Lecture V.

Arches.

Definition of an arch.—Definition of terms.—Curve of equilibrium.—Equilibrated arch with convex voussoirs.—Curve of equilibrium a catenary—Two modes of proceeding in designing an arch—Gothic vaulting.—Curve of arch governed by circumstances of locality.—Practical method of equilibrating an arch.—Minimum thickness of arch stones.—Depth of voussoirs must be as the pressure on them.—Brick arches—Arched beams of wood and iron.—Laminated beams.—Wrought iron beams.—Skew arches.—Centreing.—Three principal points for consideration.—Centres on piles.—Roman centreing.—Striking arrangements.—Abutments.—Wing walls.—Vaulting: Cylindrical vaults—Horizontal domes—Arch little used before the Roman age—Plain ribbed vaulting—Lierne vaulting—Fan vaulting—Germany and the Netherlands—Abandonment of ribbed vaulting—Works on stone-cutting-Piers: Size of columns—Crushing strength.—Weak ground.—Hollow piers—Tunnel observatories.