Applies To
	Product(s):	STAAD.Pro
	Version(s):	ALL
	Environment:	ALL
	Area:	Analysis Solutions
	Subarea:	Response Spectrum Analysis
	Original Author:	Sye Chakraborty, Bentley Technical Support Group

 

Can I use the results from a response spectrum analysis as is or do I have to generate the results on a mode by mode basis and then do a modal combination to generate the results?

 

The results from a response spectrum analysis (like the nodal displacements, element stresses, beam section forces etc. ) are all individual maximums at each node/element. These are generated by finding out the corresponding individual modal responses and then combining these, following one of the modal combination methods like SRSS/CQC etc. If you are interested in these results itself, you do not need to do anything in addition and you can use those straight away. However if you are trying to use these results to derive certain results, for example for a wall modeled with a mesh of plates, if you want to sum up all the element nodal forces at a certain level to find out the total shear at the level, and you want to do it for a response spectrum load case, then simply summing up the forces would not give you right results. This is because each of these forces are individual maximums and may not coexist at the same time. In addition due to the nature of modal combinations, these forces would be devoid of signs and so you would not know the direction in which these forces are actually acting. In situations like this, the correct approach would be to find out the total shear by summing up the nodal forces at a particular level for each mode and subsequently use modal combination to combine the results from each mode, to get the final shear at that level for the response spectrum load case.