Arch

A curved structural member that spans an opening and is generally composed of wedge-shaped blocks that transmit the downward pressure out laterally.

The top of an arch is called the extrados or back.

The underside of an arch is called the intrados or soffit.

Corbelled Arch

An arch constructed by successive layers of brick or stone projecting further towards each other from either side of the arch, until the gap is spanned.

Deck-Stiffened Arch

An arch connected by vertical elements to a deck beam that stiffens the arch in such a way that the arch carries almost no bending moments and can therefore be thin.

Elliptical Arch

The exact shape of an arch may vary hugely. The simplest is the semi-circular, but a shallow, flattened semi-circle becomes an ellipse, and many mediaeval stone bridges were built with elliptical arches spanning rivers.

Keystone

The central stone in an arch, and begins the distribution of the vertical 'load' forces down and around the arch.

Three-Hinged Arch

A curved structure with hinges usually at each abutment and at the crown.

See also: Arcade, Corbel, Corbelled Arch, Deck-Stiffened Arch, Diaphram Arch, Dome, Elliptical Arch, Keystone, Three-Hinged Arch, Vault, Voussoirs.