Applies To
	Product(s):	STAAD.Pro; Ram Elements
	Version(s):	All
	Environment:	N/A
	Area:	Analysis Solutions
	Subarea:	PDelta Analysis
	Original Author:	Bentley Technical Support Group

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 methods used to combine the responses of the individual modes in a response spectrum analysis ( for example SRSS, CQC etc.) results in loss of signs for these responses. 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 sense.