Mark Johnson wrote:
The wedged
tenons in the horizontal seat rotted out, and the end mortices obviously
failed. A better outdoor wood and a few minor design changes should last a
bit longer.

If you have people sitting on the edge of the bench, all that weight
will be transferred (with a twisting moment) to the M+T joints nearest
to that edge.

It seems like this is asking for trouble without some further form of
reinforcement.

What about some dowels or loose tenons between each outer board and the
next board in? In keeping with the "no glue" nature of the piece they
could be just left loose, but if they went a couple inches into each
board they could add a lot of strength.

Chris