arch

arch the spanning of a wall opening by means of separate units (such as bricks or stone blocks) assembled into an upward curve that maintains its shape and stability through the mutual pressure of a load and the separate pieces. The weight of the supported load is thus converted into downward and outward lateral pressures called thrusts, which are received by the solid piers (abutments) flanking the opening. The blocks, called voussoirs, composing the arch usually have a wedge shape but they can be rectangular with wedge-shaped joints between them. The underside of the arch is the intrados or soffit and the upper surface above the crown block (keystone) of the arch is the extrados. The point where the arch starts to curve is the foot of the arch, and the stones there are the springers. The surface above the haunch (just below the beginning of the curve) contained within a line drawn perpendicular to the springing line (from which the arch curves), and another drawn horizontal to the crown is the spandril. In modern fireproof construction the word arch is also used for the masonry that fills the space between steel beams and acts as a floor support. The arch was used by the Egyptians, Babylonians, and Greeks, chiefly for underground drains, and also by the Assyrians in the construction of vaulted and domed chambers. In Europe the oldest known arch is the Cloaca Maxima, the huge drain at Rome built by Lucius Tarquinius Priscus c.578 BC The Romans developed the semicircular arch, modeled on earlier Etruscan structures, in the vaults and domes of their monumental buildings. Its use was continued in early Christian, Byzantine, and Romanesque architecture. In the 13th cent. the pointed arch (used as early as 722 BC in Assyrian drains) came into general use. The contact of Europeans with Saracenic architecture during the Crusades is offered among other theories for its introduction into Europe. But it is likely that the pointed arch may have been independently rediscovered in Europe in the Middle Ages as a device for solving many of the mechanical difficulties of vault construction. Its adoption was an essential element in the evolution of the Gothic system of design. With the Renaissance there was a return to the round arch, which prevailed until the 19th-century invention of steel beams for wide spans relegated the arch to a purely decorative function. Although the circular and pointed forms have predominated in the West, the Muslim nations of the East developed a variety of other arched shapes, including the ogee arch used in Persia and India, the horseshoe arch used in Spain and North Africa, and the multifoil or scalloped arch used especially in the Muslim architecture of Spain. In the 20th cent. arches often take a parabolic shape. They are usually constructed with laminated wood or reinforced concrete, materials that give greater lightness and strength to the structure. See triumphal arch .

arch2 chief, pre-eminent XVI; (through arch rogue, etc.) cunning, waggish XVII. The prefix ARCH- used as an adj.

arch. Construction, known as an arch-ring, made of truncated wedge-shaped blocks (arch-stones or voussoirs) that by mutual pressure stay in place, set out in a curved form to span an opening and carry a superimposed load, as an alternative to a lintel: it is termed arcuated, as opposed to trabeated. Terms associated with an arch include:abutment: solid structure from which an arch springs, and which resists the outward thrust (all arches will collapse unless adequately supported);archivolt: concentric ring of mouldings round an arch, like an architrave bent around the top of the arch;chord: horizontal distance between abutments taken from the springing-line on one side to that of the other, also called the span;crown: highest point of the intrados, also called vertex;extrados: upper curve of each voussoir or outer extremity of the archivolt;flank: see haunch below;haunch: curved part on the top of the section between the crown, the portion of the arch itself, and the extremity of the span, also called flank;height: rise of an arch, or vertical distance between the chord to the crown or highest point of the intrados;impost: projecting member, often moulded, from which an arch springs, e.g. a block, bracket, corbel, or dosseret;intrados: lower curve of each voussoir, i.e. coinciding with the soffit of the arch;keystone: central large wedge-shaped voussoir in an arch, often elaborately carved as an ancon; section of the cavity: vertical plane figure bounded by the span and the intrados;springing: point at which an arch unites with its support;springing-line: horizontal plane from which an arch begins to rise.Types of arches include:acute arch: see lancet-arch;anse de panier: three-centred arch resembling a basket-handle, also known as a basket-handled arch, usually formed by a segment of a circle connected to two other segments with smaller radii, but sometimes constructed using five or seven centres to give a similar shape;back arch: see rear arch;basket arch: see anse de panier;bell arch: arch supported on two corbels with curved faces above the reveals, so that the resulting compound curve of the opening resembles a bell;Caernarfon arch: see Welsh arch;camber arch: flat arch with a slight upward curve to the intrados, or a very low segmental arch; canted arch: similar to a corbel arch, but with straight haunches set at an angle of 45°;catenary arch: formed like an inverted catenary, similar to a parabolic arch, but less sharp and more elegant;compound arch: Order or recessed arch consisting of several concentric arches with vertical supports, successively placed within and behind each other, each smaller than that in front, as in a Romanesque doorway;contrasted arch: as ogee arch;corbel arch: false or pseudo- arch formed by means of horizontal blocks corbelled out from each side of the opening to be bridged until the latter is closed;cusped arch: see foil arch;depressed arch: see anse-de-panier, four-centred, and three-centred arch;diaphragm arch: transverse arch across a nave supporting a gable between sections of a timber roof to prevent the spread of fire;diminished arch: segmental arch, less than a semicircular arch;discharging arch: see relieving arch;drop arch: pointed arch with its centres on the springing-line and with the span longer than the radius;Dutch arch: triangular false arch constructed of bricks laid on a slope of 45° starting from a skew-back at each jamb and meeting at an apex;elliptical arch: formed as half an ellipse with its axis coinciding with the springing-line;equilateral arch: pointed two-centred arch of two arcs, the radii of which are equal to the span;false arch: see corbel arch;flat arch: straight arch with a level or slightly cambered soffit, the voussoirs seeming to form a lintel;Florentine arch: semicircular arch with extrados and intrados struck from different centres, so that the voussoirs increase in length towards the top;foil arch: cusped or foliated arch associated with Gothic, Moorish, and Islamic styles. Foil arches can have trefoils, cinquefoils, or multifoils within a pointed arch, can have a series of small arcs cut in the intrados, as in the Moorish multifoil or scalloped arch, or can themselves be in the form of foils, such as the pointed trefoil or round trefoil arch;foliated arch: see foil arch;four-centred arch: depressed arch, the characteristic form of late-Perpendicular openings, with upper central arcs with centres below the springing-line, flanked by two arcs with centres on the springing-line;French arch: Dutch arch;gauged arch: flat arch, with a slightly cambered soffit, formed of voussoirs made of precisely cut stones, or, more usually, finely rubbed bricks (known as rubbers), with very fine lime-putty joints;horseshoe arch: usually associated with Islamic styles, such arches are horseshoe (semicircular, narrowing towards the base below the springing-line, on straight piers), pointed horseshoe (pointed with arcs continuing to narrow the opening below the springing-line), and round horseshoe (semicircular with arcs continuing to narrow the opening below the springing-line);interlacing arches: intersecting semicircular Romanesque arches in a blind arcade, overlapping and forming pointed arches;inverted arch: built upside-down, used in foundations;Italian pointed arch: with intrados and extrados struck from different centres, and voussoirs increasing in size towards the apex. Similar to a Florentine arch, but pointed;jack arch: segmental brick arch spanning between iron beams, thus forming a vault;keel arch: see ogee arch;lancet arch: sharply pointed two-centred or acute type with the radii greater than the span;mitre arch: triangular pseudo-arch of two flat stone slabs leaning together at a mitred apex, common in Anglo-Saxon architecture; also called a pediment arch;Moorish arch: horseshoe- shaped arch, sometimes with a pointed top;Moorish multifoil arch: see foil arch;obtuse-angled arch: pointed type formed of arcs with centres on either side of the centre-line;ogee arch: pointed keel-arch of four arcs with two centres outside it and two inside, thus producing two S-shaped curves. It first occurred around 1300. A nodding ogee has its apex projecting beyond the naked of the wall, so it is a double ogee in elevation and a single in section;Order arch: see compound arch;parabolic arch: shaped like the intersection of a cone with a plane parallel to its vertical axis, sometimes confused with a catenary arch, but sharper and less elegant;pediment arch: see mitre arch;pointed arch: any pointed arch, but especially an equilateral arch. Proportions of pointed arches are governed by the positions of the centre-points from which the arcs are struck. See acute, drop, equilateral, foil, four-centred, horseshoe, Italian pointed, lancet, obtuse-angled, ogee, Saracenic, segmental pointed, and Tudor arches;pseudo-four-centred arch: see Tudor arch;pseudo-three-centred arch: depressed type consisting of two arcs struck from the springing-line supporting a central flat or straight pseudo-arch of voussoirs with joints struck from a point well below the springing-line;raking arch: rampant arch with one impost higher than the other;rampant arch: see raking arch;rear arch: arrière voussure, back, or secondary arch spanning an opening on the inside of a thick wall, as when there is a lintel on the outside, but a splayed arched reveal inside;recessed arch: see compound arch;relieving arch: discharging or safety arch, it is usually segmental, built flush with the wall-surface over a lintel to relieve the latter from the weight of masonry above, and to discharge the forces away from the lintel;round horseshoe arch: see horseshoe arch;round trefoil arch: see foil arch;rowlock arch: has small voussoirs laid in a series of concentric rings;safety arch: see relieving arch;Saracenic arch: pointed stilted, striped arch with alternate voussoirs of contrasting colours;scalloped arch: see foil arch;scheme arch: segmental or skene arch;secondary arch: see rear arch;segmental arch: with its centre below the springing-line. A segmental pointed arch has two centres below the springing-line;semicircular arch: with its centre on the springing-line;shouldered arch: flat arch or lintel carried on quadrant-ended corbels over jambs;skene arch: see scheme arch;skew arch: has jambs at an angle other than 90° to its face, or one spanning something obliquely. The beds of the courses of a skew arch consist of spiral lines wound, as it were, around a cylinder, every part of which cuts the axis at a different angle, the angle being greatest at the keystone and least at the springing; when viewed from beneath the courses appear as straight lines. Skew is a slope, as in the abutment of a gauged-brick flat or straight arch. Skewback is the part of the abutment giving support to the arch;soldier arch: flat pseudo-arch of uncut ungauged bricks laid on end and supported by some means such as an L-shaped metal angle;squinch arch: diagonal arch or arches (see trumpet-arch) in the internal angle of a tower supporting an octagonal spire, or used instead of pendentives to carry a dome over a square compartment;stilted arch: with its springing-line raised on piers above the level of the impost;straight arch: see flat arch;strainer arch: one constructed between piers or walls to prevent them moving inwards towards each other, as at the crossing of Wells Cathedral, Som.;sub-arch: subsidiary minor arch enclosed and framed within a larger structural arch. In Gothic work it consisted of two inferior arches, under the main arch, rising naturally from the middle mullion and forming two independent arches filled with tracery;surbased arch: rises less than half its span;surmounted arch: rises higher than half its span;Syrian arch: series of small arches above a series of wider arches, centred on the arches and piers below;three-centred arch: depressed arch with two arcs struck from the springing-line with a central arc struck from below it. A depressed three-centred arch has the central arc struck from a point very much lower than the springing-line;transverse arch: divides a compartment of a vault from another, spanning from wall to wall or from wall to pier, forming a bay;trefoil arch: see foil arch;triangular arch: see mitre arch;triumphal arch: monumental arched free-standing structure, invented by the Romans, and a significant precedent for later façade treatments in which columnar and trabeated elements were mixed with arcuated forms. Many Roman examples survive, including that of Septimius Severus (AD 203), with two smaller arches flanking a wider central arch with a richly coffered vault. The form was revived during the Renaissance period, and there are many fine Neo-Classical examples, including the Carrousel arch, Paris, by Percier and Fontaine (1806–7);trumpet-arch: squinch-like part of a cone, i.e. with the arches getting wider and higher towards the extremities;Tudor arch: pseudo-four-centred late-Perpendicular arch, similar to the four-centred type, but with shanks starting as quarter-circles (with centres on the springing-line) continuing as straight lines to the apex. It is very depressed, and often expressed as a single lintel;two-centred arch: acute or lancet arch;Venetian arch: semicircular arch framing two semicircular-headed lights separated by a colonnette above which is a roundel in the space between the tops of the smaller arches and the main intrados;Welsh arch: Caernarfon arch, comprising a wide keystone resting on two corbels shaped to fit the keystone.

Bibliography

Gwilt (1903);
W. Papworth (1852);
J. Parker (1850);
Sturgis et al. (1901–2)