When performing a spectrum analysis that takes into account missing-mass and rigid-response effects, consider the following hints and recommendations:
Define the ZPA frequency value (
fZPA) corresponding to the input spectrum. (This value is the beginning of the frequency range for which the acceleration remains constant and equal to the ZPA.)Request all solutions below
fZPAin the modal analysis.Include all modes having a frequency below
fZPAin the spectrum analysis.Choose a CQC mode combination to correctly combine modes with closely spaced frequencies (if any).
Always include the missing-mass effect in the first simulation to verify its significance. (The missing-mass response is the result of a static analysis of the structure, so its computational cost is small.)
Include the rigid-response effect. The Lindley-Yow method is easier to use, as only the ZPA value must be input.