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RE: Roof Sheathing at Ridges

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Could it be possible that the two halves of the roof are working as 3 sided diaphragms and no shear transfer is really needed at the ridge? Wouldn't the nail pattern be the same as if it was designed as one continuous diaphragm?
 
Jim K. 
-----Original Message-----
From: vicpeng [mailto:vicpeng(--nospam--at)telus.net]
Sent: Tuesday, August 06, 2002 12:15 PM
To: seaint(--nospam--at)seaint.org
Subject: Re: Roof Sheathing at Ridges

1)    The archives have a lengthy discourse on this theme: about 2 yrs ago?
2)    I suggest that there will be shear forces when the roof is "forced" to carry  loads due to unbalanced (ie rigidity, length, location, torsion, etc) shear collection lines.  As I described in the previous thread, take a piece of paper, fold it and then apply counteracting forces along each edge.  The "ridge" will deform.  I.e. there will be shear forces unless it is a discontinuity.  My guess is that with current accepted practices, the redundancies in (residential) framing carry the loads elsewhere (maybe to the eaves and down the walls).  I have had instances where the rationale has needed to use the roof as a "diaphragm", and hence my 4x_ or larger detail.  I haven't gone any further with respect to any deformations at the boundaries (wall panels).  If the roof does act like a diaphragm, there will be deformations at the wall/support lines ...

Thor A Tandy  P.Eng, MIPENZ
Victoria BC
Canada
e-mail: vicpeng(--nospam--at)telus.net
 
----- Original Message -----
Sent: Tuesday, August 06, 2002 10:11 AM
Subject: Re: Roof Sheathing at Ridges

I guess I am missing something in this discussion about why the ridge would be critical in a roof diaphragm.  I am assuming the eaves on both side are where the forces eventually go to.  It would seem the ridge of a gable roof would not need to transfer shear unless all the load was being transfered to one side only.  

This would be similar to Roof X-bracing in a metal building where the loads are assumed to be near zero at the center of the roof and accumulate in the rods as you proceed to the eaves.  The rods in the center are generally smaller than the ones near the eaves since the rod load increases as you near the eave.  The only time this is not true is when we are using what is called "one-sided" bracing where one eave is assumed zero because there is not any wall bracing present and then all load must transfer to the other wall.  In these cases, the center rods do not have a lower force than the unbraced eave.

Ron Martin
Tuscaloosa, AL