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Re: LONG SPAN GIRDER

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Mr. Zaki

 I would be interested in the rational why only a time history analysis is
appropriate for an irregular structure. 

Is the time history analysis linear or non-linear?

If the analysis is linear what information is reflected in the results that
is not reflected in a response spectrum analysis?  The answer is none since
a linear time history analysis can be defined in terms of the mode shapes
just as a response spectrum analysis.  In both cases the usefullness is
dependent on the validity of the input motion and both types of input have
their limitations.

If you are doing a non-linear analysis what non-linear aspects are you
considering and which are you ignoring?

>From my point of view the most critical issue with transfer structures and
earthquakes is that the transfer structure not fail during the big one.  To
deal with this you should consider the maximum forces from the supported
structure in a major earthquake.  Given that the transfer structure won't
fail the next point to consider is whether the structure is torsionally
balanced.  The point being that with a little common sence you can safely
design an irregular structure with rather simple analysis tools.

It amazes me that people design a URM upgrade, which can asume very
non-linear behavior, using manual calculations,  while a base isolated
structure which is typically a very regular structure requires  time
history analyses. 

Mark Gilligan
markkgilligan(--nospam--at)compuserve.com