Arch and Vault

I

Introduction

Arch and Vault, a fundamental construction system in architecture used to span the space between walls, piers, or other supports and to create a roof or a ceiling. Until the 19th century the arch and vault were the only alternative to the far more limited and simpler post-and-lintel system supporting a flat or peaked beamed roof.

II

Arch

An arch, in construction, is a rigid span curving upwards between two points of support. It appears in a variety of structures, such as an arcade, formed by a row of arches, supported by load-bearing arches or a roof or a bridge, or as a single, free-standing triumphal or memorial arch. The traditional stone or brick arch, formed of many segments held in place by lateral thrust, was developed to connect a greater distance between two supports than a single horizontal beam, or lintel, could bridge. Since the 19th century, arches have also been made of single, curved spans of iron, steel, or reinforced concrete.

The masonry arch has many elements. Its supports may be walls, piers, or columns, and the capstones from which it springs are known as imposts. The upper part of the arch is the crown, the portions near the impost are the haunches, its wedge-shaped segments are voussoirs, and the central or crowning voussoir is the keystone. The inner edge of the arch is the intrados; the outer edge, the extrados; and the undersurface, the soffit. The moulded band that often is found around the opening of the arch is the archivolt. The wall spaces on either side of an arch, or between adjacent arches, are spandrels. If the space between the arch and the lintel is filled in, the resulting flat surface is the tympanum.

Arches have been built since prehistoric times. Rude prototypes were made by leaning two slabs of rock together or by constructing a stepped, or corbelled, arch in which projecting elements from a wall rise in steps from the supports to meet in the centre. The Egyptians, Babylonians, and Greeks used the arch, generally for secular structures, such as storerooms and sewers. The Assyrians built palaces with arched ceilings, and the Etruscans used arches in bridges, passageways, and gates. The Romans, however, were the first to develop the arch on a massive scale. They used the semicircular arch freely in secular structures such as amphitheatres, palaces, and aqueducts, but their temples usually had the post-and-lintel construction of Greek temples. The few vaulted exceptions, however, include the Pantheon in Rome.

During the Middle Ages, Byzantine architecture in the East and Romanesque architecture in the West retained the characteristic round Roman arch. Islamic architects developed a rich variety of pointed, scalloped, horseshoe, and ogee (S-curve) arches for mosques and palaces. In Moorish Spain, horseshoe arches were set on delicate columns, giving a characteristic airy effect. Gothic architecture in Western Europe was characterized by the pointed arch, which minimized outward thrust and thus made possible higher, thinner, window-filled walls, creating the lofty, spacious interiors of Gothic cathedrals. In the 20th century, arches of moulded reinforced concrete based on the curve of a parabola have been used in all sorts of public structures.

III

Vault

A vault, in architecture, is an arch-shaped structure, usually of masonry, used as the ceiling of a room or other enclosed space, as the roof of a building, or as the support for a ceiling or roof. Masonry vaults are usually composed of wedge-shaped pieces called voussoirs, which are held in place, like the stones of an arch, by the pressure of the neighbouring pieces. Because of the combined pressure of its components, any arch exerts an outward pressure at its base, and the base, therefore, must be so constructed as to withstand the outward as well as the downward thrust of the arch. This construction can be accomplished by using strong, heavy walls to support the arch or by supporting the walls with exterior structures, or buttresses. A temporary supporting structure must be erected within the vaulted area during construction, because a masonry vault does not become self-supporting until the central voussoirs or keystones are put in place.

A number of different types of vaults are used architecturally. The simplest of these is the barrel, or tunnel vault, the roof of which is shaped like half a cylinder and is supported by straight walls. The annular vault is similar to the barrel vault, except that the passage within it is not straight but curved, giving the entire structure the appearance of a portion of a ring. A groined vault is formed by the intersection of two vaults of the barrel type, usually at right angles to each other. The junctures at which the two vaults meet are elliptical ridges, called groins. In the simplest form of groined vault, the two conjoined vaults are of the same size and the floor of the vault is square; if the vaults are of different sizes, however, the floor of the vault is rectangular and the two areas of the ceiling between the groins are of unequal shape and size.

A dome is a hemi-spherical vault resting on a circular base wall. Pendentives are portions of hemi-spherical vaults, or curved triangles, placed in the corners of square or other polygonal structures to form a circular base for a dome above. More complicated vaults include ribbed vaults, in which the inner vault surface is subdivided by a number of independent supporting arches, or ribs. A further refinement is the fan vault, most common in English late Gothic structures, in which the ribs are multiplied and grouped in the shape of an open fan.

See also Islamic Art and Architecture; Roman Art and Architecture.