STRUCTURE magazine | June 2012

June 2012 Tall Buildings/High Rise

A Joint Publication of NCSEA | CASE | SEI

STRUCTURE ®

R E V O L U T I O N A RY W AY TO A N C H O R S E I S M I C S E N S I T I V E E Q U I P M E N T

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CONTENTS

FEATURES A Structural Facelift

26

June 2012

By William D. Bast, P.E., S.E., SECB, Kevin J. Jackson, P.E., Dziugas Reneckis, Ph.D., P.E. and Eric J. Wheeler

DEPARTMENTS

Over the next several years, the A.J. Celebrezze Federal Buildingwill be over-clad with a new glass and metal curtain wall. An outrigger and girt system consisting of structural steel framing will support the new skin.

38 Education Issues Principles for Engineering Education – Part 2

By Eric M. Hines, P.E.

Cost-Effective Solution

30

41 InSights

By Sunghwa Han, P.E., S.E., Gabe Klein, P.E. and Jacob Grossman, P.E., SECB

34

Masonry Cement Mortar in High Seismic Design Applications

Satisfying occupant comfort criteria is a common challenge in the design of slender high-rise buildings. Read how two slender towers, recently built in New York City, utilized unique solutions to resolve these issues.

42 CASE Business Practices

Condition Assessment and Repair Part 2

47 Great Achievements

By Jamie Farny

Risk Management: Elements and Tools

By R. John Aniol, P.E., S.E.

Joachim Gotsche Giaver

By Richard G. Weingardt, P.E.

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

51 Spotlight

This article is a continuation of one that appeared in the April 2012 issue of STRUCTURE. It presents a discussion and assessment of the observations and material testing described in Part 1. The project was a large sub-grade parking garage and loading dock located in Center City, Philadelphia.

Bitexco – Symbolizing Vietnam’s Global Emergence

By William J. Faschan, P.E., Anthony Montalto, R.A. and Nayan B. Trivedi, P.E.

58 Structural Forum

COLUMNS 7 Editorial

A Structural Engineer’s Manifesto for Growth – Part 3

18 Practical Solutions

Highlighting Significant Changes in ASCE 7-10

By Donald O. Dusenberry, P.E., SECB

IN EVERY ISSUE

By Seth Rogge, P.E.

23 Structural Practices

14 Guest Column Is North America Ready For Wood High-Rises?

By Nat Tocci, P.E. and Sanya Levi

36 Technology Applying 3D Laser Scanning to MEP Seismic Restraint Retrofits

By Daryl Johnson, P.E. and Ernie MacQuarrie, P.E.

By Lisa Podesto, P.E.

Erratum In the May 2012 article titled Design Considerations for Sawn Lumber Wood Studs

by Jason A. Partain, P.E., the quoted text near the bottom of the second column contained a typo. It should have read “Studs are designed for bending, spaced no more than 16-inch on center, ...”

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

STRUCTURE magazine

5

June 2012

®

By Rafik R. Gerges, P.Eng, S.E., SECB, Kal Benuska, P.E., S.E. and Colin Kumabe, P.E., S.E.

8 Advertiser Index 44 Resource Guide (Tall Buildings) 52 NCSEA News 54 SEI Structural Columns 56 CASE in Point

Backstay Effect

A Joint Publication of NCSEA | CASE | SEI

Performance-Based Seismic Design of Tall Buildings

STRUCTURE

11 Structural Performance

By Erik Nelson, P.E., S.E.

Post-Tensioned Concrete Balcony Deflections

ON

THE

COVER

The W New York Downtown Hotel and Residence under construction. The hotel/residence is a 57-story, 627-foot tall mixed-use building located one block south from the World Trade Center site.

This building is the focus of the feature article on page 30 in this issue.

June 2012 Tall Buildings/High Rise

Innovation based. Employee owned. Expect more.

photo courtesy of constructionphotographs.com (File Name: concrete_pour_rebar_cement_truck_010.jpg) Graphics Modified

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Editorial

Highlighting Significant Changes new trends, new techniques and current industry issues in ASCE 7-10

By Donald O. Dusenberry, P.E., SECB

A

s always seems to be the case each time ASCE/SEI’s Minimum Design Loads for Buildings and Other Structures (ASCE 7-10) is reissued, the new edition looks very different from the previous edition. There have been numerous editorial and format changes throughout, and a significant collection of updates based on recent research. In addition, there have been several fundamental changes that deserve highlighting.

uniform probability of exceedance throughout the country, regardless of the type of storm causing the wind. The importance factors on wind are eliminated by effectively including them directly in the wind speed maps.

That is not the only change in the wind speed maps. Following the lead of the seismic section of the standard which converted to longreturn-period seismic events about two decades ago, the magnitudes of the mapped wind speeds in ASCE 7-10 now are set at strength limit states values. This means that the strength design load factor on wind loads is changed from previous editions’ value of 1.6 to 1.0 in the new edition. For allowable stress design, wind pressures based on the mapped wind speeds are reduced by applying a factor of 0.6 to bring them into line with service load magnitudes.

These adjustments change design pressures in some parts of the hurricane region and some other areas of the country, where recent research indicates changes were necessary to create uniform risk. However, for the vast majority of the country these changes are implemented without changing design pressures significantly.

Performance-Based Design The basic philosophy of the document is adapting to the growing support for performance-based building codes. Now, for the first time, the basic requirements for determining the strength and stiffness of structures are described in the first chapter of ASCE 7-10 as being by the traditional strength or allowable stress approaches, or by newly-introduced performance-based procedures. Little has changed for the familiar strength and allowable stress procedures. However, ASCE 7-10 now sets forth the basic steps that designers might pursue to demonstrate that a structure provides appropriate reliability based on analyses and testing not directly defined in this standard. This sets the stage for new creativity in the satisfaction of the design intent that underlies this standard.

Seismic Loads The ground motion maps in the seismic sections of the standard incorporate new seismic hazard data developed by the United States Geological Survey (USGS) and related changes developed by the Building Seismic Safety Council. The maps have been updated to reflect risk-targeted magnitudes, reflecting probabilistic ground motions that are based on unifying risk, rather than hazard as has been done in the past. They also now consider revised deterministic ground motions near active faults.

The net changes (either increasing or decreasing design ground motions) typically are small in the central portion of the United States and moderate (plus or minus 10%) in the western United States. The changes are more significant in certain cities where new hazard data developed by the USGS have improved understanding about seismic hazards.

The changes highlighted above are important for the creation of reliable structures that meet the needs of our population, while advancing the technology and processes for structural design to keep pace with our rapidly advancing tools and philosophies for design. There are other important changes, including reorganizing the wind loads section into topic-oriented chapters, that are intended to simplify the process for design engineers. In making these improvements, the committee that maintains this standard continues in its quest for reliable approaches that are straightforward for engineers to apply.▪

Wind Loads

a member benefit

structurE

®

A first glance at the wind speed maps in ASCE 7-10 will suggest that this edition of the standard requires design pressures that are much higher than those in the previous edition. This is not actually the case because adjustments in the load factor and elimination of importance factors for wind loads compensate.

Previous editions of the standard published importance factors that engineers applied to the wind pressures shown on a single map of wind speeds to adjust the risk to account for the occupancy of the building.

The intent in previous editions was to change the mean recurrence interval for the design-base wind storm to adjust the conservatism of the design to suit the occupancy of the structure.

However, research has shown that return period statistics for wind speeds associated with hurricanes differ from those for wind speeds caused by other types of storms.

The use of a single table of importance factors to modify the basic wind speeds in all parts of the country did not lead to uniform probability of exceedance (i.e., risk of overload) everywhere.

To address this inconsistency, instead of just one map, ASCE 7-10 now has a series of wind STRUCTURAL speed maps that are constructed ENGINEERING INSTITUTE directly for the risk categories.

Each map is constructed to create STRUCTURE magazine

Donald O. Dusenberry, P.E., SECB is a Senior Principal of Simpson Gumpertz & Heger Inc. Consulting Engineers in Waltham, Massachusetts. He was the chair of the ASCE/SEI standards committee that maintains Minimum Design Loads for Buildings and Other Structures during the development of ASCE 7-10.

7

June 2012

ADVERTISER INDEX

PLEASE SUPPORT THESE ADVERTISERS

American Concrete Institution .............. 49 Atlas Copco Constr. & Mining USA ..... 29 Bentely Systems, Inc. ............................. 10 Computers & Structures, Inc. ............... 60 CSC, Inc. ................................................ 3 CTS Cement Manufacturing Corp........ 21 DBM Contractors, Inc. ........................... 8 Fyfe Co. LLC ........................................ 13

Geopier Foundation Company.............. 19 Halfen, Inc. ........................................... 22 Hohmann & Barnard, Inc. .................... 40 The IAPMO Group............................... 37 Integrated Engineering Software, Inc..... 43 ITW Red Head ..................................... 46 KPFF Consulting Engineers .................. 27 NCSEA ................................................... 9

Editorial Board

Polyguard Products, Inc........................... 6 Powers Fasteners, Inc. .............................. 2 RISA Technologies ................................ 59 S-Frame Software, Inc. ............................ 4 Simpson Strong-Tie......................... 17, 25 StructurePoint ....................................... 45 Struware, Inc. ........................................ 24 United States Gypsum Co. .................... 39 Wheeling Corrugating........................... 50

ADVERTISING ACCOUNT MANAGER

Chair

Jon A. Schmidt, P.E., SECB

Interactive Sales Associates

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

Craig E. Barnes, P.E., SECB

Brian W. Miller

Richard Hess, S.E., SECB

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

CBI Consulting, Inc., Boston, MA

Hess Engineering Inc., Los Alamitos, CA

Mark W. Holmberg, P.E.

Heath & Lineback Engineers, Inc., Marietta, GA

Roger A. LaBoube, Ph.D., P.E.

Davis, CA

Western Sales 951-587-2982

sales@STRUCTUREmag.org

Evans Mountzouris, P.E.

EDITORIAL STAFF

Greg Schindler, P.E., S.E.

Executive Editor Jeanne Vogelzang, JD, CAE

The DiSalvo Ericson Group, Ridgefield, CT

KPFF Consulting Engineers, Seattle, WA

Brian J. Leshko, P.E.

Stephen P. Schneider, Ph.D., P.E., S.E.

John A. Mercer, P.E.

John “Buddy” Showalter, P.E.

Mercer Engineering, PC, Minot, ND

Dick Railton

Eastern Sales 847-854-1666

CRSI, Williamstown, NJ

CCFSS, Rolla, MO

HDR Engineering, Inc., Pittsburgh, PA

Chuck Minor

execdir@ncsea.com

Editor

Christine M. Sloat, P.E.

publisher@STRUCTUREmag.org

BergerABAM, Vancouver, WA

Associate Editor

American Wood Council, Leesburg, VA

Graphic Designer

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Web Developer

Nikki Alger

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Rob Fullmer

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William Radig

webmaster@STRUCTUREmag.org

STRUCTURE® (Volume 19, Number 6). 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

reproduced in whole or in part without the written permission of the publisher.

Design/Build Earth Retention Foundation Support Slope Stabilization Ground Improvement Dewatering

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C Ink, Publishers

800-562-8460 WWW.DBMCONTRACTORS.COM Donald B. Murphy Contractors, Inc.

STRUCTURE magazine

8

June 2012

A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 P-608-524-1397 F-608-524-4432 publisher@STRUCTUREmag.org

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

ADVERTISER INDEX

PLEASE SUPPORT THESE ADVERTISERS

American Concrete Institution .............. 49 Atlas Copco Constr. & Mining USA ..... 29 Bentely Systems, Inc. ............................. 10 Computers & Structures, Inc. ............... 60 CSC, Inc. ................................................ 3 CTS Cement Manufacturing Corp........ 21 DBM Contractors, Inc. ........................... 8 Fyfe Co. LLC ........................................ 13

Geopier Foundation Company.............. 19 Halfen, Inc. ........................................... 22 Hohmann & Barnard, Inc. .................... 40 The IAPMO Group............................... 37 Integrated Engineering Software, Inc..... 43 ITW Red Head ..................................... 46 KPFF Consulting Engineers .................. 27 NCSEA ................................................... 9

Editorial Board

Polyguard Products, Inc........................... 6 Powers Fasteners, Inc. .............................. 2 RISA Technologies ................................ 59 S-Frame Software, Inc. ............................ 4 Simpson Strong-Tie......................... 17, 25 StructurePoint ....................................... 45 Struware, Inc. ........................................ 24 United States Gypsum Co. .................... 39 Wheeling Corrugating........................... 50

ADVERTISING ACCOUNT MANAGER

Chair

Jon A. Schmidt, P.E., SECB

Interactive Sales Associates

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

Craig E. Barnes, P.E., SECB

Brian W. Miller

Richard Hess, S.E., SECB

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

CBI Consulting, Inc., Boston, MA

Hess Engineering Inc., Los Alamitos, CA

Mark W. Holmberg, P.E.

Heath & Lineback Engineers, Inc., Marietta, GA

Roger A. LaBoube, Ph.D., P.E.

Davis, CA

Western Sales 951-587-2982

sales@STRUCTUREmag.org

Evans Mountzouris, P.E.

EDITORIAL STAFF

Greg Schindler, P.E., S.E.

Executive Editor Jeanne Vogelzang, JD, CAE

The DiSalvo Ericson Group, Ridgefield, CT

KPFF Consulting Engineers, Seattle, WA

Brian J. Leshko, P.E.

Stephen P. Schneider, Ph.D., P.E., S.E.

John A. Mercer, P.E.

John “Buddy” Showalter, P.E.

Mercer Engineering, PC, Minot, ND

Dick Railton

Eastern Sales 847-854-1666

CRSI, Williamstown, NJ

CCFSS, Rolla, MO

HDR Engineering, Inc., Pittsburgh, PA

Chuck Minor

execdir@ncsea.com

Editor

Christine M. Sloat, P.E.

publisher@STRUCTUREmag.org

BergerABAM, Vancouver, WA

Associate Editor

American Wood Council, Leesburg, VA

Graphic Designer

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

Web Developer

Nikki Alger

publisher@STRUCTUREmag.org

Rob Fullmer

graphics@STRUCTUREmag.org

William Radig

webmaster@STRUCTUREmag.org

STRUCTURE® (Volume 19, Number 6). 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

reproduced in whole or in part without the written permission of the publisher.

Design/Build Earth Retention Foundation Support Slope Stabilization Ground Improvement Dewatering

WWW.NCSEA.COM 3

C Ink, Publishers

800-562-8460 WWW.DBMCONTRACTORS.COM Donald B. Murphy Contractors, Inc.

STRUCTURE magazine

8

June 2012

A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 P-608-524-1397 F-608-524-4432 publisher@STRUCTUREmag.org

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

ADVERTISER INDEX

PLEASE SUPPORT THESE ADVERTISERS

American Concrete Institution .............. 49 Atlas Copco Constr. & Mining USA ..... 29 Bentely Systems, Inc. ............................. 10 Computers & Structures, Inc. ............... 60 CSC, Inc. ................................................ 3 CTS Cement Manufacturing Corp........ 21 DBM Contractors, Inc. ........................... 8 Fyfe Co. LLC ........................................ 13

Geopier Foundation Company.............. 19 Halfen, Inc. ........................................... 22 Hohmann & Barnard, Inc. .................... 40 The IAPMO Group............................... 37 Integrated Engineering Software, Inc..... 43 ITW Red Head ..................................... 46 KPFF Consulting Engineers .................. 27 NCSEA ................................................... 9

Editorial Board

Polyguard Products, Inc........................... 6 Powers Fasteners, Inc. .............................. 2 RISA Technologies ................................ 59 S-Frame Software, LLC ........................... 4 Simpson Strong-Tie......................... 17, 25 StructurePoint ....................................... 45 Struware, Inc. ........................................ 24 United States Gypsum Co. .................... 39 Wheeling Corrugating........................... 50

ADVErtIsING AccOuNt MANAGEr

Chair

Jon A. Schmidt, P.E., SECB

Interactive Sales Associates

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

Craig E. Barnes, P.E., SECB

Brian W. Miller

Richard Hess, S.E., SECB

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

CBI Consulting, Inc., Boston, MA

Hess Engineering Inc., Los Alamitos, CA

Mark W. Holmberg, P.E.

Heath & Lineback Engineers, Inc., Marietta, GA

Roger A. LaBoube, Ph.D., P.E.

Davis, CA

Western Sales 951-587-2982

sales@STRUCTUREmag.org

Evans Mountzouris, P.E.

EDItOrIAL stAFF

Greg Schindler, P.E., S.E.

Executive Editor Jeanne Vogelzang, JD, CAE

The DiSalvo Ericson Group, Ridgefield, CT

KPFF Consulting Engineers, Seattle, WA

Brian J. Leshko, P.E.

Stephen P. Schneider, Ph.D., P.E., S.E.

John A. Mercer, P.E.

John “Buddy” Showalter, P.E.

Mercer Engineering, PC, Minot, ND

Dick Railton

Eastern Sales 847-854-1666

CRSI, Williamstown, NJ

CCFSS, Rolla, MO

HDR Engineering, Inc., Pittsburgh, PA

Chuck Minor

execdir@ncsea.com

Editor

Christine M. Sloat, P.E.

publisher@STRUCTUREmag.org

BergerABAM, Vancouver, WA

Associate Editor

American Wood Council, Leesburg, VA

Graphic Designer Web Developer

Nikki Alger

publisher@STRUCTUREmag.org

Rob Fullmer

graphics@STRUCTUREmag.org

William Radig

webmaster@STRUCTUREmag.org

STRUCTURE® (Volume 19, Number 6). 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

reproduced in whole or in part without the written permission of the publisher.

Design/Build Earth Retention Foundation Support Slope Stabilization Ground Improvement Dewatering

WWW.NCSEA.COM 3

C Ink, Publishers

800-562-8460 WWW.DBMCONTRACTORS.COM Donald B. Murphy Contractors, Inc.

STRUCTURE magazine

8

June 2012

A Division of Copper Creek Companies, Inc. 148 Vine St., Reedsburg WI 53959 P-608-524-1397 F-608-524-4432 publisher@STRUCTUREmag.org

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

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riven by a passion for innovation and seeking freedom from prescriptive code requirements, Structural Engineers found in the PerformanceBased Seismic Design (PBSD) approach a perfect tool box. PBSD is not a new idea, but its application to newly constructed towers is out-of-the-ordinary. Experiences from designs and peer reviews of high-rise buildings in active seismic zones designed over the past decade have clearly shown that the PBSD approach, in lieu of a prescriptive code, significantly enhances not only the safety but also the economics of these mega structures.

This article introduces the general concept of PBSD and answers specific questions: Why should I consider PBSD in lieu of code-based seismic design? Do building codes allow for PBSD? Are there unique features in tall buildings when it comes to earthquake response? What are the available guidelines for PBSD of tall buildings?

What is PBSD? Performance-based seismic design allows the design team to choose and then explicitly verify a building’s seismic performance under different intensities of earthquake shaking. Much of the framework for PBSD in the US can be traced to work in the 1990s, such as Vision 2000 (SEAOC, 1995), ATC 40 (ATC, 1996), FEMA 273 and FEMA 356 (currently ASCE 41-06).

To perform PBSD, you need to do the following: 1) Select return periods for earthquake intensities and corresponding performance levels;

2) W ork with a geotechnical engineer to develop site-specific ground motion corresponding to selected return periods; 3) Subject the mathematical model to ground shaking and estimate structural response quantities (inter-story drift, floor accelerations, deformation demands on ductile elements, force demands on nonductile elements, etc.) for each level of earthquake intensity; 4) Evaluate global and element performance based on acceptance criteria that reflect selected performance objectives.

Typical performance goals for a tall building, expected from the building code but not actually evaluated, are: (a) Minor damage under frequent earthquakes, allowing immediate occupancy after inspection (b) Low probability of collapse under very rare earthquakes.

These objectives can be enhanced if stakeholders desire.

Why PBSD in lieu of CBSD?

• PBSD is a significant improvement over code-based seismic design (CBSD) as it provides the design team and the stakeholders with greater understanding of the building’s likely performance at different levels of seismic events.

• PBSD accommodates architectural features that may not be possible with prescriptive requirements.

• PBSD allows for innovative structural systems and materials that are not codified, resulting in more cost efficient lateral systems.

• PBSD produces safer and more serviceable buildings when compared to CBSD designs.

Codes have traditionally permitted the use of alternative analysis and design methods, provided that these methods follow well-established principles of mechanics and/or are backed up with test results. The following are excerpts from current and proposed building codes: Section 104.11 of 2009 and 2012 IBC “The provisions of this code are not intended to prevent the installation of any material or to prohibit any design or method of construction not specifically prescribed by this code, provided that any such alternative has been approved. An STRUCTURE magazine

11

performance issues relative to extreme events

Performance-Based Seismic Design of Tall Buildings

Do Building Codes allow PBSD?

777 Tower, Los Angeles, CA.

Structural Performance

Why & How?

By Rafik R. Gerges, P.Eng, Ph.D., S.E., SECB, LEED AP, BSCP, Kal Benuska, P.E., S.E. and Colin Kumabe, P.E., S.E.

Rafik R. Gerges, P.Eng, Ph.D., S.E., SECB, LEED AP, BSCP is a Project Manager at John Martin & Associates, Inc., Los Angeles, California. Dr. Gerges can be reached at rgerges@johnmartin.com.

Kal Benuska, P.E., S.E. is a Principal of John A. Martin & Associates, Inc., in Los Angeles, California.

Mr. Benuska can be reached at benuska@johnmartin.com.

Colin Kumabe, P.E., S.E. is a supervising Sr. Structural Engineer with the City of Los Angeles, Department of Building and Safety. Mr. Kumabe can be reached at colin.kumabe@lacity.org.

1.3.1.3 Performance-based Procedures.

“Structural and nonstructural components and their connections shall be demonstrated by analysis or by a combination of analysis and testing to provide a reliability not less than that expected for similar components designed in accordance with the Strength Procedures of Section 1.3.1.1 when subject to the influence of dead, live, environmental and other loads. Consideration shall be given to uncertainties in loading and resistance.”

1.3.1.3.1 Analysis. “Analysis shall employ rational methods based on accepted principles of engineering mechanics and shall consider all significant sources of deformation and resistance. Assumptions of stiffness, strength, damping and other properties of components and connections incorporated in the analysis shall be based on approved test data or referenced Standards.”

Fox Plaza Century City, Los Angeles, CA.

alternative material, design or method of construction shall be approved where the building official finds that the proposed design is satisfactory and complies with the intent of the provisions of this code, and that the material, method or work offered is, for the purpose intended, at least the equivalent of that prescribed in this code in quality, strength, effectiveness, fire resistance, durability and safety.”

Section 12.6 of ASCE 7-05

PBSD for Tall Buildings Versus Low- and Medium-rise Buildings?

• Higher modes in tall building are significantly excited by ground shaking, while low- and medium-rise respond primarily in the fundamental mode.

• Inter-story drift in a high-rise is the result of two components, namely:

“The structural analysis required by Chapter 12 shall consist of one of the types permitted in Table 12.6.1, based on the structure’s seismic design category, structural system, dynamic properties, and regularity, or with the approval of the authority having jurisdiction, an alternative generally accepted procedure is permitted to be used.”

Section 1.3 of ASCE 7-10 1.3.1 Strength and stiffness. “Buildings and other structures, and all parts thereof, shall be designed and constructed with adequate strength and stiffness to provide structural stability, protect nonstructural components and systems from unacceptable damage and meet the serviceability requirements of Section 1.3.2. Acceptable strength shall be demonstrated using one or more of the following procedures: a. the Strength Procedures of Section 1.3.1.1 b. the Allowable Stress Procedures of Section 1.3.1.2; or c. subject to the approval of the authority having jurisdiction for individual projects, the Performance-based Procedures of Section 1.3.1.3.”

One California Plaza, Los Angeles, CA.

rigid body displacement and racking (shear) deformations. In low- to medium-rise buildings, inter-story drift is dominated by shear deformations.

• Specific to tall buildings: o Large shear demand near the base due to significant contributions from higher modes o Reduced ductility due to large gravity axial demands on vertical elements o Minimum stiffness is often controlled by wind serviceability o Strength at the base may be controlled by wind survivability o Capacity-based design principles are less valid when forces need to be summed up over multiple elements o At long periods, the reliability of ground motion prediction and the availability of earthquake records are less

Comparison of Current Guidelines • LATBSDC: The first US consensus document was published in 2005 by the Los Angeles Tall Building Structural Design Council (LATBSDC) in response to the residential tall building boom in Southern California. This document, An Alternative Procedure for Seismic Analysis and Design of Tall Buildings Located in the Los Angeles Region, was significantly revised in 2008 and reached the current form in 2011 (3rd edition). This document strikes a good balance between completeness and conciseness.

777 Tower, Los Angeles, CA.

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6) D esign for seismic serviceability in order to size deformationcontrolled actions; 7) Perform capacity based design in order to size force-controlled actions; 8) Engage the peer review panel – the sooner you get their feedback, the faster the design progresses; 9) Test design for Collapse Prevention, and plan for at least one design iteration; 10) Document the design – a key to successful PBSD.

Ten Commandments for PBSD of Tall Buildings 1) M eet with the Building Officials – the design team needs to ascertain that the local jurisdiction would accept PBSD; 2) Develop a detailed design criteria – keep it alive throughout the project; 3) Ensure the structure has enough stiffness and strength for wind serviceability and survivability – this defines lower-bounds; 4) Ground motion developed and peer reviewed as early as possible; 5) Identify seismic fuses and detail them for ductility – this is the most important commandment; STRUCTURE magazine

What will the Next Generation PBSD Look Like?

• Holistic – not limited to structural elements but rather includes all Architectural, Mechanical, Electrical, Plumbing, etc. elements • Measurable in 3Ds – damage (dollars), downtime (loss of occupancy) and death (injuries, fatalities) • Probabilistic – triple integral including earthquake intensities, demand parameters and damage measures▪

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• AB-83: Faced by a number of tall building designs seeking code exceptions (mainly height limits and minimum base shear), the City of San Francisco requested the Structural Engineering Association of Northern California (SEAONC) to customize the LATBSDC’s first edition to suit local use.

These efforts resulted in the publication of the Recommend Administrative Bulletin on the Seismic Design & Review of Tall Buildings Using NonPrescriptive Procedures in April 2007.

This document was adopted by the City of San Francisco as AB-83.

This Bulletin is strongly tied to the Building Code with a minimum design base shear requirement.

• PEER: Parallel to LATBSDC and SEAONC efforts, the Pacific Earthquake Research Center (PEER) at the University of California Berkeley embarked on a four-year Tall Building Initiative that resulted in the publication of Guidelines for Performance-Based Seismic Design of Tall Buildings in 2010, and a number of task reports such as ATC 72 (2010).

• CTBUH: The Council of Tall Building and Urban Habitat summarized the best current and emerging practices worldwide in Recommendations for the Seismic Design of High-rise Buildings that tend to frame issues of importance and were published in 2008.

The online version of this article contains a comprehensive comparison of these four guidelines. Please visit the website www.STRUCTUREmag.org.

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Guest Column dedicated to the dissemination of information from other organizations

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here has been a lot of talk lately about buildings that are 8, 10, even 20 stories tall and built entirely of wood – cross laminated timber (CLT) to be precise, which is sometimes referred to as “plywood on steroids.” In Europe, CLT has been steadily gaining popularity over the past decade, due in part to a strong push by governments to lower the carbon footprint of buildings, and it’s now making inroads in North America. However, while the potential for high-rise wood buildings has been widely reported in the design media, it has also been the focus of debate in online forums frequented by structural engineers – who may love a good story of technological advancement, but approach anything new with a degree of skepticism (rightfully so).

This article has been written for the skeptics. In addition to the reasons one might consider using CLT, it examines its structural applications and some of the design considerations related to its use.

It seeks to answer the question – Does cross laminated timber have the potential to change the North American building landscape?

Is North America Ready For Wood High-Rises?

By Lisa Podesto, P.E.

First, what is CLT?

Lisa Podesto, P.E. is the national building systems technical director for WoodWorks, an initiative of the Wood Products Council established to provide free education and technical support to design and building professionals using wood in non-residential buildings. She can be reached at lisa@woodworks.org.

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

Conceived in Switzerland in the early 1990s and further developed in Austria, CLT is an engineered “mass timber” building system that complements light- and heavy-timber framing options in the arsenal of wood-based building solutions. Its prefabrication and ease of installation have drawn comparisons to concrete tilt-up.

However, it can be used as a carbon-friendly alternative to concrete, masonry and steel in a wide range of building applications.

CLT is made from layers of dimensional lumber, each stacked at right angles to the adjacent layers and glued to form solid elements.

Boards are kiln dried, prior to lamination, to a moisture content of 12% +/- 2%, which adds to their dimensional stability.

Manufactured to custom sizes, panels have an odd number of layers (three, five, seven or nine). In North America, they’re available in configurations up to 123/8 inches thick, 10 feet wide and 64 feet long. The length is usually limited by transportation restrictions.

By varying the number of layers as well as the lumber species, grade and thickness, CLT panels can be used in any assembly type (e.g., walls, floors, roofs, elevator shafts, stairways). As with plywood, the primary direction of the loadbearing capacity typically corresponds to the grain orientation of the outer layers. For example, panels developed for use in walls are oriented so

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The Stadthaus building (UK) includes eight stories of CLT over one story of concrete. Courtesy of Waugh Thistleton Architects.

the grain of the outer layers will be parallel to vertical loads while in panels used for floor and roof systems, the exterior grain is oriented to run parallel to the span direction.

Once the layers are pressed, panels are planed or sanded, and CNC routers are used to cut the required openings for windows, doors, MEP systems, etc. The addition of insulation and exterior cladding may also take place in the factory, and the completed panels are shipped to the job site ready to be erected into place.

Because panels are prefabricated and a high degree of finishing can be done off site, CLT buildings are relatively quick to erect, requiring only a small crane to lift the panels into place and lightweight power tools for on-site assembly. Construction is safer because there are few hazards on the jobsite, and there is considerably less noise and waste than with buildings made of other materials.

The Rise of CLT CLT began gaining popularity as a structural building system in the early 2000s – most notably in Austria, Germany, Norway, Sweden, Switzerland, and the United Kingdom. The tallest CLT structure to date is the Stadthaus building in London’s east end, which includes eight stories of CLT over one story of concrete. It is also gaining popularity for educational buildings such as the 102,000-squarefoot Norwich Open Academy, also in the UK. A 78-foot bell tower at Myers Memorial United Methodist Church in Gastonia, NC was the first non-residential CLT structure in North America and a two-story commercial building has since been built in Whitefish, Montana. In Canada, the University of British Columbia is building a 20,300-squarefoot Bioenergy Research + Demonstration Project

with CLT walls and roof construction, and several other buildings are in development. So far, there are three CLT manufacturers in North America, a company making nail-laminated CLT, and several CLT distributors, but the supplier landscape is changing quickly and several other companies are developing plans to enter the market.

Structural Properties CLT systems offer a number of attractive structural characteristics, including: • High dimensional stability and static strength in all directions • High axial load capacity for walls due to large bearing area of the solid panels • High shear strength to resist lateral loads coupled with good ductility • Wall systems that are rigid around openings, requiring fewer hold-downs • Failure modes that are ductile and occur at connections • Negligible settlement effects (e.g., 0.78 inches for one building with seven stories of CLT over one story of concrete after one year) • Low probability of in-plane buckling • Great span-to-depth ratio for floor panels, which allows shallow floors • Lack of susceptibility to soft-story failures CLT also offers effective and even superior performance with regard to life safety and other priorities reflected in building codes.

Fire Resistance Although it may seem counter-intuitive, CLT buildings can be designed to perform extremely well when exposed to fire. Because the panels are thick and solid, they char at a

A full-scale model of a seven-story building tested on the world’s largest shake table. Courtesy of Italian National Institute of Timber Trees (IVALSA).

slow and predictable rate. Once formed, this char protects the wood from further degradation, helping to maintain the building’s structural integrity and reducing its fuel contribution to the fire, which in turn lessens the fire’s heat and flame propagation. When used in Type IV construction, CLT assemblies also tend to have fewer concealed spaces, which reduces a fire’s ability to spread undetected.

To generate additional information for North American building designers, FPInnovations and the Canadian Wood Council have undertaken a study on the fire design of CLT systems. Researchers are considering the impact of edge-glued versus face-glued systems, performance of various adhesives, use of fire retardant treated members and other factors. Equations have also been developed to help designers determine the required thickness of a CLT system in order to accommodate structural requirements in addition to the fire rating.

Seismic Performance To evaluate the seismic performance of CLT buildings, full-scale models of threeand seven-story structures were tested by the Trees and Timber Research Institute of Italy (IVALSA) on the world’s largest shake table in Miki, Japan. The buildings performed extremely well even when subjected to motions comparable to the devastating 1995 Kobe earthquake, which had a magnitude of 7.2 and accelerations of 0.8 to 1.2 g.

At the end of the test, for example, the sevenstory building had no residual deformation.

The maximum inter-story drift was 1.57 inches and the maximum lateral deformation at the top of the building was 11.3 inches. Both buildings showed good ductile behavior and energy dissipation, which can be attributed primarily to their mechanical connections.

As with other building types, CLT structures can be designed to withstand earthquakes by adhering to capacity design principles, which are based on how the structure will sustain large deformations when subjected to severe seismic motion. For wood structures in general, this means designing so that failure is intended to occur in the connections. For a CLT structure, it is recommended that nonlinear deformations and energy dissipation occur in the brackets that connect wall and floor panels and, if used, hold-down connections and vertical step joints.

In addition to performing well during a seismic event, structural repairs after an earthquake would be relatively easy and cost effective for a CLT building as failure is

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The 78-foot bell tower at Myers Memorial United Methodist Church in Gastonia, N.C. Courtesy of Kevin Meechan, WoodWorks.

localized at the connections. New connections could be added inches from where the failure occurred using simple hand-held power tools.

Acoustic Performance Like other wood products, CLT offers acoustical advantages when used for floor and wall systems. When used in conjunction with insulation and gypsum board or resilient channels, it is possible for a CLT building to exceed code requirements related to the acoustical performance of floors and walls.

Thermal Efficiency Although properties vary based on thickness of the panels, wood’s natural thermal resistance adds value to CLT assemblies.

Insulation and cladding options are similar to a variety of concrete systems; however, the CLT itself has an R-value of about R-1.2/inch or R-4.2 for a panel that is 3½ inches thick, so less insulation is required to meet the desired level of thermal efficiency.

Precise manufacturing and dimensional stability result in tight tolerances with better energy efficiency (thanks to an airtight structure) and improved installation of doors, windows, utilities and cladding. Boards within the CLT panel’s laminations can be edge-glued to further improve thermal efficiency by further reducing air flow.

It has been observed that the potential for creep shortening due to compression under load is negligible for the walls and 0.02 inches for the floors. Likewise, the potential

North American Standards

The 102,000-square-foot Norwich Open Academy utilized 123,000 cubic feet of CLT. Courtesy of KLH Massivholz GmbH.

for moisture expansion is negligible for the walls and 0.07 inches for the floors, resulting in maximum settlement of less than 1 inch for eight stories of CLT.

Connections One of the advantages of CLT structures is that they are built with strong yet simple connection systems. However, the building’s structural performance and therefore success is dependent to a large degree on the efficient design and fabrication of those connections.

Panel-to-panel connections can be created with half-lapped, single or double splines made with engineered wood products. Metal “L” brackets, hold-downs and plates are used to transfer forces from walls to floors and foundations, and proprietary systems and innovative carpentry can also be used. In terms of fasteners, long self-tapping screws are most commonly recommended by CLT manufacturers; however, traditional dowel-type fasteners such as wood screws, nails, lag screws, rivets, bolts and dowels can also effectively connect the panel elements. Bearing-type fasters such as split rings and shear plates have also shown potential (though their use is likely to be limited to applications involving high loads), as have several innovative systems including epoxied-in rods and proprietary products.

When detailing connection systems for a CLT structure, engineers must consider strength and stiffness as well as other performance requirements such as fire protection, sound transmission, air tightness, durability and vibration. Shrinkage and swelling due to seasonal fluctuations in environmental conditions and the differential movement between CLT and other materials (if applicable) must likewise be taken into account.

Since CLT assembly configurations are customized by project, so too are the mechanical properties of the completed panels and assemblies. In Europe, mechanical properties are provided by each manufacturer and there is no European standard to date. Instead, European manufacturers are operating on a proprietary basis using European Technical Approval (ETA) reports.

In North America, an American National Standard, PRG320: Standard for Performance Rated Cross –Laminated Timber, which covers manufacturing, qualification, and quality assurance requirements has been approved and is available on the APA – The Engineered Wood Product Association’s website (www.apawood.org). The American Wood Council (www.awc.org) and FPInnovations (www.fpinnovations.ca) have also established a committee to begin developing a design standard for CLT.

Cost Competitiveness To convince developers of the Stadthaus that CLT would save money, Waugh presented concepts for two almost identical structures, one designed in wood and the other in concrete. Not only was the CLT building estimated to cost 15% less per square foot, it was projected to weigh four times less – which lowered transportation costs, allowed a 70% smaller foundation and eliminated the need for a tower crane during construction.

In a recent study by FPInnovations, researchers examined 17 building types to determine which North American market segments offer the greatest opportunity from a cost perspective. While reduced construction time (typically 25-30%) was not taken into account, the cost of a CLT structure was found to be most competitive in the following categories: • Mid-rise residential – 15% less • Mid-rise non-residential – 15 to 50% less • Low-rise educational – 15 to 50% less • Low-rise commercial – 25% less • One-story industrial – 10% less

Environmental Benefits A large part of the attraction to CLT is wood’s low carbon footprint. In addition to being the only major building material that’s renewable and sustainable, wood grows naturally, using solar energy, and doesn’t require large amounts of fossil fuels to manufacture. As trees grow, they absorb carbon dioxide (CO2) from the atmosphere and release oxygen (O2), and wood products continue to store carbon

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(C) over their lifetimes – longer if the wood is reclaimed and repurposed at the end of the product’s original service life.

Although projected cost savings convinced developers of the Stadthaus to use CLT, local building authorities were impressed by the carbon savings. Waugh estimates that the CLT design saved the equivalent of about 300 metric tons of carbon compared to the concrete design – which is the amount the building is projected to emit over 21 years of operation.

CLT also makes use of small diameter timber harvested from sustainably managed forests, contributing to efficient use of the resource.

The manufacturing process is energy efficient.

And prefabricated panels all but eliminate jobsite waste.

CLT in North America The idea that advancements in wood technology, systems and products are expanding the possibilities for wood construction is nothing new. The first industrial plywood was produced in the U.S. in the early 1900s, giving the construction industry a highstrength sheet material that could be used in many applications. The principle of bonding together cut or refashioned pieces of wood to form composite materials has since been used to create a variety other structural products – including oriented strand board, laminated veneer lumber, glued laminated timber, I-Joists, and now CLT. Each has created new, innovative opportunities to use wood.

Building on the Stadthaus experience, the Timber Research and Development Association (TRADA) published a design example for a 12-story CLT building, while IVALSA has designed a 15-story CLT and steel building, and Waugh Thistleton Architects has simulated a 25-story CLT and concrete hybrid. Closer to home, the author of a pending study says it will confirm the feasibility of a 20-story wood building in British Columbia.

The difference is one of degree. The structural performance of CLT makes it feasible in applications where wood has never before been an option – at a time when governments and the design community are seeking innovative ways to lower the carbon footprint of buildings. How high we’ll end up going is anyone’s guess, but we can safely say that CLT has the potential to significantly change the North American building landscape. APA is producing Product Reports which serve as an aid to building officials and design professionals to determine conformance with codes and standards similar to an ES Report.▪

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Practical SolutionS solutions for the practicing structural engineer

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alconies are considered a desirable feature in apartment units. They represent an important extension of the living space into the outdoors and provide homeowners their own quiet spot to enjoy the surroundings and city views. However, within the last decade, an explosion in the use of post tensioning in high rise buildings has given way to a common complaint – negative drainage of rainwater in balconies. While there are a variety of factors that can affect water draining into residential units, careful engineering should be key in maintaining a positive slope. With tight construction tolerances placed on the general contractor to maintain positive drainage away from units, the blame for poor drainage has shifted in some cases to the design engineer. While the design of a balcony may seem eerily simple, thinner slabs, longer spans, and higher post tensioning forces have given way to this recurring issue in residential construction. Careful design and review of the entire lifecycle of a balcony should be analyzed – from the time of stressing to long term deflection reviews.

Post-Tensioned Concrete Balcony Deflections Maintaining Positive Drainage

By Seth Rogge, P.E.

Seth Rogge, P.E. is an Associate at Smislova, Kehnemui, and Associates located in Potomac, Maryland. He has over 10 years of design experience of residential high-rise, post-tension buildings.

He is currently pursuing his Ph.D. at The University of Maryland.

Seth may be reached at sethr@skaengineers.com.

Balcony Types The geometry of a balcony can vary from each building, but usually fall within two categories.

The first category is the true cantilevered balcony (Figure 1). These types of balconies project approximately 5 feet from buildings and can vary in width from 8 to 12 feet. The second category includes interior balconies. An interior balcony is usually supported by the same floor system and has columns in relative close proximity (Figure 2). There is no cantilevered action occurring at these types of balconies. Regardless of the type of balcony, fall protection is either in the form of a standard railing or heavy built-up parapet walls. Architects typically determine the drainage requirements and the required step in the slabs for balconies.

Figure 2: Typical floor plan with interior balconies.

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Figure 1: Typical floor plan with a true cantilevered balcony.

Obviously, positive drainage is needed away from the residential units and off the edge of the exterior balcony. A step in the slab followed by a sloped top slab leaves 1 to 2 inches less of concrete at the ends of the balconies. A minimum required slab thickness is often stipulated by the structural engineer.

With the requirement of a step down in the slab, the end result leaves a minimal slope between 1/8 inch to 1/16 inch per foot of slope. While the two types of balconies serve the same purpose for the unit owner, the designs are somewhat different.

Design and Rotation True cantilevered balconies are often designed to account for positive bending stresses over the top of the slab. To counterbalance these top stresses, additional top steel and post tensioning cables often continue over the column and into the balcony slab. While maintaining serviceability requirements can be achieved with enough post tensioning and reinforcing bars, deflections in the cantilevered balcony are often difficult to determine. The final slope of the balcony for drainage will depend on the continued on page 20

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Figure 4: Cross-section of a true cantilevered balcony with anticipated rotation.

Figure 3: Cross-section of a true cantilevered balcony.

back span of the slab and the relative rotation (Figure 3). With a thinner slab at the ends, this has the reverse effect of less dead load over the balcony, thereby creating a horizontal plane or even a reverse slope on the balcony. If deflections in the exterior bay due to dead load are approaching the maximum tolerable limit, this in turn would provide more rotation at the ends of the balcony (Figure 4). Interior balconies, on the other hand, have the same stresses as a typical floor. The main difference is a reduced slab thickness within the balcony.

The slope of the interior balcony significantly impacts the deflection of the exterior bay.

Rotation along the exterior edge of the balcony due to the deflection in the exterior bay will certainly cause some loss in slope of drainage.

If the immediate dead load deflection was equal to the step in the slab, the rotation induced by this deflection would cause a balcony slab that was practically level. Any additional induced deflection due to the long term effect of live load will certainly reverse the drainage in the balcony slab.

Post Tensioning Concerns Post tensioning can also have a large impact on the design and deflections of balconies.

As slabs become thinner and spans become

Figure 5: Crushed PT ends at a balcony slab edge.

longer in residential units, the need to counterbalance the forces and stresses in the slab is provided with post tensioning (PT). While the PT cables can play a significant role in deflection issues within the flat plate construction, the high internal force can sometimes have the opposite effect in balconies. A careful evaluation in the pre-compressive forces needs to be performed during design. For a true cantilevered balcony, post tensioning cables are often continued over the column and into the balcony to help with downward deflections and top stresses. However, pre-compressive forces can be much higher at the balcony edge. With the step in the slab and top of slab sloping to a minimum stipulated thickness at the ends, the effective area can be reduced by as much as 15%. This potentially can cause two problems. The first problem may arise during stressing. With a reduction in area, the compressive forces acting at the edge of the balcony during stressing could cause the concrete to crush and debond (Figure 5). This can be a headache for the general contractor and the concrete subcontractor. Remedial fixing and spliced stressing may be required. The second issue with a high amount of stressing at the ends could result in an over amount of reversed deflection. While post tensioning is used to control deflections, a high amount of stressing could negate any possible downward slope required for drainage. For an interior balcony, not only is the amount of stressing force an important characteristic in controlling reversed defections, but low points within the balcony slab need to be analyzed. If too much force and too much drape are placed within the interior balcony, the force pushing up in the slab due to drape can also reverse the slope and drainage requirements. A potential of tendon pop outs is also probable with less concrete in the interior balconies (Figure 6).

and guideline for following and maintaining standards, the life cycle a balcony slab experiences should also be evaluated. From the start of concrete pouring and stressing, deflections need to be evaluated for the construction loads and attachment of doors and railings.

If the initial deflection of a balcony slab is out of tolerance, the attachments of railings become difficult and sometimes improbable.

Live loading within the balcony should also be evaluated and reviewed. Figure 7 shows a typical post tensioned floor slab with the anticipated long term deflection, including

Figure 7: Anticipated long term deflection at a true cantilever balcony with full dead and live loading applied.

Loading Figure 6: Tendon popout at a balcony slab.

Anticipated loading can also play a huge role in how a balcony slab will deflect. While long term deflection is an important benchmark STRUCTURE magazine

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Figure 8: Anticipated long term deflection at a true cantilever balcony with only dead load applied.

Corrective Measures Depending on the nature and severity of the drainage, certain corrective measurements can be taken. In a recently built apartment building in Alexandria, Virginia, several of the tenants complained about water ponding on the top of the balcony (Figure 9).

Although water was not seeping back into the units, the standing water was causing concern for the unit owners. A survey of the existing balconies was done to determine the severity of sloping. A ground penetrating radar scan was done to determine the clear cover of the top bars. After reviewing

Figure 10: Typical completed through wall flashing repair for remedial fix to balcony drainage issue.

the existing structural drawings, it was determined that minor grounding of the top concrete surface could allow for proper drainage and not compromise the required clear cover for the top bars as specified in the structural drawings. In other cases, a small amount of patching compound was applied to the top surface and skimmed down to the edge of the balcony creating the necessary sloping needed to drain the water off the balcony. The patching compound applied could only be ¼-inch thick at its maximum point due to the proximity of the door and not create a tripping hazard when tenants walk in and out of their balconies. On more severe cases, when the amount of patching compound or grinding would not be sufficient to create the necessary drainage, a thick continuous waterproofing membrane on the top surface of the concrete balcony and an adhesive waterproofing sheet was applied at the joint between the exterior of the balcony and the edge of the unit (Figure 10). While this only created a temporary fix to help protect the residential unit from ponding water seeping into the livable area, more invasive measures would be needed later.

These measurements included demolishing the balcony and rebuilding with appropriate drainage requirements.

Conclusion

Figure 9: Water ponding at an existing balcony.

While the ACI Building code specifies total long term deflections due to live load and incremental loading of structures after the attachment of non-structural components, there has yet to be a total deflection criteria adopted. Therefore engineers that look to the code for deflection requirements often overlook the entire load history a balcony may experience and not take into consideration the possibility of a reversed deflection. While designing a balcony may seem like just another extension of the flat plate slab system, prudent engineering should be maintained to ensure and verify that deflections do not interfere with the required drainage requirements.▪

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full live load on the balconies. With full live load added to the deflection load combinations, minimal positive deflections at the balconies are prevalent. In Figure 8, the long term deflection is also calculated but the live load is removed from the balconies. With the removal of live load, the balcony deflects upward due to the amount of post tensioning in the slab. While building codes stipulate a minimum live load the balcony must support, an interesting study by Johnson and Merkel reviewed 100 balconies to determine the actual live load placed on balconies. It was found that, on average, a residential balcony can experience 50% less live load during a one year study period. While long term deflections are often difficult and hard to predict, balconies should be reviewed not only for the full long term dead and live load deflection, but also take into consideration that most balconies experience far less live load demand. If there is an anticipated live load that is missing from the balcony slab, this has the direct effect of less load weighing the slab down and therefore only has the full long term dead load and self-weight associated with a downward deflection.

Petr Vaclavek/shutterstock.de

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Structural PracticeS practical knowledge beyond the textbook

O

Figure 1: Modeling options for base condition.

ne of the least understood aspects of modeling building structures is dealing with at- and below-grade components. This includes soilstructure interaction, but also the question of which below-grade structural elements should be included in a lateral model and what is an accurate representation of the base conditions.

The focus of this article is what is most commonly referred to as the backstay effect. Traditionally, lateral systems have been viewed as simple cantilever beams fixed at the base. While this analogy is reasonable for the above-grade structure, a more accurate analogy would also include the effects of the below-grade structure, which behaves like a backspan to the cantilever. In this analogy, the lateral system is viewed as a beam overhanging one support, where that support is created by the at-grade diaphragm and foundation walls.

The backstay effect is not limited to restraint at the grade level. Backstay effects are also seen at setbacks with changes to the lateral system, the most common example being lower level podiums. They are often very large in plan and introduce new lateral elements, and are therefore significantly stiffer than the set-back structure above. Backstay effects are also impacted by multiple basement levels. For simplicity of explanation, this article will focus on the most common example which is the effect of the ground floor diaphragm in contributing to backstay effects. The concepts can be extended to all conditions where backstay effects occur.

Backstay Effect Backstay effects are most noticeable in buildings with discrete lateral systems, such as shear walls, as opposed to distributed lateral systems. Building height is also a major factor in the magnitude of the backstay effects. For the purposes of illustration, this article focuses on a high rise shear wall building with a single basement.

For a typical building with one or more below grade levels, the perimeter basement walls create a very large and laterally stiff box. The ground floor diaphragm engages this box and integrates it into the lateral system. Sticking with the beam analogy, the result is an effectively larger beam section below grade. This results in shedding of lateral load from the main lateral force resisting system (LFRS) to the basement walls. Overturning and shear are shared between the perimeter walls and core rather than isolated beneath the building core. Conceptually this is fairly straightforward.

The complexity arises in properly modeling the change in section, and capturing an accurate distribution of internal forces and external reactions.

The degree to which lateral loads are transferred into the foundation perimeter is dependent on many variables, many of which there is limited certainty about, as they are not specified or controlled in a typical project. It is therefore fair to ask if it is more conservative to simply ignore any backstay effects and model the building core as an isolated element. However, it can be shown that in many cases the backstay effect will create higher demands in some structural elements, in particular shear in the main LFRS below grade as well as the backstay diaphragms, and therefore cannot be ignored.

Figure 1 is a stick diagram presenting some of the possible options for modeling the base conditions of a core wall building. The building is of height H with a basement of height B. The most traditional model, a simple cantilever, is shown in Figure 1a. It is clear that the maximum shear is V = F. The extreme case of the backstay effect is shown in Figure 1b. In Figure 1b the ground floor diaphragm and perimeter foundation are very stiff and are therefore modeled as a pin.

Statics shows that the maximum shear in the core now occurs below grade with V = 3H/2B F. The

STRUCTURE magazine

Backstay Effect

23

Basement Modeling in Tall Buildings

By Nat Tocci, P.E. and Sanya Levi

Nat Tocci, P.E. is a structural engineer in New York City and can be reached at nat.tocci@gmail.com.

Sanya Levi is an engineer at Arup New York and can be reached at sanya.levi@arup.com.

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overall base shear has not changed, but the backstay effect may create conditions with much higher demands than anticipated in certain elements. It can also be shown that the base overturning moment in the core has been reduced and redistributed to the perimeter foundation walls.

Although Figure 1b shows dramatic increases in shear, this is overly conservative for most conditions. The true restraint at the ground floor is far from rigid and may range from very stiff to almost non-existent. A more realistic model is one in which the ground floor restraint is modeled as a spring, producing results somewhere between Figures 1a and 1b. Figure 1c shows this option.

The complexity of an accurate model lies in the fact that the spring in Figure 1c represents the cumulative stiffness of numerous elements in the building structure and supporting soil. A partial list of elements represented by the ground floor spring would include: diaphragm to core connection, diaphragm stiffness, diaphragm to basement wall connection, basement wall stiffness, foundation stiffness, and passive soil resistance against the basement wall.

Ground floor diaphragms are often thick concrete plates with high relative stiffness.

However, this stiffness may be reduced by cracking, bond slip, and discontinuities such as large openings or slab elevation changes.

In addition to the stiffness of the diaphragm itself, the connections at each end must be considered for their ability to transfer the backstay shears. The same can be said for the basement walls which will have varying stiffness dependent on the same factors.

The overall stiffness of the diaphragm and basement wall system is also affected by the supporting foundation elements. Differences in relative stiffness between core and perimeter wall soil support conditions may magnify or lessen backstay effects.

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should also be considered. This component is typically small relative to the other elements and may possibly be neglected in many cases.

In addition, this force is present only in the compression cycle of loading and should be modeled as such.

Clearly there are many parameters to consider.

In most cases, the best that can be done is to model all contributing elements and make an educated estimate of the element stiffnesses.

The number of possibilities is too numerous for a prescriptive approach that will work for all buildings, which is perhaps why there is little literature on the subject. Most building codes provide requirements for loading and design of structural elements, but rarely provide detailed guidance on modeling procedures. A very good resource for an in depth discussion of the backstay effect and recommendations for modeling is Modeling and Acceptance Criteria for Seismic Design and Analysis of Tall Buildings, PEER/ATC 72-1, which is available as a free download from the PEER (Pacific Earthquake Engineering Institute) website.

The backstay concept is more familiar to engineers working in high seismic regions and has had less attention in other regions.

The concepts, however, are applicable for both wind and seismic loading.

Modeling A reasonable first step may be to assess whether the backstay effect is a consideration for the building under investigation. A quick study of the parameters that create the backstay effect may quickly rule out the need for a more in depth analysis. The building system or configuration may also determine the potential for backstay effects.

For buildings where backstay effects need to be considered, it will most likely be necessary to consider multiple scenarios. Both an overestimation and underestimation of backstay effects can produce underestimates of demand. For example, overestimating backstay restraint may underestimate the overturning demand at the base of the main LFRS. The common approach is to consider reasonable extremes for both conditions and design each element for the bounding condition. This is typically referred to as bracketing.

The backstay diaphragms must be modeled as semi-rigid elements. Semi-rigid elements have stiffness taken from the material and geometric properties of the slab. Any large discontinuities in the slabs should be modeled, and a mesh size should be chosen that produces accurate results. To account for cracking, bond slip,

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June 2012

Typical concrete core building configuration.

interface slip, and other unknowns, the stiffness of the slab should be reduced for both shear (GAv) and flexural (EI) stiffness. Similar modeling guidelines and stiffness reductions should also be applied to basement wall elements.

Soil stiffness should also be bracketed, typically starting with recommendations provided by the project geotechnical engineer. The supporting stiffness under all elements should be taken at an upper and lower bound, and passive resistance provided against the perpendicular wall should also be bracketed if it is modeled.

PEER/ATC 72-1 Table A-2 and Table A-3 provide recommended upper and lower bounds for bracketing the stiffness of the above elements. PEER/ATC 72-1 also recommends that elements outside of the backstay influence (primarily tower elements) need not be bracketed and should be modeled with the same assumptions used for their design.

Since these recommendations are intended for buildings in high seismic regions, it may be appropriate to adjust the recommendations for wind controlled design to account for primarily elastic behavior.

Due to the complexity of capturing backstay effects in the analysis, it may be desired to eliminate the phenomenon in the actual building.

This can be accomplished by isolating the LFRS from the foundation elements by providing lateral joint at the backstay diaphragms. Typically this is done by providing a corbel or similar detail at the diaphragm to shear wall interface.

Conclusion Ignoring the contribution of at- and belowgrade structural elements in lateral models may underestimate demands in key elements. A quick initial study may be enough to determine if a more in-depth model, which includes backstay elements, is justified. If backstay effects are included in the model, current practice is to bracket stiffness parameters and design for a bounding solution. Unfortunately, this approach results in overdesign of at least some members. As knowledge of the topic increases, bracketing parameters will be refined and increase the efficiency of designs.▪

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A Structural Facelift

Blast Resistant Over-Clad of 33-Story 1967 Federal Building

By William D. Bast, P.E., S.E., SECB, Kevin J. Jackson, P.E., Dziugas Reneckis, Ph.D., P.E. and Eric J. Wheeler

O

ver the next several years, the A.J. Celebrezze Federal Building, located in Cleveland, Ohio, will be over-clad with a new glass and metal curtain wall. The façade over-clad will create a new exterior double-wall, using the existing original aluminum mullion, glass window, and stainless steel panel curtain wall system as the inner wall of the new system (Figure 1). Overall, the new façade will significantly reduce the amount of energy used for heating and cooling. An outrigger and girt system consisting of structural steel framing will support the new skin and withstand the demands of wind, blast, as well as thermal loads. The new curtain wall support will be installed from the building exterior, with minimal disruption to the building tenants.

The Structural Elements The 33 story office building, completed in 1967, relies on steel moment frames in both directions and a reinforced concrete core extending up through the 13th story to resist lateral loading. Perimeter steel column and spandrel beam framing are encased in concrete for fire protection only. Story heights are typically 12 feet 6 inches, extending to a total building height of approximately 420 feet above grade.

The existing façade is an aluminum mullion, glass window, and stainless steel panel curtain wall system. The new glass and metal curtainwall system will be placed approximately 3 feet beyond the face of the existing façade (Figure 2). The glass of the new curtainwall will be treated with a frit pattern to reduce direct glare and heat loss on the building systems. One aspect of the façade over-clad involves replacing the original vision glass. For access to maintain and clean

Figure 2: New glass-and-metal façade over-cladding existing façade (background). Courtesy of Interactive Design | Eight Architects.

STRUCTURE magazine

the double wall cavity, the existing windows at three locations on each floor will be replaced with an operable window.

A steel outrigger-and-tube girt system was designed to support the new façade that will be outboard of the existing curtainwall Figure 1: Rendering of A.J. system (Figure 3). The outriggers Celebrezze Federal Building with consist of a built-up tee section façade over-clad. Courtesy of connected to the existing steel Interactive Design | Eight Architects. columns through the existing façade panels and concrete encasement (Figure 4, page 28). The stem of the tee extends perpendicular to the face of the building, with two additional flange plates that will sandwich a rectangular tube girt. The tube girt will span horizontally between the outriggers and parallel to the exterior spandrel beam. The tube outriggers are oriented to resist strong-axis bending under out-of-plane loading, and weak-axis bending under gravity load. The new façade connects to the girts with a blind bolt connection at the mullions.

The outrigger-and-tube girt system has been designed to accommodate the mill, plumbness, and position tolerances of the existing perimeter columns, as well as the fabrication tolerances of the new support tubes. The erected tolerances of the existing building structure were assumed to have met the AISC Code of Standard Practice for Steel Buildings and Bridges. The outrigger was detailed such that the built-up tee could be surveyed and set to the correct elevation.

The top and bottom flange plates were sized to accommodate the expected out-of-plumbness tolerances of the columns. The girt-tostem connection was designed as a field welded clip angle that could accommodate the expected gap between the tube and built-up tee.

The out-of-plane position of the girts developed into a struggle between architectural constraints and structural demands. The architectural and cost implication desire was to keep the girts as close to the existing building skin as possible. The structural design required outof-plane strength and stiffness to resist the wind and blast forces. An outrigger-and-tube girt system was ultimately designed to satisfy the various design criteria, while maintaining the architectural constraints.

During erection, openings will be cut in the existing stainless steel façade panels and concrete encasement to weld the outriggers to the existing columns. To appropriately size the welds and check the base metal strength, material testing was performed to confirm the steel grade of the existing columns listed in the original structural drawings – ASTM A36 and ASTM A441 steel. Attachment of the tube girts, which are continuous, will follow the installation of the outriggers.

Temporary construction joints within the continuous girt are required to accommodate the range of temperatures that the girts may experience during construction. The tube girts will be exposed for at least a year before the new curtainwall is installed and the building enclosed.

Consequently, the expected temperature variation in Cleveland throughout a full calendar year may vary from -30 degrees F to 130 degrees F. A series of temporary joints were designed to mitigate the

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Figure 3: Rendering of outrigger-and-tube girt system attached to existing building. Courtesy of Thornton Tomasetti.

construction load case. The construction load case was based on the expected temperature variation in Cleveland throughout a full calendar year, as indicated earlier, as well as the timing of certain construction operations that would introduce additional restraint to the system.

The operational load case considered the temperature variation in the cavity wall – 30 degrees F to 115 degrees F – established by a computational fluid dynamics (CFD) analysis conducted by the mechanical engineering consultants. continued on next page

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effects of the thermal load during construction and the capacity of the outrigger-and-tube girt in the final condition. The joints were located within seven feet of the outriggers, near the point of contraflexure under gravity loads.

Input from the contractor and steel erector led to the development of several alternates for the temporary construction joint.

The joint required a complete joint penetration (CJP) groove weld to connect the adjacent tubes, and to establish continuity between the tube sections. However, due to the proximity of the existing façade, there was not adequate clearance behind the tube for a successful CJP field weld. A successful solution was developed through coordination with the contractor and steel erector that satisfied construction constraints and structural demands.

Main wind force resisting system (MWFRS) and components and cladding wind load criteria were obtained from wind tunnel tests of a 1:300 scale building model. The new double wall façade increased the surface area of the office building by approximately four percent, which was an expected increase in demand. However, the wind tunnel study yielded pressures that were approximately double the original design wind pressures at several localized areas of the building elevation.

Blast loading and performance criteria were provided by a blast consultant in the form of pressures with corresponding impulse magnitudes based on building risk and hazard criteria.

Two thermal load cases were considered – an operational load case and a STRUCTURE magazine

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Design Loads and Performance Criteria

Figure 4: Typical outrigger connection to existing concrete encased steel column.

Courtesy of Thornton Tomasetti.

In addition to strength demands of blast and thermal loads, the support girt design also had to meet stringent serviceability requirements. Allowable deflection limits for the new facade were specified by the curtainwall design consultants for wind and gravity load.

However, the design was controlled by inelastic deflection limits set by the blast consultant.

Analysis and Design The existing building superstructure and foundation were modeled in ETABS© to evaluate the existing building frame for the new curtainwall weight, updated design wind pressures, and current building code seismic load. The outrigger-and-tube girt design was an iterative process that considered various load cases for blast, wind, thermal, and construction.

For the blast design of the girts, a non-linear elastic-plastic dynamic analysis was performed on a single-degree-of-freedom model of the tube. The blast load was idealized as a triangular impulse load.

Continuity of the tubes at the support locations were required to satisfy the blast performance criteria. The tubes were either HSS14x10 sections or built-up 14x10-inch tubes, with wall thicknesses varied to meet the blast load intensity. Concrete filled tubes were used to reduce the girt response where unfilled tubes yielded an undesirable response in regions of high blast.

The outriggers and tubes were modeled separately in RISA 3D© at each elevation to quickly assess the outrigger flange plates and built-up tee section. The outrigger was specifically modeled to reflect the variation in stiffness of the outrigger from the girt to the existing column.

Component dimensions, plate thickness, and steel grades of the flange plates and built-up tee were adjusted in each model until the outrigger had sufficient capacity to satisfy all of the load combinations.

The construction load case analysis evaluated the outriggerand-tube girt system for the combined effects of the welding sequence to close the temporary construction joints and withstand the thermal loads. The study revealed that the capacity of the tube girts and outriggers in the final condition were sensitive to steel temperatures during certain welding sequences. To communicate the findings to the contractor, allowable temperature ranges for welding were provided for various scenarios that could occur during construction.

The structural engineers chose to use up to 100 KSI yield strength steel for the outriggers. This enabled the built-up tee shapes to be as flexible as possible in-plane to minimize stresses due to temperature variation, while affording a robust shape to withstand large blast force effects.

A customized spreadsheet facilitated the design of the welds for the outrigger, the outrigger-to-tube connection, and the outrigger-to-existing column connection. Hand calculations of fully designed connections were used to verify the results computed by the spreadsheet. Design forces for the connection design were imported from the analysis program.

The building frame analysis results indicated that the existing steel superstructure had adequate strength to support the weight of the new curtain wall combined with the design wind and seismic loads. The outrigger-and-tube girt system was ultimately designed to accommodate the various design criteria, while meeting the architectural constraints.

Conclusion The over-cladding of the A.J. Celebrezze Federal Building presented unique engineering challenges. Aside from normal loading and design conditions, the erection procedure of the outrigger-and-tube girt system had a large influence on the final design. Additionally, great care was taken to accommodate tolerances of the new and existing construction. Ultimately, a thoughtful design was developed addressing a variety of design issues.

Construction for the new curtainwall is scheduled to begin in the summer of 2012.▪

Project Team

William D. Bast, P.E., S.E., SECB is a Principal at Thornton Tomasetti in Chicago, Illinois. Bill may be reached at WBast@ThorntonTomasetti.com.

Structural Engineer of Record: Thornton Tomasetti, Chicago, IL Building Owner: U.S. General Services Administration Architect of Record: Interactive Design | Eight Architects, Chicago, IL Blast Consultant: Hinman Consulting Engineers, Inc., San Francisco, CA Curtain Wall Consultant: Curtainwall Design Consulting, Chicago, IL MEP Engineer: Jacobs, Chicago, IL Contractor: DCK North America, LLC, Pittsburgh, PA Construction Manager: Gilbane Company, Cleveland, OH

STRUCTURE magazine

Kevin J. Jackson, P.E. is an Associate at Thornton Tomasetti in Chicago, Illinois. Kevin may be reached at KJackson @ThorntonTomasetti.com.

Dziugas Reneckis, Ph.D., P.E. is a Project Engineer at Thornton Tomasetti in Chicago, Illinois. Dziugas may be reached at DReneckis@ThorntonTomasetti.com.

Eric J. Wheeler is a Senior Engineer at Thornton Tomasetti in Chicago, Illinois. He can be reached at EWheeler@ThorntonTomasetti.com.

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Cost-Effective Solution Controlling Perception of Wind-Induced Motion in Slender Buildings

By Sunghwa Han, P.E., S.E., LEED AP BD+C, Gabe Klein, P.E. and Jacob Grossman, P.E., FACI, SECB

S

atisfying occupant comfort criteria is a common challenge in the design of slender high-rise buildings. When wind-induced accelerations exceed the acceptable limits for human comfort, structural engineers typically try to increase building mass or stiffen the building. Nevertheless, this approach may not be a cost effective or feasible option, especially when the structural system has already been determined and the option of introducing additional structural components is limited.

When a project is initiated, structural engineers estimate wind loads in compliance with building codes and standards and adjust them, if necessary, based on their past experience and knowledge of the area. In New York City, it is not uncommon to find wind loads for a nearby building of similar scale. However, when it comes to wind loads, often the information from neighboring buildings is not applicable to the building under design due to the complex nature of wind-structure interaction. In addition, even though design standards are available to structural engineers to compute wind loads, these standards are not sufficient to predict all possible issues discovered after wind tunnel testing is completed.

Two slender towers, Building A, (the real name of Building A is not identified, per the request of the building owner), and W New York Downtown Hotel and Residence (referred to as the W Downtown Hotel in this article), recently built in New York City, exemplify the aforementioned cases.

Structural System Building A is a 60-story, 650-foot tall residential building. The building footprint at the typical floors is 150 feet by 65 feet, and the building slenderness ratio is 10:1. A dual system, combining moment frames using a typical 10-inch thick flat plate slab with columns and shear walls, is used as a lateral load resisting system to resist wind and seismic loads. In order to maximize the efficiency of each structural component, shear walls extend the full 65-foot width in the northsouth direction at the base of the building and a thicker 12-inch thick flat plate slab is used at the 35th floor and above, where their frame action is more effective. This enabled engineers to increase the stiffness and mass of the building to a certain extent without elongating the building periods, which generally increase the resonance portion of wind induced responses.

The W Downtown Hotel is a 57-story, 627-foot tall mixed-use building (Figures 1 and 2) with a slenderness ratio of 11:1, located one block south from the World Trade Center site. The footprint of the typical floors is 124 feet by 57 feet. The top 24 floors are high-end condominium units and the bottom 30 floors are occupied by amenities, mostly hotel rooms and furnished residences. Two mechanical floors are strategically used for lodging full height belt walls (full height reinforced concrete spandrel beams connecting exterior columns) at the 31st floor and 100-inch deep belt beams at the 57th floor. These belt walls and beams supplement the framed tube action initially provided by the exterior columns and flat plates but reduced by transfers at the 6th floor.

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Figure 1: W New York Downtown Hotel and Residence.

June 2012

2% of the critical damping for the selected structural system of the tower, which are comprised of flat plates and shear walls coupled with shallow link beams at every level, the peak acceleration of the taller building excluding influence of hurricanes was estimated to be 19.4 mg at the top occupied residential floor (55th floor). For Building A, the peak acceleration of the 650-foot tall structure was estimated to be 22.4 mg at the topmost occupied residential floor.

Acceptable Limit in Accelerations For both Building A and the W Downtown Hotel, the wind studies indicated that accelerations were excessive at the floors where longterm occupants will reside. The commonly acceptable range for 10 year peak accelerations is 15 mg to 18 mg for residential towers and 18 mg to 20 mg for office towers. This acceptable range can be varied depending on the natural frequencies of the structure, as occupant’s sensitivity to motion decreases when the natural frequencies of buildings are lower. Therefore, buildings with longer periods can generally allow larger accelerations in terms of perception to motion.

As the initial studies indicated that the accelerations of both buildings were excessive, the structural modifications, such as increasing the stiffness and increasing the general mass, were investigated. According to the engineer’s study in cooperation with the wind tunnel testing lab, adding massive shear walls at the base of the building for Building A and improving frame tube action by enlarging exterior columns and reducing exterior spans for the W Downtown Hotel would have produced the targeted accelerations. However, these modifications would have required architectural compromises and reduction of valuable space. As an alternative option, introducing supplementary damping systems to improve the performance of both structures under the 10 year return period wind loads was explored.

Figure 2: Construction of W New York Downtown Hotel and Residence.

Wind Responses Obtained from Wind Tunnel Testing

Supplementary Damping Systems

Wind responses under two configurations of the surroundings were studied to predict the most critical wind loads for designing the structure and evaluating the serviceability performance. In both cases, the future neighboring buildings provided a shielding effect, and benefits from the future adjacent buildings turned out to be more significant in the east-west direction. Despite these benefits, both towers were designed for wind loads under the present configuration, since it resulted in the most critical wind responses.

For the initial study, stiffness of the structures under two different return periods (50 year and 10 year) were estimated based on the modified code specified wind loads (knowing that code specified wind loads do not include cross wind responses and torsional responses).

After the initially estimated wind loads were provided by a wind tunnel lab, a more precise analysis was preformed to estimate cracked sections. Then the corresponding structural dynamic properties under these estimated wind loads were sent back to the wind tunnel testing lab. The final wind loads for the design of structural members and evaluation of serviceability performance were established after several iterations, in order to reach compatible results between the wind responses and the stiffness of the structures corresponding to estimated cracked conditions.

The W Downtown Hotel was initially designed as a 55-story building, and the peak acceleration of this structure was estimated to be 17.4 mg (milli-g: 1/1000th of gravity acceleration) at the 53rd floor.

By the end of the design development phase, the owner decided to add two more floors to the building. Assuming a damping ratio of STRUCTURE magazine

Various types of supplementary damping systems (SDS) were considered: a tuned mass damper (TMD), a deep tuned sloshing damper (DTSD), a shallow tuned sloshing damper (STSD) and a tuned liquid column damper (TLCD). For both towers, a TLCD was not a feasible option because of space limitations. A tuned mass damper was also excluded due to the higher cost and maintenance requirement in comparison with a deep tuned sloshing damper (DTSD) which was eventually selected. Two levels of target performance were investigated during an initial study for the W Downtown Hotel. Level 1 performance aimed for 2.0 mg of reduction in the peak acceleration and Level 2 performance aimed for 3.0 mg to 4.0 mg of reduction.

In the end, Level 2 performance was achieved.

Tuned Sloshing Damper (TSD) A tuned sloshing damper utilizes liquid waves to absorb energy from vibrating structures through wave travel and viscous action in a partially filled tank of liquid. The tank is designed so that the liquid surface wave has a frequency “tuned” to be near the fundamental frequency of the building for the optimal performance of a tuned sloshing damper. The frequency of the liquid is determined by the density, length, width and depth of the liquid.

During the initial wind study, it was found that the majority of the excessive peak acceleration of Building A was in the north-south direction. The W Downtown Hotel was somewhat different from Building A. The acceleration in the north-south direction was also the primary contributor to the large acceleration but, due to mainly

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damper tank (Figure 5) were installed in the tank. Similarly, for the W Downtown Hotel, nine paddles were hung from the ceiling of the concrete tank (Figure 6). Each paddle consists of 1-inch thick by 10-inch wide galvanized steel plates welded to two 1-inch thick by 4½-inch wide plates to a create cross shape section. These paddles work in both directions to provide additional damping and ultimately result in reduced peak accelerations. The total construction cost of a damper for the W Downtown Hotel was estimated to be less than $200,000.

Figure 3:TSD location plan and section (Builindg A–58 th floor plan).

Frequency Measurement and Tuning

Figure 4: TSD location plan and section (W Downtown Hotel -56 th floor).

across wind responses, the acceleration in the east-west direction was not negligible. Therefore an effort was made to reduce accelerations in both directions. Considering the contribution of accelerations in each direction, a one-directional-tuned-deep-sloshing damper (18 feet x 45 feet x 11.6 feet high) (Figure 3) and a bi-directional-tuned-deepsloshing damper (27.25 feet x 23.16 feet x 8 feet high) (Figure 4) were evaluated to be the most cost-effective and space-optimal option for Building A and the W Downtown Hotel respectively.

Construction of Dampers A tuned sloshing damper consists of a damper tank, liquid and screens or vertical hangers generating turbulence of water in motion. When a wind event begins, the liquid resonates out of phase with the structure and energy is dissipated from the liquid by flowing through these devices.

For Building A, three slat screens parallel to the short direction of the

Figure 5: Scale model of one-directional TSD (Building A) for shake table performance test. Courtesy of RWDI and Motioneering Inc.

STRUCTURE magazine

Construction of the damper tanks, made of cast-in-place concrete, needed to proceed with the rest of the concrete construction.

Hence it was important to confirm the predetermined dimensions of the damper tanks prior to their construction based on the measured building frequencies. These measured building frequencies were compared with the estimated building frequencies using FEM (Finite Element Method) analysis.

Considerations for the in-situ conditions at the time of measurement had to be taken into account. Non-cracked sections were assumed under the ambient wind loads. A reduced building mass, which excluded the weight of the missing mechanical equipment, was used. Lastly, the higher strength of the tested and in-place concrete in some vertical members was incorporated in the structural model.

For both towers, the measured building frequencies were in a range of 10% of their estimated building frequencies (Table ). The concrete damper tank for each damper was built per the original design without any modifications.

For the W Downtown Hotel equipped with the bi-directionaltuned-sloshing damper, the water depth was primarily tuned for the natural frequency of the building in the north-south direction. The dimension of the damper tank in the east-west direction was left

Figure 6: Scale model of bi-directional TSD (W Downtown Hotel) for shake table performance test. Courtesy of RWDI and Motioneering Inc.

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Table: Summary of estimated and measured building frequencies of the W Downtown Hotel.

Estimated Frequencies by FEM (Finite Element Method) analysis

Measured Frequencies by Monitoring

Condition

Design phase (Assume completed structure and cladding)

As built-condition (reduced mass and 90% completion of cladding)

Interim monitoring under as-built-condition (reduced mass and 90% completion of cladding) Oct 2009

Final monitoring under as-built-condition (after completion of structure and cladding) June 2011

Purpose

Evaluate serviceability performance in terms of motion perception during an initial study

Ensure dimensions of a damper tank for construction

Ensure dimensions of a damper tank for construction

Establish frequencies of the building and the damper for tuning

X-direction (E-W)

0.208 Hz (4.8 sec)

0.253 Hz (3.9 sec)

0.278 Hz (3.6 sec)

0.266 Hz (3.8 sec)

Y-direction (N-S)

0.185 Hz (5.4 sec)

0.208 Hz (4.8 sec)

0.192 Hz (5.2 sec)

0.183 Hz (5.4 sec)

Torsion

0.322 Hz (3.1 sec)

0.377 Hz (2.6 sec)

0.418 Hz (2.4 sec)

0.402 Hz (2.4 sec)

Frequencies (Periods)

Estimated Peak Accelerations Without a tuned sloshing damper

19.4 mg (2% of inherent damping)

18.1 mg (2% damping) – 20.9 mg (1.5% of inherent damping)

With a tuned sloshing damper

15.6 mg (2% damping) – 17 mg (1.5% of inherent damping)

flexible for future adjustments, which would consist of constructing additional layers of concrete masonry unit walls at the north wall or at the south wall of the damper tank. However, interim monitoring results indicated that adjustments in the tank dimensions of the damper basin were not necessary.

After construction of the W Downtown Hotel was completed, the final monitoring was performed to ensure the performance of the TSD that was filled with 36 inches of water, predetermined from the initial study. This final monitoring indicated that the measured frequency of the damper was slightly different from the measured frequencies of the completed structure. As a result of this measurement, the water level of the damper was adjusted to 27 inches.

Design Team

Summary Within the last five years, more buildings have been equipped with supplementary damping systems. Since buildings are getting taller and more slender, conventional methods to improve their performance in terms of motion perception may no longer be cost-effective. These traditional methods such as increasing stiffness or generalized mass without negatively affecting building frequencies, result in increased construction costs and loss of valuable space. From two buildings recently designed and built in New York City, engineers have learned that a tuned sloshing damper can be a competitive alternative to those traditional means. Also, this system can be fitted to the buildings not only to decrease accelerations but also to reduce wind loads, as long as the supplementary damping system is properly tuned for the building frequencies under the considered wind loads.▪

Building A Structural Engineer: Rosenwasser/Grossman Consulting Engineers, P.C. Wind Engineering Consultant: Rowan Williams Davies & Irwin Inc. / Motioneering, Inc.

Sunghwa Han, P.E., S.E., LEED AP BD+C is an Associate with Rosenwasser/Grossman Consulting Engineers, P.C. (RGCE) and was the project manager for W New York Downtown Hotel and Residence. Sunghwa may be reached at sunghwa@rgce.com.

Gabe Klein, P.E. is a Senior Associate with RGCE and was the project manager for Building A.

W New York Downtown Hotel & Residence Structural Engineer: Rosenwasser/Grossman Consulting Engineers, P.C. Developer: The Moinian Group Wind Engineering Consultant: Rowan Williams Davies & Irwin Inc. / Motioneering, Inc.

Jacob Grossman, P.E. FACI, SECB, ACI Honorary member, is the President and CEO of RGCE and personally directs design and research for the firm. Jacob may be reached at jacob@rgce.com.

The online version of this article contains detailed references.

Please visit www.STRUCTUREmag.org.

STRUCTURE magazine

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June 2012

Condition Assessment and Repair An Existing Composite Concrete Slab and Steel Beam Framed Parking Structure Part 2

By D. Matthew Stuart, P.E., S.E., F. ASCE, SECB

I

n the fall of 2010, a property management company retained Pennoni Associates Inc. to conduct a condition assessment of a large sub-grade parking garage and loading dock located in Center City, Philadelphia. Constructed during the 1980s, the structure was exhibiting signs of significant deterioration of the wearing surface and the supporting composite metal deck. Establishing the extent and cause of the deterioration, and identifying appropriate repairs, required a thorough condition assessment of all of the sub-grade loading docks, parking areas and ramps. The following article is a continuation of one that appeared in the April 2012 issue of STRUCTURE. It presents a discussion and assessment of the observations and material testing described in Part 1.

Physical Condition

Petrographic Analysis Summary Core No.

Coarse Aggregate

Percent Entrained Air

Atypical Features

1

½-inch diameter lightweight expanded clay

None

Non air entrained

2

0.6-inch diameter lightweight expanded clay

5%

Large water voids

3

½-inch diameter lightweight expanded clay

3% – 6%

4

½-inch diameter regular weight limestone

4%

No lightweight course aggregate.

Unusual water voids. Clay particles detected in paste matrix.

5

0.6-inch diameter lightweight expanded clay

4%

Numerous water voids

6

0.40-inch diameter lightweight expanded clay

4%

Large water voids

The surface spalling and subsurface delamination observed at both levels of the garage and loading dock areas, and most of the cracking observed at the upper level, resulted from corrosion of the internal reinforcement and other embedded and exposed steel. Corrosion is an electrochemical process that requires an anode, a cathode and an electrolyte. For the existing structure, the moist concrete provided the electrolyte and the reinforcement, and other steel provided the anode and cathode. Water and oxygen must be present for any corrosion to take place, and in good-quality concrete the high alkalinity slows the corrosion process. However, if the concrete alkalinity is lowered (due to carbonization) or if corrosive chemicals (like chlorides) are present, the corrosion process will accelerate.

The electrical current that flows between the cathode and anode results in an increase in metallic volume at the anode as iron is oxidized and precipitates as rust. As the layer of rust increases due to the ongoing corrosion process, tensile forces generated by the expansion of the corrosion byproduct cause the surrounding concrete to crack, delaminate and ultimately spall. As the surface of the concrete degrades, the corrosion process accelerates because internal areas of the reinforcing that were previously protected by the concrete cover are exposed to moisture and road salts transported into the garage on vehicles. In addition, abrasion of the exposed concrete surface due to vehicular wheel traffic further damages the exposed concrete and internal reinforcement.

Eventually, if left unchecked, the corrosion process will result in the complete destruction of the concrete slab.

STRUCTURE magazine

No epoxy sealer. Paste to aggregate ratio poor.

In the case of the exposed metal deck, the corrosion process had resulted in damage and destruction even though the material was originally galvanized. The most significant aspect of this deterioration involves the deck’s contribution to the structural capacity of the concrete slab above. Composite metal deck provides two important functions: it acts as a stay-in-place formwork that supports the dead weight of the concrete until it has achieved adequate compressive strength; and, it serves as the flexural reinforcement for the composite, one-way slab section that spans between supporting steel beams.

The composite action between the metal deck and concrete slab is made possible by two factors. First, deformations located along the entire length of the fluted deck allow the concrete and metal deck to bond mechanically. Second, the location of the metal deck at the bottom of the slab, where the maximum positive flexural tensile stresses occur, enables the steel to contribute more efficiently to the composite section, while the concrete resists the maximum compressive flexural stresses located at the top of the slab.

Additional internal reinforcement within the concrete slab provides resistance to other stresses besides those induced by positive flexural bending. Welded wire reinforcement helps to prevent cracking that can result from induced thermal and shrinkage stresses. Supplemental bars located in the top of the slab over the primary beams help to resist negative flexural bending tensile stresses. However, in the absence of the positive flexural capacity provided by the composite metal deck, the ability of the concrete slab to support the imposed vehicular live loads is significantly

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June 2012

Image from Petrographic Analysis.

impaired, even though the welded wire reinforcement (via catenary action) and the top bars (by limiting moment redistribution) provide some additional capacity.

Material Testing Petrographic Analysis With the exception of some atypical features noted in the report, in general, the results of the petrographic analysis of the six core samples indicated that the concrete appeared to be of good quality. It was not clear, however, why entrained air content was not detected in core sample #1. Normally, concrete contains anywhere from 1% to 2% entrapped air; a value above 3% is indicative of the use of an air entrainment admixture. This admixture is typically recommended in concrete that will be exposed to exterior conditions because it greatly improves the ability of the concrete to resist damage that can result from freeze-thaw cycles.

The concrete in the garage is not exposed to the exterior environment, and therefore not ordinarily susceptible to freeze-thaw cycles.

However, the concrete may have been placed during winter conditions when the slab was temporarily exposed to cold weather and freezing temperatures. As indicated below, the water-soluble chloride tests suggested that this was, in fact, the case.

Water-Soluble Chloride Tests In order to prevent the corrosion of internal reinforcing steel and the subsequent deterioration of the surrounding concrete, the American Concrete Institute (ACI) recommends that the ratio of water-soluble chlorides to weight of cement not exceed 0.10% for conventionally reinforced concrete structures in a moist environment that is exposed to chlorides. Although the existing garage and loading dock are in an enclosed building, moisture is still brought into the garage via any wet vehicles that enter the facility on a rainy or snowy day. Also, as the garage entrances are open to the exterior atmosphere, the environmental humidity within the garage and loading dock is likely to be very similar to the external conditions.

In addition, during the winter months deicing salts (a source of chlorides) are brought in and deposited on the garage surface from any wet vehicles that have been driving on the outdoor road surfaces.

Therefore, the parking garage and loading dock should comply with the chloride limitations recommended by ACI to mitigate potential deterioration of the slab, embedded steel and metal deck.

STRUCTURE magazine

The results of the chloride powder sample tests indicated that the actual chloride content per mass of concrete (modified by a factor of 5 to approximate the equivalent cement content) was 25 times greater than the recommended limit of 0.10%. In addition, the chloride content at the base of the concrete where it is in contact with the metal deck and protected from exposure to deicing salts was as high as 8.5 times greater than the 0.10% limit.

Some of the chloride content in the samples taken from the top surface of the concrete likely resulted from deicing salts that were brought into the garage by vehicles that had just driven on wet and icy roadways during the winter months. However, the magnitude of chloride content at the exposed slab surface and the presence of elevated chloride content at the base of the slab, in particular at an area where the exposed wear surface had been painted, indicated that there was probably a secondary source of chlorides in the concrete.

The most plausible explanation for the elevated chloride content in the concrete slab is that a chloride-based set accelerator was used as a chemical admixture in the original concrete mix. Set accelerators are typically only employed during winter concreting operations when there is a desire for the concrete to set more quickly than normal, in order to avoid freezing of the mix water before complete hydration occurs. Therefore, it is very likely that the concrete for the garage was placed during winter conditions when the slab was temporarily exposed to cold weather and freezing temperatures.

Carbonation The high alkalinity of concrete protects any internal reinforcing from corrosion by creating a layer of passivity around the embedded steel.

When concrete is exposed to carbon dioxide from the combustion exhaust of vehicles and the surrounding polluted atmosphere, the gases are absorbed by the concrete and react with any dissolved calcium hydroxide associated with the free moisture located in the pores of the concrete. As a result of this reaction, the alkalinity of the concrete is reduced from its normal pH level of approximately 12.5. Once the pH level reaches approximately 9, the layer of passivity is lost and the internal steel is no longer protected from corrosion.

Phenolphthalein is a substance that is used to detect the effects of carbonation by reacting with the surface of the concrete if the alkalinity is above a pH of 9. If the concrete has a pH higher than 9, then the phenolphthalein reacts and turns reddish in color. If there is no reaction due to low alkalinity in the concrete, the phenolphthalein remains colorless.

The results of the phenolphthalein tests of both surface spalls and core samples indicated that only the exposed concrete surfaces, to a depth of no more than 1/8 inch, have been carbonized in the upper parking garage and loading dock areas. As there was no embedded reinforcing in the top 1/8 inch of the concrete, the presence of the carbonated concrete was therefore not contributing to the loss of any corrosion resistance of the internal steel.▪

35

D. Matthew Stuart, P.E., S.E., F. ASCE, SECB (MStuart@Pennoni.com), is the Structural Division Manager at Pennoni Associates Inc. in Philadelphia, Pennsylvania.

Part 3 of this article will appear in a future issue of STRUCTURE magazine, and will present a description of the repairs developed as a result of the condition assessment and analysis of the physical observations and material testing discussed in Parts 1 and 2.

June 2012

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

M

any buildings have been structurally retrofitted to meet current seismic codes, but what about the Mechanical, Electrical, and Plumbing or MEP systems? When the next big earthquake hits, will the sprinkler system’s Victaulic couplings fail because the supports allowed the piping to swing excessively? Will the cable tray resting comfortably above the suspended ceiling come crashing down, smashing though the fluorescent lighting fixtures? In many cases, the addition of MEP seismic restraints would have prevented much of the secondary damage we see in the aftermath of earthquakes. Today, insurance companies who provide work interruption insurance to major factories are hiring engineering firms to assess the MEP systems’ seismic adequacy. They would rather pay to increase the systems’ robustness now, than pay the factory owners millions of dollars because their factory is offline. In this article, we will look at the process required to assess MEP systems and introduce how 3D laser scanning is used to perform this task in a more cost effective manner, with the added benefit of providing as-built information of the owner’s facility.

There are a variety of standard bracing components that are routinely used for seismic restraint.

The majority of requirements can be determined by prescriptive standards defined by the insurance/risk management company for the various cases found in the field. The particular cases which apply will be determined in part by the building’s Occupancy Category, which influences the level of restraint that is required. An assortment of seismic code parameters is used in determining the matrix of what restraints are required for each type of component. Thus, the bulk of the design work consists of: identifying those objects which require seismic bracing; determining which prescriptive case they are covered by; accurately determining their location in the facility; and producing the drawings and documents required for bill of materials, approval, and construction support. Special cases not covered by the prescriptive codes would require attention from a qualified structural engineer to determine what restraints they require.

The typical approach to evaluating MEP systems is to send designers and/or engineers to the facility where they go through and confirm drawings by eyeball from a distance, and crawl (often literally) through the suspended maze of piping, wiring, and duct work to measure and document the existing MEP systems and their restraints. This information is then used to develop estimates and construction packages for the retrofit work. Because of the complexity of

Applying 3D Laser Scanning to MEP Seismic Restraint Retrofits

By Daryl Johnson, P.E. and Ernie MacQuarrie, P.E.

Daryl Johnson is Engineering Manager of Summit Engineering and Design, LLC, a firm that he established in 1998. Daryl may be contacted at djohnson@sead.com.

Ernie MacQuarrie is a Civil/ Structural, Project and Field Engineer at Summit Engineering and may be contacted at emacquarrie@sead.com.

36 June 2012

The first FARO Focus3D™ Laser Scanner delivered to the USA scanning a manufacturing plant in support of a MEP seismic retrofit. Can you imagine designing and locating over 1000 seismic braces in a busy plant like this?

MEP systems, follow-up visits are often needed, or final construction details are left to be handled in the field where costs and quality are much harder to manage.

An alternate approach is to scan the MEP systems with a 3D laser scanner and create a very detailed and accurate 3D point cloud model, which is then used as the basis of the engineering and design efforts. Today’s high speed laser scanners are quite suitable for this purpose as they can, in minutes, make tens of millions of measurements with better than 2 mm accuracy over the moderate ranges involved. Scans are taken from various positions in order to provide sufficient coverage of equipment and to fill in shadows cast by other objects. The scans are then registered together to bring them all into the same coordinate system.

By removing the appropriate ceiling tile, it is possible to laser scan above a drop ceiling, traditionally a very difficult location to gather data.

In most cases, any item you can see you can scan.

With this approach, the cost required to obtain accurate measurements is reduced while the level of detail is significantly increased. It also reduces or even eliminates the need for repeat site visits.

The engineer/designer uses the highly detailed and accurate 3D point cloud dataset to virtually fly through the cramped, elevated spaces of the above-ceiling area, allowing them to identify the MEP systems and their current support systems.

Knowing the exact distances from the suspended pipes, conduits, etc. to the ceiling above is vital; one of the key variables that determine whether or not seismic restraint is required is the distance from the supporting structure to the suspended element. If the duct, pipe or conduit is less than 12 inches below the ceiling, it may not require any seismic restraint, while those elements suspended further from the ceiling may require a series of restraints. Using laser scanning data, the designer can virtually fly through thousands of square feet of space, making accurate measurements of even hard to reach systems components.

After the designer has completed an assessment of the as-built condition of the MEP systems, he refers to the matrix of restraints

Models of seismic braces shown along with scan data that has been isolated into major systems (HVAC, piping, etc.). For more information about isolated scan data, visit www.sead.com.

determined by the prescriptive codes or, if necessary, the structural engineer, and again virtually flies through the congested MEP ceiling space identifying locations for required bracing supports. Where these supports are needed, the designer pulls a 3D model of the required support from a CAD library of supports and places it in the correct location. Clash detection can

be performed by displaying the point cloud data along with the CAD models.

At this point, it is a simple matter for the designer to extract from the 3D model the information that describes the type of supports required and where they need to be installed, and produce a plan view of these elements in the facility. We now have everything the installation contractor needs to bid ADVERTISEMENT - For Advertiser Information, visit www.STRUCTUREmag.org

STRUCTURE magazine

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June 2012

and install the restraints: a 3D model, a set of plan views showing which support goes where, and a bill of materials.

There is a growing number of contractors who are using 3D visualization programs such as Autodesk’s NavisWorks™ to take advantage of the accurate 3D models that the proper use of 3D laser scanning provides. These programs help constructors visualize work flow and planning efforts, and efficiently resolve problems as they come up. These tools also help engineers and designers provide assistance remotely, reducing the level of non-construction resources required in the field.

While it is possible to field measure, design, and install seismic restraints of MEP systems without the aid of 3D laser scanning, the cost will be more difficult to predict and control.

Accuracy will suffer, and the project schedule will most likely be extended. Laser scanning can greatly increase speed and accuracy when documenting the existing conditions in industrial facilities, hospitals, power plants or any facility with congested MEP systems. It can also be very useful in mapping the locations of existing architectural and structural elements. Employing 3D scanning in the process allows managers to move forward with confidence, removing many of the headaches caused when accurate field verification of existing construction is important.▪

Education issuEs

core requirements and lifelong learning for structural engineers

Principles for Engineering Education Part 2

By Eric M. Hines, Ph.D., P.E.

I

n the previous article (STRUCTURE® magazine, April 2012 issue), I introduced four principles that are critical for improving the “technical and practical quality of education for structural engineering students.”

1) Theory and practice are indivisible.

2) Engineering is a creative discipline.

3) Drawing is the language of the engineer.

4) There is more than one way to model every problem.

I discussed the first and second principles in Part 1 and will continue now with the third and fourth principles.

Principle 3: Drawing is the language of the engineer.

Drawings are the real product of a structural engineer’s work. A structure may well stand up if I didn’t calculate it, but it cannot be constructed if I haven’t drawn it. This principle has historical significance in the work of Karl Culmann, who founded the Department of Civil Engineering at the Federal Technical Institute (ETH) in Zurich in 1855 and published the first comprehensive work on graphic statics in 1866. Many engineers will agree that drawing is a language whose intellectual richness and power of expression matches or exceeds words and mathematics.

Unfortunately, drawing shares a similar fate to practice in the university, and has been misunderstood as either a technical skill or an expression of artistic talent.

The best English language introduction to the work of Karl Culmann and his successor, Wilhelm Ritter, can be found in David Billington’s The Art of Structural Design: A Swiss Legacy. In Culmann’s words, “Drawing is the language of the Engineers, because the geometric way of thinking is a view of the thing itself and is therefore the most natural way; while with an analytic method, as elegant as that may also be, the subject hides itself behind unfamiliar symbols.”

Following this theme, I discussed drawing as a language in a more contemporary context in my 2011 paper “Conceptual Transparency.” Colleagues of mine who are

highly accomplished designers and who have long recognized the importance of drawing for engineers, Edward Allen and Waclaw Zalewski, have made Culmann’s graphic statics accessible to a contemporary audience in Shaping Structures and Form and Forces.

When I develop a new design, I first draw what looks right and then I calculate. I encourage my students to do the same.

Their drawings direct their calculations, and in turn, their calculations allow them to develop new drawings. Once this becomes the approach students expect to follow, they find that they make choices about their calculations, i.e. even their calculations require creativity. Suddenly the virtue of simplicity begins to make sense. Simplicity facilitates creative thinking, by increasing the number and quality of ideas that are generated, expressed and judged.

Principle 4: There is more than one way to model every problem.

This principle responds to the current tension between computational methods and hand calculations as they affect undergraduate education. Questions regarding the appropriate use of the computer in practice and teaching are reminiscent of the tension between machine production and handicrafts that began over a century and a half ago.

Gottfried Semper, who was a colleague of Karl Culmann’s, visited the 1851 Crystal Palace Exhibition in London and wrote a famous essay on this tension. Semper wished to remain optimistic about machines that “encroach deeply into the field of human art, putting to shame every human skill,” and asserted that “there is no abundance of means but only an inability to master them.” By the early 20th century, the question of machine production had come to dominate not only modern architectural discourse but modern society in general.

Our current use of computers has developed all the more rapidly in light of our hindsight regarding the history of machine production.

What seems to be missing, however, is the intensive cultural discussion that flourished

STRUCTURE magazine

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June 2012

from the 1850s to the 1920s on the merits and weaknesses of the new tools. I can’t help but feel that we are missing a cultural opportunity, and perhaps also an economic opportunity, in our reluctance to discuss what it means to have mastered our tools.

Since the scientific revolution of the 17th century, we have created unprecedented wealth by systematizing, dividing and refining our approach to labor and production. In the service of this grand project, engineering has developed a reputation for acting instrumentally, for rationalizing and optimizing.

This reputation, however, misrepresents many of the stories behind the engineering that supports our modern world. What needs to be made transparent to students is that even the most mundane professional work requires a human way of thinking – drawing on experience, analogies, associations and feelings. By the way, I have been reminded by many students over the years that this is also the key to increasing participation in engineering by women and minorities.

Understanding technical rigor in context makes it more rigorous. We don’t need to make engineering more attractive, we just need to represent it as it truly is. The interaction of the human and the technical is the lifeblood of our modern world, but this interaction is hard to understand and discuss.

For this very reason, we ought to value this discussion as one of our most cherished and important intellectual disciplines.

While our trade journals are filled with articles advertising an ability to keep pace with our latest tools, a few simple observations seem to escape discussion. For instance, building professionals in general have grown more uncomfortable with drawing by hand. This makes it harder to express and discuss new ideas at meetings. Necessity no longer requires younger engineers to calculate by hand. This has removed the old safeguard that proficiency not attained in school would be acquired in practice. For the first time in history, it is possible to practice for ten years and not have advanced beyond fundamental understanding attained as a student. Superior computational power has reduced the apparent need to think long and hard about how best to model structures. This has promoted a literal approach

Eric M. Hines, Ph.D., P.E. is a Principal at LeMessurier Consultants, Inc. and Professor of Practice, Department of Civil and Environmental Engineering, Tufts University. Dr. Hines specializes in the design and renovation of building structures, renewable energy infrastructure, and the seismic performance of bridges. In 2011 he received the Henry and Madeline Fischer Award, recognizing him as “Engineering’s Teacher of the Year” at Tufts; and in 2012 he received the Designer Special Achievement Award from the American Institute of Steel Construction. He can be reached at ehines@lemessurier.com.

Part 3 of this series of articles will be included in an upcoming issue of STRUCTURE.

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June 2012

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to modeling which is highly inefficient and often incorrect. It has also indulged a culture where professionals and students alike are unable to explain their results. In response to questions regarding structural behavior, I have heard the phrase, “Would you like to see my spreadsheet?” No! I would not like to see your spreadsheet. I would like for you explain to me what is going on. Habitual work on the computer has diminished both a sense of scale and the means of expression available to engineering students working on paper.

When I calculate, my pages are filled with sketches, notes, tables, equations, numbers and graphs – each is a means of expression appropriate to its purpose. Taking Karl Culmann at his word, I often develop my force diagrams directly on top of a picture of the structure or detail. Drawing, calculation and understanding are connected. It is not enough to understand the concepts internally. An engineer must convey the same understanding to someone else.

An understanding of the creative process allows me to explain my choices of tools. As a professional, no explanation is required. As an educator, however, my job is to help my students make sense of the world, so I struggle to understand why I practice the way I do. Teaching keeps me honest. For each situation, I judge the value of my tools based on three criteria: 1) How quickly and directly can I express the idea?

2) How much does this expression facilitate judgments and inspire further ideas?

3) How well may I expect this expression to communicate?

This is especially helpful in determining the appropriate use of the computer.

While I belong to a generation of engineers proficient with all types of software, I find that many problems can be solved more quickly by hand – especially if I model them in an efficient way. Other problems are solved more quickly by the computer, but their solution offers less fundamental insight.

This poverty of insight has a tendency to obstruct the sound judgment and generation of ideas. Still other problems, however, are solved elegantly and quickly on the computer. The best tool for a given situation is not a foregone conclusion. I am responsible to judge which tool best suits my present purpose. To judge well is to have mastered my tools.▪

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Masonry Cement Mortar in High Seismic Design Applications

By Jamie Farny

T

he 2011 Masonry Standards Joint Committee (MSJC) Building Code Requirements and Specification for Masonry Structures (TMS 402-11/ ACI 530-11/ASCE 5-11) and earlier editions do not allow the use of masonry cement mortar in Seismic Design Categories D and higher for masonry walls that are part of the seismic force-resisting system. However, the latest research supports the use of masonry cement mortars in seismic structural applications.

Research affirms certain basic principles, and lays the groundwork for proposed changes to the 2013 edition of the Masonry code (TMS 402/602). Among those principles: • Structural performance of fully grouted reinforced masonry walls in response to earthquake forces is dominated by grout and reinforcement; it is unaffected by mortar formulation.

• Proper wall-tie spacing and adequate attachment to the supporting structure are key factors to ensuring stability of veneer; use of high strength mortars and joint reinforcement with seismic clips is not necessary to achieve required performance.

“Masonry cement” started appearing between 1918 and 1932 as an alternate to traditional portland cement-lime mortar. Masonry cement simplified the production of good quality, consistent mortar on masonry projects.

In 1932, the Standard for Masonry Cement (ASTM C91) debuted, and by the late 1900s masonry cement was being used in a majority of masonry constructed in the U.S. However, when the MSJC masonry code was introduced in 1988, it maintained the Uniform Building Code (UBC) limitations on the use of masonry cement mortars in lateral force resisting (participating) structural members in areas with high seismic risk. This limitation was based on historical use of portland cement-lime mortars in regions of high seismic activity and data indicating that the flexural bond strength of unreinforced masonry prisms constructed using masonry cement mortars tends to be lower than that obtained on prisms made using portland cement-lime mortars.

Research Beginning in 2005 and continuing until 2010, a project under the direction of Dr.

Franklin Moon, Drexel University, focused on the performance of reinforced masonry bearing walls and compared results of partially grouted and fully grouted masonry walls to identify differences in behavior mechanisms.

Although partially grouted and fully grouted masonry shear walls responded differently to loads, one thing was clear: mortar formulation did not have a significant effect on the strength and behavior of fully grouted walls.

In a separate project (2006 to 2010), the U.S. National Science Foundation’s Network for Earthquake Engineering Simulation (NEES) program sponsored research on Performance-based Design of Masonry and Masonry Veneer. The research team was led by Dr. Richard Klingner, University of Texas at Austin, and included academia and representatives from the masonry industry. Wall types studied included both concrete masonry unit (CMU) and wood frame assemblies with clay brick veneer on the exterior.

Experiments looked at: • Structural masonry’s response to seismic loads to compare how different grouting conditions and mortar formulations affected that response.

• Masonry veneer’s response to seismic loads over wood frame and grouted CMU backups.

Tests included full-scale walls subjected to both in-plane and out-of-plane quasi-static and dynamic loading; wall segments in a lab; wall segments on a shaking table; and fullscale prototype buildings on a shaking table.

Shaking-table tests were conducted using two ground motion records from the 1994 Northridge (California) Earthquake, one with strong acceleration pulses and the other motion more demanding in the frequency range of interest with a much longer duration of strong shaking. Repeated cycles of the earthquake loads were applied and gradually increased to 2½ times the original earthquake motions.

Careful coordination of dimensions and details of the test specimens permitted direct comparison of the quasi-static and dynamic test results.

For the CMU construction, it was observed that mortar formulation has negligible influence on the seismic response of fully grouted, special reinforced masonry shear walls; reinforcement and grout are more important.

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June 2012

Performance Experimentation showed that current MSJC requirements for veneer ties are adequate for high seismic conditions and continued use of masonry cement mortar. Both corrugated and rigid veneer ties sustained ground motions in excess of the Design Basis and Maximum Considered Earthquakes (DBE, MCE).

Shaking-table tests showed that veneers constructed over wood-stud backing, and designed in accordance with code provisions, can sustain ground motions far in excess of the DBE and MCE when adequately attached to supporting structure. One veneer failure occurred due to pullout of nails installed in wet wood. As a result, the 2011 MSJC was modified to require higher pullout strength for tie attachments to woodstuds. In the CMU building specimen and the shake-table CMU wall specimens, the in-plane veneer and its connectors performed well under repeated earthquakes above MCE without falling off the CMU. All out-of-plane CMU walls with clay masonry veneer performed well in the shake-table tests under repeated earthquakes above MCE. In all shake-table testing conducted, the out-of-plane connectors securely held the CMU wall and the veneer for well above MCE.

Conclusions Fully grouted participating elements in high seismic areas can be built with mortar formulated using masonry cement and provide acceptable performance. The veneer research also indicates that inclusion of joint reinforcement and seismic clips is not necessary to achieve required performance in high seismic design applications, validates existing wall-tie spacing criteria of the MSJC, and supports continued use of Type N masonry cement mortars in veneer. Changes consistent with these findings have been proposed for the 2013 MSJC code.▪ Jamie Farny is Market Manager, Buildings, for the Portland Cement Association. He is a member of the Construction Requirements and Seismic subcommittees of the Masonry Standards Joint Committee, which develops the masonry code and specification.

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

CASE BuSinESS PrACtiCES

business issues

Risk Management: Elements and Tools

By R. John Aniol, P.E., S.E.

M

anaging risks – from project initiation through construction completion – is critical to achieving project success and maintaining key client relationships. The Council of American Structural Engineers’ (CASE) vision is to be the leading provider of risk management and business practice resources within the structural engineering profession by improving quality, enhancing management practices and reducing professional liability.

To assist structural engineering firms in reducing risk, CASE developed the Ten Foundations for Risk Management. The first five foundations pertain to business practices, while the last five pertain to project management items.

1 – Culture Create a culture of managing risk and preventing claims: Culture is difficult to define in an organization but is a key element of any firm’s character, providing a basis for the decision-making process and operating procedures. To effectively cultivate culture, a firm must employ strategic planning (involving staff and clients), and commit to focus a substantial portion of the cultural effort on quality. High-quality client service is achieved when it is “built-in, not bolted on” and infused throughout an organization from the top-down. Producing high quality work will result in satisfied clients who provide opportunities for future business, reduced legal claims, more satisfied employees and higher profit margins.

2 – Prevention and Proactivity Employ preventive techniques: CASE has recently released two tools to assist the design professional in developing risk prevention processes within the firm. CASE Tool 2-3, Employee Evaluations, is intended to assist the structural engineering office in the task of evaluating employee performance. The evaluations not only provide a method to assess employee performance, but also serve as an integral part of the company’s risk management program.

Tool 2-4, Risk Management Plan, is a document to aid the project manager in implementing a comprehensive process that will identify risks, estimate the probability of occurrence and consequence of the risks, and create a proactive plan to mitigate the risks.

The tool is divided into four sections: 1) Risk & Opportunity Identification: CASE Tool 2-1, Risk Evaluation Checklist, can be used to indentify risks. In addition, a list of sample risks is provided. 2) Risk & Opportunity Qualification/Quantification: A risk assessment matrix is used to determine the integrated risk assessment. 3) Risk Strategy; Risk and Opportunity Leveraging Plans: This plan can both identify proactive and reactive processes.

4) Risk Management Process: Includes kick-off meetings, risk management plan, coordination meetings and appraise and control. The Risk Management Plan is element three of Tool 3-4, Project Work Plan Template.

3 – Planning Plan to be claim-free: Client evaluation, project type, staff hiring and retention, comprehensive training program and quality assurance all contribute to reducing risk.

Prepare a project work plan to document project delivery strategies and communicate them to the project team members. CASE Tool 3-4, Project Work Plan Template, serves a guide for the project manager to develop the following elements of the work plan: project metrics, financial management plan, risk management plan, resource management plan, design management plan, documentation management plan, quality management plan, and construction phase management plan.

5 – Education Educate all the players in the process: Effective training is the key element to success.

Consider a comprehensive training program including leadership skills, project management skills and technical skills. Establish a mentoring program to enable seasoned staff to nurture the career development of less experienced staff. Ascertain owner’s expectations about coordination and completeness of the contract documents, so risk can be understood.

6 – Scope Develop and manage a clearly defined scope of services: A clearly defined scope establishes a firm’s responsibilities (avoiding misunderstandings), serves as a basis for compensation and additional services, and should be used in the development of the project work plan.

Discuss the scope of the work with the entire project staff, to ensure they have a full understanding of the required work – avoiding unnecessary work and identifying when additional services are appropriate.

7 – Compensation Prepare and negotiate fees that allow for quality and profit: Adequate fees allow for adequate time to produce quality contract documents and models. Negotiate fees together with scope of services, so the client understands what is included in the basic services. Weigh contract fee versus risks to determine if the proposed fee is commensurate with scope, client, project type, complexity, schedule, delivery method and profit strategy.

4 – Communication

8 – Contracts

Communicate to match expectations with perceptions: Understanding the client and owner’s goals is the first step in effective communication, as proactive planning leads to seamless interaction. Communication must flow in both directions throughout the project team. Utilize communication tools including project status reports, meeting agendas, action item/coordination lists and design criteria document.

Identify onerous contract language: A well-written, fair and complete contract can minimize risk. Review each contract for onerous provisions; refer to CASE Tool 8-1, Contract Review. It is preferred to use in-house standard contracts, or standard contracts prepared by CASE or AIA, as a starting point of negotiations. Review the prime agreement between your client and the owner. Consider negotiating a limitation

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of liability appropriate for the scope and fee. Ensure that the terms of the contract are insurable under the firm’s professional liability insurance. “For example, most insurance policies do not provide for the defense of an indemnitee, even though that term is often found in indemnity agreements. A good contract will recognize that professional services are being provided – not a product – and therefore perfection cannot be warranted by the service provider.”

9 – Contract Documents

turn, increased firm viability will enable the firm to enjoy the benefits of a higher-quality client experience.

CASE has developed more than 16 standard agreements, more than 10 guidelines/commentaries and more than 20 tools. A complete list of all the CASE Contracts, Guidelines and Tools can be found at www.acec.org/case.

For more information regarding specifics of CASE tools contact Stacy Bartoletti, Toolkit Committee Chair, sbartoletti@degenkolb. com. All tools are free for CASE member firms.

Tools are also available to non-member firms for nominal fees. If you are interested in joining CASE, refer to the web-site or contact Heather Talbert, htalbert@acec.org.▪ R. John Aniol, P.E., S.E. is a Vice President in the Dallas office of Thornton Tomasetti.

He is serving as a member of the CASE Toolkit Committee. He can be reached at janiol@thorntontomasetti.com.

Summary By focusing on the recommendations of the CASE Ten Foundations of Risk Management, firms will achieve successful project completion through reduced professional liability; in

10 – Construction Phase Provide services to complete the risk management process: Construction quality assurance is an important element of the quality assurance plan since it is the final step in the process (and is best performed by the staff responsible for the design). Develop preconstruction meeting agendas to proactively discuss and resolve key issues. Develop guidelines for replying

STRUCTURE magazine

Also see: Douglas Ashcraft, P.E., S.E., Foundations for Risk Management, STRUCTURE magazine, August 2005, p. 41-42.

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Produce quality contract documents: The contract documents and the model are the deliverables that communicate the design intent to the construction team. In an effort to raise the document quality bar, CASE recently released CASE Tool 9-2, Quality Assurance Plan, which provides guidance to the structural engineering professional for developing a comprehensive detailed Quality Assurance Plan. A well-developed and implemented Quality Assurance Plan ensures consistent high-quality service on all projects, and includes: 1) Quality Control Review, 2) Firm-wide Standards, and 3) Construction Quality Assurance.

The quality control review may consist of three elements: Design (Jury) Review, Engineering Review and Construction Document Review. Comprehensive fi rm-wide standards (consisting of design/analysis standards {guidelines}, documentation standards and construction administration standards) enable staff to gain historical firm-wide benefits while providing the resources to ensure the design and documentation are clear, concise, accurate and consistent.

In addition, CASE Tool 9-1, A Guideline to Addressing Coordination and Completeness of Structural Construction Documents, is a great reference tool for preparing quality construction documents.

to requests for information, including issuing sketches and maintaining an RFI log.

Develop submittal review guidelines that outline the completeness of specific submittal review, including the use of the appropriate submittal review stamp and submittal log procedures. Develop guidelines for field observation and reporting procedures, and review of testing reports.

Specify and request a submittal schedule to adequately allocate submittal review resources. Reply to RFIs and return submittals within the contractually-specified time to avoid a claim for a delay in the process. Review specifications for specified submittal components. Request specified yet incomplete submittal information promptly upon receipt of submittal.

Establish a collaborative (non-adversarial) relationship with fabricators and contractors in order to work together to achieve a successful completion of the project.

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Tall Building guide

expertise in tall building design and construction

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Devco Software, Inc. Phone: 541-426-5713 Web: www.devcosoftware.com Product: LGBEAMER v8 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. ProTools include shearwalls, framed openings, X-braces, joists and rafters.

GT STRUDL Phone: 404-894-2260 Web: www.gtstrudl.gatech.edu Product: GT STRUDL Description: Structural Design & Analysis software from the Georgia Tech - CASE Center offers linear and nonlinear static and dynamic analysis features including response spectrum, transient and pushover analyses, plastic hinges, discrete dampers, base isolation, and nonlinear connections. New multiprocessor solvers enable the solution of static/dynamic models with over 300,000 DOF.

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Standards Design Group, Inc Phone: 800-366-5585 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. The software allows the user to “build” structures within the system; built-in version of the wind speed map(s) allows the user to enter a wind speed, and numerous specialty calculators.

StructurePoint Phone: 847-966-4357 Web: www.StructurePoint.org Product: Concrete Design Software from StructurePoint, Formerly PCA software group Description: Column, Slab, Mats, Beam and Wall programs for analysis and design of reinforced concrete members. Particularly suited for tall building investigation and optimization. Simple and accurate software tools to save you time and speed your design effort. StructurePoint is your gateway to vast resources of the cement and concrete industry.

All Resource Guides and Updates for the 2012 Editorial Calendar are now available on the website, www.STRUCTUREmag.org. Listings are provided as a courtesy. STRUCTURE® magazine is not responsible for errors.

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CTS Cement Manufacturing Corporation Phone: 800-929-3030 Web: www.ctscement.com Product: Professional-Grade Constr. Cement Products Description: Use Rapid Set® cement products for concrete repairs, restoration and new construction, and achieve high durability, fast strength gain and structural or drive-on strength in one-hour. Install concrete structures and industrial-size floors using Type-K shrinkage-compensating cement products with no curling, no drying shrinkage cracking and no intermediate saw cut joints.

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notable structural engineers

Great achievements

Joachim Gotsche Giaver Champion of Structural Registration Laws

By Richard G. Weingardt, P.E., Dist.M.ASCE, F.ACEC, D.Sc.h.c.

J

oachim G. Giaver always seemed to be the right person, in the right place, at the right time, beginning with his immigration to the U.S. in 1882, one year after graduating from one of Norway’s most renowned civil engineering colleges. Upon arriving in America, he was immediately employed by the Pacific Railroad Company in St. Paul, Minnesota. A year later, he moved to Pittsburgh, Pennsylvania, to work for the Shiffler Bridge Company, a J.P. Morgan venture newly founded to build bridges and furnish structural steel for all types of complex structures and buildings. The company had just secured the contract to furnish the structural steel for the framework of the Statue of Liberty.

After his stint with Shiffler, Giaver moved to Chicago to design state-of-the-art structures for the 1893 Columbian World’s Exposition.

There he met and befriended the Fair’s lead architect, Daniel Burnham (1846-1912), who along with his colleagues shortly thereafter embarked on a long and impressive run as designers of landmark buildings and skyscrapers nationwide.

Statue of Liberty, New York, NY. Courtesy of Wikimedia Commons-Misterweiss.

By the time Giaver began thinking of opening his own consulting engineering firm in 1915, commerce through the newly opened Panama Canal was having an major impact on the U.S. economy overall. Foreign trade soared to a record high, much of it also sparked by the multi-nation war developing in Europe. One year later, when Giaver formed a partnership with former Burnham associate Fred Dinkelberg to provide architectural and engineering services, the future looked promising indeed.

Joachim was born on August 15, 1856, in the tiny hamlet of Jovik, near Tromso, Norway, the ninth child of 13, four of whom would not make it to adulthood. He was only two when his newborn brother Jens, Jr. died in 1858, and was barely eight when his 16-year-old sister Anna and three-yearold brother Carl both died in the same year, 1864. Those experiences tempered him for similar tragedies that he and his wife would experience with their own offspring years later in America.

Joachim’s father Jens H. Giaver, who was from a prominent Norwegian family, was a major landholder in northern Norway and a leading figure in its fishing industry. His mother Hanna Brigitte (Holmboe) Giaver was in charge of the home-schooling of Joachim and his siblings in preparation for college.

Joachim’s university choice was Trondhjem Technical College at Trondheim, Norway, nearly 500 miles (as the crow flies) southwest of his hometown. It was next to a sizeable body of water connected to the Norwegian Sea and surrounded by high mountains, offering many opportunities for outdoor activities like boating, hiking and climbing.

On September 3, 1885, Giaver married Louise Caroline Schmedling, a native of Trondheim then living in New York. He was 29 and she was 21. They would have eight children, three of whom died as infants. Their five surviving offspring were two daughters, Astrid (Mrs. Ralph Holmboe) and Brigit (Mrs. Amasa Bull), and three sons, Erling, Finn, and Einar William “Bill.” Erling went into the construction supply business, Finn became a civil engineer like his father, and Bill studied engineering at Georgia Tech prior to a varied career first in professional football

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and the movies, and finally in the construction industry.

Around the time of Joachim G. Giaver. his marriage, Giaver Courtesy of Library of was named chief engi- Congress Prints and Photographs LC-B22neer of Shiffler. In his 206-2. five years in that position, he was in charge of the design and construction of several large bridges, including two in Pittsburgh – one over the Allegheny River and the other crossing the Monongahela River – and numerous multi-story structures. By far, Giaver’s most noteworthy Shiffler assignment was producing the structural framework for the Statue of Liberty. His work involved design computations, detailed fabrication and construction drawings, and oversight of construction.

In completing his engineering for the statue’s frame, Giaver worked from drawings and sketches produced by the famous French structural engineer Gustave Eiffel (18321923). Not only would Eiffel be remembered for the statue’s framework, he would, soon after it debuted, design and build the monumental Tower that still bears his name for the extravagant 1889 Paris Centennial Exposition.

Three-and-a-half years after President Grover Cleveland officially dedicated the Statue on October 28, 1886, President Benjamin Harrison signed into law confirmation of Chicago as the location for the Columbian Exposition, in celebration of the 400th anniversary of Columbus discovering America.

Engineers, architects, contractors and building suppliers from around the country took notice. A considerable amount of innovation, design and new construction would be required – and fast. Almost immediately, companies from all over the country began setting up operations in Chicago to get in on the action. With them came many leading structural engineers, including two daring ones from Pittsburgh, both still in their early thirties, Joachim Giaver and George Ferris (1859-1896). Of course, Ferris would erect for the 1893 Exposition the greatest observation wheel the world had ever seen.

In addition to the frantic and massive construction frenzy that the Fair generated, Chicago in the late 1880s and early 1890s

received his final papers as a citizen of the United States in 1896.

In 1898, Giaver rejoined Burnham’s company as its chief engineer, a position he held for 18 years. During that time, Giaver helped hone modern skyscraper design into a fine art, moving engineering solutions away from cast iron and wrought iron frameworks on spread footings to more costeffective structural steel bearing on caisson foundations. Among his more popular inventions was the Museum of Science and Industry (Columbian Exposition’s Palace “Giaver Belled-Caisson” footing. of Fine Arts), Chicago, IL. Courtesy of Robert B. Johnson.

While with Burnham, Giaver was was experiencing a commercial construc- in charge of over 400 of the largest buildtion boom with longer-term ramifications. ings in the U.S., among them the Flatiron, Although the 1885 Home Insurance Building Gimbel, Maiden Lane and Equitable in New stood less than 150 feet tall, its load-carrying, York City; the Field Museum, Continental iron-steel framework earned it the label of National Bank, Railway Exchange and the world’s first skyscraper. Other notable Conway Field in Chicago; the Union Station buildings with steel skeletons instead of the and Post Office in Washington D.C.; the traditional masonry bearing wall construc- Frick, Oliver, Smithfield and First National tion that were built or being completed Bank in Pittsburgh; the May Company in when Giaver arrived in Chicago included Cleveland; the Wanamaker and Land Title the Rookery, Tacoma, Rand McNally, Old in Philadelphia; and the dome of the Mount Colony, Reliance, Marquette and Republic, Wilson Observatory in California. each with its own legitimate claim to being Prior to resigning from Burhnam’s firm the nation’s first true steel-framed “skyscraper.” and opening his own consulting engineering Shortly after moving to Chicago in 1891, business, Giaver began questioning the laws Giaver became the assistant chief engineer licensing structural engineers in the State of for the Fair, for which Burnham was the lead Illinois. Up until that time, only architects architect in charge of all construction. Serving could stamp and seal drawings for obtaining Giaver well in this position with Burnham’s building permits. Giaver was the leader of the group was his experience in designing com- engineers who got this changed, securing the plicated foundation systems for difficult soils, passage of a bill by the Illinois State Legislature such as the mostly unstable swampland of the in 1915 that allowed structural engineers to Exposition’s site. Also coming into play was practice their profession on equal terms with his extensive experience with state-of-the-art architects in Illinois. This new licensing law wind bracing systems for complicated struc- made it possible for building plans to be lawtural frameworks, which the bulk of the Fair’s fully approved if bearing the signature of a buildings also required. professionally registered structural engineer.

Among Giaver’s most noteworthy A year later, Giaver formed his partnership “White City” buildings for the Fair were with Dinkelberg to provide architecture and its Administrative Building and the Palace engineering services. The most notable work of Fine Arts. For the latter’s dome, Giaver of their partnership, finished a few months designed a unique three-hinge arch, which before Giaver’s death, was the 35 East Wacker at that time was the largest truss of its kind Building. Also known as the Jewelers’ Building, in the world, having a span of 368 feet. the 40-story, 522-foot-tall East Wacker strucThe structure survives today as the Chicago ture was the tallest building outside of New Museum of Science and Industry adjacent to York City when completed in 1927. It had Lake Michigan in Hyde Park. Following the commanding views of the Chicago River and Exposition, Giaver was engaged in the general a special elevator for individual automobiles so contracting business from 1893 to 1896, and that jewelers could remain in their cars with served as the bridge designer for the Sanitary their gems while going back and forth to work.

District of Chicago from 1896 to 1898. In The building is listed on the National Register the latter position he designed various bridges of Historic Places as part of the Michiganover the main Chicago Drainage Canal. It was Wacker Historic District, and is designated a while with the Sanitary District that Giaver Chicago Landmark.

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Jewelers’ Building (35 Wacker Building), Chicago, IL. Courtesy of Robert B. Johnson.

In 1920, Giaver was decorated with the Royal Norwegian Order of St. Olaf, 1st Class,

by His Majesty King Haakon VII of Norway, in recognition of his prominence as a structural engineer and his activities on behalf of Norwegians worldwide.

For leisure, Giaver was an enthusiastic yachtsman, winning numerous prizes racing his boats on Lake Michigan. His favorite yacht was named “Mavourneen” – Irish Gaelic for “marvelous beauty” or “my sweet one.”

Giaver was a trustee of the Norwegian American Hospital in Chicago, president of the Structural Engineers Association, a director of the Western Society of Engineers, president of the Norwegian Engineers Association, president of the Chicago Norske Klub and vice-commodore of the Columbia Yacht Club. He was also an active member in Svenska Klubben, Chicago Athletic Club, and Chicago Yacht Club.

Giaver passed away on May 29, 1925, twoand-a half months shy of his 69th birthday.▪ Richard G. Weingardt, P.E. (rweingardt@weingardt.com), is Chairman of Richard Weingardt Consultants, Inc. in Denver, Colorado. He is the author of ten books, including Circles in the Sky: The Life and Times of George Ferris and Engineering Legends, both of which feature the exploits of great American structural engineers who had significant influence on the progress of the nation. His latest book, Empire Man, is about Homer Balcom, structural engineer for the Empire State Building.

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award winners and outstanding projects

Spotlight

Bitexco – Symbolizing Vietnam’s Global Emergence An Iconically Shaped, Structurally Sophisticated and Sustainable Design

By William J. Faschan, P.E., Anthony Montalto, R.A. and Nayan B. Trivedi, P.E. Leslie E. Robertson Associates, RLLP was an Outstanding Award winner for the Bitexco Financial Tower project in the 2011 NCSEA Annual Excellence in Structural Engineering Awards Program (Category – International Structures over $100M).

L

ocated in Historic District 1 and the heart of Ho Chi Minh City’s Financial District, the 863-foot (263m; 68-story) Financial Tower rises as a Beacon towards the future of Vietnam.

The tower uses as its reference the nationally symbolic Lotus flower to build upon Vietnam’s history, as well as to make a statement that Vietnam has arrived and is capable of competing in the global marketplace.

Developed in response to Ho Chi Minh City’s rapid population and business growth, the mixed use facility, which includes a highrise office building, a five story retail podium, and four basement areas, eases the demand for commercial space.

Split-Level Basement & Donut In Ho Chi Minh City, where the construction standard is two basement levels for major projects, Bitexco’s small site and tower height increased the requirement to four. Faced with an aggressive schedule, property constraints, and a high water table, the project team developed a split-four- level basement excavation method with a shallow diaphragm wall which located the top of the tower mat foundation at the second basement.

Because of poor soil conditions, closeness of surrounding buildings, and basement access needs, the structural engineers chose not to use standard temporary bracing practices of tiebacks and rakers when building the perimeter diaphragm wall. Instead, the team devised a partial top-down construction method where the ground floor perimeter slab (“donut”) stabilized the top of the diaphragm wall before basement excavation started. The decision permitted excavation of the tower mat foundation without temporary bracing. After superstructure construction, the tower mat then served as a bracing point for temporary supports for lower excavation of adjacent areas.

The estimated 123,680 ton (112.2 million kg) weight of Bitexco Financial Tower and poor soil conditions necessitated very deep foundations. Design needed as well to

address a soil-structure time analysis which predicted an approximately 100mm settlement. To compensate, a 13-foot (4m) thick bored pile supported mat distributes the tower loads from the walls of the narrow services core over the extent of the mat. At the tower, 5-foot (1.5m) diameter bored piles extend to a depth of between 262 to 295 feet (80 to 90m), while at the podium and underlying basements, 4-foot (1.2m) diameter bored piles go down as far as 207 feet (63m).

Despite the extra time required to install the deep foundations, the split-level basement and donut slab cut at least a year in excavation and construction time from the four basement design concept by enabling the tower construction to start prior to completion of all basement levels.

Helipad Test Assemblage Supporting the helipad, which protrudes from the tower near the 55th floor, are a pair of 82-foot (25m) long tapered steel cantilever girders which span from the core walls to the perimeter tower columns, and extend outward to frame the platform. Perpendicularly oriented secondary structural steel beams cantilever beyond the primary girders to support the circular edge.

While the choice of steel produced a lighter structure and sped fabrication, construction of the huge cantilever at the given height presented challenges. To ensure that parts integrated cohesively prior to hoisting in the air, workers test assembled the helipad offsite.

Unconventional Outrigger Truss The skyscraper-free views, ovo-tubular design, and high aspect ratio potentially create wind induced building sway problems for the Bitexco Tower. To minimize this possible motion, the structural engineering team developed an unconventional steel outrigger truss system.

For wind load resistance, the structural design interconnects the out-rigger trusses and belt walls to the core walls and perimeter columns at Floors 29–30, and allows the outrigger and

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June 2012

Bitexco Financial Tower rises above Saigon River, Ho Chi Minh City, Vietnam. Courtesy of the Bitexco Group.

belt wall system to function with the concrete core walls. The outrigger trusses and belt walls stabilize the core by resisting a portion of the overturning moments associated with windinduced east-west movements.

Within the outrigger system, two of the four sets of double-story trusses align with the two primary core cross walls and the other two trusses align with the fire stair walls, while connections of the outrigger trusses to the cross walls effectively transfer forces. Four one-story-tall belt walls, with two on each side of the tower, interconnect pairs of perimeter columns to each outrigger truss.

Conclusion An iconic landmark, Bitexco Financial Tower provides a model for Vietnam’s global emergence through a sophisticated and sustainable design. Innovative approaches to the foundation’s excavation, helipad assemblage, and the outrigger truss system secured completion by Vietnam’s 400th anniversary.▪ William J. Faschan, P.E. is a Partner at LERA. Mr. Faschan is directing the structural design of World Trade Center Tower 4 in New York City.

Anthony Montalto, R.A. is a Principal with Carlos Zapata Studio. Anthony is an AIA Gold Medal recipient and has been Project Director on projects in Vietnam and Singapore.

Nayan B. Trivedi, P.E., M. ASCE is a Partner at LERA. Mr. Trivedi has served as Chairperson of the ASCE Metropolitan Section Structures Group and Vice Presidentof the Society of Indo-American Engineers and Architects (SIAEA).

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News form the National Council of Structural Engineers Associations

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Over the next few months, NCSEA will provide a closer look at what the association’s committees are working on, how they are benefiting the NCSEA membership and the structural engineering profession, and how other NCSEA members may be able to help. The first two committees to report are the Advocacy Committee and the Structural Engineers Emergency Response (SEER) Committee. The Continuing Education Committee is covered as well, with its summer offerings on the opposite page.

Advocacy Committee On April 5, 2012, the Chair and Subcommittee Chairs of the NCSEA Advocacy Committee held a web-based meeting with NCSEA Board members and staff. The following subcommittee initiatives were announced and affirmed: • Clients & Prospects- Rick Boggs: A brochure explaining structural engineering to Clients and Prospects has been prepared and will be available in PDF form, along with additional brochures targeting other audiences, at the NCSEA Conference in St. Louis.

• General Public & Media – Carrie Johnson: A guideline for communication to/with each NCSEA Member Organization is being prepared, as is a Media Training Guide.

• Code Officials & Government Agencies – Kevin Westervelt: Drafts of at least two white papers will be issued by the time of the NCSEA Conference.

• Students & Educators – John Joyce: Three people are working on the next structural engineering poster and three people are working on a structural engineering video. Both the Poster Subcommittee and the Video Subcommittee plan to have products ready for review by July, with final copies expected to be ready on time for the Conference.

• Website Development – Sam Rubenzer: Sam is reviewing websites for ideas for NCSEA’s website and will define the scope for a new NCSEA website by July. He plans to seek proposals in August, but he would welcome volunteers now, to work through this with him. Contact Sam at: sam@forseconsulting.com.

The next NCSEA Advocacy Committee meeting will be a web-based meeting at 11 a.m. Central Time on Monday, June 25. All Member Organization Advocacy Committee Chairs are invited to sit in. Please send your contact information to joyce@ncsea.com for log-in information.

SEER Committee In 2012 the NCSEA Engineers Emergency Response (SEER) Committee: 1) Announced the development of a web-based database to make it easier to contact structural engineers for assessments following a disaster. Volunteers can go to the NCSEA website to add their names and contact information to the list or, if you are reading the digital copy of STRUCTURE and would like to volunteer, you can click on this link to add your name and pertinent information to the system. You will then receive timely updates regarding training, SEER program developments, and deployment opportunities, following natural and man-made disasters.

2) Published the second edition of the SEER Plan Manual and made it available, at no charge, to NCSEA Members. For your copy, go to the SEER Committee page of the NCSEA website.

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3) Together with the NCSEA Continuing Education Committee, offered the first of what NCSEA expects to be bi-annual webinar offerings of the California Emergency Management Agency (CalEMA) Safety Assessment Program. This program ran for the first time on May 18; and it will be offered again in the Fall. The program consists of three webinar segments available over one day’s time and is one of the only two postdisaster assessment programs that is compliant with the requirements of the forthcoming Federal Resource Typing Standards for engineer emergency responders.

The course was taught by Jim C. Barnes, P.E., an Associate Civil Engineer in the Recovery Division of Cal EMA. Mr. Barnes is currently the lead statewide coordinator of the Safety Assessment Program and has instructed well over 100 Safety Assessment-related classes. He also assists with engineering-related issues pertaining to rebuilding after disasters and with the statewide Preliminary Damage Assessment efforts. (Note: Someone on site must act as a proctor for the course and send CalEMA pictures of attendees, along with completed registration forms. Certificates will then be issued by CalEMA.) If you missed the May 18 course, check back with NCSEA (www.ncsea.com) in the Fall. (Charge for the May course was $500 per site.)

Next month: The Code Advisory Committee

Call for Entries

NCSEA 15 th Annual Excellence in Structural Engineering Awards The NCSEA Excellence in Structural Engineering Awards celebrates the greatest structural engineering achievements in the United States and throughout the world.

Entries are welcome, and awards will be presented, in the following award categories: • New Buildings under $10 Million • New Buildings $10 Million to $30 Million • New Buildings $30 Million to $100 Million • New Buildings over $100 Million • International Structures over $100 Million • New Bridge and Transportation Structures • Forensic/Renovation/Retrofit/Rehabilitation Structures • Other Structures Entries are due July 13. Awards will be presented at the Hilton Frontenac in St. Louis, MO on October 5, on Friday night of the NCSEA Annual conference; and winning projects will be featured in future issues of STRUCTURE magazine. For awards program rules and eligibility, as well as entry forms, see the Call for Entries on the NCSEA website: www.ncsea.com.

Save the date – NCSEA Annual Conference in St. Louis – October 4-6.

June 2012

June 14: That’s in the Code?!? Ten Overlooked Provisions in AWS D1.1

Duane K. Miller, Sc.D., P.E., is Chair of the AWS D1 Structural Welding Code Committee and a member of the Technical Activities Committee. He formerly chaired the Seismic Welding subcommittee and the AASHTO-AWS Bridge Welding subcommittee; and he has authored and co-authored chapters of many texts, including the AISC Design Guide 21 on Welding and the Mark’s Handbook of Engineering, 11th Edition.

June 21: ASCE 7-10 Significant Wind Load Provision Changes This webinar will cover the most important changes to the wind load provisions for ASCE 7-10 including new wind speed maps, organizational changes in the standard, a new simplified method for buildings less than 160 feet tall, and a discussion of changes to the occupancy classification for buildings which is key to the

use of the new wind speed maps. The purpose of this webinar is to make training available for practicing structural engineers in a timely manner so that when the ASCE 7-10 standard is adopted by local building code organizations, the user will be equipped to put the changes into practice.

William Coulbourne, P.E., SECB, is a national expert in wind and flood mitigation and has been involved in FEMA Mitigation Assessment Teams for over 15 years. He has investigated failures and mitigation design techniques for thousands of buildings and has co-authored books, written articles for journals and given presentations and provided training for homebuilders, engineers, architects and homeowners on high wind and flood design and coastal construction issues. In addition, he was one of the primary authors for FEMA’s Coastal Construction Manual and for FEMA 320, Taking Shelter From the Storm–a tornado safe room design guidance manual for homeowners and homebuilders.

News from the National Council of Structural Engineers Associations

For most steel construction projects in the U.S., work will be done in accordance with the AISC Steel Specification. This standard then invokes AWS D1.1 to address most weldingrelated requirements. When the Structural Engineer specifies conformance with AISC or AWS D1.1, they may think their task complete in terms of welding issues. This presentation will address ten often overlooked requirements from D1.1 that should be included in contract documents or on drawings.

Topics will include specification of loading type, nondestructive testing, Charpy toughness requirements, and alternate acceptance criteria. Details of groove and fillet welds and common errors and omissions on drawings will be presented, along with preferred practice. Construction details such as weld backing and weld tabs will be discussed. Presented in a checklist-type format, this presentation will explain why these issues are important and provide practical suggestions as to how potential problems can be overcome in construction documents.

Register at www.ncsea.com.

NCSEA News

Summer Courses – Programming by the NCSEA Continuing Education Committee NCSEA Webinars

July 24, 2012: Design Provisions for Lumber & Glulam Beams based on the 2012 NDS David Pollock

July 31, 2012: Design of Bolted Connections using the 2012 NDS David Pollock

August 7, 2012: Aluminum Structural Member Design Randy Kissell

August 21 2012: Aluminum Mechanical & Welded Connections Design Randy Kissell

Cost: $225 for NCSEA members, $250 for SEI/CASE members, $275 for non-members, FlexPlan option still available. Several people may attend for one connection fee. 1.5 hours of continuing education. Approved for CE credit in all 50 States through the NCSEA Diamond Review Program. Applicable for SECB recertification. No fee for continuing education certificates. Time: 10:00 AM Pacific, 11:00 AM Mountain, 12:00 PM Central, 1:00 PM Eastern. Miss a webinar that you wanted to see? Purchase the recording at www.ncsea.com.

NCSEA/Kaplan Structural Engineering Exam Live Online Review Course

June 2012

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• Key topics of structural code • Efficient analytical methods • New material in the 16-hour Structural exam • Typical exam questions • Problem solving techniques • Exam day skills • 24/7 playback – study anytime For more information or to register, see page 9 of this magazine or visit www.ncsea.com.

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Vertical – July 21-22 Lateral – August 18-19 Prepare for exam day success with this course designed by NCSEA, Kaplan Engineering Education, and leading structural engineers from across the industry.

This targeted review includes: • Over 28 hours of instruction with an emphasis on building design • New sessions on exam strategy and bridge design

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Structural Columns

The Newsletter of the Structural Engineering Institute of ASCE

Session and Abstract Proposals due by June 12, 2012 Don’t miss this opportunity to be part of the Structures 2013 Congress in the beautiful and historic city of Pittsburgh, from May 2-4, 2013. You are invited to submit session proposals and/or paper abstracts.

Visit the SEI Website at www.asce.org/SEI for details about abstract and session proposals, as well as suggested topics and subtopics.

Key Dates All Session and Abstract Proposals Due: June 12, 2012 Notification of Acceptance: September 18, 2012 All Final Publication Ready Papers: January 15, 2013 (no extensions)

How to Submit a Proposal For detailed instructions on how to upload a session proposal or paper abstract, visit the SEI Website at www.asce.org/SEI. Proceedings Authors of accepted abstracts are strongly encouraged to submit a 10-12 page final paper for inclusion in the proceedings. The proceedings will be copyrighted and published by ASCE. Questions? Contact Debbie Smith at dsmith@asce.org or 703-295-6095 For information on Sponsoring and Exhibiting, please contact Sean Scully at sscully@asce.org or 703-295-6276.

Errata

ASCE 7 Committee Call for Proposals for the 2016 Edition Deadline extended to December 31, 2012 The Structural Engineering Institute (SEI) of ASCE is currently accepting proposals to modify the 2010 edition of ASCE 7, Minimum Design Loads for Buildings and Other Structures Standards Committee to prepare the 2016 revision cycle of the standard.

Interested parties may download the proposal form from the SEI Website at www.asce.org/SEI. The committee will accept proposals until December 31, 2012.

For additional information please contact Jennifer Goupil, SEI Director, at jgoupil@asce.org.

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.

Save the Date 2012 Electrical Transmission and Substation Structures Conference Columbus, Ohio November 4-8, 2012 Mark your calendars now and plan to attend this forum where transmission and substation engineers can share technical knowledge and explore emerging issues, while bringing new engineers up to speed on core issues.

For more information visit the ETS conference website at http://content.asce.org/conferences/ets2012/index.html.

Registration Now Open Miami, Florida October 24-26, 2012 This is the first hurricane conference to focus exclusively on topics of interest to professionals who design, engineer, regulate and build projects in hurricane affected regions. This event will bring together practitioners, educators, federal and state government officials, students, and business leaders from across the nation and around the world Hurricane engineering has evolved since Hurricane Andrew wreaked havoc on South Florida and Louisiana nearly 20 years STRUCTURE magazine

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ago. Andrew taught us much about how these powerful storms affect our built environment. Much has changed in building codes, standards and products designed to resist the effects from strong hurricanes, and much more needs to be improved upon.

Join the Applied Technology Council and the Structural Engineering Institute for this informative conference at the JW. Marriott Marquis in Miami, Florida, October 24-26, 2012.

Visit the conference website at www.atc-sei.org/ for more information and to view the technical program.

June 2012

Congratulations to the Inaugural Class of SEI Fellows The Structural Engineering Institute of ASCE recently established the SEI Fellow (F.SEI) grade of membership to recognize a select group of distinguished SEI members as leaders and mentors in the structural engineering profession. SEI welcomed many of the inaugural class of SEI Fellows at Structures Congress March 31 in Chicago. See the SEI website at www.asce.org/SEI for a complete list of the Inaugural Class members and more information on how to apply.

2013 Ammann Call for Nominations The O. H. Ammann Research Fellowship in Structural Engineering is bestowed annually to a member for the purpose of encouraging the creation of new knowledge in the field of structural design and construction. The O. H. Ammann Fellowship was endowed in 1963 by O. H. Ammann, Hon.M.ASCE, and was increased in 1985 by Klary V. Ammann (widow of O. H. Ammann). The deadline for 2013 Ammann applications is November 1, 2012.

For more information and to download an application visit the SEI website at http://content.seinstitute.org/inside/ammann.html.

New ASCE Structural Webinars Available SEI partners with ASCE Continuing Education to present quality live interactive webinars on useful topics in structural engineering.

Webinars are live interactive learning experiences. All you need is a computer with high-speed internet access and a phone. These events feature an expert speaker on practice-oriented technical and management topics relevant to civil engineers.

Pay a single site fee and provide training for an unlimited number of engineers at that site for one low fee, and no cost or lost time for travel and lodging. ASCE’s experienced instructors

Date June 14, 2012 June 18, 2012 June 22, 2012 June 28, 2012 July 19, 2012 July 20, 2012 July 23, 2012

Instructor Matthew Stuart Brian Breukelman Mark Yashinsky William L. Coulbourne Michael O’Rourke Donald Ballantyne William L. Coulbourne

deliver the training to your location, with minimal disruption in workflow – ideal for brown-bag lunch training.

ASCE Webinars are completed in a short amount of time – generally 60 to 90 minutes – and staff can earn one or more PDHs for each Webinar.

Visit the ASCE Continuing Education website for more details and to register: www.asce.org/conted.

Committee Activities ASCE/SEI Disproportionate Collapse Mitigation New Standards Activity The purpose of the proposed standards activity is to develop a national consensus standard for mitigating disproportionate collapse of building structures. Users of the standard would include, but not be limited to, design professionals, building officials, building owners and building users. The scope of the proposed standards activity is to develop a standard for disproportionate collapse mitigation of building structures and publish it as an ASCE standard. The content of the standard will be based on available technical information, including the technical documents produced by the SEI Technical Activities Division committee on disproportionate collapse, the GSA/DoD Guide, other available guides and standards, and published research papers and reports. Interested parties may submit an application to join this new committee via www.asce.org/codes-standards/applicationform/. For more information, please contact Lee Kusek, Codes and Standards Administrator, at lkusek@asce.org.

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ASCE/SEI 55-10 Tensile Membrane Structures and ASCE/SEI 17-96 Air-Supported Structures New Joint Committee This new committee will combine the two standards, and eliminate duplication of requirements and removal of sometimes conflicting information. The scope of ASCE 55 includes frame-supported structures but is not all-inclusive. There is no known standard today for air-inflated structures, and there is confusion today with terms such as “membrane-covered” in the International Building Code. The new, combined standard would pertain to tensile membranes, air-supported membranes, air-inflated membranes, and frame-supported membranes. ASCE/SEI 55-10 will be the base template and will add relevant portions of ASCE 17-96. Interested parties may submit an application to join this new committee via www.asce.org/codes-standards/applicationform/. For more information, please contact Lee Kusek, Codes and Standards Administrator, at lkusek@asce.org.

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The Newsletter of the Structural Engineering Institute of ASCE

Several new webinars are available: Webinar Title Antiquated Structural Systems Damping and Motion Control in Buildings and Bridges Preventing Bridge Damage During Earthquakes A General Overview of ASCE 7-10 Changes to Wind Load Provisions ASCE 7-10 Snow Load Provisions Seismic Assessment and Design of Sewers Pier and Beam Foundation Design for Wind and Flood Loads

Structural Columns

SEI Fellows Program

CASE in Point

The Newsletter of the Council of American Structural Engineers

CASE Tool 7-1: Client Evaluation Do you know who your best clients are? Do you know where you should be focusing your marketing and sales efforts to maximize financial performance of your firm? You may be surprised.

Under Foundation 7, Compensation, CASE Tool 7-1: Client Evaluation will help you answer those questions by analyzing the amount of work and profit for each client.

1) You will need the following information: a. Name of each client, along with a client abbreviation (the shorter the better). b.The revenue of each client, either broken down by year or a total. c. The total amount of expenses incurred for each client.

2) Input the information into the spreadsheet. Additional instructions are included on the “General Information” tab on the spreadsheet.

3) Once all of the information is populated, you can generate a graph showing the performance of each client. The horizontal axis will show the total revenue generated per client. The vertical axis will show the profit associated with each client by either a percentage or total.

CASE Tool 7-1 is available 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.

You can follow ACEC Coalitions on Twitter – @ACECCoalitions.

ACEC’s 2012 Annual Convention and Legislative Summit On April 16-18, a record 1,300 ACEC members attended the ACEC Annual Convention in Washington, D.C., meeting with 300 Senators, Congressmen, and Capitol Hill staffers to urge passage of long-term transportation, water/wastewater infrastructure, and energy legislation.

600-plus attended the black-tie Engineering Excellence Awards Gala, which recognized 147 preeminent engineering achievements from throughout the world. The Lake Borgne Surge Barrier – a $1.1-billion concrete and steel response to the death and destruction of Hurricane Katrina – was named the year’s most outstanding engineering achievement in the 46th Annual Engineering Excellence Awards. At nearly two miles in length and 26 feet high, the Lake Borgne Storm Surge Barrier outside New Orleans is the largest of its kind in the world. Tetra TechINCA led the design team for the project.

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ACEC’s Annual Convention also marks the induction of a new ACEC Executive Committee. Ted Williams, Executive Vice President of Landmark Engineering, Inc. in New Castle, Del., succeeded Terry Neimeyer as ACEC Chairman for 20122013 at the spring meeting of the ACEC Board of Directors.

New members of the 2012-2013 Executive Committee are: Chairman-elect Gregs Thomopulos, Chairman/CEO of Stanley Consultants, Inc.; Ralph Christie, Jr., Chairman/ President/CEO, Merrick & Company; Michael Matthews, President/CEO, H&A Architects and Engineers; William Stout, III, Chairman/CEO, Gannett Fleming; and Peter Strub, Eastern Regional Vice President, TranSystems Corp.

ACEC/Alabama Executive Director Renee Casillas is the new NAECE representative.

June 2012

ACEC has embarked on an exciting new collaboration with the American Society of Civil Engineers (ASCE) to host a nationwide continuing education management system known as the Registered Continuing Education Program at RCEP.net.

As partners, ACEC and ASCE will establish an Independent Review Board to approve education providers while also providing excellent customer service to professionals tracking their Professional Development Hours (PDHs) and post-licensure education activities in the system. In addition, State Licensing Boards will use RCEP.net to ensure their licensees have complied with Continuing Professional Competency requirements.

RCEP’s continuing education standards for Education Providers will follow NCEES Model Law, Model Rules, and Guidelines for Continuing Professional Competency.

The Registered Continuing Education Program at RCEP.net is an interactive online system that recognizes education providers

who adhere to rigorous continuing education standards. RCEP. net serves as a portal for thousands of educational offerings, searchable on the Master Calendar of activities, and the accompanying PDH records as reported by approved providers and issued to individual professionals. Professional Engineers and Surveyors can use RCEP.net to track their professional activities by storing certificates of completion issued by RCEP providers, uploading important professional documents, maintaining a complete PDH history through the self-reporting of outside activities, and downloading transcripts.

RCEP’s educational offerings are given by Registered Providers that have been vetted against a set of established continuing education standards and thus can be reported to state licensing boards with confidence of acceptance. Visit www.rcep.net or contact Maria Buscemi at 202-682-4323 for more information.

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

Email Retention Policy An unofficial survey indicates many engineering firms opt for a 90 day limit on email retention, with some going as far as six months and some forever. Here are some factors to consider when and if a policy is established. Is there a policy just for email as opposed to all communications? If you anticipate legal action, you are bound by federal law to hold related emails for possible discovery. If you are working under a government contract, it may contain a regulation regarding retention of emails. A retention policy also may vary depending the substance of the email – whether it is financial, HR or client information. Once a policy is in place, however, it should be followed. Adherence to an inconsistent policy in the eyes of the court is even worse than having the wrong policy.

Inspection v Observation Unless you plan to provide the actual services, you should avoid the use of the word inspect in describing your basic role of observation. If the owner has it in the contract, delete it and insert observation. If that cannot be done, then carefully define the word in definitions or scope of service so it clearly means STRUCTURE magazine

observation. Clients have been known to use the word inspection when they really mean the normal level of contract administration. Inspections connote a more detailed examination and consequently more obligations than you bargained for. It could raise the standard of care and the liability implications are huge.

The Engineer during Construction The job of the engineer during construction is to interpret and clarify for the contractor the requirements of the contract documents, and work with the contractor in developing the details to supplement the design documentation. The engineer undertakes to monitor various aspects of the work as it is completed. However, the engineer does not have control over the contractor’s work and is only able to review or spot check it as it is performed. The engineer is unable to give the owner absolute assurance that the contractor has done the work in accordance with the contract documents. The only real assurance the owner has is the integrity of the contractor. Owners should not expect engineers to protect them from the consequences of contractors that lack integrity.

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CASE is a part of the American Council of Engineering Companies

CASE Business Practice Corner

CASE in Point

RCEP.net Learning System Welcomes ASCE Partner

Structural Forum

opinions on topics of current importance to structural engineers

A Structural Engineer’s Manifesto for Growth Part 3

By Erik Nelson, P.E., S.E.

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his is the third installment of what I am calling my manifesto, which presents some of my thoughts about our profession and how we can grow as individual designers. For steps 1-11, please see Parts 1 and 2 in the April and May 2012 issues of STRUCTURE®.

12: Draw with a Pen Sketch ideas of structural systems or buildings.

Buy a sketchbook and use a pen, so that you cannot erase your mistakes. Mistakes are important reminders that you are fallible (like everyone else). The best preparation for life as an engineer is the understanding of our ignorance.

In the terrific book, Structural Engineering: The Nature and Theory of Design, William Addis states: Up until the turn of the century, it was standard practice for engineers to keep their own notebooks containing annotated sketches of hundreds of interesting designs and details they have seen in their travels; this formed a body of knowledge upon which a designer could draw and provided an important link to the past. Also, until the present century, engineering textbooks and encyclopedias often used to contain many examples of successful designs, both ancient and modern. Nowadays, young engineers are generally brought up without a good knowledge of precedent and to believe that mathematics of engineering science encapsulates all they need to know.

13: Simplify Your Analysis Models The best structural engineers do not need complicated models. It is commonly said that computer software can be a valuable and reliable tool only to those who otherwise do not need it. This is true. In your work, make this true.

Software writers who work on integrating BIM with analysis models do not seem to understand this. They mistakenly think that it is useful for engineers to model the entire building – every floor slope or offset, every little filler beam around slab openings, etc.They believe that this is how we do our work! I tried to help reduce this misunderstanding when writing

BIM and the Structural Engineering Community in the December 2008 issue of STRUCTURE. Computers should be used as a tool to make design decisions, they should not make the decision. We can model base plates and foundations as shell elements, or we can do a three-second hand calculation or quick spreadsheet. This is not about trying to take shortcuts. This is about knowing what the software can and should provide, and what it cannot or should not.

If you already know that the software cannot come close to mimicking reality, where do you draw the line? Is the concrete you are modeling genuinely Hookean (linear-elastic)?

Do plane sections really remain plane? Is that foundation or base plate a true pin or a fixed point? Is the soil perfectly stable and uniform? Do our buildings never decay? Does our concrete not continue hardening over time?

I am not suggesting that we do not need to know about the state of the art in analytical modeling, I am just pressing the point that they will never achieve reality. Often complex finite element modeling is unnecessary and does not contribute to good design decisions.

14: Get into the Details Become super-technical, because actively understanding our codes and science is essential. It is also unlimited.We cannot possibly know all that is in the endless codes that we need to use. So, you can add them all as PDF files on your e-reader (Kindle, Nook, etc.) and read in bed. If you have trouble sleeping, there is nothing better! Also, you wake up with new knowledge.

Memorization is less important, since engineering is not knowledge-based, it is knowhow-based (See What Is Engineering Exactly? in the February issue of STRUCTURE).

Where to look for knowledge may be more important than the facts themselves.

15: Constantly Prod Yourself We need to keep asking questions like, “Why did you choose this over that?” or “What factors led to the decision to do that?” Avoid getting lost

in the codes, details, or loads prior to looking at the full picture. If you have trouble looking at the structure as a whole (or connection as a whole, or weld as a whole), then you are not effectively managing your time. You will have trouble seeing what you need to focus on. Determine which areas of the project need special attention and which do not.

16: Know Engineering and Architecture History Knowing our history, our leaders, our heroes, and our world’s engineering and architecture is not something that needs an explanation.

How is this not part of the curriculum?

History helps us use our long tradition of building structures to push new boundaries in our workplaces. We can stand on the structures of the past and learn to improve future design. We need to try to work daily towards rejecting the status quo, but only after we fully understand why. History will help us.

17: Seek Honesty to Achieve Beauty How do structural engineers design beautiful works of “structural art”? Pier Luigi Nervi states the importance of structural honesty or correctness: Every improvement in the functionality and the technical efficiency of a product brings out an improvement in its aesthetic quality… truthfulness is an indispensable condition of good aesthetic results.

This idea of working with honesty and clarity is similar to step 7 of this manifesto (Forget About Goals). Like the last phrase of the poem I wrote about Nervi (The Structure That Sings, in the March 2008 issue of STRUCTURE): Truth in form as the means, and beauty as the end can be our contribution towards improving the aesthetics of the built world.▪ Erik Anders Nelson, P.E., S.E. (ean@ structuresworkshop.com), is owner of Structures Workshop, Inc. in Providence, RI. He teaches one class per semester at the Rhode Island School of Design and Massachusetts Institute of Technology.

Please visit and comment on his blog at www.structuresworkshop.com/blog.

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

58

June 2012

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