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STRUCTURE magazine July 2010

The underside of Barton Creek Bridge with struts and water lines on overhang and twin shaft piers in the distance. An analysis of this bridge can be seen on page 20 of this issue.

C O N T E N T S

Publication of any article, image, or advertisement in STRUCTURE ® magazine does not constitute endorsement by NCSEA, CASE, SEI, C3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

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Columns

Features

Departments

In every Issue

on the Cover

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20 Barton Creek BridgeBy Mark W. Holmberg, P.E.Rising eighty feet above the Barton Creek streambed, this three-span fin-back bridge was recommended as the most economical alternate that met unique geometric and environmental constraints, and provided a novel gateway for the subdivision it served. The fin-back name derives from the central fins, or walls, which rise from the triangular box to peak over each intermediate pier.

5 EditorialWhat Business Are You In?

By John A. Mercer Jr, P.E.

7 InFocusEngineers Are from Aristotle

By Jon A. Schmidt, P.E., SECB

8 Guest ColumnSeismic Design of Concrete Parking Structure Ramps

Seismology Committee, Structural Engineers Association of California

12 Structural DesignPost-Tensioned Slabs on Ground Part 3

By Bryan Allred, S.E.

16 Structural PracticesSea Wall Systems

By Vitaly B. Feygin, P.E.

22 Building BlocksService Life of a Structural Retrofit

By Zachery I. Smith, P.E., Scott F. Arnold, P.E., and Guijun Xian, Ph.D.

34 Structural ForumThe Case for System-Based Structural Design

By Avinash M. Nafday, Ph.D., M.B.A., P.E.

26 InSightsCurved Steel: Means and Methods

By Erin J. Gachne Conaway, P.E., LEED AP and Jacinda L. Collins, P.E.

6 Advertiser Index27 Resource Guide

(Pre-Cast Concrete)28 NCSEA News30 SEI Structural Columns32 CASE in Point

In the Education Special Section of the May 2010 issue of STRUCTURE magazine, there was an error in the table highlighting courses available at schools not offering the full curriculum. Texas A&M University does offer, and exceeds minimum course offering requirements for, the Analysis portion of the Basic Education Requirements. A red “check mark” should have been printed in the Analysis column for Texas A&M (page 20). We apologize for this error.

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Editorial

5

What Business Are You In?By John A. Mercer Jr, P.E.CASE Chair

Author’s Note: Because of a change in employment and subsequent resignation by Doug Ashcraft, I have recently assumed the chair position of CASE. I would like to take this opportunity to thank him for his past committee participation and leadership on CASE RMP committees and as Chair of the CASE Executive Committee, and wish him well in his new endeavors. The CASE Executive Committee encourages Doug to stay engaged in our sister organizations, NCSEA and SEI, as time allows.

Looking forward as CASE Chair, I will continue to rely upon the active leadership and participation of CASE members to share their time, energy, ingenuity, and expertise with our fellow structural engineers in CASE, NCSEA, and SEI when it comes to Risk Management and Business Practices.As we continue to move forward, we are reminded constantly that

today’s economy has presented a daunting challenge to all of our country’s companies, corporations, and individuals, not the least of which includes our structural engineering firms. Firms have had to take a hard look at themselves in structure, staff, and markets to assess their survival potential until there is a turn around in the recent downward financial trends.Traditionally, firms have been grown around Finders, Minders, and

Grinders. In the past growing economy, there was a shortage of each, stimulating acquisitions to fill the gaps to grab market share.Finders are typically the firm principals responsible to feed a firm’s

hungry appetite for work. Minders are those few engineers that have moved up to a project management role to maintain contact with the client, manage firm resources including staff, and keep a project on schedule and hopefully under budget. Finally, grinders include the staff engineers and support staff that turn out the work of engineering analysis and design, document preparation, and construction services support. They typically include entry level engineering staff, itching to design something.This scenario should be familiar to you. But why is it important?

Financially, a firm must be at minimum, break-even, and profitable by design when possible. Firm CFO’s are challenged with keeping overhead rates in line using project multipliers as gauges to evaluate the performance of the firm’s staff, project type, and client.When the economic environment declines as we have recently

experienced, it may be appropriate to re-evaluate how you define and practice your business. Buggy whip manufacturers experienced this sort of situation when Henry Ford automated the auto manufacturing business. What are we missing in today’s picture? Who or what is it that is consuming our revenues and profits? I would suggest we need to take a look at the internal and external line items comprising our overhead.Internally, we can include our IT needs. We depend on computers

and software just as our predecessors relied upon the pencil and eventually calculators. But computers and software cost more than pencils. The basis of our overhead is impacted by these types of cost increases. Some of us have in-house IT departments while others outsource this capability, maximizing cost efficiency. We can make a list of our overhead line items to include equipment, software, IT staff, communications systems, cell phones, Internet bandwidth, heat, lights,

rent, vehicles, supplies, advertising, non-billable staff time and the list can go on and on.One outside influence impacting our firms today is the illusion that

BIM, perpetrated on our engineering community by the software industry, is the ultimate answer in document preparation. BIM can actually be a Trojan horse that will eventually erode the quality of our work product and increase firm risk, if we continue to allow this myth to become an unchecked part of our Culture. BIM is only a tool. BIM causes restructuring of our production departments and puts firms behind a new learning curve. We need to ask if it will it make firms money or increase our risk?Another external influence is LEED certification. LEED was created

by architects with intentions to provide our society with energy saving buildings and sustainable develop-ments. We need to evaluate the real cost to firms and our clients. It has become another way for a few to extort money out of us and our clients, as overseers of a perceived greater good.It is my intent that this editorial be

the first in a series that will intro-duce the concept of creating profit centers out of our overhead items while maintaining multipliers for government audit purposes.What if you could save just one

job in your firm? Could it be yours? Just what business are you in?▪

STRUCTURALENGINEERINGINSTITUTE

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STRUCTURE® (Volume 17, 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 $65/yr domestic; $35/yr student; $125/yr foreign (including Canada). For change of address or duplicate copies, contact your member organization(s). Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily refl ect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board.

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

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STRUCTURE magazine6

Engineers Are from AristotleBy Jon A. Schmidt, P.E., SECB

The January 2007 issue of STRUCTURE® included an “Outside the Box” article by Erik Anders Nelson entitled Architects Are from Plato. Nelson used the different philosophical priorities of Plato and Aristotle to highlight some of the distinctions between the typical approaches that architects and engineers take when carrying out their respective design tasks. I would like to elaborate on some key aspects of Aristotle’s thought that I believe are especially relevant to engineering design.Like Plato, Aristotle was concerned with resolving the tension between

the permanence and change that we observe in the world around us. Which is more basic – the one or the many? Earlier philosophers tended to take sides – for example, Heraclitus argued that permanence is an illusion, and change is the universal feature of reality; while Parmenides advocated the opposite position, claiming that change is impossible, since everything that exists is just being itself. Plato sought to harmonize the two by developing an elaborate theory of “forms” – independently existing immaterial universals in which various individual material things participate.Aristotle absorbed and adapted his mentor’s teachings, adopting the

notions of act and potency – what something is and what it has the capacity to become – and noting that potency must always be grounded in something actual. For example, that which is actually a steel billet (now) is potentially a wide fl ange beam (in the future). Aristotle also modifi ed Plato’s theory of forms, insisting that every physical object is an irreducible composite of matter and form. Matter without form is pure potency, and thus not actual; form without matter can exist only as an immaterial particular, such as an abstract concept in the mind.Change occurs when something else causes an object’s matter to tran-

sition from one form to another – to transform – actualizing a potency of that object. Aristotle identifi ed four different types of causes, which are perhaps better characterized as types of explanations: material, formal, effi cient, and fi nal. As the terminology suggests, the fi rst two correspond directly to matter and form; the last two concern how and why potency is actualized, respectively. Effi cient causes are similar to what we mean by our most common current usage of the word “cause” – that which brings something about. Final causes are ends or goals – that for the sake of which something is brought about.Aristotle believed that fi nal causes are “the cause of causes” and took

precedence over the other three kinds. Unless an object (material cause) is directed at producing certain effects (fi nal cause) by virtue of its nature (formal cause), how can we be confi dent that the object is really the (effi cient) cause of those effects? Notice that the fi nal cause is not necessarily conscious or intentional; in fact, Aristotle viewed teleology

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July 2010

Visit STRUCTURE magazine on-line atwww.structuremag.org

Visit STRUCTURE magazine on-line at www.structuremag.org

Visit STRUCTURE magazine online atwww.STRUCTUREmag.org

STRUCTURE® (Volume 17, 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 $65/yr domestic; $35/yr student; $125/yr foreign (including Canada). For change of address or duplicate copies, contact your member organization(s). Any opinions expressed in STRUCTURE magazine are those of the author(s) and do not necessarily refl ect 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.

C3 Ink

A Division of Copper Creek Companies, Inc.148 Vine St., Reedsburg WI 53959P-608-524-1397 [email protected]

ADVERTISING ACCOUNT MANAGERInteractive Sales Associates

Chuck Minor Dick Railton Eastern Sales Western Sales 847-854-1666 951-587-2982

[email protected]

EDITORIAL STAFFExecutive Editor Jeanne Vogelzang, JD, CAE [email protected]

Editor Christine M. Sloat, P.E. [email protected]

Associate Editor Nikki Alger [email protected]

Graphic Designer Rob Fullmer [email protected]

Web Developer William Radig [email protected]

SUPPORTING ORGANIZATIONSNational Council of StructuralEngineers Associations

Jeanne M. Vogelzang, JD, CAEExecutive Director

[email protected]

Council of American Structural EngineersHeather Talbert

Coalitions Director202-682-4377

[email protected]

Structural Engineering InstituteJohn E. Durrant, P.E.

ManagerASCE Engineering Programs

[email protected]

ChairJon A. Schmidt, P.E., SECB

Burns & McDonnellKansas City, MO

[email protected]

Executive EditorJeanne M. Vogelzang, JD, CAE

NCSEAChicago, IL

[email protected]

Craig E. Barnes, P.E., SECBCBI Consulting, Inc.

Boston, MA

Richard Hess, S.E., SECBHess Engineering Inc.

Los Alamitos, CA

Mark W. Holmberg, P.E.Heath & Lineback Engineers, Inc.

Marietta, GA

Editorial BoardBrian J. Leshko, P.E.

HDR Engineering, Inc.Pittsburgh, PA

John A. Mercer, P.E.Mercer Engineering, PC

Minot, ND

Brian W. MillerAISC

Davis, CA

Mike C. Mota, P.E.CRSI

Williamstown, NJ

Evans Mountzouris, P.E.The DiSalvo Ericson Group

Ridgefi eld, CT

Matthew Salveson, Ph.D., P.E. Dokken Engineering

Folsom, CA

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.AF & PA/American Wood Council

Washington, DC


InFocus thoughts from a member of the Editorial Board

7

YOUR

YourTurn

Engineers Are from AristotleBy Jon A. Schmidt, P.E., SECB

The January 2007 issue of STRUCTURE® included an “Outside the Box” article by Erik Anders Nelson entitled Architects Are from Plato. Nelson used the different philosophical priorities of Plato and Aristotle to highlight some of the distinctions between the typical approaches that architects and engineers take when carrying out their respective design tasks. I would like to elaborate on some key aspects of Aristotle’s thought that I believe are especially relevant to engineering design.Like Plato, Aristotle was concerned with resolving the tension between

the permanence and change that we observe in the world around us. Which is more basic – the one or the many? Earlier philosophers tended to take sides – for example, Heraclitus argued that permanence is an illusion, and change is the universal feature of reality; while Parmenides advocated the opposite position, claiming that change is impossible, since everything that exists is just being itself. Plato sought to harmonize the two by developing an elaborate theory of “forms” – independently existing immaterial universals in which various individual material things participate.Aristotle absorbed and adapted his mentor’s teachings, adopting the

notions of act and potency – what something is and what it has the capacity to become – and noting that potency must always be grounded in something actual. For example, that which is actually a steel billet (now) is potentially a wide fl ange beam (in the future). Aristotle also modifi ed Plato’s theory of forms, insisting that every physical object is an irreducible composite of matter and form. Matter without form is pure potency, and thus not actual; form without matter can exist only as an immaterial particular, such as an abstract concept in the mind.Change occurs when something else causes an object’s matter to tran-

sition from one form to another – to transform – actualizing a potency of that object. Aristotle identifi ed four different types of causes, which are perhaps better characterized as types of explanations: material, formal, effi cient, and fi nal. As the terminology suggests, the fi rst two correspond directly to matter and form; the last two concern how and why potency is actualized, respectively. Effi cient causes are similar to what we mean by our most common current usage of the word “cause” – that which brings something about. Final causes are ends or goals – that for the sake of which something is brought about.Aristotle believed that fi nal causes are “the cause of causes” and took

precedence over the other three kinds. Unless an object (material cause) is directed at producing certain effects (fi nal cause) by virtue of its nature (formal cause), how can we be confi dent that the object is really the (effi cient) cause of those effects? Notice that the fi nal cause is not necessarily conscious or intentional; in fact, Aristotle viewed teleology

as something that is present throughout the universe, not just confi ned to human endeavors. By contrast, modern philosophy largely abandoned both formal and fi nal causes and is still struggling with the “problems” that this created.What does any of this have to do with engineering design? Well, it

seems to me that the role of an engineer is to select the formal, material, and effi cient causes of an artifact in light of its fi nal cause, which is often dictated primarily by non-technical factors (The Social Captivity of Engineering, May 2010). This is essentially what we mean when we use the verb “design”, and the noun “design” roughly corresponds to the formal cause of the thing designed – the structure or pattern that informs the matter that ultimately constitutes the physical product or project (material cause), which serves a designated purpose (fi nal cause) after it is assembled or built (effi cient cause).Of course, in the process of designing, an engineer must determine all

four causes for various elements and subsystems – fi nal (function), formal (confi guration), material (specifi cation), and effi cient (construction). None of these component causes are inherent in the client’s overall fi nal cause, just waiting to be “discovered”; the engineer has to make decisions based on his/her knowledge of various feasible arrangements of appropriate materials and the corresponding fabrication and installation methods (Engineering as Willing, March 2010).In summary, engineering design creates roadmaps for actualizing the

potency of physical objects in order to satisfy real and perceived needs and desires. Aristotle taught that a good life was one that achieved eudaimonia – a Greek word usually equated with “happiness”, but more accurately translated as “human fl ourishing”. I would like to think that he would commend the engineers of today as enablers of eudaimonia for society as a whole.▪

Can Aristotle’s concepts of act and potency, matter and form, and the four causes be reconciled with the modern “scientifi c” worldview? Are they relevant to our understanding of engineering and its place in our

culture? Please submit your responses and see what others have had to say by clicking on the “Your Turn” button at www.STRUCTUREmag.org.

Jon A. Schmidt, P.E., SECB ([email protected]), is an associate structural engineer at Burns & McDonnell in Kansas City, Missouri. He chairs the STRUCTURE magazine Editorial Board and the SEI Engineering Philosophy Committee.


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STRUCTURE magazine July 2010 STRUCTURE magazine8

Seismic Design of Concrete Parking Structure RampsSeismology Committee, Structural Engineers Association of California

Beginning in 1959 and extending to 1996, the Seismology Committee of the Structural Engineers Association of California (SEAOC) published printed editions of Recommended Lateral Force Requirements and Commentary, which was commonly called the Blue Book. The “Requirements” portion of those publications was in large part adopted verbatim by the International Council of Building Officials as the seismic regulations of the Uniform Building Code. With the unification of the three major model building code organizations in the United States to form the International Code Council, and the nationwide use of the NEHRP seismic design provisions that are developed under the auspices of the Federal Emergency Management Agency and the Building Seismic Safety Council, SEAOC directed its focus to developing forward-looking seismic design articles. Those articles provide commentary and guidance for engineering practitioners and building officials, clarifying ambiguities in codes and standards and identifying needed improvements in them. The result is a set of articles, SEAOC Blue Book, 2009 Edition, published by the International Code Council. The SEAOC Seismology Committee is continually developing new articles, which are web-accessible at www.seaoc.org/bluebook.

that is suitable because long spans are economical with smaller member sizes. The long-span floor systems tend to vibrate, but the resulting vibrations are acceptable to uninhabited spaces such as parking garages. As a result, the structural long-span fra-ming systems often used in parking structures are not usually found in other types of building occupancies. Additionally, the open nature of parking structures has resulted in less redundant structures with fewer shear walls, frames, or other lateral force-resisting systems. Parking structures have very few interior nonstructural elements, such as partitions, ceilings, and mechanical systems. This inherently leads to lower damping than could be expected from a typical office or other building. Damping ratios ranging from 3% to 4% were ob-served in an instrumented parking structure during the Northridge earthquake.Typical parking structures differ from

office buildings in that they may not have discrete story levels. Instead the stories may be connected with long, slightly-sloping ramps, which may constitute entire parking levels and are sometimes called parked-on ramps, or shorter ramps of greater slope that provide one or two lanes of inter-level access, which are called speed ramps. Ramps can be detrimental to the intended seismic response of the building by acting as unintended diagonal braces. Additionally, ramps often create interior short columns

which are likely to be governed by shear action rather than bending. This article is confined to this important issue of the seismic design and analysis of ramps. A more complete treatment is available in the Structural Engineers Association of California Blue Book paper on Concrete Parking Structures available at www.seaoc.org/bluebook, which includes references and also covers design issues related to columns and diaphragms.

RampsWe can speak in general of a parking

structure being a particular number of stories in height, but in terms of its struc-tural actions, the concept of stories can be an ambiguous concept. Parking structures often have a spiral or split level configu-ration that is not clearly represented by discrete story levels. For example, the same segment of the deck could connect level three to level four. Ramps that connect directly to shear walls or moment frames further deviate from the idealized distinct story levels used in the current codes.

The actual performance of an integrated ramp structure may not match the ductile behavior upon which seismic factors, such as the R factor, were based. Ramps can change the stiffness and deflection patterns of the building and change the distribution of loads to the designated seismic resisting elements, in some cases attracting a significant percentage of the force. For example, the R factor in ASCE 7-05 or the 2006 International Building Code for a special moment resisting-frame (SMRF) is 8. For comparison, the R factor for a special shear wall in a building frame system is 6. In other words, the base shear for a SMRF building is permitted to be 75% of that of a shear wall building because of the relative implied ductil-ity (6/8) for the two systems by the code. If a ramp in a SMRF parking structure stiffens up the building, reducing the true flexibility and altering the hinge formation mechanism, then the use of R = 8 in this case is non-conservative.

Treatment of Parking Structures by Building Codes

Based on observations from the 1994 North-ridge Earthquake, the following code changes (Table 1) were subsequently added for concrete structures in regions of high seismicity.Current building codes do not provide

specific guidelines suitable for analyzing the complex story interactions that can occur in parking structures, nor provisions for detail-ing seismic capacity in the ramps. In some cases, assuming discrete story levels may be too simplified an approach and could cause the designer to overlook unintended struc-tural shortcomings.Shear walls and moment frames are recognized

lateral force-resisting elements in building codes, but ramps are not codified. Yet, some ramps can be stiff and massive enough to interact with the designated seismic resisting systems. A lit-eral interpretation of the 2006 International Building Code might place ramps in the “other components” category like gravity columns and non-frame beams, which are often excluded in seismic analysis models. When ramps are cat-egorized as non-seismic elements, their effect on the seismic behavior of the structure could be inadvertently overlooked.Ramps can be considered as inclined slabs,

but codes lack specificity in detailing guide-lines suitable for slabs to function as vertical elements of the primary seismic force-resisting system. Interconnected ramps are not held to the ductility detailing provisions prescribed for the shear walls and frames. The diaphragm collector and shear reinforcement is not in-tended to yield, and thus boundary member confinement would not be required. Similar concerns regarding the greater force demands

Figure 1: Collapsed parking structure, 1994 Northridge Earthquake. Courtesy of Robert Reitherman.

In the January 17, 1994 Northridge Earthquake, eight major parking struc-tures suffered partial or total collapse (Figure 1) and at least twenty others were heavily damaged. Most of these struc-tures were relatively modern, having been constructed in the 25 years prior to the Northridge earthquake.No other modern concrete building

type performed as poorly relative to the primary code objective of safety. A variety of damage occurred and was noted in the Earthquake Engineering Research Insti-tute reconnaissance report on structural damage: collapse of the gravity load-resisting systems sometimes occurred while perimeter walls and frames that were part of the lateral force-resisting system were undamaged; failure of diaphragm collectors and chords; large diaphragm deflections; and distress at precast connections due to lateral movements. On the other hand, many parking structures in the area of strong shaking received little or no damage, suggesting that some design and construc-tion practices used in these structures were inherently better than others.

Unique Seismic Issues of Parking Structures

Parking structures are usually very large in plan area, with relatively thin post-tensioned or precast concrete diaphragms as compared to a typical office building. Ar-chitectural, traffic, security, and economical demands push for long spans and large open areas. Prestressed concrete is a system

C-Guest-SEAOC-July10.indd 1 6/18/2010 11:11:23 AM

July 2010 STRUCTURE magazine July 2010

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which are likely to be governed by shear action rather than bending. This article is confi ned to this important issue of the seismic design and analysis of ramps. A more complete treatment is available in the Structural Engineers Association of California Blue Book paper on Concrete Parking Structures available at www.seaoc.org/bluebook, which includes references and also covers design issues related to columns and diaphragms.

RampsWe can speak in general of a parking

structure being a particular number of stories in height, but in terms of its struc-tural actions, the concept of stories can be an ambiguous concept. Parking structures often have a spiral or split level confi gu-ration that is not clearly represented by discrete story levels. For example, the same segment of the deck could connect level three to level four. Ramps that connect directly to shear walls or moment frames further deviate from the idealized distinct story levels used in the current codes.

The actual performance of an integrated ramp structure may not match the ductile behavior upon which seismic factors, such as the R factor, were based. Ramps can change the stiffness and defl ection patterns of the building and change the distribution of loads to the designated seismic resisting elements, in some cases attracting a signifi cant percentage of the force. For example, the R factor in ASCE 7-05 or the 2006 International Building Code for a special moment resisting-frame (SMRF) is 8. For comparison, the R factor for a special shear wall in a building frame system is 6. In other words, the base shear for a SMRF building is permitted to be 75% of that of a shear wall building because of the relative implied ductil-ity (6/8) for the two systems by the code. If a ramp in a SMRF parking structure stiffens up the building, reducing the true fl exibility and altering the hinge formation mechanism, then the use of R = 8 in this case is non-conservative.

Treatment of Parking Structures by Building Codes

Based on observations from the 1994 North-ridge Earthquake, the following code changes (Table 1) were subsequently added for concrete structures in regions of high seismicity.Current building codes do not provide

specifi c guidelines suitable for analyzing the complex story interactions that can occur in parking structures, nor provisions for detail-ing seismic capacity in the ramps. In some cases, assuming discrete story levels may be too simplifi ed an approach and could cause the designer to overlook unintended struc-tural shortcomings.Shear walls and moment frames are recognized

lateral force-resisting elements in building codes, but ramps are not codifi ed. Yet, some ramps can be stiff and massive enough to interact with the designated seismic resisting systems. A lit-eral interpretation of the 2006 International Building Code might place ramps in the “other components” category like gravity columns and non-frame beams, which are often excluded in seismic analysis models. When ramps are cat-egorized as non-seismic elements, their effect on the seismic behavior of the structure could be inadvertently overlooked.Ramps can be considered as inclined slabs,

but codes lack specifi city in detailing guide-lines suitable for slabs to function as vertical elements of the primary seismic force-resisting system. Interconnected ramps are not held to the ductility detailing provisions prescribed for the shear walls and frames. The diaphragm collector and shear reinforcement is not in-tended to yield, and thus boundary member confi nement would not be required. Similar concerns regarding the greater force demands

Structural Element Intent of Code Change ASCE 7-05 ACI 318-05

Diaphragm and Collectors

Specifi ed the minimum thickness of topping slabs.Limited the spacing and bar size at lap splices for force transfer

ACI 21.9.4ACI 21.9.8.3

Collector Design Forces Increased the collector design forces

ASCE 12.10.2

Prestress Tendons Excluded the use of prestressing tendons in boundary and collector elements, except for the precompression from unbonded tendons

ACI 21.9.5.2

Strength Factor, Φ Reduced Φ from 0.85 to 0.60 for the design of reinforcement used for diaphragm chords and collectors placed in topping slabs over precast

ACI 9.3.4

Beam-to-Column Connection

Added requirements for precast concrete gravity frames for improved beam to column connections

ACI 21.11.4

Transverse Reinforcement of Frame Members

Prescriptive requirements for transverse reinforcement for frame members not proportioned to resist seismic-induced forces

ACI 21.11.2ACI 21.11.3

Table 1: Building code changes since 1994 affecting concrete parking structures.

Figure 1: Collapsed parking structure, 1994 Northridge Earthquake. Courtesy of Robert Reitherman.

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frames in the orthogonal direction. This prac-tice allows less seismic deformation along the sloped ramps and reduces seismic loads imposed on short columns. In any event, it should be noted that ramps have two different characteristics: orthogonal and longitudinal. In the longitudinal direction, ramps act as truss elements transmitting axial forces. The con-cern in the orthogonal direction is the aspect ratio of the diaphragm and the deformation associated with it. Designers should properly account for these issues.In the absence of published guidelines, the

best approach currently being used to study these effects is project-specific computer anal-ysis, with each unique building being modeled to evaluate the effects of the particular ramping configuration. Today’s computational tools permit more complex analysis, including flex-ibility of diaphragms, and more complex defi-nitions of deck levels, including sloped ones. However, the current computer output is even more difficult to correlate with the prescribed design approach specified in the building code because seismic loads are resisted by other members of the structure such as the ramps, not just the designated lateral force-resisting system recognized by the code.

SummaryParking structures have a number of unique

characteristics, compared to conventional con-crete buildings, which affect their seismic performance. While this article has focused specifically on issues regarding ramps, addi-tional topics are addressed in the full SEAOC Blue Book article. Ramps will impact the seismic behavior of parking structures to varying degrees, depending on the interconnectivity of the ramps and the primary seismic force-resisting system. An appropriate level of analyti-cal sophistication is required to identify and properly design for these effects. A three-dimensional computer analysis, which includes consideration of the ramps, is an effective tool to capture the behavior and is highly recom-mended. The challenge, and responsibility, of the structural designer of a parking structure is to overcome the disparity between the config-uration of the structure and the current code procedures, and to demonstrate and detail a rational load path through the structure.▪

Mehran Pourzanjani, Chair of the Seismology Committee of the Structural Engineers Association of California, [email protected].

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have been raised pertaining to the discussion of highly flexible diaphragms with perimeter-only lateral restraint systems. Stiff ramps also can alter the balance of lateral resisting components, causing secondary torsion effects that redistrib-ute the story forces, potentially increasing loads to specific seismic resisting elements.

Design ApproachesIt is common practice to release ramps at

grade, but to provide positive connections at the elevated parking decks. This may result in soft and/or weak story performance in areas of high seismicity. The shift from connected to disconnected levels can cause a local redistri-bution of the shear forces, causing the second story diaphragm to act like a transfer slab with substantial load demands. This is more critical for moment frame structures than for other structures. In some configurations, the top-level floor may have shear-resisting elements on three sides only, and thus relies on canti-lever diaphragm rotation to distribute seismic forces at that level. The horizontal irregularity types noted in the building code lack guide-lines to limit cantilever diaphragm distance.It is common in the industry to neglect

the interconnectivity of the story levels in the analysis stage of design. A less common approach, due to its impracticality, is to design the ramp with a physical release at each level, using expansion joints to change the structure to match the code. While analytically possible, this construction approach is impractical as the lateral seismic loads imposed by the sloped ramps, which are connected to the horizontal diaphragms on one side only, contribute to undesirable torsional effects. Additionally, the added initial cost, ongoing maintenance, and the added aesthetic drawbacks of the expansion joints further undermine this approach.Some practitioners believe that interconnecting

sloped floors provide for structural “tough-ness,” judging that a well tied-together building is inherently more robust. While it is valid to assert that connected ramps provide reserve stiffness or redundancy to a building, it also is true that concurrent load paths are inher-ently unpredictable. Secondary systems can inadvertently absorb a disproportionate share of the load, even functioning as primary load paths. For example, stiff non-ductile ramps can dominate a moment-frame system, short-circuiting the ductile members that are designed to dissipate the energy.Many practitioners prefer to include shear

walls in the direction of the ramps, while maintaining more flexible moment-resisting


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Post-Tensioned Slabs on GroundPart 3: Proper Detailing and Quality ControlBy Bryan Allred, S.E.

Advantages in Use of RebarNumerical design using the Post-

Tensioning Institute (PTI) method is based primarily upon on the precompres-sion from the tendons in conjunction with the section and material properties of the concrete. Rebar plays a very limited role in the design for expansive soils, but is very useful as crack control reinforcement. Trim bars are typically placed around pen-etrations and re-entrant corners (Figure 1) where shrinkage cracks will most likely occur. Until the tendons have been stressed, the foundation is essentially un-reinforced, so well placed rebar is useful in minimizing shrinkage cracks. While the force of the tendons has the potential to close up small cracks that occur prior to stressing, relying on this benefit is not recommended. The typical repair for “substantially” cracked concrete is the use of structural grade epoxy; however, this fix is often very unappealing from an owner’s point of view. The crack will need to be routed out to achieve the proper width for the injector, and the epoxy rarely matches the color of the concrete. The finished repair typically looks like a spider web of dark lines, often appearing worse than the original cracked condition. The look of the repair also gives the impression that something has gone seri-ously wrong with the foundation. While trim rebar will not guarantee a crack free system, it will provide some crack control strength until the tendons are stressed. Rebar is also typically added under large hold downs or post loads to increase the footing’s flexural and shear capacity. This is often done where the foundation design does not require the use of deep footings to resist soil movement.

Placement of TendonsDuring a structural observation, the

location and path of travel of the tendons should be reviewed. Localized vertical and horizontal kinks in the strands should be removed, especially if these occur near the anchor. Unless specifically detailed, the tendons should run at the center of

the slab and essentially stay at this general position across the entire foundation. Chairs or dobies are typi-cally placed at 4-foot centers to support the tendons (Figure 2); any vertical dis-continuity in the strand is typically due to a missing or incorrect chair. Anchors that are located near penetrations should be adjusted to avoid blow outs. Provided the number of tendons installed matches the permitted plans, adjusting the loca-tion of a specific strand should not affect the performance; however, the tendons should not be placed more than 6 feet apart. If a gap larger than 6 feet is required, additional rebar or localized tendons may be required. Each tendon will be loaded to approximately 33,000 pounds during stressing, and a discontinuity near the anchor can cause cracking or a blow out. If the anchor or the penetration cannot be adjusted, schedule 40 steel sleeves have been successfully used in the past.The observer should also verify that any

rebar placed in the bottom of the footings is clear of dirt or debris. Due to the foot traffic of the contractors, it’s common to have soil fall into the trench and cover the rebar. In addition to decreasing the footing depth, the soil can reduce the rebar-to-concrete bond, which will mini-mize its effectiveness.

InspectionDuring the stressing operation, a licensed

inspector is required to observe the jack-ing procedure and record the resulting elongations. The elongation record is the primary tool for the engineer and owner to verify permitted structural drawings have been implemented correctly. The elongation record should be sent to the engineer for review prior to removing the stressing tails. If the elongations are within 10% of the calculated value, the stressing is considered acceptable and the tails can be removed. Having the inspector list the elongation out of tolerance percent-age will speed up the review process. If the elongation is outside of this tolerance, the engineer should evaluate the situation and make appropriate modifications. The author recommends taking the overall concrete section into consideration rather than focusing on a single strand.A specific tendon only has a localized

affect on the concrete for the first few feet away from the anchor, until the precompression spreads into the larger foundation area. Subgrade friction is at a minimum near the slab edge, so any re-duction in the tendon force should have a negligible effect on the foundation. As

the precompression force disperses into the whole foundation, the concrete isn’t able to determine what strands have a “low” force and which ones have a “high” force. The con-crete only feels the total load applied by the strands. Provided the overall precompression is achieved, the as-built construction satisfies the drawings and no remedial work is required. If the engineer requires the tendons to re-stressed, they will have to be de-tensioned by removing the wedges, releasing all the elon-gation and repeating the stressing procedure. De-tensioning can be dangerous and should only be done after careful consideration by qualified personnel. If the elongation errors are more systematic (generally high or low), the engineer may want to verify that the jack and the pressure gauge were calibrated together. The stressing unit should be treated as a com-plete system and not as separate pieces.

Cold Joint HazardIn the construction of ribbed foundations,

contractors will often pour the footings first, verify and make any final adjustments to the embedded hardware (Figure 3) and then place the slab over the existing footings. If the time gap between when the footings and slab are poured is large enough, a cold joint will be cre-ated, effectively disconnecting the slab from the footings. The foundation will essentially be a thin uniform thickness slab sitting on, but effectively not connected to, the footings. Without slab footing composite action, the section properties and flexural strength of the as-built system will be substantially less than design required. Having the tendons only being placed in the slab, the cold joint prevents their precompression from extending into the foot-ings. The footings are basically un-reinforced concrete and more prone to cracking. Specific details and/or notes are recommended to specify the maximum time gap between pours, or verification that the separate pours were vibrated together to replicate a monolithic system. If a cold joint is desired, rebar dowels extending from the footings into the slab are typically used to achieve composite action. The dowels should be designed to transfer the horizontal shear between the footings and slab, and address any large hold downs or post loads which may require additional reinforcing. In addition, the anchor and hold down bolts may have longer embedment requirements for a two pour system.

Concrete StrengthThe concrete used in a post-tensioned slab

on ground is the same as conventionally rein-forced foundations. The concrete will typically have a compressive strength of 2,500 to 4,500 psi. The 4,500 psi concrete is typically used

This is the third of four articles on post-tensioned slab on ground design and construction. This article will focus on detailing and quality control, while the previous two articles provided a general overview and special design considerations. Please see the January 2010 and April 2010 issues of STRUCTURE® magazine for these articles.

Figure 1: Trim Rebar.

Figure 2: Plastic Chairs Used to Support the Tendons.

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13

InspectionDuring the stressing operation, a licensed

inspector is required to observe the jack-ing procedure and record the resulting elongations. The elongation record is the primary tool for the engineer and owner to verify permitted structural drawings have been implemented correctly. The elongation record should be sent to the engineer for review prior to removing the stressing tails. If the elongations are within 10% of the calculated value, the stressing is considered acceptable and the tails can be removed. Having the inspector list the elongation out of tolerance percent-age will speed up the review process. If the elongation is outside of this tolerance, the engineer should evaluate the situation and make appropriate modifications. The author recommends taking the overall concrete section into consideration rather than focusing on a single strand.A specific tendon only has a localized

affect on the concrete for the first few feet away from the anchor, until the precompression spreads into the larger foundation area. Subgrade friction is at a minimum near the slab edge, so any re-duction in the tendon force should have a negligible effect on the foundation. As

the precompression force disperses into the whole foundation, the concrete isn’t able to determine what strands have a “low” force and which ones have a “high” force. The con-crete only feels the total load applied by the strands. Provided the overall precompression is achieved, the as-built construction satisfies the drawings and no remedial work is required. If the engineer requires the tendons to re-stressed, they will have to be de-tensioned by removing the wedges, releasing all the elon-gation and repeating the stressing procedure. De-tensioning can be dangerous and should only be done after careful consideration by qualified personnel. If the elongation errors are more systematic (generally high or low), the engineer may want to verify that the jack and the pressure gauge were calibrated together. The stressing unit should be treated as a com-plete system and not as separate pieces.

Cold Joint HazardIn the construction of ribbed foundations,

contractors will often pour the footings first, verify and make any final adjustments to the embedded hardware (Figure 3) and then place the slab over the existing footings. If the time gap between when the footings and slab are poured is large enough, a cold joint will be cre-ated, effectively disconnecting the slab from the footings. The foundation will essentially be a thin uniform thickness slab sitting on, but effectively not connected to, the footings. Without slab footing composite action, the section properties and flexural strength of the as-built system will be substantially less than design required. Having the tendons only being placed in the slab, the cold joint prevents their precompression from extending into the foot-ings. The footings are basically un-reinforced concrete and more prone to cracking. Specific details and/or notes are recommended to specify the maximum time gap between pours, or verification that the separate pours were vibrated together to replicate a monolithic system. If a cold joint is desired, rebar dowels extending from the footings into the slab are typically used to achieve composite action. The dowels should be designed to transfer the horizontal shear between the footings and slab, and address any large hold downs or post loads which may require additional reinforcing. In addition, the anchor and hold down bolts may have longer embedment requirements for a two pour system.

Concrete StrengthThe concrete used in a post-tensioned slab

on ground is the same as conventionally rein-forced foundations. The concrete will typically have a compressive strength of 2,500 to 4,500 psi. The 4,500 psi concrete is typically used Figure 2: Plastic Chairs Used to Support the Tendons.

Figure 3: Adjusting a Strap in the Footing Prior to Pouring the Slab.



cracking the slab. For smaller slab areas, rebar is recommended instead of using short tendons. The maximum length of a tendon is typically around 200 feet, due to stressing limitations and realistic pour sizes. While it’s possible to use longer tendons, the buildup of subgrade friction and increased shrinkage crack potential usually makes this practice un-economical. Tendons longer than 100 feet often require double ended pulls, unless spe-cifically designed otherwise. Double ended pulls require stressing at each end of the strand, but the stressing is not done simultaneously. The stressing system (Figure 4) is placed on one end while the wedges are hammered into the opposite anchor to resist the stressing force. The jack will be fully elongated at the one end and will generate the vast majority of the required elongation. After the wedges are installed on the first stressing end, the jack is removed and placed on the other end of the tendon. The jack is loaded to the same pres-sure as the first stressing, but a very small elongation is expected. This second stressing is referred to as a “lift off ”, and is primarily intended to relieve any slack or extra friction in the strand and to seat the wedges to correct gauge pressure. A double ended pull is typi-cally indicated by arrow heads on both ends of the strand. If the tendon location and stressing is left up to the contractor or supplier, notes requiring double ended pulls for tendons beyond a certain length are recommended.For these larger foundations, it is recom-

mended that the designer consult the concrete sub contractor to determine their preferred pour size and stressing abilities. Unlike rebar only foundations, the pour size and stressing locations should be determine during the design process so the appropriate number of tendons are specified.

New Footing RequirementsWith the new ACI appendix D requirements

for the design of concrete due to uplift loads, the typical footing details that have been used for years will most likely not be sufficient. Addition-al width and depth of the footings are typically required at the hold down bolts. If the building structural engineer is detailing the framing to concrete connection, notes are recommended on the post-tensioning plans to direct the con-tractor to the other engineer’s drawings for the additional footing requirements.▪

Figure 4: Typical Stressing Equipment.

Bryan Allred is a license structural engineer and Vice President of Seneca Structural Engineering Inc. in Laguna Hills CA. He can be reached at [email protected] with any questions.

to resist severe sulfates or is used on highly expansive sites where the higher strength can aid in satisfying allowable stresses. Some large tract home builders will require a minimum of 4,000 psi concrete with type V cement, since it provides moderate sulfate protection and sulfates have been an issue in home owner association litigation against developers and contractors. The use of higher strength concrete is typi-cally useful for a post-tensioned foundation since the minimum compressive value to begin stressing will be achieved in a shorter time. The sooner the tendons are stressed, the sooner the primary reinforcement is added to the system which should minimize shrinkage cracks. Some engineers and contractors will perform a par-tial pre-stress to place some precompression in the system in an attempt to minimize cracking. The typical practice is to stress each strand to approximately 20% of the full value the day after the foundation was poured. The author would recommend caution for new post-tensioning engineers in specifying partial pre-stressing. The more times the jack is applied to the system, the greater the chance of damag-ing the strand, anchor, wedges or the concrete. In addition, this practice is primarily used on slab-on-ground construction and is rarely performed on elevated post-tensioned systems even though the anchors, wedges, strands and concrete are exactly the same.

Construction Joints and Delay Strips

The construction of apartment complexes and industrial projects often leads to large and sometimes irregular plate configurations. These foundations will often require construction joints and/or delay strips to create manageable pour sizes and to adequately stress the tendons. Construction joints will have the tendons

continue through the joint and use shear keys with rebar dowels to connect the adjacent slabs. Ribbed foundations will typically have a center slab dowel while the thicker mat foundations will use top and bottom bars. The joints are limited to a spacing of around 100 feet which is the typical maximum length of a singled ended pull. In addition, review of the hold downs bolts and plumbing penetrations should be taken into consideration prior to selecting a joint location. Placing a joint di-rectly adjacent to a large uplift/post load or splitting a penetration is not recommended.Delay strips are typically three feet wide open

spaces between slab pours so the tendons from each pour can be stressed. The rebar is lapped for the full width of the pour strip but should not extend into the adjacent slab. Any rebar extending from one pour to the other will act as a tension tie and eliminate any independent movement of the slabs. The time the delay strip is poured is at the engineer’s discretion, but is typically around 30 to 45 days to allow the adjacent pours to shrink as a smaller unit rather than be part of a larger plate. Near the end of the project, the concrete sub-contractor will often request to place the delay strip be-fore the recommended time as occurred. From a structural point of view, there is minimal downside since the strength is not affected; however, it’s important that owner and archi-tect understand additional shrinkage cracking may occur.

Tendon LengthTendons can be manufactured to effectively

any length desired but practically range from 20 to 200 feet long. Tendons less than 20 feet long will have a very small elongation and this increases the chance of over extending the jack, over loading the tendon and possibly


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Sea Wall SystemsSea Wall vs. BulkheadBy Vitaly B. Feygin, P.E.

To properly assess the requirements for a Bulkhead or Sea Wall, the Design Professional should fully understand and differentiate the purpose of these two structures. Both structures, Sea Walls and Bulkheads, serve the purpose of vertical shoreline stabilization. They allow prop-erty owners to maximize the efficiency of their property. Both structures utilize similar construction techniques and simi-lar construction materials. However, the structures are not the same.

• A Bulkhead is a vertical shoreline stabilization structure that primarily retains soil and surcharge loads behind the wall.

• A Sea Wall is a structure that has two primary functions:

° retaining soil and surcharge loads behind the wall, and

° protection of shoreline from wave loads.

In addition, Sea Walls typically protect frontline beaches from storm surges, shore-line erosion and wave overtopping. Some waterfront properties are subject to signifi-cant wave activity during the storm surge events, even though they are not exposed to wave action for the most part of the year.The following design considerations are

normally addressed by the designer of a Sea Wall as compared to the designer of a Simple Bulkhead:

• Direct wave force action• Uplift force imposed by wave action• Wave overtopping• Storm surge• Toe scour

• Relies heavily upon the weight of the wall when that weight significantly decreases due to buoyancy effect.

• Requires a very stiff base that can prevent wall settlement, tilting or heavy toe scour that affects wall integrity and stability.

• Unviable option when bedrock elevation or elevation of other suitable base significantly varies along the wall length.

System B: L-Shaped Wall with Buttresses

• A type of wall that is more economical than a Gravity Wall and easier to construct.

• Buttress of the wall serves as a stiffening element for the wall itself, and allows some force redistribution in the wall based upon the stiffness of the tapered buttress element.

• L-Shaped wall faces exactly the same design stability issues as a Gravity Wall:

° Significant wave generated uplift force.

° Heavy reliance on soil surcharge on the hill of the wall at the time when that weight significantly decreases due to buoyancy.

° Requirement for very stiff base and possibility of heavy scour that can affect wall stability.

System C: L-Shaped Wall with Buttresses Supported by Piles

• A type of wall, a modification of System B, that has a significant advantage over System B.

• Does not rely, or relies much less, on the gravity of the heel surcharge.

• Less susceptible to distress due to scour problem.

• Stability of the wall depends upon the pile capacity to resist uplift and the effect of horizontal load.

• Variable stiffness of the buttress T-section does not allow effective span moment redistribution, particularly when resultant of the horizontal force shifts towards the top of the wall as happens in the case of wave load or Monotobe - Okabe seismic soil wedge retained by the wall.

• Price of the wall can be prohibitive.

System D: Diaphragm Wall System with Horizontally Spun Wall

A type of wall system that is easy to construct. The wall system provides a new design philosophy for Sea Wall construction. Benefits of the system include:

• Lower cost of construction and more flexibility of the system, as compared to the same features of traditional designs.

• Wall stability is not dependent on the gravity load of backfill.

• Wall stability is independent of gravity of the surcharge.

• Low effect of soil scour in front of the wall on wall system distress. Easy maintenance.

Figure 1: Diaphragm Sea Wall.

Figure 2: Section A-A of Diaphragm Sea Wall.

The uplift force imposed by wave action is an important factor that is frequently neglected by design professionals, that leads to instability and undermines the longevity of the Sea Wall structure.Many existing waterfront properties

around the country, including both East and West Coast shorelines as well as shorelines of the Great Lakes, were de-signed using a simple bulkhead approach that neglected wave forces. As a result, many waterfront properties suffered sub-stantial structural damage and incurred costly maintenance problems.

Sea Wall Systems: Advantages and Disadvantages

Many Sea Wall systems were developed to address the design considerations noted previously. The advantages and dis-advantages of several typical systems are reviewed below.

System A: Gravity Wall

• A type of wall, known from ancient times, that is extremely costly to build, especially when wall height dictates significant development of the wall base.

• Requires consideration of significant wave generated uplift force.

The following numbering indicates different wall elements in the accompanying figures:

10) Diaphragm Sea Wall11) Front column of the

Diaphragm or column of braced Soldier Pile system

12) Back column of the Diaphragm13) Web of the Diaphragm14) Continuous retaining wall15) Diaphragm web closure pour16) Retaining wall closure pour17) Caisson18) Wall drainage system20) Shaft cage21) Retaining wall splice rebar22) Diaphragm web splice rebar23) Tie Back soil/rock anchor

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Sea Wall SystemsSea Wall vs. BulkheadBy Vitaly B. Feygin, P.E.

To properly assess the requirements for a Bulkhead or Sea Wall, the Design Professional should fully understand and differentiate the purpose of these two structures. Both structures, Sea Walls and Bulkheads, serve the purpose of vertical shoreline stabilization. They allow prop-erty owners to maximize the efficiency of their property. Both structures utilize similar construction techniques and simi-lar construction materials. However, the structures are not the same.

• A Bulkhead is a vertical shoreline stabilization structure that primarily retains soil and surcharge loads behind the wall.

• A Sea Wall is a structure that has two primary functions:

° retaining soil and surcharge loads behind the wall, and

° protection of shoreline from wave loads.

In addition, Sea Walls typically protect frontline beaches from storm surges, shore-line erosion and wave overtopping. Some waterfront properties are subject to signifi-cant wave activity during the storm surge events, even though they are not exposed to wave action for the most part of the year.The following design considerations are

normally addressed by the designer of a Sea Wall as compared to the designer of a Simple Bulkhead:

• Direct wave force action• Uplift force imposed by wave action• Wave overtopping• Storm surge• Toe scour
























Figure 1: Diaphragm Sea Wall.


The uplift force imposed by wave action is an important factor that is frequently neglected by design professionals, that leads to instability and undermines the longevity of the Sea Wall structure.Many existing waterfront properties

around the country, including both East and West Coast shorelines as well as shorelines of the Great Lakes, were de-signed using a simple bulkhead approach that neglected wave forces. As a result, many waterfront properties suffered sub-stantial structural damage and incurred costly maintenance problems.

Sea Wall Systems: Advantages and Disadvantages

Many Sea Wall systems were developed to address the design considerations noted previously. The advantages and dis-advantages of several typical systems are reviewed below.

System A: Gravity Wall

• A type of wall, known from ancient times, that is extremely costly to build, especially when wall height dictates significant development of the wall base.

• Requires consideration of significant wave generated uplift force.

The following numbering indicates different wall elements in the accompanying figures:

10) Diaphragm Sea Wall11) Front column of the

Diaphragm or column of braced Soldier Pile system

12) Back column of the Diaphragm13) Web of the Diaphragm14) Continuous retaining wall15) Diaphragm web closure pour16) Retaining wall closure pour17) Caisson18) Wall drainage system20) Shaft cage21) Retaining wall splice rebar22) Diaphragm web splice rebar23) Tie Back soil/rock anchor



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• Wall stability is dependent upon the drilled caisson capacity to resist uplift and the effect of horizontal loads.

• Lack of uplift pressure on the wall base or heel, as the Diaphragm system does not have a heel.

• Effective span moment redistribution allowed by constant stiffness of the Deep Beam Diaphragm fixed at the wall base.

• Horizontally spun continuous wall supported by Deep Beam Diaphragms. Wall Diaphragm provides support for loads applied in both directions.

System E: Soldier Pile System with Horizontally Spun Wall and Tie Back Anchors (Modified Bulkhead Approach)

A type of wall system that is also easy to construct. The front of the wall is somewhat similar to the front wall of the diaphragm system; however, design of this wall is based on a different philosophy, as the wall derives its resistance from different elements, depending on direction of load application. Benefits of this system include:

• Lower cost of construction and higher adaptability of the system, as compared to the same features of traditional designs.



• Wall stability is dependent upon the drilled caisson capacity to resist the effect of horizontal load, and capacity of the soil anchors to resist the load in a seaward direction. Ability of elastic foundation (Caisson socket and granular soil backfill behind the composite width of the wall column) to resist the wave load in landward direction. Elastic foundation reaction in that case, is compared to the lateral capacity of mobilized passive


Figure 3: Section B-B of Diaphragm Sea Wall.



pressure. The designer must distinguish the difference between maximum possible soil passive resistance and mobilized passive pressure, as mobilized passive pressure frequently is only a fraction of maximum passive pressure resistance. Quite often, mobilized passive pressure does not exceed the pressure equivalent of the pressure exerted by the active pressure wedge.

• Lack of uplift pressure on the wall base.• System effective span moment

redistribution in both seaward and landward direction.

• Stiffness of the soil anchors and stiffness of specially modified backfill allows for the design of the retaining wall as a continuously spun horizontally slab.

• Attractive price of the wall.Wall Systems A, B and C are well-known

and well-described in many sources. A general

Figure 5: Section A-A.

concept of the Diaphragm Sea Wall, Wall System D, is represented in Figures 1 (page 16), 2 (page 17) and 3 (page 17). Wall System E is shown in Figures 4 and 5.Wall Systems D and E, however,

have a common requirement for behind the wall backfill. This require-ment compensates for lack of wall embedment or entrenchment into the rock or beach soil. The bottom 2-3 feet of the backfill consists of 3 to 4 inches of stone aggregate over-topped by a 2-foot thick layer of filter stone or overlaid by Geotextile filter fabric in order to prevent back-fill erosion.The final advantage of Wall System

D and E is derived from the fact that erosion of the soil around the front pile can be easily remedied by the use of flowable fly ash fill that can easily restore eroded soil around the pile to a preexisting or better condition. Erosion of the soil in

front of the wall itself is almost never critical and does not require urgent attention. Soil in front of the wall can be restored during normal beach nourishment operations.Quite often, high flexural moments are ex-

erted on the front piles of the Wall System E. Sometimes it is more economical to design front piles of that system as columns and not as beams. In that case, front pile should be designed with a post-tensioned rock anchor exerting compression force predetermined by wall designer.

2) During the wall system selection process, the Designer should understand that every flexible wall system that allows force redistribution in the horizontal direction should be designed using a set of spring values for each wall support. Each support spring value should be determined for each load combination at the level of the Horizontal Resultant force. The design should use a 3 or 5 span continuity approach, assuming pin connections at the ends of the 3 or 5 span wall. Some savings can be achieved if the designer uses spring supports only for the dynamic portion of the load. Remember that static load redistribution is a one time event causing permanent plastic deformations.

3) It is prudent to assume only half of the wave or seismic load in the mid span or alternate spans to verify the impact of the load on the supports differential movements.

4) The Designer, Owner and Contractor should collectively select the most economical Wall System. Consideration should be given to availability of materials and availability of skilled labor force.▪

Vitaly B. Feygin, P.E. is a Marine Structural Engineer. He is a Principal Structural Engineer with Marine and Industrial Consultants , Baltimore and Tampa offices. He is an author of two patents related to Sea Walls, Composite Cofferdams, Bridge Fenders and Port Structures. Mr. Feygin can be contacted at [email protected].

The easiest to use software for calculating wind, seismic, snow and other loadings for IBC, ASCE7, and all state codes based on these codes ($195.00).Tilt-up Concrete Wall Panels ($95.00).Floor Vibration for Steel Beams and Joists ($100.00).Concrete beams with torsion ($45.00).

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Figure 4: Soldier Pile Braced Sea Wall.

The stiffness of the Deep Beam Diaphragms fixed at the base, and a very rigid spring value of such support, allows the horizontally span retaining wall (14) to be designed as a multi-span continuous horizontal slab. The Author recommends a fairly conservative three-span approach for the wall design and a five-span approach for determining the wall support reactions. To design the wall properly, the designer must check the support spring values for each set of loads in order to assure the validity of the support stiffness assumption.

Sea Wall Design Guidelines1) Determine loads and load

combinations affecting the Sea Wall design. The following short list of loads should be reviewed during the design process:

° Active soil pressure wedge

° Active soil pressure wedge + Seismic rupture wedge determined from Monotobe-Okabe equation

° Direct Horizontal Wave load + Wave uplift pressure exerted on the heel of the wall


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Blank.indd 1 6/2/2010 1:59:28 PM


Barton Creek Bridge

This concrete fi n-back bridge rises eighty feet above the streambed that carries Barton Creek into Austin, Texas and eventually to the Colorado River (Figure 1). The bridge is the main

entrance to The Estates of Barton Creek subdivision, a country club community of million dollar plus homes. The bridge was constructed during 1985 to 1987, and was opened for traffi c in 1988. The design and construction of the bridge was perhaps the fi rst application of a fi n-back, balanced cantilever, cast-in-place, post-tensioned bridge in the world.The bridge is 686 feet long and consists of three spans: 156-feet, 340-

feet, and 190-feet. The unsymmetrical span arrangement was dictated to respect the environmentally sensitive gorge and stream buffer/greenway area adjacent to the creek. The basic superstructure form is a triangular box with concrete ribs and struts supporting a concrete deck. The fi n-back name derives from the central fi ns, or walls, which rise from the triangular box to peak over each intermediate pier. The fi ns encase post-tensioning ducts, which take advantage of the large eccentricity of the post-tensioning force in the negative moment regions of the structure. The bridge provides a two lane roadway with central median barrier required to accommodate the fi n.

Project HistoryDuring 1983, the developer of The Estates of Barton Creek, Barnes

Connelly Investments, negotiated with Travis County for permission to build a new road, including a landmark bridge that would minimize visual and environmental impacts to the steep slopes and fl ood plain of the Barton Creek gorge. The primary need for the road and bridge was to provide the shortest route from the subdivision to downtown Austin. In May 1984, the developer hired engineers, including Atlanta based Tony Gee + Quandel, Inc., to study a cost effective solution for the bridge.The developer and engineer were aware of problems and expense

experienced by the Texas Department of Transportation (TxDOT) in 1981 during construction of a multi-span pre-stressed con-crete (PSC) girder bridge over Barton Creek, approximately six miles downstream from the proposed crossing. Due to environmental constraints, the TxDOT contractor was re-quired to use over-the-top methods for erection of the PSC girders. This required a costly girder launching gantry in order to place the PSC girders from above.The developer and Travis County wanted to minimize the

number of piers in the area near the creek. The following alternates were considered:

• Single-span cable-stay bridge• Single-span suspension bridge• Three-span cable-stay bridge• Three-span conventional variable depth box

girder bridge• Three-span concrete fi n-back bridge

The three-span fi n-back bridge was ultimately recommended because it was the most economical alternate, limited disturbance of the creek fl ood plain due to balanced cantilever construction, accommodated the required unsymmetrical span arrangement, and provided a novel gateway for the subdivision.

DesignThe central location of the main pre-stress force presents design

challenges for the fi n-back bridge. Conventional hollow box sections require internal struts to carry loads to the center web/fi n. This is a sim-ilar design situation for cable-stayed bridges with a single plane of stays, such as the Sunshine Skyway Bridge in Tampa, Florida. To overcome the internal strut issue that would complicate cast-in-place segmental construction, the Barton Creek Bridge designers developed a constant depth triangular section with external struts supporting transverse ribs, which in turn supported an eight-inch slab spanning between the ribs (Figure 2). The triangular section allowed the central fi n to start at the apex of the triangular section. This junction also provided a suffi cient area in which to anchor the pair of main post-tension tendons required for each segment.The bridge was designed to be built as a cast-in-place balanced canti-

lever using a form traveler. A typical segment length of 11feet 4 inches was selected to accommodate a reasonable size form traveler. The deck ribs and struts were located near the leading edge of each segment, again primarily for support of the form traveler.A unique aspect of the design is that the fi n was raised as a series of

lifts above the deck. The initial lift made by the form traveler included starter bars for the fi n. As balanced cantilever construction advanced, the fi n was raised following completion of three pairs of segments.Longitudinal analysis of the superstructure indicated that shear lag,

or concentration of post-tensioning force at the center of the section, was a concern during initial stages of construction. To overcome this

situation, a high strength post-tensioning bar was added to a beam/parapet at the exterior edges of the deck. This progressively coupled bar was stressed following casting of each segment.To overcome the tension created by the strut geometry, the ribs were

post-tensioned transversely with a four 0.6-inch strand tendon, and the main triangular webs were post-tensioned with two high strength bars located at the struts.The bridge substructure consists of two abutments and two main

piers comprised of pairs of fl exible rectangular shafts, 3½ feet thick and 11feet 4 inches apart to match the superstructure segment length. The shafts are integral with the superstructure. The twin shaft design supported out-of-balance construction loads in addition to fi nal wind, live loads, and shrinkage and creep forces anticipated during the life of the structure.Foundations for the abutments and piers consist of drilled shafts founded

in sound limestone. Abutments are supported on four 36-inch diameter shafts between 15 to 25 feet deep. Each pier is supported on six 60-inch diameter drilled shafts, approximately 30 feet deep.Following completion of the design and contract documents in late

1984, Travis County hired HNTB to perform a design review of the unusual project. No major comments resulted from this review and the project was advertised to a group of pre-qualifi ed contractors.Three bids were received for the bridge, with the successful contractor

being Prescon Corporation, a subsidiary of a large French contractor, Campenon Bernard. The bid price was $3.6-million.

ConstructionFollowing execution for the construction contract in October 1985, the

contractor immediately began design of the form traveler system. Founda-tion construction began in November 1985. Superstructure construction began in March of 1986 with construction of the east pier table.Following construction of the pier table, the form travelers were

erected. Due to the limited length of the pier table (34 feet) the travelers were linked together to provide out-of-balance stability for the fi rst two pairs of segments on each side of the pier table. Following post-tensioning of the fi rst two pairs of segments, the form travelers were

Figure 3: Barton Creek Bridge Deck and Fins Condition.

Figure 1: Concrete Fin-Back Bridge Crossing 80-feet Above Barton Creek.

Figure 2: Barton Creek Bridge Typical Section.

By Mark W. Holmberg, P.E.

F-BartonCrBridge-Holmberg-July10.indd 1 6/18/2010 11:21:08 AM


The three-span fi n-back bridge was ultimately recommended because it was the most economical alternate, limited disturbance of the creek fl ood plain due to balanced cantilever construction, accommodated the required unsymmetrical span arrangement, and provided a novel gateway for the subdivision.

DesignThe central location of the main pre-stress force presents design

challenges for the fi n-back bridge. Conventional hollow box sections require internal struts to carry loads to the center web/fi n. This is a sim-ilar design situation for cable-stayed bridges with a single plane of stays, such as the Sunshine Skyway Bridge in Tampa, Florida. To overcome the internal strut issue that would complicate cast-in-place segmental construction, the Barton Creek Bridge designers developed a constant depth triangular section with external struts supporting transverse ribs, which in turn supported an eight-inch slab spanning between the ribs (Figure 2). The triangular section allowed the central fi n to start at the apex of the triangular section. This junction also provided a suffi cient area in which to anchor the pair of main post-tension tendons required for each segment.The bridge was designed to be built as a cast-in-place balanced canti-

lever using a form traveler. A typical segment length of 11feet 4 inches was selected to accommodate a reasonable size form traveler. The deck ribs and struts were located near the leading edge of each segment, again primarily for support of the form traveler.A unique aspect of the design is that the fi n was raised as a series of

lifts above the deck. The initial lift made by the form traveler included starter bars for the fi n. As balanced cantilever construction advanced, the fi n was raised following completion of three pairs of segments.Longitudinal analysis of the superstructure indicated that shear lag,

or concentration of post-tensioning force at the center of the section, was a concern during initial stages of construction. To overcome this

situation, a high strength post-tensioning bar was added to a beam/parapet at the exterior edges of the deck. This progressively coupled bar was stressed following casting of each segment.To overcome the tension created by the strut geometry, the ribs were

post-tensioned transversely with a four 0.6-inch strand tendon, and the main triangular webs were post-tensioned with two high strength bars located at the struts.The bridge substructure consists of two abutments and two main

piers comprised of pairs of fl exible rectangular shafts, 3½ feet thick and 11feet 4 inches apart to match the superstructure segment length. The shafts are integral with the superstructure. The twin shaft design supported out-of-balance construction loads in addition to fi nal wind, live loads, and shrinkage and creep forces anticipated during the life of the structure.Foundations for the abutments and piers consist of drilled shafts founded

in sound limestone. Abutments are supported on four 36-inch diameter shafts between 15 to 25 feet deep. Each pier is supported on six 60-inch diameter drilled shafts, approximately 30 feet deep.Following completion of the design and contract documents in late

1984, Travis County hired HNTB to perform a design review of the unusual project. No major comments resulted from this review and the project was advertised to a group of pre-qualifi ed contractors.Three bids were received for the bridge, with the successful contractor

being Prescon Corporation, a subsidiary of a large French contractor, Campenon Bernard. The bid price was $3.6-million.

ConstructionFollowing execution for the construction contract in October 1985, the

contractor immediately began design of the form traveler system. Founda-tion construction began in November 1985. Superstructure construction began in March of 1986 with construction of the east pier table.Following construction of the pier table, the form travelers were

erected. Due to the limited length of the pier table (34 feet) the travelers were linked together to provide out-of-balance stability for the fi rst two pairs of segments on each side of the pier table. Following post-tensioning of the fi rst two pairs of segments, the form travelers were

Figure 4: Underside of Barton Creek Bridge with Struts and Water Lines on Overhang and Twin Shaft Piers in the Distance.

Figure 3: Barton Creek Bridge Deck and Fins Condition.

separated and moved independent of one another. The travelers were anchored to each rib by means of high strength post-tensioning bars placed in small deck block-outs.There were two disadvantages of the external ribs and struts. The fi rst

involved the distance required for dropping the deck forms to clear the just-cast segment. The fi nal form traveler developed by the contractor combined partial disassembly of web, rib, and strut forms and lowering of the deck to clear all obstructions.The second disadvantage involved casting and consolidating concrete

in the relatively long, slender struts. To overcome this potential problem, the designer allowed a pre-cast strut option, which the contractor ultimately chose to use for all pier table and segment struts.

Twenty Years after ConstructionThe author visited the bridge site in May 2009. The bridge appeared to

be in excellent condition, with no obvious signs of distress. The wearing surface is sound with no evidence of cracks. It appeared that the fi ns had recently been painted and new deck joints had been installed at each end of the bridge (Figure 3). The bridge carries water lines on each side of the main triangular section (Figure 4), as well as a pair of conduits inside the main section.▪

Figure 1: Concrete Fin-Back Bridge Crossing 80-feet Above Barton Creek.

AcknowledgementsOwner: Travis CountyEngineer of Record: Tony Gee, P.E., Tony Gee + Quandel EngineersContractor: Prescon Corporation

Mark W. Holmberg, P.E. is Vice President and Civil Engineering Manager for Heath & Lineback Engineers, Inc. in Marietta, Georgia. He was Resident Engineer during construction of the Barton Creek Bridge in Austin, Texas. Mark currently serves on the STRUCTURE magazine Editorial Board and he can be reached by email at [email protected].

F-BartonCrBridge-Holmberg-July10.indd 2 6/18/2010 11:21:21 AM

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Service Life of a Structural RetrofitEngineering Judgment is a Key Element When Using FRP Advanced Composite MaterialsBy Zachery I. Smith, P.E., Scott F. Arnold, P.E. and Guijun Xian, Ph.D.

Service life is a concept always on the minds of engineers. Unfortunately, with the large majority of structures built post WWII, the engineering community is faced with a nation of structures all coming to the end of their service lives. Fiber Reinforced Polymers (FRP) Systems have been elegantly providing solutions to upgrade and extend the service life of structures for almost twenty years now. With limited financial resources and distressed structural elements, FRPs of-fer an excellent alternative to costly new structures and more obtrusive traditional repairs. While FRP systems can greatly extend the service life and performance of structures, the service life of the FRP system itself must also be considered.The service life of concrete buildings

and bridges can be 50, 100 or even 150 years. Several factors affect the perfor-mance of concrete structures and thereby limit their service life. These include, but are not limited to, the type of concrete, construction methods, coatings and envi-ronmental factors. However, there is no universal method of determining an exact service life. For example, there are no provisions in ACI 318-05 that require an explicit life-span for a building. Typically, structural durability is accounted for glob-ally with strength reduction factors and load increase factors. The assumption being that this will produce a sufficient margin of safety between demand and capacity to withstand strength degradation over time in order to reach a desired design life (Figure 1).Until model codes can incorporate time-

dependent deterioration models, the design of structural durability will largely depend on engineering judgment, as it has in the

past. This is further complicated with ret-rofit designs. Therefore, designers need to educate themselves and be conscientious of the structural elements, parameters and factors that affect an FRP retrofit design.Over the past twenty years, externally

bonded FRP systems have been used to repair and retrofit a variety of structures for a variety of reasons. FRP systems bring great qualities for retrofit designs including non-corrosive properties, lightweight, low-profile, and high strength-to-weight ratios. When properly designed, FRP can add shear strength, ductility, con-finement, flexural strength and tensile capacity to exiting walls, beams, slabs and columnsThere are numerous factors to consider

when designing an FRP system to en-

sure the retrofit meets or exceeds the intended service life. However, there are two questions any engineer should ask before commencing with an FRP design alternative: 1) is it feasible and, 2) how difficult is obtaining building permits for the specific application and municipal-ity? Feasibility depends on life safety and economics, an FRP solution should not be considered if failure of the FRP sys-tem would result in a catastrophic failure of the structure. Economics naturally weighs in on any design alternative, but FRPs are often prematurely eliminated as cost prohibitive before all the factors are considered. For example, the logistical advantages including ease and speed of installation often outweigh the increased price per unit price of FRP. And, with

owners becoming sophisticated with their capital investments, the “first costs” versus the “life-time costs” of an FRP system often well outweigh a cheaper traditional solution that will require regular maintenance over the life of the repair.Now, assuming the project is feasible and

the application is within the current industry practice to pull a permit, what are some of the many factors that impact the service life of an FRP system? Is the FRP supporting sus-tained loads or intermittent live loads, what are the environmental exposure conditions, what is the application for shear, flexure, etc., will coatings be applied? Below, each one of these topics has been elaborated to help engi-neers with the engineering judgment required when designing FRP retrofits and their relative service lives.

Sustained versus Intermittent Loads

FRP may be designed as a passive structural member or an active structural member. Passive structural strengthening includes, for example, seismic and blast mitigation retrofits. In these types of applications, the FRP will see no loads for the majority (perhaps all) of its lifetime. Only in the event of an earthquake or blast

Construction photo circa 1950.

Figure 1: Resistance of Structure vs. Age of Structure.

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SERVICE LIFE AFTER FRP RETROFIT

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C-BuildingBlocks-Smith-July10.indd 1 6/18/2010 12:09:52 PM


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23

sure the retrofit meets or exceeds the intended service life. However, there are two questions any engineer should ask before commencing with an FRP design alternative: 1) is it feasible and, 2) how difficult is obtaining building permits for the specific application and municipal-ity? Feasibility depends on life safety and economics, an FRP solution should not be considered if failure of the FRP sys-tem would result in a catastrophic failure of the structure. Economics naturally weighs in on any design alternative, but FRPs are often prematurely eliminated as cost prohibitive before all the factors are considered. For example, the logistical advantages including ease and speed of installation often outweigh the increased price per unit price of FRP. And, with

owners becoming sophisticated with their capital investments, the “first costs” versus the “life-time costs” of an FRP system often well outweigh a cheaper traditional solution that will require regular maintenance over the life of the repair.Now, assuming the project is feasible and

the application is within the current industry practice to pull a permit, what are some of the many factors that impact the service life of an FRP system? Is the FRP supporting sus-tained loads or intermittent live loads, what are the environmental exposure conditions, what is the application for shear, flexure, etc., will coatings be applied? Below, each one of these topics has been elaborated to help engi-neers with the engineering judgment required when designing FRP retrofits and their relative service lives.

Sustained versus Intermittent Loads

FRP may be designed as a passive structural member or an active structural member. Passive structural strengthening includes, for example, seismic and blast mitigation retrofits. In these types of applications, the FRP will see no loads for the majority (perhaps all) of its lifetime. Only in the event of an earthquake or blast

will it have load. Active structural strengthen-ing will see loading on a regular basis. This includes retrofitting bridges and buildings to increase their load carrying capacity, such as heavier vehicles on a bridge or the change of use in a building. Some of these applications will be for intermittent loads such as vehicular traffic or for long term sustained loads, such as high density files placed on top of a slab ret-rofitted with FRP. The different types of fibers behave differently under these types of loading conditions. Glass fibers are the most susceptible to creep rupture, and carbon fibers are the least affected. ACI 440.2R-08 addresses this issue by placing limits on the ultimate allowable stress that can be used in design. This is done to ensure a safe long term application under

Figure 3: Large Retail Space.

Figure 4: Pre-stressed Concrete Cylinder Pipe.

Table 1.

sustained stress. The viscolestic nature of the polymer matrix under sustained loads needs to be properly addressed. Short term experi-mental tests, that have traditionally been used in the aerospace industry, can be used to quickly evaluate the creep behavior of the system.One example is the Reiner-Weissenberg crite-

rion. This demonstrates that higher sustained stresses leading to associated strains closer to the composites ultimate strain significantly reduce the service life. This is illustrated in the log graph in Figure 2 and Table 1.

Environmental ConditionsEnvironmental conditions play an important

role in the service life of an FRP. Temperature,

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Figure 2: Log Service Life vs. Design Strain/Tensile Breakage Strain.

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freeze-thaw, UV radiation and humidity can all affect the performance of both the resin and fibers. To address this issue, design guidelines use a reduction factor based on both envi-ronment and fiber type. ACI 440.2R-08 has reduction factors listed in Table 9.1 of that standard; those factors range from 0.95 for carbon FRPs with interior exposure to 0.50 for glass FRPs used in an aggressive environ-ment. These reduction factors are used for both the ultimate tensile strength and ultimate strain. It is not applied to the modulus, which is typically unaffected by the environment. In the final design equations, it is the modulus that is used along with the calculated design strain. So the reduction factors ensure a factor of safety by providing upper bounds on the strain and stress. This ensures the long term performance of the FRP, and indirectly the service life.

CoatingsCoatings can provide significant protection

to the FRP, and increase the performance and service life. Due to the variety of coatings avail-able for the different FRP systems, the design should ensure that any coating that is used has been tested with the FRP System. This will ensure that the coating will stay well adhered and provide protection from the environment. It should also be noted that the FRP itself pro-vides environmental protection to the reinforced concrete member to which it is bonded. There have been several studies demonstrating that the use of FRP can reduce rates of corrosion and extend the service life of a structure.It is also important to consider coatings and

how they relate to loading type. FRP installa-tions that are designed to carry long-term sustained load must consider if a fire rating is required. Other installations designed as passive members might require a flame and smoke spread rating. It is important to check the local requirements and properly coat the FRP if required.

Figure 5: Parking Garage (rare inclined cracks). Figure 6: Parking Garage (rare inclined cracks).

Examples from the FieldNow, having taken a cursory review of the

multiple factors involved with a FRP retrofit service life, we can walk through a few examples. One of the most common applications for FRP retrofit is the strengthening for increased super imposed live loads. The project shown in Figure 3 (page 23) was a large retail store where the occupant wanted to increase the flexural strength of its slabs to accommodate more merchandise storage. In brief, the flexural strength increase was 25%; therefore, it was structurally feasible. The FRP manufacturer had a 4-hour UL rated fire protection system that could be used to pull a permit in San Diego, and the FRP design strain was only slightly reduced since the material was a primary carbon FRP, non-sustained load, interior application. The final coating was spray applied fire proofing; no other factors were considered. Qualitatively, this retrofit is expected to last as long as, or longer than, the traditional materials used in the original construction.Another project included the strengthening

of pre-stressed concrete cylinder pipe for internal and external loads (Figure 4, page 23). The pipe section had been inspected and found to have lost 30% of its pre-stressing wire from corrosion. The FRP retrofit was therefore feasi-ble, and appropriate municipality approvals were available for the FRP system. The FRP was not the primary reinforcement but would be in sustained stress; a final coating was applied to aid in the long-term protection of the FRP system. Extra conservatism was added into the design strain of the FRP composite, given the relative importance of the water supply line.The last project illustrates a construction an-

nomally with an uncertain cause. Several, if not the majority, of the the prestressed double-tees making up the parking garage shown in Figures 5 and 6 had rare inclined cracks that started approximately five feet from the supports and inclined in the opposite direction when com-pared to textbook shear cracks. A consensus

could not be reached of the cause, so it was decided that proof testing would be completed to establish the existing capacity. FRP compos-ite was used to make the difference between demand and capacity. Since there was some uncertainty of the existing double-tees capacity, the FRP was considered primary reinforcement and would require a fire protection system. The design service life of the project will be conser-vative considering the interior application, final coating, and low stress that the FRP composite was designed for.

ConclusionsService life of structures has a long way to

go before it is treated as scientifically as the rest of the structure by the engineering profes-sion. The very use of FRP systems to retrofit structures and extend their service lives inher-ently complicates the process. Thus, it will continue to depend on engineering judgment to tabulate and assess all of the parameters and factors that contribute to a structurally du-rable FRP retrofit. The sustained stress should not exceed set limits to avoid creep rupture, coatings should be considered in order to protect against UV degradation, exposure to fire must be considered, and so on. With so many parameters influencing FRP service life, engineers should be careful to choose a system that has been validated by both structural and environmental durability testing. However, when properly designed, an FRP retrofit can add significant service life to a structure and be one of the best design alternatives to our aging infrastructure.▪

Zachery I. Smith, P.E., is a Regional Manager for Fyfe Co. LLC and can be reached at ([email protected]), Scott F. Arnold, P.E., is a Vice President for Fyfe Co. LLC and can be reached at ([email protected]), and Guijun Xian is a Material Scientist for Fyfe Co. LLC.

C-BuildingBlocks-Smith-July10.indd 3 6/22/2010 10:39:41 AM

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Blank.indd 1 7/8/2009 10:02:43 AM

InSIghtSnew trends, new techniques and current industry issues


Curved Steel: Means and MethodsBy Erin J. Gachne Conaway, P.E., LEED AP and Jacinda L. Collins, P.E.

Every piece of structural steel experiences some form of bending during its life. Straightening, cambering, and curving of structural shapes are all representative of bending. W-shapes are straightened at the mill to a curvature that is within the tolerances as specified in ASTM Specification A6/A6M. Camber, or curvature, is often fabricated into structural steel beams to compensate for deflection. But it is the third reason for bending structural steel that is often misunderstood or just unknown.What is “curved steel”? The use of curved

steel in building projects is a growing trend that can benefit any type of project. Curved steel is used to increase visibility and provide more architectural freedom in aesthetics and functionality. But as curved steel has increased in popularity, so have the questions about it. “Who curves steel?” and “How is it curved?” are two common questions that many design professionals have.Bending/Rolling is carried out by a “Bender”,

who is typically a specialty subcontractor of the fabricator. Curved steel is readily available for most projects, as there are many qualified bender-rollers located across the US. Many different bending techniques exist, and each process has its advantages and specific char-acteristics. The six most widely used bending processes in the industry are included in Table 1, listed in order based on prevalence of use in the industry.It is important for design professionals to

recognize that different levels of quality and consistency are associated with each bending process, tooling and material size/thickness. Benders, if included early in a project, can help provide assistance on what is and isn’t feasible concerning a design, and can help save time and money as a project moves forward. In all cases, a qualified bending company is going to know what process is necessary to meet the design and quality requirements. Curved steel can provide many readily available options to benefit all project types – big or small – if properly understood and specified.Do you have more questions about bending?

Detailed questions regarding the visual appear-ance of a specific member with a specific bend and cost implications for a given configuration are best handled by contacting an AISC mem-ber bender-roller. For a list of AISC member bender-rollers and other bending information, visit www.aisc.org/benders.▪

Bending Process Process Description Process Distinction(s) Mainly used for: Steel Shapes

Rotary Draw / Compression Bending

Structural member is bent by rotating it around a die.The member is clamped into a form and then is drawn through the machine until the bend is formed.

Produces very tight radii (typically limited to 180 degree of bend)

Complicated bends in the machine and parts industry

Medium to smaller sections of round, rectangular, and square HSS, or pipe

Rolling or Cold Bending (a.k.a. “Pyramid Rolling”)

Structural member is placed in a machine and curved between three rolls. Also called “Pyramid Bending” because of the three rolls’ pyramid arrangement. Bending occurs when the distance between the rolls is manipulated before each successive pass.

The typical method of curving steel for construction

Usually the most economical for rolling members with tighter radii

Typically bent to larger radii than the rotary draw/compression bending

Profile rolls for bending in the 8D and above range (capable of 360 degree of bend)

Angles, flat bars, channels, W-shapes, WT-shapes, HSS (all shapes), pipe, and rails

Point Bending / Gag Pressing

Structural member is bent by applying a minimal number of point loads with a hydraulic ram or press at selected points.

This is the typical method used for cambering beams

Good for larger sections bent to larger radii

Cambering and curving to very large radii

W-shapes, channels, HSS and pipe

Synchronized Incremental Cold Bending

Structural member is bent by applying pressure in a highly synchronized fashion at several locations on the section. This method employs external restraint and internal support at the bend point.

A patented process performed by only one bender in the US

Typically this method allows for tighter radii with better levels of distortion control when compared to Point Bending / Gag Pressing

Situations where tight radii with minimal distortion is desired

HSS,W-shapes, channels and pipe

Hot (Heat) Bending

Structural member is heated directly and then bent. The heat source could be a direct flame or furnace. The application of bend pressure is performed in numerous ways; by bending around pins or forms or by short increment pushes or pulls with bending at the fulcrum point.

Expensive and rarely used as an initial bending method unless other methods cannot be used

Allows for members to be bent very tight with low levels of distortion

Repair applications

All shapes

Induction Bending

Structural member is heated over a short section with an electric coil drawn through a process similar to rotary-draw and cooled with water directly after bending.

Not commonly used and can be expensive

Produces curved steel with little distortion

Applies principles of both Rotary draw and Heat Bending, but allows the bending of larger members to very tight radii

Situations that require larger diameter shapes with heavy wall thicknesses to have a smaller, tighter radius

Large shapes with heavy wall thicknesses

Table 1: Bending process.

Erin J. Gachne Conaway, P.E., LEED AP is the Intermountain West Regional Engineer with the American Institute of Steel Construction. Erin may be contacted at [email protected].

Jacinda L. Collins, P.E. is an AISC Steel Solutions Center advisor. Jacinda may be contacted at [email protected].

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

D-InSights-Conaway-July10.indd 1 6/18/2010 11:22:54 AM

2010 PRE-CAST CONCRETE GUIDE

a listing of pre-cast concrete manufacturers/distributors and their product lines


Company Product DescriptionADAPT CorporationPhone: 650-306-2400Email: [email protected]: www.adaptsoft.com

ADAPT-PT 201ADAPT-PT is easy-to-use and versatile software for the design of prestressed beams, beam frames, one-way and two-way fl oor systems. It handles pre-cast, pre-stressed as well as cast-in place, post-tensioned members. Carries out full code checks and calculation of required reinforcement.

AltusGroupPhone: 866-462-5887Email: [email protected]: www.altusprecast.com

CarbonCast Insulated Architectural Cladding and High Performance Insulated Wall Panels

Insulated Architectural Cladding offers weight reductions of about 40% compared to solid, 6-inch thick precast concrete wall panels; engineered to deliver insulation values of R-8 or more in addition to a lower carbon footprint. High Performance Insulated Wall Panels use C-GRID carbon fi ber grid as a shear connector between inner and outer wythes of concrete.

CTS Cement Manufacturing CorporationPhone: 800-929-3030Email: [email protected]: www.ctscement.com

Rapid Set® Cement Products

Rapid Set is a brand of fast-setting cement products used in concrete applications requiring the highest durability and fastest strength gain, achieving structural or drive-on strength in one hour. Applications include structural, architectural, and ornamental precast, concrete repairs, and smoothing.

HiltiPhone: 800-879-8000Email: [email protected]: www.us.hilti.com

Expansion Anchors

With a unique wedge design, reduced requirements for edge distance and anchor spacing, Hilti Kwik-Bolt (KB) or KB-TZ Expansion Anchors can be used for many applications including pre-cast and tilt-wall construction. The Hilti Kwik-Bolt TZ Anchor is qualifi ed with ACI 355.2 and ACI 193 for use in seismic design environments.

iLevel by WeyerhaeuserPhone: 888-453-8358Email: [email protected]: www.iLevel.com

Steel and Concrete Forming Products

iLevel offers a single source for quality concrete forming materials, including TimberStrand® LSL form boards, along with rebar, remesh, anchor bolts, wire and steel construction stakes. Technical representatives are available to assist with component selection, transporting to multiple job sites upon request and just-in-time delivery.

RISA TechnologiesPhone: 949-951-5815Email: [email protected]: www.risa.com

RISA-3D

RISA-3D is the premiere choice for the design of concrete beams and columns. With fi nite element analysis, the design of both conventional and unconventional framing layouts is possible. T-Beam design, biaxial column design, custom rebar layouts, and 11 different design codes all combine to make RISA-3D your most fl exible solution.

STRUCTUREPOINTPhone: 847-966-4357Email: [email protected]: www.StructurePoint.org

spSlab, spColumn, spMats, spWall, spBeam, spFrame

Upgraded to ACI 318-08, PCA’s concrete design suite is now: spSlab, spColumn, spMats, spWall, spBeam & spFrame.

Tendon Systems, LLCPhone: 678-835-1100Email: [email protected]: www.TendonLLC.com

Post-Tension, Barrier Cable, Anchors

Tendon Systems provides and installs post-tensioning systems, Shearail® shear stud reinforcement and barrier cable to vehicular restraint.

Restoration Team Experience Since 1978

Don’t Tear it Down or Cover it with Insulation and StuccoStrengthen and stabilize masonry façades while adding veneer stiffness for added decades of protection and comfort.

CTP has engineered anchor performance solutions for claddings of brick and stone. A selection of corrosion resistant products are available to re-anchor brick to wood, concrete, steel, block, brick, metal stud, or tile back-ups.

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Contact our Technical Services Team withyour repair application needs for a cost effective and performance targeted veneer stabilizing solution.

CTP Grip-MaxMechanical Anchors for Stabilizing Stone Panel Veneers

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Company Product DescriptionADAPT Corporation

Not listed? Please contact STRUCTURE® magazine at [email protected] with your company information. Listings are provided as a courtesy. STRUCTURE magazine is not responsible for errors.

Steel Guide May 10.indd 1 6/18/2010 11:23:33 AM

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National Council of Structural Engineers AssociationsEightEEnth AnnuAl ConfErEnCE

September 30 – October 2, 2010Hyatt Regency on the HudsonJersey City, New Jersey

Next NCSEA Webinar July 15Design of Coastal Buildings – Presented by William CoulbourneThis seminar is intended to help engineers, architects and

building officials who design or oversee construction in or near coastal areas to not only better understand the magnitude of flood and wind forces but also to help them apply sound judgment about the possible siting of buildings, and about the possible consequences to the built environment when the design hurricane event occurs. The webinar emphasizes the importance of understanding the flood and wind effects and how to minimize their impacts, as follows:

1) Flood forces caused by coastal events such as hurricanes and tsunamis

2) Wind forces caused by hurricanes3) Discussion of possible mitigation measures

Mr. Coulbourne has a BS in Civil Engineering from Virginia Tech and a Masters in Structural Engineering from the University of Virginia. He is a national expert in wind and flood mitigation and has been involved in FEMA Mitigation Assessment Teams

Lectures you can expect to hear include the following:

The Future of New York City Building, presented by Robert LiMandri, Commissioner, NYC DOB

Renovation of the Guggenheim Museum, presented by Nancy Hudson, Robert Silman Associates

Lake Champlain Bridge Projects, presented by Ted Zoli, HNTB

New York Underground: Grand Central Station LIRR Terminal, presented by Colin Barratt, MTA

Protecting People and Neighborhood Property During Excavations, presented by Tim Lynch, NYC DOB

High Strength Concrete Design, including One World Trade Center, presented by Caz Bognacki, Port Authority of NY and NJ

AISC Seismic Design Provisions: Past, Present and Future, presented by AISC’s TR Higgins Lecturer and NCSEA Incoming President, Jim Malley, Degenkolb Engineers

Changes to the 2010 MSJC Code, presented by Ed Huston, Smith & Huston Consulting Engineers

Social events include an exhibitor reception on Thursday night, Friday night dinner at Carmine’s legendary Italian restaurant in the Theatre District, and the Awards Reception and Banquet on Saturday night (formal attire requested).

Register at www.ncsea.com.

Exhibitors:Courtesy of Sarah McGee Photography.

Sponsors:ACEC – New YorkBentley Systems, IncorporatedCives Steel CompanyConcrete Industry Board, Inc.Girder-Slab Technologies, LLCITW Red Head

Nicholson & GallowayPowers FastenersSimpson Strong-TieSkyline SteelWest NY Restoration of CTWheeling Corrugating

To become a sponsor of this event, please contact Erica Fischer [email protected] or Melissa [email protected].

ITW Red HeadLINDAPTER NORTH

AMERICA, INC.Singer Nelson CharlmersPowers FastenersRedBuilt, LLCRISA TECHNOLOGIES,

LLCSidePlate Systems, Inc.Simpson Strong-TieTurnaSure LLCValmont IndustriesVector Corrosion Technologies

American Institute of Steel Construction

Azz Galvanizing ServicesCMC Steel ProductsConstruction Tie ProductsConXtech, Inc.CSC IncDESIGN DATAFabreeka International Inc.FYFE COMPANY, LLCGrace Construction ProductsHardy Frames, Inc.Hilti

Visit the NCSEA website (www.ncsea.com) to view the lim-ited number of exhibitor booth spaces still available, or contact Emile Troup [email protected].

2010 NCSEA EXCELLENCE IN STRUCTURAL ENGINEERING AWARDS

Call for EntriesNCSEA’s Annual Excellence in Structural Engineering Awards program highlights

some of the best examples of structural engineering ingenuity throughout the world. Structural engineers and structural engineering firms are encouraged to enter this year’s program. Projects will be judged on innovative design, engineering achievement and creativity.Entries are due on Friday, July 9, 2010. Awards will be presented on October 2,

2010, at the NCSEA Annual Meeting at the Hyatt Regency on the Hudson, Jersey City, New Jersey. Winning projects will be featured in future issues of STRUCTURE Magazine. For award program rules, project eligibility and entry forms, see the Call for Entries on the NCSEA website at www.ncsea.com.The University of Illinois Memorial Stadium,

photo courtesy of Brad Feinknopf.

NCSEA has published a new design guide...Purchase it from ICC’s website today. Attend the course and receive the book onsite!Guide to the Design of Out-of-Plane Wall Anchorage: Based on the 2006/2009 IBC and ASCE/SEI 7-05

To date, ten cities representing eight member organizations have participated in the new NCSEA short course titled Guide to the Design of Out-of-Plane Wall Anchorage: Based on the 2006/2009 IBC and ASCE/SEI 7-05. The course and book are a direct response to over 1,500 comments received directly from our members regarding some of the most confusing issues in the code. The new course uses Dr. Mays’ concept oriented approach to instruction to carefully illustrate appropriate applications of some of the code’s most confusing requirements. If your member organization would like to schedule this 8 hour course, please contact Dr. Mays directly at [email protected].

Course Description: The 2006/2009 International Building Code (IBC) and ASCE/SEI 7-05 contain detailed design require-ments for wall anchorage systems to resist out-of-plane wind and seismic load effects. However, the provisions are scattered throughout the code and/or referenced standards, are material specific, and are often challenging for practicing structural engineers to apply for many practical building configurations. Using concept oriented instruction, Dr. Mays breaks down the analysis and detailing requirements separately for seismic and wind anchorage. Structural walls, nonstructural walls, parapets, and cladding are each considered separately as related to gov-erning provisions. Solutions for high wind areas, Seismic Design Category (SDC) B, and SDC D are provided for each problem presented in the course. Example anchorage problems for con-necting concrete, masonry, timber, and precast walls/panels to diaphragms composed of various materials are presented. Special

Plan your fall to include the NCSEA Annual Conference at the Hyatt on the Hudson, Jersey City, NJ, September 30 – October 2, 2010. While you are there, enjoy views of the Hudson River and lower Manhattan; and plan time for a visit to the One World Trade Center (formerly, Freedom Tower) construction site, via ferry or the PATH train, located only a few steps from the hotel.

NCSEA News July10.indd 1 6/22/2010 10:34:58 AM

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National Council of Structural Engineers AssociationsEightEEnth AnnuAl ConfErEnCE

September 30 – October 2, 2010Hyatt Regency on the HudsonJersey City, New Jersey

Next NCSEA Webinar July 15Design of Coastal Buildings – Presented by William CoulbourneThis seminar is intended to help engineers, architects and

building officials who design or oversee construction in or near coastal areas to not only better understand the magnitude of flood and wind forces but also to help them apply sound judgment about the possible siting of buildings, and about the possible consequences to the built environment when the design hurricane event occurs. The webinar emphasizes the importance of understanding the flood and wind effects and how to minimize their impacts, as follows:

1) Flood forces caused by coastal events such as hurricanes and tsunamis

2) Wind forces caused by hurricanes3) Discussion of possible mitigation measures

Mr. Coulbourne has a BS in Civil Engineering from Virginia Tech and a Masters in Structural Engineering from the University of Virginia. He is a national expert in wind and flood mitigation and has been involved in FEMA Mitigation Assessment Teams

Courtesy of Sarah McGee Photography.

Nicholson & GallowayPowers FastenersSimpson Strong-TieSkyline Steel

To become a sponsor of this event, please contact Erica Fischer [email protected] or Melissa [email protected].

ITW Red HeadLINDAPTER NORTH

AMERICA, INC.Singer Nelson CharlmersPowers FastenersRedBuilt, LLCRISA TECHNOLOGIES,

LLCSidePlate Systems, Inc.Simpson Strong-TieTurnaSure LLCValmont IndustriesVector Corrosion Technologies

Visit the NCSEA website (www.ncsea.com) to view the lim-ited number of exhibitor booth spaces still available, or contact Emile Troup [email protected].

2010 NCSEA EXCELLENCE IN STRUCTURAL ENGINEERING AWARDS

Call for EntriesNCSEA’s Annual Excellence in Structural Engineering Awards program highlights

some of the best examples of structural engineering ingenuity throughout the world. Structural engineers and structural engineering firms are encouraged to enter this year’s program. Projects will be judged on innovative design, engineering achievement and creativity.Entries are due on Friday, July 9, 2010. Awards will be presented on October 2,

2010, at the NCSEA Annual Meeting at the Hyatt Regency on the Hudson, Jersey City, New Jersey. Winning projects will be featured in future issues of STRUCTURE Magazine. For award program rules, project eligibility and entry forms, see the Call for Entries on the NCSEA website at www.ncsea.com.

for over 15 years. He has been involved in every major hurricane and flood disaster since 1995. Mr. Coulbourne has investigated failures and mitigation design techniques for thousands of buildings including residential struc-tures, schools used as shelters, hospitals, and other critical facilities. He holds Certifications in Structural Engineering and Building Inspection Engineering. Mr. Coul-bourne has written articles for journals and given presentations for homebuilders, engineers, architects and homeowners on high wind and flood design and coastal construction issues. He was one of the primary authors for FEMA’s Coastal Construction Manual and for FEMA 320, Taking Shelter From the Storm – a tornado safe room design guidance manual for homeowners and homebuilders.

August 5, 2010: Wind Load Design for Storm Shelters and Critical Facilities, Marc LevitanAugust 19, 2010: Wind Load Design for Industrial Structures and Appurtenances, Marc LevitanSeptember 14, 2010: Wood and Cold-Formed Steel Trusses, Ed HustonOctober 19, 2010: ATC-58, Ron HamburgerOctober 28, 2010: Design Considerations for Ponding Loads on Roofs, Tom Wallace

NCSEA has published a new design guide...Purchase it from ICC’s website today. Attend the course and receive the book onsite!Guide to the Design of Out-of-Plane Wall Anchorage: Based on the 2006/2009 IBC and ASCE/SEI 7-05

To date, ten cities representing eight member organizations have participated in the new NCSEA short course titled Guide to the Design of Out-of-Plane Wall Anchorage: Based on the 2006/2009 IBC and ASCE/SEI 7-05. The course and book are a direct response to over 1,500 comments received directly from our members regarding some of the most confusing issues in the code. The new course uses Dr. Mays’ concept oriented approach to instruction to carefully illustrate appropriate applications of some of the code’s most confusing requirements. If your member organization would like to schedule this 8 hour course, please contact Dr. Mays directly at [email protected].

Course Description: The 2006/2009 International Building Code (IBC) and ASCE/SEI 7-05 contain detailed design require-ments for wall anchorage systems to resist out-of-plane wind and seismic load effects. However, the provisions are scattered throughout the code and/or referenced standards, are material specific, and are often challenging for practicing structural engineers to apply for many practical building configurations. Using concept oriented instruction, Dr. Mays breaks down the analysis and detailing requirements separately for seismic and wind anchorage. Structural walls, nonstructural walls, parapets, and cladding are each considered separately as related to gov-erning provisions. Solutions for high wind areas, Seismic Design Category (SDC) B, and SDC D are provided for each problem presented in the course. Example anchorage problems for con-necting concrete, masonry, timber, and precast walls/panels to diaphragms composed of various materials are presented. Special

provisions for subdiaphragms, continuous ties/struts, pilasters, straps, eccentric connections, and wood ledgers are included. A detailing example for economical tilt up wall anchorage using just metal decking is presented. Comprehensive examples are provided for subdiaphragms composed of wood structural panel sheathing on wood framing and metal decking on steel joists.

Course Instructor:Timothy Wayne Mays, Ph.D., P.E. is President of SE/ES and

an Associate Professor of Civil Engineering at The Citadel in Charleston, SC. He currently serves as Chairman of the Structural Technical Group for ASCE SC Section and NCSEA Publications Committee Chairman. He has received two national teaching awards (ASCE and NSPE) and both national (NSF) and regional (ASEE) awards for outstanding research.

COURSES SCHEDULED FOR JULY AND AUGUST 2010:

July 14, 2010 – New York, NY July 19, 2010 – Nashville, TN July 21, 2010 – Tulsa, OK July 23, 2010 – Oklahoma City, OK July 28, 2010 – Tucson, AZ July 30, 2010 – Phoenix, AZ August 2, 2010 – Albuquerque, NM August 5, 2010 – Little Rock, AR August 9, 2010 – Atlanta, GA

National Council of Structural Engineers Associations Course Approval No. 100405D

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Return postmarked no later than July 31, 2010 to: SEI Board Election, 1801 Alexander Bell Dr., Reston VA 20191.

Structural Engineering Institute 2010 Award Recipients

2010 Award Recipients, left to right: Roberto Leon, Wesley J. Oliphant, Mike Ritter, Joseph Yura, Donald White, Todd Helwig, Gustavo Parra-Montesinos, Chong Zhou, Jon Peterka, and Reagan Herman.

Ed DePaola, P.E., M. ASCE is Presi-dent/CEO of Severud Assoc. Consulting Engineers PC, New York. Over the past 30 years, he has designed many projects including high-rise buildings, long-span facilities and special structures requiring innovative structural solutions. He has a B.S. in Civil Engineering and M.S. in Structural Engineering from the Univ.

of Notre Dame, and a J.D. from Seton Hall School of Law. He is one of the Founding Members and Past President of the Struc-tural Engineers Association of New York (SEAoNY), Chairman of the ASCE Tensile Membrane Structure Standards Committee, and a professor at NYU School of Continuing Education. He is Co-Chair of the Building Department’s New York City Model Code Program for the adoption of the structural portions of the IBC Building Code. He was Principal-in-charge of the American Airlines Terminal Redevelopment Project at JFK International Airport, and the roof and enclosure structures at the Denver International Airport. Currently, he is Principal-in-Charge of One Bryant Park, the 1,200-foot tall office building nearing comple-tion in midtown Manhattan. It is the second tallest building in NYC and will be the first high-rise office structure in the world to receive a LEED Platinum rating.

The Structural Engineering Institute (SEI) proudly recog-nized the following recipients at the Joint NASCC: The Steel Conference and Structures Congress in Orlando, Florida on May 15, 2010:

Jack E. Cermak AwardThis award is given jointly by the Engineering Mechanics Institute

and the Structural Engineering Institute. The 2010 award goes to Jon Peterka, Ph.D., P.E., M. ASCE, in recognition of his lifelong contributions to the field of wind engineering through education, research, and practice. Dr. Peterka is presently the President of CPP, Inc., and is one of the co-founders of the firm.

J. James R. Croes Medal (2009)The 2009 medal is awarded to Michael H. Scott, Ph.D.,

M. ASCE; Gregory L. Fenves, Ph.D., M. ASCE; Frank McKenna, Ph.D.; and Filip Filippou, Ph.D., M. ASCE for the paper “Software Patterns for Nonlinear Beam-Column Models” Journal of Structural Engineering, April 2008. Dr. Scott is currently an Assistant Professor of Structural Engineering at Oregon State University. Dr. Fenves is the Dean of the Cockrell School of Engineering at the University of Texas at Austin, and the Jack and Beverly Randall Dean’s Chair for Excellence in Engineering. Dr. McKenna is an Assistant Researcher at the University of California, Berkeley. Dr. Filippou is a Professor at the University of California, Berkeley.

Shortridge Hardesty AwardThe 2010 award goes to Dinar Camotim, Ph.D., M. ASCE,

in recognition of his sustained and substantial contributions to the field of structural stability during his career of active teaching and research, and how his scholarship and service has positively impacted many global design standards. Prof. Camotim is cur-rently a Professor in the Department of Civil Engineering and Architecture at the Technical University of Lisbon, Portugal.

Ernest E. Howard AwardThe 2010 Award is awarded to Charles Roeder, Ph.D., P.E.,

M. ASCE, for his outstanding contributions to research and prac-tice in the seismic resistant design of structural steel buildings, bridge bearing design and bridge thermal movement design. Prof. Roeder is a Professor of Structural Engineering and Mechanics at the University of Washington.

Walter L. Huber Civil Engineering Research PrizesThe 2010 recipients are Gustava Parra-Montesinos, Ph.D.,

A.M. ASCE, and Benjamin W. Schafer, Ph.D., P.E., M. ASCE. Dr. Parra-Montesinos is being honored for research on frame and wall structural systems that opened new doors of perception and enabled use of strain-hardened fiber-reinforced concrete, a highly effective composite, to improve the safety and behavior of connections and members subjected to intensive shear force. Prof. Schafer is being honored for his research on the behavior of thin walled structural members and the development of more comprehensive design methodologies. His recommendations on the Direct Strength Method have influenced several standards. Dr. Parra-Montesinos is currently an Associate Professor at the University of Michigan. Prof. Schafer is the Swirnow Family Faculty Scholar, an Associate Professor and Chair of the Department of Civil Engineering at Johns Hopkins University.

Moisseiff AwardThe 2010 award is presented to Joseph Yura, Ph.D., P.E.,

M. ASCE; Todd Helwig, Ph.D., P.E., M. ASCE; Chong Zhou, Ph.D., P.E., A.M. ASCE; and Reagan Herman, Ph.D., A.M. ASCE for the paper “Global Lateral Buckling of I-Shaped Girder Systems,” published in the September 2008 issue of the Journal of Structural Engineering. Prof. Yura is Professor Emeritus in Civil Engineering at the University of Texas at Austin; Prof. Helwig is an Assistant Professor in Civil Engineering at the University of Texas at Austin; Dr. Zhou is a Senior Specialist at Technip USA; and Dr. Herman is a Resident Assistant Professor in Civil Engineering at Johns Hopkins University.

Raymond C. Reese Research PrizeThe 2010 prize is presented to Donald White, Ph.D., M. ASCE.

He is receiving the prize for the paper “Unified Flexural Resistance Equations for Stability Design of Steel I-Section Members: Overview,” published in the September 2008 issue of the Journal of Structural Engineering. Prof. White is a Professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology.

Structural Engineering Institute AwardsDennis L. Tewksbury AwardThe Tewksbury award recognizes distinguished service to SEI.

The 2010 Award is presented to Tom Williamson, P.E., F. ASCE. Mr. Williamson has a long and distinguished history of service to SEI, ASCE, and the profession, including leadership of the stan-dard committee on LRFD for wood, the technical committee on wood, the codes and standards executive committee of SEI, and the codes and standards committee of the board of ASCE. He also served on several Structures Congress organizing committees and chaired one. His work has made a real difference for the better in our practice of structural engineering. Mr. Williamson is currently the Vice President of Quality Assurance and Technical Services at APA-The Engineered Wood Association.

Walter P Moore, Jr. AwardThis award is presented for significant contributions to the

development of codes and standards. The 2010 recipient is John Kulicki, Ph.D., P.E., M. ASCE. Dr. Kulicki is commended for his significant and career long contributions to the development of structural codes and standards which have advanced the science of bridge engineering. He has devoted considerable time to research, teaching, authoring technical publications and presentations, and has been a major force in the development of structural codes and standards which have advanced the science of bridge engineering. His work in organizing and leading the devel-opment of the AASHTO LRFD Bridge Design Specifications is a significant accomplishment, and is a testament to his skills and abilities as an engineering leader. Dr. Kuliki is the Chairman and CEO of Modjeski and Masters.

Gene Wilhoite AwardThe 2010 Award recipient is Wesley J. Oliphant, P.E., F. ASCE

for his significant contributions to the advancement of the Art and Science of transmission line engineering. Mr. Oliphant has served as a member or chair on several ASCE committees and holds multiple patents related to transmission structures. He has been in the profession for over thirty years and has authored/co-authored numerous technical papers, guides, and standards. He is President and CEO of ReliaPOLE Solutions Inc.

SEI News July10.indd 1 6/18/2010 11:26:47 AM

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Return postmarked no later than July 31, 2010 to: SEI Board Election, 1801 Alexander Bell Dr., Reston VA 20191.

Full Name: _____________________________________Member’s ASCE/SEI ID No:________________

Date:______________ Signature: _______________________________________________________________

(Please print)

Codes and Standards Activities Division

Ed DePaola

Write-in vote:_______________________________

SEI 2010 Board of

Governors Election Offi cial Ballot

Business and Professional Activities Division

Pat McCormick

Write-in vote:_______________________________

Structural Engineering Institute 2010 Award Recipients

2010 Award Recipients, left to right: Roberto Leon, Wesley J. Oliphant, Mike Ritter, Joseph Yura, Donald White, Todd Helwig, Gustavo Parra-Montesinos, Chong Zhou, Jon Peterka, and Reagan Herman.

SEI Election AnnouncementDeadline: July 31, 2010There are ten Governor positions on the SEI Board of Governors:

two representatives from each of the four Divisions (Business & Professional, Codes & Standards, Local Activities, and Technical Activities), one appointed by the ASCE Board of Direction, and the most immediate and available Past President of the SEI Board. The representatives from the Divisions each serve a four-year term. This year SEI is conducting an election for a Business & Professional and a Codes & Standards representative on the Board of Governors.The BPAD and CSAD Executive Committees have nominated

Pat McCormick and Ed DePaola as their respective candidates. In accordance with the SEI Bylaws, each ballot provides a space for a write-in vote. If you are a member of ASCE/SEI please complete and mail the ballots to the address provided. Either vote for the named candidate OR provide a write-in candidate. Because we must confi rm SEI/ASCE membership, ONLY SIGNED BALLOTS WILL BE ACCEPTED. DEADLINE JULY 31, 2010.

Ed DePaola, P.E., M. ASCE is Presi-dent/CEO of Severud Assoc. Consulting Engineers PC, New York. Over the past 30 years, he has designed many projects including high-rise buildings, long-span facilities and special structures requiring innovative structural solutions. He has a B.S. in Civil Engineering and M.S. in Structural Engineering from the Univ.

of Notre Dame, and a J.D. from Seton Hall School of Law. He is one of the Founding Members and Past President of the Struc-tural Engineers Association of New York (SEAoNY), Chairman of the ASCE Tensile Membrane Structure Standards Committee, and a professor at NYU School of Continuing Education. He is Co-Chair of the Building Department’s New York City Model Code Program for the adoption of the structural portions of the IBC Building Code. He was Principal-in-charge of the American Airlines Terminal Redevelopment Project at JFK International Airport, and the roof and enclosure structures at the Denver International Airport. Currently, he is Principal-in-Charge of One Bryant Park, the 1,200-foot tall offi ce building nearing comple-tion in midtown Manhattan. It is the second tallest building in NYC and will be the fi rst high-rise offi ce structure in the world to receive a LEED Platinum rating.

Patrick McCormick, P.E., M. ASCE is President/CEO of Brander Construction Technology, Inc., a structural engineer-ing fi rm in Green Bay, Wisconsin. Mr. McCormick has been practicing struc-tural engineering for over 25 years. Pat has analyzed structures and designed rehabilitation projects for heavy industrial facilities across the United States. After joining Brander in 1986 as a staff engineer

and holding numerous positions over the years, he was promoted to his current position in 2000. Pat currently divides his time between practicing engineering, and promotion and business development for the company. Pat holds a B.S. in Civil Engineering with a structural emphasis from the University of Wisconsin. He is a registered S.E. in Illinois. Pat volunteers his time with the American Red Cross, the YMCA, and his local church. He is married, has two children, and enjoys hunting and fi shing in his free time.

Moisseiff AwardThe 2010 award is presented to Joseph Yura, Ph.D., P.E.,

M. ASCE; Todd Helwig, Ph.D., P.E., M. ASCE; Chong Zhou, Ph.D., P.E., A.M. ASCE; and Reagan Herman, Ph.D., A.M. ASCE for the paper “Global Lateral Buckling of I-Shaped Girder Systems,” published in the September 2008 issue of the Journal of Structural Engineering. Prof. Yura is Professor Emeritus in Civil Engineering at the University of Texas at Austin; Prof. Helwig is an Assistant Professor in Civil Engineering at the University of Texas at Austin; Dr. Zhou is a Senior Specialist at Technip USA; and Dr. Herman is a Resident Assistant Professor in Civil Engineering at Johns Hopkins University.

Raymond C. Reese Research PrizeThe 2010 prize is presented to Donald White, Ph.D., M. ASCE.

He is receiving the prize for the paper “Unifi ed Flexural Resistance Equations for Stability Design of Steel I-Section Members: Overview,” published in the September 2008 issue of the Journal of Structural Engineering. Prof. White is a Professor in the School of Civil and Environmental Engineering at the Georgia Institute of Technology.

Structural Engineering Institute AwardsDennis L. Tewksbury AwardThe Tewksbury award recognizes distinguished service to SEI.

The 2010 Award is presented to Tom Williamson, P.E., F. ASCE. Mr. Williamson has a long and distinguished history of service to SEI, ASCE, and the profession, including leadership of the stan-dard committee on LRFD for wood, the technical committee on wood, the codes and standards executive committee of SEI, and the codes and standards committee of the board of ASCE. He also served on several Structures Congress organizing committees and chaired one. His work has made a real difference for the better in our practice of structural engineering. Mr. Williamson is currently the Vice President of Quality Assurance and Technical Services at APA-The Engineered Wood Association.

Walter P Moore, Jr. AwardThis award is presented for signifi cant contributions to the

development of codes and standards. The 2010 recipient is John Kulicki, Ph.D., P.E., M. ASCE. Dr. Kulicki is commended for his signifi cant and career long contributions to the development of structural codes and standards which have advanced the science of bridge engineering. He has devoted considerable time to research, teaching, authoring technical publications and presentations, and has been a major force in the development of structural codes and standards which have advanced the science of bridge engineering. His work in organizing and leading the devel-opment of the AASHTO LRFD Bridge Design Specifi cations is a signifi cant accomplishment, and is a testament to his skills and abilities as an engineering leader. Dr. Kuliki is the Chairman and CEO of Modjeski and Masters.

Gene Wilhoite AwardThe 2010 Award recipient is Wesley J. Oliphant, P.E., F. ASCE

for his signifi cant contributions to the advancement of the Art and Science of transmission line engineering. Mr. Oliphant has served as a member or chair on several ASCE committees and holds multiple patents related to transmission structures. He has been in the profession for over thirty years and has authored/co-authored numerous technical papers, guides, and standards. He is President and CEO of ReliaPOLE Solutions Inc.

SEI News July10.indd 2 6/18/2010 11:27:00 AM

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STRUCTURE magazine32

CASE Spring Risk Management Convocation in Orlando Scores Big with Attendees!

CASE Summer Meeting Planned for Boston in September

New CASE Tool Available on Website Resources

Government Affairs Update‘Retainage’NowOptionalinFederalA/EContracts;ACECSaysEliminateIt

ACEC has won a substantial change in the longstanding federal requirement that A/E contracts include a retainage of 10 percent of payments. Under the revised Federal Acquisition Regulation (FAR) policy adopted in April, retainage is no longer mandatory, but optional (up to 10 percent) and at the discretion of the federal contracting offi cer.The new rule states that no retainage is required if the contract-

ing offi cer determines that the work performed by the fi rm is satisfactory. Additionally, withheld payments for A/Es are to be paid at the successful completion of the design contract. This will, in many cases, signifi cantly reduce the time fi rms must wait for full payment. The new policy stems from an ACEC-backed recommendation included in the Small Business Administration’s Regulatory Review and Reform initiative in 2008, calling for the elimination of retainage as an unnecessary burden on cash fl ow and overhead.“While the new rule is a step in the right direction, we would

like to see this practice eliminated altogether,” said ACEC President Dave Raymond. “The Government has many other remedies for ensuring satisfactory completion that are less damaging to our businesses.”The new rule can be viewed at this link:

www.acquisition.gov/far/fac/Looseleaf_FAC%202005-39.pdf

CASE’s Toolkit Committee has just released its newest tool, Tool 3-3 – Website Resource Tool. If you have ever wondered what websites are available for the business of Structural Engineer-ing, wonder no more. This tool contains website links and descriptions of those websites that could be useful in running a structural engineering business.Examples of website links are CASE, where you can download

contracts, publications and tools, CONTRACTS AND RISK MANAGEMENT CENTRAL where you can get information on business laws, doing business across state lines, and sealing and stamping requirements, and other helpful websites.A few years ago, CASE set out to improve the practice of

structural engineering by reducing the frequency and severity of claims. One of the ways CASE planned to accomplish this was through the production of software-based tools that are made available to CASE members through e-mail and on the CASE website at www.acec.org/CASE. A summary for each tool can be found at www.acec.org/case/tools.cfm.

The CASE Summer Meeting will take place on Thursday and Friday, September 16-17, 2010, Boston, Massachusetts. On Thursday, the CASE committee breakout meetings will be held for the National Guidelines, Contracts, Programs & Communications, and Toolkit committees to continue work on their respective assignments and planning for future CASE products. The CASE Executive Committee will meet on Friday.A CASE roundtable on structural engineering issues will

be held in conjunction with the Boston Association of Structural Engineers (BASE) dinner/meeting on Wednesday, September 15th at the MIT Faculty Club. The theme of the evening will be Risks for Engineers and the roundtables will focus on the following:

• Risk vs. Award with Integrated Project Delivery• BIM Investment vs. Payback• Sustainable Design and the Risk for Structural Engineers• How to Collect Your Money Without Getting Sued

More details will follow in the August edition of CASE-in-Point.

The CASE Spring Risk Management Convocation took place on May 14th during the fi rst-ever combined NASCC/Structures Congress in Orlando, Florida. All of the CASE sessions were well received and had considerable attendance, including the CASE Breakfast which featured David Ratterman, AISC Secretary and General Counsel. His talk centered on the AISC Code of Standard Practice. Other CASE sessions held during the afternoon included Steel Design Dos and Don’ts, A Project Manager’s Day, and Managing Expectations and Risk during the Steel Detailing Process.Next year the CASE Spring Convocation will be held in

conjunction with the Structures Congress in Las Vegas, NV, April 14-16, 2011.

The next CASE Risk Management Convocation will take place during the ACEC Fall Conference, October 17–20, 2010, at the El Conquistador Resort in Fajardo, Puerto Rico. On October 18th, the CASE Convocation will include the following confi rmed sessions:

• Avoiding the Pitfalls in Working with Architects Using AIA C401

• Effective Use and Pitfalls of Building Structural Design Commercial Software

• Lessons Learned from Actual Claims (Key Cases)The ACEC Fall Conference will feature a panel discussion of

top engineering fi rm CEOs, world class educational sessions,

an innovative trade show, a full complement of tours, and entertaining networking events. The Conference will address industry trends, markets, and business practices in a continued challenging economy. For more details and to pre-register go to www.acec.org/conferences/fall-10/registration.cfm. You will receive a discounted registration price if you pre-register prior to July 31st so don’t delay!

an innovative trade show, a full complement of tours, and

CASE Risk Management Convocation Comes to Puerto Rico This October

CASE News July 10.indd 1 6/18/2010 11:28:36 AM

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July 2010STRUCTURE magazine 33

CASE Summer Meeting Planned for Boston in September

Government Affairs Update‘Retainage’NowOptionalinFederalA/EContracts;ACECSaysEliminateIt

ACEC has won a substantial change in the longstanding federal requirement that A/E contracts include a retainage of 10 percent of payments. Under the revised Federal Acquisition Regulation (FAR) policy adopted in April, retainage is no longer mandatory, but optional (up to 10 percent) and at the discretion of the federal contracting officer.The new rule states that no retainage is required if the contract-

ing officer determines that the work performed by the firm is satisfactory. Additionally, withheld payments for A/Es are to be paid at the successful completion of the design contract. This will, in many cases, significantly reduce the time firms must wait for full payment. The new policy stems from an ACEC-backed recommendation included in the Small Business Administration’s Regulatory Review and Reform initiative in 2008, calling for the elimination of retainage as an unnecessary burden on cash flow and overhead.“While the new rule is a step in the right direction, we would

like to see this practice eliminated altogether,” said ACEC President Dave Raymond. “The Government has many other remedies for ensuring satisfactory completion that are less damaging to our businesses.”The new rule can be viewed at this link:

www.acquisition.gov/far/fac/Looseleaf_FAC%202005-39.pdf

ACEC Endorses Bill to Reduce Paperwork Burden on FirmsACEC is backing legislation to repeal a provision in the

recently enacted health care law that will significantly expand the paperwork burden facing A/E firms.Under current law, a business must issue a Form 1099 to

any service provider whom it pays more than $600 in a year, unless that service provider is a corporation. Starting in 2012, the new law expands this requirement to include services or property purchased from any business, including corporations. For example, a firm that purchases $1,000 in office supplies from a retail supplier will have to issue a 1099 to the business reflecting the purchase. The provision is designed to improve tax compliance, but its primary effect will be to burden businesses with a new paperwork requirement. ACEC has endorsed legislation (H.R. 5141) introduced by Rep. Dan Lungren (R-CA) that would repeal the new mandate.For more information on the new health care law, contact

Katharine Mottley at [email protected].

The CASE Summer Meeting will take place on Thursday and Friday, September 16-17, 2010, Boston, Massachusetts. On Thursday, the CASE committee breakout meetings will be held for the National Guidelines, Contracts, Programs & Communications, and Toolkit committees to continue work on their respective assignments and planning for future CASE products. The CASE Executive Committee will meet on Friday.A CASE roundtable on structural engineering issues will

be held in conjunction with the Boston Association of Structural Engineers (BASE) dinner/meeting on Wednesday, September 15th at the MIT Faculty Club. The theme of the evening will be Risks for Engineers and the roundtables will focus on the following:

• Risk vs. Award with Integrated Project Delivery• BIM Investment vs. Payback• Sustainable Design and the Risk for Structural Engineers• How to Collect Your Money Without Getting Sued

More details will follow in the August edition of CASE-in-Point.

CASE committees have been the reason behind CASE’s success for over 20 years and they remain vital to CASE’s future. As part of the committees’ ongoing activities, face-to-face meetings and informal discussions are held twice a year to explore current issues, and work on projects like new and revised Risk Management Tools, Guidelines and Contracts, as well as Publications, and Risk Management Convocations. These meetings also allow the various CASE committees to interact across all of CASE’s activities. For more information on the CASE committees and CASE in general visit their website at www.acec.org/CASE. Contact CASE Executive Director Heather Talbert at [email protected] or 202-682-4377 if interested in joining.

an innovative trade show, a full complement of tours, and entertaining networking events. The Conference will address industry trends, markets, and business practices in a continued challenging economy. For more details and to pre-register go to www.acec.org/conferences/fall-10/registration.cfm. You will receive a discounted registration price if you pre-register prior to July 31st so don’t delay!

CASE News July 10.indd 2 6/18/2010 11:28:43 AM

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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.

34

The Case for System-Based Structural DesignBy Avinash M. Nafday, Ph.D., M.B.A., P.E.

The current code approach for structural design is member-based, where designs are checked for the safety of individual mem-bers. There is very little guidance on the overall safety, design and integrity of their assemblage except broad statements regard-ing the need for an arrangement that provides stability to the entire structural system, along with continuity, redundancy and ductility. U.S. codes do not specify how to achieve this goal, leaving its imple-mentation to the discretion and ability of the engineer.Observations from actual projects show

that competent structural engineers do in-corporate empirical strategies to limit adverse consequences to the structural system from member failures, depending on their under-standing, knowledge and experience, as well as the structure type and its vulnerability. However, there are many examples where seemingly highly redundant structures have failed due to a lack of system integrity. There are also cases where individual members that are expected to fail do not, because of interaction among members in the system. Therefore, it is of paramount importance to study structural system integrity and develop system-based design procedures, including specific code guidance to limit adverse consequences.

Measuring Structural IntegrityEfforts to pin down the structural integ-

rity concept have been thwarted due to its elusive nature, precluding development of an objective, simple and practical metric, which is a pre-requisite for rational design of systems and comparison of alternatives. Quantification of structural integrity has also proved difficult due to the diversity of systems and the various contributing causes of initiating damage. The myriad ways in which structural integrity is influ-enced – from configuration, member sizes, material properties, connection types, applied loads etc. – are all captured in the structural stiffness matrix K, where the singularity of K represents the extreme case of loss of general structural integrity.Recent research has used this fact to

quantify structural system integrity as a metric ∆ ranging from 0-1 (higher value denoting better structural integrity), defined by the determinant | KN | of the normalized

stiffness matrix KN, where KN is obtained by dividing each row of matrix K by the square root of the sum of squares of the terms in that row. This metric is easily com-puted and accounts for the contributions of configuration, geometry of members, their importance or criticality in alternative load paths, material behavior and applied loading on the structures to the system safety perfor-mance. This metric can serve as the linchpin for system-based structural design.

System-Based DesignStructural design for natural and man-

made hazards or specified loads has two components: the likelihood of the postu-lated hazard or load event (probabilistic aspect) and what happens when such an event actually occurs (consequences). Risk is determined by the combination of these factors. System-based design would neces-sarily be secondary. In the primary stage, the structure would be proportioned using the current probability-inspired, member-based code provisions, including appropriate minimum joint resistance and continuity. Thereafter, the members would be examined and, if necessary, re-designed to ensure adequate structural system integrity, based on their role and importance in contributing to adverse system consequences. These consequences can be characterized in terms of collapse or any other pre-defined per-formance criterion.The level of modification for a member is

identified through the Member Consequence Factor, Cf, which accounts for its contri-bution to the undesirable system response. The consequence factor for the ith structural member is defined as the ratio of | KN

i | to | KN |, where KN

i is the normalized stiffness matrix after removal of the ith member from the system. These consequence factors for all n members range from 0 to 1; the lower the factor, the more critical the member is for system safety. A consequence factor of 0 indicates that removal of the member results in immediate structural failure.Cf can be used as an additional partial safety

factor on the resistance side of the member-based code equations for implementation of system-based structural design. It is also possible to investigate various failure strings comprised of multiple member failures (with Cf still in range 0-1) with a

similar approach, except for the additional complexity involved in the calculations. In this formulation, even though over-all system design is consequence-based, the design of individual members is still probability-based and all requirements in current codes would still apply, with the additional proviso for consequence factors.

BenefitsAn advantage of the system-based approach

is the possibility of optimizing robustness to prevent minor damages from causing disproportionately large consequences. Robustness, a subset of structural integrity, is an important property about the form and/or connectedness of the structure and a major governing factor in system behavior, but has been neglected in modern codes due to a lack of theoretical understanding of its contribution to capacity. It provides a measure of the quality of system configu-ration and may be obtained by separating geometrical/topological properties from material properties through decomposition of the stiffness matrix K.This approach provides a tool to optimize

the assembly of members through innovative configurations, resulting in new designs limited only by the creativity of the designer. It is also possible to use member-based, probability-oriented design for service requirements and high-likelihood environmental events, while using consequence-oriented, system-based design for low-likelihood events (e.g., multi-hazard occurrence) to leverage the robustness property of configurations. This can reduce the design cost without compromising overall safety. The system-based approach is also appro-

priate for brittle materials like glass, which fail suddenly without prior warning, or for temporary structures with limited service life. Finally, the consideration of failure consequences at the design stage helps to mitigate the impact of building misuse, or design and construction errors.▪

Avinash M. Nafday, Ph.D, M.B.A., P.E., is with the California State Lands Commission, Marine Facilities Division, Long Beach, California. He can be reached at [email protected].

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

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