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[FAQ]

One shouldn’t have rigid-body modes in a model used for a PSD analysis. Basically, if we have a rigid-body mode, then the transfer function becomes 1.0, meaning that output response PSD = input PSD. Usually, we don’t see this because (a) most customers have base-excitation situations, and b) for base excitation, the direction is usually specified with the “D” command, meaning that the direction is constrained (i.e., no rigid-body modes). (We have force excitation cases or base excitation specified with SED that can bypass this situation, but these are less frequently-used, so that is why we don’t usually see such cases of rigid-body modes in a PSD analysis.) If the rigid-body mode is due to a joint that is free to move, consider either constraining the joint or adding joint stiffness, depending on which better represents the user’s situation. (A modal analysis could verify whether or not there are rigid body modes present). Basically, while harmonic and transient allow for rigid-body modes, we shouldn’t include rigid-body modes in response spectrum or random vibration analyses. In both response spectrum or random vibration analyses, we’re assuming that the base is excited in a specific manner, which means that parts shouldn’t be unconstrained.