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# Re: wood diaphragm

• To: <seaint(--nospam--at)seaint.org>
• Subject: Re: wood diaphragm
• From: "Rand Holtham, P.E." <rand(--nospam--at)sigmaengineers.com>
• Date: Fri, 7 May 2004 08:35:43 -0500

```Andrew,

At panel joints the continuous framing acts as a shear splice transferring
load from one panel to another. At the ridge in stick framed roofs the
framing is not continuous at the ridge so shear transfer is poor at best.
Wood trusses are better connected at the ridge so your assumption is correct
for wood trusses.

Rand
----- Original Message -----
From: "Andrew Kester" <akester(--nospam--at)bbma.com>
To: <seaint(--nospam--at)seaint.org>
Sent: Friday, May 07, 2004 7:15 AM
Subject: wood diaphragm

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

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