T
he user of the 2003 IBC will im-mediately notice that the seismic design provisions of Sections 1616 through 1623 look very different from those of the 2000 IBC 1and that the sections are much shorter than be-fore. The brevity is somewhat decep-tive. It is a direct result of the fact that the 2003 IBC makes extensive refer-ences to the 2002 edition of the ASCE 7 Standard Minimum Design Loads for Buildings and other Structures.2Seismic Design by ASCE 7-02It is extremely important to note that the first exception to the requirement for seismic design in 2003 IBC Sec-tion 1614.1 exempts structures de-signed in accordance with ASCE 7-02Sections 9.1 through 9.6, 9.13 and 9.14. This means that seismic design following the provisions of ASCE 7-02, rather than those of IBC 2003, is fully permitted by IBC 2003. ASCE 7-02 is organized such that Section 9,Earthquake Design, is divided into the subsections listed in Table 1. There are also the Supplemental Provisions of Appendix A, which consists of the following sections:A.9.1 Purpose
A.9.3 Quality Assurance
A.9.7 Supplementary Foundation Requirements
A.9.8 Supplementary Provisions for Steel
A.9.9 Supplementary Provisions for Concrete
A.9.11 Supplementary Provisions for Masonry
Section A.9.1, Purpose, states,“These provisions are not directly re-lated to computation of earthquake loads, but they are deemed essential for satisfactory performance in an earthquake when designing with the loads determined from Section 9, due to the substantial cyclic inelastic strain capacity assumed to exist by the load
procedures in Section 9. These supple-mental provisions form an integral part of Section 9.”
Seismic Design by 2003 IBC The 2003 IBC is organized such that the code includes all of the seismic de-sign provisions, within the code itself,for structures that can be designed with the Simplified Analysis Proce-dure of Section 1617.5. In the case of more complex structures that do not qualify to be designed by the Simpli-fied Analysis Procedure, they may be designed in accordance with 2003 IBC Sections 1613 through 1623.
Apart from items related to the Sim-plified Analysis Procedure, the deci-sion was made to retain in the seismic sections of the code only the text, ta-bles, and figures that are needed for the determinati
on of the Seismic De-sign Category. For other subjects in 2003 IBC Sections 1613 through
The Seismic Design Provisions of the 2003 Edition of the
International Building Code
S. K. Ghosh, Ph.D.
President
S. K. Ghosh Associates, Inc.Northbrook, Illinois
This article provides the reader with an overall perspective
on the seismic design provisions of the recently published 2003 Edition of the International Building Code (IBC).
Susan Dowty, P .E., S.E.
Project Manager
S. K. Ghosh Associates, Inc.Laguna Niguel, California
1623, reference is made to ASCE 7-02 for code requirements. When address-ing these subjects, it is necessary for the code user to work with the 2003 IBC and ASCE 7-02 side by side, be-cause in some instances the 2003 IBC makes modifications to ASCE 7-02. Table 2 is provided for easy reference. All of the above makes for a certain amount of potential confusion. A case in point is Section 1620, Earthquake Loads — Design, Detailing Require-ments and Structural Component Load Effects. First, 2003 IBC Section 1620.1 refers the code user to ASCE 7-02 Section 9.5.2.6 for the require-ments except in the case of structures using the Simplified Analysis Proce-
dure. If the Simplified Analysis Proce-dure is used, all of the provisions are found in the 2003 IBC itself. This is not initially obvious unless the excep-tion to 2003 IBC Section 1620.1 is read carefully, which states: “For structures designed using the simpli-fied analysis procedure in Section 1617.5, the provisions of Sections 1620.2 through 1620.5 shall be used.” Unfortunately, only the title of 2003 IBC Section 1620.2 reflects that its ap-plicability is limited to the Simplified Analysis Procedure. Seco
nd, there are modifications to ASCE 7-02 Section 9.5.2.6, which are given in 2003 IBC Sections 1620.1.1, 1620.1.2 and 1620.1.3. It is a good idea to mark these changes in the code user’s ASCE 7-02 Section 9.5.2.6, as other-wise they may be overlooked. Another case in point is Section 1617.2, Redundancy. This section refers the code user to ASCE 7-02 Sec-tion 9.5.2.4 for the requirements, ex-cept in the case of structures using the Simplified Analysis Procedure. The exception to this section reads: “Struc-tures designed using the simplified analysis procedure in Section 1617.5 shall use the redundancy provisions in Section 1617.2.2.” Section 1617.2.1 then proceeds to make important modi-fications to ASCE 7-02 Sections 9.5.2.4.2 (Seismic Design Category D) and 9.5.2.4.3 (Seismic Design Cate-gories E and F). Section 1617.2.2, Re-dundancy (for use in the simplified analysis procedure of Section 1617.5), has two subsections: 1617.2.2.1 (Seis-mic Design Category A, B, or C) and
Table 1. Seismic design requirements of ASCE 7-02.
Table 2. Partial adoption of ASCE 7-02 by reference into IBC 2003.
1617.2.2.2 (Seismic Design Category
D, E, or F). Section 1617.2.2.2 is
nearly, but not quite, identical to Sec-
tion 1617.2.1. A code user, not privy to
the background given above, is apt to
wonder why the requirements of Sec-
tion 1617.2.1 are repeated in Section
1617.2.2.2, and further why the two
sets of requirements are slightly differ-
ent here and there.
The National Council of Structural
Engineers Associations (NCSEA) has
submitted proposed code changes for
intended inclusion in the upcoming
Supplement to the 2003 IBC, which
are aimed at removing much of the
confusion discussed above.
AMENDMENTS TO
ASCE 7-02
i am aliveIt would appear from the above that
the code user would find it easier to
utilize the exception to Section
1614.1, and do seismic design by the
requirements of ASCE 7-02, ignoring
the remainder of Sections 1613
through 1623. This easy way out may
not be to the user’s best advantage,
however. This is because by going all
the way with ASCE 7-02, the user will
not get to benefit from the amend-
姚乐怡资料ments to ASCE 7-02 that have been
introduced in IBC Sections 1613
through 1623. Some of these amend-
ments are quite substantive. Discus-
sion here will be limited to arguably the most substantive amendment,which is to be found in the exception to IBC Section 1616.3, Determination of Seismic Design Categories.
Much attention has already been drawn 3to the fact that the 2000 IBC requires the seismic design category
of each structure to be determined twice: once based on the start-period design special response acceleration,S DS , from Table 1616.3 (1), and a sec-ond time based on the long-period de-sign special response acceleration,S D 1, from Table 1616.3 (2). The more severe of the two seismic design cate-
Fig. 1. Seismic design spectrum of the ies governs the design of the struc-ture. The point has been made that it is unnecessary and wasteful to require that the seismic design category of a short-period structure be determined by long-period ground motion. The exception to 2003 IBC
Section 1616.3thus allows an important relaxation.The exception reads as follows:
The seismic design category is per-mitted to be determined from Table 1616.3 (1) (i.e., based on short-period design spectral response acceleration)alone when all of the following condi-tions apply:
1. The approximate fundamental pe-riod of the structure, T a , in each of the two orthogonal directions determined in accordance with Section 9.5.5.3.2of ASCE 7, is less than 0.8T S deter-mined in accordance with Section 1615.1.4,
2. Equation 9.5.5.2.1-1 of ASCE 7is used to determine the seismic re-sponse coefficient, C S , and
3. The diaphragms are rigid as de-fined in Section 1602.
The period, T S = S D 1/S DS , is the pe-riod at which the short-period or con-stant acceleration part of the design spectrum (see Fig. 1) transitions into the long-period or velocity-governed part of the spectrum. It is the dividing line between short-period and long-pe-riod response. By requiring in Item 1above that T a be less than 0.8T S , rather than T S itself, the code is trying to min-imize the possibility that because of imprecision in period determination,the above relaxation would be applied to structures having elastic fundamen-tal period, T , that in fact is beyond the short-period range. In o
ther words, it is trying to avoid the possibility that T might equal or exceed T S , even though T a is less than T S .
Item 2 above requires that the upper-bound design base shear, as given by the constant acceleration or “flat-top” part of the design spectrum,be used in the design of a structure uti-lizing the above exception. This re-quirement is intended to impose a de-sign force penalty on a structure for which T may equal or exceed T S ,while T a is less than T S .
Item 3 above makes the relaxation in question inapplicable to structures with flexible diaphragms because the
Table 3(a). Seismic Design Category of 2000 IBC vs. Seismic Performance Category of 1999 BOCA/NBC.
flexible diaphragm may end up play-ing a decisive role in determining the elastic fundamental period of such a structure; and this period may be well in excess of the approximate funda-mental period, T a.
Tables 3(a) to 3(c), extracted from Reference 4, lists the major metropoli-tan areas of the United States where the user would benefit from the relax-ation provided by the exception to Section 1616.3. The tables indicate, for a standard occupancy (Seismic Use Group I) structure in each metropoli-tan area, founded on Site Class A, B, C, D, or E, the seismic design cate-gories: when the exception to Section 1616.3 does not apply (without paren-theses), and when the said exception does apply (within parentheses). It should be quite apparent from the ta-bles how advantageous the exception to Section 1616.3 quite often is. The reader is encouraged to consult Refer-ence 5 for further information on seis-mic and other structural provisions of the 2003 IBC.
CONCLUDING REMARKS The 2003 IBC provides the user with two distinct options for seismic design purposes. Such design may be in com-pliance with the requirements of ASCE 7-02 Sections 9.1 through 9.6, 9.13 and 9.14, disregarding the provisions of IBC 2003 Sections 1613 through 1623. Or, the designer may choose to design by Sections 1613 through 1623 of the 2003 IBC, which adopts many of the provisions of ASCE 7-02 Section 9.1 through 9.6, 9.13 and 9.14 by refer-ence, b
ut in many instances with amendments that can be substantive. Although the first option may be easier and hence more attractive, the second option may very well be preferable be-cause it enables the designer to take advantage of the amendments to the ASCE 7-02 provisions, some of which are quite beneficial.
REFERENCES
1.ICC, International Building Code,
International Code Council, Falls
Church, VA, 2000, 2003.
2.ASCE, ASCE Standard: Minimum De-
sign Loads for Buildings and Other
Structures, ASCE 7-02, American So-
陈月末
Table 3(c). Seismic Design Category of 2000 IBC vs. Seismic Performance Category of 1999 SBC.
ciety of Civil Engineers, Reston, VA,
古琴曲凤求凰2002.
3.Ghosh, S.K., “A Necessary Change in
the Seismic Design Provisions of the
2000 IBC,” PCI JOURNAL, V. 46,
No. 4, July-August 2001, pp. 76-80.
4.Ghosh, S. K., Impact of the Seismic
Design Provisions of the International
Building Code, Structures and Codes
Institute, Northbrook, IL, 2001.我们结婚了菜刀夫妇
5.Dowty, S., and Ghosh, S. K., A Quick
Start Guide to the Structural Provi-
sions of the 2003 International Build-
ing Code, International Code Council,
Falls Church, VA, to be published in
2003.
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