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RE: Diaphragm Rigidity

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Nels,
I didn't read the replies, but I would say emphatically yes - an unblocked diaphragm will usually calculate as rigid based on analyzing it for a rigid diaphragm and then applying the factor that APA recommended to convert blocked diaphagms to unblocked (I think, without looking, that it was 2.5).
There is no present analysis for unblocked diaphragms, but after applying the factor, the diaphragm deflection can be evaluated as rigid or flexible. I would guess in almost all cases other than high aspect ratio's (approaching 3:1 or 4:1).
I also don't consider T&G as a replacement for blocking any more than I would panel clips applied in lieu of blocking.
 
Dennis
 
-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)att.net]
Sent: Thursday, June 28, 2001 8:14 AM
To: seaint(--nospam--at)seaint.org
Subject: Diaphragm Rigidity

I a 3 story residential addition project that is under construction to have rigid diaphragms because the available shear walls would not work if they were bracing the diaphragms based on tributary load -- I needed a rigid diaphragm to distribute loads, past the relatively weak walls to the available strong shear walls.
 
I plywood panels of the diaphragms are specified to be blocked all edges.  My intention in requiring blocking was to assure that the diaphragms really are rigid, not flexible.  At the architect's request, the plywood was specified on the drawings to be tongue-in-groove.
 
Yesterday, I was called to observe the diaphragm nailing.  The contractor has installed no blocking.  He said that he has never constructed a residence with a blocked diaphragm, and thought that the tongue-in-groove panel edges took the place of blocking.  He didn't ask me about that.  Roofing installation, scheduled for today, has been called off because I want the blocking installed -- everyone involved is upset.
 
There is no place in the diaphragm at which the shear exceeds the allowable for an unblocked diaphragm.  The cost to install the blocking as the framing was being erected would have been nominal; the cost to install it now, with plywood, ceiling and soffit framing, and ductwork in place is going to be a big deal.  Is there any justification for considering an unblocked diaphragm rigid?  Is it possible to calculate the rigidity or displacement of an unblocked diaphragm? 
 
Nels Roselund
Structural Engineer
South San Gabriel, CA
njineer(--nospam--at)att.net