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Re: Diaphragm Chord Wood Roof

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You can block between each bottom bearing truss with studs on either side and sheathing to get your shear transfer. For each truss end, vertical stud on either side, horizontal blocking from bottom chord to bottom chord internailed to the the DBL top plate, and a horizontal block from top chord to top chord where the diaphragm nailing occurs.

Then put a coil strap on the outside of the sheathing to connect the blocking at the top chord together to create the chord. I would suggest using 3x for the blocking so you don't get any shiners.

Probably a skinny coil strap would do the trick.

Top chord bearing trusses are much better because of this very issue. Really no different than 2x rafters for the seismic load path.


On Fri, Jul 11, 2008 at 5:24 PM, Tarek Mokhtar <Tarekmokhtar(--nospam--at)> wrote:

Would a coil strap over plywood over blocking work?

Tarek Mokhtar, SE

I have a design and detail question.  I'm working on a single story wood framed building with truss rafter roof.  The lateral stability for wind is provided by wood sheathed shear walls.  The trusses are parallel chord with bottom chord bearing.  There is also a tall parapet that is part of the truss.  At the face of wall the vertical member of the truss extends up to form the parapet.

On previous typical truss rafter roofs using gable type trusses I design the top plate of the wall to be the chords of the diaphragm. I put in blocking between the trusses and transfer the shear down into the top plate of the wall.

On this project I'm wondering what is the best way to detail this so that the force comes out of the diaphragm and into the top plate of the wall, if that is possible.  If this is not practical, where else can I put the chord?  The end of the truss is 32" deep. The plywood sheathing will extend up to the top of parapet.   I thought about trying to run a ribbon at the sheathing level, but since I have this extension up for the parapet that does not seem feasible.

Anyone come across this problem before and have some creative solutions for designing and detailing the chords of the diaphragm?

Thanks for your help.



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