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Re: 1997 UBC EQ Requirements

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Are OMRF'S allowed in seismic zone 4 in the 97 UBC?
I thought they were only allowed in zones 1 and 2. Only SMRF'S were allowed in 3 and 4.

From: Parkerres(--nospam--at)
Reply-To: seaint(--nospam--at)
To: seaint(--nospam--at)
Subject: 1997 UBC EQ Requirements
Date: Thu, 1 Jul 1999 01:53:54 EDT

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
     49% increase in my loads!

Comments? What is everyone else doing?

Thanks for your input.

Bruce Resnick, SE
Parker Resnick Str. Eng.

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