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Re: Timber pitched roof as a Diaphragm[Subject Prev][Subject Next][Thread Prev][Thread Next]
- To: vicpeng(--nospam--at)vtcg.com (T)
- Subject: Re: Timber pitched roof as a Diaphragm
- From: NRoselund <NRoselund(--nospam--at)aol.com>
- Date: Mon, 20 Apr 1998 11:46:09 EDT
- Cc: seaoc(--nospam--at)seaoc.org
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"
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