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

 

I am analyzing a model with seismic load generation cases (ELFP) and getting the following error 

Can you please explain what is causing this? 

The fact that the software is coming up with a very low value for Rayleigh Frequency indicates that the structural stiffness is very low and more often than not, that is caused by modeling errors. So first and foremost, you should check the model geometry and ensure that there are no disjointed structures, orphan nodes, duplicate beams/nodes, improper connectivity between beams/plates etc. Subsequently you should check for specifications that can make the model unstable namely the releases, truss assignments, pinned support assignments etc. All of these are factors that can contribute towards an ill conditioned stiffness matrix, eventually causing solver errors. If you have a large model with lots of load cases, it would make sense to comment out all loads and analyze for a single load case with just self weight and see if the analysis completes without an error or not. Once modeling issues are sorted out and the structural displacements look ok, you may then reactivate the rest of the cases including the seismic ones.