How to implement 1st-order elastic analysis with reduced stiffness specified in AISC 360-10 section C2.3 for Direct Analysis Method in STAAD. Pro?

Hi,

As per AISC 360-10 section C2.2b.(4) for Direct Analysis Method, both maximum 2^nd-order & 1^st-order drift shall be determined with stiffnesses adjusted as specified in Section C2.3. It seems that the command “PERFORM DIRECT ANALYSIS” is only for 2^nd-order analysis (P-Delta analysis), and 2^nd-order drift can be derived from it. But how to implement 1^st-order elastic analysis with reduced stiffness for deriving 1^st-order drift in STAAD. Pro?