I'm building a small treehouse (really, a tree platform) out of locust. Platform has eight beams, each extending 5 feet radially from tree. Each beam is supported by a strut; see attached figure: [img]http://pxrox.tumblr.com/[/img]

Beam & strut each fit into blocks that are lag-bolted to the tree. Beams have a dovetail mortise/tenon (no peg) to hold them into blocks, and will be tied together horizontally with screwed-in 2x6 flooring that goes all around the tree but extends only 4 feet out (last foot of beam will stick out, for "looks"). Design load (dead + live + snow) on each beam/strut is ~1,200 lbf.

Strut fits into the beam 3 feet out from tree. Angled at 45 degrees, the strut will exert ~1,000 lbf horizontally (outward) and ~1,000 lbf vertically (upward) on the beam (strut thus takes most of the weight load; resulting moments are neutralized by the blocks above & below the beam).

Strut has an oblique tenon (no peg; see figure, in red) to fit into the beam mortise. The mortise bearing shoulder is angled at 22.5 degrees, to take the toe of the oblique tenon. The mortise has ~1 x 5 inches of shear plane to resist the horizontal force. Locust has a max. shear strength parallel to the grain of ~2,500 psi, giving (in theory) the ability to resist ~12,500 lbf of shear. Seems like enough.

But, I am concerned about possible mortise shear-out, in part because a book on timber engineering (Construction en bois: Matériau, technologie et dimensionnement, by Julius Natterer, Jean Luc Sandoz, Martial Rey) suggested 30 (!) shear-resisting nails in the tie of an analogous rafter/tie design (admittedly with ~8x higher loads; see this link: Tenon Example ).

My (aesthetic) goal is minimal hardware...trying to do it the old-fashioned way! Should I be worried?

David