STRUCTURE magazine | January 2013

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE

®

Special Section: Anchors, Piers, Foundations

January 2013 Concrete

and Underground Construction

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FEATURES 31 Special Section

Slow and Steady Upswing for Foundation Business

By Larry Kahaner

Companies involved in the foundation sector report that business generally is up and growing; this despite concerns over the economy, taxes and the “fiscal cliff”. New products and services, and revised business paradigms, have kept the industry afloat in these trying times.

By Michael Herrmann, P.E.

44 Legal Perspectives Global Patented Innovation in Structural Engineering

58 Structural Forum Meeting the Challenges of the Future Head-On

By Stephen L. Keefe, P.E., Esq.

7 Editorial How to Have a Happy and Prosperous 2013

By John A. Mercer, P.E., SECB

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

Robert Wood Johnson University Hospital

By Frank Griggs, Jr., P.E.

COLUMNS

The Proper Purpose of Engineering

51 Spotlight

Ralph Modjeski

January 2013

9 InFocus

DEPARTMENTS 42 Great Achievements

CONTENTS

By Barry Arnold, S.E., SECB

10 Codes and Standards Design of Slender Concrete Columns

By Tanner Wytroval and Robin Tuchscherer, Ph.D., P.E.

14 Structural Design Addressing Punching Failures

By Carlos E. Ospina, Ph.D., P.E. and Neil M. Hawkins, Ph.D.

IN EVERY ISSUE

Join the Discussion An article in the NCSEA News section of STRUCTURE last month (Curing the Concrete Question, December 2012, pg. 36) has garnered lots of comments. We encourage you to join in the discussion about evolving concrete coursework for the Basic Engineering Education Curricula. Visit the STRUCTURE website (www.STRUCTUREmag.org) and click on the link on the homepage to read the article and provide your input.

8 Advertiser Index 48 Resource Guide (Anchor Updates) 52 NCSEA News 54 SEI Structural Columns 56 CASE in Point

18 Structural Forensics Snow Related Roof Collapse and Implications for Building Codes

By Michael O’Rourke, Ph.D., P.E. and Jennifer Wikoff

22 Building Blocks Is Lightweight Concrete All Wet?

By David P. Martin, P.E., Alec S. Zimmer, P.E., Michael J. Bolduc, P.E. and Emily R. Hopps, P.E.

28 InSights Updated Military Criteria for Antiterrorism Design

ON Special Section: Anchors, Piers, Foundations

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE

®

By Mark Gardner, P.E. and Spencer Quiel, Ph.D.

January 2013 Concrete

and Underground Construction

THE

36 Technology

COVER

Punching shear failures in parking garage, in the aftermath of the 2011 Christchurch, New Zealand, Earthquake. Considerations to prevent concentric punching shear failures in reinforced concrete two-way slabs is the focus of the Structural Design article on page 14.

Photo courtesy of Dave Swanson, Principal, Reid Middleton Inc.

Challenging Traditional Design with BIM Tomas Amor, P.E.

38 Engineer’s Notebook Bayes’ Rule for the Practicing Structural Engineer

By James Lefter, P.E.

Publication of any article, image, or advertisement in STRUCTURE® magazine does not constitute endorsement

by NCSEA, CASE, SEI, C 3 Ink, or the Editorial Board. Authors, contributors, and advertisers retain sole responsibility for the content of their submissions.

STRUCTURE magazine

5

January 2013

Coming to a Code near you!

The Reinforced Concrete Design Manual The Manual is published in two volumes and provides design and analysis in accordance with ACI 318-11.

Information is presented in three sections: • Explanatory Material, • Design Examples, and • Design Aids.

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Available in hard copy or PDF format.

Editorial

How to Have a Happy and Prosperous 2013 new trends, new techniques and current industry issues

By John A. Mercer, P.E., SECB

I

2013

f the title of this editorial didn’t catch your eye, you must have skipped a few pages.

Truthfully, I wish for all structural engineering firms to have a Culture that exudes harmony and happiness for owners and staff, as well as their clients. Probably one of the most important traits of a firm’s culture is the way in which staff communicates with one another and with clients.

A recent conversation comes to mind that I had with the General Manager of a local sandwich shop that I frequent for lunch. Chris was behind the counter helping make sandwiches and I told him how good it was to see him working for a change. He grinned and said that he was filling in for one of the guys they moved up to manage one of their other stores in town. They manage twelve stores and this one is by far the most friendly and profitable.

Chris stated that his goal is “to train” new staff in our South Broadway store and then transfer them with ‘this store’s culture’ to our other stores to make them more appealing to customers, as well as more profitable. “ Customer loyalty is what they are actually trying to achieve. “You can get a fresh baked bread, meat, cheese, and veggies sandwich almost anywhere, but getting the customer to return is where the real value lies. Having a happy crew makes managing easier too.”

I think there is a message for us in that experience. How are you contributing to the culture in your firm? How do you see yourself with your colleagues? Do you treat them in the same way you expect to be treated? There are probably enough problems and situations that require staff time and patience. Problems create stress for all staff, so what can you contribute to lighten their load?

Just smiling while you drive to work in the morning can be the beginning of a great day. I recall noticing people smiling in their cars as they passed by me each morning. One day I intentionally made the effort to smile as I was driving. Again I noticed people smiling, but also the one’s frowning as they passed by. It really gave me a new perspective. I made it my goal to encourage one person to smile each day, just by smiling as I drove by them.

2013 is going to provide many challenges and opportunities for our structural engineering community. My wish is that all structural engineering firms and structural engineers will step up to those challenges, seek out the opportunities and be prosperous in doing so.

The Council of “AMERICAN” Structural Engineers has a Risk Management Program STRUCTURAL built upon 10 Foundations.

ENGINEERING CASE’s first foundation for INSTITUTE Risk Management is “Culture”.

CASE member firms have the a member benefit

structurE

®

opportunity to provide leadership that will pilot our country, communities, and clients into the deep-water channels of prosperity.

I challenge CASE members to seize that opportunity!

Looking forward, CASE will be having its 2013 Winter Planning Meeting preceding the NCSEA Winter Leadership Forum. I hope to see you there. I always look forward to visiting with CASE members and discussing the current issues that firms are dealing with around the country. Be sure to mark your calendars: March 5-6, 2013 at the Westin La Paloma Resort, Tucson, AZ. Likewise, CASE is a coalition under the American Council of Engineering Companies, ACEC. The ACEC Spring Annual meeting will be held in Washington DC on April 21-24, 2013. That is where Structural Engineers can participate in going up on the Hill to lobby their Congressional Delegations on issues affecting engineering businesses. Our congressional delegations truly need our assistance in understanding the technical complexities facing our nation. Mark your calendars, and make plans to attend and participate. Network with the other ACEC Coalitions as well.

The 2013 CASE Convocation will be held at the SEI Structures Congress on May 3, 2013 in Pittsburgh, PA (Westin Convention Center). Plan to attend.

Finally, you are in control of your firm’s culture, including your attitude. What is your attitude going to be in 2013? Is your glass half full, or half empty? It’s up to you to be a happy team member.

If you can achieve that, prosperity will follow. Good luck. I hope to see you at the CASE Winter Planning Meeting, the NCSEA Winter Leadership Forum, and the CASE Convocation at the Structures Congress.

Join CASE, get a CULTURE, and plan to PROSPER!▪

STRUCTURE magazine

John A. Mercer, P.E., SECB (Engineer@minot.com), is the president of Mercer Engineering, PC, in Minot, North Dakota.

He currently serves as Chair of the Council of American Structural Engineers (CASE) and is a CASE representative on STRUCTURE’s Editorial Board.

7

January 2013

Advertiser index

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Editorial Board

Powers Fasteners, Inc. .............................. 2 PT&C Forensic Consulting Serv., P.A. .. 37 QuakeWrap ........................................... 39 RISA Technologies ................................ 59 SidePlate Systems, Inc. .......................... 47 Simpson Strong-Tie............................... 17 Soilstructure.com .................................. 40 Struware, Inc. ........................................ 12 Subsurface Constructors, Inc. ................ 35

AdvErtising Account MAnAgEr Interactive Sales Associates

Chair

Jon A. Schmidt, P.E., SECB

Chuck Minor

Dick Railton

Eastern Sales 847-854-1666

Western Sales 951-587-2982

Burns & McDonnell, Kansas City, MO chair@structuremag.org

sales@STRUCTUREmag.org

EditoriAL stAFF

Craig E. Barnes, P.E., SECB

Brian W. Miller

Mark W. Holmberg, P.E.

Mike C. Mota, Ph.D., P.E.

Executive Editor Jeanne Vogelzang, JD, CAE

Evans Mountzouris, P.E.

Editor

CBI Consulting, Inc., Boston, MA

Davis, CA

Heath & Lineback Engineers, Inc., Marietta, GA

Dilip Khatri, Ph.D., S.E.

Khatri International Inc., Pasadena, CA

execdir@ncsea.com

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publisher@STRUCTUREmag.org

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Graphic Designer

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Mercer Engineering, PC, Minot, ND

American Wood Council, Leesburg, VA

Erratum STRUCTURE magazine received input regarding a past article, Antiquated Structural Systems Series – Part 4 (Engineer’s Notebook, June 2008 issue). The article describes a system called Dox Plank and attributes its development to the Nabco Plank Company. The inventor of the system was Doc Vander Heyden. STRUCTURE thanks David Vander Heyden, Doc’s son, for correcting the error. The author appreciates the information.

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STRUCTURE® (Volume 20, Number 1). 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; $90/yr Canada; $125/yr foreign. 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 reflect the views of NCSEA, CASE, SEI, C3 Ink, or the STRUCTURE Editorial Board.

STRUCTURE® is a registered trademark of National Council of Structural Engineers Associations (NCSEA). Articles may not be

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inFocus

The new trends, Proper new techniques Purpose and currentof industry Engineering issues

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

A

s Joseph Dunne has noted (“The Rationality of Practice,” September 2012), a practice as defined by Aladair MacIntyre (“Rethinking Engineering Ethics,” November 2010) is “something that can succeed or fail in being true to its own proper purpose.” Both MacIntyre and Dunne had internal goods in mind, but there is an alternative sense in which a practice may have a “proper purpose.” In a 1984 paper (“Virtues and Practices,” Analyse und Kritik, Vol. 6, No. 1, pp. 49-60, www.analyse-und-kritik. net/1984-1/AK_Miller_1984.pdf), British political theorist David Miller identified two different kinds of practices:

Aristotle would insist that the proper purpose of any worthwhile activity is to facilitate some aspect of eudaimonia (yoo-dy-mohNEE-ah), which is best translated as well-being or human flourishing (“Engineers Are from Aristotle,” July 2010). Unlike most other ancient Greek philosophers, he affirmed that favorable physical, social, and material conditions can be important factors for living a genuinely good life. With this in mind, Richard Bowen described the end of engineering as “the promotion of human flourishing through contribution to material well-being” (“Engineering Ethics as Virtue Ethics,” May 2011).

Ashvin Shah echoed this in personal correspondence, suggesting that the proper purposes of medicine, law, and engineering are physical, social, and material well-being, respectively; roughly corresponding to life, liberty, and the pursuit of happiness. In accordance with the principle that these are unalienable rights possessed by every individual, it is important to stipulate that the scope of well-being that these practices should foster is universal; i.e., engineers ought to work toward the material well-being of all people, not just a privileged group. As Miller wrote:

There is an important distinction to be drawn between practices which have no raison d’etre other than the particular excellences and enjoyments which they allow to participants (I shall refer to such practices as ‘self-contained’) and practices which have a wider social purpose (I shall refer to these as ‘purposive’).

Games, from which much of MacIntyre’s thinking about practices seems to be drawn, are the main exemplars of the first category… On the other hand, in the case of a productive activity… there is an external purpose which gives the practice its point and in terms of which it may be judged.

If a “practice” account of the virtues is going to be successful, the practices concerned must be those I have called purposive, and moreover those whose aims are fairly central to human existence. By implication it is a mistake to try to explain the virtues by reference to goods internal to practices. Although MacIntyre is quite right to draw our attention to the existence of such goods – for even in the case of purposive practices standards of excellence will develop whose achievement will be regarded as an internal good by the participants – the virtues themselves must be understood in relation to those wider social purposes which practices serve.

In other words, MacIntyre’s rigid classification of all goods as either internal or external to any given practice is difficult to maintain if it is one that Miller would describe as purposive. Such a practice has an end that is intrinsic in a way that a strictly external good cannot be; and yet that end also does not qualify as an internal good, since its benefits extend well beyond the boundaries of the practicing community.

Combining the terminology of Dunne and Miller, what is the proper purpose of a purposive practice? It seems to me that the answer is something analogous to, or perhaps equivalent with, the fundamental ideal for which the practice strives; something that is treated as an end in itself, such that the practice qualifies as a praxis (“The Social Nature of Engineering,” November 2012). Physicians pursue health; if humans ever attain perfect health, then doctors would be out of a job. Attorneys pursue justice; if humans ever attain perfect justice, then lawyers would be out of a job. Engineers pursue . . . what, exactly? What perfection would humans have to attain in order for engineers to be out of a job?

Henry Petroski contends that engineering innovation is driven primarily by dissatisfaction with the current state of affairs (“Learning from Failures,” July 2006). Would engineers be out of a job if humans ever attain perfect satisfaction? Presumably, but is satisfaction really an ideal on par with health and justice? Contentment might seem like a more suitable concept from that standpoint – except that genuine contentment is supposed to be independent of outward circumstances, and engineering deals exclusively with outward circumstances.

A better candidate might be quality of life. Would engineers be out of a job if humans ever attain a perfect quality of life? Probably, but practitioners other than engineers – including physicians and attorneys – can and do contribute to quality of life, as well. Furthermore, even within engineering, improving the quality of life for some people may diminish that of others; in fact, such tradeoffs are routine in the real world.

STRUCTURE magazine

This requires a significant adjustment of MacIntyre’s approach to virtue ethics. The proper purpose of a purposive practice constrains what will count as its internal goods, since the latter must tend to advance the former in some way. Likewise, the personal attributes that enable someone to achieve the internal goods of a purposive practice only count as genuine virtues if they also facilitate the accomplishment of its proper purpose.

Consequently, a good that is internal to the practice of engineering will be something that uniquely contributes to the material well-being of all people. What exactly is “material well-being” in this context? How do engineers enhance it? What obligations do they assume in the process? I will take up these questions next time.▪ Jon A. Schmidt, P.E., SECB (chair@STRUCTUREmag.org, twitter.com/JonAlanSchmidt), 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.

9

January 2013

Codes and standards updates and discussions related to codes and standards

Design of Slender Concrete Columns

By Tanner Wytroval and Robin Tuchscherer, Ph.D., P.E.

Tanner Wytroval is a graduate student studying civil engineering at Northern Arizona University.

Tanner may be reached at tlw85@nau.edu.

Robin Tuchscherer, Ph.D., P.E. is an assistant professor of engineering at Northern Arizona University.

Robin may be reached at robin. tuchscherer@nau.edu.

W

hen designing a column, structural engineers must evaluate the impact of second order or P-∆ effects to determine if loads applied to a structure in its deformed position significantly increase internal forces (i.e. by more than 5%). Typically, second order effects of this magnitude occur when a column is slender; that is, when its height-to-width ratio is greater than approximately 10. If a column is slender, engineers must consider either an elastic second order analysis or they may analyze the column by the moment magnification procedure contained within the Building Code Requirements for Structural Concrete (ACI 318-11). In contrast, engineers would evaluate a non-slender or short column using an elastic first order analysis.

The provisions in the moment magnification procedure allow for a column to be designed using a conventional first order analysis provided that the moments calculated by the analysis are increased to account for second order effects.

Considerable inconsistencies can exist between the results obtained from an elastic second order analysis and the moment magnification procedure. These inconsistencies cause confusion amongst practitioners and result in wide variations in their use and/or interpretation. Simply put, moments estimated by the moment magnification procedure may be upwards of five times larger than those estimated by a second order analysis. As a result, engineers often discount the moment magnification procedure in favor of the more manageable results obtained from an elastic second order analysis.

But the question remains, “Why is there such a large difference within the provisions?”

The main source of these inconsistencies can be attributed to the approximation of a column’s flexural stiffness, EI. The two methods use different base values for stiffness, and then each apply different reduction factors to the stiffness values. Figure 1 presents a summary of the current ACI 318-11 provisions for slender concrete columns. In the figure, the major steps of the slender column provisions are shown as they relate to each analysis method.

Background

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

The majority of the current provisions on slender column design first appeared in the 1971 version of ACI 318 as a result of the recommendations of ACI-ASCE Committee 441, Reinforced Concrete Columns. At the time, the new provisions allowed for the use of structural analysis methods made available with the advent of computer analysis software to assess P-∆ effects in slender columns.

10 January 2013

In lieu of a computer analysis, the 1971 code introduced the moment magnification procedure for approximating slenderness effects. The moment magnification procedure was a replacement for the reduction factor method contained in the previous 1963 version of ACI 318. In ACI 318-63, slenderness effects were accounted for by dividing the axial load and moments by a reduction factor less than one. The reduction factor method was based on the recommendations of Broms and Viest (1961), and added to

by MacGregor and Siess (1961). The researchers analytically derived the factor accounting for the inelastic buckling behavior of reinforced concrete.

The moment magnification method was based on a similar procedure used by the American Institute of Steel Construction (AISC) for approximating slenderness effects based on Timoshenko’s Theory of Elastic Stability. Overall, the basic premises introduced in the 1971 code are present in the 2011 code with slight alterations based on more recent studies.

The recommendations by Committee 441, and alterations to the code since, are derived from a series of experimental tests performed by several investigators. These tests consisted of eccentrically, concentrically, and laterally loaded sway frames, nonsway frames, and individual columns.

Columns were tested in both single and double curvature. Figure 2 (page 12) presents an example of the test specimens used to develop current reinforced concrete slender column provisions.

Researchers have used the data from these tests to support and/or derive stiffness approximations, stiffness reduction factors, sustained load factors, and slenderness limitations.

Examination of ACI 318 between 1971 and 2011 indicates that the main source of inconsistency in determining slenderness effects is the approximation of member stiffness, EI. When performing a second order analysis, the EI values used for strength design should reflect the member’s stiffness immediately before failure.

At that point, the degree of cracking varies along its length. Thus, the stiffness of the member is somewhere between its gross and fully-cracked moment of inertia multiplied by the modulus of elasticity. Fairly accurate methods exist to estimate the value of EI for a single member, but they are arbitrary for the design of a typical frame when considering the high degree of variability associated with multiple members of varying size, strength, and reinforcement. Therefore, ACI 318-11 provides the following four equations for approximating member stiffness.

Equations 1 and 2 are permitted for use with elastic second order and sway frame moment magnification analysis. Whereas, Equations 3 and 4 are used solely for the moment magnification procedure. If Equation 2 is used for the moment magnification procedure for nonsway frames,

EI = 0.70Ec Ig

(Equation 1) ACI 318‐11 Section 10.10

M P A EI = (0.80 + 25 st )(1 – u – 0.5 u )Ec Ig ≤ 0.875EIg (Equation 2) Ag Puh Po EI = 0.2Ec Ig + Es Ise 1 + βdns EI =

0.4Ec Ig 1 + βdns

Is column braced against sidesway Yes

(Equation 3)

•

Eqn. 10‐10

No

Is column slender •

No

Is column slender

Eqn. 10‐7

•

Yes Method of 2nd Order Analysis

(Equation 4) Elastic Second Order ACI 10.10.4

then it must be divided by a sustained gravity load creep factor, 1+βdns.

The equations differ from each other based explicitly and implicitly on three variables: a stiffness reduction factor, φk; an approximation of initial effective stiffness, EIeff ; and sustained load factors, βdns and βds.

•

ACI 318 requires that the stiffness value used for slender column design be multiplied by a stiffness reduction factor, φk, to account for variability in actual member properties and analysis. The code does not explicitly define a φk value. Rather, different values are implicitly included within the provisions. A stiffness reduction factor,φk, equal to 0.875 is incorporated into Equations 1 and 2 and is based on MacGregor’s (1993) recommendations. In Equation 1, the value of 0.70 is the product of both an effective stiffness of 0.80Ig and a stiffness reduction factor of 0.875 (i.e. 0.70 = (0.875)(0.80)). In Equation 2, φk is indirectly included by capping the value to 0.875Ec Ig . A separate stiffness reduction factor, φk, equal to 0.75 is implicitly incorporated into Equations 3 and 4. For these equations, φk is applied indirectly to the column by multiplying its buckling capacity by 0.75. Mirza (1987) derived this value based on a reliability study.

Sustained Load Factors, dns and ds The additional deflections resulting from creep must be accounted for because they will increase second order moments. ACI 318 addresses STRUCTURE magazine

ACI 10.10.4.1 Eqn. 10‐8

Nonsway ACI 10.10.6

Sway ACI 10.10.7

Approximate Stiffness ‫߶ܫܧ‬௄ ͳ ൅ ߚௗ௡௦

Approximate Stiffness *‫߶ܫܧ‬௄ • Eqn. 10‐14 • Eqn. 10‐15 • Eqn. 10‐8

• Eqn. 10‐14 • Eqn. 10‐15 • Eqn. 10‐8

Calculate Moment Magnifier

Calculate Moment Magnifier •

Calculate 2nd Order Moment

• •

Eqn. 10‐12

Calculate Approximate 2nd Order Moment •

Eqn. 10‐20 Eqn. 10‐21

Calculate Approximate 2nd Order Moment

Eqn. 10‐11

• •

Eqn. 10‐18 Eqn. 10‐19

Is 2nd order moment is less than 1.4 1st order moment Yes

No Redesign member

Finished with Slenderness Effects

* In the unusual case that sustained lateral loads exist, equation is divided by 1+βds

Figure 1: Summary of ACI 318-11 column provisions.

Approximation of Initial Effective Stiffness, EIeff Depending on the method of analysis, ACI 318 uses different values and expressions to estimate the stiffness of a column immediately prior to failure. Again, the value of 0.70 used for Equation 1 is the product of both an effective stiffness and stiffness reduction factor.

That is, it implicitly assumes the cracked moment of inertia, Ieff, to be equal to 0.80Ig as suggested by MacGregor and Hage (1977). The researchers derived this value based on lateral deflections measured in laboratory test frames (Figure 2, page 12 ). The EIeff used in Equation 2 is explicitly expressed as presented above and includes the influences of member loads and properties including: eccentricity; longitudinal steel percentage, ρ; and the ratio of factored axial load to nominal axial strength, Pu /Po. Khuntia and Ghosh (2004) developed this expression analytically and verified it experimentally. The EIeff used in Equations 3 and 4 is contained in the numerator of each expression. Committee 441 recommended these expressions as lower end approximations of member stiffness. They were developed using a combination of theoretical load-moment-curvature diagrams, analysis of test frames, and computer simulations. Equation 3 is highly conservative for low ρ and low Pu /Po values, but is slightly un-conservative for high ρ and high Pu /Po values. Contrarily, Equation 4 is highly conservative for high ρ and low Pu /Po values, but slightly un-conservative for low ρ and high Pu /Po values.

•

No

Moment Magnification ACI 10.10.5

Approximate Stiffness *‫߶ܫܧ‬௄

Stiffness Reduction Factor,  k

Eqn. 10‐6

Yes

this issue by dividing the column’s stiffness by the sustained load factor, 1+βdns and 1+βds for nonsway and sway cases, respectively. βdns is defined as the ratio of the maximum factored sustained axial load to the maximum factored axial load, and βds is defined as the ratio of maximum factored sustained shear within a story to the maximum factored shear within a story. In both cases, the loads should be from the same load combination. For all equations, when sustained lateral loads exist (e.g. lateral soil pressure), they are divided by 1+βds. The sustained axial load factor, βdns, is not included in Equations 1 or 2 when performing a second order analysis; however, it is included in Equations 2, 3, and 4 when using the moment magnification procedure. This point is a major discrepancy and point of confusion amongst designers because it often results in significant differences between procedures.

Discussion It is not hard to see that with, all the variations allowed for the aforementioned three variables, many different stiffness values are attainable within each method of evaluation per ACI 318-11. Figure 3 presents a comparison of these differences. The figure shows the ratio of the stiffness approximations in Equations 1 through 4 divided by the uncracked-stiffness, Ec Ig . These values are plotted as they vary with Pu/Po. Because Equations 2 and 3 vary with ρ, they are plotted once with ρ = 1% and once with ρ = 4% to envelop the practical range.

Also, Equation 2 differs with eccentricity ratio, e/h, so it is plotted

11

January 2013

(a)

(b)

(c)

Figure 2: Experimental Specimens tested by (a) Furlong & Ferguson; (b) Green & Breen; and (c) Ferguson & Breen.

1 0.9 0.8 Eqn. (1)

0.7

EI / EcIg

0.6

Conclusions and Recommendations

0.5 0.4

Eqn. (3) ρ=4%

0.3

Eqn. (4) Eqn. (3) ρ=1%

0.2 0.1 0 0.000

0.100

0.200

0.300

0.400

0.500

0.600

0.700

0.800

0.900

1.000

Pu / Po

Figure 3: Comparison of ACI stiffness approximations.

for an e/h = 0.10 and an e/h = 0.25. From Figure 3 it can be seen that, for most cases, the expressions for stiffness given by Equations 1 and 2 result in higher member stiffness than those given by Equations 3 and 4.

The code provides justification for some of these differences, but not all of them. First ADVERTISEMENT - For Advertiser Information, visit www.STRUCTUREmag.org

– depending on slenderness, eccentricity, and Pu /Po. Other research indicates that sustained loads lead to increased deflections, but any ill effects are offset by normal strengthening of concrete. It is clear that effects of sustained loading must be accounted for in determining slenderness effects, but the translation of this phenomenon to the reduction factors in the code is not clear. The question is raised why sustained axial loads are only accounted for in the moment magnification procedure for nonsway frames. The only justification provided by the code for not accounting for sustained axial loads in elastic second order and sway frame analysis is that the effects of creep are accounted for indirectly by capping the second order moments to 1.4 times the first order moments and that, in this case, a stability check in not necessary.

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of all, though it is not clearly noted in the code, the moment magnification procedure for nonsway or sway frames evaluates a single isolated slender column or single story, respectively. Frequently, an elastic second order analysis evaluates a complete frame.

This designation allows φk to take on different values in the different methods based on the conclusion of Furlong and Ferguson (1966) that “frame action as a restraint to column failure resulted in 5 to 15 percent more axial capacity for columns in single curvature than that anticipated for the equivalent isolated columns”. For this reason, a higher φk value of 0.875 is acceptable to use with an analysis that considers frame action. This point is also noted in the ACI 318 commentary.

The source of variation with βdns and βds is less pronounced. Research demonstrates that slender columns subject to sustained loads experienced unstable failure at loads significantly lower than columns without sustained loading

STRUCTURE magazine

12

January 2013

Through the examination of ACI 318 between 1963 and 2011 and supporting historical literature, several inconsistencies were identified within the current slender column design provisions. Accordingly, the following recommendations are made to clarify and simplify the current provisions. It is important to note that the purpose of these recommendations is not to alter the content within ACI 318-11 but, rather, to clarify and simplify what is currently in the provisions.

• Define the stiffness reduction factor, φk, transparently and include in all stiffness equations as opposed to the current practice of implicitly including within some of the equations and defining in the Commentary.

• Consider removing the stiffness Equations 3 and 4. Currently, the code allows the use of four different EI equations, resulting in highly variable values (Figure 3 ). Retaining solely Equations 1 and 2 in the code would be a simplification and result in less variation.

• Clarify the use of the sustained load factors, βdns and βds. If βdns is not required for an elastic second order analysis when second order forces are limited to 1.4 times the first order forces, then this point needs to be clarified within the moment magnification procedure.

Contrarily, if βdns is required, then this needs to be clarified within the provisions related to performing an elastic second order analysis.▪

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Structural DeSign design issues for structural engineers

T

wo-way slabs are unique to Reinforced Concrete (RC) construction. The most common type, due to its ease of forming and speed of construction, is the flat plate, a slab of uniform thickness supported by columns without beams, drop panels or capitals.

Flat plates are common in building construction, and can also be found as deck components in waterfront piers and wharves.

The design of RC flat plates is generally governed by serviceability limits on deflection, or by the punching shear capacity of the slab at the slab-to-column interface or at locations of concentrated loads. In practice, for the specific case where the transfer of unbalanced moment to the column is minimal, most punching failures look alike: a pyramid or truncated cone of slab remains around the column as the slab is loaded to failure. Punching may occur before (brittle) or after (ductile) a yield line mechanism has formed in the slab around the column. Brittle punching is undesirable because there is little warning of the impending failure.

For about 50 years, ACI 318 has used the equation Vn = 4√f 'c bo d (where f 'c is the specified compressive strength of concrete, bo is the critical perimeter measured at 0.5d from the column face, and d is the effective depth of the flexural reinforcement in the slab) for the nominal concentric punching shear capacity of two-way RC slabs. This expression was first introduced in the 1963 code following recommendations provided

by ACI Committee 326 (Shear and Diagonal Tension) and is based on subtle modifications to a design procedure developed by Moe (1961).

The ACI 318 equation (VACI ) has served the profession well. However, with the increasing use of higher strength steels and concretes, the equation is facing increasing scrutiny from researchers and practitioners because (1) it does not include a factor for the effect of the slab flexural reinforcement ratio, ρ, on the slab punching capacity; and (2) average shear stresses significantly lower than 4√f 'c at punching have been reported by several researchers for test slabs with ρ < 1% and also for slabs with d > 8 inches.

This article discusses qualitatively the relevance of these “perceived” deficiencies in the ACI 318 punching shear equation, highlighting its shortcomings and suggesting ways to improve the existing code provisions. This discussion concerns the concentric punching shear capacity only and does not include the effects of transferring unbalanced moments. However, the concepts suggested here can be readily extended to the moment transfer situation by use of the interaction relationship discussed in R11.11.7.2 of ACI 318.

Addressing Punching Failure Considerations to Prevent Premature Concentric Punching Shear Failure in Reinforced Concrete (RC) Two-way Slabs

By Carlos E. Ospina, Ph.D., P.E. and Neil M. Hawkins, Ph.D. Dist. M. ASCE, Hon. M. ACI

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

14 January 2013

Reinforcement Ratio Effect and Interaction Between Shear and Flexure The absence of a ρ term is often cited as a major deficiency by those who claim that the ACI 318 equation does not predict the punching shear capacities of test slabs as accurately as other equations that explicitly include this variable. This claim is, however, unfounded. Alexander and Hawkins (2005) reminded the profession that the ACI 318 equation was never intended to be used as a shear capacity predictor. Instead, it is a design equation aimed at precluding a brittle punching shear failure before the slab develops its flexural capacity. Its use assumes that the slab has already been properly designed for flexure.

The interaction between the transferred shear, V, and the shear associated with the flexural capacity of the slab, Vflex, as envisioned by Moe (1961), is qualitatively shown in Figure 1. The unbroken straight black line and the unbroken black curve represent conditions for a flexural failure and a shear failure, respectively. When the designer provides the proper amount of flexural reinforcement to resist the demand, the flexural capacity matches the design load. Vflex plots as a straight line against the flexural load capacity because it is the product of the slab design load and the area tributary to the column. Point A represents the “balanced failure” point; i.e., the point where the slab fails simultaneously in flexure and shear. To the right of point A, V > Vflex; i.e., shear failure governs. To the left of point A, Vflex < V; i.e., flexural failure governs.

The latter is the target failure zone for designers. To allow full moment redistribution and the development of sufficient slab deformation to warn of any impending failure, Moe recommended that the slab be designed for V = 1.1Vflex.

Thus, the intersection of the steeper straight line with the shear design curve leads to a slight reduction in shear capacity (point B). The plateau B-D is equivalent to the nominal punching shear capacity, VACI. In practical terms, the design envelope O-B-D separates flexural from shear failures, confirming that even though the ACI 318 equation is not explicitly set up in terms of ρ, it is tacitly based on a term (Vflex) that accounts for ρ.

Punching of Slabs with Low Reinforcement Ratios and the Issue of Ductility The effect of ρ on slab punching capacity has been discussed in the past by many researchers. Intuitively, a decrease in ρ should lead to a reduction in the depth of the compression zone available to resist transverse shear, and also to an increase in the width

Transferred Shear, V

9.0

B

8.0

Shear Failures

Flexural Failures

VMoe

A

ρ

7.0

B

6.0

C D

V b0 d

fc

5.0 ρ

4.0

VACI

3.0

V=Vflex

2.0

V=1.1 Vflex

ρ

1.0

O

0.0 0.00

Design Load (Flexural Capacity)

0.03

0.04

0.05

0.06

0.07

Figure 2: Relationship between V, VACI and Vflex for slabs with varying ρ.

< 1% may fail at shear stresses lower than 4√f 'c and display little ductility prior to punching.

The problem is exacerbated when d > 8 inches (Guandalini et al 2009). With the use of higher strength steels and higher strength concretes, many flat slabs now have ρ < 1%.

The fact that a slab with low ρ can fail at a shear stress less than 4√f 'c may seem to create the need for a new equation for Vn. Such a “necessity” is, however, unjustified because, as shown by Peiris and Ghali (2012), as long as the slab is properly designed for flexure, the slab will reach Vflex before ADVERTISEMENT - For Advertiser Information, visit www.STRUCTUREmag.org

STRUCTURE magazine

0.02

θ

Figure 1: Slab design rationale. (After Moe (1961) and Alexander and Hawkins (2005)).

of flexural cracks near the column. The increase in crack width should result in a reduction in aggregate interlock and dowel action. The combination of those three effects should lead to a reduction in the punching shear capacity.

Even though punching shear tests of slabs with low ρ are few – the vast majority have been performed on slabs with fairly large amounts of flexural reinforcement to avoid flexural failure – there is experimental evidence (Criswell 1974, Guandalini et al 2009, and Widianto et al.

2009) indicating that two-way RC slabs with ρ

0.01

15

January 2013

it punches. Hence, the maximum shear that can be transferred by a slab with a low ρ is likely to be that associated with the flexural capacity of the slab. That shear can be calculated from a yield line analysis assuming a concentric mechanism centered on the column or concentrated load; or, if using a finite element program, by extracting the shear associated with the applied load on the slab at its flexural capacity.

Historically, the mode of failure of RC twoway slabs tested in the laboratory has been determined by comparing the failure load V

against Vflex, with the latter determined from yield line analysis. Brittle shear failure occurs if V/Vflex < 1, whereas the failure is driven primarily by flexure if V/Vflex > 1. Additional refinements, to account for strain hardening effects and in-plane restraint effects, have been proposed to establish the brittle versus ductile slab failure mode boundary. Whether an RC two-way slab falls into either category depends primarily on ρ, f 'c, and the geometric characteristics of the slab-column connection.

Increased ductility is expected in slabs with larger V/Vflex ratios, but at the expense of a punching capacity reduction. For V/Vflex higher than 1.1 – i.e., low ρ – Moe’s theory assumes that such failures should be preceded by significant deflection increases due to extensive yielding of the slab reinforcement surrounding the column.

Unfortunately, direct comparisons between V and Vflex do not necessarily define whether a slab will deform significantly prior to punching.

Experience shows that it is incorrect to assume that satisfying V = VACI will result in markedly increasing deflections or rotations at a slabcolumn connection before punching occurs. The reason is shown in Figure 2, where the shear force-rotation responses per the Critical Shear Crack Theory (CSCT) of Muttoni (2008) are shown for three slabs having identical geometries and differing ρ values. Muttoni’s theory is probably the most accurate punching shear response predictor available.

Black solid circles represent punching failures per the CSCT failure criterion. Red empty circles signal full yielding of the slab based on a yield line analysis and assuming a line of contra-flexure in the slab at 0.22 times the distance between columns. The yield line capacity is that for a local failure mechanism centered on the column, and is less than the mechanism associated with full yielding of the slab reinforcement.

The slab with ρ = 0.3% was expected to reach Vflex at a strength substantially less than VACI. Punching failure in this case is driven by flexure, and the slab displayed considerable rotation before punching. The response of the slab with ρ = 0.9% shows that even though Vflex was expected to match VACI, punching occurred prematurely, in a brittle fashion, prior to developing the full local flexural capacity. This result highlights the inadequacy of defining a slab failure mode using a strength-based approach only; it implies that reaching Vflex is not enough by itself to prevent premature punching failure. The response of the slab with ρ = 1.8% shows that this slab is expected to punch at a load similar to VACI, and well below Vflex , with no ductility whatsoever prior to punching.

The most significant limitation of using the V/Vflex approach to separate shear and flexure-driven failures is that attention is concentrated on the load-resisting aspects of the slab response, and not on the associated deformations. In 1963, the primary structural design emphasis was on the accurate evaluation of strength, with little attention paid to deformations; let alone the fact that none of the slabs examined by Moe corresponded to the V/Vflex > 1.0 case, as noted by Widianto et al (2009). In fact, excluding footings, none of the test slabs considered by Moe had ρ < 1%.

The situation is even more serious for earthquake-resistant design.

Even though concentric punching is linked mainly to gravity loading conditions, the associated deformability issues can be invoked to attempt addressing those in the presence of lateral loads. Experiments have shown that the ductility under lateral loading increases as V/VACI decreases. Today, the basic concepts of seismic design, including the need for ductility and what ductility means, are widely understood and used.

For performance to be acceptable in medium and high seismic design categories (SDC), the flexural strength must be maintained through displacements that are several times those at yielding of the flexural tension reinforcement. For one-way action, there is always ductility when the flexural strength is achieved prior to the shear strength. Shear failure following development of the flexural strength is only likely if the flexural reinforcement undergoes rapid strain hardening. Even then, the deformations have increased sufficiently that adequate warning has STRUCTURE magazine

10.0 9.0 8.0 7.0 6.0

Vtest bo d

5.0

f c' 4.0 3.0 2.0 1.0 0.0

0

5

10

15

20

25

30

Figure 3: Observed size effect on RC two-way slab punching capacity.

been provided of the impending failure. Unfortunately, for two-way action in slabs, deformations do not start to develop rapidly once the flexural strength is reached at the slab-column interface, and the use of low reinforcement ratios in that region does not ensure ductility.

These observations suggest supplementing the ACI 318 equation with a design provision that explicitly addresses minimum deformability requirements for RC two-way slabs to delay premature concentric punching failure. One possible approach is a re-arrangement of Muttoni’s CSCT based on a target ratio of slab rotation at ultimate to slab rotation at first yield. Once the ductility shortage is identified, the most practical solution is the addition of shear reinforcement. However, designers should never use a punching shear strength greater than that for the development of a local yield line mechanism centered on the column. Guidance to evaluate Vflex for isolated slab-column connection tests and for slab systems is provided by the ACI-ASCE State-of-the-Art Report on punching of slabs (1974).

Size Effect Another key consideration for reliably predicting the shear capacity of two-way slabs is the so-called size effect. Figure 3 shows the effect of increasing the slab effective depth on the normalized punching capacity (4Vtest / VACI) for selected test results extracted from the ACI 445 Punching Shear Test Databank (Ospina et al. 2012). The shear capacity of two-way slabs decreases as the effective depth increases. Shear capacities less than 4√f 'c develop for d > 8 inches. The reduction in strength is substantial, especially if the slab is lightly reinforced. These observations suggest the effects of low reinforcement ratio and increasing slab depth are in large measure additive. Both are detrimental to the shear capacity of the slab.

Figure 3 shows that reasonable punching shear capacity estimates result for slabs with d > 8 inches if VACI is multiplied by 3/√d (with d in inches).▪

16

Carlos E. Ospina, Ph.D., P.E. is Senior Project Manager with BergerABAM Inc. in Houston. He is Co-Chair of ACI-ASCE Subcommittee 445C (Punching Shear) and Leader of the Task Group responsible for the development of the Collected Punching Shear Test Result Databank. He is a Fellow of ACI. He can be reached at carlos.ospina@abam.com.

Neil Hawkins, Ph.D., Dist. M. ASCE, Hon. M. ACI, is a Professor Emeritus of Civil and Environmental Engineering, University of Illinois and Affiliate Professor of Civil and Environmental Engineering, University of Washington. He may be reached at nmhawkin@illinois.edu.

January 2013

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Structural ForenSicS investigating structures and their components

T

he winter of 2010-2011 was particularly snowy in the Northeast.

Heavy snows resulted in nearly 500 problem roofs in the states of Connecticut, Massachusetts, New York and Rhode Island, of which 382 were full or partial collapses. This large number of roof problems led to questions raised by engineers and state building officials as to the adequacy of current building codes in relation to roof snow loads.

Specifically, were the 2010-2011 winter roof problems due mainly to roof components not as strong as envisioned by current codes, or were the 2010-2011 roof snow loads larger than those envisioned by building codes?

Weather data from multiple sources was used to estimate the 2010-11 ground snow loads.

Similarly, weather information – specifically snowfall, wind speed, wind direction and duration of wind storms – was used to simulate 2010-11 drift snow loads for various roof geometries at selected locations in southern New England.

Building performance databases from state officials in Connecticut and Massachusetts were gathered as well as case histories from structural engineering practitioners.

These case histories contained roof snow load measurements as well as descriptions of typical problem roofs. In turn, the measured roof loads were compared to requirements in the current American Society of Civil Engineers’ ASCE 7 load standard.

Snow Related Roof Collapse and Implications for Building Codes

By Michael O’Rourke, Ph.D., P.E. and Jennifer Wikoff

Figure 1: Estimated 2010-2011 peak ground snow load in parentheses overlaid on ASCE 7-10 map.

Ground Snow Loads Following Canadian practice, roof snow loading for structural design purposes in the U.S. is based upon the ground snow load. This approach is sensible given that historically there are many more available measurements of the ground snow loads than available measurements of roof snow loads. The ASCE 7-10 load standard has a map showing regions with what is intended to be the 50 year Mean Recurrence Interval (MRI) ground snow load (Pg)50.

Actual ground snow loads for the 2010-2011 winter were simulated using weather data for the region. Specifically, a combination of data from COOPerative (COOP) stations and Local Climatological Data (LCD) stations was used to estimate ground snow loads at 15 locations across the region. The 2010-11 ground snow loads were compared with the ASCE 7-10 map values in Figure 1. Table 1 shows that the ratio of 201011 winter loads to ASCE 7 mapped load ranged

Table 1: Comparison of COOP, LCD, and practitioner ground snow loads with corresponding values from ASCE 7-10.

Michael O’Rourke, Ph.D., P.E. is a Professor of Civil Engineering at Rensselaer. He has been chair of the ASCE 7 Rain and Snow Load subcommittee since 1997.

Michael may be reached at orourm@rpi.edu.

#

Station

Elevation [ft]

2010-2011 Pg [psf ]

ASCE 7-10 Pg [psf ]

2010 – 2011 ASCE 7-10

1

Albany, NY

280

20.3

40

51%

2

Ware, MA

475

26.6

35

76%

3

Worcester, MA

1003

22.3

50

45%

4

Boston, MA

19

27.0

40

68%

Jennifer Wikoff is a graduate of Rensselaer Polytechnic Institute.

Jennifer may be reached at wikofj@gmail.com.

5

Walpole, MA

150

29.6

40

74%

6

Norton, MA

105

32.2

35

92%

7

Middleboro, MA

141

21.8

30

73%

8

Providence, RI

52

15.4

30

51%

9

Woonsocket, RI

184

27.0

40

68%

10

Staffordville, CT

627

36.4

40

91%

11

Windsor Locks, CT

170

28.7

35

82%

12

Bridgeport, CT

8

24.9

30

83%

13

Islip, NY

82

15.0

30

50%

14

New York, NY

156

23.0

25

92%

CS

Portland, CT

180

25.0

30

83%

18 January 2013

Figure 3: Schematic of leeward roof step snow drift.

Figure 2: Measured roof snow load for heated and presumably heated structures overlaid on ASCE 7-10 design load map (Design load equal to the larger of Pf and minimum roof load).

from 45% to 92%, with an average of 71%.

The ground loads were closest to the ASCE 7 values in Connecticut, where the average ratio was 85%. The ground snow loads were less severe in MA, NY and RI where the average ratios were 71%, 64% and 59% respectively.

In no instance was the estimated simulated 2010-11 ground snow load larger than that prescribed by ASCE 7.

Return Period for 2010-2011 Winter

year MRI value. However, again due to differences between the mapped design values in ASCE 7 and individual site specific 50 year values, the 2010-2011 winter ground snow loads approached the mapped 50 year design values but did not exceed them.

Nominally Uniform Roof Snow Loads Roof collapses, due to nominally uniform snow loading, were an observed “apparent failure mechanism” during the 2010-11 winter in Southern New England. The flat roof snow load in ASCE 7-10, Pf , is a function of the ground snow load and three factors related to the building and its surroundings: Pf = 0.7CeCtIsPg

The question of the exact return period for the 2010-11 winter ground snow load is more difficult to answer. This is due in part to the fact that the ASCE 7 mapped values are different, and typically larger, than the corresponding site specific 50 year MRI ground load values within the particular map region. In addition, different sources list different values for the 50 year MRI ground snow load at various available sites.

Never-the-less, in terms of the maximum annual ground snow loads, the 2010-2011 winter was roughly a 25 year MRI event in Albany, NY, Boston, MA, and Providence, RI; a roughly 50 year MRI event in Bridgeport, CT, and a roughly 100 year MRI event in New York City. That is, as noted above, the 2010-2011 winter in the Northeast was indeed snowy with ground snow loads at a few locations larger than the site specific 50

Where: Ce is the exposure factor, Ct is the thermal factor, and Is is the importance factor. Herein the exposure and importance factors are taken to be 1.0. Furthermore, since someone was on the roof taking snow measurements, it is assumed that the roof slope is small and the flat roof design load is appropriate for comparison with measured roof loads.

A total of 33 case studies were available which provided roof snow load measurements. Twenty of the structures were heated (C t = 1.0) and four were unheated (Ct = 1.2).

For the remaining nine, the thermal condition was unknown.

A comparison was made between measured roof loads, primarily from practitioner case histories and the flat roof design load prescribed in ASCE 7-10. The comparison was made for three classes of structures: heated

Table 2: Measured roof load and estimated ground load for outlier heated structures.

#

Municipality

Measured Roof Load (psf )

Estimated Ground Snow Load (psf )

Roof Load Ground Load

16

Abington MA

35

24.85

141%

22

N. Scituate RI

35

20.65

169%

25

Chepachet RI

45

23.60

191%

27

Middletown CT

28

25.50

110%

STRUCTURE magazine

19

January 2013

and presumably heated structures, unheated and presumably unheated structures, and structures with an unknown thermal condition. For 16 of the 20 heated structures (80%) as shown in Figure 2, the ASCE 7-10 design load was larger than or only slightly below the measured roof load. For the remaining four heated case histories, the measured roof load was significantly larger (33% to 61% larger) than the ASCE 7 flat roof snow load.

These “outliers” were then compared to the estimated 2010-11 ground snow load. As shown in Table 2, the outlier roof load measurements were also significantly larger than the ground snow load. Note that for a heated building, absent drifting loads, sliding loads, and impounded water due to blocked drains, one does not expect the roof snow loads to be larger than the ground snow. Hence, assuming the measurements were made properly, the four roof snow measurements in Table 2 do not appear to represent nominally uniform roof snow loads. It is conceivable that they were taken near a parapet wall and include some drift loading. It is also conceivable that they were taken near a blocked roof drain.

The comparison for unheated and unknown thermal condition structures was similar.

Hence, if one discounts the roof load measurements which are inconsistent with flat roof snow loading (i.e. load measurements apparently include drifted snow load and/ or impounded water), the ASCE 7-10 procedures provide reasonable balanced roof snow loads in comparison to the 2010-2011 measurement.

Snow Drift Loads Roof profiles with irregular geometries create areas of aerodynamic shade. These areas often trap windblown snow, forming drifts. Snow drift loads have been a common root cause of roof structural performance problems in the past. Insurance records suggest that about 75% of past U.S. snow related roof failures were due to drifted snow. Roof snow drifts were also reported to be an apparent failure mechanism for a number of buildings during the 2010-2011 winter. continued on next page

Table 3: Weather data for drift surcharge simulation at LCD stations. * Average for times with wind speed ≥ 10 mph.

Wind Wind Simulated Wind duration speed* Vi Pg (psf ) Direction ti (hrs) (mph)

#

Station

1

Albany, NY

20.3

111

3

Worcester, MA

22.3

189

4

Boston, MA

27

66

8

Providence, RI

15.4

33

11

Windsor Locks, CT

28.7

117

12

Bridgeport, CT

24.9

72

13

Islip, NY

15

30

14

New York, NY

23

63

15.2

Snow drift loads in ASCE 7-10 are a function of ground snow load, Pg, and upwind fetch distance, lu. As sketched in Figure 3 (page 19) for the leeward roof step geometry, a triangular drift surcharge, placed atop the balanced or flat roof snow load, is prescribed.

The peak drift height, hd, in feet is given by: hd = 0.423√lu4√Pg + 10 – 1.5 Where: lu is the upwind fetch distance in feet and Pg is the design ground snow load in pounds per square foot (psf ). Again, for the roof step geometry the horizontal extent of the triangular surcharge, w, is taken as w = 4hd The peak drift surcharge load, at the roof step, Pd in psf is Pd + hdγg Where: γg is the snow density in ASCE 7 equation 7.7-1. Finally the total drift surcharge load, TS, in pounds per linear foot parallel to the roof step is TS = 1/2Pdw

Simulated Drift Loads There are three elements needed for roof snow drift formation; an area of aerodynamic shade (geometric irregularity) on the roof where the drift can form and grow, a source of “driftable” snow upwind of the geometric irregularity, and wind speed sufficient to cause transport

Available “Driftable” Snow Inches H2O

W

1.47

Load (psf ) 7.6

Dates where drifting occurred 1/9-1/15 1/19-1/23

which remains at the aerodynamic shade region) is taken to be 50% for the step roof geometry. Hence, the simulated drift surcharge is TS = 1/2Qt

Data needed to calculate roof snow drifts at each LCD station 1/9-1/14 are shown in Table 3. This includes 16.4 NW 2.22 11.5 1/21-1/27 the duration of wind with speed above the 10 mph threshold, ti, the 16.6 NW 0.73 3.8 1/21-1/24 average wind speed during ti, and the driftable snow available during 14.5 NW 0.36 1.9 1/21-1/24 ti. Also listed are the days when drifting occurred (driftable snow 1/9-1/13 available and wind speed greater 15.4 NW 2.97 15.4 1/21-1/27 than 10 mph), as well as the 20102011 peak ground snow load. In 1/11-1/13 14.7 NW 1.7 8.8 calculating the simulated snow 1/21-1/23 drifts, eight compass directions 16.2 NW 0.38 2 1/21-1/24 (N, NE, E, etc.) were considered.

1/12-1/13 Wind characteristics for the com12 W 1.27 6.6 1/21-1/24 pass direction with the largest snow flux, and hence largest simulated of “driftable” snow across the aerodynamic drift, are shown in Table 3. Note for the 2010shade region. In relation to driftable snow, 2011 winter, the peak ground snow load at a proposed set of weather conditions which the site typically occurred well after episodes preclude snow transport was used. Specifically, of drifting. That is, drifting at the LCD sites snow is considered driftable as long as none in question occurred in January 2011, while of the following apply: the peak ground snow load as well as the onset 1) Snowfall followed by rain, sleet, or of reported roof problems typically occurred freezing rain. in early to mid-February 2011.

2) Snowfall followed by temperatures As noted above, snow drift size is a function above 32° F. of the amount of available driftable snow and 3) More than 3 days since the last the ability of wind (speed and duration) to snowfall. transport the driftable snow. As shown in The size of a roof drift is related to the Table 3, during the 2010-2011 winter, some amount of snow (snow flux) flowing past the sites, such as Worcester, MA and Windsor aerodynamic shade region and the percentage Locks, CT, had comparatively large amounts of the snow flux which remains at the drift of both. Other sites, such as Islip, NY and accumulation (aerodynamic shade) region. Providence, RI, had comparatively small The snow flux, Q, having units of pounds of amounts of both. snow per hour per foot width perpendicular Table 4 presents a comparison of the Leeward to the wind direction is a function of wind roof step drifts. Specifically, the ratio of the speed above a threshold, V, (in mph): total surcharge for the 2010-2011 winter simulation to the corresponding ASCE 7 value is lu 1/2 3.8 Q = 0.00048V ( ) presented for each of the eight LCD stations 750 Herein, the threshold is taken to be 10 miles with upwind fetch distances ranging from 50 per hour (mph). If the wind speed varies over to 500 feet. Again, the simulated value is for time, the total transport, Q t in pounds per the worst wind direction, the one with the foot width (lbs/ft) is: largest resulting drift. Note that the simulated drift loads were significant at Worcester, MA Qt = ∑Qiti and the two Connecticut stations. Simulated Where: Qi is the hourly transport for wind drift loads were generally smaller in comvelocity, Vi, and ti is the duration in hours of parison to the ASCE 7 design values in New wind velocity Vi. York, Rhode Island and Boston, MA. Also, Based upon water flume studies and compar- the ratios generally decreased with increasing ison with full scale case histories, the trapping upwind fetch. There is only one instance, efficiency (percentage of transported snow Windsor Locks, CT with lu = 50 ft, where

STRUCTURE magazine

20

January 2013

Table 4: Ratio of simulated total surcharge to ASCE 7-10 drift surcharge for leeward roof step geometries.

Station

Leeward Roof Step Drift Loads

TSsimulation × 100% TSASCE

CONSTRUCTION CEMENT

lu (ft)

50

100

250

500

Albany, NY

67.1%

66.8%

47.0%

37.8%

Worcester, MA

83.0%

85.3%

96.2%

86.8%

Boston, MA

33.6%

34.1%

38.0%

40.0%

Providence, RI

21.7%

21.4%

15.0%

11.9%

Windsor Locks, CT

117.5%

83.6%

58.5%

46.7%

Bridgeport, CT

79.2%

56.0%

38.8%

30.8%

Islip, NY

22.6%

22.6%

23.1%

18.3%

New York, NY

30.4%

21.1%

14.6%

11.4%

What Went Wrong?

As shown above, the ground snow loads in 2010-11 were significant, but did not exceed those prescribed in ASCE 7. The same holds for roof snow loads including drift. So, why all the roof problems?

There are two general reasons for this “unexpected” poor roof performance. The first has to do with when modern building codes were adopted and which structures are designed per code. For example, Connecticut adopted its first state-wide building code in 1971. Of the problem roofs in the Connecticut database for which the construction date is known, 61% (107 of 175) were built prior to the 1970s.

Similarly, snow drift provisions in some older codes were arguably inadequate or non-existent. For example, modern drift provisions in which the load is related to the upwind fetch distance were first introduced into ASCE 7 in 1988. Finally, there are structures that are exempt from code provisions. For example, barns in New York State are exempt.

The second general reason for the “unexpected” poor roof performance is that significant loading reveals “hidden” structural defects. There is a long laundry list of such hidden defects. They include: a) initial design defects such as the absence of web stiffener plates at continuous girder-over-column

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connections, b) initial design/construction defects such as inadequate slope to drainage and resulting ponding loads, c) improper maintenance resulting in blocked roof drains, d) additional “unanticipated” dead loads due to post construction installation of solar panels, e) improper post construction structural modifications such as removal of inconveniently located column braces in metal buildings, f ) improper building additions resulting in a “new” unreinforced lower level roof, and g) material deterioration over time such as that resulting from wooden structural components exposed to water.▪

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Acknowledgements The project, upon which this article was based, was sponsored by the National Science Foundation and the Structural Engineering Institute of ASCE. The authors gratefully acknowledge this support. The authors would also like to thank the Connecticut Office of the State Building Inspector, the Massachusetts Emergency Management Agency and the New York State Division of Code Enforcement and Administrator for providing building performance databases. Finally, the authors would like to thank DiBlasi Associates, FMGlobal, Odeh Associates, Simpson Gumpertz and Heger, and Wiss Janney Elstner Associates for providing case histories and loss statistics. The overall study would truly not have been possible without this gracious assistance.

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the simulated drift load exceeded the ASCE value. Given the influences of the balanced snow load and dead load, as well as safety factors in structural design, it seems unlikely that a 18% overload for the drift surcharge would, of and by itself, result in roof performance problems.

FA S T ER

Building Blocks updates and information on structural materials

R

ecently, lightweight concrete has been implicated as the primary culprit in moisture-related failures of adhered flooring systems. Although fast-track construction techniques and government-mandated changes in flooring adhesives also have contributed to these problems, some critics suggest that the consequences of a finish floor failure outweigh the benefits that lightweight concrete can bring to a project. In light of this controversy, it is worth evaluating the advantages and disadvantages of lightweight concrete, considering not only its interactions with flooring systems but also how it affects building aspects such as steel tonnage, foundations, and slab fire ratings. Despite the moisture-related challenges that lightweight concrete poses, properly designed and constructed lightweight concrete floor slabs offer a number of efficiencies over normal-weight concrete slabs that project teams should consider.

Is Lightweight Concrete All Wet? The Advantages and Disadvantages of Lightweight Concrete in Building Construction

By David P. Martin, P.E., Alec S. Zimmer, P.E., Michael J. Bolduc, P.E. and Emily R. Hopps, P.E.

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

What is Lightweight Concrete?

Lightweight concrete is nothing new. It has been around in various forms for centuries. There are many types of lightweight concretes. For the purposes of this article, we are considering only structural lightweight concrete: a mixture of portland cement, water, fine (sand) aggregates, and expanded clay, shale, or slate coarse aggregates. While normal-weight concrete mixes typically weigh 145 to 155 pcf, lightweight concrete typically weighs 110 to 115 pcf. Structural lightweight concrete commonly has 28-day compression strengths comparable to normal-weight concretes.

The primary difference between normalweight concretes and lightweight concretes for structural applications is the coarse-aggregate material. Normal-weight aggregates are typically natural crushed stone, whereas lightweight aggregates are produced by heating clay, shale, or slate in a rotary kiln at temperatures on the order of 2,000°F. At these temperatures, the aggregates expand and develop a network of interconnected internal pores (Figure 1) ranging in size from 5 to 300 microns (Chua, 2009).

This internal pore network yields a lighter density than natural aggregates. Although concrete compressive strength generally is related to the compressive strength of the coarse aggregate, American Concrete Institute (ACI) 213R-03 reports that for typical building-slab compressive strengths – up to about 5,000 psi – “there is no reliable correlation between aggregate strength and concrete strength.”

22 January 2013

Figure 1: Micrograph of lightweight concrete with expanded shale aggregate and natural sand aggregate.

Note the porosity of the expanded shale aggregate.

Water in Lightweight-Concrete Mixtures Lightweight concrete is often implicated in moisture-related flooring failures because it often has a significantly higher water content than normalweight concrete. Unlike natural aggregates, which tend to become saturated with water only on their surfaces, lightweight aggregate pore networks absorb and store water within the aggregate particles, releasing it gradually over time.

To understand how water content affects concrete, we need to consider how the water reacts in the mix. ACI 304.2, Placing Concrete by Pumping Methods, considers two types of water in lightweight concrete: free water and absorbed water.

Free water influences the volume of the mix, the slump and workability of the mix, and the amount of water available for cement hydration reaction. Absorbed water is held in the pores of the lightweight aggregate. During mixing, some free water is converted to absorbed water, reducing the slump and the amount of water available for hydration. In addition, the pumping pressure drives additional free water into the porous lightweight aggregate, further reducing slump between the pump hopper and the point of discharge. To reduce the amount of mixing water absorbed by the lightweight aggregate, concrete suppliers pre-saturate the lightweight aggregates to fill the pore spaces prior to mixing. Concrete suppliers frequently use water-reducing admixtures to help reduce the total amount of mix water and, consequently, the amount of water that will potentially leave the slab over time.

In both normal-weight concrete and lightweight concrete, water that is not consumed in the hydration of the cement particles slowly evaporates through the exposed surfaces of the concrete which, as is later discussed, can create problems with floor finishes. Almost all concrete mixes contain more water than necessary for the cement hydration reaction, but the excess water facilitates placement and finishing. After the cement paste

has hardened, the hydration reaction continues, albeit at a slower pace, throughout the life of the concrete as the excess water evaporates. In lightweight aggregate, some absorbed pore water will be drawn out and contribute to more complete hydration of the cement in a layer around the aggregates, but there will still be significant amounts of absorbed water remaining in the pores which, will escape over time.

With the increasingly fast pace of construction, reducing the drying time – the time between the end of curing and when floor finishes can be installed – is often critical to the schedule. Elevated slabs on metal decks are susceptible to longer drying times because the water can only escape through the top surface of the slab (Figure 2). With the exception of some techniques that we discussed later in this article, relatively little can be done to the mixture to reduce drying time other than reducing the water in the mix. ACI 302.2R-96 reports that “there is no reason to include or exclude any concrete materials with the exception of the addition of silica fume [in place of some portland cement] in an attempt to reduce the needed drying time for a given water-to-cement ratio.” The report notes that replacing 5% to 10% of the portland cement with silica fume can decrease concrete drying time by several weeks.

However, this is often not enough time savings for fast-track construction.

Figure 2: Placing lightweight concrete on composite metal floor deck. The metal floor deck allows the concrete to dry only through its top surface.

Table 1: Slab thickness and fire rating.

Minimum Slab Thickness on 2 or 3 in. Steel Floor or Form Deck without Spray-Applied Fireproofing

Restrained Assembly Fire Rating

Lightweight Concrete (107-113 pcf )

Normal-weight Concrete (147-153 pcf )

1 hour

2⅝ in.

3½ in.

2 hours

3¼ in.

4½ in.

3 hours

43�16 in.

5¼ in.

Design with Lightweight Concrete

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Despite the moisture issues, lightweight concrete provides a number of benefits.

Lightweight concrete composite slabs are inherently more efficient than normal-weight composite slabs with nearly identical structural characteristics and design strengths.

Concrete slab-on-metal deck thicknesses are typically controlled by fire-rating requirements as opposed to strength requirements. Lightweight concrete is more fire resistant than normal-weight concrete due to its lower thermal conductivity and lower coefficient of thermal expansion. In accordance with ASTM E119, Standard Methods of Fire Tests of Building Construction and Materials, the American National Standards Institute (ANSI) and Underwriters Laboratories (UL) list minimum concrete thicknesses required for various fire ratings. Table 1 is summarized from ANSI/UL 263 Design No. J718. continued on next page

23

January 2013

Table 2: Material quantities.

Material

System A (Normal-Weight Concrete)

System B (Lightweight Concrete)

6.2 psf

4.77 psf

0.458 ft /sq ft

0.354 ft3/sq ft

68 psf

43 psf

0.09 studs/sq ft

0.12 studs/sq ft

Structural Steel Framing Concrete Volume

3

4,000 psi Composite Slab (including deck weight) Headed Stud Shear Connectors

Table 3: Cost (based on approximate unit costs).

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Material (and Unit Cost)

System A System B (Normal-Weight (Lightweight Concrete) Concrete)

Cost Ratio (A/B)

Structural Steel Framing (assume $4,000/ton)

$12.40/sq ft

$9.53/sq ft

1.30

4,000 psi Composite Slab (assume $105/CY NWC and $135/CY LWC)

$1.78/sq ft

$1.77/sq ft

1.01

Headed Stud Shear Connectors (assume $2/stud)

$0.17/sq ft

$0.23/sq ft

0.74

Total

$14.35/sq ft

$11.53/sq ft

1.24

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January 2013

For a two-hour fire-rated slab, the use of lightweight concrete results in approximately 38% concrete material savings over normal-weight concrete. Lighter slabs reduce the overall building mass, effective seismic loads, and foundation design loads, and may also reduce the required steel framing depth if deflection and vibration criteria are satisfied.

The thinner slab is not without drawbacks, but these factors rarely govern design in a typical commercial building. The thinner lightweight concrete slab has a reduced flexural capacity, but the capacity is still sufficient to support most commercial occupancy floor loads. ACI 318-08 requires a 0.75 reduction factor for lightweight concrete shear capacity. However, in composite slabs, the design is most often governed by the flexural strength. For most buildings, the diaphragm shear capacity reduction is offset by the many other benefits of the overall system.

Cost Implications of Lightweight Concrete Is lightweight concrete more expensive than normal weight? Yes and no. In order to accurately address this question, one must consider how concrete weight affects overall structural costs.

The material unit cost of lightweight concrete is typically higher than that of normal-weight concrete, but the unit cost usually is more than offset by the overall reduction in concrete volume and steel tonnage for the structural system.

Consider, for example, a typical multiuse composite structural steel building in the Boston, Massachusetts, area.

We assume a two-hour fire-rated floor assembly using 2-inch deep, 18-gauge composite metal deck with beams spaced to maximize allowable unshored “twospan condition” deck spans. Lighter wet-concrete loads on composite metal deck allow for longer deck spans between supports, and can effectively reduce the number of steel support beams and the steel framing tonnage. According to a Boston-area concrete supplier, there is typically no difference in cost for placement and finishing lightweight and normal-weight concrete, but the material unit cost of lightweight concrete ($135/ cu yd for lightweight concrete vs. $105/ cu yd for normal-weight concrete) is

slightly higher due to aggregate processing and shipping costs from nonlocal sources. In this study, we considered both cambered and non-cambered steel framing, but found little difference in quantities and cost between the framing systems. Our general design assumptions included a superimposed dead load of 20 psf, a live load of 100 psf, and maximum total and live-load deflections of L/240 and L/360, respectively. Tables 2 and 3 illustrate a comparison of material quantities and approximate costs for a typical bay (Figure 3, page 26).

While there is often a higher unit cost for lightweight concrete, there is significant structural steel tonnage and cost savings with using lightweight concrete slabs-on-metal deck. Compared with a normal-weight slab, lightweight slabs may also save inches of structural framing depth per story, which can result in a substantial savings in steel, foundations, and cladding costs for multistory buildings.

by ESCSI, it is difficult for a concrete slab (normal weight or lightweight) to reach the moisture levels currently required by many flooring material manufacturers and industry standards without months of favorable interior drying conditions.

Several elements contribute to the failure of flooring systems, whether the slab is composed of lightweight or normal-weight concrete. Topping the list is the change in flooring adhesives composition due to the now-government-regulated use of hazardous

materials and volatile organic compounds (VOCs). For example, the National Volatile Organic Compound Emission Standards for Architectural Coatings under Section 183 (e) of the Clean Air Act and the South Coast Air Quality Management District (SCAQMD) of California Rule 1168 require limitations on VOC content of floor coatings, concrete protective coatings, sealers, and stains. The most-common low- or no-VOC reformulation flooring adhesives are water-based acrylic emulsions, some of

Flooring Interactions with Lightweight Concrete

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Lightweight concrete aggregates absorb, retain, and release more moisture than normal-weight aggregates, so does that mean using normal-weight concrete in lieu of lightweight concrete will eliminate moisture-related flooring failures? No. Moisture-mitigation will likely be required for moisture-sensitive flooring systems regardless of whether the concrete is normal weight or lightweight. In a study conducted in 2000, Suprenant and Malisch reported that under controlled air, temperature, and relative humidity conditions, lightweight concrete took 183 days to reach a moisture vapor emission (MVER) of 3.0 lbs/ 1,000 sq ft/24 hr.

In a 1998 study performed under the same controlled conditions, Suprenant and Malisch reported that a normalweight slab of the same thickness took 46 days to achieve the same MVER. In 2007-2008, the Expanded Shale, Clay and Slate Institute (ESCSI) conducted a study in non-controlled environments (similar to those found on construction sites) and found that while lightweight concrete slabs did take longer than normal-weight slabs to dry, the difference in drying times was smaller than Suprenant and Malisch reported. So what does this all mean? As reported

25

January 2013

Figure 3: Example framing bays with normal-weight and lightweight concrete slabs on composite metal floor deck.

which are susceptible to re-emulsification when exposed to water and alkalinity in a concrete floor slab. Couple these moisture sensitive adhesives with construction schedule pressures to install the flooring as quickly as possible, and there is often not enough time to wait for the slab to adequately dry.

There are a number of strategies that can reduce the likelihood of a moisture-related flooring failure. One such technique is moist-curing slabs in lieu of using liquid film-forming curing compounds. Curing compounds trap moisture in the concrete to enable the curing process, but until they are removed (typically just prior to flooring installation), they prevent concrete from drying. Limited use of retardants, slag, and fly-ash content can also help reduce drying times. Also, when the slab is exposed to rain and humidity, the slab cannot dry adequately. Enclosing the building and activating the HVAC system is when the real slab drying begins. Lowering the ambient relative humidity (RH) and increasing the ambient temperature generally decreases the drying time of the concrete slab.

A Look to the Future Self-desiccating normal-weight concrete mixes are beginning to appear on the U.S. market. ACI defines self-desiccation as “the removal of free water by chemical reaction so as to leave insufficient water to cover the solid surfaces and cause a decrease in the relative humidity of the system; applied to an effect occurring in sealed concretes, mortars, and pastes.” In other words, the cement hydration reaction uses all the available free water to such an extent that not enough

water is left to cover the unhydrated particle surfaces or to maintain 100% relative humidity within the concrete. Fast-drying concrete mixes utilizing this concept of internal hydration have been used internationally for years, and even though they are gaining momentum in the U.S., there a few roadblocks including limited regional availability, lack of lightweight concrete mix options, relatively slow placement (up to approximately 85 cu yd/hr), and sticky trowel-finishing. Ready-mix suppliers are working on resolutions that could soon be available.

Waterproof flooring adhesives are becoming more prevalent. These adhesives offer limited surface preparation and resistance to moisture exposure and high pH levels. Priming is not typically required for waterproof adhesive application, but light surface grinding of the concrete slabs is necessary.

Conclusions Although lightweight concrete has the potential to cause problems with adhered flooring systems, normal-weight concrete floors can also influence moisture-related problems. Regardless of the concrete’s density, the project team needs to consider the risks of moisture-related failures and, if necessary, to evaluate potential mitigation strategies. The team needs to make realistic estimates for concrete drying time and assess the selected flooring system MVER requirements. Limiting the concrete’s water-to-cement ratio, incorporating silica fume, and limiting moisture-retaining pozzolans such as fly ash, will reduce drying time. Enclosing the building early (to limit

STRUCTURE magazine

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January 2013

environmental moisture sources) and conditioning the interior space will help dry the slab. Incorporating admixtures into the concrete that lock free moisture into the slab through a crystalline material or that promote self-desiccation may speed the installation of flooring systems, often with a cost on the order of $1 per square foot. Topically applied sealers that indefinitely keep the free water from dissipating through the top surface of the concrete have a history of good performance, but these can cost as much as $5 per square foot. Each of these strategies can add cost to the project.

The costs of a well-conceived and properly implemented moisture control system can be offset by savings in structural steel, foundations, and fireproofing. Careful planning by the entire project team – the owners, architects, contractors, and engineers – early in the design process can help reduce or eliminate the risks of moisture-related flooring failures on both lightweight and normal-weight concrete slabs.▪

David P. Martin, P.E. is a senior project manager at Simpson Gumpertz & Heger Inc. (SGH). As a member of the American Institute of Steel Construction (AISC), American Concrete Institute (ACI), American Society of Engineers (ASCE), and Structural Engineers Association of Massachusetts (SEAMass), David is active in building industry organizations.

Alec S. Zimmer, P.E. is a senior project manager at SGH. Alec is a member of AISC, ASCE, and SEAMass.

Michael J. Bolduc, P.E. is a senior project manager at SGH. Michael is a member of the Council of American Structural Engineers-Structural Engineering Institute’s BIM committee, ASCE, and AISC. Emily R. Hopps, P.E. is a senior staff engineer in the Building Technology group of SGH. Emily leads SGH’s Flooring Practice Group. She is an ICRI-certified slab moisture testing technician.

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InSIghtS new trends, new techniques and current industry issues

U

FC 4-010-01, DoD Minimum Antiterrorism Standards for Buildings, dated February 2012, outlines 21 standards that govern site planning and the design of structural, architectural, electrical and mechanical systems for Low and Very Low Levels of Protection. The current document was developed as an update to a previous version originally issued in October 2003 and modified

by Change 1, in January 2007. Though some of the revisions were incremental and provided additional clarification to existing standards, others were significant and represent a major change in approach. Implementation of the updated criteria is likely to result in levels of hardening or analysis that vary from those required by earlier editions.

The most obvious changes pertain to the standoff distances at which conventional construction may be used, the unobstructed space requirements, and the design of window and door systems.

Each of the adjustments comes with opportunities, but also potential pitfalls that could lead to unintended cost increases or criteria violations.

Updated Military Criteria for Antiterrorism Design

By Mark Gardner, P.E. and Spencer Quiel, Ph.D.

Mark Gardner, P.E. (mgardner@hce.com), is a managing engineer and Spencer Quiel, Ph.D. (squiel@hce.com), is a project engineer with Hinman Consulting Engineers, Inc. in Alexandria, Virginia.

Standoff Distances Standard 1 outlines the conventional construction standoff distances (CCSDs) that permit the structure and façade, other than glazing systems and supporting elements, to be designed without specific analysis for blast effects. In the 2007 version, CCSDs and minimum standoffs were based solely on the building category and level of protection for a corresponding explosive threat.

The 2012 version has overhauled this approach by specifying varying CCSDs for defined wall and roof construction types based on multiple construction parameters and limitations that were developed by a variety of dynamic calculations. As a result, this new version typically requires a larger CCSD for walls that are loadbearing versus non-load-bearing by allowing more damage to the latter.

This approach allows the designer to tailor a conventional construction type to the available standoff. The CCSD for heavier materials, such as reinforced concrete and masonry, are smaller when compared to the generic values in the 2007 criteria, thereby permitting the use of such construction without blast analysis when less standoff is available. However, implementation of the reduced CCSDs is limited because they are only applicable for the specified range of element parameters, including spacing, span, supported weight, boundary conditions, and material strength. For example, two-way flat slab roofs do not qualify because such boundary conditions are not included in

28 January 2013

the recognized set of parameters. Other common roof types, such as steel-framed with wide flange shapes, are also not included. Per the criteria, “Any construction type that does not fall within the specified parameters needs to be analyzed for blast loads due to the explosive threats at the appropriate standoff.” It is reasonable to expect that structural systems that are similar to or stronger than those specified in the criteria can meet the intent of the CCSD criteria. However, the use of these systems without submitting dynamic analysis calculations may leave the design team in a position of having not met the criteria as written.

The new version also states that all façade elements are assumed to conform to a pin-pin condition, which is not always the case. A cantilever condition, such as a wall below ribbon windows, is not considered. Therefore, if such elements are utilized, dynamic calculations must be performed to verify that they can resist the direct blast loads and the reaction from the window system, which can be represented as a point load at the end. This will generate the need for more analysis during design compared to the 2007 criteria, which had no restrictions based on the type of construction as long as the prescribed standoff distances were provided.

In addition, the unobstructed space now extends out to the closest applicable standoff distance for Explosive Weight II, which applies to parking and roadways within a controlled perimeter and to trash containers, but not less than the minimum standoff distance for a qualifying construction type. The 2007 criteria required 33 feet of unobstructed space regardless of construction type. This change greatly increases the required interaction with the site and landscape design in coordination with the blast protection and construction type required in Standard 1.

Windows and Doors Standard 10 outlines the design provisions for glazing systems, which are applicable even if the CCSD of the wall supporting or surrounding the window is met or exceeded, and also impose a tradeoff when the site design takes advantage of the reduced CCSDs for heavier construction types. Several significant changes were made to Standard 10 in the 2012 criteria (see Table).

The structural elements supporting windows and skylights can now be designed statically by simply accounting for their increased tributary area relative to the rest of the wall. This factor is multiplied to the moment and shear capacity of the conventional wall or roof element to determine the required capacity of the supporting element and its connections to the structure, including any load-transferring elements such as kickers.

Finally, the new version provides additional guidance for exterior doors, which must now

be tested to achieve the applicable level of protection in accordance with ASTM F2247.

Previously, the doors merely had to swing outward. This requirement will present challenges for door manufacturers; they may be required to test their products for the smaller CCSDs that are now allowed for heavier construction types. Glazed doors must also meet the glazing and bite provisions of Standard 10.

Conclusion In summary, the CCSDs have changed to allow threats closer to the building based on the exterior wall and roof types. The design team must carefully consider whether the chosen construction meets the parameters outlined in the new criteria, or if dynamic analysis will be required. Additionally, some of the parameters may have an adverse impact on project budgets.

For windows and doors in particular, smaller standoffs allowed under the new criteria will typically increase the cost relative to previous versions. There will be a period of time before vendors adjust to these changes during which the products they offer may be severely limited and expensive. If these constraints are known in advance, then the design team can make informed decisions early in the process and avoid unanticipated expenses to the building.▪

Comparison of Standard 10, v2007 and v2012.

Element

2007 UFC

2012 UFC

Glazing

Prescriptive Design w/ Table B-3

Design w/ ASTM E1300 and ASTM F2248 (based on explosive weight, standoff distance, and glazing size)

Framing

Design Loading per ASTM F2248 Design Loading per ASTM Deflection < L /160 F 2248 AND Deflection < L /160 2X Glazing Resistance per ASTM E 1300 Deflection < L /60

Connections

2X Design Loading per ASTM F 2248

Design Loading per ASTM F 2248 Deflection < L /160 AND 2X Glazing Resistance per ASTM E 1300 Deflection < L /60

Supporting Structure

8X Glazing Resistance per ASTM E1300

Increase capacity of elements relative to typical wall by ratio of tributary areas

Skylights

Same as Above

Glazing requires dynamic analysis

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STRUCTURE magazine

COLORS

JOB#

29

FILE NAME

January 2013 OK as is

FOUNDATIONS Slow and Steady Upswing for Foundation Business

By Larry Kahaner

D

espite being whipsawed by the ‘fiscal cliff’ negotiations, companies involved in the foundation sector report that business generally is up and growing, albeit at a measured pace. “Our customers tell us that there seems to be a slow but steady increase in work,” says Jim Hussin, Director, Hayward Baker, Inc. (www.haywardbaker.com), headquartered in Odenton, Maryland. “In our own company, we have seen a steady increase in business over the past two years and will perform a record volume of work in 2012.”

Hayward Baker is a contractor specializing in foundations and geotechnical construction. Their services include grouting, ground improvement, structural support and earth retention, all of which are offered as design-build services. One of their newer services is soil mixing which has gained popularity in the last few years, according to Hussin. He explains the process. “Soil mixing is a ground technique that improves the characteristics of weak soils by mechanically mixing them with a cementitious binder. The binder can be added as a slurry for dryer soft soils, or as a dry powder to very wet soft soils.

To construct columns, a powerful drill advances drill steel with radial mixing paddles located near the bottom of the drill string. The binder slurry is pumped through the drill steel to the tool as it advances and additional soil mixing is achieved as the tool is withdrawn. To perform mass soil mixing, or mass stabilization, a horizontal axis rotary mixing tool is located at the end of a track hoe arm. This technique has been used to strengthen soft soils at sites of planned buildings, storage tanks and embankments.”

Hussin adds that this soil mixing technique allows improvement of soft soils that were previously difficult to treat.

A

t Polyguard Products (www.polyguardproducts.com) of Ennis, Texas, CEO John Muncaster boasts of his company’s 20th straight year of sales growth. Polyguard does waterproofing and corrosion protection, and they’re eager for SEs to learn about their Underseal Underslab waterproofing membrane. “When you’re on a construction site and you’re about to pour a concrete slab, and you want to protect it from moisture or vapor or water, you want something that also will stand up to the

abuse of construction. Traditionally, the industry has been using poly films that become riddled with holes by the time the construction process is over. Our product not only waterproofs, which is vapor proofing plus waterproofing, but it has the ability to withstand the construction process better than anything out there,” says Muncaster.

“People talk about protecting the whole envelope from primarily moisture, but what they’ve been using underneath the slab is like Swiss cheese by the time all the equipment has rolled on it, all the welding has taken place, and people have stomped around. And, literally, contractors using poly film sometimes will punch holes in it to make the concrete slab dry faster… what I would like to emphasize is that our product is not just a vapor barrier but waterproof and damage resistant, too.” (See ad on page 32.)

B

rendan FitzPatrick, Director-North America at Geopier Foundation Company, Inc. (www.geopier.com) based in Mooresville, North Carolina, says that their latest technology innovation, Geopier Densipact, provides further cost savings by densifying loose sand with on-site or local sand aggregate to develop allowable bearing pressures upwards of 12 to 14 ksf. “The rapid densification of on-site soils combined with high bearing pressures affords considerable cost and time savings to project teams,” he adds.

Geopier prides itself on providing innovative technologies with a focus on reliable, cost-effective foundation support solutions that deliver value to the project team, says FitzPatrick. “Many engineers have experience with a traditional Geopier ‘drill and fill’ technology that has been used for decades. Many customers have also experienced the benefits of cost and time savings for their building foundations by using the displacement Rammed Aggregate Pier systems – Impact and Rampact–to reinforce loose saturated sands, or soft silt and clay, or contaminated soil where elimination of spoils generates additional cost savings to the owner.” He notes that these products, along with their new Denispact product, “provide an additional option to project teams for foundation and floor slab support, and expand our ability to serve our customers. For the right application, these additional tools provide further cost-effectiveness than other Geopier options that the design team may have previously considered.” (See ad on page 33.) continued on page 34

STRUCTURE magazine

31

January 2013

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A

nother company bringing ground improvement to customers is Subsurface Constructors, Inc. (www.subsurfaceconstructors.com), St. Louis, Missouri.

“We are a full-service geotechnical contractor. We are one of the very few companies to offer both the full range of deep foundations and earth retention, in addition to serving as a design-build contractor for vibro ground improvement solutions nationwide,” says Lyle Simonton, Director of Business Development. “Our ground improvement division, although no longer new, continues to grow its ability to be competitive in all industries and geographic locations.

We have designed and constructed vibro ground improvement for hundreds of structures of all sizes. We bring a significant amount of value to owners and developers who are seeking a lower-cost ground improvement alternative than the companies they’ve used previously.

In the past year, we have completed several ground improvement projects in the east and northeast for developers of multi-family residential and commercial facilities. Engineers and contractors are starting to realize that ground improvement for their projects do not have to be high-cost solutions.”

Simonton adds: “With some of the new equipment we’ve developed, we are becoming more mobile and even more competitive on projects that are a long way from our home office in St. Louis.”

G

ina Beim, Senior Consulting Engineer, Marketing at Pile Dynamics (www.pile.com) in Cleveland, Ohio, says that the electrical utility sector has been a growth area for their products, which includes testing and monitoring systems for all types of deep foundations.

“Two things have happened,” says Beim. “First, the sector is growing so there’s more construction. And second, the nature of the construction of these transmission lines is such that every so often a pole is supported by only one big foundation element: a monopile. It’s very important to test the quality and bearing capacity of this particular foundation element. In other cases, particularly in environmentally sensitive areas, this industry employs helical piles that up until recently had been a challenge to test (for capacity) by dynamic testing. Pile Dynamics has done some research and is now able to recommend

“Our customers tell us that there seems to be a slow but steady increase in work.” how to undertake dynamic testing for this type of pile, and that is stirring up interest on the part of this industry.”

She adds: “We have traditionally served the driven pile industry, the drilled shafts industry and the auger cast pile industry with instruments to assure quality of these types of piles. More recently, we have made certain recommendations in testing the capacity of helical piles so that they can be tested with the Pile Driving Analyzer.

That’s a relatively new development that we are quite excited about, because consultants that provide these services are embracing this new way of testing.”

Beim explains that in the past, the most often used method to evaluate the integrity of a drilled shaft was crosshole sonic logging, which is still by far the most widely-used method but it has some disadvantages. “Thermal integrity profiling is also a method of examining the quality of these drilled shafts; this process is better because it looks at the entire cross-section of the shaft. Crosshole sonic logging does not. Thermal integrity profiling evaluates the alignment of the reinforcement cage and the shape of the shaft, which crosshole sonic logging cannot do, and it’s a test that can be performed much sooner than crosshole sonic logging. With these advantages, people are excited about it. We are seeing more and more interest in our Thermal Integrity Profiler, which performs this new type of integrity test.”▪

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34

January 2013

Technology information and updates on the impact of technology on structural engineering

O

nce in a while, every engineer encounters a project that presents new opportunities for innovation and advancement. Almost seven years ago, it was a small chiller plant that tempted me to dabble with a new software called Revit, which was slowly making its way into our industry.

Today, Building Information Modeling (BIM) is a household name in the A/E community where interoperability between documentation, analysis, design, and fabrication models can be achieved.

But how close are we really from taking the architectural massing model from concept to fabrication in an efficient manner? I recently had the opportunity to address that question in the design of a 36-foot tall monumental exposed steel tower structure. Once the team decided to step away from traditional design methods and embraced the new technologies, the answers I found were surprising.

Working in a 3D Environment The basic concept is simple: Instead of independently working toward a 2D set of drawings, working models are developed simultaneously and teams can provide information to each other via software interoperability.

One of the immediate efficiencies of this model evolution is that each of the stakeholders involved is only responsible for modeling their respective area of expertise and can still use the software of their choice. Eventually, traditional 2D sheets are created, but they don’t dictate the order in which work is done. They are a byproduct of the model.

This is part of that paradigm shift we have been hearing about for some time. BIM is not the latest drafting tool; it’s a collaborative, data-rich design and visualization tool. It requires the model author to challenge the way software is being used, beyond its spatial benefits into a holistic understanding of the project and process. He or she must understand the sequencing, Level of Development (LOD) needed and modeling responsibilities required by each party. To achieve this shift, a collaborative effort is required early in the project: clearly define modeling responsibilities, establish a consistent sequence so everyone develops the same areas at the same time, and bring all those different file formats together.

Identifying a project team as soon as possible helps establish those responsibilities. Some of those relationships need to be established contractually.

For the tower structure, the steel fabricator was brought on board during the design phase while the structural model was being developed. Since most of the steel connections were exposed and required architectural input, the structural engineer was responsible for their design. But it was the steel

Challenging Traditional Design with BIM Tomas Amor, P.E.

Tomas Amor, P.E., MBA is a Senior Structural Engineer at Target Corporation in Minneapolis, MN. He chairs the National BIM for Masonry Structural Modeling Workgroup and has presented three times at Autodesk University. Tomas can be reached at tomas.amor@target.com.

36 January 2013

fabricator that modeled them. Identifying that relationship early allowed the design team to save time by not drawing lines in 2D details to communicate design intent. Clearly defining roles and responsibilities and setting up a protocol for file sharing prevented any duplication of work, which was a key objective for this project. Similar agreements were made with the architectural partners regarding the non-primary structural elements.

Interoperability and Modeling Practices When it comes to interoperability in the structural world, we have focused mainly on links between documentation and analysis models.

As engineers, we want reliable and trustworthy tools, and tend to discredit new software that is not perfect. However, is our waiting for software developers to perfect the link between documentation and analysis models preventing us from moving forward on other BIM-related progress?

The design team for the tower structure understood each program’s limitations and was able to get a number of software with very different functions to share information. The architects used Rhino first and SketchUp later for visualization.

The engineering team relied on Revit and RISA 3D for coordination, analysis and design. The steel fabricator worked in SolidWorks. All the files were linked through either direct bi-directional links or file exchange formats (IFC, DWF). Some models only shared spatial information, while others provided physical and analytical information.

Rhino and Revit There is no direct link between these two platforms. The architectural conceptual model was exported to a DWF format that could be linked into Revit, where the structural engineer modeled the primary structure. This structural file was exported so it could be used in both Rhino and SketchUp as a background. The models were adjusted and re-exported with ease several times during numerous design iterations.

Revit and RISA 3D Once the structure was defined, the analytical Revit model was adjusted to disregard miscellaneous steel, and was exported to RISA 3D using the bi-directional link available. As the architectural concept changed, several roundtrips took place to update the structure geometry in the analysis software. As that occurred, iterations of structural analysis and design were performed in RISA 3D and the most current member sizes were automatically pushed to Revit.

Revit and SolidWorks As the structural design was being finalized, the Revit file was shared with the steel detailer through

Interoperability between Revit and SolidWorks allows for the creation of complete construction documents without drawing any lines.

an IFC export. The geometry and member sizes were validated as the SolidWorks model was started. The fabricator exported that model to IFC, DWF, IGS, and DWG formats for the structural team to link into Revit once the preliminary connections were created.

Iterations in connection design occurred with model revisions and electronic markup tools.

Revit became a centralized location that was updated to reflect the latest architectural changes, which could then be shared with the structural design team and fabricator.

In essence, the architectural massing model evolved into the steel fabrication model. At that point, the project became a communication-driven effort with multiple iterations of model sharing, virtual walkthroughs, and PDF markups of model screenshots.

As important as these modeling and sequencing techniques are to drive efficiencies, communication was key to success. Setting expectations, verifying roles and responsibilities various times along the way, and sharing progress were paramount. We have all heard the old carpenter saying: “measure twice, cut once.” Well, BIM is no different: “talk twice, model once.”

Benefits The most important consideration of any innovative effort is to prove its validity and justify the risks taken with clear benefits. As

the team embarked on this venture, the hope was to achieve design efficiencies by challenging traditional workflows. But a number of unexpected benefits emerged.

The benefits of interoperability were exploited by the ability to effortlessly create construction documents. After the models were complete, plans and elevations were created, sections were cut, and sheets were built in Revit. Since all the physical information was available, 80 percent of the annotations required to finalize the drawings were smart tags that required no manual input. The rest were notes and weld symbols added manually. No lines were drawn, everything was modeled. Figure 1 shows the evolution of the model at a specific connection.

Constant information exchange allowed the design team to avoid duplication of elements, which yielded design efficiencies and cost savings. Design time was reduced considerably because no efforts were reproduced: the architects did not have to model a structure during their conceptual design phase, the engineer did not have to model or draw connections, and the fabricator did not have to start a model from scratch.

The project was designed in electronic format, making it entirely paperless. Working in models instead of sheets required no plots or prints.

Submittals were also reviewed and approved electronically, including the steel shop drawings that were generated from the SolidWorks file.

The open exchange of information resulted in a better coordinated project. Coordination is sometimes measured in the number of clashes reported during or after design, but in this case they were nonexistent.

Having accounted for all parts of the structure in the different models, accurate quantity take-offs and bills of materials were automatically created. In addition to providing the total tonnage of steel, the models yielded miscellaneous steel quantities, number and sizes of gusset plates, and number of bolts.

Conclusion In these days of economic uncertainty, it often seems like there is little time to focus on innovation. However, now more than ever, it’s important to exercise continuous improvement and find better and more efficient ways to do our work. There are many BIM tools available from different software developers, but the existence of these tools alone is not enough. It is our duty as engineers to find ways to utilize them in inventive ways to push our firms and our industry forward. What a great time to discover innovative ways to design our structures. What a great time to experiment with new technology and influence its development. What a great time to be an engineer.▪

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STRUCTURE magazine

37

January 2013

EnginEEr’s notEbook aids for the structural engineer’s toolbox

P

racticing structural engineers must make decisions on safety, cost and utility even when “hard information” is not available. “Bayes’ Rule” is a mathematical tool for using experience and judgment to calculate the probabilities that could guide these decisions. The engineer assembles data such as test results, develops a hypothesis relating the data to underlying causes, and uses Bayes’ Rule to calculate the probability that the hypothesis is correct.

Bayes’ Rule is from a paper by Thomas Bayes published posthumously and later affirmed and augmented by Pierre-Simon Laplace. Recent applications of Bayes’ Rule, coupled with computer technology, have revolutionized statistical, scientific, and medical analyses (Iverson 1984, Mcgrayne 2011). This article shows how to use Bayes’ Rule with a spreadsheet program, such as Microsoft® Excel, to evaluate quality control sampling, test reports, reliability of bridge girders under random loading, office management decisions, and the Monty Hall Problem.

Using Bayes’ Rule requires an understanding of probability and statistics, and advanced applications can be challenging. Most textbooks on finite mathematics (Goldstein et al 2007) include an introduction to probability and Bayes’ Rule.

There are also many Internet sources (Albert 2006). This article is at the introductory level.

A derivation of Bayes’ Rule is included as an Appendix to the online version of this article at www.STRUCTUREmag.org.

Bayes’ Rule offers advantages over conventional statistics. The engineer may: 1) use subjective judgment or experience, based on the availability of test data; 2) update an analysis when new information becomes available; and 3) automatically calculate the probabilities of false positives (false alarms) and false negatives (missed defects or other items). False positives may increase cost, while false negatives may be disastrous. A high probability of either alerts the engineer that the test process may be wanting.

Bayes’ Rule for the Practicing Structural Engineer

By James Lefter, P.E.

James Lefter, P.E. is a retired structural engineer with experience in industry, private practice, and federal service. He served as Director of Engineering for the Veterans Administration, Visiting Professor and Senior Lecturer in the Departments of Civil and Environmental Engineering at the University of Illinois and Virginia Tech, a Member of the ACI-318 Committee, and Project Manager for the Learning From Earthquakes Program at the Earthquake Engineering Research Institute (EERI). He can be reached at jlefter@comcast.net.

Probability and Statistics “Probability” is a “fair bet” in a world of uncertainty. From statistics, P(A) = N(A) / N ≤ 1, where N is the total number of outcomes of an event, N(A) is the number of outcomes leading to A, and P(A) is the fraction (probability) of the total number of events in which A occurs. Given S as a subset of N, and (!) as the factorial symbol, the number of ways in which S items can be chosen from N possibilities is N!/[S! (N-S)!].

38 January 2013

If a man will begin with certainties, he shall end in doubts; but if he will be content to begin with doubts he will end in certainties.

– Francis Bacon Low-strength concrete is a frequently encountered problem in construction. ACI 318 Section 5.6.2.1 (ACI 318- 08) requires at least one strength test for every 150 cubic yards (cy) of concrete placed each day, but not less than five strength tests for every class of concrete. If delivered in 5-cy capacity trucks, at least 30 trucks will be used to transport the concrete. If 10% of the concrete does not meet the ACI requirements, calculate the probability that the low-strength concrete truckloads would not be detected.

The number of ways of drawing five samples from 30 truckloads = 30!/(5! 25!) = 142,506.

Assuming three truckloads of low-strength concrete (10% of 30), the number of ways of drawing five samples without low strength concrete = 27!/(5! 22!) = 80,730. P(A) = the probability of not detecting the low strength concrete = 80,730/142,506 = 0.567.

Bayes’ Rule Analyses An “event” is the cause and its probability is called a “prior.” The engineer observes data T and uses Bayes’ Rule to calculate the probability of its relationship to a perceived cause. The probabilities of the observed effects, the data, are called “likelihoods” and may be based on experience, tests results, standard handbooks, or judgment.

A Bayes’ Rule analysis follows these general steps: 1) Propose a hypothesis P(E|T) relating priors and likelihoods.

2) List available information about the priors, as probabilities.

3) List the related likelihoods, as probabilities.

4) Multiply the priors by the likelihoods, using Bayes’ Rule to calculate the “posterior probabilities.”

The posterior probabilities represent the probability that the hypothesis P(E|T) is true. The process is shown in the example problems. If the engineer has no relevant prior knowledge, a value of P(E) = 0.5 is recommended.

Example 1 – Quality Control Sampling There are advanced applications of Bayes’ Rule in which the population data are the priors and the binomial distributions are the likelihoods. For this example, the HYPEGEOMDIST function in Microsoft Excel is more direct. In order, the

inputs are the number of “successes” in the sample, the size of the sample, the number of successes in the population, and the size of the population. In this case, a “success” is actually not detecting low-strength concrete.

HYPEGEOMDIST (0, 5, 3, 30) returns the correct value of 0.567.

Table for Example 2: Weld Inspection.

Event

Priors

Likelihoods

Product

Posterior Probability

Number of Welds

P(E|T)

E

0.1

0.8

0.08

0.31

4

P(E1|T)

E1

0.9

0.2

0.18

0.69

9

Hypothesis

P(T)=

0.26

Example 2 – Weld Inspection Report

P(E|T1)

E

0.1

0.2

0.02

0.03

1

The engineer’s experience indicated that 10% of welds inspected by the specified test method would have defects. Laboratory reports from a project indicated that 20% of the 50 welds inspected to date had defects (Jordaan 2005). The engineer used Bayes’ Rule to analyze the situation. E was that a flaw was present in a weld and T was that the flaw would be detected by the specified test process. The hypotheses P(E|T) was that genuine weld flaws would be detected.

Based on the engineer’s prior experience, P(E) = 0.1. The laboratory report indicated that P(T|E) = 0.8 and P(T1|E) = 0.2. From the Probability Sum Rule, P(E1) = 1- P(E) = 0.9, P(T|E1) = 0.2 and P(T1|E1) = 0.8.

Calculating the posterior probabilities by Bayes’ Rule equations is cumbersome. If the factors are set up using a BAYES BOX (see Appendix), the calculation is more direct and

P(E1|T1)

E1

0.9

0.8

0.72

0.97

36

P(T1)=

0.74

Welds

50

convenient. The resulting posterior probabilities are identical to those calculated by using the Bayes’ Rule equations.

To interpret the results: The engineer expected 10% of the welds to have flaws; i.e., of the 50 welds inspected to date, five would have flaws (positive test results). The laboratory indicated that 20% of the welds (10 of 50) had flaws. The laboratory tests did not calculate the probabilities of false positives and false negatives. The Bayes’ Rule analysis calculated P(E|T) = 0.31, the probability that welds that tested positive would actually have defects. Of the 50 welds examined, four welds (0.31 x 0.26 x 50 = 4.03) would have “true” defects. P(E1|T) = ADVERTISEMENT - For Advertiser Information, visit www.STRUCTUREmag.org

STRUCTURE magazine

39

January 2013

50 0.69, so about nine “defective” welds (0.69 x 0.26 x 50 = 8.97) would be false positives; that is, the test would have detected nine non-existing flaws. This was a statistical Type 1 error. Since P(E|T1) = 0.03, about one weld (0.03 x 0.74 x 50 = 1.11) had a flaw that the tests missed. This was a statistical Type 2 error. Finally, P(E1|T1) = 0.97, therefore the absence of flaws was correctly detected for about 36 of the welds (0.97 x 0.74 x 50 = 35.89).

The engineer would note that the combination of the probabilities of false positives and false negatives are indicators of the strictness of the testing system. Critical projects such as nuclear power plants need tight controls even

Table for Example 3: Bridge girder reliability.

Bayes Box P(E) = 0.95 P(E1) = 0.05 P(T|E) = 0.64 P(T|E1) = 0.36

Events

Priors

Likelihoods

Product

Posterior Probability

Number of Girders

P(E|T)

E

0.95

0.64

0.608

0.97

122

P(E1|T)

E1

0.05

0.36

0.018

0.03

4

P(E|T1)

E

0.95

0.36

0.342

0.91

68

P(E1|T1)

E1

0.05

0.64

0.032

0.09

6

Hypothesis

P(T)=

0.626

P(T1)=

0.374

Total Girders though they are costly in time and budget.

However, false negatives, flaws that were not detected in critical facilities, could be very hazardous.

If more test data becomes available, the posterior probabilities are used as priors in an updating analysis.

Example 3 – Bridge Girder Reliability (Monte Carlo Method)

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The Monte Carlo method is used to evaluate the reliability, R, of a bridge girder subjected to random simulated loads. The random loading is applied iteratively until R converges.

The number of iterations may range from hundreds to thousands. Microsoft Excel can generate over 30,000 iterations.

The bridge girder is a W36x135 spanning 60 feet, subject to a uniform dead load of 0.5 kips/foot and a concentrated live load at mid-span with a mean value of 40 kips.

The engineer calculated the girder flexural stress using 200 simulated load tests assuming a normal distribution with a standard deviation equal to the mean divided by the square root of the number of samples, but not less than 1/5 of the mean (8 kips in this case). If this stress was less than or equal to the allowable value of 24 ksi, then the girder was considered acceptable. The reliability

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200

factor R of the girders was calculated as the ratio of the number of acceptable girders to the total number of girders (200). (Elishakoff 1999 pp. 440-41). (Spreadsheet details are in the Appendix.)

The results indicated a reliability factor R of 0.64, which does not significantly vary upon increasing the number of iterations. The engineer was confident about the adequacy of the girders and assumed P(E) = 0.95 (priors) accordingly, then used the R values as likelihoods in the Bayes Box analysis. P(E|T) is the probability that a girder meeting or exceeding R would be accepted.

The reliability of the bridge girders under random loading increased from 0.64 to 0.97 for girders meeting both E and T requirements (about 122 of 200 girders). However, the engineer noted from P(E|T1) = 0.91 that about 68 of the girders were “false negatives,” indicating that the engineer may be overconfident about the test process. As always, the engineer ultimately decides on the acceptability of R.

by construction deficiencies, 0.1 by inadequate maintenance, and 0.1 by other causes.

Calculate the probability P(E|T) for each cause of failure.

The probability of failure due to Overload increased from 0.2 to 0.38.

Example 4 – Office Management 4A. Bridge Collapse Preliminary Assessment

4C. Monty Hall Problem

A bridge was being upgraded when it collapsed suddenly. A review of collapses of bridges of similar construction and vintage indicated the following probabilities of various causes: 0.2 by overload, 0.2 by design deficiencies, 0.3 by construction deficiencies, 0.1 by inadequate maintenance, and 0.2 by other causes.

However, the engineer noted that at the time of collapse, one side of the bridge was open to normal traffic while the other was loaded with construction equipment.

Accordingly, the engineer estimated a probability of 0.4 that the failure was due to an overload from the combination of construction loads and live loads. The other probabilities of failure were then assumed as 0.2 by design deficiencies, 0.2

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January 2013

4B. Project Assignment The supervising engineer considers both on-time performance and accuracy when assigning a project. Engineer A completed 70% of assigned projects on schedule, but there were major design changes required during construction on 40% of them.

Engineer B completed 40% of projects on schedule, and only 10% of them required major design changes during construction.

Assuming similar projects in scope and schedule duration, calculate the probability that each engineer would complete a project on schedule without major design changes.

Engineer A has a higher probability of completing a project on schedule and without major design changes.

Suppose you are a contestant on a TV game show and are given the choice of three doors.

Behind one of the doors is a prize, and behind the other two doors are goats. You pick a door. The host, who knows what is behind each door, then opens one of the other doors, showing a goat. You are allowed to change your door choice. Assuming you want the prize, do you change?

Using Bayes’ Rule, P(E) is the probability that the prize is behind your door = 1/3. P(E1) is the probability that the prize is behind one of the other two doors = 2/3. When the host opens a door that reveals a goat, the likelihood probability P(T|E) is the same that the prize is behind your door vs. the remaining door = 1/2.

Although it may seem counterintuitive, selecting the other remaining door, rather than the one you chose initially, actually

Example 4A: Bridge collapse.

Cause

Prior

Likelihood

Product

Posterior Probability

improves your probability of winning to 0.67. This example illustrates the broad applicability of Bayes’ Rule and the importance of using both priors and likelihoods for a complete analysis.

Design

0.2

0.2

0.04

0.19

Construction

0.3

0.2

0.06

0.29

Overload

0.2

0.4

0.08

0.38

Maintenance

0.1

0.1

0.01

0.05

Advanced Applications

Other

0.2

0.1

0.02

0.10

Sum

1

1

0.21

1

Many advanced applications of Bayes’ Rule were supported by National Science Foundation funding and are available. They include structural responses to random excitations, decision analysis in building codes, uncertainty in model analysis, reliability of damaged structures, and random loading, from wind and seismic forces or terrorist attacks. Many additional articles and computer programs are on the Internet.

Example 4B: Project assignments.

Engineer

Priors On-Time

Likelihood No-Change

Product

Posterior Probability

A

0.7

0.6

0.42

0.54

B

0.4

0.9

0.36

0.46

0.78

1

Sum Example 4C: Monty Hall problem.

Prior

Likelihood

Product

Posterior Probability

E

0.333

0.5

0.17

0.33

E1

0.667

0.5

0.33

0.67

0.5

1

Conclusions

Sum

Seattle Tacoma Lacey Portland Eugene Sacramento San Francisco Walnut Creek Los Angeles

Long Beach Pasadena Irvine San Diego Boise Phoenix St. Louis Chicago New York

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An engineer intuitively makes inferences based on available prior knowledge and related experimental data, while reserving the right to revise the inference based on new information. Bayes’s Rule is a vehicle for organizing this approach, a mathematical process for using experience and judgment to calculate the probabilities that could help guide engineering decisions. It tests both the “priors” and the “likelihoods”, while calculating the probabilities of an “E” and “T” hypothesis. In practice, the engineer would evaluate several sources of information, such as the qualifications of the testing laboratory, test records, etc., before making a critical decision.

The premise underlying Bayes’ Rule, as well as all education, is learning from experience and judgment. Bayes’ Rule reminds us that attempts to predict the future responsibly are possible only in terms of probability and that the highly improbable may occur. As always, the engineer is the one who ultimately decides.▪ The online version of this article conatains references and additional information on Bayes’ Rule. Please visit www.STRUCTUREmag.org.

SOUTHWEST WASHINGTON MEDICAL CENTER, VANCOUVER, WA

STRUCTURE magazine

41

January 2013

Great achievements

notable structural engineers

Ralph Modjeski

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

M

odjeski, (ne. Rudolphe Modrzejewska) was born in Cracow, Poland on January 27, 1861. His mother was an internationally known actress who encouraged him to become a concert pianist. But, at an early age, he determined he would become a civil engineer.

His family came to the United States to attend the Centennial Celebration in Philadelphia and start an orange farm near Anaheim, California.

His mother continued her acting career and Modjeski attended schools in the San Francisco area for a short time. In 1878, Ralph went to France to prepare for study at the École des Ponts et Chaussés, one of the leading schools of Civil Engineering in Europe. After failing admission on his first attempt, he was accepted and graduated in 1885 at the top of his class.

Shortly after, he returned to the United States.

Modjeski obtained a position with George S. Morison, a leading bridge engineer of the time (STRUCTURE February 2008) building a bridge over the Missouri River at Omaha, Nebraska. He completed his bridge training by inspecting the rolled steel at the mill.

He worked in the fabricating shop and then in the drafting room preparing shop drawings and plans. He worked with Morison on his Willamette, Nebraska City, Sioux City, Winona, and Cairo bridges. His last design project with Morison was the Memphis Bridge across the Mississippi River, which was the longest span cantilever in the country at the time. He also supervised construction of the bridge between 1891 and 1892.

Modjeski built his first bascule bridge across the Chicago River with Joseph B. Strauss as

his assistant. He then went into partnership for a short time with J. F. Nickerson, followed by his becoming Chief Engineer on a Bridge across the Mississippi River at Rock Island.

It was the fourth bridge at this site and was a seven span railroad and roadway bridge with a swing span over a set of locks.

In 1902, Modjeski went into partnership with Alfred Noble forming the firm of Noble and Modjeski with one of their largest projects being a cantilever across the Mississippi at Thebes, Illinois. After this bridge was finished, Ralph designed many bridges in the Portland, Oregon area (see Table). He replaced George Morison’s bridge across the Missouri River in Bismarck, North Dakota in 1905 with 400-foot spans, followed by a pair of swing bridges across the Columbia and Willamette Rivers in Portland, Oregon in 1908-1910. The Willamette swing bridge had a length of 521 feet, making it the longest span bridge at the time. In August 1908, he was appointed to a Board of Engineers, not a full time position, to determine the type of bridge to be built in replacement of the 1907 failed Quebec Bridge

by Theodore Cooper and the Phoenix Bridge Company. Modjeski, along with H. E. Vautelet and Maurice Fitzmaurice, were instructed to receive new tenders with the bridge companies submitting their design. Vautelet also prepared his own design which Fitzmaurice and Modjeski did not approve of. Fitzmaurice left the board and was replaced with Charles Macdonald (STRUCTURE, January 2009) formerly of the Delaware and Union Bridge Companies. After reviewing the 1910 proposals, Vautelet, Macdonald and Modjeski

Rock Island Bridge 1898 – Present.

STRUCTURE magazine

42

January 2013

could not agree on a specific recommendation, as Vautelet recommended one of the tenders on his own Ralph Modjeski design and Macdonald and Modjeski recommended a design by the St. Lawrence Bridge Company. Vautelet left the Board and was replaced by Lt. Col. Charles Monsarrat and Macdonald was replaced by C. C. Schnieder (STRUCTURE, January 2011).

It was these three engineers who oversaw the existing Quebec Bridge, then the longest span bridge in the world, that opened in September 1917. Throughout this assignment Modjeski continued to serve as consulting engineer to other clients.

Other bridges followed in 1910 with the McKinley Bridge across the Mississippi River at St. Louis, a long span of 519 feet. A partial list of other bridges by Modjeski is shown in the Table.

Ralph went into partnership with Walter Angier, under the name Modjeski and Angier, between 1912 and 1924 with several offices around the United States. Angiers worked with him since 1902 on the Thebes Bridge, and their eventual splitting evidently was not on good terms. He later partnered, in 1924, with Frank Masters, who had worked with him and Angiers between 1904 and 1914 on the Memphis and Louisville Bridges. Later Modjeski brought Clement E. Chase, who had worked under him on the Ben Franklin Suspension Bridge across the Delaware river at Philadelphia, into the partnership under the name of Modjeski, Masters and Chase. Montgomery Case came into the firm in 1933 when Chase died after a 120 foot fall from the Delaware Bridge. Modjeski mentored all these men and Joseph Strauss, just as he had been mentored by Morison and Noble.

As early as 1920, Modjeski was involved in planning for a bridge across the Delaware River at Philadelphia. In 1921, he was appointed Chief Engineer with Leon Moiseff as Engineer of design. The bridge was completed in 1926. With its 1,750-foot span, it was the longest suspension bridge in the world at the time. He broke this record in 1929 with the opening of the Ambassador Bridge over the Detroit River, which then became the longest span suspension bridge in the world with a span of 1,850 feet.

Memphis Bridge 1892 – Present.

One of his later projects was as President of the Board of Consultants for the San Francisco Oakland Bay Bridge. The design consisted of a long deck truss eastern approach followed by a major cantilever bridge followed in turn by a tunnel through the Yerba Buena Island and then by a double suspension bridge with a common middle anchorage. It was Bridge Name

being built at the same time as the nearby Golden Gate Bridge, but opened six months earlier on November 12, 1936.

His biographer, W. F. Durand, wrote, “In personal character Mr. Modjeski was inclined to be reserved rather than expansive and did not readily make close friendships. Nevertheless, he did take a generous and deep interest in his associates and in the members of the engineering profession broadly.”

An intimate personal friend of long standing, Ralph Budd, wrote to “understand him it must be appreciated that he inherited the temperament of an artist – not the artistic bias which is sometimes urged as the excuse for irrational behavior, but the delicate intuitive perception

River-Location

Type

Longest span

Illinois

Foundation Rehab

287 ft. swing

1906

Little Kanawaha

Truss

298 ft.

1907

Willamette Blvd

RR Tracks

Truss

91 ft.

1909

N. Lombard St.

Ravine

Truss

91 ft.

1909

Celilo

Columbia

Truss/ Girder/swing

320 ft.

1911

Terrebonne

Crooked

Steel Arch

460 ft.

1911

Peoria (built 1891) East St.

Cherry Street

Year

Maumee River

Conc. Arch, bascule

Broadway

Willamette

Bascule

278 ft. **

Keokuk

Mississippi

Truss

377 ft swing

1916

Harahan

Mississippi

Cantilever

770 ft.

1916

Omaha

1912 1913

Missouri

Truss

250 ft.

1916

Crooked River

2 hinged arch

340 ft.

1917

Ohio River

Truss

720 ft.

1917

ΩPoughkeepsie

Hudson River

Added truss

548 ft.

1917

†New London

Thames River

Bascule Span, 1 leaf

188 ft.

1919

ΩTanana River

Alaska

Truss

700 ft.

1922

Cincinnati

Ohio River

Truss

519 ft.

1922

Wenatchee, WA

Columbia

Cantilever

*Clark’s Ferry

Susquehanna

Concrete Arches

*Market Street

Susquehanna

Concrete arches

89 ft.

1928

ø*Tacony/Palmyra

Delaware

Steel Arch, bascule

260 ft. bascule

1929

St. Charles

Wabash

Cantilever

624

1929

Atchafalaya

Truss and Lift

Hudson

Suspension

1,500 ft.

1930

*Louisville/Jefferson

Ohio

Cantilever

820 ft.

1930

*Evansville

Ohio

Cantilever

720 ft.

1931

*Maysville

Ohio

Suspension

1,060 ft.

1931

Smithland

Cumberland

Truss

500 ft.

1931

Crooked River Metropolis

Melville +Mid-Hudson

Paducah

1925 1925

1929

Tennessee

1931

øHenry Avenue

Reading RR

Concrete Arch

øHenry Avenue

1932

Wisssahickon

Concrete Arch

288 ft.

1932

*Huey Long

Mississippi

Cantilever

790 ft.

1935

ø*Davenport

Mississippi

+ With Daniel Moran * With Frank Masters ø With Clement E. Chase

Suspension ∆ With C. H. Cartlidge Ω With Walter Angier ** Longest Bascule Span in the World when built

STRUCTURE magazine

43

January 2013

Ben Franklin Bridge 1926 – Present.

which insures balanced good taste and harmony in its outward expression, whether in music, art, architecture or engineering structures. In his professional work Mr. Modjeski always insisted on simplicity of treatment, with emphasis on function and purpose.” When he was awarded the John Fritz Medal by ASCE, Jay Kip Finch wrote, “Mr. Modjeski’s engineering designs are characterized by sincerity, which is the basis of true art. The gracefully sweeping lines of the Delaware Bridge, the Gothic treatment of the Poughkeepsie Suspension Bridge towers demonstrate the beauty which is inherent in steel construction, when freed from attempts at embellishment or concealment by means of masonry and concrete. His work will serve to lead others away from ill-considered attempts to adapt architectural tradition blindly to the treatment of steel structures without recognizing the fundamental artistic values arising from straightforward expression of the action of forces and the manner of their resistance.”

Finally, it was said of Modjeski that he had “to his credit more large bridges than any other man.” The New York Times in his obituary on June 28, 1940 hailed him as “the world’s leading bridge builder.” While that may not have been absolutely true, he was surely one of the world’s leading bridge builders over an extended period of time. He built the longest swing span, the longest suspension bridge (twice), the longest cantilever bridge and many early reinforced concrete bridges and long span simple trusses.▪ Dr. Griggs specializes in the restoration of historic bridges, having restored many 19th Century cast and wrought iron bridges.

He was formerly Director of Historic Bridge Programs for Clough, Harbour & Associates LLP in Albany, NY, and is now an independent Consulting Engineer. Dr. Griggs can be reached at fgriggs@nycap.rr.com.

LegaL PersPectives

discussion of legal issues of interest to structural engineers

Global Patented Innovation in Structural Engineering

By Stephen L. Keefe, P.E., Esq.

A

nalyzing the economic strength, patent systems, and structural engineering traditions of nations offers one way to evaluate major players in patented structural engineering innovation. In general, the leading nations for patented structural engineering innovation possess relatively strong economies, rich civil engineering traditions, and strong patent systems. The United States, Germany, Japan, South Korea, the United Kingdom, France, China, Italy, Canada, and Australia rank atop the list of patenting nations for civil and structural engineering innovation.

Figure 1: Top patenting nations for civil and structural engineering innovations.

Global Patent Law The global patent system strives to promote innovation worldwide by bridging between the national patent laws of countries around the world. The World Intellectual Property Organization (WIPO), operating under the auspices of the United Nations, administers the Patent Cooperation Treaty. Using the Patent Cooperation Treaty, WIPO shepherds the patent Applicant, including the structural engineer, through the competing patent laws of almost 200 nations.

Although WIPO and other transnational patent organizations (e.g., the European Patent Office) help to operate the international patent system, the world remains far removed from achieving the globalized dream of a world patent (or nightmare of a world patent, depending on your political stance on globalization). Currently, although WIPO and other international organizations may aid patent applicants in patent acquisition, inventors must ultimately obtain and enforce patents on a nation-by-nation basis. Nations award patents, and their court systems decide patent validity and infringement. For example, if an American structural engineer wants to enforce a patent right in the Ukraine, that U.S. citizen must obtain a Ukrainian patent from the Ukrainian Institute of Industrial Property, and enforce that patent against an accused infringer in Ukraine through the Ukrainian courts. This type of legal action is neither cheap nor certain in outcome.

However, depending on the innovation, it might be worth the trouble.

Because the global patent system ultimately distills down to national patent acquisition and enforcement, analyzing nations offers one way to identify innovative leaders and potential players in structural engineering patenting. The categories below reflect one attempt to group the major national players in structural engineering innovation.

One last preliminary note for structural engineers: Civil engineers, in general, patent much less than mechanical engineers, and vastly less than electrical engineers, according to WIPO statistics for total patent applications filed by field of technology. This fact holds true both within the United States and globally.

Although the patent system strives to promote innovation by affording legal protection that can often make inventing profitable, civil engineers simply do not patent much, relative to other engineering fields.

1st Tier United States, Germany, and Japan Large national economies with strong civil engineering traditions and good patent systems form the patenting top tier (Figures 1 and 2). These players include world powers that have topped civil and structural engineering innovation for decades. Also, not surprisingly, many patent commentators rank the American, European, and Japanese patent systems as the chief bodies of patent law in the world. The United States, Japan, and Germany also lead the world in international

STRUCTURE magazine

44

January 2013

filings under the Patent Cooperation Treaty through WIPO. Despite great advances in Asia, the United States remains the leading democratic economy, and with that, leads the world in civil and structural engineering innovation. The sheer size of the U.S. construction industry, the large number of American civil engineers (over 300,000, including environmental engineers, according to the U.S. Department of Labor), and numerous civil engineering university programs and professional organizations supply America with much potential to innovate. A two-century tradition that produced arguably the largest national infrastructure in the world, with railways and highways built by some of the most famous structural engineers in history, underpins a strong American civil engineering tradition. The vast size of the American market, supported by a large and relatively wealthy urban population, certainly also contributes to American leadership in advancing structural engineering.

The strong American patent system bolsters innovation in civil engineering. The United States Patent and Trademark Office leads the world in patent filings. A total of over two million U.S. patents remain in force, a half-million more than next-largest nation, Japan, according to WIPO estimates for 2010. Although the United States does not always lead in the total number of individual civil engineering patent applications filed worldwide, which includes large numbers of redundant filings of the same

back to at least the Meiji period following the overthrow of the Shoguns, when progressive factions rallied around the Emperor to deliberately replicate advances in the West. The Japanese government brought in top western civil engineers in the late 19th century to give advice on laying the foundation for Japan’s enduring civil engineering legacy. Today, Japan’s large economy and strong patent system globally project its civil engineering innovation.

If the 2011 tragedy temporarily knocks Japan out of the innovative 1st tier in the near future, then Japan’s large size, strong patenting tradition, and civil engineering legacy will likely ensure its subsequent return.

2nd through 4th Tiers Figure 2: Economic and patenting statistics for top civil and structural engineering innovators.

subsumed into the European Union’s patent institutions, the German patent system, particularly its court system, remains largely independent. So, Germany’s civil engineering strength, strong patent system, and strong economic market of 80-plus million people keep the Germans at the patenting forefront of civil and structural engineering.

Before its triple 2011 national tragedies of earthquake, tsunami, and nuclear disaster, Japan’s civil engineering patent resume read much like Germany’s. The 2011 Tohoku earthquake inflicted horrific loss of life and economic damage on the Japanese, and will certainly also strike a blow to the nation’s innovative contributions to civil engineering over the coming decade as it struggles to recover.

Japan’s rich civil engineering tradition goes

The 2nd tier nations, including South Korea, the United Kingdom, and France, all have attributes of the 1st tier nations, but on a smaller scale. These nations each possess strong civil engineering traditions that have yielded advanced national infrastructures. Like the United States and Germany, the United Kingdom and France root their civil engineering history back through the Industrial Revolution and into Medieval and Roman engineering achievements, while South Korea has leveraged its own traditions to play a successful game of catch-up with the West similar to the Japanese. The United Kingdom and France have slightly larger economies and richer civil and structural engineering legacies than the South Koreans. The South Koreans, though, surpass their European counterparts

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application in numerous countries, American inventors lead in the total number of distinct original civil engineering patent families filed worldwide. Accordingly, the strong tradition of American civil engineering, American economic power, and the strong U.S. patent system transform the over 300,000 American civil engineers active in the U.S. into the leading national powerhouse for patenting innovation in the field.

Although they trail the United States in overall patenting due to their smaller populations and economies, Germany and Japan likely exceed the U.S. in patented civil engineering innovations, pound-for-pound. Per capita, German civil engineers probably out-innovate their American colleagues, while Japanese civil engineers probably out-file Americans at patent offices. Americans, though, simply outnumber the Germans and Japanese by a large enough margin to make up for these shortcomings.

With about only one-quarter of the U.S. population, Germany files over half of the number of distinct civil engineering patent families as the United States. When looking at the total number of civil and structural engineering patent applications filed worldwide, though, that percentage falls to below half of American filings.

The world tends to view engineering as a German national strength. This probably explains Germany’s high number of civil engineering patent filings, relative to its population. Germany maintains a strong patent system, buttressed by the German Patent and Trademark Office, German courts well-versed in patent law, and a long patenting tradition rooted in Bismarck’s design of the German Empire and even before to legal rights granted by the medieval German princes. Though somewhat

South Korea, United Kingdom, France, China, Italy, Canada, and Australia

by more aggressively and successfully patenting their innovations in terms of overall numbers.

Ultimately, their smaller scale, rather than large qualitative differences, puts the South Koreans, British, and French into the 2nd tier of civil and structural engineering patenting.

As an entire civilization masquerading as a nation, China gets the whole 3rd tier. Though some historians argue that parity existed between Western, Indian, and Chinese civilizations around the 14th century AD (or CE if you prefer), the West accelerated beyond China and India, at least technologically, until it dominated the world by the 19th century.

Western militarism and civil war (e.g., imperial competition culminating in the World Wars) arguably drained the West, while exporting its advances around the world. Although some interpret the rise of China as the arrival of the next leading nation, it could be part of a larger historical shift, returning China to its historical role as a great civilization. China has had a few slow centuries, but the Middle Kingdom is coming back. Though today’s economic and patenting numbers still relegate it to the 3rd tier, expect China to roar into the 1st tier of civil engineering patenting soon, along with the 1st tier of many national areas. As one facet of civil engineering advance, China’s infrastructure currently advances at a tremendous rate. China today might mirror the United States at the beginning of the 20th century: a great power stepping out of the wings of history, and ready to send its own Great White Fleet around the world to prove it.

Italy, Canada, and Australia form the 4th tier, a sort of mezzanine below South Korea, the United Kingdom, and France for patenting civil engineering innovations. The Italians, Canadians, and Australians pursue slightly fewer civil engineering patent families and generally have smaller economies than the 2nd tier nations, and much smaller economies than China and the 1st tier nations. Although these nations have strong civil engineering institutions and traditions, and good patent systems, they possess them on a smaller scale than the higher-tiered countries. Canada and Australia have relatively small national populations compared to the above nations. Economic sluggishness, particularly in southern Italy, puts a drag on the Italians.

Therefore, Italian, Canadian, and Australian contributions to civil and structural engineering patenting rank behind the upper three tiers.

5th and 6th Tiers, and other Concentrations of Innovators The 5th tier includes the qualitative civil engineering strongholds of the Netherlands, Sweden, Austria, and Switzerland. These

smaller European nations have storied civil and structural engineering traditions, but lack the large populations and economies to make as much of an impact as the larger nations above. In view of their smaller size, though, the Dutch, Swedes, Austrians, and Swiss put up large numbers of civil and structural engineering patents, at least on a per capita basis.

Russia and Finland round out the big national contributors to civil engineering patented innovation, forming a 6th tier. Though arguably not having quite the strength in engineering traditions and patent systems as the higher-ranked nations, they still make noteworthy patented contributions to civil and structural engineering.

Numerous other smaller nations with solid patent systems and civil engineering establishments also make an impact on civil engineering patenting, albeit on an even smaller scale. The European Union (e.g., Denmark, Ireland, and Poland), Asia and the Pacific (e.g., Singapore, New Zealand, and Hong Kong), and South America (e.g., Chile and Peru) tend to have concentrations of these smaller innovators.

Nations Conspicuous by their Absence, and Potential Future Players India, at about 1.2 billion people, and Brazil, at about 200 million people, have large economies and enormous potential to advance in civil and structural engineering patenting. Indian and Brazilian national policies, though, tend to run counter to establishing robust patent systems at this time, and these nations currently lack concrete evidence of solid patent protection in general. India and Brazil chronically make the United States Trade Representative’s watch list for piracy–not a good thing for any intellectual property ranking. The same general assessment probably applies to Mexico and the Philippines, each with large but troubled 100+ million person economies. The Muslim powers of Indonesia and Pakistan, having large economies near 250 million and 200 million people respectively, lack meaningful intellectual property traditions and also headline piracy watch lists (although global intellectual property advocates both define and persuasively argue to criminalize piracy, some persuasive arguments justifying certain acts of so-called “piracy” also exist, particularly regarding software and business methods patents).

The final category includes relatively large nations that show promise for advancing their

STRUCTURE magazine

46

January 2013

economies, patent systems, and structural engineering foundations. Spain and the western-style nations of South Africa, Argentina, and Colombia currently lack significant civil engineering patenting, but have potential to shift toward greater patented innovation.

Muslim Turkey and Egypt, both large and often progressive nations, may embrace the secular side of their traditions and move toward greater patenting, including structural engineering. Two other relatively large nations, Vietnam and Ukraine, have been dabbling in patent law and may potentially put up larger future numbers of structural engineering patents.

Trends The top four tiers combined, including the United States, Germany, Japan, South Korea, the United Kingdom, France, China, Italy, Canada, and Australia, apparently file the vast majority of civil and structural engineering patent applications in the world.

So, seven large western nations, along with Japan, South Korea and China, will likely continue to drive patented innovation in civil and structural engineering in the coming years. Considering its enormous economic markets, strong patent system, and vast civil engineering establishment, the United States still currently possesses the greatest potential for structural engineering innovation among the top ten players.

American civil and structural engineers therefore have the opportunity to use the global patent system to protect and promote their innovations, and to lead the global civil engineering industry into the future.▪ Stephen L. Keefe, P.E., Esq. is a licensed professional engineer and a patent attorney with the Washington, D.C. law firm of Rabin & Berdo. If you have any questions or comments concerning the intersection of structural engineering and patent law, email Stephen at skeefe@keefepatentlaw.com.

DISCLAIMER: This article presents general information only and should not be regarded as legal advice. Accordingly, the author disclaims liability for any omissions or errors.

Readers should contact their own lawyer regarding their own specific legal questions, and should not take actions relying on the information presented in this article. This article does not establish an attorney-client relationship with the author or his firm.

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Web: www.deconusa.com Product: Anchor Channels Description: The exclusive representative of Jordahl in North America. Hot rolled Anchor Channels are embedded in concrete and used to securely transfer high loads. Their main application is for flexible connections of glazing panels to high-rise buildings. Anchor Channels with welded-on rebar or corner pieces are available.

Devco Software, Inc.

Web: www.devcosoftware.com Product: LGBEAMER v8 Pro Description: Analyze and design cold-formed cee, channel and zee sections. Uniform, concentrated, partial span and axial loads. Single and multi-member designs.

2007 NASPEC (2009 IBC) compliant. Pro-Tools include shearwalls, framed openings, X-braces, joists and rafters.

HALFEN USA

Web: www.HalfenUSA.com Product: Anchor Channels Description: Halfen anchor channels are cast into concrete and can support working loads up to 13,950 lbs per fastener. They are available in hot dip galvanized carbon steel or stainless steel in lengths from 1-¾ inches to 19 feet, 11 inches.

Hardy Frames, Inc.

Web: www.hardyframe.com Product: Hardy Frame Panels Description: Reinforced Concrete Anchorage solution to resolve concerns of concrete failure at hold downs from tension and shear loading. The new Hardy Frame details provide the EOR with all information needed to optimize the foundation design when specifying Hardy Frame Narrow Shear Wall Systems.

news and information from anchor companies

Hayward Baker Inc.

Web: www.HaywardBaker.com Product: Anchors Description: Permanent, temporary, and removable ground and rock anchors for support of excavations, permanent resistance of hydrostatic uplift forces on bottom slabs, and resistance of wind-induced uplift forces. Hayward Baker also provides the full range of geotechnical construction services.

Heckmann Building Products, Inc.

Web: www.heckmannanchors.com Product: Pos-I-Tie® Thermal Clip Description: Break-through in masonry construction adds thermal-break technology to all of the advantages of the Original Pos-I-Tie Veneer Anchoring System!

Easy snap-on design and proprietary plastic material offers a low thermal conductivity, over 100 times less than metals such as steel.

Hilti, Inc.

Web: www.us.hilti.com Product: Hilti HIT-HY 150 MAX-SD Description: Innovative HIT-HY 150 MAX-SD is the first and only fast cure adhesive with ICC-ES approval per AC 308 for cracked concrete. Download free Profis Anchor Design Software and BIM objects at our website.

Hohmann & Barnard

Web: www.h-b.com Product: Masonry Anchor Systems Description: Innovative design of anchor and reinforcement systems for masonry. The product line includes X-SEAL™, 2-SEAL™ and HB-213 Series Anchors as well as many restoration and moisture management systems.

Hubbell Power Systems – CHANCE

Web: www.abchance.com Product: Helical Anchor Description: A helical anchor/pile is a segmented deep foundation system with helical bearing plates welded to a central steel shaft. Load is transferred from the shaft to the soil through these bearing plates.

ITW Commercial Construction

Web: www.itwredhead.com Product: Sammys for Cracked Concrete Description: Self-tapping screw anchor for cracked concrete, seismic, and standard applications. Features an easy two-step installation process saving time and money. ICC-ES listed for all seismic zones, cracked & uncracked concrete.

Product: Epcon S7 Description: Hybrid fast cure epoxy for ALL environmental conditions! ICC-ES listed for all seismic zones, cracked & uncracked concrete.

Post-Tensioning Institute

Web: www.post-tensioning.org Product: Recommendations for Prestressed Rock and Soil Anchors Description: Technical guide providing practical guidance for the design, installation, and testing of grouted prestressed rock and soil anchors.

Powers Fasteners

Web: www.powers.com Product: Post-Installed Concrete Anchoring Description: IBC Listed/Compliant Anchors: Powers-Stud+ SD1/SD2 wedge anchors; PE1000+ /

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AC100+Gold adhesive anchors; Self Tapping Concrete Screws; Vertigo & Snake overhead anchors - Atomic Undercut anchors.

RISA Technologies

Web: www.risa.com Product: RISABase Description: RISABase uses an automated finite element solution to accurately provide exact bearing pressures, plate stresses, and anchor bolt pull out capacities, eliminating the guess work of hand methods.

Define bi-axial loads and eccentric column locations.

Choose from several connection types and specify custom bolt locations.

Simpson Strong-Tie

Web: www.strongtie.com Product: AT-XP Anchoring Adhesive Description: A fast cure, high-strength adhesive for anchoring and doweling in concrete. AT-XP cures quickly in warmer temperatures, and still dispenses easily and cures reliably in colder environments.

Rigorously tested in accordance with ICC-ES AC308 and IBC 2009 requirements, AT-XP performs in substrate temperatures as low as 14° F (-10°C).

Standards Design Group, Inc.

Web: www.standardsdesign.com Product: Wind Loads on Stuctures 4 Description: Software performs all the wind load computations in ASCE 7-98, 02 or 05, Section 6 and ASCE 7-10, Chapters 26-31, allows the user to “build” structures within the system (buildings, etc.). User can enter wind speed or use basic wind speeds from a builtin version of the wind speed maps.

Strand7 Pty Ltd

Web: www.strand7.com Product: Strand7 Description: An advanced FEA system used worldwide for a wide range of structural analysis applications. It comprises preprocessing, a complete set of solvers and post processing. It includes a range of material models suitable for the analysis of soil allowing for simulations of the complete soil/structure system.

USP Structural Connectors

Web: www.uspconnectors.com Product: USP Structural Connectors Description: The world’s leading manufacturer of code approved, structural connectors and innovative software solutions. USP’s 4000+ products are engineered, manufactured and tested to withstand Mother Nature, and are backed by our professional engineering and technical support teams and international sales.

Williams Form Engineering Corp.

Web: www.williamsform.com Product: Anchored Earth Retention Systems Description: Providing threaded steel bars and accessories for rock anchors, soil anchors, high capacity concrete anchors, micro piles, tie rods, tie backs, strand anchors, hollow bar anchors, post tensioning systems, and concrete forming hardware systems in the construction industry for over 85 years.

All Resource Guides and Updates for the 2013 Editorial Calendar are now available on the website, www.STRUCTUREmag.org. Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors.

Keep your professional license as mobile as you are.

To practice in multiple states, professional engineers need their licenses to be mobile.

NCEES records are recognized by licensing boards nationwide. Once established, your records can quickly and easily be transmitted to any state board to simplify and expedite your application for comity licensure.

You don’t have time for unnecessary paperwork.

Let NCEES keep track of your record so you can focus on what’s ahead.

ncees.org/records records@ncees.org 800–250–3196

award winners and outstanding projects

Spotlight

Robert Wood Johnson University Hospital Proton Therapy Vault | New Brunswick, NJ

By Michael Herrmann, P.E. O’Donnell & Naccarato, Inc. was an Outstanding Award winner for the Robert Wood Johnson University Hospital project in the 2012 NCSEA Annual Excellence in Structural Engineering awards program (Category – New Buildings under $10 Million).

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roton Therapy utilizes complex machines that deliver positively charged atomic particles focused precisely on small cancerous growths, without harming the surrounding healthy tissue. Embracing this promising technology, Robert Wood Johnson University Hospital has built a 4,900 square foot Proton Therapy building that houses two Proton Therapy treatment machines.

The three story, below grade concrete structure is located directly adjacent to an existing one-story medical office building, beneath its parking lot. Due to the proximity to the existing building, neighboring properties and a busy thoroughfare, the design and construction teams developed a strategy to accommodate a 40-foot deep excavation with no layback area. The team selected the soil nailing process as a means to effectively retain the earth while working from inside the building footprint. In an effort to provide an efficient exterior “basement wall” design, the team collaborated with the geotechnical engineer to integrate the soil nails into the final design, allowing for the exterior walls to be constructed as a two-way flat plate spanning between the soil nail anchor plates. The result is a cost effective exterior wall system that permits the 40-foot tall, unbraced concrete walls to be only 20 inches thick.

The team’s foundation design accounted for a water table located 8 feet below existing grade, resulting in 32 feet of hydrostatic uplift pressure at the base of the building. A series of rock anchors were strategically placed throughout the base of the building to overcome this massive uplift, and the team designed a 26-inch thick, two-way concrete hold down mat to span between the rock anchors.

The connection of the medical equipment to the concrete structure required large steel gantry embedments set into the concrete shielding walls. The steel boxes were fabricated out of 2-inch thick plates and measured approximately 11 x 5 x 4 feet. The equipment layout demanded that these embedment frames be suspended within the concrete formwork

approximately 12 feet off of the base slab, and set to a ¼-inch tolerance. To meet these parameters, the team designed a series of complex, braced steel frame supports that utilized leveling nuts in all four corners of the embedment frames to permit the precision setting.

The design team, in conjunction with the concrete subcontractor, carefully selected a concrete mix that balanced the use of fly ash required to reduce the heat of hydration with the need for accelerated set time to prevent form blow-out due to the 40 feet of concrete head. This allowed the monolithically placed 6-foot thick by 40-foot tall shielding walls to proceed in a timely and safe manner.

The team developed a creative construction sequence that would not require shoring and formwork to construct the 8-foot thick concrete lid over the 40-foot tall open vault. A series of steel beams set across the top of the vault support metal deck spanning between their bottom flanges. The lid was cast in three separate pours: the metal deck supported the first pour; the first pour supported the second; and the second pour supported the third.

In total, the lid effectively shields the medical equipment while providing support for four feet of backfill and the asphalt parking lot, along with the live loads associated with pedestrians and emergency vehicles.

A vertical shaft/areaway constructed adjacent to each vault permitted installation of the equipment after construction of the building. The team designed a concrete knock-out panel in the side wall of each vault that resists the hydrostatic and soil pressure once the shafts are backfilled, but is still easily removed for installation or replacement of the equipment. The bottom flanges of the lid support beams also doubled as supports for a complex series of rigging and crane beams to allow for the installation of the 60-ton machinery. In addition to allowing for a less expensive and faster construction of the concrete lid, this method also eliminated the need for costly secondary rigging.

The patient treatment areas presented design challenges set by the requirements of the

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Preparations for the 40-foot tall concrete vault pour.

specialized treatments and the operation of the equipment. The design team incorporated a notched cantilevered concrete slab into the floor design. This notch allows the equipment to penetrate the floor slab and easily rotate 180° from directly below to directly above the patient, providing full range treatments.

A hung catwalk system permits equipment maintenance within the treatment room without interfering with the motion of the rotating equipment and its counterweight assembly.

Adding to the complexity of the building design, many of the characteristics of the structure were governed by the requirements of the medical equipment, which was still in the final stages of being invented. Building Information Modeling (BIM) was a tremendous asset to the team, as some of these modifications were transmitted just prior to the pouring of concrete. Quick and thorough coordination by the design team was necessary to maintain the construction schedule.

Ultimately, the team delivered a successful project by combining out of the box thinking, a collaborative approach, and diligent coordination throughout the project. This was facilitated by a decisive and knowledgeable owner that had the foresight and understanding to involve the appropriate contractors in the design phase, and to encourage open communication with all involved parties.▪ Michael Herrmann, P.E. is a Senior Project Executive at O’Donnell & Naccarato, Inc. He can be reached at MHerrmann@o-n.com.

GINEERS

“The World Belongs to the Energetic.”

My love of music comes as no surprise to those who know me.

Of my many avocations, playing my trombone in a community concert band, a brass ensemble, church holiday orchestra, alumni band, etc. gives me great joy. The rehearsals are fun, the personal relationships are special, and to perform for others is priceless. Within each ensemble I belong, there is a common cycle that is inescapable: Introduce new music, rehearse, refine, perform, enjoy the afterglow, critique the performance, plan for the next concert, begin anew.

Another common theme within every ensemble is the need to introduce new players to the group. Through attrition, retirement, and people moving, there is always a need to introduce new talent into the group to fill a hole and make us complete.

We learned long ago to recruit from a variety of sources. We seek both experienced (‘seasoned’) musicians to fill a particular spot, and we seek young musicians for their enthusiasm (‘zest’) and to train and plan for future transition into leadership roles within the ensemble. What an inexperienced person may lack in immediate contribution to the ensemble, he or she can more than compensate for in future potential if given good training and the opportunity.

If you think this process sounds a lot like some of the leadership transition planning one hears about in numerous business transition seminars, you are correct! The common principles prevail, whether they are applied to your company of structural engineers or your company of musicians.

These common principles of leadership transition also apply to the 43 Member Organizations (MO’s) within NCSEA. While some of our MO’s might be fortunate enough to have secured ‘lifetime’ appointees in key positions, the reality is that our

Wind Series of Webinars This is part 1 in a webinar series to guide practicing engineers through the process of determining and applying wind loads on structures using the latest ASCE 7 standard and will cover the basic steps that an engineer must follow to calculate wind loads on any structure using the ASCE 7-10 wind load provisions, including brief examples.

February 12, 2013: ASCE 7-10: Components & Cladding Wind Load Provisions for Walls and Roofs, John Hutton & Michael Stenstrom

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ASCE 7-10 wind load provisions for components and cladding have been completely reformatted, but what has really changed? What are all of these methods and their limitations, and when can and should they be used? Learn ASCE 7-10’s answers to these questions and view worked examples of each method.

NCSEA

Diamond Reviewed

SPECIAL OFFER for the Wind Series! Buy four webinars, get the fifth FREE! These courses will award 1.5 hours of continuing education. Approved for CE credit in all 50 States through the NCSEA Diamond Review Program.

Time: 10:00 AM Pacific, 11:00 AM Mountain, 12:00 PM Central, 1:00 PM Eastern. Register at www.ncsea.com.

STRUCTURE magazine

MO’s are always seeking new leaders to emerge and assume some aspect of leadership. Each MO seeks new leaders to direct technical committees, respond to code changes, educate membership, advise on business practice issues, and assume many other key roles within the MO. I urge each of you to actively seek out potential leaders from within the younger members of your MO. NCSEA started a Young Members Group (YMG) Support Committee to encourage more of our MO’s to formally establish YMG’s. Member organizations with YMG’s directly benefit from increased membership and new energy and zest for the MO, as well as a direct source of future leaders in training. Young members benefit from taking on new and different roles, learning from one another, networking, and being given the opportunity to contribute to our profession. It is a win-win proposition.

I encourage each of you to talk about how an YMG could be started within your MO. Turn over the reins to a few of your best and brightest younger members and give them support and guidance to make it happen at the local level. In so doing, you will invigorate your MO and future leaders of your organization. I began this message with “The World Belongs to the Energetic,” a phrase made famous by Ralph Waldo Emerson that supremely speaks to my message. Look to www.ncsea.com for valuable resources in starting your YMG. Ben Nelson, P.E., SECB is the NCSEA President for 2013 and is a Principal at Martin/Martin, Inc. in Lakewood, Colorado.

He can be reached at bnelson@martinmartin.com.

February 26, 2013: Wind Loads for Signs, Other Structures, Roof-Top Structures & Equipment & Other Special Conditions, Tom DiBlasi & Bob Paullus

January 26, 2013: Basics of MWFRS & CC Loads, John O’Brien

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An overview of the reorganized wind provisions from Chapters 29 & 30 will be presented. Specific discussions will concentrate on the determination of the Main Wind Force-Resisting System for Solid Freestanding Walls and Signs, Solid Attached Signs, and Other Structures. Parts 5 and 6 of Chapter 30 will also be covered.

March 12, 2013: Wind Tunnel Applications for Buildings, Jim Swanson & Jon Galsworthy

Code-prescribed analytical procedures for calculation of wind loads may not accurately predict the response of very tall or irregular buildings to the actual load conditions they will experience. Wind tunnel testing is offered as an alternative to these procedures. A complete summary of the wind tunnel testing procedures will be presented from a user’s perspective, including code requirements, testing options, test reports, response thresholds, and adjustments for improved performance.

March 26, 2013: ASCE 7-10: Wind Loads on Non-Standard Building Configurations, Don Scott

ASCE 7-10 wind load provisions are written for “Regular-Shaped Buildings”; however, these provisions are applied every day to “NonRegular Buildings”. What techniques are applied to use the information in the standard for non-regular buildings? When can and should you use the ASCE 7-10 provisions or look for guidance in other standards?

January 2013

March 7- 8, 2013 • Westin La Paloma Resort • Tucson, Arizona

Key Financial Indicators for Leading your Firm to Success Scott Braley, FAIA FRSA, Braley Consulting & Training Much of our structural engineering work is about numbers and formulas. Most of us feel great satisfaction when it all adds up. That’s true of structural design, of projects – and of leading and managing a firm to sustained success. This interactive work session will explore and analyze those essential numbers – drivers, indicators, ratios, formulas – that mean success or failure for structural engineering firms today.

Establishing a Successful Structural Engineering Training Program Ben Nelson, PE, SECB, Martin/Martin Creating and maintaining a successful training program can be time-consuming and challenging.

Learn about key components of successful training programs around the country, and develop a program for your firm that will provide optimal training opportunities for your employees.

WEBINARS January 8, 2013

Structural General Notes & Specifications: Integration Issues & Recommendations Greg Markling This webinar will focus on the necessity of general notes and their content. It will also describe typical structural engineering work results by identifying typical Sections in the Divisions of MasterFormat® as well as examples of essential specifications content. The session will examine how these two components can work together in an integrated way to avoid duplication and redundancy, which can result in costly conflicts in the construction documents.

January 17, 2013

How To Repair Cracks – A Continuous Opportunity Kim Basham Do all cracks require repair? The answer to this depends on the type of crack, exposure conditions and expectations of the owner, architect and engineer. In this webinar, you’ll learn how to select the best crack repair option for different types of cracks. Learn about different crack repair techniques including: sealing, routing and sealing, stitching, epoxy injection, gravity filling, and grouting. Also, learn how to design a crack repair including specifying materials and procedures.

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8 Actions to Get People Goal-Directed, Self-Motivated, and Engaged in the Relentless Pursuit of Excellence Jon Stigliano, Strategic Solutions Group As leaders, we are responsible for attracting the right kind of people and creating a culture that will foster the behaviors that support our goals. Become familiar with specific actions that will motivate people to align their attitudes and behavior and to develop the skills required to achieve measurable results.

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Hard Choices in a Soft Economy: 4 Mistakes you Can’t Afford to Make Kelly Riggs, Vmax Performance Group Companies face fewer opportunities, fierce competition, the need to do more with less, the influx of the “Millennial” generation, and a general lack of workplace commitment and personal accountability. Leaders are faced with hard choices – how to sustain or increase market share, and even how to survive. Companies cannot rely on technical competence alone for success. This session will present four essential leadership skills to create stability, unlock innovation, and provide the potential to deal with challenges.

News from the National Council of Structural Engineers Associations

Developing the Next Generation of Structural Engineers Glenn Bell, senior principal & CEO, Simpson Gumpertz & Heger The future promises unprecedented opportunity and challenge for structural engineers, but new demands require more training, less specialization, greater competency, and a sharpened focus on globalization.

The entire field needs to collaborate and deliver forward-looking programs to ensure that tomorrow’s structural engineers are globally adept and committed to a lifetime of learning.

Top 10 Keys to Managing Multiple Deadlines & Expectations Jon Stigliano, Strategic Solutions Group Today’s environment makes it important to focus on items that get results. People and situations pull at us from all directions, and we do things in order of preference instead of priority. Learn how to focus on specific concepts and techniques to maximize your time and results, both personally and professionally.

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Coaching for Leadership: Transforming Potential into Performance Kelly Riggs, Vmax Performance Group Research clearly connects leadership skills to employee engagement and employee engagement to productivity, profitability, absenteeism, workplace safety, and customer satisfaction. Learn the critical skills that can transform employee potential into workplace performance. Learn the three keys to powerful coaching – clear expectations, effective feedback, and support.

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The NCSEA Winter Leadership Forum will gather together leading structural engineers in an energetic and engaging environment focused on leadership, networking and game-changing strategies for the new economy.

Designed to attract engineers at the top, and those on their way to the top, the Winter Leadership Forum offers two days of invaluable sessions, roundtables, and networking time. This event will explore the mindset and skills necessary to be a transformative leader, as well as how to best align your role with your company’s needs. Register online at www.ncsea.com.

NCSEA News

NEW!! NCSEA Winter Leadership Forum

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The Newsletter of the Structural Engineering Institute of ASCE

Structural Columns

Structures 2013 Congress Registration Now Open Make plans to attend the Structures 2013 Congress in Pittsburgh, PA, May 2-4, 2013. The focus of this highly regarded specialty conference is Bridging Your Passion with Your Profession.

The ASCE/SEI Structures Congress is your annual opportunity to broaden your technical knowledge, sharpen your business skills, deepen your understanding of cutting-edge research, and network with your peers and colleagues. There will be eleven technical tracks covering a wide range of structural engineering topics including; buildings, bridges, blast, innovative engineering, seismic, reinforced concrete, and business practice.

For more information or to register, visit the congress website at www.structurescongress.org.

Featured Technical Sessions Include Soft Skills for the Young Engineer Janel Miller, MSCE, Carnegie Mellon University Brian Krul, P.E., PTOE, LR Kimball, Pittsburgh Kyle Twitchell, P.E., M. ASCE, Robert Silman Associates, New York Matt Pierce, P.E., M. ASCE, Gannett Fleming, Inc., Pittsburgh To be successful in their careers, young engineers need to focus on adding “soft skills” to their repertoire. Join a diverse group of presenters on topics such as: writing, leadership development, public speaking, and project management. These techniques will complement their solid engineering education and give them an edge in the workplace. An increasing emphasis on being an effective communicator make this session a must-attend for young professionals.

Structural Level of Development (LOD) in BIM, Guidelines for the Practice David J. Odeh, P.E., SECB, Odeh Engineers, Inc. Will F. Ikerd II, P.E., CWI, LEED AP, IKERD Consulting – Structural Engineers R. Wayne Muir, P.E., Structural Consultants Inc. In recent years, there has been considerable interest in Building Information Modeling sessions among Structures Congress attendees. This session will include an in-depth discussion of the Level of Development (LOD) of structural models at the different stages in design and construction. The discussion will consider the new AIA BIM Protocol Exhibit E202, and the newly issued LOD BIM which can be used to define team expectations of what should be modeled. Structural modeling is used by architects, mechanical engineers, construction managers, sub-contractors and fabricators; the presentation will discuss how each of these stakeholders use these models differently and need different content. Learn from practical examples of model LOD detail issues and discuss how to move your firm forward by embracing the benefits of BIM. Three Technical Activities Division Mini-Tracks

For more information on these sessions and to see the complete matrix of Technical Sessions, visit the Congress website: http://content.asce.org/conferences/structures2013/.

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These three tracks on Special Design Issues, Concrete and Masonry Structures, and Dynamic Effects were developed to give Structures Congress attendees the opportunity to explore the advancements created by these three committee groups through their cohesive 3-4 session mini-tracks. One of the exciting sessions offered in these tracks include Green Giants: Tall and Sustainable, moderated by Sarah Vaughan-Cook, P.E., S.E., and Peter Lee, S.E., SECB, LEED® AP. This session bridges the work between the Tall Buildings and the Sustainability Technical Committees. Unconventional Reinforced Concrete Columns will be presented by the Joint ACI-SEI Concrete and Masonry Structures Technical committees and will include presentations on current practice on the design, detailing, and performance issues related to unconventional concrete columns.

January 2013

Joplin, Missouri, Tornado of May 22, 2011: Structural Damage Survey and Case for Tornado-Resilient Building Codes presents the observations, findings, and recommendations of an engineering reconnaissance team, sponsored by ASCE and SEI, which surveyed residential structures and schools in the tornado path shortly after the event.

The tornado, which was rated a 5 on the Enhanced Fujita Scale, cut a seven-mile swath through Joplin, Missouri; it destroyed more than 5,000 buildings and killed more than 150 people. The team’s data collection focused on recording the mechanisms of structural failure under tornado wind loads. One important finding is that failures could be attributed to inadequate load paths as well as to wind speeds that exceeded building code design levels.

Compliance with the building code requirements for hurricaneprone areas would have mitigated some of the damage.

Topics include: • overview of the Joplin tornado; • inspection methodology; • performance of residential buildings; • case studies of building performance; • performance of commercial buildings and critical facilities; • new design philosophy regarding tornadoes; and • conclusions and recommendations.

This ground-breaking report is must-reading for structural engineers, construction professionals, building code officials, and risk managers working in tornado-prone regions of the United States.

To order the Joplin report or to see the complete selection of structural books, visit the ASCE Bookstore website: www.asce.org/bookstore.

New Make Your Mark Poster Now Available 2012 Summer Olympic Stadium, London by Buro Happold Photo credit: Flickr

on the night sky Structural engineers design the buildings where we live, work, go to school, and play, and the bridges we cross every day.

And more.

To learn how you can make your mark, visit www.ncsea.com or www.asce.org/SEI

East Central Florida Chapter This past September, the East Central Florida SEI Chapter took a field trip to the construction site of the Dr. Phillips Performing Arts Center in Downtown Orlando. Ben Manning and Joe Brown of Balfour Beatty Construction presented a history and overview of the project, which will bring a new cultural complex to the heart of downtown, across from City Hall. In the meeting room was a scale model of the structure cut in section to show the amenities in the three planned theaters (two are currently under construction, the third will be added as part of a later expansion). After the presentation, the group was able to tour the project site, seeing major structural elements of the building’s grand stairway, steel-supported and glass-clad facade, cantilevered high roof truss, and the Disney (main) Theatre.

On October 19, 2012, the St. Louis SEI Chapter held an all-day event with vendor hall and single track session devoted to extreme events, including seismic and wind. St. Louis was honored to have among these speakers, Sam Rihani, SEI President who spoke to the attendees about SEI’s structure, SEI activities and benefits, as well as the need for SE Licensure. In addition, the group of over 80 attendees learned about the Joplin Tornado, damage to the St. Louis Lambert International Airport, the Christchurch New Zealand earthquake, and a variety of topics concerning retrofit and repair for extreme loading scenarios. Attendees and vendors enjoyed a great program and great food at the Masonry Institute of St. Louis.

To get involved with the events and activities of your local SEI Chapter or Structural Technical Group (STG) http://content.seinstitute.org/committees/local.html.

Local groups offer a variety of opportunities for professional development, student and community outreach, mentoring, scholarships, networking, and technical tours.

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

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The Newsletter of the Structural Engineering Institute of ASCE

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SEI and the National Council of Structural Engineers Associations (NCSEA) have produced a new poster to promote structural engineering careers to students. We invite you to include this exciting poster in your outreach efforts to encourage students to pursue a career in structural engineering. This “Make Your Mark” poster features the 2012 Summer Olympics Stadium in London. Limited supplies of the complimentary poster are available upon request to Suzanne Fisher at sfisher@asce.org. The poster is also available for download from the SEI Website: www.asce.org/SEI. For more resources and ideas for outreach with young students visit the SEI Kids page: http://content.seinstitute.org/SEIKidsPage.html.

Structural Columns

New Joplin Tornado Response Team Report Now Available

CASE GUIDELINE – 962-G

CASE in Point

The Newsletter of the Council of American Structural Engineers

Guidelines for Performing Project Specific Peer Reviews on Structural Projects Increasing complexity of structural design and code requirements, compressed schedules and financial pressures are among many factors that have prompted the greater frequency of peer review of structural engineering projects. The peer review of a project by a qualified third party is intended to result in an improved project with less risk to all parties involved, including the engineer, owner, and contractor.

In addition to the different types or focuses, peer reviews can be performed voluntarily by the engineer of record on a quasi-voluntary basis sponsored by the Owner/Client, or on a non-voluntary basis by the jurisdiction that is responsible for approving the project.

While it is true that the different types of peer reviews contain common elements, each has unique elements specific to the

central theme or focus. These guidelines are intended to address only technical peer reviews, which are hereafter referred to as project specific peer review (PSPR).

Many aspects of the peer review process are important to establish prior to the start of the review in order to ensure that the desired outcome is achieved. These items include the specific goals, scope and effort, the required documentation, the qualifications and independence of the peer reviewer, the process for the resolution of differences, the schedule and the fee.

The intention of these guidelines is to increase awareness of such issues, assist in establishing a framework for the review and improve the process for all interested parties.

You can purchase all CASE products at www.booksforengineers.com.

CASE is on LinkedIn LinkedIn is a great virtual resource for networking, education, and now, connecting with CASE. Join the CASE LinkedIn Group today! www.linkedin.com.

Coalitions Corner CASE is a member of ACEC’s Coalitions, groups that provide a home for communities of practice across the engineering industry.

Coalitions Joint Newsletter Available Coalitions Quarterly is the joint newsletter of the Coalitions and replaces individual Coalition newsletters. Inside, we bring together the insight, wisdom, and challenges of each of the Coalitions. Coalition members have a lot to learn from members across size and discipline spectrums. Whether you are a small or large firm, mechanical or structural, best business practices apply across the industry. This newsletter links our communities – communities of practice dedicated to smart, ethical business. Coalitions Quarterly is also a great outlet to cross-promote education and networking opportunities and highlight hot topics in the business of engineering.

Read the Fall issue of Coalitions Quarterly at www.acec.org/coalitions.

Join ACEC Coalitions on Twitter Twitter is a great way for ACEC’s Coalition to disseminate information, start conversations, and let the world know when downtown DC is shaking from earthquakes! Follow us and get the latest from Last Word, ACEC’s weekly newsletter, updates from the Capitol on hot-button issues, education opportunities, networking opportunities, and to follow us live during conferences and events! Follow @ACECCoalitions.

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You can follow ACEC Coalitions on Twitter – @ACECCoalitions.

Donate to the CASE Scholarship Fund!

The ACEC Council of American Structural Engineers (CASE) is currently seeking contributions to help make the structural engineering scholarship program a success. The CASE scholarship, administered by the ACEC College of Fellows, is awarded to a student seeking a Bachelor’s degree, at minimum, in an ABET-accredited engineering program.

We have all witnessed the stiff competition from other disciplines and professions eager to obtain the best and brightest young talent from a dwindling pool of engineering graduates.

One way to enhance the ability of students in pursuing their dreams to become professional engineers is to offer incentives in educational support.

In addition, the CASE scholarship offers an excellent opportunity for your firm to recommend eligible candidates for our scholarship. If your firm already has a scholarship program, remember that potential candidates can also apply for the CASE Scholarship or any other ACEC scholarship currently available.

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

January 2013

excellence in the engineering industry. ACEC offers three professional designations, and each has a different set of criteria for eligibility to capture an individual’s level of experience and education.

The Management Engineer – MgtEngSM – is designed for professionals working in project, program, or business management roles within an engineering firm or related to the engineering industry.

The Executive Engineer – ExecEngSM – is designed for leaders in the industry. Executive Engineers have attained the highest level of achievement in industry leadership and experience.

The Management Professional – MgtProSM – is designed for non-P.E. managers working in non-profits or government agencies related to the engineering industry or business managers within engineering firms.

Contact Kerri McGovern at kmcgovern@acec.org for more information or visit www.acec.org/education/designations/.

CASE Business Practice Corner If you would like more information on the items below, please contact Ed Bajer, ebajer@acec.org.

State Licensure on a Federal Project There is no requirement of licensure in the jurisdiction in which a federal project is located. Federal law (Federal Acquisition Regulations, FAR) requires only that the engineer be licensed in some U.S. jurisdiction unless the contract states otherwise. The principal is based on the Supremacy clause of the Constitution – all state judges must follow federal law when a conflict arises with state law. That is clear enough. However, no one knows what a local building official might do if an out of state engineer signs or seals plans for a federal project filed in their office.

Corporate Seals Not the ones engineers and architects use on their final designs… some states still require corporate seals, most do not. They are becoming obsolete. They do offer some visible evidence of legitimacy. Some clients still insist on corporate seals, even though they may not be required to make a contract binding. What is STRUCTURE magazine

required to bind a company is the signature of an individual with authority to sign. Many firms don’t have corporate seals anymore and if asked for one reply they are not required. No firm has indicated they have ever been denied work for lack of a seal. International work may be a different story.

The Expert Witness and Conflicts of Interest Regardless of the veracity of an expert’s testimony, if it is found that the expert has had a relationship with a party which could have biased the expert’s opinions, their testimony may be discounted. To avoid this, the expert should identify the organizations or individuals involved in the matter at issue and then determine whether they or their associates ever had a relationship with them. This does not necessarily disqualify them, but they should reveal such relationships to their client and permit the client or client’s attorney to determine whether it could be construed as creating or giving an appearance of a conflict of interest.

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CASE is a part of the American Council of Engineering Companies

Executives at engineering firms develop a unique skill set that transcends the technical practice of engineering – the skill and adroitness of running an engineering business. Experience managing programs, projects, personnel and budgets will drive a firm’s profitability. These vital skills are not learned in technical programs, but are acquired through company programs, from industry groups, such as ACEC, and via direct business practice experience.

ACEC, as the industry leader in best business practices, recognizes that business acumen is critical to success, but difficult to quantify for a client. ACEC is proud to offer its designation program – a way for our members to codify their experience and use it to market their services.

ACEC’s Professional Designation programs are designed to recognize a singular attainment of relevant experience and education by worthy professionals in the engineering industry.

ACEC’s Professional Designation programs set the national standards for business management and leadership

CASE in Point

GET an ACEC Designation and Set the Standard for Management and Leadership Excellence

Structural Forum

opinions on topics of current importance to structural engineers

Meeting the Challenges of the Future Head-On

By Barry Arnold, S.E., SECB

T

he entire engineering profession is buzzing about two important changes that are on the horizon.

The discussion fuels feelings of excitement and anticipation in some, while in others, it stirs feelings of fear and dread. The topics are structural engineering (SE) licensure and the Bachelor’s+30 (BS+30) initiative.

In 23 years of practice, I have witnessed many changes in the profession. Codes and standards change regularly to reflect the latest understanding of the behavior of structures and structural components. The code of ethics changed to incorporate references to sustainability and the environment in response to pressure for engineers to take an active role in planning for the future.

Change is nothing new to our profession, but it is seldom easy. Although change is often looked at dimly, sometimes it is necessary and essential. Improving and adapting to a changing environment has long-term benefits – the most important of which is survival.

I take comfort in the fact that changes in our profession must pass through a variety of organizations and individuals before being implemented. These “filters” are necessary, and to do anything less would breach the trust that society has placed in our profession.

Paramount in the process of change is our responsibility to the public. Although the steps may go by a variety of names, the process of change always involves a large number of engineers and generally flows through the following steps: Observation: Is the current system working correctly? If it is working, how well is it working?

If it is not working, how bad is the problem?

Assessment: How widespread is the problem?

Can the profession do better? What is needed to correct the problem? Should we and can we do more? What are the consequences of action or inaction? What happens when the current system fails? How is the problem addressed in the code of ethics and state laws? During this stage, input is sought and alternatives compared. The options and opportunities are carefully reviewed to identify the best possible course of action for all involved.

Engagement: In this crucial phase, the engineering community is involved in a dialogue on the best method to address the problem.

Input and insights are sought from engineers throughout the country to provide the widest possible perspective.

Action: The final stage involves doing something, moving forward and implementing the changes to meet our ethical obligation to the public and the profession.

Underscoring this whole process is the requirement that the profession take both the right and good course of action. Right and good are terms used in philosophy with very different meanings. Right focuses on the motives for a particular action. Good focuses on the consequences.

If an engineer has not been actively involved in the process, it can appear that decisions are made in a random and arbitrary fashion.

Nothing could be farther from the truth. A number of associations complete an independent review process, thus giving each proposed change thorough scrutiny from a variety of perspectives.

Each step is important and frequently takes many years to complete. For example, structural licensure has been discussed for over 90 years, while BS+30 has been on the table for almost 30 years. Both have been assessed as good and right approaches that will benefit the public and the integrity of the structural engineering profession.

As with all change, there is resistance. Some associations, companies, and individuals prefer to hold firmly to the dogma of the past, others cling to their fears about how the change will affect them, and still others focus on their ego and wonder how they will benefit.

For example, it is easy to understand why states that have no formal plan or peer review process do not see the benefit of structural licensure. They are often unaware of the magnitude of the problem of incompetent practice and the enormous benefit to be gained from separate SE licensure.

It is easy to understand why not everyone sees the benefit of BS+30. Some professional engineers may get through their careers

without ever having taken a class in concrete design. For others, it may restrict their ability to get hired and progress in their firms.

Some believe that the code of ethics canon that “Engineers shall perform services only in areas of their competence” is adequate. However, it provides no metric for evaluating competence and no means of enforcement. Those with this mindset are overlooking the first canon, which states, “Engineers shall hold paramount the safety, health and welfare of the public.”

Opponents of change should also remember the fundamental principles of the code of ethics that “Engineers uphold and advance the integrity, honor and dignity of the engineering profession by striving to increase the competence and prestige of the engineering profession.”

The world of structural engineering is rapidly changing–progressing if you will. Analysis procedures are becoming more complex, code requirements are more involved, and materials are evolving. Those adopting a limited viewpoint may not see the need to embrace and promote structural licensure and BS+30, but that does not mean that these changes are not necessary and beneficial when viewed from a wider perspective.

Accusations of professional arrogance, naiveté, and conceit will undoubtedly abound, as they always do when changes are proposed.

Regardless of the accusations that distract the profession from its highest purpose, what remains are two carefully thought-through initiatives. The need for each initiative has been determined to be consistent with our code of ethics, useful to the profession, and, most importantly, a benefit to the public.▪ Barry Arnold, S.E., SECB, is a Vice President at ARW Engineers in Ogden, Utah. He is a Past President of the Structural Engineers Association of Utah (SEAU), serves as the SEAU Delegate to NCSEA, and is a member of the NCSEA Board of Directors and the NCSEA Licensing Committee. He can be reached at barrya@arwengineers.com.

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.

STRUCTURE magazine

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