An additional comment regarding shear webs from a glider guider:
As AQ500 said, the idea is to make kind of an I-beam. In sailplanes with framed up wings, shear webs are added BETWEEN the spars maiking a real I-beam. It's a little more trouble, but not much. They are added before the second spar is glued on. Some sailplanes I have built use webbing material that is the same width as the spars. Others use 1/16th sheeting. They can be of light balsa so they add very little weight and a LOT of strength.

I remember reading articles about this years ago suggesting that the webs run tightly from rib to rib with no gaps for maximum strength. I've always installed them this way and have yet to have a wing fold. I always use them in my powered planes too, even if they were not included in the design. Cheap insurance!