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Re: Roof to top plate connection?

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Paul,
 
The unblocked diaphragm relies on the plywood's own resistance to buckling, and does not necessarily require blocking at the boundaries. So, as far as the plywood is concerned, the shear is transferred to the 2x12 joists.  
 
Now, the shear on the nails (one nail per joist) is transformed into the cross-grain bending of the joists.  The cross-grain bending apparently occurs only within the top 1-1/2" of the joist (air gap), i.e., within the effective nail length. 
 
Such condition is undesirable, even with "minimal" forces, mainly, due to the unreliability of wood in cross-bending resistance.  The unavoidable cracks (shakes) in the joists will diminish such resistance to a practical zero.  Although I cannot find the specific wording of the code not allowing the cross-section bending except for masonry/concrete cases (UBC 1633.2.9.5), I would try to avoid the detail you described.   To me, the unblocked diaphragm would be the one not nailed to the full-depth blocking still installed tightly between the joists. 
 
In the absence of the full-depth blocking, the hurricane ties obviously cannot and will not help to prevent the cross bending. 
 
I personally would not accept the described condition, and would force the contractor to fix it to my satisfaction.  At the same time, it is hard to stand your ground when you work for somebody else - even if you are right.  You have to decide if the problem is bad enough to potentially risk your job.  After all, in the absence of a specific prohibition by the code, it is the right of somebody who stamps the plans to take responsibility (if they want it) for the contractor's mistake.  If you are to stamp the plans, your opinion should not be overridden that easily.  
 
By the way, I do not quite understand how A35s can be attached to the underside of plywood - unless the screws are driven into the sillplate of the wall above.
 
HTH
 
Steve Gordin, SE
 
 
-----Original Message-----
From: Paul Skidmore [mailto:paul(--nospam--at)psarchitecture.com]
Sent: Friday, April 08, 2005 11:20 PM
To: seaint(--nospam--at)seaint.org
Subject: Roof to top plate connection?

 

Hello All--

 

Using the allowable plywood shear capacities for horizontal diaphragms from IBC 2003. The drawings called for full depth blocking at the interface between roof sheathing and top plate. In this case, the contractor left an 1-1/2 air gap between the underside of the sheathing and the blocking. The joists themselves are 2x12. Of course required edge nailing was not achieved. The lateral loads are quite minimal--unblocked diaphragm is more than acceptable.

 

I have always called for full depth blocking in this condition, but now I'm getting some conflicting information, i.e. that since the testing was for an unblocked diaphragm, this condition will still develop the full lateral capacity noted in the IBC table--as long as an alternate method of transfering the lateral load into the top plate is provided. In this case, the "hurricane ties" supply adequate lateral capacity.

 

 

This situation causes me some concern, but I'm being overruled by higher-ups. But then, I wonder if I'm too reliant on academic sources, and not being flexible enough in my thinking.  In the past I've always required additional blocking to "fill the gap", using A34's or A35's  with screws into the underside of the sheathing as necessary to maintain a continuous load path. I'm wondering what the list thinks?

 

Thanks.

 

Paul Skidmore