A masonry arch gets stronger as it does more work.
Gravity is usually the enemy of structural endeavor, as it seeks to pull structures toward the earth. But a masonry arch works because of gravity. Gravity pulls each voussoir (voo-SWAH) in an arch into contact with the one below it, which transmits the force to the one below it, and so on. The greater the loads on an arch, the more the masonry units cohere, until the compressive strength of the material is exceeded. For this reason, a masonry arch will tend to destabilize when there is a relatively small load on it, or may simply look unstable if there is relatively little masonry above it.
At its base, an arch generates an outward thrust in addition to a vertical gravitational force. This must be resisted by a large mass, such as a concrete or earthen embankment for a bridge arch or a “flying” buttress for a large church arch. When arches are placed in series, the outward thrust from each arch neutralizes the thrust from the adjacent one.
