STRUCTURE magazine March 2019

STRUCTURE MARCH 2019

NCSEA | CASE | SEI

/ Wind Seismic INSIDE: Story of a Survivor

34

San Francisco Soft-Story Ordinance 8 Performance-Based Earthquake Design 32 Seismic Design and Embodied Carbon 44

SYC

Scorpion Yielding Connector Ductile Connection

HSC

High Strength Connector SCBF and OCBF

CBB

Cast Bolted Bracket SMF and IMF

SEISMICRESISTANT SOLUTIONS

LeMessurier Calls on Tekla Structural Designer for Complex Projects Interoperability and Time Saving Tools

Tekla Structural Designer was developed specifically to maximize collaboration with other project parties, including technicians, fabricators and architects. Its unique functionality enables engineers to integrate the physical design model seamlessly with Tekla Structures or Autodesk Revit, and to round-trip without compromising vital design data.

“We’re able to import geometry from Revit, design in Tekla Structural Designer and export that information for import back into Revit. If an architect makes geometry updates or changes a slab edge, we’ll send those changes back into Tekla Structural Designer, rerun the analysis and design, and push updated design information back into Revit.”

Tekla Structural Design at Work: The Hub on Causeway

For over 55 years, LeMessurier has provided structural engineering services to architects, owners, contractors, developers and artists. Led by the example of legendary structural engineer and founder William LeMessurier, LeMessurier provides the expertise for some of the world’s most elegant and sophisticated designs while remaining true to the enduring laws of science and engineering.

Known for pushing the envelope of the latest technologies and even inventing new ones, LeMessurier engineers solutions responsive to their clients’ visions and reflective of their experience.

An early adopter of technology to improve their designs and workflow, LeMessurier put its own talent to work in the eighties to develop a software solution that did not exist commercially at the time. Their early application adopted the concept of Building Information Modeling (BIM) long before it emerged decades later.

While LeMessurier’s proprietary tool had evolved over three decades into a powerhouse of capability, the decision to evaluate commercial structural design tools was predicated on the looming effort required to modernize its software to leverage emerging platforms, support normalized data structure integration and keep up with code changes.

After a lengthy and thorough comparison of commercial tools that would “fill the shoes” and stack up to the company’s proprietary tool, LeMessurier chose Tekla Structural Designer for its rich capabilities that addressed all of their workflow needs. According to Derek Barnes, Associate at LeMessurier, ” Tekla Structural Designer offered the most features and the best integration of all the products we tested. They also offered us the ability to work closely with their development group to ensure we were getting the most out of the software.”

One Model for Structural Analysis & Design

From Schematic Design through Construction Documents, Tekla Structural Designer allows LeMessurier engineers to work from one single model for structural analysis and design, improving efficiency, workflow, and ultimately saving time. “Our engineers are working more efficiently because they don’t need to switch between multiple software packages for concrete and steel design. Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product,” said Barnes.

LeMessurier engineers use Tekla Structural Designer to create physical, information-rich models that contain the intelligence they need to automate the design of significant portions of their structures and efficiently manage project changes.

TRANSFORMING THE WAY THE WORLD WORKS

“Tekla Structural Designer has streamlined our design process,” said Craig Blanchet, P.E., Vice President of LeMessurier. “Because some of our engineers are no longer doubling as software developers, it allows us to focus their talents on leveraging the features of the software to our advantage. Had we not chosen to adopt Tekla Structural Designer, we would have needed to bring on new staff to update and maintain our in-house software. So Tekla Structural Designer is not just saving us time on projects, it is also saving us overhead.

Efficient, Accurate Loading and Analysis

Tekla Structural Designer automatically generates an underlying and highly sophisticated analytical model from the physical model, allowing LeMessurier engineers to focus more on design than on analytical model management. Regardless of a model’s size or complexity, Tekla Structural Designer’s analytical engine accurately computes forces and displacements for use in design and the assessment of building performance.

“Tekla Structural Designer offers better integration of multiple materials than we have seen in any other product.”

Positioning a large scale mixed-use development next to an active arena, a below grade parking garage, and an interstate highway, and bridging it over two active subway tunnels makes planning, phasing and engineering paramount. Currently under construction, The Hub on Causeway Project will be the final piece in the puzzle that is the site of the original Boston Garden.

Despite being new to the software, LeMessurier decided to use Tekla Structural Designer for significant portions of the project. “Relying on a new program for such a big project was obviously a risk for us, but with the potential for time savings and other efficiencies, we jumped right in with Tekla Structural Designer. It forced us to get familiar the software very quickly.”

“Tekla Structural Designer allowed us to design the bulk of Phase 1 in a single model,” said Barnes. The project incorporates both concrete flat slabs and composite concrete and steel floor framing. “Tekla Structural Designer has the ability to calculate effective widths based on the physical model which is a big time saver,” said Barnes. “On this project, the integration with Revit, along with the composite steel design features enabled us to work more efficiently. Adding the ability to do concrete design in the same model was a bonus because we had both construction types in the same building.”

“Tekla Structural Designer helped this project run more efficiently, and in the end it was a positive experience,” said Blanchet.

“Tekla Structural Designer gives us multiple analysis sets to pull from, which gives us lots of control. Most programs don’t have the capability to do FE and grillage chase-down. For the design of beam supported concrete slabs, Tekla Structural Designer allows us to separate the slab stiffness from the beam stiffness, so if we choose to we can design the beams without considering the influence of the slab. In the same model we can use a separate analysis set to review the floor system with the beams and slab engaged,” said Barnes.

Barnes also shared similar benefits with concrete column design. “Tekla Structural Designer does grillage take-downs floor-by-floor, finds the reactions and applies them to the next floor. This allows us to view column results both for the 3-dimensional effects of the structure as a whole and from the more traditional floor-by-floor load take-down point of view.

Doing both has always required significant manual intervention, but Tekla Structural Designer puts it all in one place.” “We reduce the possibility for human error because with Tekla Structural Designer less user input is required,” said Barnes. “Tekla Structural Designer automatically computes many of the design parameters, such as column unbraced lengths. The assumptions made by the software are typically correct, but we can easily review and override them when necessary.”

“Tekla Structural Designer provided the best fit for our workflow compared to other commercially available software.”

Want to Evaluate Tekla Structural Designer? tekla.com/TryTekla

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Contents MARCH 2019

34 THE STORY OF A SURVIVOR

By John A. Dal Pino, S.E.

Hurricane Michael made a direct hit on Mexico Beach, Florida, in October 2018. Almost all the homes along the beach were destroyed. One building, dubbed the Sand Palace, remained standing, alone in a field of devastation.

The owners had employed performance-based design concepts to design and build a truly sturdy building that satisfied their goals for performance and longevity.

38 ADDING A PARKING BASEMENT AFTER-THE-FACT

46

Thames River Bridge

By Carol Hayek, Ph.D., and Tony Salem

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

The picturesque Beacon Hill conservation area in Boston was the challenging location for a project involving the creation of a parking basement for a renovated property

Historic Structures

49

Professional Issues Disruption is

by excavating beneath two townhouses while preserving the existing above-ground

Coming to the Building Industry

structures. The solution – top-down, post-tensioned concrete.

By Steven Burrows, P.E.

Columns and Departments 7 8

Editorial Unleashing the Profession

24

Code Requirements for

Lessons Learned The San

Residential Roof Trusses

By Brent Maxfield, S.E.

Francisco Soft-Story Ordinance

By John A. Dal Pino, S.E., and James Enright, P.E.

14

32

Conrad (Sandy) Hohener, P.E, S.E., and Michael Valley, P.E, S.E.

Earthquake Design

By Chris D. Poland S.E.

Building Blocks Innovative 42

Structural Testing

Seismic Resiliency

Moisture and Mass Timber

By Jed Bingle, P.E., S.E.

By Evan Schmidt

Structural Systems Reaching Higher with Cold-Formed Steel

44

Structural Sustainability

Framing for Podium Structures

Structural Design and Embodied Carbon

By Robert Warr, P.E.

By Chris Horiuchi, S.E., and Nicole Wang, P.E.

InSights The Future – BIM

By Tom Winant, P.E., and Alan Jeary, Ph.D.

56

Business Practices Building Your Leadership Legacy

By Jennifer Anderson

66

Structural Forum Art of Approximation

Northridge – 25 Years Later Performance-Based

Materials to Improve Bridge

54

Structural Practices

By Frank Woeste, P.E., and Peter Nielsen

Risk Management Jobsite Safety

By Randy Lewis

Floor Design Considerations

Does Accidental Torsion Prevent Collapse?

20

28

Structural Performance

By David (Jared) DeBock, Ph.D., P.E.,

18

Codes and Standards

By Anne M. Ellis, P.E.

52

By Dilip Khatri, Ph.D., P.E.

In Every Issue 4 57 60 62 64

Advertiser Index Resource Guide – Software Updates NCSEA News SEI Update CASE in Point

On the Cover

Satellite image of a hurricane

approaching the U.S. (Elements of this photo were supplied by NASA.)

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

M A R C H 2 019

5

EDITORIAL Unleashing the Profession

How Performance-Based Design Will Shape Our Future

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

M

eeting and getting to know leaders of our profession – authors of textbooks, chairs of committees, ENR newsmakers – is one of the many perks of SEI engagement.

In sharing insights from recent conversations with leaders helping to shape our future, I intend to inspire you to join in.

Performance-Based Design (PBD) is a powerful approach anticipated to shape the future of the structural engineering profession. PBD turns the traditional design paradigm upside down as required performance is the starting point for the design. Ron Klemencic, P.E., S.E., Hon.AIA, F.SEI, F.ASCE, Chairman and C.E.O. of Magnusson Klemencic Associates and Director of the Charles Pankow Foundation (CPF), emphasizes that “Innovation in the building industry is many times hampered by prescriptive code provisions, which are arcane and based on technologies and methodologies decades old. Performance-based design unshackles the engineer, encouraging creative thinking enabled by the tools and technologies of today.”

Is PBD the answer? Is the profession ready?

Per Don Dusenberry, P.E., SECB, F.SEI, F.ASCE, Consulting Principal of Simpson Gumpertz & Heger Inc. and Chair of the SEI Committee to Advance PBD, attests, “Engineers have the tools to perform the necessary analyses and the imperative to design reliable structures that provide economy, serviceability, sustainability, and robustness.

PBD is the means to accomplish these goals.”

Dusenberry notes SEI initiatives presently underway will coordinate the activities of the many organizations that are advancing PBD and provide frameworks and guidance resources that engineers can use to pursue PBD. He predicts, “We will build structures with established reliability, are responsive to the goals – beyond life safety – that are important to our clients and society.”

However, do our model codes allow PBD? Ron Hamburger, S.E., P.E., SECB, F.SEI, Senior Principal of Simpson Gumpertz & Heger Inc. and Chair of ASCE 7 responds, “Like most contemporary design specifications, the industry loading standard ASCE 7,

STRUCTURE magazine

Minimum Design Loads and Associated Criteria for Buildings and Other Structures, has been performance-based for many years, meaning that adherence to its design recommendations, and those of its companion reference standards, is intended to meet certain performance standards. This includes a defined notional probability of failure, as well as protection of service performance for routine loadings. While these performance goals had been present for many years, they remained invisible to much of the practicing profession.” He further explains that clearly included PBD methodology – starting with ASCE 7-10 and continuing with ASCE 7-16 – provides an alternative to traditional design approaches, includes performance goals associated with the operability of critical service equipment, and PBD design criteria associated with tsunami loading and fire resistance.

Experts agree, PBD is needed, many tools are established, and there is a precedent for use. So, what comes next? SEI in partnership with CPF is leading efforts to enable PBD for wind and structural fire engineering.

Performance-based Wind Design Donald Scott, P.E., S.E., F.SEI, F.ASCE, Vice President, Director of Engineering, PCS Structural Solutions and Chair of the ASCE 7-22 Wind Loads Subcommittee, is Principal Investigator of the ASCE/SEI Pre-standard for Performance-Based Design for Wind. Scott shares, “As the use of PBD has advanced for seismic design, and in certain areas of the country, utilizing prescriptive, code-based wind loading provisions tends to ‘fight against’ the benefits of the PBD seismic provisions, resulting in overall poorer performance for these buildings.” Scott believes, “the PreStandard currently being developed will allow the designer more flexibility and creativity in the design of the lateral force resisting system for the building and advance the requirements for the design of the components and cladding systems that protect the building interior. The provisions will provide for the

same reliability for the building as if it were designed per the existing code requirements.”

Structural Fire Engineering Structural fire engineering is another area in which using PBD can make a difference. As Kevin LaMalva, P.E., M.ASCE, Senior Staff at Simpson, Gumpertz & Heger and Chair of the SEI Fire Protection Committee, explains, “Structural fire protection has not appreciably changed in a century, and there is little to no synergy between structural design and applied fire protection. Conversely, PBD structural fire engineering involves the rational allocation of resources to achieve an acceptable level of intrinsic structural fire performance.”

Structural Fire Engineering may be a new topic to many of us; however, according to LaMalva, “structural fire engineering (SFE) is about 90% structural engineering, so the bridge that a structural engineer needs to cross in order to practice structural fire engineering is shorter than most think.” To this end, LaMalva is leading an SEI/CPF effort to develop SFE exemplar designs based on actual buildings following the PBD framework of ASCE 7-16 Appendix E. Both projects will be completed this year and available for free from SEI and CPF. Thanks to these leaders, SEI members sharing expertise, and SEI with financial support from ACIF, AISC, ArcelorMittal, and MKAF via CPF, we will catapult forward. As stated

by Ron Klemencic, “We are advancing our profession’s quest for better ways to design and build, and performance-based design is key to this advancement.”

Join us at Structures Congress, April 24-27 in Orlando, to learn more from these experts and about these initiatives during the panel session titled, Unleashing the Profession, on April 26. www.structurescongress.org■ Anne M. Ellis is the Executive Director of the Charles Pankow Foundation. Anne is also a Board Member of the SEI Futures Fund and Chair of the SEI Global Activities Division.

M A R C H 2 019

7

lessons LEARNED The San Francisco Soft-Story Ordinance

By John A. Dal Pino, S.E., and James Enright, P.E., LEED AP

I

n 2013, the City of San Francisco embarked on an ambitious and groundbreaking endeavor: the mandatory seismic retrofit

of its wood-framed soft-story apartment buildings. The 1989 Loma Prieta earthquake caused considerable damage to such buildings in the Marina District (Figure 1) and exposed the vulnerability of

Figure 1. Soft-story building collapse in the 1989 Loma Prieta Earthquake.

buildings with soft and weak first stories. Yes, even wood-framed buildings, thought by most engineers to be the most naturally earthquake resistant type of structure due to their lightweight nature and reserve strength, can collapse under the right (or perhaps wrong) circumstances. According to a 2016 report by the Association of Bay Area Governments, San Francisco had 6,700 soft-story buildings, far more than the rest of the region combined.

San Francisco, despite its downtown of steel and concrete high-rises, is really the land of wood structures – long, narrow, multi-story buildings with zero side setbacks. There is almost universally nose-to-tail parking on the ground level and most of the oldest buildings, built before the automobile era, have been modified to allow for parking too. Linear parking configurations make transverse shear walls impossible, and the result was thousands of weak and soft-story buildings.

The intent of this article is not so much to describe the San Francisco Ordinance but to provide insights for other communities that intend to implement mandatory seismic programs. This article is based on the authors’ experiences at a San Francisco firm that has retrofitted over 80 such buildings.

The San Francisco Soft-Story Ordinance The Mandatory Seismic Retrofit Program (Ordinance No. 66-13) was established by the City of San Francisco in April 2013. The Ordinance addresses wood-framed buildings three-stories or taller, or two-story

buildings over a basement or crawl space, with five or more dwelling units, constructed under a permit dated before January 1, 1978, and with no seismic strengthening.

The San Francisco Department of Building Inspection (SFDBI) published Administrative Bulletins 106 and 107, outlining the technical requirements of the retrofit ordinance. Buildings were grouped into four Tiers, with the largest and most vulnerable in Tier 1 (special, institutional, and educational), then Tier 2 (15 or more units), then Tier 3 (5 to 14 units), and then Tier 4 (buildings with ground floor commercial spaces) (Table 1).

The Ordinance requires retrofit work in the weak/soft or “target” story only. The target story is considered weak/soft if the number of walls and the wall layout are significantly different from the typical stories above. San Francisco’s residential buildings commonly have identical or nearly identical plan layouts in the upper stories with a large number of interior walls around small rooms, and open ground levels consisting of undeveloped crawl spaces or developed ground levels with large unobstructed areas used for parking or storage. The lateral force resisting system in the target story must be wood framed elements to be subject to the Ordinance.

Community Outreach and Owner Education

Figure 2. Retrofit fair.

8 STRUCTURE magazine

The City undertook a rigorous community outreach campaign. The campaign began by informing owners of buildings that were believed to be part of the program and continued with a number of additional notices and media outreach. The SFDBI website also provided a good description of the program, with links to all important documents.

The City’s Office of Resilience and Recovery team worked with stakeholders to develop and host several financing workshops, annual Earthquake Retrofit fairs attended by over 3,000 people, and a postcard-noticing program (Figure 2). Working directly with SFDBI, the Office of Resilience and Recovery also hosted several public information meetings, giving the public a chance

Table 1. Wood-frame seismic retrofit program compliance timeline and tier.

Compliance Tier

Submittal of Permit Application with Plans for Seismic Retrofit Work

Completion of Work and Issuance of Certificate of Final Completion

1

September 15, 2015

September 15, 2017

2

September 15, 2016

September 15, 2018

3

September 15, 2017

September 15, 2019

4

September 15, 2018

September 15, 2020

to speak directly with SFDBI staff, experts in disability access and structural engineering, and Rent Board staff (most of the buildings are rent controlled) directly about their questions and concerns.

These outreach efforts were intended to educate building owners from a “zero” starting point regarding the Ordinance and to put them in contact with engineers and contractors who were focusing on these projects.

Based on recent data from SFDBI (Table 2), compliance has been excellent for the Tier 2 buildings, the construction for Tier 3 buildings should be complete by September 2019, and Tier 4 is just underway.

Compliance was also aided by an “Earthquake Warning” placard (Figure 3) affixed near the building entrance that alerted tenants and owners alike to the fact their building was out of compliance.

The outreach programs could be improved for future ordinances by more effectively focusing on specific educational needs: selecting the right engineer, selecting the right contractor, financing, and dealing with commercial tenants.

Project Costs Retrofit costs were initially estimated to be $10,000 to $20,000 per unit. This cost was approximate given the varying sizes of individual units, overall building configurations, and the levelness of the site (San Francisco is very hilly). Due to many factors, including the strength of the regional economy, those costs are very low today.

Straightforward projects cost $20,000 to $25,000 per unit, more complicated ones more. In hindsight, it would have been better to estimate the cost of construction on a square foot basis, with annual updates that include current market costs, and let the individual owners calculate their own cost per unit.

The Right Engineer

The Right Contractor Most of the contractors were small residential contractors and new firms formed specifically to serve the soft-story retrofit market. Work quality varied widely; unfortunately, many contractors do not understand enough about seismic retrofit issues. Costs can vary widely and detailed bid break-out was not always provided, even when requested.

Building owners got sticker shock if the original cost figures were cemented into their thinking and therefore selected their contractor based on cost without a basis to do otherwise. Owners needed educational outreach to review contractor competence along with guidance from their engineer.

Financing and Cost Recovery The City offers public financing through Alliance NRG/ Counterpointe Sustainable Real Estate. If used, the NRG financing approach permits the entire cost of the retrofits (100%) and the cost of the financing to be passed on to tenants as approved by the City’s Rent Board (for rent controlled properties). If the owner chooses to self-finance or get a loan from a bank, there are more restrictions on what costs can be passed on to the tenants. Even when the interest rates were lower on a bank loan, the NRG program might have been the better option, but many owners did not grasp this, focusing on the interest rates alone. Some owners were also squeezed by the inability to pass on rent increases to renters who claimed economic hardship. A takeaway is that legislators need to understand the ability of building owners to make the projects work financially and show them how to do it.

Commercial Tenants The Tier 4 buildings are properties with commercial tenants. These tenants cannot survive in business during the disruption created by extended construction projects. A significant amount of planning and negotiation is necessary to create a workable retrofit approach that addresses temporary relocation and phasing and maintains tenants.

Many building owners are not prepared for these tasks, but the projects cannot proceed until these issues are addressed. The Ordinance also triggered ADA upgrades. Finding qualified ADA specialists who are willing to work on small projects has proven to be difficult, although SFDBI has developed a list of such firms.

Technical Provisions There are three analysis and design methodologies that can be used: California Existing Building Code (2016 CEBC Appendix 4), FEMA P-807, and ASCE 41-13.

• CEBC Appendix 4 – Chapter A4, Earthquake Risk Reduction in Wood-Frame Residential Buildings with Soft, Weak or Open Front Walls, was written to address the soft-story retrofit program.

The design is based on 75% of design base shear for a new building. One caveat is that any existing strength contributions from plaster and gypsum board walls within the target story are to be ignored.

Skilled building owners and their professional representatives have trouble enough evaluating engineering proposals, but the average apartment building owner is not equipped, even after outreach, to make a reasonable decision. In the beginning, the “early bird” owners selected from a handful of engineers based on interviews and qualifications. Fees were adequate to do proper engineering and the results were good. Over time, more engineers entered the market as the number of buildings requiring retrofit grew. The Table 2. Current status of compliance. impact of outreach seemed to fade and Tier Buildings inexperienced owners started to select 1 6 engineers based almost exclusively on 2 508 design fee. Design fees fell as a result of the competition, making it harder 3 3398 to do a proper job. Owners could not 4 985 tell what they were getting.

Permits

Completed

Non-Compliant

3

3

3

508

508

0

3254

1306

144

518

203

NA

M A R C H 2 019

9

• FEMA P-807 – FEMA P-807, Seismic Plan Review Process Evaluation and Retrofit of Multi-Unit Wood Frame Buildings with Weak First SFDBI data shows that approximately Stories, is a performance-based approach 3,700 building permits of all kinds were for the seismic evaluation and retrofit issued in 2016 for projects with individual of wood-framed “soft-story” buildings. estimated construction costs exceeding The downside to this method is that the $100,000. To date, approximately 3,300 software requires some degree of experisoft-story permits have been issued from ence (by both engineer and plan checker), 2015 to 2017, or about 1,100 per year. and judgment is needed in deciding on Assuming the 2016 data is a reasonable the final retrofit scheme. estimate of annual permit activity, the soft• ASCE 41-13 – This is a great tool, but story program has increased the workload it is not often used because of its greater on plan reviewers by 30% per year. That is complexity compared to the other two a significant increase, and the SFDBI staff methods and engineering fee constraints. should be commended for their efforts.

Over time, it became evident that the SFDBI reviews projects by formal submittechnical requirements were not as clear tal and also by using an over-the-counter as hoped, which led to a problem with procedure with review time limited to only the consistency in the application of the one hour. If one hour is not enough, formal program’s technical provisions. The SFDBI submittal is required. Most owners and looked to the Existing Building Committee engineers try the over-the-counter process of the Structural Engineers Association of first. However, this puts the plan reviewer Northern California (SEAONC EBC) for in a difficult position because there is conguidance, which improved the situation but siderable pressure to approve the projects did not eliminate inconsistencies. in one hour. Inevitably, some inadequate • Strength of the Story Above – The Figure 3. Non-compliance placard. designs may slip through the cracks despite “strength of the story above” is calcuthe reviewer’s best efforts. lated by determining the lengths of interior and exterior walls Having greater plan review staffing during the life of the Ordinance, above the target story and multiplying by their shear strength. an increase in the time allotted for over-the-counter reviews, and Most buildings have interior wood lath and plaster finishes and creating special checking procedures for soft-story projects would be exterior wood siding, sometimes with stucco. Strengths are listed beneficial options to consider. Over-the-counter reviews are likely to in California Historical Building Code. In most cases, the total shear become more problematical when the Tier IV buildings are submitted strength added up to much more than the 75% design base shear. because they are structurally more challenging and involve ADA issues.

As a result, the design base shear was insufficient to eliminate the soft- or weak-story condition.

Lessons Learned • Multiple R-Values – The retrofits usually involve multiple systems with different R-values. Plywood shear walls are almost 1) There can never be enough education focused on key issues and always used (R=6.5) but, due to the parking issues, ordinary decisions that building owners will need to make. The audience is steel moment frames (R=3.5) and cantilevered column systems diverse and largely uninformed on technical and financial topics.

(R=2.5) are common. After much debate, R values could be The task is difficult and, when the end result does not cover the considered on a line by line basis for an A4 analysis, largely needs of all stakeholders, the effectiveness of the program may because of the assumption of flexible diaphragms. This is allowed suffer. Only studies of building performance after the next large

by P-807 and ASCE 41. earthquake will show the overall result.

• Cantilever Column Systems – To maintain parking clearance, 2) The Ordinance writers attempted to consider as many issues engineers designed inverted one-story moment frames with tube and factors as possible but, understandably, overlooked some columns and concrete grade beams. The SFDBI considered these things. Moreover, there will always be differences in interpretation. frames as cantilever column systems, which have a low R-value An entity implementing an ordinance needs to have a dedicated because the CEBC envisions tall columns standing up through advisory panel to educate engineers, engage questions, stimulate multiple stories. After debate, the SEAONC EBC and SFDBI discussion, and refine technical requirements to achieve a result Structural Subcommittee recommended the R-value correspond consistent with the ordinance goals. to the R-value for an equivalent moment frame system when the 3) A mandated ordinance affecting a large number of buildings will columns carry no gravity load and are connected by a concrete put a burden on the plan review organization. Increased grade beam designed to yield the column in flexure. So, even after staffing and training are required (internal or external), with clarification, it was still up to the engineer to designate the R. procedures tailored to the unique aspects of the program.▪ • Foundations – Continuous footings below the wood bearing walls are commonly unreinforced concrete. Some are brick. Although there John A. Dal Pino is a Principal at FTF Engineering and is a current member of was no consensus, standard practice was not to replace or strengthen the STRUCTURE magazine Editorial Board. (jdalpino@ftfengineering.com) the foundations unless there was a large overturning moment that the existing foundations could not resist. Providing new foundations to James Enright is a Project Engineer at FTF Engineering and an Adjunct resist overturning moments or to address ACI anchor bolt provisions Lecturer at San Francisco State University. (jenright@ftfengineering.com) would increase construction costs astronomically.

10 STRUCTURE magazine

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structural PERFORMANCE

Does Accidental Torsion Prevent Collapse?

How Collapse Potential is Affected by the Method of Considering Accidental Torsion

By David (Jared) DeBock, Ph.D., P.E., Conrad (Sandy) Hohener, P.E, S.E., and Michael Valley, P.E, S.E.

S

tructural Engineers have long observed that torsional building response is an indicator of earthquake collapse risk. The Building Code’s explicit treatment of torsion dates back at least to the 1961 Uniform Building Code (UBC), which introduced the requirement of adding 5% eccentricity to any inherent torsion when distributing lateral earthquake forces to the vertical seismic force-resisting elements. Although today’s code includes additional penalties for torsionally irregular structures, the treatment of “accidental torsion” remains much the same. This often-maligned but critically important provision Figure 1. Plan views of the baseline and generic archetype configurations. Thickened prohibits the design of cruciform-type structures without any lines represent lines of lateral resistance. Courtesy of FEMA P-2012. torsional strength. It also offers increased collapse protection by indirectly accounting for the non-uniform degradation of the vertiThe second method “explicitly” accounts for accidental torsion by cal seismic force-resisting elements that occur in the true non-linear physically offsetting the mass in a three-dimensional model, thereby response of structures. modifying the dynamic characteristics of the structure and the direct results of the eigenvalue analysis. ASCE/SEI 7-16's Commentary (as well as ASCE/SEI 7-10 with Supplement 1) implies that the second method State of Practice is preferred: “The advantage of this approach is that the dynamic effects Much of today’s linear design, particularly on the West Coast, employs of direct loading and accidental torsion are assessed automatically,” while the use of Modal Response Spectrum Analysis (MRSA) to proportion the same commentary notes that the, “[…] ...practical disadvantages are the vertical seismic force-resisting elements in a structure. ASCE/SEI the increased bookkeeping required...” However, today’s commercially 7-10, Minimum Design Loads for Buildings and Other Structures (as well available software (notably RAM Structural System and ETABS 2017) as ASCE/SEI 7-16), allows practitioners to use one of two methodolo- include functions that automate this process. If we know that accidental gies to account for accidental torsion when conducting MRSA. The torsion is an important consideration in linear design, why would we first method is similar to how one would consider accidental torsion not use this “direct” method in all of our MRSA designs? in an Equivalent Lateral Force analysis; apply a static accidental torsion The supposition that underlies the “dynamic mass offset” method of moment to the results of a concentric eigenvalue analysis scaled to the considering accidental torsion is that “repositioning the center of mass response parameter of interest. This method is automated in many increases the coupling between the torsional and translational modal structural design software and is commonly used for MRSA designs. responses, directly capturing the amplification of the accidental torsion.” While this supposition is correct in many cases, it is not true if the structure has a high degree of torsional irregularity as a result of high torsional flexibility relative to translational flexibility. In such structures, minimal coupling occurs between the translational and torsional responses, and the torsional response is virtually lost in the modal combination of the translational response; this observation has been made by de La Llera and Chopra (1994) and more recently in FEMA P-2012 (FEMA, 2018), Assessing Seismic Performance of Buildings with Configuration Irregularities.

Figure 2. Maximum lateral seismic design forces for a Figure 3. Maximum drifts for a symmetric 2:1 aspect To illustrate this phenomenon, symmetric 2:1 aspect ratio building with varying degrees ratio building with varying degrees of torsional flexibility. consider the archetype buildof torsional flexibility. Torsional to lateral period ratios at Torsional to lateral period ratios at the ASCE/SEI 7-16 ing plans shown in Figure 1. The the ASCE/SEI 7-16 thresholds for torsional irregularity and thresholds for torsional irregularity and extreme torsional baseline building is as torsionally extreme torsional irregularity (for this particular configuration) irregularity (for this particular configuration) are overlaid are overlaid for reference. for reference. regular as possible – square in plan 14 STRUCTURE magazine

probabilities of collapse given large earthquakes. Consequently, while the previous example indicates that the dynamic mass offset method can produce substantially a Required Strength Relative to Baseline Required Stiffness Relative to Baseline Plan weaker designs than the static method, it Aspect TIR Static acc.

5% mass Static acc.

5% mass could still be a valid design method if it Difference Difference Ratio Tors. offsets Tors. offsets produces designs that achieve the collapse 1.31 1.15 1.12 -3% 1.05 1.05 0% reliability intended by ASCE/SEI 7.

Recently, a study conducted by the Applied 1 1.56 1.66 1.40 -16% 1.47 1.00 -32% Technology Council (Project 123, FEMA 2.25 2.71 1.41 -48% 4.42 1.00 -77% P-2012) used the FEMA P-695 incremental 1.22 1.14 1.45 27% 1.00 1.40 40% dynamic analysis approach (FEMA, 2009) to quantify the effect of various irregularities 2 1.53 1.35b 1.20b -11% 1.42 1.20 -15% on the collapse performance of structures.

1.14b -50% 3.74 1.12 -70% 2.04 2.27b With respect to torsional irregularity, the 1.26 1.17 1.55 32% 1.00 1.50 50% FEMA P-2012 study evaluated the collapse resistance of more than 2,000 archetype 1.24b -13% 1.65 1.30 -21% 4 1.60 1.43b buildings with varying degrees of torsional 1.17b -51% 3.99 1.20 -70% 2.09 2.37b irregularity with the intent of recommending a code design provisions that produce designs The stability coefficient and story drift of the baseline are 0.07 and 1.2%, respectively, so the required stiffness does not increase immediately as torsional irregularity is introduced. with consistent probabilities of collapse. This b Design forces are determined using a redundancy factor ρ = 1.0, per the proposed FEMA 2012 torsion design research explicitly included torsionally flexprovisions, rather than ρ = 1.3, as required by the ASCE 7-16 torsion design provisions. ible buildings designed using the dynamic mass offset method versus the static method with lines of lateral resistance at the perimeter – and is proportioned per to observe whether these weaker designs produce structures with ASCE/SEI 7-16 rules that permit torsionally regular structures to be greater collapse rates. designed neglecting accidental torsion. For the strength and stiffness of Design results showing the strength and stiffness of structures prothe lateral systems to be directly comparable, the generic building has portioned using the dynamic mass offset method for accidental torsion the same seismic mass as the baseline building, but with variable plan compared to the static torsional method are shown in Table 1. These aspect ratio and variable locations of the lines of lateral resistance. Figures designs include symmetric archetypes with plan aspect ratios of 1:1, 2 and 3 show the maximum wall shear and displacement demands at the most PHOTO BY EDMUND BARR critical location, relative to the baseline version, when accidental torsion is applied to the generic model by the two methods – static accidental torsion moment versus the dynamic mass offset method (i.e., 5% CM offsets with MRSA). The results shown in Figures 2 and 3 are for 2:1 aspect ratio archetypes that are symmetric in plan (i.e., α = β = γ = δ in Figure 1) and have firstmode translational periods in the constant velocity portion of the response spectrum.

The most critical locations for wall shear demands are in resistance lines 3 and 4, and the critical locations for displacements are at the left-hand and right-hand edges of the building. Figures 2 and 3 show that the dynamic mass offset method amplifies the accidental torsion effect when the torsional and translational building periods are similar, but the amplifying effect is lost Saugus High School Performing Arts Center when the torsional period separates from Santa Clarita, CA the translational period.

SAN FERNANDO VALLEY Seattle San Francisco Des Moines Effect of Dynamic

Table 1. Differences in strength and stiffness required when the dynamic mass offset method (i.e., 5% CM offsets) are used in lieu of static accidental torsion moments for torsionally irregular symmetric archetype buildings (in the more critical direction). Courtesy of FEMA P-2012.

Different design methods produce different results, and the intent of ASCE/ SEI 7 is to produce designs that have low

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Figure 4. Collapse performance of symmetric archetypes proportioned with static accidental torsion moments. Courtesy of FEMA P-2012.

designs that have greater collapse resistance than the baseline (regular) building; values above 1.0 indicate collapse resistance greater than the baseline. In contrast, Figure 5 shows a steady decline in collapse resistance for extremely torsionally irregular buildings proportioned with the dynamic mass offset method. Ironically, the dynamic mass offset method amplifies accidental torsion demands where they are least needed, causing a spike in collapse resistance at low levels of torsional irregularity. However, the method fails to amplify accidental torsion where it is needed most (at high levels of torsional irregularity), resulting in significant declines in collapse resistance for buildings that are extremely irregular. Therefore, building systems that meet the ASCE/SEI 7 collapse reliability criteria when they are torsionally regular cannot generally be expected to still meet the collapse reliability criteria when they are extremely torsionally irregular if they are proportioned with the dynamic mass offset method.

Future Code Provisions

Figure 5. Collapse performance of symmetric archetypes proportioned with the MRSA “dynamic mass offset” method (i.e., accidental torsion applied directly through offsetting the mass ± 5% of the perpendicular building dimension in the structural model). Courtesy of FEMA P-2012.

2:1, and 4:1 (refer to Figure 1 for archetype layouts). The archetype buildings reflected in Table 1 have translational periods on the order of two seconds, placing them in the constant velocity portion of the response spectrum. MRSA results are scaled so that base shear is 100% of the equivalent lateral force procedure base shear per ASCE/SEI 7-16 requirements. Torsional irregularity is quantified by a Torsional Irregularity Ratio (TIR), which is the ratio of the maximum story drift at a building’s edge to the average story drift, given a lateral force with 5% eccentricity; this is identical to the ratio used for determining the presence of torsional irregularity in Table 12.3-1 of ASCE/ SEI 7-16 (TIR > 1.4 means extremely torsionally irregular). Table 1 shows that a significant discrepancy in the force and displacement design parameters develops as torsional irregularity increases. As the symmetric-in-plan buildings become highly torsionally irregular, the force and displacement demand parameters actually decrease rather than increase when the dynamic mass offset method is used. This finding is consistent with the trends observed in Figures 2 and 3 and by de La Llera and Chopra (1994).

Figures 4 and 5 summarize the collapse resistance of symmetric archetype buildings proportioned with MRSA using the static torsional method and the dynamic mass offset method for accidental torsion. Collapse performance is quantified as the ratio of the median spectral acceleration that causes the building to collapse to the median spectral acceleration causing collapse of the baseline (regular) building (Collapse Resistance Relative to Baseline). Figure 4 shows that the static accidental torsion method is somewhat conservative, leading to 16 STRUCTURE magazine

Several code change provisions stemming from the work done on the FEMA P-2012 project are planned for ASCE/SEI 7-22. While a full examination of the FEMA P-2012 recommendations, the associated code changes, and an explanation of the research justifying those code changes are beyond the scope of this article, it should be noted that current torsion design provisions generally produce designs that meet ASCE/SEI 7’s collapse target standard. The 5% accidental eccentricity is around to stay, and the structural designs that are conducted in the 8 years between now and when ASCE/SEI-22 is adopted into the building code will generally be “safe.”

There is a notable exception to “safe” designs arising from the current code provisions that are examined in this article. This article highlights that the dynamic mass offset method for simulating accidental torsion with modal response spectrum analysis can lead to unsafe designs for buildings that are extremely torsionally irregular. When the same structural layouts are proportioned using the static application of torsional moment, the resulting design meets the collapse reliability intended by ASCE 7, as demonstrated in Figure 4.

Consequently, an emergency Supplement #2 to ASCE/SEI 7-16 is planned that will prohibit the use of the dynamic mass offset method for extremely torsionally irregular structures. Before the adoption of ASCE 7-16, and in jurisdictions that do not adopt supplements to ASCE 7, engineers are strongly encouraged to use the static method of applying accidental torsion when conducting modal response spectrum analysis on any building that is extremely torsionally irregular. As engineers, it is incumbent on all of us to understand the implications of what today’s analysis software allows us to do with ease. Just because we can check the box does not always mean we should.■ The online version of this article contains references.

Please visit www.STRUCTUREmag.org.

David (Jared) DeBock is an Assistant Professor at California State University Chico. He is also a team member at Haselton Baker Risk Group. (ddebock@csuchico.edu) Conrad (Sandy) Hohener is an Associate with Degenkolb Engineers. (shohener@degenkolb.com) Michael Valley is a Principal with Magnusson Klemencic Associates. (mvalley@mka.com)

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building BLOCKS Innovative Materials to Improve Bridge Seismic Resilience

By Jed Bingle, P.E., S.E.

T

he current seismic design philosophy for bridges in high seismic risk areas is prescribed by nationally accepted

design standards that have been based on laboratory testing since the 1960s. For ordinary bridges that are expected to experience strong seismic ground motions, this philosophy prescribes structural details that have been shown to provide a low probability of collapse.

To construct a modern, economical bridge that meets this criterion, it is acceptable for elements of the structure to suffer significant damage. A column cage ready for erection with SMA bars spliced in the plastic hinge region.

As long as the bridge does not collapse and users can safely get off the structure, the design is considered successful. Using conventional (SMA), and conventional concrete can be replaced with engineered construction methods to construct a bridge without significant damage cementitious composite (ECC). is typically considered to be uneconomical.

The typical energy dissipation mechanism for modern economical Shape Memory Alloy multi-span bridges is to allow plastic hinging. For bridges, the plastic hinge is designed to be a short portion of the length of the steel rein- Shape memory alloys (SMA) have historically been used in robotics, forced concrete column, where the column connects to the crossbeam automotive, biomedical, and aerospace applications. Superelastic and where the column connects to the foundation. shape memory alloys for this type of bridge application are a special The plastic hinge mechanism using conventional materials requires type of metal that is manufactured from a combination of nickel and permanent damage to elements that are critical to the serviceability titanium. The metal is shaped into smooth round bars that replace and stability of a bridge. A bridge experiencing the ground motions longitudinal steel rebar in the plastic hinge region of bridge columns. of a design earthquake may experience permanent damage, such as Conventional steel rebar used in bridges will return to its original yielding of the longitudinal steel reinforcement in the plastic hinge and undeformed shape if not stressed beyond its yield point. Rebar is region and crushing or spalling of the conventional concrete. This sized for strength and service limit states to remain below this limit. If type of damage may lead to restricting the types of vehicles that can stressed beyond the yield point, conventional rebar will exhibit residual use the bridge or require complete closure. The damage may require deformation after unloading. Deformation of the conventional rebar costly repair or replacement of the column, and may even require beyond the yield point is essential to dissipate energy during a seismic replacement of the bridge. event. The challenge of relying on deformation beyond the yield point With research support from the University of Nevada, Reno (UNR) is that the structure will not be operational after a seismic event. – one of the top earthquake engineering laboratories in the U.S. – For SMA, the alloy will deform like steel beyond the yield point, the Washington State Department of Transportation (WSDOT) but it will return to its undeformed shape. This means the alloy has constructed a first real-world is superelastic. Energy can still be pilot project bridge. The project dissipated by stretching the SMA incorporated innovative materiand, once the earthquake motions als in the plastic hinge region that subside, the SMA will return to its will not only eliminate the need for original shape. permanent damage of the structure Superelastic shape memory alloys but also provide a necessary energy undergo crystalline structure phase dissipation mechanism. transformations, between the ausReplacing conventional materitenite and martensite phases when als in the plastic hinge region with stressed. When the alloy is loaded innovative materials can improve in the austenite phase, the material the seismic resilience of bridge will behave linearly, similar to steel structures. Conventional steel reinup to the yield point. The alloy will forcement can be replaced with a transform into the twinned, marsuperelastic shape memory alloy tensite phase as loading continues.

Superelastic shape memory alloy stress-strain diagram.

18 STRUCTURE magazine

The first seismic resilient bridge in Seattle, WA to incorporate SMA and ECC.

Bridge column with SMA and ECC. Courtesy of Washington State Department of Transportation.

In the twinned state, the crystalline structure is oriented to allow the microstructure to shear without breaking molecular bonds when stressed. As loading continues beyond the yield point, the martensite will begin to de-twin, allowing the alloy to behave plastically. When unloading follows, the martensite will transform back to austenite and the material recovers to its original undeformed shape.

The SMA is only needed for energy dissipation in the plastic hinge region. Therefore, the SMA is not needed for the entire height of the column or elsewhere in the bridge. As compared to conventional steel rebar, SMA alloys are expensive, so the SMA is only used in the plastic hinge regions of the column. Due to the difficulty of machining this specific alloy, headed couplers have been shown to be the only feasible means to couple the SMA to steel rebar.

earthquake motions from the shake table equipment in the Earthquake Engineering Laboratory at UNR. WSDOT decided to incorporate SMA and ECC into a new bridge that is part of the State Route 99 Alaskan Way Viaduct Replacement project in the Sodo district of Seattle, WA. The primary overall goal of this mega-project is to replace the seismically damaged and compromised Alaskan Way Viaduct structure with a single-bore tunnel under the core of the city (See the tunnel article in the January 2019 issue of STRUCTURE ). This bridge is one of many in the vicinity of the south portal of the new tunnel. It will serve as a northbound off-ramp that will span over two roadway alignments providing access to the tunnel.

The bridge is a three-span, 400-foot long structure carrying two lanes of traffic and has a roadway width of 30½ feet. The bridge superstructure consists of two lines of prestressed, precast concrete tub girders. The abutments have a semi-integral end diaphragm and are founded on 8-foot diameter drilled shafts.

The superstructure type and foundation type of this specific bridge is not necessary for the SMA and ECC to improve seismic resilience.

Generally, these materials can be used where any conventional plastic hinges are currently being used.

To encourage new research technologies to be incorporated into practical projects, the Federal Highway Administration created the Innovative Bridge and Research Deployment (IBRD) program. This program granted funds in the amount of $400,000 for this WSDOT project. These funds were intended to cover the costs of lab testing at UNR, procuring the innovative materials, and project documentation costs.

After the completion of construction of the bridge, the Transportation Research Board published NCHRP Research Report 864: Seismic Evaluation of Bridge Columns with Energy Dissipating Mechanisms. This report provides guidelines for quantitative analysis and design methods and construction procedures to incorporate SMA and ECC in columns.

By incorporating shape memory alloy and engineered cementitious composite in a typical highway bridge, WSDOT has demonstrated that bridges in high seismic regions can be improved upon with a small investment in new innovative materials. Bridges subjected to earthquakes can perform without the need to rely on permanent damage to dissipate energy. This means that new bridges can remain operational and useful after an earthquake without costly repair or replacement.■

Engineered Cementitious Composite Engineered cementitious composite (ECC) is a cement-based mixture that is similar to conventional concrete used in modern reinforced columns. A typical ECC mix contains water, cement, fine sand, polyvinyl alcohol fibers, and chemical admixtures. The key difference between conventional concrete and ECC is that the coarse aggregates are replaced with fibers to provide improved ductile performance.

The inclusion of the fiber gives ECC the ability to have a tensile strain capacity as high as six times that of conventional concrete.

The high tensile and compressive strain ductility cause ECC to not spall like conventional concrete. The ductility, tensile strength, and unique cracking behavior make ECC a much more resilient material compared to conventional concrete.

First Implementation Together, these innovative materials work to provide an energy dissipation mechanism similar to that which is incorporated into modern bridges, but will significantly reduce the risk of damage to the structure and eliminate respective repair costs. This will create a bridge that is more likely to remain serviceable after significant earthquake ground motions and reduce or eliminate the need for repair or replacement.

M. ‘Saiid’ Saiidi, Ph.D., P.E., F.ASCE, at UNR, has performed extensive research and testing to validate the use of these innovative materials in bridges. There have been several levels of testing completed, including small scale testing of the SMA and ECC to establish material properties, testing of individual columns for specific plastic hinge performance, and full-scale multiple-span bridges subjected to

Jed Bingle is a Senior Bridge Engineer with HDR Inc. in Vancouver, WA and was the Engineer of Record for this project while employed with the Washington State Department of Transportation. (jed.bingle@hdrinc.com) M A R C H 2 019

19

structural SYSTEMS Reaching Higher with Cold-Formed Steel Framing for Podium Structures

By Robert Warr, P.E.

B

uilding designers are often challenged with an increasing demand for high-density, combined-use buildings in urban locations. The

most common scenario is a base structure, commonly referred to as the podium. The base structure is generally used for parking or retail space, with an upper structure that is of another use, such as apartments, dorms, senior living, hotels or other private spaces. This upper structure lends itself well to individually framed walls and floor systems.

With this increased density comes the need to provide a safe, noncombustible framing system that is also strong to achieve the desired overall building height. Combining non-combustible materials such as cold-formed steel with concrete and hot-rolled steel results in higher building heights for these structures at an economical price. Building height limits also affect building material selection, the treatment of fire safety, sound considerations, architectural unit space planning, and egress. Cold-formed steel provides an ideal solution for all of these design challenges.

How High Can Cold-Formed Steel Go? The International Building Code (IBC) requires the upper and lower structures to be separated for fire and other safety reasons. There are three sections in IBC 2015 Chapter Five pertaining to podium designs that describe permissible building heights and story limits and are critical to understand when designing cold-formed steel upper structures over a concrete podium to maximize the benefits of non-combustible steel construction.

The first provision, Section 510.2, allows an upper structure of any construction type to be built over a lower podium where the two structures are treated as separate and distinct structures. This permits a separate and distinct determination of each of the areas based on the allowable area 20 STRUCTURE magazine

Figure 1. Concrete podium with CFS above.

limitation, continuity of firewalls, type of construction, and number of stories. This provision only applies when four criteria are met: 1) A horizontal assembly separates the building portions with a minimum three-hour fire resistance rating; 2) The building below is of Type IA construction and is protected throughout with sprinklers; 3) Shafts, stairways, ramps, and escalator enclosures penetrating the horizontal assembly have a two-hour fire resistance rating; and, 4) The maximum building height above grade is not exceeded.

Starting with the last point, Type IA, IB, IIA and IIB types of construction require non-combustible materials within a fire-rated assembly, except for IIB which does not have a minimum fire rating for the non-combustible material. However, with each increase in fire protection, the building is allowed to be taller and have a greater number of stories. For example, residential occupancy classification ranges from five stories to unlimited stories and heights ranging from 75 feet to unlimited heights for non-combustible materials in these construction types. This is achieved by providing a fire protection system for the bearing walls that meets an hourly rating between one and three hours. For cold-formed steel, there are various UL assemblies as outlined in A Guide to Fire and Acoustic Data for Cold-Formed Steel Floor, Wall and Roof Assemblies (www.steelframing.org).

The most common range for Figure 2. CFS over parking structure podium. mid-rise non-combustible building

materials is five to twelve stories and 85 feet to 180 feet. These heights are associated primarily with Type IIA and IB, which require one-hour and two-hour fire-rated assemblies, respectively. For example, by using the residential occupancy and cold-formed steel framing over the top of retail space, one could get the maximum stories of floors and still be under the overall building height limitations when coupled with a multi-story podium. Figure 1 shows an example of a concrete podium for retail use below a multi-story cold-formed steel framed residential upper structure.

The second provision, Section 510.4, is not used as often as 510.2 but offers a similar opportunity for stacking buildings and gaining the ability to add an additional floor. Specifically, for buildings with parking below (S-2 occupancy) and any residential group occupancy above, this section allows a podium of Type I construction but only requires a two-hour fire separation that can be further reduced to a one-hour separation if sprinklered per Table 508.4. Assuming a parking level meeting Type I construction (either Figure 3. Nested jamb with studs and track.

IA or IB), the limiting height would be based on the cold-formed steel framing assembly above. Using the construction type that limits the height the most, a building is allowed to be 75 feet with sprinklers which could be seven stories with this provision alone. Figure 2 depicts this provision where a cold-formed steel superstructure is positioned atop a parking structure below.

The third provision, Section 510.6, governs Group R-1 and R-2 buildings of Type II-A construction.

It presents a rare opportunity for a nine-story, 100-foot-tall, Type II-A building when there is at least a 50-foot lot line separation. This provision does not require a podium level separation but does require a 1½-hour fire-rated first elevated floor Figure 5. Overlapping shear straps system. Below-ground parking would still require a Figure 4.

Nested LDM with steel plate. for lateral resistance. three-hour fire separation. This design would likely be most cost-effective when the building has no need for a parking deck and ample space in the lot on which it is being built. elements as MWFRS (www.aisistandards.org). Section B1.1 allows for live load reduction of the framing along with a check for combined MWFRS loading and axial loading. Separately, there is a bending Seismic and Wind Load Considerations check for components and cladding loading without the effect of Most podium buildings are designed for seismic loads using the axial loading. This helps to clarify the use of the framing element Two-Stage Analysis Procedure described in Section 12.2.3.2 of ASCE as both the main frame of the building and a component. This is a 7-10, Minimum Design Loads for Buildings and Other Structures. The unique and necessary check for cold-formed steel as it often receives two-stage analysis procedure recognizes the unique performance exterior components connected directly to it, unlike other structure characteristics of a lightweight and flexible superstructure over a stiff types that provide a skeleton frame with in-fill framing where cladbase which is ten times stiffer than the superstructure. This analysis ding is attached to the in-fill framing only. procedure treats the upper and lower portions of the structure as two For the lateral force resisting system, shaft assemblies constructed distinct structures, with the base shear of the superstructure applied to from CFS could be the best option. Shaft assemblies are typically the base in a second analysis. The lightweight nature of cold-formed placed in the central core area of a building and are inherently strong steel provides a direct benefit for seismic design (less mass) but also due to the construction and fire-rating requirements. However, they offers a significant benefit since the inherent stiffness of cold-formed alone may not be capable of providing the torsional resistance due to steel makes it easy to comply with the ASCE 7-10 requirement that their dimensions and location within the building’s footprint. When the fundamental period of the entire structure not be greater than this is the case, cold-formed steel shear walls can be used to supple1.1 times the period of the upper structure alone. ment or even provide the entire lateral force resistance. If the shear For wind loading, the cold-formed steel framing is the building’s walls are non-bearing walls, the seismic forces may be significantly main frame and therefore the primary wind force resisting system higher than gravity loads. This is where cold-formed steel is uniquely (MWFRS). In this case, AISI S240, North American Standard for qualified. Since it is relatively thin, many members can be nested Cold-Formed Steel Structural Framing, allows for the design of these together, overlapped, or even built-up to form an efficient assembly M A R C H 2 019

21

to resist the loads. Figure 3 (page 21) depicts an example of creative overlapping of thin cold-formed steel lateral strap bracing. In this situation, the design called for more lateral resisting elements than what was available in open wall locations, so an overlapping method was employed. This is achievable with cold-formed steel. As seen in Figure 4 (page 21), connections to the podium can range from post-installed anchors to welded embed plates. More discussion on this topic is found in AISI D110-16, ColdFormed Steel Framing Design Guide, 2016 Edition (www.steel.org).

Designing with Steel

When specifying a construction material, the engineer should be prepared to educate the client on the positive attributes of the recommended materials. Cold-formed steel framing provides many benefits for construction teams and building owners.

An essential benefit of cold-formed steel is its flexibility, giving architects the opportunity to be creative in their designs. Cold-formed steel can be curved and offers many options for indoor space planning.

Steel is also a sustainable material.

It is the only building material recognized by LEED as having a default Connection and Detailing minimum value of 25 percent recycled Considerations content and is 100 percent recyclable.

Unlike other building materials, cold- Figure 6. Steel embed in concrete at podium to CFS wall above.

Steel is routinely collected in aggreformed steel framing is unique in its gate quantities from construction and strength-to-weight ratio and connection to other steel and concrete demolition sites and is recycled into new steel products.

Steel is a resilient material too. Resiliency is the measure of a buildvia traditional commercial building methods, which allows it to marry well with other trades, tools, and techniques already employed in ing’s ability to serve its intended purpose with minimal disruptions. commercial applications. Cold-formed steel framing is dimensionally Cold-formed steel framing meets these requirements of a resilient and geometrically stable, which can be a major consideration when building material – safety in the face of a natural hazard; security in evaluating the longevity and durability of a mid-rise building. a man-made event; energy efficiency; reduction in environmental Often, it is the detailing and concentration of load into a single impacts over the life of the building; durability resulting in a long life isolated connection that creates the greatest challenge for the designer. with minimal maintenance and resistance to deterioration, predictable It is easy to achieve uniform loading that is evenly spaced, and many performance, serviceability, repairability and adaptability.

Cold-formed steel framing has a proven track record of providing materials can handle this load scenario. Unlike some alternates, cold-formed steel can also resolve concentrated loads into singular cost-effective benefits over the entire construction cycle due to lower or closely grouped resisting elements when appropriately detailed. insurance rates, shorter project cycles, predictability and accuracy of As discussed in CFSEI Tech Note G200-15, Chase the Loads: Load components, and design efficiency.

Path Considerations for Cold-Formed Steel Light-Frame Construction, For additional information on cold-formed steel framing, visit the goal of every framing system is to provide a concise and direct www.cfsei.org. load path (www.cfsei.org).

Another challenge is the design of gravity framing around stacked Conclusion openings like doors and windows. The loading from a single floor can be handled by the header over the opening. However, in Cold-formed steel framing has both the strength and non-comframed walls, it is the jamb that transfers the forces vertically from bustible attributes to support the increasing need for denser urban floor to floor. Jambs can accumulate quite large forces in taller housing construction typified by podium building construction. buildings when openings align. With cold-formed steel framing, Cold-formed steel allows for tall buildings, high capacthere are many ways to keep the framing concentrated into an ity elements, efficient connections between the base and area that is relatively narrow in the wall without dimensional superstructures, and durable, quality construction.â– build-up that occurs with other materials. This allows more room for electrical, plumbing, and other necessary items that occupy The online version of this article contains references. the wall cavity. Figure 5 (page 21) shows an example of nested Please visit www.STRUCTUREmag.org. studs and tracks to form a compact jamb condition. Once the jamb load reaches the podium level, it can be transferred via a Part 1 of this series on podiums, Mid-Rise Wood Frame Buildings, base plate or other assembly to transfer the force to the podium ran in the February, 2019 issue of STRUCTURE. efficiently. If the podium is concrete, then an embed plate is a common way to transition the force to the concrete. Likewise, with hot-rolled steel beams and columns, a plate can be employed Robert Warr is the Managing Principal at Frameworks Engineering, LLC and welded into place for a steel-to-steel connection. Figure 6 in Marietta, GA, where he practices cold-formed steel framing structural shows a nested wall top, called a load distribution member, with engineering design. (rwarr@frameworksengineering.com) a steel plate inside to stiffen the elements.

22 STRUCTURE magazine

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CODES and standards

Code Requirements for Residential Roof Trusses

By Brent Maxfield, S.E.

This article outlines the related 2018 International Residential Code (IRC) and the 2018 International Building Code (IBC) requirements for residential truss engineering and delivery.

T

here are many roles played in the design and delivery of residential wood roof trusses. Engineers can play various roles in this process, and it is essential to understand which role you play. This article discusses the scope of work required of the various roles as defined by the various codes and standards for residential roof truss. If a building falls within the IRC, all roles can be played by non-engineers, unless the jurisdiction requires the construction documents to be prepared by a Registered Design Professional.

Code Requirements The International Residential Code (IRC) is the governing code for one- and two-family dwellings. It is a prescriptive code. For those elements that fall outside of the prescriptive criteria, engineering design (i.e., using the IBC) is required (See IRC R301.1.3). The IRC does not have prescriptive provisions for the design and installation of prefabricated wood trusses, but they are allowed per Section R801.10.

The applicability limits for trusses are found in Section R802.10.2.1.

These must be followed in order to stay within the purview of the IRC. The limits that apply when snow loads control the design are: • Building width not greater than 60 feet perpendicular to the truss span • Truss span not greater than 36 feet • Minimum roof slope of 3:12 • Maximum roof slope of 12:12 • Maximum design wind speed of 140 miles per hour (63 m/s), Exposure B or C • Maximum ground snow load of 70 psf (3352 Pa), with roof snow load, computed as 0.7pg The IBC becomes the governing code for the truss design and associated load paths if the structure falls outside of these limits (See IRC R301.1.3).

The following is a summary of the IRC requirements for wood Trusses (capitalized terms are defined by ANSI/TPI 1-2014, National Design Standard for Metal Plate Connected Wood Truss Construction, Section 2.2, published by the Truss Plate Institute (TPI)): • Wood Trusses shall be designed in accordance with accepted engineering practice, and the design and manufacture of metal-plated wood Trusses shall comply with ANSI/TPI 1 (R802.10.2). A read-only version of the full ANSI/TPI 1 document can be downloaded for free at https://goo.gl/j7cK9E. • The Truss Design Drawings shall be prepared by a Registered Design Professional where required by the statutes of the jurisdiction in which the project is to be constructed in accordance with Section R106.1 (R802.10.2). Note that under the IRC, both the residence and the wood Truss design could be performed by persons who are not Registered Design Professionals.

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There may be times when the Building Official will require the Truss Design Drawings to be prepared and stamped by a Registered Design Professional even though the structure was not. The key to this IRC provision is that if the jurisdiction requires the Construction Documents to be prepared by a Registered Design Professional, then the Truss Design Drawings shall also be prepared by a Registered Design Professional.

• Truss Design Drawings shall be provided to the Building Official and approved prior to installation (R802.10.1).

• Truss Design Drawings shall be provided with the shipment of the Trusses delivered to the job site (R801.10.1).

• Truss Design Drawings shall include the following information: o Slope or depth, span, and spacing o Location of all joints o Reaction forces and required bearing widths o Top and bottom chord uniform and concentrated loads o Joint connector type and description such as size, thickness, and the dimensioned location of each joint connector o Lumber size, species, and grade for each member o Adjustments to lumber and connector design values for conditions of use o Connection requirements for Truss to girder and Truss ply-to-ply o Calculated deflection ratio and/or maximum description for live and total load o Information to allow the Building Designer to design the size, connections, and anchorage of the permanent continuous lateral bracing o Required permanent Truss member bracing locations • Truss bracing requirements are found in Section R802.10.3.

This section requires Trusses to be braced to prevent rotation and to provide lateral stability. It allows the bracing requirement to be specified in the construction documents or on the individual Truss design drawings. It also states, “In the absence of specific bracing requirements, Trusses shall be braced in accordance with accepted industry practice such as the SBCA Building Component Safety Information (BCSI) Guide to Good Practice for Handling, Installing & Bracing of Metal Plate Connected Wood Trusses.” See the Building Component Safety Information Book (BCSI), which has the above reference guide as a section. (https://goo.gl/phc1gj or https://goo.gl/c9YWGb) The requirements for wood Trusses in the IBC (2303.4) are very similar to the IRC. Some of the differences include: 1) The IBC specifically addresses environmental design criteria such as wind, rain, snow, and seismic.

2) The IBC requires the consideration of drag strut loads and other lateral loads.

3) The IBC requires specifically listing maximum uplift loads with the reaction forces.

4) The IBC not only requires the Truss Design Drawings to show the location of permanent individual Truss member restraint locations, but it also requires the method and details of restraint/bracing to be used (2303.4.1.1.14).

5) The bracing requirements of IBC Section 2304.1.2 specifically allow the use of T-reinforcement or L-reinforcement, and proprietary reinforcement, so that the buckling of any individual Truss member is resisted internally by the individual Truss. (These are also in ANSI/TPI 1 and the BCSI document.) The IBC also allows a project-specific permanent bracing design by any Registered Design Professional.

6) Trusses spanning 60 feet or more require a Registered Design Professional to design the temporary installation restraint/ bracing and the permanent individual Truss member restraint/ bracing (2303.4.1.3). Special inspection of the Truss member bracing is also required where a Truss clear span is 60 feet or greater (1705.5.1).

7) IBC Section 2303.4.5 specifically requires written concurrence and approval of a Registered Design Professional before Truss members and components can be cut, notched, drilled, spliced, or altered.

ANSI/TPI 1 is the Standard required by both the IRC and the IBC. It establishes the minimum requirements for the design and construction of metal-plate-connected wood Trusses. Chapter 2 of this Standard defines the roles and responsibilities of the various

players (Owner, Building Designer, Truss Manufacturer, and Truss Designer), and it is essential to know which role you are playing.

Section 2.2 defines the Building Designer as, “Owner of the Building or the Person that contracts with the Owner for the design of the Building Structural System and/or who is responsible for the preparation of the Construction Documents. When mandated by the Legal Requirements, the Building Designer shall be a Registered Design Professional.” Under the IRC, if the jurisdiction does not require the Building Designer to be an engineer, an Owner or a non-engineer may play the role of the Building Designer. This could be problematic because there are technical responsibilities placed on the Building Designer by ANSI/TPI 1. The Truss Designer is defined as, “Person responsible for the preparation of the Truss Design Drawings.” When the Truss Designer is required to be a Registered Design Professional, the Truss Manufacturer engages this engineer. ANSI/TPI 1 also references the BCSI document noted above. It is important to understand the bracing details in this document. A few key elements of ANSI/ TPI 1, with reference sections in parenthesis, are listed below: 1) The Owner is required to engage a Building Designer in preparing the Construction Documents and reviewing the Truss Submittal Package (2.3.1.3).

2) The Owner or Owner’s representative shall be responsible for ensuring that the Truss Submittal Package is reviewed by the Contractor and the Building Designer (2.3.1.5 and 2.3.4.2).

3) The Construction Documents shall show in detail that they conform to the Legal Requirement, including the Building Code (2.3.2.1).

4) The Construction Documents shall list the Truss design as a Deferred Submittal, and the Building Designer shall review the

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Truss Submittal Package for “compatibility” and “general conformance” with the design of the Building (2.3.2.2 and 2.3.2.3).

5) The Construction Documents shall provide information sufficiently accurate and reliable to be used for the design of the Trusses and shall provide among other things “… the location, direction, and magnitude of all dead, live, and lateral loads applicable to each Truss, including … snow drift and unbalanced snow loads” (2.3.2.4.d). (Note that ANSI/TPI 1 puts the burden of calculating the load on each Truss, including the snow drift load, on the Building Designer.)

6) The serviceability criteria shall be included in the Construction Documents (2.3.2.4.g).

7) Permanent Individual Truss Member Restraint/Bracing shall be per the BCSI unless the Building Designer specifies a project-specific bracing design (2.3.3.1.1, 2.3.3.1.2, 2.3.3.1.3, and 2.3.3.2).

8) Several requirements must be met by the Contractor, including reviewing the Truss Submittal Package and then forwarding it to the Building Designer for review. The Contractor shall not proceed with the Truss installation until the Truss Submittal Package has been reviewed by the Building Designer (2.3.4.2 and 2.3.4.3). The contractor must also check the Trusses for damage both prior to installation and after installation (2.3.4.6, 2.3.4.7, 2.3.4.8, and 2.3.4.9).

9) The Contractor shall provide to the Truss Manufacturer a copy of all Construction Documents pertinent to the Building Structural System and the design of the Trusses, including the name of the Building Designer if not noted on the Construction Documents (2.3.4.1).

10) Where the Legal Requirements mandate a Registered Design Professional for the Building, each individual Truss Design Drawing shall bear the seal and signature of the Truss Designer (2.3.5.3). An exception allows only the Cover/Truss Index Sheet to be stamped.

11) The Truss Designer is only responsible for “individual” Trusses, not the roof system. Section 2.3.5.2 states, “The Truss Designer shall be responsible for the design, in accordance with this Standard, of each singular Truss depicted on the Truss Design Drawing.” It is critical to understand that, per the TPI Standard, the Truss Designer does not have the responsibility to calculate loads for individual Trusses, nor does the Truss Designer have the responsibility for the roof system.

12) The Truss Submittal Package consists of each individual Truss Design Drawing, the Truss Placement Diagram, the Cover/Truss Index Sheet, Lateral Restraint and Diagonal Bracing details, and any other structural details germane to the Trusses (2.2).

13) The Truss Placement Diagram is only an illustration identifying the assumed location of each Truss. It does not need to be stamped because it does not have engineering input (2.3.5.4).

Implementation 1) Building Officials, Contractors, Owners, and Building Designers should be cognizant of and enforce the requirement that the Contractor and the Building Designer review the Truss Submittal Package prior to the installation of the Trusses. Building Officials should establish procedures to ensure that this code requirement is followed.

26 STRUCTURE magazine

2) Many engineering drawings have general notes that require the Trusses to be designed and stamped by a registered engineer. It is important to understand that the stamp is for individual Trusses and not for the Trusses acting together as a system. Many engineers falsely assume that this stamp is for the individual Trusses as well as for the roof system.

3) Truss web bracing locations are provided on the Truss Design Drawings in the Truss Submittal Package. The BCSI document usually provides the bracing details. Many Truss webs do not align with adjacent Trusses, making continuous Lateral Restraint bracing impossible to install. In these cases, T or L bracing will be required. Construction Documents should provide details and instructions for when T or L bracing is required.

4) Truss web bracing is critical to the stability of the roof system, yet very few residential projects have engineering observation of completed roof systems. Unless the Truss spans 60 feet or more, special inspection of the Truss web bracing installation is not required. This is an area where the code requirements could be improved.

5) Many projects have general notes that state that snow drift and unbalanced snow loading are required to be considered in the Truss design, but the Construction Documents do not provide the actual values of the snow drift loads and the unbalanced loads for each Truss. This is contrary to ANSI/ TPI 1, Section 2.3.2.4(d). It is important to understand that the responsibility for calculating and providing the loads applied to each Truss rests with the Building Designer.

6) A functioning roof system is the responsibility of the Building Designer and consists of Trusses, bracing, blocking, connections to structure, diaphragms, and an understanding of the load path of all forces. The Truss Submittal Package is only one piece of the system.

7) If a portion of the roof system falls outside of the scope of the IRC, then that portion, including the associated load paths, will require engineering analysis. If the Building Designer is not an engineer, then an engineer who is not filling the role of the Building Designer could be engaged for a limited scope to design and stamp the elements that fall outside of the scope of the IRC. This article intends to educate engineers about the roles and division of responsibilities for residential wood Trusses. It is critical to understand the specific scope of the Truss Designer as defined in ANSI/TPI 1. The Truss Designer is responsible for individual Truss Design Drawings using loading information obtained from the Truss Manufacturer, who gets information from the Contractor in the form of selected information from the Construction Documents. The Building Designer is responsible for ensuring that the Truss loads given to the Truss Designer are accurate. The Building Designer is also responsible for ensuring that all Trusses act together as a roof system. All players need to understand and fulfill their responsibilities as outlined in ANSI/TPI 1 in order to achieve a safe and code-conforming building.

Look for a forum piece in an upcoming issue of STRUCTURE where the author will share his experiences, opinions, and recommendations to improve the practice.■ Brent Maxfield is a Civil/Structural Engineer with the Special Projects Department of The Church of Jesus Christ of Latter-day Saints in Salt Lake City, Utah. (maxfieldba@ldschurch.org)

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structural PRACTICES Floor Design Considerations Preventing Tile and Stone Cracks

By Frank Woeste, P.E., and Peter Nielsen

T

wo kinds of designers are sometimes involved in home construction – design professionals responsible for the structure and interior-focused designers responsible for the final appearance.

Although these roles can overlap, it is important for design professionals to be aware of in-service demands that will result from interior finish choices.

There are three aspects of floor design likely to impact the performance of ceramic tile and stone installations in-service: 1) The use of an adequate design dead load, which includes the weight of the hard-surface installation method, 2) The relative importance of joist stiffness and sheathing stiffness in preventing cracks, and 3) Accounting for substantial concentrated loads, such as the weight of kitchen islands with stone or polished concrete surfaces.

This article reviews these three aspects and offers guidance for designers and others interested in the structural performance of their products after construction.

Floors Supporting Tile and Stone If the dead load (weight) of tile or stone “Installation Methods” by Design Professionals (DPs) are inadequately specified, wood-frame floors can be over-stressed in-service and excessive “creep deflection” can occur that causes cracked grout and tiles in-service. Specifying an adequate dead (sustained) load is essential for a reliable hard surface installation.

One way to think of “creep deflection” is to imagine an overloaded shelf in a bookcase. Over time, the weight of the books can cause the shelf to bow. The deformation is not visible immediately, but the damage can be permanent. In the case of a floor, not only is the effect on the floor itself an issue, but also the effect on the flooring. The “creep” occurs beneath the flooring, so the flooring is no longer on top of a level surface and the result can be cracking of tiles and grout.

Weight of Hard-Surface Installation Methods Dead load data for the weights of tile and stone installations have been tabulated in Appendix B of the 2017 Tile Council of North America (TCNA) Handbook for Ceramic, Glass, and Stone Tile Installation (https://goo.gl/9acfZE).

Appendix B is an invaluable design resource that contains the typical weight of dozens of “Installation Methods” for ceramic and glass tile (hereafter, tile) and stone installed on woodfloor framing.

Figure 1. A portion of IRC Table R502.3.1(2).

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In Table 1, the Installation Method Dead Load is the weight of the materials above the subfloor. Total Dead Figure 2. Depiction of stresses induced from the Load in the rightmost bending action of joists and sheathing/substrates. column is based on an assumed dead load of 10 psf for the subfloor, joist, and ceiling.

As shown in Table 1, a design dead load of 10 psf is not adequate when the floor covering is tile or stone. This structural issue requires the attention of all professionals involved in the specification of the hard surface/ installation method, floor system design, and for framing inspectors.

An excerpt from the IRC, the International Residential Code, indicates that an inadequate assumed dead load could be a “code issue” with respect to maximum span (Figure 1) and, in addition, the extra loading can contribute to additional creep deflection that can result in tile, stone, and grout cracks.

The content of the IRC table suggests why the 2015 IRC span table for joists has two sections – one for an assumed dead load of 10 psf (carpet, vinyl, wood) and the other for a dead load of 20 psf (typical residential hard surfaces).

Tile Cracks Tile cracks typically form on top of and parallel to the joist. The primary practical question is: With respect to floor design, what is more important for protecting tile or stone from potential cracking: joist deflection/stiffness (L/xxx) or floor sheathing thickness/stiffness?

From engineering science for a beam (Bretzfield and Woeste, 2003), 1/ρ = M/EI where ρ is the “radius of curvature” of the beam at the location of bending moment (M), and EI is joist stiffness.

To translate the expression, the amount of “bend” at any cross-section of a floor joist (or sheathing) span is proportional to the bending moment divided by the product of the modulus of elasticity (E) and moment of inertia (I) of the section. Referring to Figure 2 and the tile depicted, joists and subfloor/substrates with the largest possible ρ will minimize the stresses induced in the tile from bending action of the joists and sheathing/substrates, respectively.

Figure 3. Cross-sectional view of the test specimen assembly of the Robinson-Type Floor Tester (ASTM C627).

Bend of Joists Verses Sheathing Joist Bend Calculations Using a total floor load of 60 psf, the authors calculated the radius of curvature, ρ, for several IRC joist spans at maximum bending moment. The results are given in Table 2, page 30.

The average radius of curvature for the joists is about 7,000 inches.

Sheathing Bend Calculations The ASTM C627 standard for testing tile installation methods includes a rotating platform that applies up to 300 pounds at three locations (Figure 3).

The test does not include the deflection of the joists and butt joints in the subfloor and underlayment that are present in an actual construction. Likewise, the calculations below represent a best case scenario for an actual tile installation because a constructed floor contains joists that deflect under loads and the floor sheathing is butted at the 4- x 8-foot panel ends.

The authors calculated ρ for four (wood sheathing) installation methods using the following assumptions: 1) A 300-pound concentrated load in the first sheathing span at the point of maximum moment, and 2) No composite action between the subfloor and underlayment.

For the first wood sheathing span under a 300-pound concentrated load, ρ (calculated) for four TCNA Methods was about 300 inches.

The following statements summarize the relative importance of joist stiffness (L/xxx) versus sheathing stiffness.

Table 1. Sample of Installation Methods from the 2017 TCNA Handbook.

Page

TCNA Method Number

Description

138

RH122-17

16” oc, 23⁄32” T&G Ply. Subfloor ¾” Gyp Underlayment Hydronic Tubing

150

F141-17

16” oc, 19⁄32” T&G Ply. Subfloor 1¼” Mortar Bed

158

F150-17

16” oc, 19⁄32” T&G Subfloor, 15⁄32” Ply. Underlayment

162

F144-17

16” oc, 19⁄32 or 23⁄32” T&G Ply. Cement Backer Board, ¼, 7⁄16, or ½”

174

F149-17

24” oc, 23⁄32” T&G Ply. Subfloor, 19 ⁄32” Underlayment

358

F250-17 (Natural Stone)

16” oc, 19⁄32” T&G Ply. Subfloor, 15⁄32” Ply. Underlayment, and (various) Backer Board Products

182

F180-17 Ceramic Tile, Glass Tile

16” oc, 23⁄32” T&G Ply. Subfloor, Poured Gypsum Underlayment (Min. ¾”), Bonded Membrane

1) Floor sheathing bend, as determined by (1/ρ) under a concentrated load, is about 23X greater than the bend of the joist under 60 psf uniform live and dead load.

2) Based on field evidence and engineering analyses (presented above), the structural designer for floor areas supporting tile or stone should place the greater emphasis on subfloor/sheathing specification relative to joist framing specifications.

3) The analyses and recommendation as summarized above did not include the impact of end butt-joints in the subfloor and the impact of differential joist deflection due to concentrated loads, girders, and points of bearing that can cause differential deflections of framing.

Practical Specifications for Concentrated Floor Loads For ceramic tile, the 2017 TCNA Handbook has “deflection requirements” listed for all framed floor system Installation Methods that cites the IRC and International Building Code (IBC), as well the following requirement: “For ceramic tile installations, maximum allowable floor member live load and concentrated load deflection for framed floor systems shall not exceed L/360, where “L” is the clear span length of the supporting member per applicable building code.”

Given this language, uniform live load and concentrated load deflections in residential/multi-family applications are a requirement for the “supporting member” or floor framing system. The TCNA “concentrated load deflection” requirement is in addition to the requirements of the applicable IRC, as the authors are not aware of a concentrated dead-load deflection requirement in the 2015 IRC (or earlier editions). Knowing that concentrated loads such as kitchen islands may be shown on the Construction Documents, it is incumbent upon the design team to specify concentrated loads for the Component Manufacturer’s (CM) use (if trusses or I-joists are specified) or other measures to address the expected deflection from concentrated loads.

An example of a modern and large kitchen island in residential construction is shown in Figure 4, page 30. The framing design question is – given the footprint of the island, what is a reasonable dead load specification?

The density of natural stone can vary between 160 and 200 lbs/ft2.

As such, a stone thickness of 1¼ Installation Total Dead Load Method Dead (Framing, drywall, inches (30 mm) can weigh between Load above and subfloor, 16.7 and 20.8 psf. Considering the “Subfloor” (psf ) 10 psf assumed) weight of the cabinet, doors, and shelves, the island weight without 19 29 contents approaches 30 psf (a sustained load).

21 31 The fact that it is a “sustained load” is significant, as sustained 7 17 loads cause creep. Assuming the total dead load of the island with 8, 9, or 10 18-20 contents is 40 psf, it should not be considered equivalent to a 40 7 17 psf live-load due to creep deflection from sustained loads. Simply stated, an L/360 design based 11-14 21-24 on a 40 psf live-load can experi12 (Add 2¼ psf ence additional creep deflection per additional ¼” 22 in-service due to a 40 psf “suspoured gypsum) tained load” with the actual joist M A R C H 2 019

29

to differential deflection of the joists (and to improve the likely vibrational performance of floors as well).

Live Load Dead Load Joist Spacing Span (max) M ρ = EI/M 5) Offer Customers (homebuyers and 1 (psf ) (psf ) (in.) (ft-in) (in-lb) (in.) owners) floor framing and subfloor 40 20 16 12-10 19,761 6,983 “upgrades” for added protection against the likelihood of tile and 40 20 19.2 11-8 19,600 7,041 grout cracks and annoying floor 40 20 24 10-5 19,531 7,066 vibrations.

1 EI for 2x10 No.2 Southern Pine equals 98.93 in4 x1,400,000 lb/in2, or 1.38x108 in2-lb. See 2015 NDS Supplement, Historically, the design rules in resipp.14 and 40, for 2x10 design data. dential construction building codes addressed a concern for plaster crackTable 3. Radius of curvature (ρ) for first sheathing span at maximum bending moment when loaded by a 300 lb ing (L/360 live-load only deflection) concentrated load as applied in the ASTM C627 test of installation methods. (Note: For joists spaced 16 inches and a floor collapse (40 live load plus on-center, M=927 in-lb; for joists spaced 24 inches on-center, M=1301 in-lb.)

10 psf dead load) when solid-sawn joists and wood flooring were the norms.

2017 TCNA Joist Spacing Subfloor T&G Underlayment Uncoupling ρ = (EI/M) Fast-forward 100 years, longer joist Method (in.) (in.) (in.)

Membrane spans, hard surfaces (ceramic, glass, F144 CBB 19 16 -221 ⁄32 and stone), and large kitchen islands Residential ¼ are common, yet the current IRC does F144 CBB 23 not include provisions to address the 16 -345 ⁄32 Light Comm. ¼ intrinsically inelastic nature of hard surface flooring. To do so, the design team 3 23 F147 24 ⁄32 ⁄8 Yes 284 must take into account concentrated 19 23 floor loads and total load deflection to ⁄32 ⁄32 Optional 404 F149 24 mitigate the serviceability issues that stem from the use of hard surface floordeflection approaching L/180 (using a creep factor of 2.0 for trusses ing. Hopefully, this article will motivate a coordinated effort by design from ANSI/TPI 1-2014). For seasoned lumber, structural glued teams to address serviceability issues for modern construction with laminated timber, prefabricated wood I-Joists, or structural composite hard surface flooring.■ lumber used in dry service conditions, a creep factor of 1.5 is required. A version of this article written for wood-framing/truss designers appeared Table 2. Radius of curvature (ρ) for 2015 IRC maximum spans for 2x10 No. 2 Southern Pine joists and 40-20 psf loading (and L/360 live-load deflection limit).

in the June 2018 issue of The Component Manufacturing Advertiser.

Conclusions The design of residential and multi-family floors has become more complicated due to the widespread use of hard surfaces and the placement of heavy items, such as large kitchen islands, in the center section of framing spans. When the uniform and concentrated loads, including creep deflection, are not adequately accounted for in the floor system design, in-service performance issues of the hard surfaces can be expected.

Based on analyses and experience, the following suggestions are offered for consideration by the design team.

1) Prepare Construction Documents that contain: a) the TCNA tile/stone Installation Method, b) the weight of the installation method, and c) the footprint and weight of kitchen islands (and other heavy equipment such as large front-loading washers).

2) Require floor system designs based on a “total load” that includes the actual weight of the TCNA Installation Method for the hard surface listed in the Construction Documents.

3) Upgrade the subfloor thickness above the thickness specified for the TCNA Installation Method in the Construction Documents to improve the predicted bending behavior of the floor sheathing under a 300-pound concentrated load. (For example, a 23⁄32-inch T&G plywood panel has a bending stiffness (EI) of 320,000 (lbf-in.2/ft) when installed as a subfloor, whereas a 7⁄8-inch panel has an EI of 500,000 (lbf-in.2/ft). The EI of a 7⁄8-inch subfloor panel is 1.56 times greater than the EI of the 23⁄32-inch panel, thus providing substantially more protection for the hard surfaces.)

4) Require strongback bracing for floor trusses to protect tile and stone floors against potential hard surface damage due 30 STRUCTURE magazine

The online version of this article contains a detailed reference.

Please visit www.STRUCTUREmag.org.

Frank Woeste is a Professor Emeritus, Virginia Tech, and a wood construction consultant. (fwoeste@vt.edu) Peter Nielsen is a co-founder of MGNT Products Group, LLC, a consulting and product design company for the tile and construction industry. (nielsentile@gmail.com)

Figure 4. Kitchen island oriented parallel to the floor framing. For this case, almost the entire weight of the island would be supported by two framing members at 24 inches on-center.

NORTHRIDGE

25 YEARS LATER

Performance-Based Earthquake Design

Lessons Learned from a Building Code Option

By Chris D. Poland S.E., FASCE-SEI, NAE

P

erformance-Based Earthquake Design (PBD) has become a standardized process in Structural and Earthquake

Engineering with the publication of the American Society of Civil Engineers Standard ASCE 41-13, Seismic Evaluation and Retrofit of Existing Buildings (ASCE 41). It represents a shift from the prescriptive code provisions of the past that are silent about what they achieve, to a design tool that meets the varying needs of the public by targeting specific performance objectives.

Figure 1. Damage to the Northridge Meadows Apartments.

The need for a new approach was formally recognized after the 1989 Loma Prieta Earthquake and became a nationally funded effort after the 1994 Northridge Earthquake (Northridge). Those moderate earthquakes proved that the then current design standards saved lives.

However, the public declared on multiple fronts that the extent of damage was unexpected and too expensive to repair, and therefore unacceptable. Thanks to the tireless work of the Applied Technology Council (ATC) and their hundreds of volunteer experts and millions of dollars of funding from the Federal Emergency Management Agency (FEMA), ASCE 41 stands today as a significant PBD tool that can be used by Structural Engineers to achieve acceptable performance and improve the resilience of communities.

Moving the profession from the prescriptive, non-committal, seismic design rules of the 1960s to PBD was, and continues to be, very controversial and contentious. Some believed that the simplicity of the existing codes was enough to guide the expert judgment of the designers. Others argued that there was too much uncertainty in the entire PBD process to declare the anticipated performance.

Unspoken was a need to hide under the building code’s mandate to protect the public. Still, others believed that while designing to various performance levels was possible; the potential liability was too high. Many feared to be sued out of business after the next major earthquake if expectations were not met. Fortunately, the earthquake design profession is protected by consensus-based codes and standards, including ASCE-41, that define the “state of the practice” the courts use to judge liability.

While structural engineers recognized that their design provisions had achieved life-safety, there were a handful of specific examples of performance that got everyone’s attention. Excessive damage to multi-story wood frame buildings and the collapse of the three-story Northridge Meadows Apartments (Figure 1) demanded new analysis techniques, design procedures, and detailed provisions. The damage to the apartments not only illustrated the collapse potential of code permitted soft-first story construction, but also the deficiencies in the design of shear walls and diaphragms. While concrete buildings

designed after the 1971 San Fernando Earthquake generally performed well, there was excessive damage and even collapse for newly designed parking garages with precast elements (Figure 2). Precast elements, while tied together for the then-code-level forces, did not perform as well as the cast-in-place concrete structures where the interconnection is continuous and not the weak link. New code provisions were needed to assure that the precast elements were adequately designed and interconnected. Within a year of the Northridge earthquake, over 100 steel moment resisting frame buildings were found to have a significant number of weld fractures in the beam-column connections that compromised their strength and ability to resist future earthquakes. A multi-year research and testing program followed, completely changing analysis, design, and detailing procedures.

Damage to existing non-ductile concrete buildings as shown in Figure 3 demanded that attention be given to older, non-conforming buildings. Vision 2000, Performance-Based Seismic Engineering of Buildings (Structural Engineers Association of California, 1995), cataloged these lessons. ASCE 41 has incorporated the evaluation of these and many other deficiencies in its four styles of evaluation and material specific acceptance criteria. The earthquake occurred just months before the needed seismic retrofit of this building was scheduled to begin.

The Vision 2000 committee was organized in 1992 and, after Northridge, received a grant from the California Office of Emergency services to formally develop PBD recommendations. By 1995, the team of 20 academic and design professionals published a conceptual framework and interim recommendations in their report, Vision 2000. They knew it was feasible to design and construct buildings that would not experience damage in the most severe earthquakes, but believed it was unnecessary and uneconomical. It was judged to be more prudent to design to varying levels of performance depending on the occupancy of the building, its importance to community response and recovery, and the economic viability of investing in reducing future losses. The committee formalized the process and vocabulary used today to define earthquake design levels, performance levels, and performance objectives Their design framework begins with

32 STRUCTURE magazine

Figure 2. Collapse of the precast concrete parking garage at Northridge Fashion Center.

Figure 3. Damage to this non-ductile concrete frame building.

site selection followed by a three-stage design process (Tiers 1 to 3), design review, and quality assurance during construction. The Vision 2000 report stands as the foundation for ASCE 41.

Vision 2000 also looked closely at the performance of buildings in and around strong motion recording stations as a means of testing the consistency and accuracy of the analysis and design provisions used in the past. The structural and non-structural performance of almost 200 building were surveyed and cataloged using a comprehensive data gathering form that included 10 levels of damage (10 being fully functional and 1 being collapse) that matched the Vision 2000 performance descriptions. While the sample size is not statistically balanced and focuses on damaged buildings, the results were judged acceptable enough to draw general conclusions. Of the most significant conclusions related to structural performance was the observation that buildings designed to modern codes had little impact on the average performance of the structural system. Also, for non-structural performance, no correlation was observed between the damage recorded and the ages of the building or their design to modern standards; Vision 2000 is filled with additional information. Refined analysis and design techniques were needed for PBD’s goal of producing predictable performance. ASCE 41 provides 2 force-based and 2 displacement analysis procedures that better predict the expected damaged to a wide variety of structural systems and material types. A new generation of non-structural evaluation procedures are also provided.

The 1971 San Fernando Earthquake demonstrated that the provisions being used for the design of new buildings did not yield the desired results. The earthquake also generated hundreds of strong motion records that cataloged the variation in strong shaking across the region.

The ground motion records showed that the shaking was much larger than expected and demonstrated the need for a new process for defining the design ground motions based on the relationship between the strong motion records and the observed damage. By 1980, such a process had been developed and has since formed the basis of the International Building Code (IBC) in use today.

The 1971 earthquake also signaled the need to do something about the seismic resilience of the existing building stock. With the updated design process for new buildings in place, FEMA began to focus on design standards for the evaluation and rehabilitation of existing buildings. A landmark workshop held in Tempe, Arizona, produced an action plan that would eventually result in ASCE 41. It began as a conceptual framework followed by a development guide that resolved the major controversial issues (ATC 28), a published guideline (ATC 33/FEMA 273), a pre-standard suitable for the America National Standards Institute (ANSI) balloting process (FEMA 356) and, finally,

published as an ANSI approved standard in 2006 as ASCE 41-06. It was immediately recognized as an acceptable PBD tool by the State of California for use in the SB 1953 Hospital Retrofit Program and by the IBC a few years later.

In parallel with the development work that led to ASCE 41-06, FEMA sponsored the transition of FEMA 178, Handbook for the Seismic Evaluation of Existing Buildings, from a guideline to the pre-standard FEMA 310, Handbook for the Seismic Evaluation of Buildings – A Pre-standard. ASCE subsequently balloted the prestandard and published ASCE 31, Seismic Evaluation of Existing Buildings. ASCE 31 and 41 then became widely used standards for the evaluation and rehabilitation of existing buildings. As they gained acceptance and popularity, engineers also used ASCE 41-06 as a PBD tool for the design of new buildings under the alternate design procedures clause in the IBC. Because ASCE 31, ASCE 41, and ASCE 7 were developed under different programs and did not share a common origin, they were not universally compatible. In the worst cases, a building found to meet the life safety standards of ASCE 31 when evaluated did not meet the same performance objective specified in ASCE 41.

Further, the characterization of seismic hazards in ASCE 41 did not match the characterization in ASCE 7, the basis for the IBC. These inconsistencies became barriers to the use of ASCE 41-style PBD but were eventually reconciled. ASCE 41 now stands as a consistent PBD tool that is accepted by the IBC as suitable for new building design.

Following the lessons learned in the San Fernando and Northridge earthquakes, Vision 2000 saw that a single set of design provisions that would be used for both new building design and existing building rehabilitation was important. Unfortunately, since they were developed under different programs, only ASCE 41 became a PBD standard. The controversy continues as to whether ASCE 7 and the IBC should fully embrace and incorporate the PBD process as outlined in ASCE 41. Fortunately, engineers can and should choose to use ASCE 41’s PBD on all their projects, either as a design tool or validation tool after the design is complete, because of the defined performance expectations. Either way, it is critical to the resilience of buildings and communities that structural engineers determine and declare the performance that is expected during design level and extreme level earthquakes, so their clients can invest wisely and their communities can plan accordingly.■ Chris D. Poland is a Consulting Engineer in Canyon Lake, California. (cpoland@cdpce.com)

M A R C H 2 019

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STORY SURVIVOR THE

OF A

SAND PALACE OF MEXICO BEACH

By John A. Dal Pino, S.E.

H

urricane Michael, one of the strongest Atlantic hurricanes to ever make landfall in the contiguous United States, made a direct hit on Mexico Beach, Florida, on October 10, 2018. The Category 4 storm strengthened unexpectedly as it raced

through the Gulf of Mexico with maximum wind speeds of 155 mph. At landfall in Mexico Beach, the measured storm surge was nearly 16 feet and, if wave height is added, the height of the wall of water was over 20 feet. The storm caused 45 deaths and resulted in damage of approximately $15 billion.

In Mexico Beach, almost all of the homes along the beach were destroyed by the wind and waves and swept away. However, one building along the shoreline (Figure 1) remained standing, alone in a field of devastation. Several news reports, including coverage in the

Figure 1. Sand Palace after the hurricane.

34 STRUCTURE magazine

New York Times, suggested that, from an engineering perspective, something unique had occurred and was worthy of further investigation. This attracted STRUCTURE’s attention.

So, John Dal Pino, Chair of the STRUCTURE magazine Editorial Board, tracked down the building’s owners, Russell King of Chattanooga, Tennessee, and Lebron Lackey of Cleveland, Tennessee, and their structural engineer Southeastern Consulting Engineers of Wewahitchka, Florida, to learn the full story. The Southeastern Consulting Engineers (SCE) team consisted of co-owners Jack Husband, President, Lance Watson, Vice President, and Matthew DeVito, Project Manager and lead engineer.

Unbeknownst to the owners, working closely with SCE, performance-based design concepts were employed to design and build a genuinely sturdy building that satisfied their own goals for performance and longevity while also conforming with the local building code. The evolving trend toward using performance-based concepts to increase the likelihood of achieving a specific set of goals is being used more frequently and something worth highlighting when it is performed successfully, all the more so when it is done by non-specialist owners (Mr. King is an attorney, and Mr. Lackey is a physician).

As you might imagine, the owners are very busy recovering as is SCE with their other clients, so we are grateful for their time. Lance Watson mentioned that his own home was flooded with three feet of water.

Figure 2. Concrete base – beams and piles.

Figure 3. ICF walls.

What first got you concerned about building performance?

worry about the code. We had our engineers confirm that we had satisfied the code.

Lackey/King: We have each owned several homes, but we had always purchased existing buildings. The house in Mexico Beach was going to be our first new, ground up building. We had decided that we wanted a vacation house and wanted to build something that would stay in the family for several generations, say 100 years or so. Russell had visited Costa Rica in the past and had noticed that a lot of the properties were constructed from concrete. Media coverage of climate change and the expectation that tropical storms were going to be more frequent and more intense also got our attention.

Were you concerned that the building code was not adequate for your situation?

Lackey/King: No. We did not believe that the building code was inadequate, but we did believe it only represented a minimum. We knew the code was a compromise document that was drafted by many different groups that did not necessarily share our goals.

What was your overall performance goal? Did you do your own research, or did you rely on the advice and expertise of your engineers?

Lackey/King: The survivability of the building in a major hurricane, you might call it the Big One, was our primary concern and the driver behind all of our decisions. We came up with that goal ourselves as we thought about the longevity we desired and factored in the news stories about storms.

SCE: The base of the building consists of 12-inch square precast, pre-stressed concrete piles that cantilever upward from the bearing stratum below the surface beach sands. Atop the piles, there are precast concrete beams that support the exterior and interior walls.

(See Figure 2 for the base of the pile base of the structure and the concrete beams at the first elevated floor.) The beams are attached to the piles with epoxied #10 reinforcing steel bars for uplift and shear transfer. The perimeter walls are constructed using Insulated Concrete Forms (ICF) with a seven-inch thick concrete wall inside with a single layer of #5 bars at 18 inches on center, each way (Figure 3). There are 2½ inches of insulation on each side for a total thickness of 12 inches. The interior walls are non-bearing wood stud.

The floors and roof are constructed with pre-manufactured wood trusses (Figure 4 ).

Wind naturally controls lateral force design. What were the pressures?

SCE: The building code wind pressures are based on a 140 mph wind speed. On an allowable stress basis, the windward and leeward wind pressures are 34 pounds per square foot (psf ) and 19 psf respectively. The sidewall pressures were 24 psf. The roof uplift pressure was 30 psf.

Would you describe the interaction between the owners and the engineers? How did you decide what changes to make and what parts of the building were modified?

Did you start with a completed design and then make changes, or did the process evolve differently?

Lackey/King: We worked with local designers to come up with an architectural floor plan that we liked. Then we worked with our structural engineers to come up with a design for the building focused on survivability as the goal and what made sense to us. Only then did we

Would you describe the building’s structural system?

Figure 4. Wood floor trusses.

Lackey/King: We started at the foundations and went through all of the major elements and evaluated them in terms of their contribution to survivability.

We started with the piles. It turned out that 40-foot piles did not cost much more and allowed us to penetrate the more solid ground beneath the sand, so M A R C H 2 019

35

foot basis over the cost of an equivalent, existing building in the same area. We evaluated many options and, if the extra cost was reasonable and we thought it represented an upgrade, we went with it, while always keeping an eye on the overall cost. We actually found that some elements, such as the ICF walls, did not cost that much more at all because the extra material costs are offset by lower labor costs. The ICF also had the side benefit of better insulation and resulted in lower insurance costs. We calculated that the on-going cost savings would offset the original extra costs in six to eight years. There was no noticeable impact on the project schedule. The cost of the lot is also the same regardless of what is built. (The building at the time of near completion is shown in Figure 5.)

What advice would you have for other owners seeking to mitigate specific risks?

Figure 5. Nearing completion.

that is what was built. 50-foot piles were much more expensive, because of handling and trucking, so we decided against them. The extra length in the piles, since they are cantilevers, added significantly to the lateral strength of the building and better protected against scour for a small cost. We wanted to have a solid slab-ongrade but learned that everything in the path of the storm surge needs to be designed to break away to protect the piles and the superstructure. Some of the only damage the building sustained was where the slab did not break away cleanly and contacted the piers, producing some cracking.

Then we focused on the walls. Our engineers told us that plywood sheathed wood stud walls would work but, thinking back to Costa Rica, we opted for insulated concrete form (ICF) walls. Our engineers told us that they would withstand 235 to 240 mile per hour winds, which sounded good to us. The doors and windows were custom made in Ocala, Florida, to the 140 mph, 4-pound-projectile Miami Dade County criteria. This is not 235 mph, but it was the best available.

We focused on potential weak points and made sure that the wall piers had two-foot-wide minimum widths and that a door or window failure would not compromise the entire building.

Lastly, we focused on the roof. It was robustly designed using multiple factors of safety to account for any construction errors, unprecedented storm events, and more to provide an exceptional design in the event of an unforeseen act of nature. Particular focus was placed on the eave design because our engineers told us that the eave design had a significant influence on survivability. It turned out that a hipped roof is better than a gable roof. We made the eaves as small as possible, about 12 inches, and enclosed the soffits to minimize the forces on the roof.

Then we had the roof trusses secured to the concrete walls with Simpson Strong-Tie hurricane tie-downs which were conservatively designed.

Lackey/King: We did not have any special skills going into this process (it was our first house remember). It just takes asking the right questions and the ability to analyze alternatives rationally and not emotionally. Spend time talking with your structural engineer so that you understand why things are the way they are and what you get with minimum code compliance. Understand the local risks (wind, earthquake or flood) and then build to match your own goals and comfort level so that you can sleep at night. Also, we recommend that owners stay personally involved throughout the construction process. If you live far away, hire a local representative. Just do not be an absentee owner. You need to have eyes and ears on the site continuously to make sure that the contractor follows the plans and builds in the quality that the structural engineer designed. We installed cameras on site so we could watch the contractor and monitor progress. These cameras also let us watch the building go through the storm.

Any last words?

Lackey/King: For us, function always won out over form. Before Hurricane Michael, we would not have said that we had the prettiest house in Mexico Beach but, after Michael, it would seem we have the only house still standing in Mexico Beach.â–

If we might ask, how much more did the design changes cost and what impact was there on schedule?

Lackey/King: The cost premium turned out to be 15% to 20% on a per square 36 STRUCTURE magazine

Sand Palace owners, Mr. Lackey (left), Mr. King (right).

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ADDING A

Parking Basement After-the-Fact

Top-Down, Post-Tensioned Concrete Solution Drives Success in Boston Congested site between adjacent State House and condominiums.

By Carol Hayek, Ph.D., MBA, and Tony Salem

T

he picturesque Beacon Hill conservation area in Boston is home to some of the most historic and prestigious buildings in the city. The area is characterized by narrow streets

and Federal-style façades that are protected as part of Boston’s rich heritage.

This was the challenging location for a project involving the conversion of two townhouses – numbers 6 and 7 Mount Vernon – into luxury 21st-century properties, along with a building to the rear which was converted into luxury condos. Sea-Dar Construction was the general contractor for the project. The developer’s vision was to create a parking basement for the renovated properties by excavating beneath the two townhouses while preserving the existing above-ground structure.

The Challenges The plan for a basement parking lot was viewed as the ideal solution to provide parking spaces. Due to the limited space of the location and the restrictions on any exterior modifications to the properties because of their conservation area location, restrictions had been placed on creating exterior, ground-level parking.

The underground parking solution presented numerous challenges, however. While plans for the townhouses included internal remodeling, there was a project-critical requirement to retain the façades and some of the internal structural walls, so it was vital that the excavations did not cause any adverse 38 STRUCTURE magazine

structural movement. Moreover, the buildings share structural walls with neighboring properties and are sandwiched between the Parkman House, where the Mayor of Boston entertains guests, and the golddomed Massachusetts State House.

Consequently, any movement or structural damage caused by the excavations would affect these buildings too.

The location also involved design and buildability challenges. Modifications to the external aspect of the properties had to be ‘in keeping’ with the local architectural context and original design, and the narrow streets severely restricted vehicle movements, size of the equipment, and any cranage requirements.

The underground parking lot was designed with a ground-level entrance and a vehicle elevator to accommodate the heritage restrictions. The engineering and construction team needed to develop a solution that would avoid structural movement and support the structure above both during the excavations and post-construction. The design also had to overcome the logistical restrictions and enable large spans to allow 14 parking bays and associated vehicle movement.

Interwoven post-tensioning tendons.

Initial Proposals

A New Approach Sea-Dar Construction began brainstorming alternative approaches and proposed a top-down construction program that would enable the above-ground structure to be supported without underpinning during the excavations. A post-tensioned slab was the obvious answer, and SeaDar Construction brought in CCL to advise on feasibility and design a suitable solution.

It became clear that, by combining mini piles with a flat post-tensioned transfer slab tied into the structural walls, the above-ground structure could be supported both during the excavation and post-construction, in addition to providing the large span required for an open-plan basement with just two support columns. This option overcame the logistical issues of transporting large pieces of steelwork and heavy equipment to the site, offered a much faster construction schedule, and reduced excavation depth and the cost of materials.

As the major structural risk element of the construction process was the

Build Methodology The original wooden ground-level floor was removed, an opening was created in the wall to enable the piling works, and closely drilled mini piles were drilled around the internal perimeter of the external wall. These piles were specified to enable post-excavations of 10 feet

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The initial parking garage design proposed the use of structural steel beams to hold the existing structure in place. This required installing the beams after the structure had been temporarily shored by carrying out an extensive needling process of the existing structural brick walls and underpinning of the existing foundations to allow for the excavation. It was clear that this initial plan did not take into consideration the implications of the structure’s integrity, the significant logistical constraints, or the project schedule.

Needling through existing old brick structural walls would have been very risky. Also, the weight of the structural steel beams would have made it close to impossible to get them into the building and maneuver them into position.

The time and cost implications were extremely onerous. Sea-Dar Construction estimated that the underpinning works alone would take up to six months, and there was likely to be a wait and a cost premium for the right expertise. Added to those program and labor costs was the excessive price of the steel itself, due to the weight and number of structural beams required.

Finally, there was a significant design issue. Due to the depth of the steel members and the height clearance needed for the basement, the initial design proposal would also have required deeper excavations.

excavation of the parking basement, a top-down approach was an ideal way to protect the façade and adjoining buildings, as the excavation would be carried out after the post-tensioned slab was already in place, enabling it to support the above-ground structure and brace the retaining elements.

M A R C H 2 019

39

Beacon Street garage interior showing one of two interior columns.

of the longitudinal and latitudinal post-tensioning tendons was also carefully mapped out, and post-tensioning tendons were crossed through the wall openings, ensuring that post-tensioning of the slab was tied into the existing structure.

The concrete pour was carried out in two phases, filling some of the wall openings with concrete before casting the remaining with the inter-connecting slab. When all the concrete had cured to the required strength, the tendons were post-tensioned, resulting in a self-supporting slab that was fully connected and running through the existing walls. Excavations could then begin, with earth and all foundations of the existing structure removed from underneath the post-tensioned slab, creating the new level space that would later become the parking garage.

Shotcrete walls were created around the perimeter of the excavated area connecting the (now exposed) upper third of the mini piles and ensuring a permanent and effective retaining structure. Foundation walls and the basement slab-on-ground were then finally put in place.

Maximizing Space One of the early ideas was to have a U-beam underneath the slab to embrace the walls and thereby avoid movement of the structure above. This proposal entailed the loss of space, reducing vehicle clearance. After rigorous calculations and modeling, and the idea of intertwining the post-tensioned slab with the existing walls, CCL was able to present a two-way post-tensioned transfer slab solution that provided adequate capacity to support the self-weight and the movements of the above-ground structure while allowing for topdown construction. This was very appealing to the project team because it left an open-plan parking garage with limited drops and just two columns, essentially yielding a transfer slab of 32-foot spans intertwined with the existing walls.

Summary of Advantages

Post-tensioning tendons running through the walls.

of headroom for the parking garage – while further anchoring into the ground.

The idea was to brace the top of the piles with a post-tensioned slab, providing a structure that could support the force of the earth during the excavations and carry the weight of the existing aboveground structure.

To tie the post-tensioned slab into the existing structure, small openings were created in the existing load-bearing brick walls at regular intervals at the height of the proposed slab, creating a castellated effect with gaps in the brickwork for concrete.

The size and location of these openings were modeled by the CCL team to ensure that the risk of structural wall movement and deflection of the slab were managed. All structural walls were also monitored throughout the work to corroborate the expected design values.

Extensive hand calculations were performed to verify the feasibility and demonstrate to the project’s Engineers of Record that the solution would deliver against all structural and risk management considerations.

With the castellated openings in place, the steel reinforcement was installed into the brickwork and through the gaps. The positioning 40 STRUCTURE magazine

Thanks to the creative thinking and collaborative approach of the project team, the project was not only viable from a cost and buildability perspective but also enhanced the spatial effectiveness of the newly-excavated basement.

In total, the change in approach reduced the time required to create the basement parking lot by around six months and avoided the need for any fire protection installation as the post-tensioned slab already met the required fire rating.

An extremely unusual project with some particular challenges, this small underground parking lot demonstrates the viability of a topdown approach to basement excavation using post-tensioned concrete, which could be replicated to resolve similar challenges for other confined site projects and heritage buildings.■ Dr. Carol Hayek is Chief Technical Officer at CCL and is a lecturer in prestressed concrete design at Johns Hopkins University, Baltimore. A fellow of the Post-Tensioning Institute (PTI) and a member of the PTI’s Technical Advisory Board, Carol also chairs the PTI’s Building Design Committee. She is a member of the American Concrete Institute ACI-ASCE 423 Committee, chairs the ACI 423F Subcommittee for Sustainable Prestressed Concrete, and is a member of the International Federation of Concrete (fib). (chayek@cclint.com) Tony Salem is Vice President and Principal at Sea-Dar Construction. Under his leadership, several Sea-Dar projects have received numerous awards. (tsalem@seadar.com)

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structural TESTING Moisture and Mass Timber

Large Scale Research into Durability of Cross Laminated Timber (CLT) Connections

By Evan Schmidt

M

oisture management plays an essential role in the serviceability and preservation of buildings, particularly when considering wooden structures. This is because wood’s durability can be compromised by the sustained presence of liquid water, while large moisture fluctuations can also impact dimensional stability and The MCMEC (Multi-Chamber Modular Environmental Conditioning System), where CLT samples will be exposed to varying environmental conditions during the first stages of the mechanical performance. Like any material, wooden new USDA and TDI funded research. structures perform excellently when designed and maintained properly, with many examples that have stood the test of time and lasted centuries (e.g., the Horyu-ji temple TallWood Design Institute (TDI), recently demonstrated that in Japan, stave churches in Norway, etc.). While moisture man- exposure to rain, followed by simulated interior conditions, can agement principles are generally well understood for light frame result in higher levels of checking and other stress-relief disconticonstruction, there are still many questions to be answered nuities, particularly at the edges where connections would occur. regarding moisture performance of large scale, “mass timber” While this study explored moisture performance in terms of structures, including how fast they wet and dry and how exposure wetting, drying, and checking, there is still a need to understand affects long term durability. the effects of different exposures on the short- and long-term This is particularly relevant because, despite requirements for performance of connections. protection of mass timber elements during the service life of the In response to this need, collaborators at OSU and Portland State structure (i.e., that they should not be exposed directly to weather), University (PSU) have been awarded large research grants from the contact with moisture is common during construction and, in USDA ($500,000) to explore the effects of moisture exposure on the the case of leaks, could happen anytime. Moisture monitoring durability of CLT connections. Another grant from the TallWood projects at Brock Commons (Vancouver), Carbon12 (Portland), Design Institute ($250,000) was awarded to a different team to and George W. Peavy Science Center (Corvallis) have demonstrated that CLT elements exposed during construction can generally dry to or remain below acceptable moisture levels during the envelopment process and in service.

However, there is still a need for more data and, particularly, research on the propensity for such exposure to affect long term connection durability and service life.

CLT connections are uniquely affected by weathering, in part as a result of the restraining action of the cross laminations. Specifically, while the cross laminations result in higher in-plane dimensional stability, they also induce higher internal stresses between boards and subsequent checking (due to the restraint of moisture-related differential swelling and shrinking). The SMART CLT project, led by Oregon State The edge of a CLT panel at a half-lap joint after Moisture monitoring equipment – part of the “Living Lab @ Peavy University (OSU) Assistant Professor controlled exposure to heavy rain and subsequent Hall” study at OSU – installed on a shear wall at the George W. Mariapaola Riggio and funded by the drying – part of the “SMART CLT” project at OSU. Peavy Hall construction site.

42 STRUCTURE magazine

assess CLT durability and develop numerical models to characterize dissertations, and educational seminars. Reporting will the degradation. be in stages, annually, and the total duration of the project Both projects are led by Arijit Sinha, Associate Professor of Wood will be 4 years, with expected completion in 2022.■ Science and Engineering at OSU. The research team for the USDA Evan Schmidt, B.A. Architecture and M.S. Wood Science, is the Outreach study consists of people with expertise in the fields of wood durability Coordinator at the TallWood Design Institute. TDI represents a collaboration (Jeff Morrell), non-destructive testing (Thomas Schumacher), accelerbetween the College of Forestry (Oregon State University), College of ated weathering (Fred Kamke), and timber mechanics and structural Engineering (Oregon State University), and the College of Design (University of engineering (Arijit Sinha). For the TDI project, Professors John Nairn Oregon). TDI’s mission is to advance the capabilities and usage of engineered and Andre Barbosa join Sinha and Morrell. wood products through applied research, testing, education, and workforce “Lack of data and models for this behavior is a significant hindrance development. For general inquiries about this and other mass timber research, to the understanding of the long term behavior of structures, and contact the TallWood Design Institute at tdi@oregonstate.edu. thus effective service life,” Sinha says.

The team has identified the need to systematically correlate the influence of moisture exposure and wood species on short- to long-term connection durability and mechanical performance.

The research, which aims to quantify degradation, will be conducted in three main stages. In the first stage, Douglas-fir and Lodge-pole Pine CLT samples will be exposed to various wetting conditions and monitored in the Multi-Chamber Modular Environmental Conditioning system (MCMEC) at OSU, a state of the art accelerated weathering device.

Wetting conditions will include simulated construction conditions, in-service leaks, and full immersion soaks (to mimic the worst-case scenario). Some samples will be inoculated with decay fungi, From a Space Program Hall while others will be cycled between wet and dry conditions. The samples will be of Fame induction to one closely monitored to observe wetting and of the tallest, mixed-use drying trends and durability, all using buildings in San Francisco, non-destructive methods (including Taylor devices continues to ultra-sonic wave propagation and full provide the most efficient, CAT scans).

The second stage will involve destructive effective and innovative assessment of the exposed connections structural protection through quasi-static loading at OSU products on the planet.

College of Forestry’s structural testing laboratory to determine changes in strength, stiffness, ductility, energy dissipation, and maximum load/deflection.

The data from this stage will be used to create a hygro-mechanical model update for the Seismic Analysis for Woodframe Structure (SAWS) 10 parameter model, which will have inputs for moisture and biological conditions.

The third stage will validate the developed model through full-scale testing and will assess a new structural health monitoring methodology for in situ testing of buildings using ultrasonic and acoustic emissions equipment.

The data from this project will be pub716 694 0800 | seismicdamper.com licly available and will be reported in the form of engineering design guidelines, peer-reviewed publications, student

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structural SUSTAINABILITY Structural Design and Embodied Carbon Considerations over a Building’s Service Life

By Chris Horiuchi, S.E., LEED BD+C, and Nicole Wang, P.E.

T

he structural engineering design profession needs to carefully reconsider design approaches. Embodied carbon of structural systems in buildings has been established to be a considerable influence on the detrimental environmental impact of structures. Embodied carbon is defined as the CO2-equivalent emissions into the atmosphere caused by the production of a material, product, or system. Embodied carbon impacts of a building’s structural system are primarily associated with the different life cycle stages: material extraction, manufacturing and production, construction, damage and repair during service life, and end-of-life considerations.

The ASCE/SEI Sustainability Committee is focused on reducing the global climate change impact of structural components and has completed significant research into the many opportunities for structural engineers to reduce the embodied carbon of buildings. It is critical to consider these options early in the decision process such that they become important aspects of the project. Some possible strategies include the use of alternative materials, particular structural system selection, and incorporation of enhanced seismic-resisting systems.

Alternative Materials

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Material decisions can help reduce embodied carbon. For example, cement is the most significant contributor to the embodied carbon of concrete. Replacing cement with supplementary cementitious materials (SCM) like fly ash and slag can reduce the overall environmental impact without negatively affecting concrete performance.

Additionally, wood is a renewable material and can be considered a

44 STRUCTURE magazine

Normalized comparison of embodied carbon values of various lateral systems based on survey of previously-designed buildings.

carbon sink as it sequesters carbon when sourced from a sustainably managed forest. While timber has been traditionally used for low-rise construction, mass timber components have become more common in taller construction and longer span conditions, and serve to reduce the amount of steel and concrete used in construction. For additional information on embodied carbon of structural materials, refer to the ASCE/SEI technical report Structural Materials and Global Climate.

Structural System Selection Surveys of previous buildings have been analyzed to determine trends in structural system selection. Based on previous studies, it is estimated that utilizing a slab and beam system over a flat slab system can save 15 – 20% of embodied carbon.

The more efficient gravity framing depths translate into lower material quantities in the framing and lower lateral demands.

Other considerations are required when selecting structural framing systems, but the intensity of structural embodied carbon should also factor into the decision process.

Lateral system selection can also affect overall material quantities. The Figure shows average structural embodied carbon values based in a database of previously designed buildings with particular lateral force-resisting systems as compared to buildings of similar height and seismicity.

For steel structures, systems incorporating axial resistance (e.g., braced frames) can

reduce embodied carbon impacts by 20% when compared to similar structures using moment frames. For concrete structures, shear wall buildings show trends of higher structural embodied carbon over moment frame buildings. Even though moment frames are a less efficient method of resisting lateral load, shear walls may include underutilized concrete material.

The examples of trends exhibited by this data are not expected to be absolute rules for lateral system selection. Rather, they should serve to encourage engineers to use embodied carbon as another decision variable in structural design. The results for any particular project may vary depending upon many factors, including the seismicity, the geographic location, the assumed service life, and the building program and configuration.

Resilient Seismic Systems Buildings have a probability of experiencing a design-level earthquake over their service life. Damage resulting from a seismic event requires repair and designs include a particular failure probability of complete demolition and replacement of the structure, thereby causing further use of natural resources and emitting additional carbon.

Enhanced seismic systems can limit the expected damage during a future earthquake. These systems enable the more efficient use of the required structural materials by reducing the likelihood of repair or demolition and replacement in the event of a collapse. These effective seismic systems localize displacements to allow the majority of the structure to behave as essentially elastic. Isolators reduce seismic demands on the superstructure and fuse systems localize ductility to a particular location which can be easily replaced after a damageinducing seismic event. A previous building study on a mid-rise

residential building in San Francisco showed a possible 15–20% reduction in overall embodied carbon when considering probabilistic seismic damage using base isolation. Considering these long-term structural impacts with respect to embodied carbon impacts reinforces the importance of seismic design methods that target improved performance beyond code minimums.

One could also argue that structural engineers are among those who can have the most significant impact on climate change since they decide the structural materials and system performance. As a profession, we need to create greater awareness of the damaging carbon emissions in the construction and maintenance of building structures negatively impacting global climate change. It is important for the engineering profession to embrace the most advanced systems available and create a holistic awareness of their benefits to building performance, damage repair cost and extent, and environmental impact. With a goal of minimizing the built environment’s impact on the natural environment, engineers need to focus on intelligent use of materials and incorporating resilient systems.

Structural engineers interested in participating in the further development of sustainable structural design ideas are encouraged to connect with the ASCE/SEI Sustainability Committee (www.seisustainability.org) or their local organization.■ Chris Horiuchi is an Associate at Skidmore, Owings & Merrill LLP in San Francisco and is chair of the ASCE/SEI Sustainability Committee’s Disaster Resilience Working Group. (christopher.horiuchi@som.com) Nicole Wang is a Project Engineer at Skidmore, Owings & Merrill LLP in San Francisco and is chair of the SEAONC Sustainability Committee. (nicole.wang@som.com)

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historic STRUCTURES Thames River Bridge New London, Connecticut, 1889

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

A

lfred P. Boller (STRUCTURE, November 2011) had already designed swing bridges across the Pequannock

River, Harlem River, Hudson River, Ft. Point Channel, and the Arthurkill when he was called to be the designer of the longest swing bridge in the United States across the Thames River in Connecticut.

Preliminary surveys of the site to replace a train ferry started in 1859 for the Stonington Railroad, but the Connecticut legislature Artist’s Conception, Scientific American, June 8, 1889. did not pass an act approving the New York, Providence, and Boston Railroad to build the bridge until January 1882. A commission of for his original estimate for a single-track bridge. It was not until 1888, Army Engineers was appointed that approved of the site at the nar- four years later, that all the financing was approved by the two railroads rowest point along the river. Boller was authorized to proceed with and bids received. On April 6, 1888, the construction contract was his first design in the summer of 1882 for the bridge between New awarded to the Union Bridge Company run by Charles Macdonald London and Groton, Connecticut, across the Thames River. It utilized and others. The bridge as built differed from the initial design with Whipple double intersection trusses and had a swing span of 500 feet single intersection trusses and two fixed spans of 150 feet, two fixed with two 310-foot approach through-spans and two 150-foot deck- spans of 310 feet, and a swing span 503 feet long with four panels spans on each shore. His design was placed before an independent adjacent to the swing towers of the top chord on a parabolic curve. panel of engineers that issued a favorable report on March 28, 1883.

Foundation problems were compounded by the fact that the river The plans were submitted to a select panel of engineers consisting of was 40 to 60 feet deep and bedrock was at a depth of 130 feet, making C. C. Martin, Octave Chanute, and Col. J. Albert of the Corps of pneumatic caissons questionable. Boller arrived at the following Engineers. Congress passed an act in May 1883 approving the bridge, construction procedure. with the proviso that another panel of military engineers approve of 1) Wooden cribs, built with two walls of 12 x 12 inches with a the final plans. Congress approved the bridge and designated the space of 8 feet apart and cross-braced, were sunk and the mud road as a Post Road. dredged out with the cribs sunk to a bottom of 70 feet below The long, two-track swing span was later fixed in the spring of 1884 the water level. The 8-foot space was filled with rock to help by a panel consisting of three Army Engineers and two Navy Engineers in the advancement of the cribs and extensions added on top to ensure there was adequate clearance for ships using the United to keep the top of the crib above the water level.

States Naval Station just upstream. The clear passage on each side of 2) The cribs were supported by interior cross walls that also left the swing pier was 225 feet, and the vertical clearance on the three pockets in which to drive the piling. central spans was 32 feet. The weight of the swing span was 1,000 3) Wood piles 85 to 95 feet long were then driven to bedrock in tons. They approved of the location on July 25, 1884, but raised the the pockets. spans six feet and called for pointed icebreakers on the upstream faces of the piers.

The railroad approaches were approved by the cities of Groton and New London on May 15, 1884. The railThe first design of the Thames River Bridge, early 1880s, with 500-foot swing span. roads then got into a debate as to whether the bridge should be single-tracked or double-tracked and ordered Boller to prepare an estimate for both possibilities. He found that, with improved designs and lower prices then prevalent, he could build a double-track bridge The second design of the Thames River Bridge, 1888, with a 503-foot swing span.

46 STRUCTURE magazine

Thames River Bridge.

4) The piles were cut off at the new mud line and sand in deep water.

5) The tops of the piles were encased in tremied concrete.

6) Caissons were then floated into place and sunk until they set on the concrete, and then pumped out.

7) The masonry was placed in the dry in these caissons.

After the swing pier was completed, up to three courses from the top, a test load of 2,672 tons of iron ingots was placed to ensure its load carrying capacity. A maximum settlement of 5 inches was observed. At the same time, it was discovered that the stonework was not coming up level requiring the last three courses to be cut to ensure the top of the pier was level, necessary to make the swing table work properly.

Boller’s colleague, Alexander McGraw, placed the foundations and masonry. Construction on the foundations started in June 1888. The entire project was completed in 16 months, with its grand opening on October 10, 1889. It carried two tracks and, in profile, resembled Boller’s Eighth Avenue span across the Harlem River. Mr. Babcok, President of the Railroad at the dedication, commended Boller, stating, “The bridge will stand as a monument to your labor and skill.

Although the Company furnished the money, neither the Directors nor the stockholders had the brains to do the great work. It is indeed a work the Company has every reason to be proud of.” The total cost of the project, including the approach trackage, was $1,600,000.

The Railroad and Engineering Journal wrote of the bridge, The aesthetic features of the structure have evidently been studied, as well as its purely engineering features. It is gratifying to know that the design is not only graceful and pleasing, but that it is also economical. The excuse for making structures hideous and unsightly is that it would be too expensive to make them otherwise. As a matter of fact, the ugliness of bridges is due generally to the absence of a sense of beauty or grace in their designers. In the present instance, the Engineer of the Thames River Bridge was an artist as well and, as a result, both the engineering and the artistic effects are good and neither was sacrificed for the other, and, in fact, the science of the engineer seemed to improve the work of the artist, and vice versa.

Boller wrote a lengthy report on the bridge entitled New York, Providence, and Boston Railroad-Report to the General Manager upon the Completion of the Thames River Bridge and Approaches at New London, Conn. It had 43 pages of text and many drawings and test results. Engineering News wrote of the report,

The total length of new work required by the bridge was 5.13 miles. The chief difficulty in location and construction lay in the piers and abutments and their foundations, the adopted plan calling for a draw-span of 503 feet flanked on either side by spans of 310 feet and 150 feet each. Careful soundings made at the site of the main foundations only reached rock and boulders at depths of 130, 100, and 120 feet below mean low water, and the material overlying this rock was from 60 to 75 feet deep. Pneumatic foundations were out of the question and the ingenious methods adopted and carried out by Mr. BOLLER are fully described and illustrated in this report with an engineering minuteness that makes them of great professional value. The superstructure, with its great swing draw-span of 503 feet, is treated in a similarly careful manner, and is made plain in all its details of design and erection by concise description and complete drawings, with dimensions and sections. As we have before mentioned in this journal, the bridge is not only one of the latest and best examples of American steel bridge design, but its treatment is artistic in general form and detail, and the result proves conclusively that even so utilitarian an object as a railway bridge can be made a pleasing object to the eye without practical increase in cost. The bridge proper only cost $658,489, though the entire cost of land, approaches and general account swelled the final cost to $1,293,939. In appendices are given the official tests of the bridge, the full specifications of foundations and superstructure, the weight of metal in the truss spans, full-sized eye-bar tests, and the various other finished material tests. The illustrations are from photographs taken during and after erection, and the various detail sheets of foundations and superstructure are facsimiles of Mr. BOLLER’s own plans, these latter including the turntable and gearing and end-locking gear. Taken altogether, it is a model report and proves how useful a complete record of a difficult and important piece of work can be made to the engineering profession when the engineer in charge goes conscientiously to work. The final regret is that reports of this kind are too rarely issued by the executive officer, who is alone able to fully detail the true history, and that it is still more rare to find corporations willing to publish such reports when made.

It remained the longest swing span in the country until 1893 when the Omaha Bridge and Terminal Railroad built a bridge, by J. A. L. Waddell, across the Missouri River at East Omaha, Nebraska, with a span of 520 feet. Despite all of Boller’s efforts, the easterly pier of the Thames River Bridge started to settle and shift to the south. In 1908, rail traffic was limited to one track and, in 1919, when a new railroad bridge was built, it was converted into a vehicular bridge carrying U.S. Route 1 across the river. It was replaced in 1943 with a high-level bridge, the Gold Star Bridge, that is now part of I-95.■ Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an Independent Consulting Engineer. (fgriggsjr@twc.com)

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professional ISSUES Disruption is Coming to the Building Industry

By Steven Burrows, CBE, P.E., FICE, FASCE, MIStructE, LEED-AP

T

he construction industry is on the cusp of significant disruption that will forever change how buildings are bought, designed, made, and assembled. The summation of the technological forces being applied to the construction industry is termed PropTech.

PropTech investment in 2016, according to Re:Tech, was a record at $4.2 billion which tripled to $12.6 billion in 2017. Data from 2018 is expected to outstrip 2017 easily. The reason is that construction employs 7% of the world’s working population and around 50% of manhours on construction sites goes towards unproductive tasks. Construction also has the lowest digitization index, a measure of technological adoption, of any major industry, creating a target for PropTech. The rewards for success are therefore huge and investment capital across the world understands that.

Buildings have also become unaffordable, with the cost of construction outstripping the consumer price index by a significant margin in major economy in the world. This can be seen in major cities by looking at increased commute times or by comparing the average wage to the average cost of housing. By all measures, this trend must reverse to be sustainable.

The concept of technology disrupting established high cost/low productivity industries is not new. Airbnb disrupted the hospitality industry in 2008, Uber did the same to taxis in 2009, Amazon has disrupted retail, and WeWork has disrupted the workplace. Their rewards have been huge, with Airbnb now valued at $25 billion and Uber at $50 billion in sectors that have revenues that are a mere fraction of those in construction.

The AEC industry is seen as the last major industry to be disrupted. Investment dollars are a sure sign that change is coming, and it is coming fast. Some examples, just in the last months, include PlanGrid (formed just 7 years ago) sold for $875M, Building Connected founded in 2012 and sold in 2018 for $275M, and Katerra formed in 2015 and now employing over 5,000 people. There are hundreds of examples of new players entering the AEC space all with one goal in mind, to increase cost effectiveness and disrupt the status quo.

What are Investors Seeking?

McKinsey reported in Reinventing Construction that the global productivity gap between construction and the average for

other industries equates to $1.6 trillion per annum. That would require construction to increase overall productivity by around 50% to align with the average for all industries.

Improvement is predicted to come from five big moves (and their share of the productivity increase): 1) Collaboration 7.5% 2) Design 10% 3) Supply chain 7.5% 4) Onsite execution 10% 5) Technology 15% Technology underpins the changes that are coming but, to understand how such a major move can be made, it is worth looking at other industries that have been transformed such as automotive, shipbuilding, or aerospace.

In the automotive industry, cars have broadly maintained the same relative price over the last 50 years, meaning that, in terms of 2018 dollars, a car in 1960 costs about the same as one today. However, that car is now considerably more advanced, of higher quality, and far greater reliability than in 1960. By way of comparison, homes cost around double what they cost in 1960, in present-day dollars.

However, buildings are not significantly more advanced now than they were 100 years ago.

They still use wood and nails, doors still use keys invented in 1860, and the loss of the artisan tradesman to the journeymen means they have not improved in terms of quality, life expectancy, or reliability. At a different scale, we marvel at the speed of construction of the Empire State Building and cannot replicate it today while using substantially the same structural materials used for the last 100 years. To build better, cheaper, and faster requires a transformation and that is what PropTech intends to achieve.

Shipbuilding is another excellent example of the type of transformation that can be expected to occur in the AEC industry. The world’s largest vessels are around 1,500 feet long. These massive structures were mostly stick built in shipyards 30 years ago. Unlike the car, the scale of these ships is such that their length would put them alongside the world’s top 20 tallest buildings. In shipbuilding, the industry moved to offsite construction. Today, ships are made of assemblies of huge modular components largely factory built and then merely connected at the shipyard like giant Lego® blocks.

Aerospace used to build passenger aircraft in one place. It became the first major industry to standardize using a common modeling protocol to create a single coordinated source of truth. This decision allowed suppliers anywhere in the world to make standard components that fit in the model and sell those components for multiple buyers, effectively creating a global supply chain model. The automotive industry followed that approach.

Now, many cars are built on the same chassis with the same drivetrains but produced by assorted brands. Can buildings be created that way, instead of each building being bought out on a one-off basis?

The benefits to these approaches being applied to the AEC industry will lower cost, increase speed, and improve quality.

Investment is bringing these thought processes to the AEC industry at the speed Uber arrived on the scene to rival taxicabs.

The AEC industry has long argued that the unique nature of the product, together with the fragmented nature of the market, prevents this revolution occurring. However, technology thrives on complexity. The time taken to reach 50 million users for the radio was 75 years, television did it in 13 years, the Internet took just 4 years, but PokemonGo did it in 19 days. Technology now enables us to deal with complexity at a speed never before possible. We now use smart devices that allow apps to be developed anywhere by anyone; Tesla is using that same approach to communicate with its cars. The age of technology disrupting the AEC industry is here.

Where is the Investment Money Going?

Investment into AEC has been going into 10 primary places: 1) Offsite manufacturing 2) New building materials 3) 3-D printing and additive manufacturing 4) Autonomous construction 5) Visualization 6) Data and analytics 7) Wireless connectivity 8) Collaboration tools 9) Scanning and sensors 10) Building Information Modelling (BIM) continued on next page M A R C H 2 019

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Several new companies have also emerged, as have alliances between manufacturers and constructors. The competitive landscape is changing as is the process for buying buildings. The way customers will interface with building providers will be more like buying a car online.

Some of these changes were inevitable; for example, automation on site. Autonomous cranes, dirt scrapers, bricklayers, drywall finishers, and other trades are obvious candidates, as there is insufficiently trained labor to meet demand. Automation simply closes the labor gap.

For other parts of the supply chain, the impact will be more than just lost jobs; engineering is one of them.

The Professional Engineer 2.0 The author firmly believes that now is the greatest time in history to be an engineer.

There have been several other great times such as in ancient Egypt, Rome, or during the industrial revolution, but today is the best of them all. The disruption to AEC gives the engineer the greatest opportunity for transformation.

The engineering profession needs to take advantage of these three opportunities: 1) Designers must design for constructability much as the ancient engineers did in Egypt and Rome.

2) Designers must design for manufacturing so that the deliverable is not a construction drawing but machine code that can go straight to the toolset.

3) Designers must design for whole life cost, understanding maintenance and how to replace elements of buildings to allow them to be renovated easily.

Designing for Constructability For most of history, engineers determined the means and methods by which buildings were built. The engineer had to understand how technology impacted the design in order to design it. Tomorrow’s AEC industry will be much the same; as manufactured modules, components, and assemblies become the building blocks of tomorrow’s buildings, so will the engineer need to understand how automated systems on construction sites will place these blocks within the building. Process engineering will sit alongside structural engineering as a design discipline, and the separation of engineer and builder will blur if not disappear.

In recent years, engineers have increasingly relied upon pre-construction services from contractors to de-mystify the logistics of building. However, in a future where vertically integrated firms manufacture the majority of 50 STRUCTURE magazine

parts and pieces offsite, engineers will need to look to the ways of the past to determine how assemblies will be put together.

Designing for Manufacturing Designing for manufacturing will also bring a need for change. Today’s engineers deliver documents that are sufficient for permitting, and for contractors to tender and complete the design and deliver it back for review via shop drawings. In a world in which the engineering product is a machine file sent directly to manufacturing, the role of the engineer will become one of designer-maker. Shop drawings will disappear. The product of an engineering design office will be a single coordinated model from which components can be extracted in the form the factory needs it.

No more will there be many versions of the building in models held by architects, engineers, subcontractors, and general contractors.

There will be just one coordinated source of truth, a model not just used for gaining a permit but a model that is manufacture-ready.

Engineers need to step into the world of product design, understand the capabilities and needs of equipment, and coordinate the components that make up the building systems as a set that fits together as one.

Designing for Whole Life Cost The cost of a building has 3 parts: the first cost, the operational cost, and the personnel cost. The ratio of these is 2:6:92, yet today we almost exclusively focus on the first 2%.

Taking the first cost; building construction tolerances are poor and lack of accuracy is far below that in industries where manufacturing methodologies are used to create the product.

In construction, these tolerances and lack of fit are remedied by many means such as shims, welds, wedges, caulking, sealants, cover strips, or finishes. All these things add unnecessary cost that high-quality construction would require less of, or in some cases will just not be needed. Manufacturing demands that tolerances are tighter to make it efficient. Quality of construction has to improve.

Operational costs also have to come front and center. Buildings built as a series of components will have the ability to unplug these elements for renovation. Today we see small examples of this in items such as bathroom pods. However, disaggregation based on life expectancy will soon be a key design consideration. Additionally, the life-to-firstmaintenance and the means of doing this will become the responsibility of the designer.

Engineering decisions around material compatibility, tolerances, maintenance, and

lifespan impact the cost the customer pays.

Engineers will need to take a leading role in this for the transformed industry, as design decisions will need to be made holistically.

How will engineers take on all this additional work and responsibility? The answer is design automation. Many designers have 30% or more of their staff undertaking routine tasks that algorithms will replace. Studies suggest that one-third of the entire U.S. workforce will need to find new occupations by 2030 as automation comes to the AEC industry, creating a lower ratio of staff headcount to fee revenue. This will create space for these changes to occur at no cost to the project.

Too Much, Too Soon?

In a transformed industry, where buildings will be built twice as fast for half the cost and at much higher quality, the role of the engineer is going to be key.

The future will be data-driven, automated, and component based. Now is the most significant time in history to be an engineer in AEC, but the profession must change to meet these demands.

The great news is that many disruptive firms in PropTech are led by engineers who already see the opportunities that lie ahead. However, many consultants are only now beginning to consider the impact of change. Some firms are investing in parametric capabilities, allowing staff to go back to school to learn how to script code and attend manufacturing conferences to understand the tools being developed and the potential of new products. Some of these firms are late to the changes, but not too late.

If we remain alive to the possibilities and stay open-minded, not passive, then we should be able to play a crucial part in tomorrow’s disrupted industry.

So my message is this: Change is coming, and it is coming fast, so get your business future ready.

To do this, you need to do four things: 1) Change your mindset and be open to the possibilities.

2) Consider the future, not as a destination but a place of disruption.

3) Prepare yourself for a range of scenarios; stay current and relevant.

4) Invest in your people, your processes, and your future.

It is an incredibly exciting time to be in the AEC industry.■ Steve Burrows was a Director of Arup, EVP at WSP, and Executive at Katerra. He is now a Principal Consultant at Cameron MacAllister Group and a Founding Partner of Baumatix Building Automation. (burrows@cameronmacallister.com)

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risk MANAGEMENT Jobsite Safety

Actions Speak Louder Than Words

By Randy Lewis, CPCU

F

or design professionals, assuming any responsibility for a contractor’s means or methods – either by contract or by the A/E’s actions – can have serious consequences.

Jobsite safety is typically the primary responsibility of the general contractor. The contractor has actual, physical control of its employees and the site, and is usually the overall coordinator of the work. Sometimes, however, a design professional’s actions can result in the assumption of liability for jobsite safety. For example, if an engineer were to visit the construction site three or four times in a week, giving direction regarding the installation of the HVAC system she designed, her actions could result in an assumption of liability.

A 2016 court decision highlights how A/Es can protect themselves from an assumption of liability for jobsite safety and its negative consequences. In McKean v. Yates Engineering, the Supreme Court of Mississippi affirmed a 2015 Court of Appeals decision concluding that a design firm had no duty to notify or warn workers or employees of hazardous conditions on the site unless the design firm had assumed by conduct or contract to supervise a construction project.

A Case in Point It was a complex case and not without controversy. Boiled down, though, it involved an engineer hired by the contractor to design scaffolding. The design was flawed in that it called for materials that were not commercially available. Without seeking guidance or clarification from the engineer, and ignoring some essential features of the design requirements, the contractor erected the scaffolding using different materials. The scaffolding collapsed, injuring four of a subcontractor’s workers.

The workers sued the engineer and the project architect (who had an AIA B141™ agreement with the project owner), alleging they were “negligent in inspecting the scaffold[ing] and failed and/or refused to correct known deficiencies and defects in the construction [that] made it dangerous to use prior to the subject incident.”

The trial court ruled in favor of the architecture firm, stating it was not involved in actual

52 STRUCTURE magazine

supervision and control of the contractor’s work. The court also pointed out that the architect’s AIA B141 agreement with the owner was unambiguous in stating that the architect was not responsible for construction methods or safety precautions in connection with the work. The trial court later ruled in favor of the engineer, too, quoting an earlier decision: “Unless [the engineer] has undertaken by conduct or contract to supervise a construction project, he is under no duty to notify or warn workers or employees of the contractor or subcontractor of hazardous conditions on the construction site.”

Seven Factors Can Undermine Your Contract In upholding the trial court’s decision, the court of appeals and the supreme court cited earlier decisions that had listed seven factors that determine whether supervisory powers go beyond the terms of the contract: 1) Actual supervision and control of the work 2) Retention of the right to supervise and control 3) Constant participation in ongoing activities at the construction site 4) Supervision and coordination of subcontractors 5) The assumption of responsibilities for safety practices 6) Authority to issue change orders 7) The right to stop the work The supreme court also went a step further to clarify its position “that an architect has no affirmative duty to supervise safety absent contract or conduct.”

The court’s guidance is a reminder that your actions on the site can undermine any contractual protections. For example, if you perform supervisory and inspection tasks over and above the scope of your contract, a court may find you have “assumed a duty of safety,” which could leave you liable for damages… never mind what your contract says.

Let’s suppose that the architect in the above case had been continually at the site offering advice regarding the contractor’s means and methods. In that instance, the court may have found he had assumed some control of the work, therefore failing the above test.

Similarly, had the engineer visited the site, watched the erection of the scaffolding, and given the contractor suggestions regarding a construction barrier, he, too, may have been seen as assuming “supervisory powers” and the liability that went with them.

The Takeaway The main thing to remember here is that you should have a good contract and take care to stay within the terms of that contract and the bounds of your scope of services. Your staff should be briefed on the scope, duties, and responsibilities, and other critical issues of your contract so that your employees, while observing the work at the job site, do not undertake responsibilities for jobsite safety.

At the same time, you cannot ignore your duty as a licensed professional to step forward in the face of imminent threats to life or safety.

Each firm should develop a field manual for staff to follow if your on-site representatives observe unsafe conditions on a project site, including situations that pose an imminent danger to others.■ Randy Lewis brings over 20 years working in the insurance industry. Currently, Randy manages the risk management and client education programs of AXA, XL a Division of AXA, a recognized leader in the AEC community. (randy.lewis@axaxl.com)

M A R C H 2 019

INSIGHTS The Future – BIM Building Structural Monitoring

By Tom Winant, P.E., and Alan Jeary, Ph.D., DSc

C

odes of practice and standards for structural design have traditionally been regarded as the ultimate security for making sure that structures comply with requirements for safety. The advent of new technology and higher computing power allows engineering design to be supplemented by measurements of the actual performance of a structure during and after construction. This would allow for feedback to validate whether buildings are built as intended and to determine if existing buildings have acceptable levels of capacity. If done on a broad level, it would give insight into community resilience and ultimately allow poorly performing buildings to be identified prior to shock events.

As buildings are subjected to storms, earthquakes, and natural events, tracking degradation is also critical to understand the structure’s residual capacity, risk profile, and suitability for re-occupancy after natural events. In a few hours, an assessment from very small dynamic movements can be processed into a structure’s Dynamic Signature and yield a risk profile of the building. With cloud computing, this assessment is made in near real time after data is collected.

Using accelerometers that are extremely sensitive, it is possible to use low amplitude excitation from natural sources (such as wind or traffic) to obtain information about the in-service performance of the structure. This analysis of tiny movements is analogous to the use of an electrocardiogram to judge the state of health of a human heart. The measurements at low amplitude allow for a prediction of the response at high amplitudes, which is where damage can occur. A recent awardwinning paper issued by ASCE (Spence and Kareem) gives a detailed explanation of the predictable nature of non-linear damping, allowing for extrapolation of low values of damping to higher values. The paper summarized decades of international research on the topic of non-linear damping, providing a fundamental understanding of the previously poorly-understood mechanism. This understanding, applied appropriately using classic structural dynamic analysis, becomes a powerful tool to compare all structures with the 54 STRUCTURE magazine

code under which they were built, as well as to each other, based on their measured response and associated risk profile. The measurements not only give a direct measure of the capacity of the structure but also identify the presence of any anomalies or weaknesses and their location. The techniques and analysis have been used on buildings in the U.S., Australia, New Zealand, and other countries to support decisions about the viability of continuing to use a building, what repairs/improvements need to be made, and even whether construction work is safe to continue.

Already, this approach has been used extensively and has identified a building in an earthquake zone that had damage in areas hidden from sight. In this example, 10 buildings of various design types, heights, construction quality, and uses were measured, and the response of each was put into a bell curve based on their relative risk profile to compare each structure based on both their risk and the local codes. One building was identified as an ‘outlier,’ which under any design criterion was substandard. It had been damaged mildly in several earlier earthquakes, but the damage was covered cosmetically.

Using classical engineering techniques, investigators could not identify its fundamental weaknesses; however, the objective measurements clearly identified the weakness as well as the location of damage.

In another example, the techniques were used to give assurances that a school building’s newly developed cracks were merely cosmetic and did not pose a risk to the children. Also, the techniques were used to modify a proposed addition to the roof of a kindergarten to allow safe working loads where no existing drawings were available.

The technology is also applicable to work under construction. It has been used by a piling contractor to avoid costly stop-work orders by giving near real-time feedback about the acceleration and vibration effects of the pile installation on adjacent buildings. The baseline behavior of the existing buildings was also established as a reference, before the pile driving began, to compare with the condition after construction. In another case, the

On-site monitoring.

technology was used to modify the approach to demolition or nearby blasting very close to an adjacent structure. The baseline measurements showed the adjacent structure had an unusual type of behavior in which a resonance-on-a-resonance type of response was probable. When this type of condition occurs, the response of a structure can increase dramatically. The modifications included raising the elevation of soil berms intended to catch the structure being demolished, therefore reducing the impact forces.

In all instances, the objective capacity-related measurements were critical to engineers to give them actual performance measurements, in addition to engineering judgment. The measurements proved critical and allowed the engineers to provide the best service to their customers.■ The online version of this article contains a reference. Please visit www.STRUCTUREmag.org.

Tom Winant is the President of STRAAM Group. (twinant@straamgroup.com) Dr. Alan Jeary is CTO and Partner for STRAAM Group. He is a fellow of IStructE (UK), Engineers Australia, The Australian Institute of Building, The Royal Society (NSW), and an Officer of the Order of Australia. (ajeary@straamgroup.com)

M A R C H 2 019

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STRUCTURES CONGRESS 2019

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Orlando, Florida April 24–27

Experience the Premier Event in Structural Engineering Visit the Exhibit Hall for the latest resources and opportunities

JOIN US IN ORLANDO TO LEAD AND INNOVATE: •NEW: Performance-Based Design Trends, New Smart Technologies, Career Development, and Leadership Skills •View program detail and use interactive planner online •Special Sessions: Grenfell Tower | Workshop on Conceptual Design | Improve your Communication/ Presentation Skills •A fantastic evening Celebrating the Future of SE hosted by CSI •Register by March 20 for best rate

Check out demos of ASCE 7 Online and the Hazard Tool at the Bookstore, and enter to win a free six-month individual subscription to ASCE 7 Online. www.asce.org/asce-7

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business PRACTICES Building Your Leadership Legacy

By Jennifer Anderson

A

s a leader in your company and in many other roles of your life, you have an opportunity to build a leadership legacy that will not only make a difference in how you will be perceived by your team, your peers, and the community at large today… but also how you want to be perceived in the future.

Leadership is defined as the position or function of a leader, a person who guides or directs a group. How you lead today will make a difference for how people will remember you in the future –your leadership legacy.

Legacy is often associated with an endowment or a large sum of money bequeathed to a favorite university that honors the donor with naming a building on the engineering campus. Another way to look at it is that legacy is your personal brand, or your mark on the world. Impressive as it sounds, few people in leadership roles will amass a fortune in their lifetime sufficient to have a building named for them. For those in a leadership role who realize that how they lead impacts the lives of team members, they have a significant way for their leadership legacy to shine… one that might be even more impactful than their name on a building.

If you are not satisfied with how your leadership legacy is being built today, you can improve and make changes to how people perceive you as a leader. Here are three ways to build a leadership legacy of which you can be proud:

Start With the End When it’s the end of your life, for what do you want to be known? What do you want people to say about you at your funeral? You might think that end-of-life questions are too morbid for a career development article but, in answering these profound questions, the author’s coaching clients – and the companies they run – are positively affected. If you are not clear about what you want to be known for at the end of your life, no one else will be clear about your personal brand either. That means your leadership legacy is messy and needs to be adjusted to reflect the true and best leader within you. Start with the end in mind.

56 STRUCTURE magazine

Perception If you are trying to get to New York, you need to know where you are now. If you want to be known as a “visionary leader,” start with surveying how people perceive you. This can be done with an extensive 360-degree review, or you can simply ask people to describe you in one word. Compile that list of one-word responses and you will get a snapshot of your current reputation based on how people perceive you. If you like what people have to say, then continue your path to New York. If you do not like what they have to say, then identify where you are and make course corrections to get yourself on the right path to New York.

Understand how others perceive you.

You Can Change In the author’s career coaching, clients tell her, “I am not going to change. I am who I am.” You do not need to change yourself to please other people, but, if you have noticed that your work relationships are not as strong as you want, then there is an opportunity to evaluate your part in the relations. As a leader, take note of how people are responding to your leadership style – are they happy working for you, or resistant? You cannot change much about other people, but you

can make changes in yourself if you want to.

If you are not keen on how your leadership legacy is developing, then there is a perfect opportunity to start making some incremental changes today, if you want to.

Finally, think about the people who have had a positive influence on your career – your professors, the senior engineers who mentored you when you were a junior engineer, the client who patiently allowed you to make a mistake and then fix it, the architects, other partner engineers, previous firm owners, etc.

Likely there have been many people who have made an impression upon you. The author would be willing to assume that you have a good opinion about the leadership legacy of each of those key people. Be certain that other people have an opinion about you too.

How you are leading people is building your personal brand – your mark, your legacy. Are you happy with how people perceive you today? Are you content with your leadership legacy?■ Born into a family of engineers but focusing on the people side of engineering, Jen Anderson has over 21 years of helping leaders and their teams build stronger careers. (www.CareerCoachJen.com)

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Tensar International Corporation

Victaulic

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NCSEA

NCSEA News

NCSEA Corporate Members National Council of Structural Engineers Associations ASSOCIATE MEMBERS

Nationally recognized bodies that are associated with the practice of structural engineering, regardless of location and membership, who are approved for this status by the Board of Directors.

AISC American Wood Council Fabreeka International, Inc. Insurance Institute for Business & Home Safety

International Code Council Metal Building Manufacturers Association Precast/Prestressed Concrete Institute Simpson Strong-Tie

Steel Tube Institute USG Corporation – Structural Solutions

AFFILIATE MEMBERS

Companies who provide supplies or services to structural engineers, including vendors of structural engineering applications software, insurance, and structural products used for construction.

Allplan, Inc. Atlas Tube AZZ Galvanizing Services BDS VirCon Bekaert Blind Bolt Cast Connex Corporation Chicago Clamp Company Cold-Formed Steel Engineers Institute Concrete Reinforcing Steel Institute Construction Tie Products, Inc. CoreBrace DECON USA, Inc. DeWALT Freyssinet, Inc.

Geopier GIZA Steel Graitec GRM Custom Products Hayward Baker Headed Reinforcement Corporation Hexagon PPM Hilti, Inc. IAPMO Evaluation Service International Masonry Institute ITW Commercial Construction North America Kinemetrics Lindapter USA MeadowBurke Mitek Builder Products

New Millennium Building Systems Performance Structural Concrete Solutions, LLC Pieresearch Qnect RISA Technologies SE Solutions, LLC SidePlate Systems, Inc. SkyCiv Stabil-Loc Inc. Steel Deck Institute Steel Joist Institute Strand7 Trimble Vector Corrosion Technologies Voss Engineering

SUSTAINING MEMBERS

Structural engineering firms, firms that employ structural engineers, or individual professional engineers practicing structural engineering.

ARW Engineers Barter & Associates, Inc. Blackwell Structural Engineers Burns & McDonnell Cartwright Engineers Collins Engineers, Inc. Construction Consulting Associates, LLC Cowen Associates Consulting Structural Engineers Criser Troutman Tanner Consulting Engineers CSA Knoxville DCI Engineers Deems Structural Engineering Degenkolb Engineers DiBlasi Associates, P.C. Dominick R. Pilla Associates DrJ Engineering, LLC ECM Engineering Solutions, LLC

Gerald E. Kinyon Gilsanz Murray Steficek Glotman Simpson Consulting Engineers GRAEF Haskell Holmes Culley IBI Group Engineering Services (USA) Inc. James Ruvolo Joe DeReuil Associates Katerra KBR KOMA Krech Ojard & Associates LBYD, Inc. LHB Inc. Mainland Engineering Consultants Corp.

Mainstay Engineering Group, Inc. Martin / Martin, Inc. Mercer Engineering PC

Morabito Consultants, Inc. Mortier Ang Engineers O'Donnell & Naccarato, Inc. Omega Structural Engineers, PLLC Professional StruCIVIL Engineers, Inc. Rimkus Consulting Group Ruby & Associates, Inc. Scovis PLLC SES Group LLC Simpson, Gumpertz & Heger Sound Structures, Inc. Stability Engineering Structural Engineers Group Inc. STV, Inc. TEG Engineering, LLC TGRWA, LLC The Harman Group, Inc. Thornton Tomasetti Wallace Engineering

PARTNERING ORGANIZATIONS ASCE Structural Engineering Institute (ASCE SEI) 60 STRUCTURE magazine

Coalition of American Structural Engineers (CASE)

International Code Council (ICC)

News from the National Council of Structural Engineers Associations

The Benefits of NCSEA’s Corporate Membership

As a Corporate Member of NCSEA, you are not only part of a unique group of over 112 organizations sharing a higher level of involvement, but also more connected to a national network of over 11,000 professional structural engineers. When you join, you are encouraged to use this enhanced level of engagement with NCSEA to virtually “reach out” within our structural engineering network. Specifically, your online presence within NCSEA will increase recognition of your organization that comes with the membership investment, such as an acknowledgement at the annual Structural Engineering Summit, special logos that feature your connection with NCSEA, or a “spotlight” on the NCSEA website that showcases your engineering firm.

Add to that, discounts for all NCSEA events are included in every membership level: live and recorded webinars, the Summit, and the Trade Show at the Summit. All members also have the opportunity to save on a yearly webinar subscription, which includes all live webinars (25+) within a 12-month period, unlimited access to the recorded webinar library (over 120), and unlimited certificates for all webinars attended.

As an NCSEA Corporate Member, you have the unique opportunity to connect with engineers nationwide in a variety of ways! Learn more about how you can join, as well as the increased money saving benefits included within each Corporate Membership level, by visiting www.ncsea.com/members/more.

NCSEA Forms New Ad Hoc Committee

The NCSEA Board has voted and approved the formation of an ad hoc Sustainable Design Committee. Proposed by Kelly Roberts, P.E., Principal at Walter P. Moore’s Atlanta offices, the committee’s main mission will be to promote sustainable design practices within the profession of structural engineering through leadership, advocacy, outreach, and education. With her level of interest and commitment to the concept, Roberts was installed as the committee’s first chair.

Stephanie Young, Principal at Mattson Macdonald Young and Director on the NCSEA Board, worked with Roberts to secure the approval needed for the group’s formation. She noted, “Such a committee fits well within the NCSEA structure due to the strong network of Structural Engineers Associations (SEAs) across the country that are members of NCSEA.” Initial deliverables of the committee will include creating a web page on the NCSEA site dedicated to education on sustainable design – the best in class resources – as well as articles and direct outreach to the SEAs to introduce the committee and the topic at the local level.

NCSEA Webinars

Save the Date for NCSEA's Structural Engineering Summit November 12-15, 2019 Disneyland ® Hotel Anaheim, CA

Register by visiting www.ncsea.com.

March 26, 2019

2018 IEBC – Part 1: Prescriptive & Performance Methods Chris Kimball, S.E., P.E., MCP, CBO

This two-part series will go through the structural provisions that are included in the 2018 IEBC. Part 1 addresses the specific provisions for the prescriptive and performance compliance methods.

April 9, 2019

2018 IEBC – Part 2: Work Area Compliance Method

Learn more about the NCSEA Summit by visiting www.ncsea.com!

Chris Kimball, S.E., P.E., MCP, CBO

Part 2 addresses the specific provisions for the work area compliance method which comprises the majority of the code.

April 18, 2019

Masonry Infill Design Charles J. Tucker, P.E., Ph.D.

This course will cover the behavior of masonry infills and the 2016 TMS 402 Building Code Requirements design provisions.

Courses award 1.5 hours of continuing education after the completion of a quiz. Diamond Review approved in all 50 states.

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SEI Update Membership

Join or Renew SEI/ASCE

For innovative solutions and learning, to connect with leaders and colleagues, and to enjoy member benefits such as SEI Member Update monthly e-news opportunities and resources – visit www.asce.org/myprofile or call ASCE Customer Service at 800-548-ASCE (2723).

ASCE Member Get a Member Offer

Invite your peers to join ASCE/SEI and earn rewards for recruiting new members. message.asce.org/mgam

Advancing the Profession

2018 O.H. Ammann Research Fellowship

SEI Standards

Visit www.asce.org/SEIStandards to: • View ASCE 7-22 Committee Meeting schedule and archive • Submit proposals to revise ASCE 7

Congratulations to recipients of the 2018 O.H. Ammann Research Fellowship in Structural Engineering: Jame Alexander, S.M.ASCE, New Mexico State University Mohammad Alipour, S.M.ASCE, University of Virginia Kien Quang Nguyen, P.E., A.M.ASCE, University of Kansas Mohammad Taghi Nikoukalam Mofakham, S.M.ASCE, Texas A & M University Yujie Yan, S.M.ASCE, Northeastern University Learn more at www.asce.org/structural-engineering/ammann-research-fellowship.

Become an ASCE Key Contact

Key Contacts influence public policy at the state and federal levels. By building relationships with policymakers, you can influence issues important to the profession. https://www.asce.org/keycontacts/

Svend Ole Hansen presented the Eurocodes in Wind Actions to the ASCE 7-22 Wind Loads Subcommittee meeting January 17 at ASCE in Reston, VA.

SEI Online

Engage via SEI Social Media

We want to hear from you! Share what inspires and informs you in structural engineering with your colleagues and contacts via social media. When you tweet, make sure to add @ASCE_SEI, so we see it. And remember to connect with your SEI/ASCE colleagues via ASCE Collaborate. https://collaborate.asce.org/integratedstructures/home

ASCE Guided Online Courses

Starting in March/April on GIS, crane lift plans, seismic design, and construction. Earn up to 23 PDHs per course with 6 or 12-week options. www.asce.org/guided-online-courses

Errata 62 STRUCTURE magazine

Follow SEI on Twitter @ASCE_SEI

SEI News Read the latest news items including SEI New Orleans Chapter events at www.asce.org/SEI

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI-Errata.

If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org.

News of the Structural Engineering Institute of ASCE Learning / Networking

Announcing ASCE 7-16 SEI Futures Fund Lectures

Goals include professional development, increasing ASCE/SEI membership and participation, presenting the ASCE/SEI standard development process and future of performance-based codes, and promoting opportunities to get involved. Made possible by the SEI Futures Fund in collaboration with the ASCE Foundation. Presented by leaders that develop the standard.

March 5 – ASCE/SEI 7-16 Overview Presentation in Boston Hosted at Tufts University by BSCES and SEI Boston Chapter https://goo.gl/5Chhz2

March 7 – ASCE/SEI 7-16 Wind Loads Presentation in Miami

Hosted by the SEI Graduate Student Chapter at Florida International University and SEI Miami-Dade Chapter https://goo.gl/x92XRZ

April 9 – ASCE/SEI 7-16 Seismic Presentation in Portland, OR Hosted by SEI Oregon Chapter – Details to be announced

June 5 – ASCE/SEI 7-16 Wind Loads Presentation in Chicago Hosted by SEI Illinois Chapter – Details to be announced

Register by March 20 for the best rate at www.structurescongress.org.

• New: Learn from the Experts on Performance-Based Design Trends, Smart Technologies, Career Development, Leadership Skills, and more • View program detail and use interactive planner online • Make sure to include your tickets for the CSI Special Evening Reception April 26 • If you can’t attend, Livestream up to 3 sessions Friday, April 26; sponsored by CSI

Visit SEI at NASCC April 3-5 in St. Louis Join structural engineers, architects, and project stakeholders for in-depth case studies exploring differing international design and construction demands, the challenges that lie ahead, and how we can find globally connected solutions.

Key themes: • Unusual structures, unique challenges • Large occupancy venues, sports stadia design • Tall buildings, performance-based design Structures featured will include Shanghai Tower (China), Jeddah Tower (Saudi Arabia), Singapore Sports Hub (Singapore), London Eye (UK), and Salesforce Tower (USA).

Registration opens in March.

See us at Booth 211 for membership benefits and opportunities, and enter to win a six-month individual subscription to ASCE 7 Online.

Free eLearning Webinar

In case you missed it! Check out the ASCE Free eLearning webinar with SEI leaders from December 11: ASCE Made Me – How SEI Young Professional Involvement Leads to Career and Personal Success. https://goo.gl/YjniUr M A R C H 2 019

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CASE in Point Save on CASE Membership!

Can you ever really be too successful? Keep your business thriving – no matter what your competition or the economy is doing – and say YES to membership in ACEC’s Council of American Structural Engineers (CASE). An “Association within an Association” that complements your ACEC National benefits.

CASE, the oldest of ACEC’s four discipline-specific Coalitions, is a professional community for, of, and by structural engineers who want relevant, useful information – on BIM, international building codes, risk management, and more – to run their businesses better.

Join CASE today, and you’ll qualify for: • Education: CASE offers a track of 3 dedicated education sessions at both the ACEC Fall Conference and Annual Spring Convention to keep members current with best practices and trends in structural engineering. As a member, you will also receive a discounted rate to ACEC webinars focused on structural engineering issues. CASE also provides education sessions at the AISC Steel Conference and the ASCE-SEI Structures Congress.

• Resources: Coalition members get free access to over 145 contracts, tools, and publications (a total value of over $5,000! ). CASE developed over 65 documents geared toward structural engineering firms. You will also receive our weekly A/E/C Digest, an online compilation of current articles and information about our industry.

• Advocacy: Your voice matters! Coalition members are often the first ones contacted to share their expertise with Congress and government agencies in response to current legislation and relevant regulatory agendas.

As an active member of CASE, I have found great networking opportunities within the structural engineering industry and consistently reference Coalition documents to aid in my business’ development.

Save $75 off your first year’s dues through June 30, 2020! Join CASE by March 31, 2019, and get 15 months for the price of 12!

Questions? Contact CASE’s Executive Director, Heather Talbert, at 202-682-4377 or email her at htalbert@acec.org.

Together, we can take your business to the next level!

Corey Matsuoka CASE Chairman SSFM International

Keep Your Firm Thriving

ACEC’s Business of Design Consulting Course, April 3-6 ACEC’s Business of Design Consulting course in Phoenix, Arizona, April 3-6, provides a proven playbook for building leadership and managing your firm.

The 3½-day program delivers strategies for a wide array of critical, need-to-know business topics to maximize your firm’s performance in today’s churning business environment, including change management, strategic planning, finance, leadership, ownership transition, contracts and risk management, and marketing.

The experienced faculty of industry practitioners includes Rod Hoffman of S&H Consulting; Brett Stewart of XL Catlin Design Professional; Colvin Matheson of Matheson Financial Advisors; and David Stone of blüStone Marketing.

The course carries 22.5 professional development hours (PDHs).

For more information and to register, https://goo.gl/thz46A.

Donate to the CASE Scholarship Fund!

The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at a minimum, in an ABET-accredited engineering program. Since 2009, the CASE Scholarship program has given $29,000 to help engineering students pave their way to a bright future in structural engineering.

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

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

64 STRUCTURE magazine

News of the Council of American Structural Engineers Did you know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills young engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need! CASE has several tools available for firms to use to enhance their business development processes: CASE 962-F CASE 962-H CASE 976-A Tool 5-4 Tool 7-1 Tool 7-2

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

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

CASE Practice Guidelines Currently Available Structural Engineer’s Guide to Fire Protection

This publication is intended for structural engineers with no prior experience or training in fire protection engineering. It is a comprehensive and concise treatment of prescriptive and performance-based methods for designing structural fire protection systems in an easy to understand format.

CASE 504 – Proposal Preparation Spreadsheet

The CASE Proposal Preparation Spreadsheet was developed to assist project managers and administrators in developing cost proposals for a project. The spreadsheet may be easily customized for any organization or project type. It also may be used as a checklist to see that all phases of a project are adequately staffed.

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

Fresh EJCDC Contracts to Meet Modern Market Demands

EJCDC’s newly released 2018 Construction (C-Series) Documents are a significant modernization, revision, and expansion of the 2013 C-series and now the state-of-the-art in construction contract documents.

The updated edition comprises 25 integrated documents, including: • Fundamental contract documents such as the Standard General Conditions, the Small Project agreement, and Supplementary Conditions • Forms for gathering information needed to draft bidding documents • Instructions for bidders and a standard bid form • Bonds including bid, performance, warranty (new for 2018), and payment bonds • Administrative forms, such as change orders and a certificate of substantial completion EJCDC C-700, Standard General Conditions of the Construction Contract, has been extensively refreshed and updated, too. The new EJCDC 2018 C-Series also includes expanded and updated, “Notes to Users” and “Guidelines for Use” to provide more specific instructions and it eliminates the need for notary and corporate seals.

You can purchase these and other EJCDC documents at www.acec.org/bookstore.

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

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structural FORUM Art of Approximation

By Dilip Khatri, Ph.D., P.E.

M

easure the distance between two points using a ruler, and you may read “12 inches.” This implies that the relative accuracy is to the nearest inch. If I write “12.00 inches,” then this implies accuracy to nearest 1/100th of an inch. For a foundation, the accuracy of ± ½ an inch may be appropriate. The tools of that construction trade do not have a high level of precision (i.e., a backhoe, shovel, or excavator). In contrast, for an Aircraft Wing on a fighter jet, the accuracy may be measured in Mills (0.001 inches) or 1/1000th of an inch. Understanding the level of accuracy required for a given task involves familiarity and judgment.

Structural Engineers approximate wind and earthquake loads based on data provided by seismologists, then use factors of safety on materials to design buildings that will hold our world’s inventory for an unknown period of time, giving our client’s a vote of confidence that will assure their tranquility. We represent the trust of society, the honor of integrity, and are responsible for millions of people’s lives because our structures store their memories, house their loved ones, and transport the world’s treasures across the nation’s highways.

At both the beginning and the end, it is all based on approximation because engineering is a system of approximations, based on judgment, experience, and past errors that become lessons learned.

Where can a young engineer learn such skills? The first way is to learn from interaction with experienced engineers. Another way is to spend time on construction sites familiarizing oneself with the tools, fit-ups, and methods of the workers. When in doubt, review drawings produced by others and recommended tolerances for construction such as those in ACI 117 or AISC 303. The best learning experience for young engineers is to get construction experience. Work on something “real,” build “stuff,” get your hands dirty, and immerse yourself in the trades before becoming an “office junky.” You will experience how the paper design turns into a real structure.

Masters level students now graduate with extensive training and theoretical background on Finite Element Methods. Feats of analysis 66 STRUCTURE magazine

that were impossible in earlier generations are now done in matters of seconds using powerful software, hardware, and sophisticated algorithms. No doubt, impressive progress from this author’s days when the PC was just emerging on the horizon, and the first fourfunction calculator was being sold for $100.

However, how can we teach the “common sense” and “engineering judgment” to guide us in design, and know that the answers provided by the software are right?

Teaching our engineering students about Strain Energy Density, Non-Linear Dynamic Analysis using the Galerkin Method, Bete Reciprocal Theorem, Heisenberg’s Uncertainty Principle, Fourier Transform, Jacobian Matrix, and the Duhamel Integral Formulation are useful tools for engineering analysis. But where’s the judgment?

Universities cannot teach it because it is not really from textbooks and you cannot learn it from Timoshenko’s Treatment on Plates and Shells. You have to “live it” and learn through osmosis with experienced engineers in the profession who have designed many structures over many years.

The profession of Structural and Civil Engineering deserves its own School to match that of other professions: Dentistry, Medicine, Architecture, and Law. We need a practice degree at the Master’s Level that is a “Master of Structural Engineering” that teaches Structural Engineering Design and is taught by Professional Structural Engineers from industry. These teachers can introduce students to “Instinct,” the Engineer’s ability to see the structure perform in the extreme event and foresee the failure before it happens, then intuitively design for it so that it does not happen. They can teach scale, both in terms of proportions of members and the production of drawings which describe the structure.

They can teach Judgment, the “common sense” to “feel” the right answer. Yes, “feeling” whether an answer is right is also part of the engineering experience. Our judgment matters because an undersized footing will lead to a sinking building, an over-reinforced concrete roof system will cause sudden collapse when overloaded by snow, and a poorly designed bridge truss will lead to hundreds of deaths.

These are skill sets that are best conveyed by practicing engineers or those with extensive design experience. Practicing Engineers have this wealth of experience, accumulated through years/decades of toil working with construction contractors, owners, building departments, and perhaps lawyers on projects that went well, wrong, sideways, and sometimes won an award. All of this matters, because Wins and Losses are both learning experiences that should be shared. Classroom learning is limited to textbooks, charts, tables, graphs, and research papers. There is a boundary to your educational envelope when you limit your circle to only Ph.D. level educators, and you lose the perspective of the real world.

Research is still essential, as are tenured Ph.D. professors, grants, publications, and experimental testing of components and systems.

Our profession is being asked to stamp and qualify older structures with retrofit systems that are unproven and be even more economical with designs. We need researchers to help bridge the gaps in our understanding, but we also need them to understand that their students need us too.■ Dilip Khatri is the Principal of Khatri International Inc, Civil and Structural Engineers, based in Las Vegas, NV, and Pasadena, CA. He was a Professor of Civil Engineering at Cal Poly Pomona for 10 years. He previously served as a member of the STRUCTURE Editorial Board. (dkhatri@gmail.com)

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