Return to index: [Subject] [Thread] [Date] [Author]

RE: Roof to top plate connection?

[Subject Prev][Subject Next][Thread Prev][Thread Next]

I agree with other responses in that you must establish boundary nailing to something, whether it’s blocking over the shearwall, or a structural 2x fascia (if there’s an eave).  The hard part if you use the fascia for boundary nailing is that you must be able to demonstrate a shear transfer path from diaphragm to shearwall, which may or may not be feasible based on the shear forces, overhang, members, connectors, etc.   As Bill Allen mentioned, “unblocked” has nothing to do with the transfer from diaphragm to wall.

 

So most structural engineers never bother with calcs that clients won’t want to pay for and simply specify the full-depth blocking (with vent notches or holes).   

 

Here in Oregon, the State Building Codes division issued a residential “interpretive ruling” a few years ago that allows blocking to be installed anywhere within “d” (joist depth) of the shearwall.  That is, the blocking receives the diaphragm boundary nailing, but doesn’t transfer it directly to the shearwall – the blocking is only there to resist joist rolling and “cross-grain bending”.  Does that work?  Well, for a lightly loaded diaphragm, if everything is nailed presciptively, you have a diaphragm to blocking to joist-in-flatwise-bending to ceiling joist to toenail to wall plate.  OK for light loads.

 

If you’re not calculating the load path all the way through, then you’re using “engineering judgement”.  And I think that conservatism is rightly due in that case.

 

Ed Tornberg

Tornberg Consulting, LLC

503-551-4165

-----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