The output is the 1 sigma response (displacement, stress etc) which is the square root of the area under the response PSD curve. The RPSD curve shows the degree to which various natural frequencies of the structure are contributing to the response. For example, you might apply a PSD input that goes from 100 to 200 Hz in the Z direction. A natural frequency (mode) may be at 140 Hz but it may not be in the Z direction (very low participation factor in Z direction). So you won't see a peak at 140 Hz in the RPSD curve.
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