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

Can I use a response spectrum load case in a DIRECT ANALYSIS ?

 

In a response spectrum analysis, you are essentially finding out the maximum responses at various nodes/members. So the various responses are not at the same time instant. One cannot carry out a second order analysis without knowing the actual real time values for the displacements at all nodes. Also due to the modal combination, the results lose their signs which makes it impossible to find the true nature of the force ( tension/compression) or the direction of displacement. In addition, in direct analysis, the structural stiffness changes between iterations depending on the forces in the members which makes the analysis type nonlinear in nature. As of now there is no way to handle nonlinear dynamic analysis in STAAD.Pro.

 

One can do a DIRECT ANALYSIS with a response spectrum load case and in such a scenario, at least the stiffness reduction factor of 0.8 would be applied to the axial and flexural stiffness by STAAD while analyzing the structure against the response spectrum loading. The Tau-b part of the flexural stiffness of course cannot be computed or applied for a spectrum load case.

 

So essentially one can do a proper Direct Analysis for all cases other than the response spectrum case. The response spectrum cases would then have to be combined with the solved Direct analysis cases using LOAD COMBINATION.

 

Alternately one may use an equivalent static approach for generating the seismic load instead of using a response spectrum. The generated equivalent static seismic cases can then be combined with gravity loads using REPEAT LOADs. DIRECT ANALYSIS would then be able to account for the Pdelta effects due to seismic plus gravity loads.

When a response spectrum load case is subjected to a DIRECT ANALYSIS, does the software apply the reduction in stiffness like 0.8EA and 0.8*Tau*EI as is required for a DIRECT ANALYSIS?

 

Yes the software does apply the 0.8 reduction factor to the area ( A ) and Moment of Inertia ( Ix, Iy and Iz ) but it cannot account for Tau as for that the software needs to know the nature of the axial force which is not possible for a response spectrum analysis because the forces are devoid of any signs. Moreover it would not make sense to apply the Tau-b anyway because the member forces in the various members from a response spectrum analysis are not forces that occur at the same time instant.