Why are the calculated effective masses for the deformable modes in a free-free modal analysis equal to zero?
March 17, 2023 at 1:12 pmFAQParticipant
If rigid body (near zero frequency) and deformable modes exist in a modal analysis, the participation factors associated with the rigid body modes will be very large compared to those associated with the deformable modes (which will be almost zero). The effective mass is the square of the participation factor. Thus, the calculated effective masses for the deformable modes will be very small numbers. The output may report them to be zero because the format used to list them may not be able to express very small numbers. You cannot alter the participation factor calculations because they are calculated from eigenvectors normalized to the mass matrix. For example, specifying a frequency range that does not include the rigid body modes will not affect the calculated participation factors for the deformable modes. If you need to assess the relative behavior of the deformable modes by comparing their effective masses, you could extract the calculated participation factors using the *GET command (*GET,,mode#,pfact) and square them to obtain the effective masses. Even if the calculated participation factors are near zero values, you should be able to obtain them with the *GET command. See the attached simple test case.
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