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.

One can do a DIRECT ANALYSIS with a response spectrum load case and in such a scenario, at least the stiffness reduction factor of 0.8 would be applied to the axial and flexural stiffness by STAAD while analyzing the structure against the response spectrum loading. The Tau-b part of the flexural stiffness of course cannot be computed or applied for a spectrum load case.

So essentially one can do a proper Direct Analysis for all cases other than the response spectrum case. 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.