out promote the flood mitigation system’s overall success. Deployment-related risks can be decreased by minimizing the number of components assembled in the field.

Focus on the Workforce Deployable flood barriers are workforce intensive; therefore, design teams should select these systems considering the emergency management organization responsible for the deployment and plan accordingly. Things might not always go according to plan in times of crisis or as seamlessly as factory testing implies. To mitigate this risk, the design team should consult the end user (e.g., the owner, operator, or staff deploying the flood mitigation system) to understand key variables such as workforce availability, time to deploy, and other logistics. If the deployment window is short, additional workforce may be necessary. This evaluation may also include a qualitative assessment of the end user’s capacity for robust training, accessibility of the site, and whether there is onsite personnel qualified to deploy the system in the face of an emergency. Without end user support, deployable flood barriers are rendered ineffective.

Robust training programs arguably are the most critical factor for successful deployment. At a minimum, an owner’s Flood Operation Response Plan should specify annual exercises that fully or partially deploy the system (Figure 1). The training frequency should also be customized to account for end-user staff turnover and the system’s complexity.

It is also vital to understand how much time is available, prior to a storm landfall, for deploying the flood barrier system. Site-specific temporal constraints leading up to a storm event inform the overall system’s design and influence the selection of barrier types. These constraints include but are not limited to storage locations and the workforce needed to deploy the system. Temporal variables should also be considered, such as the permitted duration of suspended

Figure 2. Flood barrier location signage.

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facility operations (e.g., subways are taken out of service 2-6 hours prior to landfall depending on the line), the designated threshold for shut down of equipment (e.g., power down industrial processes), and the evacuation procedure for occupants. For example, a building may need to be fully evacuated at the time of deployment to facilitate the safety of building occupants. Figure 1. Training exercise: Deployment of an aluminum flood plank system. Courtesy of However, in some Angelica Chan, STV. instances, the system may be partially erected while the building remains occupied.

The design team should also consider various workforce availability scenarios. For example, if janitorial staff deploy the flood barriers, will those same workers have other responsibilities during an emergency?

Workforce availability may be impacted by inaccessibility to the site due to flood-related road closures or increased traffic resulting from evacuation orders. Ask: “If the workforce was limited to two workers, how many shifts would it take to deploy?” Conversely, if the workforce was tripled, is the time to deploy reduced by a third? The time to erect each component and the number of available workers should influence the number and type of deployable flood barriers, the overall timing of the closure, and the deployment sequence – all critical inputs to the Flood Operation Response Plan.

The design team must be cognizant that workforce availability considerations are not limited to the next event; instead, they are essential over the lifespan of the asset being protected. Therefore, the owner must commit to promoting workforce sustainability over the flood mitigation system’s lifecycle. For example, if a third party will provide operations support during an emergency, when will that contract expire? Will there be funding available for continued support post-contract expiration? Note that it is inherently riskier to rely on an outside entity to deploy flood barrier systems because there are many variables that an owner cannot control. For example, the third-party contractor may go out of business or have other commitments that could jeopardize the availability of a workforce at the time of deployment.

The design team can effectively deploy the flood mitigation system by incorporating logistics planning into the project’s conceptual phases. Using an intuitive order of operations, such as deploying the system counterclockwise around the subject building, will contribute to efficient deployment. If the storage of barriers is organized to facilitate counterclockwise deployment, the end user may find

it easier to deploy. For example, one may Intuitive Storage is Key choose to number the stored flood barriers in ascending numerical order correspondA well-conceived and organized storage ing to counterclockwise deployment. system is key to the flood mitigation sysIn general, simple naming conventions tem’s operational success. Note, however, for flood barriers (Figure 2) make it easier that every time the system is deployed, the for the workforce to deploy to the proper likelihood of misplacing or losing pieces locations without continually consulting increases. For this reason, design of the a location map. It is helpful to label the storage space is fundamental to the success opening (e.g., doorway) with a unique of the flood mitigation program. corresponding identifier. Other logisticsAdministrative items such as well-docurelated value-adds include: consideration mented inventory lists, labels on both the for a supply of battery-powered radios in components and the shelves (Figure 3), case communication infrastructure (i.e., cell posted storage plans, and a clearly articutowers) is down when the system must be lated process for reporting missing pieces deployed, waterproofing the pages of the contribute to an organized storage space.

Flood Operation Response Plan and other Spare parts are essential and should be essential documents, and including spray specified as part of the design. If tools bottles filled with a water-dish soap mixture are needed to assemble flood barriers, to lubricate the jamb seals or other edges standard tools, rather than proprietary that need to be slid into place in the field. tools, should be specified early in the Engaging the end user early also has the Figure 3. Organized Storage. Courtesy of Yoko Stilwell, STV. design process. Tools provided by the added benefit of garnering stakeholder manufacturer need to be included in the buy-in and impresses upon the end user the importance of training, inventory and stored with the flood barrier system. An end user’s organized inventory and storage, and proper erection of flood barriers. worst nightmare would be having all the components to deploy a The design team should prioritize the Flood Operation Response Plan’s flood barrier except the custom tool needed to assemble it. Store preparation as much as designing the actual barriers. Considering dollies or hand trucks ready for deployment, and in appropriate deployment constraints, capitalizing on site-specific attributes, and quantities, in the storage room. involving the end user in the design are vital to creating the right Depending on how complex the flood mitigation system is, the design system for a particular client’s needs. team may want to examine the most effective stacking procedures for ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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Table 1. Example deployable parts list for a segmental flood panel barrier.

Item #

Description

Quantity

1

Flood Panels (Planks)

9

2

Threaded Hand Knobs

2

3

Latch Brackets

2

4

Latch Locks

2

Total Deployable Parts

15

components. An example flood storage diagram is shown in Figure 4.

For instance, it may be beneficial to stack the segmental flood panels in the deployed orientation. That way, the user can off-load the parts onto the dolly or hand truck from top to bottom, then deploy them for erection (from bottom piece to top).

Beyond the storage room’s interior organization, the design team should also consider its proximity to the deployment location and travel path to the deployment location. If, for example, two workers must carry a segmental flood panel, the design should consider turning radii in corridors and pinch points along the route. In addition, the flood barrier storage location should be near the deployment site to reduce the travel distance.

If storage is not available onsite, many other variables should be considered. Determine the storage fee over the flood mitigation system’s lifecycle and manage the budget. Consider equipment rentals and availability or exclusivity of trucking agreements to reduce the risk of materials not being transported to the site on time.

Detail the complete process for transport in the Flood Response Operation Plan.

Less (Parts) is More Numerous flood barrier parts have the potential to prevent the successful deployment of flood mitigation systems. Deployable flood Figure 4. Storage design schematic. barriers have two general groupings of parts. The first is the main barrier pieces, whether a fabric or metal material that serves as the The binomial cumulative distribution function (CDF), given by the protective surface (the barrier). The second is the additional com- following equation, may be used to approximate the probability of ponents needed to install, assemble, seal, and lock the barrier (the system failure, given a selected number of parts for which, if failure components). All parts should be stored securely and be identifiable were to occur, would trigger an overall system failure. and transportable to designated locations prepared to receive them for N proper installation. Operational pitfalls associated with an increased N pi (1−p)N−i CDF(M,N) = i number of parts include: i=M 1) Misplacement or loss of a component required to maintain barrier effectiveness/functionality Based on the following parameters: 2) Damage to parts during storage or deployment M – total number of parts that can fail 3) Restocking the parts incorrectly after a training exercise N – total number of parts making up the system 4) Errors in maintenance procedures or storage p – probability of failure for a single part 5) Faulty or incomplete deployment The probability of failure of the segmental flood panel barrier Not being able to deploy a single flood barrier part may result in under different scenarios is described in Table 2, where the assumed the entire flood mitigation system’s failure. Since manufacturers for per-part probability of failure (p) varies. In Table 2, there are three individual products typically provide guidance to maintain and operate conservative failure probabilities: 0.5% (1 incorrect deployment flood barriers, flood mitigation systems encompassing multiple products (especially if sourced from mul- Table 2. Probability of system failure given different numbers of deployable parts. tiple manufacturers) require overarching operation Total number of and maintenance measures integrated into the Flood components (N) Scenario P = 0.5% P = 1.0% P = 2.0% Operation Response Plan. This is key to minimizing 30 2 assemblies 13.96% 26.03% 45.45% the chance of pitfalls relating to individual parts while 15 1 system, all parts 7.24% 13.99% 26.14% addressing potential conflicts and efficiencies for the 10 1 system, less 5 parts 4.89% 9.56% 18.29% system’s coherent and reliable operation as a whole.

Table 1 includes a hypothetical deployable parts list 5 1 system, less 10 parts 2.48% 4.90% 9.61% (15 total parts) for a segmental flood panel barrier for 1 One part 0.5% 1.0% 2.0% an approximately 8.5 feet wide and 4.5 feet tall opening.

∑( )

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in 200 attempts), 1% (1 incorrect deployment in 100 attempts), and 2% (1 incorrect deployment in 50 attempts). The scenarios presented in Table 2 assume that each flood barrier part is mutually independent, equally essential to functionality, and equally likely to fail. This demonstrates how the probability of system failure increases as the number of parts in the system increases and how the probability of system failure decreases as the system becomes more resilient (e.g., greater redundancy or lesser interdependencies of components within the system).

Considering this data, selecting flood barriers with fewer parts, particularly for projects with multiple barriers, is always prudent.

In cases where a barrier typology with numerous parts is selected, a thorough inventory and maintenance system should be developed, with schedules and parts lists, and communicated to the end user. Adequate planning and training can also reduce the probability of pitfalls associated with each part’s successful deployment.

References are included in the PDF version of the article at STRUCTUREmag.org.

Breanna Gribble is STV’s Senior Resilience Manager. (breanna.gribble@stvinc.com) Robert Fields is an Environmental Engineer at STV in New York. (robert.fields@stvinc.com) A. Christopher Cerino is the Director of Structural Engineering in STV’s New York office and the firm’s Resilience Practice Lead. (anthony.cerino@stvinc.com)

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This article identifies various ways to manage risk of failure of deployable flood barriers. Risks fall into three categories: parts, people, and storage.

Systems should be designed to limit human interaction where feasible.

Designers should also work with manufacturers to reduce the number of operable parts per flood barrier and make the deployment procedures as easy as possible. While the flood mitigation system’s focus tends to be on the emergency condition, the end user will more likely operate it in a nonemergency condition (i.e., training exercises) during its tenure. Because of this, anticipate, at minimum, annual human contact with barriers that may result in operational pitfalls.

Designers should engage with the end user early and iteratively in the development process to address limitations or constraints such as workforce availability, workforce sustainability, training capacity, and off-site storage.

Communicate those needs to the owner upfront and specify the Flood Response Operation Plan’s approach.

Lastly, do not underestimate the importance of well-organized storage. The flood mitigation system must be maintained and stored properly so that the system can be efficiently deployed in an emergency situation.

The design team’s responsibility is to provide a user-friendly, site-specific system that addresses the end users’ needs when successfully deployed in an emergency.■

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Up Ashes FROM THE

Rebuilding the Sperry Chalet: Part 2

By Ian Glaser, P.E., Jeffrey Schalk, P.E., S.E., and Michael Schuller, P.E.

T

he backcountry Sperry Chalet was constructed over several summers in the 1910s using timbers harvested within Glacier National Park. The stone was quarried from the talus slope just uphill of the chalet. Over two feet wide at the base and tapering to sixteen inches wide at the roof level, the masonry walls consist of two interwoven leaves of argillite, a fine-grained sedimentary stone consisting primarily of lithified muds. The masonry is purposefully laid in random ashlar coursing, and the massive exterior leaf stones project by varying degrees conveying a natural aesthetic and textural ruggedness. Around the perimeter, log knee braces are supported on projecting exterior stones that act as corbels. Smaller stones and thinner mortar joints on the walls’ exposed inside faces define a more uniform finish than the exterior. Opening heads are supported by unreinforced concrete lintels on the interior and arched rowlock stone coursing on the exterior (Figure 6). Only the fire-damaged masonry chimneys and perimeter walls remained after the historic building was lost to the

Figure 6. Completed roof and balcony framing as well as rehabilitated masonry.

Note unremovable soot stain over door. Courtesy of Mark Bryant Photographics.

Sprague Fire in 2017, as described in Part 1 (STRUCTURE, May 2021). [Note: Figure numbering continues from Part 1.]

High temperatures damage metals and cementitious materials, but the effects on stone are less well known. Thermal differentials in stone are known to create fracture planes. The initial site investigation sought to understand the fire’s intensity and ascertain the extent of masonry damage. Glass, aluminum alloys, lead, iron, and steel debris were extant in the debris within the building footprint when the design team first visited the site. These materials were studied to estimate the temperature of the fire. Knowing the melting temperatures of these materials, the fire’s sustained temperature was determined to fall between 1350°F and 2100°F. Since the fire burned from within the structure, the masonry’s exterior leaf remained essentially undamaged except at the jambs and the top of the walls. The design team mapped the intensity of the fire based on the level of damage to the interior leaf of masonry. The most intense areas of fire were at the tops of the walls and around wall openings where the most oxygen was available. The hot inside faces of the stones fractured when they were quenched during firefighting.

Most of the chalet’s stone units are naturally bedded, with their sediment layers oriented horizontally.

However, there are face-bedded units, and face-bedded units turned normal to the plane of the wall that helped define the natural vernacular. Over the building’s lifespan, the face-bedded stones on the exterior had not delaminated, as is common with other sedimentary stones in cold environments. However, the stone delaminated on the wall’s inside face where the fire burned the hottest. The face-bedded stone delaminated somewhat cleanly along bedding planes to a uniform depth, the depth varying depending on the intensity of the fire.

The face-bedded stones turned normal to the wall and the naturally bedded stones fractured unevenly. Fracture planes crossed bedding planes and converged within the depth of the stones to create faceted voids.

The depth of delamination varied and, in the worst cases, was just over 2 inches. Structural analysis of the wall assembly indicated that an average of 2 inches of material could be lost without compromising the wall strength for gravity and lateral loads in the new design. continued on page 28

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Figure 7. Color changes through the cross-section of spalled concrete lintel provides clues on the intensity and duration of fire. Effect of temperature on concrete strength keyed to observations (Chart: Bilow, S., Kamara, M. “Fire and Concrete Structures.” Structures 2008: Crossing Borders, ASCE.)

The masonry walls’ inside faces were first descaled of unbonded material from top to bottom. Mortar joints were raked back to match the new inner wall plane. Jamb stones that were exposed to the fire on three sides and consequently suffered through-unit cracks were replaced. The tops of the walls were reconstructed where the setting mortar between the smaller stone units had severely degraded in the fire. Replacement stones were transported by helicopter from the original quarry to the masons.

Another component of the investigation was understanding the fire’s effect on the concrete lintels. Concrete, mortar, and other cementitious materials undergo color changes when subjected to high temperatures.

Color changes progressively from gray to a subtle pink color, whitegray, and finally to buff as the temperature rises. After the fire, the concrete lintels’ faces were observed to be a buff color, suggesting that maximum fire temperatures exceeded 1700°F. In one location, the concrete spalled to reveal a multicolored cross-section that provided more clues. For concrete with siliceous aggregate, the observed depths of color gradation suggested a high-temperature duration of 1½ to 2 hours and an approximate halving of concrete compressive strength in the outer 1 inch of material (Figure 7 ).

The fire-weakened concrete lintels were retrofitted with galvanized steel lintels. WT webs were fit into vertical slots cut into the underside of concrete lintels, and the flanges bear on galvanized plate steel at

the jambs concealed by window trim. Spalled sections of concrete were patched with repair mortar.

The exterior masonry walls’ condition was unknown when the lateral design for the reconstruction was initiated. The building is classified as Seismic Design Category C. The masonry walls have abundant perforations allowing each dorm-sized room to have a window and balcony door. The historic stonework had to remain visible, eliminating common retrofitting options such as shotcrete or carbon fiber. A steel frame or brace system was similarly ruled out because of the constructability challenges they would present.

Instead, the designers decided early on to use flexible wood-framed diaphragms and wood-framed shear walls as the lateral force-resisting system. These new interior walls were designed to take the full seismic demand and are founded on new reinforced concrete footings that bear directly on the natural rock below the crawlspace. This design decision created a challenge to attract load away from the stiffer existing masonry walls.

In the transverse direction, the new wood diaphragms cantilever to the outboard masonry walls. Recognizing the diaphragm’s inherent stiffness, the location of the shear walls was fine-tuned to limit the load that would be delivered to the masonry gable end walls. Although the transverse stone walls were checked to have the requisite strength to carry a tributary portion of the lateral load, they are not relied upon

Figure 8. Construction crew fishes second-floor balcony’s joist flitch through wall pocket. Courtesy of A Boring Photo.

Figure 9. Second-floor balcony joist flitch grouted in wall pocket. Joists are supported on inside face of wall by a steel angle ledger.

28 STRUCTURE magazine

in the load path. The transverse lateral system was designed as though the end walls would lose integrity during a significant seismic event. The lateral design in the longitudinal direction was designed following the same philosophy.

The masonry walls’ inner and outer leaves were stitched together with a matrix of ½-inch-diameter stainless steel pins set in Hilti HIT-HY 270 epoxy adhesive that supplements the bonder stones. The pins ensure that the masonry leaves act compositely and that the walls satisfy minimum height to thickness ratios. The repaired masonry walls offer redundancy in the lateral system and resistance to eccentric loading.

While the perimeter stone masonry walls are stiffer than the new interior wood shear walls, the slenderness of the masonry piers counteracts the disparity to some degree.

When the reconstruction was designed, it was Figure 10. The roof endures its first load test after the first construction season. assumed that the original log floor joists at the second Courtesy of Gravityshots.com. floor cantilevered through the exterior masonry walls to form the balconies. It was discovered during the initial site visit the exterior appearance. The flitches are grouted solid into the walls, that the original balcony framing was separate from the interior with logs tight to masonry on either side and serve as seismic wall framing. The balcony joists were simple-spanning, supported only ties that transmit masonry anchorage into the floor diaphragm. by the wall pockets and diagonal log knee braces. Without posiInterior log floor joists that are not part of the balconies were sized tive anchorage of the balconies through the wall into the interior without flitches and were held short of the wall pockets to avoid floor, the original design lacked seismic connectivity. The knee decay susceptibility, bearing instead on a ledger angle. The abanbraces exerted lateral thrust mid-height on the stone walls, and the doned interior pockets were grouted solid, and the steel angle was framing was extremely prone to decay in the harsh environment. designed to double as a continuous diaphragm chord and seismic Formerly, the balcony framing was actively maintained and periodi- collector (Figure 9). cally rebuilt by the park to address decay conditions. A part of the One of the most obvious structural challenges was the roof design, as design challenge was to increase the longevity of the balconies and the chalet is almost entirely buried by snow during the winter months integrate life safety.

(Figure 10). Stakeholders were clear in their desire to reframe the roof Since the wall pockets did not align from exterior to interior, inte- with exposed logs having the same proportionality as the original rior log joist framing was respaced to facilitate the new cantilevered ones. The original roof framing was certainly undersized. It was later joist design. The interior joists were located to exit the building learned that the original roof survived the winter months via internal in alignment with the original balcony joists to favor the historic props installed during the end-of-season shutdown. The roof framing exterior appearance. The new continuous joists are fitted with gal- also relied on a set of interior knee braces along the heavily loaded vanized steel plate flitches (Figure 8). The flitches were designed to purlin lines that induced horizontal point loads on the masonry end take the full cantilevered outer-span load, allowing the logs to stop walls and interior posts. short of the masonry where they would otherwise be prone to decay.

The interior knee braces were eliminated in favor of clear-spanning The balcony logs act essentially as jackets that can be periodically HSS tube steel purlins wrapped in log jackets to create a cleaner roof replaced. The balcony knee braces are not relied upon for strength framing load path. Like the balcony framing design, the purlins were but are incorporated into the design to limit deflection and recreate designed to cantilever to the rake edges without structural reliance on the exterior knee braces. The log jacketing is omitted through the plane of the wall to avoid decay and allow for periodic replacement of the outboard ends. The purlin ends are foam-insulated to lessen the potential for condensation inside the building envelope (Figure 11). The long-span log rafters and valley beams were outfitted with galvanized steel plate flitches. Unlike the balcony logs and roof purlins, the rafters are better protected by the roof, and they run uninterrupted over the top of the walls wrapped in a vapor barrier in stone saddles.

Like all successful projects, this project required commitment, teamwork, and ingenuity. It presents a valuable case study in responding to disaster with creativity and finesse. The rebuilt Sperry Chalet offers the same charm but with a more resilient backbone.■ Ian Glaser is JVA, Inc.’s Historic Preservation Director.

Jeffrey Schalk is a Senior Project Manager at JVA, Inc. Figure 11. Tube steel purlin with log wrap.

Michael Schuller is President of Atkinson-Noland & Associates.

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The Tsubaki Tower

By Steven M. Baldridge, P.E., S.E., LEED AP, Mark Hirschi, S.E., and Yuriy Mikhaylov, S.E.

T

he U.S. territory of Guam is a small, 210-square-mile island in the western Pacific Ocean with a population of 169,000 residents. Guam is home to one of the harshest building environments in the world. The nearby subduction of one major tectonic plate under another along the Mariana Trench creates frequent and intense earthquakes. The resulting design spectral accelerations are similar in intensity to those present in more familiar earthquake hotspots like Los Angeles. Guam also finds itself situated in “Typhoon Alley,” an area of the western Pacific Ocean that experiences the highest frequency of tropical cyclones on earth, four times more active than similar areas in the Atlantic Ocean.

These dueling hazards result in near-constant extreme events for Guam that regularly push its structures to their limits. For example, in the 10 years between 1992 and 2002, Guam experienced four major earthquakes ranging from 6.5 to 7.8 magnitude and direct hits from five major typhoons. In addition to these extreme events, the aggressive tropical environment daily batters structures with humidity, salt spray, and mold. In these respects, to a structural engineer, Guam is in many ways comparable to Darwin’s studies of the Galapagos Islands, where only the fittest structures survive.

Building Description The Tsubaki Tower is a 26-story (297-foot), 476,000-square-foot luxury hotel with 325 guest rooms. The Tower is situated on an escarpment overlooking Tumon Bay. While Guam’s population is relatively small, it is a tourist destination for Japan and Korea. Over 1.6 million tourists visited Guam in 2019 and the hotel was positioned to attract many of these visitors.

The tower floor plan is 310 feet long, 49 feet wide, and is in the form of a 75-degree arc with every guest room facing the ocean. The two concrete stair cores located on each side of the tower protrude 33 feet towards the interior of the arc. The centrally located elevator core, with a fully glazed front, protrudes 25 feet into the interior of the arc. The hotel also includes all-day dining facilities, specialty restaurants, and administrative areas in addition to the guest rooms.

The 26th floor is dedicated to amenities and includes a reflecting pool that cantilevers off the tower towards the ocean, making it an ideal location for an outdoor chapel for spectacular sky weddings.

Planned initially at 25 stories, the design process began as normal, with all building elements designed to standard building code requirements. A wind tunnel study was carried out to analyze the wind loads for the building, including the effects of local topography and nearby structures. A modal response spectrum analysis was performed to 30 STRUCTURE magazine

Completed Tsubaki Tower at dusk. Courtesy of Kevin Frias.

determine the seismic loads on the structure. These analyses were then combined with the gravity loads to design and detail the tower’s special reinforced concrete shear walls, concrete gravity columns, pile caps, and precast pile foundation. The slabs were planned to utilize unbonded post-tensioning but local resources were limited, and the Japanese contractor decided to source these systems outside the U.S. where bonded systems are more common.

The resulting analysis and BIM models were also shared with the contractor and their subcontractor to assist in the detailing and designing of the bonded post-tensioned floors, with BASE providing a detailed review of the design. Shear wall link beams were designed as special composite plate link beams, a system that BASE has pioneered in the Pacific region. The one exception to this plan was a height limit for shear wall lateral systems included in more current editions of the building codes.

Performance-Based Design Like many smaller communities, Guam has had difficulty finding the resources to keep up with ever-changing building codes. In 2010, Guam started transitioning from the 1994 edition of the Uniform Building Code (UBC) to the 2009 edition of the International Building Code (IBC). This major leap between code editions introduced sudden changes in restrictions on building systems previously allowed by the UBC. For example, the newer IBC introduced a height limit on buildings using shear walls to resist lateral forces, a system used in most hotels in Guam and the most desirable and economic system for The Tsubaki Tower. With this height limit in place, the structural system would have to be reconfigured to incorporate special moment frames and their deep beams and large columns to augment the shear walls. These changes would have reduced ceiling heights in guest rooms, restricted ocean views, slowed construction, and added significant cost to the structure. The only way to avoid these considerable detriments to the project was to go beyond traditional design procedures with a more detailed performance-based design.

With performance-based design, an extra story could be added to

the building without increasing the overall building height, creating quite a bit of additional value to the client.

The Tsubaki Tower was the first performance-based design project administered through the Guam Department of Public Works.

Fortunately, the project’s architect represented the American Institute of Architects on the Guam Building Code Council and was therefore closely involved in assisting the building department with code updates. They were able to leverage their experience to set up meetings with the Guam Department of Public Works to explain the process and assemble teams to carry out the performance-based design and its independent review for the Government of Guam, who lacked the in-house technical expertise to evaluate the design.

With the typical design process complete, the performance-based design began. The structure was exported from ETABS, the software package primarily used for standard analysis and design, and imported into PERFORM 3D for more in-depth performance-based design.

As sufficient ground motion records were not available for this site, a site-specific response spectrum was developed. Ground motions were generated and scaled to the Maximum Considered Earthquake (MCE) level. The structural elements were then discretized into fiber models and digitally instrumented to capture the strains, rotations, and other deformations required to gauge acceptance under criteria provided in Los Angeles Tall Buildings Structural Design Council (LATBSDC) guidelines and ASCE 41, Seismic Evaluation and Retrofit of Existing Buildings. Nonlinear dynamic analyses were then carried out using stiffness modifiers and expected material properties per LATBSDC guidelines and results compared to acceptance criteria to determine if structural performance met the desired benchmarks.

The performance-based design revealed minimal changes required to meet the desired performance under the MCE. For example, wall rotations were generally within desired ranges without any additional reinforcing or boundary elements and gravity components did not undergo any significant yielding. Also, the composite plate link beams performed admirably and met or exceeded the levels of ductility and energy dissipation that would be expected from more traditionally reinforced link beams. The primary alterations that resulted from the performance-based design were to the diaphragms. The changes primarily resulted from lower stiffnesses assumed in the LATBSDC guidelines for MCE-level events than are traditionally used for design under standard building codes.

Wind and seismic load comparisons; Guam vs. Los Angeles.

Topping off of structural framing.

Energy dissipation charts revealed a somewhat surprising aspect that is unique to environments like Guam. Under extreme seismic loading, it is anticipated that a significant amount of seismic energy is dissipated through inelastic deformations as the structural ‘fuses’ inherent in specially detailed seismic systems yield. Smaller levels of energy dissipation are typically expected from damping and elastic strain. However, The Tsubaki Tower dissipated energy primarily through elastic strain and damping with significantly less inelastic dissipation than would normally be expected. Inelastic energy instead typically made up roughly ten percent of the energy dissipated through all time histories analyzed.

The culprit is the extremely high winds in Guam. Under standard design, structures are not allowed to yield under wind loads as they are under seismic loads, and no performance-based design guidelines for wind design were available at the time of design. For the Tsubaki Tower, wind loads exceeded design seismic loads and wind story shears were generally over half of MCE story shears, conditions not typically found in other high seismic zones in the U.S. By comparison, Los Angeles’ wind loads are typically significantly lower than its design seismic loads, similar in magnitude to the seismic loads in Guam.

Since the Tsubaki Tower’s wind loads are significantly higher than the design seismic load, very little inelastic behavior could be utilized for

Tower nearing completion of the structure.

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Wind tunnel testing. Courtesy of RWDI.

Bonded post-tensioning placed along the arc of the floor slab.

energy dissipation under a high seismic event, and significantly less nonlinearity was encountered than would normally be anticipated in a high seismic zone. It was as if the entire building was designed for seismic loads with overstrength.

Unfortunately, this also included numerous sand-filled cavities and areas of weaker zones in the coralline limestone formation. As a result, the foundations took 18 months to complete and required significant redesign for many of the structure’s pile caps to account for pile field modifications. This included areas too hard or too soft for the pile installation, resulting in piles that could not be driven to the required depth or ones that hit subterranean voids, ending up in soft areas. Both conditions meant they could not develop the required compressive or tensile capacities. In the end, these conditions required some piles to be abandoned and additional piles to be added, resulting in the placement of 966 piles, many that had to be cast in two sections and spliced in the field. The final tally included approximately 1,300 pile sections, equivalent to about 16 miles in total length.

Construction Challenges Beyond the significant design challenges, there were substantial hurdles to clear during construction as well. Guam is a particularly remote U.S. territory situated on the other side of the international dateline in the eastern hemisphere, further from the U.S. mainland than any other U.S. territory. Islanders proudly proclaim to be where “America’s Day Begins.” Due to Guam’s remoteness, its primary sources for construction labor and supplies are typically drawn from multiple nearby countries in Asia. The owner acted as the construction manager, and their ties to Japan led to a Japanese contractor being selected for the shell construction. The rest of the team was rather diverse. Materials were also diverse, with formwork imported from Korea, bathroom pods from Japan, bonded post-tensioning from Singapore, and workers from many nations and territories around the Pacific.

The construction of the Tsubaki Tower was hampered initially by difficult and varying subsurface conditions that significantly slowed the completion of the project’s precast pile foundation. The tower was underlain with shallow soils over a native coralline limestone formation.

Completion As the Tsubaki Tower went vertical, changes in reviews of temporary H-2B worker visas shrank the construction labor pool on the island severely, creating more delays. Finally, after almost four years of continuous construction, the project topped off and finish work continued. As the hotel underwent final preparations for its planned public opening in April 2020, the COVID-19 pandemic reached the island resulting in travel restrictions and mandatory quarantines in Guam. However, the Tsubaki Tower now stands finished and open to the public, ready and waiting for the full brunt of Guam’s burgeoning tourism to return, and prepared to protect its inhabitants and withstand the many anticipated extreme conditions over its lifespan.■ Steven M. Baldridge is the President at BASE. (sb@baseengr.com) Mark Hirschi is an Associate at BASE and is based in its Chicago office. (mhirschi@baseengr.com) Yuriy Mikhaylov is a Structural Engineer at BASE and is based in its Guam office. (ymikhaylov@baseengr.com)

Project Team

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32 STRUCTURE magazine

Owner: P.H.R. Ken Micronesia, Inc. / Premier Hotel Group Structural Engineer of Record: BASE Architect of Record: RIM Architects General Contractor: P.H.R. Ken Micronesia, Inc./ Asanuma Corporation

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34 STRUCTURE magazine

Lady Liberty Gets a Precast Concrete Neighbor

By Monica Schultes and PCI Staff, Tom Bagsarian, and Becky King

S

choolchildren learn very early that the Statue of Liberty was not manufactured at its location in New York Harbor. Instead, it was fabricated in France and shipped piece-by-piece across the Atlantic Ocean to where it currently stands as a beacon to freedom and liberty for all the world to see.

History has repeated itself; the Statue of Liberty Museum’s 83 precast concrete pieces were also manufactured offsite and shipped from 150 miles away in Denver and Pennsylvania. Crews then assembled the pieces at the northern tip of Liberty Island.

The museum exhibits are about freedom, immigration, and the entire story of the statue: why and how it was made and transported, how money was raised to have it built in New York, the gateway to America, the experiences of immigrants entering the harbor and seeing the lady of the harbor, and watching maintenance and renovation activities over the years.

The museum’s concept stemmed from respecting the site, the statue, and its history. “We wanted to defer to the statue and coordinate with existing materials on the island,” said Dan Piselli, a senior associate with FXCollaborative (FXC) Architects, designer of the museum.

The museum provides a new setting for the statue’s torch, offering panoramic views of Lady Liberty and Lower Manhattan. The green roof is planted with native grasses. In addition to the striking precast

Because the project was on an island in the middle of New York Harbor, logistics were challenging. Credit: Iwan Baan, courtesy of FXCollaborative.

concrete panel façade, the structure uses the same granite, bronze, and copper used for the statue’s pedestal more than 130 years ago.

Stony Creek granite was selected for the project because the material was used on the original pedestal. It was retrieved from the same quarry in Connecticut.

General contractor Phelps Construction was forced to handle many logistical issues on this project, which was in the middle of New York Harbor. Precast was the ideal construction material to resolve these issues. Precast concrete allowed each piece to be individually shipped to the site rather than creating an area onsite to store and stage construction materials. The speed of erection was also much faster than it would have been with concrete poured The museum’s exterior’s vertical ribbed at the worksite. Once the pieces pattern took some cues from the nearby arrived on site, they were quickly Palisades cliffs in New Jersey that placed and assembled with a crane have a vertical rhythm. Credit: Frank Linemann, courtesy of FXCollaborative. and a small crew of ironworkers.

Crucial Early Coordination

Helping to build a museum on tiny Liberty Island is no ordinary task. High Concrete Group manufactured and erected 83 pieces of precast concrete for the striking building. Credit: Iwan Baan, courtesy of FXCollaborative.

The nature of prefabrication leaves little room for error in the field, so coordination during the design phase was critical. “We were able to talk in real-time, viewing the models and envisioning what would happen in the field,” says Piselli. The museum’s complex geometry required intricate contours. It is an irregular shape with numerous cuts and angles and not very repetitive; forms could not be used repetitively.

Corner panels were very custom and had acute angles. High Concrete Group shared Tekla models showing how the panels would connect to the steel frame.

In addition to aesthetics and geometry, the precast concrete panels had to be designed to provide lateral stability for the building structure.

This was realized during early coordination between the construction team and the design team, resulting in the elimination of cast-inplace shear walls initially specified. Each panel was engineered to resist lateral shears and moments specified by the Structural Engineer J U L Y 2 0 21

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and subsequently anchored to the finish, we looked at all the variables roof diaphragm. At the exterior, the related to the mix design: aggregates, team designed pockets and corbels to pigments, sand, and cement,” Piselli receive the steel beams, eliminating says. The mix design for the structhe need for perimeter columns and tural wythe was a 28-day, 7,000-psi dispersing the weight of the green mix. The mix design for the face mix roof above to the building perimeter. was a 28-day, 5,000-psi mix. Both Energy efficiency was crucial to the mixes used a corrosion inhibitor. museum’s enclosure. It uses a low The architect worked extensively window-to-wall ratio, highly insuwith High Concrete Group, which lated walls, extensive thermal bridge manufactured the precast concrete, to mitigation, and internal thermal mass test different formulations to achieve to moderate energy use for heating the right color and texture. and cooling. Continuous insulation A 4-inch-deep formliner imparted achieved R-19 by enclosing 4 inches A form-liner imparted a deep vertical relief creating shadow that the design texture and shadow to the 21-inchteam wanted, its darkness to accentuate the panels and complement the of rigid insulation in the precast con- context and site. Courtesy of Timothy Schenck Photography. thick panels and added the darker crete panels. tones the design team wanted to Using precast concrete sandwich panels allowed the project team complement the context and site. In addition, the vertical ribbed to thermally isolate the building more than with a single wythe and pattern echoed the striated look of the nearby Palisades cliffs. “The eliminated the need to insulate on site. The building benefited from overall effect is that the precast concrete looks like stone lifted out of the thermal mass and eliminated thermal bridges. the ground, which makes it feel like part of the landscape,” says Piselli.

The project team and the Statue of Liberty-Ellis Island Foundation committee, which raised money to construct the new museum, proDesign Considerations vided opinions on the numerous samples and mock-ups that were The new building was set 10 feet above the adjacent plaza, which is cast. The mock-up panel was ferried around the island to view it in 5 feet above the base flood level. Resiliency requirements included various sun and shade angles to provide differing perspectives. surviving a 100-year flood or storm. This was especially important since Superstorm Sandy struck this region in 2012. FXC’s design Working Between Tourists focused on selecting resilient materials in conjunction with durable and appropriate construction methods to withstand major disasters High Concrete Group manufactured the precast concrete at its plant and climate change. Drainage slots were incorporated into the base in Pennsylvania. The pieces were shipped individually on a trailer and to ensure future resiliency and reduce the large hydrostatic loads that transported on a barge to the island. As the trailers drove onto the would have resulted from a fully enclosed condition. Flood holes island, a crew of ironworkers used a crane to pick the pieces off the varied from 2 feet high to 5 feet, 8 inches high by 4 inches wide. trailer and place them on the foundation. While assembled on-site, Storm surge water should come into the flood zone and flow back each piece was already a fully insulated architectural panel that needed out. The area below the first floor is an open, unusable space, enclosed minimal work after being placed. by precast walls with openings cast into them. The panels on the lower Precast concrete was transported by truck to Jersey City, NJ, where a level in the flood zone are 21 inches thick to resist the force of waves barge ferried four truck trailers at a time with panels, some weighing breaking against the building while supporting the structure above. 50,000 pounds, to the site. The only cargo service to Liberty Island The panels at the flood zone, which is 19 feet above sea level, are was from Staten Island, two miles away. Since construction could “horizontal” in orientation, then topped by vertically placed precast not disrupt tourism, Phelps Construction built a temporary dock concrete panels. The project is located in multiple flood zones, the to receive construction materials and personnel. With no staging most stringent being Zone VE. Federal National Park Service flood zone requirements are more stringent than New York City’s requirements. The panels, weighing up to 60,000 pounds, were made as large as practical for shipping, handling, and alignment with the design.

The building’s foundation is comprised of concrete grade beams spanning between pile caps founded on piles driven into stable earth.

“In addition to the in-depth design coordination required between teams, the foundations required an extraordinary amount of on-the-fly coordination to build the elements in and around the maze of existing utilities and historic sea walls encountered on the site,” according to Jarret Johnson, Associate Principal of DeSimone Consulting Engineers.

This required close coordination to ensure the required connections between the precast panels and foundation elements could still be installed on-site.

Meeting Aesthetic Requirements Selecting the precast concrete finish was essential. Designers wanted to make the museum walls darker while keeping in the color family of the Fort Wood sea wall at the statue’s base. “To achieve the right 36 STRUCTURE magazine

High Concrete Group shared Tekla models showing how the panels would connect to the steel frame.

The architect wanted to defer to the statue and coordinate with existing materials on the island. Courtesy of David Sundberg/ESTO.

area on Liberty Island, an average of eight panels were installed per day. The fully insulated architectural panels provided enclosure for the primary building as they were erected, which helped speed construction.

Precast concrete can be an excellent solution for the durable, resilient, highperformance building enclosures found on this project, says High Concrete Group President J. Seroky. Precast concrete was the only material possible, considering the logistics and speed required.

The new 26,000-square-foot museum opened in May 2019.■

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Project Team Owners: Statue of Liberty and Ellis Island Foundation working in partnership with the National Park Service/U.S. Department of the Interior Engineer: DeSimone Consulting Engineers, New York, NY Designer: FXCollaborative, New York, NY Contractor: Phelps Construction Group, Boonton, NJ PCI-Certified Precast Concrete Producer: High Concrete Group, LLC, Denver, PA PCI-Certified Erector: Precast Services Inc., Morgantown, PA

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J U L Y 2 0 21

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structural ANALYSIS BRBF Global Stability The Real Failure Mode

By Brandt Saxey, S.E.

A

young researcher studying Buckling Restrained Braces (BRB) once commented that a change made in their design had improved the failure mode such that fatigue of the yielding core no longer controlled the BRB’s performance. Upon investigation, the change had simply created a failure mechanism that developed BEFORE the steel would have fatigued had the change not been made. The fatigue life of the BRB had not changed – the rest of the system simply no longer had the capacity to survive until fatigue limits would have been reached. Thus, what was thought to be a benefit, was not. But the point is an important one.

BRBs are tested primarily to confirm the relationship between strain and the overstrength generated in the steel core (known as a backbone curve), to measure the cumulative inelastic ductility (CID) capacity (an item related to fatigue life), and to confirm the ability of the BRB to undergo inelastic rotations. Even with the damaging nature of AISC-type qualification tests, BRBs will commonly survive tests as high as 3% strain (a 6% strain range) and achieve CID values of 800 to 1000 – 3 or 4 times the code required value. BRBs often achieve even higher strains and CID values when tests are performed that mimic expected earthquake records. For example, in recent tests of full-scale BRBs subjected to earthquake records scaled to at least 100% of the Maximum Considered Earthquake (MCE), each specimen survived more than 10 EQ records while accumulating between 5 to 8 times the code required CID and experiencing peak strain ranges of up to 6.3%.

Such tests indicate that the CID and fatigue capacity of the BRB will likely not be the limiting factor in a BRB’s ability to resist even multiple earthquake events. These tests, however, are designed for just that – to check the fatigue life of a properly designed BRB’s core in a properly designed frame. But this fatigue life can only be realized if the frame surrounding the BRB has sufficient strength to allow the BRB’s core to reach its full overstrength while keeping demands within safe limits in the remainder of the BRB, the gusset, and the frame. BRB testing provisions do not intend to test the frame itself, and only limited requirements are made for the connections and gussets. For example, the commentary of AISC 341, Seismic Provisions for Structural Steel Buildings, makes statements such as “While the subsequent design of the gusset plate connection is itself a complicated issue and the subject of continuing investigation, it is not intended that these connections become the focus of the testing program.”

And “For the purposes of utilizing previous test data to meet the requirements of this section, the requirements for similarity between the brace and subassemblage brace test specimen can be considered to exclude the steel core extension connection to the frame.” (See AISC 341-16 commentary section K3.3 – emphasis added.)

The AISC 341 seismic provisions contain limited requirements that certain elements of the testing program be preserved in design, such as for gusset bracing used in tests, but, as noted, the commentary is clear that it is not intended to extend these to the interaction of the BRB with the surrounding frame. Doing so would dramatically increase the required testing parameters to the extent that testing would become impractical.

38 STRUCTURE magazine

Figure 1. Global out-of-plane buckling of a BRB Test Specimen with views showing: plastic hinge at neck facilitated by restrainer local yielding (top); additional plastic hinge at gusset (bottom).

Global Stability The interaction between the structural frame, the gusset plate, the BRB neck, and the BRB restrainer is complex and challenging to model adequately in testing. If these components are not accounted for adequately, they can form hinges creating a “global buckling” mechanism that occurs before the BRB core can reach its full overstrength potential – like the young researcher’s experience described above. Figure 1 shows a global stability failure of a BRB in testing.

Notably, this specimen neither buckled at midspan nor did it experience local plate buckling, but instead plastic hinges formed at the gusset and neck, allowing the global mechanism to form.

To better predict when global buckling will occur, analytical models have been developed accounting for the strengths of the components affecting stability and their interaction with each other. Much of the work developing these models has been performed over the last 15+ years by a group of researchers under the direction of Professor Toru Takeuchi at the Tokyo Institute of Technology in Japan. Their model accounts for the rotational stiffness and strength of the many components involved, the reduced capacities of these components with increased axial forces, and the interaction of these components with each other. It is a plastic method that amplifies the initial geometric imperfection along the elastic buckling path until one of two plastic collapse mechanisms is reached. The method is understandably complex

and can be difficult to implement. Iteration is also required to solve for the axial load at which the system becomes unstable.

However, a known force (the compressive overstrength of the BRB, for example) can be used without iteration to determine whether it is above or below the stability point.

A simplification to this method, called the Notional Load Yield Line (NLYL) method, has been developed by Bo Dowswell with Arc International. This method combines many parts of the Takeuchi method with a simplified notional load model for the gusset plate capacity. This method has been adopted further by a group at the University of Canterbury, NZ, under the direction of Professor Charles Clifton, to apply the deformed shapes and associated strain energies of the Takeuchi method. This method is a specialized case of Takeuchi’s method with several simplifying assumptions to improve the usability for practicing engineers. It also adds a local gusset plate buckling check, which may govern for extended unstiffened gussets.

Strength and Embedment The NLYL and Takeuchi’s methods both address global stability by accounting for the formation of hinges within the system. These hinges may form at either end of the brace in the gusset plates, in the neck, or at the neck’s insertion into the restrainer. Once three Figure 2. Asymmetrical mode of global instability: a) plastic failure with one hinge hinges form, a mechanism will develop, leading to global instability. at the gusset plate and a second hinge at the BRB neck/restrainer interface, or; An asymmetric mode of instability, as shown in Figure 2, typically b) plastic failure with both hinges forming in the gusset plate (gusset buckling). requires the least energy to form and therefore usually controls design. Hinges that form in this mode are located at and based upon direction that stability is being considered) to its height. The the strength of the plastic moment capacity of the gusset (Mgp) and Takeuchi research notes that embedment ratios around 2.0 are the moment transfer capacity of the restrainer (Mrp). The latter is considered sufficient to develop the neck such that its full moment controlled by the axial-flexural capacity of the BRB neck and the capacity can be achieved. However, ratios less than 2.0 may also flexural capacity of the restrainer. Which one governs depends on develop in the neck, especially in the presence of a stiff restrainer. the embedment length of the neck into the restrainer.

As this ratio is reduced, however, it becomes increasingly diffiThis embedment length introduces a cult to develop the strength of the neck. critical element to BRB design that has Embedment ratios around 1.0 are likely to not been considered expressly before the result in a premature plastic failure of the development of analytical global stability restrainer wall with the capacity dictated methods. This embedment is similar to by the retainer strength. An embedment cantilevering a steel beam from a concrete ratio nearing 0.5 will act like having a pin wall. If a deep enough insertion into the at the face of the restrainer cap plate, perwall exists, the beam’s full moment capacity mitting rotations in the neck and with little can be developed. However, if the insertion or no moment capacity. These embedment length is short, the material surrounding ratios are shown schematically in Figure 3. the hole may not have sufficient strength Note that both methods calculate the to develop the beam completely. In the moment capacity expressly using the insercase with sufficient embedment length, the tion length (Lin) and the neck/restrainer moment capacity of the beam itself dictates strength. As such, specifying a value of the the capacity. But when the embedment embedment ratio itself is not necessary. length is insufficient, the strength of the The NLYL method involves two general material surrounding the beam limits the mechanisms – plastic failure above, or capacity, and a premature failure of this “over” the yield line (OYL) and gusset buckmaterial “blowing out” around the inserling “under” the yield line (UYL). These tion zone occurs. can be seen in Figure 2, where Figure 2a The cantilevered element’s moment shows an OYL-type mechanism and Figure 2b capacity is related to its height, and so Figure 3. Embedment ratio depictions showing: a) deep shows a UYL-type mechanism. In the the “embedment ratio” can be introduced embedment allowing the development of the BRB neck; OYL failure mechanism, one plastic hinge as a means to express the relative level b) intermediate embedment with capacity controlled by forms at the yield line in the gusset, and of embedment. This ratio is taken as the restrainer strength; (c) shallow embedment approximating a second plastic hinge forms in the neck length of the embedded element (in the hinge near restrainer end. of the BRB. Thus, the hinging is above or

J U L Y 2 0 21

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“over” the yield line. In the UYL mechanism, a plastic hinge forms at the yield line in the gusset at the tip of the brace, and a second forms in the gusset below and the gusset buckles between the two.

Thus the hinging is below or “under” the yield line. This latter mechanism is independent of the BRB, as it develops with hinges only in the gusset and is identical to local gusset plate buckling studied for years. The capacity side of the global stability equation is the summation of the effective moment transfer capacity of the restrainer (Mrp) plus a reduced gusset capacity (M gp) for the OYL mechanism, or twice the reduced gusset capacity for the UYL mechanism. The reduced gusset capacity is a function of the gusset bending strength, including the axial force effect plus the

destabilizing effect of the neck misalignment. (In the case of the UYL mechanism, the destabilizing effect on the reduced gusset capacity is included only once.)

In both failure mechanisms, the strengths of the elements and their initial out-of-straightness are considered to determine the total imperfection angle. The notional load used in the NLYL methods is a function of the total imperfection angle multiplied by the axial load (including overstrength) in the BRB core. The demand on the system is then generally in the form of the notional load multiplied by a moment arm and a second-order term (similar to the B2 factor used in stability analysis). When the demand exceeds the capacity, global instability is assumed to occur.

Conclusion

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40 STRUCTURE magazine

Global stability calculations can tell us a lot about a BRBF’s performance. For example, a flexible gusset may remain stable in the presence of a strong neck with proper embedment into a stiff restrainer since a hinge would need to form in each for instability to occur.

Similarly, a weaker neck may be acceptable in the presence of good embedment, a strong restrainer, and a stiff gusset. In the absence of global stability checks, assessing the individual components in isolation might have suggested that the system was not acceptable when it was.

Conversely, investigating the global stability of the system may reveal a weakness that would not be apparent when considering only the limit states of individual elements in isolation. These checks analyze the BRB frame holistically and help ensure that undesirable failure modes will be precluded, allowing energy dissipation to remain focused in the BRB core as intended.■ The significant contributions of the following individuals are acknowledged in furthering the research into BRB global stability and their assistance in the writing of this article.

Bo Dowswell – ARC International Ben Sitler – Tokyo Institute of Technology Charles Clifton – University of Canterbury Behnam Zaboli – University of Canterbury Brandt Saxey is the Technical Director for CoreBrace. He is a member of the AISC 341 TC-9 Seismic Systems Committee, TC-6 Connection Design Committee, and M3 Seismic Manual Committee. (brandt.saxey@corebrace.com)

structural DESIGN

An Overview of Slope Stability

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

G

entle slopes are usually stable. As the slope’s inclination angle increases, the risk of failure increases accordingly. This can be attributed to the instability of the soil mass when the geometry results in the soil strength being unable to provide adequate support and its natural tendency to achieve stability and equilibrium. Failures in slopes usually occur in Figure 1. Type of slip circles. the form of soil movement, where the unstable mass topples or slides downwards or sideways to achieve stability. slips for homogenous soils. In non-homogenous soils, the slip surfaces may take on other shapes or geometry. Many ground conditions, in reality, are heterogeneous.

Key Principles When an unstable mass of soil slides, the sliding surface tends to The first step in approaching a slope stability problem is to understand follow the path of least resistance. If there are any existing faults, whether there is loading or unloading. Loading problems are typically fractures, fissures, or weaknesses between or within the soils’ mass embankment and reclamation works where the slope is built up from or weak soil layers, the failure surface or plane will likely be along an existing grade and load is added to the soil. Unloading problems these weaknesses. occur when soil is excavated from an existing ground such that the load on the soil decreases. Over time, soils tend to get stiffer in loadAnalysis and Design ing and weaker in unloading due to pore pressure build-up and the dissipation of the excess pore pressure. The designer needs to consider Many slope stability problems are analyzed using finite element the type of soil present, groundwater conditions, and soil permeability. methods (FEM) or limit equilibrium methods. Due to the tedious For embankment and reclamation, the designer has the advantage of nature of the calculations, software or spreadsheets are often required. choosing suitable backfills in his design, whereas for excavation, the Codes generally recommend an appropriate FOS against failure. For designer has to consider the soils found on site. example, in the Eurocode 7 design approach 1, there are two combinaSlope stability problems are a consideration of soil mass stability and tions to be checked – Combination 1 to factor up the loadings and satisfying force and moment equilibrium. An inclined mass of soil needs Combination 2 to factor down the soil strengths. to withstand its own weight, surcharge, and water conditions, either It is prudent to check the validity of the FEM results. For undrained flow or hydrostatic. The soil shear strength along a sliding plane provides problems, the designer can refer to Taylor stability charts where a the stabilizing force. The failure plane or surfaces need to be assumed, minimum FOS can be obtained based on soil strength and geometry. and the corresponding factor of safety (FOS) against sliding evaluated. For drained problems, designers can use the method of slices where Types of slip circles include shallow, deep, and translational. The a sliding soil mass is divided into small slices to find FOS against failure modes must be identified and checked to ascertain the lowest sliding by considering force and moment equilibrium. Examples FOS. In design, it is often easy and convenient to choose circular of these methods include Fellenius, Bishop, Spencer, MorgensternPrice, and Sarma. Each method differs in consideration of the inter-slice forces and the extent of compliance with force and moment equilibrium.

Although codes allow such methods to be used in a design, equilibrium must be satisfied, regardless. The drawback to using the method of slices is that it is necessary to test a large number of failure circles with varying centers of rotation; understanding the types of slip circles and making sensible judgments regarding where the potential slip circles might lie is critical.

Designers should also note the differences between a two-dimensional plane strain model commonly used to design and analyze slopes and a real-life threeFigure 2. Taylor’s stability coefficients. dimensional problem on site. continued on next page J U L Y 2 0 21

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Figure 3. FEM results for undrained analysis – a single layer of soil, constant shear strength.

Figure 4. FEM results for undrained analysis, using layered soil with increasing shear strength for each layer.

Types of Slip Circles There are generally two main types of slip circles, as shown in Figure 1 (page 41). Shallow slip circles are typically slope circles and toe circles where the circles are found above the toe level. A deep slip circle is identified by the circle cutting below the bottom of the slope. In sand, where the relevant type of analysis is drained, the slip circle is shallow. But in clay, using an undrained analysis for the short term, the slip circle is deep. The shear strength of sand comes from its friction angle; the greater the depth, the higher the friction due to the vertical pressure. Thus, the failure surface would not likely go deep, resulting in a shallow slip circle. The undrained shear strength for clay is unaffected by the confining pressure.

Case Study

analysis due to the unrealistic assumption of using a constant cu for the entire soil layer. In reality, even for very soft soils, the value of cu increases with depth.

The same problem was re-analyzed using layers of soil such that the undrained shear strength increases with depth, as shown in Figure 4, and a more realistic slip circle is obtained, with a lower FOS. Figure 5 shows a typical drained analysis, where the slip circle cuts the slope above the toe and has a shallow failure zone.

In FEM analysis, it is important to note that, even for drained analysis, c´ is not inputted as zero because zero is prone to numerical errors, being indivisible. Pure sand in a drained analysis is seldom encountered, and most real soils in practice have a small value of c´.

Use of Taylor Chart

Taylor’s stability coefficient, Ns for a homogenous slope in the undTaylor’s stability coefficients can be used to find the minimum FOS rained case, without a firm stratum below the toe of the slope, is for the case of a fully saturated clay under undrained conditions (e.g., 0.18 for slopes up to 55°. Beyond that, Ns increases approximately for a condition immediately after excavation). linearly to 0.26 for a 90° slope. The minimum FOS = cu/(NsγH).

Consider the case of a 45° slope excavated to a depth of H = 26 For slopes gentler than 55°, the FOS is essentially the same: a ratio feet (8 m) in a deep layer of saturated clay of unit weight γ = 122 pcf of undrained shear strength against a product of soil unit weight (19 kN/m3), with the relevant undrained shear strength parameter and slope height. For a higher slope, the shear strength required 1350 psf (cu = 65 kN/m2). The minimum FOS can be found using to achieve the same FOS is higher. For slopes steeper than 55°, an Taylor’s stability coefficients (Figure 2, page 41), where the slope angle, even more significant increase in shear strength is required due to ß = 45°, and, assuming that the depth factor, D, is large (i.e., no firm the increase in Ns. The Ns value of 0.26 for a 90° slope suggests that stratum), the value of Ns is 0.18. The FOS is = cu/(NsγH), = 2.37. a vertical cut slope is possible.

The problem was analyzed using Plaxis The Table shows FOS for a vertical cut Table of FOS for a vertical cut with varying shear strength.

FEM software, as shown in Figure 3. slope with undrained shear strengths of The FOS obtained from FEM is simi20 kPa, 50 kPa, and 100 kPa. Values γ = 19 (kN/m3) Ns = 0.26 lar to that predicted by Taylor’s stability highlighted in yellow correspond to a H cu = 20 cu = 50 cu = 100 coefficients. From the deep-seated slip value above 1.5 (a commonly adopted (m) (kPa) FOS (kPa) FOS (kPa) FOS circle, it can be deduced that this soil FOS), and the retained height can be is undrained. However, the slip circle read under the “H” column. Designers 2 2.02 5.06 10.12 appears too large to be realistic. The need to be cautioned that FOS estimated 4 1.01 2.53 5.06 slip circle is limited by the confines of from Taylor’s stability number are for 6 0.67 1.69 3.37 the problem boundary. Enlarging the undrained, homogenous, pure cohesive boundaries does not solve the issue, as soils that are optimistic and seldom 8 0.51 1.27 2.53 the slip circle will simply continue to encountered in practice. A higher FOS 10 0.40 1.01 2.02 extend to the edge of the boundary. This should be adopted if one were to rely 13 0.31 0.78 1.56 problem can occur in an undrained FEM on these values for preliminary design.

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Figure 5. FEM results for drained analysis – using effective stress parameters.

Figure 6. FEM results for a vertical cut.

Figure 6 shows FEM results for a vertical cut; the failure surface is approximately 45°. This coincides well with the theoretical sliding wedge formulation.

Figure 7 shows three idealized cases of slopes with gradients 90°, 0°, and 45°. Based on geometry and mechanics, the FOS are estimated and compare well against Taylor’s FOS. It can be seen that the slope FOS normalized by cu/(γH) ranges from about 4 to 5.5. Do not be tempted to extend these formulas to drained cases by considering the shear strength of soil to be the sum of cohesion and normal stress multiplied by the tangent of soil friction angle. This does not work. It must be emphasized that these formulas are applicable for undrained cases for cohesive soil where the consideration of groundwater is not applicable, thus allowing calculations to be simplified. Pure sand

without cohesion would not stand vertically, and, for sand in its loosest state (minimum density), the angle of slope is the angle of repose.

Translational Slip For purely frictional sand, a potential failure surface is parallel to the surface of the slope and is known as an infinite slope failure. As the failure surface is shallow, the depth of the slip is small compared with the length of the slope. The FOS for such a case is given by tan φ divided by tan α, where φ and α are the sand friction angle and slope angle, respectively. For pure sand, a slope steeper than the friction angle is likely to be unsafe.

Conclusion Slopes are often an economic consideration in construction.

The safety and stability of slopes depend on many factors such as soil type, loading condition, duration of construction, the permeability of the soil, presence of water, and weaknesses in the ground. When assessing the FOS of slopes, designers should always keep in mind the likely type of slip surfaces that could develop. Taylor stability coefficients and translational slip surface/angle of repose slopes are benchmark cases of an undrained case for clay and a drained case for pure sand, respectively. These are useful aids for the designer to consider when looking at a real-life problem, which is likely to be more complicated.■

Figure 7. Three idealized cases of slopes with gradient 90°, 0°, and 45° for cohesive soil.

Hee Yang Ng is a Principal Engineer with a building control agency in the Asia-Pacific region.

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building BLOCKS Shotcrete Today – Not Your Father’s Gunite

By Charles Hanskat, P.E.

S

hotcrete has been in active use since 1907, predating the introduction in 1913 of central plant batched ready-mix concrete. With its evolution over the last 100+ years, shotcrete has become ubiquitous for concrete delivery on today’s construction sites. So how has shotcrete evolved over the last century of use?

Shotcrete is simply a placement method for concrete. Early shotcrete equipment conveyed dry concrete materials via air through a flexible delivery hose and added water at the exit nozzle, in effect making low water/cement ratio (w/cm) A dry-mix shotcrete (gunite) crew in the 1920s.

Shooting a structural shotcrete wall in the 1940s. concrete immediately before placement.

This was a method that allowed remote placement of relatively high Also, since shotcrete does not impose fluid concrete pressure against strength, low w/cm concrete mixtures. the form, a much lighter form can be used. This allows constructing As the materials were shot from a dry-mix gun, the Cement Gun curved shapes much more efficiently and at a lower cost. As formwork company, which controlled the use of shotcrete equipment in the may approach 40% of a construction project cost, this provides both early days, adopted the tradename “Gunite.” Many still use the old time and cost savings and sustainability benefits. tradename when referring to what we now call dry-mix shotcrete.

Excellent Bond The American Concrete Institute (ACI) wrote a standard, Recommended Practice for the Application of Mortar by Pneumatic Whether using shotcrete to repair existing concrete, shooting in mulPressure (ACI 805-51), and suggested the term shotcrete. After tiple layers to build out thicker structural sections, or shooting against adopting the term, the introduction of high-velocity placement of construction joints, shotcrete inherently provides an excellent bond. ready-mixed concrete using concrete pumps and high-velocity airflow With proper surface preparation, shotcrete can easily provide over 150 pushed ACI to refine the terminology to dry-mix shotcrete (the original pounds per square inch (psi) tensile bond strength. This equates to a gunite) and wet-mix shotcrete for the pumped concrete accelerated by bond shear strength of 300 to 400 psi and allows the shotcrete to act air at the nozzle. This is the terminology in use today. monolithically with the concrete substrate. Thus, shotcrete placed in layers acts monolithically. There is no need for a bonding agent due to the high-velocity impact and inherent paste-rich environment at Benefits of Shotcrete the bonding interface.

So, what differentiates shotcrete placement from conventional formEasily Create Curved or Tapered Sections and-pour construction?

1) High-velocity placement Since shotcrete does not need to carry the fluid pressure of concrete 2) Minimal formwork along the full length of a wall, much thinner and lighter forms are 3) Excellent bond to existing concrete substrates used. Also, a one-side form is all that is required to define the back 4) Ability to more easily provide curved and tapered sections surface application crews are shooting against. This reduced formwork 5) All placement immediately visible makes it much easier to create curved sections. Also, since the wall is built-out from one side, it is easy to produce tapered wall sections High Velocity that make the most efficient use of the concrete.

The key to quality shotcrete is high-velocity acceleration and then Placement is Fully Visible impact of the concrete. High-velocity shotcrete equipment projects concrete from the nozzle at 60 to 80 miles/hour. The concrete then Since shotcrete is placed against a form or existing substrate, every impacts the substrate (form or previously shot concrete) and is fully cubic foot is visible during placement. An experienced nozzleman, compacted and consolidated without the need for external vibration. inspector, superintendent, or project manager can continuously This allows shotcrete to be placed vertically or overhead against a know whether the concrete being placed meets the compaction receiving surface. and encasement of reinforcement needed for structural concrete sections. Any problems experienced during placement (slugs of Minimal Formwork material, tears, sloughs) can be cut out and reshot immediately.

In new construction, a one-sided form may be used to define the back There is no waiting until the forms are stripped days later to surface of the shotcreted section. In soil-supported and underground discover rock pockets or other issues that may occur inside doublework, no form is required. In repair, often no formwork is required. sided form-and-pour work.

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Advances in Shotcrete Though shotcrete has been used for over a century, the industry has progressed substantially in concrete materials, shotcrete equipment, and placement techniques.

In either dry-mix or wet-mix, today’s shotcrete materials are some of the most sophisticated concretes used in the concrete industry. Shotcrete often includes supplemental cementitious materials (SCM), like silica fume, fly ash, slag, or metakaolin. The shotcrete industry was Bing Concert Hall – Stanford University. A shotcreted truncated cone. Courtesy of Joseph J. Albanese, Inc. one of the first significant users of silica fume as it increased the adhesion and cohesion of the concrete, Placement Techniques allowing crews to shoot thicker layers with a better bond and less waste. Using fly ash has allowed pumping low-slump concrete In the 1950s, ACI began developing standards on shotcrete. In the through smaller delivery pipes and hoses more easily. Alternative early years of shotcreting, quality concrete placement was possible, cements are also able to be used efficiently and effectively with although it depended on the expertise and commitment of the conshotcrete placement. tractor for the work. As the ACI standard evolved, they emphasized Shotcrete concrete mixtures use common concrete admixtures like the specific techniques required to get maximum concrete compachigh-range water-reducers, air-entraining, summer-time retarders, tion with high-velocity impact and full encasement of embedded winter set accelerators, shrinkage-reducers, and corrosion inhibitors. reinforcement.

Additionally, the shotcrete placement process provides the unique Twenty years ago, both the American Shotcrete Association (ASA) ability to use rapid-set accelerators, added just as the concrete leaves and ACI worked together to develop the ACI Shotcrete Nozzle the nozzle, with the concrete set in a few minutes and providing Certification program. ASA developed an education program to early strength gain. These admixtures assist applications that are support the certification efforts. Simultaneously, ACI has a shotcrete time-sensitive and need to provide structural stability quickly. Many certification committee that maintains the policies and procedures to shotcrete contractors have benefited from using hydration control provide consistency, relevance, and quality of the certification process. admixtures to extend the time concrete stays fresh for placement.

In 2019, ACI added a Shotcrete Inspector certification. This cerShotcrete mixtures have also used nano-particle technologies to tification is for inspectors who have both concrete and shotcrete enhance the fluid flow during pumping yet allow stability when experience. ASA provides an education program for inspectors who impacting in place. Additionally, fibers, both steel and synthetic, may be very well versed in concrete and introduces them to the speare routinely used in shotcrete. These increase resistance to plastic cifics of inspecting shotcrete. Though shotcrete is just a placement shrinkage cracking, toughness, and ductility. method for concrete, there are many details for materials, equipment, and technique that need to be done correctly for quality shotcrete.

Equipment

ACI Technical Documents on Shotcrete ACI Committee 506 is ACI’s shotcrete committee. They developed and now maintain many documents, including ACI 506R Guide to Shotcrete; ACI 506.2 Specification for Shotcrete; ACI 506.4R Guide for the Evaluation of Shotcrete; ACI 506.5R Guide for Specifying Underground Shotcrete; and ACI 506.6T Core Quality Evaluation.

Several new ACI 506 documents are on the near horizon, including

Demos at www.struware.com Wind, Seismic, Snow, Rain, etc. Struware’s Code Search program calculates these and other loadings for all codes based on the IBC or ASCE7 in just minutes (see online video). Also calculates wind loads on rooftop equipment, signs, walls, chimneys, trussed towers, tanks and more. ($295.00).

CMU or Tilt-up Concrete Walls Analyze solid walls for out of plane loading and panel legs next to or between openings by automatically calculating loads to the wall leg from vertical and horizontal loads at the opening. ($75.00 ea) Floor Vibration Program to analyze floors with steel beams and/or steel joist.

Compare up to 4 systems side by side ($75.00).

Concrete beam/slab Program to provide bending, shear and/or torsional reinforcing.

Quick and easy to use ($45.00).

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The original dry-mix equipment proved durable and allowed effective placement. However, dry-mix guns have evolved from sizeable dualpressure chamber guns to smaller rotary guns. The rotary guns are easier to operate and have made dry-mix very portable. The smaller size lets contractors place their gun, air compressor, and hoses on a single truck. This allows them to roll up on a site and be productive almost immediately.

With the advent of reliable dual-cylinder hydraulic concrete pumps, the shotcrete industry moved to the wet-mix process. This predominately uses ready-mixed concrete delivery and provides increased production rates when compared to dry-mix. Over the years, concrete pump manufacturers serving the shotcrete industry have refined the design to increase pumping pressures, making pumping low-slump concrete mixes through small diameter delivery lines more reliable.

Nozzle designs have evolved as well. Enhanced dry-mix nozzles and pre-dampening systems are available that can decrease dust and waste. Nozzles come in various shapes and sizes to accommodate hand nozzling or remote nozzle manipulation with robotic equipment.

Lidar and laser scanning systems are available to evaluate shotcrete placement. It is expected that systems will provide real-time feedback to the nozzleman on thicknesses during placement in the near future.

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Shotcrete placement on the Bing Concert Hall. All concrete placement fully visible during shooting. Courtesy of Joseph J. Albanese, Inc.

ACI-certified wet-mix shotcrete nozzleman shooting a structural mockup panel.

Courtesy of Joseph J. Albanese, Inc.

a Guide for the Construction of Pools Using Shotcrete, a Guide to Qualification for Specific Projects, and a TechNote on Shotcrete Preconstruction Mock-ups.

The ACI 506R Guide to Shotcrete is non-mandatory language document that is an excellent primer of all aspects of shotcrete. The ACI 506.2 Specification provides terse, mandatory language that is readily able to be used in a project specification or adopted by reference. The fiber document (see below) gives specifics for using fibers in shotcrete and helps both the engineer and contractor. The Guide for Specifying Underground Shotcrete is a unique document in ACI. It is not a mandatory language specification but a guide for creating a project specification. It provides extensive commentary and example language that helps write an appropriate project-specific specification for an underground job. The Core Quality Evaluation TechNote is useful for engineers to evaluate a shotcrete nozzleman’s performance by evaluating cores taken from a mockup panel.

Several other ACI documents include some aspects of shotcrete or refer to the ACI 506.2 document, including: Codes and Specifications ACI 318-19, Building Code Requirements for Structural Concrete and Commentary ACI 301-20, Specifications for Concrete Construction ACI 350-06, Code Requirements for Environmental Engineering Concrete Structures ACI 350.5-12, Specifications for Environmental Concrete Structures ACI 563-18, Specifications for Repair of Concrete in Buildings ACI 376 11, Code Requirements for Design & Construction of Concrete Structures for Containment of Refrigerated Liquefied Gases & Commentary Guides CCS-4(20), Shotcrete for the Craftsman 372R-13, Guide to Design and Construction of Circular Wire-andStrand-Wrapped Prestressed Concrete Structures ACI 506.1-08 Guide to Fiber-Reinforced Shotcrete 544.4R-18, Guide to Design with Fiber-Reinforced Concrete 334.3R-05, Construction of Concrete Shells Using Inflated Forms (Reapproved 2020)

Addition of Shotcrete Provisions to ACI 318-19

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Of particular interest to many practicing engineers is the addition of shotcrete requirements into the ACI 318 Code. You can find a past article from ASA’s Shotcrete magazine that provides much more detailed coverage of the changes at https://bit.ly/381CpJg. This was a reprint of an article from the December 2019 Concrete International magazine from ACI. ACI 318-19 made extensive additions to directly include shotcrete in the Code. As a result of ACI 318’s thorough inclusion of shotcrete, future versions of the International Building Code will remove their specific shotcrete requirements and instead refer to ACI 318 for shotcrete provisions. This will provide more consistency on the requirements for the use of shotcrete placement in structural concrete.

Summary Shotcrete is well over 100 years old. Through constant improvement in concrete materials, equipment, and placement techniques, the quality of the concrete placed is equal or superior to conventional cast-in-place concrete in strength, low permeability, and durability.

Shotcrete offers the engineer and architect a creative and efficient way to use concrete in a wide variety of structures. Indeed, some applications are uniquely suited to shotcrete placement and offer the best, or sometimes the only way to place quality concrete cost-effectively and efficiently. Many codes, specifications, guides, and reports from standards-developing organizations recognize the successful performance of shotcrete placement and including shotcrete provisions in their documents. As a result, designers should feel comfortable including shotcrete in their projects in North America and other countries worldwide.■ Charles Hanskat is a Fellow member of the American Concrete Institute, American Society of Civil Engineers, and the Florida Engineering Society.

He is Executive Director and Technical Director for the American Shotcrete Association. Hanskat is active in developing codes and standards from many professional and technical engineering societies, including ACI, ASTM, AREMA, and ICRI.

structural CARBON Structural Engineers and the Climate Crisis Reducing Embodied Carbon through the SE2050 Commitment

By Chris Horiuchi, S.E., LEED AP BD+C, Mark D. Webster, P.E., LEED AP BD+C, Mike Gryniuk, P.E., and Megan Stringer, S.E., P.E., LEED AP BD+C

I

n response to the scientific consensus that man-made emissions are driving warming global temperatures and other climate changes already causing havoc worldwide, the structural engineering community has created a voluntary program called the SE 2050 Commitment designed to reduce the emissions associated with building structures.

This article shares details about the program, what’s driving it, its overall goals, and how structural engineers can participate.

Structural Materials Impacts Globally, the building and construction sectors account for nearly 40% of global energy-related carbon dioxide emissions in constructing and operating buildings. Current building codes and rating systems focus on addressing operating energy but do not typically give as much focus to the impacts embodied in building materials and products.

However, more than half of all greenhouse gas (GHG) emissions are related to materials management (including material extraction and manufacturing). As building operations become more efficient, these embodied impacts related to producing building materials become increasingly significant. Structural elements, especially those of steel and concrete, serve as the significant carbon contributors of building material embodied carbon.

Climate scientists have determined that we must reduce man-made emissions to net-zero by 2050 to avoid the most calamitous impacts of climate change. Moreover, during the 30 years leading up to 2050, construction-related building emissions carry an even greater weight

relative to the emissions associated with building operation (such as heating and cooling) since the construction emissions happen upfront.

What is SE2050?

SE 2050 stands for the Structural Engineers 2050 Commitment Program that encompasses the SE 2050 Challenge issued in 2019 by the Carbon Leadership Forum (CLF) and the SE 2050 Commitment Program developed by the Sustainability Committee of the Structural Engineering Institute (SEI) of the American Society of Civil Engineers (ASCE).

The SE 2050 Commitment Program is being developed with SEI’s support in response to the SE 2050 Challenge, which states: All structural engineers shall understand, reduce, and ultimately eliminate net embodied carbon in their projects by 2050.

The Program’s goal is to provide an accessible sustainability program, for individual structural engineers and structural engineering firms, with an accountable commitment strategy of active engagement on projects and sharing of information, all in the name of achieving zero net carbon structures by 2050.

Through this process, embodied carbon impacts of structural systems will be able to be tracked, trends assessed for various systems, and then achievable reduction targets established over time. This concept is modeled after the Architecture 2030 reduction targets for operational energy; SE 2050 will run parallel to Architecture 2030 and address structural embodied carbon. continued on next page

Terminology and Concepts Embodied carbon

Net-zero embodied carbon Carbon sequestration

The general team used to quantify the total impact of all greenhouse gases emitted (measured in CO2-equivalent or CO2-e since the measurement includes greenhouses gases beyond carbon dioxide) into the atmosphere by the extraction, production, transportation, construction, and maintenance of a material, product, or system. In buildings, structural materials generally account for 50% or more of the total embodied carbon. As buildings’ and infrastructures’ operational emissions trend towards net-zero, embodied carbon becomes drastically more critical to address and reduce. Carbon released during the production of materials and construction of buildings/infrastructure is emitted earlier than carbon released during operation and has a more immediate impact on climate.

When the upfront embodied carbon is reduced to the greatest extent possible. Then, the remaining embodied carbon is offset or sequestered so that the emissions over the building’s lifecycle are effectively eliminated.

The process of capturing and storing atmospheric carbon dioxide. Wood and other renewable materials, as well as concrete to some extent, can be used to sequester or “store” carbon within the material.

Life cycle assessment (LCA)

A method of evaluating the environmental impacts, including embodied carbon, associated with various stages of a product’s or building’s life. These stages include raw material extraction, manufacturing, distribution, construction/assembly, maintenance/repair, and disposal/recycling. For a holistic view of a structural system’s embodied carbon, it is essential to conduct a whole building LCA (WBLCA).

Environmental product declarations (EPDs)

Third-party verified reports measuring the environmental impacts, including embodied carbon, of a product or material from a life cycle assessment. The location of a material or product’s extraction and manufacturing can significantly influence the magnitude of its embodied carbon, both due to the mix of electricity production at the point of manufacture (e.g., renewable vs. fossil) and transportation impacts. EPDs can be industry-average, representing the impacts of product manufacture as a whole within a region, or product-specific, representing the impacts of a particular product from a particular manufacturer.

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The program has four components: 1) Education: Educate the structural engineering profession on the best practices of sustainable structural design and construction that will lead to net-zero embodied carbon by 2050.

2) Reporting: Engage in an embodied carbon tracking program within the structural engineering profession, thereby enabling the establishment of appropriate embodied carbon reduction targets until net zero is realized. Report on the current embodied carbon impacts and trends of various structural systems for different regions throughout the country.

3) Reduction: Reduce embodied carbon in building structures.

4) Advocacy: Advocate for embodied carbon reductions among engineers, architects, contractors, owners, and regulators.

Why Should Structural Engineers Participate?

Among the compelling reasons to participate in SE 2050 are to: • Improve the health and welfare of humankind in accordance with the ASCE Code of Ethics.

• Address social inequities posed by climate change.

• Reduce negative environmental impacts of climate change, including loss of biodiversity and species extinction.

• Reduce property damage and costs associated with climate impacts, including sea level rise, storm intensity, and wildfires.

• Meet client demand for consultants that are addressing climate change.

• Improve staff recruitment and retention by addressing a deep concern for many younger engineers.

The Role of Structural Engineers Society will not stop building with steel, concrete, timber, etc., any time soon. But building with improved versions of those materials and materials using emerging technologies will allow us to get to net-zero by 2050.

Getting to net-zero will require changes in both design and construction methods as well as in areas where structural engineers may feel “there isn’t anything we can do.” Structural engineers are responsible for educating the industry on opportunities for embodied carbon reductions, including demanding the availability of both production and documentation of lower impact materials. After all, any changes to the structure related to embodied carbon must be approved by the Structural Engineer of Record.

The SEI Sustainability Committee published the white paper, Achieving Net Zero Embodied Carbon in Structural Materials by 2050 (https://bit.ly/3vPtp3M) to explain how this necessary goal can be achieved. It is estimated that design improvements by engineers and architects, combined with decarbonizing the electrical grid and advances in material production, can get us there. Sequestration in materials such as timber and concrete will be necessary to avoid relying on carbon offsets.

The paper reports that concrete accounts for over three-quarters of the carbon emissions associated with the three primary structural materials used in buildings (steel, concrete, and timber).

Furthermore, over half the emissions associated with these materials are related to residential construction (see Figure). Structural engineers can help reduce concrete emissions by specifying mixes with less portland cement. Supplementary cementitious materials (SCMs) such as fly ash and slag can replace cement. Blended cements such as Portland-limestone (Type IL), Portland-slag (Type IS), and Portland-pozzolan (Type IP) with reduced carbon 48 STRUCTURE magazine

Annual CO2e emissions associated with structural materials used in new construction in the United States by building sector.

emissions are becoming increasingly available. New technologies such as CarbonCure and Blue Planet, which sequester carbon in concrete and aggregates, respectively, are further helping drive down the embodied carbon of concrete. Concrete suppliers also are developing lower-carbon concretes, including ECOPact (LafargeHolcim) and EF Technology (US Concrete). Structural engineers can decarbonize concrete and drive market innovation by specifying SCMs, blended cements, and new technologies where appropriate and available.

Substituting responsibly grown and harvested timber for other structural materials is also a promising strategy. Although the carbon cycle of forest ecosystems is complex and not fully understood, many studies suggest that sustainably managed forests can produce wood products that store enough carbon during a building life to offset emissions associated with harvesting, manufacturing, and construction, resulting in net-negative embodied carbon.

The white paper and other references point to numerous other strategies structural engineers can employ to reduce carbon, including material optimization and the use of salvaged materials. The reader is encouraged to review these resources to learn more.

Current Status Forty-five engineering firms have signed on to the SE 2050 Commitment as of mid-June 2021. The SE 2050 team continues to develop the program and resources.

• The SE2050 team is currently developing specification guidance for reducing embodied carbon, a guide to embodied carbon in green building rating systems in both the U.S. and Europe, and case studies of projects that achieved embodied carbon reductions. Additional resources, including updates to Embodied Carbon Intensity Diagrams (ECIDs) and the ECOM tool, should be published this year.

• The SE 2050 database, made possible through funding by SEI Futures Fund, is currently in development and anticipated to rollout mid-year. A user guide for the database will be published prior to the database release.

• SE 2050 team members and advisors are working to help spread the word on the SE 2050 Commitment Program. Look for local and national presentations from SEI and NCSEA organizations to learn more about embodied carbon and the SE2050 program.

• The team is working to help guide initial signatories and make the process as simple as possible for all engineering firms. The signatory firms’ commitment progress is being tracked, and periodic check-ins are scheduled.

• A sponsorship program will soon be published to encourage contributions to the program, allowing improvements to the database, website, and service to participants.

Conclusions We encourage readers to sign up for updates at https://se2050.org and encourage their firm leadership to sign on to the program. Participants in the first year will need to: • Submit a letter of commitment.

• Within 6 months (and annually thereafter), prepare an ECAP. • Within one year (and annually thereafter), submit data for some of their projects to the SE 2050 Database. The number of required projects is capped at five in the first year and only embodied carbon needs to be reported. In the future, the database will also collect structural material quantities. All submitted projects will remain anonymous.

Resources https://se2050.org https://carbonleadershipforum.org/the-carbon-challenge

The ECAP must include the following elements: • Education, including assigning an embodied carbon champion and offering an introduction to embodied carbon for all staff.

• A reporting plan describing how your firm will measure, track, and report project embodied carbon.

• Embodied carbon reduction strategies setting embodied carbon reduction goals and implementation plans (qualitative goals focused on education are appropriate for the first year).

• Advocacy, such as outreach to clients about the firm’s participation in the program The SE 2050 Commitment will be good for the climate, good for the profession, and good for your firm. Join us!■ Chris Horiuchi is an Associate with Skidmore, Owings & Merrill in San Francisco, CA. He is chair of the SEI Sustainability Committee’s Disaster Resilience Working Group and a member of the SE 2050 Commitment subcommittee.

Mark D. Webster is a Senior Consulting Engineer with Simpson Gumpertz & Heger in Waltham, MA. He is co-chair of the SEI Sustainability Committee and leads the Resources Group for the SE 2050 Commitment Program.

Mike Gryniuk is an Associate with LeMessurier in Boston, MA. He is chair of the SE 2050 Commitment subcommittee and serves on the steering committee of the SEI Sustainability Committee.

Megan Stringer is an Associate Principal with Holmes Structures in San Francisco, CA. She is active in the SEAOC sustainable design community and also serves as co-chair of the SEI Sustainability Committee.

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business PRACTICES Post-Pandemic Career Realignment

Avoid These 7 Mistakes To Best Propel Employees Post-Pandemic Growth

By Ryan Curtis, P.E.

T

he Covid-19 Pandemic has been lifechanging and life-altering since its stronghold took reign early in 2020. A workfrom-home approach became the norm, and many professional industries wrestled with keeping employees in or away from the office. As a result, career evolutions (or stagnation) have run rampant over the past 12-18 months. Without taking intentional time to pause and record the impacts on one’s life, employees have ridden the COVID-19 wave and need introspection on what the necessary steps forward look like.

As employees continue to face the current work-modified environments and erratic market movements, they should consider the following mistakes to evaluate their current state of self and the impact on those immediately around them within their respective organizations.

Leaders, managers, and production staff should pay keen attention to avoid these critical 7 mistakes to ensure they are challenging and propelling themselves forward as the workforce turns the corner on the Covid-19 season.

Mistake #1: Disregarding Goal Setting Career growth is still possible amidst a pandemic and especially after a pandemic. Without goal setting and forward-facing areas of technical or leadership growth, employees are prone to get stuck in the reoccurring cycle of a constant task-driven schedule, better known as the daily grind. Ponder the tasks performed on every project, assignment, or the routine meetings teammates have with their co-workers.

As employees continue to get the necessary work done and meet all the requirements of a demanding position, they are likely not pausing to seek areas of improvement. How has staff taken the simple day-to-day production and/ or managerial assignments and improved upon them? When employees set short-term (0-3 years) and long-term (3+ years) goals, they are immediately aware of what target to run after and the most needed areas of improvement.

Consider a holistic approach to a career while setting goals. What can a teammate accomplish in their 20s, 30s, 40s, 50s, and beyond? By 50 STRUCTURE magazine

Career growth is still possible amidst a pandemic and especially after a pandemic. considering personal and professional growth decade by decade, this reverse engineering approach can help place goals more sensibly and practically. Employees will see the roadmap of the leader they want to become and then start growing into that leader.

Mistake #2: Pushing off Employee Reviews Both Junior and Senior Staff need to be aware of their career progression. If there are a handful or significant number of direct reports within a manager’s role, do not lose sight of the needs of those individuals. At a minimum, younger staff absolutely need bi-annual or annual performance reviews to ensure they are growing in the technical areas to achieve licensure, certifications, or generally increased project responsibilities.

When reviewing a direct report’s body of work since Covid-19 began, consider the adjustments the employee has had to make to continue their productivity while balancing the everyday “zoom” and “teams” calls within work-from-home environments. Those who have excelled should be recognized for their adaptability. These teammates have proven that they should be considered for future leadership positions, having demonstrated resiliency and the ability to serve their peers

and clients amidst the chaos. Those individuals who have struggled with job performance since the pandemic are certainly not alone.

Continue to develop and lead these teammates into scenarios of “easy wins” to regain their confidence and momentum.

If formal personal performance reviews have been paused due to pandemic-driven reasons, encourage leadership and direct superiors to review their teammate’s performance regardless of financial compensation outcomes. Everyone benefits from feedback, especially when it is meaningful, encouraging, and honest.

Mistake #3: Forgetting about Employee Self-Needs Inhale, exhale, repeat. It is reported that Jeff Bezos, founder of Amazon, keeps his weekly calendar clear until 10 am so he has the time and space to maintain AND grow his bandwidth once the days and weeks get moving. Take a cue here from Mr. Bezos.

Consider employees’ self needs. Where have employees missed opportunities to have time to reflect and recharge? Perhaps it is yoga, running, spiritual, or spending time with a spouse. Whatever it is, encourage staff to make space in their calendar and hold fast to it. These hours or days of self-focus will make

employees more productive and consistent when it is needed most. In addition, if a company’s direct labor ratio has remained high thru the pandemic season, it is likely that teammates are fatigued and have not taken the necessary time away from work.

Encourage time away for a substantially healthier and rejuvenated team.

Overall mental health and well-being have dropped 33 percent since the pandemic began, according to a survey conducted in the Fall of 2020 by Hibob, an HR service company based in New York. This likely impacts employees, their teammates, and significant others. Reach out and ask for help at all times, not just in an emergency. Organizations benefit from healthy employees, and so do the employees!

Mistake #4: Dismissing Employees Work-Life Balance Turn it off. Create space to allow for more bandwidth and more clarity in decisionmaking moments. Many employees remain in a work-from-home setting and find themselves at the workstation on many occasions past dinner time. Without physically separating themselves from personal workstations, more and more employees are blurring personal time and work time. Smartphones have always challenged this separation, and with new setups (laptops, large screen monitors, desk space), employees face the risk of burnout every day. With large social gatherings being limited, many find themselves generally at home and within arm’s reach of their workstation. In their State of Remote Report 2020, Buffer states that not being able to unplug was the third biggest struggle when working remotely.

Without this physical and emotional separation of work and life, employees and teammates become dangerously overwhelmed and overstressed. As a result, work outcomes and deliverables may be performed at a lower level of quality, and, even worse, employees

may become disengaged. Make firm commitments to logging off home computers or keeping phones for personal use at the end of the workday. Encourage teammates to use their PTO and get away at times to fully commit to their personal lives and duties.

leaders, and studio managers with purposeful intent. All benefit from intentionality or personalized conversations.

Mistake #5: Devaluing Employees Skillsets

Embrace areas where employees have grown.

At a minimum, the last year has amplified teammate’s shortcomings yet has put on display a company’s ability to react and adapt. While individual shortcomings have been exposed, employees have also found ways to improve upon methods of completing specific duties. Revised communication with others and the ability to lead and coach from an isolated setting are just a few. Also, the methods of connecting with clients in more meaningful ways while being socially distant are just as critical. All these areas of newfound growth are signs of more nimble, efficient, and socially connected employees.

Consider the years ahead when the organization will gain back physical connectedness with assistants, peers, leaders, and clients.

Employees will be placed in a scenario of incredible growth beyond what they have achieved to date; they must accept the challenge and embrace it.

Remember, the skill sets that made employees great teammates before the pandemic also gave employees the ability to survive the pandemic and excel as leaders in a postpandemic work setting. The tools listed above are culture drivers. The hope is that organizations are committing to concepts that yield healthier teamwork and outcomes. Always remember, being the culture driver starts with self.■

Muscle memory becomes mind memory. What made a great employee pre-pandemic likely made an excellent employee during the pandemic and will propel staff’s careers postpandemic. The skillsets that set employees apart from their peers are also the skillsets that have managed to carry employees through COVID-19. The talents of communication, technical knowledge, leading a small team, and ability to adjust to unforeseen scenarios will continually remain an excellent asset to the business. Consistent physical and mental repetition will lead to self-automation. When employees do things the right way repeatedly, the processes become natural habits.

Encourage staff to capitalize on good habits and remain close to them.

Mistake #6: Lessening Communication and Connectedness Employees are created to be in community.

In the same Buffer Report mentioned earlier, the biggest struggle with working from home is Collaboration and Communication and Loneliness. Consider the number of contacts leadership teams are making with staff on a daily or weekly basis. Without prioritized close and consistent communication, the lack of day-to-day physical interactions can slowly degrade an employee’s working relationships and wreak-havoc on the efficiencies employees worked so hard to build upon before the pandemic.

Encourage staff to check in with teammates,

Mistake #7: Ignoring Employees Growth Areas

References are included in the PDF version of the article at STRUCTUREmag.org.

Ryan Curtis is a Senior Structural Engineer and Project Manager for LEO A DALY in Omaha, NE. (rbcurtis@leoadaly.com)

The skill sets that made employees great teammates before the pandemic also gave employees the ability to endure the pandemic.

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51

NCSEA

NCSEA News

National Council of Structural Engineers Associations

Call to Action: One Year of Progress

On June 12, 2020, NCSEA, in partnership with SEI and CASE, issued a Call to Action to denounce racism and commit to improved diversity, equity, and inclusion (DEI) in the Structural Engineering profession. In addition to the Call, NCSEA publicly pledged to actionable DEI goals, listed below.

While the Call to Action represented a critical step to establishing a vision and a commitment, it is actions that lead to meaningful progress.

On the one-year anniversary of the Joint Call to Action, NCSEA is proud to report on our progress in collaboration with our SEA Member Organizations.

Joint DEI Committee NCSEA, CASE, and SEI formed a joint committee to collaborate and coordinate on actions to improve equity and opportunity. The committee is actively meeting and the work is ongoing.

Resource Sharing Read.Watch.Listen. is a monthly feature in NCSEA's newsletter, Structural Connection, that promotes conversations on DEI within the structural engineering profession.

Unconscious Bias Training In September, NCSEA held a series on cultural humility and bias awareness for all SEA members that was facilitated by expert Shani Barrax Moore.

Summit Session “How Do We Progress Towards Racial Equity in the Structural Engineering Community?” was led by the SE3 Committee at the 2020 Virtual Summit.

Diversity of Leaders NCSEA facilitated a diversity visioning session, improving policy and process for outreach to members regarding leadership positions.

SE3 Survey The 2020 NCSEA SE3 Survey analysis and reporting prioritized additional dimensions of demographics and discrimination in the profession.

Advocacy Formalized partnerships with the National Society of Black Engineers (NBSE) and the Society of Hispanic Professional Engineers (SHPE) to share initiatives, programs, speakers, and membership.

Scholarships The first ever NCSEA Diversity in Structural Engineering Scholarship attracted nearly 60 applicants, with four recipients: Juan Vera-Bedolla, Jessica Brown, Jessica Gonzalez, and Aime Nacoulma.

Mentoring A working group is studying potential focused mentorship programs and opportunities through NSBE, SHPE partnerships.

University Engagement Local SEAs have had success through NSBE University Chapters and focused University outreach to increase the diversity of engagement in programs that lead to careers in structural engineering.

We acknowledge that real change takes long-term commitment and intention. This is just the beginning. To be truly effective, focused actions must be implemented at a national, state, and local level. We are thankful for the SEAs, firms, and members who have begun this work with us, and we are looking forward to more SEAs, firms, and members joining us in our collective efforts.

2021 Grant Program Open for Application The NCSEA Grant Program was been developed to assist Member Organizations in growing their Association and promoting the structural engineering field. The Grant Program is supported by the NCSEA Foundation, which supports the non-profit activities of NCSEA and its Member Organizations, and funds initiatives and activities that aid in the advancement of the science and practice of Structural Engineering as well as promote technical development, education, outreach, and engagement. Last year's recipients were awarded funding for: • SEAC (Colorado) to launch a local SE3 Committee.

• SEAOI (Illinois) to create a cohesive library of STEM videos and funding to enhance the association's remote site visits.

• SEAU (Utah) 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) 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) to further support their students by establishing a new scholarship.

• SEAKM (Kansas/Missouri) to enhance their Mola Model Initiative and Student Outreach.

The 2021 Grant program is now open for submission; members can submit applications by visiting www.ncsea.com/awards/grants.

52 STRUCTURE magazine

SEAKM leading a STEM activity at a local elementary school using Mola kits.

News from the National Council of Structural Engineers Associations

2021 Structural Engineering Summit: In-Person and Virtual

The 2021 Structural Engineering Summit will be an immersive in-person and virtual event. The In-Person Summit will host attendees in New York City at the Hilton Midtown on October 12-15. The Virtual Summit will span from September 27 to October 21. Attendees at both events will have access to an industry leading Trade Show, NCSEA’s unrivaled educational options, and unique networking opportunities.

To honor the 20th anniversary of 9/11, the keynote at this year's event will feature panelists who were involved in the initial recovery efforts and the subsequent building performance study. They will discuss the response of the engineering community, building performance, structural design, fire protection, emergency response legal issues, and building code changes, focusing on what has changed and what changes are still recommended.

Join us for this one-of-a-kind event that draws the best of the structural engineering profession, highlights structural engineering innovation and ingenuity, honors outstanding service and commitment to the organization, and illuminates the necessity of the practicing structural engineer. Register at www.ncsea.com.

Announcing: Susie A. Jorgensen Presidential Leadership Award 2021 Special Awards Nominations Due July 16

The Special Awards are annually presented to NCSEA members who have displayed outstanding service, exceptional dedication, and notable commitment to the association and to the structural engineering profession. These prestigious awards include the NCSEA Service Award, the Robert Cornforth Award, the Susan M. Frey NCSEA Educator Award, the James Delahay Award, and now the Susie A. Jorgensen Presidential Leadership Award. This new award was created to honor the late NCSEA Board President, Susie A. Jorgensen, who passed away in November 2020, and advocate for the profession.

NCSEA Service Award 2020 Recipient: Brian Dekker Presented to an individual who has worked for the betterment of NCSEA, the member organizations, and the profession, to a degree that is beyond the norm of volunteerism.

Robert Cornforth Award 2020 Recipient: Jerry Maly Presented to an individual for exceptional dedication and exemplary service to a member organization and to the profession.

Susan M. Frey NCSEA Educator Award 2020 Recipient: Duane Miller Presented to an individual who has a genuine interest in, and extraordinary talent for, effective instruction of practicing structural engineers.

James Delahay Award 2020 Recipient: Kevin Moore Presented at the recommendation of the NCSEA Code Advisory Committee, to recognize outstanding contributions towards the development of building codes and standards.

Susie A. Jorgensen Presidential Leadership Award The Susie A. Jorgensen Presidential Leadership Award will be presented to an individual who has demonstrated exceptional leadership potential through their activities within NCSEA and/or their SEA. The award is to be bestowed on candidates who embody Susie’s passion, vision, and legacy of leadership.

Nominations for the Special Awards are due July 16, 2021, visit www.ncsea.com to learn more about the awards.

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J U L Y 2 0 21

53

SEI Update Membership

Vote in Online Election for the SEI Board of Governors by July 31 The SEI Board of Governors consists of two representatives from each of the five SEI Divisions (Business and Professional, Codes and Standards, Global, Local, and Technical Activities), one young professional appointee, one appointee from ASCE, the SEI President, SEI Past President, and the SEI Director as a nonvoting member. The Division representatives each serve a four-year term. In accordance with the SEI Bylaws, this year, SEI is conducting an election for one Technical Activities Division representative to the Board of Governors, term effective October 1.

Current SEI members (dues fully paid) above the grade of Student will receive a notice July 1 via ASCE Collaborate on how to verify and submit your secure ballot online. Ballots are due no later than July 31, 11:59 pm US ET.

Call for Volunteer Members

The SEI Design Practices Committee seeks interested and energetic committee members. The committee’s purpose is to consider issues related to code implementation and national standards and to improve the practice of structural engineering. The committee works at developing practice problems, presentations, and articles related to improving practicing engineers’ understanding of structural codes, standards, and specifications.

Engineers of all experience levels are invited to apply at www.asce.org/structural-engineering/sei-business-and-professional-activities-division.

The SEI SE Licensure Committee invites members to apply to join the committee. The committee’s purpose is to further the mission of SEI relating to licensing, regulatory issues, and professional development activities for individual structural engineers. The committee promotes legislation for structural engineering licensure in all jurisdictions by creating a plan for working proactively with local engineers, stakeholders, and engineering organizations. The committee develops resources such as statistical data, white papers, case studies, etc., to support the efforts of local structural engineers. Engineers of all experience levels are invited to apply at www.asce.org/structural-engineering/sei-business-and-professional-activities-division.

Advancing the Profession

Participate in Public Comment for ASCE/SEI 7

• Public Comment for Supplement 3 for ASCE/SEI 7-16, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, is open through July 11, 2021. This Supplement updates section 11.4.8 provisions and commentary of the standard: Section 11.4.8 Site-Specific Ground Motion Procedures, with Table 11.4-1 Short-Period Site and Table 11.4-2 Long-Period Site, and revisions to Chapter 11 Commentary, Section C11.4.8. To participate, contact James Neckel at jneckel@asce.org.

• Public Comment for ASCE/SEI 7-22, Minimum Design Loads and Associated Criteria for Buildings and Other Structures, is open through August 2, 2021. Access with your ASCE user account, details at www.asce.org/structural-engineering/asce-7-and-sei-standards For additional questions, contact James Neckel at jneckel@asce.org.

Congratulations The joint ASCE/SEI and Charles Pankow Foundation Performance-Based Structural Fire Design: Exemplar Design of Four Regionally Diverse Buildings Using ASCE 7-16, Appendix E, and the results of the tall building chapter authored by the Thornton Tomasetti team, have received the 2021 CTBUH Audience Award in Innovation Category. Access the free publication at https://bit.ly/2TpVvo0.

Errata 54 STRUCTURE magazine

Now Available

A primer on ASCE Future World Vision: An Innovative, Forward-looking Tool for Resilient Future Infrastructure. Access resources at www.futureworldvision.org/#get-involved.

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 kdooley@asce.org.

News of the Structural Engineering Institute of ASCE

Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering!

SEI Online

SEI Virtual Events

www.asce.org/SEI/virtual-events • SEI Annual Meeting Wednesday, July 14, 4:00 – 5:00 pm US ET Join for the latest on programs, standards, and strategic initiatives, including Performance-Based Design/Resilience, CROSS-US Collaborative Reporting for Safer Structures, and SE2050 Commitment to Net-Zero. Interact with leaders and participate for a chance to win the new ASCE/SEI 7-22. Register by Wednesday, July 14, Noon ET at https://tinyurl.com/SEIAnnualSTRUCTURESMAG. • 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 and participate in Live Q&A. Each session is LIVE and only available 1:00 – 2:30 pm US ET. JULY 15 - ASCE/SEI 72 Athletic Field Lighting (expected to be published summer 2021) Registration deadline July 13 @10:00 pm US ET. Join SEI Host Jennifer Goupil for a discussion with the chair of the ASCE/SEI 72 committee, Brian Reese, P.E., M.ASCE ASCE/SEI 72 Design of Steel Lighting System Support Pole Structures has been specifically written to unify the core body of best practice knowledge available in the structural engineering community and to provide public and private agencies, practicing engineers, installers, and facility owners a consistent roadmap for the safe specification, design, fabrication, installation, and ongoing maintenance of structural supports of this type. Lighting system support structures (primarily cold-formed single and multi-pole tubular steel structures) differ from buildings in many performance-related characteristics. But, like buildings, lighting systems support structures are a critical public safety issue if they are not properly specified, designed, fabricated, installed, and actively inspected and routinely maintained by competent professionals. Current practices related to support structures of this type have been inconsistently developed and even more inconsistently applied, rendering many of those practices confusing – and even worse, ill-advised. The catastrophic failure of dozens of lighting system support structures around the country and the removal from service of hundreds more as a precaution in view of faulty or incomplete specifications for design, fabrication, installation, or ongoing maintenance prompted several members of ASCE’s Structural Engineering Institute to begin development of this document in the interest of improving public safety related to these structures.

SEPTEMBER 16 – ASCE/SEI 59 Blast Protection of Buildings NOVEMBER 18 – ASCE/SEI 8 Specification for the Design of Cold-Formed Stainless Steel Structural Members Individual session: Member $49, Nonmember $99. Student member: Free registration.

REGISTER NOW at https://cutt.ly/9hQDTEo • #SEILive Conversation with Leaders on Code Development August 4

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle J U L Y 2 0 21

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CASE in Point News of the Coalition of American Structural Engineers CASE Tools and Resources Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills their 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 business development processes: CASE 962-F CASE 962-H CASE 976-A Tool 5-4 Tool 7-1 Tool 7-2

A Guideline Addressing the Bidding and Construction Administration Phases for the Structural Engineer National Practice Guideline on Project and Business Risk Management Commentary on Value-Based Compensation for Structural Engineers Negotiation Talking Points Client Evaluation Fee Development

You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

Welcome to the 2021-2023 CASE Executive Committee Members Chairperson: Brent White, ARW Engineers Chair-Elect: Kevin Chamberlain, DeStefano & Chamberlain, Inc. Committee Member: Bruce Burt, Ruby + Associates, Inc. Committee Member: Nils Ericson, M2 Structural LLC Committee Member: Roger Parra, Degenkolb Engineers

Committee Member: Tina Wyffels, BKBM Engineers Member at Large: Jeffrey Morrison, Lynch Mykins Structural Engineers P.C. Past Chairperson: Stacy Bartoletti, Degenkolb Engineers Ex-Officio: Smith Michael, ACEC Utah

WANTED: Engineers to Lead, Direct, and Engage with CASE Committees!

If you are looking for ways to expand and strengthen your business skillset, look no further than serving on one (or more!) CASE Committees.

Join us to sharpen your leadership skills – promote your talent and expertise – to help guide CASE programs, services, and publications.

We currently have openings on all CASE Committees: Contracts – responsible for developing and maintaining contracts to assist practicing engineers with risk management.

Guidelines – responsible for developing and maintaining national guidelines of practice for structural engineers.

Programs – responsible for developing program themes for conferences and sessions that enhance and highlight the profession of structural engineering.

Toolkit – responsible for developing and maintaining the tools related to CASE’s Ten Foundations of Risk Management program.

To apply, your firm should: • Be a current member of ACEC • Be a member of the Coalition of American Structural Engineers (CASE); or be willing to join the Coalition.

• Be able to attend the groups’ normal face-to-face meetings each year: August, February (hotel, travel partially reimbursable) • Be available to engage with the committees via email and video/conference call • Have some specific experience and/or expertise to contribute to the group Please submit the following information to Michelle Kroeger, Coalitions Director (mkroeger@acecl.org): • Letter of interest indicating which committee • Brief bio (no more than a page) Thank you for your interest in contributing to advancing the structural engineering profession!

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

56 STRUCTURE magazine

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structural FORUM Ethics Instruction: Are We Covering What We Need To?

By Scott Civjan, Ph.D., P.E.

S

tructural engineers make ethical choices every day. Many decisions are engrained in engineering practice and become second nature, such as ensuring the safety of the public through sound design and engaging in honest business practices. In addition, engineers work to ensure equitable and inclusive work environments and uphold professional codes of ethics.

We do our best. But is that good enough?

Some ethical dilemmas become touchstones in the profession and are canonized in case studies. Most engineers are lucky never to face these decisions but like to think that we would be as ethical as the heroes presented are. In reality, ethics is not as straightforward or monumental as these case studies lead us to believe. Daily design activities bring a barrage of decisions with impacts to projects, personnel, and stakeholders – with many conflicting interests. The ability to navigate these decisions ethically defines good practice. Unfortunately, there are signs that the profession does not always uphold these standards. Professional Engineering Advisory Boards regularly post examples of disciplinary actions, and monthly examples are collected in the American Society of Civil Engineers (ASCE) column, A Question of Ethics.

Since 2000, accredited civil engineering programs have been required to include ethics instruction. However, requiring ethics instruction and influencing ethical behavior can be very different things. Do teaching methods impress the practical importance of ethics in a meaningful way? Studies have reported higher incidences of ethical transgressions among engineers than in other majors, with cheating in university situations a strong predictor of unethical behavior in the workforce. In fields where public safety is at risk, this is especially troubling.

Meeting accreditation requirements typically includes ethics modules discussing the Codes of Ethics, published by ASCE and the National Society of Professional Engineers (NSPE), and example case studies. More innovative instruction methods are sometimes included, like reading or writing stories with ethical cliffhangers, assigning philosophical readings, or technical discussions of micro/meso/macro ethics (personal behavior through societal, ethical issues). I propose that ethics instruction may not be effective when focusing on infractions, heroic actions, relying solely on philosophy/ethicist views, or relying on nonrealistic scenarios to engage students.

58 STRUCTURE magazine

Codes of Ethics-based instruction can imply that avoiding violations equals ethics, which is misguided. Future engineers should be people you want to do business with, not those looking for loopholes in morality. Case studies create challenges when presenting scenarios that ask us to pontificate on the scurrilous offender, perhaps discussing variations that would result in a different action. Having students cognitively evaluate situations they cannot fully imagine themselves in, due to inexperience, completely misses the intuitive and reactive nature of many ethical decisions.

When construction issues arise, decisions must be made to replace a piece, modify it, or check capacities, often by the end of the day, if not sooner.

How do we make the decision? It is pointless to discuss whether the fabricator should have notified us before it got to the field or assign blame for missing a design error. We are engineers. We fix problems. We quickly focus on the problem, decide whom to bring into the decision, and identify impacts to the schedule and final product. Other stakeholders and society at large are of concern but not often at the forefront of these decisions. Our preconceptions and history of previous decisions come into play. The approach to these decisions relies on personal ethics, experience, and the company culture, a culture that subtly shifts over the years based on the ethics of the individuals.

Perhaps a more significant issue with case studies, or fictional ethics stories, is that a clear tragic outcome or heroic action is often used to grab student interest. This suggests that ethical decisions are a once-in-a-career event of major consequence, which is likely to give students an arms-length perspective on ethics as events that happen to others and a belief that one can learn ethics purely through observing the behavior of others.

The more removed the situation is from their current life, the easier it is to think abstractly without relating personally to the decision. I have given an assignment of two situations, one a student using online resources during a closed book exam, and another where an early career engineer finds online sample calculation spreadsheets posted for a design. I see the former as directly violating an ethical agreement and the latter as a typical office scenario using blogs or posting useful templates. Many students could not relate to the latter, thinking it a much bigger ethical lapse since the stakes of

public safety and company reputation could be at risk. They misconstrued the ethics because the situation is not in line with their current experience. Case studies expecting the perspective of a project or construction manager are situations even further removed from their experiences. This leads to learning “correct” decisions but not evaluating and modifying their own ethical behavior.

So, what to do? I propose that ethics instruction should include awareness of how individuals make decisions, slowly expanding scenarios from current student experiences to what they might experience later in their careers. We need to move the conversation away from blame and toward understanding different perspectives and competing goals. Everyone working on a project wants to get the job done safely, on time, and on budget. No one wants to get bogged down in RFIs, arbitration, and lawsuits. We want to include the broader impacts our decisions have on stakeholders and society-at-large but often find it hard to see the direct connection to our decision. We can disagree on decisions because of our background and perspective. There are temptations to defer decisions, place the responsibility on others, or let something slide because “that is how it has been done before” or to avoid confrontation.

Ethics is often the simple matter of what battles we choose to engage in and what perspectives, aside from our self-interest, are included in the decision. A subsequent article will follow up with some ideas on guiding students and early-career engineers toward a career where they think about the ethics and impacts of their decisions.■ Scott Civjan is a Professor at the University of Massachusetts Amherst Department of Civil and Environmental Engineering. He teaches structural engineering, including design classes, where he has been introducing and modifying ethics content.

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SPOTLIGHT Mighty Mac’s New Coat of Paint

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he Mackinac Bridge or “Mighty Mac” is a 5-mile-long suspension bridge that connects Michigan’s Upper and Lower Peninsulas. The bridge crosses the Straits of Mackinac, the waterway connecting Lake Huron and Lake Michigan. It opened in 1957 and is the third-longest suspension bridge in the world – the longest in the Western Hemisphere.

A repainting job is no small undertaking and requires significant planning, as might be expected with a bridge of this age, size, and location. The two towers from which the 8,614-foot-long suspension portion of the bridge hangs are 552 feet in height. As part of the bridge’s repainting project, Ruby + Associate’s team was tasked with designing platforms to enable a team to remove and collect the original lead-based paint and repaint the bridge’s towers with a new zinc-based paint. In addition to serving as SER, Ruby provided erection engineering, construction engineering, heavy lift engineering, and steel and aluminum fabrication detailing services to complete the project.

Creativity of Structural Design Without any precedent of similar systems, the platforms were custom designed and built for painting crews to move up and down the bridge’s towers above the roadway deck, to strip and repaint the bridge’s north and south towers in phases. The platforms encircle the tower legs and allow workers to adjust to accommodate the towers’ tapering with another platform allowing access to struts.

The system needed to be enclosed to allow workers to sandblast and paint a complete section during a work shift to maximize efficiency. The resulting design was a lightweight, two-story movable steel and aluminum platform system that encircles the tower’s leg.

Typically, bridge painting projects require scaffolding. The environmentally responsible and innovative platform design eliminated the need for almost 400 feet of scaffolding, resulting in significant savings in materials, material transportation, and labor.

Constructability Solutions “It’s not like building something at ground level. At every step of the process, we had STRUCTURE magazine

to ask ourselves, how do you actually carry, connect, tighten or build this 500 feet in the air?” said Project Manager Andrew Twarek, P.E., S.E. “The outriggers had to be made of light aluminum pieces that would fit within the tower’s tiny elevator and could be bolted together at the top of the bridge. Those could then be used to hoist larger pieces,” he added.

The system itself is 24 feet tall, comprised of two rigid aluminum platforms, attached to the outriggers by four separate traction hoists. The contractor had previously selected and purchased the 5800-pound rated capacity hoists, so the platform weight was limited. The platform was designed to be pre-assembled and erected into place in two sections and is formed from 4-foot-deep by 4-foot-wide “C” shaped steel box trusses. They encircle three sides of a tower leg and support a two-story-tall aluminum upper works. The two ends of the “C” truss are attached to each other with cabling so that, in plan, the system resembles a “D” during painting operations. The upper work surface is made of grating to allow sand and paint particles to fall to the bottom platform for vacuum extraction, centralizing the debris gathering process. Canvas walls ensure that debris is contained, and both blasting and painting work can be performed in high winds, regularly exceeding 100 mph at the top of the towers.

Unique Challenges and Complex Criteria “Another complexity was designing for these anticipated wind loads,” said Bruce Burt, P.E., V.P. of engineering with Ruby, and Principal in Charge. “The 40-foot canvas shrouds that contain the blasting sand and paint act as a giant sail, and wind loads are magnified 500 feet above the water. We designed the platform as a lightweight space frame and incorporated spring-loaded guide rollers to limit side movements during climbing operations.”

A strut platform with its own dual 4-footdeep steel box truss is used to work on the tower’s struts. The platforms are tied together with hinged sway frames that swing out of the way to allow the platforms to pass by the tower struts as they travel vertically up the tower.

A davit-like “outrigger” system mounted to the top of the tower supports the platforms, which raise and lower using traveling hoists and cables to allow painting the upper 320 ft of the towers. The outriggers are transported in sections to the top of the tower using a tiny existing elevator that barely accommodates three people.

“As this [was] the first time in the bridge’s history when the towers have been stripped to bare metal and repainted, it makes sense that the team would need new, innovative equipment to get the job done,” said Mackinac Bridge Authority Chief Engineer Kim Nowack, P.E. “We... thank them for their work preserving this infrastructure icon.”■ Ruby + Associates, Inc was an Outstanding Award Winner for the Mackinac Bridge Paint Platforms project in the 2020 Annual Excellence in Structural Engineering Awards Program in the Category – Other Structures.

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