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Re: earthquake in STAAD-III

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I do not have a STAAD manual but as I look at the input, I see the same
data used as input for the time history analysis and for the response
spectrum analysis.  This is wrong.  The input for the response spectrum
analysis consists of values of period or frequency combined with
acceleration values.  The input for a time history analysis consists of
pairs of time and acceleration values.  You cannot use the same input for
both analyses.

A basic difference between a time history and a response spectrum method is
in the way the forces from each mode are combined.

In a time history analysis, the forces at a particular location are
algebraically combined at each time step for each mode.  In a response
spectrum analysis the program calculates the maximum forces in each mode
and then combines the results.  The method of combining modal results in
all of the response spectrum values being reported as positive values thus
making it meaningless to draw a moment diagram.

I believe that the more basic problem here is that the program is being
used without a basic understanding of the method of analysis.  I would
recommend that you study a basic book on dynamics before proceeding.

Mark Gilligan