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Re: 1997 UBC EQ Requirements

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Roy-
That is all well and good, but it is my understanding (I am sure someone
will correct me if I am wrong) that even if you get a site specific
response spectra that differs greatly from the 97 UBC spectra, you are
still obliged to use AS A MINIMUM 80% of the 97 UBC spectra.  

So no matter how sophisticated and exact an analysis you do, the  best
you can hope for is a 20% reduction in your design spectra.

Based on this, it may not be so advantageous to do a site specific
spectra.

Lynn





Roy Kroll wrote:
> 
> 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
> known
> high seismic activity/past history and documented seismic hazards, such as
> liquefaction
> 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
> obtained
> downhole seismic shear wave velocity and SPT blowcounts that were extremely
> low.
> 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.
> roykroll(--nospam--at)velocityhsi.com
> 
> Parkerres(--nospam--at)aol.com 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.
> >
>