Will,
Thanks for offering to take a look and clarify.
Essentially the entire page in the article is devoted to demonstrating the change in sheer forces on a tenon as a result of the reduction. (At the end of the section, Ed writes of the drop-in tenon, "this joint is not acceptable.") According to Ed's calculations, the sheer value ranges from 35 psi for a soffit tenon (entire 6" depth of joist housed in sill) to 182 psi for a drop-in dovetail (4" deep tenon after a 2" reduction at the joint - experiencing a six-fold increase in sheer forces as a result of dovetail and the 2" reduction). In the middle of the range is the tusk tenon (my preferred joint) with a 1.5" reduction coming in at 83 psi (still nearly 3 times the force calculated for the soffit tenon).
I've been aware of the need for tapering the reduction, but this is the first source that I've seen claiming an exponential rise in force at the reduction. Any help at understanding the mathematical basis for the calculations will be greatly appreciated!
