The voussoir arch

node · earth · Mediterranean, Near East and beyond

A curved span built from wedge-shaped blocks (voussoirs) set on a temporary timber former called centring and locked by the last stone, the keystone. Because each voussoir is wedged against its neighbours, the whole ring is in compression and the load runs down through the curve to the ground - which is exactly what stone, brick and unreinforced concrete are good at. The price is horizontal thrust at the springing, which must be resisted by a heavy pier, an abutment, buried fill, or by the next arch in an arcade pushing back. The arch was known in Mesopotamia and Egypt well before Rome, but Roman builders industrialised it: standard voussoir geometry, reusable centring, arcades that cancel each other's thrust, and the extension of the same logic into barrel vaults, groin vaults and domes. Once the arch exists, span stops being set by the tensile strength of a beam and starts being set by how much thrust you can absorb.