The he elastic deformations calculated under reduced forces (see Section C12.8.6) are multiplied by the deflection amplification factor, Cd, to estimate the deformations likely to result from the design earthquake ground motion.
The application of the factor R/Ie to the design response spectrum reduces not only design forces but also displacements. To compensate for this, ASCE 7 requires that the deflections at level x at the center of mass, δx, be determined in accordance with where Cd is from Table 11.7, Ie is from Table 1.1, and δxe is the deflection determined by elastic analysis with seismic forces reduced by factor R/Ie. Using a value Cd equal to R would result in the displacement δx being equal to δxe, which would be consistent with the equal displacement rule.
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