Response spectrum analysis is a method for estimating the seismic response of a structure from a set of modal dynamic or Ritz analysis results along with a response spectrum which capture the frequency content of an expected earthquake.
The basic response cases are set up in the data. This page confirms which output is required. If no modal results are present the user is warned: “There are no modal results present. This analysis will terminate”.
Unlike most other analysis options derived results from a response spectrum analysis cannot be derived from the basic response spectrum results. Instead the derived modal results must be calculated and then these combined.
A pseudo response spectrum analysis is based on mode shapes arrived at by static analysis. In order to combine these for a response spectrum analysis the modal information must be supplied.