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Re: Report on Wood Diaphragm Issues

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It is a sad day when we allow attorneys and building officials to dictate
how we are to design structures. Anytime the code allows for a choice in the
engineering design, the building officials will always opt for the more
conservative method. Case in point is the wind exposure definition of case b
or case c which has resulted in Exposure "c" being required as the standard
throughout Southern California. But to argue for Exposure "b" is futile
(aerial topo maps,etc). Along with some jurisdictions requiring 90 mph wind
speed and Exposure "c" results in designing for a Class 2? hurricane. The
City of Big Bear Lake, which has a non-engineer building official, has
required the following criteria for lateral design for projects close to the
lake: 80mph Exposure "d" , 100psf snow load and no 75% reduction for snow
loads for seismic design ( dl + 100% snow).

So if we allow the Building Officials the choice between flexible or rigid
diaphragms, they will always opt for the more conservative method. To use
the UBC Standard 23-2 for the calculations of the diaphragm and shear wall
deflections along with their wide range of assumptions and parameters can
never accurately determine rigidity or flexibility.

To try and explain the 1997 UBC earthquake design chapter 16, along with
sections 2320.5, 2320.11.4, 2:1 aspect ratio for plywood walls to the
general public results in appearing to be stupid.

Somehow sound engineering judgement, common sense and  practical design
experience, have been replaced by code formulas and ambiguous requirements
that just don't work in the real world.

----- Original Message -----
From: Shafat Qazi <seaint-ad(--nospam--at)>
To: <seaint(--nospam--at)>
Sent: Monday, January 03, 2000 4:43 PM
Subject: Re: Report on Wood Diaphragm Issues

> Ken:
> This subject is very hot. It is my understanding the SEAOC is now taking a
> position to "Exempt" single family dwellings from Rigid diaphragm
> Tim McCormick is one of the few building officials that has impressed me.
> He is very knowledgeable on this subject. He is also open to new ideas. I
> am sure he will consider the current SEAOC position on this issue.
> Last but not the least, you may want to include rigid diaphragm analysis
> just to cover yourself from future law suits. What SEAOC says does not
> matter in the court. What matters is what the building code says.
> That is 1 cent worth.
> Shafat
> At 01/03/00 12:18 PM, you wrote:
> >Tim McCormick the Building Official at the City of Santa Monica wants a
> >Rigidity Analysis (1630.6) and a Story Drift Analysis (1630.10) for a one
> >story residence no matter what Seaoc or LA City says.  I used the
> >Design Base Shear Method. A lot of Mr. McCormick's  information comes
from a
> >SEAOC Seminar, Wood Diaphragms and Shear Wall Defections,.authored by
> >Douglas Thompson S.E. in which is stated "Past earthquakes have shown
> >lateral forces are distributed by relative rigidities."
> >
> >I would appreciate any help and how to handle this situation?
> >
> >Ken Wilkinson S.E.
> >
> >
> >----- Original Message -----
> >From: <RShreenan(--nospam--at)>
> >To: <seaint(--nospam--at)>
> >Sent: Saturday, December 11, 1999 1:05 PM
> >Subject: Report on Wood Diaphragm Issues
> >
> >
> > > I just received this e-mail.  It looks like Dennis's long, hard fought
> >battle
> > > is producing some encouraging results.  My hat is off to Dennis and
> > > who have pursued this issue and my thanks to Ron Hamburger and others
> >the
> > > Seismology Committee that responded.
> > >
> > > In this business it is hard to find time to get involved in complex
> > > when one doesen't fully understand them.  I find it necessary to
> >more
> > > seminars each year just to keep up with new  code changes materials
> > > methodology.  When business is good you have to make hay while the sun
> > > shining.------My excuses.
> > >
> > > I hope that this latest report on diaphragms will be accepted by plan
> >check
> > > agencies
> > > on the intermin until the prescribed code changes are made official.
> > >
> > > From one of the many silent observers,
> > > Ray Shreenan
> > >
> > >
> >
> >