A picture IS worth a thousand words! [img]http://pxrox.tumblr.com/[/img]

From top to bottom, pics of the strut mortise, the "dovetail" tenon and its mortised support block (assembled & disassembled), and my home "test" of shear Fv (described in the pics). I get an "Fv at failure" of ~500 psi, and my test specimen held at an experimental Fv of 200 psi, so 200 psi (tested rather amateurishly on the actual wood!) seems safe.

Roger, we're on the same page RE: relish. In the picture of the dovetail, the shear plane I'm referring to is horizontal, from the base of the triangular face (away from the camera; this is where the stress-relieving curved cut needs to go) back to the end of the beam (outlined in red) --- about 1.25 inches across the breadth of the beam by 2.5 inches along its length (the block is 2.5" thick). ~3.125 sq. in * 200 psi = ~600 lbf. Not quite what I need (~1200 lbf), but as I mentioned before, the decking should take quite a bit of the tension load (please let me know if you think not!).

For the dovetail, I can't multiply by 2, as you said, due to the unusual geometry.

For the strut mortise/tenon, I can multiply by 2. But, why are there considered to be only two shear planes in a mortise? I can see why a peg has two, but doesn't a mortise have *three* defining shear planes---both sides and the bottom?

David