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Re: Dynamic base shears, is it considered?

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In a message dated 98-02-20 21:01:38 EST, you write:

<< Subj:	 Dynamic base shears, is it considered?
 Date:	98-02-20 21:01:38 EST
 From:	MRodrig273(--nospam--at)aol.com
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org
 
 I am almost done with my Seismic design class here in Cal Poly, and I would
 like to get some input from practicing engineers.
 
 From the Uniform building code, we base our lateral designs by comparing 2
 base shears, wind and seismic.  From my understanding, the seismic base shear
 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 to
 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 is
 considered, where do designers get their response spectra from?
 
 Thanks for all your input.
 
 Marlou
 Arce 98
 Cal Poly, SLO
  >>

The lateral design of a structure requires determining the base shear (V) for
both wind and seismic forces, using the larger of the two forces for design as
you mentioned.  It should be noted that even if wind governs for the building
base shear in a seismic region, detailing and limitations for seismic forces
must still be evaluated (UBC 1624.2).  Depending upon building height,
eventhough wind governs for base shear, the overturning forces may still be
governed by seismic because of the force distribution over the height of the
building.  Depending upon the building being designed, the UBC may require
that a dynamic analysis be performed instead of a static analysis.

Dynamic analysis is the preferred method for the 3 dimensional analysis of a
building using a computer model.  Keep in mind that a static analysis can also
be 3 dimensional.  The Uniform Building Code (UBC) has a normalized response
spectra which is adjusted by a multiplier depending upon the seismic region
(zones 1-4) and the soil type at the site.  Also a geotechnical engineer can
prepare a site specific response spectra for the design of the building
instead of using the UBC response spectra.  Site specific response spectra are
required by the UBC in certain cases.

Because of the concern about how a engineer might model a building using
response spectra and to help building officials plan check the design, the UBC
requires a minimum base shear level for the building based upon a static
equation in the code.  This static equation takes into account some factors
that influence the seismic forces which act upon the building (soil type,
building height, type of lateral resisting system being used).  The UBC
minimum static base shear force level equation approximately represents the
expected dynamic first mode response of the building which contributes the
largest shear component of the total building base shear of all the
participating mode shapes of the building. 

The reason for doing a dynamic analysis as opposed to a static analysis is to
obtain a more realistic distribution of lateral forces over the height of the
building and evaluate possible higher mode effects on the building.  The
dynamic analysis usually results in lower overturning forces to design for as
opposed to the static analysis method.

In the actual application of the dynamic analysis in practice,  the calculated
lateral forces from the response spectra (or time history) are scaled down by
a lateral system factor (Rw) so the building base shear is no less than the
base shear calculated by the UBC static equation.  The Rw represents the
expected ductility of the lateral resisting system to resist the seismic
forces through inelastic deformations of the framing system should the
earthquake be large enough.  If the response spectra analysis results in
forces less than required by the UBC static equation, then the building base
shear must be scaled up to this force level.  In private practice, few
buildings are actually designed for building base shears greater than required
by the UBC static equation because of the associated higher costs of
construction.

Since you have been using the UBC, you may have noticed that the building code
also rewards doing a 3 dimensional static analysis over a 2 dimensional static
analysis by allowing the building base shear (V) to be reduced since the
actual expected period of the building is being calculated. But, again a limit
is placed upon the amount of reduction relative to the code static base shear
equation (The 3 dimensional model building period (T) used to calculate the
building base shear (not drift) can not exceed 1.3 times the building period
calculated using the code static equation requirements). 

So in a sense you are designing for dynamic base shears (scaled down) even
when you use the code static equation.  In actual practice, I believe most
firms do a 3 dimensional static analysis, unless required by the code to do a
dynamic analysis.  The main reason for not doing a dynamic analysis, when not
required by the code, is because it is more time consuming, and the minimum
building base shear (V) remains the same as the 3 dimensional static analysis
limitations, therefore your final design will most likely be very similar
whethere you did a static or dynamic analysis.

When doing a dynamic analysis,  remember that you still need to do a static
analysis to help understand the force distribution through the building since
in a dynamic analysis you are combining the various mode responses of the
building and lateral load paths can be hard to follow.


Michael Cochran S.E.
Brian L. Cochran Associates