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RE: Diaphragm Chord Wood Truss Roof

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If the chord HAS to be directly at the diaphragm, what about the 90% of roofs that have eaves, where the top chord of the truss runs past the top plate to form the eave.  The normal detail has roof boundary nailing into blocking which subsequently transfers to the top plates (aka – the diaphragm chord).  With the parallel chord truss, we have simply a little deeper blocking with mini shearwalls in each bay, rather than full depth solid blocking.  Is there really any difference?  When is the deep blocking too deep to consider using the top plate as the diaphragm chord?




From: robert.freeman(--nospam--at) [mailto:robert.freeman(--nospam--at)]
Sent: Friday, July 11, 2008 5:37 PM
To: seaint(--nospam--at)
Subject: Diaphragm Chord Wood Truss Roof


Hi Rich:


Difficult problem.


The chord needs to be at the diaphragm, and the diaphragm is supported by wood trusses. 


An option is to switch to top chord bearing for your wood roof trusses.  This would allow you to use your conventional chords at the top plate level.  You would be looking at forming the parapet on top of the roof plywood using separate bracing, rather than having the truss folks include the parapet as part of the truss. 


This option would be conventional, easy to review, and easy to bid, and easy to get a permit.


Another approach may work.  It's unconventional You can keep the bottom chord truss bearing.  You may want to consider using fiber wrap (FRP).  Using several FRP layers on top of the plywood along the  edges of the plywood diaphragm would add the tensile capacity you need.  It would not interfere with the wood trusses, and would not be a huge waterproofing problem.  You may have blocking  for edge nailing on the edge of the plywood between the trusses.



IDS Group Inc.

Bob Freeman, AIA, EIT

(949) 387-8500