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RE: Overturning under seismic load

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Ben,

The seismic code development is charged developing methods for eningeers to design buildings that will have a demand predicated non-uniform ground motions that will have a nonlinear response. The "proper" way to design a structure is to perform a nonlinear dynamic analysis. I am not smart enough to do that and make money for what is routinely charged. The code developers understand structural response and take out the majic R factor dart board and assign R values (or Rw depending on what year you are in) in order for us to simplify our design lives, and give us all a false sense of security until the ground shakes again.

Another factor that is definitive is that the ground shaking frequency due to an earthquake is generally greater than a building response. If we take the straight seismic pseudo-static forces and apply them to a structure to calculate overturning on a structure, all the buildings in California would be 2 stories or less. The result is that in order to half way mimic how buildings respond in overturning due to the dynamic movements caused by seismic ground motion, we have to "condition" our numbers. Taking the Rw out by dividing by Rw for overturning is as good of a guess as any. And it is within standard R value (1 decimile point) dart board accuracy.

Regards,
Harold Sprague

From: Zhoub002 <zhoub002(--nospam--at)hawaii.rr.com>
Reply-To: <seaint(--nospam--at)seaint.org>
To: "Sherman, William" <seaint(--nospam--at)seaint.org>
Subject: Overturning under sesimic load
Date: Wed, 25 Aug 2004 10:37:42 -1000


I don't understand why the building code like UBC 1997 allows reduced
seismic load (i.e. 1/Rw) to be used for evaluating overturning. "Rw" is
a coefficient associated with ductility, but I can't imagine a
"ductile" overturning.

Ben Z.


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