On 19-Jun-2006, "todd" wrote:

My credentials: degree in mechanical engineering with a specialization in
strength of materials.

Civil eng (Structures) with one old prof that used to toss the textbook and
point out the basics occasionally.

If it was an isotropic material, I might agree, but it's wood and the grain will
dominate the behavior. Wood can exhibit significantly different strengths
depending on orientation relative to grain.

How that joint will behave will depend on factors beyond just the wood or the
orientation of the tenon. If the joint is glued with a seriously stiff and strong
adhesive that prevents any motion in the joint, either will be strong. If the
adhesive can flex, the shape and orientation of the tenon will affect the overall
strength (given the grain). Since I've seen more joints fail due where no
strong adhesive is used or the adhesive lets go, I'd tend to design ignoring the
adhesive and then B is stronger. I wouldn't go with a tenon crossing grain
lines significantly. If the tenon in the drawing is to scale, then the rail is
thin on the periphery - Use B.

If the wood selected has crap grain, either A or B would be equally weak.

Mike