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Re: Timber pitched roof as a Diaphragm

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Thor Tandy,

I apologise for the long delay in responding to your post about pitched
diaphragms (it was posed January, '98).

Pitched diaphragms act in their plane, so that the planes of a gable roof are
two separate diaphragms.  As the roof responds to horizontal forces, the plan-
view defromation appears to be like that of an ordinary diaphragm.  However,
the plane that rises in the direction of deformation will have an upward
component of deformation, while the plane that slopes downward in the
direction of deformation will have a downward component of deformation.  The
result is that the the deformations of the two planes at the ridge tend to be
incompatible.  This incompatibility must be overcome by vertical shear
capacity at the ridge that restrains the two planes from separating.  This
vertically-acting restraint then forces the two planes to warp as it prevents
separation at the ridge.  The "traditional" strap-tie bent across the ridge
that connects opposing rafters together is ineffective for this kind of
restraint.

Another interesting effect of the vertical component of deformation of a
pitched diaphragm is that the diaphragm tends to lift the top of the wall off
by lifting the anchors; the anchors, then need to have vertical load carring
capacity as well as horizontal.  In an unreinforced masonry retrofit, this
kind of action may cause a vertical separation that allows the portion of the
wall under the lower ends of the anchors to displace without restraint.  I've
seen this kind of earthquake damage: its a horizontal offset of the wall near
the top, along a horizontal plane in the wall just under the lower ends of
anchor bolts.  The mitigation for this effect in a URM building retrofit is to
be sure that the anchors extend deeply enough into the wall to engage a mass
of wall at least equal to the vertical component force.  To be realistic, the
calcualtion of required mass needs to be based on strength analysis, not
working stress.

With the above considerations taken into account, I believe that (except for
practical design considerations) there is no limit to the pitch of a roof that
may be used as a diaphragm.

Nels Roselund
Structural Engineer
"Only works on buildings that are older than him"