Why do modal FSUM results differ when all nodes are selected vs only constrained nodes? This isn't the case for a static analysis. Shouldn't unconstrained nodes be in a state of equilibrium and not contribute to FSUM?
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Based on the 14.0 release note below, at 13.0 we were doing a static expansion that didn't included the inertia forces resulting in an apparent imbalance in the dynamic equilibrium at unconstrained nodes. At 14.0 the FSUM with all the nodes selected matches the one with just the fixed nodes. 14.0 Release Note: An option is now available on the mode-combination commands (CQC, DSUM, GRP, NRLSUM, PSDCOM, ROSE, SRSS) to combine the summed modal static and inertial forces. The default (and prior release behavior) is to combine the modal static forces (that is, only the stiffness multiplied by mode shape forces, both of which are the stress-causing forces). An option is now available to combine the summed modal static forces and inertia forces (both stiffness and mass forces, which are the forces acting on the supports). |
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