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Re: UBC97 question

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Dear Michael,

In general buildings with longer periods, frame buildings, will result
in higher base shears than the static design method.  Conversely
buildings with short periods may result in base shears less than the
static method.  Please review section 1631.5.4.

The reason this occurs I believe (there other guys on this board who
have more knowledge) is that a response spectra is not actually an
earthquake.  It is all the accelerations possible at this site, and
the responding mode shapes of the structure.  The forces and or
stresses are combined in either CQC or SRSS method to give the final
stresses.(this an over simplification but idea is right)

Often engineers consider these loads as the ultimate loads. 
Especailly when investigating collpse potential.  However, gernerally
this is only done with a site specific or uniform hazard reponse specta.

Ken Tarlow 
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