STRUCTURE magazine | May 2017 by structuremag

May 2017 Masonry

Inside: University of Notre Dame

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE ®

Coming This Summer PCI Design Handbook 8th Edition

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TRANSFORMING THE WAY THE WORLD WORKS

CONTENTS Columns and Departments EDITORIAL

7 The NCSEA SEER Committee: Relevance Today By Jonathan Hernandez, P.E., SECB.

Cover Feature

BUILDING BLOCKS

30 University of Notre Dame Campus Crossroads

10 Anchoring Rocks! By Scott W. Walkowicz, P.E. STRUCTURAL SYSTEMS

By Andy Greco, P.E. and Peter Heeringa, P.E., S.E. The large Crossroads building initiative includes three new buildings and numerous renovations to Notre Dame’s iconic football stadium. Masonry played an integral role and required attention to detail for reinforcement, anchorage, and masonry relief.

14 Masonry to the Second Order By W. Mark McGinley, Ph.D., P.E. PRACTICAL SOLUTIONS

18 A Masonry Cage Fight By Richard M. Bennett, Ph.D., P.E.

Features

SPOTLIGHT

43 Structural Innovations of Lotte World Tower By SawTeen See, P.E., C.E., M.Eng.,

INSIGHTS

24 Earthquake Resiliency – Where Do We Stand?

Leslie Earl Robertson, P.E., C.E., S.E., D.Sc., D.Eng., and Edward J. Roberts, P.E.

By John A. Dal Pino, S.E.

STRUCTURAL FORUM

HISTORIC ENGINEERING

50 Lessons at Yellowstone National Park

34 Methods of Structural Analysis By Charles Sanders Peirce

By Samantha Fox

ENGINEER’S NOTEBOOK

37 Building Cladding By Steven Judd S.E. BUSINESS PRACTICES

38 Minimize Scope Creep by Learning Wants and Needs By Stuart G. Walesh, Ph.D., P.E.

IN EVERY ISSUE 8 Advertiser Index 40 Resource Guide – Steel/CFS 44 NCSEA News 46 SEI Structural Columns 48 CASE in Point

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement by NCSEA, CASE, SEI, C 3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

STRUCTURE magazine

May 2017

20 CRUISE TERMINAL EXPANSION AT THE PORT OF GALVESTON By Jon D. Jelinek, P.E. More than half of the Port of Galveston’s proposed $30 million cruise terminal expansion was impacted by an abandoned grain silo foundation. Read how engineers took advantage of this 1930s relic for support of the superstructure of the new terminal.

26 TREATING HISTORIC MASONRY VERY GENTLY AT DRAYTON HALL By Craig M. Bennett, Jr., P.E. Age, hurricanes, weather, and even a major earthquake have left their mark on this 1740s-plantation house in South Carolina. Careful study resulted in a plan for rehabilitation, and an experienced contractor factored into the success of the masonry upgrades to this historic structure.

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Editorial

The NCSEA SEER Committee: new trends, new techniques and current industry issues Relevance Today By Jonathan Hernandez, P.E., SECB

t was Tuesday, September 11, 2001, fresh from the Labor Day holiday, when our innocence was shattered by attacks on the World Trade Center and the Pentagon. My colleagues and I watched in horror on TV, and on the street in front of our office on University Place in Manhattan, as the Trade Center Buildings burned and collapsed. At the request of the New York City Department of Buildings (DOB) and the New York City Department of Design and Construction (DDC), the Structural Engineers Association of New York (SEAoNY) called for volunteer member engineers to assist in the rescue and rapid evaluation of the damaged structures at Ground Zero. The National Council of Structural Engineers Associations (NCSEA) also heeded the call for volunteers, and soon fellow engineers from all over the country joined the teams. Deployed at ground zero as part of one of the safety assessment teams during the search and rescue, my colleagues and I saw firsthand the horror of the destruction, felt the heat of the burning buildings, and experienced the pain and frustration of the first responders. SEAoNY performed damage assessment inspections on approximately 400 buildings around Ground Zero to determine which buildings were safe to reoccupy and which buildings required restricted access. Rapid assessments were performed on September 17 and 18 based on ATC-20 methodology successfully applied during the rapid structural assessments after the 1989 Loma Prieta and 1994 Northridge earthquakes in California. The ATC-20 methodology was adapted for the conditions found at Ground Zero. The results of the rapid assessment were reported to the DDC, the agency tasked with the work at ground zero in collaboration with the New York City Building Department. In the midst of this tragedy, NCSEA recognized the need to form the Structural Engineering Emergency Response (SEER) committee. The purpose of this group was to prepare and plan for structural engineers to respond to any significant disaster where Civil Authorities need additional assistance in rapid structural evaluation and safety assessment. The committee was formed in November 2001 with Gus Domel as the first Chair, appointed by the NCSEA Board. Gus authored the SEERPlan Manual together with the support of the ATC (Applied Technology Council), CASE (Council of American Structural Engineers), FEMA (Federal Emergency Management Agency), the State of California Governor’s Office of Emergency Management, as well as the Structural Engineers Associations of California (SEAOC), Washington (SEAW), and Oregon (SEAO). The manual was used as a guide for the NCSEA member organizations in the formation of their State level SEER Committees. From these groups, a Structural Engineers Volunteer roster was compiled by NCSEA.

Structural Engineering practitioners in the city to have an immediate rapid assessment response post-disaster. The Department of Buildings decided that the rapid post assessment, as well as the post-disaster detailed assessment, would be performed by structural engineering firms under contract to the Department. Structural engineers willing to devote their time to this enormous effort would be subcontracted to those structural engineering firms. The NYC DOB prepared rapid assessment forms based on ATC20/45 and the structural engineering teams used these forms for rapid evaluation safety assessment. The teams were composed of one or two structural engineers and an inspector from the Department of Buildings. The service performed by the structural engineers during this emergency allowed the community to bounce back quickly. The rapid inspection permitted residents to re-occupy their homes tagged with the GREEN placard, and set in motion recovery and rebuilding.

The Future The NCSEA SEER Committee has embarked on developing a webbased database so that qualified structural engineers can be identified and mobilized to assist local authorities in the event of a widespread disaster. The 2nd Responder Roster, part of the Structural Engineers Volunteer System, summarizes an individual engineer’s qualifications, physical condition, and geographical location so that local authorities can quickly contact and mobilize rapid assessment teams within the vicinity of a disaster. NCSEA has recently offered an on-line course for the California Office of Emergency Services (CalOES) Safety Assessment Program. This program provides basic skills required to perform a structural safety assessment based on ATC-20/45. It is a six-hour course that delivers a 7-unit program including practical tests. Upon completion, the course enables licensed design professionals and certified building officials to be certified as an SAP Evaluator. Structural Engineers can add their name to the SEER 2nd Responder Roster found on the NCSEA Volunteer System website at http://ncsea-seer.com. It is part of our DNA as structural engineers to assist communities post-disaster. Structural engineers are the most qualified professionals to perform rapid assessments. There is much discussion now about resiliency and how communities can bounce back quickly after a disaster. The rapid assessment performed during 9/11 and Hurricane Sandy allowed New York to bounce back. Our profession can provide that skill wherever it is needed in the country and the world. The SEER Committee is here to make that a reality.▪ Jonathan Hernandez is a Partner at Gilsanz Murray Steficek LLP Engineers and Architects, New York, NY. He is on the NCSEA Board of Directors and is a Liaison to the SEER Committee.

Sandy Post Disaster Assessment On October 29, 2012, with Hurricane Sandy bearing down on New York City, the NYC Department of Buildings collaborated with

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ADVERTISER INDEX

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EDITORIAL BOARD Chair Barry K. Arnold, P.E., S.E., SECB ARW Engineers, Ogden, UT chair@structuremag.org

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Erratum

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We have received a revision and clarification to the March 2017 Construction Issues article. The updated information relates to FM Approval Standard 4470 regarding steel decks and the use of a Factor of Safety of 2. For your reference, the online article (www.STRUCTUREmag.org) has been updated.

Linda M. Kaplan, P.E. TRC, Pittsburgh, PA Dilip Khatri, Ph.D., S.E. Khatri International Inc., Pasadena, CA Jessica Mandrick, P.E., S.E., LEED AP Gilsanz Murray Steficek, LLP, New York, NY

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Greg Schindler, P.E., S.E. KPFF Consulting Engineers, Seattle, WA Stephen P. Schneider, Ph.D., P.E., S.E. BergerABAM, Vancouver, WA John “Buddy” Showalter, P.E. American Wood Council, Leesburg, VA Diane Throop, P.E. Sunset, SC C3 Ink, Publishers A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 Phone 608-524-1397 Fax 608-524-4432 publisher@structuremag.org May 2017, Volume 24, Number 5 ISSN 1536-4283. Publications Agreement No. 40675118. Owned by the National Council of Structural Engineers Associations and published in cooperation with CASE and SEI monthly by C3 Ink. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $75/yr domestic; $40/ yr student; $90/yr Canada; $60/yr Canadian student; $135/yr foreign; $90/yr foreign student. For change of address or duplicate copies, contact your member organization(s) or email subscriptions@ STRUCTUREmag.org. Note that if you do not notify your member organization, your address will revert back with their next database submittal. Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board. STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

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Building Blocks updates and information on structural materials

Anchoring Rocks! By Scott W. Walkowicz, P.E., NCEES

any buildings are partially or wholly clad with stone. There are many aspects of stone anchorage that can, and do, affect the longterm performance of the building envelope and the initial and long-term costs of the envelope and building operation. This article presents critical stone anchorage issues with a focus on anchor and fastener selection and design based on the author’s experience. Two of the most important things to consider when selecting and designing stone anchorage are: • The size of the stone being installed: face dimensions, thickness, and shape. • The backup system; this impacts anchor capacity and placement and will influence the amount of movement in the stone. This article focuses on large-scale stone and is generally applicable to both natural quarried/cut stone and cast stone, although minor differences may exist between the two based on performance property differences. The code content included has been drawn from the 2015 International Building Code (IBC), the 2013 Building Code Requirements for Masonry Structures – TMS 402-13/ACI 530-13/ASCE 5-13 (TMS 402), and the 2013 Specification for Masonry Structures – TMS 602-13/ ACI 530.1-13/ASCE 6-13 (TMS 602).

Stone Size Considerations

Scott W. Walkowicz owns Walkowicz Consulting Engineers, in Lansing, Michigan. Scott is a Past President of The Masonry Society (TMS) and is a member of multiple subcommittees within the TMS 402/602 Committee (formerly the Masonry Standards Joint Committee (MSJC)). He can be reached at scott@walkowiczce.com.

The IBC addresses stone veneer and slab type veneer in separate sections within Chapter 14 for Wall Coverings. Those sections, however, rarely are directly applicable to the large stone cladding that is considered in this article. For the purposes of this discussion, the assumption is that the stone veneer is to be anchored and that the stones require specific engineered anchorage. Large-scale stones are often used for monumental or significant projects where scale and aesthetics require the use of larger, often significantly larger, stones rather than those that can be secured with veneer type mortar bedded anchors. These large stones are usually anchored with four or more stainless steel anchor straps per stone fastened to the backup system. Anchors for largescale stones can receive load from more than 20 square feet of cladding and must transmit several hundred pounds of force to the backup system. The anchors, also, must typically be placed at very specific locations, such as at the ¼ points of the units, to allow for loading by the stone above and below the anchor while maintaining consistent stone support and behavior. These potentially high anchor loads and specific location requirements typically preclude the use, or at least the easy use, of direct connection to stud framing. Therefore, a masonry or another solid backup

10 May 2017

Figure 1. Large scale stone – institutional project. Courtesy of Indiana Limestone Institute.

Figure 2. Large scale stone – institutional project. Courtesy of International Masonry Institute.

system is preferred. Figure 1 shows a completed large-scale stone project and Figure 2 shows a large-scale stone project under construction showing strap anchors in place.

Anchorage Basics Simple span type stone anchorage can generally be designed by hand analysis methods, although software applications can more accurately and efficiently accomplish the work. The effect of anchor location on anchor loading is based on tributary area, and that makes analysis easier for the anchor designer. Stones with reasonable size, geometry, and bonding allow the use of four anchor points per stone. The lateral load is distributed to the anchors based on tributary area. In rare cases where differential stiffness exists in the anchors or backup for a particular stone, it is desirable to employ finite element analysis (FEA). The same is true for longer stones with slender profiles, thin section stones, or stones too large to be anchored with only four anchors. When using FEA, both the stone and the anchor stiffness must be estimated as accurately as possible. This requires knowledge of the stone. Properties can vary by stone type and

(a)

(b) (a)

Figure 3. a) 2- x 8-foot limestone reactions (lbs.) with 4 anchors; b) 2- x 8-foot limestone reactions (lbs.) with 6 anchors.

also within varieties of a particular stone type. Natural stone suppliers and cast stone manufacturers can provide the appropriate material information. Then, using FEA, the anchor reactions and internal stresses can be compared to the anchor and stone limits. Figures 3a and 3b contain reactions for a rectangular, large-scale stone where an additional anchor is added at the mid-point. It is often observed that simply adding an extra anchor may not relieve a minor over-stress and could, in fact, increase localized stresses or create a stress reversal in the anchors. This example demonstrates that adding an anchor may have little effect and could have even had a significant detrimental effect. When checking stone stresses, the use of Von Mises stress is a convenient check for natural (unreinforced) stone whereas

Figure 4. a) 2- x 8-foot Limestone Stresses (Von Mises, psi) with 4 anchors; b) 2- x 8-foot limestone stresses (Von Mises, psi) with 6 anchors.

large-scale cast stone will be reinforced and needs to be evaluated using reinforced concrete analysis. Figures 4a and 4b show the same stone pieces from Figures 3a and 3b but with the internal stresses highlighted based on anchor placement. Note that while there is little change, the addition of the extra anchors slightly increased the internal stress within the stone. Anchor reactions and stone stresses can be significantly affected by anchor location, stone thickness, and stone geometry – so the designer should be careful! When evaluating stone stresses, keep in mind that many types of natural stone can have tested modulus of rupture values more than 1,000 psi. Safety factors must then be applied. The Indiana Limestone Institute (ILI) recommends not less than ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org

STRUCTURE magazine

(b)

May 2017

an 8:1 factor. For the 4-inch thick stone illustrated in Figures 4a and 4b, the minimum tested ultimate modulus of rupture value for limestone would need to be 586 psi and 594 psi, respectively – not a problem for most natural stones. If the values were higher, more anchors or thicker stone could be required.

Anchor Type Selection and Design Most stone anchors are created from stainless steel flat stock. Analysis of the straps can be conducted using methods outlined in the Design Manual for Structural Stainless Steel (2006 Euro Inox and The Steel Construction Institute) or the Specification for Structural Steel Buildings (2010 AISC). Both provide

(a)

Example – Anchor Design

(b)

Figure 5. a) Manufactured strap anchor with dowel; b) Manufactured strap anchor with splittail turns. Courtesy of Wire-Bond.

methods and address strength/stress limits, buckling, etc., but neither document contains specific content for the design of simple flat strap elements. The author prefers, and typically uses, the AISC approach for better recognition of simple rectangular elements and its relative modernity. Most common anchor straps are 1/8-, 3/16or possibly ¼-inch thick with widths that vary from 1¼ inches to 3 inches or more. The straps can have a single turn at the end or a ‘split-tail’ where one tab points up and the other down to engage stones on each side of the joint – typically bed joints but sometimes head joints in which the anchor is being placed. Other approaches include straps with dowel pins that engage holes in the stone, or straps with holes to allow the insertion of anchors into the stone edges. It is not recommended to use fasteners that impart expansive forces to the stone when anchoring into or near an edge. Figures 5a and 5b show examples of how these anchors tabs and dowels engage the stone by being placed into cut slots or holes in the stone. Fixity at the stone is determined by the material used to fill around the dowel or tab, as well as the joint filler material which ‘pinches’ the end of the anchor strap. If flexible materials are used, then a simple ‘pinned’ connection can well represent the joint. Fixity can be obtained using firm fill and mortar materials. Based on the stone end fixity, and with a reasonable degree of fixity at the backup end, a proper k-factor can be determined and tension/compression analysis conducted. Flexure and shear in the tabs at the stone embedment and at the fastener end must be considered in addition to the axial force analysis. Prying considerations should also be included to address resulting strap and fastener load increases. Most straps can be designed thick enough to eliminate the prying concern. Punching shear should be evaluated in the stone, in accordance with the appropriate standards, although it will rarely control anchor design for 3- or 4-inch thick stone.

Consider a split-tail anchor strap similar to that shown in Figure 5b. The strap includes an offset dimension from the center of the fastener hole to the center of the outstanding strap of 1 inch, and it features ¾-inch high tabs, one turned up and one turned down. Typically, half of the anchor load will originate in each of the tabs although stone coursing of different heights can create some imbalance in the tabs. Usually, the flexure between the outstanding strap and the fastener will control the design due to the greater distance between lines of action. Using the anchor reactions from the diagrams shown in Figure 3a, let’s look at a strap design for 200 pounds in tension and compression. Remember that the anchor is also receiving a load from the stone above (in this case, an equal size stone), so the load applied is 2 x 200 pounds, or 400 pounds. The use of ASCE 7-10 will result in a wind load factor of 0.6 for allowable stress design, so the final ASD design load for the straps and fasteners would be 240 pounds. Expect one fastener to be required and try a 3-inch wide x 3/16-inch thick strap to moderate the steel stresses and prying action. Assuming rigid fill and checking axial plus flexure at the backup surface bend, base the calculations for axial compression on a ‘k’ factor of 0.8 to allow for some movement. Using AISC Section E3, the axial compression capacity with an 8-inch dimension from the face of backup to the back of the stone is 5,051 pounds. The axial tension capacity is 8,421 pounds. With the ¾-inch offset creating a ¾-inch moment arm, the flexure in the elbow is 180 in.-lbs which would be compared to a capacity of 395 in.-lbs. As you can see, in this, as in many cases, flexure is the dominant aspect in the anchor design. Combined stresses indicate a utilization of approximately 50 percent under these conditions. Deflections can be checked but will usually not control, especially when the stresses are this low. Checking manufacturers’ product information shows several options that can work. As mentioned before, prying should also be checked, at least until a feel is obtained for the limits where it will not control. In this case, the combination of the fastener diameter and plate width/thickness indicate that prying does not need to be considered when using AISC Part 9 for the evaluation of connecting elements.

Other Considerations Overhead Stone Anchorage is an art form within the larger stone anchorage discipline. Gravity

STRUCTURE magazine

May 2017

Figure 6. Split-tail strap anchor with a horizontal distribution angle for anchoring to a metal stud backup system. Courtesy of Indiana Limestone Institute.

load support generally demands higher safety factors and specific overhead anchor types. Mechanical and plate type anchors are most common and often include an adjustment in one or more planes to allow field fitting and tuning of stone placement. Epoxy anchors can aid in the installation of overhead anchors, but restrictions and concerns about longterm capacity and fire/heat degradation of the adhesive should be carefully evaluated before implementation. Opening Surrounds and Other Special Applications create unique anchor load and stone stress conditions. Out-of-plane, gravity, and rotational stability of the stone pieces must be considered with solutions including anchors and mortaring. The sequential fit-up of the stones in unique assemblies can be challenging in the field, and the anchor placement should be coordinated with the contractor’s installation sequence.

Backup Types for Stone Anchorage Stone cladding may be applied to any backup system, but the most common applications for large-scale stone include concrete masonry and or concrete. The primary points of interest related to stone backup should be anchor capacity and location, as well as backup load capacity and deflection under load. Another consideration would be system longevity. Nobody wants to invest in a ‘forever’ cladding of stone for their building only to have it fail due to the poor structural performance of the backup, or to have to remove it to repair or replace a backup system that hasn’t withstood the test of time. Fastening methods for connecting stone anchors to the backup vary based on the type of backup system being used. Direct tension screw fasteners work well and are most common in solid substrates such as concrete masonry and concrete. Fastener capacity values are high even for modest size fasteners.

greater longevity and less maintenance. Solid backup systems often provide stiffness that is an order of magnitude better than framed backup systems.

Conclusions When evaluating all the criteria above, several things seem obvious. Stone anchorage has multiple facets to consider and evaluate. Masonry provides a backup system that, along with the stone cladding, completes

Backup Load Capacity and Deflection While the backup system can significantly impact the installation and performance of fasteners, the stone anchorage designer doesn’t often design the backup system – it is usually designed by the Engineer of Record or Specialty Engineer. Because large-scale stone anchors can collect force from many square feet of tributary area, the anchor loads may not have been fully or well considered during the backup design. These forces must be communicated to, and reviewed by, the backup designer. Solid substrates such as masonry typically can disburse the anchor forces over several square feet and rarely require supplemental strength. Individual backup members such as studs within a framed system, however, may require strengthening for load and/or stiffness considerations. Stiffer backups inherently provide better cladding performance and thereby lead to

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an authentic ‘masonry’ wall assembly while allowing for modern detailing such as ventilated and pressure equalized drainage cavities. Stone stresses, anchor capacity, and fastener capacity, along with proper backup behavior, are all essential to good anchorage design. The good news is this – there are many long-standing technical guides and design methods to apply that, when combined with modern analysis through FEA software, can provide acceptable stone anchor designs.▪

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Locational concerns are generally not present due to the solid and continuous substrate presented by masonry and concrete. This is because anchors may be located as needed by the stone layout for optimized efficiency to provide secure and permanent anchorage of the stone veneer. When working with solid substrates, multiple anchors can easily be placed side-by-side or at stone-specific locations while retaining their effectiveness, subject to possible fastener capacity reductions based on the fastener spacing. Direct tension anchor fasteners are much less desirable for use with metal or wood stud backup systems. ILI recommends against using self-drilling/tapping screws in tension, when connecting to metal stud flanges, and instead recommends through bolts with washers. Unpredictably spaced studs or studs not located where needed for stone anchor/fastener installation often drive the need for supplemental horizontal members. Figure 6 shows an ILI detail for a supplemental horizontal member to allow for proper anchor location. Additional members may be placed within the stud space or may be applied angles, hat channels, or other members capable of transferring the cladding loads to the backup. These solutions add significant coordination effort, time, and cost. Timing becomes critical for any members located within a stud space before being sheathed, as does consideration of AVB application and performance.

Structural SyStemS discussion and advances related to structural and component systems

he masonry code, TMS 402/ACI 530/ ASCE 5 Building Code Requirements for Masonry Structures (TMS 402), requires that reinforced masonry walls designed using Strength Design (SD) procedures be analyzed for second-order effects. This article discusses the design of reinforced, load-bearing masonry wall systems under the action of combined vertical and out of plane loading using the new moment magnifier provisions in TMS 402 referenced in the 2015 International Building Code (IBC). Since its inception, TMS 402 has had Allowable Stress Design (ASD) provisions for the design of load bearing reinforced wall systems subjected to combined axial forces and bending moments. SD provisions were added starting in the 2002 version. These SD provisions generally result in more efficient reinforced masonry wall designs but require second-order effects to be included in the analysis for walls subjected to axial forces and outof-plane loading. Second-order effects must be addressed in SD design as lower global factors of safety are used, and a more accurate prediction of the maximum load effects are needed to maintain adequately low probabilities of failure. Before 2013, the TMS 402 SD chapter provided provisions for estimating second-order effects using procedures developed by Amrhein and Lee [Amrhein, J. and Lee, R., Design of Reinforced Tall Slender Walls, Western States Clay Products Association, 1984]. However, these procedures are limited to simply supported walls subjected to uniform out-of-plane loads with axial load stresses less than or equal to 0.20 f'm, and h/t ratios less than or equal to 30. To address these limitations and provide a simplified alternative second-order analysis methodology, the 2013 version of TMS 402 now includes a moment magnifier method. This new methodology is based on the moment magnifier method for slender concrete columns and is generally easier to apply than the slender wall method of Amerhien and Lee. In addition, the moment magnifier method is not limited to axial load stresses of 0.20 f'm, simple support conditions, h/t ratios of 30, nor uniform loads. It should be noted that the just-published 2016 version of TMS 402 has made no significant changes to this section of the standard. The moment magnifier method requires that first-order ultimate factored moments (Mu,0) be increased using a moment magnification factor, ψ, to account for second-order moments as shown below.

Masonry to the Second-Order The New Masonry Moment Magnifier Method By W. Mark McGinley, Ph.D., P.E.

Dr. W. Mark McGinley is a Professor of Civil Engineering and Endowed Chair for Infrastructure Research at the University of Louisville. He has served as the Chair of the Flexural and TMS Axial Load, Flexure, and Shear Subcommittee and is one of the Prime authors on all eight of the Masonry Designer Guides. He can be reached at m.mcginley@ louisville.edu.

Mu = Mu,0Ψ

TMS 402 – Eq. 9-31

14 May 2017

The moment magnification factor is to be determined using the following equation: 1 Pu Pe

Ψ=

TMS 402 – Eq. 9-32

Where Pu is the factored axial load applied to the top of the wall for the load combination being considered and Pe is the Euler buckling load. This buckling load is determined using the code-defined equation below and an effective moment of inertia, Ieff. Pe =

π 2EmIeff h2

TMS 402 – Eq. 9-33

Ieff varies depending on whether the section is cracked or not. TMS 402 assumes that if the wall is subjected to a maximum moment that is less than the cracking moment of the wall, (Mcr), it will have an effective moment of inertia of 0.75 In where In is the net moment of inertia of the uncracked wall system. If the wall is subjected to moments in excess of the cracking moment, it is assumed to be cracked fully along its length and Ieff is taken as Icr. Note that this is a conservative estimate for the stiffness of the masonry wall. Mcr is determined based on the modulus of rupture of the masonry wall assembly (fr ), the axial stress and the section modulus, S, of the wall, as shown below.

(

)

Mcr = fr + P S Ag fr is determined based on the type of unit and mortar used in the assembly, the extent of grouting, and is typically assumed to be oriented normal to the masonry assembly bed joints. Table 9.1.9.2 in the TMS 402 lists fr values. The value of S varies with the size of the units in the wall and the extent of grouting. TEK Note 14-B from the National Concrete Masonry Association [http://ncma-br.org/pdfs/56/TEK%201401B.pdf] is an excellent resource for determining the effective section modulus of concrete masonry walls of various sizes and distance between grouted cores. Chapter 9 of TMS 402 also provides procedures for determining Icr and lists an expression that can be used when the neutral axis is in the face shell of the masonry assemblies. These equations are shown below. As the neutral axis is often in the face shell at the critical design condition, this expression applies to most designs. This expression can also be conservatively used in conditions where the neutral axis is not in the face shell. c=

Asfy + Pu TMS 402 Eq. 9-35 0.64 f'mb continued on page 16

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(

Icr = n As +

Pu tsp bc 3 (d-c)2 + TMS 402 Eq. 9-34 fy 2d 3

)

TMS 402 also has provisions that allow the moment magnifier to be used to determine the second-order effects on deformations under service load conditions.

A modulus of rupture ( fr ) of 163 psi was obtained from TMS 402 Table 9.1.9.2 for a fully grouted wall with Type S PCL mortar (normal to the bed joints). This produces a cracking moment for the given axial load as follows.

(

Mcr = fr +

Examples

P 5,640lb S = 163psi + (12in.)(7.63in.)2/6 Ag 12in.(7.63in.)

) (

)

= 26,150lb.in/ft

8-inch CMU Bearing Wall The following two examples show how the moment magnifier method can be applied to typical wall designs. The first case addresses a fully grouted 8-inch CMU wall, with PCL Type S mortar and a specified assembly compression stress (f'm) of 2000 psi, that is used to support a roof. The wall system is assumed to be reinforced with a Grade 60, #5 rebar, centered in the wall thickness at a 16-inch vertical spacing.

As required by the TMS 402 provisions, as Mu = 14,600 lb-in is less than Mcr = 26,100 lb-in then Ieff = 0.75 (In). Thus, Pe =

π 2EmIeff π 2900(2,000psi)(443in4)(0.75) = = 93,000lb h2 (21 ft x 12in./ft)2

and Ψ=

P 1- u Pe

5,640lb 193,000lb

= 1.06

This results in the design second-order moments of Mu = 1.06 (1,210 lb.ft per foot of wall) = 1,283 lb.ft per foot of wall. The above calculations show the second-order effects only increase in the applied moments by 6% using the conservative moment magnifier analysis. As this level of loading and wall configuration are typical of most masonry wall systems, this increase due to second-order effects is also typical. It should be noted that second-order analysis need only be done for where vertical loads are applied to the walls and act to increase applied moments. Typically uplift due to wind loadings will act to reduce out-of-plane moments and need not be addressed. Wall Section with Higher Moment

Figure 1. Example – 8-inch CMU load bearing wall.

One critical loading condition on the wall is shown in Figure 1. The roof loading produces (1.2D + 1.6Lr + 0.5W), an axial load of 5,640 lb per foot of wall, and the maximum moment at mid-height of 1,210 lb-ft (14,600 lb-in) per foot of wall. Using a 1-foot design width and the provisions in TMS 402, the net and cracked moments of inertia can be determined as follows. In =

bt 3 12in.(7.63in.)3 = = 443.in3 12 12

E s 29,000,000psi = = 16.6 E m 900(2,000psi)

For the second example, a wall section with higher moments is addressed. In this example, it is assumed that a fully grouted, 8-inch CMU masonry wall is located below a lintel and subjected to a peak factored axial load of 2,000 lb per foot of wall and a maximum factored moment of 1,942 lb.ft (23,300 lb.in) per foot of wall. This wall section is also assumed to be simply supported over a height of 18 feet. The wall is constructed of masonry with an f 'm of 2000 psi, with #5 rebar at 2 feet vertical spacing centered in the wall thickness. Using a 1-foot design width and the provisions in the TMS 402, the net and cracked moment of inertias can be determined as follows. As = 0.31in.2(12in.) = 0.155in.2 16in.

0.31 in.2(12 in.) = 0.232 in.2 16 in. As fy + Pu 0.232in.2(60,000psi) + 5,640lb c= = = 1.273 in. 0.64f'mb 0.64(2,000psi)(12in.) As =

(

Icr = n As +

Pu tsp bc (d-c)2 + fy 2d 3

)

c = Asfy + Pu = 0.155in.2(60,000psi) + 2,000lb = 0.736in. 0.64f'mb 0.64(2,000)(12in.) bc 3 Icr = n As Pu tsp (d-c)2 + 3 f 2d

(

Icr = 16.1 0.155in.2 +

5,640lb 7.63in. Icr = 16.1 0.232in. x (3.81in.-1.27in.)2 60,000psi 2(3.81in.)

(

)

= 38.2in4 / ft

= 33.9in4 / ft STRUCTURE magazine

May 2017

2,000lb 7.63in. x (3.81in. - 0.736in.)2 60,000psi 2(3.81in.)

)

Summary As before, fr is 163 psi (TMS 402 Table 9.1.9.2) for a fully grouted wall with Type S PCL mortar (normal to the bed joints). This produces a cracking moment for the given axial load as follows.

(

Mcr = fr +

P 2,000lb S = 163psi + (12in.)(7.63in.)2/6 Ag 12in.(7.63in.)

) (

)

= 21,522 lb.in per foot of wall Unlike the previous example, the Mu of 23,300 lb-in is greater that Mcr = 21,522 lb-in. Therefore, the standard provisions would require that Ieff be taken as = Icr.

Examination of the equations used in the moment magnifier method shows that significant increases in second-order moments can occur when the applied moments exceed the cracking moment, and the applied forces are close to the Euler buckling capacity (Pe). This is more likely to happen with partially grouted walls with type N masonry cement mortars, as the fr and S values can be significantly reduced. Also, where wall configurations and loading produce conditions where the cracking moment is exceeded and the axial force is close to the Euler buckling force, a significant reduction in predicted secondorder effects are possible using the original slender wall procedures, as compared to the moment magnifier approach. This can be explained by examination of the TMS equation for the slender wall approach shown below.

Thus,

δu =

π 2EmIeff π 2900(2,000psi)(38.2in4) Pe = = = 14,545 lb 2 h (18 ft x 12in./ft)2 and Ψ=

1 1 = = 1.16 1- Pu 1- 2,000 lb Pe 14,545 lb

This would produce a design second-order moment of Mu = 1.16 (1,942 lb.ft per foot of wall) = 2,253 lb.ft per foot of wall. The second-order effects in this second example were much higher than the first.

5Mcr h 2 5(Mu - Mcr)h2 + 48EmIn 48EmIn

TMS 402 Eq. 9-30

In these provisions, when Mu is greater than Mcr (Eq. 9-30), only the portion of the applied moment above cracking is used to determine the deformation using Icr unlike moment magnification where Icr is essentially used to amplify all moments whenever the wall is subjected to moments higher than Mcr. Thus, second-order moment effects can be significantly reduced using the slender wall approach, which should promote its use if conditions warrant. Designers are encouraged, however, to use this new moment magnifier method as it provides simple, efficient procedures to conservatively estimate second-order moments and deflections for most masonry structures subjected to combined out-of-plane and vertical loads.▪

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structure-half-h-3-2017.indd 1

STRUCTURE magazine

May 2017

3/7/17 9:05 AM

Practical SolutionS solutions for the practicing structural engineer

o maybe this is not exactly a rendition of Fight Club, but engineers often wonder which masonry analysis method comes out better in a headto-head duel. Historically, masonry has been designed using allowable stress design (ASD). Strength design (SD) was added to the TMS 402 masonry code in 2002 and thus has only been in the code for about 15 years. Designers often ask which method is preferable – ASD or SD? Let’s explore this question. This article compares the design methods for beams, bearing walls, and shear walls using the 2013 TMS 402 Building Requirements for Masonry Structures code.

Round 1: Beams Figure 1 compares the allowable moment in a masonry beam as a function of the area of reinforcement and is the key to comparing the two design methods. In this figure, the nominal moment from strength design has been divided by an assumed load factor of 1.6 to obtain an “allowable” moment so the design methods can be compared. For smaller amounts of reinforcement, the allowable steel stress controls in ASD, so ASD and SD require approximately the same amount of reinforcement. At higher amounts of reinforcement, the allowable masonry stress controls the design, and the graph becomes quite flat. The reinforcement is not being used efficiently. For example, using Figure 1 and a design moment of 500 k-in, SD would require 0.97 square inches of reinforcement, while ASD would require 1.48 square inches of reinforcement or 50% more than SD. Even though the allowable masonry stress is controlling the design, the failure would still be a ductile failure; the balanced reinforcement where the masonry crushes as the reinforcement yields is 1.78 square inches. This trend holds for combined axial and flexure (interaction diagrams). When the allowable steel stress controls the ASD design, ASD and SD are reasonably similar. When the allowable masonry stress controls the design, ASD becomes quite conservative. The above comparison was based on an assumed load factor of 1.6. This would be appropriate for non-bearing-walls under out-of-plane load, where the load would be either wind or seismic. For beams, much of the load is often dead load. SD has the advantage in this case, due to the smaller dead load factor of 1.2. For example, if half the load were dead load, SD requires only 75-80% of the reinforcement as ASD, even with the allowable steel stress controlling. For round 1 of the cage fight, beams, the advantage is given to SD.

A Masonry Cage Fight ASD vs. SD By Richard M. Bennett, Ph.D., P.E., FTMS

Richard Bennett is a Professor of Civil and Environmental Engineering at the University of Tennessee. He was the chair of the 2016 TMS 402/602 Code Committee and Vice Chair of the 2013 and the 2022 TMS 402/602 Code Committees. Richard can be reached at rmbennett@utk.edu.

18 May 2017

Round 2: Bearing Walls So, going into Round 2, SD is ahead of ASD by almost a knock-out. However, let’s not count ASD out just yet. For most masonry bearing walls, the allowable steel stress will control the ASD design. No second-order, or P-delta analysis is required with ASD. When the allowable stresses were recalibrated in the 2011 TMS 402 code as a result of the removal of the one-third stress increase, an allowable steel stress of 32 ksi was chosen for Grade 60 steel rather than 36 ksi (0.6fy ) in order to avoid having to perform a second-order analysis in ASD. By contrast, SD requires a second-order analysis of all bearing walls, no matter how small the height/thickness ratio or how small the axial load. SD also requires a deflection check under allowable stress level loads, while there is no deflection check in ASD. Due to the allowable steel stress usually controlling, and the primary load being wind or seismic, the difference in required reinforcement using ASD and SD is usually less than 5%. The two design methods give approximately the same amount of required reinforcement, but ASD design is much simpler due to not requiring a second-order analysis. (Note that, in the future, the TMS 402 Code Committee is considering adding a trigger to SD design and only requiring a second-order analysis for truly slender walls.) For round 2 of the cage fight, bearing walls, the advantage is given to ASD.

Round 3: Shear walls Heading into this third round, ASD and SD have both scored a win. So, what happens with shear wall design? Three aspects of shear walls are considered to answer this: flexure (overturning), shear, and special reinforced shear walls. With regard to flexure, SD has a distinct advantage over ASD whenever there is distributed reinforcement. Most, if not all, intermediate reinforcement in tension will be at yield with SD. Although the smaller lever arm makes the intermediate reinforcement less effective than reinforcement at the ends of the wall (jamb steel), the intermediate reinforcement still contributes significantly to the flexural capacity. With ASD, the distributed reinforcement not only has a smaller lever arm, but it will also have a smaller stress due to stress being directly proportional to strain in ASD. For example, in one trial design of a 16-foot long shear wall that had a high in-plane load, ASD required a #6 bar in each of the three end cells and #6@40 inches for the intermediate bars. Designing the same wall using SD required a #5 bar in each of the two end cells and #5@40 inches for the intermediate bars. The results of the trial design are shown as an interaction diagram

Figure 1. Comparison of ASD and SD for a masonry beam.

in Figure 2. SD required only 55% of the reinforcement that was required using ASD. There was a major change to the shear provisions in ASD recalibration effort done in the 2011 TMS 402 Code. As the SD shear design procedures have been shown to be an excellent predictor of shear strength of fully grouted shear walls, the ASD provisions were harmonized with the SD provisions. The ASD provisions are written in terms of stress instead of force and have a factor of safety of 2 (approximate load factor of 1.6 divided by a shear strength reduction factor of 0.8). Thus, the amount of shear reinforcement required in shear walls will be approximately the same in ASD and SD. Special reinforced masonry shear walls have shear capacity design provisions. These provisions require an increase in shear strength to minimize the probability of a brittle shear failure. In SD, the design shear strength (φVn) is required to be greater than the shear corresponding to 1.25 times the nominal flexural strength, Mn (increases shear at least 1.39 times), except that Vn need not be greater than 2.5Vu (doubles shear). In ASD, the shear is required to be increased by 1.5 and the contribution of the masonry to the shear strength is reduced by a factor of 2. Although the intent in developing the code provisions was that both methods would require approximately the same shear reinforcement, SD typically requires less shear reinforcement in special reinforced masonry walls. For the trial design mentioned above and designed as a special reinforced shear wall, the required shear reinforcement was #5@16 inches for ASD. For SD, there was barely any shear reinforcement required, #5@269 inches. However, it

Figure 2. Shear wall with distributed reinforcement.

should be noted that the prescriptive reinforcement (at least 0.0007 of the gross area of the wall) and spacing limits (⅓ the height, ⅓ the length, or 48 inch for running bond) will often control the horizontal reinforcement in special shear walls. Thus, in the trial design, the horizontal reinforcement was required to be #5@40 inches in SD. Except for highly loaded shear walls, the prescriptive requirements generally result in the same horizontal reinforcement whether using ASD or SD. There are two other differences in the design of shear walls with ASD and SD. One is the maximum reinforcement requirements, which are to ensure ductility and minimize the probability of toe crushing. In ASD there are only maximum reinforcement requirements for special walls, while in SD there are maximum reinforcement requirements for all reinforced shear walls (ordinary, intermediate, and special). The SD maximum reinforcement requirements tend to be more onerous. This has caused some designers to use ASD instead of SD. However, using ASD results in even more reinforcement being required in the wall, defeating the purpose of the maximum reinforcement requirements. The other difference is that SD calls for the horizontal bars needed for shear reinforcement to be bent around the edge of the vertical reinforcing bar with a 180° hook, while ASD is silent and presumably allows a 90° hook with the hook extension being turned down in the cell of the wall. Because of this, some designers have indicated a preference for ASD, citing the potential difficulty with 180° hooks. However, if the 180° is turned, so the hook lies in a 45° plane from the horizontal, there are usually no construction problems.

STRUCTURE magazine

May 2017

Masonry Design: ASD vs. SD Official Score Card

Round

ASD



1. Beams 2. Bearing Walls 3. Shear Walls Final Decision

  

To recap, SD requires much less flexural (overturning) reinforcement and requires less shear reinforcement for special reinforced masonry shear walls. Although the maximum reinforcement requirements for SD are more stringent, the recent increase in f 'm from 1500 psi to 2000 psi for Type S mortar and minimum strength CMU units has made the maximum reinforcement provisions less likely to control. For round 3 of the cage fight, shear walls, the advantage is given to SD. And the winner is… Strength Design, but in a split decision. To summarize, SD offers several advantages over ASD and can result in more efficient designs. Designers are encouraged to consider using SD for the design of masonry structures.▪

All archive articles available online: www.STRUCTUREmag.org

Cruise Terminal Expansion at the Port of Galveston Strange Bedfellows – The Liberty of the Seas® and an Abandoned Grain Elevator Foundation By Jon D. Jelinek, P.E. and Ashish Patel, P.E. Berthed Vessel @ Port of Galveston Cruise Terminal. Courtesy of Webber.

he Port of Galveston has served as a hub for the cruise industry since the early 1990s and ranks as one of the busiest cruise ports in the U.S. Its facilities handle more than one million cruise passengers each year. When the industry demanded increased terminal capacity and throughput, the Port was quick to oblige. However, could consulting engineers with Horner & Wyatt in Kansas City, Missouri, who were responsible for the design of a grain elevator complex for the Galveston Wharf Company in 1930, have possibly envisioned their part in accommodating the Port’s expansion of Terminal 2? For nearly seven decades, the grain elevator played a pivotal role in Galveston. Operations of the grain elevator commenced in 1931 and continued until 1998. Three “banks” of silos with a storage capacity of 5.2 million bushels enabled Galveston to be among the nation’s largest exporters of cotton, flour, sulfur, fertilizer, chemicals and grain while the Wharf Company operated. The 168 reinforced concrete silos, each 20 feet in diameter and over 100 feet tall, were supported on 40-inch thick reinforced concrete mat slabs and more than 12,000 thirty-foot long timber piles spaced at 30-inches on center, each way. Figure 1 illustrates an elevation view of one of the original grain elevators. In June 2003, the abandoned grain elevator site made way for the Port’s $30 million cruise terminal expansion plan with the demolition of the elevator, railcar unloading facilities and offices, and the implosion of the 236-foot tall “head house.” The mat slabs were abandoned in place. In 2013, Royal Caribbean Cruises, Ltd., announced its desire to operate one of its Freedom-Class vessels, the MS Liberty of the Seas®, from the hub port at Galveston. The Liberty of the Seas has a length of 1,112 feet, a beam of 184 feet, a height of 209 feet above the waterline, a draft of 28 feet, an average crew size of 1,300, and fifteen passenger decks accommodating as many as 4,370 passengers. The existing Cruise Terminal 1 and Cruise Terminal 2 facilities encompass approximately 90,000 square feet. To accommodate larger STRUCTURE magazine

vessels and to process the ever-increasing number of passengers efficiently, the Port initiated plans to expand the Terminal 2 facility by approximately 60,000 square feet and solicited tenders from designbuild teams in 2014. Webber, LLC, a construction firm with headquarters in The Woodlands, Texas, was awarded the design-build contract in late 2014. They partnered with BEA Architects, Inc. (BEA) and Lockwood, Andrews & Newnam, Inc. (LAN), a planning, engineering and program management firm, to design the facility and to prepare the construction documents. Professional Service Industries, Inc. (PSI) was engaged to provide supplemental geotechnical engineering services. The $12.7 million project includes a two-story, 55,000 square foot main building that accommodates security screening, ticketing, and check-in, with seating accommodations for approximately 2,500 passengers. A two-story, 5,000-square-foot connector bridge accommodates the security-controlled passenger embarkation and disembarkation process.

Figure 1. Elevation view of Grain Elevator Storage C. Courtesy of Horner & Wyatt.

May 2017

Figure 2. Plan view of the Port’s Envisioned Building Configuration. Courtesy of LAN.

Figure 3. FLAC3D modeling – mat deformations. Courtesy of Webber.

The main building is comprised of a pre-engineered metal building (PEMB) with a full mezzanine framed using open web bar joists and wide-flange steel beams. The connector bridge is also a PEMB with a second level framed using wide-flange steel beams. The ground floor of the main building consists of a conventionally reinforced concrete slab supported on select fill. The ground floor of the connector bridge consists of precast concrete hollow core planks with a topping slab supported by cast-in-place concrete beams and plinths founded on shallow spread footings.

varied between 5.2 feet and 7.8 feet. It was necessary to import five to eight feet of select fill contained by perimeter closure walls to enable a soil-supported ground floor slab. A storm surge event would impose hydrostatic, hydrodynamic, and wave loads, as prescribed by ASCE/ SEI 7 and ASCE/SEI 24, on the perimeter closure walls. When detailed design commenced, geotechnical engineering analyses, including laboratory testing for consolidation of a weak soil stratum located between 8 and 25 feet below existing grade, were not yet complete. When consolidation testing was completed, the structural

Project Challenges

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The bridging documents prepared by AECOM, on behalf of the Port’s alternative delivery strategy, programmed the terminal expansion immediately adjacent to existing terminal buildings whose foundations consist of auger-cast piles. The team was aware of the existence of an abandoned grain silo foundation beneath the footprint of slightly more than half of the proposed expansion and was keen on taking advantage of it for support of some of the superstructure. The remaining area of the expansion was undeveloped. The site is identified by the Federal Emergency Management Agency (FEMA) as a flood hazard area, and the current Flood Insurance Rate Map (FIRM) lists a Base Flood Elevation of 11.0 feet using the North American Vertical Datum of 1988. When the FIRM is updated in the near future, the base flood elevation will be increased to 12.0 feet. As such, the ground floor of the main building was established with a finished floor elevation of 13.0 feet to provide the necessary freeboard above a storm surge event, with a probability of occurrence of one percent each year. Existing grades across the proposed site

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Seattle Tacoma Lacey Portland Eugene Sacramento

San Francisco Los Angeles Long Beach Pasadena Irvine San Diego

Boise St. Louis Chicago Louisville New York

Figure 4. Settle3D modeling – Undeveloped Area Settlements. Courtesy of PSI.

Figure 5. Existing silo mat foundation. Courtesy of LAN.

engineering team was presented with the potential for significant settlements given the magnitude of the anticipated surcharge loads from the superstructure and the overburden from the imported fill. The overburden would impose down drag effects on both new and existing piles, significantly reducing the capacity of the piles to resist compressive loads. Moreover, the potential for significant differential settlement across the proposed new building footprint required design solutions to ensure compliance with the Americans with Disabilities Act and Texas Accessibility Standards. As with any expansion or renovation project, design teams are always challenged with maintaining the form, function, aesthetics, structural integrity, and life safety systems of the existing facility. This project was no exception. Also, the construction team had to accommodate on-going operations at the existing cruise terminals, which created numerous logistical challenges. The Port had only a limited budget to facilitate expansion and renovation of Cruise Terminal 2. Webber tendered a firm, fixed price offer to execute the program.

Figure 6. Plan view of the revised building configuration.

Collaborative and Innovative Solutions The design team developed a number of solutions to accommodate the Port’s envisioned building configuration. Figure 2 outlines the original foundation design that accommodated the existing conditions and the compressible subsoil stratum, and prevented adverse down drag effects on existing pile foundations. Supplemental augercast pile-supported mats were incorporated to accommodate portions of the main building that fell beyond the footprint of the existing silo mats. A structural slab supported by auger-cast piles and pile caps was utilized for a previously undeveloped portion of the site. Moreover, minimum pile-setback distances were maintained along the interface with the existing facilities to mitigate otherwise adverse vibration transmission during pile driving operations. The team collaborated to ensure that the existing grain silo mat foundation, located 13 feet below finished floor, had the capacity and integrity to accommodate 1,500 psf of soil overburden and the anticipated dead, live, and environmental loads imposed on the superstructure. The assessment engaged the geotechnical and structural engineers and Webber’s technical design services and construction teams. Uniform and differential settlements were estimated using sophisticated three-dimensional modeling software (FLAC3D and Settle 3D; refer to Figure 3 (page 22) and Figure 4). An analysis of the load-carrying capacity STRUCTURE magazine

Figure 7. BIM rendering – foundation and ground floor systems. Courtesy of LAN.

of the existing grain silo mat foundation was performed based on a review of the grain elevator design drawings. A non-destructive testing program was initiated to substantiate the integrity of the existing reinforced concrete mat foundation and timber piles. The program included a visual inspection of the exposed mat slab (Figure 5) and measurement

May 2017

of its surface hardness and penetration resistance. Rebound hammer test results were correlated with concrete core sample testing. A timber pile was exposed and visually inspected for signs of deterioration or distress. The results of the assessment were positive, and design documents were finalized for construction. Unfortunately, accommodating the unanticipated site subsoil conditions was cost-prohibitive. With the Port’s cooperation, BEA initiated a complete reconfiguration of the terminal expansion. With the exception of the connector bridge, the entire main building was positioned above two of the existing grain silo mat foundations, as illustrated in Figure 6. The team was determined to utilize a grade-supported ground floor slab for the entire main building. However, the pair of silo mat foundations are separated by 30 feet, and the potential for subsoil consolidation between the pair of mat foundations had to be mitigated. PSI prepared a narrative for pre-loading that area of the site with a soil embankment 20 feet high. PSI further recommended the installation of vertical wick drains to accelerate the consolidation effort. The construction team implemented this action plan a few months before construction commenced in the affected area. The connector bridge was designed with a structural ground floor slab to preclude the need for any imported select fill. Furthermore, shallow spread footings were sized with an allowable bearing capacity of only 1,500 psf. An expansion joint was introduced between the connector bridge and the main building to accommodate differential movements. Refer to Figure 7 for a BIM rendering of the final structural solution for the foundation and ground floor systems.

Project Benefits/Successes Webber and its subcontractors executed the seven-month long construction program with no cruise line disruptions, security breaches,

or safety “recordables.” The expanded and renovated facility has been operational since July 2016. The United States Customs and Border Protection is one of the Department of Homeland Security’s largest and most complex components. The required segregation of disembarking and embarking cruise passengers can often substantially increase passenger “cycle” times. BEA successfully incorporated an interior ramp (Figure 8), coupled with elevators and escalators, that resulted in a significant improvement in passenger throughput. Passengers can navigate from curbside to the ship’s gangway within nine minutes. Most importantly, the Webber team was able to deliver the project within the Port’s approved budget. The Port, Royal Caribbean (et. al ) and cruise passengers can enjoy the benefits of a well-conceived and executed expansion initiative for years to come. Furthermore, BEA maintains that subsequent site expansion efforts are viable and can ultimately accommodate even larger passenger vessels. Royal Caribbean’s MS Harmony of the Seas®, with accommodations for up to 6,360 passengers, was launched on June 19, 2015, and is currently deployed to Port Everglades. There is no doubt that the abandoned grain elevator foundations can accommodate any future expansion. Could Messrs. Horne and Wyatt have envisioned such a legacy?

Conclusion Despite the many challenges, the success of the Cruise Terminal 2 expansion project was made possible because the Port of Galveston, Webber, BEA, LAN, and PSI collaborated on solutions that incorporated imagination, technology, and innovation. With collective diligence and persistence, the team delivered a benchmark for efficiency. When you next embark on a cruise via the hub port at Galveston, know that you are standing above robust foundations designed by engineers in the 1930s.▪ Jon D. Jelinek, P.E., is a Senior Structural Engineer and Associate with Lockwood, Andrews & Newnam, Inc. in Houston, Texas. Mr. Jelinek is a Past President of SEAoT and can be reached at jdjelinek@lan-inc.com. Ashish Patel, P.E., provides structural engineering services for a variety of projects at LAN including municipal buildings, water and wastewater treatment plants, hospitals, institutional buildings, and marine ports and terminals. Mr. Patel can be reached at APatel@lan-inc.com.

Project Team Owner: Port of Galveston, Galveston, TX Structural Engineer: Lockwood, Andrews & Newnam, Inc. (LAN), Houston, TX General Contractor (Design-Builder): Webber, LLC, The Woodlands, TX Design Manager: Webber, LLC, Technical Services, The Woodlands, TX Architect: BEA Architects, Inc., Miami, FL Geotechnical Engineer: Professional Service Industries, Inc., Houston, TX MEP and Civil Engineers: LAN, Houston, TX Figure 8. Disembarking passengers ramp. Courtesy of Webber.

STRUCTURE magazine

May 2017

InSIghtS new trends, new techniques and current industry issues

esiliency is “the capacity to recover quickly from difficulties; toughness.” To a structural engineer, this means a strong yet ductile structure that is survivable and repairable in the face of severe environmental loads, such as major earthquakes. In terms of earthquake resiliency, where does our building inventory stand? We have come a long way as you will see, but we have a long way to go. A generation ago, farsighted policymakers in California, urged on by leading structural engineers, implemented measures to strengthen unreinforced brick masonry buildings, eliminate brick parapet falling hazards, and retrofit concrete tilt-up buildings. Many private companies and public agencies embarked on ambitious programs to strengthen and improve existing structures. Today, cities throughout California, including San Francisco, Los Angeles, and Oakland, have implemented programs to retrofit soft-story wood framed residential buildings of which there are several thousand in San Francisco alone. On the existing building front, excellent progress is being made.

the top, but even that was misleading in that there is significant variability in the data. With the current focus on climate change, global warming, and recycling/conservation, as evidenced by the establishment of LEED design, the United States Resiliency Council (USRC) and the REDi Rating System, the public, architects, and engineers have started to focus on smarter design processes. Using certain kinds of lumber, or recycled steel, or waste coal products in construction certainly help the environment, but when the big earthquake hits and the resulting damage requires the building to be torn down because it lacks resiliency, how “green” was it really? Perhaps the greatest contribution that structural engineers can make to green design is to engineer a resilient structure – a structure that can handle a range of extreme environmental loadings and stay in service. Getting back to the building code, how earthquake resilient are buildings being designed to the code today? Until the past few code cycles, most engineers probably did not know. Fortunately, the commentary to Chapter 1 in ASCE 7-10, Minimum Design Loads for Buildings and Other Structures provides insight. Because the code permits design using PerformanceBased Procedures (i.e. alternative means of compliance) in Section 1.3.1.3, the code writers had to give engineers a performance standard by which their designs could be evaluated. With regard to earthquakes, Table C.1.3.1b, titled Anticipated Reliability (Maximum Probability of Failure) for Earthquake, provides the information and a window into how resilient our structures are. For Risk Category II buildings (most buildings), the commentary states that society can expect total or partial collapse to occur in 10% of new buildings in the maximum considered earthquake and failures that could result in endangerment of individual lives to occur in 25% of new buildings in the MCE. The actual damages will most likely be far greater because existing buildings designed to lower standards should fare worse. Moreover, there is always the possibility of earthquakes that are not considered (i.e. greater than the MCE). How resilient then are our buildings in high seismic regions? You can be the judge of that. A 10% rate of total or partial collapse may not be a concern if you believe an MCE earthquake will not happen. But in reality, they do. The Christchurch area of New Zealand experienced one in 2011 and suffered tremendous devastation. Perhaps the 25% endangerment of individual lives will sharpen our focus? Hopefully, this insight into the expected resiliency of our code-compliant new buildings, and that of our older existing buildings, will motivate structural engineers to educate their clients about the expected performance of perhaps their greatest financial assets, and the ones that protect their friends and families. For a small premium, should we design new buildings to standards in excess of the code minimum to achieve resilience?▪

Earthquake Resiliency – Where Do We Stand ? By John A. Dal Pino, S.E.

John A. Dal Pino is a Principal with FTF Engineering located in San Francisco, California. He serves as a member of the STRUCTURE Editorial Board and may be reached at jdalpino@ftfengineering.com.

However, what about new buildings? The building code is more comprehensive than ever and has incorporated many of the lessons that were learned from past earthquakes, so we are clearly better off. But the building code is a prescriptive document (you shall do this, and you shall not do that) and remains somewhat of a black box. Some argue that the more comprehensive the code becomes, the more of a black box it becomes. Exactly what does a code compliant building provide society regarding earthquake resiliency? The California Building Code states that its purpose is to “establish the minimum requirements to safeguard the public health, safety, and general welfare…” This type of language has been in the code for a long time and is in most other state building codes. However, in terms of safety (or life-safety, being the term used by structural engineers), what does it mean exactly and what structural performance and reliability does it imply? Building codes of the past were generally silent on the issue and did not provide the public with an explanation of the performance that a building owner might expect from a compliant building. Unfortunately, most engineers probably gave the topic little thought either. Code compliance is generally equated with safety and solidity, and that settled that. We all know that in terms of earthquakes, the maximum considered earthquake or MCE is not the largest earthquake that can happen at any given site. The word “considered” in the name should be warning alone that there may be larger earthquakes that have not been considered. The “C” used to stand for credible, which implied something that is near

24 May 2017

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Treating Historic Masonry Very Gently at Drayton Hall By Craig M. Bennett, Jr., P.E. Figure 1. Drayton Hall.

rayton Hall is considered one of the finest, if not the finest, example of Palladian architecture in America (Figure 1). This 1740s plantation house sits on the banks of the Ashley River, near Charleston, South Carolina. It is owned by the National Trust for Historic Preservation and is maintained in roughly the same condition in which it was received from the Drayton family in the 1970s. The house was beautifully put together by skilled craftsmen when originally constructed. However, 270 years of weathering, numerous hurricanes, a major earthquake, and deterioration of materials have all left their marks on this majestic structure. Not surprisingly, some efforts to “fix” things have been less successful than others. Over the last several years, it has been a privilege to both evaluate and design significant repairs to this National Historic Landmark. The lessons learned, or in some cases re-learned in doing this work, are especially valuable to all professionals doing maintenance and repairs on such historic structures. As with most historic structures, work on Drayton Hall began with careful study. The design team was first asked to study the west, or land side, portico of the building (Figure 2) and to determine how serious a problem the deteriorating 1920s concrete supporting the first floor of the portico was. Following that study, the team did a general structural study of the whole building, focusing strongly on the effects of visitorship on the structure.

supported timber joists carrying a timber deck with a stone and concrete walking surface. The first floor was much more interesting. The original timber structure had been replaced because of deterioration, once in timber and a second time, in the early 20th century, with a concrete beam and slab system supporting the brownstone and Portland stone pavers. Unfortunately, some of the concrete beams landed directly in the middle of the relatively flat 1740s brick masonry arches (Figure 3). To make matters worse, the early 20th-century reinforcing was corroding and destroying those same masonry arches. The study of the portico turned up a couple of other things that were relatively interesting. First, the brick piers that supported the arches appeared to have settled. Tracing the elevation of an original mortar joint on the wall above at the first floor showed that the columns beneath had settled just over 3 inches (Figure 4). This unquestionably indicated the need for both foundation investigation and load calculation. Drayton Hall archaeologist Sarah Stroud carefully excavated the soil next to one of the footings. Her work showed that the footing stepped

Study

Figure 2. West Portico.

These two initial studies had several interesting findings. The study of the portico showed that the columns were segmental columns of whitewashed limestone masonry. At the second floor, those columns supported brownstone lintels which themselves STRUCTURE magazine

Figure 3. 20 th Century concrete damaging 1740s arches.

May 2017

Figure 4. Tracing a mortar joint to determine settlement.

Figure 5. Questionable stability of the columns if the concrete were to be moved.

out only half the width of a brick, or two inches, all the way around. Calculation of the loading on the soil indicated that the bearing stress was roughly 6300 pounds per square foot, in an area normally designed for about 1500 psf. And that was just the dead load! Adding full live load resulted in bearing stresses up to about 9,000 psf. Typically this would seem to be a cause for immediate action. However, one must remember that the columns had been in this condition for roughly 270 years. The conclusion was that the settlement should be carefully monitored but that no immediate action need be taken. Additional study of the portico showed that the two outermost columns were supported on tall masonry piers and that those piers had moved slightly away from the wall to which they had been originally attached. These two piers would have to be tied back to the wall and house so they could not move away from either. While the portico had its share of problems, the house itself was generally in better shape, with the exception that the south wall was deformed outward. A simple laser scan confirmed that it had moved outward approximately three inches in the first ten feet from the ground floor to the first floor. Tying the south wall back to the house, particularly at the first-floor level, will be important in the future. It was deemed not as high a priority as was dealing with the damage to the 1740s brick masonry arches caused by the 1920s concrete.

Design With the decision made that the 1920s concrete had to be removed, the question became, “What are the problems associated with doing so?” Further study of the portico indicated that the concrete slab, and even the beams, had been poured tight up under the stone columns. When determining how that could have been done without lifting the columns, it was ultimately concluded that the team did not want to know the answer to that question. Construction was significantly more risky in the early 20th century than it is today, and yet it had been done. The task at hand was to determine a repair method. A simple 3D model of the structural system (Figure 5), without the concrete shown in place, indicated that removing the concrete would cause a tremendous stability issue. It was clear that the stone columns would have to be lifted, but lifting more than a couple of millimeters risked breaking the stone lintels above and destabilizing the second-floor columns as well. So how should one lift a house of stone cards (Figure 6)? STRUCTURE magazine

Figure 6. Segmental stone masonry – a potential house of cards.

The decision–lift from the bottom, remove the concrete, rebuild the masonry, then insert a timber frame and deck to support a well-hidden modern drainage mat under the limestone and brownstone deck.

Construction The contractor for this delicate work was Richard Marks, who not only has a career of experience in historic preservation construction but has done graduate work in historic preservation at the University of Pennsylvania. Architect Glenn Keyes was instrumental in the design of what amounted to a plaza deck to support the stone pavers. The first design for the shoring met with disapproval from the contractor – he was right…right for the wrong reasons, but still right. It was reengineered. Together, an agreement was made on a process to shore the columns off of the existing wall beneath them. This required significant investigation of the wall, then sequencing so that the masonry stayed stable at all times. The sequence would be tricky: • Cut away enough concrete to rebuild the masonry supports next to the columns, but still keep the structure stable; • Rebuild enough masonry to provide support for the lifting mechanism; • Let it cure; • Lift the columns only enough to take the load off of the concrete under the columns;

Figure 7. Lifting mechanism in place.

May 2017

Figure 9. Timber joists taking loads to the sides of masonry arches.

Lessons Learned

Figure 8. Freshly rebuilt masonry under a column base.

• Remove the concrete under the columns and replace it with masonry; • Lower the stone column back onto the rebuilt masonry; and, • Install the timber joists, the deck, a waterproofing layer, a drainage mat and copper drains, a setting bed, a slip system, another setting bed, and finally the brownstone and Portland stone pavers. After reconstruction of the masonry wall on each side of the bases of the columns, the team was able to instrument the plinth blocks, then lift the columns, using the surrounding masonry (Figure 7, page 27) to get the load off of the masonry directly beneath the column bases. The first column was lifted 500 micrometers and the second only 200 micrometers. With the load off of the concrete, removal of the concrete beneath was easy, and reconstruction of the masonry under the bases proceeded (Figure 8). The masons were able to build joist pockets into the masonry wall and line them with copper boots to receive the ends of the timber joists (Figure 9). The timber deck soon followed, as did all of the layers up to the stone pavers (Figure 10). The columns were lowered onto the cured masonry beneath their bases. The portico was ready for another hundred years.

For this job, it became apparent that the principles of working on historic structures were no different from those which are found in so many other structures from the eighteenth and nineteenth century. First and foremost, while Bennett Preservation Engineering was the prime design professional on the project, the work on such important structures is not the work of one person or even one firm. It is the work of a large team that includes other preservation engineers (Andrew Dutton in London), civil engineers, architects, archaeologists, historians, preservation contractors, and most important, an ownership team absolutely dedicated to historic preservation. Other lessons were more mundane: it became acutely evident that later additions, such as 20th-century concrete, were not necessarily a good thing. While early 20th-century concrete holds up very well, early 20th-century rebar does not. It is also evident that the early 20th-century concrete was integrated very well into the 18th-century masonry, and that removing it had the potential to destabilize the whole structure. The team was further reminded that, as Jacques Heyman noted in his simple, clear primer on historic masonry, The Stone Skeleton, historic masonry is almost always controlled not by strength but by stability. Finally, there was a clear reminder that if you have to make a decision between saving 1740s masonry and 1920s concrete, go with the masonry.▪

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Figure 10. Rebuilt Portico floor.

Craig M. Bennett, Jr., P.E., is head of Bennett Preservation Engineering in Charleston, South Carolina, where he additionally teaches in the Clemson/College of Charleston joint graduate program in historic preservation. He can be reached at cbennett@bennettpe.com.

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STRUCTURE magazine

May 2017

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UNIVERSITY OF NOTRE DAME Campus Crossroads

By Andy Greco, P.E. and Peter Heeringa, P.E., S.E.

he Campus Crossroads Project is the largest building initiative in the history of the University of Notre Dame. The $400 million project includes construction of more than 750,000 square feet of classroom, research, student facility, digital media, performance, meeting, event, and hospitality space. Notre Dame University is recognized around the world for its first class academics, athletics, and campus architecture. The campus buildings are notable for their collegiate gothic style. The scope of the project includes three new buildings in addition to numerous renovations of the existing stadium envelope and interior. The new structures wrap themselves around three sides of the existing football stadium with broader University programs, such as student life initiatives located in the West Building, the psychology department and digital media in the East building, and the Department of Music making its home in the South building. The massing of the exterior masonry was detailed aesthetically to match Knute Rockne’s original stadium. The facade is layered and set back as it ascends over multiple stories. As a result, complex shaped built-up brick and block masonry walls and piers rest on multiple framed floors.

Reinforcement Detailing Due to the complexity of the façade, sdi-structures (sdi) established a system on the structural drawings where each unique concrete masonry unit (CMU) pier was detailed and then tagged on elevation views (Figure 1). Altogether 65 individual piers were detailed in the East and West buildings, with each detail highlighting the CMU reinforcing requirements and the bracing requirements for CMU back to the superstructure. Red lines were included in the elevations to highlight locations of brick relief.

Masonry Pier and Wall Reinforcing Analysis Walkcowitz Consulting Engineers (WCE), a consulting structural masonry expert, was hired by the mason contractor to conduct the final masonry backup designs, which were reviewed by sdi-structures and processed through the construction change system. Once the STRUCTURE magazine

Figure 1. A sample of structural exterior elevation.

sub-contracts were awarded for the masonry packages, revised wind loading criteria, evaluation of the materials to be used, and a comprehensive construction document package allowed for further investigation and final design of the masonry backup systems used throughout the three building projects. WCE applied finite element analysis to approximately 25 masonry piers that were found to be generally representative of the 65 piers identified in the project structural drawings.

Masonry Anchorage to Superstructure The CMU piers were anchored to the steel beams at each floor level. sdi-structures, in consultation with WCE on behalf of the Michigan Structural Masonry Coalition, reviewed available products for anchoring the CMU backup to the structural frame. Due to the hundreds of anchor locations, sdi thoroughly researched backup anchorage solutions that could resist the concentrated wind forces while maintaining cost efficiency and speed of installation. The concept for the final anchor detail (Figure 2), which included a slotted channel and strap anchor, was commercially available but had low capacities for pull-out and compression. Given the tall floorto-floor heights up to 16 feet, parapet heights as much as 7 feet, and discrete pier construction, the load demand on each anchor required a more robust channel-strap system than was commercially available.

May 2017

Figure 2. Typical CMU backup anchor detail.

The chosen anchor design allowed for 8 inches of vertical play to accommodate unique CMU coursing conditions at each floor. And yet, the design maintained a consistent detail. When installed, the anchor was designed to provide adequate out-of-plane bracing of the wall for wind and seismic loads while maintaining vertical slip with respect to the floor construction. The anchor also had to have the capability to reach out over variable distances, as dictated by the layered gothic façade, to secure the walls and piers. This meant that the tee strap component of the system needed to resist buckling in compression at variable lengths. Per AISC 360-10 Table User Note E1.1, section E3 of the specification was used to evaluate the flexural buckling capacity of the strap anchors in compression. Given the uncertain level of rotational restraint at each end of the strap, a K factor of 1.0 was used to determine anchor compression capacity. The “neck” of the T-strap was sized to maintain adequate tension capacity at the slot. The geometry of the T-strap, as a whole, balanced strength requirements with constructability tolerance. Multiple thicknesses of the tee strap were designed so that the mason could choose the appropriate one based on the distance needed. The “slotted channel” was selected from a readily available HSS and fabricated with a full height vertical slot to allow for easy insertion of the tee (Figure 3). The contractor was given the option of using bent plate steel in place of the slotted HSS.

continued on next page

Figure 3. Details of slotted anchor channel and anchor strap.

STRUCTURE magazine

May 2017

Figure 5. Sample relief angle FEA model.

Figure 6. Sample “split pier” floor framing.

Figure 4. Von Mises results for anchor channel FEA model.

In addition to the tradition buckling analysis performed for the strap anchor (per AISC Specification Chapter E), a finite element model was used to validate the anchor channel (Figure 4 ). The model included shell elements 5/16-inch thick, with the element mesh kept small enough to capture local bending effects. Von Mises stresses in the plate elements were considered to evaluate the principal stresses in the plate steel due to bending and axial loads, with the final stress levels kept low enough to prevent yielding or fatigue failure of the channel. Where possible, the anchor channels were shop installed on the beam webs and delivered to the site ready for CMU installation. To avoid conflicts with mechanical trades in the ceiling, it was also critical that the perimeter structural details did not require kickers. By locating the wall bracing connection near the top of the perimeter beams, kickers were avoided across the project and bracing was achieved with only discrete “tie angles” tight to the composite deck at each pier location.

Masonry Relief Given the complex brick patterns, relief angle design and attachment posed a unique challenge. Relief angles were supported using a combination of post-installed epoxy anchors, screw anchors, through bolts, and welded embed plates. Cavity dimensions varied across the project to help create the layered appearance of the brick, with some cavities more than 8 inches. To limit the number of visible horizontal joints in the brick, certain relief angle assemblies supported as much as 35 feet of brick above. To determine accurate relief angle deflections and anchorage forces, sdi created finite element models of each relief condition (Figure 5). These models allowed the team to design with confidence and efficiency, which would not have been possible with traditional hand calculations. STRUCTURE magazine

The geometry of the building, with brick supported on multiple levels of the superstructure, required careful consideration of differential expansion of brick on each level. Each time brick wrapped over the top of roof levels, it was necessary to create a horizontal joint in both the backup CMU and the brick veneer. These “split piers,” as they came to be known, required careful coordination of multiple items including structural steel framing, the edge of the concrete deck, and the extent of the horizontal joint to ensure the joint’s performance without compromising the look of the veneer. Figure 6 highlights a typical split pier condition as the masonry wraps over an exterior terrace on the North end of the East building. At split piers, the stub cantilevers were mitered at 60 degrees to allow the block from below to extend uninterrupted to the beam top flange. The horizontal relief joint was then created by embedding a flat plate in the masonry block coursing that was cut to the specific profile of the surrounding brick. Additionally, the plate size was set by determining the weight of CMU required to counter the overturning moment applied by the brick relief. The Campus Crossroads masonry work is nearly complete at Notre Dame. When finished, it will stand as an example of the highest levels of tradition, elegance in design, craftsmanship, and engineering.▪

Andy Greco, P.E., is the President and Co-founder of sdi-structures in Ann Arbor, Michigan. He can be reached at andy@sdistructures.com. Peter Heeringa, P.E., S.E., is a Partner at sdi-structures in Ann Arbor, Michigan, and founder of iStructures, LLC. He can be contacted at peter@sdistructures.com. May 2017

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Historical EnginEEring

revisiting theory and problem solving of the past

Methods of Structural Analysis By Charles Sanders Peirce Editor’s Note: The following is an excerpt from an original text by Charles Sanders Peirce. Please see the sidebar for additional information.

first made approximate calculations following the method developed by Prof. Melan in the second edition of the Handbuch der Ingenieurwissenschaften [Handbook of Engineering Sciences] (Vol. II. chap. xii.). But a practical application of this method led me to consider it inconvenient both in its analysis and its modes of numerical computation. Melan endeavors to adhere, as far as possible, to the method of moments. I believe it ought to be entirely abandoned at the outset. A problem involving n elements of any kind and capable of some sort of solution almost always becomes essentially simplified when n = 2. By saying it becomes essentially simplified, I mean that some special theorem then becomes applicable which practically revolutionizes the solution. For example, an algebraic equation of the nth degree becomes conveniently soluble for n = 2. Usually, in such cases, even though n exceeds 2, as long as it remains very small, it is still possible to employ a generalization of the theorem for n = 2. But as n increases, the advantage of any such generalization soon becomes converted into an enormous disadvantage. Thus, the solution of an algebraic equation of the nth degree, which is so exceedingly handy for n = 2, is also possible for n = 3 and even for n = 4, though it is seldom practically wise to employ it in the last case. The method of moments is somewhat analogous. For a beam of infinitesimal flexibility resting on two supports, nothing could be happier. Even when there are more than two supports, Clapeyron’s theorem of three moments enables us to use this method. But as the structure becomes more complicated, and especially when the effect of flexures upon the moments have to be taken into account, it is far better, in my opinion, to leave moments out of account until the deformations have been ascertained. If the deformations are insensible, each piece can be considered separately, and the usual graphical methods are in most cases perfectly satisfactory. But in cases in which the deformations are sufficient sensibly to displace the lines of action of the forces, although they do not take place so swiftly that the momentum of the masses has to be taken into account, I would recommend equations so formed that the only unknown

quantities are displacements and deformations. As this method, which is substantially that of Lagrange, is not used by engineers, I will endeavor to describe it that it may be readily understood by those who are rusty in their mathematics. The first step, whenever a practical problem is set before a mathematician, is to form the mathematical hypothesis. It is neither needful nor practical that we should take account of the details of the structure as it will exist. We have to reason about a skeleton diagram in which bearings are reduced to points, pieces to lines, etc. and [in] which it is supposed that certain relations between motions are absolutely constrained, irrespective of forces. Some writers call such a hypothesis a fiction, and say that the mathematician does not solve the real problem, but only a fictitious one. That is one way of looking at the matter, to which I have no objection to make: only, I notice, that in precisely the same sense in which the mathematical hypothesis is “false,” so also is this statement “false,” that it is false. Namely, both representations are false in the sense that they omit subsidiary elements of the fact, provided that element of the case can be said to be subsidiary which those writers overlook, namely, that the skeleton diagram is true in the only sense in which from the nature of things any mental representation, or understanding, of the brute existent can be true. For every possible conception, by the very nature of thought, involves generalization; now generalization omits, means to omit, and professes to omit, the differences between the facts generalized. In following out these logical reflections, I am not wandering from the matter in hand so far as might be supposed. For there are many men who will fancy that the question whether a balanced force can be said to exist or not to exist depends upon whether force is a “reality,” or “fact,” or “entity,” or whether it is a “mere mathematical expression.” Such persons can certainly not be accused of any unusual confusion of ideas when we find such a mind as Prof. Tait arguing the similar question of whether energy has “objective existence,” or not. But all such persons confound the quite anti-rational experience of an outward reality, or force, with any possible rational conception of a reality.

STRUCTURE magazine

May 2017

Transcriber’s Comments This text comes from Peirce’s manuscript 1357 as maintained by the Houghton Library at Harvard University, cataloged in 1967 by Richard S. Robin, and made available online by the Scalable Peirce Interpretation Network (SPIN) at http://fromthepage.com/collection/ show?collection_id=16. It constitutes a partial draft of the “Report on Live Loads” that he prepared in the mid-1890s for George S. Morison’s proposed span across the Hudson River, and immediately follows the excerpt that appeared in the Editorial (The Esthetics of Structures) in the February 2017 issue of STRUCTURE. Peirce suggested that “the method of moments” used by most engineers in his day was not well-suited to complicated structures, such as suspension bridges. He advocated the now-familiar theory of virtual work instead, although he did not use that term, instead referring to it as “the method of Lagrange.” The equation that Peirce provided for elastic strain energy, the area under the linear force-deformation curve, uses (q-q) for the virtual displacement and “cessiciosity” – from the Latin cessicius, which means ceding, conceding, or surrendering – for the inverse of stiffness, or what we today call flexibility; i.e., σ = L/EA for an axially loaded member. In describing the first step of any engineering analysis, Peirce characterized it as diagrammatic reasoning, just as he did in the 1898 article that I quoted throughout Part 2 of my recent “Outside the Box” series on The Logic of Ingenuity (STRUCTURE, October 2016). He acknowledged that by employing “a skeleton diagram,” the process “does not solve the real problem, but only a fictitious one.” His subsequent digression into “logical reflections” is worth pondering for its insights into how engineers think and why it (usually) works. Jon A. Schmidt, P.E., SECB, (jschmid@ burnsmcd.com) is an associate structural engineer in the Aviation & Federal Group at Burns & McDonnell in Kansas City, Missouri. He serves as Secretary on the NCSEA Board of Directors, chairs the SEI Engineering Philosophy Committee, and shares occasional thoughts at twitter.com/JonAlanSchmidt.

if it exists, another equal upward acceleration exists; while if the former does not exist, neither does the latter. We can save the appearance of verbal absurdity either by saying that the component accelerations exist in any case, but “may be neglected” in the statical problem, or by saying that the component accelerations, or other vectors, which balance one another, are mere mathematical expressions without real existence. Both of these are mere façons de parler [manners of speaking], which do not affect the meaning of what is said at all. In the method, I propose every force is looked upon as the acceleration of a mass coupled with that mass. In the statical problem, every force is balanced. If any independent element of the deformation of the structure is known, we can if we please take cognizance of it and call it a constraint against which there are no forces. If, however, we wish to determine one of the forces which are balanced in that element of the deformation, we annul the constraint and use our knowledge of the amount of that deformation to eliminate the sum of the forces balanced against the one we wish to determine. The maxim will be: where there is constraint there is no force, meaning by “where” at the same points and in the same directions. A corollary from that maxim is this: whatever relative displacement does not take place may be treated as impossible.

Whenever there is a numerous series of parts near to one another and in all important respects regularly related to one another in their serial order, there will be an essential simplification in generalizing the description so far as to represent the whole as a continuously deformable body. Thus, a chain will be treated as a cord and a Warren girder as an elastic line. It is true that the resulting differential equation may make a puzzle for the computer [problem solver], from which a temporary resort to the discrete hypothesis may help to extricate him. Nevertheless, there are conclusions which can be drawn from a differential equation which would not follow from the corresponding equation of finite differences, and such conclusions almost always hold good for the structure. The second step would consist in forming an expression for the work required to produce any possible given deformation of the structure. By a possible deformation, I mean any deformation which under the conditions we can suppose to be the state of equilibrium that we wish to study. It is not necessary that the expression should hold good for any other deformation. Moreover, the expression may have a constant term which we need not trouble ourselves to determine. continued on next page

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That is just the confusion into which Bayle and a hundred others have fallen in accusing the great Cynic [Diogenes] of committing an ignoratio elenchi [fallacy of ignorant conclusion] when in answer to the Velean’s [Zeno’s] argument against motion he just got up and walked. He could not have committed a fallacy of any kind since he did not reason at all. He simply repeated, for his part, the experience which blindly and brutally forced the individual fact upon him. Zeno having argued that motion was unreal because unintelligible, Diogenes recognized that, whether intelligible or not, as a fact it compels recognition. But he said nothing because he saw that that sort of reality has to be experienced here and now. If a man carrying a pole on his shoulder knocks one in the eye, there is a reality in that force which I am brutally compelled to see. But when it is a question, not of an experience here and now, but of a general description of things, we can attach no other meaning to the reality of that, than that the statement that it exists is true; and to say that it is true means that there is no understanding the facts without acknowledging this statement. If a ball rests upon a table, it is perfectly true to say that ball receives continually a component acceleration downward from gravity plus an equal component acceleration upward from the elasticity of the table. For we cannot state the behaviour [sic] of the ball under gravity without enunciating a general proposition from which this is a corollary. Since, then, it is true to say there is a component acceleration downward, that component acceleration is a real fact. Its reality consists in its being a case, though only a limiting case, of a general law involving continuity. Generality and continuity are almost the same thing. We might even allow ourselves to say that the ball has a component acceleration east plus an equal component acceleration west, provided there was any general truth from which this statement could receive any meaning. On the other hand, there is never any falsity in keeping silence about, or otherwise not referring to, wholly irrelevant circumstances. Now if we are exclusively concerned with the statical problem of the ball on the table, and if the table neither suffers any sensible compression nor is [in] danger of breaking under the weight, then the fact that the ball would fall under other circumstances is utterly irrelevant, and it is perfectly true to say that it receives no acceleration whatever. Here, then, is [an] acceleration of 32 feet a second [per second] which really exists or does not exist, just as we choose to think it. Only

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The work required to overcome a force, F, acting along a curve so as to tend to increase, s, the arc of that curve, is -∫Fds. Hence, if P is a weight at a depth y below a fixed level, the part of the work due to this is -Py. If one of the deformations of the structure consists in changing the value of a quantity q and such deformation is in the line of an elatistic [sic] force tending to give q the special value, q, and if what I call the elastic cessiciosity, or yieldingness, of q is σ, then the corresponding term of the work is (q-q)2/2σ. The third step is as simple as the second. In a statical problem treated in the manner proposed, the work is always a continuous function of the elements of the deformation and has a differential coefficient. Where there is equilibrium, every differential coefficient relative to a possible deformation vanishes. Applying this principle, we obtain an equation for each degree of freedom of the rigid parts and a sufficient number of differential equations to determine the deformations of the continuously deformable parts. The latter have to be treated with circumspection, or the wrong signs will be obtained. Attention must be paid also to the terminal conditions. The fourth step consists in solving the equations so obtained, which will be of a complicated nature. The best way will be to substitute for each unknown the sum of an assumed approximation plus an infinitesimal unknown, and so reduce the equations to linear forms, which will give a new approximation. This method may fail, though it can hardly do so if the first assumptions are sufficiently near the truth. For the exceptional cases in which it cannot be made to work, I have no general recommendation to make. The fifth step will consist in determining from the displacements the values of such moments and forces as may be wanted. I repeat that there is nothing novel about this method, except that it is not in practical use…▪ Charles Sanders Peirce was an internationally acclaimed scientist, engineer, and philosopher who lived from 1839 to 1914. He is widely recognized as the founder of pragmatism, the only major school of philosophy native to the United States, as well as semiotics, the study of signs and sign-action.

aids for the structural engineer’s toolbox

EnginEEr’s notEbook

Building Cladding The Role of the Specialty Structural Engineer By Steven Judd S.E.

he Structural Engineer of Record (SEOR or EOR) is responsible for the design of the primary structure and possibly, to some extent, to define or prescribe the elements of the exterior enclosure (cladding) system for the structure. The enclosure system may be cold-formed light gauge framing elements, architectural precast concrete, some sort of panelized (either site built or prefabricated) wall system, other systems, or combinations thereof. The Specialty Structural Engineer (SSE) is generally responsible for the final design and installation drawings of the aforementioned exterior enclosure system(s). The SSE is most often retained by one of the sub-contractors supplying the enclosure system. It is not uncommon for the exterior cladding design of a typical commercial or institutional structure to be the overlooked piece of the primary structural design, and for the SEOR to only make a few assumptions about the impact of the cladding itself: uniform weight of the skin and assumed load paths of the exterior skin of a building. Detailing might consist of a note here and there about 16 gauge studs spaced at 16 inches on-center, with a slip joint located somewhere near the floor line or window head elevation to represent the preferred design in the bid documents – with the details to be flushed out in the deferred submittal process by the SSE, if specified. For the design of the primary lateral force resisting elements of the structure, it may be adequate for the SEOR to assume a uniform averaged weight for the cladding and exterior wall system, and assume it is applied in a linear fashion along (or on top of ) the edge of the slab. However, the reality is that there are foreseeable instances where a uniform load is not the correct assumption for slab edge or spandrel beam designs, particularly when multiple glazing systems are utilized, such as curtainwall in some areas and punched or ribbon windows in others, interspersed with solid walls. It is also not uncommon for the SEOR to place somewhat unrealistic prescriptive requirements on the cladding design, such as: “Structural elements at the perimeter of the building have been designed for vertical loads only. Cladding attachments shall not apply

moments to the slab edge or lateral loads to steel beams, or introduce torsional loads into steel beams or columns.” (Paraphrased from a recent project’s structural general notes.). Furthermore, the structural General Notes may state, “Braces, added reinforcing, and ties shall be designed and supplied by the contractor [meaning the cladding sub-contractor] for load eccentricities and lateral loads.” For by-pass cold-formed light gauge steel stud framing – exterior wall framing “passes by” the edge of the slab (for continuity of the framing) – the standard stud clip used to attach the stud to the slab edge creates eccentricities and induces torsions to the slab edge. This is a foreseeable condition for the SEOR. The SEOR should address these common conditions, not push that responsibility to design slab reinforcing to the SSE. Parapet framing and spandrel framing for ribbon windows frequently require kickers from the exterior wall stud framing to either the slab or the spandrel beam bottom flange (or spandrel beam web) to stabilize the wall framing that lacks vertical continuity. This is another condition where the lateral loads and moments induced on the primary structure are foreseeable and should be the responsibility of the SEOR to resolve (design and detail). For phased or fast-track projects, by the time the cladding is bid, awarded, designed, and detailed, the structural framing might well be underway and perhaps past the point where added reinforcing in the slab, if necessary, is even achievable. Added reinforcing in a slab is generally not the purview of the cladding subcontractor, nor are copious amounts of structural bracing elements. The most efficient cost model and most efficient construction schedule would have the concrete subcontractor include all concrete reinforcing in their scope (as defined in the bid documents), and for the structural steel contractor to include all hot rolled bracing elements (for spandrel beams and/or columns, as defined in the bid documents) in their scope. So how does one resolve this dichotomy of design responsibility? Possibly the best example would be to include the responsibility of the cladding

STRUCTURE magazine

May 2017

design as an additional scope (with appropriate additional fee) for the SEOR, or to engage an SSE during the design phase to work out at least one viable solution to the cladding design so that what goes out to bid is a full package of information, including all concrete reinforcing required to resolve loads and eccentricities, added bracing to resolve induced torsions, and more. At the very least, it would be prudent to carry a reasonable allowance for concrete reinforcing and structural bracing elements for a specific cladding solution which can then be applied toward the final solution, as represented in a deferred submittal process. If the cladding design is provided via a deferred submittal process, and that process generates a different solution with regard to added reinforcing and/or bracing requirements, at least the base bid documents can be used as a design guide or as a definitive source for material and labor allowances, or credits for an alternate solution. As structural engineers, it would be prudent to take opportunities, as they arise, to enlighten owners and clients to the advantages of providing a more developed cladding design for bid documents which does not bifurcate portions of other sub-contractors’ scope of work by having the cladding subcontractor add concrete reinforcing and/ or miscellaneous hot rolled steel bracing. The optimum way to do this is to engage the SEOR to provide the added scope of a more fully developed cladding design or to engage an SSE to provide the necessary cladding design services for the bid documents. At some point, one way or the other, the owner will be paying for the cladding design and associated connections design impact, whether it is provided by the SEOR on the front end or the SSE on the back end. The advantage of front-end design is that there is cost and schedule savings when all elements are identified and bid as part of a larger volume of work and materials.▪ Steven Judd is the Director of Engineering for KEPCO+, an architectural cladding systems design-build subcontractor. He can be reached at sjudd@kepcoplus.com.

Business Practices

business issues

Minimize Scope Creep by Learning Wants and Needs By Stuart G. Walesh, Ph.D., P.E.

cope creep is a constant plague on some if not every project. As typical projects proceed, the client (Owner, Architect, or Contractor) requests alternatives to the original design, more features, attendance at additional meetings, presentations to stakeholders, and other efforts beyond those defined in the Scope of Work contained in the contract. Too often, engineers react to these requests and forge ahead without considering the consequences. Perhaps performing the “freebies” is rationalized by claiming that each is innocuous or part of marketing the client for future work. Over time, performing freebies gradually diminishes project profitability and destroys budgets. Unfortunately, pushing back runs the risk of alienating the client and jeopardizing future work – both awkward predicaments every engineer should actively avoid.

Ways to Avoid Client-Driven Scope Creep With sufficient individual and organizational will and discipline, engineers can proactively, mostly in a pre-contract mode, apply practical methods to prevent most client-driven scope creep. For example, earn the trust of those we serve early in the relationship, prepare a comprehensive project plan, define quality, “front end” participation by our organization’s experts, conduct a risk analysis, identify “their” and “our” project responsibilities, carefully draft an agreement, and prepare and share some deliverables. The preceding scope creep minimization list intentionally omits striving to learn client wants and needs. The wants-needs topic is singled out because it is the most important element in providing professional services in the private and public sectors. Engineering is ultimately a people-serving profession. Starbucks CEO Charles Schulz said, “We are in the people business serving coffee, not in the coffee business serving people.” Let’s paraphrase that for our profession: “We are in the people business serving engineering, not in the engineering business serving people.” This is an important paradigm shift, and the subtle distinction can profoundly affect how engineers go about their daily professional activities, including minimizing scope creep which arises when wants and needs are not adequately

defined. Wants and needs missed or misunderstood before a project starts are likely to appear later – because they are important to the client. When they do, they often drive scope creep.

Ask – Ask – Ask So how can engineers understand a potential client’s wants and needs? Obviously, review past experiences with the client; study the RFP thoroughly; examine the client’s/organization’s website to discover their values, mission, and vision. Also, talk to colleagues who know and have worked with the client, and knew them well. Most importantly, ask the client many and varied questions. An initial reaction may be that the need to ask questions is obvious, so why dwell on it? The author’s experience indicates that many engineers are reluctant to go beyond superficial, innocuous, and obvious queries. Polls conducted during the author’s webinars and workshops repeatedly reveal that the principal reason engineers give for their reluctance to ask questions is the fear that they will appear uninformed and poorly prepared. On the contrary, asking probing questions indicates that a person is well informed and prepared, and that the engineer knows the questions to ask. Consider the following three question-asking techniques.

Mix Closed-Ended and Open-Ended Questions Use some closed-ended questions – questions that can be answered with a yes, no, or statements of fact. For example, how much is budgeted for the new bridge? Also ask openended questions, which often begin with “why,” “how,” or “what.” An example: “Why are you considering a new bridge rather than upgrading and widening the existing bridge?” Mixing closed and open-ended questions are likely to stimulate a very enlightening conversation, one that reveals wants and needs.

Five Whys This persistent tactic enables a “drill down” – to get to the bottom of things – to move past symptoms and get to causes. Diplomatically and persistently ask “why,” or variations of “why” questions up to five times. There is nothing magic about five, but at least several

STRUCTURE magazine

May 2017

“why” questions are needed to get to the cause of the problem and the reasons the client is seeking engineering services. For each question asked, seek to determine and understand the client’s wants and needs by listening to their answers and probing deeper by asking why. Why questions deepen and widen thinking, and sometimes lead to surprising results, brilliant insights, trustful life-long relationships, and a better understanding of the scope that can be agreed upon and included in the contract.

Kipling’s Six In saying “I had six honest serving men – they taught me all I knew. Their names were Where and What and When and Why and How and Who,” English writer Rudyard Kipling offers another method for effective questioning. Hard to conceive a challenging potential project that does not connect with Kipling’s six elements. During meetings, negotiations, and discussions with the client, use Kipling’s Six to guide questions and learn more about the client’s wants and needs.

Conclusion Scope creep is typical on most projects and frequently costly for the engineer. However, scope creep can be minimized by the systematic application of appropriate methods. The prime scope creep control measure is learning the client’s wants and needs before a project begins, which is accomplished by diplomatically and proactively asking probing questions, mixing closed- and open-ended questions, using the five “whys,” and applying Kipling’s Six. Once an engineer has asked the questions and understands the client’s wants and needs, include them in the Scope of Services and prevent scope creep from draining profits.▪ Stuart G. Walesh is an Independent Consultant providing management and education/training services. Also an author, his most recent books are “Engineering Your Future: The Professional Practice of Engineering,” Wiley 2012, and “Introduction to Creativity and Innovation for Engineers,” Pearson 2016.

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Trimble Solutions USA, Inc.

Phone: 800-925-5099 Email: web@strongtie.com Web: www.strongtie.com Product: CFS Designer™ Software Description: Design CFS beam-column members according to AISI specifications and analyze complex beam loading and span conditions. Intuitive design tools automate common CFS systems such as wall openings, shearwalls, floor joists, and, with the newest software update, up to eight stories of load-bearing studs.

Phone: 919-845-1025 Email: support@steelnetwork.com Web: www.steelnetwork.com Product: ThermaFast Description: ThermaFast is an engineered installerfriendly set of steel framing tracks and angles designed to be an integral part of the continuous rigid insulation, and at the same time provide a stable component for direct substrate attachment.

Phone: 770-426-5105 Email: kristine.plemmons@trimble.com Web: www.tekla.com Product: Tekla Structures Description: Models carry the accurate, reliable and detailed information needed for successful Building Information Modeling and construction execution. Tekla links with major AEC, MEP, and with plant design software solutions thanks to open BIM approach and IFC compliancy. Also integrates with industry leading construction management and analysis and design software, and most major advanced production or resource planning and machine automation systems.

Strand7 Pty Ltd Phone: 252-504-2282 Email: anne@beaufort-analysis.com Web: www.strand7.com Product: Strand7 Description: An advanced, general purpose, FEA system used worldwide by engineers and analysts for a wide range of structural analysis applications. Strand7 can be used as a standalone system, or with Windows applications such as CAD software. It comprises preprocessing, solvers (linear and nonlinear static and dynamic), and postprocessing.

Product: SigmaStud Description: Provides installation and design advantages. Each bend made to a flat LSF element increases load capacity over a standard stud section with the same material thickness. The return lips present in SigmaStud also increase capacity, delivering the most efficient LSF load-bearing stud member available. Product: VertiClip Description: High quality vertical deflection connectors for all light steel framing applications that require a movement-allowing connection to the structure. Provides both an anti-friction and antiseizure connection between the clip and the stud web surface preventing transfer of vertical forces.

USG Structural Solutions Phone: 312-436-4260 Email: jmestrada@usg.com Web: www.usg.com Product: USG Structural Panels Description: High-strength, reinforced concrete panels for use in noncombustible construction. Lighter than precast or poured concrete, USG Structural Panels install like wood sheathing and are mold-, moisture- and termite-resistant. Providing a faster, easier, and more efficient way to build floor, walls, and roofs.

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May 2017

Steel/Cold-Formed Steel ProduCtS Guide a definitive listing of steel/cold-formed steel product manufacturers/distributors and their product lines Simpson Strong-Tie

The Steel Network, Inc.

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May 2017

award winners and outstanding projects

Spotlight

Structural Innovations of Lotte World Tower

By SawTeen See, P.E., C.E., Dist. M.ASCE, M. Eng., Leslie Earl Robertson, P.E., C.E., S.E., D.Sc., D.Eng., NAE, Dist. M.ASCE, AIJ, JSCA, AGIR, and Edward J. Roberts, P.E. Leslie E. Robertson Associates was an Award Winner for its Lotte World Tower project in the 2016 NCSEA Annual Excellence in Structural Engineering Awards Program in the Category – New Buildings over $100M.

t 1,820 feet (555 meters) tall, the 123-story Lotte World Tower stands as the first supertall building in South Korea, more than 1.8 times taller than the previous tallest building (located in the nearby city of Incheon). The building topped out in December 2015 and the tower’s mast, dubbed the “Lantern,” was complete in March 2016. Designed by architects Kohn Pedersen Fox Associates and structural engineers Leslie E. Robertson Associates (LERA) of New York, the $2.5 billion tower and adjacent development features a variety of usages, including office, retail, hotel, officetel (a combination office and apartment common in Korea), parking, museum, and observation space. The tower’s tapered shape – with sloping concrete core walls in the middle third of the building and columns sloping in two directions – creates a unique environment on every floor. Occupying the majority of the tower (from the ground level to the 71st floor), the office and officetel floors are steel framed with a slab-ontruss deck, whereas the hotel floors (from levels 87 to 101) are concrete flat slabs with drops. The diagrid structure at the top of the building contains premium office, museum, and observation floors, which are also steel framed with a slab-ontruss deck. The tower sits on top of a 21.3-foot thick mat (6.5 meters), with piles reinforcing the ground. These piles are not connected to the mat, to conform to Korean building regulations. Though the tapered shape of the building led to challenging structural complexities, it was effective at minimizing wind loads. The tower’s primary lateral load and gravity system consists of eight concrete mega-columns, concrete core walls and a series of outriggers and belt trusses located at the mechanical, refuge, sky lobby, and hotel amenity floors. The belt trusses transfer the diagrid “lantern” structure to the column configuration of the hotel floors, as well as the columns of the hotel floors to the mega-columns at the officetel and office floors. Level 5 to Level 7 are hung from the lowest belt truss. Only two levels of outriggers tying the perimeter mega-columns to the concrete

core were needed to control the tower’s drift and lateral accelerations due to wind loads. The 10-foot 9-inch by 10-foot 9-inch (3.3-meter by 3.3-meter) mega-columns at the ground level (unbraced for the first eight levels) are comparatively small compared to other towers of similar height and even qualify as slender members. LERA worked closely with the architects to strike a balance between the structural efficiency gained by adding columns and the need to preserve open floor plans. Several structural designs were studied: a system of concrete mega-columns with relatively small intermediate steel columns at the perimeter; a system of long-span spandrels with clear spans between concrete mega-columns; and a combination of the two. The owner, Lotte, selected a system of long-span spandrels for the office and officetel floors, with spans of up to 80 feet (24.5 meters) between mega-columns. At the building corners, the long-span spandrels cantilever 46 feet (14 meters) beyond the mega-columns while bending to follow the building’s curved floors. These corners posed significant challenges for meeting the stringent deflection and vibration floor criteria. In response, LERA designed a series of 1-story-high deflection control posts at every other floor, aligned with the cladding mullions. Since these small members are not required for strength, they were not fireproofed. Higher up the building, the hotel floors are supported by perimeter steel columns that are spaced at the module of the hotel room partitions and transferred through belt trusses. The design for gravity and lateral loads from wind and earthquakes is only one element of a grander structural design. For the Lotte World Tower, robustness and redundancy were foremost considerations in the design. At the outset of the project, LERA’s team of engineers studied various disproportionate collapse scenarios, including the loss of members in either the belt trusses or the steel perimeter columns. As a result, some belt truss members were increased in size where required. The effects of creep and shrinkage also had to be taken into account, such as differential

STRUCTURE magazine

May 2017

settlements between the perimeter and the services core. The potential imbalance of the floor was partly addressed by vertically cambering the mega-columns during construction, allowing forces to be transferred through the outrigger members as the megacolumns settle further than the concrete core. LERA recommended delaying the final connections of the outriggers to mitigate the force transfer in the short term, while also designing for the potential force transfer due to longterm creep and shrinkage. The tower topped out in 2016 and stands as the fifth tallest building in the world.▪ SawTeen See is the Managing Partner at LERA. She led the structural design of the Lotte World Tower, Seoul, South Korea. Dr. Leslie E. Robertson, an internationally acclaimed structural engineer, has transformed urban and rural landscapes through pioneering and innovative designs. Among a plethora of notable projects, Dr. Robertson led the structural design of the World Trade Center, New York, NY. Edward J. Roberts is the Project Manager at LERA. His expertise in 3D modeling has advanced the development of LERA’s analysis software.

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NCSEA News

News form the National Council of Structural Engineers Associations

NATIONAL

TNSEA Holds Seismic Workshop

NCSEA Special Awards

In March, the Tennessee Structural Engineering Association (TNSEA), in conjunction with the Building Seismic Safety Council (BSSC), hosted a regional seismic mapping workshop in Memphis, Tennessee. This workshop presented the history of ground motion development by the BSSC, through the Project 17 Committee, which is tasked with providing seismic design value recommendations for use in the development of ASCE 7-22. Professionals and students were in attendance for Kevin Moore’s presentation on the technical background of acceptable risk, precision and uncertainty, multiperiod spectral parameters, deterministic maps, and seismic design category. Kevin Moore, Senior Principal and Head of the San Francisco Structural Engineering Division of Simpson Gumpertz & Heger, is a nationally recognized expert in seismic performance engineering with special experience in California healthcare facility development as well as in structural steel design and construction, with unique experience with special steel moment frames and buckling restrained braced frames. The seminar included dynamic question and answer sessions after each topic. Attendees were able to discuss, in length, their opinions as well as gain additional insight. They expressed concerns about unrealistic precision on seismic hazard data, overly restrictive design requirements that are associated with ground motion related to long return periods, and the variability of seismic design category and associated seismic forces from code cycle to code cycle. The Project 17 Committee is working with this data as well as data collected at the April 11, 2017 workshop in Burlingame, California. More information can be found through Project 17 publications as they are released by NIBS/BSSC.

NCSEA’s core vision is to be the leading advocate for the practice of structural engineering. It is with that statement in mind that we created the NCSEA Special Awards. These awards honor those that work to better NCSEA as an association, but also work toward the betterment of the structural engineering profession. The NCSEA Special Awards are granted to worthy recipients during an awards banquet at the Structural Engineering Summit. Nominees are submitted to NCSEA by various individuals or groups. Nominees can be entered into one of four categories: • NCSEA Service Award Awarded to an individual to recognize their work for the betterment of NCSEA beyond the norm of volunteerism. • Robert Cornforth Award Awarded to an individual for exceptional dedication and service to an NCSEA Member organization and to the profession. • Susan M. Frey NCSEA Educator Award Awarded to an individual who has an extraordinary talent for effective instruction for practicing structural engineers. • James Delahay Award Awarded at the recommendation of the NCSEA Code Advisory Committee, to recognize outstanding individual contributions towards the development of building codes and standards. Additional information on each award can be found on www.ncsea.com. Special Award nominations for 2017 are due July 18th. Although nominations are accepted for each award every year, they are only granted to worthy recipients. It is decided by the NCSEA Board if there is a deserving awardee. It is possible that a recipient may not be declared in every category in a given year.

NCSEA Hosts Two-Part SE Review Course This March, NCSEA hosted its first of two SE Review Courses for 2017. This is the first year NCSEA has set out to complete these courses alone. With the support and encouragement from loyal speakers, a successful program was held. Over the course of four days on two separate weekends, ten speakers taught 28 hours of structural engineering material. Attendees viewed presentations on topics commonly examined on NCEES’s SE Exam that covered both vertical and lateral subject matter. All ten speakers are familiar names across NCSEA curriculum. Having trustworthy speakers was mandatory to the success of this program and several students said they were impressed with their presentations. “I took another course last fall...So far this course has been much more thorough and assumed a more reasonable amout of background knowledge.” reported one student.

It was clear that the students came prepared to learn and were involved from the time they registered until they received access to the recorded courses. The active audience for the course, as well as the variety of students, was encouraging. The course hosted a diverse range of young engineers preparing for the SE Exam and experienced engineers looking for a refresher on SE basics and continuing education hours. The course included an online discussion forum for additional student-instructor interaction. Did you miss the NCSEA SE Review Course? Purchase each session separately or as a group at www.ncsea.com. The recordings offer up to 28 hours of continuing education in every state except New York. The second live course will be held later this year, keep an eye on the NCSEA website for the announcement.

NCSEA Committees are accepting new members! For a listing of committee openings and the application,visit www.ncsea.com. All SEA members are qualified! STRUCTURE magazine

May 2017

PLATINUM

Structural Engineering Summit Registration & Hotel Reservations

May 16, 2017 Structural Welding: Special Inspections with the IBC & AISC Robert E. Shaw, Jr., P.E.

See your company here! The 2017 NCSEA Structural Engineering Summit offers plenty of opportunities to showcase your company. For more information on how you can become a sponsor, an exhibitor or for other advertising opportunities, visit www.ncsea.com.

SILVER

BRONZE

Webinar Subscriptions Offer PDHs at a Great Value Did you know you can view NCSEA’s live webinars for as low as $37 each for a whole year? That’s just $25 per credit hour when you sign up for the live subscription, and much lower if you opt for live & recorded! These two exclusive annual plans are available to NCSEA Corporate Members & SEA Members only. Choose between: Live & Recorded Webinar Subscription Plan with access to all live webinars and the entire recorded webinar library, hosting over 180 webinars, or the Live Webinar Subscription Plan. Visit www.ncsea.com to purchase your subscription today!

June 15, 2017 Seismic Design of Large Wood Panelized Roof Diaphragms in Heavy Wall Buildings John Lawson, S.E.

July 11, 2017 Repair of Construction Defects David Flax

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More detailed information on the webinars and a registration link can be found at www.ncsea.com.

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June 27,2017 Special Inspections for Existing Buildings Chris Kimball, S.E., P.E.

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June 6, 2017 Ethics: Stamping & Sealing: Satisfying Statures & Standard of Care David Ericksen, J.D.

STRUCTU

Upcoming NCSEA Webinars

GOLD

News from the National Council of Structural Engineers Associations

Registration for NCSEA’s 2017 Structural Engineering Summit is now open! This year’s Summit will draw in the best from the structural engineering profession, from experienced engineers to students to companies offering the software or materials needed on the job each day. The Summit is an educational experience through and through, offering sessions throughout the day and receptions at night which provide the perfect opportunity for extra mentoring from our more experienced attendees. In addition to the NCSEA Special Awards presented at the Summit, NCSEA also recognizes some of the most impressive structures across the world with its Excellence in Structural Engineering Awards. The Summit not only draws in the best of the structural engineering field, but it provides an opportunity to award it as well. The 2017 Structural Engineering Summit is being held at the Washington Hilton in Washington, D.C. The hotel is conveniently located only a few blocks from public transit and is a pickup location for the Big Bus Open-Top Sightseeing Tour Bus that takes visitors to over 40 of the city’s best landmarks and attractions. For more information about the hotel and what type of sessions you can expect at this year’s NCSEA Summit, visit www.ncsea.com.

NCSEA News

2017 Summit Sponsors

COUNCI L

The Newsletter of the Structural Engineering Institute of ASCE

Structural Columns

Structures Congress 2018 Call for Abstracts and Sessions CALL FOR PROPOSALS Abstracts and sessions are invited addressing topics listed below, with emphasis on presentations that highlight innovative topics with practical application. Additional types of sessions that encourage broader participation and a more dynamic and interactive approach are available. Submit your sessions at www.structurescongress.org. Abstracts & Session Proposals due June 5, 2017

Abstracts Single Abstracts address a single subject. These are reviewed and grouped with other single abstracts to create a full session, but may also be used in other session types described (right). Part of the submission process includes telling NTPC how this abstract/session impacts and improves the Structural Engineering profession. Think about how this can help create leaders in the Structural Engineering Profession. How does this benefit the profession? What impact does this have globally or socially? How does this bring together the broader project teams? How does this help SEI realize the Vision for the Future of Structural Engineering?

Invited Topics Blast and Impact Loading and Response of Structures Bridges and Transportation Structures Buildings Business and Professional Practice Education Forensic Natural Disasters Nonbuilding and Special Structures Nonstructural Systems and Components Research

At submission, you must select your first choice of one of these options. • Innovative Executive Sessions (IES) – 10 speakers present for 3 minutes each, each with one PowerPoint slide. Then all move to an area for small group discussion with each presenter. This consists of three 20 minute blocks so the audience can rotate to different speakers. • Comprehensive Sessions – may extend beyond the traditional 90 minute time frame and will provide detailed information as well as practical or technical application. An example of this might be a session on a new Code or Standard. Audience participation and dividing into small working groups, if appropriate, is encouraged. • Panel Sessions – multidiscipline project team members discuss project’s success and lessons learned, etc. Panel sessions can also include debates or other formats. • Case Studies – presenter(s) describe in detail an entire project including issues like management structure, design, issues encountered, solutions to overcome problems, and outcome. • Other creative sessions – encourage audience participation and even breakout groups • Traditional Sessions – a moderator and 4 to 5 presentations/papers.

Questions ? Contact Debbie Smith at dsmith@asce.org or 703-295-6095

Refer a New Member, Receive an Amazon Gift Card

Take One of ASCE’S New Guided Online Courses

Do you know a colleague who would benefit from ASCE? During ASCE’s member referral program for 2017, Member Get a Member, current members will receive a $50 Amazon.com Gift Card for each new professional member they refer. Our goal is to continue increasing the safety, health, and welfare of the public by enhancing professionalism through ASCE membership. We invite our members to play an active role in helping us achieve this goal. Visit our Member Get a Member landing page at http://message.asce.org/mgam for more information.

STRUCTURE magazine

Session Types

ASCE offers asynchronous online instructor-led programs in which you move through a 6 or 12-week learning experience with your peers. The Guided Online Course content includes video lectures, interactive exercises, case studies, live webinars and weekly discussion topics to help you master the course material. With unlimited, 24/7 accessibility to weekly modules, you can complete coursework at the time and pace that is most convenient for you. Current courses include Seismic Loads and Fundamentals of Forensic Engineering. Learn more and register today on the ASCE website www.asce.org/guided-online-courses.

May 2017

The 2017 Infrastructure Report Card has been released. Every 4 years, civil engineers provide a comprehensive assessment of the state of our nation’s infrastructure across 16 categories. Aviation Energy Ports Schools Bridges Hazardous Waste Public Parks Solid Waste Dams Inland Waterways Rail Transit Drinking Water Levees Roads Wastewater Explore the condition and performance of our nation’s infrastructure in the 2017 Infrastructure Report Card. Access the Report Card to see the grades and solutions at www.infrastructurereportcard.org.

Call for New Members for SEI Membership Committee The SEI Membership Committee, a Board Level entity of SEI, is seeking to expand its membership pool. The committee is actively engaged in several important activities such as SEI Fellow selections, younger member activities, student chapters, sustainable organization memberships, strategic global

initiatives, and digital/social networking. The committee is looking for new members with a vision for expanding SEI membership, experience on working with students and young professionals, and networking with major structural engineering companies.

Apply today on the SEI website: www.asce.org/structural-engineering/sei-board-committee-application.

SEI Local Activities Pittsburgh Chapter

The SEI Mohawk-Hudson Chapter hosted its annual Model Bridge Competition on February 16th during the Capital District’s 37th Anniversary Celebration of National Engineer’s Week. Several local schools sent teams and brought 65 bridges made of balsa wood and wood glue. Bridges were “broken” using a testing apparatus, and several new records for efficiency and load ratings were established. Chapter members also attended the CalOES Safety Assessment Program (SAP), hosted by NCSEA on March 24, 2017. Based on ATC-20/45 methodologies, and forms for rapid evaluation and detailed assessment procedures of structures damaged by earthquakes and other natural disasters, this is one of only two post-disaster assessment programs that will be compliant with the requirements of the forthcoming Federal Resource Typing Standards for engineer emergency responders. Learn about these activities and more from the Mohawk-Hudson Chapter’s newsletter at http://sections.asce.org/mohawk-hudson/newsletter.

The SEI Pittsburgh chapter recently held a presentation on UAVS (drones) and their use in the structural engineering industry and construction industry. About 100 attendees had a great time learning about this new technology and how it can be utilized for engineering projects. A lively discussion ensued on future developments in this field and how regulations and code requirements are hurting further promotion and use of drones in this industry. Learn more about this and other civil engineering activities on the Pittsburgh Chapter blog at www.asce-pgh.org/Blog.

Get Involved in Local SEI Activities

Join your local SEI Chapter, Graduate Student Chapter (GSC), or Structural Technical Groups (STG) to connect with colleagues, take advantage of local opportunities for lifelong learning, and advance structural engineering in your area. If there is not an SEI Chapter, GSC, or STG in your area, review the simple steps to form a SEI Chapter at www.asce.org/structural-engineering/sei-local-groups. Local Chapters serve member technical and professional needs. SEI GSCs prepare students for a successful career transition. SEI supports Chapters with opportunities to learn about new initiatives and best practices, and network with other leaders – including annual funded SEI Local Leader Conference, technical tour, and training. SEI Chapters receive Chapter logo/branding, complimentary (from left: Brian Eseppi, Pat McFadden, Jeanne Rice, SEI MohawkHudson Chair Paul Byrd, Adam Bailey, John Rizzo, proctor David Biggs) webinar, and more.

Errata SEI posts up-to-date errata information for our publications at www.asce.org/SEI. Click on “Publications” on our menu, and select “Errata.” If you have any errata that you would like to submit, please email it to Jon Esslinger at jesslinger@asce.org. STRUCTURE magazine

May 2017

The Newsletter of the Structural Engineering Institute of ASCE

Mohawk- Hudson Chapter

Structural Columns

2017 Infrastructure Report Card

JUST RELEASED

CASE in Point

The Newsletter of the Council of American Structural Engineers

Updated Guidelines for Addressing the Bidding and Construction Administration Phases for the Structural Engineer Council of American

Engineers www.acec.org/case Structural The Strength Behind the Beauty

CASE

CASE 962-F A Guideline Addressing the Bidding and Construction Administration Phases for the Structural Engineer

Copyright © 2017 by the American Council of Engineering Companies (ACEC). All rights reserved. No part of this document may be reproduced, stored in any form of retrieval system, or transmitted in any form or by any electronic, mechanical, photographic, or other means without the prior written permission of ACEC.

This document was originally developed to assist all parties associated with bidding and construction administration phases of a project, with the primary emphasis on those issues associated with the structural engineer (SER). This is a guide to the SER’s roles after the construction documents have been issued for construction. It provides guidance on pre-bid and pre-construction activities through the completion of the project. The appendices contain tools and forms to assist the SER in applying the guide to their practice. The committee has done a comprehensive update to this document and brought it up to 2017 industry standards, with specific references to latest codes and appropriate CASE documents including specific tools and contracts. To view the updated practice guideline, go to www.acec.org/case/getting-involved/guidelines-committee.

CASE Risk Management Tools Available Foundation 5: Education – Educate all of the Players in the Process • Educate to form realistic expectations. • Educate management, staff, and clients • Have established education and training policies and procedures • Coaching and mentoring is an important part of the education process Tool 5-1: A Guide to the Practice of Structural Engineering This Guide is intended to teach structural engineers the business of being a consulting structural engineer and things they may not have learned in college. While the target audience for this tool is the young engineer with less than 3 years of experience, it also serves as a useful reminder for engineers of any age or experience. The Guide also includes a test at the end of the document to measure how much was learned and retained. Sections deal with getting and starting projects, schematic design, design development, construction documents, third party review, contractor selection/project pricing/delivery methods, construction administration, project accounting and billing, and professional ethics. Tool 5-2: Milestone Checklist for Young Engineers The tool helps engineers understand what engineering and leadership skills are required to become a competent engineer. It also provides managers a tool to evaluate engineering staff.

STRUCTURE magazine

Tool 5-3 Managing the Use of Computers and Software in the Structural Engineering Office Computers and engineering software are used in every structural engineering office. It is often a struggle to manage and supervise these tools. Software availability is in constant flux, software packages are continually updated and revised, and few software packages fully meet the needs of any office. This tool is intended to assist the structural engineering office in the task of managing computers and software. Tool 5-4 Negotiation Talking Points This tool provides an outline of items for consideration when you are pressured to agree to lower fees. The text is subdivided into situations that are commonly experienced in our profession. This document is purely advisory and designed to assist you in your individual negotiations and business practices. Foundation 6: Scope – Develop and Manage a Clearly Defined Scope of Services A well-developed scope of services: • Avoids later misunderstandings • Provides clarity for future additional services • Clarifies the work effort and fee • Establishes many contract terms • Functions as a guide for your staff • Helps to mitigate claims You can purchase these and the other Risk Management Tools at www.acec.org/bookstore.

May 2017

June 15 – 16, 2017; Boston, MA

August 2 – 3, 2017; Chicago, IL

Engineers are often asked to serve as expert witnesses in legal proceedings – but only the prepared and prudent engineer should take on these potentially lucrative assignments. If asked, would you be ready to say yes? Developed exclusively for engineers, architects, and surveyors, this unique course will show you how to prepare for and successfully provide expert testimony for discovery, depositions, the witness stand, and related legal proceedings. Applying Expertise as an Engineering Expert Witness is a focused and engaging 1½ day course that will run you through each step of the qualifications, ramifications, and expectations of serving as an expert witness. For more information about the course, please contact Katie Goodman at 202-682-4377 or kgoodman@acec.org.

The CASE Summer Planning Meeting is scheduled for August 2nd to the 3rd in Chicago, IL. A popular feature of the planning meeting is a roundtable discussion on topics relating to the business of Structural Engineering, facilitated by the CASE Executive Committee members. Topics have included the Business of BIM, using social media within your firm, Peer Review, and Special Inspections. Attendees to this session earn 2.0 PDHs. Please contact CASE Executive Director Heather Talbert (htalbert@acec.org) if you are interested in attending this roundtable or have any suggested topics for the roundtable.

Seeking Innovative Ideas!

Does your firm have an innovative idea or method of practice? Looking to get more involved in short duration projects? We are inviting you to “share the wealth” and submit a proposal for a web seminar topic, publication, or education session you would like to see CASE present at an upcoming conference. Our forms are easy to use, and you may submit your information via email. Go to www.acec.org/coalitions and click on the icon for Idea Sharing to get started. Questions? Contact us at 202-682-4332 or email Katie Goodman at kgoodman@acec.org. We look forward to helping you put your best ideas in front of eager new faces!

First Annual CASE Risk Management Seminar PLAN TO ATTEND! August 3 – 4, 2017; Chicago, IL

Once again, CASE will put on the industry’s only seminar dedicated solely to improving your firm’s business practices and risk management strategies. Come and join us and learn about Time-Tested Techniques for Managing Your Firm’s Risk in Chicago on August 3 and 4. Gather for training and collaboration with industry leaders and project managers from firms of all sizes intended to improve your structural engineering practice. Immerse yourself in topics designed to help engineers learn better ways of reducing areas of risk and liability on projects while learning about tools that are available to implement better practices immediately in your firm. The Seminar is geared towards Owners, Principals, Project Managers, and Risk Managers – if you are concerned with risk management, new trends, and profitability, you cannot afford to miss this event! Registration for the event opened in mid-April; seats will be limited. Registration is now open and can be found at the following link; however, space is limited so register early at: www.acec.org/calendar/calendar-seminar/case-risk-management-seminar. For more information about this seminar, contact Heather Talbert at htalbert@acec.org or 202-682-4377.

Follow ACEC Coalitions on Twitter – @ACECCoalitions. STRUCTURE magazine

May 2017

CASE is a part of the American Council of Engineering Companies

CASE Summer Planning Meeting

CASE in Point

Applying Expertise as an Engineering Expert Witness

Structural Forum

opinions on topics of current importance to structural engineers

Lessons at Yellowstone National Park By Samantha Fox

he start of a career as a young engineer is a continual information overload. The facts to remember and considerations to take when making design decisions are overwhelming. Keeping up with tight deadlines leaves little extra time to spend understanding the process and building confidence in making decisions. Brand new engineers lack the basis to distinguish which design checks are necessary, which are extraneous, and how to be efficient in calculations and designs. Knowledge can come slowly over time with repetition, or it can come from first-hand experiences. In my case, knowledge about snow loading came in the form of experiencing winter in Yellowstone National Park. When I started as an intern, I was the fourth person in a small branch office. At that time, I was working on several log and timber structures in heavy snow regions and getting the hang of snow loads, log design, and timber frame connections. Unbalanced snow, eave loading, drifting, and sliding snow – developing these loads seemed daunting at the time. When should these conditions be considered? What is the appropriate way to handle valleys and dormers? I worked through these questions, trying to include critical conditions but also trying to be efficient. This often meant spending less time in developing loads and more time in designing mortise and tenon connections. Several of these projects happened to be located in Yellowstone National Park. The design ground snow load for much of Yellowstone is 200 pounds‐per‐square‐ foot. With an approximate density of 19 pounds‐per‐cubic‐foot, that is over ten feet of snow on the ground. Before seeing this snow accumulation for myself, I imagined that the design ground snow value had a large degree of conservatism and that designing structural members to near capacity was appropriate. My attempts at efficient design were complicated by the high snow load since timber truss designs were controlled by large unbalanced snow loads and seismic forces were driven by the roof snow.

During the winter months, the only way into many remote project sites in Yellowstone is through a gate with a combination lock and often on a snowmobile or snow coach if the plows have not started for the season. This is truly an amazing time to see the park. During my first winter trip, I was shocked at the amount of snow on the ground and even more amazed by the amount of snow on the roofs of the structures. There was easily ten feet of snow on the ground in several locations. Unbalanced snow, drifting on low roofs, drifting against parapets and walls, and eave loading could all be observed. It was not an unusually big snow year or even the most snow that the structures had seen in Yellowstone in that particular season. Additionally, many of the structures in Yellowstone are winterized every season, meaning that they remain unheated for nearly six months out of the year. One project in our office involved splicing new log rafter tails to existing framing as part of a roof structure retrofit. The existing tails were deteriorated and were to be cut off so that new, matching tails could be spliced on and blended into the existing structure. The depth of snow that I witnessed on the eaves in Yellowstone justified the loading conditions that previously seemed conservative. Site visits to Yellowstone National Park were excellent learning experiences in my young career. Those experiences helped me gain understanding and confidence to make practical design decisions and taught many lessons that I continue to incorporate into practice: • The creation of snow maps for more complicated roof structures as part of the analysis. Sketching out each snow load condition on a roof plan can be helpful to visualize the potential behavior and effects of the snow loading on the roof structure. • The inclusion of several snow cases in finite element models. Spending additional time at the front-end of a project in creating several load cases can help ensure that certain checks are not missed in design, particularly when changes are made.

Spring time eave loading in Yellowstone National Park.

• Adoption of tools and spreadsheets that have made these checks easier and more efficient. In particular, drift load checks are simplified by comparing each drift condition side by side in a spreadsheet. • Placing importance on observing engineering loading conditions and construction practices in the field. Any of these opportunities are valuable for a young engineer. • Understanding that engineering judgment is gained from first-hand experiences, as well as traditional learning from textbooks and training received from other engineers and managers. It takes time to develop engineering judgment.▪ Samantha Fox is a Project Engineer at BCE Structural in Bozeman, Montana, and sits on the NCSEA Young Member Group Support Committee. Samantha can be reached at sam@bceweb.com.

Structural Forum is intended to stimulate thoughtful dialogue and debate among structural engineers and other participants in the design and construction process. Any opinions expressed in Structural Forum are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C 3 Ink, or the STRUCTURE® magazine Editorial Board. STRUCTURE magazine

May 2017

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