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Re: Timber pitched roof as a Diaphragm

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I repaired a lot of church roofs pitched after the earthquake.(newcastle)
those that performed best had tie rods between the walls.  They can look a
decorative feature.  I was amazed at the difference in damage levels.

John Nichols



At 11:46 20/04/98 -0400, you wrote:
>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"
>
>
>
>