Seismic Design Of Wood Light-Frame Structural Diaphragm .

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NIST GCR 14-917-32NEHRP Seismic Design Technical Brief No. 10Seismic Design of WoodLight-Frame StructuralDiaphragm SystemsA Guide for Practicing EngineersKelly E. CobeenJ. Daniel DolanDouglas ThompsonJohn W. van de Lindt

NEHRP Seismic DesignTechnical BriefsNational Earthquake Hazards Reduction Program (NEHRP) TechnicalBriefs are published by the National Institute of Standards andTechnology (NIST) as aids to the efficient transfer of NEHRP andother research into practice, thereby helping to reduce the nation’slosses resulting from earthquakes.National Institute ofStandards and TechnologyNIST is a federal technology agency within the U.S. Departmentof Commerce that promotes U.S. innovation and industrialcompetitiveness by advancing measurement science, standards, andtechnology in ways that enhance economic security and improve ourquality of life. NIST is the lead agency of NEHRP. Dr. John (Jack) R.Hayes, Jr., is the Director, and Dr. Steven L. McCabe is the DeputyDirector of NEHRP within NIST’s Engineering Laboratory.Applied Technology CouncilThe Applied Technology Council (ATC) is a nonprofit corporationadvancing engineering applications for hazard mitigation. Thispublication is a product of Task Order 13-486 under ContractSB134113CQ0009 between the ATC and NIST. Jon A. Heintz servesas the Program Manager for work conducted under this contract.Consortium of Universities forResearch in Earthquake EngineeringThe Consortium of Universities for Research in EarthquakeEngineering (CUREE) is a nonprofit organization advancingearthquake engineering research, education, and implementation.This publication was produced under a cooperative agreementbetween ATC and CUREE. Robert Reitherman served as AssociateProgram Manager overseeing production. Reed Helgens and DarrylWong served as report production and report preparation consultantsfor this work.About The AuthorsKelly E. Cobeen, S.E., is an Associate Principal at Wiss, Janney,Elstner Associates, Inc. in Emeryville, California. Ms. Cobeen has30 years of experience in structural design, working on a widerange of project types, sizes, and construction materials. She hasbeen involved in code development, research, and educationalactivities, including the NEHRP Recommended Provisions forSeismic Regulations for New Buildings, and International BuildingCode (IBC) and International Residential Code development. Shehas taught wood design at University of California at Berkeley, coauthored The Design of Wood Structures textbook, and co-authoredthe Recommendations for Earthquake Resistance in the Design andConstruction of Woodframe Buildings, a guideline developed for theCUREE-Caltech Woodframe Project.Applied Technology Council (ATC)201 Redwood Shores Parkway - Suite 240Redwood City, California 94065(650) 595-1542www.atcouncil.orgJ. Daniel Dolan is a Professor of Civil and Environmental Engineeringand Director of Codes and Standards for the Composite Materialsand Engineering Center at Washington State University. Dr. Dolanhas been involved in development of several of the building codes anddesign standards used in the United States, as well as France andCanada. He holds positions on the Building Seismic Safety Counciland IBC Technical Update Committee for the Structures Section. Hehas conducted extensive research in the area of the dynamic responseof timber structures, especially their response to earthquakes andhurricanes. He has published over 300 technical publications andhas given over 500 technical presentations dealing with these topics.Douglas Thompson, P.E., S.E., SECB, is president of STB StructuralEngineers, Inc. in Lake Forest, California, and he is also the 20132014 president of the Structural Engineers Association of SouthernCalifornia. He has authored several articles and publications, includingthe light-frame design examples in the Seismic Design Manuals, theGuide to the Design of Diaphragms, Chords and Collectors, andFour-story /Five-story Wood-frame Structure over Podium Slab. Hehas been involved with code changes to the Uniform Building Codeand IBC for over 25 years and is a voting member of the AmericanWood Council’s Wind & Seismic Task Committee.Dr. John W. van de Lindt is the George T. Abell Distinguished Professorin Infrastructure in the Department of Civil and EnvironmentalEngineering at Colorado State University. He has led more than 30research projects, with many focused on seismic performance of woodstructures, with 275 technical publications to his credit. While servingas the Project Director for the NEESWood Project from 2005-2009,he led the research on the full-scale six-story building tested on theE-Defense shake table in Miki, Japan and recently served as theProject Director for the Natioanl Science Foundation-funded projectSeismic Risk Reduction for Soft-Story Woodframe Buildings. Heis an Associate Editor for the Journal of Structural Engineering forwood topics and past Technical Administrative Chair for the AmericanSociety of Civil Engineers Structural Engineering Institute Committeeon Wood.About the Review Panel(see inside back cover.)CUREEConsortium of Universities for Research inEarthquake Engineering (CUREE)1301 South 46th Street - Building 420Richmond, CA 94804(510) 665-3529www.curee.org

NIST GCR 14-917-32Seismic Design of Wood Light-FrameStructural Diaphragm SystemsA Guide for Practicing EngineersPrepared forU.S. Department of CommerceNational Institute of Standards and TechnologyEngineering LaboratoryGaithersburg, MD 20899-8600ByApplied Technology CouncilIn association withConsortium of Universities for Research in Earthquake EngineeringandKelly E. CobeenJ. Daniel DolanDouglas ThompsonJohn W. van de LindtSeptember 2014U.S. Department of CommercePenny Pritzker, SecretaryNational Institute of Standards and TechnologyWillie E. May, Acting Under Secretary of Commerce forStandards and Technology and Acting Director

Contents1.2.3.4.5.6.7.8.9.10.11.Introduction .1The Roles of Diaphragms.4Diaphragm Components.5Diaphragm Behavior and Design Principles.12Diaphragm Seismic Design Forces.20Modeling and Analysis Guidance.25Design Guidance.29Detailing and Constructability Issues.32References.36Notations and Abbreviations.39Credits.41DisclaimersThis Technical Brief was prepared for the Engineering Laboratory of the National Institute of Standards and Technology (NIST) under theNational Earthquake Hazards Reduction Program (NEHRP) Earthquake Structural and Engineering Research Contract SB134113CQ0009,Task Order 13-486. The contents of this publication do not necessarily reflect the views and policies of NIST or the U.S. Government.This report was produced by the Applied Technology Council (ATC) in association with the Consortium of Universities for Research inEarthquake Engineering (CUREE). While endeavoring to provide practical and accurate information, ATC, CUREE, the authors, and thereviewers assume no liability for, nor express or imply any warranty with regard to, the information contained herein. Users of informationcontained in this report assume all liability arising from such use.Unless otherwise noted, photos, figures, and data presented in this report have been developed or provided by ATC staff, CUREEstaff, or consultants engaged under contract to provide information as works for hire. Any similarity with other published information iscoincidental. Photos and figures cited from outside sources have been reproduced in this report with permission. Any other use requiresadditional permission from the copyright owners.Certain commercial software, equipment, instruments, or materials may have been used in the preparation of information contributingto this report. Identification in this report is not intended to imply recommendation or endorsement by NIST, nor is it intended to implythat such software, equipment, instruments, or materials are necessarily the best available for the purpose.NIST policy is to use the International System of Units (metric units) in all its publications. In this report, however, information is presentedin U.S. Customary Units (e.g., inch and pound), because this is the preferred system of units in the U.S. earthquake engineering industry.Cover photo. Construction of a five-story, wood-frame apartment building.How to Cite This PublicationNIST (2014). Seismic design of wood light-frame structural diaphragm systems: A guide for practicing engineers, NIST GCR 14-917-32,prepared by the Applied Technology Council for the National Institute of Standards and Technology, Gaithersburg, MD.

1. IntroductionOther Structures (ASCE 7) (ASCE 2010). In most cases,the diaphragm construction will also serve as the floor orroof surface and resist gravity, wind uplift, and other loadsin addition to the loads associated with earthquakes. Wherea solid floor or roof surface is not required, elements such ashorizontal trusses or space frames can serve the same functionas solid diaphragms.The seismic force-resisting system (SFRS) of a buildingconsists of a three-dimensional collection of elements thattransmit loads and forces from the point of occurrence tothe foundation and supporting soils. This system typicallyconsists of horizontal and vertical elements (Figure 1-1).When resisting seismic forces, the horizontal elements(i.e., roof and floors) are classified as diaphragms and act totransmit the forces horizontally from the point of origin tothe vertical elements. The vertical elements (i.e., walls orframes) transmit the forces down to the next lower level or tothe foundation. Together, these elements function as a systemto provide a complete load path for seismic forces to flowthrough the building to the foundation and supporting soils.Diaphragms not only act to distribute the forces horizontallyto the vertical elements of the system, they also tie the verticalelements together to act as a system so that they share theload rather than respond individually. For seismic forces,the diaphragms are an integral part of the SFRS and deservesignificant attention during the design process.This Guide addresses wood light-frame diaphragms used inbuildings of all wood light-frame construction, as well as woodlight-frame diaphragms used with other vertical elements ofthe SFRS, including concrete or masonry walls, steel momentframes, and steel braced frames. “Light-frame” refers to therepetitive, closely spaced wood framing (e.g., joists or rafters)to which the diaphragm sheathing is attached. Of the buildingsconstructed entirely of wood light-frame construction, manyare small buildings, with single-family homes of three or lessstories being a majority (Figure 1-2). Medium-size buildingsconstructed entirely of wood light-frame construction includemulti-family residential buildings (Figure 1-3), hotels, schools,and small commercial buildings (Figure 1-4). These buildingsare of varying sizes. Buildings of up to three stories have beencommon for many years. Buildings of up to five or six stories arenow being constructed with increasing frequency. A number ofSeismic design of diaphragms is required for buildings inSeismic Design Categories (SDC) B through F, as definedin the International Building Code (IBC) (IBC 2012) andASCE/SEI 7 Minimum Design Loads for Buildings andRoof diaphragmHorizontal floordiaphragmSeismicforceShearVertical end wall(shear wall)ShVertical end wall(shear wall)earVertical side wallloadSeismicforceDiaphragm supportof side wallVertical side wallloaSeismicforcedHLFoundationreactionBFigure 1-1. Wood light-frame building with load path illustrated (FEMA 2006).Seismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers1

commercial and light-industrial buildings constructed entirelyof wood light-frame construction often have a large plan areaand are primarily of single-story construction.Buildings constructed using wood light-frame diaphragmswith concrete and masonry walls, steel frames, or othervertical element types, include commercial, institutional, andlight-industrial buildings predominantly of one or two storiesand “big-box” retail buildings with a large plan area andpredominantly of single-story construction. Concrete tilt-upwall buildings (Figure 1-5) represent a significant portionof wood light-frame diaphragm building inventory in theseismically active western states, while steel deck diaphragmsare more prevalent in other regions. Additional seismicperformance concerns and seismic design requirementsare applicable to wood light-frame diaphragms used withconcrete or masonry walls; brief discussions of these additionalconcerns and requirements are provided in this Guide.Figure 1-2. Single-family residential wood light-frame construction.Figure 1-5. Concrete tilt-up wall building with wood light-frameroof diaphragm.Although many of the ideas and analysis methods covered inthis Guide are applicable to a wider scope of diaphragm types,this Guide focuses on diaphragms consisting of wood framing(dimension lumber, structural composite lumber, I-joists, metalplate connected wood trusses, or wood nailers attached tosteel bar joists) sheathed with wood structural panels (orientedstrand board (OSB) or plywood). The sheathing is commonlystructurally fastened with nails or staples. Adhesives may beprovided between the sheathing and framing to reduce floorsqueaking but are not relied on as a structural connection. ThisGuide addresses platform construction (Figure 1-6(a)), wherethe wall framing extends a single story in height from the topof the foundation or floor below to the bottom of the floor orceiling/roof above, such that the floor and roof framing areconstructed to bear on top of the walls. Balloon framing (i.e.,the wall studs are continuous for multiple stories, and the floorsare suspended off or let into the inside of the walls) is not atypical framing method for modern construction in the UnitedStates and is not addressed in this Guide (Figure 1-6(b)).Figure 1-3. Multi-family residential wood light-frame construction.This Guide is written primarily for practicing structuralengineers and should also be useful to architects, buildingregulators (building officials and plan checkers), andcontractors. Students, educators, and others interested inFigure 1-4. Commercial wood light-frame building.Seismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers2

Roof and ceilingframing seatedon wallRoof and ceiling framingmay be seated on wallor supported at insideface of wallStud wall floor-to-roofor ceilingFloor framing supportedat inside face of wallFloor framingseated on wallStud wallfloor-to-floorStud wall full heightof building(b) balloon frame construction(a) platform constructionFigure 1-6. Typical wall sections.understanding the basis for the common design methodsused for wood diaphragms will find this document a usefulbeginning step in expanding that understanding.covered in Section 8. References are provided in Section 9;notations and abbreviations are in Section 10.This Guide Does Not Address the FollowingDesign Requirements for Wood-FrameDiaphragms in IBC, ASCE 7, SDPWS, and NDS Wood structural panel diaphragms with sheathingattached with structural adhesives that areintended to provide capacityT h e d e s i g n r e qui r e m e nt s fo r wo o d - f r am ediaphragms are in the IBC, ASCE 7, the SpecialDesign Provisions for Wind and Seismic (SDPWS)(AWC 2008), and the National Design Specificationfor Wood Construction (NDS) (AWC 2012). The IBCand ASCE 7 primarily define the seismic demand,while the SDPWS and NDS primarily addresscapacity and related design requirements fordiaphragm members, sheathing, and connections.It is recommended that the reader of this Guidehave these documents available. Diaphragms with sheathing other than woodstructural panel sheathing (straight and diagonalboard sheathing, gypsum wallboard, structuralinsulated panels (SIPs) and other stressed skinpanel systems) Proprietary diaphragm systems Proprietary framing (structural composite lumber(SCL), laminated veneer lumber (LVL), or I-joists) Proprietary sheathing and/or its fastening Proprietary fastening methods for either theconnections within the framing or for connectionsto transfer the forces into the vertical elementsof the buildingSection 2 of this Guide provides an introduction of the role ofthe diaphragm in the overall building structure and the waythe diaphragm resists both gravity and lateral loads. This isfollowed in Section 3 with a description of the components thatcomprise the diaphragm and the way each component functionsindependently and together to transfer loads. Diaphragmseismic behavior is described in Section 4, along with designphilosophy and principles for acceptable seismic performance.Seismic design forces are discussed in Section 5. Detailedguidance for analysis and for design are included in Sections6 and 7, respectively. Detailing and constructability issues are Horizontal trusses Steel-panel-sheathed diaphragms Heavy timber construction Concrete and timber composite floor systemsSeismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers3

2. The Roles of Diaphragms Provide lateral support to vertical elements—Diaphragmsare connected to the vertical elements of the SFRS. Becauseof this connectivity, diaphragms provide lateral supportto, and therefore improve lateral stability of, the verticalelements. In addition, the diaphragm connects all of thevertical elements associated with the story above and thestory below and provides the ability for the building torespond to lateral loads as a three-dimensional systemrather than as individual elements. This provides alternativeload paths (or redundancy) should one or more of thevertical elements become overloaded.Diaphragms perform a number of different roles as an elementof the building structural system including: Resist vertical loads—Most diaphragms act as the floor,ceiling, or roof of the building. Therefore, the verticalgravity loads (i.e., dead load, live load, snow load)associated with these elements of the building are supportedby the diaphragm. The vertical inertial forces that areinduced by earthquake vertical accelerations must also beresisted by the diaphragm. Resist horizontal inertial forces and distribute them to thevertical elements of the SFRS in the story below—Oneof the principal functions of the diaphragm is to resisthorizontal inertial forces due to the self-weight of thediaphragm and supported components and contents and todistribute these forces to the vertical elements of the SFRSat the story below. Influence dynamic building behavior—The fundamentalperiod of vibration of buildings with long-span woodlight-frame diaphragms is often strongly influenced bythe diaphragm, resulting in a longer fundamental buildingperiod than is the case with buildings with short-spandiaphragms. Resist out-of-plane forces—Exterior and interior walls andcladding that are oriented perpendicularly to the directionof seismic accelerations develop out-of-plane seismicforces (as well as out-of-plane wind loads). The wall orcladding connection

Seismic Design of Wood Light-Frame Structural Diaphragm Systems: A Guide for Practicing Engineers 2 Figure 1-2. Single-family residential wood light-frame construction. commercial and light-industrial buildings constructed entirely of wood light-frame construction often have a large plan area and are primarily of single-story construction.

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