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Fw: Dynamic base shears, is it considered?

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Speaking from the perspective of the UBC (1994), the answer to your
question depends on the structure you are designing.  Wind should always be
considered.  The UBC has spectra that can be used for an RSA, but it also
requires that the results of the dynamic analysis (in terms of base shear)
be at least equal to 90% of the static base shear for regular buildings,
and 100% for irregular buildings.  The UBC doesn't preclude the use of
higher base shears.

The UBC requires a dynamic analysis for taller structures and for irregular
structures.  A simple static distribution may not give a good indication of
potential problems.  As far as I know, you can always use a dynamic
analysis procedure, and you can sometimes use a static analysis procedure. 
It is important to note, I think, that the response of a structure under
seismic loading is very much dynamic in nature, and that the UBC's
simplified static approach is just that, a simplified approach (although
with proper design performed by an experienced engineer it is more than
adequate in many cases).

Just remember that the design base shears for earthquakes are almost always
much (or very much) smaller than the anticipated initial base shear
expected during an actual earthquake.  Comparing wind to the design
(reduced) base shears can be very misleading to a person unfamiliar with
the basic philosophy underlying current seismic design procedures. 
Ductility requirements are an integral part of the current seismic design
procedures, since they give the structure the ability to deform under the
large displacements expected during an actual earthquake (as opposed to the
small displacements one would get under the reduced design base shears).

T. Eric Gillham PE

> From: MRodrig273(--nospam--at)
> To: seaoc(--nospam--at)
> Subject: Dynamic base shears, is it considered?
> Date: Saturday, February 21, 1998 11:59 AM
> I am almost done with my Seismic design class here in Cal Poly, and I
> like to get some input from practicing engineers.
> >From the Uniform building code, we base our lateral designs by comparing
> base shears, wind and seismic.  From my understanding, the seismic base
> is a static base shear.  From my seismic class we have been analyzing and
> designing structures for dynamic base shears.  So that means we now have
> consider 3 base shears for our lateral designs, wind, static seismic, and
> dynamic seismic.
> Does anyone consider this in any of their designs? if dynamic base shears
> considered, where do designers get their response spectra from?
> Thanks for all your input.
> Marlou
> Arce 98
> Cal Poly, SLO