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Re: dynamic analysis

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In a message dated 97-08-22 13:37:19 EDT, you write:

<< Subj:	 dynamic analysis
 Date:	97-08-22 13:37:19 EDT
 From:	Julio.Guerra(--nospam--at)theaustin.com (Julio Guerra - Chief Struct. Eng.)
 Reply-to:	seaoc(--nospam--at)seaoc.org
 To:	seaoc(--nospam--at)seaoc.org
 
 I think I need some help from the structural community. 
 I am working on a dynamic analysis of an irregular structure and it is my 
 understanding that I have to use the Rw factor to reduce the base shear.
 UBC 1994 Section 1629.2.4.3 refers to the use of this factor, applicable 
 only for soil profile type S4. Is it a different procedure for other soil 
 profile types, like S2 and S3?
 Thanks in advance for your response
 
 Julio Guerra
  >>

Section 1629.2 deals with the requirements for the ground motion to use in
the dynamic analysis.  The UBC does not provide a response spectra for a S4
soil type (Figure 16-3) and requires a site specific response spectra be
developed by a geotechnical engineer.  The results of a response spectra
(example: base shear V) can not be less than what is obtained from the static
analysis methods A and B. (section 1629.5.3)  Therefore, in some cases you
may have to scale up the results of the dynamic analysis to not be less than
the static method results.  You can also scale down the results of your
dynamic analysis to the static force levels (section 1629.5.3.2). 

If you are using the UBC response spectra, you will note that you are first
scaling the  response spectra by the Z value since this is a normialized
curve (2.5 x 0.4 =1.0g  effective peak ground acceleration  for zone 4).
  The building response parameters (shear, bending moments, etc.) to this
response spectra may be scaled down by a scaling factor to the building
response parameters developed using the static analysis method A and method
B.  This scale factor that you use is determined by several trial and error
computer runs since the scale factor is applied to the spectral accelerations
used in the computer input. You can not scale the results directly because of
the combination methods used (SRSS, CQC). The resulting scale factor may not
be the same as the Rw of the building system that you are using for the
statics analysis.  I think in most cases this scale factor will be less than
the Building Rw factor because of the minimum force level or deflection
requirements that you must satisfy for static analysis methods A and B.

By doing this scaling down (or up) you are meeting the minimum requirements
of the building code.  There is nothing in the code that says you have to
scale down the results of your dynamic analysis.  By scaling down you are
depending upon the ductility and overstrength of the framing system to handle
the actual earthquake forces instead of requiring the system to remain
essentially elastic (Rw=1.0).

Hope this helps,

Michael Cochran