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Re: Dynamic base shears, is it considered?

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(This message is being re-sent since it did not appear to go thru the first

There have been several excellent detailed descriptions of design practice
response to this question.  This reply is intended to be more of a "brief
summary" to clarify normal practice.

UBC wind and seismic forces must both be considered - often times a rough
estimate of each type of load will show which will govern design, so a full
analysis of each condition is not always required.  In most cases the
lateral force procedure" is used for seismic design, unless the structure is
complex or operability after an earthquake is critical (such as power plants
or other utilities).  (This method used to be referred to as the
method", recognizing that the forces are not really static.)  Where a
analysis is required for critical structures, the base shear must also be
determined by the static force method as the minimum allowable base shear -
but the full structure need not be analyzed in detail by the static force
procedure.  Thus most designs require comparison of more than one lateral
force method but may only require detailed load development in all members
one design lateral force procedure.

For critical structures, the response spectrum is often determined by a site
specific seismic review by the geotechnical engineer.  Where resulting base
shears exceed the code minimums, the UBC allows scaling the results down to
the code design level - however, I have designed critical utility structures
for higher than code levels where the client wants improved probability of
operability after a seismic event.