Originally Posted by gaRCfield

Just FYI, they are called 'sheer' webs because they are there to counteract sheer forces, not compression. When air puts lift on the wing and pulls it up, the top spar goes into compression (forces pushing toward the fuse) and the bottom in tension (forces pulling away from the fuse). A wing rib attached to the top and bottom spar would end up twisting (top in, with right-side-up lift) without proper reinforcement. The sheer webs basically keep the wing ribs square to the top/bottom spar. The wing ribs can handle whatever compressive loads would be present, but they have to be perpendicular/square to do so. This is why diagonal cables can be used in place of ply or whatever else.

If you have this plane or problem and can access the wing structure, a little 3/4oz glass cloth laid diagonally to the grain of the sheer web would be a quick and easy fix. It shouldn't need much. You could also glue a stick of balsa diagonally across each web. Remember that the diagonal bracing is in tension. Balsa is plenty strong for that application.

If you remember learning why triangular patterns are used in bridge structures, it is the same principal.