Symbols
Cd = Deflection amplification factor
Ie= Seismic importance factor
R = Response modification coefficient
12.9.1.2 Modal Response Parameters
The value for each force-related design parameter of interest, including story drifts, support forces, and individual member forces for each mode of response, shall be computed using the properties of each mode and the response spectra defined in either Section 11.4.6 or 21.2 divided by the quantity R ∕ Ie. The value for displacement and drift quantities shall be multiplied by the quantity Cd ∕ Ie.
C12.9.1.2 Modal Response Parameters
The design response spectrum (whether the general spectrum from Section 11.4.6 or a site-specific spectrum determined in accordance with Section 21.2) is representative of linear elastic structures. Division of the spectral ordinates by the response modification coefficient, R, accounts for inelastic behavior, and multiplication of spectral ordinates by the Importance Factor, Ie, provides the additional strength needed to improve the performance of important structures. The displacements that are computed using the response spectrum that has been modified by R and Ie (for strength) must be amplified by Cd and reduced by Ie to produce the expected inelastic displacements.
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