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[SEAOC] Re: [SEAOC] ridge vent in wood roof diaphragm

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Regarding question 1, in "post-frame" diaphragm design, we treat the two
roof halves as two independent diaphragms.  We assume that the chord forces
are carried by the roof purlins.   
Regarding question 2, I do not understand the purpose of the blocking per
UBC when no other code to my knowledge requires blocking between residential
roof-truss bearing points.  Some will probably say "transfer of chord
forces", but we have learned in post-frame that simple models for chord
forces are just that--simple.  They are not realistic.  If you research
chord forces in residential roof trusses, I suspect that you could establish
that the plywood sheathing is carrying the chord forces.

Frank Woeste, P. E., Ph. D.
03:52 PM 9/11/96 -0400, you wrote:
>On pitched (gabled) wood roofs using plywood sheathing over either 
>prefabricated wood trusses or stick-built rafter framing there quite 
>often is a continuous ridge vent, which requires a continuous opening 
>in the diaphragm a couple of inches wide along the ridgeline of the 
>1. Does anyone have advice on how forces in the diaphragm are handled at 
>this opening?
>In conjunction with the ridge vent (for air flow out), there often are 
>continuous soffit vents (for air flow in) to provide the code-required 
>ventilation of the attic space.  Blocking in the spaces between the 
>trusses for tranfer of diaphragm forces to the top plate unfortunately 
>blocks this air flow.
>2. Does anyone have advice on how these two conflicting requirements 
>(structural force transfer vs. architectural ventilation requirements) 
>can be handled? 
>Rick Burch
>Columbia, SC
Frank E. Woeste, Ph. D., P. E.
Biological Systems Engineering
Virginia Tech
Blacksburg, VA 24061-0303