diaphragm accidental eccentricity on response spectrum analysis

[xarz]

Hello everyone,
I'm using sap2000 software to perform a linear dynamic analysis of a simple two storey building using a response spectrum. For each storey I have defined a diaphragm.
Referring to sap2000 help, I should be able to assign diaphragm eccentricities by using the "Overrides" button in the Diaphragm Eccentricity area of the form "Load Case Data - Response Spectrum". The problem is that such a button does not exist. How can I workaround this problem performing the response spectrum analysis?
thanks

 

[Guest262653]

Hello,

This is an old question on this matter and, to my knowledge, in SAP2000 it is still not possible to automatically take this into account. In fact, you would need to have additional modal analysis cases for each analysis, which is not practical at all.

What I usually do is a simplification which consists first on running the basic response spectrum analysis and determining the frequency associated with the dominant mode and the corresponding acceleration from the response spectrum.

Then, knowing the mass on each storey, we apply the Rayleigh-Ritz procedure for equivalent static analysis to get the forces acting on each storey. With these forces, the torsional moments are determined from the diaphragm eccentricities and applied at any point on the diaphragm.

Hope it helps.