STRUCTURE magazine | September 2021 by structuremag

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INSIDE: Wave One Anchoring-To-Concrete Provisions Strengthening Reinforced Concrete The Historic Witherspoon Building

SEPTEMBER 2021

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4 STRUCTURE magazine

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 9, © 2021 by The National Council of Structural Engineers Associations, all rights reserved. Subscription services, back issues and subscription information tel: 312-649-4600, or write to STRUCTURE magazine Circulation, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606.The publication is distributed to members of The National Council of Structural Engineers Associations through a resolution to its bylaws, and to members of CASE and SEI paid by each organization as nominal price subscription for its members as a benefit of their membership. Yearly Subscription in USA $75; $40 For Students; Canada $90; $60 for Canadian Students; Foreign $135, $90 for foreign students. Editorial Office: Send editorial mail to: STRUCTURE magazine, Attn: Editorial, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. POSTMASTER: Send Address changes to STRUCTURE magazine, 20 N. Wacker Drive, Suite 750, Chicago, IL 60606. STRUCTURE is a registered trademark of the National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

Contents SEPTEM BER 2021

Cover Feature

WAVE ONE By Anantha Chittur, P.E., S.E., and Steven M. Baldridge, P.E., S.E.

Located in Noida, India, Wave One is a 41-story mixed-use building with more than 2 million square feet of floor area. Post-tensioning helped simplify many challenges by allowing for the removal, transfer, and shifting of columns. It also permitted slabs to span longer where required. Cover photo courtesy of Nostri Architects.

Feature

Columns and Departments 7

Editorial

By Donald O. Dusenberry, P.E., SECB

By James LaBelle, P.E.

Structural Anchorage By Richard T. Morgan, P.E.

By Ammar Motorwala, P.E., and J. Brent Stephens, P.E.

Large Anchor Grout Pockets for Foundations

This four-part series discusses the adaptive 20

Structural Analysis

reuse of the historic Witherspoon Building

Rigid and Non-Rigid Base Plate Assumptions

in Philadelphia, PA. Part 1 includes the building history and description of the additions and alterations.

By Arif Shahdin

Spotlight

2021 SEI and ASCE Structural Award Recipients

The Point Ellice Bridge Failure

Preschool Engineering Books

Insights

Does Building Taller with Wood Make Sense? By Paul Fast, P.Eng., Struct. Eng., P.E.

Business Practices

The High Cost of Poor Leadership

By Rebecca Zucker

Structural Forum

Building Design Collaborator or Implementing Technician? By Julie Mark Cohen, Ph.D., P.E., SECB

Structural Licensure

Georgia Passes P.E., S.E. Practice Act

By Michael Planer, P.E., S.E.

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

September 2021 Bonus Content Available Only at – STRUCTUREmag.org

By Silky Wong, Ph.D., S.E., P.E., CEng MICE,

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

InFocus

By Linda Kaplan, P.E.

Structural Design

and Widianto, Ph.D., P.E.

structure, highlighting previous structural

Structural Renovation

Strengthening of Reinforced Concrete Structures

ADAPTIVE REUSE OF THE HISTORIC WITHERSPOON BUILDING – PART 1

Building Blocks

Test-Based Available Strengths for Aluminum Structures – Part 1

Integrating Shear Lug Design with Anchoring-To-Concrete Provisions

9/11 in the Conversation about Disproportionate Collapse

Structural Connections

Wood Framed Residential Construction

By Becky Havel, S.E., Rose McClure, S.E., P.E., and Matt Johnson, P.E.

In Every Issue

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

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. S E P T E M B E R 2 0 21

EDITORIAL 9/11 in the Conversation about Disproportionate Collapse By Donald O. Dusenberry, P.E., SECB, F.SEI, F.ASCE

n this month’s anniversary of the attack on 9/11, we remember while many lives were tragically lost that day, many others were saved the lives lost on that day and in the subsequent years while by the inherent robustness of those buildings. addressing the aftermath. We remember the shock and anger we felt Disproportionate collapse or not, the events of 9/11 truly moved when our world changed. We grapple with our role in preventing the entire AEC industry to be proactive to directly consider the postragedies like that. sibility of serious damage causing the unreasonable collapse of certain Structural engineers always have and always will put public safety at significant buildings. the highest priority. As a profession, we have been tenacious in our misNow we arrive at 2021, and the unthinkable collapse in the middle of sion to find the best, cost-effective ways to protect lives and property the night of a high-rise condominium in Surfside. People in their beds, from the forces that would destroy structures. secure in the knowledge that their homes had While we have anticipated that wind and for 40 years, were killed by a sudden, The performance of WTC1, stood earthquakes could cause widespread damage, ghastly collapse. This happened in the United we have generally shied away from considering States. It really comes home now. It does not WTC2, and the Pentagon that some unforeseen threats could challenge get more personal than this. our designs. To the extent that we expressly Undeniably, the profession and the world are far exceeded what I would thought about those unforeseen threats, we different now than they were before Ronan might have reasoned that the general qualiPoint, before L’Ambiance Plaza, before guess most structural ties of ductility and continuity would offer Oklahoma City, before 9/11, before Surfside, protection. and before any of the other shocking and engineers would have bet 9/11 changed that. Explosions, disproportiontragic failures along the way. We know this, ate collapse, general robustness, and resilience and the structural engineering profession is possible before 9/11. are very much in the conversation now. doing more than just talking about things like But it did not start in 2001. Instead, the disproportionate collapse, blast resistance, Ronan Point collapse in 1968 comes up in every conversation. That performance-based design, robustness, resilience, and life-cycle perevent sparked research by the pioneers of disproportionate collapse formance. We are a new profession facing new challenges, and we resistance – John Breen, Eric Burnett, Bruce Ellingwood, Edgar are working to conquer them. Leyendecker, William McGuire, and others – to contemplate unforeSEI is leading in these efforts. In 2011, SEI issued the first edition seen threats and keep structures erect when damaged. Their papers in of SEI/ASCE 59, Blast Protection of Buildings. SEI/ASCE 59 provides the 1970s started serious discussions about serious issues. guidance for designing buildings to resist nearby explosions. The first Then came L’Ambiance Plaza, a building that collapsed entirely while revision is nearing completion and likely will be released early next year. under construction in Bridgeport in 1987. The profession took its next Also nearing completion is a new standard for mitigating disproporincremental steps toward addressing the problem of disproportionate tionate collapse potential, inspired by the series of unacceptable losses collapse. But, like many of the most shocking failures through the years, over the past decades. This new standard strives to be performancethis was a failure during construction rather than one while in service. based, giving guidance about risk assessments and the scope of the We reasoned that it was a fluke, a one-off. We decided that design and structural responses engineers should consider when contemplating construction processes needed to change rather than structural design. resistance to collapse. It should be published early next year as well. Next was the 1995 bombing of the Alfred P. Murrah Federal Building In addition to developing these new documents, SEI has formed in Oklahoma City. This event awakened American society to two facts: committees to advocate for performance-based design, advance resilexplosions are a real threat to structures, and people in this country ience, and study life-cycle performance. These critical initiatives should will actually bomb structures to bring them down. Since then, we have help the structural engineering profession respond to ever-changing needed to combat active, calculating, ever-changing enemies to structural societal and environmental demands. performance in addition to the familiar natural, statistically quantifiable Incremental steps are not enough. We cannot be watching the news threats. That event started a serious discussion about blast resistance. about the next shocking, heart-wrenching loss without being able to The world changed on September 11 when the Pentagon was say we are doing something about it. It is time to be proactive and undercut by an airplane, and the iconic 1,350-foot twin WTC1 and directly acknowledge rare but high-impact events. One action you can WTC2 towers in New York City were struck and collapsed, obliterat- take is to share a safety issue and knowledge to help create a safer built ing buildings for hundreds of feet in all directions and causing the environment via Collaborative Reporting for Safer Structures nearby 47-story WTC7 tower to collapse a few hours later. All with US at www.cross-us.org. Then, sign up for the CROSS-US unimaginable and unacceptable human, physical, and cultural tolls. newsletter and access the reports and lessons learned.■ Except for WTC7, none of the structural failures on that day were Donald O. Dusenberry, Consulting Engineer, is a past President of SEI and the result of disproportionate collapse. In fact, the performance of chairs the committees developing SEI/ASCE 59 and the new standard for WTC1, WTC2, and the Pentagon far exceeded what I would guess disproportionate collapse mitigation. most structural engineers would have bet possible before 9/11. Thus, STRUCTURE magazine

S E P T E M B E R 2 0 21

structural ANCHORAGE Integrating Shear Lug Design with Anchoring-To-Concrete Provisions By Richard T. Morgan, P.E.

shear lug is a steel embedment used as part of an anchorage to transfer shear loads into a concrete foundation. In the context of this article, an anchorage is comprised of 1) a structural member subjected to tension and shear loading, 2) the base plate to which the member is attached, 3) one or more shear lugs welded to the base plate, and 4) cast-in or post-installed anchors. Depending on how the connection is detailed, anchors may share in resisting applied shear loads, or they may be designed only for tension load on the connection such that the entire shear demand is assigned to the shear lug. Shear lug provisions have been introduced into the American Concrete Institute (ACI) Figure 1. Types of shear lugs. publication Building Code Requirements for Structural Concrete (ACI 318-19). This article discusses these provi- a square shape can be used to resist the shear load component in sions and how they can be integrated into anchorage design. each direction. ACI 318-19 Chapter 17 design provisions for cast-in-place and post-installed anchors consider possible tension and shear failure Shear Lug Provisions modes, for which an anchor design strength is calculated. The design ACI 318-19, Chapter 17, addresses anchoring-to-concrete. Section strength is checked against a factored load. The Table summarizes 17.11 – Attachments with shear lugs has been added in this code these anchor design provisions. version. It references two types of shear lugs welded to a base plate: ACI 318-19 Section 17.11.1 includes general provisions for designcast-in-place shear lugs and post-installed shear lugs. Cast-in-place ing shear lugs. Section 17.11.1.1.2 requires a minimum of four shear lugs are cast into the concrete, as shown in Figure 1a. Post- anchors to be provided in the anchorage. This section also states that installed shear lugs are grouted into a keyway or “blockout” in the anchor failure modes associated with shear loading do not need to be concrete, as shown in Figure 1b. Shear lugs are oriented perpendicular checked; therefore, the shear lug is assigned the entire shear demand. to the direction of the shear load. If shear load acts in more than However, for base plates with rigidly attached anchor elements, Section one direction, shear lugs having a cruciform (i.e., a cross shape) or 17.11.1.1.3 and the commentary R17.11.1.1.3 note that displacement of the plate in response to Table of ACI 318-19 anchoring-to-concrete provisions. shear loading induces anchor deformations that must be accounted for. Anchors subjected to tension loads must still be designed for the relevant tension failure modes noted in the Table. Section 17.11 shear lug provisions include two design checks: one for bearing failure and one for concrete breakout failure. Bearing failure provisions are given in Section 17.11.2, and concrete breakout failure provisions are given in Section 17.11.3. Per Sections 17.11.1.1.4 and 8 STRUCTURE magazine

Figure 2. Bearing area for shear lugs.

17.11.1.1.5, a nominal strength for bearing failure (Vbrg,sl) is calculated and multiplied by a strength reduction factor (φ-factor) of 0.65 to obtain a design strength (φVbrg,sl). Per Sections 17.11.1.1.6 and 17.11.1.1.7, a nominal strength for concrete breakout failure (Vcb,sl) is calculated and multiplied by a strength reduction factor (φ-factor) of 0.65 to obtain a design strength (φVcb,sl). Each design strength is checked against the factored shear load (Vu) acting on the anchorage. If φVbrg,sl ≥ Vu and φVcb,sl ≥ Vu , the provisions are satisfied. Section 17.11 provisions only consider concrete failure modes for shear lugs; reference the American Institute of Steel Construction (AISC) publication Steel Design Guide 1: Base Plate and Anchor Rod Design, Second Edition, for information on sizing the shear lug and designing the welded connection between the shear lug and plate.

Bearing Strength in Shear of Attachments Bearing failure for a shear lug occurs at the shear lug/concrete interface such that a fracture surface develops in the concrete in front of the shear lug. Section 17.11.1.1.2 notes that shear failure modes for anchors are not considered; however, the commentary R17.11.1.1.3 notes that anchors welded to a plate are placed in bearing as the fracture surface develops because the anchors displace with the plate. Therefore, if tension and shear (bearing) loads act on an anchorage consisting of anchors welded to a plate/shear lug assembly, Section 17.11.1.1.3 requires a combined load interaction check on the anchors. This check is only noted for anchors that are welded to a plate/shear lug assembly. Equation (17.11.2.1) defines the nominal bearing strength of a shear lug (Vbrg,sl) as: Vbrg,sl =1.7 f c´ Aef,sl Ψbrg,sl Equation (17.11.2.1) where: f c´ = specified concrete compressive strength Aef,sl = effective bearing area of the shear lug Ψbrg,sl = modification factor for axial load acting on the anchorage The parameter (Aef,sl) is defined by the width of the shear lug perpendicular to the direction of the applied shear load and a projected distance based on twice the shear lug thickness (tsl). Aef,sl for a typical cast-in-place shear lug (red shaded area Figure 2a) does not include the embedded plate. Aef,sl for a typical post-installed shear lug (red shaded area Figure 2b) does not include the portion of the shear lug above the surface of the concrete or the plate. If stiffeners are installed perpendicular to the face of the shear lug, the leading edge (i.e., thickness) of the stiffener would be included in the Aef,sl calculation.

The modification factor (Ψbrg,sl) considers the influence on shear lug bearing strength from confinement effects. Part D.11.1 in the ACI publication Code Requirements for Nuclear Safety-Related Concrete Structures (ACI 349-13) defines confinement effects as “the confinement afforded by the tension anchors in combination with external loads acting across potential shear planes.” ACI 318-19 calculations for Ψbrg,sl only consider external axial load (Pu) acting on the anchorage. An external moment that creates tension in some anchors and compression in front of the shear lug across the potential fracture surface is not considered in the calculation of Ψbrg,sl. Equation 17.11.2.2.1a is used to calculate Ψbrg,sl when axial tension acts on the anchorage. Pu is negative in this equation, and the calculated value for Ψbrg,sl is less than or equal to 1.0. Equation 17.11.2.2.1c is used to calculate Ψbrg,sl when axial compression acts on the anchorage. Pu is positive in this equation, and the calculated value for Ψbrg,sl is greater than 1.0. If no axial load acts on the anchorage, Ψbrg,sl equals 1.0 per Equation 17.11.2.2.1b. Axial tension load (-Pu): Ψbrg,sl =1 + -Pu ≤ 1.0 nNsa Equation (17.11.2.2.1a) where: Nsa = anchor nominal steel strength in tension n = number of anchors in tension

No axial load: Ψbrg,sl = 1 Equation (17.11.2.2.1b) +Pu ≤ 2.0 Axial compression load (+Pu): Ψbrg,sl = 1+4 Abp f ć Equation (17.11.2.2.1c) where: Abp = area of base plate Per Section 17.11.2.4, if multiple shear lugs oriented perpendicular to the direction of the applied shear load are used, Vbrg,sl for each shear lug may be summed to give a total bearing strength, provided the stress on a shear plane in the concrete at the bottom of the shear lugs, and extending between them, does not exceed 0.2f ć .

Concrete Breakout Strength Concrete breakout failure for a shear lug occurs at a concrete edge when the shear load acts towards the edge. ACI 318-19 Section 17.7.2 is titled Concrete breakout strength of anchors in shear. Equation (17.7.2.1a), used to calculate nominal concrete breakout strength in shear of a single anchor (Vcb), is given as: A Vcb = vc Ψed,V Ψc,V Ψh,V Vb Equation (17.7.2.1a) Avc,0 continued on next page

S E P T E M B E R 2 0 21

Per Section 17.11.3.1, this equation’s Ψc,V is a modification factor for cracked parameters are modified with shear lug or uncracked concrete conditions. parameters to calculate the nominal conReinforced concrete is assumed to crack crete breakout strength for a shear lug (Vcb,sl). under service loads. Uncracked concrete The parameter A Vc in Equation can be assumed where analysis indicates (17.7.2.1a) corresponds to the projected no cracking under service loads. The failure area on a fixed concrete edge in default Ψc,V value equals 1.0 for cracked the direction of the applied shear load. concrete conditions. Ψc,V can be increased AVc for a shear lug is defined by projected to 1.4 if uncracked concrete conditions distances of 1.5ca1 on each side of the are assumed. shear lug and 1.5ca1 beneath the effecΨh,V is a modification factor for a thin tive embedment depth of the shear lug concrete member, such as a slab (Figure 3). (hef,sl). The distance in the direction of If ha is less than (hef,sl + 1.5ca1), Ψh,V is the applied shear load from the bearing calculated per a modified ACI 318-19 surface of the shear lug to the fixed edge is Equation (17.7.2.6.1) that includes the represented by ca1. The parameter AVc0 in parameter hef,sl. Equation (17.7.2.1a) is given in Equation Ψh,V = hef,sl +1.5ca1 ≥1.0 (17.7.2.1.3) as: ha AVc0 = 4.5 (ca1)2 Equation (17.7.2.1.3) modified Equation (17.7.2.6.1) where ca1 for a shear lug design is defined above. When calculating nominal concrete The red shaded areas in Figure 3 illustrate breakout strength for a shear lug how the geometry to define AVc for a shear (Vcb,sl), the parameter Vb (basic concrete lug is determined. The effective bearing breakout strength) is calculated using area (Aef,sl) for each type of shear lug shown Equation (17.7.2.2.1b). This equation is shaded in white to emphasize that it is is an anchor equation, but, for shear lug not included in the calculation of AVc. The design, the parameter ca1 corresponds embedded portion of the plate above the Figure 3. Concrete breakout area for shear lugs. to the distance in the direction of the shear lug (white shaded area Apl in Figure applied shear load from the bearing sur3a), is likewise not included in the calculation of AVc. face of the shear lug to a fixed edge. λa is a modification factor for Concrete thickness (ha) must be considered when calculating AVc. If ha is lightweight concrete. less than (hef,sl + 1.5ca1), the calculated value for AVc will be limited by ha. Vb = 9λa √f c´ ca11.5 modified Equation (17.7.2.2.1b) Ψed,V is a modification factor for a fixed edge distance perpendicular to the direction of the applied shear load (ca2) that is less than 1.5ca1. Special cases for concrete breakout occurring at a fixed edge paralIf ca2, as shown in Figure 3, is less than 1.5ca1, Ψed,V is calculated per lel to the direction of the applied shear load (Figure 4a), shear lug ACI 318-19 Equation (17.7.2.4.1b): located in a corner (Figure 4b), and multiple shear lugs (Figure 4c) ca,2 are covered in ACI 318-19 Sections 17.11.3.2, 17.11.3.3, and Equation (17.7.2.4.1b) Ψed,V = 0.7+0.3 1.5ca1 17.11.3.4, respectively. Note that the actual direction of Vu in If there are multiple fixed edge distances perpendicular to the direc- Figure 4a is in the y+ direction, but Vcb,sl is calculated as if Vu acts tion of the applied shear load that are less than 1.5ca1, the smallest towards the x+ edge and ca1 is taken from the center of the shear value is used to calculate Ψed,V. lug. Vcb,sl for shear load acting towards the x+ edge and parallel to the y+ edge would have to be considered for the case shown in Figure 4b. Vcb,sl towards the x+ edge is calculated for each shear lug in Figure 4c, and the smaller value is used for design.

√

When to Use Shear Lugs?

Figure 4. Special conditions for calculating concrete breakout.

10 STRUCTURE magazine

Shear lugs can be used in an anchorage 1) when a calculated shear design strength for the anchors is less than the factored shear load assumed to be acting on them, 2) when the number of anchors needed for shear transfer is impractical, or 3) when alternative means for shear transfer into the concrete is desired.

Per the Table, concrete breakout is a failure mode considered in ACI 318 anchoring-to-concrete provisions. When concrete breakout in shear occurs, a failure surface in the concrete develops from anchors resisting shear load to a free edge in the concrete. Anchorages to pedestal foundations often involve small anchor edge distances, such that anchor resistance for shear loads is severely limited by the possibility of concrete breakout occurring at an edge. If the calculated design strength for concrete breakout in shear (φVcbg) is less than the factored shear load assumed to act on the anchors (Vua), ACI 318-19 anchoring-to-concrete provisions for concrete breakout in shear are not satisfied. The use of a shear lug in the center of the pedestal, with the anchors devoted solely to resisting uplift tension, offers a design solution for the shear demand. Figure 1b illustrates anchors installed with standoff. When anchors are installed with standoff and subjected to shear load, bending stresses develop in the anchors. Anchor resistance to bending depends on the material properties of the anchor element, its geometry, the amount of standoff, and the amount of rotational restraint acting on the anchor. An anchor bending resistance can be calculated as a steel design strength in shear using these parameters and checked against a factored shear load. ACI 318-19 Section 17.10, Earthquakeresistant anchor design requirements, includes provisions for designing anchors to undergo tension yielding in response to earthquake loading. Designing anchors with a stretch length permits energy dissipation via the anchors. Stretch length can be obtained by designing an anchor chair in conjunction with a grouted standoff. Shear load acting on the anchorage would subject the anchors to bending stresses. If the calculated anchor bending resistance is less than the factored shear load assumed to act on the anchorage, a post-installed shear lug offers a design solution for the shear demand.

Summary ACI 318-19 has included a new section (Section 17.11) titled Attachments with shear lugs that contains design provisions for shear lug concrete failure modes. This article provided an overview of the provisions given in Section 17.11 and discussed how a shear lug could be integrated into an anchorage design when the shear demand on the anchors is exceeded.■ Richard T. Morgan is the Manager for Software and Literature in the Technical Marketing Department of Hilti North America. He is responsible for PROFIS Engineering and PROFIS Rebar. (richard.morgan@hilti.com)

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Putting it Into Practice Anchoring-to-concrete calculations can be quite involved. Consideration must be given for a multitude of anchor failure modes and design parameters. Many engineers use anchor design software to perform these calculations. Shear lug design requires sizing the shear lug, designing welded connections, and calculating design strengths for bearing and concrete breakout failure. Software capable of performing anchoring-toconcrete and shear lug calculations can reduce the time and effort spent designing an anchorage.

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S E P T E M B E R 2 0 21

structural RENOVATION Strengthening of Reinforced Concrete Structures

Coordination and Quality Control for the Use of FRP Composites By Ammar Motorwala, P.E., and J. Brent Stephens, P.E.

ince its introduction to the commercial construction industry in the 1980s, the use of Fiber Reinforced Polymer (FRP) systems to strengthen/retrofit reinforced concrete and other structures has increased dramatically from a few early experimental projects to currently being the material of choice for many renovation projects. FRP strengthening techniques have gained popularity due to the ease of installation (particularly in occupied spaces), minimal impacts on structural appearance and geometry, costeffectiveness, and other benefits. The development of codes and standards for externally bonded FRP systems is ongoing in Europe, Japan, Canada, and the United States. For the United States, the publications and standards regarding FRP design procedures for reinforced concrete remain limited to guidelines such as the American Concrete Institute’s ACI 440.2R, Guide for the Design and Construction of Externally Bonded FRP Systems for Strengthening Concrete Structures. These have not been included in the enforceable building codes. This leaves the design and quality assurance processes for FRP strengthening scope up to the consultant teams and local jurisdictions for individual projects.

Design

to develop the FRP design to meet the project performance criteria. However, even with this delegation of design responsibilities, the overall structural design integrity lies with the SEOR. Hence, they must maintain a sufficient understanding of the externally bonded FRP reinforcement design, various FRP systems available in the market, and their limitations. This knowledge helps the SEOR determine the feasibility of FRP on their project and provide clear information on their docuFRP Specialty Engineer’s Responsibilities ments regarding FRP design Strengthening Design to satisfy the requirements performance requirements. specified on the construction documents by SEOR. That said, the lack of universally adopted standards has led to significant variation in the In general, design and detailing shall be develmethods for specifying and oped in accordance with ACI 440.2R guidelines. denoting FRP strengthening Consult with SEOR whenever recommended limits requirements on construction and provisions of the guidelines are not met and documents. when this may be acceptable based on engineerCertain delegated design ing judgment and the particular application. items like guardrail systems or building façade and cladding Provide feedback to SEOR regarding specific systems are independent of the requirements for quality assurance and testing base building structure, except based on final design and assist with site obseranchorage, and do not require vations as needed. significant SEOR involvement beyond providing performance criteria and review of

The detailing of FRP for structural strengthening is a specialization and somewhat dependent on the product selected by the contractor. Therefore, it is typical for the project’s Structural Engineer of Record (SEOR) to delegate the FRP design scope to a third-party engineer working with the product manufacturer and installation contractor Table 1. Recommended distribution of responsibilities.

SEOR’s Responsibilities Have a general idea and understanding of different FRP strengthening systems and available techniques. Work with the project architect and local building officials to confirm acceptable materials and fireproofing requirements (if any). Evaluate the feasibility of FRP system application: – Existing concrete compressive strength (min. 2,500 psi concrete) – Concrete tensile strength (200 psi min.) – Un-strengthened load combo check (in accordance with ACI 440.2R Section 9.2) – Fire load combo check (in accordance with ACI 216.1 or ACI 562 as referenced in ACI 440.2R section 9.2.1) Provide FRP specifications and design criteria for FRP engineer to calculate the amount of reinforcement and other details for construction. Establish and enforce quality assurance and quality control measures to ensure construction meets design goals.

12 STRUCTURE magazine

Figure 1. Sample FRP schematic layout plan for slab flexural strengthening.

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loads exerted on the base building structure. On the other hand, delegated design items like structural steel connections and posttensioned reinforcement become part of the primary structural system by directly affecting the structure’s load-carrying capacity and serviceability. Similarly, as externally bonded FRP strengthening design is a structural reinforcement, it requires significant input and involvement of the Figure 2. Typical flexural FRP strengthening section. SEOR in the design process to ensure a successful project. Recommended responsibilities b) Provide sufficient information to the delegated design for the SEOR and FRP specialty engineer for an FRP strengthening engineer of existing structural properties needed to perform project are outlined in Table 1. appropriate calculations. A tabulated FRP As a first step, it is recommended that prior to specifying externally schedule scheme for FRP reinforcement design requirebonded FRP reinforcement as a viable solution for strengthening ments, as shown in Table 2 ( page 14 ), where an example concrete structures, the SEOR should consider performing a feasibility of slab flexural strengthening is shown, is generally recomstudy including these steps: mended to communicate criteria on bid documents. A a) The structural element to be strengthened shall be analyzed similar scheme can be applied to other structural strengthand reviewed for capacity to support minimum new loads ening applications. Information that should be provided, without FRP contribution per ACI 440.2R Section 9.2: 1.1 when available, includes: Dead Load + 0.75 Live Load (use 1.0 if sustained Live Load). • Concrete compressive strength b) Structural elements to be strengthened shall be analyzed and • Structural element geometric properties (slab thickness, reviewed for capacity to support minimum loads considereffective width, etc.) ing reduced steel and concrete strength in case of fire event • Existing reinforcing details, including effective depth and (without FRP contribution) in accordance with ACI 216.1 grade of steel methodology: 1.0 Dead Load + 1.0 Live Load. • Design strength requirements (for example, required c) Confirm existing concrete compressive strength is higher factored moment capacity and service moments for than 2,500 psi (required minimum for proper bonding flexural strengthening) of FRP systems). If existing drawings are not available to confirm this information, then testing of core samples is Construction and Quality Assurances recommended. d) Check the condition of the existing structure and conSimilar to other proposed structural systems on a project, the SEOR firm concrete tensile strength to be a minimum of 200 psi is also responsible for defining within the structural construction/ (required minimum for proper bonding of FRP systems). permit documents the requirements for Special Inspections and Perform pull-off testing if uncertain. Testing for the FRP installation during construction. This can be In addition to this structural feasibility review, it is highly recom- challenging as codes and standards do not provide details regarding mended that the SEOR have discussions with the project architect FRP inspection and testing criteria. According to the International and local jurisdiction building official to confirm acceptance of the Building Code (IBC) Chapter 17, Required Special Inspections and use of FRP for the proposed application and any fireproofing requirements that may arise. As previously noted, the incorporation of FRP strengthening into design and building codes is still evolving. Therefore, an early understanding of the local jurisdiction’s stance regarding this strengthening technique is important to avoid permitting and other issues. After confirmed feasibility, the next task for the SEOR is incorporating FRP strengthening criteria into the bid, permit drawings, and START WRITING YOUR DCI STORY other construction documents. Current methods for communicating these criteria vary significantly. The following is recommended We’re Hiring! to maximize the FRP specialty engineer’s understanding of design intent and application of FRP: Visit our website a) Provide a schematic layout plan depicting areas/elements for more details requiring externally bonded FRP reinforcement with typical details clarifying the design intent to the delegated WASHINGTON | OREGON | CALIFORNIA | TEXAS | ALASKA | COLORADO | MONTANA design engineer (Figure 1 and Figure 2).

S E P T E M B E R 2 0 21

Table 2. Sample FRP schedule for slab flexural strengthening.

EFFECTIVE DEPTH "d" (in.)

(E) MILD STEEL REINF. "As" (sq. in.)

EFFECTIVE SLAB WIDTH "bf" (in.)

SERVICE SELF-WEIGHT MOMENT (k-ft)

SERVICE SUPERIMPOSED DEAD LOAD MOMENT (k-ft)

SERVICE LIVE LOAD MOMENT (k-ft)

174"

165"

187

250

MARK

FRP LOCATION & ORIENTATION

(E) SLAB THICKNESS "h" (in.)

FRP-1

BOTTOM (N-S)

10"

8.3"

3.41

FRP-2

BOTTOM (E-W)

10"

8.9"

3.1

FRP-3

TOP (E-W)

10"

(+8.5" DROP)

9" (17")

164"

8.5

Tests, FRP strengthening would likely be considered “special cases” per Section 1705.1.1, where requirements are dictated by the building official and/or design professional. The lack of detailed standards often leads to an SEOR providing either too little or too superfluous information regarding FRP inspection and testing requirements in their documents. This may result in minimal oversight of the quality of FRP installation. The ACI 440.2R guideline provides a long list of recommendations regarding a Quality Assurance and Quality Control Program for FRP installation. The guideline recommends that the SEOR becomes familiar with these recommendations and selects a program based on the project type, scale, etc., and ensures the completeness of third-party inspection reports. The following are recommended steps for an FRP inspection and testing program, with a summary provided in Table 3: 1) Material Storage Inspection and Product Data Verification, including material testing and sampling. 2) On-Site Layout Verification and general observations of existing conditions. 3) Surface Preparation Inspection and Concrete Substrate Compressive and Tensile Strength Verification to check if proper surface roughness is provided, cracks are treated per manufacturer requirements, imperfections or obstructions are removed, etc. Consider testing of prepped surface to confirm adequate tensile strength. 4) FRP Installation Procedure Observation including saturation of fabric, priming, epoxy application on substrate, and finishing. 5) Installed FRP Inspection – cured FRP review for any defects like bubbles, damaged FRP fibers, etc., and recommend repairs at any defective installation locations. 6) Final Inspections – Pull tests of installed FRP and inspection of fireproofing (if required).

(INC. 96" DROP)

Conclusions FRP reinforcement is generally considered a supplementary reinforcement instead of primary reinforcement like mild steel or embedded post-tensioned cables. Hence, strengthening limits enforced by ACI 440 guideline checks ensure that the un-strengthened base structure has sufficient capacity to temporarily support imposed loads without failure in case of vandalism, accidental damage to the FRP reinforcement, or in a fire event until the supplementary reinforcement is repaired/re-installed. However, fireproofing of FRP systems is a complicated discussion topic that is not covered in this article. Fireproofing requirements should be discussed with the local jurisdiction and appropriately provided to meet their requirements. It is also imperative that, after successful installation of the FRP system, the management and Owner are made aware of the presence of FRP reinforcement and that steps should be taken to ensure that any future additions or modifications do not accidentally damage the FRP reinforcement. Externally bonded FRP composite systems provide one of the most efficient solutions for strengthening concrete structures. While the commercial construction industry overwhelmingly accepts the use of FRP, it is still a work in progress as far as establishing codes and standards regarding design, inspection, and testing. Following the recommendations provided from feasibility studies, coordination during design and defining inspection/testing criteria will help to ensure a smooth process through each step of an FRP strengthening project.■ Ammar Motorwala is an Assistant Project Manager with Smislova, Kehnemui & Associates, P.A. (SK&A) Structural Repair and Restoration Division. (ammarm@skaengineers.com) Brent Stephens is a Principal with SK&A’s Structural Repair and Restoration Division. (jbrents@skaengineers.com)

Table 3. Potential inspections programs.

LARGE SCALE PROJECTS

SMALL SCALE PROJECTS

Visual inspections (accompanied by Non-Destructive Testing) during and after installation, particularly the cured laminate, are crucial. Most of the required information can be obtained from this inspection by a well-qualified, experienced inspector. Visual inspections ascertain fiber orientation, voids, delamination, and wrap dimensions, which pull-off tests cannot determine.

Visual inspections during and after the installation are critical and should be strictly enforced, including layout verification.

Pull-off tests are a good tool, but over-reliance on pull-off tests alone is misleading. In addition to visual inspections, pull-off tests should be specified with a minimum recommended frequency of 1 test set (3 samples) per 500 sq. ft. (or per day or per roll/package).

Excessive reliance on pull-off tests is not prudent. Frequency of pull tests could be limited to 1 random test set (3 samples each) per every 500 sq. ft. of FRP fabric with a minimum of 1 test set (3 samples) for a small project.

Test panels are useful for verification of material properties. Passing rate is usually high, and they are relatively expensive. Hence, limit the frequency to a set-aside of 1 sample for each new roll of material (FRP fabric) and randomly select 10% of the samples for lab testing.

Test panels for small projects may be waived if familiar (frequently tested) materials are specified.

14 STRUCTURE magazine

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structural DESIGN Large Anchor Grout Pockets for Foundations A Review of Design Considerations

By Silky Wong, Ph.D., S.E., P.E., CEng MICE, and Widianto, Ph.D., P.E., M.ASCE

rout pockets are man-made holes in concrete structures (pre-installed before concrete placement or drilled after concrete placement) to allow the installation of anchors. The main benefit of using grout pockets is to allow equipment or structures to be installed after the concrete placement, providing more construction/installation schedule flexibility. In many non-modular projects, the equipment/machinery packages are typically completed and arrive at the construction site after most of the civil works at the site (including foundations) are completed. The grout pockets also provide extra installation tolerances and eliminate the risk of castin-place anchor movement during a foundation placement. Figure 1. Large grout pocket construction (Ref: Figure 8.4 from EFRC report, 2017). From a size perspective, there are generally two different grout pockets: required depth, diameter, and surface preparation, is typically • Large grout pockets: The pocket size is much larger than the provided by the post-installed anchor manufacturer as part of anchor, and pockets are typically pre-formed prior to the the installation procedure and is not discussed in this article. concrete placement, or they may be cored after the concrete Chapter 17 of the American Concrete Institute’s ACI 318-19, placement. An example of the use of this pocket is with heavy Building Code Requirements for Structural Concrete and Commentary, machinery foundations (Figure 1). Typical diameters of large does not provide the specific guidelines for designing large anchorgrout pockets are at least 3 to 4 times the anchor bolt diameter age grout pockets. Without a detailed procedure or design guideline + 2⅜ inches [60 millimeters (mm)]. The use of 3 to 4 diamfor designing and constructing large grout pockets in any industry eters approximately equates to the minimum size of the anchor standard, it is important to provide practical large grout pocket design head to resist a minimum of 70% of pretensioning force in methods to ensure safe designs. high-strength anchor bolts (ASTM A-193 B7), which are comThe American Institute of Steel Construction’s (AISC) Design Guide 1, monly used for heavy machinery. 2nd Edition: Base Plate and Anchor Rod Design (Guide 1), covers another • Small grout pockets: the size of the pocket is slightly larger type of grout pocket that is not discussed in this article: shear lugs than the anchor and typically drilled. A common use is with embedded in a grout pocket in a concrete pedestal. Unfortunately, post-installed bonded/adhesive anchors or reinforcing bars. Guide 1 does not provide specific guidance on grout pocket design Typically, optimal performance of adhesive anchors is achieved and sizing, other than recommending that the grout pockets be of with a relatively thin bond line, that is, with an annular gap sufficient size for ease of grout placement. Guide 1 also recommends of 1⁄16 to ⅛ inch. The design of small grout pockets, including that non-shrink grout have a flowable consistency.

Figure 2. Large grout pocket example – section detail (rebars sketch is provided as an illustration only, not for showing details).

16 STRUCTURE magazine

This article presents methods for designing a typical large anchor grout pocket in reinforced concrete foundations supporting equipment/machinery. It presents a load transfer analysis model, addresses the failure modes to be checked (pullout strength, interface strength between grout and concrete), and discusses the effect of the clamping force of a skid beam to restrain grout-pocket pullout. Only headed anchors in large grout pockets are discussed herein (Figure 2 ).

Using Large Grout Pockets Absent industry standards and available literature with design guidance and procedures, there is little consistency in designing large grout pockets. From the authors’ experiences, large grout pockets are typically used to set equipment bases and are often specified by European equipment vendors. From a structural design perspective, no serviceability, performance, or strength benefit has been observed for grout pockets. The Table provides a summary of advantages and disadvantages of using large grout pockets.

Grout Pockets Construction Grout pockets can be constructed in several ways: pre-installed during concrete placement using permanently installed corrugated pipes to form the pockets (common for large pockets) or drilled/cored (common for small pockets). Before grouting, the pockets must be properly cleaned (from debris, oil, dust, etc.) to minimize the risk of voids or air-pocket formation around the anchor bolts. The pockets should be roughened, such as using steel brushes, to ensure proper bonding between grout and concrete. Corrugated pipes typically do not require roughening. In general, cementitious grouts generally bond best to damp concrete, and polymer adhesives bond best to dry concrete (BASF, 2004).

Design Considerations Tensile Load Transfer Analysis Model

Summary of advantages and disadvantages of using large grout pockets.

Advantages Schedule benefit: Allows for foundations to be cast first and then, equipment to be set and fit later (with anchor bolts) Allow for some additional adjustment (because the grout pocket is significantly larger than the bolt).

Disadvantages Grout may not be as robust as structural concrete (especially in tension). Note that there are many grout products (both cementitious and epoxy) that are stronger than concrete. Uncertainty of interface strength between grout and concrete dependent on the surface preparation. When the distance between the edge of the grout pocket and the edge of concrete foundation is small, the use of grout pocket results in higher density (more congested) reinforcement between the grout pocket and the foundation edge, which increases the risk of “honey combing” (See Figure 2). Formwork is required to form the grout pocket (i.e., additional cost)

Interface Strength between Grout and Concrete For cement grout in the grout pocket, laboratory and field tests by Felt (1956) showed that bond strengths greater than 400 pounds per square inch (psi) might be achieved, regardless of the bonding medium, but that strengths of 200 psi or less may be adequate. Although these tests were performed based on cement grout, regardless of bonding types, the bond strength of 200 psi is generally used as a guide in designing bonding media (Suprenant, 1988). Thus, it is suggested to use 200400 psi as the ultimate bond strength between grout and concrete. European Forum Reciprocating Compressors (EFRC) report (2017) indicates that the cementitious or epoxy grout bond to the concrete foundation is stronger than the bond of the concrete to itself. Typically, concrete will separate next to the bond line of the grout concrete. Therefore, the weakest link in the bond of the grout (cementitious or epoxy) to concrete is the concrete itself. The force required to pull the concrete apart is called its shear strength Fv, and the minimum required grout pocket sizes, based on this bond strength, are as follows: Fv = Interface surface area × vc where Interface Surface Area is the surface area of the interface between grout pocket and surrounding concrete foundation.

For headed anchors, it is generally assumed that the entire tensile force is transferred through the anchor head bearing on the grout. The design of headed grouted anchors generally follows the procedures for cast-in headed anchors, assuming that the governing failure mode is concrete breakout. The bearing stresses at the head of the anchor typically create sufficient outward pressure to generate substantial friction at the grout/concrete interface. Where the hole size is large, and there is a question about the bond between the grout and the concrete, a separate design check for bond failure may be appropriate (Zamora et al., 2003). Where a threaded rod is used for the anchor, a sleeve or bond breaker is recommended. A sleeve may be used to provide an unbonded length for tensioning (Figures 2 and 3) and to avoid spalling at the concrete surface. When grouts have a higher compressive strength than the foundation concrete, it is reasonable to use the 8 times fć, (where fć = specified 28-day compressive strength of concrete) for the bearing strength at the anchor head similar to the cast-in-place headed anchors pullout strength (i.e., Equation 17.6.3.2.2a of ACI 318-19) and the strength reduction φ-factor of 0.7 per Table 17.5.3(c) of ACI 318-19. The pullout strength determined from Equation 17.6.3.2.2a (ACI 318-19) corresponds to the force at which crushing of the concrete occurs because of the Figure 3. Large grout pocket load transfer analysis model. bearing of the anchor head.

continued on next page S E P T E M B E R 2 0 21

For headed anchors, the interface surface area can be estimated from the intersection of the concrete breaking cone (35-degree angle per the Concrete Capacity Design [CCD] method of ACI 318) to the side of the grout pocket, as shown in Figure 3. For square grout pockets: Interface surface areasquare = 4 × bp × hp For cylindrical grout pockets: Interface surface areacylindrical = π × Dp × hp where, bp = square side dimension of the grout pocket Dp = cylinder diameter of the grout pocket hp = depth of the grout pocket, measured to the intersection of the breakout cone (Figure 3) vc = the concrete shear strength For typical normal concrete, vc can be taken as 2√f c´ (in psi) as per ACI 318, where √f c´is the square root of specified compressive strength of concrete (psi). The above formula provides the minimum required grout pocket size to avoid pulling out the entire pocket together with the anchor bolt. When corrugated pipes are used, the interface strength can be higher. For example, test results show that the minimum ultimate interface strength between the winding pipe (one of the manufacturers is Kurimoto) and surrounding concrete is 460 psi or 3.2 MPa (Kouei Japan Trading Co, 2020). For strength design, it is reasonable to use the strength reduction factor (φ) of 0.75.

Clamping Force to Restrain Pullout In most machinery foundations, the largest tension force in the anchor bolts typically occurs during pretensioning. The purpose of the bolt pretensioning in machinery foundations is to keep the skid-frame tight to the foundation and maintain a sufficient friction force for any lateral component of the dynamic unbalanced forces and, therefore, reduce the risk of vibration. The bolt pretensioning force should also be larger than the maximum peak value of the sum of the dynamic loads in the vertical directions (EFRC Report, 2017). The EFRC report summarizes pre-load values from various references and recommends 70% of the bolt material yield stress for heavy machinery. Unlike the net tension force on the anchor bolt during operation (i.e., due to uplift of the equipment), the pretensioning of the bolts of the machinery skid clamps down the skid beam to the concrete foundation, providing downward vertical force on top of the grout pocket, as shown in Figure 2. The design engineer can consider accounting for the clamping force to reduce the required interface strength due to initial the pre-tensioning of anchor bolts, provided that: 1) The skid beam is rigid, and the flange totally covers the grout pocket. 2) There is no net tension or uplift under any load combinations during operation. Still, as the skid beam flange often covers only part of the grout pocket, the grout pocket should be designed for the total pre-tensioning force.

Shear Load Transfer Analysis Model When anchor bolts are pretensioned (common for machinery foundations), the friction between the bottom of the steel base plate and the top of the concrete foundation is commonly considered to resist shear force (due to wind, bundle-pull, seismic, etc.). However, it is generally not preferred to resist shear by the anchor bolt because of the flexibility of the anchor bolt and typical oversized holes on the steel base plate. 18 STRUCTURE magazine

However, in cases where welded washers are used, and the anchor bolts are relied upon to resist shear, it is reasonable to use the CCD Method presented in Section 17.7 of ACI 318-19, provided that the grout strength is higher than the surrounding concrete foundation.

Minimum Edge Distance The minimum distance between the edge of the grout pocket and the edge of concrete foundation should consider the following: • Sufficient clear spacing to minimize the risk of honeycombing due to congested reinforcement (Figure 2). Depending on the magnitude of tensile and shear forces on the anchor, reinforcement surrounding the grout pocket may be required, and hence, sufficient space should be provided. • The minimum edge distance of the headed anchors (Section 17.9 of ACI 318-19). • Construction tolerance of grout pocket (e.g., 10 mm [⅜ inch] recommended in EFRC report (2017)). EN 1992 Eurocode 2 specifies that the distance between the outer edge of the bolt pocket and the reinforcement steel is the minimum reinforcement diameter + 10 mm (⅜ inch).

Grout Materials Grout materials with the following properties are recommended: • Low peak exothermic temperature and low coefficient of thermal expansion grout. These properties are essential to minimize the expansion of grout and the associated risk of cracking a concrete foundation, especially between the edge of the grout pocket and the edge of the concrete foundation. • A strength that is at least equal to the strength of concrete foundations. This strength property is important to minimize the risk of pre-mature failure of the headed anchor in the grout pocket (e.g., due to bearing/pullout failure). • Flowable grout consistency is important to minimize the risk of voids in the grout pocket and at the underside of the equipment skid beams.

Summary This article presents design considerations for large grout pockets, typically larger than approximately (3d to 4d ) + 60 mm (2⅜ inch), where d is the anchor diameter. Large grout pockets in foundations supporting equipment/machinery provide more flexibility for the construction/installation schedule because it allows for equipment to be installed after the concrete foundation placement. In addition, large grout pockets increase available installation tolerances. One of the critical parameters for designing large grout pockets for tension is the strength of the interface between the grout and the concrete foundations. Several recommended values are presented and illustrated by an example calculation. For future research, it is suggested that these recommended interface strength values be verified by large-scale tests.■ An example calculation, acknowledgments, and references are included in the online PDF version of the article at STRUCTUREmag.org. Silky Wong is a Lead Civil/Structural Engineer at Dow, Inc. She is a member of the ASCE Energy Division Executive Committee and Anchorage Design for Petrochemical Facilities Committee. Widianto is a Lead Structural Engineer at ExxonMobil. He is a member of Anchorage Design for Petrochemical Facilities Committee.

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structural ANALYSIS Rigid and Non-Rigid Base Plate Assumptions Use of CBFEM for Validation By Arif Shahdin

ave you ever stopped for a second and The purpose of this article is to further the thought, “do we ever validate our rigid understanding of non-rigid base plate design by: plate assumption when designing anchorage 1) Developing a non-rigid base plate to concrete?” The answer is simple. There is no criterion analytical method for validation and, because 2) Linking the (above) criterion with prethe design codes mandate it, the design viously explained CBFEM parameters engineer abides by it as it is a Building and 3) Using the criterion to define a “NonSafety (Plan check) requirement. This article Rigid base plate Close-To-Rigid” demonstrates how Component Based Finite 4) Understanding the power of CBFEM Element Modeling (CBFEM) can validate methodology for validating rigid or any base plate behavior, rigid or non-rigid. non-rigid base plate assumptions So, regardless of what base plate assumption you make as a design professional, now you Non-Rigid Base Plate Design can get validation. Design of anchorage per the American Figure 1. Bi-linear stress-strain curve. A criterion for Non-Rigid Base plate assumpConcrete Institute’s Building Code tion must first be developed. When a design Requirements for Structural Concrete (ACI 318) is mandatory for engineer thinks about a non-rigid base plate, what comes to mind structural engineers and design professionals in general. When dis- first? For instance, some say “a thin plate,” so thickness can be the first cussing anchorage to concrete, a non-rigid plate assumption is a assumption. But then, very quickly, it is concluded that “thickness of rarity, predominantly because the anchorage design codes mandate the plate” itself does not entirely paint the picture until plate loadthat the anchor forces be determined based on a rigid plate assumping is examined. Furthermore, since Finite Element Analysis (FEA) tion. In a previous article (Morgan and Shahdin, Structural Analysis, is the common computational approach, the stiffness and material STRUCTURE, January 2020), this and other consequences of non- type of the profile/fixture is also a key ingredient to obtain a realistic rigid plate assumptions such as increased anchor forces were discussed. load transfer. Finally, do not forget the anchors. In reality, the anchor CBFEM as a methodology to analyze the non-rigid plate behavior was types, sizes, and positions can also influence the load transfer and be also discussed, and the Bi-Linear Stress-Strain curve was introduced considered input to a criterion. to depict the behavior of the two-dimensional (2-D) shell elements To recap from the previous article, CBFEM methodology helps the of the plate. The article closed by highlighting some key CBFEM design professional make an engineered decision on whether or not parameters that would help a structural engineer make an engineer- to accept the resultant anchor forces from a non-rigid plate assumping judgment as to whether it is ok to go ahead with the non-rigid tion and proceed with an ACI anchorage design. This premise was plate-based anchor forces for anchorage design. developed on the three parameters.

Plastic Strain Limit

Figure 2. Defining Close-to-Rigid base plate behavior.

20 STRUCTURE magazine

The plastic strain limit depicts the behavior of the plate beyond the yield point. To understand this further, look at the defined material behavior curve. A material behavior curve is typically a stress-strain curve that determines the behavior of the steel elements. In this case, the elements of choice for the plate are 2-D shell elements, and a bi-linear stress-strain curve is the material behavior curve of choice. A bi-linear stress-strain curve eases the understanding of material behavior. The behavior remains linear after yield, with a slight slope associated with it, as shown in Figure 1. Until yield strength is reached, the plastic strain remains zero. This shows classic linear elastic behavior as defined by the American Institute of Steel Construction (AISC). Once the yield point is reached, the plastic strain counter starts. Now connect this parameter to the non-rigid plate criterion that was established above. In conjunction with the bi-linear stress-strain curve, the plastic strain limit percentage helps to understand the force level on the structure by relating it to the von Mises stress. The von Mises stress criterion is used to depict whether a given material will yield or fracture.

Per this criterion, the material is assumed to be elastic before reaching the yield stress, Fy. CBFEM provides the stress (von Mises) related to a particular plastic strain limit, which can also be seen as an account of the loads applied on the plate. Therefore, it can be deduced that the plastic strain limit parameter relates to the loads on the plate.

Absolute Plate Deflection Now the second parameter is examined to see how it can be related to the established Figure 3. Design example. criterion. As the plate yields, it deforms, and the CBFEM captures that absolute deformation. This closely relates to the first criterion for a non-rigid plate, i.e., “thickness of the plate.” A thinner plate would ideally deform more than a thicker plate and vice versa. So, it can be concluded that the CBFEM parameters (relating to the plate itself ) that help us decide whether or not to use the non-rigid anchor forces for anchorage design, in fact, relate to the simple but logical criterion initially set.

Anchor Forces The third parameter, an important one, is the anchor force. In CBFEM, as discussed in the previous article, the fixture, plate, and anchors are modeled as components held together by the mesh. Now, look at the load path through this model. The base plate receives the load from the fixture (column) and distributes it to the anchors. The load transfer in any CBFEM model is through the material stiffness and, hence, anchor

stiffness plays a key role in the analysis. Since the non-rigid plate tends to yield, this methodology/assumption can help model the effects of prying action in the anchorage design. However, as a consequence, higher anchor forces should be expected compared to what would be expected from a rigid plate assumption. Therefore, the CBFEM methodology compares non-rigid anchor forces and equivalent rigid anchor forces to monitor the increase in the anchor forces that are determined as a consequence of assuming a non-rigid plate. CBFEM methodology assumes the plate as non-rigid. However, the advantage of CBFEM is that, once the rigid plate behavior is validated, it is no longer necessary to use the higher anchor forces resulting from the non-rigid plate assumption. In that case, the forces obtained from any rigid analysis can be utilized. Therefore, using the above-developed criterion for a non-rigid plate and the associated (three) CBFEM parameters, a non -rigid plate can now be defined as “Close-to-Rigid.” This is illustrated in Figure 2.

continued on next page

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S E P T E M B E R 2 0 21

A Design Example

Table 1. Results (Example: Validation of Rigid Plate Behavior).

The following design example illustrates how it all comes together and how the CBFEM helps validate rigid/non-rigid plate behavior. Start with a fixture and loading criteria, as shown in Figure 3 (page 21).

Validation of the Rigid Plate Behavior

Component-Based Finite Element Analysis (CBFEM) - Rigid Anchor Plate

Component-Based Finite Element Analysis (CBFEM) - Non-Rigid Anchor Plate

Anchor 1

0 kip

5.6 kip (∞%)

Anchor 2

0.8 kip

5.7 kip (613%)

Anchor tension forces

In the first part of the design example, run this Anchor 3 1 kip 0.9 kip (-10%) fixture as is, i.e., given a one-inch-thick plate. Anchor 4 1.7 kip 1.7 kip (0%) Table 1 shows the results of the CBFEM. Anchor 5 1.9 kip 1.7 kip (-11%) Interpretation of Results: Anchor 6 2.1 kip 0.3 kip (-86%) a) Higher anchor forces as expected with a non-rigid assumption via CBFEM. None 0% Anchor plate plastic strain (max) b) % Plastic strain limit is zero. This indicates 0.043 in 0.044 in Anchor plate deformation (max) that the yield stress, Fy, has not been reached on the bi-linear stress-strain curve. That depicts a rigid plate The CBFEM gives an option to set that limit and, by default, behavior. However, this can be further proved by looking at the places an upper limit of 5% plastic strain. However, again, von Mises stress calculated by CBFEM. That value is around 18 examine the von Mises stress. This value is 36.028 ksi, which kips per square inch (ksi), which shows that, load-wise, the results shows that the result is just past the yield point (36 ksi) on the fall about halfway on the linear portion of the curve below Fy. bi-linear stress-strain curve. From a stress/loading perspective, it This is classic linear elastic behavior. is positioned very close to the start of the second portion of the c) A bsolute deflection for the non-rigid case is precisely the curve. Position-wise, it is close enough to the inflection point same as that of the rigid case. between rigid and non-rigid behavior; this can be classified as d) Summarizing the results from the parameters above, this is Close-to-Rigid. a true rigid plate behavior, even though it has been analyzed c) A bsolute deflection for the non-rigid case is higher than that using CBFEM. This is how CBFEM methodology can be of the rigid case. This is expected as the yielding of the plate has used to validate rigid plate behavior. In this case, once valiinitiated. However, it is not higher by a factor of 10. Therefore, dated, the design engineer could revert to the anchor forces for all practical purposes, it is still within an acceptable limit. obtained from any rigid analysis and use those anchor forces d) Summarizing the results from the parameters above, this nonfor anchorage design. rigid base plate behavior can be classified as Close-to-Rigid. Engineering judgment based on experience is a crucial factor here, Value Engineering the Base Plate without a doubt. The software can be utilized (as illustrated above CBFEM allows the design professional to value-engineer the base plate with the examples) to assist the design professional in understanding (achieving Close-to-Rigid behavior). In this case, assume that the one- the parameters associated with a non-rigid base plate analysis, thus inch-thick plate seems a bit excessive for this application, and so, as helping to qualify a non-rigid plate behavior as Close-to-Rigid. shown in Part 2 of this design problem, reduce the plate thickness and see where Close-to-Rigid Behavior can be achieved. So, reduce the Summary plate thickness to half an inch. Table 2 shows the results. Interpretation of Results: CBFEM assumes a non-rigid plate behavior and provides an alternative a) Higher anchor forces as expected with a non-rigid assumpto the classical rigid base plate design methodology. This methodology tion via CBFEM. can easily handle any combination of base plate loading and profile b) % Plastic strain limit is 0.1. This indicates that the inelastic eccentricity and is not as limited as the classical rigid plate methodology. behavior has initiated, and yielding of the base plate should be It also provides validation of rigid or non-rigid plate behavior, unlike expected. How much inelastic behavior and yielding can be any other method. allowed for plate behavior to be categorized as Close-to-Rigid? However, since steel design codes mandate loads to be calculated using a rigid plate assumption, understanding the Table 2. Results (Example: Value Engineering the Base Plate). CBFEM parameters is mandatory as CBFEM assists Component-Based Finite Component-Based Finite the design engineer in qualifying a non-rigid plate Element Analysis (CBFEM) Element Analysis (CBFEM) as Close-to-Rigid. This approach can then be used - Rigid Anchor Plate - Non-Rigid Anchor Plate to design the anchorage per the Building Code Anchor tension forces Requirements for Structural Concrete (ACI 318), Anchor 1 0.5 kip 8.2 kip (1,540%) which is the primary goal.■ Anchor 2

0.8 kip

9.2 kip (1,050%)

Anchor 3

1 kip

0 kip (-100%)

Anchor 4

1.7 kip

0.9 kip (-47%)

Anchor 5

1.8 kip

0.9 kip (-50%)

Anchor 6

2 kip

0 kip (-100%)

Anchor plate plastic strain (max)

None

0.1%

Anchor plate deformation (max)

0.052 in

0.064 in

STRUCTURE magazine

Calculations performed using HILTI PROFIS Engineering Software. Arif Shahdin is a Steel Design Expert/Software Product Manager with Hilti North America. He is a degreed former practicing structural engineer and was an adjunct faculty of Civil Engineering at California State University, Los Angeles. (Arif.Shahdin@hilti.com)

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historic STRUCTURES The Point Ellice Bridge Failure By Frank Griggs, Jr., Dist. M.ASCE, D.Eng, P.E., P.L.S.

he Point Ellice Bridge crossed the Upper Harbor from Victoria, British Columbia, to Esquimalt. The first wooden pile bridge at the site was built in 1861 and was replaced in 1872. This was replaced by an iron bridge built in 1885 by the San Francisco Bridge Company for regular carriage, wagon, and pedestrian traffic. It was turned over to the City of Victoria by the Provincial Government in 1891. Engineering News described the bridge: “It consisted of two 120-foot deck spans of Pratt combination trusses, 15 feet deep, with panels 17 feet 6 inches long; two 150-foot through spans of Whipple combination trusses, 25 feet deep, with eight panels 18 feet 9 inches long; and a short trestle approach…The piers were pairs of iron cylinders filled with concrete. The trusses were 20 feet apart, center to center, carrying a roadway 19 feet wide in the clear, with felloe or wheel guards 3 x 6 inches. Two 5-foot sidewalks were added after the bridge was commenced, each having three lines of 2- x 12-inch joists and 2- x 12-inch floor planks. The floor was about Point Ellice Bridge. 20 feet above the water.” Even though the trusses were iron, the entire deck was wood. handle the number of people attending the festivities. They even Crossbeams on the 150-foot spans were 12 ×18 inches with wooden brought out car No. 16, their heaviest at 16,000#, to handle the stringers. They were hung from the trusses with 1⅛-inch bars passing traffic. When the car with a capacity of 60 people, but now carrying through the ends of the beams. 143, crossed the bridge, the span collapsed into the harbor. Evidently, In 1889, The Victoria Tramway Company, later the Consolidated one of the old wooden beams sheared off due to decay and reduced Electric Railway Company, obtained a charter to use the bridge for the section, which caused the car to tip side-wards and strike the iron their streetcars, possibly without checking to see if the bridge was truss work, which then collapsed, leading to the failure of the entire strong enough for additional loading. When the San Francisco Bridge span. The car fell into the water with some of the truss falling behind Company heard of this, they visited the bridge. They determined the it. Some of the passengers were killed by the falling ironwork, but bridge was not designed for that loadmost drowned. Of the estimated 143 ing, as they had used a static load of 600 passengers in the car, 55 died. pounds per foot and a rolling load of A local report stated, 1,000 pounds per foot in their design. “The central span of Point Ellice Those who are to blame are the However, they were told by the engibridge had again given way, precipitatneer of the Tramway Company that ing the car into the waters of the Arm, designers of structures and those who he had checked, and the bridge was where a majority of the imprisoned safe. Initially, the Tramway Company passengers – men, women, and little employ them and insist on a trivial had simply spiked iron straps to the children – to whom the world had a planks and placed them close to one moment before been all sunshine, were economy at the expense of safety. side of the bridge with a clearance at drowned before aid could reach them. the outer rail of 2 feet 7 inches and a The crashing timbers and ironwork of gauge of 5 feet. They added no addithe bridge piled upon the ill-fated car tional stringers. as the waters received it, and doubling In 1893, one of the cross beams broke under their heaviest car, No. up, pierced it also from below so that many were killed even before the 16, and the deck sagged at the breakage. The city hired a local car- water was reached, while the others were less mercifully held below the penter/blacksmith to repair the bridge. He replaced five of the seven muddy waters…So many victims as it claimed that there is scarcely beams with new 12- × 16-inch crossbeams on each of the 150-foot a home in Victoria that has not lost some relative or friend. Ours is a spans and 1¼-inch hangers with the ends upset to 1⅛ inch. The beams city of desolation and of sadness, and in its mourning, Seattle, Tacoma, were drilled and notched to receive the bolts, cutting down on the New Whatcom, Port Townsend, and the other cities of the Sound are strength of the ends of the beams. He left two existing beams in place, joining, for each has contributed among the holidaymakers who formed which proved to be penny wise and dollar foolish. At the same time, the burden of the submerged car some of its well-known citizens.” they replaced the straps with 30# T rails. These rails rested on two A coroner’s inquest was held, and after interviews and investigation 10- × 12-inch stringers that were two panels long with broken joints. by the ten-man jury, found in part, May 26, 1896, was a day of celebration in Victoria, as people were “That the said accident was the result of the sudden collapse of the celebrating Queen Victoria’s 76th birthday at a carnival in Esquimalt. eastern Whipple truss of said bridge, and was caused by the weight The Consolidated Electric Railway Company ran additional cars to of car No. 16 of the Consolidated Electric Railway Company and

24 STRUCTURE magazine

its immense load of passengers, which was in excess of the capacity of the bridge in question as originally constructed, and that the said Consolidated Electric Railway Company is guilty of negligence in not having taken proper precautions for the safe conduct of its passengers accordingly; That car number 16 was dangerously overloaded with passengers, and in the interest of public safety, it is imperative that restrictions should be imposed upon the traffic of this and similar corporations in the future… Furthermore, it is manifestly the duty of all corporations of this kind who are entrusted with the safety of human lives to see that all roads and bridges over which it passes are in a safe condition and to take such steps as are necessary to ensure this condition of things being carried on by the proper authorities… That the bridge in question was adequate in strength to the ordinary traffic for which it was constructed and was under ordinary circumstances suitable for the ordinary railway traffic for which the railway company obtained permission to use it from the government department for whom it was constructed; but the design was poor, the system of construction obsolete, and the contract was not carried out according to specification by the contractors. We desire to call attention to and to condemn the system of public works which has been, and we believe now is in vogue in the public works department of the city. We find that the city engineer and heads of departments under him who should be held personally responsible for the good and efficient execution of the details of their department are so hampered and interfered with by un-technical, elective superiors that they are without authority necessary to carry out their work, and are consequently without responsibility, which is certainly not conducive to good results… We find that the specifications call for weldless iron, but that the ironwork in almost all cases were welded, and in many cases of inferior quality, and that the factor of safety provided for in the specifications is of an unknown quantity. It is quite evident from the evidence produced before us that the primary cause of the accident was the breaking of one certain hanger, shown as number 5 on the diagram produced in evidence, resulting finally in the collapse of the bridge; said hanger being part of the original construction. We find therefore that the Consolidated Electric Railway Company are primarily responsible for the accident and that the city council is guilty of contributory negligence.” (Victoria Daily Colonist, June 13, 1896, 6) Engineering News ran a lengthy article on the collapse and wrote in part, "There are some lessons in this accident which those in responsible charge of bridge structures will do well to profit by. It appears that the bridge in question was built for ordinary highway traffic and was designed to carry a live load of 1,000 pounds per lineal foot. As the combined width of roadway and sidewalks was 31 feet, this was equivalent to a live load of only 32 pounds per square foot, a figure which speaks for itself to any engineer. Manifestly the proper thing to do, if economy was necessary, was to narrow the roadway and sidewalks enough to enable the bridge to carry a crowd without exceeding the load per lineal foot for which it was designed. If a bridge is to be built of a strength suitable for a country highway, then it should be made of a corresponding width; but it is recklessness which cannot be too strongly characterized to make a bridge of a width for a city street and dimension its members as if it were of a width for a highway… It is not the bridge-building companies who are to blame for this dangerous economy in laying down live loads for structures. Every one of the bridge builders, we venture to say, would much prefer to build structures designed to carry the greatest load that can be placed

Failed 150-foot span, search for bodies underway.

upon them. Those who are to blame are the designers of structures and those who employ them and insist on a trivial economy at the expense of safety. We are aware that it is hard, oftentimes, for an engineer to stand up for what he knows is good and safe practice. To the average alderman, city councilor, or other layman, it may look like a piece of theoretical nonsense for the city engineer to insist on designing bridges to carry the weight of a crowd which may very probably never come upon the structure. But the only safe rule for the engineer, notwithstanding such opposition, is to stand out for what he knows to be safe and the only safe practice. We believe that the engineer who is in such difficulties may make good use of the descriptions of such accidents as that at Victoria, in this journal, to show to those who oppose him the results which may follow a neglect of sound engineering principles with respect to bridgework. The second point deserving attention, with respect to the Victoria bridge, is that although it was only designed for a live load of 1,000 pounds per lineal foot, several years after its erection, an electric car line was allowed to lay its tracks across the bridge, and no investigation, or at least no adequate investigation, was made to see whether the structure was strong enough to take the added load… Another point which may be noticed in this connection is the extent to which the overcrowding of cars is permitted on our street railways, and this has been a direct or indirect cause of many accidents, and the cause of much injury in many accidents not attributable in any way to overcrowding. The companies make little or no attempt to check this, or even to provide additional men in charge of the crowded cars, and some of them (whose roads carry enormous crowds of people) make the absurd claim that if the number of persons in the car was limited to its proper number or if they provided extra men, the profits would be so reduced that the company would have to go out of business. Such a claim cannot be seriously considered. In the accident at Victoria, noted above, about 140 persons were crowded in and upon a car seating only 60. Filling the standing room on days of exceptional traffic is permissible, according to universal American practice. But crowding passengers in cars without limit is barbarous and ought to be prevented by city authorities.” (Engineering News, June 18, 1896, 394) Court cases of the families of the killed ran on throughout 1897 and 1898 in the Supreme Court of British Columbia. The cases were finally settled in the Privy Council in June 1899 when the Railway was found liable for the deaths. In summary, the cause of the failure was likely the compounding of bad design, bad construction, lax inspection, and inadequate oversight of the electric railway operation.■ Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19 t h Century cast and wrought iron bridges. He is now an Independent Consulting Engineer. (fgriggsjr@twc.com)

S E P T E M B E R 2 0 21

Wave One

A Story of TransfEr GirdErs By Anantha Chittur, P.E., S.E., and Steven M. Baldridge, P.E., S.E.

he Near Capital Region (NCR) around India’s capital, New Delhi, witnessed a decade-long construction boom starting in 2008. Several high-rise apartments were constructed to cater to the growing need for housing in Noida, which was followed by new office construction to lure large corporations away from the urban sprawl of New Delhi. Wave One is a $276 million, 41-story mixed-use building with more than 2 million square feet of floor area located in the heart of Noida. It includes three levels of retail, two-story cinemas on the fourth floor, seven levels of parking above the cinema, and three levels of underground parking for a total of 2,600 cars. In addition,

Basement construction.

26 STRUCTURE magazine

an extensive amenity deck, tennis courts, and a health club on Level 15 serve the tenants of the 26-story office building that rises above the podium. The structure has a footprint of 528 feet by 228 feet on the podium levels and is constructed with no expansion joints. The building is characterized by a large rectangular aperture created to allow the “flow of energy” through the property. This aperture breaks the large office mass into two distinct towers, which are reconnected between Levels 32 and 41. The project is located in a region of moderate high seismicity, requiring special detailing for concrete elements, including additional

checks for deformation compatibility of the slab-column gravity system. Shear walls at the cores provided lateral force resistance. The design is in accordance with India’s National Building Code (NBC) and other applicable local codes. Due to the building’s height and unique geometry, a wind tunnel study was conducted to determine the structural loads and occupant comfort.

Zero Lot Line Construction To maximize the building’s footprint, the basements are constructed with zero lot lines using a contiguous bored pile (CBP) wall earth Two-stage transfer girder scheme. retention system tied back using post-tensioned soil anchors until the basement slabs were installed. Traditionally, concrete retaining walls are constructed to act as the permanent structure in front of these piled walls. However, on this project, these CBP walls also act as a permanent earth retention system and support the basement slabs along the perimeter. In addition, masonry walls are built in front of the piled walls to conceal the drainage channels. This scheme resulted in a significant cost saving.

Cinema transfer girder.

Post-Tensioning as a Driver Concrete is the material of choice for constructing buildings in India. When speed is a primary driver, as it was for this project, post-tensioned slabs and beams reign supreme. This is due to the load-balancing provided by the post-tensioning tendons that allow formwork to be removed and reused as soon as the tendons are stressed. When this project was conceived, post-tensioned concrete for buildings was in its nascency in India. It required a collaborative effort between the design team, contractor, and developer to devise the layout of vertical framing organized around an efficient post-tensioned slab system. The mixed-use building is apportioned into various uses such as parking, retail, offices, cinemas, outdoor amenities, mechanical floors, etc. The multi-story parking levels determined the layout of the columns in the podium, which, in turn, resulted in a square grid of 28 feet by 28 feet for the majority of areas, except for some longer spans at drive aisles. This layout also allowed the columns to continue vertically through the office levels with minimal impact. The typical slab thickness is 8 inches at the parking and office levels, while the retail levels have a thicker 9-inch slab. Traditionally, buildings in India are constructed with 2 to 3 inches of screed on top of the structural slab to allow for the lack of sophisticated finishing methods and allow tenants to pick their flooring as part of tenant improvement. However, on this design-assist project, it was decided to forgo the layer of screed on the parking levels due to the contractor’s ability to deliver a superior finished surface. In addition, the removal of the screed reduced the seismic mass of the building and helped to optimize the post-tensioned slab. The project’s scale and varied occupancies triggered a “Type 1” construction in accordance with the NBC, necessitating a 3-hour fire rating for S E P T E M B E R 2 0 21

Aperture transfer girder.

the slabs. The conventional practice was to consider the concrete clear cover of 1⅜ inches to the post-tensioning duct even though the tendons are grouted and are often higher in the duct at midspan. BASE collaborated with the PT supplier to determine the elevation of the tendon within the duct, thereby gaining valuable drape in the tendon profile while still meeting the clear cover intent of the code.

Transfer Girders Galore At Level 4 of the podium, above three levels of retail, the floor area was allocated for cinemas that required 56 feet by 84 feet of doubleheight, column-free space for each of the five cinemas. This could only be achieved through a five-span shallow post-tensioned transfer girder that spanned 56 feet while supporting seven levels of parking, one level of office, and one level of outdoor amenities. A portion of the office tower also hovered above the cinemas, requiring a different strategy to transfer the tower columns. The limited height available over the cinemas required a double transfer beam to support the office tower columns spanning between Levels 15 and 41. This was achieved by introducing one-story-deep transfer girders between Levels 14 and 15 and using the transfer beam over the cinemas to only support the podium loads. Due to the unique nature and length of the 280-foot multi-span transfer girder over the cinemas that were required along two column lines, the design team collaborated with the PT supplier to stress the tendons outside the cross-section of the girder. This facilitated pour strips and allowed the entire girder to be constructed simultaneously while also providing intermediate stressing to minimize post-tensioning losses. The external stressing strategy is often used in bridges but is not very common in buildings. BASE worked closely with the PT contractor and formwork subcontractor to minimize the number of reshoring levels by continuing the column removed in the cinemas as a low-strength sacrificial column supporting the transfer girder. This strategy enabled the early removal of beam formwork and three levels of reshoring below the transfer girder. Due to the zero lot line construction, the parking ramps are integrated within the building footprint. Two octagonal spiral ramps, one on each side of the building, provide vertical circulation for the parking levels. The ramps stop at the underside of Level 13 with 27 levels above it. Like the columns transferred over the cinemas, the tower columns above the ramp are supported on 6-foot-deep transfer girders

28 STRUCTURE magazine

extending between Levels 14 and 15. The total length of the transfer girder on Level 14 is 310 feet. The building’s characteristic rectangular aperture measures 200 feet by 103 feet and extends between Levels 15 and 32. The aperture is framed at the top by offices between Levels 32 and 41. The top of the building steps up from one end of the building to the other. The structure above the aperture is supported by three profiled posttensioned girders that span 112 feet between each tower. These sizeable concrete transfer girders were partly conceived due to the contractor’s desire to use a tried and tested construction methodology instead of introducing structural steel trusses. The contractor’s message to the design team was, “No Discovery Channel stuff, please.” However, the concrete option came with its own challenges from transfer girders that are 3.3 to 5 feet wide and constructed at 200 feet in the air. Due to the immense weight of the three transfer girders and the associated formwork requirements, the girders were shaped to reduce overall weight such that it was 20.5 feet deep in the middle to maximize drape and stiffness for the 112-foot span and 15 feet at the ends to satisfy shear requirements. Despite the shape optimization, the weight of wet concrete during placement would be problematic. BASE worked with the contractor and PT supplier to construct the beam in two segments with a horizontal construction joint with layered tendons that could be independently stressed. Once stressed, tendons in the lower half were designed to support the weight of the upper half during construction. Tendons in the second half were profiled such that maximum drape was utilized for final loading. The overall process was done in six stages: three bottom segments and three top segments.

Conclusion A 2 million square foot high-rise project with multiple occupancies often comes with many challenges that require creative engineering solutions and close collaboration with the architect and other consultants. Post-tensioning often helps to simplify many of these problems by allowing for the removal, transfer, and shifting of columns and allowing slabs to span longer where required, giving the architect the desired flexibility.■ Anantha Chittur is a Senior Associate at BASE and is based in its Chicago office. (achittur@baseengr.com) Steven M. Baldridge is President at BASE and is based in its Honolulu office. (sb@baseengr.com)

Project Team Owner: Wave Infratech Structural Engineer of Record: BASE Architect of Record: Nostri Architects Design Architect: BBG-BBGM General Contractor: Leighton-Infra Joint Venture Wind Tunnel Consultant: RWDI

www.iesweb.com Your practical structural analysis & design tools. iesweb.com/model

his four-part series discusses the adaptive reuse of the Witherspoon Building in Philadelphia, PA. Part 1 includes the building history and description of the structure, including previous structural additions and alterations. In addition, numbered photos are provided with the print version of the articles, and additional lettered photos are provided with the online version of the articles.

Building History The 11-story Witherspoon Building (not including full and sub-basement levels) is on the National and Philadelphia Registers of Historic Places. It is considered Philadelphia’s first “skyscraper” and was constructed with Carnegie Steel beams between 1895 and 1897 for the Presbyterian Board and Sabbath School for use by various Presbyterian Church groups (Figure 1). As indicated on the available historical Sanborn Map for the building, the exterior brick masonry curtain wall varies in thickness from 28 inches to 12 inches, from bottom to top of the exterior façade, respectively. The Sanborn Map also indicates that the floors and roofs were framed with hollow clay tiles. The building was named for John Witherspoon, the first president of Princeton University (then known as the College of New Jersey). It was designed by Architect Joseph M. Huston, a graduate of Princeton, and constructed by William Steele and Son, Carpenters and Builders.

Figure 1. Witherspoon Building south elevation. Courtesy of JKRP Architects.

Adaptive Reuse   Historic W B

riveted steel plate and angle girders (Figure A, online) or steel B section girders. Built-up, story-high, riveted parallel top and bottom chord steel transfer trusses (Figure 3) support the south portion of the 4th and 5th floors and the columns that support the floor and roof framing above. Also, similarly constructed roof trusses clear span across the north end of the building above the 11th floor. In addition, except at the mechanical roof penthouse and standalone 1st floor shed, the main building columns consist of built-up riveted angle sections known as Gray columns (Figure 4 , page 34 ). Portions of the basement floor framing over the sub-basement were constructed with brick arches spanning between steel beams. In contrast, the roof over the mechanical penthouse was constructed with 4-inch hollow clay “book” tiles that simple span between inverted tee or “bulb” steel purlins supported by steel B section beams (Figure 5, page 34). Round, hollow cast iron columns support the beams. There is also an ornamental, cast-iron framed, single-story gabled roof structure that occupies what was originally an open courtyard area at the 1st or ground floor on the west side of the building. Details of notable original documented structural features of the building include:

Gray Columns

It is interesting to note that fabricated columns in the Witherspoon Building appear to have been used because the engineer and contractor had not transitioned from employOriginal Structure ing rolled steel “I” sections as In general, the floors and main horizontal beam elements to roof are constructed with vertical column elements, as By D. Matthew Stuart, P.E., S.E., P.Eng, F.ASCE, F.SEI, A.NAFE, SECB 12-inch-deep, hollow clay is done today. The Gray coltiles arranged in end-to-end, umns used in the Witherspoon flat arch construction with voussoir, or “keystone,” hollow clay tiles Building were a patented section that could be fabricated with strucplaced parallel to and on each side of the supporting 12-inch-deep tural shapes, in this case, angles that any steel mill could roll. Carnegie Steel B section “I” beams (Figure 2). The voids of the keystone Similar patented sections, like Phoenix columns, could not be fabritiles are also oriented parallel to the beam span and therefore perpen- cated with rolled sections. Therefore, because Carnegie Steel supplied the dicular to the main arch tiles. This is referred to as a combined end beams for the building, it was assumed that the Gray column components and side configuration. The beams are typically supported by built-up were likewise rolled by Carnegie (cross-section (4) in Figure B, online).

Part 1: Building History and Structure

32 STRUCTURE magazine

In addition, it was confirmed via field measurements that the vertical angles used in the Gray columns conformed with the available standard angle sizes provided in the 1896 Carnegie Steel Pocket Companion. It is also interesting to note that both the Fisher Building in Chicago and Guaranty Building in Buffalo, constructed at the same time as the Witherspoon Building, used Gray columns.

Main Roof Trusses

Figure 2. Voussoir keystone hollow clay tiles on each side of a typical Carnegie Steel beam.

At the north end of the building, to provide a column-free space at the 11th floor, 4-foot-deep Warren steel trusses, fabricated with angles, plates, and rivets, were constructed to clear span from the east to west sides of the building (Figure C, online). The trusses support the main roof framing, the original mechanical penthouse floor and roof framing, and the 11th-floor walk-on attic/ceiling in this same area of the building.

4th Floor Transfer Trusses At a portion of the south end of the building, to provide a partial column-free space between the 2nd and 4th floors, story-high steel trusses, fabricated with angles, plates, and rivets, were constructed to span from east to west across the main rectangular footprint of the main building. The trusses support all floor and roof framing at and above the 4th floor in the area impacted by the same transfer members.

Figure 3. Riveted steel plate and angle transfer truss at the 4 th floor.

The open area below the transfer trusses was initially used as a large assembly space called Witherspoon Hall.

11th Floor Ceiling Because the story height from the 11th floor to the main roof structure is almost equivalent to a two-story space, steel beam framing was provided to support the 11th-floor plaster ceiling, which also served as an accessible attic/plenum space. This same ceiling framing extends over the entire building footprint and serves as the floor framing for the original mechanical penthouse. The attic/plenum is accessible from a spiral staircase adjacent to the passenger elevators that extends from the 11th floor to the main roof and a knee wall hatchway in the original mechanical penthouse. A portion of the attic/plenum was also originally accessible from the stair located in the northwest corner of the building. continued on next page

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rooftop dunnage beams that span between the main building columns, except at the south end of the eastern beam, which is supported by the northernmost roof truss (Figure E , online). In addition, drawings provided by the Otis Elevator Company, dated 1950, indicate that the original two shafts at this elevator, shown on the Sanborn Map, were converted to a single shaft elevator as a part of the renovations.

Northwest, Main, and South Stair Renovations The extent of structural renovations that occurred at the top of the referenced stairs is unclear; however, the current configuration of the roofs of these stairs appears different from that shown on the Sanborn Map. In addition, it appears the southern stair may have been renovated as a part of the 2nd-floor mezzanine and 3rd-floor infill framing in the 1960s. Figure 5. Book tiles supported by bulb tees spanning between Carnegie Steel beams at the penthouse roof.

Structural Additions At some point during the life of the building, several structural renovations and additions were constructed. The exact age of these renovations is not known and include the following:

2nd Floor Mezzanine and 3rd Floor Infill A mezzanine above the 2nd floor and infill framing at the 3rd floor were constructed between the 2nd and 4th floors at the south end of the building, within the same large assembly space originally called Witherspoon Hall. According to the National Register of Historic Places (NRHP) Inventory Nomination Form, this portion of the building may have been constructed in the early 1960s. The existing 2nd-floor mezzanine and 3rd-floor framing consist of a 2½-inch concrete slab on a 1½-inch cellular metal deck for a total of 4 inches. Historical information obtained from the Steel Deck Institute (SDI) indicates that the metal deck was a Q-Deck product manufactured by H.H. Robertson Company. The slab spans between W12 purlins and W14 girders. The 3rd-floor infill was reused for the latest adaptive reuse project; however, the 2nd-floor mezzanine was demolished to make room for a 2nd-floor loft that was subsequently not incorporated into the final project.

Passenger Elevators Upgrade According to the NRHP Inventory Nomination Form, the original passenger elevators were upgraded in the early 1960s. However, a comparison of the Sanborn Map for the original facility to the current building indicates that the only structural manifestation of this renovation appears to be the construction of a rooftop machine room penthouse located above the original elevator shafts (Figure D, online).

Roof Top Dunnage At some point during the life of the facility, steel dunnage frames were constructed above the south end of the main roof that are supported by steel wide flange columns that post down to the top of the original main building columns. There are two sets of dunnage frames that interface slightly. The southernmost dunnage frame supports an active prefabricated electrical room, while the northern dunnage frame supported mechanical cooling towers that were replaced as a part of the adaptive reuse project.

Mechanical Penthouse A one-story mechanical penthouse was constructed southwest of the passenger elevator penthouse sometime during the life of the facility. The space is accessed from the same spiral staircase described for the 11th-floor ceiling and provides the only access to the top of the main roof through the penthouse. The floor is framed with a solid, one-way 6-inch concrete slab supported by steel wide flange beams at an elevation slightly above the main roof level, which created a shallow crawl space that can be accessed via two floor hatches. The floor framing is supported by steel columns that extend down to the main building columns. The roof consists of precast concrete planks supported by open web steel joists that clear span to steel beam and column supports at the perimeter of the penthouse. A portion of the penthouse was also constructed above a large mechanical opening that was partially enclosed for the full height of the shaft with hollow, masonry pyro-bloc units that contain asbestos. This same shaft opening was infilled at each affected floor level as a part of the adaptive reuse.■ The author would like to thank Sara Wermiel, Ph.D. as the source for most of the historical information provided in Part 1.

Freight Elevator Upgrade

Part 2 of this series will provide more details on the adaptive reuse and structural investigation.

Similar to the passenger elevators, it appears that the freight elevator was upgraded during the life of the building. The structural renovations included a new machine room penthouse supported by

D. Matthew Stuart is Senior Structural Engineer at Pennoni Associates Inc. in Philadelphia, PA. (mstuart@pennoni.com)

34 STRUCTURE magazine

Figure 4. Typical Gray column.

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structural CONNECTIONS Wood-Framed Residential Construction Member and Connection Design Considerations By Becky Havel, S.E., Rose McClure, S.E., P.E., and Matt Johnson, P.E.

s architects, owners, and contractors continue to push the limits of wood-framed residential construction, manufacturers have responded by providing an increased number of specialized wood and connector products. However, in addition to specialized wood products, these same complex residential projects increasingly rely on reinforced concrete and structural steel to achieve the intended design, frequently necessitating custom connections. This article reviews the various wood products available today for framing floors and roofs and discusses connection considerations to wood, steel, and reinforced concrete.

Engineered Wood Products

I-joists are lighter, stronger, and stiffer for equivalent depths. Manufacturers have developed prefabricated connector products specific to I-joists. Though the adhesives used in the manufacturing process are rated for temporary moisture exposure, they are not intended for prolonged or permanent exposure to moisture. Therefore, they are generally more susceptible to moisture damage than traditional sawn lumber.

Glue-Laminated Timber

Prefabricated wood I-joists.

Though sawn lumber remains a common choice for traditional wood-framed building construction, engineered wood products help architects, engineers, and contractors provide wood solutions to a variety of projects where traditional sawn lumber could not otherwise be considered. Typical products include I-joists, glue-laminated timber (glulam), structural composite lumber such as laminated veneer lumber (LVL) or parallel strand lumber (PSL), and metal-plate-connected wood trusses (MPCWT). In addition, cross-laminated timber (CLT) is an increasingly popular product in commercial construction but is not discussed in this article.

I-Joists Prefabricated wood I-joists are an engineered wood product comprised of two flanges and a continuous plywood web. The flanges are typically dimensional lumber or laminated veneer lumber, and the web is typically traditional plywood or oriented strand board (OSB). I-joists are commonly available in depths of 9½, 11⅞, 14, and 16 inches, though available up to 24 inches from certain manufacturers. Flange widths vary from 1¾ to 3½ inches. Compared to solid sawn lumber,

Metal plate connected wood trusses and punched sheet metal plate connectors.

36 STRUCTURE magazine

Glulam timber is an engineered product made by bonding individual pieces of lumber, typically, along their flat face. Individual pieces of timber can be end-joined to create continuous laminations. Laminations are then face-joined to create a finished wood cross-section. Individual laminations can, and often do, vary in strength from the strongest laminations at the outer surfaces to weaker or lower quality wood at the neutral axis. Glulam comes in a variety of species with varying glue and preservative treatments. Glulam beams offer higher strengths and more dimensional stability than solid sawn lumber, primarily due to their manufacturing process. They use dimensional lumber typically dried to a moisture content of 16% or less. Glulam beams can also be created in custom shapes, including curved, to create unique profiles, and are often used in exposed, vaulted ceiling applications. Glulam beams are commonly available in widths of 3⅛, 3½, 5⅛, 5½, and 6¾ inches, but many custom manufacturers can create unique sizes and shapes.

Structural Composite Lumber Structural composite lumber is a class of engineered wood product that includes LVL, PSL, laminated strand lumber (LSL), and oriented strand lumber (OSL). They are often used as headers or transfer girders, or in long-span beam conditions. PSL can also be used as posts. Structural composite lumber is created by layering stress-graded wood veneers, strands, or flakes with moistureresistant resin. The layers are then molded into blocks and sawn to specified sizes. Like glulam, structural composite lumber members are more dimensionally stable and have higher, more uniform strength properties than traditional sawn lumber. Though the adhesives used in the manufacturing process are rated for temporary moisture exposure, they are not intended for prolonged or permanent exposure to moisture. Therefore, they are generally more susceptible to moisture damage than traditional sawn lumber.

Metal Plate Connected Wood Trusses

beams and girders introduced for these purposes are typically supported on reinforced concrete foundation walls or steel columns. An important design consideration for these types of connections is the differential movement between the wood and steel, as wood will expand and contract with seasonal ambient moisture variations. In contrast, steel will remain relatively dimensionally stable. Steel connections that confine the wood can result in splits, cracked finishes, and potential connection failures.

Metal plate connected wood trusses consist of individual timber elements connected by punched sheet metal plates, pressed into each face of the jointed connection. They are a popular engineered solution used widely in floor and roof construction. MPCWT offers a high strength-to-weight ratio, can span 25 feet or more and can accommodate various roof profiles. The deflection behavior of metal plate connected wood trusses depends on connector plate installation. These systems Wood-to-Steel Beam Connections are often more susceptible to floor bounce and vibration than a traditional sawn lumber Wood framing may be connected to steel system. Metal plate connected wood trusses beams in three primary ways: 1) face mount are typically specified with design criteria hangers connected to the side face of wood by the project structural engineer of record blocking attached to the web of the steel (SEoR) and designed by the manufacturer beam, 2) top flange mounted hangers conand their Specialty Structural Engineer (SSE). nected to wood nailers affixed to the beam In specifying the trusses and outlining the Wood joist connection at steel beam. top flange, and 3) top flange mounted hangdesign criteria, the SEoR must be clear on ers welded to the top flange of the steel beam. who will be responsible for the permanent bracing of the installed Face mount hangers are affixed to continuous wood blocking bolted trusses. The authors typically require permanent bracing to be designed to the web of the steel beam. Depending on the size of the steel beam by the manufacturer’s SSE but provide details of the permanent and the depth of the wood element, multiple layers of continuous connections at each bearing location in the base building drawings. wood blocking may be required. Engineers must design the wood blocking attachment to the steel beam for the required joist loading (gravity, lateral, and uplift), as the joist hanger manufacturers specifiPrefabricated Connectors cally do not qualify those attachments. Additionally, wood blocking attachments, typically through bolts and face-mount hangers, need Wood-to-Wood Connections to be coordinated to avoid conflicts between bolt heads, nuts, and In modern wood-framed construction, it is common to see a variety exposed bolt end and joist hanger flanges. Finally, certain hanger of sawn lumber and engineered wood products on the same project. nailing requirements may require at least three inches of blocking. However, when combining sawn and engineered wood in the same Top flange mounted hangers can be used with continuous wood plates floor system, care should be taken as they can have different moisture bolted to the top surface of the steel beam top flange. Depending on the contents and corresponding shrinkage. Sawn lumber, I-joists, or metal- width of the beam and the type of hanger, the wood plates may need to plate-connected wood trusses often comprise a building’s primary be cut to match the steel beam flange width. Engineers must design the floor framing system. Glulam or structural composite lumber beams wood plate attachment to the steel beam to transfer the required loading often function as headers and transfer girders. In some applications, to the steel beam. Wood plates can be bolted to the steel beams with glulams are used as exposed elements due to their aesthetic qualities. pre-welded threaded rods, through bolts, or powder-actuated fasteners Manufacturers have developed prefabricated metal connector prod- for thinner steel flanges. As with face-mount hangers, coordinating ucts to efficiently facilitate connections for the variety of sawn and the wood plate attachment with the top flange hangers is essential to engineered wood elements. These connectors accommodate a range prevent installation conflicts. Some hanger nailing requirements may of framing applications. They include options to mount on member- necessitate at least three inches of wood plate depth. side faces or top flanges, connect skewed and sloped members, and conceal connector flanges where desirable aesthetically or required geometrically. Prefabricated metal connectors are efficient to install due to their template holes for nails, screws, and bolts. They are also easy to determine based on load demands and easy to specify with manufacturer tabulated permissible loads and logical naming conventions. Designers need to consider the required size, quantity, and embedment of the fasteners and confirm they are compatible with the pieces to be connected as well as other connections in proximity to the connection being considered. Some manufacturers also offer replacement fastener options with load adjustment factors for commonly used nails and screws.

Wood-to-Steel Connections Steel is typically introduced in an otherwise wood-framed project to provide greater strength or stiffness in a depth-constricted area, to support discontinuous loads from multiple floors above, or to create a larger open area than can be efficiently developed in wood. Steel

Wood-to-steel column connection. Weld material reaction to paint.

continued on next page S E P T E M B E R 2 0 21

Custom wood connection at a concrete wall.

Certain top flange-mounted hangers can be welded directly to steel beams. This can be advantageous when a deep steel beam is required in a limited floor-to-floor height or depth-limited ceiling space. However, for plywood sheathed floors, consideration must be given to how the edges of the plywood diaphragm will attach to a steel beam to transfer diaphragm force if required. This is especially important where the steel beam is in place to transfer a discontinuous shear wall above. Powder-actuated fasteners, carriage bolts, or sex bolts are options. Top flange mount hanger welds to steel beams are typically ⅛- to 3⁄16-inch fillet welds with minimum lengths specified by the manufacturer. Wood blocking is not typically required behind the welded joist hanger. However, web stiffeners may be required for the steel beam based on loading configuration, the loading magnitude, and steel flange and web thicknesses. Depending on the type of construction, welder quality can vary widely. Therefore, engineers should consider both the quality of welding available and the additional labor/trade required when specifying top flange mounted, welded hangers.

Wood-to-Steel Column Connections When steel beams are introduced to a building in the floor framing, it frequently results in steel columns being introduced as well. For this discussion, the authors are not including lally columns used in traditional basement construction. Steel columns can be wide flange sections, but they are frequently hollow structural sections (HSS) to fit within wood stud framed walls. Sometimes, wood elements connect to steel columns by prefabricated welded joist hangers. Depending on the column flange width, welded joist hangers attach directly to a base steel member and can have concealed or extended flanges. For HSS columns, the joist hanger width should be coordinated with the workable flat-tube dimension and not the nominal dimension. The welds are typically 1 ⁄16-inch fillet or flare bevel groove stitch welds, symmetrically placed. The small weld size prevents burn-through of the metal hangers; as noted above, this requires skilled, qualified welders. Therefore, even for small projects, specifying an AWS-certified welder to perform these welds is valuable. Not every joist hanger can be welded, so check the manufacturer’s product literature before specifying a welded joist hanger. The manufacturer’s product literature will also provide the tested weld sizes and weld configurations along with their allowable load carrying 38 STRUCTURE magazine

capacities. The load-carrying capacity is typically the lesser value between the joist hanger capacity and the weld capacity. In seismic regions, consideration should be given to lateral forces at these hangers since manufacturers typically do not publish allowable lateral values for weldable joist hangers. As an alternative, custom wood-to-steel connections can also be designed and utilized for situations where load demand, connection geometry, or architectural requirements preclude the use of readily available prefabricated joist hangers. In this case, the options are endless. However, the downside to these connections is the high material and labor costs and engineering time relative to traditional joist hangers. Therefore, custom hangers should be designed by the structural engineer and not delegated to the contractor. For all welded connections, base metal preparation is key to a successful connection. Base steel that has surface rust should be cleaned before welding. Welding over paint is discouraged by welding codes and can cause unsightly blemishes, release toxic fumes, and compromise the quality of the weld. Steel that has been primed requires cleaning prior to welding. Engineers should recognize the potential for less sophisticated welders in residential construction and be clear in welding requirements. Consider requiring weld-specific inspections by qualified inspectors for critical welds on a project.

Wood-to-Concrete Connections Designers must pay special attention to moisture, where wood is attached to concrete elements, typically at foundation walls and footings. Wood sill plates and rim boards attached to concrete foundation walls are typically pressure treated to protect against moisture migration from the concrete. The coordination of concrete anchors, post bases, steel embed plates, beam pockets, and ledges is also important. Locating these elements before concrete foundation placement requires up-front coordination but saves time and cost overall for the project. If the contractor prefers to post-install these anchors, they should scan for reinforcement before drilling. Wood nailers and plates are typically anchored to concrete through cast-in threaded rods or J-bolts. They are often doubled up to provide adequate nailing for plywood-sheathed shear walls and diaphragms. Where double sill plates or rim boards are used with cast-in anchors, contractors must coordinate the anchor’s location and extension beyond the concrete surface to ensure adequate thread length for the nuts.

Summary As manufactured wood product options continue to increase, architects, contractors, and engineers should understand the advantages and disadvantages of each system, material, and product option. There are numerous configurations in which wood elements connect to other wood elements, structural steel, and concrete. Engineers can design with prefabricated and custom connectors and create ever-more complex and exciting structures. However, to do so successfully, engineers need to understand both the opportunities and limitations of each material product, the contractor’s experience with different systems, and the available trade people.■ All authors are with Simpson, Gumpertz & Heger in Chicago. Becky Havel is a Consulting Engineer. (rshavel@sgh.com) Rose McClure is a Senior Consulting Engineer. (rfmcclure@sgh.com) Matt Johnson is a Principal. (mhjohnson@sgh.com)

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building BLOCKS Test-Based Available Strengths for Aluminum Structures Part 1

By James LaBelle, P.E., Doc.E.

This is Part 1 of a two-part series. This installment summarizes two methods of determining available strengths from testing of aluminum structures, presents equations, and includes selected plots of calculated safety factors. The next part will discuss and compare the results from the two methods.

et’s say you have some test data for an aluminum component or structure. Will it make a difference which method is used to establish an available strength with that data? In the Aluminum Association’s 2020 Aluminum Design Manual, Part 1 – Specification for Aluminum Structures (Appendix 1: Testing [§1.3]), there are two methods for determining available strength from test data. This study included both the Allowable Strength Design (ASD) and Load and Resistance Factor Design (LRFD) procedures to compare the results from these two methods. This was an analytical study only – no tests were conducted. However, the author’s prior use of these two methods, for a few individual sets of test data, indicated that allowable strengths from Method 1 tended to be more conservative than those from Method 2. The number of samples considered ranged from 7 to 50 and the coefficient of variation from 4% to 20%. For Method 1, safety and resistance factors from the Specification are applied to calculated nominal strength values based on test statistics. Method 2

Figure 1. Statistical coefficient (K) vs. the number of samples (N).

available strengths are found by applying calculated (using test statistics and other parameters) safety and resistance factors to test averages. Of the many possible input values for Method 2, the current study was restricted to default values for the parameters. Plots of Method 2’s calculated safety factors provide a sense of the wide range of possible values. For the set of variables considered, this study (including Part 2) shows that available strengths based on Method 1 are generally, but not in all cases, less (by widely varying percentages) than corresponding values from Method 2.

Method 1

Figure 2. Safety factor SF2: rupture of beams.

40 STRUCTURE magazine

This method is simpler to use than Method 2 and has fewer input parameters. As given in the Specification for various limit states, the pertinent safety factors (Ω) for ASD range from 1.95 to 3.0, and the resistance factors (φ) for LRFD from 0.75 to 0.50. To find the allowable strength (R1Ω), this method uses a calculated nominal strength (RN1), which per Method 1 is based on test statistics, divided by a safety factor chosen separately depending on the limit state. Similarly, the design strength (R1Φ) is the product of the resistance factor and RN1. The nominal strength (RN1) is a statistical lower bound (99% exceedance, with 95% confidence) on strength, which is based on test average (R TM), sample standard deviation (σx), and a statistical coefficient (K; Figure 1) that is based on the number of samples (N ). Minimum N is 3. For 7 ≤ N ≤ 50, K varies from 4.641 to 2.863. K accounts for uncertainty about the possible difference between the sample and population

Figure 3. Safety factor SF2: rupture of tension members.

Figure 4. Safety factor SF2: tapping-screw connections.

standard deviations; it increases at an increasing rate as N becomes smaller, especially for N less than about 20. To compare Method 1 available strengths with Method 2 available strengths, for ASD and LRFD, the applicable equation for the nondimensional ratio of available strength to average test strength was determined for each method. Variables include: K = statistical coefficient N = number of samples RN1 = calculated nominal strength RTM = test mean (average) strength σx = sample standard deviation φ = resistance factor for LRFD Ω = safety factor for ASD The Method 1 allowable strength (ASD) is: R1Ω = RN1 / Ω = (RTM - Kσx) / Ω (Eqn. 1) Now divide the allowable strength by the test average. Note that the coefficient of variation CV = σx / RTM: R1Ω / RTM = (1 - KCV) / Ω (Eqn. 2) The Method 1 design strength (LRFD) is: R1Φ = φRN1 = φ (RTM - Kσx) (Eqn. 3) Divide the design strength by the test average: R1Φ / RTM = φ (1 - KCV) (Eqn. 4)

• VF (= 0.05 for structural members and mechanically fastened connections; 0.15 for welded connections): fabrication variation • VQ (= 0.21): load variation • βo (= 2.5 for beams and columns, 3.0 for tension members and 3.5 for connections): target reliability index. To determine allowable strengths (R2SF) for Method 2, the average test strength is divided by a calculated safety factor (but not less than a minimum), which depends on many statistical variables. To distinguish the safety factor (Ω) in Method 1 from that in Method 2, the notation SF2 is used here for the Method 2 safety factor. SF2 = e ψ (1.05α + 1) / [MmFm (α + 1)] (Eqn. 5) where, ψ = βo (VM 2 + VF 2 + CNVP 2 + VQ2 )0.5 Here, VP = CV, which is the coefficient of variation for the test results, and CN = (N 2–1) / (N 2–3N ). The minimum N is 4. For ASD, SF2* is the greater of SF2 and the applicable value of Ω in the Specification. The allowable strength is: R2SF = RTM / SF2* (Eqn. 6) For Method 2, the ratio of the allowable strength to the test average is: R2SF / RTM = 1 / SF2* (Eqn. 7) The calculated resistance factor in Method 2 is denoted here as φ2 to distinguish it from the resistance factor (φ) used in Method 1. φ2 = 1.5 MmFm / e ψ (Eqn. 8) For LRFD, φ2* is the lesser of φ2 and the applicable value of φ in the Specification. The design strength is: R2Φ = φ2* RTM (Eqn. 9) For Method 2, the ratio of the design strength to the test average is: R2Φ / RTM = φ2* (Eqn.10)

Method 2 See the Specification for further Method 2 details. As an example, this method had previously been applied to data for screw pull-out from screw chases. For the current more general study, the default values of various parameters were employed: • α (= 0.2): dead-to-live load ratio • Mm (= 1.00 for rupture): material factor • Fm (= 1.00): fabrication factor • VM (= 0.06): material variation

Safety Factors Plots of calculated safety factors (SF2) and the required minimums are shown in Figures 2 through 5. Each plot is based on a different S E P T E M B E R 2 0 21

Where a calculated safety factor (SF2) exceeds the corresponding minimum (Ω), then SF2* equals SF2. In this case, Method 2 determines an allowable strength that is less than RTM / Ω.

Resistance Factors Plots are not shown for calculated resistance factors (φ2), but the Table provides a sampling of results for each condition. If a calculated resistance factor (φ2) is less than the corresponding upper limit (φ), then φ2* equals φ2. In this situation, Method 2 provides a design strength that is less than φRTM. For both ASD and LRFD in Method 2, the available strengths are based on the test averages. However, the main body of the Specification bases available strengths on nominal strengths (RN), which are in most cases less than test averages. This means that a test average divided by SF2* could produce an allowable strength that exceeds RN / Ω. Similarly, a test average multiplied by φ2* could result in a design strength that is greater than φRN.

Method 1 vs. Method 2

Figure 5. Safety factor SF2: welded connections.

combination of βo, Mm, and VF. VF equals 0.05, except for Figure 5, where it is 0.15. SF2 decreases as N increases (CV constant) and as CV decreases (N constant) for all plots. For large N and small CV, the decrease in SF is imperceptible in the figures. In these figures, a small number of samples (combined with intermediate or large CV values), or a large CV, typically results in a relatively large SF2. In each Figure, the range of calculated safety factors (SF2) is: • Figure 2 (beam rupture): 2.39 to 1.78, but the required minimum is 1.95. • Figure 3 (rupture of tension members): 2.84 to 2.00, which exceeds the minimum of 1.95. • Figure 4 (tapping-screw connections): 3.37 to 2.24, but the minimum is 3.0, which governs over most of the calculated values. • Figure 5 (welded connections): 3.71 to 2.58, all of which exceed the minimum of 1.95.

Note that KCV > 0 for CV > 0. Given this, Equation 2 (Method 1’s ratio of allowable strength to test average) is less than Equation 7 (Method 2’s ratio for allowable strength) if SF2* equals Ω. Similarly, Equation 4 (Method 1’s ratio of design strength to test average) is less than Equation 10 (Method 2’s ratio for design strength) if φ2* equals φ.

Acknowledgments The author acknowledges the constructive comments during article preparation from J. Randolph Kissell, P.E., and Scott Walbridge, Ph.D., P.Eng.■ References are included in the online PDF version of the article at STRUCTUREmag.org. James LaBelle is a Consultant with experience in the design and investigation of aluminum and other structures. He is retired from CSD Structural Engineers, Milwaukee, WI, and is a member of the Aluminum Association’s Engineering Design Task Force, FGIA (formerly AAMA), and ASTM. (jlabelle@csd-eng.com)

Table of bounding values of φ2 for LRFD.

Case

Calculated φ2

Upper limit

Comment

Min.

Max.

Beam Rupture

0.63

0.85

0.75

Limit < max

Tension-Member Rupture

0.53

0.76

0.75

Limit < max

Tapping-Screw Connections

0.45

0.68

0.50

Limit < max

Welded Connection

0.41

0.59

0.75

Max < limit

42 STRUCTURE magazine

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INFOCUS Preschool Engineering Books By Linda Kaplan, P.E.

hy? Why? Why? Every preschooler’s favorite question. Annoying at times but important for building a knowledge foundation of how the world works – also key to how engineers, including structural engineers, approach new challenges and advance the profession. Rather than shutting down these questions, encourage problem-solving and creative thinking. Just as books for the youngest readers presented in Part 1 of this series of reviews introduced the language of engineering, several books aimed at preschool- to early elementary-aged children start to introduce engineering principles and approaches. With the recent overall push to introduce STEM fields, including engineering, to younger children and especially young girls, there has been an influx of children’s books published on the subject in the last ten to fifteen years. Many particularly focus on female characters with encouraging “you can do anything” type messages. Older publications seem to be more male-dominated and take a “cool technology” approach. Ultimately, there is a place for all different types of stories, with both boys and girls pursuing their dreams, problem-solving, and being introduced to the tools and language of the field. It would also be good to see increased diversity in future publications as minorities are not well represented.

The Most Magnificent Thing by Ashley Spires (Kids Can Press) explores the ideas of problem solving “When she is finished, she steps and perseverance in a spirited story of a little girl and her assistant (her dog) as they try to make back to admire her work…. They The Most Magnificent Thing. are shocked to discover that the Despite knowing exactly what she wants to build, try after try. the little girl cannot get it right. She thing isn’t magnificent. Or good. grows increasingly frustrated as her prototypes pile up until she finally must take a break and go It isn’t even kind-of-sort-of okay. for a walk. Coming back, she sees all her failed It is all WRONG. The girl tosses it attempts and realizes that there are parts of each that she can use. Ultimately, she is able to make aside and gives it another go.” the magnificent thing! While the book does not use the language of engineering or call it out by name, the methodologies presented and the iterative design approach are core engineering principles. The “can-do” attitude of the little girl, along with her frustration when things do not go smoothly, are easy for kids to relate to and a good reminder to everyone that setbacks are a normal part of life, are not an indication that you are a failure, and do not need to end a project. Introductory Calculus for Infants by Omi M. Inouye (omionline.ca) is an alphabet book telling the story of lonely x and fabulous f as they become friends. Despite the title, the language is beyond infants and best suited for the preschool crowd. This book introduces 26 mathematical terms or concepts that form the basis of advanced math, including calculus, utilizing a minimalistic plot of building friendship as f(x) explores their possibilities. In much the same way the books for babies served to make the language of engineering familiar, this serves to establish the language of math, taking away the scary factor that comes from the unknown. There could be long-term positive impacts to engineering and STEM fields overall if the average high school senior was able to approach Calculus not as a scary, hard class reserved for nerds but instead as an approachable subject with a language they grew up with. 44 STRUCTURE magazine

“You can be fun if you want to be! With me by your side you can be anything! You can be Absolute! You can be Boundless!”

Rosie Revere, Engineer by Andrea Beaty (Harry N. Abrams) is another story with a core message of perseverance and a “follow your dreams” approach aimed at young girls. Rosie dreams of becoming a great engineer but hides her inventions away, afraid of failure and ridicule. Finally, her aunt sees her work and teaches her that failure is the first step to success. While engineering is the example used, the book is as much about believing in yourself and not worrying about what others think as it is about engineering. Rosie’s story holds the “girls can do anything” message at the forefront of the narrative, both through Rosie’s dream of becoming an engineer and through the aunt, also an engineer, who encourages her. The simple rhyming prose keeps the book to an appropriate level for preschoolers and helps hold their interest.

“But when no one saw her, she peeked in the trash for treasures to add to her engineer’s stash. And late, late at night, Rosie rolled up her sleeves and built in her hideaway under the eaves.”

Mike Mulligan and His Steam Shovel by Virginia Lee Burton (HMH Books for Young Readers) is a classic story that has stood the test of time better than most. Mike Mulligan and his steam shovel “When people used to stop Mary Anne have been involved in some great conand watch them, Mike struction projects, but no one wants to hire them with new technology available. Mulligan and Mary Anne Although an old-fashioned setting focused on friendship and hard work, Mike and Mary used to dig a little Anne still prove themselves useful and relatable. faster and a little better.” Engineering is more of a backdrop to the story here; however, the example projects that Mike Mulligan and Mary Anne worked on, referenced at the beginning, offer an excellent segway for side conversations and further discussion. Additionally, having a piece of equipment as the main character engages and teaches at the preschool level in much the same way it does for younger audiences. It is hard to know what bit of information will strike a child’s curiosity and lead them down the path to an engineering or construction career.

Look at That Building! A First Book of Structures by Scot Ritchie (Kids Can Press) “Martin is watching the construction has a specific focus on structural engineering. Five friends wanting to build a workers pour concrete. This foundation doghouse head on a trip to the library for is going to be very strong. A foundation more information and observe the buildings they see on the way. is the lowest part of a building. It Far more technical and less of an engaging storyline than the other books discussed, keeps the rest of the building stable by the engineering concepts are front and anchoring it securely in the ground.” center. Foundations, beams versus columns, framing, and even green roofs are introduced. This book will be more appropriate for the early elementary age range and for a child who has already shown some interest in these topics. Pull it out when you try to explain how to get that perfect Lego® tower to stand just a little bit taller. The selection of engineering-themed books for preschool and early elementary ages is diverse in storyline and approach. Some use engineering in the background to tell a classic story of friendship or perseverance, and others put the language of engineering front and center. Since children in this age group will have stronger opinions on which stories they will listen to than babies will and come back to again and again, picking the best fit is more important. As previously discussed, the goal is to have engineering and technical

concepts and language be enough a part of everyday life that it does not seem new or scary when they are older. The right book may just inspire a future engineer.■ Linda Kaplan is a Project Engineer with Pennoni in Pittsburgh, PA. She has 2 daughters, ages 3½ and 1½. The Lego table has already replaced the coffee table. (lkaplan@pennoni.com)

S E P T E M B E R 2 0 21

INSIGHTS Does Building Taller with Wood Make Sense? By Paul Fast, P.Eng., Struct. Eng., P.E., FIStructE, IngKH

or centuries, structural engineers have been intrigued by the unique allure of designing buildings that rise higher, span longer, and assemble materials in new and counterintuitive ways. One of the current frontiers is building taller with wood. The introduction of cross-laminated timber has accelerated this pursuit, most notably in Europe and North America. While pushing the envelope is a noble objective, doing so just to secure bragging rights misses the mark. Just as the structural engineering community asks if it makes sense to build ever taller with concrete and steel, the same question can apply to tall wood towers. Having been involved in the design of several tall timber buildings, including the 18 story TallWood House student residence at the University of British Columbia (at the time of completion, the tallest wood high rise in the world), the author offers some thoughts toward answering this question, beginning with some of the challenges in building tall with wood. The taller the building, the more heavylifting that has to be performed by the columns and shear walls. With timber falling

well shy of steel and concrete in the strength category, this results in larger columns and shear walls and decreased useable floor area in a tall wood building. In addition, if wood columns are exposed, charring requirements for fire protection will exacerbate this differential. Tall timber buildings typically weigh less than their concrete and steel cousins; however, these advantages begin to diminish when breaking through the 30 to 40 story barrier, where controlling wind accelerations can benefit from the increased mass. Using CLT shear walls in buildings taller than 10 to12 stories, especially in severe seismic regions, can quickly become a futile exercise as steel connections between The Arbour, George Brown College, Toronto. Courtesy of wall panel lifts become unduly Moriyama & Teshima Architects. large, long, and labor-intensive. This begs the question, “Why not use contin- The building is being monitored to underuous steel columns or zone steel in concrete stand long-term behavior better. However, instead to manage overturning moments?” as structural engineers break through the If we allow ourselves to be nudged toward a 25 to 30 story barrier, rigorous analysis and hybrid solution with a steel or concrete core fine-tuning becomes a more pronounced chalfor lateral resistance in taller buildings, then lenge. Introducing further hybridization by the problem of differential vertical settle- substituting steel columns for wood columns ment between the concrete or steel core and neatly circumvents the differential deformawood columns arises. The author’s team tion problem and still preserves the large found this a manageable problem on the volume of wood in the building. Another 18-story TallWood House, compensating alternative that the author’s firm investigated for future differential elastic, shrinkage, and for a 450-foot tower was to essentially stack creep deformation by ‘cambering’ the floors three identically-framed TallWood House slightly between cores in the upper stories. blocks on each other. A steel-perimeter truss transfer structure was then introduced at the one-third and two-thirds building height levels supported by four steel corner columns, effectively allowing us to skate around the vertical deformation problem. Prolonged exposure to rain can pose additional challenges when building taller with timber in wetter climatic regions. While CLT is dimensionally quite stable under wet weather conditions, protecting fit-out work that wants to progress at lower levels of the building Tallwood House, University of British Columbia. Left photo courtesy of Michael Elkan; right photo courtesy of Seagate Mass Timber Inc. against moisture ingress requires

46 STRUCTURE magazine

lend themselves to timber construction, with off-site component prefabrication resulting in faster construction. In addition, office buildings with warmer ambiance and ample daylight also can be expected to secure higher rental income. In light of these factors, does it make sense to build taller with wood? In the right location for the right building type, it absolutely does. And from a sustainability perspective, the most current research suggests we should keep moving forward.

However, let’s not force a square peg into a round hole. When your timber structure starts groaning, remember that other materials will gladly help out.■ Paul Fast is a Partner with Fast + Epp. Paul was recently awarded the Institution of Structural Engineers’ Gold Medal for 2021 to recognize his world leadership in the design of architecturally exposed structures. (mail@fastepp.com)

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additional rain protection management. As a minimum, the structure should be constructed and wrapped with the building envelope as quickly as possible. However, notwithstanding these challenges, many factors speak in favor of building taller with mass timber. Allowing for material fabrication lead times, erection schedule savings can be one of the leading benefits. For example, it took only nine weeks to erect seventeen stories of mass timber construction at TallWood House. With a carefully detailed, efficient frame and a dedicated crane, mass timber structures can be erected at a pace of 10,000 square feet per day. Mass timber construction can be erected in a quieter manner, which is increasingly desirable in urban environments, where neighboring building residents often must tolerate prolonged periods of excessive noise on tall building projects. Wood is also a sustainable building choice when considering the renewability of the resource and metrics such as embodied carbon. Current trends in office building construction relating to interior ambiance will also drive more tall buildings toward wood construction. The biophilic nature of building with wood has undoubtedly caught the attention of clients and users alike, not to mention the sustainability signal that a building constructed of material grown by the sun sends to the community at large. Time will tell how long this trend lasts. The bottom line to all these considerations will, in the end, likely be largely influenced by cost. Interestingly, when tasked with designing the structure for TallWood House, the question was not “Can it be done?” (yes), or “Should we do it because it would be the tallest timber tower in the world?” (vain), but rather “Can we do it economically?”. This forced the team to determine, very near the outset of design, “Can we build a timber structure, with all its proposed sustainability advantages, for the same price as a more conventional steel or concrete building?” If the answer were no, the project would be constructed with concrete. Well, we ended up coming close enough to see the project built with timber. However, this answer is a function of many variables, including, most importantly, the cost of concrete and steel construction in a given region. Certainly, tall buildings in dense urban centers with high construction costs can

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S E P T E M B E R 2 0 21

business PRACTICES The High Cost of Poor Leadership By Rebecca Zucker

s an executive coach, I am often brought into organizations to help leaders develop critical leadership and management skills – notably “soft skills” that may have been lacking in the past but were not necessarily a derailer. Often, these leaders have strong technical expertise (i.e., engineering, finance, sales, law, etc.) and are promoted based on their technical capability and performance, not their leadership ability or potential. A recent study showed that the costs of such inefficient promotion decisions are often high (https://bit.ly/3iAKnOZ). Most leaders, when promoted, are elevated to a position that requires a different skill set than what has made them successful in the past. These increasingly important required skillsets are typically more peoplefocused – motivating, inspiring, developing, and empowering others, creating conditions for others to succeed, and managing performance. Unfortunately, either unsure of what the job really requires or fearful that they will not be good at new job responsibilities and requisite skills, these leaders often end up doubling down on what they know and do best – which is getting the work done. My colleagues and I refer to this pattern as a “doom loop” that only gets them into trouble with their team members and superiors. Task-focus is what these leaders know best and made them successful in the past and made them feel productive. It is tangible progress they can show to themselves and others. However, as explained in my Harvard Business Review article, Why Highly Efficient Leaders Fail, many overly task-focused leaders feel that having greater people-focus, like taking time to connect with team members and inspiring and developing others, slows them down. On the contrary, research shows that more people-focused leadership styles, such as a coaching leadership style, results in tangible improvements to bottom-line financial results (https://bit.ly/3s5M0ra). So “soft” skills have a “hard” or tangible benefit to all involved. Likewise, overly task-focused leadership styles, like the pace-setting leadership style, have been shown to negatively impact organizational climate and bottom-line results. If you are concerned that showing a greater focus on people slows you down, think of the elite, high-performing U.S. Navy Seals 48 STRUCTURE magazine

unit, whose maxim is “Slow is smooth. Smooth is fast.” Taking the time to focus on your people – coaching and giving feedback, and ensuring team members are aligned and have the resources and support they need – gets you further faster. Since a leader’s job is to get results through people by creating the conditions for their team to be successful, and their people ultimately perform the tasks, it pays to focus equally on the people side of the equation, not only the task at hand. Below are some ways that you can do this. Listen for emotions and show empathy: Most task-driven leaders listen for facts and information. While useful, this ignores other critical information about team members and the team as a whole. Are they tired, bored, frustrated, angry, ambivalent, annoyed, defeated – or excited and optimistic? Much of this is conveyed by their voice tone, word choice, what is being said, and what is not being said. Practice reflecting the emotion you believe you are hearing from them. If you are incorrect, it allows them to correct you and share what they are feeling. For example, suppose they sound angry but deny feeling angry. In that case, you can also challenge them appropriately and probe further about what is going on for them to clarify and surface their underlying feelings. Understanding the emotions that others are experiencing ultimately allows you to both express empathy and build deeper connections with your team members. Take an interest in others’ career development: Know your team members’ goals and aspirations. This knowledge should inform the work you delegate to them and can result in greater motivation, engagement, and retention. Incorporate these goals into your regular one-on-ones to go beyond discussing work status. Taking an interest in their career development also involves giving ongoing, real-time coaching and feedback, especially when it includes delivering a tough message. These situations create genuine learning opportunities

Taking the time to focus on your people…gets you further faster. if communicated openly and focused on mutual goals and the individual’s behaviors (versus the person). You are doing no one a favor – the individual, the team, or yourself – by shying away from difficult conversations. You can frame your motivation in giving them the feedback as wanting them to grow and be successful, which shows the feedback is coming from a caring place and that you want them to succeed. Notice your own impatience and manage it: Self-observation and self-management are two related, critical skills for leaders to develop, especially those trying to balance people-focus with task-focus. First, notice what triggers your impatience. Then, ask yourself some good reflective questions, like “What is my concern about slowing down?” “Where am I driving too hard or fast unnecessarily?” Pausing to notice and reflect also allows you the opportunity to try a different approach. For example, you might try to teach a team member struggling with a new skill or a new way of doing something, explain how their task fits into the larger vision for the project, or even acknowledge their contribution. Balancing task- and people-focus is not a “one and done” item to check off your list. Instead, it is an ongoing effort that requires intention, regular practice, and continuous rebalancing. The above-mentioned strategies can help you achieve and maintain this balance.■ Rebecca Zucker is an Executive Coach and Partner at Next Step Partners. She is a regular contributor to Harvard Business Review and Forbes. Follow her at @rszucker.

S E P T E M B E R 2 0 21

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structural FORUM Building Design Collaborator or Implementing Technician? By Julie Mark Cohen, Ph.D., P.E., SECB

n April 29, 2021, I attended a virtual meeting of SEAoNY’s President’s Breakfast Roundtable: Engineering PostPandemic. In my assigned break-out room, I listened to the attendees talk about the push for “sustainability” and, to achieve this goal, “optimization” of structural framing. This article discusses the evolution of engineering over the past decades and notes the consequences of misunderstanding “optimization.” It reiterates my observation that a lack of 3-D structural framing systems resulted in damage to many structures during the 1994 Northridge earthquake. These structures were pulled apart in the same manner as they were designed – that is, as a collection of two-dimensional vertical and horizontal planes of framing. The 1960s brought us: • The first civil engineering papers were published on the optimization of the physical infrastructure for electric power transmission. • The widespread introduction of master’s degree programs in structural engineering. • The continuation of the post-war construction boom. • External pressures on structural engineers from tight budgets and short schedules were unprecedented. • Changes in structural safety criteria in codes from allowable stress design to ultimate strength design for concrete and load and resistance factor design for steel. • The disproportionate progressive collapse of London’s 22-story Ronan Point residential building in May 1968. • Education of university structural engineering students was conducted under the umbrella of ASCE (1950-1982), which required general education courses in place of practice-relevant courses in related engineering fields. • In most cases, structural engineers collaborated with architects during schematic design and, often, conceptual design. During the 1970s: • Bay sizes of office buildings of all heights were increased, up to 30 feet by 40 feet, to provide building owners with the flexibility of larger open spaces for tenants. • Often, the floor framing spanned large distances from the center core to the exterior walls. 50 STRUCTURE magazine

• Architecturally, individual offices were replaced with cubicles to take advantage of open floor plans. The expectation was that each tenant could change the layout of their cubicles as needed. In reality, cubicle partitions were rarely moved, showing that the arguments about needing large open spaces did not necessarily hold true at the expense of more columns and fullheight walls, which could have been constructed without interfering with office activities and communications. By the 1980s, the following occurred: • Textbooks continued to emphasize the design and analysis of 2-D planes of structural framing, with little emphasis on 3-D framing. As such, the notion of 3-D structural framing design and optimization never penetrated structural engineering education, research, or practice. • Master’s degree programs geared for practitioners were commonplace. • Recessions came and went as in previous decades, but external financial pressures that shaped the design decision-making of structural engineers were omnipresent. • Structural failures were increasing and recurring, more and more without recognizing and using well-established knowledge published in various documents in structural engineering and related engineering fields. For example, knowledge on fatigue and fracture dates back to at least the late 1800s, but little, if anything, appears in structural steel design textbooks. Also, the following failures due to fatigue and fracture at geometric discontinuities in steel structures have rarely, if ever, been mentioned in structural steel design textbooks: 1943 Liberty Ships, 1954 Comet De Havilland airplanes, 1967 nonredundant Silver Bridge, 1979 Kemper Arena, 1980 Alexander L. Kielland semi-submersible offshore drilling platform, etc. • The standards for structural engineering education (in the same format as prior decades) were transferred in 1983 to the oversight of ABET (an engineering accreditation entity) with

“

Structural engineers have been failing to understand the difference between structural optimization and minimization.

ASCE’s guidance. However, no successful attempts were made to establish structural engineering as a separate undergraduate degree program. Undergraduate students interested in structural engineering were therefore unable to take many relevant courses but instead were required to take many non-engineering courses that subtracted from the engineering education. • The collaboration of structural engineers with architects was waning due to architects’ perceptions (often relayed to them in undergraduate university education) questioning the need for participation early on by structural engineers compounded with budgetary constraints of owners. The 1994 Northridge earthquake brought dozens of examples of structural engineers having minimized locations of lateral load resistance. However, many of them referred to this design decision as having “optimized” the sizes. This is not correct. Optimization requires a 3-D structural framing system. Optimization includes simultaneous assessment of (1) structural efficiency, which includes, in its simplest form, uniformity and regularity of horizontal and vertical framing (i.e., stress and deformation) with minimal use of abrupt stress-flow physically-adjacent lateral load-resisting and gravity-supporting framing, (2) 3-D system reliability which includes but is not limited to structural redundancy and alternate load paths, and (3) cost of materials and labor. Instead of 3-D structural framing systems, the result was a lesser number of momentresisting frames with very large columns and beams and the cost savings of fewer un-tested welded beam-to-column connections. Again,

(Analyses without experimental data do not validate anticipated performance.) Clearly, over time, the role of most structural engineers has shifted from building design collaborators to becoming implementing technicians. However, structural failures have increased in the same time period, at least 40 percent of which have been caused by recurring shortcomings in engineering design decision-making (i.e., cognitive errors). Are these failures not as important as “sustainability”? The trajectory is undeniable. Is it acceptable?■

References are included in the online PDF version of the article at STRUCTUREmag.org. Julie Mark Cohen is a Consulting Structural and Forensic Engineer and a Science and Technology Studies Scholar specializing in the History of Engineering Design. Her research is entitled “Cognitive Errors in Recurring Failures of Engineered Artifacts.” Her eventual book is entitled “Unintentional Engineering Failures by Design.” (jmcohen@jmcohenpe.com)

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this is minimization, not optimization. “Size effects” and risk-laden extrapolation to larger sizes, as well as evaluation of test results, are essential parts of the overall failure story, as noted in the Federal Emergency Management Administration’s (FEMA) FEMA 355E, The State of Art Report on Past Performance of Steel Moment-Frame Buildings in Earthquakes. That is, structural engineers have been failing to understand the difference between structural optimization and minimization. Lateral load-resisting framing members became relatively large while gravity loadcarrying framing was smaller, resulting in an inefficient distribution of structural resistance and abrupt changes in resistance (often not tested). Minimization was shown to affect structural performance adversely. By the early 2000s, structural engineers rarely, if ever, participated in the conceptual design and even the schematic design of buildings. In 2018, Angie Sommer, S.E., summed this up as follows: “We [structural engineers] come on usually after a building is somewhat formed and we are working with the architect and the owner to figure out how we can fit [structural framing] into the allotted spaces.” This lack of early-on participation in the design process has also been observed in university education. “Design” courses either do not include architecture students or, if they do, they mimic practice with little, if any, early-on collaboration between structural engineering and architecture students. At the SEAoNY meeting, the New York City structural engineers in attendance presented an argument that optimizing structural framing by providing larger open spaces for building owners, an architectural requirement, promotes sustainability by increasing longevity. They claimed that this would provide alternatives for the buildings’ future use. If this was optimizing, why did it take until 2021 to do this? Why has no one questioned the viability of the “open space” concept? This sustainability, however, minimizes the locations of lateral load resistance and also minimizes locations of gravity load-carrying framing. As a result, the possibilities are lessened for structural redundancy and alternate load paths unless something costly is introduced, such as using multiple, deep transfer trusses or deep beams, thus increasing the heights of the buildings. In addition, risk is introduced by using larger connections and other sub-assemblages that likely have not been tested.

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S E P T E M B E R 2 0 21

NCSEA

NCSEA News

National Council of Structural Engineers Associations

NCSEA's Grant Program Can Support Your SEA's Next Initiative The NCSEA Grant Program, supported by the NCSEA Foundation, awards SEAs funding for projects that grow and promote their SEA and the structural engineering profession in accordance with the NCSEA Mission Statement. The goal of the Foundation and the Grant Program is to advance the profession through technical development, education, and outreach.

SEAs have been previously awarded grants to: • Create a cohesive library of STEM videos and funding to enhance the association's remote site visits. • Enhance their Mola Model Initiative and Student Outreach. • Support the EERI San Diego-Tijuana Regional Earthquake Scenario Study and a Special Wind Region Study. • Launch a diversity and inclusion initiative. • Establish an Engineers Alliance for the Arts and Student Impact Project at a local high school. • Strengthen their Young Member Group's student outreach programs. Applications are due November 9, 2021. Application requests must be reviewed and approved by the Member Organization before being submitted to NCSEA for consideration.

In Need of High-Quality, Expert-Led, Affordable Education?

NCSEA's Webinar Subscription Plan is an cost-effective option for members and nonmembers seeking high-quality continuing education. By subscribing to NCSEA webinars, you are subscribing to webinars developed by leading experts at an incredible value (as low as $30/ hour). With at least 30 live webinars per year and a recorded library of over 170 webinars, NCSEA's Webinar Subscription is designed for the individual engineer as well as the firm; no matter the size, this subscription plan can work for you! Webinars are available whenever, wherever you need them. Multiple users at the same office, together or remote, can take advantage! Subscribe now by visiting www.ncsea.com and don't miss another webinar in 2021!

Are You Prepared for the Next NCEES PE Structural Exam?

The next NCEES PE Structural Exam is October 21 and 22; start preparing with NCSEA’s on-demand course. The SE Refresher & Exam Review Course is the most economical PE Structural Exam Preparation Course available with 30 hours of instruction, preparation tips and problem-solving skills to pass the exam. All lectures are up-to-date on the most current codes with handouts and quizzes available. This NCEES PE Structural Exam Preparation Course allows you to study at your pace but with instant access to the material and instructors through the exclusive virtual classroom. Several registration options are available; visit www.ncsea.com to learn more.

ESCN Virtual Career Fair

NCSEA is a partner in the Engineering & Science Career Network (ESCN), an alliance of associations built exclusively for professionals working within engineering and science industries. Annually, the ESCN hosts a Virtual Career fair that is targeted specifically to those looking for a career in the engineering field. Register to learn about the companies recruiting through informational booths, live chat interactions, and other features. The Career Fair will take place September 23, 2021. For more information visit: https://bit.ly/3kjmmfh. Are you an employer looking to engage with and meet a high number of skilled engineering candidates? This virtual career fair is the solution for you! Visit the link above to learn more.

follow @NCSEA on social media for the latest news & events! 52 STRUCTURE magazine

News from the National Council of Structural Engineers Associations

NCSEA is Excited to See You Next Month in New York In today’s world, you need dynamic solutions that can adapt to their environment. The 2021 Structural Engineering Summit is a solution designed for you. This year's event is an immersive in-person and virtual event perfect for reconnecting with other structural engineering professionals. Join us in-person in New York City at the Hilton Midtown on October 12-15, or virtually from September 27 to October 21. Attendees of both events will have access to engaging educational sessions led by experts in the field, riveting networking opportunities, and access to our industry leading Trade Show. We understand that things are changing and evolving on a daily basis. To ease any worries, not only is Summit registration worry-free guaranteed, but if attendees purchase a Virtual registration now, they will have the opportunity to upgrade to All Access later (locking in the current rate). Additionally, we are in weekly contact with our host hotel to stay on top of any procedures that will be in place in New York City, as well as the hotel. At NCSEA, we will be abiding by the CDC recommendations/rules, as well as the city/state of New York. As the situation evolves, so will our approach to keeping attendees safe. Register before fees increase on September 18th to save $100 on your All Access registration. If joining us in New York, don't forget to reserve your stay at the Hilton Midtown. Learn more and secure your spot on www.ncsea.com.

Call for NCSEA Committee Volunteers

Are you interested in volunteering with NCSEA? The Council depends on its members to get involved to help advance our mission and further develop our partnership. Our volunteers help educate on codes and standards, develop publications, create courses, advocate for safe structures and post-disaster recovery, and so much more. If you are a new volunteer interested in serving on an NCSEA committee, please visit www.ncsea.com to complete the Committee Volunteer Application. Most committees admit new members on a rolling basis while others add members only once per year. More information about NCSEA committees can be found by visiting www.ncsea.com/committees.

NCSEA Webinars

September 16, 2021

Timber Engineering for Structural Engineers Jim DeStefano, P.E., AIA, F.SEI

This informative and entertaining presentation will describe some of the challenges of timber engineering and how to avoid common pitfalls and mistakes. September 30, 2021

Allowable Stress Design vs. Strength Design: A Masonry Cage Fight Richard Bennett, Ph.D., P.E.

This presentation compares the differences, advantages, and disadvantages of each design between allowable stress design (ASD) and strength design (SD) methods, with the comparison being made through examples. October 26, 2021

Design of Insulating Concrete Form Walls for High Winds Lionel A. Lemay, P.E., S.E., and Scott Campbell, Ph.D., P.E.

The presentation will discuss preliminary wall sizing and placement along with structural design considerations including design details and construction inspections. Design for tornadoes and hurricanes and seismic forces, including storm shelters, will also be discussed.

Courses award 1.5 hours of Diamond Review-approved continuing education after the completion a quiz. S E P T E M B E R 2 0 21

SEI Update Advancing the Profession

Access Progressive Collapse and Structural Health Monitoring Collection In response to the collapse of the Champlain Towers South in Surfside, Florida, the ASCE Library has assembled papers highlighting the importance of condition assessment of existing buildings. Available for free through September 15 at https://ascelibrary.org/SHMcollection. Share knowledge to help create a safer built environment. Share or browse structural safety info or contribute a safety issue. Sign up for email and newsletter updates. www.cross-safety.org/us

Membership

2021-2022 SEI Board of Governors

The SEI Board is composed of two representatives from each of the five SEI Divisions (Business & Professional, Codes & Standards, Global, Local, and Technical Activities), one young professional appointee recently added through SEI Bylaws amendment, one appointee from ASCE, the SEI President, SEI Past President, and the SEI Director as a nonvoting member. The following includes recently elected SEI Board officers. Welcome to Governors starting terms October 1 – shown in italic: Victor E. Van Santen, P.E., S.E., F.SEI, F.ASCE, SEI President Randall P. Bernhardt, P.E., F.SEI, F.ASCE, SEI Treasurer Laura E. Champion, P.E., F.ASCE, SEI Secretary John Cleary, Ph.D., P.E., F.SEI, M.ASCE Aimee Corn, P.E., M.ASCE Joseph G. DiPompeo, P.E., F.SEI, F.ASCE, SEI Past President Jerome F. Hajjar, Ph.D., P.E., F.SEI, F.ASCE Edwin Huston, P.E., F.SEI, M.ASCE

Takahiko Kimura, P.E., F.SEI, M.ASCE Chad Schrand, P.E., F.SEI, M.ASCE Donald Scott, P.E., F.SEI, F.ASCE, SEI President-elect Stephanie Slocum, P.E., M.ASCE J. Greg Soules, Ph.D., P.E., P.Eng, S.E., F.SEI, F.ASCE Elaina Sutley, Ph.D., A.M.ASCE James Wacker, P.E., M.ASCE

Thank you for your service and leadership! Finishing terms September 30: www.asce.org/mgam Glenn R. Bell, P.E., S.E., CP, F.SEI, Dist.M.ASCE, Satyendra K. Ghosh, Ph.D., F.SEI, F.ASCE SEI Past-President Robert E. Nickerson, P.E., F.SEI, M.ASCE

Join us this year in celebrating 25 years of SEI – advancing and serving structural engineering! Refer a New Member and Earn Rewards ASCE encourages members, like you, to earn rewards for each newly joining professional member you refer to ASCE membership. Invite your peers to become part of the largest civil engineering network. Start earning rewards and refer colleagues today! www.asce.org/mgam

SEI News Read the latest at www.asce.org/SEINews SEI Standards Visit www.asce.org/SEIStandards to view ASCE 7 development cycle 54 STRUCTURE magazine

News of the Structural Engineering Institute of ASCE Learning / Networking

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

• SEI Standards Series – Interact with ASCE/SEI Standard developers on state-of-the-market updates. Participants will learn about technical revisions, review a design example, and are invited to participate in Live Q&A. Each session is LIVE and only available 1:00 – 2:30 pm US ET. Thursday, September 16 – ASCE/SEI 59 Blast Protection of Buildings Join the standard committee chair Donald Dusenberry, P.E., F.SEI, F.ASCE. ASCE/SEI 59 Blast Protection of Buildings provides minimum requirements for planning, design, construction, and assessment of new and existing buildings subject to the effects of accidental or malicious explosions. The Standard includes principles for establishing appropriate threat parameters, levels of protection, loadings, analysis methodologies, materials, detailing, and test procedures. It provides a comprehensive presentation of current practice in the analysis and design of structures for blast resistance. Commentaries on the requirements are also included. The Standard supplements existing building codes, standards, and laws but is not intended to replace them. Session sponsor: Redguard Individual session: Member $49, Nonmember $99. Student member: Free registration. REGISTER by September 14 at https://cutt.ly/9hQDTEo. • #SEILive Conversations with Leaders ICYMI – View Code Development on SEI YouTube Next on Licensure – Wednesday, October 13, 12:30 pm ET • ETS (Electrical Transmission Structures): Powering Past the Pandemic – Wednesday, November 3, 1:00 pm ET Join moderator Ken Sharpless, P.E. F.SEI, F.ASCE, for a big-picture panel discussion on the state of the electrical transmission structures industry as it emerges from pandemic mode. How well did essential businesses adjust to the challenges of COVID, and where does this experience take the industry going forward? Listen to perspectives focusing on essential employees from utilities, contractors, manufacturing, supply chain, engineering, and design. Panelists: Sarah Beckman, ULTEIG; Archie Pugh, American Electric Power; Alex Richards, Aquawolf, LLC; Bill Sales, Sabre Industries, Inc. Register for the live program (1.5 PDHs) or the post-event recording (no PDHs). • Save the Date Structures Congress – April 20-23, 2022 in Atlanta Electrical Transmission and Substation Structures Conference – October 2-6, 2022, in Orlando Students and Young Professionals: Apply for SEI Futures Fund Scholarships to participate.

VIRTUAL • OCT 6-8

ASCE’s 2021 Convention is going virtual Oct. 6-8 Registration includes access to the v i r t u a l O PA L Aw a r d s c e r e m o n y a n d t h e A S C E Te c h Ta l k s s e r i e s .

ASCE Convention Registration Includes ASCE Tech Talks Tuesdays, October 12 – November 16. Check out SEI Sessions on October 19: ASCE 7-22 Wind & Tornado Load Provisions, Designing with Data

Engineering> Innovating> Leading asceconvention.org #ASCE21

Errata

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. S E P T E M B E R 2 0 21

CASE in Point CASE Tools and Resources Did you know? CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural

engineering firms face daily. So whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has several sample contracts, practice guidelines, and risk management tools for firms to enhance their construction management practices. CASE #4 An Agreement Between Client and Structural Engineer for Special Inspection Services CASE #6 An Agreement Between Client and Structural Engineer for a Structural Condition Assessment CASE #7 An Agreement for Structural Peer Review Services CASE #8 An Agreement Between Client and Structural Engineer for Forensic Engineering (Expert) Services CASE #12 An Agreement Between Structural Engineer of Record (SER) and Contractor for Transfer of Digital Data or Building Informational Model File Commentary C Commentary on AIA Document A201, “General Conditions of the Contract for Construction,” 2017 edition CASE 962-D CASE 962-E CASE 962-F CASE 962-G

Tool 2-4 Tool 4-3 Tool 9-1 Tool 10-1 Tool 10-2

A Guideline Addressing Coordination and Completeness of Structural Construction Documents Self-Study Guide for the Performance of Site Visits During Construction A Guideline Addressing the Bidding and Construction Administration Phases for the Structural Engineer Guidelines for Performing Project Specific Peer Reviews on Structural Projects Project Risk Management Plan Sample Correspondence Guidelines A Guideline Addressing Coordination and Completeness of Structural Construction Documents Site Visit Cards Construction Administration Log

Check Out this CASE Publication CASE Tool 2-6: Structural Engineer Job Descriptions When targeted to people outside the firm, well-written job descriptions entice the most qualified people to apply with your firm. To get the most qualified candidates, list both quantitative and qualitative requirements such as experience, education, and desired personality traits. These types of qualifications help to eliminate undesirable candidates. When targeted to people inside the firm, job descriptions can be utilized as a powerful management tool. The details contained in a well-written job description form the basis for developing a clear understanding between the employee and the manager of what is expected of the employee. Managers can also use the terms in the job description to determine how the employee performed when conducting performance appraisals. The criteria used for performance evaluations ideally would match the expectations listed in the employee’s job description. The job description for the position above the employee’s current position can be used to explain what is required for that person to earn a promotion. The job descriptions contained within this tool are intended to be used as a template to create job descriptions specific to your firm. Word files are provided with detailed descriptions and a matrix with abbreviated descriptions when comparing engineering levels. You can purchase these and the other Risk Management Tools at www.acec.org/bookstore. 56 STRUCTURE magazine

News of the Coalition of American Structural Engineers 2021 CASE Summer Meeting Recap

CASE members got together in Nashville, TN, for one of the first in-person events for most attendees in over 18 months. CASE members attended risk management and business practices educational sessions, held committee meetings, and engaged with each other and with other ACEC Coalition members during the two-day meeting. Some of the discussions included: • An update on the infrastructure package from ACEC Chair Robin Greenleaf and ACEC President and CEO Linda Bauer Darr • A roundtable discussion about Building Assessments and Risk Management • Education sessions on CyberSecurity, Current Trends and Best Practices in Risk management, and an overview of the 2020 ACEC Grand Conceptor Award-winning Cooperhill Watershed project CASE committees all met in person the afternoon of August 10; look in next month’s edition for a re-cap of all committee work and what new/updated publications are coming to an inbox near you! We are also looking forward to getting together this October for the Fall Conference in Marco Island, FL.

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@acec.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. S E P T E M B E R 2 0 21

structural LICENSURE Georgia Passes P.E., S.E. Practice Act By Michael Planer, P.E., S.E.

eginning in April 2011, the National Council of Examiners for Engineering and Surveying (NCEES) recommended the 16-hour structural exam as the standard for demonstrating minimum competence in the practice of structural engineering. As a result, structural engineering became the only professional engineering discipline requiring more than an 8-hour exam in many jurisdictions. Prior to this date, some states already had some form of structural engineering legislation and, since 2011, more states have either passed or attempted to pass S.E. legislation. For Georgia, the Structural Engineers Association of Georgia (SEAOG) started looking at developing an easily understandable and compelling reason for structural engineering legislation soon after April 2011. This included making sure complex structures were being designed by qualified engineers and addressing a competitive disadvantage that engineers in Georgia who were designing structures faced, compared to engineers from states that had a P.E., S.E. designation. On August 4, 2020, the Governor of the State of Georgia signed a bill recognizing Structural Engineering as an addition to the current Professional Engineering license for specifically designated structures. The legislation is a practice act and requires passing the 16-hour exam for a structural engineer to receive the P.E., S.E. designation. Georgia engineers who had a P.E. designation before 2011 and who could demonstrate substantial structural design experience also received the P.E., S.E. designation without the need to take the exam. Passage of the Georgia structural engineering practice act was a lengthy process requiring the coordination and efforts of various local organizations, including SEAOG, the American Society of Civil Engineers (ASCE), and the Georgia Chapter of the American Council of Engineering Companies (ACEC Georgia). In 2012, SEAOG started reviewing the current Georgia licensing statute and how the wording could potentially be revised to require a Professional Engineer, Structure Engineer (P.E., S.E.) to be the Engineer of Record in responsible charge of any designated structure established by Rule by the Professional Engineers and Land Surveyors Board. During these initial efforts, SEAOG anticipated that there might be some objections from other interest groups, so they worked closely with ASCE, ACEC, and other 58 STRUCTURE magazine

local organizations as well as the Georgia Department of Transportation to determine a list of designated structures. The wording of the designated structures was necessary so that these other organizations developed a comfort level as to when a P.E., S.E. stamp would be required versus only a P.E. stamp. For example, the Georgia Department of Transportation developed the wording for what types of bridges would require the P.E., S.E. stamp. The group also used the Oak Brook Accords Consensus Document Regarding Geo-Structures, and S.E. Licensure developed within ASCE in November of 2015 to establish the list of Geo-Structures that can be designed by an appropriately qualified and licensed professional engineer. This initial work was difficult; however, actively getting the legislation introduced through committees, voted on, passed, and signed into law was more challenging. At the beginning of the process, SEAOG enlisted the assistance of ACEC Georgia to help establish the initial wording of the legislation, navigate the legislative process, and set strategies for finding sponsors in the House and Senate to introduce the bill. During these planning sessions, it became apparent that the legislative process is not easy to predict. Therefore, working with the ACEC Georgia lobbyist was essential to moving the bill forward. The bill was first introduced in 2015 but did not make it out of the committee process. ACEC Georgia met with legislators and, now knowing who may be opposed to the bill, provided guidance on strategy on the proper timing to reintroduce the bill, which committee meetings to attend, and which local legislators to contact. Educating and communicating with the legislators was also crucial to the bill’s passage. In particular, the legislators needed to understand the merits of the legislation and the logic for supporting the bill. During this process, ACEC Georgia also met with the Governor’s office to educate the Governor and his staff on the bill’s merits. In 2016, the bill was reintroduced in the House, went through committees, and ultimately passed. Unfortunately, the bill stalled in the Senate Regulated Industries Committee due to the Committee’s focus on other issues at that time. Between 2016 and 2018, the bill was not submitted due to challenges within the various committees or other ongoing

political priorities in the legislature. In 2019, it was determined that the timing seemed right, and the bill was again reintroduced. The bill easily passed the House but had more difficulty in the Senate, ultimately passing in a close vote late in the evening on the last day of the session. Excitement over the bill’s passage was short-lived as the Governor vetoed it due to a technical requirement that the legislation needed to be reviewed by the Georgia Occupational Regulation Review Council (GORRC) prior to it being signed into law. ACEC Georgia met with the Governor’s office to make sure it understood the proper procedure. With that understanding in place, the bill would be reintroduced in January 2020 legislative session only to have the session cut short due to the COVID 19 pandemic. However, with ACEC Georgia’s guidance, the bill was reintroduced once the legislators reconvened in the summer of 2020. With a shortened session, the organizations working toward passage acted quickly to move the bill through the process and a vote in both the Georgia House and Senate. Again, the bill was successful in the committees and both chambers, and, with the technicality from the previous year resolved, Governor Kemp signed the bill into law. Many engineers were involved in the 10-year process of gaining passage of the bill. They, including the author, learned so much about the legislative process and the importance of engaging industry organizations early. The author would like to thank Ashley Jenkins, Director of Government Affairs of ACEC Georgia, Rob Weilacher, P.E., S.E. of Uzun+Case, and Angelina Stasulis, P.E., S.E. of Davis & Church for their help with this article.■ Michael Planer serves as a Principal and President of PES Structural Engineers, Inc. He is currently the Chair of ACEC Georgia, serves on the Programs Committee for the Council of American Structural Engineers (CASE), and is a Past President of the Structural Engineers Association of Georgia. (mplaner@pesengineers.com) S E P T E M B E R 2 0 21

Arka Business Centre Post-Tensioned Beams & Slabs | Transfer Slab Structural Engineer | Strandeck

WHERE VISION BECOMES STRUCTURE RISA offers a comprehensive suite of design software that work together to simplify even the most complex projects. As a result, engineers can work efficiently on a variety of structures in a mix of materials including steel, concrete, wood, masonry and aluminum. risa.com

SPOTLIGHT Congratulations to 2021 SEI and ASCE Structural Award Recipients SEI PRESIDENT’S AWARD

WALTER P MOORE, JR. AWARD

SEI GRADUATE STUDENT CHAPTER OF THE YEAR AWARD

Donald R. Scott P.E., F.SEI, F.ASCE

Therese McAllister Ph.D., P.E., F.SEI, M.ASCE

SEI Graduate Student Chapter at Northeastern University

W. GENE CORLEY AWARD

ALFREDO ANG AWARD

Carl H. Josephson P.E., F.SEI, M.ASCE

Paolo Gardoni Ph.D., A.M.ASCE

SEI CHAPTER OF THE YEAR AWARD SEI Philadelphia Chapter

SHORTRIDGE HARDESTY AWARD

GENE WILHOITE INNOVATIONS IN TRANSMISSION LINE ENGINEERING AWARD

Leroy Gardner Ph.D., M.ASCE

Majid R.J. Farahani P.E., M.ASCE

MOISSEIFF AWARD

RAYMOND C. REESE RESEARCH PRIZE

T.Y. LIN AWARD Benjamin Z. Dymond Ph.D. Catherine French Ph.D., P.E., Dist.M.ASCE, Hon. ACI, Fellow PCI

Shengzhe Wang S.M.ASCE

Maria Garlock Ph.D., P.E., F.SEI, M.ASCE

Branko Glisic Ph.D., M.ASCE

Kevin McMullen Ph.D., A.M.ASCE

NATHAN M. NEWMARK MEDAL

2020 T.Y. LIN AWARD

Ahsan Kareem Ph.D., NAE, F.EMI, Dist.M.ASCE

Peter Bischoff Ph.D., P.Eng, F.ACI, F.CSCE

Arash E. Zaghi Ph.D., P.E., S.E.

Clay J. Naito Ph.D., P.E., M.ASCE

Carol K. Shield Ph.D., M.ASCE

Joseph P. Ingaglio EIT, A.M.ASCE

JACK E. CERMAK MEDAL Gregory A. Kopp Ph.D., P.E., M.ASCE

2020 ERNEST E. HOWARD AWARD William F. Baker, Jr. P.E., F.SEI, F.ASCE STRUCTURE magazine

Join us at Structures Congress April 20-23, 2022, in Atlanta to celebrate the winners! Learn more and nominate for 2022 SEI/ASCE Awards by November 1 at www.asce.org/SEI.

S E P T E M B E R 2 0 21 B O N U S C O N T E N T

STRUCTURE magazine | September 2021

Articles inside

Structural Forum

Business Practices

Insights

Structural Connections

Structural Design

Historic Structures

Structural Analysis

Structural Renovation

Building Blocks

Structural Anchorage

Editorial