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The International Code Council’s 2003 International Building Code (IBC) Section 1617.6 Seismic-Force- Resisting Systems and the American Society of Civil Engineers’ Minimum Design Loads for Buildings and Other Structures (ASCE 7-02) Section 9.5.2.2.4.1 Increased Building Height Limit for SDC D and E, both address the topic of height limits for specific structural systems in a specific seismic design category.

ASCE 7-02 Table 9.5.2.2 sets forth 160-foot height limits for bearing wall systems or building frame systems with steel-braced frames or special reinforced cast-in-place concrete shear walls, which are assigned to seismic design category D or E.

While the basis of the height limits has been questioned, particularly in recent times, they represent mandatory requirements of the code and cannot be violated unless specific jurisdictional approval is obtained under Section 104.10 of the 2003 IBC. However, in seismic design categories D and E, ASCE 7-02 Section 9.5.2.2.4.1 allows these height limits to be relaxed from 160 feet to 240 feet, provided that two criteria are met: the steel-braced frames or cast-in-place special reinforced concrete shear walls in any one plane do not resist more than 60 percent of the seismic design force excluding torsional effects, and the configuration of the lateral-force-resisting system is such that torsional effects result in no more than a 20-percent contribution of the strength demand on the walls or frames. The intent is that each of these shear walls or braced frames are in a different plane and that the four or more planes are spaced throughout the plan or on the perimeter of the building in such a way that the premature failure of one of the single walls or braced frames will not lead to excessive inelastic torsion.

Although a structural system with lateral force resistance concentrated in the interior core is allowed by the IBC, it is highly recommended that the use of such a system be approached with extreme caution, particularly for taller buildings.

Preference should be given to systems with lateral force resistance distributed along and across an entire building.

Answers to FAQs:

Q: Please comment on the concrete shear wall layouts in Figure 1.We would like to know which layouts would qualify under ASCE 7-02 Section 9.5.2.2.4.1 to allow a 240-foot building height.

A: Scheme A, although there are multiple lines of resistance in each direction, will probably not qualify because of the 20-percent torsional shear limitation.

A specific check needs to be made.

Scheme B does not qualify because 100 percent of the seismic design force is resisted along one plane (Line B).

Scheme C qualifies because each line of shear walls resists approximately 50 percent of the seismic design force and because there are no or minimal torsional effects.

S.K. Ghosh Associates, Inc., is a structural, seismic, and code consulting firm located in Palatine, Ill., and Laguna Niguel, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the development and interpretation of national structural code provisions. They can be contacted at skghosh@aol.com and dowtyskga@cox.net, respectively, or via the firm’s website at www.skghoshassociates.com.

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