An arch is a curved vertical structure spanning an open space underneath it. Arches may support the load above them, or they may perform a purely decorative role. As a decorative element, the arch dates back to the 4th millennium BC, but structural load-bearing arches became popular only after their adoption by the Ancient Romans in the 4th century BC.
Arch-like structures can be horizontal, like an arch dam that withstands a horizontal hydrostatic pressure load. Arches are usually used as supports for many types of vaults, with the barrel vault in particular being a continuous arch. Extensive use of arches and vaults characterizes an arcuated construction, as opposed to the trabeated system, where, like in the architectures of ancient Greece, China, and Japan (as well as the modern steel-framed technique), posts and beams dominate.
The arch had several advantages over the lintel, especially in masonry construction: with the same amount of material an arch can have larger span, carry more weight, and can be made from smaller and thus more manageable pieces. Its role in construction was diminished in the middle of the 19th century with introduction of wrought iron (and later steel): the high tensile strength of these new materials made long lintels possible.
Basic concepts Terminology
A true arch is a load-bearing arch with elements operating under compressive forces. The term false arch has a few meanings. It is usually used to designate an arch that has no structural purpose, like a proscenium arch in theaters used to frame the performance for the spectators, but is also applied to corbelled and triangular arches that are not based on compression.
A typical true masonry arch consists of the following elements: Keystone, the top block in an arch. Portion of the arch around the keystone (including the keystone itself), with no precisely defined boundary, is called a crown Voussoir (a wedge-like construction block). A compound arch is formed by multiple concentric layers of voussoirs. The rowlock arch is a particular case of the compound arch, where the voussoir faces are formed by the brick headers. Extrados (an external surface of the arch) Impost is a block at the base of the arch (the voussoir immediately above the impost is a springer). The springing line or springing level is where the arch begins to curve; it need not coincide with the top of the impost or capital. A portion of the arch between the springing level and the crown (centered around the 45° angle) is called a haunch. On a column, an arch can rest on the abacus; a dosseret provides an alternative in the form of a deep rectangular impost above the capital. Dosserets were common in Late Antique, Early Christian, and Byzantine architecture. Intrados (an underside of the arch, also known as a soffit) Rise (height of the arc, distance from the springing level to the crown) Clear span Abutment The roughly triangular-shaped portion of the wall between the extrados and the horizontal division above is called spandrel.
The thickness of an arch is measured between its intrados and extrados and may vary along the curve. Dividing the rise by the span gives its rise-to-span ratio.
A (left or right) half-segment of an arch is called an arc, the overall line of an arch is arcature (this term is also used for an arcade). The geometrical centre of an arc is the point from which its circular curve is drawn in elevation. Archivolt is the exposed (front-facing) part of the arch, sometimes decorated (occasionally also used to designate the intrados). If the sides of voussoir blocks are not straight, but include angles and curves for interlocking, the arch is called "joggled". Alternating colours of stone can emphasize the interlocking pattern. Joggled arches are common in Islamic architecture, including at the Bab al-Nasr in Cairo (1087).
Arch action
A true arch, due to its rise, resolves the vertical loads into horizontal and vertical reactions at the ends, a so called arch action. The vertical load produces a positive bending moment in the arch, while the inward-directed horizontal reaction from the spandrel/abutment provides a counterbalancing negative moment. As a result, the bending moment in any segment of the arch is much smaller than in a beam with the equivalent load and span. The diagram on the right shows the difference between a loaded arch and a beam. Elements of the arch are mostly subject to compression (A), while in the beam a bending moment is present, with compression at the top and tension at the bottom (B).
In the past, when arches were made of masonry pieces, the horizontal forces at the ends of an arch (so called thrust) caused the need for heavy abutments (cf. Roman triumphal arch). The other way to counteract the forces, and thus allow thinner supports, was to use the counter-arches, as in an arcade arrangement, where the horizontal thrust of each arch is counterbalanced by its neighbors, and only the end arches need to buttressed. With new construction materials (steel, concrete, engineered wood), not only the arches themselves got lighter, but the horizontal thrust can be further relieved by a tie connecting the ends of an arch (bowstring arch).
Funicular shapes
When evaluated from the perspective of an amount of material required to support a given load, the best solid structures are compression-only; with the flexible materials, the same is true for tension-only designs. There is a fundamental symmetry in nature between solid compression-only and flexible tension-only arrangements, noticed by Robert Hooke in 1676: "As hangs the flexible line, so but inverted will stand the rigid arch", thus the study (and terminology) of arch shapes is inextricably linked to the study of hanging chains, the corresponding curves or polygons are called funicular. Just as the shape of a hanging chain will vary depending on the weights attached to it, the shape of an ideal (compression-only) arch will depend on the distribution of the load.
While building masonry arches in the not very tall buildings of the past, a practical assumption was that the stones can withstand virtually unlimited amount of pressure (up to 100 N per mm2), while the tensile strength was very low, even with the mortar added between the stones, and can be effectively assumed to be zero. Under these assumptions the calculations for the arch design are greatly simplified: either a reduced-scale model can be built and tested, or a funicular curve (pressure polygon) can be calculated or modeled, and as long as this curve stays within the confines of the voussoirs, the construction will be stable (a so called "safe theorem").
Early studies of arch thrust included Philippe de La Hire's work of 1712 and Charles-Augustin de Coulomb's 1773 treatment of sliding and rotation between voussoirs. developed a graphical method for determining the line of thrust in 1828 and published it in 1840.
Classifications There are multiple ways to classify arches: by the geometrical shape of its intrados (for example, semicircular, triangular, etc.); for the arches with rounded intrados, by the number of circle segments forming the arch (for example, round arch is single-centred, pointed arch is two-centred); by the material used (stone, brick, concrete, steel) and construction approach. For example, the wedge-shaped voussoirs of a brick arch can be made by cutting the regular bricks ("axed brick" arch) or manufactured in the wedge shape ("gauged brick" arch); structurally, by the number of hinges (movable joints) between solid components. For example, voussoirs in a stone arch should not move, so these arches usually have no hinges (are "fixed"). Permitting some movement in a large structure allows to alleviate stresses (caused, for example, by the thermal expansion), so many arch bridge spans are built with three hinges (one at each support and one at the crown) since the mid-19th century.
Arrangements A sequence of arches can be grouped together forming an arcade. Romans perfected this form, as shown, for example, by arched structures of Pont du Gard. In the interior of hall churches, arcades of separating arches were used to separate the nave of a church from the side aisle,Günther Wasmuth (ed.): Wasmuths Lexikon der Baukunst, vol. 4: P - Zyp. Wasmuth, Berlin 1932, . or two adjacent side aisles.: Baustilkunde - Europäische Baukunst von der Antike bis zur Gegenwart. Sonderausgabe, Orbis Verlag, München 1988, , .
Two-tiered arches, with two arches superimposed, were sometimes used in Islamic architecture, mostly for decorative purposes.
Arches can be combined with lintels in a serliana, where the central opening is arched and the openings on either side have horizontal heads. Repeated serlianas form the loggias of Andrea Palladio's Basilica Palladiana in Vicenza.
An opening of the arch can be filled, creating a blind arch. Blind arches are frequently decorative, and were extensively used in Early Christian, Romanesque, and Islamic architecture. The space under the head of a blind arch is its lunette. A containing arch forms an outer frame around one or more smaller arches; the inner and outer profiles need not match. The galleries at Jumièges Abbey and Amiens Cathedral illustrate this arrangement, which was widely used in Romanesque and Gothic churches.
Multiple arches can be superimposed with an offset, creating an interlaced series of usually (with some exceptions) blind and decorative arches. Interlaced arcades were popular in Romanesque and Gothic architecture. The interlaced motif also has pre-Islamic precedents in Roman mosaics and the carved decoration of early Christian churches in Syria. An external wall may have a rear-arch (also rere-arch) around an opening on the side facing into the building. In a doorway, this arch can also hold the wooden door in place.
Structural Relieving arches carry loads above an opening or a thinner section of wall. They can accommodate large windows and gateways or direct loads towards stronger parts of a building, as in the Roman Pantheon. They were common in Roman brick construction and in Ottonian and Romanesque architecture, and also occur in Islamic buildings.
Transverse arches cross an interior at right angles to the side walls, subdividing it into bays and supporting vaults. They were common in Western architecture from the Carolingian period. A diaphragm arch also crosses the interior, but carries an exposed section of wall above it. The wall and arch can provide intermediate support beneath a high roof or vault, divide the roof into sections, or brace the side walls. Examples occur in Islamic buildings as well as in Carolingian, Ottonian, and Romanesque work.
A strainer arch prevents a wall or masonry pier from moving inwards by bracing it against another. Such arches often formed part of medieval repairs: Wells Cathedral () and Canterbury Cathedral (1495) have decorative examples that perform this structural role. This bracing also made it possible to use higher supports at the Great Mosque of Córdoba. Strainer arches can be "inverted" (upside-down) while remaining structural. When used across railway cuttings to prevent collapse of the walls, strainer arches may be referred to as flying arches. A counter-arch is built adjacent to another arch to oppose its horizontal action or help to stabilize it, for example, when constructing a flying buttress.
In foundations and at the base of tunnel linings, inverted arches can resist upward pressure from the ground.
Shapes
The large variety of arch shapes (left) can mostly be classified into three broad categories: rounded, pointed, and parabolic.
Rounded