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RE: Valley Rafters

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Thor,

 

The design of a valley has more to do with the connections than with design of the member itself.  And it has to do with the roof structure as a system. The components of the system include the rafters, the ridges, valleys and hips, and the continuous double top plate.

 

In a rational and properly designed roof system, only the rafters need to be designed for flexure.  If the rafters are tied in each direction, their axial components of force resolve into an axial force in the valley or hip, so that neither the valley nor the hip acts as a beam.  However, as you follow the axial forces through the system, the forces at the connections can become pretty large, and even unmanageable.  And it can become impractical to tie rafters in each direction.  If you can’t follow the axial forces through the system and resolve them all with designed connections, you should not hesitate to design ridges, valleys and/or hips as beams.

 

The conventional light-frame construction rules for framing can result in roof systems that cannot be rationalized.  Nevertheless, they seem to work O.K..  If they are not magical, perhaps, as Barry suggested, it has something to do with the buttressing or diaphragm action of the sheathing.  I have not figured out how to take advantage of the action of the sheathing in a gravity-load design of a roof-framing system.  

 

Nels Roselund, SE

South San Gabriel, CA

njineer(--nospam--at)att.net


From: Avicpeng [mailto:vicpeng(--nospam--at)telus.net]
Sent: Friday, September 09, 2005 3:04 PM
To: SEAINT
Subject: Valley Rafters

 

And just to start ... does anyone design the valley rafter of a roof, or do you write it off as "standard framing"?  If not, why not?  If so, why?


Thor Tandy P.Eng MIStructE
Victoria, BC
Canada
vicpeng(--nospam--at)telus.net