As I stated, with low shears the continuous ridge vent can be detailed
effectively. With proper design and detailing the second discontinuity you
refer to is eliminated and there is no difference. This is for an
engineered design, I don't do prescriptive.
Typical air space requirements are only for about an inch over the
insulation, not the full joist depth. With a full depth ridge beam and
hangers you can chamfer the top corners of the ridge beam to provide air
space flow. Provide blocking between the joists below the airspace to
provide lateral restraint at the hangers if required. The ridge provides
continuity from one side to the other. For the other direction provide
straps across the ridge at required intervals, just as would normally be
done. How is this different from an un-blocked panel joint?
There is a discrete point of contact at each joist, same as an unblocked
diaphragm. There is a positive shear transfer between the discrete points
just like an unblocked diaphragm.
In the traditional case the unblocked joint is tied by the perpendicular
joist below, in the case above continuity of the "joist" is provided by the
transfer through the hanger, blocking and ridge. Is this not essentially
the same thing?
I can expand on a couple other ways to achieve the same result, but let's
use this one for now. A typical roof has the ridge in the center or nearly
so (i.e. midspan). The diaphragm shear in the parallel direction is minimum
at the mid-span as you said. For the perpendicular direction the tie
continuity is no different from any of the standard construction details,
continuity across the joint is provided by the joists which are strapped
across the ridge the same as if there was not a ridge vent.
----- Original Message -----
From: John Rose <jrose36(--nospam--at)earthlink.net>
Sent: Friday, June 16, 2000 8:16 PM
Subject: Re: wood diaphragm edge / roof venting
> I don't agree that a pitched roof diaphragm with a ridge is no different
> unblocked diaphragm. There are *two* discontinuities at the ridge; one for
> sheathing edge joint, and also the framing (rafter or truss chords) are
> at this location. There is shear force parallel to the ridge which is
> at the ends of the diaphragm, and linearly reduce to the midspan of the
> diaphragm. There may need to be a formed sheet metal shear strip stapled
> sheathing on each side of the ridge, when the shear force exceeds about
> lb/ft. (engineered design, not prescriptive conventional construction).
> For continuous ridge vent applications, suggest doing the same thing at
> ridge as suggested at the eaves. Namely, sheathe and apply strips (or
> blocking at ridge) in every other framing bay, to develop required shear
> which would be double the regular force if the full length of
> was used. Then the ventilated areas can be cut into the sheathing at the
> where there is no diaphragm shear transfer at the ridge.
> John Rose/APA
> Paul Feather wrote:
> > Paul Franceschi wrote:
> > A continuous ridge vent is not an option because it would cause a
> > diaphragm
> > discontinuity.
> > This is only the case for a blocked diaphragm. The joint at the ridge
> > different from the joint along all the other panel edges for an
> > diaphragm. For residential construction where the diaphragm shears are
> > continuous ridge vents can be detailed effectively.
> > Paul Feather