39287615 2006 IBC Seismic Design Provisions

18 Structural Engineer January 2006

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2006 IBC seismicdesign provisions

The dramatic difference between thetwo codes can be attributed to the factthat the 2006 IBC references theAmerican Society of Civil Engineers’Minimum Design Loads for Buildingsand Other Structures (ASCE 7-05) forvirtually all of the seismic design require-ments. The provisions that remain in the2006 IBC are not different from those inASCE 7-05 (with the exception of twomodifications) and are related to the localground motion and soil parameters aswell as the definitions of terms actuallyused within those provisions. Provisionsregarding additions, alterations, andchange of occupancy in an existing build-ing have been relocated to Chapter 34.

It is expected that code users willwelcome the straight forward approachthe 2006 IBC uses to reference ASCE7-05 for seismic provisions, especiallybecause the optional approach used bythe 2003 IBC to reference ASCE 7-02was found to be confusing. Section1613.1 of the 2006 IBC reads as follows:

1613.1 Scope. Every structure, andportion thereof, including non-structuralcomponents that are permanently attachedto structures and their supports and attach-ments, shall be designed and constructed toresist the effects of earthquake motions inaccordance with ASCE 7, excluding Chapter14 and Appendix 11A.

ASCE 7-05 Chapter 14 is entitled

“Material Specific Seismic Design andDetailing Requirements” and ASCE 7-05 Appendix 11A is entitled “QualityAssurance Provisions.” Because theserequirements are addressed specificallyin other chapters of the 2006 IBC, theyare excluded in 2006 IBC Section1613.1 Scope.

The remainder of this article is dedi-cated primarily to discussing the seismicprovisions of ASCE 7-05 because that iswhat the designer will be using. Only thediscussion on Seismic Design Category

refers back to the IBC, because seismicdesign category determination is still inthe IBC. In the case of the 2003 IBC, ifthe simplified method was used, ASCE 7was not used. This is no longer the case.The simplified method can only be foundin ASCE 7-05. It is expected that thedesigner will have little need to refer backto the 2006 IBC for seismic design —perhaps only to Section 1613.6, whichincludes two modifications to ASCE 7-05. One modification is for the definitionof flexible diaphragms (Section 1613.6.1)

2 0 0 6 ■ PA R T 1 O F 6

Significant improvements come from reorganization

By S. K. Ghosh, Ph.D., and Susan Dowty, S.E.

t first glance, the seismic provisions of the 2006 International Building Code (IBC), publishedby the International Code Council, look very different from those in the 2003 IBC. Mostnoticeable is how short the section has become. In the 2003 IBC, the seismic provisionsextended from Section 1613 to 1623 and were 43 pages long. In the 2006 IBC, they are

completely contained in one section, Section 1613, and are expected to be only 22 pages long.A

Table 1: The latest editions of the commonly used materials standardare referenced in the updated ASCE 7-05.

Material Referenced document

Structural steel Standards developed by the American Institute of Steel Construction (AISC),as follows:• AISC 360 (2005)• AISC 341 (2005)

Concrete Standards developed by the American Concrete Institute (ACI), as follows:• ACI 318 (2005)

Masonry Standards developed by the ACI, the ASCE, and The Masonry Society (TMS),as follows:• ACI 530/ASCE 5/TMS 402 (2005)• ACI 530.1/ASCE 6/TMS 602 (2005)

Wood Standards developed by the American Forest & Paper Association (AF&PA),as follows:• AF&PA National Design Specification (2005)

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January 2006 Structural Engineer 19

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and the other modification is an excep-tion applicable to seismically isolatedstructures (Section 1613.6.2).

Changes in the seismic design provi-sions of ASCE 7-05 were the subject ofan earlier article published in theFebruary 2005 issue of StructuralEngineer. Only a few of the most signifi-cant changes are discussed here.

Reference standardsASCE 7-05 became available in mid-

November 2005. The traditional tancover has changed to navy blue. Theformat, including typeface, resemblesthat of the 1997 Uniform BuildingCode, published by the InternationalConference of Building Officials. Moreimportantly, it includes Supplement No.1 to the white-covered edition publishedin late 2004. The supplement updatesthe referenced material standards totheir latest editions (see Table 1).

Reformatting the provisionsThe seismic provisions of ASCE 7-

05 not only include substantive, techni-cal revisions, but they have beenreformatted completely and reorganized.

The reorganization project was accom-plished through a two-year effort fundedby the Federal Emergency ManagementAgency through the Building SeismicSafety Council and the ASCE.The goalof this effort was to recreate the ASCE7-05 seismic provisions so that they areuser-friendly and can be understood andinterpreted correctly and easily by anaverage engineer designing an averagestructure. By taking what used to be inone section (ASCE 7-02 Section 9.0)and expanding the material into 13chapters and two appendices (ASCE 7-05 Chapters 11 through 23, Appendices11A and 11B), the organization of theprovisions became more transparent andunderstandable. Also, by expanding theprovisions into chapters, the number ofsubsections is reduced. For example,ASCE 7-02 Section 9.14.7.3.10.6.2(Structure Period) is Section 15.7.10.6.1in ASCE 7-05.

One way to achieve the goal of makingthe seismic provisions easy to use was torelocate those provisions that are notfrequently used to later sections. Forexample, most design engineers will notuse the materials found in new Chapters

16 through 21, while all engineers will usethe material found in Chapter 11. AllSeismic Design Category (SDC) Arequirements are located up front inSection 11.7 for the convenience of theuser designing a structure assigned toSDC A. Also, a common complaint wasthat the ground motion maps took up toomany pages and interrupted the flow ofthe provisions. The remedy was to relo-cate the seismic maps to Chapter 22.

Another significant formattingchange is the manner in which referencesare handled. In ASCE 7-02, referenceswere listed in different sections andreferred to by a reference number withinthe text. In ASCE 7-05, they are referredto by their common names within thetext and are all listed in one central loca-tion, Chapter 23. The reference docu-ments listed in Chapter 23 are classifiedtwo ways, either as consensus standardsor as other reference documents.

Ground motion mapsMaps showing the contours of 5

percent damped spectral response acceler-ations at 0.2-second and 1-second periodscorresponding to the Maximum


Sa=SD1

T

Sa=SD1TL

T2

Period, T (sec)1.0

SDS

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T0 Ts TL

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Figure 1: In ASCE 7-05, the constant velocity branch of the design spectrum terminates at the long-period transition period, TL, beyond which anew constant-displacement branch starts.

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Considered Earthquake (MCE) havebeen updated whereby Figures 1615(1)through (10) in the 2003 IBC are nowreplaced by Figures 1613.5(1) through(14) in the 2006 IBC. Besides includingfour additional enlarged maps for the NewMadrid fault area and the Charleston,S.C., area, the new maps incorporate threemajor updates as follows:

1) The new maps are based on the2002 U.S. Geological Survey (USGS)probabilistic maps, rather than the 1996USGS maps that formed the basis of the2003 IBC. This results in some changesin spectral acceleration values in someparts of the country.

2) A deterministic area is nowincluded around New Madrid, whichsomewhat reduces the size of the highground motion region near the NewMadrid fault.

3) MCE maps now provide contoursof varying spectral values in Puerto Ricoand the Virgin Islands, as opposed to theolder practice of assigning a single valueto the entire region.

Design spectrum, Design baseshear

In ASCE 7-05, the constant velocitybranch of the design spectrum (see Figure1) terminates at the new long-period tran-sition period, TL, where a new fourthbranch to the design spectrum, propor-tional to 1/T 2, starts.This is the constant-displacement part of the design spectrumthat will govern the seismic response ofstructures with periods in the rangebeyond TL. The period TL is given onnew contour maps for all 50 states. Onemust locate one’s site on this contour mapto determine TL, which ranges between 4and 16 seconds, depending upon the loca-tion. It should be noted that the values ofTL are much larger than has been usedtraditionally by many engineers.

To be consistent with the new designspectrum discussed above, the followingbase shear equation for long-period struc-tures (where T>TL) is added in the equiv-alent lateral force procedure as follows:

where Cs is the seismic response coeffi-cient, SD1 is the design spectral response

acceleration at the 1-second period, R isresponse modification factor, and I is theimportance factor.

Determination of SeismicDesign Category

The term “Seismic Use Group”(SUG) has been omitted from the 2006IBC, and the SDC of a building is nowdetermined directly from its OccupancyCategory. The new tables defining SDCsimply replace SUG I with OccupancyCategories I or II, replace SUG II withOccupancy Category III, and replaceSUG III with Occupancy Category IV.The exception in 2003 IBC, whichpermitted the determination of SDCbased on short-period ground motionalone, has been modified. The followingthree additional criteria have beenincluded in the 2006 IBC’s alternativeSDC use qualifications to make itconsistent with the same provision inASCE 7-05 Section 11.6:• the 1-second mapped spectral acceler-

ation S1 must be less than 0.75g;• in each of two orthogonal directions,

the fundamental period of the structureused to calculate story drift must be lessthan the period, Ts, which marks theend of the constant-accelerationplateau of the design spectrum; and

• for buildings with flexible diaphragms,the distance between the verticalelements of the seismic force-resistingsystem must not exceed 40 feet.

RedundancyThe basic premise of the new redun-

dancy provisions that have been adoptedinto ASCE 7-05 is that the most logicalway to determine lack of redundancy isto check whether a component’s failureresults in an unacceptable amount ofstory strength loss or in the developmentof extreme torsional irregularity.

In ASCE 7-05, the redundancyfactor, �, is equal to either 1.0 or 1.3,depending upon whether or not an indi-vidual element can be removed (deemedto have failed or lost its moment-resist-ing capabilities) from the lateral-force-resisting system without causing theremaining structure to suffer a reductionin story strength of more than 33


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percent or creating an extreme torsionalirregularity (Plan Irregularity Type 1b).

Braced frame, moment frame, andshear wall systems have to conform toredundancy requirements. Dual systemsare included also, but in most cases areinherently redundant. Shear walls with aheight-to-length ratio greater than 1.0have been included in redundancyconsiderations to help ensure that anadequate number of wall elements areincluded or that the proper redundancyfactor is applied.

ASCE 7-05 adds a new user-friendlyfeature of conveniently listing when �may be taken as 1.0 for any of thefollowing situations:• structures assigned to SDC B or C;• drift calculation and P-delta effects;• design of non-structural components;• design of non-building structures that

are not similar to buildings;• design of collector elements, splices,

and their connections for which theload combinations with over-strengthof Section 12.4.3.2 are used;

• design of members or connectionswhere the load combinations withover-strength of Section 12.4.3.2 arerequired for design;

• diaphragm loads determined usingEquation 12.10-1; or

• structures with damping systemsdesigned in accordance with Section 18.

Simplified designThe simplified design procedure has

been revised completely and standsalone in Section 12.14. The procedureapplies to structures in SDC B, C, D,and E, but is not permitted for structures where the design is typicallydrift-controlled. It was felt that theapproach should be limited to certainstructural systems to avoid problemsthat may arise from omitting the driftcheck for drift-controlled systems(steel moment frames for example).The simplified procedure is allowed for bearing wall and building framesystems, provided that several prescriptive requirements are followed,which result in a torsion-resistant,regular layout of lateral-force-resistingelements.

ConclusionThe 2006 IBC references ASCE 7-

05 for virtually all of its seismic designrequirements. This change represents asignificant improvement to the seismicdesign provisions of the 2003 IBC andshould make it remarkably easier toimplement seismic design when usingthe 2006 IBC.


S.K. Ghosh Associates, Inc., is a struc-tural, seismic, and code consulting firmlocated in Palatine, Ill. and LagunaNiguel, Calif. President S.K. Ghosh,Ph.D., and Susan Dowty, S.E., are activein the development and interpretation ofnational structural code provisions. Theycan be contacted at [email protected] [email protected], respectively.

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39287615 2006 IBC Seismic Design Provisions

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