Describing a joint with word is at times tricky to put out and receive. How about drawings? Hard to misinterpret a simple sketch. The first picture was perfect.

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

I take it you will not be using any fastener at the dove tail located at the tree trunk? Question would that hold with out the decking, No. So you will be relying on the decking. I am trying to picture that.

I wasn't planning on fasteners between beam and tree...just the dovetail, the mortised block below, and the block above (to counteract the moment). New sketch, showing a top view, to illustrate how I think the deck will serve to counterbalance the outward tensions in the beams, and hold things together. [img]http://pxrox.tumblr.com/[/img]

I could add a vertical peg or a lag bolt linking the beam to the support block. Limited opportunities for placement: center of hole would be about 1" from end of beam. Calculation with a 1/4" lag bolt gives what seems like an absurdly small load capacity:
AWC Connection Calculator
ASD lateral loading, lag screw, single shear; mixed oak of 2.5" thickness for both members; main loaded at 90, side at 0 degrees to grain; 1/4 x 4" lag bolt gives 140 lbs. Value doesn't change when both angles are 90 degrees, which seems very wrong...

Thoughts? (Wrong use of the calculator?)

Calculation based on shear out of a bolt gives about: 2 (shear planes) x 1" (from end) x 2.5" (deep) x 200 psi (Fv) = 1,000 lbs. That would get me where I need to be.

How would a peg do? I haven't paid much attention to how big a peg should be, but 3/4 seems like a minimum size, and there seems (intuitively) not enough relish---in the 2.5" total width I have to work with---to take a peg.

I would be tempted to drill a hole through the beams, 10-12 inches from the tree, thread a cable and pull it taunt via some manner. Perhaps even better would be far out at the ends or in a notch at the end of the beam. A tension band?

That would be pretty straightforward, and it certainly wouldn't hurt, but I wonder if its overkill.

The deck will be Doug Fir 2x10's. Screwing a 2x10 to beams with three #10 screws at each end (>1.5" of relish) gives ~500 lbs of lateral (tension) load. There will be five rows of 2x10's, laid side by side, going out from tree...giving something like a 2,500 tension load.

Do you think that such a properly-screwed-on decking will itself serve as the tension band?

Nothing wrong with a little over kill.

Right you are!

I recommend overkill. Something American architects and engineers today don't value as highly as they should.

Overkill in my opinion is the difference between a house that will stand for 100 years and one that can stand for 500 years.
We can engineer it to the requirements of the materials and conditions as they exist now, but conditions change, materials degrade.

Especially in a situation like I see here, tying into a live tree and such, conditions will change significantly over time.