The Roman Arch, Vault, and Concrete

A flat stone lintel spanning a gap has to resist bending: the bottom edge gets stretched, and stone is weak in tension, so it cracks. An arch sidesteps that problem entirely. Instead of bending, each stone block in the arch, called a voussoir, pushes against its neighbors, sending the load outward and down along the curve as compression, the one thing stone handles well. The keystone at the top is the last voussoir set in place; it wedges the whole ring tight so every block locks against its neighbors and none can fall inward.

To build one, Romans first built a wooden frame called centering, shaped like the underside of the arch, and stacked voussoirs on top of it starting from both sides and working up to the keystone. Once the keystone was set, the ring supported itself, and workers struck out the centering. Stretch one arch forward in a straight line and you get a barrel vault, a continuous tunnel of compression. Cross two barrel vaults at right angles and you get a groin vault, which concentrates the thrust into four corner points instead of along two long walls, opening up the space between.

The Colosseum shows this logic at full scale: stacked rings of arches form arcades, each level passing its load down through the arch below to the ground, letting the building rise four stories and hold tens of thousands of spectators without solid walls. Roman concrete, poured into wooden formwork from volcanic ash, lime, and rubble, pushed the idea further still, letting builders form continuous curved vaults and domes as one mass rather than assembling them block by block.