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RE: Seismic Zones in Texas

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The seismic accelerations obtained from the USGS web site are the same
values that will be in the IBC 2000 maps. (USGS created the maps also.)
There might be some refinement, but generally the values are the starting

It is kind of true that the values are based on bedrock.  (How's that for
ambiguity?)  The values must be modified for the actual soil at the building
site, proximity to a fault, etc.; those modifications are contained in the
1997 NEHRP Provisions.  All this will still boil down to a base shear.

I would strongly urge you to look at the Provisions and the Draft 2000 IBC.
I would like to receive your comments.

Harold Sprague
The Neenan Company

-----Original Message-----
From: Kenneth Tarlow [mailto:ktarlow(--nospam--at)]
Sent: Tuesday, January 05, 1999 3:51 PM
To: seaint(--nospam--at)
Subject: RE: Seismic Zones in Texas

Dear Harold,

The USGS maps are not very accurate.  Even in there website they
indicate that the acceleration values are based on bedrock.  The
actual ground accelerations can be much higher.

Will there be any other kind of refinement short of structural
Engineers dealing with attenuation equations?

Ken Tarlow

---Harold Sprague <harold.sprague(--nospam--at)> wrote:
> The IBC 2000 (which will soon be the only national building code in
> US)will be using the 1997 NEHRP as its basis.  There will be no more
> "seismic zones".  The seismicity will be determined using seismic
> ordinates.  Check out the USGS site at
> You will note in the left hand column under "Seismic Hazard" a
listing for
> Hazard by Zip Code and another listing for Hazard by Lat/Lon.  Use
these to
> determine the seismicity of the site and question.  Then follow
along in the
> 1997 NEHRP to see what provisions apply.  
> Regards,
> Harold Sprague
> The Neenan Company
> harold.sprague(--nospam--at)
> -----Original Message-----
> From: Powers, Tony [mailto:tpowers(--nospam--at)]
> Sent: Tuesday, January 05, 1999 10:18 AM
> To: 'seaoc'
> Subject: Seismic Zones in Texas
> According to the Fig. 16-2 in the 1994 UBC (and I believe also the
> UBC), most of the state of Texas is in Seismic Zone 0.  I understand
> this is subject to change as the next code becomes more similar to
ATC 32
> (or we go to a national code), and that areas that are currently
Zone 0 may
> no longer be so in the future.  Does anyone out there know if/where
I can
> find some information on these changes?  A revised seismic
> coefficient map would be ideal.
> Thanks in advance.
> Tony Powers
> HDR Engineering, Inc.
> 916-351-3837
> tpowers(--nospam--at)

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