ASCE 7-05 Sections 12.2.2 and 12.2.3: Combinations ofseismic force-resisting systems

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October 2009 » Columns » CODE SIMPLE

Susan Dowty, S.E.

It is rather common for different seismic force-resisting systems to be combined in the same building toresist the seismic forces. Two or more systems may be used such that there are horizontal combinations orvertical combinations, as shown in Figure 1 — on relatively rare occasions, there may be horizontal as wellas vertical combinations in the same building.

Answers to FAQsQ: What R, Cd, and Ω0 values should be used in the design of the building when there is a combination ofhorizontal and/or vertical seismic force-resisting systems?

A: ASCE 7-05 Section 12.2.2 clearly states that for horizontal combinations of systems in differentdirections, the respective R, Cd, and Ω0 values are to be used.

ASCE 7-05 Section 12.2.3.2 sets forth R, C d, and Ω0 values for when different systems are used in thesame direction, but they do not form a dual system. Table 1 summarizes this section, and we includednotes for situations not addressed by the code language.

ASCE 7-05 Section 12.2.3.1 sets a rule for vertical combinations of systems that the value of R used in thedesign of any story must be less than or equal to the value of R used in the given direction for the storyabove. Also, the values of Cd and Ω0 used in the design of any story must be greater than or equal to thevalues of Cd and Ω0 used in the given direction for the story above.

Figure 2 below illustrates how to properly select R, C d, and Ω0 for a vertical combinations of seismic load-resisting systems. However, ASCE 7-05 makes three exceptions to this requirement for small rooftopstructures; other supported, small structural systems; and detached one- or two-family dwellings of light-frame construction.

Table 1: R, Cd, and Ω0 values are evaluated differently for horizontal combinations in the samedirection.

Figure 1: ASCE 7-05 provisions address how to properly select R, C d, and Ω0 values when acombination of seismic force-resisting systems exists within the building.

Figure 2: When evaluating verticalseismic force-resisting system combinations, R cannot increase asyou go down and Cd and Ω0 cannot decrease as you go down.

S.K. Ghosh Associates Inc., is a structural seismic, and code consulting firm located in Palatine, Ill., andAliso Viejo, Calif. President S.K. Ghosh, Ph.D., and Susan Dowty, S.E., are active in the developmentand interpretation of national structural code provisions. They can be contacted at skghosh@aol.com andsusandowty@gmail.com, respectively, or at www.skghoshassociates.com