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Re: Design of Wood Top Plates

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There are several "factors" that may help for flatwise horizontal shear.

The listed values typically have been for stouter solid-sawn members having
one or more flatwise splits parallel to grain at the end of the member where
the shear is highest, and where no concentrated vertical load bears nearby
that has the effect of pinching the split surface (or unsplit member)
strongly in crossgrain compression.

Your flatwise 2x6's are unlikely to have splits separating the 2-inch
direction, and you do have nearby concentrated loads and few nearby ends.

See what you can find in current NDS and commentary. Provisions come and go
like guests at an office Christmas party, and some of what I remember may
have gone home.

On the other hand, the lack of control of framing layout and unsympathetic
Architect you allude to suggests being wary of the whole business. Also,
wood stress allowables are statistically based from data scatter, not exact
and certain.  

Charles O. Greenlaw, SE   Sacramento CA

At 11:38 AM 11/10/98 -0800, you wrote:
>What is the common practice for the design of top plates for a load 
>bearing wood stud wall (non-hurricane or seismic area).  The NDS has a 
>Flat Use factor for 2 x 6's used on their weak axis.  The problem is the 
>factor only applies to the Allowable Bending stresses (Fb) and not to the 
>Allowable Shaer Stresses (Fv).  The problem is I have trusses with a 2.8k 
>reaction on a 2 x 6 @ 16 " o.c. stud wall.  A double top plate will not 
>calc. out (2 x 6 SPF Stud Grade Fv=70 psi per NDS 1991)  Based on the 
>calcs, I would need approximately 5 plates (or if No 2 Southern Yellow 
>Pine was used 4 plates).  The architect would not accept this (over 
>engineered)  and the chances of having the workmen locating a stud 
>directly under the truss is not good.  Are there any factors I am 
>missing?  I understand that the shear check for wood is for a horizontal 
>shear and wood think a flat member would resist it better.  Anyone have 
>any suggestions?