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Residual Vectors and Effective Modal Mass Table




I was under the impression that the inclusion of residual vectors added the “missing modal” modal mass back to the model for frequency response run. In my model this is verified since the dynamic response at very low frequencies matches that which you would expect for a static analysis i.e. all the mass is present.


However I am confused.

In the modal effective mass table, the sum of the mass fractions does not sum to 100% the mass. The effective mass matrix is still less than the A-set rigid body mass matrix, but better than if residual vectors have not been accounted for. For example, the actual model mass is 9.8kg, in the X direction of the A-set rigid body mass matrix, the mass is 9.8kg, but the effective mass matrix is 6.908kg without resvec, and 8.52kg with resvec.

Why does the residual vectors not correct the effective modal mass table (via correcting the effective mass matrix), but corrects the transfer functions eg the SPC force at the base for the frequency response run?