Typically, for an impulsive type load analyzed in the frequency domain (i.e. a spectrum analysis), damping has little impact on the results.

When damping must be considered, it must be factored into the spectrum itself. CAESAR II provides such spectra for earthquakes from Reg Guide 1.60 (i.e. spectra 1.60h2 (horizontal 2% damping), 1.60h5 (horizontal 5% damping), etc.).

For other types of spectrum analysis, damping shouldn't really have an effect. If your load duration is longer (i.e. non-impulsive) then damping may be more important, but then a time history approach would be required instead of the spectrum approach.


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Regards,
Richard Ay (COADE, Inc.)