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RE: Valley Rafters

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Nels,
I think that what yoy say is true if there is a certain degree of 
symmetry, i.e. loads on both side are approx equal.  However without 
that, the valley must be designed taking into account the unequal 
loadings and support.
Gary


On 10 Sep 2005 at 11:02, Nels Roselund wrote:

> 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
> 
> 



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