Arch

A masonry arch
1. Keystone 2. Voussoir 3. Extrados 4. Impost 5. Intrados 6. Rise 7. Clear span 8. Abutment

An arch is a structure that spans a space and supports structure and weight below it. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture^[1] and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.

Basic concepts

An arch is a pure compression form.^[2] It can span a large area by resolving forces into compressive stresses and, in turn eliminating tensile stresses. This is sometimes referred to as arch action.^[3] As the forces in the arch are carried to the ground, the arch will push outward at the base, called thrust. As the rise, or height of the arch decreases, the outward thrust increases.^[4] In order to maintain arch action and prevent the arch from collapsing, the thrust needs to be restrained, either with internal ties, or external bracing, such as abutments.^[5]

Fixed arch vs hinged arch

Rossgraben bridge (Rüeggisberg) near Berne, Switzerland, showing the hinge at mid-span of this three-hinged arch.

The most common true arch configurations are the fixed arch, the two-hinged arch, and the three-hinged arch.^[6]

• The fixed arch is most often used in reinforced concrete bridge and tunnel construction, where the spans are short. Because it is subject to additional internal stress caused by thermal expansion and contraction, this type of arch is considered to be statically indeterminate.^[5]

• The two-hinged arch is most often used to bridge long spans.^[5] This type of arch has pinned connections at the base. Unlike the fixed arch, the pinned base is able to rotate,^[7] allowing the structure to move freely and compensate for the thermal expansion and contraction caused by changes in outdoor temperature. Because the structure is pinned between the two base connections, which can result in additional stresses, the two-hinged arch is also statically indeterminate, although not to the degree of the fixed arch.^[5]

• The three-hinged arch is not only hinged at its base, like the two-hinged arch, but at the mid-span as well. The additional connection at the mid-span allows the three-hinged arch to move in two opposite directions, and compensate for any expansion and contraction. This type of arch is thus not subject to additional stress caused by thermal change. The three-hinged arch is therefore said to be statically determinate.^[6] It is most often used for medium-span structures, such as large building roofs.

Another advantage of the three-hinged arch is that the pinned bases are more easily developed than fixed ones, allowing for shallow, bearing-type foundations in medium-span structures. In the three-hinged arch, "thermal expansion and contraction of the arch will cause vertical movements at the peak pin joint but will have no appreciable effect on the bases," further simplifying the foundation design.^[5]

Types of arches

Arches have many forms, but all fall into three basic categories: Circular, pointed, and parabolic. Arches can also be configured to produce vaults and arcades.^[5]

• Arches with a circular form, also referred to as rounded arch, were commonly employed by the builders of ancient, heavy masonry arches.^[8] Ancient Roman builders relied heavily on the rounded arch to span large, open areas. Several rounded arches placed in-line, end-to-end, form an arcade, such as the Roman aqueduct.^[9]

Roman aqueduct near Nîmes, France: An example of an arcade, employing the circular arch.

Horseshoe arches (9th century) in the Mosque of Uqba, in Kairouan, Tunisia.

• Pointed arches were most often used by builders of Gothic-style architecture.^[10] The advantage to using a pointed arch, rather than a circular arch, is that the arch action in a pointed arch produces less thrust at the base. This innovation allowed for taller and more closely spaced openings, typical of Gothic architecture.^[11][12]

Cathedral of St. Michael and St. Gudula in Brussels, Belgium, with its central, pointed arch window, typical of Gothic architecture.

• Vaults are essentially "adjacent arches [that] are assembled side by side." If vaults intersect, complex forms are produced with the intersections. The forms, along with the "strongly expressed ribs at the vault intersections, were dominant architectural features of Gothic cathedrals."^[8]

Interior vaulted ceiling of Notre Dame de Paris, showing the ribs at the intersection of several arches.

• The parabolic arch employs the principle that when weight is uniformly applied to an arch, the internal compression resulting from that weight will follow a parabolic profile. Of any arch type, the parabolic arch produces the most thrust at the base, but can span the largest areas. It is commonly used in bridge design, where long spans are needed.^[8]

Tyne Bridge in Newcastle upon Tyne, England: An example of a parabolic arch used in bridge design.

Pont d'arcades in Móra d'Ebre, Catalonia: the bridge is designed as a series of parabolic arches.

Types of arches displayed chronologically, roughly in the order in which they were developed:

• Triangular arch

• Round arch or Semi-circular arch

• Segmental arch or arch that is less than a semicircle

• Unequal round arch or Rampant round arch

• Lancet arch

• Equilateral pointed arch

• Shouldered flat arch -see also jack arch

• Trefoil arch, or Three-foiled cusped arch

• Horseshoe arch

• Three-centered arch

• Elliptical arch

• Inflexed arch

• Ogee arch

• Reverse ogee arch

• Tudor arch

• Parabolic arch

History

True arches, as opposed to corbel arches, were known by a number of civilizations in the Ancient Near East, the Levant, and Mexico, but their use was infrequent and mostly confined to underground structures such as drains where the problem of lateral thrust is greatly diminished.^[13] A rare exception is the Bronze Age arched city gate of Ashkelon (modern day Israel), dating to ca. 1850 B.C.^[14] An early example of a voussoir arch appears in the Greek Rhodes Footbridge.^[15] In 2010, a robot discovered a long arch-roofed passageway underneath the Pyramid of Quetzalcoatl which stands in the ancient city of Teotihuacan north of Mexico City, dated to around 200 AD.^[16]

The ancient Greeks introduced the arch in Italian visible today in the south of the peninsula and in Sicily.

The ancient Romans learned the arch from the Etruscans, refined it and were the first builders to tap its full potential for above ground buildings:

The Romans were the first builders in Europe, perhaps the first in the world, fully to appreciate the advantages of the arch, the vault and the dome.^[17]

The segmental arch was first built by the Romans who realized that an arch in a bridge did not have to be a semicircle,^[18][19] such as in Alconétar Bridge or Ponte San Lorenzo. They were also routinely used in house construction as in Ostia Antica (see picture).

The first example of early Gothic arch in Europe is set in Sicily in the Greek fortifications of Gela. 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. The semicircular arch can be flattened to make an elliptical arch as in the Ponte Santa Trinita. Both the parabolic and the catenary arches are now known to be the theoretically strongest forms. Parabolic arches were introduced in construction by the Spanish architect Antoni Gaudí, who admired the structural system of Gothic style, but for the buttresses, which he termed "architectural crutches". The catenary and parabolic arches carry all horizontal thrust to the foundation and so do not need additional elements. The first examples of the pointed arch in the European architecture are in Sicily and dates back to the Arab-Norman period.

The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. The first known built horseshoe arches are known from Aksum (modern day Ethiopia and Eritrea) from around the 3rd–4th century, around the same time as the earliest contemporary examples in Roman Syria, suggesting either an Aksumite or Syrian origin for the type of arch.^[20]

Construction

Since it is a pure compression form, the arch is useful because many building materials, including stone and unreinforced concrete can resist compression but are weak when tensile stress is applied to them.^[21]

An arch is held in place by the weight of all of its members, making construction problematic. One 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). The interior and lower line or curve of an arch is known as the intrados.

Old arches sometimes need reinforcement due to decay of the keystones, forming what is known as bald arch.

In reinforced concrete construction, 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, such as tensile or torsional stress, it has to be resisted by carefully placed reinforcement rods or fibres.^[22]

Other types

The Delicate Arch, a natural arch near Moab, Utah

A blind arch is an arch infilled with solid construction so it cannot function as a window, door, or passageway.

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.

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

Gallery

• The dry stone bridge, so called Porta Rosa (4th century BC), in Elea

• Arch of Constantine, Rome, Italy commemorating a victory by Constantine I in 312 AD

• The Gateway Arch in Saint Louis, Missouri; a sculpture based on a catenary arch

• Stonework arches seen in a ruined stonework building – Burg Lippspringe, Germany

• Several arches at the Casa Simón Bolívar in Havana, Cuba

• Arches in the Armenian monastery of Geghard.

• Arches in the nave of the church in monastery of Alcobaça, Portugal

• The Arc de Triomphe, Paris; a 19th-century triumphal arch modeled on the classical Roman design

• The Second Wembley Stadium, in London, built in 2007

• Catenary arches inside Casa Milà in Barcelona, Spain by Antoni Gaudí

• Lucerne railway station, Switzerland

• Chinese Eastern Han Dynasty (25–220 AD) tomb chamber

• The First San Mamés Stadium, in Bilbao, arch built in 1953, demolished 2013.

See also

• Arcade (architecture)
• Arch bridge
• Blind arch
• Corbelled arch
• Golden Arches
• Natural arch
• Order moulding
• Skew arch
• Suspension bridge
• Triumphal arch

References

Notes

• Boyd, Thomas D. (1978), "The Arch and the Vault in Greek Architecture",  
• Galliazzo, Vittorio (1995), I ponti romani, Vol. 1, Treviso: Edizioni Canova,  
• O'Connor, Colin (1993), Roman Bridges, Cambridge University Press,  
• Rasch, Jürgen (1985), "Die Kuppel in der römischen Architektur. Entwicklung, Formgebung, Konstruktion", Architectura 15: 117–139 
• Roth, Leland M (1993). Understanding Architecture: Its Elements History and Meaning. Oxford, UK: Westview Press.   pp. 27–8

External links

• NOVA: Physics of Stone Arches
• InteractiveTHRUST
• DIYinfo.org's Constructing Brick Arches Wiki - A wiki on how to construct brick arches around the house
• DIYinfo.org's Constructing Timber Framed Arches Wiki - Similar to the brick arches but extra information for timber arches