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[OM] Re: [OT] any structural engineers on the list

Subject: [OM] Re: [OT] any structural engineers on the list
From: Andrew Fildes <afildes@xxxxxxxxxxxxx>
Date: Fri, 28 Mar 2008 09:54:33 +1100
By width, do you mean depth? Rafters can be quite skinny so long as  
they are deep and braced.
But then, I'm feeling a bit thick this morning and I'm supporting far  
too much weight.

With a tenon joint, it's weight bearing properties depend on position  
of the tenon in the joint - the tenon does not have to be the full  
depth of the joint but the lower down  it is on the joint, the more  
of the depth of the rafter or cross-piece is available for bearing  
load. Any shoulder on the bottom of the joint tenon negates that  
amount of the depth of the beam. For instance, the second image here -
http://en.wikipedia.org/wiki/Mortise_and_tenon
  - 'stub-tenon' with no shoulder at the base is load bearing for  
furniture so that my ample rear does not meet the floor suddenly. The  
depth of the tenon reduces torsion but it is the lower part that  
takes the weight and a 'haunched tenon' holds the tenon in the  
mortice better, preventing sag. So does a short top shoulder.
Ancient technique required a over-length through tenon locked with a  
tapered pin through the stub of the tenon projecting through the back  
of the mortice.
Andrew Fildes
afildes@xxxxxxxxxxxxx



On 28/03/2008, at 9:29 AM, Jeff Keller wrote:

> Chuck explained common US construction. The only thing I can add is
> that the width of the rafter depends upon it's length. The width
> prevents excessive bowing when weight is applied to the center of the
> rafter. It probably doesn't take a very large tenon to support the
> weight  that typical toe-nailing will support.



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