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Re: 1997 UBC EQ Requirements

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Just a little bit of Geotechnical insight, before you make too many gaffs with your
statements about Northridge, San Fernando Valley, So Cal in general.  The UBC 
seismic book is only a starter tool, per se.  I don't know how many times I've
told civils 
and structurals that seismic hazard evaluation (yes, including proper
selection and 
application of the 97 UBC chapter 16 seismic coefficients) is not a cookbook procedure.
An engineering geologic/geotechncial evaluation is recommended in all areas of
high seismic activity/past history and documented seismic hazards, such as
potential or seismically induced landslide potential.

I just drilled a site in Canoga Park about 3 months ago for a 4 story
apartment "bee-hive" 
structure with semi-subterranean parking.  We gave them coefficients based on
downhole seismic shear wave velocity and SPT blowcounts that were extremely
Additionally, Division of Mines & Geology's SP 117 was used to "accurately"
place the surface expression of the '94 blind thrust and determine the site
was only 4 kilometers from ground 
zero.  Historic groundwater records were researched on the web, and the client
got highly
site-specific 97 UBC coefficients, not to mention a tri-partite spectral
response graph that the strucutral knows goes far beyond the "cookbook" parameters.

If you're truly concerned about the 49% increase that the cookbook method
yields in your loading calcs, do your clients a favor and advocate an
experienced full service geotechnical consultant go the way I outlined above
(sorry folks, it's the future after the Northridge nightmare and it's now in
effect for us to live with).  Savvy structurals understand that a good geotech
can save their butts and make them look good in the process.  Start developing
those symbiotic relationships with a good experienced geotech, and in the
process realize . . . . . not all geotechs are equal (just as all structurals
are not equal).

Roy Kroll, CEG / Associate
Youngdahl & Associates, Inc.

Parkerres(--nospam--at) wrote:
> To all:
> As you all know, the local Building Dept's here in Southern California (LA,
> etc.) are enforcing the 1997 UBC starting tomorrow (7/1).  We are a firm that
> does primarily Type V (wood) structures with OMRF steel frames.  I have been
> trying to get some feel from the local Cities as to what guidelines to
> follow, but they do not seem as well informed as one might like.
> Currently, we are going to go with the following approach.  I would
> appreciate the comments of others before we head too far down the path.
> 1.  Use "Simplified Static Design" per 1630.2.3 so that we can ignore all the
> fancy deflection checks, etc., required for regular "Static Design".  This
> gives an equation of V = (3.0 Ca / R) / 1.4 W.
> 2.  Assume Ca = 0.44 Na for worst case soil, Sd, without a soils report in
> seismic zone 4.
> 3.  Assume Na = 1.3 for ALL of the LA area which assumes <2km from a Seismic
> Source Type B fault.  We bought the special seismic zone map book, but it is
> hard for us to justify that some areas of LA need the 1.3 factor, but areas
> across the street can use Na = 1.0 since they are more than 2 km from a
> "known fault".  Ironically, Northridge is squarely in one of the 1.0 zones.
> 4.  Use R = 4.5 for light framed buildings or OMRF Steel Frames.
> 5.  This yields: V = 3.0 * 0.44 * 1.3 / 4.5 / 1.4 = 0.272 W
>      The 1994 UBC gave V = 0.183 W for similar worst case scenarios.  This is
> a
>      49% increase in my loads!
> Comments? What is everyone else doing?
> Thanks for your input.
> Bruce Resnick, SE
> Parker Resnick Str. Eng.