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RE: Plywood Roof Diaphragms

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> -----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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