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RE: Plywood Roof Diaphragms

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I thought about playing Devil's advocate on this one, but would not do good
trying to support something I don't believe in. Blocking provides two
values - it prevents rotation of the joists or trusses until the sheathing
is securely nailed in place and provides a shear transfer equal to or
greater than the capacity of the boundary nailing of the sheathing.

The issue of venting the roof is dealt with pretty easy. I'm not sure where
I found the detail I use - it might even have been the Simpson Catalog, but
an "V" is cut out from the center of the block - not exceeding 1/2 the depth
of the solid blocking. This provides sufficient air transfer into the roof
cavity. The block is then free to transfer shear from the diaphragm to the
double top plate by either toe-nailing (where allowed) or by mechanical
connectors such as the Simpson A35f clip.

If the architect wants to jog the blocking so that it lies outside the plane
of the wall, this can be accommodated by using a Simpson A35 at the back of
the block to the top of the double plate or from the bottom of the block to
the face of the double plate.

Inasmuch as there are very few openings in the area above the double plate
and the roof, you can't ignore the capacity of stucco when used alone. For
trusses, you can do a number of things including a flat 2x block at the top
against the roof sheathing and at the bottom against the double plate. He
the face is sheathed in stucco. This might be best in lower risk areas. In
high risk areas, the two blocks can be gusseted and a hole entered through
the center of the gusset to allow for air flow.

Where trusses or joists are parallel to the wall below, these members can
replace blocking if designed as a drag element. We do this all the time with
Gable trusses designed to accommodate the shear and finished only in lath
and stucco.

In my opinion, anything less is unacceptable in an engineered design. The
purpose of full compliance is to provide the additional performance
protection which exceeds that provided by the conventional construction
methods. Why argue over the use of blocking when there is just as much a
means to provide venting as long as you are willing to be creative.


> -----Original Message-----
> From: Bill Polhemus [mailto:bill(--nospam--at)]
> Sent: Monday, May 21, 2001 3:21 PM
> To: seaint(--nospam--at)
> Subject: RE: Plywood Roof Diaphragms
> > -----Original Message-----
> > From: CDaniels [mailto:ced(--nospam--at)]
> > Sent: Monday, May 21, 2001 4:56 PM
> > To: 'seaint(--nospam--at)'
> > Subject: Plywood Roof Diaphragms
> >
> >
> > * How do you transfer diaphragm forces from the roof sheathing
> to the wall
> > plates without blocking between the trusses or rafters?  The
> > blocking is the
> > first thing that the architect wants to eliminate because of
> roof venting
> > and here in the East no drawings I've seen ever shows blocking at this
> > location.  If you use hurricane ties at the trusses it is possible to
> > develop enough strength to justify the design but if you have a
> > truss with a
> > raised heel is roll over of the trusses a problem?
> Note that the manufacturers of "hurricane ties" (i.e. Simpson) states that
> you may NOT use only the ties in lieu of blocking.
> > * On a roof with pre-engineered trusses, how is an interior shear wall
> > handled?  Can the bottom chord of the truss be used as a drag strut or
> > collector if the shear wall is only a fraction of the building width and
> > does not extend to the underside of the roof deck?  This would
> assume that
> > the pre-engineered truss(es) above the wall would have to behave
> > as a rigid
> > element.
> Perhaps I misunderstand your question, but if you let the truss
> manufacturer
> know that there will be drag-strut forces in the lower chord of a given
> truss, and what magnitude, they can do their design on that basis.
> William L. Polhemus, Jr., P.E.
> Polhemus Engineering Company
> Katy, Texas
> Phone 281-492-2251
> Fax 281-492-8203

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