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>I want to work out from the time history using modal analysis instead
>of the modal spectral acceleration obtained from the diagrams.
There is software for converting response spectra to time histories at  
<> I've used SIMQKE when I've had to 
do it. 

What you'll get is a time history with a frequency/amplitude content that 
more-or-less matches the spectra. Since the spectra are composites the 
time history won't necessarily match any particular set of site spectra. 
The time history is really a set of randomly phased fourier components 
which you can make a zero base correction to. You can make several runs 
on the horizontal spectra to get east-west and north south time histories 
or take a guess at the critical direction 

You're going to be wasting a lot of time doing it this way unless you 
have some specific non-linearity you didn't mention. Your time history 
run will require several thousand oad steps and you'll need to figure out 
exactly how you'll determine the critical instant in time. This won't be 
easy because you'll need to juggle a great many quantities like base 
shear, maximum deformation, overturning moment and stress. Whatever 
software you're using for the time history will need to be able to 
calculate such results and plot them in the time domain. Unless you know 
something about doing time-history analysis, take it very easy, do some 
simple example problems first and work out methodology for examining 
results. Doing it right  and interpreting the results properly is not 
easy, so get set for a long, involved piece of work. 

>Can I use a Uniform acceleration and check the maximum response,I need some 
>informations .

Christopher Wright P.E.    |"They couldn't hit an elephant at
chrisw(--nospam--at)        | this distance"   (last words of Gen.
___________________________| John Sedgwick, Spotsylvania 1864)

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