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wood diaphragm

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 Dave (and listees):
I don't understand one thing (for now...). When wind load is perpendicular to the ridge of a rectangular roof, the shear is perpendicular to the ridge, and for most roof truss layouts the trusses are perpendicular to the ridge as well. The way I have understood it, the hole at the ridge vent does leave a gap in the diaphragm. However, how is this different then a panel edge of the sheathing, but with a much larger gap? Either way, the integrity of the diaphragm relies on the truss or rafter members to transfer the shear via the nails in the sheathing from one sheet of sheathing to the adjacent sheet, so on and so forth from the middle of the roof to the end of the roof. The only difference I see is if you require a BLOCKED diaphragm, then the ridge vent would present some issues. I see the shear parallel to the trusses chords (perp to the ridge) acting like a drag strut at the ridge opening.
 
Parallel to the ridge, in theory, the ridge has zero shear if it is in the center of the roof.
 
Now hipped roofs are a whole other issue.....
 
Andrew
 
 

There has been considerable discussion on this forum

about discontinuous roof diaphragms at ridges.

Suggest searching the archive.

Also, under lateral loads perpendicular to a ridge,

diaphragm shears at the ridge act perpendicular to the

truss top chords. The chords and ridge nail plate

*may* be able to carry the diaphragm shear across the

ridge joint via weak axis bending/shear, but I

wouldn't call that "drag strut" behavior.

d a v e e v a n s