STRUCTURE magazine | August 2022

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INSIDE: Moynihan Train Hall CFS Trusses in Flat Roof Applications Cantilever Design Extrapolated Floating Fireboat Station

AUGUST 2022

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AUGUST 2022

Contents AU GUST 2022

MOYNIHAN TRAIN HALL

By Brian A. Falconer, P.E., S.E., and Ick Kim, P.E.

Cover Feature

On New Year’s Day 2021, Moynihan Train Hall opened to the public and restored a grand entrance to New York City. The new facility is an adaptive reuse of the landmarked James A. Farley General Post Office Building. The train hall’s central feature is the main boarding concourse. Located in Farley’s former mail sorting room, the 150- by 200-foot space is column-free due to three existing steel roof trusses, uncovered and reinforced to become a significant focal point of the design.

F E AT U R E S A ONE-OF-A-KIND FLOATING FIREBOAT STATION

By Craig Lewis, P.E., S.E., Leah Olson, P.E., and Daniel Silva, P.E., S.E.

San Francisco’s new Fireboat Station No. 35 is a floating two-story building supported by a steel float moored by four guide piles behind San Francisco’s historic Fire Station 35 building. The station includes a new steel pier and associated steel support piles, a vehicular and pedestrian ramp between the steel pier and float that supports the building, and a gangway between the float and historic timber pier for the firefighters.

PARK UNION BRIDGE: A BALANCING ACT

By Lana Potapova, P.E., M.Eng., B.Eng., and Matt Carter, P.E., MA M.Eng., C.Eng

The Park Union Bridge connects the U.S. Olympic and Paralympic Museum to America the Beautiful Park and Downtown Colorado Springs. The bridge is called the rip curl for its cresting design; the footbridge spans 245 feet above active rail lines. The form of the Park Union Bridge, composed of a structural steel shell simply supported on concrete abutments, is one of the most unique bridge typologies.

STRUCTUREmagazine

REHABILITATION OF THE HISTORIC McELMO FLUME

By Carlo Citto, P.E, Ronald W. Anthony, FAPT, and Douglas Porter

The McElmo Creek Flume is the last remaining flume of a water delivery system designed to irrigate the flat, open Montezuma Valley in Colorado. The original wooden flume supported by a wood substructure and was in service until 1921, when it was replaced with a semi-circular wood-stave flume. By 2012, the Flume was at risk of collapse. After a comprehensive assessment and the development of a preservation plan, the Flume was repaired, maximizing the retention of historical material.

COLUMNS and D E PA RT M E N TS Editorial

Structural Engineers Must Bridge the Gap

By A. Christopher Cerino, P.E.

Structural Components

Cold-Formed Steel Trusses

By Tim Liescheidt, P.E., and David C. Dunbar, P.E.

Project Delivery

and Fabrication

Bridging the Gap Between Engineering

By Darren Hartman, P.E., and Mara M. Braselton, P.E.

Structural Design Extrapolated

Precedent Deflected: Cantilever Design

By Julie Mark Cohen, Ph.D., P.E.

Structural Practices

Common Sense is Not Always Common

By Alfredo (Al) Bustamante, P.E.

Professional Issues

Ethical Decision Making in Structural Engineering

By Joe Brejda, P.E.

InSights

Mitigating Seismic Risk

By Jiqiu Yuan and Mai Tong

Spotlight

The Carrier Dome’s New Cover

Spotlight

Pavilion in the Park 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. STRUCTURE magazine is not a peer-reviewed publication. Readers are encouraged to do their due diligence through personal research on topics.

In Every Issue

Advertiser Index Resource Guide – Wind/Seismic NCSEA News SEI Update CASE in Point AUGUST 2022

EDITORIAL Structural Engineers Must Bridge the Gap

By A. Christopher Cerino, P.E., F.SEI, DBIA

T

here have been many articles and discussions about the misconceptions between the public and the profession regarding the performance level of code-compliant buildings. Historically, Building Codes (and ASCE 7, Minimum Design Loads and Associated Criteria for Buildings and Other Structures) have focused on ensuring life safety. For hazards that do not offer much advanced warning (e.g., earthquakes, tsunamis, and tornadoes), a focus on life safety saves lives. Still, it is not sufficient to make communities resilient to these hazards. After an earthquake, for example, many buildings are tagged as needing substantial work before they are safe to enter, leaving communities displaced and distressed for long periods.

Looking at ASCE 7 today, a practitioner sees many pages dedicated to the design of elements beyond the primary structural system, which may not tie as directly to life safety. These provisions, such as components and cladding wind loads, seismic anchorage of mechanical systems, and even flood loads, receive industry pushback when increases are proposed, with the most common objection being that this is “beyond life safety.” While it is true that ASCE 7 says right in the title that it represents the minimum design loads, advocating for owners to go beyond the Code is often a hard sell. Designers should be having informed performance discussions with owners to decide if going beyond the Code is merited, but often recommendations are ignored or removed for cost reasons. But herein lies a fundamental problem – arguably, the majority of persons living and working in the structure are not part of that decision process and are unaware of the performance level of their building, which leaves communities vulnerable post-event.

This divergence is even more apparent as engineers discuss the possible ramifications of the changing climate to the defined hazards.

Currently, ASCE 7 only looks backward in terms of hazard data.

Certainly, factors of safety serve to alleviate major problems between the 6-year cycle of the Standard, when additional historic data is used to update considerations. The 2022 version of Chapter 5, which is still in the Supplement voting process, is the first to look forward for potential hazards, adding provisions for future relative sea-level change. While there are many projections for sea-level change, with a wide variation in impact, the Standard has cautiously proposed including only the historical change rate defined for the project site over the intended service life. While many consider this inadequate, this was the compromise to not push the envelope regarding what is considered necessary for life safety.

Today, some building owners are requesting a resilient building from their designers. While it is encouraging that this is becoming a common topic, there is little guidance on what this means and how to accomplish it. Are we striving for a building that is operational 3-days after the maximum considered earthquake? Is it a dry building after a 500-year flood in a coastal A-Zone? Is it an office with no broken windows or roof loss in a category 2 hurricane? The possibilities are endless, and the industry is looking for a champion for these project discussions. Traditionally, the architect has served as the master builder, directing the project criteria and holistic programming choices via their discussions with the owner. But in the field STRUCTURE magazine

of resilience, which is rooted in the probabilities and performance of very different hazard types with future local projections, I do not think the architect should lead this charge. Structural engineers understand the mean recurrence intervals and probability associated with the load combinations provided in ASCE 7. In addition, ASCE is an industry leader in designer guidance on hazards and the changing climate and has dozens of additional resources to supplement the minimum load standard.

Structural engineers are the most equipped to lead project resilience discussions.

When owners request a resilient building, they are not looking for a Code minimum design that simply allows people time to exit the structure safely but subsequently requires a complete rebuild. Instead, owners envision a building where most of the architectural façade or roofing system is intact, which is a function of the structural response of the system components and their connections. They are envisioning a building where the mechanical system needs only minor repairs for isolated sections to be up and running, also a function of the structural response of the system components and their connections. They envision a plumbing system with only minor ruptures leading to local repairs…you get the point. The gorilla glue that ties the entire discussion together is the structural engineer!

Building owners are looking to expand discussions on how to make their buildings more resilient, but the public is also demanding it because of the skyrocketing costs of annual damage and utter destruction to towns and cities worldwide. Whether this performance guidance belongs in ASCE 7 will be a long debate, but the guidance is needed, and structural engineers are the most equipped to fill this void. Structural engineers lament commoditized engineering fees and are striving to increase exposure via the We See Above and Beyond campaign (weseeaboveandbeyond.com), but here lies the opportunity of the new century. Today, many project organization charts now have a defined position titled “Resilience Lead.” This position should always be filled by a Structural Engineer who is the most qualified to bridge the gap in the emerging realm of resilience performance.

Become part of the discussion via firm organization and Code committees and help elevate the profession to meet the needs of an ever-changing hazard landscape.■ A. Christopher Cerino is the Director of Structural Engineering in STV’s New York office and a member of the NCSEA Board of Directors (anthony.cerino@stvinc.com).

AUGUST 2022

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structural COMPONENTS Cold-Formed Steel Trusses The Case for Flat Roof Applications

By Tim Liescheidt, P.E., and David C. Dunbar, P.E.

S

teel bar joists have been and will likely continue to be the primary choice for structural engineers, architects, and building developers/owners in large commercial and industrial building construction – and for a good reason. Steel bar joists have been around for a very long time: a mature industry that is better than 100 years old. The Steel Joist Institute (SJI) was formed in 1928 and has done an excellent job developing and maintaining quality standards for its products. And steel bar joists have typically been readily available for use on projects.

Projects these days move very quickly. Once a develTypical truss to steel connection. oper/owner has decided that it is time to move forward on their project and dollars are committed, timing becomes critical over the next 140 years. However, in the 1990s, the first standards for since the owner wants or even needs to get a financial return on their CFS design were created, and the industry has been progressing at a project as soon as possible. Likewise, contractors benefit from com- lightning pace ever since. So, CFS is far from brand new. pleting the project quickly for the owner, not only because a happy Myth: Cold-formed steel is too expensive owner means the possibility of future work, but because the costs for Truth: The actual cost of steel is in the weight of the component. the contractor are reduced if the project duration is minimized. This In many cases, a CFS truss system can be designed to be a lighter pressure always flows to the design team – nothing can be done until overall system; the designer needs to be cognizant of the limitations the design is completed. If that design depends on products that are of CFS. For instance, by maximizing the depth available between not available in a reasonable amount of time and a project is delayed, the required ceiling line and the roof deck, a designer can reduce the it could cost many people a lot of money! weight of the CFS truss, thus lowering the cost. CFS trusses used in Such has been the case with many products over the past couple of years. a building may be deeper than what would have been required using The COVID-19 effect has taken hold on many items that people could other systems; however, some additional depth in the roof system assume were readily available before the pandemic. But unfortunately, does not matter in many structures. The adage that deeper is cheaper demand outpaced production. As a result, many owners, contractors, and is probably more accurate for CFS trusses than almost any other type design professionals quickly discovered that what they had been able to of material used in truss construction. Also, dropping a ceiling from rely on for decades now came with a significant wait time. the joists or beams above is often required for a retail building due Once the viable options were apparent, owners, contractors, and to the depth of the truss or beam. For a CFS truss, sloping the top designers started scrambling for alternative products to support their chord does not require a sloped bottom chord and does not necessarily flat roof projects to keep projects moving forward. increase the cost of the truss. In fact, a flat bottom chord increases Cold-formed steel (CFS) trusses have been used for decades on truss depth. It makes the truss more efficient (and less expensive) projects that require a sloping roof. Many building types benefit while providing a level surface for ceiling attachment and possibly from the residential, sloping-roof look but require non-combustible eliminating the need to drop a ceiling. Ductwork, piping, and other construction, and CFS trusses seemed to have found a niche there. mechanicals, which generally run between the bottom of the truss But why have they not gained more acceptance in floors and flat roof and the dropped ceiling, can easily be relocated between CFS truss applications? The author believes the simple answer is that it just webs with some planning. has not been necessary to consider them. With structural engineers Myth: CFS trusses are not as strong as steel bar joists designing flat roof structures using steel bar joists for literally a century, Truth: Engineers should cringe anytime they are asked this question. why change? Unfortunately, it sometimes takes a big event to affect Like any other building component, CFS trusses are designed for the necessity for change. the specific application they are to be used for. For optic’s sake, it Besides the ‘I’ve always done it that way’ objection, here are some other is very easy to justify the strength of CFS trusses by pointing to the common misconceptions that historically have created hesitancy in projects that structural engineers and architects have trusted them on the design community to move toward what the author would like for decades. There is no shortage of schools, military facilities, and to refer to as Light Gauge Bar Joists. medical buildings all over the country that are safely insulated from Myth: Cold-formed steel is too new a product wind and heavy snowfall by a CFS roof structure!

Truth: The first use of cold-formed steel products was in small homes Today’s CFS trusses are a viable alternative because roll forming and other basic structures in the 1850s. CFS products’ use was sporadic technology has progressed as the demand for roll forming equipment

8 STRUCTURE magazine

(across many industries) has increased. As a result, structural engineers have developed new solutions using cold-formed steel that may not have been able to be produced just a few years ago. Trusses, specifically flat trusses, are no different.

The shapes that could be produced in the early days of cold-formed steel were relatively simple. While they were outstanding in some applications, they did not have the properties required to be an alternative to traditional bar joists. With the roll forming equipment available today, the limitation is no longer the ability to produce a section – giving structural engineers the freedom to develop the most efficient shape for any required application.

General Guidelines for CFS Trusses Trusses are nothing more than lightweight, efficient alternatives to beams with a predictable load path transferring expected forces. One of the advantages of a truss is that Roof truss systems – CFS truss and sloping bar joist in similar application. individual truss members can be sized appropriately for the forces the members are anticipated to carry, as opposed to a beam, Design documents are available from at least two major profor example, where the entire cross-section is sized for the worst-case prietary materials suppliers for CFS trusses (www.trussteel.com; forces in the member. www.advantsteel.com). These documents provide loading capacities As structural engineers learned in statics, the top and bottom chords for their respective steel truss sections based on truss span/depth. of a truss are designed to carry the bending moment of the span of a In addition, the CFS truss supplier still provides shop drawings truss, and the webs are designed to carry the shear. The moment over with specific truss designs for the given loads, including loads for the span of the truss can be calculated by the simple span moment mechanicals, snow drift, equipment screens, etc., as required for a equation, wl 2/8. This moment is resisted as an axial load by the top given project. and bottom chords of a truss based on the resisting moment arm of the chords as determined by the effective depth of the truss.

Readily Available Products Because the bending moment over a span is exponential and the axial forces in the chord members of a truss are based on this bend- Building designers may not be aware that a network of regional ing moment, the axial forces that truss chords are required to carry cold-formed steel truss fabricators exists across the country. Most increase exponentially as the truss span increases. cold-formed steel truss fabricators today are bigger and stronger than A cold-formed steel truss creates design limitations as with any ever and have decades of experience under their belts. In addition, material type. While cold-formed trusses have very high strength-to- due to the nature of the product, do not be concerned about shipping weight ratios, the sections that comprise CFS trusses are relatively thin. distances as many thousands of square feet of roof can be managed When designing with CFS trusses, it is helpful to use the following on a single truckload. guidelines to ensure a successful project: For the past year, numerous details have been developed to assist • Truss Spans below 30 feet: with installation to ensure a smooth transition to flat cold-formed Min Truss Depth (ft) = Truss Span (ft) / 24 steel trusses. These include direct welding at steel bearing supports • Truss Spans between 30 feet and 50 feet: and optional bracing schemes. The connection of metal deck to the Min Truss Depth (ft) = Truss Span (ft) / 20 top chord of cold-formed steel trusses is limited to mechanical means • Truss Spans above 50 feet: per Steel Deck Institute (SDI) recommendations.

Min Truss Depth (ft) = Truss Span (ft) / 16 The cold-formed steel truss industry has come a long way in the The above numbers may be used for floor trusses and flat (or almost flat) 30 years that it has existed and enjoys an ever-expanding market in roof trusses. Assume floor trusses with a typical live load of 40 pounds healthcare and education while it continues to gain traction in small per square foot (psf ) spaced 2 feet on centers or less or roof trusses retail projects, fast food buildings, gas stations, and even a handful designed for a live load of 20 psf and spaced 4 feet on centers or less. of grocery stores and larger warehouse-type buildings. As the understanding of the capabilities of CFS trusses continues to increase, the use of CFS trusses on many types of flat roof projects (and floors) will become the norm. Knowing what types of projects make sense for each product, structural engineers can give themselves and their clients a distinct advantage by utilizing the most efficient products for the application they are designing for.■ Tim Liescheidt is an Engineer for Advant Steel LLC (tim@advantsteel.com).

Forces in truss chords over increasing span/constant depth.

David C. Dunbar has spent his entire career in the truss component industry. After serving 10 years as a Regional Chief Engineer for Alpine Engineered Products, he helped develop the TrusSteel product line (ddunbar@alpineitw.com).

AUGUST 2022

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project DELIVERY Bridging the Gap Between Engineering and Fabrication Rethinking Delivery Methods

By Darren Hartman, P.E., and Mara M. Braselton, P.E.

C

hange is necessary for progress and growth, but it is often met with resistance. In recent years, technological advances have led to greater efficiencies and increased creativity for projects of all shapes and sizes. However, despite the apparent benefits, many in the design and construction community still cling to doing things the way they have always been done, namely siloed teams and workflows.

Under conventional project delivery methods, architects, engineers, and fabricators rely on drawings to share data, while each group creates separate 3-D models for their own needs. Having the various stakeholders working individually and with multiple models opens the door to errors and a litany of problems that can ultimately lead to cost and schedule overruns and increased risk for owners and contractors. An integrated approach, in which the project team leverages technology to share information in a single model, is needed to move us forward.

Increased collaboration and greater attention to detailing and erection items earlier in the lifecycle of a project can shorten schedules, improve constructability and minimize risk. With widespread BIM adoption and cloud collaboration, forward-thinking firms are transforming project delivery.

Early Steel Connection Design No matter what the project, collaboration produces better outcomes.

The earlier that collaboration begins, the better. Considerable schedule and cost efficiencies can be achieved when design, fabrication, erection, and construction experts weigh in at the onset of a project.

Incorporating steel connection design and detailing in the design phase gives the team the necessary information to build the project sooner than conventional delivery methods. Early constructability feedback to the design team and conceptualizing and designing structural steel connections means fabricators have more – and better – information at the start of the bid process. This results in fewer unknowns and, thereby, greater cost certainty.

Under the traditional project delivery framework, the design engineer, detailer, and connection designer are separate entities with different responsibilities and deliverables. When these teams work independently, it can often lead to miscommunication and project delays.

The key to success lies in the early integration of the construction engineering team (those skilled in connection design, detailing, fabrication, and erection) with the structural design engineers. Instead of multiple drawing sets and models, the structural engineer can deliver a 3-D model to the steel fabricator produced in coordination with structural steel detailers and connection designers. This model becomes the base from which the fabricator can execute final shop drawings and drive CNC (computer numerical controlled) equipment during fabrication. The model can also be converted into other formats and used for detailed coordination, early clash detection, and as part of a post-construction building management model.

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A Tekla model (top) and as-built (bottom) connection of the drop-in steel truss panels to the pipe shores for Climate Pledge Arena in Seattle, WA. Advanced Project Delivery™ (APD) helped accelerate the steel procurement and detailing processes, thereby reducing the project schedule by four months.

A Domino Effect of Downstream Value In addition to eliminating many challenges architects, engineers, and fabricators face, a project delivery process that brings construction engineering and detailing into the early design phase has several benefits. These include: Schedule Savings and Schedule Certainty. Open communication between the design and construction teams enables the steel to be procured, fabricated, and erected on time with minimal change orders and schedule impacts.

Cost Certainty. 3-D models can be issued with the bid documents. Although preliminary and created early in a project, these models provide a 3-dimensional visualization of the structure and can contain representative, detailed examples of more complex connections. Combined with the tonnage schedule provided with the bid documents, bidders understand the project’s scope and complexity more easily, removing some of the uncertainty and contingencies for the unknowns. Thus, steel bids typically have a low percentage spread between high and low bidders.

Increased Trade Coordination. When steel members are embedded in concrete, 3-D modeling of the concrete envelope, concrete reinforcing, and structural steel allows for the detection and elimination of clashes between steel and concrete reinforcing between trades. Additionally, the model can be imported into programs such as Navisworks so that clash detection can be performed for other trades, such as curtain wall and MEP, throughout the construction process.

Reduction in RFIs. Collaborating with the design team during the model’s production allows for seamless, real-time resolution of engineering and construction issues as they are discovered. Most of this work occurs before a steel fabricator gets involved in the project. With weekly coordination meetings throughout the design and model development, formal RFIs are dramatically reduced.

Reduced or Validating Change Orders. By preparing and utilizing a 3-D model throughout the design and construction process, quantities and complexity are identified as the design progresses.

Once a model or drawings are issued, changes can be tracked and compared and quantities verified. Because all information is shared utilizing a collaborative design/construction approach, potential change orders are identified earlier in the process, and discussions on the necessity of the change are evaluated before implementation, thus leading to fewer change orders.

time-consuming communication between connection engineers and the design team. Engaging connection designers in parallel with the design team in a collaborative environment enables a more constructible product.

This minimizes the need to revisit it once the designer has established the components and controlling forces.

Using contractor fabrication and modeling preferences obtained from either experience working with fabricators on past projects or consulting with them during development, an engineering team experienced in connection design generates detailed sketches for each connection on the project and tags them in the model. This makes it possible to begin addressing erection sequences for optimizing final member selection, splices, pick weights, shipping dimensions, and other critical issues in the normal flow of the design.

Frequently, engaging a fabricator during connection design development to incorporate their individual shop and erection preferences can produce substantial economic and scheduling value for the project.

Connected Modeling

The connection sketches tagged in the model are then used to generate the final detailing. These show all plate material, welds, bolts, surface preps, weld preps, and other information required for a complete connection. In addition, the modeling considers the physical aspects of assembling and completing the connection, including the accessibility of bolts for tightening and the accessibility of joints for welding.

Thornton Tomasetti’s Tekla Structures modeling is performed as an iterative process with the optimization The Advantages of an Early Start of connections working in tandem with the fabricators Incorporating construction modeling and detailing at or when physical interferences or constructability issues the beginning of a project’s life cycle helps deliver better are uncovered during the modeling process. Typically, projects faster. The following is how this approach can the modeling of critical representative connections positively impact various phases of project delivery. is done during the completion of the design documents to identify the level of complexity for the steel Advanced Bill of Materials (ABM) procurement process.

The delivery of an ABM model for the purpose of Modeling connections in 3-D smooths the transiordering materials from the mill, leads to numerous tion between design and construction by ensuring that efficiencies. For example, developing a Tekla model For New York City’s One Vanderbilt, complex connections are designed so that geometry and that contains accurate geometry, member sizes, built- a 1.7 million-square-foot tower, fabrication are understood and constructible. The fabriup column configurations, material grades, Charpy Tekla models were used for the cator is presented with an electronic file that contains a V-Notch (CVN) requirements, and other intelligence bid process and detailing. The 3-D model or shared model access that the fabricator’s virtually eliminates the traditional RFIs for these items steel framing package was split detailer can use to create shop drawings. The fabricaduring the early procurement phase. This allows the into seven sequences, each with a tor’s detailer remains an integral part of the process to scheduled advanced bill of materials model to be in development long before fabricator date and release for detailing date. help ensure that the information conveyance meets the selection. This information can be readily shared at fabricator’s specific needs and preferences. In addition bid time for more accurate takeoffs and a greater understanding of to accelerating steel fabrication, providing connection designs in a the overall project. In addition, the fabricator no longer needs to 3-D model fully communicates design intent and project complexity. build an independent 3-D model from a set of contract drawings to estimate or bid on the project. The mill order can then be placed as Advanced Project Delivery™ in Action soon as the drawings are ready to be released and the steel contractor is engaged, shortening the schedule and reducing project risk.

Thornton Tomasetti’s Advanced Project Delivery (APD) brings construction engineering and detailing into the design phase early in Connection Design a project’s lifecycle, using Tekla Structures as the deliverable from Connection design can be one of the most significant schedule risks in conception in design, through fabrication, and to completion of the steel procurement. It requires a clear understanding of forces, controlling project. This integration can help the entire team collaborate more load cases, force flow, and transfer forces. Complex projects can have effectively and be used for projects of any scale or complexity. numerous load combinations, and enveloping these into a simplified American Airlines Hangar 2 at Chicago’s O’Hare International force format often leads to information gaps, resulting in inefficient and Airport was a massive project consisting of 194,000 square feet of AUGUST 2022

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issued and a fully connected model of the first area just two weeks later. From there, LeJeune immediately began creating the shop drawings, which were turned around and issued to the design team within a couple of weeks. Working with LeJeune, the structure was split up into 10 sequences following the order of erection. The required dates were identified to keep the shop drawing and fabrication schedule continuously moving and on track. All 10 sequences were turned over to LeJeune in three months.

With connection design and detailing integrated into the design phases, the time for the design team to review connection calculations and shop drawings submittals was significantly reduced. Steel erection was complete 10 months later.

This collaborative approach to project delivery reduced the steel erection schedule by more than American Airlines Hangar 2 at O’Hare International Airport in Chicago, IL, was completed 14 weeks three months. There were only five structural steel early due to incorporating construction engineering and detailing early on in the project.

RFIs, and any construction delays related to steel maintenance space and 7,000 tons of structural steel, including trusses fabrication and erection were eliminated. None of this could have that span more than 900 feet. An early traditional project delivery happened without the APD process and the forward-thinking of the schedule had bid drawings issued in February, with the steel fabricator entire team. selection and connection design starting in March. Steel erection was estimated to be completed 12 months later (the following March).

The Road Ahead The steel fabricator, LeJeune Steel Company, and the authors’ in-house connection designers and steel detailers, as part of the APD process, Pioneering and leading the change in the industry is a Thornton were involved early during the design phases. The team began working Tomasetti trademark delivery process. Without evolving processes together some six months before the traditional process was scheduled alongside the adoption of BIM and model-sharing technologies, the to start. During this time, the team could get input from LeJeune early industry fails to move forward and realize the true, intended value and design connections that were efficient and met the fabrication and of those tools. Working collaboratively in a connected construction erection preferences of LeJeune and steel erector Danny’s Construction environment, where the right people have access to the right informaCo. LLC. The detailing and connection design team collaborated with tion at the right time, is key to reducing rework and risk. It ensures the design team. Questions were addressed early on that traditionally that inefficiencies and potential problems are curtailed from would be handled through the RFI process at a later date, such as forces, the onset, and the team can focus on what really matters – geometry, and information that needed to be clarified. Early Tekla models the project’s success.■ and concepts were shared with general contractor W.E. O’Neil, LeJeune, Darren Hartman is Managing Principal and Construction Engineering and Danny’s Construction to communicate design intent and complexPractice Leader (dhartman@thorntontomasetti.com), and Mara M. ity, which meant better cost certainty and reduced risk related to steel.

Braselton is Vice President (mbraselton@thorntontomasetti.com) at Thornton Because of this early coordination, the team could issue an ABM Tomasetti in Kansas City, MO. or mill order Tekla model one week after the permit drawings were

The architect’s rendering, top left (Courtesy of Ghafari Associates); structural rendering, top right; Tekla rendering mill order, bottom left; and Tekla rendering connection model, bottom right.

12 STRUCTURE magazine

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structural DESIGN Precedent Deflected: Cantilever Design Extrapolated

By Julie Mark Cohen, Ph.D., P.E.

C

antilevers in many modern buildings exceed historical precedents and proportions. These cantilevers can be unenclosed balconies and enclosed occupiable rectangular volumes of buildings. Enclosed portions of buildings are found in New York City buildings in which the Owners have purchased air rights over adjacent properties or within property lines and above such ground features as driveways.

The cantilevers are typically steel-framed for air rights buildings and integrated with diagonals or Vierendeel trusses that extend into the overall building framing. For enclosed occupiable cantilevered stories constructed within property lines, the protruding structural framing is typically steel and attached to the building framing with beam-to-column connections. The last type of framing is of particular interest. The design of long cantilevers, cantilevered enclosed occupiable stories, and atypical back-span conditions require consideration and caution on the part of the designer. Engineers should carefully review layouts, bracing, stiffness, deflection compatibility, detailing, and vibration that can affect more than one cantilevered floor to avoid problems during construction and the long-term performance of cantilevered structures.

Background At first glance, structural engineers may think of enclosed cantilevered building construction as simple, straightforward structural framing not worthy of an in-depth discussion. They may recall that, in the 1950s, the Brutalist architectural style emerged in Great Britain and spread to other European countries, such as The Netherlands. In 1958, Herman Hertzberger, Tjakko Hazewinkel, and Henk Dicke

Figure 1. Studentenhuis Weesperstraatm, University of Amsterdam, The Netherlands.

Courtesy of Gerardo Brown-Manrique, photographer.

won the design competition for a University of Amsterdam student housing building, Studentenhuis Weesperstraatm, with a Dutch structuralism design that paralleled Brutalism in the early post-War period. For this building, constructed from 1959 to 1965, the two large enclosed volumes are cantilevered beyond the outermost row of columns. The visual impression of these cantilevered floors is one of substance and structural integrity (Figure 1).

From the early 1960s through the 1970s, several U.S. buildings were also designed in the Brutalist style. The predominant feature of this style continued to be the use of concrete, at least in façades. Another distinguishing feature was large rectangular boxes that protruded (cantilevered) from the main portion of the buildings, typically along one or two bays and over one to two stories in height. Concrete structural members framed the protruding volumes with minimal glazing in the façades. In these boxes, the structural floor depths of the cantilevers and back spans followed long-standing rules of thumb. The ratios of cantilever lengths to back span lengths were no more than 1:3 and sometimes closer to 1:4 (the maximum ratio the author learned as an architecture student). These rectangular protuberances read as rigid sub-assemblages. An example of a U.S. Brutalist-style building is the Folsom Library at Rensselaer Polytechnic Institute in Troy, NY, which was designed by Quinlivan, Pierik & Krause and constructed from 1972 to 1976 (Figure 2). Here, some of the cantilevered volumes appear to be supported from framing just above, but their actual cantilevered lengths are relatively short.

Enclosed Cantilevered Construction Today Figure 2. Folsom Library, Rensselaer Polytechnic Institute, Troy, NY. Courtesy of Folsom Library, AC19 Institute Archives and Special Collections, Rensselaer Polytechnic Institute, Troy, NY.

14 STRUCTURE magazine

By 2000, buildings reminiscent of the Brutalist architectural style started to appear in newly-constructed residential, hotel, and office buildings of about five to twelve or so stories. The protruding rectangular volumes are larger than in the older buildings; they include many more stories and extend along most or all of the bays in the

façades. For these buildings, the primary framing is either structural steel or reinforced concrete. The cladding typically comprises a significant amount of glazing or is blank (windowless) stucco or other lightweight walls supported on light gauge steel studs.

The cantilevers may provide clearance for vehicular and/ or pedestrian traffic and other activities, reach out over low-rise construction or landscaping at ground level, or take advantage of air rights to extend building cantilevers over shorter, existing buildings. In some cases, the lengths of cantilevered floors increase with increasing elevations.

This kind of structural framing configuration may present unique and not often encountered design issues and loadings. The structural designers must also consider longterm performance under gravity loads (i.e., deflection and creep, especially for concrete framing) and torsional response during strong winds and seismic excitation.

In some of these buildings, typically the air rights buildings, the set of cantilevered floors is designed as one multi-story cantilever of multiple bays. However, in others, the cantilevered floors are designed as independent floors, such as enclosed occupiable canFigure 3b. Structural section of building.

Figure 3a. Structural plan of typical floor with tilevered stories. The floors are not structurally tied building-length cantilever. together vertically through the façade. The bottom cantilevered floor and its back span are the same depth as the upper floors; the lowest cantilever floor does not partially support the gravity loads from the upper floors. Also, the bottom cantilevered floor is not supported by diagonal members from below with clearance for ground activities. A structural plan of cantilevered floors that includes features of this and several other similar buildings is shown in Figure 3a without stairs, an elevator, and holes for vertical HVAC ducts. A cross-section without the foundation, basement parking, bulkhead, and elevator shaft is given in Figure 3b. The cantilevered W12 beams and back-span W12 beams are drawn as “moment connected” to the webs of W10 columns. Still, these connections are semi-rigid, given the flexibility of the W10 columns globally and locally. The beams on Line 2 are subjected to torsion from patterned Live load. The vertical frames on Lines A, B, C, and D provide little lateral load resistance with just one moment connected column.

(The adverse effect of the continuous CMU block wall on Line 3 to lateral loads is outside the scope of this article.) Instead, the cantilevered floors are connected Figure 4b. Structural section of building. to the exterior column with moment connections, many Figure 4a. Localized line load at exterior. without the stiffening effect of back-span framing on the inboard side of the columns.

In another example in Figures 5a and 5b (page 16), the kitchens are On a typical floor, there are two apartments, one between lines A (over)loaded with heavy pots, pans, and equipment. Again, the loadand B.5 and the other between lines B.5 and C. The façades along ing patterns in plan and elevation may differ from kitchen to kitchen.

Lines A and E contain windows. In Floors 2 through 6, bedrooms The loads adversely affect the deflections of the cantilevers and result are between Lines 1 and 2, between A and B, and between Lines C in differential deflections being imposed on the façade material and and D. The kitchens are located back-to-back between Lines B and the glazing in the façades on Lines A and D. In a third example, an occupant may decide to have a dance party B.5 and between Lines B.3 and C. Although several loading scenarios may occur, they are not included in one of their unit’s bedrooms (Figure 6a). The vibration adversely in building codes. For example, in Figures 4a and 4b, fully loaded affects their floors and the others that are “ganged” to it through the 6-foot-tall bookshelves can be placed very close to the tips of the light gauge steel framing behind the façade (Figure 6b). Again, the cantilevered steel beams. The bookshelves may be placed in various loads adversely affect the deflections of the cantilevers and result in patterns both in plan and elevation. Their line loads can be significant, differential deflections being imposed on the façade material and the glazing in the façades on Lines A and D. perhaps on the order of 72 lbs/ft. continued on next page

AUGUST 2022

15

Figure 5a. Localized interior line load.

Figure 5b. Structural section of building.

Figure 6a. Localized heavy area live load.

Figure 6b. Structural section of building.

Performance As far as the author has been able to ascertain, no evidence exists of structural stability problems or collapse during the construction of the cantilevered concrete boxes in the older buildings highlighted as precedents. The author notes that, historically, the American Concrete Institute (ACI) and the American Institute of Steel Construction (AISC) focused on the analysis and detailing of structural framing. For example, ACI 318-56, Building Code Requirements for Structural Concrete and Commentary, introduced requirements for the maximum allowable deflections of reinforced concrete cantilevered beams and slabs. Seven years later, ACI 318-63 introduced a replacement requirement for cantilever design, specifically minimum 16 STRUCTURE magazine

thicknesses for deflection control (if deflections are not calculated) for cantilevered one-way slabs and beams or ribbed one-way slabs. In contrast, the 1936 AISC Specifications focused on end restraint and stresses from moment, shear, and “all other forces.” However, neither ACI nor AISC has provided design guidelines for enclosed cantilevered structural framing or published documents with examples of cantilevered structural framing that possess adequate strength and stiffness for short- and long-term performance. In addition, neither entity has published a collection of failures.

The analysis and design of the structural framing of enclosed, multi-bay, multi-story cantilevered floors present unique challenges for structural engineers, three of them being the lack of redundancy, the complex nature of structural behaviors of members and connections, and the potentially extreme consequences of failure. While rare, failures of these components have occurred during construction, shortly after the buildings have been placed into service, or during longer-term building use.

Extrapolation is conjectured knowledge that entails reaching beyond historically-established knowledge. In any engineering field and even in life, extrapolation inherently elevates risk. Concerning the design of cantilevers, extrapolation occurs when no codified information is available to guide structural engineering design decision-making. This may pertain to structural member and sub-assemblage length and depth dimensions and structural framing configurations.

Some newer buildings have suffered collapses during construction or exhibited structural stability problems, but few have been publicized. Others with potential problems have been discovered before or during construction.

The structural framing of cantilevers exposed to humidity, intense rainstorms, and salty air requires more frequent inspections and maintenance of balconies. The ability to maintain these cantilevered architectural features depends on knowledge and budgets for repair. The knowledge pertains to the original design work (i.e., material selection, structural analysis and design, structural and architectural detailing, and construction quality).

Ongoing Issues When a structural failure occurs, a forensic investigation is conducted. The objective is to determine technical causes of failure (i.e., lateral-torsional buckling, etc.). Unfortunately, these investigations are often part of a legal matter, and the results are either posted in the public domain or locked away by attorneys. Since the purpose of litigation is to place blame and award monetary sums to injured parties, less attention is paid to how and why these technical failures have occurred than perhaps should have been. In this process, the external and in-house forces on structural engineering designers are overlooked. As a result, opportunities to develop effective feedback loops are missed, and the undue risk of structural failure is neither ameliorated nor reduced.

and cantilevered reinforced concrete slabs will affect the exterior wall framing. Spandrel beams, especially those supporting facia materials, do not lend themselves to cambering.

8) The detailing of the trim beams behind the façade (perhaps W10s) that frame into the cantilevered members may require on-site modifications because the cantilevered members are not necessarily horizontal or at the same elevation at their tips.

9) Cladding, such as all glazing, requires specialized claddingto-frame connections (often proprietary) to accommodate the static, vibrational, and time-dependent random cyclic movement of the cantilevered construction.

10) Cladding is directly exposed to outside temperatures.

Its thermal movements from extreme cold and extreme heat need to be accommodated without imposing any stresses on its support framing or the cantilevered floors.

11) Over time, there is a chance that cantilevered steel or reinforced concrete members may exhibit short- and longterm deflections due to overloading the cantilevered framing, patterned live loads not initially taken into account, shrinkage of various materials, creep (of concrete), and thermal effects.

Excessive deflection introduces potential performance problems of brittle cladding material such as stucco and glazing, detrimental deformations in joints, sealants, and window gaskets, and buckling of mullions.

The structural design and detailing of enclosed cantilevered stories are not as simple as the trivial task of calculating moments, shears, and deflections. However, relevant knowledge is available to modify the architectural design to reduce risk. Hopefully, this article will serve as the initial framework to establish a feedback loop into structural engineering practice.

Acknowledgments This article was sponsored, in part, by Grant No. 1724829 from the Science, Technology, and Society program of the National Science Foundation and by a donation from an anonymous senior U.S. structural engineering practitioner.■ Julie Mark Cohen is a Consulting Structural and Forensic Engineer and an Historian of Engineering Design. Her research is entitled “Cognitive Errors in Recurring Failures of Engineered Artifacts.” Her eventual book is entitled Unintentional Engineering Failures by Design (jmcohen@jmcohenpe.com).

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Architects determine overall building massing and 3-D geometry during conceptual design. Increasingly, fewer structural engineering designers participate in conceptual design and even some or all of schematic design. That is, the role of the structural engineer in building design has been devolving (see Building Design Collaborator or Implementing Technician? by Cohen, STRUCTURE, September 2021).

For newer buildings with cantilevered boxes, historically-established precedents from theory and laboratory testing for design, bracing, and detailing are not being adhered to.

Often overlooked issues include: 1) Unsupported ends of cantilevered beams can move or rotate with respect to each other within one floor and among all of the floors vertically, thus imposing varying forces on cladding and its lightweight support framing.

2) Not abiding by historically established rules of thumb for maximum ratios of cantilever spans to the back span lengths (1:3 ratio). Longer cantilevers or shorter back-spans may result in stiffness and vibration problems in the cantilevered area. For vibration response prediction of cantilevers from human occupancy and equipment, see AISC’s 2016 Design Guide 11: Vibrations of Steel-Framed Structural Systems Due to Human Activity (Second Edition) which recommends finite element analysis.

3) Not providing back-spans for cantilevered beams. Cantilevered steel beams are framed into structural steel columns, often into column webs. In this configuration, the rotational restraint, stiffness, and local deformations must be accounted for in a design. These cantilevered beams may also be framed into girders acting in torsion. The torsional strength of the beam and rotational stiffness also affect the cantilever strength and deflection. In some cases, the architectural design does not lend itself to column locations that would allow back spans for the cantilevered beams. Cantilevered reinforced concrete slabs occur without being framed into perimeter spandrel beams; these slabs are cantilevered from interior slabs that often have no perimeter or interior beams.

4) Bracing the compression (bottom) flanges of steel beams leads to structural instability. To determine if bracing is needed, the 4th Edition of the Guide to Stability Design Criteria for Metal Structures published in 1976 provides guidance founded on previous work dating back to 1960.

5) The constructability of complicated connections. Often, the difference between member sizes is insufficient to implement clean, simple, structurally efficient details. For example, flangebolted beam-to-girder moment connections require bending bottom plates and possibly using shims. In another example, for beam-to-HSS-columns, the beam flanges can only be partially welded to columns because they are wider than the (flat portion of the) columns.

6) Determining the deflections at the ends of cantilevers is much more complicated than the simple calculation with a uniformly distributed load on the cantilever. Various load combinations need to be taken into account: 1) dead, 2) dead + uniform live, 3) dead + patterned live load, 4) dead + live load that includes concentrated loads at/near the tips of the cantilevers, 5) dead + dynamic vertical loads from human activity, 6) dead + code-specified live + vertical response of cantilevers from seismic excitation, and so on.

7) Camber, likely more than “natural camber placed upward during erection,” must be specified on structural drawings.

Differences in fabricated camber for cantilevered steel beams

AUGUST 2022

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structural PRACTICES Common Sense is Not Always Common

By Alfredo (Al) Bustamante, P.E.

M

any buildings and other structures include structural components that are parts of suspended scaffold and rope descent systems used to gain physical access to elevated areas. The Occupational Safety and Health Administration (OSHA) federal regulations include many provisions related to the design and evaluation of suspended scaffold systems and related items such as equipment tieback and lifeline support anchorages. While many of the relevant OSHA provisions are clear and consistent, several important structural requirements are either unclear, inconsistent with one another, or both. However, the International Building Code (IBC) is very clear on the structural requirements for the design and load testing of façade access support equipment (FASE). The different types of FASE consist of davit and davit bases (Figure 1) and fall arrest anchors (Figure 2). The purpose of the FASE is to support the suspended system, including powered platforms (Figure 3), rope descent systems (RDS) (Figure 4 ), and lifelines. This article provides specific commentary concerning the OSHA and IBC structural provisions regarding FASE design and load testing and presents proper approaches to load testing of FASE.

Standards OSHA operates under a federal mandate to regulate workplace safety and develop minimum safety standards nationwide; all applicable FASE operations fall under their jurisdiction. Load testing requirements are also specified in the IBC. Table 1 summarizes the key OSHA standards that most likely apply to most FASE-related installations.

Note that OSHA requires that FASE equipment be used under the supervision of a Qualified person, i.e., a licensed Structural Engineer, based on OSHA’s definition. This requirement has practical difficulties because FASE users would have to essentially hire a Structural

Figure 1. Davits and davit bases.

Engineer to be present on-site with them the entire time the FASE is being used.

The IBC started incorporating façade access support equipment design and load testing requirements in its 2015 version. Section 1607.9.3 indicates that façade access equipment used to support hoists must be designed for the larger of the hoist’s stall load (allowed to be a maximum of 3 times the rated load) or 2.5 times the rated load of the hoist. In other words, from a structural engineering standpoint, the service load for FASE design is 3,000 pounds for a 1,000-pound rated hoist. This is in addition to other applicable live loads. Also, the design should include a live load factor of 1.6. For example, if the hoist rated load is 1,000 pounds, the design load should be 3 x 1,000 x 1.6 = 4,800 pounds. The design load for lifeline anchorage is 3,100 pounds times a live load factor of 1.6 for each attachment in each direction the fall arrest load may be applied. Also, Section 1708.3.2 of the IBC and the American Society of Civil Engineers (ASCE) Facade Access Equipment: Structural Design, Evaluation, and Testing guideline indicate that test loads shall be full factored design loads. Table 2 summarizes the FASE design and load testing requirements by OSHA and the IBC 2015.

Load Testing

Figure 2. Fall arrest anchors.

18 STRUCTURE magazine

Since July 23, 1990, OSHA has required that all FASE components be verified by load testing prior to initial use. OSHA does not provide specific test requirements, but the load testing is valid only by applying a load that demonstrates conformance to minimum strength requirements (a stated OSHA testing objective). Therefore, load test demands must, at the very least, reflect the equivalent of full factored loads. This requirement is clearly indicated in the IBC. Also, suppose proof testing to factored loads is performed. In that case, testing a sample of the total FASE population will not lead to reliable statistical inferences regarding the strengths of the untested FASE elements. For example, testing 8 out of 10 davit bases to factored loads provides only an 80 percent degree of confidence that the

Table 1. Key OSHA Standards for FASE-related installations.

FASE System

OSHA Standard

Testing Requirement

Inspection Requirement

Davits, Davit Bases, and Tie-Backs

1910.66 (M) 1926 (C)

4X rated load of the hoist prior to initial use 1.5X stall load of the hoist (i.e., up to 3x rated load) = 4.5x rated load

Annually (Qualified person – i.e., a P.E.) Monthly (Competent person – i.e., contractor)

Lifeline Anchors

1910.66 (M) 1926 (C)

5,000 pounds prior to initial use

Annually (Qualified person – i.e., a P.E.) Monthly (Competent person – i.e., contractor)

Rope Descent System (RDS) Supports

1910.27

5,000 pounds prior to initial use and every 10 years

Annually (Qualified person – i.e., a P.E.) Monthly (Competent person – i.e., contractor)

Horizontal Lifeline (used as fall restraint)

1926.502

None

No requirement

Horizontal Lifeline (used as fall arrest in conjunction with powered platforms)

1910.66 (M) 1926 (C)

5,000 pounds prior to initial use

Annually (Qualified person – i.e., a P.E.) Monthly (Competent person – i.e., contractor). Must also be used under the supervision of a “Qualified” person.

Horizontal Lifeline (used as fall arrest in conjunction with RDS)

1910.27

5,000 pounds prior to initial use and every 10 years

Annually (Qualified person – i.e., a P.E.) Monthly (Competent person – .e., contractor).

Must also be used under the supervision of a “Qualified” person.

(M) = Maintenance activities (C) = Construction activities

remaining bases had adequate strength, which is not acceptable for to 2 times service load is adequate. This approach would not satisfy structural elements. The reasons are numerous: the connections of OSHA or IBC testing requirements. This IWCA Standard lost its the FASE component to the building structure can vary, the building ANSI accreditation due to this and several other technical shortcomstructure configuration can also vary, there could be hidden construc- ings and is therefore no longer considered a relevant standard. tion deficiencies that affect the behavior of some but not all FASE Despite the very clear full-factored design load requirement for load components, there could be distress of steel components of some of testing indicated by the IBC, the author has learned of instances the FASE components due to corrosion from water infiltration, etc. where the load test of the FASE was done at half (or less) of the Even if all the FASE components are exactly the same, the results minimum load required by OSHA and the IBC. Clearly, this “halfof testing a sample of the populations do not mean the rest of the load approach” is inadequate. No logical scientific algorithm can be populations will achieve the same result. used to justify that a structural system load tested at 2,500 pounds is In the cases where full load testing is not feasible to perform, addi- capable of handling 5,000 pounds. The only possible way to justify tional measures could be implemented, including the following: that a FASE meets the minimum load requirements other than a load • Proof testing FASE elements to loads beyond minimum test to the minimum load requirement is by thoroughly inspecting factored loads: Testing a sample of elements to Table 2. FASE design and load testing requirements. higher loads can provide high levels of confidence that untested elements have sufficient strength.

FASE System OSHA IBC However, such test programs must be carefully 4 to 4.5 x rated 2.5 x rated load x 1.6 (i.e., 4) or stall load x Davit and Tie-Back developed and consider the degree of similarity load of the hoist 1.6 (i.e., 3x rated load x 1.6 = 4.8) Anchorage between tested and untested elements and the 5,000 pounds 3,100 pounds x 1.6 (i.e., 4,960 pounds) Lifeline Anchorage minimum acceptable degree of confidence.

• Perform thorough visual inspections and structural analysis of untested FASE elements: This is needed to ensure the tested elements adequately represent the critical components of the untested elements.

The analysis provides theoretical assurance that untested components are adequate. The structural analysis does not have to be for each single FASE component as long as it has been confirmed visually that they are all built and installed the same way. This level of inspection and analysis is typically not cost-effective. Load testing usually is the most economical approach to verify the structural adequacy of FASE components.

There is also a standard often referenced by some engineers: the International Window Cleaning Association (IWCA) I-14.1-2001. This standard indicates that testing Figure 3. Powered platform.

AUGUST 2022

19

all components and performing a structural analysis. Load testing is generally a more cost-effective approach to verify minimum load capacity than the visual inspection and structural analysis approach.

The real problem with the half-load testing is that building owners are misled by this approach, which could present a life safety concern.

To be clear, just because a FASE is load tested to half the minimum required loads does not mean the FASE is unsafe. However, if the FASE is deficient for the minimum load requirements or even for service loads, there is a life safety concern that the half-load testing approach would not be able to address.

Some other important considerations regarding load testing of FASE include making sure the load is applied in the direction of actual use during service, does it provide a means to protect the roofing system and building structure from damage during testing (Figure 5), and has load testing been done for all FASE components? Load testing a sample of the fall arrest anchors, for example, does not provide any information regarding the remaining untested fall arrest anchors, regardless of whether those other anchors are the exact same type. The reason is that the building envelope components hide the connections of the anchors to the building structure. Some anchor connections to the building structure could have been installed in a deficient manner. They could be different due to the building frame configurations, or they could have been compromised due to corrosion from water infiltration, etc. Also, it is essential to have the means to accurately measure the load applied during testing and deflections of the FASE components (Figure 6 ).

Figure 4. Rope descent system.

Case Study – Inadequate “Half-Load” Testing A defective davit base at a high-rise building failed while in service supporting a powered platform with two hoists, each rated for 1,000 pounds. The subject davit base was load tested prior to the failure event by a “half-load” test. The minimum design load for davit bases per OSHA is 4 times the rated load of the hoist (1,000 pounds in this case) – i.e., 4,000 pounds. The load test previously performed was for 2,000 pounds which is half the minimum. The davit base was reportedly claimed to comply with applicable OSHA standards based on the 2,000 pounds load test.

So, what was the big deal? The davit base was load tested to only half the minimum required load; that does not mean it was unsafe.

Well, in this case, it was. The issue was that a hoist on the powered platform stalled because it was caught on a ledge of one of the floors of the building. This means the hoist imposed a stall load on the davit base. That stall load was greater than 2,000 pounds. OSHA allows the stall load of hoists to be up to 3 times the rated load of the hoists.

The davit base failed at a load greater than 2,000 pounds and less

Figure 5. Davit base testing.

than the full factored load. The failure of the davit base led to one of the workers on the powered platform at the time falling and being seriously injured. If the davit base were tested to the full factored loads required by the IBC, the deficient davit base would have been identified during the load testing. Testing to full factored loads, in turn, would have likely prevented the tragedy of the injured worker.

Conclusions The following are some key points for Structural Engineers regarding FASE requirements and load testing.

1) Always test all FASE components to full factored loads to verify compliance with minimum required strengths.

2) Perform the load test to simulate, as close as possible, the actual in-service conditions.

3) If load testing is not physically possible, thorough inspection and structural analysis could be a way to verify the strength of FASE components. Also, proof load testing to loads beyond minimum factored loads could also be an option.

4) Some OSHA requirements do not make much practical sense, so engineering judgment is required.■ References are included in the PDF version of the online article at STRUCTUREmag.org.

Figure 6. Dial gage measuring davit base movement during load testing.

20 STRUCTURE magazine

Alfredo (Al) Bustamante is a Senior Vice President, Managing Director, and Board Member for Walker Consultants . He has extensive experience in assessing, repairing, and load testing structures (abustamante@walkerconsultants.com).

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Floating Fireboat Station

By Craig Lewis, P.E., S.E., Leah Olson, P.E., and Daniel Silva, P.E., S.E.

Figure 1. Floating Fireboat Station facility.

S

an Francisco’s new Fireboat Station No. 35 is a floating twostory, 16,000-square-foot building at Pier 22 ½ supported by a 173-foot-long by 96-foot-wide by 9-foot-deep steel float moored by four guide piles behind San Francisco’s historic Fire Station 35 building. The station includes a new steel pier and associated steel support piles adjacent to the Embarcadero and historic timber pier, a vehicular and pedestrian ramp between the steel pier and float that supports the building, and a gangway between the float and historic timber pier for the firefighters (Figures 1 and 2).

A Collaborative Effort The success of this project is due to the collaborative efforts of the owner, San Francisco Public Works (SFPW), and the Design-Build team. The project schedule required the building and marine design to be performed simultaneously, rather than the more typical progression of completing the building design before the float design. This required close coordination between the building and marine designers to ensure the various structures were compatible and met the project criteria.

A Unique Project When first conceived, the project’s primary objective was to develop a one-of-a-kind floating fire station facility meeting modern standards and resilient to sea-level rise. As an essential facility, Fireboat Station No. 35 is designed to remain operational after the 100-year design storm and the Design Earthquake as defined by the American Society of Civil Engineers Standard 7-10, Minimum Design Loads and Associated Criteria for Buildings and Other Structures. Having an essential facility on the water will allow the San Francisco Fire Department (SFFD) to immediately deploy marine units to locations in the Bay Area that might experience extensive damage from seismic or fire events and are not accessible by roads. The facility also serves as SFFD’s marine-based Emergency Operations Center and includes a large multi-purpose Boson/Ambulance room, shops, and boat/dive gear storage. The new building design is considerate of the existing station and other buildings within the Embarcadero National Historic District.

22 STRUCTURE magazine

Figure 2. Plan view of the facility.

Fabrication also required close collaboration between many parties. The steel float, pier, access ramp, and pilings were fabricated in China and transported on a barge to Treasure Island in San Francisco. While the building was constructed on the float, the piles and pier were installed at Pier 22 ½ in San Francisco. After the building was constructed, the float with the building was towed to Pier 22 ½, where it was connected to the piles, and the access ramp was installed.

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bulkhead, with a stub extending above deck for the building column connection. Steel curbs were welded to the float deck to support the building walls.

The building design accommodates the stresses created by thermal deformations of the steel float from daily solar differential heating and seasonal water temperature changes. In addition, the design accounts for axial forces and moments due to daily, seasonal, and storm waves, as well as wind and seismic loads.

The building’s gravity system is composed of a 3-inch metal corrugated deck with 6 Design and inches maximum of lightweight concrete Structural Systems fill, supported on wide flange composite The design team’s initial work included beams and girders connected to hollow workshops with SFFD to discuss project structural steel (HSS) columns. programming, operational needs, and safety The building’s lateral system is composed requirements for the new facility. With that of buckling restrained braced steel frames in knowledge, they developed a program both the longitudinal and transverse direcnarrative and basis of design consisting of tions to prevent excessive lateral deflection design criteria such as float motions for of the structure. The columns and braces are occupant comfort, seismic performance connected to one another by gusset plates. criteria, design standards, and applicable A simple seismic base shear comparicodes and requirements. son between a floating structure and a Figure 3. Pile collar dampener system.

A float moored with “guide” piling is a land-based structure indicates a notable common structural system on the water to accommodate changing reduction in base shear for the floating structure; this behavior is water levels due to waves, tides, and sea-level rise, and to facilitate related to the water damping and being supported by the long period access to berthed vessels. The float is a rectangular steel box with float-piling structure. A time history analysis of the float-piling watertight longitudinal and transverse bulkheads and transverse system confirmed the design storm motions controlled over the trusses. The box shape is frequently used for stationary floating struc- earthquake motions. The 100 Year Storm dynamic accelerations for tures in the water. The length, width, and depth were based on the building were combined with the response modification factor, multiple factors, e.g., fire department operations such as fireboat R = 8, to account for yielding, as allowed by the code. berthing and ambulance access, buildcontinued on next page ing size and access, stability, pile loads, and cost. Multiple float compartments were ballasted with fixed or water ballast to level the float and meet the required operation freeboard.

Compared to other projects with floatpiling systems, one unusual aspect of the project was the float motions needed to typically meet allowable “Cruise liner” accelerations and rolls to accommodate the firefighters living in the facility during their shifts, including eating and sleeping. It was challenging to meet these limits while providing the required pile strength to resist the wave, wind, and berthing loads. To meet both requirements, a dampener system was introduced between the pile collar and ultra-high molecular weight polyethylene (UHMW) bearing pad to provide significant cushioning, reducing the float-pile impact by up to 90 percent.

The system is composed of various reinforced and unreinforced rubber pads between steel plates (Figure 3).

The float and building elements were coordinated during design to facilitate the construction. The bulkheads are located to support many of the building walls. The building columns extend through the float, typically adjacent to a

AUGUST 2022

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Figure 4. Pier and seawall elevation.

Figure 5. Pier piling plastic hinging and free body diagram.

Special load combinations were implemented to include all the possible load case scenarios with load case factors coordinated with the marine structural engineers; wind load combinations mainly govern the building’s structural design.

The entire structure was designed to maintain a stress ratio of less than 0.3, which represents the fatigue threshold specified by the steel code. This criterion will help ensure the continued safe performance of all the connections through the full design life of the structure.

Seismic Systems and Sea-Level Rise Protection One of the most challenging marine engineering items was designing the steel pier piles for earthquake movements of the existing seawall. The piles are installed through about 50 feet of seawall rock embankment (Figure 4 ). No seawall design or strengthening was performed for this project. Instead, the pier piles were designed to deform plastically without buckling from the embankment lateral earthquake movement and still resist the earthquake inertial loads and design vertical loads, and remain functional.

An extensive analysis estimated that the rock embankment would move about 11 inches toward the Bay in the controlling Contingency Level Earthquake due to the relatively weak soil layer beneath it. To better understand the soil layer’s strength, sonic drilling methods were used to complete borings through the rock embankment. The analysis also indicated that the rock embankment was strong enough to remain mostly intact so that two plastic hinges would form in the pile, one below and one in the embankment. Figure 5 shows the hinge locations and free body diagram.

The facility is designed to accommodate the seawall, pier, and access ramp movements and to remain operational after the controlling Contingency Level Earthquake. Some repairs may be required landside of the seawall due to the waterside movement of the seawall and the settlement of the sandy fill. Still, the gap and settlement can be accommodated with temporary bridging plates across the gap to provide access to the facility.

Due to storms and earthquakes, the structures need to accommodate significant relative displacements and rotations. These are accommodated in many ways: a seismic gap at the pier-to-land interface, a kingpin and vertical support Teflon sliding bearings at the pier-toaccess-ramp interface, wheels at the access-ramp-to-float interface, and transition or bridging plates at all interfaces. In addition, the access 24 STRUCTURE magazine

ramp supports undergo continual movement, so they are designed with low-friction, self-lubricating marine bearings.

The utilities running from the land to the building on the float must also accommodate relative movements of up to several feet.

As a result, flexible conduits were used at multiple locations, and additional structure was required to support and protect the conduits.

The pier is designed to integrate with the existing San Francisco Embarcadero, so the deck elevation matches the existing sidewalk and historic Fire Station 35 timber pier deck. However, those elevations are below the projected Total Water Level for the 50-year project design life. So if the deck elevation needs to be raised for future sea-level rise, the pier superstructure can be removed as it was installed (i.e., by lifting with a floating crane), pile extensions added to the existing piles, and the pier superstructure reinstalled onto the extended piling.

A Facility for the Future The successful collaboration of the numerous stakeholders on this complex project resulted in a facility that will protect the San Francisco Bay Area well into the future.

The project adds to the structural engineering body of knowledge in several areas. The project work determined the following: 1) Earthquake loads on floating structures can be significant and should be considered.

2) Multiple plastic hinges in pier piling may be acceptable, depending on the ability of the soil to stabilize the pile.

3) A practical method to dampen the movement of a float on piles by using a dampener system between the pile and pile collar.■ Reference included in the PDF version of the online article at STRUCTUREmag.org.

Craig Lewis is a Project Manager at GHD in San Francisco, California (craig.lewis@ghd.com).

Leah Olson is a Principal at Liftech Consultants Inc. in Oakland, California (lolson@liftech.net).

Daniel Silva is an Associate at Pannu Larsen McCartney in San Francisco, California (dsilva@plmse.com).

Moynihan Train Hall A N E w E r a f o r t h E Ja m ES A . Fa r l E y B u i l d i n G

By Brian A. Falconer, P.E., S.E., and Ick Kim, P.E.

O

n New Year’s Day 2021, Moynihan Train Hall opened to the public and restored a grand entrance to New York City. In the mid-1960s, the original Pennsylvania Station, a McKim, Mead & White masterpiece constructed in 1910, was demolished. The new facility is an adaptive reuse of the landmarked James A. Farley General Post Office Building, also designed by McKim, Mead & White in 1912, located across Eighth Avenue to the west of the current Penn Station head house.

The five-story building sits on a superblock bounded by Eighth and Ninth Avenues and 31st and 33rd Streets. Before work started, the floor area totaled 1.9 million square feet, including the original eastern half and the Annex, constructed in 1933, which extended the building to the west. Almost 30 percent of the total, or about 550,000 square feet, was removed, added, or significantly altered to create this major redevelopment. Further, 1,000 tons of the building’s existing steel were removed, 4,000 tons were modified, and 6,000 tons of new steel were added. The project was led

by New York State’s Empire State Development via a public-private partnership.

Due to its massive scale, the project was constructed in phases. The first phase, completed in 2017, was the West End Concourse, which Train hall roof (aerial). Photo by Lucas Blair Simpson and Aaron Fedor ©Empire State Development | SOM. expanded and widened an existing

Redevelopment transforms an outwardly grand but mostly functional 20 th-century postal building into a 21st-century transportation hub that is elegant, both inside and out.

26 STRUCTURE magazine

concourse beneath the Farley Building parallel to Eighth Avenue and now provides new entrances and access to all tracks served by Amtrak and Long Island Rail Road. The commercial component, the Farley Building redevelopment, converted most of the existing floor area, including all of the Annex, into retail at street level and offices on the second through fifth floors. Core and shell work for this phase was completed in late 2020 through early 2021.

But it is Moynihan Train Hall, the 255,000-square-foot intermodal transportation hub, that most people have heard about and, a year and a half after its successful opening, have probably seen for themselves. The combined passenger count for Moynihan and Penn Station is expected to eventually exceed 650,000 travelers a day.

A Dramatic Space The train hall’s central feature is the main boarding concourse, designed by architect Skidmore, Owings & Merrill (SOM designed all three phases). Located in Farley’s former mail sorting room, the 150- by 200-foot space is column-free due to three existing steel roof trusses – invisible a century ago – that were uncovered and reinforced to become a significant focal point of the design. Their latticed configuration and riveted connections are reminiscent of framing in the old Penn Station and add delicacy of detail and a sense of lightness, despite their large scale.

Beyond its Beaux-Arts exterior and ornate retail post office, the Farley Building is, for the most part, more function than form. The mail sorting room was for post office employees and not a public space. Its floor was two levels above the tracks, only one level below the trusses, and the long clear spans afforded workers the flexibility needed for sorting racks, carts, conveyor belts, pneumatic tubes, and other equipment; in practice, the space was quite cluttered. The skylights were pragmatic, providing ample lighting from the sun but not intended as aesthetic elements. A ceiling of filigreed diffusers closely followed the truss bottom chords where viewing galleries were concealed, along with the full depth of the trusses. Removing the mail room floor and most of the existing roof, combined with

Main boarding concourse – section at girder.

Mail sorting room – looking Southwest.

placing the arched skylights above the truss top chords, fully utilized the available height to create a dramatically open space.

Stability and Elegance The existing trusses had sufficient capacity to carry a new roof – the loading would be essentially unchanged. However, all existing framing between the trusses had to be removed to maximize the skylights’ function and appearance. This left them unbraced at their ends and for the full length of their gabled top chords. Restoring the trusses’ stability was a central component of the structural design

by Severud Associates.

Each truss is composed of two identical and parallel bents, spaced about three feet apart, initially to form an observation gallery for postal inspectors. The bents are tied together with diaphragm plates and latticed straps that terminate about six feet above the bottom chords. A box beam 36 inches wide by 24 inches deep, composed of 3.5-inch-thick steel plates and located along the top of each truss, provides sufficient lateral support without extending beyond the width of the existing chord members. The box beams also deliver lateral loads from the skylights to the ends of the trusses and eliminate the need to provide bracing between them.

The skylights were designed by SOM in collaboration with schlaich bergermann partner, structural engineers. Four modules, each measuring 50 feet by 150 feet and arched in cross-section, follow the gabled truss top chords to enclose the concourse. The structures are lightweight grids of steel tees that vary in depth and are spaced with three to four feet between them. The frameworks are internally braced with in-plane diagonal cables and transverse “spider webs” of cables at the existing truss third points.

The trusses required additional reinforcements to maintain stability under the skylight loading. First, diaphragm plates were welded between each pair of existing truss bents, at the top of the top chords and just below them. Next, diagonal bracing plates were welded to AUGUST 2022

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Main boarding concourse – truss elevations.

the diaphragms to prevent rotation where the bracing cables connect. Finally, plates were welded to tie together pairs of truss bottom chords at each panel point to replace the removed gallery framing.

Existing double-bent trusses also frame the perimeter of the train hall and, as the main trusses, serve as observation corridors. They support the low roof between the skylights and Farley Building office wings and are now exposed to view. With a uniform horizontal profile but located at about the main truss bottom chord level, the perimeter trusses are too low to support the new skylights. So instead, existing columns were extended up to the box beam elevation, and new framing was installed between them. At the ends of the box beams, steel tube diagonals were welded from each side, down to the first perimeter truss panel point, to prevent rotation and transfer lateral loads into the building frame.

Main boarding concourse – truss top chord section.

28 STRUCTURE magazine

Existing trusses and reinforcement.

Reaching Great Heights Demolishing the existing sorting room floor created a lofty space and a logistical challenge: the highest point of the skylights is 92 feet above concourse level. In response, Skanska, who, with Vornado Realty Trust and The Related Companies formed the developerbuilder team, created a temporary intermediate platform 30 feet above the concourse level to facilitate reinforcement of the existing roof trusses and installation of the skylight components. The platform was designed to withstand all working loads, including the scaffold frame that held the skylight segments in place during installation, and provided workers with a secure environment while also protecting personnel below. As a result, contractors could perform multiple scopes of work simultaneously, with the added benefit of significant cost and schedule savings.

Main boarding concourse skylight installation.

Restoring Load Paths

What Was Old Is New Again The 110-year-old James A. Farley building had become antiquated and largely abandoned. Its continued use without modifications would have been inefficient, while the complete demolition necessary for a traditional redevelopment

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The existing concourse level framing consists of nine-foot-deep, riveted steel plate girders spaced at 20-foot intervals and spanning up to 70 feet in the north-south direction, perpendicular to the platforms. Because the concourse framing transferred the weight of the demolished floor over the tracks, its capacity increased proportionally – enough to support the new concourse loading without global reinforcement. However, the openings necessary for escalators – rectangular, with the long direction parallel to the platforms and hence, perpendicular to the girders – interrupt the direct load path to existing columns. Therefore, existing girders had to be cut and new transfer girders installed.

The clearances for the escalator trusses and headroom meant that notches had to be cut in the top or bottom (respectively) of existing girders. At each notch, new flange plates were welded on both sides of the existing web, and the West end concourse – section at escalator pit. remaining flange elements – double angles and cover plates – were welded to each other and the web. Where new would have been wasteful. Alternatively, renovating the building loads exceeded the shear capacity of the existing web, additional web – taking full advantage of its original strengths – and repurplates were added, usually extending beyond the notch to provide a posing it for transit, retail, and offices created a 21st-century transition from the full-depth to notched section. facility that maintains its early 20th-century grandeur.■ At the ends of girders, reinforcement of the notches was usually sufficient to restore capacity and deliver loads to existing columns.

Brian A. Falconer (bfalconer@severud.com) is a Principal and Ick Kim Conversely, a new column was necessary where a notch occurred (ikim@severud.com) is a Senior Associate with Severud Associates. at a girder’s mid-span. At these locations, a new footing was placed underneath the existing platform, and a column was erected and shimmed tight to the underside of the girder before the notch was cut.

Andrew Mueller-Lust, a former Principal of Severud Associates, In one location, an escalator conflicted with an existing column, also contributed to this article. which had to be removed. This existing column supported a girder on the opposite side that in turn transferred other existing columns and girders, complicating the condition further. New footings were placed on both sides of the existing column’s grillage, supporting a new steel girder that bridges it.

Next, a hammerhead column was erected and shimmed tight to the underside of the transfer girder. Unfortunately, there was insufficient space to connect the notched girder to the remaining framing adequately. Instead, a new column was added on the other side of the notch, between tracks, before the notch was cut.

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Figure 1. Park Union Bridge spans 250 feet over active rail lines. Courtesy of Jason O’Rear.

D

esigned by Diller Scofidio + Renfro (design architect) and Arup (engineer of record), the Park Union Bridge opened to pedestrians in July 2021, connecting the U.S. Olympic and Paralympic Museum to America the Beautiful Park and Downtown Colorado Springs. The bridge is called the rip curl for its cresting design; the footbridge spans 250 feet over active rail lines (Figure 1).

One of the key project successes for the design team was producing a design that balanced achieving the aesthetic vision and minimizing the impact on the railroads below.

From bridge inception, the designers leveraged their parametric 3-D work environment to communicate visually and explain crucial aspects of the bridge behavior among themselves and with key collaborators of the project team.

Structural Analysis Long-span footbridges are challenging by nature due to their high span-to-width ratios; dynamics become a significant consideration. Additionally, the form of the Park Union Bridge, composed of a structural steel shell simply supported on concrete abutments, is one of the most unique bridge typologies. To realize this structure, the design team conducted global, modal, spectral, footfall, buckling analysis, and wind tunnel testing.

Figure 2 demonstrates an output from the robust analysis used to validate the concept; the buckling arch analysis confirmed that the slender area of the steelwork in compression would remain stable under all applicable load cases and load combinations following AASHTO LRFD Bridge Design Specifications.

The design team collaboratively massed, rationalized, and managed the steelwork 3-D model in Rhino with a Grasshopper parametric plug-in. All analysis was completed in Oasys GSA, which linked parametrically to the 3-D steelwork model. This allowed for fast updates to structural analysis models as the design iterated and facilitated rationalizing the steelwork. While the rip curl geometry is captivating in its unusual form, the shape is predominantly built from flat and single curvature plates, which was key to simplifying fabrication.

Figure 2. Buckling analysis modal shapes.

30 STRUCTURE magazine

Park Union Bridge A Balancing Act

By Lana Potapova, P.E., M.Eng., B.Eng.,

and Matt Carter, P.E., MA M.Eng., C.Eng.

Rowan Williams Davies & Irwin Inc. (RWDI), located in Ontario, Canada, was commissioned to design and build an aeroelastic model to evaluate the bridge’s wind-induced responses.

The aeroelastic model was tested in a fully turbulent boundary layer wind tunnel that accurately simulated the approaching wind conditions. Arup observed the test to determine that the bridge was aerodynamically stable (Figure 3).

The view inside the bridge (Figure 4 , page 32) demonstrates the close relationship between architecture and bridge engineering: pedestrian headroom clearances and the desire to minimize the depth of the bridge elevation aesthetically were closely coordinated with arch span-to-depth ratios and arch cross-section shaping.

Another barely visible element in Figure 4 is the protective parapet; a significant design success lies in its translucency. Railroad parapets on bridges have strict performance criteria and are notoriously known to impact bridges’ aesthetics negatively. The Arup Facades team produced a performance specification for a tension net structure that met the performance requirements for Union Pacific Railroad without impacting the aesthetic vision for the crossing; this included a maximum opening size of one and a half inches and the ability to resist a climber. The stainless-steel mesh is held by a cable spanning from steel shell to parapet post; the sag of the cable is specified such that a minimum ten-foot height requirement for railroad parapets is always maintained.

The superstructure comprises the steel shell, floor beams, and concrete deck on stay-in-place (SIP) metal deck form. The pinnacle of the design effort was the designers collaborating on shaping the 300-ton steel superstructure. The steel shell is further rationalized into three elements: the edge girder, the asymmetrical arch, and the stiffened steel web (steel shell) linking the two.

Shallow box construction was critical for achieving the architectural vision while providing a robust structural solution. The edge girder and arch sections are built up similar to a lidded box: three pieces of the box are built up assuming access from the inside is available, and the final capping plate is welded on from the outside only as the box is too small to access from inside. Fillet welds are used where possible to minimize welding requirements.

The derivation of the asymmetrical arch box required close collaboration with the architect. The arch carries approximately 5000 kips in

Structural Details Building over railroads presents unique constraints both in design and construction. Maintaining railroad operation unimpeded during erection was a crucial driver in the design; the yard includes thirteen rail tracks including two through tracks with freight train traffic of approximately ten trains per day.

Figure 3. Wind tunnel testing of the bridge. Courtesy of RWDI. AUGUST 2022

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adjacent to the rail yard. In October 2020, the 550-ton superstructure was lifted by self-propelled modular transporters (SPMTs) and driven (by Mammoet) into its final position in under five hours, ahead of the allowed railroad outage window (Figure 5). Railroad track protection was provided for the drive to ensure SPMT wheel loads did not damage the railroad infrastructure. The bridge was then fit-out from the inside, minimizing additional railroad closures.

Conclusion The gravity-defying bridge enhances the landscape of Colorado Springs. The collaborative communication style was continually enhanced by cutting-edge design tools and allowed the preservation of the bridge’s sculptural floating nature while balancing the aesthetic with a pragmatic and sensible design over the railroad. The Park Union bridge is a recipient of the American Public Works Association Colorado Award and two Structural Association of New York “Excellence in Structural Engineering” awards (first place in Other Structures and Engineer's Choice Award).■ Lana Potapova is an Associate in the New York Bridge Group of Arup.

Lana has assisted in teaching graduate architecture studios at Columbia University and published work in collaboration with the Harvard Graduate School of Design.

Matt Carter is a Principal in Arup’s New York office. He has extensive experience designing long-span and complex bridge structures in North America, East Asia, Europe, Africa, and Australia.

Figure 4. Inside view showing translucency of parapets required over rail tracks.

Courtesy of Jason O’Rear.

Project Team Owner: City of Colorado Springs Structural Engineer of Record – Superstructure: Arup Structural Engineer of Record – Abutments: KL+A (as a subconsultant to Arup) Design Architect: Diller Scofidio + Renfro Lighting Designer: Tillotson Design Associates Architect of Record: Anderson Mason Dale (executive architect) General Contractor: Kiewit SPMT Erector: Mammoet Steel Fabricator: King Fabrication

compression. The arch deviates from the natural parabolic shape to meet architectural aspirations; therefore, a gravity bending moment is induced into the section. The asymmetry in plan causes an additional lateral bending moment. The size of the arch box directly influences head heights and internal cladding requirements. 70 ksi steel was used in the arch section to minimize the overall size and plate thickness of the highest loaded member on the bridge. Higher-strength steel allowed plate thicknesses to remain below 2 inches, making welding and building up the section more cost-effective.

The structural solution facilitated erection over the railway, assuming all components would be assembled in a staging area before erecting the nearly finished bridge in a single railroad outage.

The steelwork was fabricated and painted offsite at King Fabrication in Houston, TX. This ensured the quality of production and limited the impact of construction on-site. In addition, the fabricator fully fit up the bridge before the shipment of pieces to validate the fabrication geometry and camber. Finally, the design team strategically located bolted splices in floor beams and permitted welded field splices in the arch and edge girders to facilitate transport to the site and assembly. The pieces were delivered on multiple trucks from Houston to Colorado Springs.

Kiewit started the construction of the bridge in early 2019. The concrete deck was cast once the superstructure was assembled in a staging area Figure 5. Bridge erection using SPMTs. 32 STRUCTURE magazine

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Rehabilitation of the Historic McElmo Flume

T

By Carlo Citto, P.E, Ronald W. Anthony, FAPT, and Douglas Porter

he Montezuma Valley Irrigation Company Flume No. 6, also supported by a wood substructure. At the time, a wooden box flume referred to as the McElmo Creek Flume (the Flume), located near or rectangular flume was a popular choice in the West. Standard practhe City of Cortez in Montezuma County, Colorado, is the last remain- tice was to cover the surfaces with paint, hot asphalt, or tar to delay ing flume of a water delivery decay – creosoting was known system designed to irrigate to add life to the timber but the flat, open Montezuma was generally too expensive in Valley. The Flume is histhe late nineteenth century. torically significant as an The original flume was in serirrigation system of early vice until 1921, when it was Euro-American settlement. replaced with a “semi-circular Located in southwestern wood-stave flume, 10-foot Colorado, the Flume is in an diameter” (Engineering arid area with archaeological World, 1922). The new evidence suggesting that the flume was constructed with earliest permanent settlement creosoted Douglas-fir staves began around AD 400 when supplied by the Continental Ancestral Puebloan people Pipe Manufacturing Company began to occupy the Mesa Figure 1. Section view, wood-stave flume design (from Continental Pipe Manufacturing based in Seattle, Washington; it Verde area. They depended on Company’s Catalog No. 18, 1923). spanned the arroyo on wooden agriculture as their primary trestles and passed over low source of food, but due to the lack of a permanent water source in land on timber foundations. The flume consisted of timber bents – each the area, rain and snow were critical to the growth of crops and their comprised of two segments, two struts, one base, and a sill – providing long-term survival. In the 1760s, Spanish expeditions brought the first support to the semi-circular stave configuration that was reinforced Europeans to this part of the Southwest, and ranchers, farmers, and with spreaders and steel tie rods matching the diameter of the flume Pueblo traders settled the Montezuma Valley in the 1870s. Although (Figure 1). The Flume was supported initially on timber girders, which many natural drainages occur in the valley, including the McElmo were replaced by steel beams and concrete piers in 1955. In its current Creek, these drainages are seasonal and could not provide a steady configuration, the Flume is approximately 10 feet wide by 100 feet long water supply to farmers and settlers. As a result, several entrepreneurs and constructed of 18 timber bents resting on steel I-beams and timber began to develop a water system to irrigate the valley. stringers supported by concrete-encased steel columns and diagonal When the McElmo Flume was first constructed, it was described as braces. Although the irrigation system was no longer operational, water a “4-feet by 18-feet [sic] wooden flume” (Engineering World 1922) continued to flow through the McElmo Flume until 2007.

Over time, the Flume suffered from lack of maintenance and damage by severe natural events, including runoff from a heavy rainstorm in 2006 that collapsed its northern section and strong winds in 2010 that damaged staves and bents on the southern section. Prolonged exposure to severe weather resulted in corrosion of the steel elements, concrete spalling, and scour of the foundation. As a result, by 2012, the Flume was at risk of collapse.

The Evaluation of the Flume

Figure 2. The McElmo Flume in 2012 in a state of partial collapse due to damage from severe natural events.

34 STRUCTURE magazine

It was critical to document the existing structure to develop a preservation plan (Anthony & Associates, Inc., 2013). The extant elements and those displaced but still nearby

were recorded via LIDAR scanning, providing a record of the Flume site in 2012 (Figure 2). For the basis of design used in the structural analysis, the flume was analyzed to no longer carry water, so the loads were less than when operational and determined to be the dead load (weight of the structure) and live load (wind) for the preserved structure. The sizing of elements and design properties were based on satisfying the anticipated loads over a 50-year service life. The material assessment and evaluation were used to establish minimum sizing and material properties.

Wood Condition Assessment The wood assessment was carried out to determine the suitability of the elements for reuse and continued service. Although the Flume no longer carried water, the wood elements still needed to support their own weight and resist wind loads to reduce the likelihood of additional collapse and loss of historical material. Initially, a visual inspection of the accessible elements was conducted to identify missing, broken, or deteriorated components. Visual inspection and probing with an awl quickly identified areas that needed further investigation to determine the extent of material loss due to wood decay. This was accomplished using resistance drilling – a quasi-nondestructive technique used to detect internal voids by determining the relative density of the wood as a small diameter needle penetrates the element. Next, wood samples were removed for species identification, and in-situ visual grading was conducted to determine the grade of structural elements. Identifying wood species and structural grade made it possible to assign design properties for conducting a structural analysis and identifying compatible material for repairs.

Wood condition varied from good to poor, with 40% of the timber in the bents in an advanced state of deterioration. Approximately

Figure 3. A typical repair of a bent by splicing new timber.

50% of the staves required replacement because they were missing or substantially damaged. The staves were identified as Douglas-fir and had evidence of creosote visible in the cellular structure, consistent with documentation that the flume box was purchased from the Continental Pipe Manufacturing Company. All other sampled elements (stringers, bents, and spreaders) were identified as a western yellow pine, suggesting that these elements may have been cut from locally available Ponderosa pine. The staves were free of knots and slope of grain and assigned the grade of Select Structural Douglas-fir.

The timbers were assigned No. 2 Western Woods, excluding sills and stringers that did not meet structural grade requirements due to large knots or steep slope of grain.

continued on next page

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Figure 4. The McElmo Flume after rehabilitation.

Concrete and Steel Evaluation The evaluation of concrete and steel focused on concrete cracking and spalling, steel corrosion, and foundation erosion. Condition assessment was based on visual methods supplemented by nondestructive techniques. Sounding of the concrete was done on representative elements to determine the presence and extent of delamination. A metal-detecting pachometer was used to scan concrete elements to locate embedded metals such as ties and reinforcing steel. An ultrasonic thickness meter was used to measure the steel thickness of the existing structural elements and evaluate the degree of section loss due to corrosion.

Extensive concrete spalling and cracking of the concrete-encased steel columns and diagonal braces were observed throughout the structure.

However, the condition was particularly severe at the diagonal braces, where the full length of the steel beams was left exposed. Concrete columns were also affected by vertical cracking, typically in line with the embedded steel. The corrosion of the embedded steel caused the concrete deterioration. Minimal concrete cover, measured at ¾ of an inch at most locations, did not adequately protect the steel. The level of corrosion varied from minor (surface pitting) to moderate (flaking, less than 15% loss of original material) to severe (flaking, more than 15% loss of original material).

Girders were in fair to good condition, with minor surface corrosion and an average thickness loss of 12%. However, severe corrosion of the columns and braces resulted in reductions in the original thickness of 25% or more (based on measurements with an ultrasonic thickness gauge). In addition, severe corrosion was observed at all the plates that supported the wooden stringers, which, in some cases, produced substantial uplift of the stringers.

The foundation of the Flume was compromised due to severe runoff of the creek, with the bank of the arroyo eroded from beneath the abutment on the southeast end of the Flume, leaving the supports hanging in space.

Preservation Plan and Repair Work Because of the National Register status of the Flume, the preservation plan followed the principles of minimal intervention as defined in the Secretary of the Interior’s Standards for the Treatment of Historic 36 STRUCTURE magazine

Properties. Withholding treatment was not an option since it would result in the loss of the Flume. The preservation goals were to maintain the original appearance of the Flume, allowing for interpretation of the history of water use in Montezuma Valley, with a service life of 50 years with minimal maintenance. Two intervention options were identified: stabilizing the Flume in its current condition and mitigating future deterioration, or, through rehabilitation, repairing the structure using the remaining original elements and augmenting it with new, in-kind elements as necessary. The rehabilitation option was selected by Montezuma County officials, the stewards of the Flume.

The work was completed in two phases: The treatment of the substructure included the construction of concrete footings for the unsupported bents and the placement of rip-rap to stabilize the arroyo bank during high runoff events. The concrete and steel elements were repaired so that support for the wooden flume superstructure was maintained. Deteriorated concrete was removed, encased steel cleaned, and the exposed surfaces prepared to receive patching material; new concrete was used to restore the original cross-section. The heavily corroded steel plates that supported wood stringers were replaced with self-weathering steel plates.

Finally, the flume timbers were repaired, and the stave pipe was reinstated using salvaged and supplemental staves. Surviving historical elements were numbered and disassembled, so deteriorated elements could be repaired with dutchmen and splices or replaced in kind (Figure 3, page 35). Salvaged staves, supplemented with new staves matching the original profile, were installed to complete the half-pipe and engage the steel rods and spreaders.

Conclusions The McElmo Flume is a significant infrastructure feature that facilitated the settlement of the Montezuma Valley in southwest Colorado. It is the last remaining flume of an extensive irrigation system developed in the late 1800s. After a comprehensive material and structural assessment and the development of a preservation plan, the Flume was repaired, maximizing the retention of historical material (Figure 4 ).

This engineering achievement helps to tell the history of water in the development of Montezuma County.■ References are included in the PDF version of the online article at STRUCTUREmag.org.

Carlo Citto is a Structural Engineer and Principal at Atkinson-Noland & Associates (ccitto@ana-usa.com).

Ronald W. Anthony is the President and Wood Scientist at Anthony & Associates, Inc (woodguy@anthony-associates.com).

Douglas Porter is a Research Faculty at the School of Engineering at the University of Vermont (douglas.porter@uvm.edu).

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professional ISSUES Ethical Decision Making in Structural Engineering

By Joe Brejda, P.E.

S

tructural engineering is one of those jobs where people joke, “if you see me running, try to keep up.” This may not seem dark until we experience tragedies like the recent Florida International University’s pedestrian bridge collapse or the Surfside condo collapse.

In some ways, the phrase is meant to express the importance of what structural engineers do and the catastrophic consequences of failure.

Fortunately, such events are exceedingly rare.

In general, the majority of the population does not know that structural engineers exist or at least do not understand what structural engineers actually do. If a structural engineer does their job correctly, it is not exciting. There are true structural emergencies, however, and there is a lot that we can learn from these emergencies. This article focuses on the decision-making process during a structural emergency. In this unique event, engineers and other team members necessarily streamline decision-making and remove many of the factors that are typically at play. Of course, money is always a concern, but it drops way down the list of priorities in a case such as this. This example also does not involve other engineering disciplines, so the structural engineer has full authority due to the critical and immediate nature of the event.

During a morning walk-through of a medical supply manufacturing facility in the rural United States, a plant worker noticed a large circumferential crack in the base of a 60-foottall, 6-foot-diameter, freestanding fiberglass industrial scrubber. A scrubber is a piece of equipment that facilitates chemical reactions to reduce emissions from industrial processes.

In this case, the scrubber used sulfuric acid and was filled to approximately 15 feet. The acid in the structure has a pH of less than 1. The plant owner immediately acted and called a structural engineer to the site to investigate the crack and determine the stability of the structure. The owner also took proactive steps to shut down the plant, send all non-essential workers home, de-energize equipment within the fall radius, and make other general safety precautions.

Before the structural engineer was on site, he had already reviewed design documents, viewed pictures, and offered preliminary consultation to mitigate the risks of the hazard. For instance, the engineer recommended leaving the liquid in the scrubber since there were no visible leaks and it was likely serving as ballast, maintaining the stability of the structure.

38 STRUCTURE magazine

Table of a simple risk management matrix.

Once on-site, the engineer assessed the surrounding structures for a suitable system to brace the tower. Each surrounding structure had to be evaluated, including creating analytical models and designing emergency bracing using only materials on hand. The team was fabricating the bracing on-site as the engineer was designing it and doing so around a potentially weakened structure; this was some of the highest-risk work required.

The engineer also had to convey to the owner that this was emergency bracing and not a permanent fix. It was designed only to secure the tower until a hazardous materials team could drain the scrubber, and inspecting personnel could more safely approach the base to examine the crack. Given the urgency, it was not feasible to create a permanent system.

19 hours later, with the engineer satisfied that the bracing was sufficient, the team removed the insulation from the scrubber and determined that only the insulation was cracked. A further forensic investigation followed to determine the probable cause of the damage and give reasonable assurance that the tower would not experience further issues that could cause another shutdown or worse.

As for the decision-making, many of the typical design considerations were taken out of the equation. The goal was safety and reasonable certainty, not optimization, and the team was required to work quickly with only the materials on hand. Meeting code minimums was not the goal of design. With the limited time and information, the design utilized much more conservative loading. Cost

was very far down the list of considerations.

Authority was more concentrated in the structural engineer, as all stakeholders understood that with time and safety of the essence, decisions could be questioned later if necessary.

Even with these typical roadblocks removed, the decision-making process was still difficult.

The owner informed the team that financial losses were in the millions for each day the plant was not operational. They rightfully shut down the entire plant once the crack was found, as the collapse of a 60-foot tower could have caused extensive damage, including rupturing gas lines and damaging electrical infrastructure, in turn causing fires. Industrial processes such as this also involve the controlled use of hazardous materials. The release of those materials could have immediate and long-term effects on the environment and the people in the area. The plant is integral to the economic livelihoods of local residents and critical to the medical supply chain, two factors that were both compounded by the global pandemic. Life safety was obviously the most immediate concern, but the local and global socio-economic impact was also considered.

Once the engineer and the contractors had adequately secured the tower, drained it, and removed its insulation, the engineer faced the ultimate decision of whether the tower was safe to operate. A second team was working on contingency in case the tower needed to be condemned. The best solution that the team could identify would be to close the plant for a minimum of two weeks. Stopping the medical supply chain for that long could create supply

likelihood is equal. We can mitigate the likelihood through training, physical barriers, etc., but if a fall occurs, then the impact is the same.

This increased risk has been weighed against the necessity of workers performing tasks at such heights. Without this work, construction would be extremely difficult to complete, if not impossible. Therefore, the decision has been made to accept the greater risk.

As for the scrubber situation, based on the considerations listed earlier, most of which are not explicitly quantifiable, the team prioritized and addressed the risks and made final recommendations.

Immediate risk to human life is the top priority.

The importance of financial impact decreases rapidly as safety risk increases since, for most individuals, life holds no quantifiable monetary value. The emergency bracing remediated an urgent safety risk and potentially limited damage to the facility. Once the safety risk was mitigated, the decision-making became more complicated. Trying to quantify and rank the successive priorities was challenging because the factors were not as clearly defined, especially while in the field with limited time.

Ultimately, with no visual evidence of damage to the structure and the economic, business, and other potential impacts, the engineering team decided that the structure could be put back

in service for its limited remaining lifespan.

However, the team also recognized that although no visual indications were present, the structure could still be under additional stress from the work that had caused the damage to the insulation. Since risk is a factor of both the likelihood and the impact, the owner implemented a monitoring plan to track the structure’s behavior so that plant management could promptly address worsening issues should they occur.

To be clear, this was an incredible team effort.

It was one of those moments where egos were put aside, and different groups with otherwise different priorities came together for a clear common purpose. For structural engineers, lives are always on the line. In an instance such as this, the risks are more immediate and recognizable. Still, as those risks are addressed, the subsequent risks enter greyer and greyer areas where ethical decisions take a more significant role. Engineers are ultimately risk mitigators who are dealing with human life and livelihood. Therefore, every decision we make is a matter of ethics.■ Joe Brejda is a Project Manager and Senior Structural Engineer with Argus Consulting, a multidisciplinary design firm specializing in aviation fuels infrastructure (jbrejda@argusconsulting.com).

Photo Courtesy of Jeff Durkin at Bread Truck Films

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shortages. In addition, the plant has widereaching economic and social impacts, and a shutdown of such duration would have been detrimental to an area already struggling with the pandemic. The engineer had to consider all of this, along with the potential for injury or death if the tower failed after reentry into service.

Had there been clear structural damage, the call would have been easy. In this case, there was no visible damage to the structure itself. The engineering team had to weigh all risks and make a judgment call based on available information.

How do engineers make such decisions, though? And how does ethics play a role?

The public may be familiar with triage methods in the medical field or perhaps with risk management/mitigation practices in the business world. There are many methodologies for these assessments, but they all generally revolve around the same basic principles. A fundamental way of defining risk is to evaluate the impact of a potential outcome versus the likelihood that the outcome occurs. A simple risk management matrix looks like what is shown in the Table.

While still a judgment call, likelihood is generally based on data and experience.

Likelihood, therefore, is more quantifiable and agreed-upon. In most regards, it is a stand-alone factor, and mitigation of likelihood can generally be approached by concrete identifiable measures.

Impact is where ethical considerations come into play. The impact varies based on several factors such as financial considerations, safety/human life, and overall economic consequences. Different cultures place different values on human life; some even determine that value based on age, gender, race, and socio-economic standing. In America, we tend to place a very high value on human life across the board.

Financial impact is weighed differently by different stakeholders, as is the economic impact. These factors are more easily quantified in exact dollar amounts. However, how significantly those dollar amounts affect different people varies greatly.

Take OSHA regulations for fall protection as a concrete example. These regulations are based on the likelihood and severity of the injury. According to OSHA, the height at which fall protection is required is 4 feet for the general industry but increases to 6 feet for construction. Why are these different?

The simple answer is that construction workers accept greater risk. Four feet is the height at which the risk of significant injury greatly increases, 6 feet is the height at which the risk of death jumps up. This increase is one of impact. Are you more likely to fall from 4 feet than 6 feet under the same circumstances? No, so the

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INSIGHTS Mitigating Seismic Risk

Seismic Code and Standard Development and NEHRP Recommended Provisions

By Jiqiu Yuan and Mai Tong

M

ajor earthquakes are rare compared to other natural hazards such as wind, floods, and snowstorms; however, the destructive power of major earthquakes can be devastating. Thousands of lives and billions of dollars in economic investment could be lost in poorly prepared communities. Our nation’s seismic risk can be largely reduced through earthquake-resistant buildings designed and constructed in compliance with modern building codes.

Protecting the Built Environment In dealing with earthquakes, it is essential to understand that the risk of building failure or collapse is real and more significant than many other natural hazard impacts.

Buildings are designed to survive damage and be robust enough to provide sufficient stability to minimize the risk of collapse.

However, designing structures resistant to major earthquakes is a complex process, and developing seismic design provisions and codes applicable across the nation is even more daunting.

Seismic requirements in the building codes can be traced back to the 1927 Uniform Building Code (UBC) with non-mandatory provisions. Since then, seismic design has evolved significantly with many milestones like (not limited to) the development of the Recommended Lateral Force Requirements and Commentary (the Blue Book) in 1959 and subsequent updates by the Structural Engineers Association of California (SEAOC) and the publication of the landmark ATC 3-06 report, Tentative Provisions for the Development of Seismic Regulations for Buildings by the Applied Technology Council (ATC) in 1978. In addition, the establishment of the National Earthquake Hazards Reduction Program (NEHRP) (Public Law 95-124) in 1977, and subsequent reauthorizations (latest reauthorization in 2018 Public Law 115-307), provided significant support for the development of nationally applicable seismic regulations and modern building codes and standards.

40 STRUCTURE magazine

Collaborative process of NEHRP provisions development and paths into building regulations.

The NEHRP Recommended Seismic Provisions for New Buildings and Other Structures (Recommended Provisions), developed by the Building Seismic Safety Council (BSSC), supported and published by the Federal Emergency Management Agency (FEMA) on behalf of NEHRP, provides a consensusbased code resource for improving nationally applicable standards and model building codes. Since its inception, the Recommended Provisions have evolved from a code language document adopted by the regional model building codes and the first two editions of the International Building Code (IBC) to a code resource document that feeds into the ASCE/ SEI 7 Minimum Design Loads and Associated Criteria for Buildings and Other Structures and then the IBC. The Recommended Provisions result from a successful collaboration with public and private sectors for translation of research results for best practices and implementation in codes and standards.

The NEHRP Provisions Update Committee (PUC) involves leading practicing engineers, code and standard specialists, university researchers, construction material industry professionals, and NEHRP agency representatives. Through the two-tiered BSSC consensus

process, the PUC and BSSC member/professional organizations evaluate and resolve all proposed technical changes to be included in the Recommended Provisions. The NEHRP Recommended Provisions have become a well-known brand in the United States and internationally.

Topics Considered The Recommended Provisions are regularly updated in a three- to six-year cycle, and each update engages over 100 top experts and around 40 relevant organizations in the field of earthquake engineering. FEMA’s resource support and close association with USGS and NIST have led to significant technical changes.

These include updated U.S. seismic design maps (by USGS), new seismic force-resisting systems, qualification of design coefficients and response modification factors, integration of occupancy with seismic mapping and seismic design categories, design requirements for structural irregularities and deflection (drift), improved design force formula for nonstructural components, and more.

Besides developing important code changes, Recommended Provisions introduce innovative

concepts and new procedures for trial use by the design community, researchers, and standard- and code-development organizations (often called NEHRP resource papers).

Supporting and educational documents are also produced to provide design examples to walk practicing engineers, code officials, students, and other stakeholders through the Recommended Provisions and corresponding standards and codes.

such as how wind or flood can be consistently treated, how the building industry supports community resilience and adapts to climate change, and how federal agencies and the private sector can collaborate.

Acknowledgment Steven McCabe from the National Institute of Standard and Technology provided a peer review of the article.■

Jiqiu Yuan is the Executive Director of MultiHazard Mitigation and Building Seismic Safety Councils, National Institute of Building Sciences.

Mai Tong is a Physical Scientist and Project Officer with the National Earthquake Hazards Reduction Program (NEHRP), Federal Emergency Management Agency.

Current Edition

Moving Forward One may consider that seismic design is one of the complicated fields in engineering design. The process by which the seismic design requirements are developed to take form in the building code has been a very successful model and will continue to support seismic design advancement and innovation. It could also provide insight into other hazards,

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The 2020 NEHRP Recommended Provisions (FEMA P-2082) mark the tenth edition of this technical resource document since its first publication in 1985. Most of the recommended code changes in Part 1 of the document are adopted by ASCE/SEI 7-22, which will be adopted by reference by the 2024 IBC. The 2020 NEHRP Recommended Provisions: Design Examples (FEMA P-2192) were developed by BSSC for FEMA to illustrate and explain the major changes of the 2020 NEHRP Recommended Provisions, ASCE/SEI 7-22, and the material design standards referenced therein in design applications.

The Design Examples were developed primarily for design practitioners. However, college students learning about earthquake engineering and engineers studying for their licensing exam or designing in regions of moderate and high seismicity will find this document’s explanation of earthquake engineering, the 2020 NEHRP Recommended Provisions, and ASCE/SEI 7-22 seismic provisions helpful.

A National Institute of Building Sciences webinar series (https://bit.ly/3yypsmD), organized following the eight chapters in the Design Examples, was conducted in the first half of 2022 and serves as part of the FEMA NEHRP/BSSC outreach and education effort. A series of technical articles based on the webinar series will appear in future issues of STRUCTURE.

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WIND/SEISMIC guide The Masonry Society

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SPOTLIGHT The Carrier Dome’s New Cover

T

he University of Syracuse Stadium (a.k.a. Carrier Dome) opened in 1980 and is covered by an air-supported PTFE coated fiberglass fabric roof. It was the first and remains the only domed stadium in New York State. The 50,000-seat stadium is prominently located on campus, home to Syracuse basketball, football, and lacrosse, while serving the larger community by hosting an array of events.

Although economical to construct, the stadium’s original air-supported roof had significant operating demands, including the need to maintain constant positive interior pressure to support the roof and a limited ability to support snow. In addition, any significant snow accumulation had to be melted or manually removed to prevent the roof from inverting. As the roof fabric was approaching the end of its design service life, the University chose to replace the roof with a new nonair-supported structure and tasked Geiger Engineers with the design.

In addition to the complexity of replacing an air-supported roof with a dissimilar structure, the University placed significant limits on the period that the facility could be closed.

Syracuse Athletics heavily utilizes the stadium, requiring that the renovation allowed for facility use from September through February. In response, the new roof design was conceived such that the construction of the crown truss, which is the majority of the structure’s mass, is outside of the building envelope at the perimeter of the roof so its construction could proceed while the stadium remained in use.

This required that the partially completed structure be engineered and the building verified safe for each event occupancy throughout the construction of the crown truss. The balance of the roof primary structure, a twoway cable truss, was developed to be rapidly erected following the closure of the facility and demolition of the air-supported roof.

The new roof is a creative solution addressing several other major challenges. First, it accomplishes the goal of passively resisting the very heavy snow loads of the region. The concept takes advantage of the skewed-symmetric layout of the original roof to achieve structural efficiency. More inspired, though, is how it repurposes the original compression ring as a key component of the new structure. Further, the unique nature of the STRUCTURE magazine

Geiger Lynch MacBain Campbell Engineers, PC, d.b.a. Geiger Engineers was an Outstanding Award Winner for the Syracuse University Stadium New Roof Project in the 2021 Annual NCSEA Excellence in Structural Engineering Awards Program in the Category – Forensic/Renovation/Retrofit/ Rehabilitation Structures over $20M.

design concept limited the new demands to be generally within the existing capacity of the supporting stadium structure. Optimization of the cable truss form and prestress capitalized on this concept in developing the design.

The original structure required minimal modification to be incorporated into the new system. The benefit of this clever and resourceful design technique is partly evidenced by noting that the required reinforcement of the existing structure comprises less than 5% of the new work.

The crown truss upper ring is a primary component of the structure. Therefore, numerical optimization techniques were applied to determine the best geometry of the upper ring and cable truss form and prestress for a least-weight, efficient design while limiting the demands on the existing structure. The result is not just structurally efficient concerning materials and labor; it is also visually engaging and compliments the architecture of the existing stadium.

The crown truss bottom chord tension ring is laterally released at its supports on the stadium structure. It is located within the footprint of the original compression ring and rests on slide bearings, allowing the two to perform their respective duties independently of each other. The structural separation of these two major components in nearly the same space is a design feature that is visually inconspicuous

yet enormously beneficial to the structural behavior of the combined system.

An innovation developed specifically for this project is the custom in-house software created to model the unique connections of the cable truss where certain cables cross and slide while remaining connected, a release condition not found in existing commercial software. This feature manifests in the use of compact bronze slide-saddles at these cable crossings, resulting in distinctive details throughout the structure that accommodate the expected displacements without engaging the cladding.

The new roof dramatically reduces the energy usage of the Stadium. It no longer requires 24/7/365 continuous fan operation to keep the roof inflated, does not necessitate snow melt, and has dramatically reduced the heat loss and gain compared to the old airsupported roof.

It is a testament to the construction team and the design that the new roof was completed on time without the need to delay or postpone any scheduled Dome events. The finished structure efficiently overcomes complex challenges and now passively supports snow without the need for monitoring or intervention. It is a creative, innovative, and iconic addition to the University campus and the Syracuse skyline that will be utilized for many years.■ AUGUST 2022

43

NCSEA

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News from the National Council of Structural Engineers Associations

2022 PRE-SUMMIT EDUCATION ON 11/1 Structures Symposium Presented by SEAOI 8:00am to 4:30 pm The Structural Engineers Association of Illinois (SEAOI) is pleased to announce the 12th annual Structures Symposium, which provides a forum for engineers to share analysis, design, and construction information from recent projects with unique and/or distinguishing characteristics.

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The California Office of Emergency Services (Cal OES) Safety Assessment Program (SAP), hosted by NCSEA, is highly regarded as a standard for training emergency second responders. FEMA’s Office of Domestic Preparedness has reviewed and approved the training.

Atlas Tube Mill Tour and Discussion 11:00am to 3:00pm

Interested in learning how steel HSS is manufactured? Join us on a FREE tour of the Chicago Atlas Tube mill, the leading North American producer of HSS. Lunch and transportation to and from the Hilton Hotel Chicago will be provided.

Visit www.ncsea.com/education for the latest news on upcoming webinars and other virtual events.

Special Webinar Series - August 30 -31:

Ductility, Brittle Fracture

Hear from leading expert Duane Miller, P.E., as he discusses steel ductility and how to avoid brittle fractures on your projects.

First, understand the role of shear stresses and ductility for effective structural designs, especially in seismic conditions. Then, examine case studies of brittle fracture to avoid this problem in your designs. This web-based seminar will be delivered over two days in two 1.5-hour webinars by one of the industry’s best and brightest minds.

August 23, 2022

Frequently Asked Wind Questions Answered

September 13, 2022

What to Do When a Client Asks for a Resilient Building

September 27, 2022

Leveraging BIM & Technology for Structural Design

Purchase an NCSEA webinar subscription and get access to all the educational content you’ll ever need! Subscribers receive access to a full year’s worth of live NCSEA education webinars (25+) and a recorded library of past webinars (170+) – all developed by leading experts; available whenever, wherever you need them!

Courses award 1.0-2.0 hours of Diamond Review-approved continuing education after the completion a quiz.

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

AUGUST 2022

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SEI Update SEI Online

Civil Engineering Online How engineers combat conditions within the ‘red zone’ of global warming As climate change makes the world warmer, wildfires and heat waves are becoming more intense. Engineers are on the front lines to protect and preserve our threatened infrastructure. Read the article at https://bit.ly/3NZs4Qc.

The 2022 SEI Standards Series

Review ASCE 7-22, changes from ASCE 7-16, the Digital Products/Hazard Tool, and join the discussion with the expert standard developers. 1.5 PDHs per session.

• September 8, 2022: How & Why to Use ASCE 7-22 in Your Practice Join SEI host Jennifer Goupil, P.E., F.SEI, M.ASCE, for a discussion with guests Alexander Griffin, P.E., S.E. and Michelle L. Wilkinson, P.E., S.E., M.ASCE. Learn more and register https://collaborate.asce.org/integratedstructures/sei-standards.

Students and Young Professionals

Undergraduate Student Scholarships Available Learn about ASCE academic scholarship opportunities and apply at www.asce.org/career-growth/awards-and-honors/scholarships.

Learning / Networking

ASCE Week

Las Vegas, NV | September 25-30, 2022 • New lower pricing and livestream options • Earn over 40 PDHs Structural highlights: • Wind Loads for Buildings and Other Structures Using ASCE 7-22 Instructors: Timothy A. Reinhold, Ph.D., P.E., M.ASCE, and T. Eric Stafford, P.E., M.ASCE

go.asceweek.org

• Structural-Condition Assessment of Existing Structures Instructors: Brian K. Brashaw, Ph.D., Thomas M. Gorman, Ph.D., P.E., M.ASCE, Larry D. Olson, P.E., M.ASCE, Dennis A. Sack, P.E., and Steven Smith, CWI

Follow SEI on Social Media: 46 STRUCTURE magazine

News of the Structural Engineering Institute of ASCE Advancing the Profession

Congratulations to 2022 SEI and ASCE Structural Award Recipients Dennis L. Tewksbury Award Ron Klemencic, P.E., S.E., NAC, NAE, F.SEI, Dist.M.ASCE Gene Wilhoite Innovations in Transmission Line Engineering Award Meihuan Zhu Fulk, Ph.D., P.E., M.ASCE SEI Chapter of the Year Award SEI East Central Florida Chapter SEI Graduate Student Chapter of the Year Award SEI Graduate Student Chapter at University of Central Florida SEI President’s Award Matthew B. Kawczenski, P.E., F.SEI, M.ASCE Walter P. Moore, Jr. Award Otto Lynch, P.E., F.SEI, F.ASCE W. Gene Corley Award Chun Lau, P.E., S.E., P.Eng., F.SEI, F.ASCE Arthur M. Wellington Prize Sunyong Kim, Ph.D., M.ASCE; Baixue Ge; and Dan M. Frangopol, Sc.D., F.SEI, F.EMI, Dist.M.ASCE George Winter Award Edmond Saliklis, Ph.D., P.E., M.ASCE John E. Cermak Medal Timothy A. Reinhold, Ph.D., P.E., M.ASCE Moisseiff Award Liang Liu, Ph.D., Aff.M.ASCE; David Y. Yang, Ph.D., A.M.ASCE; and Dan Frangopol, Sc.D., P.E., F.SEI, F.EMI, Dist.M.ASCE

Nathan M. Newmark Medal Giovanni Solari, P.E., F.EMI, M.ASCE Raymond C. Reese Research Prize Andronikos Skiadopoulos, Ph.D., Aff.M.ASCE; Ahmed Elkady; Dimitrios G. Lignos, Ph.D., M.ASCE Shortridge Hardesty Award Sherif El-Tawil, Ph.D., P.E., F.SEI, F.ASCE T.Y. Lin Award Chungwook Sim, Ph.D., A.M.ASCE; Michael Asaad, P.E., Ph.D.; David Gee; and Maher Tadros, P.E., Ph.D., M.ASCE Walter L. Huber Civil Engineering Research Prize(s) Amir H. Gandomi, Ph.D., A.M.ASCE; and Gaurav Sant, Ph.D., A.M.ASCE O.H. Ammann Research Fellowship • Claire Gasser, S.M.ASCE, Texas A&M University • Eduardo Montalto, S.M.ASCE, University of California, Berkeley • Lily Polster, S.M.ASCE, University of Notre Dame • Babak Salarieh, Ph.D., A.M.ASCE, University of Alabama in Huntsville • Arman Tatar, S.M.ASCE, Michigan Technological University Learn more at www.asce.org/SEINews and nominate for 2023 Awards at www.asce.org/SEIAwards.

Award winners at Structures Congress

Errata

SEI Standards Supplements and Errata including ASCE 7. See www.asce.org/SEI. To submit errata, contact sei@asce.org.

AUGUST 2022

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CASE in Point Events for Structural Engineers ACEC Fall Conference

October 16 -19, 2022, Colorado Springs, Colorado Each year, ACEC sponsors two major national meetings: the Annual Convention and the Fall Conference. National meetings provide attendees an opportunity to obtain information about issues that affect the industry through informative education, networking, and exhibits.

The Coalition of Structural Engineers (CASE) hosts a roundtable at each meeting to discuss issues specific to the field of structural engineering. Hope to see you there!

Business of Structural Engineering Bootcamp: Claims and Contracts November 1, 2022, Chicago, IL The CASE Coalition and the National Council of Structural Engineers Associations (NCSEA) have teamed up to present the Business of Structural Engineering Bootcamp. This series is dedicated to helping you further develop your management and business skills, as well as how to apply those skills to your career to enhance your firm's profitability.

Structural Engineering Summit November 1- 4, 2022, Chicago, IL

The National Council of Structural Engineers Associations (NCSEA) hosts an annual Summit that offers education, networking events, and expert speakers. As part of the summit, CASE and NCSEA have teamed up to present the Business of Structural Engineering Bootcamp.

This series is dedicated to helping you further develop your management and business skills, as well as how to apply those skills to your career to enhance your firm's profitability. CASE members receive a discount on registration! www.ncseasummit.com/registration

Invest in the future of our workforce.

Donate to the CASE scholarship fund!

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!

Donate today: www.acecresearchinstitute.org/scholarships

Announcing 2022 CASE Scholarship Winner Congratulations to the 2022 CASE Scholarship winner, Taylor Drahota from the University of Nebraska-Lincoln!

View information about the scholarship and winners at acec.org under “Awards.”

Get to Know the Executive Committee

Before the ACEC Annual Convention in May, members of the CASE Coalition elected the following members of the Executive Committee to serve the next term.

Kevin Chamberlain DeStefano & Chamberlain Chair

Jeff Morrison Lynch Mykins Structural Engineers Member at Large

Roger Parra Degenkolb Engineers Toolkit Committee

Bruce Burt Ruby + Associates, Inc. Chair-Elect, Contracts Committee

Justin Naser ARW Engineers Programs & Communications Committee

Brent White ARW Engineers Past Chair

48 STRUCTURE magazine

Tina Wyffels BKBM Engineers National Guidelines Committee

News of the Coalition of American Structural Engineers CASE Tools and Resources Did You Know?

CASE has tools and practice guidelines to help firms deal with a wide variety of business scenarios that structural engineering firms face daily. Whether your firm needs to establish a new Quality Assurance Program, update its risk management program, keep track of the skills engineers are learning at each level of experience, or need a sample contract document – CASE has the tools you need!

Check out some of the CASE Contract Documents developed by the Contracts Committee…

CASE #1 – An Agreement for the Provision of Limited Professional Services.

This is a sample agreement for small projects or investigations of limited scope and time duration. It contains the essentials of a good agreement including scope of services, fee arrangement, and terms and conditions.

CASE #2 – An Agreement Between Client and Structural Engineer of Record for Professional Services.

This agreement form may be used when the client, e.g., owner, contractor developer, etc., wishes to directly retain the Structural Engineer of Record. The contract contains an easy-to-understand matrix of services that will simplify the “what’s included and what’s not” questions in negotiations with a prospective client. This agreement may also be used with a client who is an architect when the architectowner agreement is not an AIA agreement.

CASE #9 – An Agreement Between Structural Engineer of Record and Consulting Design Professional for Service.

The Structural Engineer of Record, when serving in the role of Prime Design Professional or as a Consultant, may find it necessary to retain the services of a sub-consultant or architect. This agreement provides a form that outlines the services and requirements in a matrix so that the services of the sub-consultant may be readily defined and understood.

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

If you are a member of CASE, this tool and all publications are free to you. NCSEA and SEI members receive a discount on publications.

Use discount code – NCSEASEI2022 when you check out.

Get to Know the CASE Committees The CASE Coalition has several committees that meet regularly to develop documents that help guide engineers in their business practice. The work of these committees is an important part of what coalitions do and are one of the biggest values of CASE membership. The Guidelines Committee is responsible for developing and maintaining national guidelines of practice for structural engineers.

This Month, the Guidelines Committee is working on a peer review of guideline documents to be updated or published later this year.

We are currently seeking two to four new members to join the Guidelines Committee. Do you know someone in your firm that is looking for ways to expand and strengthen their business skillset, gain experience serving on a committee, sharpen their leadership skills, and travel to interesting places? Please consider applying for a position on the committee. Committee member commitments include a monthly virtual meeting, a few hours a month working on relevant documents, and travel to the Coalitions winter and summer meetings!

To apply, your firm should: • Be a current member of ACEC • Be a member of the Coalition of American Structural Engineers (CASE); or be willing to join the Coalition • Be able to attend the groups’ regular face-to-face meetings each year: August, February (hotel, travel partially reimbursable) • Be available to engage with the committees via email and video/conference call • Have some specific experience and/or expertise to contribute to the group AUGUST 2022

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SPOTLIGHT Pavilion in the Park

T

he DC Southwest Library is a beautiful, natural, and functional aesthetic solution for the community’s new neighborhood library. The Library elegantly addresses its unique site and connection to the adjacent park. It showcases the world’s first self-supporting timber folded plate roof, using Dowel Laminated Timber (DLT). This, along with other crucial sustainability protocols that influenced the design and construction of the structure, contributed to the LEED Platinum accreditation bestowed on the Library.

The new library was designed as a “lowcarbon” heavy timber structural system composed of exposed wood columns, beams, and dowel-laminated floors and roof panels. The library is the first building in the Washington, D.C. area to use this unique mass timber system, setting it apart within the neighborhood and the city.

History DC Southwest Library honors the MidCentury Modern architecture and mass timber construction for which this neighborhood is known. The design drew inspiration from another community landmark, the nearby Arena Stage. The exposed timber and its resulting form, along with the textured charcoal brick façade, make the building a memorable and unique addition to the District and neighborhood architecture. The crinkle folded plate bears similarities to historical cast-in-place concrete roof outlines of the 1950s but without the burden of framework and with a nod towards modern connection technology and low-carbon materials. The result is a series of dowel-laminated wood panels that create a unique building massing and a memorable space within.

Challenges The entire library superstructure is mass timber, with glulam beams and columns and DLT floor and roof slabs. The city building officials needed to be introduced to and educated about this new form of construction before acceptance.

This crinkled roof presented engineering challenges in how it would be detailed, fabricated, installed, and erected. Connection detailing, prefabrication, the assembly jig, and erection planning were keys to its speed 50 STRUCTURE magazine

StructureCraft was an Outstanding Award Winner for the DC Southwest Library Project in the 2021 Annual NCSEA Excellence in Structural Engineering Awards Program in the Category – New Buildings under $30M.

of erection and successful installation. The unique shape of the long-span folded plate roofs created a challenge for the structural engineering and construction of these complex elements. The use of dowel laminated timber in a folded-plate structure was a world first. It required complex nonlinear finite element analysis to predict the folded plate’s stresses and structural behavior.

Structural engineering utilized Rhino, hsbCAD, and Revit for the design, engineering, fabrication, and production of the glulam beams, columns, DLT panels, and steel plate connections. The rest of the design team worked inside these models to ensure all elements of the structure were coordinated well. By the time fabrication began, the structural model was highly detailed.

Building Information Modeling Building Information Modeling (BIM) allows for a 3-D fabrication model with a high level of detail to be used by both designers and constructors – the onsite team used it extensively.

Also, BIM allows proactive clash detection and penetration coordination amongst all trades, which is particularly important in a prefabricated, highly exposed timber structure.

The BIM model drove the manufacturing of Glulam, DLT, and steel and produced detailed piece shop drawings for each element. Small chamfers were added to the edges of the half

modules fabricated in the shop to ensure the elements remained within legal width limits for trucking cross-continent.

Fabrication The folded plates consisted of DLT panels sheathed in glued plywood to create the necessary diaphragm stiffness. The plywood was in turn fastened and glued in the StructureCraft shop to the folded plate glulam chords at the ridge and trough. For shipping, the chords were split in half, and full folded plate “trusses” were assembled, along with tension rods connecting the four trough corners, to allow the erection of each 65-foot-long x 20-foot-wide folded plate structure on site.

Erection The erection of the timber structure was rapid, taking a small crew of timber specialists only a matter of weeks to erect the glulam beams and columns, DLT floor slabs, and all sections of the DLT folded plate roof. Because of the extensive prefabrication, all the complex work was done off-site, and elements were shipped ready to “click into place.”

From initial form and design to engineering, prefabrication, and installation, the entire team came together around this complex challenge and delivered successfully on the unique architectural form.■ AUGUST 2022

THE MOST SUSTAINABLE HSS AVAILABLE

Sustainability drives decision-making more than ever before. Nucor Tubular Products is your partner for achieving your sustainability goals and bringing your projects to life. And because all of our steel is made with electric arc furnace (EAF) technology, our HSS is the greenest produced in North America.

Whether you need HSS for a long span truss or a handrail, take your project to the next level with Nucor Tubular Products.

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