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Re: Seismic loads using ASCE 7

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We work in similar areas (we're based in Modesto) and have found the
same thing.  My understanding is that the UBC was far too conservative
in assigning that part of California to zone 3, and the new code
reflects more a more accurate assessment of seismic hazards in the
area.  We're following the new code and always assuming a RHO of 1.3,
even though on many of the light framed residential jobs we do, RHO is
more likely 1.   That bumps up the value a bit, and makes us feel more

We typically saw Wind loading govern for central valley jobs even
under the old code since many of the houses are in open areas where
Exposure C is required.  With the new code, we're finding that Wind
still governs (just by a bit more).  The wind loads are sometimes a
bit less than what they were under the UBC, though not by much.  So
our designs wind up being about the same as they were before.

Hope that helps.


On Fri, Jun 6, 2008 at 9:43 AM, Dave Adams <davea(--nospam--at)> wrote:
> Happy Friday, everyone!
> One of the things that we are sort of struggling with here (old "zone 3"
> country) is that we are finding seismic loads via ASCE 7 wind up being less
> (sometimes uncomfortably) than what we used to calculate using the 1997
> UBC.  As a result, we've sort of adopted our own internal "company minimum
> seismic coefficient" that we are comfortable with.  Are most people
> discovering the same thing and what are your thoughts?
> Thanks!
> Dave K. Adams, S.E., P.Eng.
> 979 N. Blackstone Street
> P.O. Box 1059
> Tulare, CA  93275
> Direct Line:  (559) 688-5263
> Direct FAX:  (559) 688-8388
> E-Mail: davea(--nospam--at)
> Web:

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