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RE: Plywood rigid diaphragms

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How about when one side of the building is an open front with steel moment
resisting frames, and in the back of the building you have plywood shear
wall with stucco.  We all know that shear walls with stucco are much stiffer
than regular plywood shear walls.  But not for many cycles during the EQ.
After the design event bits the heck out of stucco, it behaves more like the
regular shear walls.

So if we disregard stucco, then the shear wall will be underestimated.  If
we consider the stiffness of stucco ( and how are we going to do that?),
then shear at the moment frame will be underestimated, since the rigidity of
the shear wall would be reduced significantly after the first several

The separate issue is how to calculate the deflection of the shear walls.
Mr. Dieckman suggested to distribute the shear based on the shear wall
capacity  times the length.  We now know that the slender shear walls
deflect differently then the wider ones.  Is rigidity of OSB shear wall the
same as of plywood s.w.?  I don't believe there is enough experimental data
to take all of these factors into account.

My question is this:  How far the solution using the rigid diaphragm theory
will be from the actual lateral force distribution.  May be before we all
start spending hours calculating eccentricities, torsional moments etc., we
need to look at the uncertainties that we deal with in the design, and ask
the industry to provide us with a little more experimental data.

I agree we Bill Cain's opinion that we need to make a distinction between
complex buildings with offsets, large eccentricities etc. and simple wood
structures.  We also need to educate our clients about the effect of the
architects design on engineering fee, and overall cost of construction.

Sasha Itsekson, PE
-----Original Message-----
From: John Buchanan [mailto:jjb(--nospam--at)]
Sent: Friday, August 28, 1998 8:54 AM
To: seaint(--nospam--at)
Subject: Re: Plywood rigid diaphragms


The rigid diaphragm suggestion was mentioned in two papers. One provided by
Kelly Cobeen and the other by Ed Diekmann. There was no design guide lines
provide only the mention that the diaphragms should be considered as rigid.

They are citing the 1997 code section 1630.6 and the 1994 code section

Again this was only mentioned as the method to be used and no real design
examples or other information regarding stiffness calculations were

Dolan agreed that the small diaphragms are rigid but there limited
information available on calculation of the relative stiffness of the shear

Does any one else have any comments?

john b