Can anybody explain this from the Abaqus documentation to me?
"Reaction force output in response spectrum analysis using eigenmodes extracted with the non-SIM-based Lanczos eigensolver provides directional combinations of so-called, modal reaction forces weighted with maximal absolute values of corresponding generalized displacements. Directional and modal combination rules used for the reaction force calculation are the same as for other nodal output variables. Modal reaction forces are calculated in the frequency extraction procedure. They represent static reaction forces calculated for the normal mode shapes. Generally, they cannot adequately represent reaction force in dynamic analysis. For models with diagonal mass and diagonal damping matrices the superposition of the modal reaction forces can provide a reasonable approximation of a nodal reaction force in mode-based analyses other than response spectrum analysis. In response spectrum analysis the model response can be better represented by requesting section stresses and section forces in structural elements containing supported nodes."
Are the section stresses and forces not calculated based on the superimposed displacements of the connected element nodes? How would recovering those be any different than the reaction force from a node rigidly tied to each node of a support?
