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Re: steep pitcheed roof diaphragms

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Nels,
Ask APA's Help Desk (help(--nospam--at)apawood.org or 253-620-7400) for a copy of the APA
Research Report on folded plate roofs (diaphragms).  It explains how diaphragm
forces are analyzed for a pitched roof diaphragm where there are edge members at
the eave and ridge. Of course, if the roof is of conventional construction where
there are no "edge members" at the ridge, then blocking between roof framing
members may need to be added at the ridge in high shear areas to transfer
diaphragm shear along one edge of the diaphragm to the other edge, along the
ridge. There also may need to be tension ties needed between the eaves to
prevent the diaphragm plates from unfolding. This is also explained in the
report on folded plates.

John Rose
APA (retired)
Tacoma, WA

"Nels Roselund, SE" wrote:

> Paul,
>
> The force that a diaphragm applies to a wall as it restrains the wall  acts
> in the plane of the diaphragm.  Thus, a steeply sloped diaphragm apples a
> force with a vertical component to the wall that is anchored to it -- this
> needs to be taken into account in the connections.  Its deformation is also
> in its own plane.  At the ridge, the horizontal shear component of the force
> can be transferred from one diaphragm plane to the other, but the two planes
> tend to displace vertically in opposite directions -- each displaces in its
> own plane.  Connections are needed at the ridge to handle the vertical
> component.
>
> I have played around a little in trying to figure out how the diaphragm
> forces are distributed through a hip roof, but still don't have a very good
> understanding.  Maybe when Thor Matteson finishes with his book all about
> plywood shear walls, maybe he'll write a book all about plywood diaphragms
> that will explain sloped diaphragms.
>
> Nels Roselund
> Structural Engineer
> South San Gabriel, CA
> njineer(--nospam--at)att.net
>
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