This is an older bridge I came across when browsing through my old Science Olympiad photos. I used both Balsa and Basswood to build this bridge. This bridge was part of my design process which ended up with my SO winning bridge in 2004.
It is pretty easy to see how this bridge failed: the bottom fell out. This is a fairly easy fix, and might have been due simply to poor wood selection. Had I paid more attention to the quality of wood I was using, this failure might have been avoided.
I used two pieces for the arch. I had a square 1/16″ x 1/16″ piece of Basswood for the bottom of the arch. Then I glued a rectangle piece of Balsa wood on top of that to form a T beam. This method worked pretty well, but Balsa is harder to bend than Basswood.