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RE: Diaphragm Rigidity

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Homebuilders hate blocking and many will not even block walls in areas without enforcement.  I would buy your friend who ignored the plans a nice fruit basket.
1997 NEHRP Recommend Provisions    section  "...Diaphragms constructed of wood structural panels shall be considered rigid in light-frame structures using structural panles for lateral load resistance....".   The aspect ratio would have an impact on the stiffness of the diaphragm. 
For floor sheathing 1997 UBC Table 23-II-E-1 footnote 4 "Panel edges shall have approved tounge-and-groove joints or shall be supported with blocking....."

Scott M Haan P.E.
Plan Review Engineer
Building Safety Division
Development Services Department
Municipality of Anchorage
phone:907-343-8183  fax:907-249-7399

-----Original Message-----
From: Nels Roselund, SE [mailto:njineer(--nospam--at)]
Sent: Thursday, June 28, 2001 7:14 AM
To: seaint(--nospam--at)
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