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For other uses, see Arch (disambiguation).

An arch is a curved structure capable of spanning a space while supporting significant weight (e.g. a doorway in a stone wall). The arch was first developed in the Indus Valley civilization circa 2500 BC [1] and subsequently in Mesopotamia, Egypt, Assyria, Etruria, and later refined in Ancient Rome. The arch became an important technique in cathedral building and is still used today in some modern structures such as bridges.

Technical aspects

The arch is significant because, in theory at least, it provides a structure which eliminates tensile stresses in spanning an open space. All the forces are resolved into compressive stresses. This is useful because several of the available building materials such as stone, cast iron and concrete can strongly resist compression but are very weak when tension, shear or torsional stress is applied to them. By using the arch configuration, significant spans can be achieved. The arch is a very useful structure as it is completely self-supporting. This is because all the compressive forces hold it together in a state of equilibrium. This even applies to frictionless surfaces.

This same principle holds when the force acting on the arch is not vertical such as in spanning a doorway, but horizontal, such as in arched retaining walls or dams.

Even when using concrete, where the structure may be monolithic, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, it has to be resisted by carefully placed reinforcement rods or fibres. (See Arch bridge.)

The following gallery shows examples of arch forms displayed in roughly the order in which they were developed.

Construction

An arch requires all of its elements to hold it together. This raises the interesting question of how an arch is actually constructed. One simple answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. The voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would in any case be required by the builders, so the scaffolding can be combined with the arch support. Occasionally arches would fall down when the frame was removed if construction or planning had been incorrect. (The A85 bridge at Dalmally, Scotland suffered this fate on its first attempt, in the 1940s).

History

Arches were used by the Harappan, Egyptian, Babylonian, Greek and Assyrian civilizations for underground structures such as drains and vaults, but the ancient Romans were the first to use them widely above ground although it is thought that Romans learned it from the Etruscans. The so-called Roman arch is semicircular, and built from an odd number of arch bricks (in modern architectural parlance, these are called voussoirs). The capstone or keystone is the topmost stone in the arch. This shape is the simplest to build, but not the strongest. There is a tendency for the sides to bulge outwards, which must be counteracted by an added weight of masonry to push them inwards. The semicircular arch can be flattened to make an elliptical arch. The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. It was used in Spanish Visigothic architecture, Islamic architecture and mudéjar architecture, as in the Great Mosque of Damascus and in later Moorish buildings. It was used for decoration rather than for strength. The semicircular arch was followed in Europe by the pointed Gothic arch or ogive, whose centreline more closely followed the forces of compression and which was therefore stronger. This design had been used by the Assyrians as early as 722 BC. The parabolic and catenary arches are now known to be the theoretically strongest forms.

The arch was especially important to the construction of the great systems of aqueducts by the ancient Romans. These consisted of long series of arches, a much more economical mode of construction than, say, a continuous wall of support.

The arch was used in some bridges in China since the Song dynasty.

Other types

A dome is a three-dimensional application of the arch. Igloos are notable early structures making use of domes.

A special form of the arch is the triumphal arch, usually built to celebrate a victory in war. The most famous example of this is the Arc de Triomphe in Paris, France.

Natural rock formations may also be referred to as 'arches'. These natural arches are formed by erosion rather than being carved or constructed by man. See Arches National Park for examples.

References

• Roth, Leland M (1993). Understanding Architecture: Its Elements History and Meaning, Oxford, UK: Westview Press. ISBN 0-06-430158-3. pp. 27-8

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