Cd = Deflection amplification factor
Ie= Seismic importance factor
R = Response modification coefficient
18.104.22.168 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, is 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 is multiplied by the quantity Cd ∕ Ie.
C22.214.171.124 Modal Response Parameters
The design response spectrum is simulated 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 must be amplified by Cd and reduced by Ie to produce the expected inelastic displacements.