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

I am doing an analysis with response spectrum loads. I also need to define compression only springs to simulate soil support. How can I do this ?

Response Spectrum loading, due to it's very nature, does not lend itself to any iterative type analysis ( Tension/compression, Pdelta etc.). So the answer to the question is, response spectrum loading cannot be used in conjunction with compression only springs.

However here are a couple of suggestions which may help 

First option is to define the support springs without compression only attribute to analyze the response spectrum cases and other primary cases ( live load, equipment load etc.). Use PERFORM ANALYSIS and CHANGE after that. Subsequently re-define the support springs with compression only attribute followed by REPEAT LOADs to combine the primary cases. For combining the response spectrum cases with other loads, you still have to use LOAD COMBINATION. Once all REPEAT LOADs and LOAD COMBINATIONs ( for response spectrum combos only ) are defined, follow it up with another PERFORM ANALYSIS and CHANGE. It is however worth mentioning here that by following this process you are able to account for the compression only springs when it comes to load combinations involving non-Response Spectrum load cases (like DL + LL, DL +/-WL, DL + LL +/-WL etc. defined as REPEAT LOADs). However when it comes to the Response Spectrum load case or a load combination involving a response spectrum load case (like say DL+/- SL, DL+LL+/-SL) you are still not able to account for the compression only behavior. 

Other option is to use equivalent static seismic loads to represent seismic loads as opposed to using a response spectrum load in which case you would be able to use REPEAT LOADs and compression only springs to combine gravity+seismic loads. You may scale the equivalent static seismic loads by the ratio of the base shears ( in other words use a multiplying factor equal to response spectrum base shear/equivalent static base shear).