As you clarified in "STAAD.Pro Response Spectrum [FAQ]", if seismic effect combination method is SRSS, the effect of damping is built into the spectrum values. How about CQC method? How to reflect the effect of damping upon the spectrum values that is being input?
Furthermore, if CDAMP is specified, as we know, each mode may have the unique damping value, how to deal with this case?
The spectral acceleration versus period curve entered into STAAD is developed for a particular damping. This is true for all combination types. SRSS, ABS, and TEN do not use the damping entered on the Spectrum command for any calculation.
However, the CQC and ASCE methods do use the damping supplied to form an interaction matrix for determining the response magnification due to closely spaced modal frequencies.
In general there is no need to reflect the effect of damping. The curve has the acceleration that you want for a given modal period.
If you know the curve for each of several damping values, then enter a file containing a curve for each damping value. STAAD will interpolate the curve data to get a curve for each modal frequency used. See the description of the file data containing the curves.
CDAMP is used to compute the weighted average damping ratio based on the strain energy and damping ratio of each material type for each mode. There is no need for CDAMP unless you have two or more material damping ratios represented in the structural model. The CDAMP values will not be used except in CQC and ASCE4 combinations.