Analyse your structure for possible combinations of spectra and take the envelope of forces. Now a days almost all finite element packages support enveloping. Envelope means to take the maximum response in each member for a particular response spectra.

Hi Bannu!
Thank U for your reply.
I would like to clarify my question.

For a particular seismic spectra(say in Vertical direction),the response in different modes can be comined by
methods like SRSS(Square Root of Sum of Squares),CQC etc to arrive at the net response FOR THAT PARTICULAR SPECTRA.

Now lets say there are three such Seismic spectra in East-West,Vertical and North_South directions.We can get net response(combined effect of all modal responses)for EACH spectra by the methods like SRSS etc.In some FEA packages there are options to get the net stuructural response for
ALL THE THREE SPECTRA(E_W,N_S,Vertical),again combining the responses from each seismic spectra by SRSS etc

Is there any such option in ANSYS to get the total response of a structure due to ALL THE THREE SPECTRA?
I am using ANSYS 5.7

Thanks in advance and regards.

I am looking for somebody to answer my  above question"how to do multi spectra seismic analysis using ANSYS?"

Thanks in advance

I am looking for somebody to answer my  above question"how to do multi spectra seismic analysis using ANSYS?"

Thanks in advance