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Re: LONG SPAN GIRDER-Seismic analysis

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Mr. Mark

There are a difference between the results of the linear response spectrum
analysis and time history analysis.

1- Finally in a response spectrum analysis you solve a static problem with
a lateral story shear forces that correspond to the instant where the base
shear is maximum. Some elements may have maximum response at different
instant, so its response may be under-estimated.
2- In a response spectrum analysis, the response is restricted to the
number of modes specified by the engineer.

3- In a response spectrum analysis you can not take the effect of soil
structure interaction, which may cause significant change to the structure

4- Some Codes do not allow the use of the response spectrum analysis for
irrigular structures (e.g. The 1995 Egyptian Code). 

Structural Engineer

At 03:12 Õ 28/10/98 -0500, Mark Gilligan wrote:
>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