2x6 T&G Roof Decking - Diaphragm Capacity? 1

[marinaman]

I've got a client who wants me to provide the structural design for a park picnic shelter. Its framed of heavy timber columns supporting heavy timber beams and heavy timber wood trusses.

He's showing the whole thing sheathed in 2x6 T&G roof decking. The decking is horizontally oriented atop the truss top chords, not diagonal oriented.

The roof pitch is 9:12.

I know I read somewhere that the roof decking can not be counted on for diaphragm capacity, but for the life of me, I can't recall where I read that. I've been in my AITC book, the AWC's output on tongue and groove roof decking, as well as in the IBC, but I can't find it.

Am I crazy?

As I recall, T&G decking of this type, in this orientation, has to be sheathed in plywood in order to get diaphragm capacity from it.

Can someone guide me as to where this data is located?

 

[EngineeringEric]

I thought you could use it. I believe that if the decking is installed at an angle to the joists as opposed to perpendicular there are differences (capacity at an angle is larger, acting more like T/C members as opposed to shear). You may need to confirm the T&G part but I think that shouldn't hurt anything (?)

Nominal Capacity
Horizontal Wind = 140 plf
Diagonal Wind = 840 plf

I believe the values are in the AWC (american wood council) special design provision for Wind and Seismic. AF&PA SDPWS-year. in 2008 it is table 4.3D and there are some important notes in there.

 

[JAE]

We've never found an approved source/testing for T&G decking and always add a layer of plywood on top to deal with diaphragm forces.



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[marinaman]

Thanks JAE

 

[msquared48]

I do the same as JAE, unless the roof diaphragm it is diagonally sheathed.

I believe the older Douglas-Fir Use Book editions have values for these diaphragms.

Mike McCann, PE, SE (WA)