STRUCTURE magazine October 2019

STRUCTURE OCTOBER 2019

NCSEA | CASE | SEI

Bridges

INSIDE: The Edlangeni Footbridge

26

FRP Composites for Bridges Broadway Bridge Replacement Penn Street Viaduct

8 22 32

2019 NCSEA SUMMIT – Anaheim, CA • NOV 12-15

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

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

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

Tekla Structural Design at Work: The Hub on Causeway

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

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

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

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

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

One Model for Structural Analysis & Design

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

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

TRANSFORMING THE WAY THE WORLD WORKS

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

Efficient, Accurate Loading and Analysis

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

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

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

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

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

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

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

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

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

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

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

Contents O CTO BER 2019

Cover Feature

26 THE EDLANGENI FOOTBRIDGE

By Jay Arehart

The 200 families that make up the Edlangeni community worked with students from the University of Colorado Boulder and the Engineers in Action Bridge Program to construct a 240-foot pedestrian suspended bridge.

Features 22 CONSTRUCTION ENGINEERING FOR BROADWAY BRIDGE REPLACEMENT

Columns and Departments 7

By David Byers, Ph.D., P.E., John Boschert, P.E., S.E., and Paul D. Scharmer, P.E.

The Broadway Bridge was replaced using an incentivized bidding process to minimize the closure durations. By pre-erecting two 440foot tied arch spans, the total roadway closure was 152 days.

8

The Penn Street Viaduct project, the oldest bridge of its kind in

11

respecting its historic integrity.

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

Construction – Part 5

16

By Scott Reeve and Andy Loff

Structural Design

Parking Structures and the Northridge Earthquake

Structural Specifications Flexural Strength of Extruded Aluminum Mullions

glass structure and façade system for the Jewel Changi Airport, Singapore, an elaborate, toroid-shaped gridshell enclosure.

By J. Randolph Kissell, P.E.,

40 MAUI’S KAHULUI AIRPORT CONRAC

and James LaBelle, P.E.

By Craig Meierhoffer, P.E., S.E., and Mark Hirschi, S.E.

20

Historic Structures

columns supporting post-tensioned beams and girders that span

Gasconade Bridge Failure 1855

60 feet by 40 feet.

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

end. The main structure’s cast-in-place concrete structure included

and Michael Mota, Ph.D., P.E., SECB

Northridge – 25 Years Later Concrete

and Jennifer Acton, P.E.

The authors discuss the development and design for the steel and

The new ConRAC facility is nearly a quarter-mile from end to

By David A. Fanella, Ph.D., S.E., P.E.,

Building Blocks 48

By Nathan C. Gould, D.Sc., S.E., Mikael K. Kallros, S.E., and Susan M. Dowty, S.E.

By Jeremy Salmon, P.E., S.E.,

36 ENGINEERING AN ICON

Construction Issues Recommended Details for Reinforced Concrete

Designing for Future Expansion

Berks County, rehabilitated the bridge to extend its useful life while

By Cristobal Correa, P.E., and Craig Schwitter, P.E.

44

Technology is not a Substitute for Experience

Using FRP Composites for Bridges and Bridge Decks

32 RESTORING THE HISTORIC GATEWAY TO READING, PA

By Kamlesh Ashar, P.E., Brian Teles, P.E., and Michael Urban, P.E.

Editorial

58

Structural Forum Is Modular a Good Fit for You?

By Taeko-Karyn Takagi and Wally Naylor

In Every Issue 4 Advertiser Index 50 Resource Guide – Seismic/Wind 52 NCSEA News 54 SEI Update 56 CASE in Point

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

O C T O B E R 2 019

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EDITORIAL Technology is not a Substitute for Experience

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

O

ur firm is fortunate to be celebrating our 50th anniversary this year. We have grown from a one-person firm to a dedicated staff of over 30. That growth has not been exponential, but it has been steady and consistent. Recently, while preparing a presentation for our employees about our 50-year history, I had the chance to reminisce about some of the successes and challenges since 1969. I have been around for 36 of those 50 years, so I have some knowledge of our history, but I was reminded of some of the work our firm did in those earlier years. The founder of our firm still visits with us occasionally and attends company social functions, so we still have a connection to our founding, and it is great to have that connection.

As I have reviewed files and photos dating back to our founding, I am amazed by the projects that were undertaken and completed from that very first year. From the perspective of a structural engineer today, some of those projects are even more amazing considering the tools that an engineer today takes for granted and expects to be available. 50 years is insignificant when considering engineering projects accomplished throughout history. The tools available today have grown exponentially in sophistication, reasonable availability, and actual use by today’s engineer. From that perspective, knowing that projects completed 50 years ago in the average engineering office were completed using slide rules, pencil and calculation pad, and hand drafting is a bit awe-inspiring.

I have never used a slide rule. As a college freshman, I entered school the first year that students were required to have a scientific calculator.

By the way – those calculators cost a fortune when compared to the much more sophisticated versions available today.

My brief historical review reminded me of the concerns I have regarding engineering design in today’s environment. In a competitive, capitalist environment, the efficient use of available tools is essential. It would be impossible to compete and provide the expected engineering services if we were all still using slide rules and drafting tables. However, I am concerned that we are losing, or may have lost, something along the way. Using those early tools required an engineer to develop a rapid understanding of not only the necessary engineering principles but a feel for how things really worked. Not only having a general feel for if the calculation is correct, but is the solution reasonable? The founder of our firm, acting as a mentor, always reminded me and others to “put on your contractor’s hat” as we designed structural systems and elements, especially as we prepared construction documents and details.

With the prevalence of useful engineering programs, computers, BIM, and other tools, do the engineers of today develop the same sense of what is correct and what is actually doable as was the case with our predecessors? The likely answer to this question causes me serious concern. We can and should develop a sense that the calculations and designs we complete are correct and right using all of these modern tools.

Precision does not guarantee accuracy. While attending university, I had a professor that confused many students when he said the only digit STRUCTURE magazine

after a decimal when completing a calculation is 7. This was his way of helping us understand that there is always a certain level of inaccuracy or unknown in calculations.

When we begin calculations with established live loads that may or may not be actual, and dead loads that are determined to the best of our knowledge and then add a little extra, completing calculations to three decimals does not make sense. Computers do not understand this situation. Therefore, it is easy to assume that precision guarantees accuracy and that accuracy is actual knowledge to be utilized without question. I have reviewed structural elements and systems designs with less experienced engineers and within a few seconds point out that something isn’t correct.

“How do you know? You didn’t do the calculation.” My response is, I just know; experience tells me something is out of place. Developing this experience base is essential. We must not allow technology to prevent the development of this essential experience.

Just because you can does not mean you should. The engineers in our office have heard me say this many times. Considering the tools mentioned previously and the presumed ease of using those tools, it is easy to fall into a trap assuming they must be used for everything.

There are times when wl2/8 or wl2/2 and the steel manual is all you need – and faster. Culturally it seems that, as engineers, we tend to be developing the idea that everything needs to be “modeled.” For complex and sophisticated analysis, this is the case; but in my opinion, it is used more than necessary and may be preventing the development of the “feel” for correct answers. Just because the computer is sitting there, and software is available, does not mean it should be used in every instance.

Know when it is good enough. Related to the accuracy discussion above, understanding when solutions are adequate based on our understanding, as well as schedule and budget constraints, is essential.

Almost any problem can be refined indefinitely without providing additional value. Obviously, there are complex solutions to complex problems that require iterative design processes. Complexity, however, is not generally true. This also ties into the idea of complicating the analysis and design more than necessary.

The technology available to us today as structural engineers is wonderful and exciting. I do not want to be misunderstood that I may think it is not important, necessary, and essential. However, just as essential is developing our experience base and understanding so that we are not entirely dependent on a computer program as engineering professionals.

With all that said, I shared this not because it is precise or accurate. I did it because I can, and it is good enough.■ Brent L. White is President of ARW Engineers. He is also Past President of the Structural Engineers Association of Utah and current chair of the CASE Toolkit Committee.

O C T O B E R 2 019

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building BLOCKS Using FRP Composites for Bridges and Bridge Decks A Basic Primer

By Scott Reeve and Andy Loff

F

iber-reinforced polymer (FRP) composites are engineered to resist corrosion and provide high strength-to-weight and mod-

ulus-to-weight ratios compared to steel and concrete. In simplest terms, a composite material is produced when two or more substances are combined to take advantage of their unique strengths while overcoming their weaknesses. The first use of composite materials can be traced back to 1500 B.C. where Egyptians and Mesopotamians mixed mud and straw to create strong buildings.

FRP cross-section.

In the 1900s, the development of plastics birthed synthetic resins. Owens Corning introduced the first glass fiber combined with polymer resins. Following WWII, composites made their way into cars, airplanes, and boats, among other things.

When it comes to construction applications, FRP has mostly been confined to piping and tanks for corrosive environments. The last 15 years have seen this trend begin to shift along with the development of a modern polymer matrix material reinforced with fibers. Several factors have influenced these changes.

Corroding concrete and steel, the need for lighter weight, shorter installation times, and reduced maintenance costs have prompted the adoption of FRP for mainstream infrastructure applications like bridges and bridge decks. Engineers tasked to select building materials have to make sure their choices will benefit construction and installation crews, as well as the end-user. The ability to mix the right combination of fiberglass fabric, core material, and resin means FRP products can be tailored to the requirements of individual bridge projects, for example.

FRP bridge elements is fiberglass. There is a big difference between fiberglass and carbon fiber in terms of properties and cost. For example, say you have a building in an earthquake zone that is held up by concrete columns. These columns are wrapped with carbon fiber to hold them in place. For a full bridge deck, the carbon cost would be prohibitive. Carbon fiber is roughly 15 times the cost of fiberglass FRP. In terms of weight, the difference between the two materials is minor. On an aircraft where weight is critical, the customer is willing to pay the higher cost for carbon fiber. However, for bridges and bridge decks, fiberglass composites are already five times lighter than conventional materials. Typically, owners are not willing to pay the higher price for carbon fiber when they are already getting significant weight savings with fiberglass FRP. So how does an engineer who is new to FRP begin the design process for a bridge or bridge deck application? Start with design requirements.

Fiberglass and Carbon

Design Requirements

Carbon fiber wrap is considered the standard choice for repairing steel and concrete structures because it provides high stiffness for applications that require bonding these materials together. In new bridge and bridge deck construction, carbon wrap does not deliver the same pay off because decks and bridges do not work compositely with steel and FRP components are prefabricated.

Engineers familiarizing themselves with composites are often asked to evaluate whether the reinforcing fiber for a project should be fiberglass or carbon. The reinforcement material for prefabricated

Deck design loads are based on AASHTO LRFD Bridge Design Specification (6th Edition) and the LRFD Guide Specification for the Design of Pedestrian Bridges (Dec. 2009).

Typical pedestrian bridge deck requirements call for an unfactored uniform live load of 90 psf pedestrian loading. Midspan deflection is limited to the support span length divided by 500 (L/500). Vehicle maximum loading is rated at H-5 for a rear axle load of 8,000 pounds and a wheel load of 4,000 pounds to consider maintenance vehicles. Midspan deflection under the vehicle is limited

8 STRUCTURE magazine

Fiberglass fabric is rolled out into the molding tool in the required direction, and a Fiber-reinforced foam core is placed before vacuum infusion.

to L/300, and the mid-span deflection of a superstructure is limited to L/360. The uplift load is rated at 30 psf, with a minimum crushing strength of 150 psi.

Although the supplier provides written specifications that define loads, an engineer also needs to think about how FRP will respond. Unlike concrete, engineers are not working from a standard base of design. In the case of FRP, agencies like the Department of Transportation (DOT) issue structural requirements while the FRP supplier submits plans on how to Example material properties determined during testing. build it. The FRP supplier is also tasked with the requirement to show compliance and demonstrate that the material can handle loads with adequate safety factors. This of 4 per AASHTO’s Guide Specification for Design of FRP Pedestrian is accomplished through design calculations and shop drawings.

Bridges. These safety factors are applied after statistical and environFor engineers who want to know how FRP is used compositely with mental reductions. Since FRP has a lower stiffness than steel and steel, the answer is short and straightforward. The FRP supplier does reinforced concrete, deflection tends to drive the design of FRP not design for composite action. The stiffness and mass of the two panels and generates higher strength safety factors of more than 10. materials are too different, and connection methods for composite Sandwich construction is used for FRP bridge decking because it action require tight tolerances. An FRP deck transfers the live loads provides the most efficient structural configuration for applications to the support structure (steel beams or concrete piers), and the that require high stiffness and low weight. Multiple internal shear webs superstructure transfers the load to the ground. are molded with top and bottom face sheets to create a redundant There are other types of forces and stresses to consider, the largest being pattern of “I-beams.” This anatomy is what allows panel depth and bolted connections. Wind shear is the other critical design criteria. laminate thicknesses to be tailored to a project’s requirements while Typically, wind uplift and seismic load are accounted for during the maintaining light weight. design phase for bridge and bridge deck connections. Wind shear is a The structural calculations used in traditional beam equations to component used to ensure a bridge or bridge deck’s safety factors are met. generate a design’s overall safety factors starts with FRP properties Since FRP materials are linear elastic to failure and do not yield like and continues with strength. steel, the strength safety factors for FRP need to be higher than steel.

It is important to remember that the physical properties of FRP Safety factors on bending and shear capacity should be a minimum structures can be directionalized based on a ‘rule of mixture’ approach.

continued on next page

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Example wind uplift and connection calculation.

What you add in one direction takes away from the other. Almost all structures should have some fibers in the 0°, 45°, 90° or -45° directions.

This provides basic fiber strength in all directions of a panel.

Standard test methods for FRP include ASTM D3039 for tensile strength and modulus; ASTM D6641 for compression strength and modulus; ASTM D7078 for in-plane shear testing; ASTM D732 for punch through shear testing; ASTM D5961 for pin bearing testing; ISO MAT-2209 for thermal expansion; and ASTM D5868 for adhesive lap shear.

Following material testing, other factors include material scatter that occurs during testing. To account for this, a statistical reduction factor based on ASTM D7290 is applied to strength values only, not to the modulus. Environmental reduction factors are then applied to account for long term exposure to saltwater, ultraviolet light, and thermal cycling. Since FRP does not show a decrease in properties even with high fatigue cycles, there are no reduction factors for fatigue loading.

Bridge elements have been tested to 2,000,000 cycles. The resulting design properties are then carried forward into the calculations.

Design Calculations The primary design calculations performed for FRP bridge decking include: • Panel analysis for uniform and concentrated loadings o Deflection o Bending Stress o Shear Stress • Deck-to-support connection analysis o Wind uplift o Shear transfer between stringers • Thermal expansion analysis • Railing connection Like traditional materials, engineers can calculate section properties and moment capacities with material design properties and geometry. The most complicated component of FRP deck design is determining effective deck width with concentrated or vehicle loading applied. There is no standard or rule of thumb for this because of the variations in deck design. The effective width can range from 40 to 90 percent of the support span. This variation is based on the internal structure of the FRP deck section. Unidirectional designs (in that they only have shear webs going in the primary load direction) tend to have effective widths closer to 40%, while bi-directional structures (in that they have shear webs in both directions) tend to have effective widths closer to 90%. There are two ways to determine the effective width of a design: FEA analysis or full-scale testing.

10 STRUCTURE magazine

Connection holes are drilled and tapped in panels before shipping. On-site, the contractor screws the bolts through the clip plates to capture the steel superstructure.

Deflection is another critical design requirement for FRP decks. The design engineer must properly account for the loading and span conditions and verify that calculated deflections fall within project specifications.

Connections Generally, FRP decks are joined to superstructure supports using mechanically fastened clips that capture the underside of the beam flange. This type of connection restrains the deck for vertical loads, both live and wind uplift, while allowing for construction tolerances and thermal expansion variables. This connection does not provide composite action of the deck and beams as typically happens with concrete decking. The mass of the FRP is so low that it is not worth using a more complicated and costly connection detail to gain only a slight benefit of composite action.

An FRP fabricator should design for mechanical connections analytically but test for capacity. Clips are sized for compaction against the beam flange. Steel plates are embedded in the deck to receive the bolts and handle the concentrated loads.

The light weight of FRP decking makes certain calculations more critical. The uplift load determines the number of connections needed to keep the deck attached. Seismic and vibration concerns are less critical because of the material’s light weight.

Sandwich construction means that compression (crushing) loads and shear loads from concentrated loads must be evaluated. Concentrated loads are typically vehicle loads generated by a vehicle or truck’s wheels on the bridge. In the case of a pedestrian span, compression or crushing load could refer to a maintenance vehicle. Wider pedestrian bridges must handle either maintenance vehicles or emergency vehicles.

When it comes to crushing capacity, one option is to design for a 4,000-pound wheel load over a 10-inch by 10-inch wheel area, for 40 psi crushing force. The engineer then specifies a panel with a 160-psi crushing strength for a 4 times factor of safety.

FRP bridge decks (and other elements) can be modified in the field.

Threaded holes can be tapped, or holes can be drilled, and pieces cut.

Upfront engineering coordination and prefabrication minimizes the potential for field issues. It is another advantage of working with FRP. Prefabrication saves the owner and contractor time and money.■ Scott Reeve is President and Founder of Composite Advantage. (sreeve@compositeadvantage.com) Andrew Loff is the Chief Technology Officer of Composite Advantage. (aloff@compositeadvantage.com)

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structural DESIGN Designing for Future Expansion

By Jeremy Salmon, P.E., S.E., and Jennifer Acton, P.E.

H

ow many times have you designed an addition to an existing building and wondered, “What was the engineer thinking?�

Many types of structures are designed for future expansion in anticipation of the ever-changing needs of the owner and community.

Hospitals are frequently designed for additional floors. Structural engineers play an essential role in designing and detailing the structure to accommodate future expansion, as well as helping the owner and design team plan ahead for the intended use and functionality of the additional floors.

Building Codes The city, county, or state in which a building resides specifies the building code used by the structural engineer. The local jurisdiction may enforce an older version of the International Building Code (IBC), even though a more recent edition has been published. When designing for future expansion, the most recent edition of the IBC and material codes should be consulted, as well as the version adopted by the authority having jurisdiction.

Most of the codes used in design publish a draft version for public review before being formally issued. Draft versions of wind speed and seismic hazard maps reveal if the wind pressures and spectral response accelerations will increase or decrease in the next code cycle.

Predicting unwritten code changes is unreasonable. However, owners should not think they are paying for a building designed for multiple floors, only to find out a few years later that not all of the future floors can be added because the engineers did not consider the most recent codes available at the time.

Figure 1. Cap plate detail at future steel column.

be kept open or if knock-out slabs will be used so the spaces can be used for storage or other purposes.

Medical office buildings are frequently located adjacent to a hospital. Office buildings may be a business occupancy, Type IIA or IIB construction type, and be classified as Risk Category II in which case a firewall is sometimes needed. The neighboring hospital is commonly institutional occupancy, Type 1A construction, and Risk Category III or IV. If hospital programming might spill over into the medical office building in the future, the office building should be designed for the same construction type, occupancy, and risk category as the hospital.

When planning for future expansion, the architect may have renderings depicting the final height and look of the building. The architect needs to explicitly define the future floor heights, cladding, and which stairs or elevators will need to serve the roof. Parapets and screenwalls

Design Team Coordination During design, the architect, mechanical engineer, and other consultants typically focus on the building at hand and not the future expansion of the building. For the structural engineer, the number and extent of future floors are just the beginning of the information needed to provide a structure capable of handling a proposed future addition.

The architect must provide guidance regarding the use of future floors. If the current roof/future floor will be used for operating rooms, check the current roof for sensitive equipment vibration criteria. When future floors require a large column-free space (e.g. ballroom), design the appropriate columns and foundation for a larger tributary area. If future floors require thick-set tile, identify the locations and depths of any slab recesses. If additional stairs or elevators will be added with the future floors, decide if they will be inboard or outboard of the building. If they are located within the footprint of the current building, discuss whether the spaces will

Figure 2. A section at future concrete column.

O C T O B E R 2 019

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can add a significant amount of wind load to the building. They should be discussed with the owner and architect to ensure the lateral load-resisting system is designed properly.

In high-rise construction (buildings with occupied space over 75 feet above the lowest level with fire truck access), the stairs must be pressurized per IBC 403.5.4. A building without the future floors may not be classified as high-rise. However, stair pressurization may be required after the structure is expanded vertically. During the original design, space should be allocated on each floor for the stair pressurization chases and not treated as an afterthought when the future floors are added.

Elevator hoistway sizes and pit depths should accommodate both the present and future travel distance, weight, and speed requirements.

The roof framing above the elevators should be laid out to match the typical floor framing; a knock-out slab should be provided so the contractor can easily saw cut the slab to match the hoistway size below and extend the elevator vertically. If a doghouse is required in the elevator penthouse to meet the elevator overrun requirements, the penthouse roof should be designed for the same loading as the penthouse floor; only one floor may be added in the future even if the building is designed for multiple additional levels.

The mechanical design of the built-out project has structural implications that need to be addressed in the original design. Will there be a mechanical room on each floor? Will there be a mechanical penthouse or rooftop units on the future roof? The size, location, and anticipated equipment weights are required for the column and foundation gravity design and the seismic lateral design. Furthermore, rooftop equipment and whether it is screened can have a significant effect on the lateral design due to increased wind loading.

Figure 3. A section at future concrete-encased steel column.

The roof beams of a mechanical penthouse may need to be designed for the same equipment loading if the penthouse moves up in the future instead of staying in place. Knock-out slabs may be needed at future mechanical chases. The phasing of the future construction must be considered as well. A rooftop unit should not be located over one of the building columns because vertically extending the structure will require shutting down and removing the unit.

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Special Detailing

Figure 4. Framing plan depicting future beam layout.

Design Considerations Once the scope of the future expansion has been finalized with the design team, the structure can be properly designed and detailed. For the sake of the owner and subsequent design team (hopefully you!), the structural drawings should clearly depict the future expansion capabilities of the structure via a future key plan and supplemental notes. On a recent healthcare facility in Florida, the authors designed a one-story portion of a hospital for six future floors. The design team allocated space for a mechanical room on each floor, and the columns and foundations were designed accordingly. The future key plan on the structural drawings clearly indicated the locations of those future mechanical rooms in addition to the extent of the expansion.

The design of the current roof structure needs to consider both the present and future conditions; the future use of the framing as a floor does not always govern the design. Designing roof framing for snowdrifts is one example. A one-story building that is designed for only a portion of the footprint to expand vertically will create snowdrifts on the roof areas that have no potential future expansion.

On a project the authors designed in Alaska, a one-story building was designed for one future floor. The design of the roof beams was controlled in some areas by large snowdrifts adjacent to stair and mechanical penthouses in the present case. Similarly, when a roof level has mechanical equipment and screenwalls in the present case, the design of the structure considering only the future floor loading may be inadequate.

When designing the exterior wall studs or furnishing exterior wall component and cladding wind pressures for the stud supplier, the wind pressures should be based on the future building height. Exterior wall studs designed for just the current building height may be inadequate for the higher leeward wind pressures after the additional floors have been added.

The topped-out structure will control the design of most of the lateral system components. However, there are instances when the present case impacts lateral design. The chord and collector diaphragm forces at the current roof in the present model may be larger than the forces at that level when it is an intermediate floor in the future model due to a different distribution in lateral forces. There may be a torsional irregularity in the present case that is not apparent in the built-out model but would warrant using modal response spectrum analysis.

Columns designed for future expansion are typically detailed to extend above the top of the roof slab. Column extensions of just a couple of inches will be contained within the roofing material and will not be visible. Extending the column in the future will require disturbing the roofing at each column location. Column stubs that extend 18 to 24 inches above the roof slab will not compromise the roofing system during future construction but will be visible.

For steel columns, a cap plate is usually placed on top of the column to allow for tolerances during the erection of the future column (Figure 1, page 13).

For projects that must comply with AISC 341, Seismic Provisions for Structural Steel Buildings, Section D.2.5 requires all column splices be located four feet away from the beam-to-column flange connection. One of the exceptions allows the splice to be located closer if the column splices are made with complete penetration welds, but cannot be less than the depth of the column.

Concrete structures can be designed for future floors of either structural steel or concrete. For future concrete floors, mechanical couplers can be used at the top of the column, as shown in Figure 2 (page 13).

The roof of a concrete structure is typically not designed for shoring loads, and re-shores are not permitted because the uppermost floor of the building is occupied. Therefore, the future formwork will need to span between the future columns after they are placed. Alternatively, concrete structures can be designed to expand vertically in structural steel. The size of the future steel base plate relative to the concrete column and how the lateral loads are transferred between the steel and concrete structures will determine the column detail. At moment frames, it may be beneficial to encase the first lift of steel columns in concrete and locate the anchor rods inboard between the column flanges as depicted in Figure 3.

The detailing of the cladding must consider future expansion. The head of a curtainwall system is often located below the roof slab and metal studs used to form a parapet. If the curtainwall system extends vertically in the future, the slab edge condition that suited the metal stud parapet may be incompatible with the curtainwall.

When the stud parapet is removed, the size of the curtainwall may require the slab edge to be sawcut to allow the curtainwall to bypass the slab edge.

Ideally, penthouses will be extensions of the building columns and not require cantilevered roof framing or stub columns. For cantilevers or stub columns, the drawings should clearly denote how the roof framing was designed. For example, a cantilevered penthouse roof beam could either be designed for a future point load from a new shear connection, designed to be cut and removed, or it could be complete penetration-welded to a new beam and become continuous. A simple diagram on the drawings can provide the design intent as depicted in Figure 4. Similarly, stub columns used to support penthouse framing can either be designed to remain or be removed in the future.

Successful Expansions Thoughtfully addressing the design and detailing of structures with future expansion capability increases the likelihood of a successful addition down the road.â– Jeremy Salmon is a Principal at Structural Design Group in Nashville, TN. (jeremys@sdg-structure.com) Jennifer Acton is an Engineer with Structural Design Group in Nashville, TN. (jennifera@sdg-structure.com)

O C T O B E R 2 019

15

structural SPECIFICATIONS Flexural Strength of Extruded Aluminum Mullions

By J. Randolph Kissell, P.E., and James LaBelle, P.E., Doc.E.

C

urtain walls use extruded aluminum mullions to support glass lites, which are subjected to wind pressures acting inward and outward on the building exterior (Figure 1). The primary loading on mullions is flexure, but determining their flexural strength is complicated by several factors: • Mullions are usually unsymmetric about their bending axis.

• Mullions are often composed of an assembly that employs a snap-in piece.

• Mullions often use a thermal break, resulting in a composite beam.

• Glass attached to the mullions partially restrains them against twisting or lateral movement.

• Transverse loads are usually not applied at the mullion’s shear center.

• Either extreme fiber of the mullions may be in compression.

Consequently, approximate methods have been devised to predict flexural strengths, but approaches vary widely, and testing is often required. The 2015 and 2018 editions of the International Building Code (IBC) require that aluminum construction complies with the 2015 edition of the Aluminum Association’s Aluminum Design Manual, Part I – Specification for Aluminum Structures, which addresses flexural strength of aluminum extrusions; however, many designers have difficulty applying the Specification’s provisions to mullions. This article discusses the Specification’s provisions for mullions’ flexural strength. References below are to the Specification.

Mullions are usually 6xxx series aluminum alloy extrusions, often 6063 in T5 or T6 temper. For 6063-T6, the Specification gives the minimum tensile yield strength Fty as 25 ksi, minimum tensile ultimate strength Ftu as 30 ksi, and modulus of elasticity E as 10,100 ksi. Thus, the mullion material has slightly less strength and considerably less stiffness than mild carbon steel.

Figure 2. Local buckling of a square tube.

16 STRUCTURE magazine

Chapter F identifies four flexural strength limit states listed in Table 1.

Allowable Strength Design (ASD) or Load and Resistance Factor Design (LRFD) Figure 1. Extruded aluminum mullion assembly methods may be used without and with thermal break. to determine the available strength; the nominal strength is the same for both. The available flexural strength is the least of the available strengths of the limit states.

As Table 1 shows, Section F.4.1 prescribes safety factors Ω of 1.95 for rupture and 1.65 for all other flexural limit states, and resistance factors φ of 0.75 for rupture and 0.90 for all other flexural limit states.

Nominal strengths are divided by the safety factor or multiplied by the resistance factor to determine the available strength.

Rupture and Yielding Section F.2 addresses rupture and yielding. The yielding and rupture limit state strengths (the plastic moment Mnp = ZFty and the ultimate moment Mnu = ZFtu /kt , respectively) are straightforward to determine once the plastic modulus Z is calculated (kt is a factor greater than or equal to 1 that accounts for notch sensitivity of certain aluminum alloys).

For ASD with 6063-T6, Fty /1.65 = 15.2 ksi and Ftu /1.95 = 15.4 ksi, so ASD available strength is governed by the yield limit state. For LRFD, 0.9Fty = 22.5 ksi and 0.75Ftu = 22.5 ksi, so available strengths for rupture and yielding are equal. Because the lateral-torsional buckling strength is limited to the plastic moment, the plastic moment affects the lateral-torsional buckling strength.

Local Buckling

Table 1. Flexural strength limit states.

Limit state strength Section F.3 addresses local buckling (buckling of affected by which elements of the cross-section). Figure 2 shows local ASD safety LRFD resistance flange is in buckling of a square tube bent about a diagonal axis Limit State factor Ω factor φ compression? with the upper sides in compression.

Rupture 1.95 0.75 no Specification Section B.5.4 provides local buckling strengths for elements in uniform compression and Yielding 1.65 0.90 no Section B.5.5 for elements in flexural compression.

Local Buckling 1.65 0.90 yes Strictly speaking, no element in a beam is in uniform Lateral-torsional Buckling 1.65 0.90 yes compression, but elements that are thin relative to their distance from the neutral axis, such as the flange of I shapes, can be idealized as such. The Specification addresses four to determine the elastic local buckling strength of the entire shape. cases of uniform compression and two cases of flexural compression This elastic buckling strength is then used to determine the actual for flat elements of constant thickness like those encountered in compressive strength, whether it be yielding, inelastic buckling, or mullions. Ignoring small grooves or projections common in mullions elastic buckling. CUFSM determines the elastic buckling strength usually does not significantly affect accuracy. of any assembly of rectangular elements, accounting for interactions In reality, each element’s local buckling strength depends on the sup- between elements. Designers can include any of the extrusions’ grooves port given to that element by other elements of the shape; separately or projections that they wish. determining each element’s strength does not account for this. This CUFSM also determines section properties for any open shape, includinteraction is approximated, however, by averaging the elements’ ing the warping constant Cw, location of the shear center, and coefficient strengths using Section F.3.1. This approach requires dividing the of monosymmetry β, which are often difficult to determine but needed shape into flange elements (elements in uniform compression) and to calculate the lateral-torsional buckling strength. CUFSM determines web elements (elements in flexural compression) and determining the coefficient of monosymmetry about the two principal axes – not the moment of inertia of each group. the geometric axes – but these are usually nearly the same for mulSection F.3.2 provides another way to determine the shape’s local lions. CUFSM does not account for which side of the neutral axis is in buckling strength: the Direct Strength Method (DSM), also used in the compression, so the user must determine the sign. For unsymmetric I AISI cold-formed steel specification. This method uses software such beams, for example, β is positive when the larger flange is in compresas CUFSM (Constrained and Unconstrained Finite Strip Method) sion and negative when the larger flange is in tension. continued on next page ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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Figure 3. Lateral-torsional buckling.

debridged," is created by pouring an elastomer into a cavity in the extrusion and then removing the aluminum bridge at the bottom of the cavity. If the barrier's modulus of elasticity is known and it remains intact until the mullion reaches another limit state, calculating the composite beam’s flexural strength is straightforward.

Equation F.4-9 can approximate the barrier’s effect by factoring its width by its modular ratio, n, the elastomer's modulus of elasticity divided by aluminum’s modulus (10,100 ksi). For a commonly used elastomer with a modulus of about 250 ksi, n = 250/10,100 = 0.025.

Alternatively, CUFSM can be used to analyze shapes with multiple

Lateral-Torsional Buckling (LTB)

Because the tension side of the neutral axis is pulled straight while the compression side of the neutral axis buckles laterally, lateraltorsional buckling twists the cross-section (Figure 3).

As many mullions are unsymmetric about the bending axis, the only Specification Section that applies to all mullions is F.4.2.5, which determines the slenderness from the elastic LTB moment for such shapes. Section F.4 can then be used to determine the actual LTB moment, which may be elastic or inelastic, from the slenderness.

Either equation F.4-9 (below) or the Direct Strength Table 2. Section properties required by equation F.4-9 to determine LTB strength.

Method can be used to determine the elastic LTB moment. While equation F.4-9 requires the section propSymbol Description erties listed in Table 2, these can be determined for any Iy Minor axis moment of inertia open shape using CUFSM or other software. Equation Unbraced length Lb F.4-9 addresses beams with the transverse load applied at any location; CUFSM always applies the transverse Vertical distance from the load application point to the shear center go load at the shear center. xo, yo Shear center location 2 2 π EIy U + U 2+ 0.038JLb + Cw Major axis coefficient of monosymmetry βx Me = Iy Iy , Eqn. F.4-9 Lb2 J Torsion constant where U = C1g0 + C2 βx /2 Warping constant Cw

[ √

]

Complicating Factors Thermal breaks (thermal barriers) are often provided in aluminum mullions. One type, called "poured and

Figure 4. Local buckling of a snap-in assembly.

18 STRUCTURE magazine

C1 and C2 Factors that depend on loading; values are given in Section F.4.2.5 C1

Factor on the transverse load location

C2

Factor on the coefficient of monosymmetry

Table 3. DSM pros and cons.

Complicating Factors

Addressed without DSM?

Addressed

by DSM?

Load not at the shear center

yes

no

Thermal breaks

yes

yes

Snap-in parts

no

yes

Restraint from glass

no

yes

materials. The American Architectural Manufacturers Association’s (AAMA) technical information report, TIR-A8, further addresses structural aspects of thermal breaks.

Aluminum extrusions can be snap-fit together without fasteners or adhesives by elastically flexing the parts past a catch point, so they lock. The coefficient of friction between aluminum parts varies significantly depending on surface conditions, and part clearances vary due to tolerances. Consequently, snap-fit joints provide little resistance to rotation at the joint. The snap-in part, however, effectively restrains relative movement of the parts in the two orthogonal directions in the shape’s cross-section. It thus significantly contributes to the mullion’s torsional stiffness and LTB strength, as well as affecting the local buckling strength. CUFSM can model the assembly with these restraints as shown in Figure 4.

The same wind load acts on the glass on each side of a typical mullion; this symmetrical loading restrains the mullion from twisting about its longitudinal axis at the glass pocket. This restraint can

be modeled by DSM and may be on the compression side of the neutral axis or the tension side of the neutral axis, with differing effect on flexural strength. In one example, the LTB strength with rotational restraint was 3.5 times the strength without restraint.

The glass is not assumed to restrain the mullion against translation in any direction.

Table 3 lists the pros and cons of using DSM for determining flexural strength. While neither addresses all issues, DSM addresses all but the location of the load with respect to the shear center. If the transverse load acts towards the shear center, the LTB strength is less than if the transverse load acts at the shear center; neglecting this effect overestimates the LTB strength. Conversely, if the transverse load acts away from the shear center, the LTB strength is greater than if the transverse load acts at the shear center. Equation F.4-9 can be used to determine the sensitivity of the LTB strength to the location of the transverse load. Thus, DSM provides a powerful tool to address the complicating factors affecting the flexural strength of aluminum mullions.■ J. Randolph (Randy) Kissell is a Managing Consultant for Trinity Consultants.

He serves on Aluminum Association, ASTM, Canadian Standards Association, American Welding Society, and American Petroleum Institute committees that address aluminum structures, and teaches ASCE’s aluminum structural design seminar. (rkissell@trinityconsultants.com) James LaBelle is a Consultant at CSD Structural Engineers. (jlabelle@csd-eng.com)

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19

historic STRUCTURES Gasconade Bridge Failure 1855

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

T

he Pacific Railroad was chartered in Missouri in 1849 to build a railroad

from St. Louis to the Pacific Ocean. Due to financing problems and outbreaks of cholera, construction did not begin until 1851 with a groundbreaking at which prominent citizens of St. Louis turned out to celebrate the start

Gasconade bridge site.

of construction of the line. It included speeches, a national salute, and the reading of a poem written for the moment in history.

Due to the time required to make tunnels and build bridges, the line did not reach the nearby city of Pacific until 1853. Much of the line between a point just east of Washington and Jefferson City was along the Missouri River. Over the next two years, it would reach the capital city of Jefferson City with a planned grand entry on November 1, 1855. All of their bridges were of wood with the largest one being across the Gasconade River, a tributary of the Missouri River.

The river was about 760 feet wide at the crossing, and stone abutments and five piers were placed on a skew of about 30°. The spans were four of 130 feet and two of 92 feet that were to be Howe trusses with iron verticals. The masonry piers were about 82 feet high and built by Saler, Schulenburg & Co. The wooden truss spans were to be built by Stone, Boomer & Co., of Chicago. Amasa Stone was a brother in law of William Howe, the developer of the Howe Truss. Lucius Boomer started his first bridge company in Chicago in 1852 and was building the first bridge across the Mississippi River at Rock Island. They were one of the leading bridge building firms in the Midwest. It became apparent that the bridge would not be completed by November for the line reaching Jefferson City. Stone & Boomer, who would be building the falsework on which six Howe trusses would be built, decided to beef up the falsework until it could carry the railroad traffic as well as the dead load of the bridge with the approval of Thomas O’Sullivan, the Chief Engineer of the line. They chose to place bents supported by three to four wooden piles cross-braced with a wooden cap beam, all tied together with wooden dowels. They were spaced about 14 feet apart and were also braced together with longitudinal cross bracing similar to trestlework on other parts of the line. The lower chords of the bridge were 13 x 28 inches, were spaced 15 feet 1 inch apart, and rested on the cross pieces of the bents. Floor beams, 7 x 13 inches, rested on these chords and were spaced 4.2 feet on-center. The twotrack stringers, 10 x 12 inches, of 28 feet long with breaking joints, supported the iron rails. The lower chords, floor beams, and stringers were to become a part of the Howe truss spans.

Many of these members were placed only a day or two before the failure. Also, the easterly approach fill was not complete for 80 feet east of the abutment, so a temporary trestle was built to span this distance.

On the surface, it appeared that the structure, being similar to other trestle bridges on the line, would be safe for the passage of trains.

On the previous day, a heavily loaded gravel train crossed the bridge at what was an estimated speed of five miles per hour as a test of its 20 STRUCTURE magazine

load-carrying capacity. O’Sullivan had crossed the bridge and believed it to be safe.

An account of the November 1st failure published in St. Louis newspapers is as follows, But how soon was the scene destined to be changed! How soon were so many of those bounding hearts to be pulseless. No one dreamed that death was near, and yet it lurked for us only a few miles further on. At 1 o’clock, we left Hermann, preceded by a locomotive and tender, which had been sent forward, to see that the way was clear and no danger impending.

Soon we came in sight of the bridge across the Gasconade River, about nine miles from Hermann, and about thirty-five from Jefferson City.

The bridge is approached by an embankment, thirty feet high, which terminates in a massive stone abutment. Forty yards from the abutment, and just at the edge of the river, stands another staunch stone pillar, three more of which reach to the other side of the stream and support the bridge.

The river is about two hundred and fifty yards wide, and the bridge thirty feet high, at least. The Pioneer locomotive had crossed the structure safely and was waiting on the other side to see the result of our attempt. There was no fear of danger and no apprehension of peril.

We slowly moved along the embankment and came on the bridge. The locomotive had passed the first span, and had its fore wheels above the first pillar – beyond the abutment – there being then, resting on the first span, the locomotive, baggage car, and two heavily loaded passenger cars. The weight was too much for the long, slender timbers which supported the rails and the enormous load above. Suddenly we heard a horrid crash – it rings in our ears now – and saw a movement amongst those in the car in which we were seated; then there came crash - crash - crash as each car came to the abutment and took the fatal plunge. The affair was but the work of an instant. Six cars fell in one mass, each on the other, and were shivered into fragments.

The seventh fell with its forward end to the ground, but the other end rested on the top of the abutment. Those in it were only bruised. The eighth and ninth cars tumbled down the embankment before they reached the abutment.

Immediately after the accident, the heavens grew dark and black, as though the night had come. The wind shrieked from the leafless trees; the heavens were rent in twain, and from the crevice gleamed the white lightning, and the hoarse thunder bellowed its cruel mocking’s at the woe beneath. It seemed as if the elements were holding high carnival over the scene of slaughter.

Immediately after, the Railroad Company appointed a Commission to investigate the collapse. They held hearings and took testimony

from various builders and Julius Adams, a well known Civil Engineer. The majority in their Report of the Committee appointed to Investigate the Causes of the Accident (available online), dated November 9, 1855, concluded in part: The approach to the bridge from the East was on a curve of 1,432 feet radius, which terminated at or about the end of the bank, there being some 80 feet of tangent line before coming unto the bridge. The train from the East, consisting of one baggage car and ten passenger cars, being with the engine some 600 feet long, covered the tangent and was partly on the curve.

The engine and tender, certainly, and perhaps the baggage car and part of one passenger car (for the evidence on this point is not clear) were on the trestles of the first span of 130 feet when Sketch of disaster, note temporary bent for approach and masonry piers. it gave way, precipitating the forward part of the train to the bottom, which consists in this place of the low bank of the river and a short as not to condemn the recklessness, ignorance, and incompetence of the distance of waterway to the first pier. The engine was found on the left of men who will subject 600 passengers to more than 99 chances out of 100 the center, bottom upwards and reversed; that is to say, the forward part of being killed. Thanks to one man who is intelligent enough to perceive of the engine toward the rear of the train, lying partly in the tender with the cause not only of this but of future accidents…and who dares to tell drawbar unbroken, the forward truck of the engine detached and lying the truth and to condemn the ignorance and carelessness which cause the uninjured on the tender, and the drivers, both forward and rear, bearing death of travelers. the appearance of having suffered a violent contact with the stonework of He then restated a portion of the minority opinion by Henry Kayser, the pier. It would appear from this that the forward part of the engine had followed by Kayser’s description of the bents: reached the pier and the span of 130 feet, in consequence, was covered by Now behold these support or bents, and keep in mind that they rise, twelve the train when the tender and the rear of the engine fell through, dragging to 15 feet apart, rows of posts or rows of piles, to a height of twenty-five the train after them. After a critical examination of the portion of the to thirty-five feet or more above the bed of the river – the piles standing structure now standing, the Commission proceeded to examine in detail in a yielding mud bed, both piles and posts standing out of plumb, and the witness brought before them, and, from the evidence adduced, which overhanging in different directions, with but a dowel pin connection was reduced to writing and accompanies this report, combined with the between them! result of their observation of the structure itself are of the opinion that And do you wonder, if such frail “false works” evidently erected without although they consider it unsafe for general use, yet that its strength might plumb or square, which as a whole, or in their different sections, or in have been sufficient for the passage of the train at a speed not exceeding their component parts, present to the eye not one continuous horizontal five miles per hour. This is sufficiently proved by the passage of a heavily or perpendicular line and resemble more a field of cornstalks after corn loaded gravel train at about five miles per hour, which recrossed empty gathering, than anything in the way of building… at about twelve miles per hour, the weight of which when loaded was Vose then went on to criticize the managers of the railroad, the Chief one hundred and fourteen net tons for the length of the broken span, Engineer, and the road-masters, concluding, whilst the weight of the passenger train, which would have covered the There ought to be an Examining Committee appointed by the State or same span, was but seventy-one net tons; and although the engine of the General Government, for the purpose of testing severely the qualifications passenger train weighed on the drivers three net tons more than did that of engineers, road-masters, and all personnel employed on railroads. A of the gravel train, yet the testimony goes to show an excess of strength for lawyer must be admitted to the bar before he can practice, a physician must a deadweight more than equivalent to this difference. have a medical examination; but a man who is trusted with hundreds We are therefore of the opinion that the immediate cause of the disaster of human lives daily needs only a brazen face and plenty of influence to was the high rate of speed at which the train was moving at the time of be Chief Engineer Road-master or Superintendent. the accident.

The real cause of the accident was, and is, hard to determine. It A minority report was issued by one member who criticized the appears to have been a result of excessive speed, a roadbed with varydesign, believing it wasn’t the speed but the size of the members that ing support, and maybe even an undersizing of the trestlework with caused the collapse. He concluded, after a very detailed description minimal falsework. The local newspapers were filled with people of the trestle work, “The cause of said disaster was the breakage of the believing the railroad was guilty of poor and shoddy construction, and wooden structure in, and the superstructure over the bay between the that traffic was permitted on the unsafe bridge as the Pacific Railroad eastern abutment and next pier west, a consequence of their entire was trying to impress the state government to provide funds for the insufficiency in foundation, material, and construction, to bear the construction of the line. pressure of the locomotive and car running over the same. The fact In the end, 31 of the estimated 600 people on the train, many leading of the said attempt having been made, and particularly at a speed of citizens of St. Louis, died in the collapse. Stone & Boomer rebuilt the about fifteen miles per hour, can only be ascribed to the management bridge and the line finally reached Jefferson City four months later. The of the affairs of the company-defective in system, supervision, and accident, the worst in the United States at the time, served as responsibility.” a wakeup call for trained engineers to be involved not only in George Vose, a graduate of the Lawrence Scientific School of Harvard truss design but also in the design of trestles and falsework.■ University and an experienced railroad man, wrote a long letter to Colburn’s Railroad Advocate on December 8, 1855, very critical of the Majority of the Coroner’s report and praising the minority report.

Dr. Frank Griggs, Jr. specializes in the restoration of historic bridges, having After discussing the report, and its findings, he wrote, restored many 19 th Century cast and wrought iron bridges. He is now an Of the eight men chosen as a committee of investigation, only one dares Independent Consulting Engineer. (fgriggsjr@twc.com) to tell the truth; the other seven are so stupid as not to see or so dishonest O C T O B E R 2 019

21

CONSTRUCTION ENGINEERING for Broadway Bridge Replacement

By David Byers, Ph.D., P.E., John Boschert, P.E., S.E., and Paul D. Scharmer, P.E.

Span configuration for float-in operation.

T

he Broadway Bridge, connecting Little Rock and North Little Rock in Arkansas, was replaced in 2016 using an incentivized bidding process to minimize the duration of the bridge closure. Each bidding Contractor was required to add to their base bid a cost

of $80,000 per day for each day the bridge would be closed. Since the new bridge is located along the same alignment as the original bridge, the ideal sequence of completing new construction and switching traffic was not possible. Based on the winning bid proposed by Massman Construction Co., the duration of roadway closure was limited to 180 days starting the day the existing bridge was closed. The roadway closure duration guarantee was part of Massman’s $98.4M contract price. As such, the planning and sequencing of construction activities were critical to accomplish both bridge demolition and new bridge construction within the allotted time limit.

New Arch Spans The highlight of the new bridge was two 440-foot network-tied-arch spans over the Arkansas River. The tight project schedule dictated that these spans would be erected before the bridge closure and at a location away from the final alignment so that they could be installed as quickly as possible following removal of the existing bridge. Also, the final height of the bridge over the river is significant which added to the complexity of replacing these spans.

The new tied arch spans were each constructed on a system of custom falsework towers supported on barges immediately downstream of the existing bridge. The barge system for each arch span consisted of four 35-foot x 195-foot deck barges connected together. Fabricated steel towers were utilized to support each steel arch span on the barges for erection.

Following arch erection, the spans were individually floated into position and installed on the permanent bearings. The height of the towers 22 STRUCTURE magazine

and the height of steel erection above the barges varied between 56 and 68 feet, based on the final bridge profile. The height of the system, including the falsework towers and steel grillage system, allowed for span float-in at nearly the final elevation and the span lowering was accomplished using only barge flotation and ballasting (no hydraulic jacks were required).

Falsework Towers and Barge System The towers were developed considering future use and constructed for modular assembly. The main column members were 24-inch x ½-inch round pipe members (ASTM A252 Gr. 3). Diagonal and “k-frame” angle bracing members were included in the tower modules for stability.

The steel towers were supported by a steel grillage system and reinforced connections within the existing barges. The supporting members of the system were connected into the frame of the barges

to avoid concentrated load effects applied to the deck of the barges.

Local reinforcing of the internal barge framing consisted of vertical column members, stiffeners, and bearing plates welded to the side and center longitudinal bulkhead plates.

Lateral stability of the barge system was a significant design consideration and was facilitated through the use of significant bracing between towers. The system was constructed to develop continuity and rigidity between individual barges to create a stable flotation system, forcing the set of four individual barge units to work together as a group.

approximately 625,000 pounds. Each of the floor system field segments and arch structure field segments were pre-assembled on staging barges and subsequently lifted as units into position on the falsework towers.

Each half-span was assembled independently on separate barge pairs including the steel tie girder segments, connected floor system, arch ribs, and associated bracing members.

Large-diameter steel pipe struts with length and load adjustment capability temporarily supported the arch ribs. Temporary cables were utilized to support the floor system that briefly cantilevered from the falsework towers. The use of the axial compression struts and the temporary tension Steel Erection cables allowed for system position and The large and heavy steel arch spans member force adjustments during the were assembled in field segments erection process. on the falsework towers. Massman Following the independent assemConstruction Co. used a large bargebly of each half-span, the supporting supported ringer crane to do all steel barges were moved together, and the erection, and the crane’s significant Arch span during erection with adjustable temporary struts. span segments were joined by erectlifting capacity allowed for steel to ing the center deck field segments. be erected in large, pre-assembled segments. The first and most sub- This was a critical stage, as the complete span was supported temstantial segment to be erected was the end floor segment, with weight porarily on variable barge supports. Following placement of the ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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demolition operations were carefully planned in consideration of the stability of the temporarily-remaining structures as well as the safety of the nearby urban infrastructure. The stability of the spread-footing foundations on the existing bridge limited the load imbalance that could occur at the piers, so the explosive demolition sequence was planned based on these limitations.

A full demolition engineering study of the structure and proposed demolition techniques was completed to determine that the sequence of demolition would be feasible and the operations would be safe.

New Spans Float-In Operations Arch span prior to closure.

center floor system segment, the center keystone arch segment was erected to close the tied-arch structure.

The temporary conditions during erection were carefully studied with a staged erection computer analysis using LUSAS Finite Element software. Results of the computer model included member forces, deflections, and reactions which were used for the design of the temporary support system, barge floatation analysis, and geometry controls. Temporary load demands for the arch structure were computed to confirm adequacy for each construction stage as a part of the erection engineering effort.

Hanger Installation and Tensioning Hangers consist of two 2⅜-inch-diameter ASTM A586 strands at each permanent hanger location. Each cable is pin-connected to the arch and tie girder with an open “prolite socket” and pin provided by CBSI, Clodfelter Bridge and Structures International, Inc. The girder connection of the hanger is made using an adjustable rod. Detailed length calculations were required for each hanger to determine the fabrication lengths and geometry conditions during installation and tensioning. The geometry of the hanger connection details facilitated the need for a custom-designed and fabricated tensioning bracket which included tensioning bars, custom steel frames, and center-hole hydraulic jacks.

Original Bridge Demolition Constructed in 1923, the original bridge was comprised of 37 castin-place concrete girder spans and five 200-foot concrete arch spans.

In 1974, two of the concrete arch spans were replaced with a single 412-foot steel arch span. The entire existing structure had to be removed as a part of the project.

As noted, the bridge demolition was on the critical path, and swift completion was essential to the overall project success. Due to the project timing restrictions, the substructure units for the new bridge had been constructed under the existing bridge at the time the demolition operations were to be undertaken. The presence of the newly-constructed substructure complicated the operation and required detailed planning to avoid damaging these elements.

The concrete deck for all spans was removed using a team of excavators working together in a planned operation. The excavators started at the steel arch river span and worked toward the ends of the bridge, proceeding in opposite directions. The team of excavators also removed the spandrel columns for the concrete arch spans as a part of the deck removal process.

The multiple concrete arch spans and the single steel arch span were all removed using explosive demolition techniques. The explosive 24 STRUCTURE magazine

The weight of each assembled arch span was more than 4,000,000 pounds and had a center-of-gravity nearly 100 feet above the river, so the operation of transporting the span to its final location was critical. The travel distance from the erection location to the final bridge location was about 1200 feet. Since the travel distance was relatively short, winch lines were installed to assist multiple tug boats to pull and push each arch span into position. With the use of both the winch lines and tug boats, the barge-supported arches were slowly maneuvered into position without incident.

Geometric restrictions were tight for the float-in operation due to the minimal clearance over preset bearings and the limited lowering distance available based on barge freeboard. Once in the final position, the barges were flooded with ballast water, effectively lowering the system to engage the permanent bearings. The float-in operations were successful – no significant geometric conflicts were encountered, and the spans were both successfully delivered to their final locations.

Final Construction Activities Following the span float-in operations, the adjustable struts were removed and the remaining cables were installed and tensioned.

The new concrete deck was then constructed in six separate pouring sequences. Final cable hanger forces were adjusted to achieve the intended deck geometry and bridge profile, and construction of the bridge was completed. This project successfully replaced the Broadway Bridge by pre-erecting two 440-foot tied arch spans to facilitate span placement within the short-duration road closure.

The total duration of roadway closure was 152 days, which included the safe demolition and removal of the original existing bridge and complete reconstruction of the new bridge. Extensive planning and execution resulted in the safe and successful replacement of the bridge within the allotted timeframe.■ David Byers is a Principal for Genesis Structures in Kansas City, MO. John Boschert is an Associate and Senior Structural Engineer for Genesis Structures in Kansas City, MO. Paul D. Scharmer is Vice-President of Operations for Massman Construction Co. in Overland Park, KS.

Project Team Owner: Arkansas Department of Transportation Design Engineer-of-Record: HNTB Corporation General Contractor: Massman Construction Co. Erection Engineer: Genesis Structures, Inc. Analysis Software: LUSAS Bridge Plus Finite Element Software

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The Edlangeni Footbridge Rural Isolation in Eswatini

By Jay Arehart

The Edlangeni Footbridge completed in July 2017. Courtesy of Madison Sankovitz.

T

hombile Tsela is celebrating the fact she will no longer have to worry about her children being swept away by the Mbuluzi River on their way to school. Of the 1.4 million people in Eswatini, a small kingdom in southern Africa, approximately 75% walk as their primary mode of transportation. And, like Thombile, many of these people become isolated during the long rainy season as rivers swell and become impassable. This isolation is a root source of poverty not just in Eswatini, but around the world.

For ten weeks in 2017, the 200 families that make up the Edlangeni community worked together with students from the University of Colorado Boulder and the Engineers in Action Bridge Program to construct a 240-foot (73-meter) pedestrian suspended bridge. Being only the third engineered footbridge at the time in Eswatini, the

Preparing to pour an anchor after completion of towers. Courtesy of Madison Sankovitz.

26 STRUCTURE magazine

Edlangeni Footbridge provides year-round safe access to essential resources such as education, healthcare, farm fields, and markets.

Footbridge Technology Working in a remote community is a challenging endeavor due to a lack of electricity, skilled labor, and materials. The Edlangeni Footbridge design and construction team responded to these challenges by utilizing local materials and simple construction methods.

The bridge design used at Edlangeni is a modification of a standardized suspended bridge design produced by Bridges to Prosperity.

Due to the absence of a governing code, a variety of design checks utilizing the allowable stress design (ASD) methodology were performed to ensure the safety of the structure. The primary design checks included an analysis of the cable geometry to determine cable size and configuration, a check of the overturning moment induced at the top of the tower, a sliding check of the abutment, and a check for uplift of the anchor, among others. Unlike a project in the United States, much of the data required to verify design criteria, such as design wind speeds or allowable soil bearing pressures, was unavailable or too expensive to acquire. As a result, conservative assumptions for these design values are made for the worst possible conditions and factors of safety are increased. While acquiring the information could lead to a more efficient design, the relatively low cost of the construction materials and labor did not warrant it.

The walkway platform consists of a wooden deck, wide enough for livestock, motorcycles, wheelbarrows, and pedestrians. The platform is attached to wood crossbeams through a nailer. The crossbeams are attached to three walkway and two handrail cables by two pieces of rebar which are bent around both the handrail and walkway cables. The five 11â „8-inch (28mm) steel cables are suspended between two 15-foot-tall (4.5-meter) stone masonry abutments. Masonry materials

are sourced within walking distance of the bridge to reduce material costs. Rock is collected from the river or a nearby quarry and is then carried by hand or tractor to the bridge site.

Similarly, sand is collected from the river, screened and cleaned, and then hand-mixed with cement to make the mortar. The steel cables are anchored within the abutments by a reinforced concrete gravity anchor, each of which is comprised of 6.2 cubic yards (4.7 cubic meters) of hand-mixed concrete. The horizontal tension force in the cables is resisted by the mass of the entire abutment, while the vertical tension component is opposed by the mass of the anchor and associated overburden.

The majority of the construction process is spent collecting University of Colorado Boulder student, Maddie Philips, carrying sand alongside and moving materials to the job site, excavating the foundations community members. Courtesy of Madison Sankovitz. by hand, and laying the masonry abutments. After the abutments are complete and the anchors are poured, the cables are raised with a lever hoist.

The cable and corresponding deck shape are set using a simple automatic level.

Hoisting the cables and setting the deck shape is the most critical aspect of the construction process and requires the most precision of any of the other construction processes. After the cables are correctly positioned and secured with wire rope clips, the crossbeams are launched. The wooden deck boards and fencing are then secured to the deck with lag screws and u-nails, respectively. The simplicity of the design and associated Use for all types of concrete and grout applications, from slabs-on-grade to construction process allows for relatively containment tanks, multi-story post-tension structures to bridge decks. fast implementation. The Edlangeni Footbridge took a total construction time of ten weeks between the groundADVANTAGES breaking and inauguration ceremonies ยก Maximize joint spacing (up to 300 ft, L/W 3:1) ยก Enhance compressive and flexural strengths and cost less than $100,000 (U.S.).

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A multitude of stakeholders are associated with any project, and the Edlangeni Footbridge is no exception. While the bridge design and construction were led by the student chapter of Engineers in Action from the University of Colorado Boulder, under the supervision of a professional engineer, its completion would not have been possible without the support of the community and local partners. While the direct monetary cost of the bridge for the community members was close to zero, their primary contributions came in the form of providing unskilled labor for the project and their annual tax contributions to the national government which funded most of the materials. The families shared the labor responsibilities, each working an average of one to two days per week to carry rocks, collect sand, and handmix concrete. Eight students from the

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while the student chapter fundraised in the U.S. to contribute the steel cables and clamps.

Student Engagement and Cultural Exchange

The completed Edlangeni Footbridge provides safe access for children to attend school year-round. Courtesy of Madison Sankovitz.

University of Colorado Boulder and two professional engineers traveled to Eswatini for the duration of the project and became part of the Edlangeni community, sharing the labor each day.

In addition to the community and students, the project was supported by a pseudo-governmental organization, Eswatini Microprojects, that is responsible for managing and developing the country’s rural infrastructure. The Microprojects organization consists of municipal engineers and builders who, in addition to assisting with the implementation, trained alongside the students in the design and construction of the footbridge. Microprojects, who is funded through the national government, was also responsible for contributing most of the materials, such as the wood and cement,

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A unique aspect of the Engineers in Action Bridge Program is their partnership with university students in the U.S., Canada, and the UK. This partnership engages students in interactive design-build projects that sharpen technical skills learned in the classroom. Beyond refining their design skills, students learn how to communicate across cultures with community members and local engineers from different backgrounds. The singing and dancing that accompanies each day of a footbridge project create a space for this cross-cultural exchange of ideas. Not found in most engineering curriculums, this exchange allows students to gain a global perspective on how structural engineering is really about people and not just equations and calculations. By working on resource-limited projects like the Edlangeni Footbridge, students become better equipped to enter the engineering workforce. On-site, students are faced with unique problems that require them to think outside the box for solutions, an experience not typically encountered in summer internships. For example, while digging an anchor pit with just picks and shovels, and encountering a 1000-pound boulder, creativity is necessary for either finding safe ways to move it without heavy machinery or adapting the design to accommodate it. Because the students participating in these projects are not siloed into either design or construction, they are empowered to create and evaluate each alternative to make the best decision while in the field.

Celebration of New Beginnings On a sunny July afternoon, the completion of the Edlangeni Footbridge was celebrated by the community, students, and partners who made the project possible. Children ran back and forth over their community’s new bridge, testing out their new, elevated path to school. Parents relaxed, knowing that their children and grandchildren would have a safe river crossing for years to come. Moreover, the students from the University of Colorado Boulder reflected on all that was learned through the year-long design-build project. The Edlangeni Footbridge marked the start of the EIA Eswatini Bridge Program, led by university students and recent alumni, which aims to provide safe access over impassable rivers to all citizens of Eswatini. Since 2017, six bridge projects have been completed in Eswatini and many more are in the planning process. For more information and pictures about the Edlangeni Footbridge Project, please visit https://bit.ly/2k4zZno.

Become a Bridge Builder As a small but efficient non-profit organization, Engineers in Action thrives on the passion and commitment of volunteers. Whether you are a student seeking ways to use your studies to make a difference in the world, or a professional interested in mentoring the next generation of global change-makers, you can find out more by visiting www.engineersinaction.org.■

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28 STRUCTURE magazine

Jay Arehart is the Education Manager for the Engineers in Action Bridge Program and a Ph.D. student at the University of Colorado Boulder. (joseph.arehart@colorado.edu)

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2019 STRUCTURAL ENGINEERING SUMMIT TUESDAY, NOVEMBER 12, 2019 7:00–6:00 8:00–4:00 9:00–5:00 1:00–5:00

5:00–6:00 Registration Open 6:00–7:30 Committee Meetings Timber-Strong Design Buildsm Competition The SE3 National Symposium: Engagement & 7:30–9:30 Equity in the Structural Engineering Profession Sponsored by DeWalt

Young Engineer Reception Grand Opening Reception on the Trade Show Floor Welcome to California, Sponsored by the Structural Engineers Association of California

WEDNESDAY, NOVEMBER 13, 2019 7:00–5:00 7:30–7:00 7:30–8:50 9:00–10:30

Registration Open Trade Show Floor Open Coffee on the Trade Show Floor A Perspective on the Future of Consulting Engineers Stacy J. Bartoletti, S.E., Degenkolb Engineers 10:30–12:00 Talk Nerdy to Me: Science Not Communicated is Science Not Done Melissa Marshall, Present Your Science 12:00–1:30 Lunch

1:302:30

2:403:40

3:504:50

Frequently Misunderstood Lessons Learned from Building The SEAOC Blue Book: Seismic Provisions Performance and Earthquake Past, Present, Future Emily Guglielmo, S.E., P.E., F.SEI, and Response and Recovery from the 2018 Kevin Moore, P.E., S.E., SECB; Ryan Kersting, S.E. Anchorage Earthquake Ben Mohr, P.E. and Jeremy Callister, S.E. David Ojala, S.E. and Than Do, Ph.D. Resetting the Economics of Seismic Close Quarters: Protecting Neighboring Assessment & Strengthening in Areas of Lateral Analysis Overview/A Closer Properties While Building in the Lower Seismicity Look at Lateral Load Path Urban Environment Gwenyth R. Searer, P.E., S.E. and Matt Thomas, P.E., S.E. Steven D. Hall, P.E. Terrence F. Paret Tall Wood Structures: Current Trends and Effective Coordination of Buckling ACI 318-19: Building Code Requirements Related Code and Standards Restrained Braced Frame Connections for Structural Concrete Michelle Kam-Biron, P.E., S.E., SECB and Design Parameters Matt Senecal, P.E. and Brandt Saxey, S.E.; Rafael Sabelli, S.E. SEER Risk Mitigation for S.K. Ghosh, Ph.D. and Zac Vidmar, P.E. Structural Engineers Involved with How to Engage and Retain the Next Emergency Responses Inequities Within the SE Generation of Structural Engineers Profession: How to Address Diversity and John J. Lewis, P.E.; David Troxell, S.E., P.E. Jonathan Bayreuther, P.E. and and Jeff Coleman, P.E., F.ACI, JD Inclusion in Today’s Changing Sabrina Duk, P.E. Work Environment Public Outreach 101: Giving a Voice Rose McClure, S.E. and Structural Engineering - Claim Sharing to the SE Profession Andrea Reynolds, S.E., P.E. Moderator: John G. Tawresey, S.E., Moderators: Ed Quesenberry, S.E. F.TMS, F. SEI and Rick Boggs, P.E., SECB, LEED AP Limitation of Liability Clauses in Engineering Contracts Charlie Geer, P.E., F.ACEC

5:00–6:00

SE3 Committee Reception

5:00–7:00

Cocktails on the Trade Show Floor

7:00–10:00 A Celebration of Structural Engineering Hosted by Computers Structures, Inc.

REGISTER NOW! WWW.NCSEA.COM/REGISTER

NOVEMBER 12–15, 2019 · DISNEYLAND® HOTEL · ANAHEIM, CA THURSDAY, NOVEMBER 14, 2019 7:00–2:00 7:30–2:00 7:30–8:50 9:00–10:00

Registration Open Trade Show Floor Open Coffee & Raffle on the Trade Show Floor The Power of Connection Avery Bang, Ph.D, Bridges to Prosperity

1:402:40

10:00–11:00 Moving Beyond Life Safety for Community Recovery, Lucy Jones, Ph.D., Dr. Lucy Jones Center for Science and Society 11:20–1:20 Vendor Education Sessions 11:30–1:00 Lunch on the Trade Show Floor

2:503:50

Recent Updates in ATFP BlastResistant Design Criteria for DoD, VA, and GSA Facilities Marlon Bazan, Ph.D., P.E., S.E. Best Practices in Post-Tensioned Concrete Design Miroslav Vejvoda, P.E. Academy Museum of Motion Pictures – Innovative and Unprecedented Structural Solutions Andrew Rastetter, P.E. and Derrick Roorda, S.E. Young Member Groups: Leading to Success Angelina Stasulis, P.E., S.E. Training, Technology, and Talent – Focusing Internally to Advance NCSEA Awards Celebration External Business Development Sponsored by Atlas Tube Kristin Killgore, P.E., S.E.

Rain Loads – The Forgotten Natural Hazard?

Michael O’Rourke, P.E., Ph.D. Economic Design of SMF Connection Continuity Plate Welds Kevin Moore, P.E., S.E., SECB Wilshire Grand Building – Downtown Los Angeles Peter J. Maranian, P.E., S.E. and Leonard Martin Joseph, P.E., S.E. PE/SE Exam Panel Discussion Moderators: Brian E. Adorno, Jr., P.E., S.E. and Sarah Scarborough, P.E. Ethics for 2019 and Beyond Moderator: Barry Arnold, P.E., S.E.

5:45–9:30

4:005:00 ASCE 7 Approach to Snow Drift Loads – Current & Future Michael O’Rourke, P.E. , Ph.D. A New Path Forward for Tall Wood Construction: Code Provisions and Design Steps Ricky McLain, P.E., S.E. Governor George Deukmejian Courthouse & The Long Beach Civic Center: Project Highlights Elie-Issa El-Khoury, S.E. and Scott Stewart, S.E. Mentor Roundtable: Business Development Moderator: Isabella Horton, P.E., S.E. The Present and Future Roles of Drafters and Engineers: The Discussion Marcello Sgambelluri, S.E.

FRIDAY, NOVEMBER 15, 2019 7:00–9:30 Registration Open

7:30–8:30 Coffee

8:30–9:15 Structural Engineering: Indispensable to Civilizations – So Why Don't We Have More Influence?

Why Don't We Make More Money?, Ashraf Habibullah, S.E., Computers & Structures, Inc.

9:4510:45

10:5011:50

11:5512:55

ASCE 7 Wind Design Emily Guglielmo, P.E., S.E. and Don Scott, S.E., F.SEI, F.ASCE A Series of Unfortunate Events – Unexpected Contributors to Structural Failures Ross J. Smith, P.E., LEED AP Structural Engineers in Public Policy: We are Not (Just) in Code Hearings Any More Ryan Kersting, S.E. Existing Buildings and the “10% Rule”: Are We In Agreement Kevin O’Connell, S.E.

Does Accidental Torsion Prevent Collapse?

Conrad (Sandy) Hohener, P.E., S.E. and David (Jared) DeBock, Ph.D., P.E. Seismic Performance of New Code – Conforming Buildings in California Katherine Wade, P.E. Resilience and What it Means to the Structural Engineer Moderator: Kevin Moore, P.E., S.E., SECB Developing Acceptance Criteria and Evaluation Reports to Address Products Not Addressed in the Codes Melissa Sanchez, S.E., LEED AP and Manuel Chan, S.E.

Seismic Design Issues of a Wood Cantilever Diaphragm Terry Malone, P.E., S.E. Seismic R=1, 1.5, and 3 – When and Where…and is it Prudent?

Thomas F. Heausler, P.E., S.E., SECB Resilience and What it Means to the Structural Engineer – Part 2 Moderator: Kevin Moore, P.E., S.E. Solar Photovoltaic Arrays: Wind, Seismic & Gravity Loads Emily Guglielmo, S.E., F.SEI and Gwenyth Searer, P.E., S.E.

RESTORING the Historic Gateway to Reading, PA

By Kamlesh Ashar, P.E., Brian Teles, P.E., and Michael Urban, P.E.

T

he Penn Street Viaduct in the City of Reading and Borough of West Reading is an example of the “City Beautiful Movement” of the late 19th and early 20th centuries that emphasized the aesthetic treatment of civic architecture and urban planning. As the oldest bridge of its kind in Berks County, Pennsylvania, it is an iconic example of a concrete arch bridge worthy of restoring for the benefit of future generations. The Pennsylvania Department of Transportation (PennDOT) and their design team, led by Gannett Fleming, rehabilitated the bridge to extend its useful life while respecting its historic integrity.

The 1,337-foot-long concrete viaduct, constructed in 1913, spans across the Schuylkill River, Norfolk Southern railroad, Reading Area Community College, Front Street, a local trail, and a utility access road.

The bridge is adjacent to a limited-access freeway, SR422, locally known as the West Shore Bypass. The bridge serves as the transition from the West Shore Bypass to the city via five open-spandrel arch spans, nine closed-spandrel arch spans, a two-span concrete T-beam ramp attached to the main bridge, and a concrete pile-supported slab structure.

The bridge serves as the vehicular, pedestrian, and communications/ utility link between the Borough of West Reading and the City of Reading. The community cherishes this landmark, which serves as the “Gateway to Reading.”

Honoring the Past, Preparing for the Future The bridge is eligible for the National Register of Historic Places, so the rehabilitation balanced preserving character, bringing the bridge up

to current standards, and not precluding future work planned around the bridge. The project team first identified the character-defining and aesthetic features of the bridge; namely, the open-spandrel and closed-spandrel arches with architectural detailing throughout.

In consultation with stakeholders early in design, the project was to restore the bridge’s original architectural detailing by using the original bridge drawings as a guide. The most notable change is the reconstruction of the reticulated balustrades with outlooks, which were closed in the 1950s due to deterioration. The reticulated balustrades were constructed using a combination of precast and cast-in-place elements to allow for utilities and to ensure the complex balustrade geometry could be constructed to a very high quality, which the contractor (J.D. Eckman, Inc.) successfully executed via their on-site precast facility built specifically for this project.

Because the structure was modified in many ways over the years, each portion of the bridge required evaluation. Some later additions were removed, like an added retaining wall which shortened the balustrades.

Further, the original bridge contained obelisks with gas lighting; in 1972, the obelisks were replaced with ornamental electric lighting. Consulting parties came to a consensus to restore the ornamental electric lighting. To preserve the original features of the bridge and to enhance the gateway to Reading, the obelisks were added on the eastern side of the bridge with aesthetic LED up-lighting to allow for colors to be changed for various holidays or events. Aesthetic lighting was added beneath the bridge to illuminate the open-spandrel arches creating a dramatic visual effect at night without impairing the historic character of the bridge.

The project team first identified the character-defining and aesthetic features of the bridge; namely, the open-spandrel and closed-spandrel arches with architectural detailing throughout.

32 STRUCTURE magazine

The most notable change is the reconstruction of the reticulated balustrades (right) with outlooks, which were closed in the 1950s due to deterioration.

To extend the life of the bridge for at least another fifty years, this project reconstructed the upper portion of the bridge due to deterioration and to meet current standards.

PennDOT also accounted for future plans for a bike lane by modifying the cartway to 11-foot-wide A new crashworthy barrier at the curb allows for reticulated lanes with five-foot-wide During construction, the reticulated balustrades used a combination of precast and cast-in-place elements to allow for utilities and to produce balustrades and sidewalks narrowed from 11 feet to 7 feet shoulders with bicyclecomplicated concrete details to a high quality. to allow for adequate geometrics for bridge inspections. safe grates, that could easily be converted later into bike lanes. A new, crashworthy evaluated various types of fill to meet the requirements of permeability barrier at the curb allows for reticulated balustrades and sidewalks as well as to support traffic during staged construction. A permeable narrowed from 11 feet to 7 feet, allowing adequate geometrics for lightweight cellular concrete fill was used between the closed-spandrel bridge inspections. arches and the full-width moment slab. The cellular concrete’s unit weight was designed to maintain the existing structure weight, thus avoiding substructure rehabilitation. Cellular concrete incorporates Assessing the Existing Condition a specially designed foam into a concrete mix which also contains Starting with the in-depth inspection report, designers inspected the fine aggregate. As the concrete cures, the bubbles from the foam bridge, focusing on poorly performing details, deteriorated elements, create air pockets that allow water to permeate. The balancing act and conditions that required frequent maintenance. of incorporating cellular concrete into the project ensured that the Drainage issues plagued the bridge, so the team designed a new and strength, weight, and permeability achieved desirable specifications redundant drainage system. Full-width moment slabs with traditional and that the mix could be produced in the field by the contractor. inlets were installed in a way that allowed for maintenance. The top The 1950s rehabilitation encased most of the bridge in shotcrete to repair of the arches were waterproofed and concrete added above the piers deterioration; however, water trapped behind the de-bonded shotcrete to provide positive drainage to weep holes in the arches. The team accelerated the deterioration. This project removed the shotcrete allowing ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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Staged construction showing the superstructure removed (left side of bridge), temporary paved lane (center) and new construction (right side of bridge).

The design incorporated a way to excavate fiber optic lines from the existing sidewalk, temporarily support the lines on the side of the bridge via a temporary support, reconstruct the bridge deck, and cast the utilities back in the new sidewalk including a vault to access splices.

original concrete to be patched. This original surface, the patched areas, and newly cast concrete resulted in many textures and colors. Knowing this would be the case and understanding the historical significance, consulting parties requested a uniform appearance. The project applied a two-part coating to provide a uniform texture, concrete protection, and a uniform appearance. The base coat applied to the repaired surface was a two-part breathable, polymer-modified, cementitious coating that provided texture to the new smooth concrete and patches while filling in the deeper profiles of the original weathered concrete. A breathable acrylic topcoat was used to provide concrete protection and the desired color, as selected in the field by the consulting parties.

From a strength perspective, the designers analyzed the existing bridge and determined that the West Shore Bypass on-ramp and the eastern pile-supported slabs could not support today’s heavier vehicles. The pilesupported slab on the east end consisted of side walls enclosing a series of columns supporting floor beams and a slab. Beneath this structure was a cobblestone roadway used prior to this bridge. To preserve the cobblestone roadway, designers placed a geosynthetic fabric on top and filled the area with lightweight cellular concrete to minimize settlement.

The outer sidewalls were retained to maintain the original appearance.

A similar approach was taken on the western end of the bridge, where the closed-spandrel span was closed off when the West Shore Bypass was built in 1958. To eliminate the unnecessary span, but still preserve the historic integrity, the span was filled below and above the arch with lightweight cellular concrete. The sharply skewed non-historic on-ramp super-structure was replaced, which required an integral superstructure to maintain clearance to a utility access road as well as strengthening a pier with micro-piles to allow the ramp structure to support today’s heavier vehicles.

throughout construction. The design incorporated a way to excavate the lines from the existing sidewalk, temporarily support the lines on the side of the bridge, reconstruct the bridge deck, and cast the utilities back in the new sidewalk, including a vault to access splices.

The coordination for these tasks was incorporated in the concrete and rebar details for construction.

Thermal Expansion Beneath the aesthetic and operational improvements, the design team evaluated the thermal movements in the existing bridge to understand its behavior with the lack of expansion joints. With no signs of thermal damage, the rehabilitated bridge was designed with additional deck reinforcing steel to resist thermal effects. The roadway slabs on the closed-spandrel arches consisted of traditional roadway joints to allow for expansion leading up to the open-spandrel arches.

The reticulated balustrades also were detailed to allow for expansion with special details where they connect to balustrade posts.

Accounting for Critical Utilities The fiber optic utilities encased in the original sidewalk serviced most of the critical City services and thus had to be maintained 34 STRUCTURE magazine

Construction Considerations With 22,000 daily commuters, the Penn Street Bridge services a congested urban area; therefore, maintaining vehicular, pedestrian, and utilities during construction was critical. This included a comprehensive traffic control plan with a partial detour for a ramp, a temporarily relocated ramp, continuously maintaining one sidewalk, and maintaining three of the four lanes of traffic. Working with all of these critical features drastically limited construction access and required six stages for the 3 years of construction.

The five main spans of the bridge each consist of three arch ribs. This required an analysis of the ribs supporting traffic during construction stages where traffic was shifted, allowing for the deck, floor beams, and selected spandrel columns to be replaced. The closed-spandrel spans required temporary sheeting to support fill carrying traffic while replacing the adjacent fill and installing moment slabs.

With three of the 120-foot open-spandrel spans over the Schuylkill River and the limited construction access noted, a causeway was designed to access two of the spans and allow equipment large enough to reach all areas of the bridge. Understanding the need for a large causeway, necessary permitting and approvals were planned early in the design phase, eliminating the need for the contractor to submit permit modifications for a larger causeway during construction. The causeway was critical to accessing the new construction as well as for crews to complete the labor-intensive concrete repairs below.

Rehabilitation of the Penn Street Bridge restored and preserved many of the original features, unified past modifications, brought the bridge to current standards, and extended the bridge’s life for another 50 years. Constant coordination from all consulting parties and a respect for the historical significance of the bridge left the city with an enhanced “Gateway to Reading.”■

Kamlesh Ashar is the District Bridge Engineer for PennDOT District 5-0. (kashar@pa.gov) Brian Teles is a Senior Project Manager for Gannett Fleming. (bteles@gfnet.com) Michael Urban is a Senior Engineer for Gannett Fleming. (murban@gfnet.com)

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ENGINEERING AN ICON Complex Structural Systems for the Jewel Changi Airport

By Cristobal Correa, P.E., and Craig Schwitter, P.E.

A toroidal form was chosen by the design team for the 1.4 million-square-foot glass-enclosed building.

BuroHappold and Safdie Architects conceived a “gridshell” concept for Jewel’s roof and façade system that sits at the edge of level five of the ring-shaped steel and concrete base building. This perimeter support defined Completed structure. a span of more than 650 feet (200 meters) at its widest point. With only intermittent supports in the garden, ewel Changi Airport , a nature-themed entertainment this resulted in a nearly column-free interior. The geometry of the and retail complex on the landside at Changi Airport, Singapore, roof also accommodated an indoor rain-fed waterfall at its center, is their newest development, opening this year. The unusual activated by pumped water during dry weather. building, conceived by the firm Safdie Architects, extends the airport’s principal function as a transit hub to enfold an interactive The Initial Idea civic plaza and marketplace, combining landside airport operations with expansive indoor gardens and waterfall, leisure facilities, retail, The initial “gridshell” concept was a single layer structural system restaurants, a hotel, and other spaces for community activities. The that would maximize light and transparency within the space. This combined intensive marketplace and paradise garden create a new civic steel and glass shell is made of prismatic steel elements intersecting center – “the heart and soul” of Changi Airport, as the airport leaders at solid steel nodes. call it – and a new paradigm for community-centered airport design.

The offset toroidal roof shape of the Jewel is the product of an elaborate architectural program that incorporated a forest valley, entrances to adjoining terminals at the gateway gardens, an indoor waterfall, and an oculus. The roof asymmetry and offset of the oculus were created to allow for the passage of the airport train through the project. The steel gridshell meets the base building at its 5th level where a steel ring beam circles the building, arching up at each of the gateway gardens. This ring beam allows for a uniform transition of the thrust and vertical forces from the springing of the gridshell Building cross-section. into the base building.

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36 STRUCTURE magazine

Traditional steel gridshells generally strive for thinness by resisting compression and tension forces only in the plane of the shell (with only modest bending capacity). This can be seen, for example, in the British Museum’s Great Court Roof.

The Jewel at Changi Airport goes beyond the traditional shell by accommodating not only the membrane forces in the shell but also the out-of-plane forces that bend the shell. To resist this combination of forces, the design team examined the forces in the Aerial view of the exterior. shell and introduced deeper elements at locaStress diagram. tions subjected to bending, making the shell expressive of the forces flowing through the different zones.

For example, the inner zone of the toroid moving from the internal the quality of the work without restricting the improvements and support columns toward the oculus functions as a tension cone, with changes to design that might be suggested by the specialty contractor. the waterfall and oculus suspended and pulling down on the structure.

These membrane surface tensions act in two directions: hoop and Discretizing the Design meridional. With no risk of buckling and minimal bending, these steel elements are the shallowest in the project with only 8-inch depths.

After resolving the overall form, the shape was discretized using In contrast, the outer section of the shell – between the perimeter triangular panels made of steel and glass. A series of horizontal hoop support and moving toward the internal supports – has primarily elements along the surface is overlaid by continuous bias or vertical compression hoop and meridional membrane forces with a force elements, or both, to discretize the form as triangles. Larger triangles distribution similar to a dome. This area was designed for buckling required larger steel elements, impacting the aesthetics. The project and is made up of members 12 inches deep. leaders traveled to visit gridshells in Europe and the United States to The most significant complexity is found in the zone around the understand scale and to review similar triangulated forms in person and intermediate supports. Here, the tension and compression fields that best understand how proportions and scale would affect the aesthetics are pulling and pushing towards the center converge and are resisted and user experience. Another variable was the impact on procurement by a compression ring zone. This is analogous to the spokes of a and manufacturing; the project leaders hoped for numerous competitive bicycle pulling inward and being resisted by the compression ring of bids to reduce installed cost, resulting in the engineers recommending the wheel rim. Also, the intermediate column supports of the shell in an upper-bound dimension on the triangle sizes. this ring create bending out of plane. Bending demands are highest Also, as the continuous bias lines arch from the perimeter to the center, here with very deep elements of up to 750mm (almost 30 inches) in the triangles become progressively smaller, creating congestion and the final constructed system. practical construction issues at the element intersections. The design In the end, this gridshell geometry with various shell depths cre- team pruned the bias lines – removing elements as they got closer to ates a form that expresses the structural force patterns within the the center, giving rise to different transitional geometries. This had a gridshell roof. practical benefit, easing fabrication by keeping the scale of the triangles Using SAP2000 software, the engineering team experimented with visually consistent. The flow of the surface design appears smooth and varying the depths in the shell, rerunning analyses, and adding material where needed. These analyses included elastic as well as inelastic behavior. Buckling was addressed in an analysis that employed the provisions of the Eurocodes with Singapore amendments and was reviewed by the Singapore Building Authority as well as the Singapore EOR RSP-S. As the design progressed, different shapes and discretizations were reviewed. The amount of information was very large, necessitating the development of an approach that could rapidly synthesize meaningful insights into the behavior of the structure. The team coined the phrase “big data” – referring not to structural analysis per se, but instead to the geometrical complexity of the form, the geometry of each connection, and the impact on fabrication and procurement. Taken together, it presented something beyond the typical scope of the structural engineer and required the involvement of other experts in the fields of computing, geometrical modeling, and mathematics. This allowed for an investigation into the methods of advanced fabrication and construction technology that would be required to build the project. It was possible to create a bid set of drawings and specifications that would allow enough definition of Discovery slides at canopy. Courtesy of Jewel Changi Airport.

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Sketch of distribution of the forces.

Schematic of gridshell triangles.

natural, and the transfer of forces is unimpeded. The steel elements present a consistent width throughout, though their depths are varied as required by the stress distribution, including at column intersections.

The project team defined and developed a precision steel node that was machine fabricated to connect sections of the diagrid roof system.

Using mathematical analysis, the engineers developed standardized families of components to facilitate efficient fabrication and detailing, depending on the tolerances that might be possible to achieve in fabrication and detailing. In the end, based on the specialty contractor capability post-bid, unique nodes were found to be competitive and about 5,000 nodes and 14,000 steel elements, all unique, were produced to construct the toroidal shell assembly. The hardware elements all share a consistent geometrical ruleset so that their connection detailing is based on a consistent, project-wide system strategy.

Manufacturing processes available for this kind of work helped simplify the complexity of Jewel’s roof structure. All steel elements, for example, are straight, precisely cut from plate stock, and welded at angles by robots in Singapore’s steel manufacturer Yong-Nam shop.

Many time zones away, in Germany, the company MERO employed a 3-D computer-numeric-controlled (CNC) milling machine to create nodes with stubs at the exact angles required to meet these connecting steel elements. On-site, one of five of the node-to-steel-piece connections are welded, and about 80% were pre-drilled with bolt holes. In China, the glass was fabricated, laminated, and assembled into panels, all cut from standard rectangular sheets based on a packing study by the engineers to optimize efficiency and reduce waste.

While the result offers a highly uniform and consistent appearance, the architect and the structural engineering team methodically tailored the enclosure to heighten the destination’s experience. The welcoming transparency of the gateway garden areas, for example, contrasts with the opaque metal panels used to define and express the lower levels. The glazing has been hardened at vehicle dropoff points for improved security and blast resilience. Glass fritting, which adds texture and opacity to the glass surfaces, helps reduce and finetune daylight transmission to allow optimal ultraviolet (UV) penetration for the garden areas while also helping to control solar heat gain for occupant comfort. The glass attachments were engineered to allow for the thinnest possible roof aesthetics, yet the nodes also integrate a durable system of flow-through gaskets and effective waterproofing, a dually redundant approach that helps ensure resistance to moisture leakage.

To ensure the final system would work as intended – and to facilitate assembly and erection processes – the project team created a 5-story visual mockup during the design phase. The visual mockup assisted in determining the level of workmanship required for various aspects of the enclosure. Also, several performance mockups were assembled 38 STRUCTURE magazine

to study waterproofing requirements; the testing of the mockups included spraying water and applying negative pressure inside of the performance specimens’ test chamber.

Construction and Review Based on the design team’s analysis, BuroHappold suggested a construction sequence for the gridshell that began with the installation of the perimeter ring beam and support columns on the base building.

This was to be followed by the building of the compression ring zone of the shell over the columns and then the sequential installation of pie-shaped wedges spanning between perimeter ring beam and compression ring. Last, the central oculus would be completed. The contractors Woh-Hup along with Yong-Nam and MERO, however, had a better idea. Based on their extensive experience and evaluation of the task ahead, they decided to begin with the oculus and then to assemble pie-shaped sections from oculus to perimeter ring beam using a moveable crash deck, which helped accommodate sequencing required for the construction of the base building.

The project received a Platinum rating from Singapore’s GreenMark program for environmentally sustainable buildings. Its integrated system of glazing, static and dynamic shading, and an innovative and efficient displacement ventilation system achieved the required high level of comfort for a diversity of activities, as well as the ability to sustain the vast array of plant life.

Safdie Architects’ vision, linked to advanced engineering analysis, state-of-the-art automated fabrication, precision manufacturing, and skilled on-site assembly methods, has created an unparalleled experience for world travelers and the people of Singapore. “Jewel weaves together an experience of nature and the marketplace, dramatically asserting the idea of the airport as an uplifting and vibrant urban center, engaging travelers, visitors, and residents, and echoing Singapore’s reputation as ‘The City in the Garden,’” says the architect, Moshe Safdie.■ The online version of this article includes a sidebar, Designing with Big Data. Please visit www.STRUCTUREmag.org.

Cristobal Correa is the Project Director at BuroHappold Engineering Cristobal is also a Professor at Pratt Institute and has previously served as a guest critic at several universities.

Craig Schwitter founded the first North American office of BuroHappold Engineering in 1999.

Maui’s Kahului Airport ConRAC

By Craig Meierhoffer, P.E., S.E., Courtesy of Hawaii Department of Transportation

T

he new $340 million Consolidated Rent-A-Car (ConRAC) facility at Kahului Airport is part of an ambitious airport modernization program with the goal of upgrading the state’s airports to increase operational efficiency and improve the traveler experience.

The Kahului Airport ConRAC accomplishes this goal by consolidating most of the rental car companies within one state-of-the-art structure and connecting to the main airport terminal via tram.

The ConRAC facility is a three-story structure, including a partial basement area and small enclosed roof structures for stair egress and elevator access to the roof parking areas. The building has an overall area of approximately 1.9 million square feet, measures nearly a quarter-mile from end to end, and includes more than 3,700 parking stalls dedicated to the rental car companies and an additional 700 stalls for employee parking. There are also 72 fuel positions, 12 carwash bays, and 11 maintenance and mechanic stations for rental car servicing.

The main structure was built using cast-in-place concrete with columns supporting post-tensioned beams and girders that span 60 feet by 40 feet, respectively. The beams were spaced at 20 feet on-center, providing a regular and repetitive framing system to simplify formwork.

The structural slab consists primarily of a one-way postTypical column detail. tensioned slab, typically 5 40 STRUCTURE magazine

and Mark Hirschi, S.E.

inches in thickness, and spans 20 feet between the beams. In addition to the requirements of the building code, the garage was also designed to meet the recommendations provided in ACI 362.1R-12, Guide for the Design and Construction of Durable Concrete Parking Structures, which designated the Kahului Airport ConRAC as being within Coastal Chloride Zone I due to its location between half a mile and three miles from the Pacific Ocean.

The Hawaii Department of Transportation and the architect of record both wished to maintain an open floor plan for the main structure to maximize ventilation, circulation, and flexibility. No interior bearing walls were provided, and the structure was designed as a moment frame to accommodate this desire. Due to the high seismicity present on Maui (Ss = 0.989g, S1 = 0.254g), the moment frames were designed as special reinforced concrete moment frames with a response modification factor, R, of 8. As the state of Hawaii was still operating under the 2006 version of the International Building Code (IBC), a variance was successfully sought to allow the structure to be designed in accordance with the relatively newer ACI 318-08, Building Code Requirements for Structural Concrete, which was the first instance of that code to include or allow for the effects of post-tensioning in special moment frames.

The effects are limited,

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however, as ACI 318 prohibits utilizing more than 25% of the post-tensioning strength capacity to resist seismic loads.

However, in accordance with the strong column-weak beam capacity design phiPT beam/column picture during construction. losophy, ACI 318 still requires the engineer to account for 100% of the probable moment capacity, including the full effects of post-tensioning, when designing the columns and joint regions.

The structural design of the horizontal framing endured many iterations to balance the beneficial impacts of post-tensioning on Moment frame column cage during construction Soffit view of the typical floor framing. the performance and durability of the gravity system and the detrimental effects of post- minimal conflicts, and to limit the amount of unnecessary “throwtensioning on the demands placed on columns and joints to maintain away” or “feel good” reinforcement that can inadvertently inflate the strong column-weak beam behavior. The result of this iteration capacity of the horizontal framing elements and thus detrimentally yielded non-frame beams typically 30 inches high by 16 inches wide affect strong column-weak beam behavior. continued on next page with approximately 360 psi of post-tensioning, moment frame beams typically 36 inches high by 16 inches wide with approximately 300 psi of post-tensioning, and girders typically 42 inches high by 28 inches wide with approximately 260 psi of post-tensioning. Moment frame beams are typically increased in size near building edges to resist higher forces due to the inherent and accidental eccentricity of the building; however, perimeter frames were designed to not contribute to lateral resistance due to a lack of joint confinement.

While smaller columns were sufficient for gravity design, the typical interior column size was increased to 36 inches square to improve ductile behavior, decrease the amount of reinforcement required, prevent the use of expensive Photo by Jeremy Bittermann #18 rebar, and alleviate congestion.

All joints were designed to meet the Excellence in joint shear limits of ACI 352R-02, Structural Engineering Recommendations for Design of BeamColumn Connections in Monolithic Providence Park Stadium Expansion • Portland • OR Reinforced Concrete Structures, in place Seattle Sacramento Boise of the less stringent requirements of ACI Tacoma San Francisco Des Moines 318. The design of the post-tensioned Lacey Los Angeles St. Louis slabs, beams, and girders were all careSpokane Long Beach Chicago Portland Irvine Louisville KPFF is an Equal Opportunity Employer fully coordinated to allow reinforcement Eugene San Diego New York www.kpff.com and post-tensioning to be placed with

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The location and length of the main building required it to pass over an existing 70-foot-wide box culvert that could not be relocated. To avoid loading the culvert as it ran under the building, a post-tensioned transfer slab was provided over the culvert to transfer column loads across an approximately 75-foot span between lines of 52 auger cast-in-place pile foundations located on either side of the culvert. The concrete transfer slab is between 36 and 42 inches thick, with typically 2,500 kips of post-tensioning forces under each column that was stage-stressed to avoid overbalancing the transfer slab before the structure above the culvert was built. To ensure that no column load was imparted onto the existing culvert, cardboard void forms were provided between the

top of the culvert and the bottom of the transfer slab to support the self-weight of the transfer slab during its construction. Once the transfer slab cured and was stressed and self-supporting, the cardboard void forms were injected with water to dissolve them, leaving a void space between the top of the culvert and the bottom of the transfer slab.

At the central service bay where the tram exits and all rental company counters are located, a series of 80-foot-long open web structural steel trusses and glass canopy were incorporated to provide an airy, columnfree lobby and unique structural feature that is highly visible to the public. The trusses cantilever 40 feet with a 40-foot back span and are just over 8 feet tall at their deepest point. The trusses are comprised of hollow structural section (HSS) chord-and-web members and utilize all welded connections to provide a clean and architecturally appealing solution.

The HSS trusses are spaced at 60 feet on-center with steel wide flange girders spanning between the trusses and HSS purlins located at 10 feet on-center supporting the glass canopy and spanning between the girders. To maintain a column-free area over the tram station, the cantilever portion of the canopy was designed as a horizontal moment frame to distribute earthquake and wind forces back to the remainder of the canopy and the primary lateral force resisting system.

The Kahului Airport ConRAC opened to the public on May 15, 2019. Through careful coordination and attention to constructability, this state-of-the-art facility was completed on-time and under-budget despite numerous challenges. Maui’s primary airport now has a facility worthy of its distinction as one of the world’s premier vacation destinations and is ready to welcome the millions of domestic and international travelers who visit the Valley Isle annually.■ Craig Meierhoffer is an Associate at BASE and is based in its Honolulu office. (cmeierhoffer@baseengr.com) Mark Hirschi is an Associate at BASE and is based in its Chicago office. (mhirschi@baseengr.com)

Project Team Owner: Hawaii Department of Transportation Structural Engineer of Record: BASE Architect of Record: Demattei Wong Architects General Contractor: Hawaiian Dredging Construction Company 42 STRUCTURE magazine

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construction ISSUES Recommended Details for Reinforced Concrete Construction Part 5: Foundations

By David A. Fanella, Ph.D., S.E., P.E., F.ACI, F.ASCE, F.SEI, and Michael Mota, Ph.D., P.E., SECB, F.ACI, F.ASCE, F.SEI

Spread Footings Flexural Reinforcement Requirements for the distribution of flexural reinforcement in two-way footings are given in Sections 13.3.2.2 and 13.3.3.3 of ACI 318-14, Building Code Requirements for Structural Concrete. For square footings, the reinforcement is to be distributed uniformly across the entire width of the footing in both directions. In the case of rectangular footings, the reinforcement must be distributed in accordance with the requirements in Section 13.3.3.3 of ACI 318-14, which are illustrated in Figure 1. Reinforcement in the long direction is uniformly distributed across the entire width. A portion of the reinforcement in the short direction is uniformly banded over the column with the remainder uniformly distributed outside of the band width.

To facilitate bar placement in the field, a common practice is to increase the amount of reinforcement in the short direction by 2β/(β+1) (where β is the ratio of the long side to the short side of the footing) and space it uniformly across the long dimension of the footing instead of distributing the bars as shown in Figure 1.

Figure 1. Distribution of flexural reinforcement in a rectangular footing.

the longitudinal bars of the supported member under all applicable load combinations.

Dowels are commonly used as interface reinforcement between columns or walls and footings. The dowels are set in the footing prior to casting the footing concrete and are subsequently spliced to the longitudinal bars in the column or wall.

Illustrated in Figure 2 are dowels across the interface between a column Reinforcement Across the Interface and footing. For the case where all the column bars are in compression, The amount of reinforcement that is required at the interface between the dowels must extend into the footing a compression development a column or wall and the footing depends on the type of stress in length ldc determined in accordance with Section 25.4.9.2 of ACI 318-14. The dowel bars are usually hooked and extend to the level of the flexural reinforcement in the footing. According to Section 25.4.1.2 of ACI 318-14, the hooked portion of the dowels cannot be considered effective for developing the dowel bars in compression. The following equation must be satisfied to ensure adequate development of the dowels in the footing: h ≥ ldc + r + (db)dowel + 2(db)f + cover In this equation, r is the radius of the dowel bar bend, (db)dowel is the diameter of the dowel bars, and (db)f is the diameter of the flexural reinforcement.

Tensile forces (either direct or transferred by a moment) must be resisted entirely by reinforcement across the interface. Tensile anchorage of the dowel bars into a footing is Figure 2. Dowel bars where all the longitudinal bars Figure 3. Dowel bars where the longitudinal bars in the in the column are in compression. column are in tension. typically accomplished by providing 44 STRUCTURE magazine

90-degree standard hooks at the ends on the long face of the column and some (a) of the dowel bars with the developon the short face. This is the least preferment length of the hooked bar, ldh, able arrangement because there is more determined in accordance with Section potential for difficulties during instal25.4.3 of ACI 318-14 (Figure 3). lation; lowering the column cage over The location of the dowels protruding the dowels will be challenging because from a footing can have an impact on the ties will not allow the same degree (b) the installation of preassembled column of flexibility as will the hooks at the cages. Dowels should be positioned so ends of ties. as not to interfere with the longitudinal bars or the tie hooks in the column.

Mat Foundations Consider the arrangements of the (c) column longitudinal bars and the Once the thickness of a mat foundation dowel bars depicted in Figure 4a. has been established and the required Except for the bars located adjacent to amounts of reinforcement are calculated the center crosstie, the dowel bars are at the critical sections, a suitable bar size offset 45 degrees from the column’s lonand spacing must be selected. gitudinal bars relative to the long side For deep mats, the reinforcing bars can Figure 4. a) Ideal arrangement of dowels; b) Dowels arranged on of the column. There is no interference be placed in two layers at both the top and the long face of column; c) Least preferable arrangement of dowels. between the dowel bars and the 135bottom faces or in four layers. Bars that degree tie hooks in this arrangement. are in the interior layers should be aligned At the center crosstie, the dowel bars are located 90 degrees inboard with those in the outer layer (Figure 5, page 46). This helps reduce voids relative to the long side of the column so that no interference occurs in the concrete because it provides a clear passage for concrete placement. between them and the hooks of the crosstie. Depicted in Figure 4b The size of the bars in the interior layers should be the same size as, or is the same column but, in this case, all the dowel bars are adjacent smaller than, the bars in the outer layers. It is recommended that a 3-inch to the tie on the long side of the column. This arrangement is not as spacing be provided between the bars to facilitate concrete placement. preferable as the one in Figure 4a; however, it is manageable because In cases where additional bars are required in localized areas that the ends of the hooks are relatively flexible and can be maneuvered are heavily loaded, these bars should be spaced as a multiple or subaround the dowels. Some of the dowel bars in Figure 4c are located multiple of the spacing for the typical flexural reinforcement. continued on next page ADVERTISEMENT–For Advertiser Information, visit STRUCTUREmag.org

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Figure 5. Typical reinforcement configuration in a deep mat foundation.

Where the column spacing is not on a regular, symmetric grid, the layout of the reinforcing bars in the mat should be placed on an orthogonal grid and should not be skewed to follow the column layout.

Additional bars can be placed at locations in the regular grid wherever required. This dramatically simplifies placing the bars in the field.

Staggered splices should be avoided because of their negative impact on placement and constructability. Using the maximum straight bar length as often as possible usually minimizes the number of lap splices.

Like columns supported by spread footings, the dowels from the columns and walls that are supported by the mat should extend to the bottom layer of flexural reinforcement in the mat (Figure 6). The dowels should have a 90-degree standard hook at the bottom end; this allows the dowels to be tied to both the top and bottom layers of reinforcement in the mat, which secures the dowels from displacing before or during concrete placement.

Drilled Piers Recommended reinforcement details for drilled piers subjected to uniaxial compression loads are given in Figure 7. A minimum

Figure 6. Dowels in a mat foundation.

longitudinal reinforcement ratio of 0.005 should be provided; this corresponds to the ratio that is permitted in Section 10.3.1.2 of ACI 318-14 for columns with cross-sections that are larger than required for the applied loads.

Grade Beams A unique challenge occurs at the grade beam/drilled pier joint. If a grade beam is too shallow, congestion problems can occur at the joints. Some of the options to alleviate this problem are: • Extend the column dowels directly into the drilled pier • Provide a deeper grade beam • Provide a deeper grade beam only at the drilled pier • Provide a pile cap under the grade beam at the drilled pier • Hold back the concrete from the pile top • Provide a blockout at the top of the drilled pier One or more of these options may not be viable in every situation, and there may be other options not listed that may be more suitable for the situation at hand. Option 2 is illustrated in Figure 8.

More information on economical detailing of foundations can be found in the CRSI publications Design Guide for Economical Reinforced Concrete Structures, Design Guide for Drilled Piers, and Design and Detailing of Low-Rise Reinforced Concrete Buildings. Design and detailing requirements for pile caps can be found in Design Guide for Pile Caps and Design Guide for AASHTO Pile Caps.■ The online version of this article contains references. Please visit www.STRUCTUREmag.org.

David A. Fanella is Senior Director of Engineering at the Concrete Reinforcing Steel Institute. (dfanella@crsi.org)

Figure 7. Reinforcement details for drilled piers subjected to axial compression loads.

46 STRUCTURE magazine

Figure 8. Option 2 – Providing a deeper grade beam at a grade beam-drilled pier joint.

Michael Mota is Vice President of Engineering at the Concrete Reinforcing Steel Institute. (mmota@crsi.org)

NORTHRIDGE

25 YEARS LATER

Concrete Parking Structures and the Northridge Earthquake Performance and Resulting Building Code Changes

By Nathan C. Gould, D.Sc., S.E., Mikael K. Kallros, S.E., and Susan M. Dowty, S.E.

O

ne of the most iconic images of the 1994 Northridge Earthquake is the photograph of a collapsed precast con-

crete parking structure at California State University, Northridge.

While it was only one of many precast parking structures that suffered extensive damage as a result of the earthquake, it illustrates the juxtaposition of the incredible ductility exhibited by the perimeter columns with the collapse of the overall structure. It epitomizes one of the primary performance issues highlighted by the earthquake. The failure of numerous concrete parking struc-

Figure 1. Precast concrete garage failure at Cal State Northridge. Courtesy EERI.

tures during the earthquake, both precast and cast-in-place, led to an in-depth examination of the current design practices and ultimately led to several building code changes to improve the performance of these types of structures.

Representative Failures California State University, Northridge This partially collapsed garage at California State University was a relatively new, four-level precast concrete garage, approximately 18 months old at the time of the Northridge event. Given the age of the structure, it is likely that it was designed in conformance with the 1991 Uniform Building Code (UBC) requirements.

The design of the garage included a perimeter “ductile” concrete lateral force-resisting frame, with the exterior columns designed to carry all the lateral loads and the interior columns designed to carry only the vertical loads. As shown in Figure 1, the exterior columns exhibited a significant degree of ductility; however, it is likely that the interior columns, designed for vertical loads only, were unable to accommodate the loads imposed as the structure experienced significant lateral displacement.

The failure of vertical-load-only columns in structures where the lateral resistance was concentrated in perimeter lateral load-resisting frames was a valuable lesson learned from the Northridge event and was the impetus for future building code changes.

Northridge Fashion Center At the Northridge Fashion Center, two large, precast, prestressed concrete garages collapsed (Figure 2). The garages were relatively new in that the mall had just opened in 1991. The garage at the southwest corner of the shopping plaza had a vertical load-resisting system comprised of precast concrete columns supporting precast concrete beams, while the lateral load-resisting system was comprised of concrete shear walls in each of the structure’s principal directions. There was visual evidence of damage to the precast columns as well as a loss of support 48 STRUCTURE magazine

for the precast beams. It was interesting to note that that the concrete shear walls, which were likely intended as the primary lateral elements, suffered little if any damage. A garage in the northwest corner of the plaza, with similar construction, failed as well. There were likely several contributing factors to the collapse of the garages. The connections between the precast elements were likely insufficient to allow the elements to maintain continuity as the structure underwent significant displacement. Also, the lack of continuity reinforcement across the construction joints in the concrete slab may have limited the ability of the diaphragms to transfer the loads to the shear walls adequately.

A one-story cast-in-place concrete structure at the Center had damage to the circular concrete columns that supported the concrete drive ramp. The column’s transverse reinforcement, which was spaced at 12 inches on-center, was likely inadequate to provide the needed confinement for these columns; the columns were subjected to a high level of shear due to their increased stiffness which can be contributed to their relatively “short” length.

In addition to the practice of utilizing “independent” lateral loadresisting frames, with strength and detailing different from the standard vertical frame elements, there were several other factors that may have contributed to the significant level of damage in the concrete parking structures. These factors include the practice of designing parking structures at a minimum code compliant level, the irregularity of structural systems often found in multi-story parking structures with interior ramps, and the marginal connection of precast elements.

Changes to the Building Code After the 1994 Northridge Earthquake, there were two code change cycles (1995 and 1996) that provided opportunities to incorporate

lessons learned into the 1997 Jokerst). The code change was UBC, which was the premier “approved as revised” by the code for seismic design at that ICBO Lateral Design Code time. Most of the lessons appliDevelopment Committee cable to parking structures fell with further amendments into three basic categories: submitted by the SEAOC Deformation Compatibility, Seismology Committee and Design of Collectors, and approved at ICBO’s 1996 Design of Diaphragms.

Annual Education and Code The online version of this Development Conference in article contains a table that proSt. Paul, Minnesota. vides the 1997 UBC approved 1997 UBC Section code changes in these three 1921.6.12, Diaphragms, was categories, with the reason added because topping slabs given for the code change. over precast concrete members, The information in the table Figure 2. Failure of precast garage at the Northridge Fashion Center. Courtesy EERI. typically intended to be used as was collected from a variety of the diaphragm to transfer the resources, including the 1997 lateral loads, performed poorly Analysis of Revisions for the Uniform Building Code published by the during the Northridge Earthquake. Minimum thickness requirements International Code Council’s legacy organization, the International were added as well as requirements for mechanical connectors used to Conference of Building Officials. transfer forces between the diaphragm and the lateral force-resisting 1997 UBC Section 1633.2.4, Deformation Compatibility, system. The diaphragm code change (cited in the table in the online was added because of the deformation-induced damage during the version of this article) was submitted by the California Division of Northridge Earthquake to parking garage elements not part of the the State Architect (Vilas Mujumdar) and the Portland Cement lateral force-resisting system. The added language required elements Association (Mark Kluver). It was approved by the ICBO Lateral not part of the lateral force-resisting system, regardless of material type, Design Code Development Committee without amendments at their to be designed and detailed to maintain support of the design dead meeting in Sparks, Nevada, in February 1996. plus live loads when subjected to the expected deformations caused by seismic forces; plus, additional considerations were stipulated. For Conclusion concrete and masonry lateral force-resisting elements, the assumed flexural and shear stiffness properties were limited to a maximum of The failure of numerous concrete parking garages during the one-half the gross section properties unless a rational cracked-section Northridge earthquake highlighted several major issues with the lateral analysis was performed. Also, new design and detailing requirements force-resisting systems of these types of structures. In “bare” structures, for concrete were added to Chapter 19 to improve deformation such as parking garages, the significant lessons learned included the ductility and ensure their ability to continue to support gravity loads. importance of ductile interconnections between the different elements These new provisions were submitted by the SEAOC Seismology of the lateral force-resisting system, deformation compatibility between Committee (Chair Bob Chittenden) for the 1995 code development “vertical only” elements and the lateral force-resisting system, the cycle. The code change was “approved as revised” by the ICBO importance of designing for ramp and diaphragm discontinuities, Lateral Design Code Development Committee. There were further and designing non-seismic systems for the full expected seismic amendments to the code change approved at ICBO’s 1995 Annual drift. Following the 1994 Northridge earthquake, several structural Education and Code Development Conference in Las Vegas, Nevada. engineering and building code organizations worked together The Chapter 19 deformation compatibility provisions cited in the to quickly develop and adopt code modifications to address table (online version of this article) were submitted by the Portland these issues in future building codes.■ Cement Association (Mark Kluver). This code change was approved by the ICBO Lateral Design Code Development Committee without The online version of this article includes a Table referencing amendments at their meeting in Des Moines, Iowa, in February 1995. several changes to the 1997 UBC resulting from the 1997 UBC Section 1633.2.6, Collector Elements, was added performance of parking garages in the Northridge earthquake. because collector elements failed in parking garages during the Also, the online version contains references. Please visit Northridge Earthquake, and lateral loads were not delivered to www.STRUCTUREmag.org. the shear walls as intended by design. The new provisions required that collector elements, splices, and their connections to resisting elements be designed to resist forces increased by the new overstrength Nathan C. Gould is a Director in ABS Group's Advanced Engineering factor introduced in the 1997 UBC. The overstrength factor was Division, while also serving as the General Manager of the ABS Group St. Louis office. introduced in recognition that forces generated in the lateral forceresisting system can be two to three times the design seismic forces.

Mikael K. Kallros is a Director in ABS Group’s Advanced Engineering Division.

Failures of collectors in the Northridge Earthquake resulting in Susan M. Dowty is a Regional Manager in the Government Relations disconnection of the building from the lateral force-resisting system department of the International Code Council. From 1994-2000, Ms. and, in some cases, a loss of a portion of the vertical load-carrying Dowty was Senior Staff Engineer in ICBO’s Codes and Engineering system demonstrated these higher design forces were warranted.

Department and served as staff liaison to the ICBO Lateral Design Code The collector element code change was submitted during the 1996 Development Committee during the development of the 1997 UBC. code development cycle by Forell/Elsesser Engineers Inc. (Mark O C T O B E R 2 019

49

SEISMIC/WIND guide American Wood Council Phone: 202-463-2766 Email: info@awc.org Web: www.awc.org Product: Special Design Provisions for Wind and Seismic Description: Criteria for proportioning, designing, and detailing engineered wood systems, members, and connections in lateral force resisting systems.

Engineered design to resist wind or seismic forces either by allowable stress design (ASD) or load and resistance factor design (LRFD). Nominal shear capacities of diaphragms and shear walls provided for reference assemblies.

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Concrete Masonry Association of CA & NV Phone: 916-722-1700 Email: info@cmacn.org Web: cmacn.org Product: CMD18 Design Tool for Masonry Description: Structural design of reinforced concrete and clay hollow unit masonry elements in accordance with provisions of Ch. 21 of 2010 through 2019 CBC or 2009 through 2018 IBC and 2008 through 2016 Building Code Requirements for Masonry Structures (TMS 402).

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50 STRUCTURE magazine

IES, Inc. Phone: 800-707-0816 Email: info@iesweb.com Web: www.iesweb.com Product: VisualAnalysis Description: Thousands of engineers use VisualAnalysis to design structures for both lateral and gravity loading, including ASCE 7 wind and dynamic analysis for seismic. Customer-proven for over 25 years. the latest version updates material specifications and will help you beat deadlines. Try it today at no cost.

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Meca Enterprises, Inc Phone: 918-258-2913 Email: rachel@mecaenterprises.com Web: www.mecaenterprises.com Product: MecaWind Software Description: A cost-effective program used by Engineers and Designers to perform wind calculations per ASCE 7-16, ASCE 7-10, and ASCE 7-05. Simple to use and offers a professional looking output with all necessary wind calculations. The software will save you time and is a great tool to have in your business.

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NCSEA NCSEA News

National Council of Structural Engineers Associations

NCSEA Awards Record 15 Summit Scholarships to Young Members

2019 celebrates the seventh year of NCSEA’s Young Member Scholarship program. NCSEA is excited to announce that a total of fifteen young engineers are being awarded scholarships to NCSEA’s Structural Engineering Summit this year, making the 2019 Scholarship program the largest since its start! NCSEA thanks Computers & Structures, Inc. (CSi) for their sponsorship of this year’s scholarship recipients. Visit www.ncsea.com to view each of the recipients’ essay answers.

Congratulations to the 2019 Scholarship Winners!

Victoria Flys is a Structural Engineer in Training with Meyer Borgman Johnson and a member of the Structural Engineers Association of Arizona.

Emily Garrison is a Project Engineer with Silman and is a member of the Structural Engineers Association of Metro Washington.

Ilias Gibigaye, P.E., S.E., is a Structural Engineer with Wallace Engineering and a member of the Oklahoma Structural Engineers Association.

David Hackney, S.E., P.E., is a Structural Engineer with Hollis+Miller Architects and a member of the Structural Engineers Association of Kansas & Missouri.

Kayla Hampe, P.E., is a Structural Engineer with Hoyle, Tanner & Associates and a member of the Structural Engineers of New Hampshire.

Chelsea Hoplin, P.E., is a Project Engineer with GEI Consultants, Inc., and a member of the Structural Engineers Association of Massachusetts.

Sharon Jankiewicz is a Project Engineer with Silman and is a member of the Structural Engineers Association of Metro Washington.

Jeena Jayamon, Ph.D., is a Project Designer with John A. Martin & Associates and a member of the Structural Engineers Association of California.

Dorian Krausz is an Engineer in Training II with Martin/Martin Consulting Engineers and a member of the Structural Engineers Association of Colorado.

Maggie Mahoney is an Engineer in Training with Buehler Engineering and a member of the Structural Engineers Association of California.

David Marshall, P.E., is a Project Manager with Calder Richards Consulting Engineers and a member of the Structural Engineers Association of Utah.

Devon Minich is a Structural Design Associate II with The Haskell Company and a member of the Florida Structural Engineers Association.

David Nauheimer, P.E., S.E., is a Senior Associate I with Sargent & Lundy and a member of the Structural Engineers Association of Illinois.

(Returning Scholarship)

Andrew Podojil, P.E., is a Project Engineer with e2 Engineers and a member of the Connecticut Structural Engineers Coalition.

Anthony Wulfers, S.E., MSCE, is a Structural Engineer with Caruso Turley Scott and a member of the Structural Engineers Association of Arizona.

Did you know NCSEA offers discounted registration to the Structural Engineering Summit for young members? Not only can you save on registration, but there are special events, resources, and even an entire education track dedicated to young engineers. Visit www.ncsea.com/register to secure your deal!

52 STRUCTURE magazine

News from the National Council of Structural Engineers Associations 2019 Young Member Group of the Year Finalists Announced Each year, NCSEA awards the Young Member Group of the Year award at the Structural Engineering Summit. This award recognizes Young Member Groups that are providing a benefit to their young members, member organization, and communities. Each finalist group receives a complimentary registration to send a representative to the Summit, and a $1,000 travel stipend (sponsored by Computers and Structures Inc.). The winning Young Member Group will be announced at the Summit, and, will receive an additional $2,500 for their Young Member Group to use for future activities. Visit www.ncsea.com to view each groups essay answers.

Congratulations to this year’s Young Member Group finalists: Minnesota Structural Engineers Association, Structural Engineers Association of Georgia, Structural Engineers Association of Massachusetts, and Structural Engineers Association of Metro-Washington

2019 STRUCTURAL ENGINEERING SUMMIT DISNEYLAND® HOTEL · ANAHEIM, CA NOVEMBER 12–15, 2019 The 2019 Structural Engineering Summit is just over a month away; have you secured your spot? Register today for the only event designed by structural engineers for practicing structural engineers. The NCSEA Summit draws the best of the structural engineering field together for the outstanding practical education, a dynamic trade show, and compelling peer-to-peer networking at an event designed to advance the industry.

Be a part of this dynamic and growing event! Attendance at the Summit has increased by more than 100% in the last three years and the Trade Show has increased its exhibitors by more than 30%. This year, we have even increased our educational offerings and networking events! Attendees have the opportunity to earn over 16 continuing education hours from our 5 tracks offered over 3 days.

On top of our regular events, the Structural Engineers Association of California (SEAOC) is sponsoring a Welcome to California celebration for all Summit attendees to enjoy the magic of Disney along with a piece of California hospitality!

Visit www.ncsea.com/register before it is too late!

National SE3 Symposium at the Summit This half-day program is the first Structural Engineering Engagement and Equity (SE3) Symposium to be held in conjunction with a national engineering conference. This event is for engineers of all levels, business owners, human resource managers, and anyone within the AEC industry who is interested in promoting dialogue on engagement and equity in the structural engineering profession.

As part of this program, attendees will participate in three separate sessions focused on various aspects of engagement, retention, diversity, and inclusion. They will learn more about findings from the 2018 SE3 survey, hear from industry panelists on the state of our profession, and acquire practical strategies and best practices for improving retention within their organizations.

Those interested in attending can register for this event alone, or may add it to their Summit registration by visiting www.ncsea.com.

NCSEA and the SE3 committee thank DeWalt for their sponsorship of the National SE3 Symposium.

NCSEA Webinars

Register by visiting www.ncsea.com.

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October 29, 2019

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Douglas L. Fell, P.E.

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October 24, 2019

November 19, 2019

Fundamentals of Fire Resistance Michael Rzezznik, P.E.

Ground Improvement for Structural Engineers Jeffrey Hill, P.E.

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

O C T O B E R 2 019

53

SEI Update Learning / Networking

Apply/Nominate by November 1 for: • O.H. Ammann Research Fellowship - stipend at least $5,000 • SEI Fellow to be recognized at Structures Congress, April 5-8 in St. Louis • SEI/ASCE Awards including Dennis Tewksbury, Walter P Moore, W. Gene Corley, and more Learn more at www.asce.org/SEI

STRUCTURAL ENGINEERING INSTITUTE

STRUCTURES CONGRESS 2020

Sponsor/Exhibit to showcase your brand.

St. Louis, Missouri I April 5-8

Apply for an SEI Futures Fund Student/Young Professional Scholarship to participate. www.structurescongress.org

Advancing the Profession

Announcing 2020 SEI Futures Fund Initiatives

Thanks to generous donor contributions, the SEI Futures Fund Board has committed more than $120,000 in funding for these strategic SEI programs in 2020: • Increased support to forty Student Scholarships to Structures Congress • Continued support for twenty-five scholarships for Young Professionals to Structures Congress • Continued support for Young Professional Engagement on SEI Standards Committees • Continued support for SEI Standards lecture for SEI Chapters • Support to develop structural fire engineering curriculum Learn more and give www.asce.org/SEIFuturesFund.

My Experience with the Grand Challenge

By Yasser Darwish, S.M.ASCE, Missouri University of Science and Technology

We were honored to be selected as the winner for the New Construction Materials and Methodology category and received an invitation to participate in the ASCE Grand Challenge. There we had the chance to pitch our idea in front of industry leaders and engineers, engage in discussions, and receive beneficial feedback from the audience. Also, we had the chance to engage with ASCE leaders and industry leaders in closed meetings to get some advice on our idea and how to develop it. The event was very valuable for our team and opened the way for new opportunities for us. We are looking forward to participating in upcoming ASCE/SEI events. Our future goal is to start a company, get our products to market, and see them used on different structures on highways and roads. Read more at www.asce.org/SEINews.

Confidential Reporting on Structural Safety – US Improve Structural Engineering Practice and Public Safety Through Learning From Failures and Near Misses

CROSS US is a confidential reporting system that captures and shares lessons learned from structural safety issues which might not otherwise be available to the public. Anyone is is a confidential reporting invited to confidentially submit reports of structural failures, near misses, concerns, and incidents for anonymouse CROSS-US development of analysis system that captures and shares lessons Improve commentary by subjecct matter experts, and to use the valuable information posted. Learn more atstructural www.cross-us.org. learned from structural safety issues engineering practice which might not otherwise be available to the public. and public safety Anyone is invited to confidentially submit SEI Standards Supplements and Errata including ASCE 7. through See www.asce.org/SEI-Errata. learning reports of structural failures, near misses, If you would like to submit errata, contact Jon Esslinger at jesslinger@asce.org. concerns, and incidents, for anonymous from failures and development of analysis commentary by near misses. subject matter experts, and to use the

Errata

54 STRUCTURE magazine

valuable information posted.

f Learn more www.cross-us.org | www.asce.org/SEI

News of the Structural Engineering Institute of ASCE Membership

Announcing 2019-2020 SEI Board of Governors, effective October 1

Thank you to SEI members for voting in the recent online election for new SEI Board member Jerry Hajjar, representing SEI Technical Activities, and to SEI Local Chairs for electing John Cleary to represent SEI Local Activities. Congratulations to Jerry, John, and Taka Kimura on his recent appointment by SEI Global Activities to the SEI Board. The following roster indicates SEI Board officers elected by the Board at their April 27 meeting.

Glenn R. Bell, P.E., S.E., SECB, F.SEI, F.ASCE, SEI President Aimee Corn, P.E., M.ASCE David W. Cocke, S.E., F.SEI, F.ASCE, SEI Past President Satyendra K. Ghosh, Ph.D., F.SEI, F.ASCE Joseph G. DiPompeo, P.E., F.SEI, F.ASCE, SEI President-Elect Jerome F. Hajjar, Ph.D., P.E., F.SEI, F.ASCE Robert E. Nickerson, P.E., F.SEI, M.ASCE, SEI Treasurer Ronald O. Hamburger, P.E., F.SEI Laura E. Champion, P.E., M.ASCE, SEI Secretary Takahiko Kimura, P.E., F.SEI, M.ASCE Randall P. Bernhardt, P.E., S.E., F.SEI, F.ASCE Donald R. Scott, P.E., S.E., F.SEI, F.ASCE John Cleary, Ph.D., P.E., M.ASCE Victor E. Van Santen, P.E., S.E., F.SEI, M.ASCE

For those finishing terms September 30, thank you for your service and leadership on the SEI Board: Cheng Lok Caleb Hing, Ph.D., P.E., F.SEI, F.ASCE

Satish Nagarajaiah, Ph.D., F.SEI, F.ASCE

Sivaji Senapathi, P.E., F.SEI, F.ASCE

Welcome to the SEI Board

John Cleary, Ph.D., P.E., M.ASCE, is a faculty member at the University of South Alabama. His research includes structural performance and behavior during storm events, load behavior of bridges subjected to wave loading, evaluation of construction vibrations, and LRFD calibration of deep foundations. He teaches a wide variety of materials, mechanics, and design courses. In addition to teaching and research, John works as a consulting engineer designing steel and timber structures, analyzing and designing foundations, providing forensic analysis services, and conducting forensic investigations.

John’s involvement with SEI/ASCE started as the Graduate Advisor for the ASCE Student Chapter at Case Western Reserve University. At Western Illinois University, he became the Chair of the Quad Cities SEI Branch. Then, in Alabama, he founded the SEI Mobile Chapter and became President of the ASCE Mobile Branch. He is a past chair of the SEI Local Activities Division (LAD) Executive Committee and Chair of the SEI Student Initiatives Committee.

John comments, “Being involved in leadership roles with SEI has been an amazing experience and opportunity to learn, grow, advance, and give back to the future of the profession.” He is honored and humbled to be elected to the SEI Board and looks forward to working with the Board and membership to advance the practice of structural engineering, elevate that status of structural engineering, and foster the development of the next generation of structural engineers. Read more at www.asce.org/SEINews.

Jerry Hajjar, Ph.D., P.E., F.SEI, F.ASCE, is the CDM Smith Professor and Department Chair in the Department of Civil and Environmental Engineering at Northeastern University. His work includes analysis, experimental testing, and design of steel and composite steel/concrete building and bridge structures, regional modeling and assessment of infrastructure systems, and earthquake engineering. Jerry chaired the SEI Technical Activities Division Executive Committee, currently chairs the ASCE Department Heads Coordinating Council, and is a member of the ASCE Committee on Education, an ASCE Board-level committee that oversees much of the interactions of ASCE with educational institutions and ABET accreditation. Jerry is a Fellow of SEI and ASCE, an Associate Editor for the ASCE/SEI Journal of Structural Engineering, and served on the planning committee for the ASCE Civil Engineering Education Summit, the first summit on education under the auspices of ASCE in over two decades.

Jerry comments, “The ongoing technology revolution and globalization within the economy are pivotal issues for the profession and provide significant new pathways for education, research, and practice in structural engineering. The ASCE Future World Vision provides a bold vision for the profession at a time of great opportunity in civil engineering. And, with the Joint Vision for the Future of Structural Engineering, providing several key recommendations to strengthen the profession, SEI is well-positioned to continue to lead on these important topics nationally and globally.” He looks forward to serving on the Board and addressing these and other topics in the field. Read more at www.asce.org/SEINews.

SEI Online

New SEI T-shirt

Available by popular demand https://bit.ly/2kcjQwe.

SEI Standards

Visit www.asce.org/SEIStandards to: View ASCE 7 development cycle

SEI on Twitter

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SEI on Facebook Follow us: @SEIofASCE O C T O B E R 2 019

55

CASE in Point Did you know?

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

CASE has several tools available for firms to use to enhance their internal policies and procedures – from office policy guides to employee review checklists.

Tool 1-3 Tool 2-2 Tool 2-3 Tool 2-5 Tool 3-2

Sample Policy Guide Interview Guide and Template Employee Evaluation Templates Insurance Management Staffing and Revenue Projection

Tool 3-5 Tool 4-3 Tool 5-3 Tool 5-5 Tool 5-6

Staffing Schedule Suite Sample Correspondence Guidelines Managing the Use of Computers and Software Project Management Training Lessons Learned

CASE Risk Management Tools Available Foundation 9: Contract Documents: Produce Quality Contract Documents • • • • • • • •

Understand the definition of contract deliverables Include staff in the work planning effort Develop written design criteria Capitalize on similar designs without starting over on each project Establish reasonable schedule expectations Share agreement/contract information with staff Tailor project documents to project delivery method Integrate the BIM/CAD team

Tool 9-1: A Guideline Addressing Coordination and Completeness of Structural Construction Documents

This guide discusses the purpose, the background behind the issue, the important aspects of design relationships, communication, coordination, and completeness, guidance for dimensioning of structural drawings, effects of various project delivery systems, document revisions, and closes with recommendations for development and application of quality management procedures. A Drawing Review Checklist is attached.

The key to achieving the desired level of quality throughout the profession is for each structural engineering firm to focus on and develop its own specific quality management plan and to implement that plan on each project. This guideline will assist the structural engineering profession in achieving that goal.

Tool 9-2: Quality Assurance Plan

High-quality client service – from project initiation through construction completion – is critical to both project success and maintaining key client relationships. This tool guides the structural engineering

professional in developing a comprehensive, detailed Quality Assurance Plan suitable for their firm.

Foundation 10: Construction Phase: Provide Services to Complete the Risk Management Process

• Train staff for the CA work • Clarify SE’s role during submittal review and construction site visits • Get to know the Superintendent and other essential players • Document efforts well • Make site visits and reports meaningful • Follow up on changed construction tasks • Strive toward the goal of a successful project

Tool 10-1: Site Visit Cards

This tool provides sample cards for people in your firm who make construction site visits. These cards provide a brief list of tasks to perform as a part of making a site visit, such as: what to do before the site visit; what to take to the construction site; what to observe while at the site; and, what to do after completing the site visit.

Tool 10-2: Construction Administration Log

Construction administration is a time when good record-keeping and prompt response are essential to the success of the project and to limit the risk of the structural engineer. For this reason and many others, a well-organized and maintained construction administration log is essential.

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

Follow ACEC Coalitions on Twitter – @ACECCoalitions.

56 STRUCTURE magazine

News of the Council of American Structural Engineers ACEC FALL CONFERENCE

October 13-16

ACEC Fall Conference Features Additional Risk Management Case Risk Management Strategies For Bottom-Line Convocation and More!

Results October 13-16, ACEC is holding its Fall Conference at the Sheraton Grand, Chicago, IL. CASE will be holding their convocation on Monday, October 14. Sessions include: 10:45 am Risk Management – Thinking Past the Contract Speaker: Susan Winslow, Tela Vuota, PLLC 2:15 pm That Means What? Contractual Provisions That May Surprise You Speaker: Robert Hughes, Ames & Gough 4:00 pm Tackling Today’s Business Practice Challenges – A Structural Engineering Roundtable Moderator: Stacy Bartoletti, Degenkolb Engineers

In addition to the CASE sponsored sessions, the ACEC Fall Conference will feature more than 30 advanced business programs, including the following sessions focused on managing firm liability and risk: Read the Dang Contract T. Wayne Owens, T. Wayne Owens & Associates Beyond Professional Liability; Environmental Health & Safety Risk in the Geoprofessional Community David Duke, S&ME, Inc. BIM for Infrastructure: Are you Ready to Sign and Seal 3D Deliverables?

Will Sharp, Andy Lauzier, Shawn Rodda, and Daniel Prokop, HDR

The Conference also features: • CEO roundtables; • Exclusive CFO, CIO, tracks; • Numerous ACEC coalition, council, and forum events; and • Earn up to 21.75PDHs

The Conference will also feature: • Robert Costa – National Political Report, Washington Post • Keller Rinaudo – Founder and CEO of Zipline, International • Anirban Basu – Chairman and CEO of Sage Policy Group, Inc. • Sekou Andrews – Creator of “Poetic Voice” and Inspirational Speaker

For more information and/or to register, go to www.acec.org/conferences/fall-conference-2019.

CASE Member Recommendations

The following documents are ones that CASE members use most in their businesses: Sample Contact Documents #1: An Agreement for the Provision of Limited Professional Services #2: An Agreement Between Client and Structural Engineer of Record for Professional Services #9: An Agreement Between Owner and Structural Engineer for Professional Services Practice Guidelines 962: National Practice Guidelines for the Structural Engineer of Record (SER)

962-C: Guidelines for Int’l Building Code-Mandated Special Inspections and Tests and Quality Assurance 962-D: A Guideline Addressing Coordination and Completeness of Structural Construction Documents Sample Toolkits 3-5: Staffing Schedule Suite 5-2: Milestone Checklist for Young Engineers 9-1: A Guideline Addressing Coordination and Completeness of Structural Construction

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

Manual for New Consulting Engineers An HR Favorite for New Hires

ACEC’s best-seller, “Can I Borrow Your Watch?” A Beginner’s Guide to Succeeding in a Professional Consulting Organization offers new engineers a head start in the business of professional consulting. This essential guide is tailored to the unique needs of engineering firms, and the skills and experiences rookie consultants need to be successful in a large organization, including: • Proposal Preparation • Financial Management • Client Relationships • Project Management • Staff Management With over 140 pages of consulting expertise, this resource is the perfect addition to any new staffer’s welcome pack or in-house orientation.

It can even be a useful resource for more seasoned engineers looking to refine their skills. To order this book, go to www.acec.org/bookstore.

Bulk ordering is available; for more information, contact Maureen Brown (mbrown@acec.org).

O C T O B E R 2 019

57

structural FORUM Is Modular a Good Fit for You?

By Taeko-Karyn Takagi and Wally Naylor

D

evelopers, owners, general contractors, and subcontractors all face a similar dilemma. How can fantastic projects be created when they cost so much? Competition for land, difficult entitlements, and rising construction costs remain ongoing challenges facing the mandate to build, particularly housing. Material and equipment scarcities, code and regulation intensifications, and the lack of available skilled labor all contribute to decreased productivity. Inadequate housing and transportation create shortages of local labor while insatiable demand continues to outpace supply, all leading to soaring construction costs.

Shifts in policy and, especially, attitude have created openings for progress and solutions for this dilemma. While many search for a “silver bullet,” others are turning to off-site construction for cost and schedule savings, safety, and quality assurance. Off-site construction can also benefit projects in less obvious ways through reduced impacts to the site and surrounding neighborhoods, and employment of a much wider range of labor than traditional construction. Adapting to new processes required to best utilize off-site construction is challenging, but critical to its successful implementation. The industry has begun to truly adopt this construction means and methods, yet it still experiences growing pains while learning exactly what off-site construction is and how to use it correctly.

Off-site construction exists on a continuum of prefabricated elements including smart products, single trade and multi-trade components, and volumetric modular systems.

CLT, electrochromic glass, rebar cages, wall panels, concrete precast, MEP racks, riser assemblies, bathroom pods, and fully equipped and furnished volumetric modules are a few examples used today. Some items, like prefabricated MEPS system components, are already being installed in your project without much involvement beyond the typical general contractor and subcontractor coordination, and do not require owner and architect buy-in. The utilization of BIM has enabled MEPS subcontractors to prefabricate considerable portions of their work using shop labor in a controlled environment, which improves both quality and schedule while generally reducing the overall cost. Some panelized framing systems are implemented 58 STRUCTURE magazine

without full team involvement. However, once off-site construction begins to affect the structure of the building, everyone needs to be involved. Everything becomes extremely interconnected, and the overall team needs to integrate and coordinate the volumetric modular construction with the rest of the building. This requires early decision making and commitment to collaborate early in the design process to incorporate the strengths of modular fabrication.

Modular fabrication is ultimately a manufacturing process; construction can finally tap into increased productivity by integrating the advancements of other industries and technology. The fabrication assembly line operates with multiple stations and utilizes shop drawings that are transcribed from traditional plans and details to very specific assembly pieces and connections.

This eliminates a considerable amount of time and skill traditionally required for site-built construction and allows for a broad base of labor opportunity. Full volumetric modular is not just building inside of a warehouse but a production line taking full advantage of a controlled safe environment unconstrained by typical site restrictions such as weather, noise, allowable construction times, labor shifts, and premium costs. Factories can control these variables while simultaneously reducing material and labor waste.

The final manufactured product must achieve design-lock very early in the process, which is very different from the traditional site-built iterative process and can have a steep learning curve for the project team if not properly planned. To fully realize the benefits of prefabrication, the full team must adopt the new processes and continually adapt their modes of operation. When fabrication of the volumetric components is complete, there are still a substantial amount of the building components that need to be assembled onsite.

The grey area between what is produced and what is left to be assembled and built onsite continues to evolve through adoption, adaptation, lessons learned, and execution.

Selecting a modular fabricator early in the process is essential to a successful project.

Limited qualified facilities are currently fabricating on a mass scale, requiring critical early feasibility review. Before modular fabricator engagement, the project team must analyze

production capacity, factory location, shipping logistics, financing, and insurance. Equally important is selecting the architectural, structural engineering, and MEPS members of the team. While more and more firms are gaining experience with modular construction, which is an overall benefit to the industry, selecting an experienced team on your project has real benefits. Different permitting and inspection processes require training and experience with implementation. Selecting a general contractor with modular experience will help “glue” the entire process and team together and minimize the overall risk.

Getting the right “fit” of team members, requesting their early involvement, sharing with them “why” you are choosing modular (is it quality, price, time, or all three?), developing an overall project schedule to incorporate modular into the project, sharing the overall Target Value Design budget, and allowing ample preconstruction funds for the design development process are all factors that need to be considered.

Changes in construction are notoriously slow to be adopted, especially for those looking for a quick fix, a disruptive switch, or complete reinvention of real estate and construction which are some of the oldest and most regulated industries in this country. Adopting new technologies and innovation takes smart retraining and integration of the existing and the new. Teams need continuing education and training to plan for modular construction and a lot sooner than you might anticipate.

Done successfully, modular construction can provide real-time and cost savings while eliminating risk and improving productivity in construction.■ Taeko-Karyn Takagi (ttakagi@pankow.com) is Senior Project Manager, and Wally Naylor (wnaylor@pankow.com) is Project Executive with Pankow.

O C T O B E R 2 019

A Steel Joist Is A Beautiful Thing.

Why Not Treat It Right?

“The design of open web steel joists is based on concentric loading.

Asymmetrical loading can reduce joist strength.”

- Steel Joist Institute Treating a steel joist right requires loading joists in accordance with their design criteria.

Loads must be applied to both chord angles without creating a torsional moment. With that fact in mind, Chicago Clamp Company has designed and tested their Tube Framing Clamp System.

Load applied through the center line of the top chord

Chicago Clamp is uniquely designed to load joists concentrically Load applied symmetrically during uplift The clamp’s strength and stiffness transfer the load vertically to the center of the joist’s top chord in download and transfer symmetrically during uplift.

Testing was performed using rocker end-attachments to avoid reliance on torsional strength of the joist chord.

Clamping to joists avoids possible detrimental effects of welding to joists that are under stress due to loading.

Clamping has the economic benefits of reducing labor costs, reducing installation mistakes, and reducing job delays (associated with locating and sizing curbs and openings prior to steel joist installation).

Fall protection test using rockers

Phone: 708.343.8311

Test with hydraulic ram

Email: info@chicagoclampcompany.com

Electronic data acquisition

Location: Broadview, IL

Visit us at the 2019 NCSEA Expo in Anaheim, CA, Nov. 12-14 in Booth # 205