At a recent woodworkers' get-together held in Brisbane, Australia, we tested a zigzag frame to destruction; the intention was to ascertain the ultimate strength of the frame, which has a similar consruction to some of my zigzag chairs. The frame's 22.5 degree miter joints have an aligning domino and twin reinforcing spline keys glued into saw kerfs with epoxy. The cross section of the lumber (Tasmanian blackwood, which has similar properties to American walnut), was 35 mm x 40 mm (about 1 3/8" x 1 5/8")
The first photo shows two woodworkers with a combined weight of about 500 lbs climbing onto the frame, which easily supported their weight.
The second photo shows four woodworkers with a combined weight of about 800 lbs standing on the frame, which supported their weight for a few seconds before collapsing.
On one side of the frame, the joint itself remained intact, but the wood split. On the other side, the joint itself was torn apart, with the keys being pulled out of their kerfs. In this case, perhaps the joint was starved of epoxy because of the tight fit of the keys in their kerfs.
The test showed that a cantilevered chair with well-made joints should easily be able to support the weight of even an obese sitter with a wide margin of safety.
David Dundas