STRUCTURE magazine | July 2016

A Joint Publication of NCSEA | CASE | SEI

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STRUCTURE SPECIAL SECTION WIND/SEISMIC

July 2016 Wind/Seismic

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EDITORIAL

7 Risky Business! Who Knew?

By David W. Mykins, P.E.

CONSTRUCTION ISSUES

43 What is a 10d Common Nail? – Part 1

By Williston L. Warren, IV,

INFOCUS

S.E., SECB

24

9 Stealing like an Artist

By Barry Arnold, P.E., S.E., SECB STRUCTURAL ANALYSIS

10 Form follows Physics

STRUCTURAL PRACTICES

46 An Error in Timoshenko’s Theory of Plates and Shells

By Angus Ramsay, M.Eng.,

By Jon Galsworthy, Ph.D., P.Eng., P.E.,

Ph.D., C.Eng. and Edward

John Kilpatrick, Ph.D., P.Eng., C.Eng.

Maunder, Ph.D., C.Eng. HISTORIC STRUCTURES

14 Seismic Design of Nonstructural Building Components

By Daniel C. Duggan STRUCTURAL PERFORMANCE

18 Critical Wind Mitigation for Mega-Tall Structures

By Ben Eder

50 Covington-Cincinnati Bridge

By Frank Griggs, Jr., D.Eng., P.E. ENGINEER’S NOTEBOOK

By Jeremy L. Achter, S.E. PROFESSIONAL ISSUES

SECB, Al Ghorbanpoor, Ph.D., P.E. and

INSIGHTS

63 Permanent Wood Foundation

By John “Buddy” Showalter, P.E. SPOTLIGHT

24 Repair of Corrosion Protection for Cables

Seismically Isolated Building Bares All

FEATURE

Kevin Saldivar, E.I.T. STRUCTURAL REHABILITATION

FEATURE

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61 The Women of QLIC

STRUCTURAL TESTING

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

July 2016

By Steven B. Tipping, S.E. and Gina T. Phelan Imagine a compact, two-story wood-framed structure less than a mile from one of the most active faults in the U.S. Now imagine engineers developing a seismic isolation system with exposed bearings. Unique, yet effective and cost-efficient.

59 How Effective Are Your Arc Spot Welds?

By Vicki Arbitrio, P.E., SECB

20 Magnetic Flux Leakage for NDE

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and Derek Kelly, M.Eng., P.Eng. STRUCTURAL DESIGN

STRUCTURE

67 Dolphin Towers Condominium Remediation

By Frank Morabito, P.E., SECB

Repair for Low Concrete Breaks on Downtown Denver Project

By Orville “Bud” Werner II, P.E. In downtown Denver, CO, those fortunate enough to have a location are required to find creative ways to utilize small sites to their full potential.

Unfortunately, even with great designs, things go wrong – like when, during construction, a second-floor deck did not achieve the specified design strength.

By Paul A. Gossen, P.E.

55

STRUCTURAL FORUM CODE UPDATES

28 Special Reinforced Concrete Shear Walls

74 Five Tips for Young Engineers

By Stan R. Caldwell, P.E., SECB

By S. K. Ghosh, Ph.D. LESSONS LEARNED

33 Hybrid Masonry Connections and Through-Bolts – Part 2

By Gaur Johnson, Ph.D., S.E. and Ian Robertson, Ph.D., S.E.

FEATURE

IN EVERY ISSUE 8 Advertiser Index 64 Resource Guide (Concrete Products) 68 NCSEA News 70 SEI Structural Columns 72 CASE in Point

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

STRUCTURE magazine

5

Business Strong for Seismic Companies

By Larry Kahaner Companies offering structural seismic products are busy adapting to new codes with increasing complexities, along with a resurgence in new building starts. Read how these companies are upgrading offerings and developing new solutions.

On the cover The wind tunnel model of the Burj Khalifa testing in RWDI’s laboratory in Guelph, Canada. Through the process of optimizing the shape for the wind loading to improve its aerodynamic efficiency, the building team was able to construct a much taller building. See Structural Analysis article on page 10.

July 2016

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Editorial

Risky Business! Who Knew? new trends, new techniques and current industry issues

Protect your Firm with a Culture of Risk Management

By David W. Mykins, P.E., Chair CASE Executive Committee

I

are then reinforced by observing the daily actions of the leadership.

So to create a culture of risk management and claims prevention, it is important that it be included as one of the firm’s core values.

One method to create the right culture at the firm is to have a separate Risk Management Commitment Statement. If a specific reference to risk management does not fit into your overall mission statement, create a separate statement to emphasize this core value.

One way to create such a statement is to have each manager develop their version independently. Then meet as a group and choose one or combine the suggestions to form the final version. Then promote this on a daily basis. Make it such a part of the company culture that all employees know it and can repeat it, or else have it readily available to them in some other form.

Once the firm has committed the inclusion of risk management in its culture, there are several ways to reinforce it in daily practice.

The first is to create awareness and show commitment with constant reminders to the staff and management of its importance to the firm. This can be done by including risk management and/or quality control items on the agenda for all internal manager and project meetings. Promote open and free discussion of any lessons learned or quality control issues among the participants, not to judge or assign blame, but with the sole purpose of improving the internal business practices.

Reinforce your commitment by providing opportunities for training and continuing education on topics related to risk management.

Take advantage of the presentations on risk management provided by CASE. Each year CASE conducts a Risk Management Convocation in conjunction with SEI’s Structures Congress and produces a number of webinars. Also, next month CASE is providing a full day program of risk management sessions in Chicago, IL on August 5, 2016, titled Managing Risk for High Stakes Success. Sessions for this day are geared toward engineers from project manager level to principals. This full day of sessions kicks off with a welcome dinner presentation on Thursday, August 4, featuring Dr. Ahmad Rahimian of WSP | Parsons Brinkerhoff, who will discuss trends in tall buildings and talk about his experiences designing the new One World Trade Center tower.

For more detailed information about the sessions and how to sign up, contact me or Heather Talbert at htalbert@acec.org or visit www.acec.org/education/seminars. For more information about creating a culture of risk management and other resources for claims prevention, visit the CASE website at www.acec.org/CASE. Remember that awareness and training to improve risk management practices and avoid potential claims may be just the insurance you need to protect the future health of your firm.▪

a member benefit

STRUCTURE

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was recently having lunch with an insurance broker and mentioned that I was involved in CASE. I explained that CASE provides risk management tools and guidelines to help structural engineering firms improve their business practices and reduce their liability exposure. He looked confused and asked me what risk management meant for a structural engineering firm. He was in the health insurance business, and he understood quite clearly how risk management was applied in the process of underwriting insurance.

There is a wealth of statistical data available to help underwriters predict the risks associated with insuring an individual based on objective data like age, gender, and current health. However, what do structural engineers need to know to limit their business risk?

To answer that question, let’s first examine some basic facts about structural engineering claims. Here’s a slightly scary statistic: structural engineers have the highest claims-to-revenue ratio among practitioners in the Architectural-Engineering (A/E) field. It is not that structural engineers have more claims made against them. Rather, these claims tend to be higher, per claim, than claims for other types of engineers or architects. In fact, one study showed that for every dollar of professional service fees earned by a structural engineering firm, the frequency of a claim being filed was almost three times that of other engineering disciplines.

So where do these claims come from? Of the claims filed against structural engineering firms, about 13 percent come from claims related to on-site construction phase services. Most of these allege some form of negligence in performing observations or inspections.

However, a whopping 70 percent are the result of allegations of negligence in the performance of design services.

As far as types of projects are concerned, residential projects are near the top of the list. Single-family homes and townhouses rank highest in the number of claims made (about 23 percent of all claims) but are almost equal with condominiums in terms of claims paid (12 percent for houses and townhouses to 11 percent for condos).

So how can we reduce the risk of someone making a claim against us? Let’s start with the biggest contributor of claims: design services.

The best way to begin to manage risk during the design phase of the project is first to create a culture of risk management and claims prevention within the firm. This starts with an overall awareness of the current culture and how a culture develops within a firm.

Cultures within a company are developed and nurtured by the leadership. For new firms, there is usually a conscious decision about what type of firm you want to be and what your core values are. These are often memorialized in a firm’s mission STRUCTURAL statement. New employees and ENGINEERING outside observers often first learn INSTITUTE about a firm’s culture from their mission statement. These values STRUCTURE magazine

David W. Mykins is the President and CEO of Stroud, Pence & Associates, a regional structural engineering firm headquartered in Virginia Beach, VA. He is the current chair of the CASE Executive Committee. He can be reached at dmykins@stroudpence.com.

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July 2016

ADVERTISER INDEX

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Applied Science International, LLC....... 75 Cast ConneX........................................... 4 Construction Specialties ........................ 23 CTP Inc. ............................................... 45 CTS Cement Manufacturing Corp........ 27 Decon USA Inc. .................................... 54 Deicon .................................................. 19 Dlubal ................................................... 47 Geopier Foundation Company.............. 51 Hardy Frame ..................................... 2, 60 Hayward Baker, Inc. .............................. 37 Hohmann & Barnard, Inc. ...................... 6 Integrated Engineering Software, Inc..... 62 Integrity Software, Inc. ............................ 8

ITT Enidine, Inc. .................................. 42 KPFF Consulting Engineers .................. 13 Legacy Building Solutions ..................... 53 Lindapter .............................................. 25 MAPEI Corp......................................... 58 New Millennium Building Systems ....... 56 RISA Technologies ................................ 76 SidePlate Systems, Inc. .......................... 57 Simpson Strong-Tie......................... 17, 31 Structural Technologies ......................... 65 StructurePoint ....................................... 32 Subsurface Constructors, Inc. ................ 49 Trimble ................................................... 3 Williams Form Engineering .................. 41

ADVERTISING ACCOUNT MANAGER INTERACTIVE SALES ASSOCIATES sales@STRUCTUREmag.org Eastern Sales Chuck Minor 847-854-1666 Western Sales Jerry Preston 480-396-9585

EDITORIAL STAFF Executive Editor Jeanne Vogelzang, JD, CAE execdir@ncsea.com Editor Christine M. Sloat, P.E. publisher@STRUCTUREmag.org Associate Editor Nikki Alger publisher@STRUCTUREmag.org Graphic Designer Rob Fullmer graphics@STRUCTUREmag.org Web Developer William Radig webmaster@STRUCTUREmag.org

ERRATUM In the 56 Leonard article (STRUCTURE, June 2016), an ‘s’ was inadvertently dropped from the spelling of a company name in the Project Team credits (page 30). The correct spelling for the Structural Engineer is WSP | Parsons Brinkerhoff.

WANTED

Put pen to paper and write about what you know – structural engineering. Visit the STRUCTURE website (www.STRUCTUREmag.org), click the “For Authors” tab, and download the Author’s Handbook, and send an article abstract to publisher@STRUCTUREmag.org.

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

John A. Dal Pino, S.E. FTF Engineering, Inc., San Francisco, CA Dilip Khatri, Ph.D., S.E. Khatri International Inc., Pasadena, CA Roger A. LaBoube, Ph.D., P.E. CCFSS, Rolla, MO Brian J. Leshko, P.E. HDR Engineering, Inc., Pittsburgh, PA

Important news for Bentley Users

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STRUCTURE magazine

EDITORIAL BOARD

Jeremy L. Achter, S.E., LEED AP ARW Engineers, Ogden, UT

STRUCTURAL ENGINEER AUTHORS

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July 2016

Evans Mountzouris, P.E. The DiSalvo Engineering Group, Ridgefield, CT Greg Schindler, P.E., S.E. KPFF Consulting Engineers, Seattle, WA Stephen P. Schneider, Ph.D., P.E., S.E. BergerABAM, Vancouver, WA John “Buddy” Showalter, P.E. American Wood Council, Leesburg, VA C3 Ink, Publishers A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 Phone 608-524-1397 Fax 608-524-4432 publisher@structuremag.org July 2016, Volume 23, Number 7 ISSN 1536-4283. Publications Agreement No. 40675118.

Owned by the National Council of Structural Engineers Associations and published in cooperation with CASE and SEI monthly by C3 Ink. The publication is distributed free of charge to members of NCSEA, CASE and SEI; the non-member subscription rate is $75/yr domestic; $40/yr student; $90/yr Canada; $60/yr Canadian student; $135/yr foreign; $90/yr foreign student.

For change of address or duplicate copies, contact your member organization(s) or email subscriptions@STRUCTUREmag.org. Note that if you do not notify your member organization, your address will revert back with their next database submittal. Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily reflect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board.

STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be reproduced in whole or in part without the written permission of the publisher.

InFocus

Stealing like an Artist new trends, new techniques and current industry issues

Theft, and Fraud – Do You Know the Difference?

By Barry Arnold, P.E., S.E., SECB

S

tealing is bad. From a young age and throughout life, this principle is reinforced and emphasized. Regardless of whether it is petty theft or grand theft, and regardless of whether it involves burglary, embezzlement, larceny, looting, robbery, shoplifting, library theft, or fraud – stealing is bad.

As quickly as we learn this principle, we also learn that certain types of stealing are acceptable. For example, A young child working on their first finger-painting starts their composition with original ideas.

However, after the child sees a classmate paint a big yellow sun with streaming rays of sunlight cascading onto a house with a gabled entry and smoke wafting from a chimney, he or she adds those elements to his or her painting. The painting may be accented with a tree or picket fence, so the finished work is an original – albeit, influenced by other children – but, nevertheless, an original.

Researcher Anders Ericsson assures us that we are all “pre-wired to imitate.” Copying minimizes the steep learning curve and accelerates the evolution of an idea. Because the term ‘stealing’ carries with it a negative connotation, this type of copying – using someone’s ideas and adding to them – is referred to as “stealing like an artist” and has been popularized by Austin Kleon in his book, Steal Like an Artist.

“Stealing like an artist” can be seen in all facets of our lives, including the arts, music, entertainment, science, and engineering. Leonardo Da Vinci worked as an apprentice and learned the techniques of high art under the influence of his master, Andrea del Verrocchio. When Leonardo was ready to embark on his career, he took the lessons of his master, and the influences of other artists, and combined them with his unique style and methods to produce great paintings and sculptures. The Beatles worked as a cover band imitating and playing other musician’s music until they perfected their unique sound and style. Paul McCartney acknowledged fellow musicians as having a profound influence on the Beatles and their music.

Great scientists like Einstein and Feynman acknowledged the influence other scientists had in their lives. The book On the Shoulders of Giants, edited by Stephen Hawking, does a beautiful job of tracing the evolution of scientific ideas and how scientists form new ideas influenced by the work of their predecessors and peers.

Engineers are not immune from “stealing like an artist.” Because a person cannot un-see and un-learn what they have seen and learned, engineers who engage in plan reviews, peer reviews, or value engineering cannot help but be influenced by the work of their peers.

Often the things that engineers see, hear, and learn may passively, or actively, be introduced into their firms, spread to other engineers, and become the new norm.

Embedded in the concept of “stealing like an artist” is the principle that it is acceptable for a person to use the ideas of another and add to them, to create something new. Those who do so are considered original, innovative, and visionary. Those that simply take the ideas and work of another are considered thieves, charlatans, hacks, or frauds.

STRUCTURE magazine

Theft is defined as taking something from another person without the consent of that person. An engineer recently complained that after he had designed a 30-foot x 100-foot canopy, the architect he worked with used the engineer’s plans for a variety of similar projects across a large geographic region. Although the original roof structure consisted of joists supporting 30psf snow loads and spanning 30 feet, the architect used the same joist designation for joists supporting 45psf snow loads and spanning 50 feet. The architect also used the beam, column, and footing sizes shown on the original plan without verifying the sizes were appropriate for the new site conditions and gravity and lateral loads.

Fraud is defined as the intentional deception, deceit, or trickery over another to gain profit and an untruthful advantage. Engineer A was bewildered when, after completing an ASCE 31/41 investigation and upgrade on a 50-year-old, single story, unreinforced masonry building, she was not selected as the EOR for seismic investigation and upgrade of two identical buildings located nearby which were owned by the same client. Engineer A was further perplexed when her former client requested Engineer A’s CAD drawings and calculations. Engineer A declined the client’s request.

Engineer A was amused when the architect called and, after making friendly small talk and apologizing for not using Engineer A on the two new projects, also requested the drawings and calculations.

Engineer A declined the architect’s request. Engineer A was furious when Engineer X, the EOR on the new projects, called and after explaining that he “bid the job thinking he would just alter the original title block and put his stamp on the drawings,” asked for the drawings and calculations. Engineer X further explained that he “had never used ASCE 31/41 and needed the drawings and calculations as a go-by reference.” After pointing out Engineer X’s unethical behavior, Engineer A declined the request stating that she would not be an accomplice to fraud.

“Stealing like an artist” is acceptable and the basis for much of the innovation we enjoy. Theft and fraud are counterproductive, damaging to the profession, and jeopardize the public. One thing is certain: If we do not protect our intellectual property, it sends a message that our intellectual property is not worth protecting.

What are your thoughts? How do you protect yourself, your firm, the public, and the profession from theft and fraud? Would you like to share your ideas? The discussion continues at www.STRUCTUREmag.org.▪ Barry Arnold (barrya@arwengineers.com) is a Vice President at ARW Engineers in Ogden, Utah. He chairs the STRUCTURE magazine Editorial Board and is the Immediate Past President of NCSEA and a member of the NCSEA Structural Licensure Committee.

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July 2016

Structural analySiS discussing problems, solutions, idiosyncrasies, and applications of various analysis methods

How wind tunnel shaping workshops have become an invaluable tool for architects and engineers in search of high rise buildings’ optimal form.

T

oday, a great deal is known about how wind affects tall buildings differently than it does shorter structures.

However, wind-related design issues are usually considered much later in the design process than they should be to achieve structures that have optimal performance with minimal cost. Wind engineers traditionally interact with structural engineers after the conceptual design phase when basic decisions about the building form and structural system have already been made. A better approach is to bring the entire building team up to speed on potentially challenging wind effects very early and resolve them when the design is still malleable. Early efforts have proven to be highly advantageous for some of the world’s tallest buildings, discussed later in this article.

Form follows Physics

By Jon Galsworthy, Ph.D., P.Eng., P.E., John Kilpatrick, Ph.D., P.Eng., C.Eng., FICE and Derek Kelly, M.Eng., P.Eng.

Jon Galsworthy is a Principal at RWDI. He is active in a number of technical committees, including the National Building Code of Canada, ASCE 7 Wind Load standard, and the ACI 307 Concrete Chimney wind loading standard. Jon can be reached at jon.galsworthy@rwdi.com.

Basic Aerodynamics of Buildings

Structural engineers who design tall buildings understand that the wind’s effect on a tall, slender structure is much more complex than just the effect of wind on a sail. Differences in complexity are due in part to differences in the load-resisting behavior of squat structures compared to slender structures, but it is also influenced by the different aerodynamics of these structures and their dynamic response.

For many buildings, a good initial design can be developed using the building code. The structural engineer then typically relies on wind tunnel testing for fine tuning the final design. However, as buildings increase in height, their dynamic response becomes more significant which makes them much more challenging from a structural design perspective. Code based calculations may not be such a good initial guide.

One consideration is the building’s aspect ratio, which reflects its slenderness. There are codes of

practice that suggest any structure with a heightto-width aspect ratio over 4:1 or 5:1 would be sensitive to wind effects. Many of the buildings that have benefited from in-depth wind tunnel testing have aspect ratios of 10:1 or 12:1, values well beyond a code’s applicability. With such high slenderness ratios, the structure has a very high degree of flexibility, which also means it has long periods of vibration. Generally speaking, structures with longer periods of vibration attract more energy from the wind, which increases their motion during a windstorm.

Crosswind effects can also be significant. Shorter, squatter buildings tend to be dominated by wind induced drag, so the form drag of the building is what governs the loads. However, for tall structures, crosswind excitation becomes more important. These effects also can vary a great deal, based on the plan geometry of the building.

In essence, as the wind passes a building it generates a wake, which results in a fairly organized pattern of vortices being shed downstream (Figure 1). Tall and slender buildings with longer periods of vibration are more susceptible to the forces generated by that wake. Again, unlike the drag loading on a sail, these organized wakes generate their largest forces at 90 degrees to the direction of the wind. Because these forces are generated in an oscillating pattern, the structural loading may be amplified due to resonance.

The strength of the crosswind force is related to the plan geometry of the building, as well as the variation of geometry with height. Very prismatic buildings that show little variation in the plan form with height are the most susceptible to crosswind-induced motion.

Strategies to reduce these wind-induced motions include tapering structures and changing the plan form with height. Plan form changes can be things like changing building corner geometry or introducing apertures – or holes – into the structure.

Any number of modifications can have beneficial effects. For example, introducing slots at building corners may provide some pressure relief. The challenge is putting potential improvements on the table, finding mitigating shape adjustments

John Kilpatrick is a Principal at RWDI. John is active in the field of wind engineering and, from 2012 to 2014, was Chair of the United Kingdom’s Wind Engineering Society. John can be reached at John.Kilpatrick@rwdi.com.

Derek Kelly, also a Principal, is a senior project manager and wind engineering specialist with RWDI. He can be reached at Derek.Kelly@rwdi.com.

Figure 1. Sketch showing vortex shedding flow patterns. These regularly spaced vortices generate their largest forces perpendicular to the direction of flow, causing crosswind vibrations.

10 July 2016

Figure 2. Burj Khalifa wind tunnel model. Stepped setbacks into the building were introduced, and the base was rotated to improve its aerodynamic efficiency in the direction of strongest winds.

or other changes acceptable to the architect, and then working through them to determine what is optimal for the structure.

A Little Expertise to the Rescue Experience is a great teacher and, as a result, engineers specializing in wind design have developed in-depth expertise in this field.

RWDI, one of the several firms specializing in wind engineering, has served as the Wind Engineer on many of world’s tallest buildings since its founding in 1986 – including record-breaking Petronas Towers, Taipei 101, the Burj Khalifa and the Shanghai Tower.

At 2073 feet (632 meters), the Shanghai Tower is China’s tallest building and the second tallest in the world. The design team wanted to build a twist into the building’s form, in addition to a tapering with height.

The question was a matter of how much. The optimal form was found by testing more than 12 options that looked at different rotations of the building floorplan with height as well as the building’s orientation relative to the strongest winds. The final selected design reduced wind loads by 24 percent. This, in combination with an optimized structural design, resulted in significantly reduced material costs.

Wind engineering for the 2716-foot (828meter) Burj Khalifa, in Dubai, United Arab Emirates, had an even greater benefit. The original design had been based on a much shorter building, similar in basic concept to the final product but approximately 984 feet (300 meters) shorter. Construction had already begun on foundations based on the original design, but the project goal changed

Figure 3. One of the prototype shapes that was explored for the Shanghai Tower before selecting the final design. This option has a 180-degree twist as it rises.

while it was still under construction. Through the process of optimizing the shape for the wind loading, which included introducing stepped setbacks into the building and rotating the base to improve its aerodynamic efficiency in the direction of strongest winds, the building team was able to construct a much taller building that still had the same wind loads at its foundation level.

Much like the insights and technology from Formula 1 racing makes their way into the design of more common automobiles, these and other experiences designing super tall buildings have enabled the development of a shaping workshop approach for the optimization of more common tall structures.

Formally launched at RWDI in 2012, the

shaping workshop brings together the key players in the design team – the architectural designer, structural engineer, wind engineer, and owner’s representative – and typically accomplishes in a day or two what used to take months. The keys to success are today’s enabling technologies and the design team’s willingness to come together to participate in the workshop.

How it is Done Although the shaping workshop is best held as early as possible in the design process, it frequently is convened after a competition has established a base architectural form for the building. The owner likes the design, the

Figure 4. Large scale wind tunnel testing of the Shanghai Tower. The final design has a 180-degree twist as it rises and its base is rotated to be aerodynamically efficient in the direction of the strongest winds.

STRUCTURE magazine

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July 2016

architect obviously has some passion for it, and the structural engineer has developed a baseline structural system.

That is when wind engineers are brought on board to do a first wind tunnel test of the proposed structure. The test simulates the airflow associated with the atmospheric boundary layer and the localized influence of surroundings to identify the wind forces on a scale model. Using the most common approach, the high-frequency-force-balance technique, data collected from the simulation are combined analytically with the structure’s dynamic properties to determine its windinduced motion.

Particularly with tall and slender buildings, this motion is a governing factor in the design of the structure. Building deflections affect the overall serviceability of the building with respect to façade performance and other items such as interior partitions, so general guidelines have been established to limit the deflections of the building. However, occupant comfort is the more significant design challenge for most tall buildings. Through experience, wind engineers have established benchmarks of acceptable magnitudes of building motion considering their effect on occupants. It is not unusual for the first wind tunnel test to reveal that the architectural design sits well above those criteria.

The goal is then to get within these benchmarks through design modifications. One option is to make slight modifications to the architecture that can yield large aerodynamic improvement, such as rounding the corners or introducing some taper into the building.

Another approach is to determine how sensitive the motion is to the structure’s natural periods and identify ways to move the needle in a positive direction. A third alternative is to look at the building’s natural ability to dissipate energy, i.e., its damping capability. Often it is feasible to add supplemental damping into the building and enhance performance in that way.

In the context of a shaping workshop, these discoveries occur in real-time when the design team is together. After that first evaluation to quantify the scope and scale of the challenge, the wind engineers typically suggest modifications to the architecture that would result in certain benefits. These suggestions inevitably lead to a discussion – some suggestions are palatable, some are not.

The opportunity for real-time feedback and collaboration is one of the most significant benefits of having the design team all together.

For example, there may be a suggestion to modify a particular aspect of the design that

Figure 5. Initial wind tunnel model of 432 Park Avenue that was tested before the exploration of openings in the shaping workshop.

the architect does not want to change. In this type of forum, they can explain their architectural passion about the item, which enables others on the team to better understand the architectural perspective.

There also may be economic drivers to consider. The building may have a particular shape or orientation because it lends itself to a better unit layout, or it may be oriented a certain way to maximize views – for example, overlooking New York’s Central Park. With such considerations on the table, there is typically a good discussion resulting in a few options to be considered. At that point, it is back into the wind tunnel to quantify the benefit of those different changes to the shape.

Fortunately, advances in rapid prototyping now permit testing of numerous models costeffectively and in rapid succession. In one day at a recent shaping workshop, the team ran through a series of 10 different iterations in succession, each showing slight improvements over the others.

A Challenging Structure in NYC One of the more challenging and creative recent applications of wind engineering expertise is nearing completion in the form of 432 Park Avenue. This 1396-foot (426meter) structure in the center of Manhattan is the tallest residential tower in the western hemisphere and the third tallest building in the U.S.

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Architect Rafael Viñoly’s vision for the tower was pure prismatic form, something that was very compelling for the owner as well. It was meant to be this square shape, extruded straight into the sky, the exposed concrete expressing structure in a pure and simple elegant form.

When RWDI’s wind engineers began working with the architect and the structural engineer, WSP Cantor Seinuk, part of their job early on was to educate the rest of the design team on the special challenges in this tall, slender, prismatic structure. Its dynamic properties, when combined with the site’s known wind characteristics, meant that about once a month the wind speed would cause a significant wind-induced motion. That type of event is similar in magnitude to a once a year event in more traditional structures, but this building would get into this resonant condition with the wind fairly frequently.

That was new and somewhat unexpected territory for the design team to consider.

Recognizing the comfort of occupants is the factor that is most critical, the team had to set the limits of motion – and more importantly of acceleration – to be achieved.

There are well-established criteria based on a great deal of practice working on more conventional structures over the years.

However, they are based on the assumption of infrequent wind-induced motion. On this project, they would be much more frequent, if not the norm, for some periods of time.

all members of the design team that they would have to look to tuned mass dampers and other technologies to make the project a success. With the limits of motion coming out of the simulation experience, it became clear that the two 600-ton mass tuned dampers already planned for installation at the top of the tower would not be sufficient working on their own. It was time to address architectural modifications.

The strong commitment of the architect and owner to maintaining a pure prismatic form led to a shaping workshop exploring the concept of opening up some of the floors to allow wind to pass through the building and break up the coherence of the vortices.

Ultimately, opening up two adjacent floors at five locations evenly spaced throughout the building’s height was found to be the optimal arrangement. Working in concert with the tuned mass dampers, this achieved the necessary reduction in wind effects on the structure without compromising the architectural expression.

Figure 6. 432 Park Avenue with double story openings at five levels.

Project Partners For the projects identified in this article, consulting engineering firm RWDI (Guelph, Ontario, Canada) worked in close collaboration with the respective design teams. For the Burj Khalifa, this included Skidmore, Owings & Merrill, who were the design architect and structural engineers for the project. For the Shanghai Tower, the design team included the design architect Gensler and the structural engineers Thornton Tomasetti. The design team of 432 Park Avenue included the design architect Rafael Viñoly Architects of New York City, WSP Cantor Seinuk, who was the structural engineer, and SLCE, the architect of record.▪

Conclusion Wind effects are undoubtedly forces to be reckoned with, especially in designing tall, slender structures. Using an orderly and

The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

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The traditional ways of thinking about wind effects and the rules of thumb normally used didn’t apply in this case, which meant rewriting the rulebook.

The design team looked to a marine simulation center in Newfoundland for a better understanding of the problem.

This group’s unique motion simulator is designed to help train marine captains.

Because the natural frequency of the 432 Park Avenue tower aligns fairly well with the period of ocean waves, the facility could easily simulate the sort of motion predicted for the structure.

First, the standard rules of thumb were translated into a motion simulation that could be experienced by the design team.

The simulation replicated the building response during a wind event at the more frequent occurrence levels anticipated.

Next, the design team – the architect, the structural and wind engineers, and the owner – went through a series of exercises to experience the actual predicted motions that occupants would feel in the building, looking at different scenarios for mitigating that motion. It was an exercise in translating rules on the page – the benchmark values gleaned from years of experience – into rules based on simulations of the occupants’ anticipated experience.

After an early study had quantified the wind-induced motion, there was an acceptance and understanding among

rational approach, such as a shaping workshop, addressing wind-related concerns early in the design process enables all involved to push the limits of possibility while still producing a building that is comfortable, economical, and safe.

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Structural DeSign design issues for structural engineers

T

his article discusses Federal Laws, Executive Orders, Building Codes and Standards that apply to the seismic design of nonstructural building components. It highlights various aspects of the seismic design requirements. The discussion is not all-inclusive; however, it does include requirements that are either new or commonly overlooked. References to specific nonstructural building components and seismic sway brace components are provided as examples.

Executive Order 13717 On February 2, 2016, President Obama signed an Executive Order (EO 13717) titled Establishing a Federal Earthquake Risk Management Standard. It sets out to establish requirements for earthquake safety of new federal buildings, improvements to existing federal buildings, and federally leased, financed, or regulated buildings. EO 13717 also indicates that federally financed buildings include those that are financed through federal grants, loans, or guaranteed financing through loan or mortgage insurance programs.

EO 13717 goes on to order that every new building covered by the order, and for which project assumptions, scope, budgets and strategy for building start after May 2, 2016, shall comply with the seismic provisions of the 2015 or later edition of the International Building Code (IBC).

According to EO 13717, the National Institute of Standards and Technology (NIST) currently leads the Interagency Committee on Seismic Safety in Construction (ICSSC) for The National Earthquake Hazards Reduction Program (NEHRP) and shall assist the federal agencies with implementing these earthquake risk-reduction measures. Each federal agency shall also designate a Seismic Safety Coordinator to serve as the focal point for compliance with the EO and to participate in the ICSSC. Among other things not mentioned above, EO 13717 also revokes EO 12699, which was required by Public Law 101-614 to be signed by President Bush in 1990 and made the Federal Emergency Management Agency (FEMA) the lead agency in implementing NEHRP. Public Law 101-614 was a 1990 amendment to The Earthquake Hazards Reduction Act of 1977 (Public Law 95-124), the effects of which are still dealt with today. While there have been additional amendments to Public Law 95-124 over the past one-quarter of a century, most of those have dealt primarily with funding issues.

Public Law 101-614 required implementation of NEHRP, including the requirements of NEHRP Standards, and required the signing of EO 12699 to accomplish its implementation.

Seismic Design of Nonstructural Building Components Federal Laws, Executive Orders, Building Codes, Standards

By Daniel C. Duggan

Daniel C. Duggan is Vice President of Vibration & Seismic Technologies, LLC in St. Louis, MO. He is a member of the ASCE 19 Structural Applications of Steel Cable for Buildings Committee, the NFPA 13 Hanging and Bracing Committee and the UL 203 (203A) Standard Technical Panel. Daniel can be reached at dduggansr@vstseismic.com.

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Federal Law, NEHRP, IBC and ASCE 7 Public Law 101-614 and EO 12699 apply NEHRP to new building construction started after January 4, 1993, and existing federal buildings, federally leased or regulated buildings, and buildings financed through federal grants or loans or guaranteed through Federal loan or mortgage insurance programs.

Application of the new EO 13717 is virtually the same in this regard. Since the NEHRP standards are seismic provisions and not complete building codes, the most practical way to implement them was through the model building codes. The model building codes were the ICBO Uniform Building Code (UBC), National Building Code (BOCA) and Standard Building Code (SBCCI). Since FEMA would be judging eligibility for disaster-relief funding, the model code organizations wanted to make adjustments to their seismic requirements to demonstrate their compliance with NEHRP. Each of the model codes was allowed to self-certify their NEHRP compliance.

In the years following the passage of Public Law 101-614 and the signing of EO 12699, successive revisions were made to the NEHRP Standards, the three model codes, and their referenced standards. By 1996, they agreed not to publish updates to their codes and consolidate into one International Code Conference (ICC) and promulgate the use of one International Building Code (IBC) starting in 2000.

The 2000 IBC included the seismic maps and requirements of the then-current NEHRP standard, as did subsequent editions of ASCE 7 Minimum Design Loads for Buildings and Other Structures. Because of the required application of NEHRP to federal buildings, federal agency specifications, such as the Corps of Engineers Guide Specifications (CEGS) and now the Unified Facilities Guide Specifications and others, have references to NEHRP standards and ASCE 7. Today ASCE 7 is referenced by the IBC and virtually all the federal documents dealing with building construction requirements.

In 1990, it was not known that there would someday be an International Building Code, with ASCE 7 as its referenced standard, containing the seismic requirements that satisfy the federal laws and executive order implementing NEHRP, but that is the condition today. So, the requirements of the current EO 13717 do not differ greatly from the 1990 EO 12699. However, the new EO does make it clear that the 2015 IBC or more recent IBC edition is to be used to comply with the federal laws implementing NEHRP, and that NIST is the lead agency for NEHRP rather than FEMA.

ASCE 7 Seismic Design Requirements for Nonstructural Building Components A significant portion of the work that is required to be done by NEHRP and the IBC is the earthquake protection of nonstructural building components, and these requirements are in the structural design requirements of the building code. Section 1613.1 of the 2015 IBC requires nonstructural building components to be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7. These nonstructural building components are architectural, mechanical, electrical, and plumbing equipment and systems, and Chapter 13 of ASCE 7 contains the seismic design requirements for them.

The Seismic Design Category (SDC) for the nonstructural building components is the same as that of the building that they occupy or to which they are attached. However, the Component Importance Factor (Ip) is not the same as the Building Importance Factor.

The Importance Factor can be 1.0, 1.25, or 1.5 for the building but it is either 1.0 or 1.5 for the nonstructural building components.

ASCE 7-10 requires that Ip shall be taken as 1.5, if the component is for life safety, including fire sprinkler systems; or, if the Authority Having Jurisdiction (AHJ) finds that component contains sufficient quantities of toxic or poisonous materials to pose a threat to the public if released; or, if the component is required for continued operation of a Risk Category IV structure; or, if the component contains enough hazardous material for the AHJ to classify the structure as hazardous.

The Ip shall be taken as 1.0 for all other nonstructural building components.

There are general exceptions from the seismic design requirements of Chapter 13 of ASCE 7-10. These exceptions include: • most storage cabinets six feet or less in height • temporary or movable equipment • architectural components with an Ip of 1.0 in SDC A or B other than parapets and shear walls • MEP equipment in SDC A or B regardless of Ip • MEP equipment in SDC C when Ip = 1.0 • MEP equipment in SDC D, E, and F when Ip = 1.0 o and it is positively attached to the structure o and flexible connections are provided between the component

Figure 1. Prying.

and associated ductwork, piping and conduit o and either ▪ the component weighs less than four hundred pounds with its center of gravity four feet above the floor, or ▪ the component weighs twenty pounds or less, or ▪ the component is a distribution system that weighs five pounds or less per foot.

Chapter 13 of ASCE 7 also contains additional exceptions from its seismic design requirements based on the size of the nonstructural building component or the method of attachment, such as hanger length.

Designated Seismic Systems Some Designated Seismic Systems require special certification. Designated Seismic Systems are those nonstructural building components with Ip = 1.5 and require seismic design in accordance with Chapter 13 in SDC C through F. Special certification by the manufacturer is required for active mechanical and electrical equipment that must remain operational after the design earthquake.

According to ASCE 7 C13.2.2, “active” designated seismic equipment is a limited subset of Designated Seismic Systems that has parts that rotate, move mechanically or are energized during operation, and as a rule of thumb can be limited to equipment with electric motors greater than 10 horsepower or greater than 200 MBH (one MBH equals 1000 BTU/hr). Pipes, ducts, conduits, and similar apparatus are not active equipment.

Special certification by the manufacturer to maintain containment following the design earthquake is required for components with hazardous substances and Ip = 1.5 per Section 13.1.3. Special certification shall be certified

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based on either shake table testing, such as ICC-ES AC156, or based on experience data unless it can be shown that the component is inherently rugged. ASCE 7 C13.2.2 also states “Past earthquake experience has shown that most active equipment is inherently rugged.”

Concrete Anchor Capacities Prying and Overstrength Among the other seismic design requirements in Chapter 13 are a consideration of allowable stresses on piping, based on ductile or non-ductile materials and the connection methods used, and more conservative design requirements for anchorage to concrete. Post-installed anchors in concrete have been required to be seismically prequalified in accordance with ACI 355.2 since the publication of ASCE 7-05. Post installed anchors in concrete have been required by ASCE 7 for many years to be designed in accordance with ACI 318 Appendix D and to consider the effects of prying and eccentricities, although the effects of prying have been overlooked in the design of seismic sway braces for nonstructural building components.

Prying (Figure 1), due to the geometry of the seismic sway brace brackets or fittings that are used for anchorage, can significantly increase the applied tension load on the anchor. This condition could influence the selection of sway brace brackets with more beneficial geometry.

In addition, ASCE 7-10 Supplement No. 1 adds a column for Overstrength Factors (Ω0) to Tables 13.5.1 and Table 13.6.1, which assign Component Amplification Factors (ap) and Component Response Factors (Rp) to the range of nonstructural building components for which the seismic design requirements of Chapter 13 apply, with an Ω0 value of 2.5 for most of them. ASCE 7-05 required anchors in concrete be proportioned to carry at least 1.3 times the earthquake load. ASCE 7-10

Figure 2. Tension/compression sway brace.

(Strut, pipe or angle iron type, hanger not shown for clarity.)

Supplement No. 1 virtually doubles it with this change to using Ω0.

The NFPA 13 Standard for the Installation of Sprinkler Systems has been revised over the last four change cycles to include the applicable seismic design requirements of ASCE 7 through each of its change cycles. This change includes tables that address allowable stresses on piping, the requirement to use ACI 355.2 seismically prequalified concrete anchors, along with the effects of prying, and tables for reduced allowable earthquake loads based on Ω0. Accordingly, NFPA 13 has maintained its status as an ASCE 7 complying standard.

direction on suspended piping but the same principals apply for other suspended nonstructural building components. These sway braces commonly use strut, pipe or angle iron for the primary sway brace element.

The second type resists back and forth shaking motion of the horizontal earthquake load in tension only. This second type uses steel cable for the primary sway brace element. Figure 3 shows an example of a tension-only (cable type) sway brace installed in the transverse direction on suspended piping, but the same principles that apply to cable-type sway braces also apply for other suspended nonstructural building components.

An upward vertical reaction results when either type of sway brace resists the horizontal earthquake load in tension because both types are free to rotate about their connection to the structure. However, a downward vertical reaction results from seismic sway braces that resist the horizontal earthquake load in compression and adds a substantial load to the vertical support for the nonstructural component. Tension only (cable type) seismic sway braces cannot resist the horizontal earthquake load in compression; therefore, they cannot have a downward vertical resultant load on the nonstructural component hanger.

Seismic Sway Bracing

Code-Referenced Sway Brace Standards

The nonstructural building components that are required to be designed in accordance with the seismic design requirements of ASCE 7 Chapter 13 are architectural, mechanical, electrical and plumbing equipment that are floor, roof, and wall mounted, or suspended from the building structure.

Generally, the earthquake protection of suspended nonstructural building components is provided by installing seismic sway bracing that is required to be designed in accordance with ASCE 7 Chapter 13 and includes consideration of both horizontal and vertical earthquake effects. While the vertical earthquake effects are both upward and downward, the downward vertical effect is often overlooked. ASCE 7-10 Section 12.4.2 requires that the seismic load effect includes the vertical effect, both upward and downward, and applied simultaneously with the horizontal load effect.

There are two types of seismic sway brace.

One type resists back and forth shaking motion of the horizontal earthquake load in tension in one direction and compression in the other. Figure 2 shows an example of such a sway brace installed in the transverse

There is no IBC or ASCE 7 referenced standard for the strut, pipe or angle iron type seismic sway brace components. However, there is an IBC and ASCE 7 referenced standard for the steel cable type seismic sway brace components, and it is ASCE 19 Structural Applications of Steel Cable for Buildings.

Section 102.4 of the 2015 IBC states that the standards referenced in it are considered part of its requirements, but that the requirements of the IBC take precedence if there are any conflicts between them. Section 2208 of the 2015 IBC requires that steel cables for buildings be in accordance with ASCE 19, and only ASCE 19.

Section 1613.1 of the 2015 IBC requires nonstructural building components to be designed and constructed to resist the effects of earthquake motions in accordance with ASCE 7. Chapter 13 of ASCE 7-10 defines the seismic design requirements for nonstructural components, and Section 14.1.6 requires steel cables to be in accordance with ASCE 19.

ASCE 19-10 Section 4.1 Cable Specifications includes ASTM A1023 Standard Specification for Carbon Steel Wire Ropes for General Purposes,

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Figure 3. Tension only (cable) brace. (Cable type, hanger not shown for clarity.)

Table 9, which is galvanized steel aircraft cable, and refers to Commentary Section C4.0 for the specific application. Commentary Section C4.0 states that ASTM A1023, Table 9, is listed to allow small-diameter galvanized steel cables to be used for seismic sway bracing of nonstructural building components.

ASCE 19-10 Section 3.3.1 requires a reduction in the available cable strength caused by cable end fittings based on the fitting reduction factors in Table 3-1, which no longer includes U-shaped cable clips or wedge type fittings. Also, Section 3.3.2 requires end fittings to develop an ultimate strength greater than the nominal cable strength. There are cable fittings on the market which make it possible to secure a cable loop by sliding the cable through one side and then back through the other side with a wedge, usually with serrated teeth that bite into the outer wires of the cable when tension is applied.

U-shaped cable clips and wedge type fittings are prohibited by ASCE 19 because they “damage the cable and/or loosen over time.” Such cable fittings do not develop an ultimate strength greater than the nominal cable strength, and ASCE has issued a formal interpretation regarding their prohibition along with the reasons for it. This formal interpretation to ASCE 19-10 was published online in the ASCE Library, and it can be viewed here http://ascelibrary. org/doi/pdf/10.1061/9780784411247.int.

Conclusion In conclusion, while the Executive Orders of 1990 and 2016 are not dramatically different, the requirements for seismic sway bracing of nonstructural building components have changed significantly. Since the new EO requires compliance with the 2015 (or later) IBC, the referenced standards mentioned above have become critically important in the design and construction of federally owned, leased, regulated or financed buildings – and, of course, all buildings where International Building Code compliance is required.▪

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Structural Performance performance issues relative to extreme events

I

n developed cities around the world, space on the ground is becoming scarce. The new trend is to build slender, “mega-tall” skyscrapers.

These structures require special considerations to mitigate the effects of wind forces that develop against the building’s façade. Wind forces can cause significant sway or twist, particularly in the upper floors of the skyscraper, and windinduced accelerations can cause motion sickness for many individuals inside these structures.

Tuned Mass Damper (TMD) systems have gained popularity in recent years as a cost-effective solution to mitigate wind-induced vibrations in slender, mega-tall structures. A TMD is a system that is tuned to the building’s natural frequency and acts 90 degrees out-of-phase with external forces to add damping, dissipate dynamic energy, and ultimately reduce accelerations to imperceptible levels to maintain occupant comfort of the structures. Traditionally, these mega-tall structures have been designed with conventional structural materials and stiff components within the lateral load-resisting system to overcome the accelerations due to external wind forces. However, when using a design philosophy that begins with the intent to incorporate a TMD system, engineers can design the structural system with lighter-weight, more flexible components, which can translate into significant cost savings even once the cost of the TMD system is included.

One common TMD often considered for megatall skyscrapers is the pendulum system. There are many different ways to configure such a system to control structural accelerations. The pendulum system itself is a large mass, usually a cube of steel weighing several hundred to one thousand tons, hung by cables through a central shaft or room within the structure.

Critical Wind Mitigation for Mega-Tall Structures Tuned Mass Damper Systems Deliver

By Ben Eder

Ben Eder is the Business Development Manager at ITT Control Technologies / ITT Enidine Inc. His primary focus is on the application of hydraulic energy dissipation devices. Ben can be reached at Ben.Eder@ITT.com.

Why are TMD Systems Ideal for Wind Mitigation?

Based on many years of data, we can reasonably predict the severity and incidence of wind events that act upon a building in a particular location and using engineering analysis we can estimate a building’s natural frequency. TMD systems could theoretically be used to mitigate demands due to other lateral forces such as from seismic excitations; however, ground motion excites a structure at multiple frequencies and therefore a TMD system is not as effective to counter earthquake motion. If there was an earthquake near a skyscraper, a TMD system might provide some protection, but it would only mitigate the effects of excitations near the tuned frequency. Thus, when looking to mitigate the effects of wind forces on a structure with one natural frequency, TMD systems are an ideal solution.

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TMD technology offers the ultimate in wind load mitigation in high rise applications.

However, a building’s frequency can change over time, resulting in a need to service TMDs periodically. Although the structure of a building and external wind forces typically remain the same, certain floors may be used for purposes other than their original design. For example, if a room was originally dedicated for a gymnasium, but is being converted to accommodate a swimming pool, a significant amount of weight is added to that floor. The additional weight can, in turn, change the building’s natural frequency. So the ability to adjust the tuning of a mass damper system over time is essential to keeping it matched with the building’s frequency.

The Benefits of FVDs in TMD Systems TMD systems may incorporate several proprietary damping technologies, and fluid viscous dampers (FVDs) can be used in combination with the pendulum system to provide easier, safer and more cost-effective servicing and tuning of the TMD. FVDs add damping to a structure and therefore reduce the structural response caused by external wind forces.

FVDs are passive energy dissipation devices that convert kinetic energy into heat, typically over multiple cycles of deformation. The basic relationship governing the behavior and performance of a fluid viscous damping device is defined by the Constitutive Law: Constitutive Law – “A relation between two physical quantities (especially kinetic quantities as related to kinematic quantities) that is specific to a material or substance, and approximates the response of that material to external stimuli, usually as applied fields or forces.”

• F: Damping Force [kN] • V: Relative Structural Velocity [m/sec] • C: Damping Coefficient [kN/(m/sec)α] • α: Velocity Exponent F = CV α A resultant force produced by an FVD is related to a power function of the velocity (non-linear FVD), where a Velocity Exponent of 1 results

in a force that is proportional to the relative structural velocity (linear FVD). FVDs use silicone fluid and custom orifice geometry to obtain linear or non-linear FVD performance.

Velocity exponents of 0.1 to 2.0 are possible.

FVDs are attached to the structural system on one end, and the pendulum mass damper on the opposite end, an ideal location for FVDs to enhance system damping due to the large differential displacements cause by building deformations being out-of-phase with pendulum displacements. The function of the FVD is to dissipate wind energy, via kinetic friction, by creating heat energy which is easily managed by the building’s HVAC system. Kinetic friction is produced from the hydraulics within the piston by the actuation of a piston rod pushing internal hydraulic fluid through engineered orifices. The resultant resistance produced by this actuation through the damper stroke is the damping performance.

It is important to note that a skyscraper with a mass damper system will still deflect up to a few feet at the top, where the accelerations are the highest. The primary purpose of the TMD system is too slow down the building’s acceleration, so it is imperceptible to the human occupants inside the building, preventing motion sickness. Unfortunately, where a TMD system is needed – at the highest points of the

structure – is also some of the most valuable realestate in developed cities like New York City.

While the primary purpose of the FVD is to dissipate energy, new design features are being incorporated into these devices that may allow easier maintenance and tuning, which can ultimately help reduce the lifecycle cost of the TMD system. A traditional TMD system is mechanical and, to stop the pendulum swinging motion, an engineer has to attach a steel clamp to the large, moving mass to slow and stop the swing. This can be dangerous to the operator because of the potential to become entrapped or pinched between the moving TMD mass and other components of the TMD system. FVDs can include a “lock-out” that allows the damper movement to be stopped and locked from outside the range of travel of the TMD mass for safe servicing and inspection of the TMD system. FVDs can also include the ability to “tune” the damping performance, which can allow for simple routine tuning of the TMD system to ensure it is always operating as designed. These specialty FVD features are as simple as turning a knob. These features are critical because of the inevitable need for the TMD system to be “locked-out” and “tuned” for the continued safety and comfort of individuals inside these structures.

Design of TMD systems using ITT Enidine fluid viscous damper technology increases occupant comfort, reduces life cycle costs, and increases safety.

Conclusion It is important to match the TMD and FVD design to the application requirement and to work with the right manufacturer, designer, and engineer to provide the ideal system for the structure. There are many benefits of incorporating a tuned mass damper system to mitigate the effects of wind on infrastructure, specifically skyscrapers. Fluid viscous dampers are the solution to improve a TMD system’s performance further and improve the long-term safety and quality of a structure.▪

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Structural teSting issues and advances related to structural testing

T

his article is a follow-up to one of the precast, prestressed thin slab parking garage structures described in Precast, Prestressed Thin Slabs in Parking Garage Structures, published in the January 2016 issue of STRUCTURE magazine.

As a result of the findings of the previous investigation of the upper four levels of an existing six-story parking garage structure, and the identification of a significant number of unsafe prestressed slabs (planks) that had to be temporarily shored, Pennoni recommended that Non-Destructive Evaluation (NDE) of the first framed level be conducted. The purpose of the NDE was to determine if additional unsafe planks also existed at the lower level of the garage.

The NDE investigation was required because access to the underside of the framing at the lowest level of the garage was not practical or, in some cases, even possible. In addition, because all of the other unsafe planks in the upper levels of the garage were impacted primarily by corroded and or failed strands in the slabs, it was reasonable to assume that the lowest level of the garage (which is exposed to the most traffic from vehicles entering the facility immediately from streets treated with deicing salts) would also be adversely affected by strand corrosion. Because the ground floor spaces immediately below the first framed level could not be disturbed, the plank soffit cracking and spalling at corroded strands could not be easily observed or accessed for closer inspection. Therefore, it was necessary to use extraordinary means to detect any unseen strand corrosion from the driving surface of the first framed level.

Initial research into possible NDE resources first involved contacting Pennoni’s Intelligent Infrastructure Systems (IIS) Division, which performs NDE and installs a number of different types of structural monitoring systems for highway bridges. IIS indicated that, of the many NDE methods available, the best approach would be to use Magnetic Flux Leakage (MFL) to detect the presence of a loss of cross-sectional area of the strands. MFL involves magnetizing the embedded reinforcing and scanning the same areas to detect anomalies. Anomalies detected during the survey would be an indication of the likely section loss of a strand, which in turn would allow for the identification of areas of strands likely to be significantly damaged or completely consumed by the corrosion process.

Although IIS did not have access to MFL equipment, the Center for Advanced Infrastructure and Transportation (CAIT), Infrastructure Condition Monitoring Program (ICMP) at Rutgers University did have the recommended equipment.

Magnetic Flux Leakage for NDE

By D. Matthew Stuart, P.E., S.E. P.Eng., F.ASCE, F.SEI, SECB, Al Ghorbanpoor, Ph.D., P.E., F.ASCE and Kevin Saldivar, E.I.T.

D. Matthew Stuart (MStuart@ Pennoni.com) is the Structural Division Manager at Pennoni Associates Inc. in Philadelphia, Pennsylvania.

Al Ghorbanpoor (algh@uwm.edu) is a Professor & the Director of the Structural Engineering Laboratory at the Department of Civil & Environmental Engineering, College of Engineering & Applied Science, University of Wisconsin-Milwaukee.

Kevin Saldivar (KSaldivar@ Pennoni.com) is a Graduate Engineer at Pennoni Associates Inc. in Philadelphia, Pennsylvania.

20 July 2016

Dr. Nenad Gucunski, Ph.D., the ICMP Program Director, indicated that the MFL equipment at the CAIT did not have the capability to detect the strand, which was as deep as 5 inches from the top surface of the slab. He directed Pennoni to contact Dr. Al Ghorbanpoor, Ph.D., the Director of the Structural Engineering Laboratory at the University of Wisconsin-Milwaukee. After further discussions, Dr. Ghorbanpoor was subsequently retained for the MFL survey of the lower level of the garage.

Based on the results of the previous investigation of the upper levels of the garage, which revealed that corroded and failed strands had occurred predominately adjacent to the joints of the planks, the MFL scan was performed on each side of the plank joints. The approximate locations of the plank joints were determined based upon the location of the corresponding concrete topping control joints, which could be observed on the driving surface. The approximate total linear feet (LF) of joints located at the lower level of the garage was 2,500 LF; therefore, a total of approximately 5,000 LF of MFL scans were required.

Due to the presence of existing #5 top reinforcing bars in the concrete topping, which occurred parallel to and near the plank strands, it was not clear before the MFL survey was completed if it would be possible to magnetize and assess the condition of the underlying plank reinforcing. It was anticipated that the most likely area for potential interference would be above and near the existing steel beam supports. In addition, it was anticipated that the proximity of the vertical lattice or “filigree” reinforcement could also potentially make it difficult to identify the adjacent 3/8-inch diameter strand using the MFL system.

MFL Investigation A novel NDE method based upon the concept of MFL was used in the assessment of the prestressing strands in the thin precast concrete slabs in this parking garage structure. The MFL concept is based on detecting magnetic flux leakage in a magnetized strand where there is a loss of crosssectional area. Figure 1 illustrates this concept as the response of a Hall-effect sensor signifies a section loss and its location in a strand or rebar embedded in concrete. Hall-effect sensors are transducers that produce a varying output voltage in response to a changing magnetic field. The sensor output, or the shape of the MFL signature, including its amplitude and period, is influenced by the size, configuration, and orientation of the section loss as well as the distance between the steel reinforcing and the magnetic sensors.

The primary components of the MFL system are two strong permanent magnets and a set of Halleffect sensors that are installed in a testing cart to

Figure 1.

allow for portable testing of parking garages and bridge decks (Figure 2). The system is also equipped with an encoder device that allows recording of the precise location of each data point along the length of each scan. The system is designed to be durable for field use and requires only a small DC power source for its operation.

Before the MFL test at the site, a laboratory investigation was performed to verify the capability of the MFL system to detect section loss in steel strands. The MFL system was verified to have the capability of detecting losses exceeding approximately 40 percent of the cross-sectional area in strands and rebars that were located no more than five inches from the surface. Smaller cross-sectional area losses could not be detected reliably due to the depth of embedment and the effect of magnetic interference from other steel rebars within the garage slabs.

Figure 3 illustrates representative laboratory MFL signatures from multiple side-by-side sensors (sensors 1 to 7 covering a width of 8 inches) for steel rebars and strands with varying cross-sectional area loss. These strands and rebars were located a distance of 5.5 inches from the top surface. Figure 3 indicates the scan length, in feet, and signal amplitude, in

volts, along the horizontal and vertical axes, respectively. Note that the larger amplitude MFL signals from sensors 6 and 7 indicate a complete fracture in the lowest bar shown in Figure 3.

After the completion of the laboratory verification, the MFL equipment was transported to the site for field testing. Daniel Strohfeldt, an Electrical Engineering undergraduate research student at the University of Wisconsin – Milwaukee, assisted Dr. Ghorbanpoor with the field investigation.

The system was used to scan the surface of the lowest framed level of the structure to assess the condition of prestressing strands in the thin precast slabs. The construction of the framed garage structure included a layer of concrete topping over the precast slabs. Each scan was made by moving the testing cart directly above a strand near the topping joint located above the precast slab joints below.

Figure 3.

Figure 2.

Figure 4.

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July 2016

The approximate location of each strand was marked on the garage deck with a chalk line before the start of the MFL tests, which was used to guide the direction of each test scan.

The field testing was preceded with an additional verification test on strands with known conditions (i.e., with and without corrosion) at the second framed level of the garage structure in which the precast slabs were visible from the level below. Figure 4 shows side-by-side representative graphs of the MFL tests for these strands, with the resulting data (i.e., amplitude vs. scan data position) indicating the strand conditions.

The multiple peaks and valleys seen in the graphs are from the responses of the sensors due to the interference effects of crossing rebars in the concrete topping. The higher amplitude signals in the graphs are associated with the location of the steel beams supporting the precast slabs.

The MFL test at the first framed level of the parking garage structure included a total of 106 scans along the edges of adjacent precast slabs. These tests were completed over a period of 1.5 days. The use of the MFL system in the garage structure allowed for a successful detection of corrosion in multiple areas. The MFL test scan locations, corroded strand lengths, and strand fracture for each strand were marked on the plan to aid the condition assessment of the garage deck (Figure 5, page 22). continued on next page

X'-X" (X)

#

Figure 5.

As indicated in Figure 5, there is evidence of local and extended corrosion, as well as a partial or full fracture of the strands within the precast slabs for the first framed level of the parking garage structure. In total, the MFL test results indicated 37 partial or full strand fractures and 81 areas of local or extended strand corrosion. Using the results from the MFL tests, appropriate temporary support methods were designed and implemented by Pennoni Associates to extend the service life of the structure.

Repairs As previously indicated, the majority of the underside of the lowest framed level of the garage was not accessible. However, some areas of the framing were visible, but not easily accessible, from the ground floor loading dock and entrance ramp of the garage. Observations made of surface cracks and spalls in the soffit of the planks were used as the basis for confirmation of the conclusions of the MFL survey. Areas of strand section loss identified by the scans

corresponded with the same areas of cracked and spalled plank soffit.

Fortunately, the majority of the unsafe plank locations identified by the MFL survey were above these same limited-access areas, which made it possible to install shoring beams rather than having to block off the same planks above from further vehicular access.

The method of installing the shoring beams followed the previously established criteria used in the upper levels of the garage, which minimized the impact of the repairs on the operation of the facility. Unsafe areas that were not accessible, including parking spaces and a small area of the driving aisle, were permanently blocked off from further vehicular access via barricades that were attached to the concrete topping.

The design of the shoring beams was based on a tributary width of 2 feet, which corresponded to the strand edge distance from the plank joint of 3 inches and one-half of the uniform spacing of the strand within the plank of 18 inches on center, on each side of the plank joints. This allowed for the shoring beams to be located near the

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July 2016

plank joint centerline, thereby providing support for the portion of both adjacent planks that had already exhibited, or had the potential to be subjected to, significant strand corrosion.

Conclusions As a result of the overall investigation and temporary shoring of the garage at all five framed levels, and in acknowledgement of an estimated remaining service life of the structure of six to nine years (i.e. the point at which all planks would likely have to be shored), it was recommended to the owner of the facility that visual and exploratory demolition investigations of the accessible framed levels and MFL NDE, at the lowest framed level only, be conducted annually. Annual inspections would identify additional unsafe planks that needed shoring over the remaining life of the building. Based on the results of the additional future investigations, the owner plans on determining when it would be economical to demolish the existing structure and construct a new replacement garage.▪

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Structural rehabilitation renovation and restoration of existing structures

I

n 2009, maintenance workers at Canada Place observed brown spots on several stay cables of its large fabric membrane roof.

Based on this observation, Canada Place Corporation, the authority for maintaining the facility, launched an evaluation of the cables and the protective coating of the wires that make up the cables.

Wires that make up the cables are protected against corrosion as they generally are used at high-stress levels. Corrosion of the wires causes surface discontinuities that can result in stress risers and reduce the load capacity of the wires and, thus, the cable.

the roof. The stay cables are anchored along the edge of the promenade and cross over it to the top of the masts (Figure 2). The stay and ridge cables are 27/8-inch diameter wire strands and are comprised of one hundred ¼-inch diameter wires.

The ridge cables are positioned external to the fabric and thus are exposed to the environment.

Stainless steel loops wrap over the cables and are bolted to a clamp system that holds the fabric (Figure 3). Due to the proximity to the salty sea water of the bay, a Class C coating (see below) was specified for all cables.

Canada Place

Wires for cables are fabricated from high-strength steel that are rolled to a circular or Z-shaped section and then drawn through a mandrel to their final size. The cold working of the steel during this process increases the strength of the wires. A protective coating is applied after the cold working process.

There are several corrosion-resistant systems for cables. The most common one is a process of coating its wires. Various protective wire coatings include epoxy, zinc-aluminum-mischmetal, and pure zinc coatings, though the zinc coating is by far the most common one. Three levels of zinc protection are listed in the American Society for Testing and Materials ASTM A475-03 (Standard Specification For Zinc-Coated Steel Wire Strand) and ASTM A603 (Standard Specification For ZincCoated Structural Wire Rope), the specifications for strand and rope cables.

Class A is the most widely used and sufficient for the most common applications. Class B is used

Canada Place was built in 1986 in Vancouver, British Columbia. It houses a hotel and convention center that also serves as the terminal for cruise ships that frequently dock in Vancouver’s harbor. It is owned and operated by the Vancouver Fraser Port Authority, which is responsible for the stewardship of federal port lands in and around Vancouver. The main feature of the building is a large tent-like fabric membrane structure that covers most of the convention center space (Figure 1).

The roof spans 170 feet and is 478 feet long. Its system of cable-stayed masts are spaced 80 feet apart and are 125 feet high. The masts anchor the ridge cables that support the membrane roof and are located along promenades at each side of

Repair of Corrosion Protection for Cables A Case Study

By Paul A. Gossen, P.E., F.ASCE

Paul A. Gossen is a Member of the ASCE Committee 19 – Structural Application of Steel Cables for Buildings, as well as the ASCE Committee 55 –Tension Membrane Structures Standards Committee. He can be reached at pag@geigerengineers.com.

Figure 1. Canada Place.

24 July 2016

Cable Fabrication and Protection

corrosion resistance than Class A coatings, their coating is less abrasion resistant than Class A coatings.

Cables can be fabricated with a combination of Class B or Class C for the outer wires and Class A for the interior wires, or Class A, B, or C for all the wires of the cable.

Thick layers of zinc coating, like the one used for the wires of Class B and C, are generally deposited through an electrolytic process, while the coating for the Class A protection is commonly applied through a “hot dip” zinc bath. The temperature of the zinc bath, 840ºF, does not change the metallurgical structure of the steel that was obtained during the cold deform process because the exposure time is very short. Thus, there is no or very little loss of strength in the wires.

Figure 3. Ridge cables supporting the fabric.

Cable Observations and Remedial Measures Figure 2. Promenade along the convention center.

in more corrosive environments such as frequent water exposure. It consists of a thicker zinc coating on the wires. Class C, with an even thicker zinc coating, is prescribed for the most severe corrosion-causing environments.

Though Class B and C coatings have better

The observations of the brown spots on the stay cables in 2009 caused concern that the cables may be corroding. As a result, the Canada Place Authority launched a detailed investigation to evaluate their condition. Unfortunately, there is no reliable method to detect corrosion within a cable other than through destructive testing. Destructive testing (cutting the cable

Figure 4. Depleted zinc coating on cable.

open) implies that it must be replaced. Thus, the option is either just to replace the cable outright or to visually inspect them for signs that could indicate corrosion of the hidden inner wires. Rust stain (bleeding) or bulging

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from expanding corrosion can be an indication of corrosion within the cable.

A close examination of the surface of the cable showed no rust stains or bulging, but the zinc coating was delaminating in little flakes on the exposed surfaces of the outer wires (Figure 4, page 25). The area of the wires, where the coating had delaminated, turned brown, indicating the onset of corrosion. There was no observed cratering or pitting from corrosion. Based on these findings, the consultant recommended using a cold galvanizing process that called for a recoating of the cables with a zinc-rich compound. A detailed preparation procedure and specification were drafted. It specified that the cables were wire-brushed with a soft wire wheel, with the brushing applied in the same lay, the twist direction of the wires, to avoid creating any scarring that could cause stress risers. The cables then were to be cleaned by air blasting and wiped to remove any dust.

The surface had to be free of any contaminants that could diminish the bond of the zinc compound. The specification called for a coating thickness of 2.5 to 3.5 mils with 95% metal zinc content after drying.

The execution of the treatment followed in 2010. The stay cables on one side of the roof were treated first because both promenades could not be closed off due to fire escape considerations. The limited loading capacity of the promenade slab did not allow using large equipment with the required reach to cover the full length of the cables. As an alternative, scaffold towers that reached the top of the masts were erected. Tarps wrapped these towers to protect the work from rain and wind during the coating process. A concern with using a coating was the sealing of all surfaces. No coating is 100% waterproof over time, particularly cables composed of round wires. Even though there may be fill materials, such as grease or mischmetal to fill the voids between the wires of the inner construction, there can still be areas in which moisture may accumulate. Thus, pinholes in the coating were deliberately included to allow the venting of the inner structure of the cable.

Two years later, the stay cables on the other side of the roof were treated. During this work, brown spots were noticed on the exposed ridge cables of the roof. Again, an inspection was conducted in 2015 to evaluate these cables. The inspection was not as straight forward as that of the stay cables.

Unlike the stay cables that could readily be observed from the floor of the promenade, these ridge cables were not as readily accessible. Not only do they span away from the promenades, but they also have a very steep incline at the masts. They start 125 feet above

the promenade’s floor and span across the convention center. For the inspection, the owner engaged Pacific Ropes, a specialty firm that is equipped and experienced in inspecting cables. They accessed the ridge cables by climbing up the stay cables (Figure 5). The inspection was conducted by taking pictures of the ridge cables for evaluation. The pictures revealed that the same condition existed as previously observed on the stay cables. Again, a recoating with a zinc rich compound was chosen as a remedial treatment.

The recoating of the ridge cables is scheduled for this year. The recoating process will be more difficult and tedious due to the difficult access to these cables. Also, the stainless steel loops that anchor the fabric clamps will have to be unbolted to reach the covered area for treatment. Care will have to be taken not to damage the fabric.

Possible Cause of the Coating Failure What could have caused the flaking of the coating on the outer wires? As mentioned earlier, the wires had a Class C coating in which the zinc is electrolytically deposited on the wires. In the electrolytic process, zinc is bonded to the steel through adhesion. The flaking of the coating from the wires suggests a bond failure between the zinc and the steel.

We rarely see this problem in Class A coating, which is the hot- dipped galvanization protection. During the Class A process, a steel zinc alloy layer forms that bonds firmly to the steel. Only steel with a phosphorus content higher than 0.03% has the tendency to flake.

The answer of the delamination may lie in the fabrication process of the wires. When drawn through a mandrel to obtain the high strength, the wires are lubricated. This lubrication needs to be fully removed for wires that are electrolytically coated so that a strong bond between the steel and the zinc is obtained. It takes more than a solvent to remove the lubricant. A solvent only dilutes the lubricant but does not remove it fully. In the Class A coating process, the wires are also cleaned, but any remnant lubricant burns off in the zinc bath.

Life Span of Cables The coating failure was caught in time before any damaging corrosion occurred. Thus, the remedial measure of a cold galvanizing coating was applicable. The alternative would have been to replace the cables, which essentially would have shut down the facility for an extensive period. The coating of the cables in place allowed the facility to be kept

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July 2016

Figure 5. Inspection of ridge cable.

Figure 6. Applied coating after 6 years.

open, though minor inconveniences such as blocking of portions of the promenades were required. The treatment of the ridge cables will again require the closing of some sections of the promenade but less than when scaffolds were used to coat the stay cables.

Inspections of the coating of the stay cables that were coated in 2010 showed it to be in excellent condition (Figure 6). A coating with a cold galvanizing compound is not as durable as a coating of the wires by hot dipping or even electrolytic depositing, but it is a viable alternative to replacing the cables.

It is understood that the cables need to be monitored and that their recoating in the future may be necessary.▪

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Code Updates code developments and announcements

T

he American Concrete Institute (ACI) published the Building Code Requirements for Structural Concrete (ACI 318-14) and Commentary (ACI 318R-14) in the Fall of 2014. ACI 318-14 has been adopted by reference into the 2015 International Building Code (IBC). There are very significant organizational as well as technical changes between ACI 318-11 and ACI 318-14. A two-part article on the changes was published in the April and May 2016 issues of STRUCTURE magazine.

This current article discusses one of the most significant technical changes – located in the seismic design provisions for special or specially detailed shear walls, which are the only shear walls that can be used as part of the seismic force-resisting system of a building assigned to Seismic Design Categories (SDC) D, E, or F.

Introduction to the Changes ACI 318-14 Section 18.10, previously ACI 318-11 Section 21.9, has been extensively revised in light of the performance of buildings in the Chile earthquake of 2010 and the Christchurch, New Zealand earthquakes of 2011, as well as performance observed in the 2010 E-Defense full-scale reinforced concrete building tests [Gavridou, et al., 2012]. In these earthquakes and laboratory tests, concrete spalling and vertical reinforcement buckling were at times observed at wall boundaries. Wall damage

Special Reinforced Concrete Shear Walls Design and Detailing Requirements of ACI 318-14

By S. K. Ghosh, Ph.D.

was often concentrated over a wall height of two or three times the wall thicknesses, much less than the commonly assumed plastic-hinge height of one-half the wall length. Out-of-plane buckling failures over partial story heights were also observed; this failure mode had previously been observed only in a few, moderate-scale laboratory tests.

The following are the significant changes.

Applicability of Displacement-Based Design The displacement-based design procedure in Section 18.10.6.2 has all along been applicable only to a cantilever wall with a critical section at the base. Another requirement is now added for the displacement-based design procedure to be applicable – the total height to total length ratio (hw/ℓw) of the wall must be no less than two; in other words, the wall must be reasonably slender.

Trigger for Requiring Specially Confined Boundary Zone In the displacement-based approach, special confinement is required over a part of the compression zone, if: c≥

ℓw 600(1.5δu/hw)

Where c is the largest neutral axis depth calculated for the factored axial force and nominal moment

S. K. Ghosh is President, S. K. Ghosh Associates Inc., Palatine, IL and Aliso Viejo, CA. He is a long-standing member of ACI Committee 318, Structural Concrete Building Code, and its Subcommittee H, Seismic Provisions. He can be reached at skghoshinc@gmail.com.

The online version of this article has detailed references. Please visit www.STRUCTUREmag.org.

Figure 1. Specially confined boundary zone of special shear wall.

28 July 2016

ACI 318-14 Equation (18.10.6.2)

Figure 2. Minimum thickness of compression zone of special shear wall that is not tension-controlled.

strength consistent with the direction of the design displacement δu (Figure 1). Note that the 1.5 factor is inserted in the denominator in ACI 318-14. Thus, more shear walls will require confined boundary zones under ACI 318-14 than under ACI 318-11. There were four considerations behind the insertion: (1) The deflection amplification factor, Cd, of ASCE 7 may underestimate displacement response. (2) Since collapse prevention under the Maximum Considered Earthquake (MCE) is the prime objective of IBC/ASCE 7 seismic design, maybe displacements caused by the MCE, rather than the Design Earthquake (DE), should be considered; the MCE is 150% as strong as the DE. (3) There is a dispersion in seismic response, making it desirable to aim at an estimate that is not far from the expected upper-bound response. (4) Damping may be lower than the 5% value assumed in the ASCE 7 design spectrum.

The 1.5 factor is applied to the design displacement to emphasize that it is the design displacement that is modified (versus changing the constant in the denominator to 900).

The lower limit of δu/hw = 0.007 in Equation 21-8 of ACI 318-11 is changed to 0.007/1.5 = 0.0047 (0.005) to be consistent with the above change. The Commentary already stated: “The lower limit of 0.005 on the quantity δu/hw requires moderate wall deformation capacity for stiff buildings.” The following new sentence has been added: “The lower limit of 0.005 on the quantity δu/hw requires special boundary elements if wall boundary longitudinal reinforcement tensile strain does not reach approximately twice the limit used to define tension-controlled beam sections according to 21.2.2.”

Minimum Wall Thickness ACI 318-14 Equation 18.10.6.2 shown above is based on the assumption that yielding at the assumed critical section occurs over a plastic hinge height of one-half of the wall length.

To achieve this spread of plasticity, the wall

Figure 3. Lateral instability of wall boundary previously yielded in tension [NIST, 2014].

section should be either tension-controlled, or the compression zone must remain stable when subjected to large compressive strains (transition or compression-controlled section). Observations from the 2010 Chile earthquake, supported by the 2010 E-Defense tests, indicated that brittle failures are possible for thin walls. Two changes have been made in light of this observation. First, the sentence noted at the end of the preceding section has been added to Commentary Section R18.10.6.2. Second, a minimum wall thickness of 12 inches is imposed throughout the specially confined boundary zone where the wall section is not tension-controlled (see 18.10.6.4(c), Figure 2).

cross-sectional area of transverse reinforcement, there is no difference now between an SMF column hinging region and the specially confined boundary zone of a special shear wall. Also, the maximum center-tocenter horizontal spacing of crossties and hoop legs, hx, of 14 inches has been found not to provide sufficient confinement to thin walls. Based on laboratory tests by Thomasen and Wallace, the maximum center-to-center horizontal spacing of crossties and hoop legs is now restricted to the lesser of two-thirds the wall thickness and 14 inches (Section 18.10.6.4(e)).

Confinement Requirements for Specially Confined Boundary Zones

No slenderness limits existed in 318-11 Section 21.9 for specially confined boundary zones, primarily because this failure mode had only been observed in moderate-scale laboratory tests. Observations of wall instabilities following the recent earthquakes in Chile and New Zealand prompted a reexamination of this issue.

The 1997 Uniform Building Code (UBC) included a limit of ℓu/16 for Special Boundary Elements. Both the Canadian and New Zealand codes include more restrictive limits.

Observations of wall performance in recent earthquakes and laboratory tests indicate that slender walls, which typically have low shear stress, are susceptible to lateral instability failures. In ACI 318-11 Section 21.9.2, a single curtain of web reinforcement was allowed as long as Vu did not exceed 2Acv λ√fc. Use of a single curtain of web reinforcement makes these walls more susceptible to instability failure because, following yielding of the longitudinal reinforcement in tension, a single layer of vertical web reinforcement lacks a mechanism to restore stability (Figure 3).

Two changes have been made to address the issues identified above, (a) Limit the slenderness ratio at all specially confined boundary zones

Required transverse reinforcement for specially confined boundary zones of special shear walls has traditionally been determined using provisions for potential hinging regions of special moment frame (SMF) columns. In the plastic hinge region of an SMF column, ACI 318-11 required the minimum cross-sectional area of transverse reinforcement to be the larger of amounts given by Equations (21-4) and (21-5). In the case of specially confined boundary zones of special shear walls, however, two exceptions were made. Equation (21-4) was declared inapplicable. Moreover, the maximum spacing limitation of one-quarter the minimum plan dimension was relaxed to one-third. In ACI 318-14, instead of referencing the SMF section of the code, ACI 318-11 Equations (21-4) and (21-5) are now reproduced in Table 18.10.6.4(f ), and both of them are now applicable for the special boundary zone confinement of special shear walls.

However, the second relaxation (on the maximum spacing of transverse reinforcement) remains intact. As to the minimum

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Slenderness Considerations

Figure 4. Minimum thickness of compression zone of special shear wall.

to ℓu/16 (Section 18.10.6.4b, Figure 4), and (b) Require two curtains of web reinforcement in all walls having hw/ℓw ≥ 2.0 (Section 18.10.2.2).

Buckling Restraint Cyclic load reversals may lead to buckling of boundary longitudinal reinforcement even in cases where the demands on the boundary of the wall do not require special boundary elements. For walls with boundary longitudinal reinforcement ratio exceeding a certain threshold value, ties are required to inhibit buckling (Figure 5). The longitudinal reinforcement ratio is intended to include only the reinforcement at the wall boundary, as indicated in Figure 5.

The following changes have been made in the non-special confinement requirements of Section 18.10.6.5(a) [Underlining is used to indicate text that was not in ACI 318-11, but has been added in ACI 318-14; strikeout has been used to indicate text that was

Figure 5. “Local” reinforcement ratio at shear wall boundary.

included in ACI 318-11 but has been deleted from ACI 318-14.]: “The maximum longitudinal spacing of transverse reinforcement in at the wall boundary shall not exceed the lesser of 8 in. and 8db of the smallest primary flexural reinforcing bars, except the spacing shall not exceed the lesser of 6 in. and 6db within a distance equal to the greater of ℓw and Mu/4Vu above and below critical sections where yielding of longitudinal reinforcement is likely to occur as a result of inelastic lateral displacements.”

Summary of Boundary Confinement Requirements The Commentary to ACI 318-14 has added two very useful figures summarizing the boundary confinement requirements for walls with hw/ℓw ≥ 2 and a single critical section controlled by flexure and axial load.

One figure (Figure 6, which is a reproduction of ACI 318-14 Figure R18.10.6.4.2a) is for

Figure 6. Summary of boundary confinement requirements for walls with hw/ℓw ≥ 2, a single critical section controlled by flexure and axial load, and designed by the displacement-based approach of Sections 18.10.6.2, 18.10.6.4, and 18.10.6.5.

STRUCTURE magazine

walls designed by the displacement-based approach of Sections 18.10.6.2, 18.10.6.4, and 18.10.6.5. The other figure (Figure 7, which is a reproduction of ACI 318-14 Figure R18.10.6.4.2b) is for walls designed by the traditional approach of Sections 18.10.6.3, 18.10.6.4, and 18.10.6.5.

Conclusions The design and detailing requirements for special reinforced concrete shear walls have undergone significant changes from ACI 318-11 to ACI 318-14. The changes are a result of the unsatisfactory performance of many shear walls in the Chile earthquake of 2010 and the Christchurch, New Zealand earthquake of 2011. Most of the changes make shear wall design and detailing more stringent.▪ This article was originally published in the PCI Journal (March/April 2016), and this condensed version is reprinted with permission.

Figure 7. Summary of boundary confinement requirements for walls with hw/ℓw ≥ 2, a single critical section controlled by flexure and axial load, and designed by the traditional approach of Sections 18.10.6.3, 18.10.6.4, and 18.10.6.5.

30

July 2016

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(a)

(b)

(c)

Lessons Learned problems and solutions encountered by practicing structural engineers

Figure 1. Hybrid masonry systems: (a) Type I; (b) Type II; (c) Type III.

I

n the May 2016 issue of STRUCTURE, Part 1 of this series addressed through-bolts in masonry walls. That article was based on hybrid masonry research funded by the National Science Foundation’s Network for Earthquake Engineering Simulation Research (NEESR), but the information provided is useful to all designers of masonry construction where through-bolts are useful. The goal of this article is to further help practitioners gain a better understanding of the behavior of three particular types of connectors; these are hybrid “link” and “fuse” connectors, as well as headed stud anchors. While the “link” and “fuse” connectors are specific to hybrid masonry, the headed stud anchors are not.

Hybrid Masonry Overview Refer to the Part 1 article for an overview of hybrid masonry. The system is composed of a structural steel frame and reinforced concrete masonry panels.

Hybrid Masonry offers a design alternative to braced frames and moment-resisting frames that is appropriate for low and mid-rise construction. It is best suited for projects where a structural steel framing system and masonry walls would naturally be chosen due to structural and architectural efficiency.

Figure 1 shows, graphically, the three distinct types of hybrid masonry. In Type I hybrid masonry (Figure 1a), steel connectors transfer in-plane shear between the steel frame and the top of the masonry panel. These connectors can be either rigid “link” plates or ductile “fuse” plates.

The connectors do not transfer any vertical load to the masonry wall, but their design can have a significant influence on the overall performance of the system. This particular situation makes the wall design a non-loading bearing shear wall.

In Type II and III hybrid masonry (Figure 1b and 1c), headed studs are used to transfer shear from the beam and/or columns to the masonry panel.

Vertical load is also transferred directly through contact from the beam to the top of the masonry panel. This instance makes the wall design a load bearing shear wall.

Hybrid Masonry Connections and Through-Bolts Part 2

By Gaur Johnson, Ph.D., S.E. and Ian Robertson, Ph.D., S.E.

Type I Connections As mentioned, the steel connector plates used in Type I Hybrid Masonry can be designed either as elastic “link” connectors or as ductile “fuse”

Gaur Johnson is an Assistant Professor at the University of Hawaii at Manoa. Guar can be reached at gaur@hawaii.edu.

Ian Robertson is a Professor of Structural Engineering in the Department of Civil and Environmental Engineering at the University of Hawaii at Manoa. Ian can be reached at ianrob@hawaii.edu.

Figure 2. Hysteretic responses for tapered ductile fuse connectors (1k = 4.448kN, 1in = 25.4mm).

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The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

Figure 3. Original bent plate concept with non-ductile weld failure mechanism. Courtesy of International Masonry Institute, www.imiweb.org.

connectors. In both cases, the intent is for the connector plates to transfer only lateral and in-plane shear from the steel frame to the top of each concrete masonry panel. “Link” connectors are designed to remain elastic during a design level seismic event, while the concrete masonry wall is designed with sufficient ductility to absorb the necessary lateral displacement during the earthquake. Post-earthquake repair will likely require replacement of some of the ductile masonry walls.

Ductile “fuse” connectors are designed to remain elastic during wind and low seismic events, but to yield and provide hysteretic energy absorption during moderate to high seismic events. Fuses which are tapered to provide an equal potential for yielding along a significant length of the connector provide the best energy dissipating behavior. These fuses provide a high degree of ductility to the system.

The masonry wall is designed with an appropriate overstrength factor so that it remains essentially undamaged during the earthquake.

Post-earthquake repair will involve only the replacement of damaged fuses.

The tapered ductile fuse design was one of the several prototypes initially evaluated using

cyclic quasi-static testing of individual fuses. To study the behavioral effects of Figure 4. Successful bent welded fuse connector plates on either side of beam to simultaneously load- masonry panel connection. ing multiple fuses, wall tests using both four and six tapered duc- panel. A third alternative developed was to tile fuses to transfer the load were conducted. weld side plates to the flanges of the steel Figure 2 (page 33) compares the hysteretic beam and then either bolt (with slip critical response of the wall test using six fuses with bolts) or weld the fuse or link plates to these hysteretic responses of other tests using one or side plates as shown in Figure 5 (Ozaki-Train four fuses. The loads were scaled, as indicated, et al., 2011). The advantage of the bolted to facilitate a direct comparison. configuration is the ease of fuse replacement after a damaging seismic event.

Steel Connector For each of these connectors to function correctly, it is essential that the through-bolt Plate Attachment connection between the masonry panel and Several fuse and link plate configurations were the fuse or link plate can transfer the required considered and tested as part of the NEESR load without premature failure. The throughproject on hybrid masonry. The original con- bolts were discussed in Part 1. cept of a bent plate welded to the bottom flange of the steel beam, as shown in Figure Type II Connections 3, showed an undesirable non-ductile weld failure (Goodnight et al., 2011). A second Type II hybrid masonry requires a connection bent plate configuration shown in Figure 4 between the steel beam and the top of the proved more successful at transferring the concrete masonry panel that transfers both inlarge shear loads from steel frame to masonry plane shear and vertical load. One approach

(a) Figure 5. Link connectors (a) or ductile fuse connectors (b) and (c) attached-to-beam side plates using slip critical bolts or welds (Dimensions in mm; 1mm = 0.0394 in). Figure 5a courtesy of International Masonry Institute, www.imiweb.org.

(b) STRUCTURE magazine

(c)

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Figure 6. Hybrid masonry type II test wall showing the location of reinforcement and headed studs.

crushing at anchors, bearing, and shear and ACI 318-14 limit state of shear loading of anchors were considered. The AISC headed stud specification, and limitations, as described in the commentary, are summarized below.

AISC 360-10 Section I8.2a Shear Studs specifies the in-plane shear transfer capacity for studs embedded in solid or composite slabs. This section is not intended for use when there is an edge in the vicinity of the studs unless concrete breakout in shear can be prevented by confinement (AISC 2010) – there is no specific guidance to determine if there is sufficient confinement.

Qn = 0.5Asa √f'c Ec ≤ R g R p A sa F u

Figure 7. Type II headed stud connection testing setup.

to achieving this load transfer is the use of headed studs welded to the bottom flange of the steel beam and embedded in a grout beam at the top of the masonry wall (Figure 6 and 7). Figure 7 shows the test setup used to conduct cyclic quasi-static loading tests on the Type II connections made with headed studs.

Non-linear behavior is restricted to the ductile masonry panel, so the headed stud connection should be designed for elastic response using an appropriate overstrength factor.

Headed Studs Embedded in a Grout Beam The American Institute of Steel Construction’s (AISC) 360-10 provides design guidance for the use of headed studs to transfer load for composite structural steel beams and columns. Headed stud embedment in a grout beam at the top of a CMU wall is the primary load transfer mechanism for Type II hybrid masonry systems. Limited testing was conducted to observe local failure mechanisms associated with the load transfer from the steel beams through headed studs, the grout beam, and into the top course of a CMU wall.

Three tests were conducted to verify if the

AISC specifications were valid for this case.

Details regarding the reinforcement and testing protocol were documented in a research report and paper at the 12th NAMC (Aoki and Robertson, 2013; Aoki et al. 2012; Johnson and Robertson 2015).

The results were compared with the AISC 360-10 provisions as well as code specified limit states from TMS 402-13 (The Masonry Society) and ACI 318-14 (American Concrete Institute). The TMS 402-13 limit states of masonry breakout at anchors, masonry

For the hybrid masonry case, the grout beam was formed using the grout which filled the CMU. The grout compressive strength was tested in accordance with ASTM C1019 and used in place of f'c in the AISC equations including the calculation of Ec. The AISC equation predicts a nominal capacity of 21.5 kips based on the shear studs (right hand side of the inequality), which was less than 28.5 kips that AISC predicts based on the capacity of the grout (left hand side of the inequality).

Table 1 indicates the AISC predicted total capacity of the headed stud connection based on grout failure, stud failure as well as the maximum experimental load, and the average experimental load per stud.

A user note in AISC Section I8.3 states that the provisions are not intended for hybrid construction where the steel and concrete are not working compositely, such as with embed plates (AISC 2010). The section accounts for steel anchor failure and concrete breakout in shear. Geometric limitations of the studs are formulated to preclude anchor pry out and concrete breakout in tension. The commentary states, “…if these provisions are to be used, it is important that the engineer deem that a concrete breakout failure mode in shear is

Table 1. AISC headed stud capacity.

Number AISC of Grout Failure Studs [28.5 k /stud]

Average Failure AISC Maximum Experimental Mechanism Stud Failure Experimental Load Per [21.5 k /stud] Load Stud

4

114 k

86 k

60.1 k

20.0 k*

Break-out Stud Failure

5

143 k

108 k

89.0 k

17.8 k

Masonry Shear Wall Failure

10

285 k

215 k

95.2 k

9.5 k

Masonry Shear Wall Failure

*Based on 3 studs – the stud at the end was assumed to be ineffective due to a breakout failure.

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Figure 8. Assumed geometry applied to the ACI 318 shear loading of anchors provisions.

Table 2. ACI318 and TMS 402 breakout predictions for headed studs.

Number ACI 318-14 of Studs Shear Loading of Anchors 4

7.30+2(8.99)+2.74 = 28.0 k

5

3(8.99)+2(7.30) = 41.6 k

10

9(5.73)+2.74 = 54.3 k

TMS 402-13 Masonry Breakout at Anchors

Experimental Maximum Load

3(24.5)+6.13 = 79.6 k

60.1 k

5(24.5) = 123 k

89.0 k

9(24.5)+6.13 = 227 k

95.2 k

to transfer shear forces parallel to the wall. The C-shaped weld is required to resist shear, torsion, and flexure.

The AISC Manual does not provide tabulated capacities for this case. Even weld analysis by the instantaneous center of rotation method or elastic method may not provide conservative capacities considering the relatively large deformations within the bend of the plate that are immediately adjacent to the beginning of the weld.

• Details which eliminate the 90-degree bend in the plate and transfer the loads directly to either the top flange or web of the beam can provide the restraint and capacity needed to transfer in-plane shear loads.

• Highly ductile fuses can be designed to limit the force transferred through these connections, thereby reducing damage to the masonry panels. Regions of equal potential for yielding within the fuse are critical to achieving a ductile response.

Use of Headed Studs Embedded in a Grout Beam

Figure 9. Breakout failure of test specimen with 4 headed studs.

directly avoided through having the edges perpendicular to the line of force supported, and the edges parallel to the line of force sufficiently distant that concrete breakout through a side edge is not deemed viable… the determination of whether breakout failure in the concrete is a viable failure mode for the stud anchor is left to the engineer. Alternatively, the provisions call for required anchor reinforcement with provisions comparable to those of ACI 318 [for anchors]” (AISC 2010).

Applying anchor shear loading provisions of ACI 318 results in breakout failures shown in Figure 8. The figure is a cross-section through the length of the wall specimens. The circles show the locations of the studs. The concrete which is assumed to breakout is also shown.

Note that in-plane loading results in breakout of the concrete at the parallel edge. Table 2 shows the ACI and TMS predictions controlled by the breakout of anchors compared with the maximum experimental loads. The ACI 318 provisions are conservative, while the TMS provisions overestimate the capacity of the specimen with 4 studs, which failed at the grout beam and appeared to be initiated at the headed studs. The other two specimens failed within the CMU wall after the load

was transferred through the headed stud connection. The authors suspect that the high average force required of each stud caused the breakout type failure for the specimen with 4 studs, and the lower average force per stud precluded a stud initiated failure for the specimens with more studs. Figure 9 shows the breakout and splitting failure of the grout beam with 4 studs.

Conclusions Practitioners who would like to use connection details described in this article will not be able to find code language or limit states that directly address the behavior, boundary conditions, and loading which can make these connections cost effective for hybrid masonry systems. They will need to rely on engineering judgment and should consider the following information.

Steel Connecter Plates & Fuses • Welded connections commonly used in conjunction with bent plates attached to the bottom flange of W sections, to provide lateral restraint at the top of masonry walls, should not be used

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• Do not count on the full capacity of the stud as documented in AISC 36010, which is intended for shear transfer from a beam into a slab which provides significant lateral confinement to the concrete in the vicinity of the shear stud. Indeed, the AISC commentary expresses the importance of using a design which precludes a breakout failure as defined by ACI 318.

• TMS 402-13 anchor related limit states do not account for the thin geometry of the grout beam detail and resulted in unconservative predictions of failure load for the limited number of tests reported here.

• ACI 318-14 shear loading of anchor bolts limit states provide a conservative estimate of the capacity.

• Too few tests with headed stud failures are available to make any code change recommendations. However, observed failures indicate that the following details are good practice to reduce the potential for early break-out failure of the grout beam. ° Headed studs should be placed at least 12 inches (305mm) from the end of the CMU. ° Headed studs should be placed in the center of the grout beam to provide maximum edge distance to each side of the grout beam.▪

SEISMICALLY ISOLATED BUILDING BARES ALL

By Steven B. Tipping, S.E. and Gina T. Phelan 1908 and 1906 Shattuck.

I

t is not often that a new building’s structural engineer is also its developer – and the synergies arising from this pairing are equally rare. How can seemingly opposing priorities and objectives combine positively? How can a developer’s challenges transform into innovative seismic engineering opportunities? The Tipping Structural Engineers (TSE) mixed-use project at 1908 Shattuck Avenue in Berkeley, CA, is that unique result of the interdependence between creative engineering and real-estate development strategies.

borders a residential lot), losing TSE almost 2,500 square feet of usable space. To clear this zoning hurdle and maximize the size of the new building, the developer-engineers decided to merge 1906 and 1908 Shattuck into one property, which drastically reduced the setback requirements. Furthermore, the merger allowed both buildings to share a common entry (from the street to the second floor) as well as the existing building’s elevator and above-ground parking garage.

Idiosyncratic Isolation

1908 Shattuck Avenue TSE – whose existing offices at 1906 Shattuck are immediately adjacent – developed and designed the project to accommodate their increasing numbers. The structure accommodates a restaurantbrewery at street level and an office space designed for 25 people on the second and third floors. Its superstructure comprises two stories of wood-frame construction, totaling about 4,800 square feet, with a lateral system consisting of a series of distributed wood shear walls sheathed with half-inch plywood. Supporting the superstructure are six cantilevered columns and a 14-inch-thick post-tensioned concrete podium slab. Lastly, a series of crossing grade beams measuring 6 feet wide by 3 feet deep make up the building’s foundation system.

The downtown Berkeley infill site, measuring only 39 feet by 90 feet in plan, is less than a mile west of the Hayward Fault, considered to be the most active fault in the country, with a 31 percent chance of releasing a magnitude 6.7 earthquake in the next 30 years. A primary goal of the project was to develop a structural system that reflected quintessential Tipping engineering – effective, cost-efficient, projectspecific design thinking. Given this project’s location, seismic isolation was quickly identified as the best way to achieve this goal, resulting in perhaps the smallest isolated commercial building in the United States.

Once the basic geometry of the building was set and the planning and development issues resolved, the engineers turned to designing the seismic isolation system, which became the single most compelling feature of 1908 Shattuck.

Isolation bearings are typically installed below grade, hidden from view, requiring intensive – and expensive – foundation work. As it happened, previous construction had left retaining walls on three sides of the lot to accommodate a grade change of about eight feet over the depth of the site. This fortuitous condition spurred the engineers to envision alternative isolation configurations. They decided to expose the bearings. Spliced into the ground-floor columns ten feet above grade, they are visible to the patrons of the restaurant. The façade was designed so that one column stands outside, viewable by the public from the street.

Planning and Development Challenges The City of Berkeley is well-known for its stringent zoning codes and building requirements. For 1908 Shattuck, a combination of these limited the size of the building to a maximum of 7,500 gross square feet, its height to 40 feet, the number of stories to 3, and the use of the ground floor space to only retail, which constrained the office space to the upper two floors. Also, the zoning code would have dictated a 25-foot setback at the west side of the property (which STRUCTURE magazine

Column and bearing are exposed to public view. The isolator will be painted yellow to match the yellow stripe, indicating movement and the isolation plane.

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Building the column and isolator assembly.

Column embedment at the grade-beam intersection. PVC tubes hold posttensioned high-strength bars.

Traditional base isolation systems require a ground-level “moat” that allows the isolated building freedom to move during an earthquake.

Splicing the isolation bearings into the columns above grade resulted in an atypical isolation plane requiring a different approach: groundfloor walls cantilever from the foundation and an isolation moat was designed at the second story. All in all, this idiosyncratic isolation design resulted in a more cost-efficient solution than traditional below-grade installations, an outcome sweet to any developer’s ears.

Because of the site’s proximity to the Hayward fault, practice would dictate isolation bearings with an allowable horizontal movement of roughly 36 inches. Installing such bearings would have resulted in a building 33 feet wide at the maximum. Given the small lot size and the architectural program, TSE was reluctant to forego so much real estate – having originally planned for a 35-foot-wide building. The engineers recognized that the only way to achieve this would be to employ isolators with a smaller allowable movement, specifically, 24 inches. To that end, they worked with Earthquake Protective Systems (EPS) in Vallejo, CA, to customize six stock triple-pendulum bearings.

both faces, to achieve the desired elastic behavior. Lastly, to allow the footings to yield, displace, and then self-center in the event of an earthquake force larger than the lock-up, the foundation’s grade beams were reinforced with post-tensioned high-strength steel rods. All these design strategies added cost to the project; however, the gain in the programmable square footage was a tradeoff well worth the expense.

Statewide Seismic Monitoring TSE submitted 1908 Shattuck to the California Strong Motion Instrumentation Program for inclusion in the statewide monitoring system.

It was accepted on the basis of two unique aspects: 1908 will be the first isolated building employing a wood shear-wall lateral system; and the adjacent existing office building, constructed about twenty years ago, is structurally identical to 1908, with the exception of the isolation bearings.

Both buildings are hard-wired to transmit data via satellite to a data center in Sacramento, CA. The new building is equipped with sixteen, three-axis sensors, the older building with six. The sensors were positioned to capture the translation and rotational components of each building’s seismic response.

Lock-Up Potential and Mitigation The next step was to test a customized bearing. EPS ran multiple ground motions at maximum-displacement earthquake (MCE) and design-basis earthquake (DBE) hazard levels. They confirmed that the median displacement under DBE simulations would be 11 inches with an associated shear force of 40 kips, and under MCE simulations, 22 inches with an associated shear force of 50 kips. Significantly, two of fourteen analyses caused the isolator to “lock up,” or reach the 24-inch allowable displacement capacity, resulting in a shear force of 150 kips.

To account for the slight probability of lock-up, the engineers designed the cantilever columns with a diameter of 24 inches instead of 20 inches, which would allow them to remain elastic at lock-up. Further, to analyze the effect that locking up would have on the superstructure, they modeled every wood wall. Based on those results, they concluded that all the walls had to be sheathed with plywood, in some cases on

Lessons for Everyone The knowledge and advantages gained in planning, designing, and constructing 1908 Shattuck need not benefit only the developer-engineers at TSE. In fact, Steven Tipping is hopeful that each innovation and design strategy inspires others, and ultimately informs the profession.

As engineers go about their practice, learning to think like a developer and being empathetic to a developer’s project goals can multiply the possibilities at their disposal – both in serving clients and in creating cutting-edge engineering solutions. “When structural engineers abandon their silos to ask the bigger questions, the profession can only grow and flourish,” advises Tipping. “What’s best for the whole project? What’s best for all the players – the owner, the architect, the other disciplines, the trades? How can our work improve the built environment at large?”▪ Steven B. Tipping is founding Principal at Tipping Structural Engineers in Berkeley, CA, and has many of the firm’s awardwinning innovations. He can be reached at s.tipping@tippingstructural.com.

Project Team Owner: Steven B. Tipping Structural Engineer: Tipping Structural Engineers (initiated as Tipping Mar) Architect: Fernau and Hartman Architects General Contractor: Oliver and Company STRUCTURE magazine

Gina T. Phelan is an Associate at Tipping Structural Engineers and serves as strategic development director. She can be reached at g.phelan@tippingstructural.com.

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REPAIR FOR LOW CONCRETE BREAKS ON DOWNTOWN DENVER PROJECT

By Orville “Bud” Werner II, P.E.

I

t is an unprecedented time to be working in Denver. Individuals and businesses are relocating at a swift pace to the city, recognizing its natural beauty, strong business climate, and forward-thinking leadership. In fact, the city’s projected growth rate over the next five years is more than four times the national rate, according to The State of Downtown Denver, a report released last year by the Downtown Denver Partnership.

As the city’s density increases, the number of parcels available for development in the downtown area will shrink. According to Denver’s Open Data (City and County), only about 3 percent of the parcels within the City and County are vacant land. As a result, developers are willing to consider more extensive remediation of brownfield sites for their projects.

Against this backdrop of facts, the design team wrestled to identify solutions for a notable downtown project: 16M. This LEED-certified, mixed-use commercial site houses 115,000 square feet of office space over five floors, 36 luxury apartments on the top four floors and 13,000 square feet of retail space. This project’s layout speaks to how developers can manage tighter spaces and smaller lots – and overcome the issues that may result.

When work was well underway on 16M, a concrete producer for the project contacted CTL|Thompson after the concrete at the secondfloor parking deck did not achieve the specified design strength. The structural engineer of record carefully analyzed the slab using the lesser strength achieved and found a region of the slab that would not provide adequate support for the design loads, as built. The flexural strength in this area of the concrete slab was insufficient when analyzed using the reduced concrete strength.

STRUCTURE magazine

No one on the project anticipated this problem. In fact, many steps were taken to ensure a smooth build. Early tests indicated that the concrete met the required design strength, yet some pours did not perform as expected. The project team faced practical – and potentially costly – concerns as to whether to tear out and replace the existing slab, which would disrupt and delay work on the building.

When CTL|Thompson’s materials lab team received the call, most of the project design team, with years of experience in high-rise construction, considered slab replacement to be the presumptive repair option. CTL|Thompson expected this option to come with a high price. Another option considered included new support beams below the problematic area. This option was also expensive and potentially unacceptable, as it would limit clearance in the main driveway location of the lower level parking lot.

Prior work with Department of Transportation and road and bridge developers led to the suggestion that carbon fiber be used to improve the strength of the area in question. This solution could be implemented with the concrete in situ, preventing the need for a teardown.

Commercial construction rarely needs such a fix, as it is primarily used to reinforce roads and bridges, but the firm believed it would work and careful study bore out the results.

Carbon fiber material has been used for more than two decades and possesses a strength that is significantly higher than steel reinforcement in concrete. The Colorado Department of Transportation uses it occasionally on bridge repair and highway construction projects. The strong, thin layer of carbon fibers, encapsulated in a resin film, can be adhered with epoxy to the surface of concrete, akin to putting a cast on a broken arm. In this case, when installed

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below reinforced post-tensioned concrete, the carbon fiber places additional tensile reinforcement at the bottom of the slab. This lower layer of tensile reinforcement increased the effective depth of the slab sufficiently to reduce the compressive stress at the top of the slab to within the capacity of the reduced concrete compressive strengths. The unique application provided a structurally sound repair that, in the small area, required less than 500 square feet and was far less costly than replacement.

Most importantly, the application allowed the existing concrete to stay in place and work to continue. External tensile reinforcement uses strong, flexible fibers that are not subject to corrosion – a key consideration for structures such as parking garages, which are exposed to aggressive levels of sand and road salt in the Denver climate.

There are other advantages of using carbon fiber in this type of application, such as: • There is no compromise in the floor-to-ceiling clearance in the area of repair.

• The repair is nearly invisible to passing occupants or visitors.

Anyone looking closely at the underside of the slab in the area of the understrength concrete would see only a slight difference in texture and color compared to the rest of the underside of the slab.

• Where other repair methods include a risk of damaging steel during concrete removal, this repair required no cutting or chipping into the concrete that encases the existing unbounded tendons.

After additional discussion and investigation, including the structural engineer of record’s evaluation and endorsement, the developer

recognized this solution as the best possible scenario – one that was safe, efficient, and kept construction on schedule.

The case of 16M has shown that viable solutions to initially perplexing problems exist, even if they are borrowed from other fields.

Success is based on the ability to develop a strong coalition among the contractor, the supplier, specialty repair contractors and the engineer of record. This healthy working relationship among all parties involved led to an outcome at 16M that minimized the expense of repair and retained the structure’s value.▪ Orville “Bud” Werner II is Principal of CTL|Thompson & President of its division, CTL|Thompson Materials Engineers Inc. He can be reached at owerner@ctlthompson.com.

Project Team Owner: Integrated Properties Structural Engineer: Monroe and Newell Architect: Gensler General Contractor: Milender White Construction Repair Contractor: Restruction Corporation Materials Consultant (for repair): CTL|Thompson, Inc.

DESIGNED not to be seen

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• Using sizes from 32mm – 76mm, these bars offer up to 407 kips of strength, up to 3.88in2 of cross sectional reinforcement area, and their selection modulus provides considerable bending resistance.

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After you finish the engineering on a project and it goes out the door, what happens to it? Who is responsible? How will it perform versus the engineering assumptions?

O

ver the years, the reality of what gets engineered versus what gets constructed has become more concerning. Performing site visits to observe construction configuration and specifics of the contractor’s interpretations of the permitted drawings has been, to say the least, enlightening.

The author’s most significant experiences have been in California, where the majority of the engineering effort is to reduce the seismic hazard of timber-framed buildings. You would think that knowing this, builders would try to comply with the intent of the Structural Engineer. Builders point to the successful inspection by the governing agencies as a testament to the quality of their work. However, in loading conditions other than gravity, shortcomings of the construction are not immediately obvious.

The author’s experiences in the investigation of the as-built conditions of buildings have also contributed to these types of observations. Just what is the point? Conversations with some Structural Engineering professionals, as well as builders and their framing subcontractors, reveals there is an amount of “flexibility” in the construction that has not been included in either the code development and the engineering assumptions made by the structural designer. For example, wood structural panel sheathing with nail fasteners to the framing: first of all, what is the tolerance for the fastener spacing and the fastener size?

Construction authorities suggest that the fact that there is nailing is a compliant result.

For years, the building code defined a 10d common nail as 0.148 inches in diameter and 3 inches long, and that definition occurred in one location in the code. In the shear panel table of the code, Figure 1 (page 44), International Building Code (IBC), the minimum penetration was listed for each of the nail specifications.

Upon careful examination, many users conclude that the length of the nail in 3/8-inch thick sheathing is the 15/8 inches of minimum nail penetration plus the 3/8-inch sheathing thickness to be 2 inches long, even though the code specifically notes a 10d common nail is 3 inches in length. This convenient confusion was addressed in the shear panel table in the 2006 International Building Code (IBC) and is reflected in Figure 2 (page 44). This “flexibility” is further supported by the fact that nail manufacturers make 10d common nails, as well as other types, in length increments of ¼ of an inch and in some cases 1/8 inch. The inspection agency typically does not pick up the incorrect nail sizes used, whether it is size or fastener diameter or length.

The Codes have referenced many editions of the National Design Specification® (NDS®) for Wood Construction and recently referenced editions of the American Wood Council’s (AWC) Special Design Provisions for Wind and Seismic (SDPWS) which also includes a length specification of nail fasteners into framing. Further, lengths and diameters of common, box, and sinker nails in accordance with ASTM F 1667-00, Standard Specification for Driven Fasteners: Nails, Spikes and Staples, are tabulated in SDPWS Appendix Table A1. So how could this still be in question?

Structural Engineering professionals use the history of “flexibility” as a justification for current interpretations that allow for what clearly is a misinterpretation. What is the impact of varying interpretations of code requirements? To understand this impact, it is prudent to know the source and intent of the code requirement. What is the source and intent of specific shear panel capacities for the different sheathing thicknesses, nail sizes, and spacing?

What are the nailing capacities for different connections and where do they come from? One can use AWC’s NDS® yield limit equations to calculate the capacity of wood structural panels nailed to wood framing. The recent edition of the NDS Table 11Q describes common wire nail lateral design values for sheathing. Everyone should go through this exercise.

When you do, you see that the numbers you generate are not those in the code tables for wood structural panel shear walls. How can this be?

Then where do the tabulated values come from?

As with many construction materials and types that are used, testing the components and configurations is the basis of allowable requirements in the building code. After the 1994 Northridge earthquake, the City of Los Angeles passed ordinances that required certain building types to be investigated and retrofit, if necessary. There were committees established that included building department members as well as Structural Engineers Association of Southern California (SEAOSC) members who examined many facets of building types subjected to the earthquake and the resultant performance. One specific item examined by a dedicated committee was the performance of wood structural panel sheathed shear walls subjected to cyclic loading.

The City required cyclic testing of components going forward when those components were used in the City of Los Angeles. Much of this required testing was to confirm the component’s ability to provide a factor of safety that resulted from static testing. Many hardware and component manufacturers scrambled to test and provide testing documentation; some were required to revise

STRUCTURE magazine

ConstruCtion issues discussion of construction issues and techniques

What is a 10d Common Nail?

43

Part 1

By Williston L. Warren, IV, S.E., SECB

Williston L. Warren, IV is Principal Structural Engineer, SESOL, Inc., Newport Beach, California.

Treasurer of the National Council of Structural Engineering Associations (NCSEA), Member of the NCSEA Code Advisory Committee (CAC), Chair of the CAC Evaluation Service Committee, Member of the Applied Technology Council (ATC) Board of Directors, and ATC Board Representative on the Project Review Panel for ATC 58-2, ATC-110 and ATC-124.

manufacturing or installation guidelines. One specific committee was directed to cyclically test wood structural panel sheathed shear wall assemblies to confirm and document performance and failure modes. This requirement was quickly included in the Evaluation Service criteria for affected components.

Results of the testing can be found in a full report to the City of Los Angeles, Report of a Testing Program of Light-Framed Walls with Wood-Sheathed Shear Panels, by the Research and COLA-UCI Light Frame Test Committee of the Structural Engineers Association of Southern California, December 2001 (www. icclabc.org/uploads/2001-12_COLA-UCI_ Shear_Wall_Test_Report.pdf). This testing specifically used ASTM F 1667-00’s descriptions of common nails, which uses the length of 10d common nails as 3 inches long. There are other references that include discussion of a shorter than specified nail length, but it was one test specimen, and the test showed lower values and recommended further testing. Shear wall behavior was observed during the testing with specific notice to the nails. Nails were observed to be withdrawing along with the typical “shear” force resistance.

Nails withdrawing during loading leads one to believe that a shorter nail would have a reduced capacity when compared to a nail with the length specified by ASTM F 1667.

All the testing aside, the building code specifies nail diameters and lengths, and some professional Structural Engineers are accepting and excusing non-code conforming nail size and spacing in construction. This extends to litigation, where not only the spacing of the nails but the size of the nails is excused because of the factor of safety used in these materials.

Load capacities do have these factors, but stiffness is not one of them. Certainly, fabrication variances need to be considered for nail spacing, but one would think a craftsman would be able to get nailing within a ¼ of an inch to 3/8 of an inch average spacing of that specified over a 24-inch length of nails. Put another way, when an apple is specified, you would not expect that a pear would be used even though you like pears.

Many builders and engineering professionals also excuse closer nailing than specified as acceptable because it provides greater capacity, neglecting any effect that closer spacing has on the stiffness of the wall and how it affects the load distribution throughout the building.

Short nails get excused because they seem to meet criteria that were not included in the testing that provided building code requirements for cyclic loadings. The position of builders and framing subcontractors is that any errors or mistakes they make are usually

Figure 1. Portion of 2000 IBC Shear Wall Capacity Table.

Panel Grade

Panels Applied Direct to Framing Minimum Minimum Fastener Nominal Fastener spacing at panel Nail Panel Penetration edges (inches) Thickness in Framing (common or galvanized box) or staple size 6 4 3 2 (inches) (inches) 5/16

3/8 Structural I Sheathing

7/16

1¼

6d

200

300

390

510

1

11/2 16 Gage

165

245

325

415

13/8

8d

230

360

460

610

1

11/2 16 Gage

155

235

315

400

13/8

8d

255

395

505

670

1

11/2 16 Gage

170

260

345

440

8d

280

430

550

730

10d

340

510

665

870

1

11/2 16 Gage

185

280

375

475

11/2

10d

340

510

665

870

13/8 15/32

Figure 2. Portion of 2015 SDPWS Table 4.3A.

Panel Grade

Panels Applied Direct to Framing Minimum Minimum Fastener spacing at panel Fastener Nominal edges (inches) Panel Penetration Nail Thickness in Framing (common or galvanized 6 4 3 2 (inches) (inches) box) or staple size 1¼

6d (2 x 0.113" common, 2" x 0.099" galvanized box)

200

300

390

510

1

11/2 16 Gage

165

245

325

415

13/8

8d (21/2" x 0.131" common, 21/2" x 0.113" galvanized box)

230

360

460

610

1

11/2 16 Gage

155

235

315

400

13/8

8d (21/2" x 0.131" common, 21/2" x 0.113" galvanized box)

255

395

505

670

1

11/2 16 Gage

170

260

345

440

13/8

8d (21/2" x 0.131" common, 21/2" x 0.113" galvanized box)

280

430

550

730

1

11/2 16 Gage

185

280

375

475

11/2

10d (3" x 0.148" common, 3" x 0.128" galvanized box)

340

510

665

870

5/16

3/8

Structural I Sheathing

7/16

15/32

STRUCTURE magazine

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July 2016

apparent immediately because they are very familiar with construction for resisting gravity loads. They think that distress due to gravity loads occurs during their construction and distress due to lateral loads occurs sometime in the future, if at all.

Some engineers take the position that building inspectors can authorize changes in construction for the specified requirements.

Does this extend to the size of nail fastener sizes and spacing specified by the structural engineer? Where is the line? If an engineer specifies nails spaced at 3 inches, can the inspector allow 2-, 4- or 6-inch spacing? What about Section 106-Inspections, “Approval as a result of inspection shall not be construed to be an approval of a violation of the provisions of this code or other ordinances of the jurisdiction.”? What about specifying a 16d nail to attach a 2x to a 4x? Does this mean the length of the nail of 3¼ inches can be used?

Who in the field performs the addition for nail lengths for each condition, or does the Code provide this by specifying one length of nail for each nail size?

So what does this say about the Structural Engineering profession, where individuals cannot only accept and even excuse non-conformance with building code requirements due to a general lack of understanding?

What does this say about the excusing professional, whether or not they are aware of the misinterpretation?

Many structural engineers spend countless hours in building code development, and can appreciate and understand the subject. However, there are those who do not understand and do not appear to care. Investigation after the 1994 Northridge earthquake discovered that in certain applications, small differences in what is provided versus what is required can make a substantial difference. One can encounter this situation in other types of loading cases such as snow, wind and foundation movement, and is usually not seen in gravity design. What this finally ends up as is a question as to the capacity of the existing configuration in an attempt to determine if there is sufficient capacity to call it good. The author has seen some attempts to determine this, however, generally, it becomes a case of “it is OK because I say so” and those left to judge do not understand that there is no basis for this position other than the building has yet to collapse.

Also, the general public does not understand what affects a building and what doesn’t. Their experience every day with buildings is that they are hard when you hit your head against them. With this being the case, “how can what I am doing affect or damage a building.” The ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org

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differences between loading cases are not easily understood by those not trained to see the difference. When someone inquires about the existing capacity versus the anticipated demand, the obvious response is that the professional has not tested every combination or permutation of incorrect installation of components to determine their capacity. That is why the building code exists, to establish the minimum criteria for the construction so that testing is not needed.

Currently, many structural engineers in California are working to encourage and assist in the development of retrofit standards and ordinances to increase the safety of certain types of buildings that have proven to be a hazard. Resistance to this in the past came from building owners, but, currently, there is a confluence of forces apparently going to make it happen. So what does it say about our profession when one portion is willing to accept below code required construction while others are working to advance code compliant construction? In the author’s opinion, it makes it hard for others to see us as a profession.▪ The online version of this article includes a copy of the 2000 IBC Shear Wall Capacity Table and the 2015 SDPWS Table 4.3A. Please visit www.STRUCTUREmag.org.

STRUCTURAL PRACTICES practical knowledge beyond the textbook

The authors recently conducted a study into the elastic behaviour of thin (Kirchhoff) plates using commercial finite element (FE) software. In attempting to verify the FE solution, it was compared to results presented in Timoshenko’s text and a significant difference was observed. This article presents the work conducted to uncover the reason for this difference and reveals an error (probably typographical) in the text. The source of the error is identified and it is demonstrated how such errors might propagate into other texts on the subject of plates. The significance of the error to the practising engineer is also discussed.

Plate Configuration The plate considered is rectangular with an aspect ratio b/a. It is simply supported on two opposite sides and loaded with a uniformly distributed load (UDL) as shown in Figure 1.

This problem is considered in Article 48 (p 214) of the Theory of Plates and Shells Figure 2. Moments and stresses at center of plate (point A) (Timoshenko, 1989) and the deflections and and definition of moment ratio. moments at points A and B are reported in the text (Table 47, p 219) for a Poisson’s ratio of of the moment ratio (defined in Figure 2) with v = 0.3. This table has been reproduced in Figure both mesh refinement and span to thickness 1 where D is the flexural rigidity of the plate ratio (a/t). The reason for considering converand w is the transverse gence with span to thickness ratio was that the displacement. finite element system used only provided thick In Figure 2, an infini- (Reissner-Mindlin) plates, and it was therefore tesimal region around necessary to ensure that the chosen thickness the center of the plate is was small enough to have removed the influence shown, together with the of shear deformation which is not considered in moments and stresses. the thin (Kirchhoff) formulation which is being The uniformly distributed load causes sagging investigated. moments in both longitudinal and transverse The results from this convergence study are directions which induce stresses in the plate, lin- summarized in Figure 3, where an initial mesh early distributed across the thickness, as shown of 1x2=2 elements was used with uniform mesh with the stresses on the top surface both being refinement. Span/thickness ratios between 2 and compressive. Note that the moment mx causes a 2000 were studied. Both studies converge to a direct stress in the x direction (σx). moment ratio of 10.19 as shown in the figure.

The ratio of the moments presented in Timoshenko’s text is 12.11, so there is a signifiFinite Element Analysis cant difference, approaching 20%, between the The aspect ratio of the plate considered was 0.5 FE values and the published moments. This needs and the authors chose to study the convergence further investigation.

An Error in Timoshenko’s Theory of Plates and Shells

By Angus Ramsay, M.Eng, Ph.D., C.Eng, FIMechE and Edward Maunder, MA, DIC, Ph.D., CEng, FIStructE Angus Ramsay is the owner of Ramsay Maunder Associates, an engineering consultancy based in the UK. He is a member of the NAFEMS Education & Training Working Group and acts as an Independent Technical Editor to the NAFEMS Benchmark Challenge. He can be contacted at angus_ramsay@ramsaymaunder.co.uk.

Edward Maunder is a consultant to Ramsay Maunder Associates and an Honorary Fellow of the University of Exeter in the UK. He is a member of the Academic Qualifications Panel of the Institution of Structural Engineers and acts as a reviewer for several international journals, such as the International Journal for Numerical Methods in Engineering, Computers and Structures, and Engineering Structures. He can be contacted at e.a.w.maunder@exeter.ac.uk.

Development of a Computer Program

Figure 1. Plate configuration and Timoshenko’s results.

46 July 2016

Timoshenko’s text provides an expression for the plate transverse displacement (w) as a single series (attributed to Levy and presented on p 217, eq(h)), which may be twice differentiated to produce the moments – see page 39 of Timoshenko’s text for example.

The expressions for the moments were coded into a small program so that they could be evaluated at a given point within a plate of arbitrary aspect ratio (b/a). The summation implied by the series solution is carried out in a loop for which only odd indices are considered and the upper value of the index is maintained as a variable in the program. Rapid convergence

Table 1. Summary of moment ratios at point A from different sources.

Figure 3. Convergence of moment ratio at point A with span/thickness ratio and mesh refinement.

is observed with the moment ratio for 26 terms in the series being 10.17843 as shown in Figure 4 (page 48).

The program produces values of displacement and moment at any point. These values may be used to plot distributions across the plate, and inspection of these distributions for satisfaction of the kinematic and static boundary conditions will provide verification that the program is correct.

The displacement field, not shown in this article for conciseness, demonstrates that the zero displacement condition along the simply supported edges is satisfied and that the field

possesses the expected symmetry about the lines x = a/2 and y = 0.

The Cartesian components of moment are shown in Figure 5 (page 48). The static boundary conditions require there to be zero bending moment along all edges, and this is clearly seen. The torsional moments are not required to be zero along the boundary, as Kirchhoff theory is assumed, but they should be zero along the two lines of symmetry and this is the case. The principal moments and the von Mises moment field, Mvm, are shown in the figure. The point of first yield is at point B, the center of the free edges.

Source

Moment Ratio at Point A

Timoshenko

12.11

FE (Reissner-Mindlin)

10.19

Program

10.18

FE (Kirchhoff)

10.18

An additional finite element result was produced using a pure Kirchhoff finite element.

This gave a moment ratio of 10.1784 which, to four decimal places, is identical to that produced by the program, thus independently verifying the program. The moment ratios from the four independent sources considered are shown in Table 1.

The results shown indicate that there is something amiss with the values published in Timoshenko’s text, at least for an aspect ratio of 0.5, and further investigation of the individual moment components used in the moment ratio show that it is the value of My at point A which is in error, with the value in the text being 0.0102 and the value from the program being 0.0122. continued on next page

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July 2016

Figure 4. Convergence of moment ratio at point A.

Figure 5. Moments from program.

The table of point results produced in Timoshenko (and reproduced in Figure 1) is attributed to a 1936 publication by D. L. Holl studying the problem presented in this article. In the era when the original publication was prepared, digital calculators/computers were not available and so it is likely that the moment values were calculated by hand, using tabulated data to obtain the hyperbolic trigonometrical functions and taking only a small (but presumably sufficient) number of terms in the series. The difference between the published values and those produced by the program is reasonably small for all but My for an aspect ratio of 0.5 as shown in Table 2.

As already noted, the values of My at point A for the book and program are, respectively, 0.0102 and 0.0122. It is interesting to surmise that there is a typographical error in the book value, since if the last two digits are transposed then it becomes 0.0120 and the error reduces to -1.64% which is much more in line with the error in the other values reported in Table 2.

Table 2. Percentage difference in displacements and moments.

Practical Conclusions This article has uncovered, by chance, an error in the published result for the transverse moment at the center of the plate configuration considered when the aspect ratio is 0.5. It illustrates the sort of care required by practicing engineers when taking published data at face value, even when it comes from such revered texts as Timoshenko’s. With the wide availability of finite element systems, the practising engineer can, and should, check the values he or she is going to use in the design or assessment of a structural member. It is also interesting to note the fact that published errors can propagate. In this case, erroneous data published in 1936 was still being used in the 28th reprint of Timoshenko’s text published in 1989 and also appears in Szilard’s 2004 publication on the theory and application of plate analysis (case number 103). The authors of this current

article have contacted the publishers of 1989 printing of Timoshenko’s text regarding this error, asking whether it might be corrected at a future reprint. However, it is understood that no further reprints are likely. This raises the question of how one then might protect practising engineers against the propagation of erroneous published data. One way to do this would be to have an online repository of such errors which engineers can access to check that there are no reported discrepencies in the data they are proposing to use. In the absence of such a facility, the best one can do is publish the finding, as here, with the hope that it will reach the intended audience.

With regard to the engineering significance of this finding, the error leads to an under-prediction of the minor (transverse) component of the moment at the plate center.

The engineer designing a steel plate might use the moments to calculate the von Mises moment and ensure that this is below the yield moment for the steel being used, since the von Mises moment is greater at the center of the free edge (point B) than at the center.

Then, provided the engineer notices this, the erroneous value in the table would never be used. For a designer of a reinforced concrete slab, however, this number may well be used to size the reinforcement lying parallel to the y axis and an under-prediction of some 16% might lead to a situation where the structure

STRUCTURE magazine

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July 2016

is pushed out of the elastic region and into the plastic region. The degree to which this will occur should, however, be well within the ultimate capacity of the slab, but may be undesirable in terms of serviceability issues such as cracking of the concrete.▪

Acknowledgement The authors would like to thank Professor J. P. B. M. Almeida of the Department of Civil Engineering & Architecture, IST, and University of Lisbon, Portugal for running the plate through his Kirchhoff equilibrium finite element software and Professor Husain Jubran Al-Gahtani of the King Fahd University of Petroleum & Minerals, Dhahran, in the Kingdom of Saudi Arabia for independently verifying the results produced by the program using Mathematica.

A similar article was published in the NAFEMS Benchmark Magazine, January 2016. Content is reprinted with permission.

The online version of this article contains detailed references. Please visit www.STRUCTUREmag.org.

Historic structures significant structures of the past

A

bridge across the Ohio River connecting Cincinnati, Ohio, and Covington, Kentucky, was suggested in the 1820s.

A charter for the bridge was granted in 1826 by the State of Kentucky and a second charter approved by Kentucky in 1840. Charles Ellet, Jr. (STRUCTURE, October 2006) submitted a plan for a wire cable suspension bridge in the same year. He was at the same time promoting wire cable suspension bridges across the Schuylkill River at Philadelphia and the Mississippi River at St. Louis. Nothing much happened until 1845 when the local newspapers were writing that Col. Steven H. Long had proposed a bridge at a cost of $185,000 that would be 85 feet above low water.

In February 1846, the Covington and Cincinnati Bridge Company was chartered again by the state of Kentucky, but the Ohio legislature would not approve it based upon objections by steamboat operators and others. It was then that John A. Roebling (STRUCTURE, November 2006) went to Cincinnati to advise Covington promoters on a bridge. At the time, he had only finished his Allegheny Aqueduct Bridge and was competing with Ellet for a bridge across the Niagara River below the Falls and one over the Ohio River at Wheeling, both of which he lost to Ellet. On September 1, 1846, he wrote a long, detailed report which began with: REPORT AND PLAN FOR A WIRE SUSPENSION BRIDGE, PROPOSED TO BE ERECTED OVER THE OHIO RIVER AT CINCINNATI.

By JOHN A. ROEBLING, Civil Engineer The object of the following report, is to explain the principal features of the accompanying plan of a Wire Cable Suspension Bridge, proposed to be erected over the Ohio river between Cincinnati and Covington, and to present such data to the consideration of the public, as to enable it fully to discuss the merits of the enterprise, and to decide, whether it is deserving of a general support, or not.

Whatever we undertake in opposition to public opinion, if that opinion is the result of a careful investigation, must turn out a fatal enterprise, and can not succeed in the end.

In this report, therefore, we will endeavor to be guided by truth and facts alone. We invite those who interest themselves, either for or against, to meet us in the same spirit, in those discussions, which no doubt will be carried on, before the public mind can rest satisfied.

[After listing all of the sections of his report, Roebling continued:] The idea of bridging the large rivers of the west could not be entertained before the system of suspension bridges was fairly introduced.

Covington-Cincinnati Bridge John A. Roebling Bridge

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

Dr. Griggs specializes in the restoration of historic bridges, having restored many 19 th Century cast and wrought iron bridges. He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an Independent Consulting Engineer.

Dr. Griggs can be reached at fgriggsjr@verizon.net.

50 July 2016

Bridge in 1867.

An attempt at this mode of building in the United States was made about forty years ago, when a number of chain bridges were erected upon a rude and insufficient plan.

Although these attempts failed, they clearly demonstrated the practicability of the system.

That no further efforts were made to perfect the plan, was not so much owing to the difficulty of construction, as to the great abundance of good timber in most parts of the country, which greatly facilitated the construction of wooden bridges, and reduced their first cost.

The solution of the problem of crossing large and deep rivers with great spans and at high elevations was left to modern engineering. It has been fully solved by the application of the principle of suspension. Numerous structures of the kind have been reared in different parts of Europe in the course of this century…

It would appear that spans of 400 feet are ample for all purposes of navigation. But there is no necessity of adopting this limit on the Ohio river. The construction of suspension bridges is now so well understood, that no competent builder will hesitate to resort to spans of 1500 feet and more, where localities may require it, and where the object will justify the expense…

The length of the bridge, from center to center of the abutments is 1570 feet, total length, including approaches, 2070 feet. Two spans are proposed, which will meet in the center of the river upon a gigantic stone pier of 200 feet high.

Three-fourths of the whole suspended weight of the bridge will be supported by this pier.

A large stone pier in the middle of the river did not appeal to the shipping interests. Without the approval of the Ohio State Legislature, nothing was accomplished. In 1849, Ohio approved a charter with the proviso that the bridge not connect directly with any street in Cincinnati. On January 15, 1849, Charles Ellet made another proposal in the form of a letter with the title: LETTER ON THE PROPOSED BRIDGE ACROSS THE OHIO RIVER AT CINCINNATI, WITH A SINGLE SPAN OF 1400 FEET, AND AN ELEVATION OF 112 FEET ABOVE LOW WATER. It is now nine years [1840] since I gave formal assurances to many of your citizens that

work until July 1857 on the masonry. Work stopped completely later in the year due to the financial panic of 1857. During this time, the two towers were built up just above water level and stood as stark evidence of the financial situation of the company. Work did not start again until January 1863, during the civil war, when it became clear to some investors that the bridge would be a good investment. Two pontoon bridges across the river were insufficient to support the war effort. New bonds were sold in 1863, and work began. With the lack of supplies and manpower, progress was

slow until after the war ended in April 1865.

Roebling then sent his son, Washington A. Roebling, to take charge of the bridge. He had graduated from Rensselaer Polytechnic Institute in Troy, New York, in 1857 and had worked with his father on the Allegheny River Suspension Bridge. He then served in the Civil War until March 1865, resigning as a Lt. Col. At that time, the towers and anchorages were complete, and they were ready to receive the wire cables. The spinning was completed in June 1866 after which the deck was hung. continued on next page

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July 2016

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it was quite within the present state of art and mechanical knowledge, to throw a bridge over the Ohio, which should offer no obstruction to the current, nor appreciable impediment to the navigation…

I propose to place no masonry, or work of any sort whatever, nearer to the center of the channel than the stores now standing on the landing; or to keep the abutments 700 feet back from the center of the river on each side – thus spanning the whole stream with a gigantic arch of fourteen hundred feet opening…

The elevation to be given to the flooring is a question of equal moment; but legislation in other states has already established certain limits which will serve in some degree as a guide in this part of the inquiry, here can certainly be no injury done to the navigation, if the boats that pass under the platform are obliged in their trips to pass beneath other bridges as low or lower.

[Ellet concluded with:] Estimating all these circumstances at their value, and making fair allowances for reasonable contingencies, I have no hesitation in assuring you that this work can be completed in a style worthy of its position, and the grandeur of the project, for the sum of three hundred thousand dollars…

By the adoption of a single arch, spanning the whole river at once, and giving ample space above, the navigation will be fully protected; and there exists no other interest that has a right to object.

In March 1849, the Scientific American wrote, “Mr. Ellet proposes to build a suspension bridge over the Ohio, between Cincinnati and Covington, to cost $300,000, and not to interfere with the navigation. The gigantic arch is to be 120 feet above the center[sic] of the river at low water, or fifty-two feet above the great flood of 1832 – the towers for the suspension of the wire cables 230 feet high – twenty cables four inches in diameter, capable of sustaining a weight of 7000 tons...”

Ellet then presented his designs to the Ohio legislature, but his proposal was rejected and the project put on hold. In 1856, the legislatures dropped the required clear span length from 1,400 to 1,000 feet, and Roebling submitted a new design, with a 1,057-foot span and with no tower at mid span. This proposal was accepted. There is no record that Ellet submitted a plan with the 1,000-foot clear span.

Roebling began work on the Covington Tower in September 1856. After stopping work for the winter, he was not able to start

Roebling wrote a long final report to the Trustees of the bridge, dated April 1, 1867, in which he not only described the bridge he built but why he built it the way he did.

All civil engineers and engineering students should read this report, so they learn the conditions under which Roebling and Charles Ellet worked just before and after the civil war. Excerpts from the report include: Thus united by strong cords of wire, we all fervently hope, for the sake of a common country, that these two great commonwealths will forever continue to use this national highway as a perpetual link of mutual interests and amicable relations, commercially, as well as socially and politically.

To comply with this act, the distance from center to center of towers measures 1,057 feet, which leaves a clear space of 1,005 feet between the bases of masonry.

The two small spans left open between the abutments and towers are each 281 feet from face to center of tower… As the bridge stands now, its elevation is 103 feet in the clear at a medium temperature of 60 degrees, rising one foot by extreme cold and sinking one foot below this mark in extreme heat.

[On the towers, Roebling wrote:] It is a difficult task to produce a proper architectural effect when designing towers for a suspension bridge of large dimensions. Highly ornamented masonry may be built, but it looks out of place, when the general impression should be that of simplicity, massiveness and strength. On the other hand, a public work, which forms a conspicuous landmark across a great river which separates two large cities, both abounding in highly ornamental facades, should also serve as a model of appropriate architectural proportions. Public works should educate public taste; at any rate, should not violate it. In the erection of public edifices, therefore, some expense may and ought to be incurred in order to satisfy the artistical aspirations of a young and growing community.

[On the anchorage chains and cables:] Each chain is composed of 9 links, of an aggregate length of 92 feet. The lower links consist of 14 and 15 bars, alternately, each bar 10 feet long from center to center of eye, 9 inches wide and 1½ inches thick, making a solid section of 12 square inches.

The eighth and ninth links are formed of 17 bars, with an aggregate section of 190 square inches, while the lowest link has a section of only 168 square inches…

The bridge floor is suspended to two cables. Each cable is composed of 5,180 wires, No. 9 gauge, and forms a cylinder of

12½ inches in diameter. Eighteen feet of this wire weigh one pound, and 60 wires have an aggregate metal section of one square inch. The deflection of the cables is 89 feet at a medium temperature…The cables of the Ohio bridge are wrapped from end to end with No. 10 wire.

[On the subject of suspending:] Except one hundred in the center of the main span, they are all made of wire rope.

To obtain more stiffness the short ones in the center are made of solid rods 1¾ inches diameter. The rope suspenders are 5 inches circumference [1.6-inch diameter], equivalent to 45 tons ultimate strength.

Those next to the towers are greatly relieved by the action of the stays, and consequently reduced to 36 tons strength.

[On the deck system:] The total width of the floor is 36 feet between the outside railings. The cables are suspended between the roadway and sidewalks, and they form, therefore, together with the suspenders and stays, division lines. Inside of the suspenders two lines of iron trusses 14 feet high, extend over the whole length of the bridge from abutment to abutment... Every 5 feet, corresponding to the suspenders, iron beams, 39 feet long are attached to the latter. These beams are made of two pieces, spliced in the center; their section is of the usual beam pattern, 7 inches deep, weighing 20 lb. per foot lineal… Under the roadway, each beam is further strengthened by truss rods, which pass under the center girders…

[On stays:] The general plan which I have always pursued in my works insures, by the heavy contraction of the cables in the center of the span, great lateral stability at this point. The larger and heavier the span, the greater will be its comparative stability at the center… Aside from simply stiffening the floor, the stays are rendering another and very important service; they effectually insure equilibrium between the main and half spans.

Without the stays and trusses, the elevation of the bridge floor would be too light in appearance, as compared to the massiveness of the towers. As it is, the whole has a pleasing effect, and at the same time presents strong and reassuring proportions, which inspire confidence in the stability of the work.

Where the cables enter the towers on the river side, 19 wire ropes, each 2¼ inches diameter, will be noticed descending in straight lines to different points in the floor. Crossing the suspenders diagonally,

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Bridge after rehab in 1896, note sidewalks wrapping around the towers.

the two are connected by wire wrappings, which keep them firmly in their position.

Thus a network is formed that occupies the same inclined plane, which coincides with that of the cables. The total number of stays in the main span is 76.

[On decking:] The roadway for vehicles is divided into two tracks, which compels wagons to keep to the right, and to follow each other in succession. By this arrangement all confusion and turning out is avoided, and more teams can be passed than would otherwise be possible.

The bridge opened for pedestrian traffic in November 1866. The official opening was held on January 1, 1867, even though finishing up work went on for another six months. Its total cost was $1,491,110, making it the most expensive bridge built in the United States at the time.

What Roebling had done was build a 1,057-foot long suspension bridge taking the record from Ellet’s Wheeling Bridge at 1,010 feet. It would remain the longest bridge until the Brooklyn Bridge opened in 1883, with its span of 1595 feet 6 inches. Between 1895 and 1899, Wilhelm Hildenbrand, John A. Roebling’s assistant in the preliminary plans for the Brooklyn Bridge and Washington A. Roebling’s assistant on the final plans and construction of the Brooklyn Bridge, added two 10½-inch diameter cables and anchorages while widening and replacing the suspended structure without stopping traffic. Tolls were removed in 1963 after the Commonwealth of Kentucky purchased the bridge in 1962. Kentucky purchased it as the state line ran to the northerly bank of the Ohio making most of the bridge in Kentucky.

In 1982, the bridge was officially named the John A. Roebling Bridge. A statue of Roebling was placed in a park in 1988 on the Covington side, with Roebling holding a ruler in his left hand with plans laying on a rock at his feet. It was named a National Historic Civil Engineering Landmark in 1982 and added to the National Register of Historic Places in 1975.▪

Business Strong for Seismic Companies New Products and Services Help SEs Do More For Less

By Larry Kahaner

A

t RISA Technologies (www.risa.com), Amber Freund, Vice President, Operations, is seeing slow and steady growth since coming out of the recession. From that time, she says, a lot of small engineering startups have popped up, and most have been turning to RISA for their structural analysis needs. RISA has responded, in part, by adding a powerful time history analysis capability to the latest version of RISA-3D. “Simply put, this is the ability to load and analyze structures as a function of time. The primary application of this feature was intended for the analysis of vibrating equipment on steel/concrete frames, but we have also seen users utilize time history for other applications, including wind and seismic.”

Freund says that in a recent project, one of their users had his proprietary structure analyzed in a wind tunnel which produced plots of deflection versus time for a number of nodes on the structure. “The user was then able to import that data into RISA-3D to essentially ‘shake’ the structure exactly as it had been shaken in the wind tunnel. RISA-3D was then able to back out the forces and stresses caused by the shaking.

This type of analysis allowed the user to identify stresses in parts of the structure where strain gauges had not been placed during testing.”

As for trends, Freund has noticed the high frequency of new codes being released and the increasing complexity of those codes. “For example, by the time we got the 2011 Masonry Code implemented in our software, the 2013 Masonry Code was already being published.

We have worked hard to get ahead of the curve so that new codes are already in the program by the time they are adopted by the states, but it is almost a full-time job for us. The complexity of the new codes is also unprecedented. For example, if you follow the ASCE-7 and AISC codes to the letter, it is virtually impossible to satisfy them with hand calculations. As a staff of mostly professional engineers, we at RISA sympathize with the engineer on the ground who is trying to comprehend all of this. That is why we provide transparency in our software and educational webinars on the ever-evolving nature of the design codes,” she says. (See ad on page 76.)

Frank Metelmann, President of Decon (www.deconusa.com), a Jordahl Company, would like SEs to know about their two product groups. “We have stud rails which provide punching shear reinforcement, and then we have Jordahl anchor channels, which are used to connect high loads and concrete wherever you cannot drill into the concrete. As for new products – it is more software-related. We have a new software that calculates the design of the anchor channels and also for the stud rails.” He says the software is a tool being offered to customers to make it easier for them to design and calculate the loads that can be used with their product. He adds: “For the anchor channels, we have an ICC report and evaluation report which we just received last month.”

How’s business? “Business is doing well. We have full order books for the next year or two,” says Metelmann. “The [U.S] elections coming up can always throw a wrench into the system, and that is one concern we have, but everything looks positive up to now.”

Sideplate (www.sideplate.com) has been growing at about 25 to 30 percent per year, says Jason Hoover, Eastern Regional Business Manager, Industry Outreach Executive. “This year started a little STRUCTURE magazine

slowly, particularly in the eastern U.S., but we’ve been picking up some momentum recently and are on pace to hit a similar target.

SidePlate has been in business for almost 22 years, and we still have a lot of growth opportunities. In fact, we are currently looking to hire structural engineers in California and the middle of the U.S.” Hoover notes: “The SidePlate Bolted SMF [Special Moment Frame] is our latest innovation, which is applicable for high-seismic projects.

We completed the full-scale testing earlier this year, and this bolted connection performed better than our fillet-welded connection ever did. In general, the SidePlate stiffness controls frame drifts better than any other connection, but the biggest advantage of our field-bolted connection is the speed of construction. It simply drops in place and gets bolted, so there is no field welding at all. Our motivation is always to make structures more efficient, from design through construction.

We’ve consistently heard that welding is expensive and slow, and welders are difficult to find. Developing a field-bolted option was a natural progression for SidePlate.”

He adds: “We continue to see structural engineers asked to do more for less. Design schedules are shorter and expectations are higher, but the fees are not following suit. I do not know the answer to this, but SidePlate has over 20 engineers, and we design well over 100 steel buildings each year. Accordingly, SidePlate’s assistance takes some of the burdens off of the structural engineer of record, and we do this at no charge to them.”

Hoover says that there is a perception that SidePlate is only for seismic projects. “We still fight this consistently. Frame stiffness matters in ‘wind world’ also, and much of our recent growth has been from wind-governed jobs where SidePlate designs save money versus conventional steel frames. I’d also like SEs to keep an open mind and acknowledge that, while we have a new way of doing things, we are partners with them. We do not have anything to sell them as our fees are paid by the construction team, and we can help make their lives easier.” (See ad on page 57.)

Says Rich Madden, Marketing Director at New Millennium (www.newmill.com), “To optimize building design and safety, FlexJoist tension-controlled open web steel joists are engineered to exceed standard steel joist design for strength, reliability and ductility. This alternative design approach gives building owners and specifiers the option of consistently higher steel joist performance at an affordable cost. Increased strength, higher reliability index, and improved ductility provide an enhanced timeframe for emergency management in the case of an overload situation. The Flex-Joist system can be coupled with a third party overload sensoring system to enhance building safety further.”

Madden continues: “A Flex-Joist project is ideally suited for the optional, post-erection installation of electronic sensors by a thirdparty provider. In the event of an overload, the bottom chord and end webs of a Flex-Joist will be highly stressed prior to collapse. Sensors and alarms installed along these components by the third-party provider can establish an early warning system for possible overload removal, roof shoring, and evacuation of personnel.” He adds a disclaimer that “no joist will withstand sudden and catastrophic impact forces that

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exceed system capability. Flex-Joist design offers the probability of high ductility and time delay under static gravity overload conditions.”

Says Madden: “The open web steel joist industry produces millions of joists each year that safely support roofs and floors in hundreds of thousands of buildings. Due to the range of potential overload condition, including unusual snow and rain levels, it is inevitable that some percentage of roofs and floors will be overloaded beyond anticipated worst-case load conditions during the lives of these structures. The FlexJoist Gravity Overload Safety System offers a safety feature to building owners and managers seeking additional protection against potential overload conditions and related risks of damage, injury and exposure to liability. Under most overload conditions, Flex-Joist introduces important advantages including engineered overload safety for floors and roofs, a steel joist structure designed to flex under extreme static gravity overloading, a time delay for possible evacuation and injury prevention, and possible shoring, removal and collapse prevention.”

Emory Montague, Director of Engineering at Simpson StrongTie (www.strongtie.com), suggests that with the increase in urban light-rail infrastructure and a younger demographic of environmentally-conscious consumers, there is an increase in dense urban infill rental units under construction nationwide. “This trend is expected to be steady or increase over the next ten years. Some cities are also looking at ways to make their communities more resilient, giving them the ability to bounce back quicker after a natural disaster. This push for better performing structures has increased the focus on providing more robust connections to create a continuous load path, tying the structure together from the roof to the foundation. These multi-story structures have demanded new solutions, where the continuous rod and screw systems have gained wider acceptance over the last five years because of their relative low labor costs,” he says.

To help meet these and other challenges, Simpson Strong-Tie recently released the Strong-Rod Systems Seismic and Wind Restraint Systems Guide (F-L-SRS15). Says Montague: “the guide provides insight into a variety of special design considerations when using continuous rod systems for shearwall overturning restraint. It also outlines how to take advantage of working with us as your full-solution design partner. By contacting Simpson Strong-Tie early in your project and sharing specific design loads and structure geometry, we can collaborate to create the most cost-effective and project specific solution for shearwall overturning restraint using the Strong-Rod Anchor Tiedown System (ATS).”

Montague says that one reason they offer this service is because Simpson Strong-Tie is constantly innovating to find better products that make installations faster, save cost, or provide additional benefits to their customers. “One of the main considerations to address when using continuous rod systems in multi-story, light-frame construction is wood shrinkage. Take-up devices are used to keep the rod system components tight and ready to function in an earthquake or storm after the wood framing has settled. We offer several styles of take-up devices to suit the application and load requirements. Recently, we extended our ratcheting take-up device (RTUD) line of products from sizes that accept 3/8-inch and ½-inch diameter threaded rod to include the RTUD5 for 5/8-inch and RTUD6 for ¾-inch diameter rods. The RTUD is extremely cost effective, code listed (ESR-2320) and allows for unlimited shrinkage compensation.”

Business is strong, concludes Montague. “With our new, more streamlined rod system assemblies and lower costs to the installer, we have seen a significant increase in Strong-Rod Systems sales in 2016. Our innovative truss and floor-to-floor screws are also gaining popularity in high-wind areas.”

(See ads on page 17 & 31).▪

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1/5/16 1:56 PM

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Elastocolor beats the elements

After

Before

The facades of the 608 units in the Blue Lagoon Condominiums complex in Miami, Florida, are subject to aggressive environmental effects from heat, humidity and high levels of sea salt as well as carbon emissions from proximity to expressways and the Miami International Airport. The condo association’s board of directors turned to MAPEI’s Elastocolor Coat protective/decorative coating for defense against the elements.

MAPEI supplied three custom colors of Elastocolor Coat to meet aesthetic requirements.

And thanks to superior performance characteristics – such as carbonation resistance, high elongation, chloride resistance and water-vapor permeability – Elastocolor Coat offered protection against the environment.

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aids for the structural engineer’s toolbox

EnginEEr’s notEbook

How Effective Are Your Arc Spot Welds?

By Jeremy L. Achter, S.E., LEED AP

A

n effective diaphragm is an essential component of a structurally sound building. The diaphragm provides lateral stability for the columns and/or bearing walls, braces the compression edge of floor framing members, and distributes wind and seismic forces to elements of the vertical lateral force-resisting system.

Diaphragms are divided into two main categories: rigid and flexible. Rigid diaphragms are often idealized as infinitely rigid plates that rotate about the center of rigidity and distribute loads based on the relative stiffness of each vertical lateral force-resisting element.

Concrete slabs are often analyzed as rigid diaphragms. Flexible diaphragms experience distortion under the application of lateral loads and distribute those forces based on the geometric layout of the vertical lateral forces-resisting elements. Cold-formed steel decking without a concrete topping slab is generally considered a flexible diaphragm.

Whether rigid or flexible, the connection of the diaphragm to the supporting structure is critical to the performance of the system.

Several different methods are used to attach cold-formed steel decking to the supporting structure: arc spot welds, shot-pins, or screws are the most common.

For flexible diaphragms, the demand is calculated for the required earthquake and wind loads in accordance with the adopted code.

Manufacturer’s literature and ICC/IAPMO report’s contain shear capacity and flexibility factors the engineer can use to determine the needed metal deck gage and fastening pattern to achieve the required diaphragm stiffness and shear resistance.

Read either the footnotes of the Support Fastening section of the manufacturer’s literature or the corresponding ICC/IAPMO report and you’ll likely find that the published values are based on a fusion area resulting from a ½-inch effective diameter, de, arc spot weld. An arc spot weld can be thought of like a mushroom with a short stem rooted in the supporting structure.

Achieving the correct effective fusion area is key to the performance of the diaphragm.

What does effective fusion area mean and how can it be correlated to the information contained in the structural drawings? In my experience as a designer and having reviewed structural drawings from many different firms, most engineers show the industry standard for

Effective Diameters of ¾" Visible Arc Spot Welds (d=0.75" ) Deck Gage and Thickness

One Layer of Sheet Steel (Typical in Field of Sheet)

Two Layers of Sheet Steel (Typical at End Laps)

Gage

Thickness (inches)

t (Inches)

de (inches)

2t (Inches)

de (inches)

22 gage

0.0295

0.0295

0.48"

0.059

0.44"

20 gage

0.0358

0.0358

0.47"

0.072

0.42"

18 gage

0.0474

0.0474

0.45"

0.095

0.37"

16 gage

0.0598

0.0598

0.44"

0.120

0.35"

diaphragm construction and indicate in the structural drawings that a ¾-inch diameter nominal arc spot weld is required.

Per section 2210.1.1 of the 2012 International Building Code, steel roof decks shall be designed and constructed in accordance with ANSI/SDI-RD1.0. Section 3.2 of ANSI/SDI-RD1.0 states that all welding of steel deck shall be in accordance with ANSI/ AWS D1.3. Figure 2.4 of AWS D1.3 provides the following equation for calculating the effective weld size of arc spot welds. de = 0.7d-1.5t where de = Effective diameter of fused area at the plane of maximum shear transfer d = Visible diameter of the outer surface of the arc spot weld t = Total combined base steel thickness of sheets involved in shear transfer above the plane of maximum shear transfer The effective diameter is calculated, using the above equation, as follows: This table indicates that the effective diameter of a ¾-inch visible arc spot weld does not comply with the ½-inch minimum required by the independent evaluation reports.

Perhaps, through mathematical rounding, one can justify that the specified ¾-inch diameter arc spot weld achieves the ½-inch required effective weld; however, this type of reasoning may be harder to justify with 18 gage deck or thicker, and is unreasonable at lap conditions where the design thickness (t) is twice the single sheet thickness. In these cases, a nominally larger diameter weld, or the use of weld washers may be required by design.

Should the ubiquitous ¾-inch diameter arc spot welds be replaced with larger 7/8-inch or 1-inch diameter welds? Should published deck shear values be provided with nominal

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or visible weld diameters instead of effective diameters? If their values are based on testing, then yes they should. The design values should be based on verifiable data, not prescriptive provisions. Should construction drawings specify an effective weld diameter as opposed to a visible diameter and let the contractor figure it out? This option is not viable because a contractor has no idea what effective means and the welds cannot be verified by the inspector, contractor, or engineer.

An engineer might consider that calculated diaphragm shears rarely result in a demandto-capacity ratio of 0.99; deck spans are often rounded to the next highest ½-foot increment to achieve published load values; or, that a 0.47inch effective diameter spot weld through 20 gage material is 0.03-inch less than the required ½-inch effective diameter, approximately 6%.

These are reasons to believe the ¾-inch diameter weld is within the realm of acceptable “construction tolerances.” However, it is also worth considering that any attorney can make a big deal out of such a small number.▪ Jeremy L. Achter is an Associate Principal at ARW Engineers in Ogden, Utah. He currently serves on the UBC Commission Structural Advisory Committee and serves as a member of the STRUCTURE Editorial Board. He can be reached at jeremya@arwengineers.com.

Since the writing of this article, AISI has published a new report on the resistance of arc spot welds loaded in shear and tension for building and construction. The information in this article has not been compared to, or reviewed against, the new report. The report can be found at www.steel.org.

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issues affecting the structural engineering profession

Professional issues

The Women of QLIC

For additional information

…and Why We Need More Women in Design and Construction

By Vicki Arbitrio, P.E., SECB, F.SEI

I

n August 2015, The American Society of Civil Engineers (ASCE) finally recognized Nora Stanton Blatch Barney as an ASCE Fellow, 110 years after she graduated from Cornell with a Civil Engineering degree and 99 years after the ASCE Board of Directors turned down her application for membership (reference blogs.asce.org).

Although nationwide 17% of civil engineers are women, in New York City women leading project teams of design, construction or real estate disciplines is now not uncommon.

Throughout my engineering career, I was one of very few women in my classes at college, the only woman on a construction site, and one of just a few women at any design team meetings. I got used to explaining whom I was, proving that I knew my stuff, and like Sisyphus (in Greek mythology, he is condemned to an eternity of rolling a boulder uphill then watching it roll back down again), I just kept going and going and going. Recently, I heard about a project where the developer’s representative, the structural engineer, the mechanical engineer and the contractor were represented by women!

Okay, not every employee was a woman, but women were in the primary leadership roles.

The project is called QLIC and it was completed in August 2015. It is a 400,000 square foot mixed-use development at 41-42 24th Street, in Long Island City, NY on the north side of Queens Plaza. The 21-story tower consists of 421 rental units, double height retail spaces at grade, and parking for 120 cars below grade.

The building’s 28,000 square feet of amenity space includes a rooftop pool, cabanas, a roof deck with an open-air theater and barbecue, a landscaped courtyard with a fire pit, a media lounge, a game room, a fitness center, and other amenities on an occupied terrace.

The structural system is conventionally reinforced concrete flat plate floors with concrete columns and shear walls for lateral load resistance.

This system is very common for high-rise residential construction in New York City. The 8-inch concrete slabs accommodate services for mechanical, plumbing and electrical trades. Shear walls are located around building cores that house elevator shafts and egress stairs to provide a structural and fireproof enclosure. Locating the shear walls at the elevator and stair shafts minimize their architectural impact, and for this project additional walls were not needed for lateral resistance.

The developer was World Wide Holdings; the architect, Perkins Eastman; the structural engineer, Gilsanz Murray Steficek (GMS); the mechanical engineer, MG Engineering; and the contractor, Lettire Construction. None of these firms are Minority or Women Owned, but most of the Project Managers for this project were women. Rachel Loeb ran the project for World Wide; Cathy Huang designed the structure for GMS; Masha Dinaburg coordinated the mechanical, electrical and plumbing work for MG; and Jessica Licata got everything built for Lettire.

This project is one of many projects across the country which is successful because the client’s needs were fulfilled by a diverse team who conceived, nurtured and delivered the project.

So how can all of us make our projects more successful? Here are 4 suggestions: 1) Set the tone for collaboration. Do all your staff members feel engaged and empowered to make decisions – regardless of gender? Do they feel a sense of ownership for each project or task to which they contribute? Are they corrected promptly and courteously when they make mistakes (because we all make mistakes)? Do they have mentors? Do they have people, besides their direct supervisor, from whom they can learn and ask for help?

2) Remove unconscious bias. Create a company culture that is inclusive and encourages the expression of different points of view. It is not enough to hire a diverse workforce if staff members are not engaged. If engineers do not feel trusted, respected and included, they will find someplace else to work. Even small biases are demeaning – do you send both male and female engineers out to the field? Do women and men get an equal shot at designing the coolest projects in the office? Treat them equally, pay them equally and provide them with similar project opportunities.

3) Build consensus. Listen to each other and work together to solve issues, instead of pointing fingers.

Remember that most of the people working in our industry love their work. So give them the benefit of the doubt and listen while

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Women and Leadership – www.pewsocialtrends.org/2015/01/14/ women-and-leadership McKinsey 2015 Women in the Workplace – Lean In www.mckinsey.com/businessfunctions/organization/our-insights/ women-in-the-workplace https://foreignpolicy.com/2016/04/19/ how-to-get-tenure-if-youre-a-womanacademia-stephen-walt Peterson Institute for International Economics, Working Paper Series WP 16-3, Is Gender Diversity Profitable?

Evidence from a Global Study What do Millennials Really Want at Work? https://hbr.org/2016/04/what-domillennials-really-want-at-work they brainstorm solutions to field conditions or to whatever other issues that may arise. You might be surprised by the solutions.

4) Encourage your entire staff – women and men – to ask for help, and to offer to help others.

Engineers, including structural engineers, must do more to bring more women and minorities into our profession. From an article in The Economist (www.economist.com/ node/15174418): “This growing cohort of university-educated women is also educated in more marketable subjects. In 1966, 40% of American women who received a BA specialized [sic] in education in college; 2% specialized [sic] in business and management.

The figures are now 12% and 50%. Women only continue to lag seriously behind men in a handful of subjects, such as engineering and computer sciences, where they earned about one-fifth of degrees in 2006.”

A July 2011 study by Forbes finds that innovation and business growth are driven by diversity in the workforce. There are numerous other studies that show a diverse workforce is stronger, similar to having a diverse investment portfolio. Structural Engineers provide an invaluable service to our communities and society. So let’s not leave out 50% of our society – we need all the talent we have available to solve issues like building resiliency. Want to know more? Please ask me.▪ Vicki Arbitrio is an Associate Partner at Gilsanz Murray Steficek in New York and a Past-President of NCSEA. She can be reached at vickiarbitrio@gmsllp.com.

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new trends, new techniques and current industry issues

InSIghtS

Permanent Wood Foundation

By John “Buddy” Showalter, P.E.

T

he 2015 Permanent Wood Foundation (PWF) Design Specification addresses structural design requirements of wood foundations for light-frame construction. The standard for designing wood foundations is most commonly used in residential structures in the upper Midwest. The 2015 PWF standard is adopted by reference in the 2015 International Building Code (IBC) and the 2015 International Residential Code (IRC).

Updated from the 2007 version, minimal changes are included in the 2015 PWF. These include updated references to the following standards: • 2015 National Design Specification® (NDS®) for Wood Construction • 2015 Special Design Provisions for Wind and Seismic (SDPWS).

• American Softwood Lumber Standard, PS 20-10 • Construction and Industrial Plywood Standard, PS 1-09 • Performance Standard for Wood-Based Structural-Use Panels, PS 2-10 • Use Category System: User Specification for Treated Wood (Use Category 4B: Permanent Wood Foundations), American Wood Protection Association (AWPA) U1-14 A PWF system consists of a load-bearing wood-frame wall and floor system designed for both above and below-grade use as a foundation. Properly designed, a PWF can be engineered with stress-graded lumber framing and plywood sheathing to support lateral soil pressures as well as dead, live, snow, wind, and seismic loads. The 2015 PWF includes criteria for materials, preservative treatment, soil characteristics, environmental control, design loads, and structural design.

Moisture control measures based on foundation engineering, construction practice, and building materials technology are employed to achieve dry and comfortable living space below-grade. The most important of these moisture control measures is a granular drainage layer surrounding the lower part of the basement that conducts ground water to a positively drained sump, preventing hydrostatic pressure on the basement walls or floor.

Similarly, moisture reaching the upper part of the basement foundation wall is deflected

downward to the gravel drainage system by polyethylene sheeting, or by the treated plywood wall itself. The result is a dry basement space that is readily insulated and finished for maximum comfort, conservation of energy, and utility.

Framing used in the PWF system is required to be lumber in accordance with PS 20 and needs to bear the stamp of an approved grading agency or inspection bureau which participates in an accreditation program, such as the American Lumber Standard Committee (ALSC) program or equivalent.

Sheathing used in the PWF Properly designed, a PWF can be engineered with stress-graded system is required to be plywood lumber framing and plywood sheathing to support lateral soil manufactured with all softwood pressures as well as dead, live, snow, wind, and seismic loads. veneers, bonded with exterior Courtesy of the Southern Forest Product Association. adhesive (Exposure 1 or Exterior), and grade-marked indicating conformance interior basement walls), hot-dipped galvanized with PS 1, PS 2, or applicable code evalua- (zinc-coated) steel fasteners conforming to the tion reports. requirements of ASTM A153 are permitted in All exterior foundation wall framing and lumber-to-lumber connections. sheathing (except the upper top plate); inteStructural design of a PWF is required to be in rior bearing wall framing and sheathing, accordance with the NDS, SDPWS, and proviposts or other wood supports used in crawl sions of the PWF standard. Reference design spaces; sleepers, joists, blocking and plywood values for sawn lumber, plywood and connecsubflooring used in basement floors; and all tions are provided in the NDS. Nominal unit other plates, framing and sheathing in contact shear capacities for shear walls and diaphragms with the ground or in direct contact with are provided in the SDPWS standard. concrete are required to be pressure treated The 2015 PWF is available for download on with preservatives. Treatment is in accordance the AWC website (www.awc.org). It was first with AWPA U1: Commodity Specification developed in 2007 and is based on information A, Section 4.2 Lumber and Plywood for developed cooperatively by the wood products Permanent Wood Foundations. Each piece industry and the U.S. Forest Service, with the of treated wood is required to bear the qual- advice and guidance of the Department of ity mark of an inspection agency listed by Housing and Urban Development’s Federal an accreditation body complying with the Housing Administration.▪ requirements of the ALSC Treated Wood Program or equivalent.

John “Buddy” Showalter is Vice Wood foundation sections of lumber framing President of Technology Transfer for the and plywood sheathing may be factory fabriAmerican Wood Council and serves as a cated or constructed at the job site. Fasteners member of the STRUCTURE magazine and connectors used in preservative treated Editorial Board. He can be reached at wood are required to be of Type 304 or 316 bshowalter@awc.org. stainless steel. However, when framing lumber is treated with Chromated Copper Arsenate (CCA) and the moisture content of the framThis article was previously published in ing remains at 19 percent or less (such as the June 2015 issue of The Construction studs, blocking, and top plates of exterior and Specifier and is reprinted with permission.

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CONCRETE PRODUCTS GUIDE ADAPT Corporation Phone: 650-306-2400 Email: florian@adaptsoft.com Web: www.adaptsoft.com Product: ADAPT-Builder Description: A complete solution for 3D FEM design of post-tensioned floor systems that combines global, multi-level lateral analysis capabilities with the detailed design requirements of single levels. Offers integrated column design module that automatically considers the hyperstatic moments from post-tensioning. PT Shop Drawing module reports chair heights and elongations of tendons.

Product: ADAPT-PTRC Description: Popular software for design of posttensioned slab systems and beams. A simple and easy-to-learn 2D equivalent frame analysis solution.

Automatically calculates optimized tendon layouts for any design strip configuration, eliminating the usual guess work and iterations when designing posttensioned projects. Ideal for parking structures. PT and RC design.

American Wood Council Phone: 202-463-2766 Email: info@awc.org Web: www.awc.org Product: National Design Specification® (NDS®) for Wood Construction and Special Design Provisions for Wind and Seismic Description: Contains design provisions and tabulated anchor bolt design values for wood-to-concrete connections. AWC’s Special Design Provisions for Wind and Seismic contains design provisions for lateral, shear, uplift, and hold-down connectors used to attach wood frame shear walls to concrete.

Product: Connection Calculator Description: AWC’s Connection Calculator allows design of wood-to-concrete anchor bolts per the National Design Specification® (NDS®) for Wood Construction.

Bentley Systems Phone: 800-BENTLEY Email: Samantha.Langdeau@bentley.com Web: www.bentley.com Product: RAM Concrete Description: Efficiently obtain reinforcement quantities your gravity and lateral frames. Quickly compare alternative design schemes with accurate material takeoffs . Increase productivity and provide superior service to clients, from conceptual design through construction, by producing detailed drawings and accurate cost estimates even in the most preliminary stages of projects.

Product: RAM Concept Description: Quickly design post-tensioned and reinforced concrete floors economically, with exceptional visibility into compliance, efficiency, and practicality while designing slabs, mats, and rafts. Design floors and foundations reliably and efficiently, saving time and money using features that specifically target the most common concerns.

Additives, Lightweight Concrete, Post-Tensioning, Precast Concrete, Reinforcement Products, Add-ons

Product: STAAD Foundation Advanced Description: Get efficient foundation design and documentation using plant-specific design tools, multiple design codes with U.S. and metric bar sizes, design optimization, and automatic drawing generation.

Provides a streamlined workflow through its integration with STAAD.Pro or as a stand-alone program.

CTS Cement Manufacturing Corporation Phone: 800-929-3030 Email: jong@ctscement.com Web: www.ctscement.com Product: Rapid Set® Cement Products and Type K Shrinkage-Compensating Cement Description: CTS Cement manufactures Rapid Set fast-setting hydraulic cement and Type K shrinkagecompensating cement. Make structural repairs and rehabilitation, and return the concrete to full use in just one hour. Install industrial-size floors with no curling, no cracking, and no control joints.

Decon® USA Phone: 707-996-5954 Email: frank@deconusa.com Web: www.deconusa.com Product: Studrails® Description: The North American standard for punching shear enhancement at slab-column connections. Studrails are produced to the specifications of ASTM A1044, ACI 318-08, and ICC ES 2494. Decon Studrails are also increasingly used to reinforce against bursting stresses in banded posttension anchor zones.

Product: JORDAHL® Anchor Channels Description: Decon USA is the exclusive representative of Jordahl in North America. Hot rolled Anchor Channels are embedded in concrete and used to securely transfer high loads. Main application is for flexible connections of glazing panels to high-rise buildings. Anchor Channels with welded-on rebar or corner pieces are available.

Dlubal Software, Inc. Phone: 267-702-2815 Email: info-us@dlubal.com Web: www.dlubal.com Product: RFEM Description: Performs the required ultimate and serviceability limit state designs of reinforced beams, columns, slabs, and walls according to ACI and other international standards. Further advanced capabilities include non-linear analysis of reinforced concrete elements in the cracked state for a realistic view of deformations, stresses, and crack widths considering non-linear behavior.

Halfen USA Phone: 800-423-9140 Email: info@halfenusa.com Web: www.halfenusa.com Product: Halfen Anchor Channels Description: Embedded in concrete, HALFEN Anchor Channels and T-bolts provide the most durable, reliable and adjustable connection to concrete elements. Available in a range of hot-rolled profiles, including toothed channels to provide mechanical load transfer in all directions, they are superior to post-installed anchors and weld plates for many applications.

Product: Halfen Reinforcement Products Description: HALFEN offers a wide range of reinforcement products connecting concrete members; HALFEN Shear Dowels and Rebend Connections are innovative and time saving structural slab-to-slab and slab-to-wall connections. A wide range of capacities is available; HALFEN insulated connections secure concrete balconies to the main building structure while forming a thermal break.

Hilti, Inc. Phone: 800-879-8000 Email: us-sales@hilti.com Web: www.us.hilti.com Product: Hilti HIT-RE 500 V3 Adhesive Anchoring System with SafeSet™ Technology Description: Delivers ultimate performance and safety in design while making installation even easier and faster than ever before. Teamed up with Hilti’s SafeSet self-cleaning hole system and PROFIS Anchor and Rebar design software, HIT-RE 500 V3 is nothing short of revolutionary.

IES, Inc. Phone: 800-707-0816 Email: info@iesweb.com Web: www.iesweb.com Product: VisualAnalysis w/Design Description: Rebar helps with cracks and strains, while VisualAnalysis lessens pains, as you design to optimize, that concrete frame we analyze.

Intergraph Phone: 800-766-7701 Email: joe.harrison@intergraph.com Web: www.coade.com Product: GT STRUDL Description: Structural analysis software with a high-quality, fully integrated, and database-driven system for comprehensive frame and finite element analysis, and steel and reinforced concrete design.

All Resource Guide forms for the 2016 Editorial Calendar are now available on the website, www.STRUCTUREmag.org. Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors.

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The Masonry Society Phone: 303-939-9700 Email: info@masonrysociety.org Web: www.masonrysociety.org Product: Publications including TMS402/602 Description: The Masonry Society is a non-profit, professional organization of volunteer Members, dedicated to the advancement of masonry knowledge.

Through Members, all aspects of masonry are discussed. The results are disseminated to provide guidance to the masonry and technical community on various aspects of masonry design, construction, evaluation, and repair.

Mitek Builder Products

Product: USP CIA-GEL 7000-C Description: The new 7000-C Epoxy Adhesive for post-installed holdowns in high seismic zones (SDC C-F) and other anchoring applications is approved in ICC-ES ESR-3609. The 7000-C complies with the 2012 IBC Code for use in concrete that is or may become cracked due to wind or cyclic earthquake loading.

Powers Fasteners

STRUCTURAL TECHNOLOGIES provides a wide range of custom designed systems which restore and enhance the load-carrying capacity of reinforced concrete and other structure types, including masonry, timber and steel. Our products can be used stand-alone or in combination to solve complex structural challenges.

V-Wrap™

Carbon Fiber System

DUCON®

Micro-Reinforced Concrete Systems

VSL

External Post-Tensioning Systems

Tstrata™

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Phone: 510-364-6263 Email: jacob.olsen@sbdinc.com Web: www.powers.com Product: Ac100+Gold Description: One of the most versatile adhesive anchoring systems on the market for bonding threaded rod and reinforcing bar into concrete and masonry base materials. ICC-ES approved for grouted & hollow masonry (ESR-3200); cracked & uncracked concrete (ESR-2582); seismic design categories A-F; approved for dispensing down to 14°F.

Engineered Solutions Our team integrates with engineers and owners to produce high value, low impact solutions for repair and retrofit of existing structures. We provide comprehensive technical support services including feasibility, preliminary product design, specification support, and construction budgets. Contact us today for assistance with your project needs.

RISA Technologies Phone: 800-332-RISA Email: info@risa.com Web: www.risa.com Product: RISAFloor ES Description: RISA offers everything you need for concrete design. For concrete floors, including beams and two way slabs, nothing beats RISAFloor ES for ease of use and versatility. The design of columns and shear walls with RISA-3D offers total flexibility.

Integration between RISA-3D and RISAFloor ES provides a complete building design. continued on next page

www.structuraltechnologies.com

+1-410-859-6539 To learn more about Structural Group companies visit www.structuralgroup.com DUCON® trade names and patents are owned by DUCON GmbH and are distributed exclusively in North America

by STRUCTURAL TECHNOLOGIES for strengthening and force protection applications.

VSL is the registered trademark of VSL International Ltd.

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ADVERTISEMENT–For Advertiser Information, visit www.STRUCTUREmag.org

Phone: 800-754-3030 Email: DLopp@mii.com Web: www.mitekbuilderproducts.com Product: Hardy Frame/Z Anchorage Solutions Description: New Hardy Frame pre-engineered anchorage details for Unreinforced, Reinforced and Back-to-Back Reinforced Anchorage Solutions are available to download. Embed details are also available for Z4 continuous tie-down systems.

Details are organized to be used as supplemental sheets for plan submittals.

State-of-the-Art Products

CONCRETE PRODUCTS GUIDE

Additives, Lightweight Concrete, Post-Tensioning, Precast Concrete, Reinforcement Products, Add-ons

S-FRAME Software Inc.

Simpson Strong-Tie

Strand7 Pty Ltd

Phone: 604-273-7737 Email: info@s-frame.com Web: www.s-frame.com Product: S-CONCRETE Description: The only truly interactive design solution for reinforced-concrete columns, beams and walls. Design and optimize a single section or evaluate thousands of concrete sections at once. S-CONCRETE’s comprehensive and multi-code support generates detailed reports that include clause references, equations employed, intermediate results and diagrams.

Phone: 800-925-5099 Email: web@strongtie.com Web: www.strongtie.com Product: Speed Clean™ DXS Dust Extraction System Description: Developed in conjunction with the Bosch Alliance partnership, the Simpson Strong-Tie Speed Clean DXS dust extraction system reduces dust while producing precise, clean holes for adhesive anchor installation. Speed Clean drill bits work in conjunction with Bosch and other commonly available vacuum systems and rotohammers to offer best-in-class concrete drilling.

Phone: 252-504-2282 Email: anne@beaufort-analysis.com Web: www.strand7.com Product: Strand7 Description: An advanced, general purpose, FEA system used worldwide for a wide range of structural analysis applications. It comprises preprocessing, solvers (linear and nonlinear static and dynamic capabilities) and postprocessing. Features include staged construction, a Moving Load module and quasi-static solver for shrinkage and creep/relaxation problems.

Product: S-FOUNDATION Description: Quickly design, analyze and detail foundations with a complete foundation management solution. Run as a stand-alone application, or utilize S-FRAME Analysis’ powerful round-tripping integration links for a detailed soil-structure interaction study. S-FOUNDATION automatically creates and manages the meshed foundation model. Includes powerful import/export 3rd party links.

Product: FX-70® Structural Repair and Protection System Description: Repairs concrete pile damage in-place without the need to dewater or take the structure out of service. It repairs and protects from tidal action, river current, salt water exposure, chemical intrusion, floating debris, marine borers, electrolysis and general weathering.

Product: S-LINE Description: Design and detail continuous reinforced concrete beams for both strength and serviceability to multiple codes. The interactive GUI updates results, including capacity envelopes on shear, moment and torsion diagrams. Comprehensive reports incorporate equations employed, clause references and diagrams.

All Resource Guide forms for the 2016 Editorial Calendar are now available on the website, www.STRUCTUREmag.org. Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors.

Trimble Solutions USA, Inc. Phone: 770-426-5105 Email: kristine.plemmons@trimble.com Web: www.tekla.com Product: Tekla Structures Description: Move from design-oriented to construction-oriented engineering and offer improved additional services for your clients. Through our open and collaborative software environment, you can work with other disciplines and reduce RFIs. From concept to completion, Tekla software gives you collaboration and control.

Product: Tedds Description: A powerful software that will speed up your daily structural and civil calculations, Tedds automates your repetitive structural calculations.

Perform 2D Frame analysis, utilize a large library of automated calculations to US codes, or write your own calculations while creating high quality and transparent documentation.

2016 Annual Trade Show in Print The definitive buyers’ guide for the practicing structural engineer

Get your submissions in soon for this year’s issue!

Visit www.STRUCTUREmag.org.

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award winners and outstanding projects

Spotlight

Dolphin Towers Condominium Remediation

By Frank Morabito, P.E., SECB Morabito Consultants, Inc. was an Outstanding Award Winner for Dolphin Towers Condominium – Remediation project in the 2015 NCSEA Annual Excellence in Structural Engineering Awards program (Category – Forensics/Renovation/Retrofit/Rehabilitation Structures under $20M).

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n June 24, 2010, the residents of the Dolphin Towers, a 12-story residential condominium constructed over a 3-story parking garage in Sarasota, FL, heard a large BANG. The residents of the plaza level condominium units discovered large cracks in walls and floors, along with doors that would not open that caused much concern. Five days later, the building was condemned by the City of Sarasota and the residents began a 5-year ordeal to get their homes back. After independent structural assessments and evaluations, over a 30-month period, that identified many structural deficiencies, Morabito Consultants, Inc. (MC) and Concrete Protection & Restoration, Inc. (CP&R) were hired to repair the 40-yearold building. The work included strengthening of the failed 24-inch thick concrete transfer slab, enlargement of the supporting concrete columns, installation of additional foundation piles, punching shear strengthening of numerous apartment tower columns, and installation of new concrete shearwalls to resist Category 4 hurricane wind loading.

Though the apartment building remained vacant, the street level retail shops and Florida Power and Light’s (FP&L) electric room had to remain in service during construction. To expedite the construction process and to assure the safety of all working personnel, the design/ build team first repaired the failed transferred slab, followed by the balance of the work.

MC designed and detailed the construction documents and all temporary shoring and column jacking. Also, MC prepared all mild steel reinforcing and post-tensioned tendon shop drawings.

To proceed with the transfer slab repairs, it was necessary to install shoring under all tower columns. At 5 locations where the stress in the existing transfer slab far exceeded the allowable punching shear stress, MC designed a jacking scheme that lifted 5 tower columns sufficiently to allow a drop panel + column encasement solution to eliminate the slab punching shear overstress. The repair of all spalled and cracked concrete in the transfer

slab was completed per ICRI standards. The major failure cracks were filled with 80 gallons of low-viscosity pourable epoxy placed over 2 days.

The strengthening of the failed 24-inch thick concrete transfer slab required the installation of continuous drop panels around the entire tower building footprint and under the new tower shearwalls, along with the enlargement of the garage tower columns and the installation of a bonded overlay over the entire fourth level transfer slab. Depth and size of drop panels were determined to eliminate punching shear and transfer slab overstress, and prevent excessive residual stress in the existing concrete slabs. Post-tensioning reinforcing was added to the drop panels to reduce mild steel reinforcing, reduce shear and residual stresses, and control deflection in the existing transfer slab. The design of the transfer slab considered the existing transfer slab, 15-inch drop panels, and 6-inch concrete overlay to be one composite section throughout the entire floor footprint. To assure that the drop panels, existing transfer slab, and overlay worked as a composite section, it was necessary to install shear lugs throughout the drop panel footprint anchoring these 3 elements together.

The shear lugs consisted of (610) 6 x ¾-inch steel channels installed in 8-inch diameter holes that were drill thru the existing 24-inch slab. Where new tower shearwalls stopped on top and at the bottom of the 4th level transfer slab, the horizontal shear stress and uplift were transferred to the new composite slab by installing rebar dowels in (86) 5-inch diameter cores drilled thru the 24-inch slab. The 8-inch diameter shear-lug holes and 5-inch diameter shearwall cores provided a place to pour the drop panel self-consolidating concrete which created a natural head pressure to assure full volume placement of the drop panel concrete.

The bonded overlay included sufficient flexural reinforcing to resist all negative moments in the composite transfer slab.

To increase the existing foundation capacity, (168) 30-ton helical piles were installed and tied to the existing pile caps. MC designed

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numerous eccentric pile caps to keep all new piles at the tower perimeter inside the garage footprint, allowing occupancy of the retail shops to remain undisturbed. Since access to the FP&L electric room was forbidden, MC designed a 2-story tall 60-foot long post-tensioned beam to transfer the overstress portion of the column and foundation in the electric room to the adjacent building columns which were strengthened for the additional load.

A testing program initiated by the City of Sarasota found that the existing concrete strength in the framed tower slabs was typically less than the specified 3,000 psi and as low as 1820 psi. The MC punching shear repair solution included grouting the exterior block walls and jacking up the 6-inch tower slabs in 4 locations adjacent to the new interior shear walls that transferred the framed slab live and dead loads to the new shearwalls and eliminated column punching shear overstress.

The design by MC and construction by CP&R saved the unit owners over $5,000,000.

The Remediation of the Dolphin Towers won the 2015 ICRI Award of Excellence in the Repair of High-Rise Structures and was a finalist for 2015 ICRI project of the year.▪ Frank Morabito is President at Morabito Consultants, Inc. He can be reached at frank@morabitoconsultants.com.

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NCSEA News

News form the National Council of Structural Engineers Associations

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SEER Committee Report: Refocus and Expand When NCSEA first developed and published its SEER Program Manual, it helped define and set the standard for what has become today’s Damage Assessment Professional, i.e., a design professional or code professional who assesses and provides input to Authorities Having Jurisdiction (AHJ) on damages to structures and infrastructure within post-disaster environments. Their services are provided in support of Emergency Management and long-term recovery (ESF-14).

There are two basic categories of Damage Assessment Professionals: First Responders and Second Responders. First responders assist with emergency efforts focused on victim rescue and recovery.

Second Responders assist with post-event recovery and reconstruction efforts by performing damage assessments of hundreds or potentially thousands of structures in a short period of time.

Challenges Within the U.S., First Responder Damage Assessment Professionals are utilized exclusively by State and Federal Urban Search and Rescue (US&R) programs. These programs are well organized, municipally funded and self-contained, with a cadre of trained and certified individuals capable of participating in emergency response operations.

For Second Responder Damage Professionals, however, there has not been a nationally organized and recognized database of trained and certified individuals; and local and regional municipalities, along with most States, have been left to fend for themselves, accepting help where they could get it from individuals with unknown training or experience.

SEER’s Expanded Focus The goal of NCSEA’s SEER committee is to work with NCSEA and its Member Organization SEER committees to expand and further develop the SEER Program into a comprehensive nationwide program with four main components: Training – this component consists of facilitating and/or delivering requisite training to NCSEA’s member organizations and members. This is currently being offered through NCSEA webinars, namely the CalOES Safety Assessment Professional classes.

It is also being offered in on-site training seminars, i.e., the ICC Disaster Response Inspector classes. For more information or to set up either of these programs, visit the NCSEA SEER webpage.

Assistance Coordination – this component consists of coordinating and/or providing 2nd response assistance to AHJs or other stakeholders.

This assistance will range from providing lists of properly trained individuals to providing coordination and logistical assistance when needed. It will also include the strengthening and developing of ties with allied organizations and associations in both the private and governmental sectors.

Advocacy – this component consists of advocating through educating of AHJs, allied associations, members and the public on the benefits of having engineers participating in 2nd response.

In short, the message is: All too often after a disaster, affected communities are left on their own to struggle with assessing damage and determining whether buildings can be safely reoccupied. When evaluations are not performed in a rapid fashion by properly qualified individuals, residents can and most likely will reoccupy potentially unsafe buildings. The key to adequately evaluating the extent of damage to a community and keeping residents from occupying unsafe structures is to ensure that sufficient numbers of qualified Disaster Assessment Professionals exist, working as 2nd responders to rapidly and appropriately perform building damage evaluations.

Update Your Record Today If you wish to remain on the SEER 2nd Responder Roster, you must access your record, review your information for accuracy and provide all of the requested updates. Those who do not update their record will be removed from the roster. The contact and certification information in this SEER 2nd Responder Roster will be shared with local, state and federal agencies requesting post-disaster assessment assistance, on an as-requested basis. NCSEA will utilize the contact information contained in the database to disseminate information relevant to Disaster Response professionals, including event notices, training opportunities and credentialing initiatives.

For more information or to get involved, please visit the NCSEA SEER webpage or contact either of the NCSEA SEER co-chairs, Scott Nacheman or Bill Bracken.

NCSEA Webinars July 19, 2016 Design Considerations for Mid-Rise Structures

Roster Management – this component consists of compiling and maintaining a comprehensive national roster of trained 2nd responders. Formerly the Volunteer Database, it has been renamed the SEER: 2nd Responder Roster and has been recently updated and significantly expanded. It is now a fully interactive database that can be updated as needed by the participant. In addition, enhanced search features are being added that will enable NCSEA and/or MO SEER committees to obtain a listing of participants by certification and location. The goal is to create and maintain a roster of properly trained individuals who can and will be called upon to assist communities in post-disaster environments.

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July 28, 2016 Communication Between the SE & Masonry Contractor August 9, 2016 Multi-Hazard Design of Blast-Resistant Facades August 25, 2016 Assessment of Seismic Performance of Reinforced Masonry Wall Structures Detailed information on the webinars and a registration link can be found at www.ncsea.com. Subscriptions are available! 1.5 hours of continuing education. Approved for CE credit in all 50 states.

July 2016

Disney’s Contemporary Resort · Lake Buena Vista, FL · September 14th-17th

© Disney

STAY CONNECTED BY USING THE HASHTAG #NCSEASummit

The slate of educational sessions for the NCSEA Structural Engineering Summit, September 14 –17, features two tracks of sessions on both technical and non-technical subjects specific to structural engineers. Sessions include:

Has THIS Ever Happened to You? NCSEA Young Member Group Support Committee, moderated by Jera Schlotthauer, EIT, chair, Martin/Martin Wyoming

Keynote: Structural Engineering for Walt Disney Theme Parks Kent Estes, Ph.D., S.E., Walt Disney Imagineering

So you Want to Delegate Connection Design – How to Do It Right Kirk Harman, P.E., S.E., SECB, President, The Harman Group

ASCE 7-16 Wind: How it Affects the Practicing Engineer Don Scott, P.E., S.E., F.SEI, F.ASCE, Vice President/ Director of Engineering, PCS Structural Solutions

Upcoming Changes to AISC 341 – Seismic Provisions for Structural Steel Buildings Jim Malley, S.E., SECB, Senior Principal, Degenkolb Engineers

Presenting TMS 402-13, The Masonry Design Standard Edwin Huston, P.E., S.E., Principal, Smith & Huston, Inc., Consulting Engineers

SEAOC Structural / Seismic Design Manual Ryan Kersting, S.E., Buehler & Buehler Structural Engineers, Doug Thompson, SECB, STB Structural Engineers, and SEAOC Seismic Committee members Becoming a Trusted Advisor: Communication and Selling Skills for Structural Engineers Annie Kao, P.E., Senior Field Engineer, Simpson Strong-Tie

Summit Hotel room block over 70% reserved Don’t delay in securing your hotel reservation for NCSEA’s 2016 NCSEA Structural Engineering Summit!

The Summit will be held at Disney’s Contemporary Resort, which is just a short monorail ride, water-launch trip or walk to the Magic Kingdom Park.

Hotel reservations are accessible through a link online at NCSEA’s website. Airport transportation is included in the hotel amenities, and transportation can be booked through a special NCSEA microsite, which also has information on the parks, including special group discount tickets.

Register now for the best discounts and secure your hotel room, as we expect a sell-out.

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2015 IBC, ASCE 7-10 and SDPWS Seismic Provisions for Wood Construction Michelle Kam Biron, P.E., S.E., SECB, Director of Education, American Wood Council

Detailed information on each session and speaker can be found at www.ncsea.com.

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New ACI Standards & the Repair of Existing Concrete Structures Gene Stevens, S.E., P.E., J.R. Harris & Co., and Chuck Larosche, P.E., WJE

Top 10 Useful Lessons for Structural Engineers Lawrence Novak, S.E., F.ACI, F.SEI, Director of Structural Engineering, Portland Cement Association

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Great (and Horrible) Masonry Design Practice Donald Harvey, P.E., Associate Vice President, Atkinson-Noland & Associates

News from the National Council of Structural Engineers Associations

Florida SE Licensure: How the Bill Was Created and Almost Became Law Tom Grogan, P.E., S.E., Florida Structural Engineers Association, The Haskell Company

Wind Loads on Non-Building Structures for the Practicing Engineer Emily Guglielmo, P.E., S.E., F.SEI, Principal, Martin/Martin, Inc.

Strut and Tie Design: What They Didn’t Teach You in School Thomas Mendez, S.E., Structural Engineer, WSP | Parsons Brinckerhoff

NCSEA News

2016 Structural Engineering Summit

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SEI Election

Structural Columns

The Newsletter of the Structural Engineering Institute of ASCE

July 31, 2016, Deadline The current Board of Governors positions on the Structural Engineering Institute Board of Governors are: representatives from each of the five Divisions (Business and Professional, Codes and Standards, Global Activities, Local Activities, and Technical Activities), one appointed by the ASCE Board of Direction, the current SEI President, the most immediate and available Past President of the SEI Board, and the SEI Director as a nonvoting member. The representatives from the Divisions each serve a four-year term. In accordance with the Structural Engineering Institute Bylaws, this year SEI is conducting an election for a Business and Professional Activities Division (BPAD) representative and Codes & Standards Activities Division (CSAD) representative on the Board of Governors. The BPAD and CSAD Executive Committees have nominated Joseph G. Di Pompeo and Ronald Hamburger as their candidates, respectively. If you are a member of SEI, please complete and mail your ballot to the address provided. Because we must confirm SEI membership, only signed ballots will be accepted.

DEADLINE JULY 31, 2016 Joseph G. Di Pompeo, P.E., SECB, F.SEI, M.ASCE, is the President of Structural Workshop LLC, a Mountain Lakes, NJ-based Structural Engineering and Building Consulting Firm. Joe is the chair of the SEI Business Practices Committee, which considers the role of the structural engineer in the business environment and the public at large. In addition, he serves on the SEI Business & Professional Activities Division (BPAD) Executive Committee (Excom). He will be the incoming Chair of the BPAD Excom. Joe has also served on the SEI Interdivisional Coordination Committee, which works to encourage the five SEI Divisions to collaborate on projects and initiatives. He serves as the President of the Stevens Institute of Technology Alumni Association and is on the Board of Trustees of Stevens Institute of Technology. Joe is a licensed engineer in 22 states, is certified by SECB, and is an SEI Fellow.

Ronald Hamburger, S.E., SECB, F.SEI, is a Senior Principal at Simpson Gumpertz & Heger with more than 40 years of experience in design, failure investigation, research, and building code and standards development. He has personally investigated the effects of more than 15 major earthquakes as well as tsunamis, hurricanes, and terrorist attacks. He was a member of the ASCE/FEMA Building Performance Assessment Team following the September 11, 2001, terror attacks on the World Trade Center. In the past, he served on the SEAOC Existing Buildings and Seismology Committees, served as Vice Chair of the AWS Seismic Committee, served as chair of the Building Seismic Safety Council’s Provisions Update Committee and the NCSEA Code Advisory Committee. He served as technical director for the FEMA/SAC Steel Moment Frame project and the recently completed ATC-58 project to develop next-generation performance-based seismic design criteria. Mr. Hamburger is the current chair for the 2016 edition of ASCE 7 Minimum Design Loads for Buildings and Other Structures, he received the 2014 Walter P Moore Award from SEI and is a member of the SEI Codes and Standards Activity Executive Committee.

Full Name: _____________________________________Member’s ASCE/SEI ID No:________________ (Please print) Date:______________ Signature: _______________________________________________________________

Return postmarked no later than July 31, 2016 to: SEI Board Election, 1801 Alexander Bell Dr., Reston VA 20191.

Business and Professional Activities Division SEI 2016 Board of Governors Election – Official Ballot

q Joseph Di Pompeo Codes and Standards Activities Division

q Ronald Hamburger STRUCTURE magazine

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to incorporate life-cycle concepts into structural design codes and standards. Before the workshop, the Task Group conducted a survey to collect information from researchers and experts involved in the development and the implementation of criteria, methods, and tools for life-cycle design and assessment of civil structure and infrastructure systems. A final product of the special project will be the development of a state-of-the-art report outlining the current status and research needs in the fields of life-cycle of civil structure and infrastructure systems.

The Technical Council and its three Task Groups provide a forum for reviewing, developing, and promoting the principles and methods of life-cycle performance, safety, reliability, and risk of structural systems in the analysis, design, construction, assessment, inspection, maintenance, operation, monitoring, repair, rehabilitation, and optimal management of civil infrastructure systems under uncertainty. Task Group 1 focuses primarily on the life-cycle performance of structural systems under uncertainty.

To learn more about the workshop, visit the SEI news page at www.asce.org/structural-engineering/news/20160121-lifecycle-performance-of-civil-structure-and-infrastructure-systems.

Two SEI Members Named as Flood Resistant Design ASCE Distinguished Members Standard Committee Call for New Members SEI is proud to congratulate Gary Y.K. Chock, S.E., D.CE, F.SEI, Dist.M.ASCE, and Kenneth C. Hover, Ph.D., P.E., Dist.M.ASCE, who have been named as ASCE Distinguished Members.

Distinguished Membership is the highest honor ASCE can bestow. It is reserved for civil engineers who have attained eminence in some branch of engineering or related arts and sciences, including the fields of engineering education and construction. The 2016 class of Distinguished Members will receive their honors at the ASCE 2016 Convention, September 28 through October 1, in Portland, Oregon. Learn more from ASCE News at http://news.asce.org/class-of-2016-addsnine-to-asces-roster-of-distinguishedmembers.

STRUCTURE magazine

The third edition of ASCE/SEI 24 was published in November 2014. The Committee is being re-established for the next cycle and is seeking members. ASCE/SEI 24-14 was adopted by reference in the 2015 I-codes, and the next edition of the Standard is being planned for inclusion in the 2021 I-Codes.

Please consider submitting an application for membership, via the on-line application at www.asce.org/codes-and-standards/ standards-committee-application-form. If you have any questions, contact the Chair, Christopher P. Jones, P.E., at chris.jones@earthlink.net.

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

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The Newsletter of the Structural Engineering Institute of ASCE

Task Group 1 of the Technical Council on Life-Cycle Performance, Safety, Reliability and Risk of Structural Systems recently organized an International Workshop on Life-Cycle Performance of Civil Structure and Infrastructure Systems. The workshop program included invited plenary lectures addressing the current state of research and practice, as well as breakout working sessions and group reports. More than 30 invited participants from several countries attended the workshop, held in Reston, Virginia, at the ASCE headquarters. This was a very successful and fruitful effort to assemble information on the development and implementation of criteria, methods, and tools for life-cycle design and assessment of civil structure and infrastructure systems.

Civil infrastructure systems are the backbone of modern society and among the primary drivers of the economic growth and sustainable development of countries. Hence, it is a strategic priority to consolidate and enhance criteria, methods, and procedures to protect, maintain, and improve the safety, durability, efficiency and resilience of critical structure and infrastructure systems under uncertainty. The workshop organizers hope that the ongoing effort within the Technical Council and the Task Groups will contribute to promoting the application of life-cycle concepts in design practice, influence the development of structural design codes and standards, and enhance the state of the civil structure and infrastructures to protect the public safety and improve the quality of life.

The workshop is part of an SEI Technical Activities Division special project that is overviewing advances in the field of life-cycle civil engineering, promoting a better understanding of life-cycle concepts in the structural engineering community, and discussing methodologies and tools

Structural Columns

Workshop on Life-Cycle Performance of Civil Structure and Infrastructure Systems

CASE Heads to Chicago this Summer

CASE in Point

The Newsletter of the Council of American Structural Engineers

CASE Summer Planning Meeting August 3 – 4, 2016; Chicago, IL The CASE Summer Planning Meeting will again be scheduled for August 3 – 4 in Chicago, IL. A popular feature of the planning meeting is a roundtable discussion on topics relating to the business of Structural Engineering, facilitated by the CASE Executive Committee members. Topics have included the Business of BIM, using social media within your firm, Peer Review and Special Inspections. Attendees to this session will earn 2.0 PDHs. Please contact CASE Executive Director Heather Talbert (htalbert@acec.org) if you are interested in attending this roundtable or have any suggested topics for the roundtable.

This new CASE Convocation Seminar is geared towards Owners, Principals, Project Managers, and Risk Manager. If you are concerned with risk management, new trends, and profitability, you cannot afford to miss this event! Join us August 4–5 in Chicago for the updated CASE Risk Management Convocation! To register for this event, go to www.acec.org/education/seminars.

CASE Risk Management Seminar August 4 – 5, 2016; Chicago, IL This summer, for the first time in 9 years, CASE will put on the industry’s only seminar dedicated solely to improving your firm’s business practices and risk management strategies. Join us and learn to Manage Risk for High Stakes Success in Chicago on August 4 – 5 for training and collaboration with industry leaders and project managers from firms of all sizes. This seminar is intended to improve your structural engineering practice.

Immerse yourself in topics designed to help engineers learn better ways of reducing areas of risk and liability on projects, while discovering tools that are available to implement better practices immediately in your firm.

Join us for the following sessions/speakers: Thursday, August 4 6:00 pm – 8:00 pm – Dinner with Speaker World Trade Center: Then & Now Ahmad Rahimian, Ph.D., P.E., S.E., F.ASCE USA Director of Building Structures Friday – August 5 7:45 am – 9:00 am – Breakfast with Speaker Brian Stewart, Attorney at Law, Collins, Collins, Muir & Stewart, LLP 9:00 am – 9:15 am Break 9:15 am – 10:30 am – Session 1 Professional Negligence (Karen Erger, Eric Singer) 10:30 am – 10:45 am Break 10:45 am – 12:00 pm–Session 2 How to Succeed without Risking it All! (Panel of Experts) 12:00 pm – 1:15 pm Lunch 1:30 pm – 2:45 pm – Session 3 Secrets to Communicating Technical Topics to NonTechnical Audiences (Shelley Row) 2:45 pm – 3:00 pm Break 3:00 pm – 4:15 pm – Session 4 Recapping New Risk Management Concepts Moderator: Brian Stewart, Collins, Collins, Muir & Stewart, LLP 4:15 pm – 4:30 pm Wrap-up / Adjourn STRUCTURE magazine

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Follow ACEC Coalitions on Twitter – @ACECCoalitions.

Pathways to Executive Leadership – NEW! A practical, focused program for new leaders facing the challenges of a continuously evolving business environment.

To be successful at taking on higher levels of leadership responsibility and preparing for the demands of being owners, new practice builders need specific and relevant training in the intricacies of leading an AE firm in ever-changing, always uncertain economic times.

Pathways to Executive Leadership is an intensive leadership program for early-career elites and promising mid-career professionals with 8 to 12 years of experience who are just beginning to lead and think strategically about their practices and careers. The reality-based curriculum focuses on the core skills necessary to becoming more influential (in team development, coaching, and client relationships) and more strategic (in business forecasting, team building, and client development), including: • High-Level Business Development • Leading Teams of Teams • Managing Uncertainty • Personal and Career Visioning • Strength Identification and Self-Awareness • Building Personal Resiliency • Coaching, Managing Others and Intentional Influence • Strategic Market Analysis Pathways to Executive Leadership will span 7 months beginning October 18 – 21, 2016 at the ACEC Fall Conference in Colorado Springs and ending April 22 – 25, 2017 at the ACEC Annual Convention in Washington, DC. To register for this program or get more information about schedule click, on the following link: www.acec.org/ calendar/calendar-seminar/consulting-by-design-pathwaysto-executive-leadership.

July 2016

Foundation 9 Contract Documents – Produce Quality Contract Documents Tool 9-1 A Guideline Addressing Coordination and Completeness of Structural Construction Documents It is recommended that engineers read this Guideline and take the test at the end of the document. More experienced engineers should then sit down with the engineers to go over the various subjects and answer any questions. The CASE Drawing Review Checklist will be a valuable tool to take away from this experience and implement for regular office use.

Wanted: Engineers to Get Involved with CASE Committees!

If you’re looking for ways to expand and strengthen your business skillset, look no further than serving on one (or more!)

CASE Committees. Join us to sharpen your leadership skills – promote your talent and expertise – to help guide CASE programs, services, and publications.

We have a committee ready for your service: • Risk Management Toolkit Committee: Develops and maintains documents such as business practices manuals and policies for engineers under CASE’s Ten Foundations for Risk Management.

Please submit the following information to htalbert@acec.org • Letter of interest • Brief bio (no more than 2 paragraphs) EXPECTATIONS AND REQUIREMENTS To apply, you should • be a current member of the Council of American Structural Engineers (CASE) • be able to attend the groups’ two face-to-face meetings per year: August, February (hotel, travel reimbursable) • be available to engage with the working group via email and conference call • have some specific experience and/or expertise to contribute to the group Thank you for your interest in contributing to your professional association!

STRUCTURE magazine

Tool 10-1 Site Visit Cards This tool provides sample cards for the 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, What to do before the site visit; What to take to the construction site; What to observe while at the site; What to do after completing the site visit. The sample cards include several types of structural construction, plus a general guide for all site visits.

Tool 10-2 Construction Administration Log Construction administration is a time when good record keeping and prompt response is essential to the success of the project and to limiting 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 other CASE products at www.acec.org/bookstore.

A/E Industry’s Premier Leadership-Building Institute Filling Fast for September Class Since its inception in 1995, the American Council of Engineering Companies’ prestigious Senior Executives Institute (SEI) has attracted public and private sector engineers and architects from firms of all sizes, locations and practice specialties. Executives – and up-and-coming executives – continue to be attracted by the Institute’s intense, highly interactive, energetic, exploratory, and challenging learning opportunities.

In the course of five separate five-day sessions over an 18-month timeframe, participants acquire new high-level skills and insights that facilitate adaptability and foster innovative systems thinking to meet the challenges of a changed A/E/C business environment.

The next SEI Class 22 meets in Washington, D.C. in September 2016 for its first session. Registration for remaining slots is available.

Executives with at least five years’ experience managing professional design programs, departments, or firms are invited to register for this unique leadership-building opportunity. As always, course size is limited, allowing faculty to give personal attention, feedback, and coaching to every participant about their skills in management, communications, and leadership.

SEI graduates say that a significant benefit of the SEI experience is the relationships they build with each other during the program. Participants learn that they are not alone in the challenges they face both personally and professionally, and every SEI class has graduated to an ongoing alumni group that meets to continue the lifelong learning process and provide support.

For more information, visit http://sei.acec.org or contact Deirdre McKenna, 202-682-4328, or dmckenna@acec.org.

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CASE is a part of the American Council of Engineering Companies

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. Elements of ensuring quality service include: • Client and project ownership by the individuals responsible for the project.

• Continual staff education including both leadership and technical skill development Firm-wide standard of care.

• Quality control process with a complete communication loop.

• Written Quality Assurance Plan.

As part of the Ten Foundations of Risk Management, CASE Tool No. 9-2: Quality Assurance Plan provides guidance to the structural engineering professional for developing a comprehensive, detailed Quality Assurance Plan suitable for their firm.

Foundation 10 Construction Administration – Provide Services to Complete the Risk Management Process

CASE in Point

CASE Risk Management Tools Available

Structural Forum

opinions on topics of current importance to structural engineers

Five Tips for Young Engineers

By Stan R. Caldwell, P.E., SECB

A

fter forty-five years of managing and mentoring dozens of young structural engineers, I have seen firsthand the various struggles they face in building successful careers in our high-liability profession. To assist a larger group of young engineers, I would like to offer advice on five important topics.

Mind the Gap Always track your load paths and close any gaps you find. Reliable load paths are essential for all structures, and their absence is one of the leading causes of failures. A complete load path defines how your carefully calculated vertical and lateral loads are going to find their way to the foundation of your structure.

Recently, I witnessed a project with multiple lateral load path issues. While failure had not occurred, more than $12M in repairs was necessary to bring the structure up to code.

Do not rely on computer software to detect gaps. That is your job as a structural engineer.

Think about nature. It is all about first principles and could care less about codes and equations. Unlike humans, it always chooses the path of most resistance. That is, stiffer elements always receive proportionally more load than relatively flexible elements nearby, regardless of your design intent.

Ensure Stability You have been trained to size beams and columns accurately, but the devil is not there – it is in the details. Structures rarely fail because beams or columns are substantially undersized. More often, failure is due to unanticipated loads, inadequate load paths, inadequate connections, or instability – especially instability, which can take many forms.

Stability is essential, not just when a structure is in service, but also during construction.

Last year, I witnessed steel structures that failed due to missing or insufficient diagonal braces, lateral braces, bridging, and local stiffener plates. Five years ago, I witnessed a seven-level precast concrete parking garage

collapse like a house of cards. At the time of the collapse, lateral bracing had not been installed and the intended moment connections between the precast beams and columns had not been grouted. You will likely find yourself in an uncomfortable situation if your structure becomes unstable while it is being built. So pay attention to the stability of your structure, not just when it is completed, but also while it is under construction.

Design First, Then Compute You should deliberately avoid your computer until after you have manually designed your structure. Lay out the geometry and initially size all of the principal elements. If you are not able to roughly design your structure by hand, you certainly have no business relying on your computer to do so. After you have completed an initial design, then turn on your computer, access your favorite structural engineering software, and verify or refine the design as appropriate.

After decades of looking, I have yet to find any structural engineering software that can actually think. Thinking, after all, is your primary responsibility as a structural engineer. No one should ever mistake computing for engineering.

Be a Sponge In college, you learned how to analyze and design beams, columns, connections, and other structural elements. You probably did not learn how to design economical buildings and bridges, comprehend the project workflow process from concept through completion, or understand the role of a structural engineer within a firm and within a multi-discipline design team. All of this and more must be learned in the workplace.

Mentoring is arguably the most important aspect of workplace training. This is the process by which young engineers are actively coached by the experienced engineers around them. It is a critical process because it is the most effective way to transfer knowledge and wisdom from one generation to the next.

Unfortunately, formal mentoring is not always available, so be proactive and absorb knowledge like a sponge. Start asking questions of those around you from the moment you walk in the door. While “how” questions are obviously necessary, “why” questions usually yield better learning opportunities.

Other than questions asked repeatedly, the only dumb questions are those not asked.

Keep an old-fashioned notebook handy and write down every tip and trick you learn. Your notebook will become a helpful resource, and it might come in handy in a few years when you become a mentor.

Own Your Work Hopefully, you have one or more mentors and your work is being regularly reviewed – that is, after all, the way the system is supposed to work. I was supervised by two structural engineers during my first year after college. Then I was promoted, and the oversight ended. Working in industry, there were no peer reviews, plan checks, or P.E. seals; I was on my own. Whatever I designed would go to the drafting room at the end of the hall, and then straight out to the field for construction. If I made an error, I would have to face the consequences. Lacking any opportunity for further mentoring, and working in an environment where overdesign was frowned on, I quickly became self-reliant.

I encourage you to adopt a self-reliant attitude and to “own your work.” Invest the time necessary to be sure of yourself and your designs. Structural engineering is a high-liability profession. Accept that reality, and act accordingly.▪ Stan R. Caldwell (StanCaldwellPE.com) is a consulting structural engineer in Plano, Texas. He currently serves on the SECB Board of Directors, the SELC Steering Committee, and the SEI Task Committee on Digital Presence.

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

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