Can I use a response spectrum load case in a DIRECT ANALYSIS ?

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. Also due to the modal combination, the results lose their signs which makes it impossible to find the true nature of the force or the direction of displacement. One cannot carry out a second order analysis without knowing the actual real time values for the displacements at all nodes. Moreover in direct analysis the structural stiffness changes between iterations depending on the forces in the members which makes the analysis type nonlinear in nature. As of now there is no way to handle nonlinear dynamic analysis in STAAD.Pro. The closest you can get, would be to carry out a PERFORM ANALYSIS for the response spectrum load case and do a Direct Analysis for all other cases. The response spectrum cases would then have to be combined with the solved Direct analysis cases using LOAD COMBINATION.

Alternately one may use an equivalent static approach for generating the seismic load instead of using a response spectrum. The DIRECT ANALYSIS would then be able to account for the Pdelta effects due to seismic plus gravity loads.