Discrepancies in results obtained from SAP

[ankit2609]

I need to carry out time history analysis of a frame(simple 2D frame) in SAP2000.
There are various scenarios..lets consider one by one.

1) When I am running linear time history analysis by direct integration and mode super position method, I am finding quite a lot of difference in the base shear obtained by both methods. Can somebody please explain why is it so?
2) When I am running non-linear time history analysis, base shear obtained from nonlinear case is coming out to be more than that will be obtained from linear case,which is not possible . Please explain how shall I provide non-linearity in SAP2000.I am supposed to consider material non-linearity only.

 

[jdgengineer]

How are you modeling the non linearity? Are you using plastic hinges or link elements?

 

[ishvaaag]

I can visualize at least one scenario where the nonlinear time history analyses give more base shear than the linear case. It is when the lower stiffness brought about by the nonlinear behaviour brings the building vibration more in resonance with the excitation, what may happen for some buildings and excitations. Just then keep the excitation time enough acting on the more resonant structure and you will get higher base shear for the nonlinear case by one effect that the simpler linear model would have kept invisible.

So, if something, the nonlinear model should be thought more accurate, rather than a warrant for lower overall, or general, or particular local response to the earthquake.