Can I use a response spectrum load case in a DIRECT ANALYSIS in RISA 3D?
In a response spectrum analysis, you are essentially finding out the maximum responses at various nodes/members. So the various responses are not at the same time instant. One cannot carry out a second order analysis without knowing the actual real time values for the displacements at all nodes. Also due to the modal combination, the results lose their signs which makes it impossible to find the true nature of the force ( tension/compression) or the direction of displacement. In addition, in direct analysis, the structural stiffness changes between iterations depending on the forces in the members which makes the analysis type nonlinear in nature.
The steel frame system is for the support of a Coker furnace and other petrochemical equipment - non-building structure with an R=2.
Mike McCann, PE, SE (WA)