Shear Forces

Shear force is stress that acts sideways across a joint or material, sliding one layer or piece past another, rather than pulling them straight apart (tension) or squeezing them together (compression). A bookshelf side panel resisting a heavily loaded shelf trying to slide down and out of its dado is being loaded in shear at that joint line, distinct from the panel simply being pulled apart.

Racking force -- a frame or case being pushed sideways, distorting a square shape toward a parallelogram -- is closely related, essentially shear acting across an entire assembly's joints at once rather than one joint in isolation. It's why diagonal bracing, plywood panels, or well-fitted joinery at the corners all matter for keeping a frame or case square under lateral load, not just how strong any single joint is in isolation.