Forces at work in a stone vault

The tall, narrow, long interior of Salisbury Cathedral (slightly over 450 feet) is composed of rectangular cubes called bays. A bay is a square or rectangular unit with vertical supports (piers)at the four corners. The supports carry a stone ceiling or vault. Because the vaults are made of stone, they exert forces of weight and outward thrust that the master mason must consider when designing the building. The outward thrust of the vaults is greatest at the outer corners. To counteract the thrust and stabilize the wall, the exterior walls are thickened at this point with a structural element called a buttress. Beneath the aisle roofs is a quadrant arch (like a "flying" buttress but hidden from view) that abuts the clerestory wall to counteract the outward thrust from the high vaults.