Can I do a Pdelta analysis using STAAD for Response Spectrum load cases?
A response spectrum analysis by definition provides the user with joint displacements which are the absolute maximum displacements which can occur at the joint when the structure is subjected to dynamic loads represented by the spectrum. This also means that these displacements do not necessarily occur at all joints at the same instant of time. In other words, the absolute maximum at joint 1 and that at joint 2 are most probably occurring at different instances of time. Another fact to be considered is that the response of the individual modes is combined using either the SRSS or the CQC methods depending on the input provided by the user. Both of these are approximate methods. The result is that the joint displacements and consequently, the member forces cannot be used as a basis for obtaining the secondary forces on the structure. Hence, doing a P-Delta Analysis under such conditions does not make any sense.
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