P-delta + Modal Spetral Analysis

[vaquers]

Is there an option in SAP2000 to take into account the p-delta effect when you are working with load combinations which have a Response Spectrum load case?

I know that it makes no sense to combine p-delta with a response spectrum result because it loose the sign. But how can I have this amplification into account?

Currently I'm scaling up the drifts and forces by the ASCE 7-05 Section 12.8.7 coefficient. Is there another way? Regards,

Sebastian

 

[masomenos]

This isn't a full answer to your question, but modal analysis cases can be based on the stiffness of a nonlinear P-delta case. Those "stressed" modal cases can then be used as the basis for response spectrum or modal time history cases. For example, you could define a static nonlinear p-delta load case which includes selfweight, equipment loads, and maybe even a percentage of live load. You could then define a modal case to use the stiffness of that nonlinear load case. In some designs, it can make a significant difference in modal results.

As you already pointed out, there isn't a reasonable way to include response spectrum (RS) results in a P-delta case. You could define nonlinear P-delta load cases which include static earthquake loads, but not RS loads. Alternatively, a direct integration time history analysis for seismic loads could consider P-delta

I don't have it in front of me at this moment, but doesn't 12.8.7 refer to amplification of static results?

 

[vaquers]

Dear masomenos,
in my opinion it doesn't say that this amplification is only valid for static results.

According with ASCE 7.05 Section 12.8.7: "Where the stability coefficient (teta) is greater than 0.10 but less than or equal to the incremental factor related to P-delta effects on displacements and member forces shall be determined by rational analysis. Alternatively, it is permitted to multiply displacements and member forces by l.O/(l - teta)."

Regarding you answer, according with ASCE I need to take p-delta effect into account if teta is > 0.10. The "stressed" modal cases could amplify the forces but I think that it isn't the same. Thanks, regards.

Sebastian