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Re: Valley Rafters

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From:  Jan I. Blok <jblok(--nospam--at)thestructuralgroup.com

To:  seaint(--nospam--at)seaint.org

Subject:  Re: Valley Rafters

 

In a simple beam analysis of a typically sized valley rafter, the member almost never works.  I suspect the actual action is a V shaped beam with the valley rafter as the bottom chord of two intersecting plywood diaphragms.  The top chord action for this analysis is more complicated as the two top chords are obviously not parallel to the bottom chord.  If this is correct, the capacities would be orders of magnitude higher. Of course, then there are all the nailing requirements to be sure that the diaphragm action of the V beam is achieved.

 

Has any one out there run this analysis?

 

Jan I. Blok, S.E.
The Structural Group, Ltd.

Deerfield, Illinois

In a message dated 2/7/09 7:38:39 PM, ENGRLAINES(--nospam--at)aol.com writes:

 

An architect client asked me to answer a building official's request to verify the strength of a valley rafter (or beam) in a wood roof.  The architect specified a valley rafter 2" deeper than the rafters (as usual), but this time he is being challenged to prove it. I analyzed it as a simple beam taking half the tributary load from the jack rafters framing into it and it is in fact way off! The architect maintains that he and everyone he knows have always spec'd 2" deeper for decades with no failures or even challenges. He even showed me text books and other reference books with this recommendation. Am I missing something?

Charles O.  Laines, S.E.

LAINES ASSOCIATES, LONG BEACH, CA