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RE: Site Specific Seismic IBC 2000

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There are really two separate issues here: a) development of
site-specific seismic response coefficients in lieu of using code
prescribed seismic coefficients; and b) using a dynamic analysis in lieu
of using the equivalent lateral force procedure. 

The site-specific seismic investigation should be able to provide values
that are essentially equivalent to Fa*Ss and Fv*S1; i.e., it should
account for seismicity of the geographical area as well as effects on
local soil conditions.  The seismicity should be based on seismic
probabilities/ methods similar to those used to develop the seismic maps
in ASCE 7/ IBC.  These seismic values may be presented in the form of a
site-specific acceleration response spectrum.  Fa*Ss would correspond to
the peak value of the response spectrum and Fv*S1 would correspond to
the spectral acceleration at a period of 1-second.  The design response
spectrum would be reduced to 2/3 of this general response spectrum,
similar to Sds and Sd1 being reduced by a factor of 2/3 relative to Ss
and S1.  The values derived from this design response spectrum could be
used with either the equivalent lateral force procedure or with a
dynamic analysis. 

The dynamic analysis is intended to better account for distribution of
seismic forces relative to the simplified equivalent lateral force
procedure and to account for multi-modal effects.  The full response
spectrum would be used in the dynamic analysis (including the reduced
short-period curve vs a flat-top curve).  (The flat-top curve is used
with the equivalent lateral force procedure.) 

There are limits set on how much the effective base shear can be reduced
relative to the code prescribed base shear using the equivalent lateral
force method.  This prevents these methods from allowing drastic changes
from code prescribed forces.  Since a site-specific seismic study
provides a more accurate evaluation of site seismicity, it makes sense
that some reduction in forces should be permitted.  However, a dynamic
analysis is not so much intended to be used to reduce the level of
seismic forces as it is intended to provide a more accurate distribution
of seismic forces.  Nevertheless, some codes allow a small reduction in
seismic forces when performing a dynamic analysis, since the detailed
analysis should also provide a more accurate evaluation of seismic
effects on the structure. 

William C. Sherman, PE
(Bill Sherman)
CDM, Denver, CO
Phone: 303-298-1311
Fax: 303-293-8236
email: shermanwc(--nospam--at)

-----Original Message-----
From: Brian Stanley [mailto:bstanley(--nospam--at)] 
Sent: Friday, March 03, 2006 11:27 AM
To: seaint(--nospam--at)
Subject: Site Specific Seismic IBC 2000

I was hoping someone could provide some clarification regarding
site-specific procedure for determining ground motion accelerations (IBC
1615.2) and it's correlation with Dynamic Analysis.

If a site classification of Class F has been identified, then a
site-specific procedure for determining ground motion accelerations will
be required of the geotechnical engineer.  What information is provided
by the geotechnical engineer in this procedure.  Would the Response
Spectrum - sim. to Figure 1615.2.2 be provided?  Would Fa and Fv be
provided?  How is the SDC determined using dynamic analysis?

In performing a dynamic analysis (IBC 1618), the resulting base shear
shall not be less than that determined from Equivalent Lateral Force
Procedure (per Table 1616.6.3).  Question - how do you determine the
base shear for the Equivalent Lateral Force Procedure if there is no Fa
or Fv values listed in tables 1616.3(1) or 1616.3(2) because the soil is
Class F?

Also, what is the norm for satisfying the requirements of IBC
1807.2.23.1 - do others utilize the slab on grade to brace piles in SDC
C or are grade beams added.

Brian T. Stanley
Richmond, VA

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