Аркалар: Құрылыс және Архитектуралық Элементтер

Curved structure that spans a space and may support a load
the architectural construct

An arch is a curved vertical structure spanning an open space underneath it. Arch can either support the load above it or perform a purely decorative role. The arch dates back to fourth millennium BC, but became popular only after its adoption by the Romans in the 4th century BC. Arch like structures can be horizontal, like an arch dam that withstands the horizontal hydrostatic pressure load. Arches are normally 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. Arches had several advantages over the lintel, especially in the masonry construction: with the same amount of material it can have larger span, carry more weight, and can be made from smaller and thus more manageable pieces. Extrados (an external surface of the arch)
Impost is block at the base of the arch (the voussoir immediately above the impost is a springer). The tops of imposts define the springing level. A portion of the arch between the springing level and the crown (centered around the 45° angle) is called a haunch. If the arch resides on top of a column, the impost is formed by an abacus or its thicker version, dosseret. 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 triangular shaped portion of the wall between the extrados and the horizontal division above is called spandrel. 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). 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".

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 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.

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 like 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.

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, or two adjacent side aisles. Two tiered arches, with two arches superimposed, were sometimes used in Islamic architecture, mostly for decorative purposes. 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. Alternatively, the opening can be filled with smaller arches, producing a containing arch, common in Gothic and Romanesque architecture. Multiple arches can be superimposed with an offset, creating an interlaced series of usually (with some exceptions) blind and decorative arches. Most likely of Islamic origin, the interlaced arcades were popular in Romanesque and Gothic architecture. Rear arch (also rere arch) is the one that frames the internal side of an opening in the external wall.

Structurally, relieving arches (often blind or containing) can be used to take off load from some portions of the building (for example, to allow use of thinner exterior walls with larger window openings, or, as in the Roman Pantheon, to redirect the weight of the upper structures to particular strong points). Transverse arches, introduced in Carolingian architecture, are placed across the nave to compartmentalize (together with longitudinal separating arches) the internal space into bays and support vaults. A diaphragm arch similarly goes in the transverse direction, but carries a section of wall on top. It is used to support or divide sections of the high roof. Strainer arches were built as an afterthought to prevent two adjacent supports from imploding due to miscalculation. Frequently they were made very decorative, with one of the best examples provided by the Wells Cathedral. 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.

The large variety of arch shapes (left) can mostly be classified into three broad categories: rounded, pointed, and parabolic.

"Round" semicircular arches were commonly used for ancient arches that were constructed of heavy masonry, and were relied heavily on by the Roman builders since the 4th century BC. It is considered to be the most common arch form, characteristic for Roman, Romanesque, and Renaissance architecture. The advantages of a pointed arch over a semicircular one are flexible ratio of span to rise and lower horizontal reaction at the base. This innovation allowed for taller and more closely spaced openings, which are typical of Gothic architecture. Equilateral arch is the most common form of the pointed arch, with the centers of two circles forming the intrados coinciding with the springing points of the opposite segment. Together with the apex point, they form a equilateral triangle, thus the name. If the centers of circles are farther apart, the arch becomes a narrower and sharper lancet arch that appeared in France in the Early Gothic architecture (Saint Denis Abbey) and became prominent in England in the late 12th and early 13th centuries (Salisbury Cathedral). If the centers are closer to another, the result is a wider blunt arch. The intrados of the cusped arch (also known as multifoil arch, polyfoil arch, polylobed arch, and scalloped arch) includes several independent circle segments in a scalloped arrangement. These primarily decorative arches are common in Islamic architecture and Northern European Late Gothic, can be found in Romanesque architecture. A similar trefoil arch includes only three segments and sometimes has a rounded, not pointed, top. Common in Islamic architecture and Romanesque buildings influenced by it, it later became popular in the decorative motifs of the Late Gothic designs of Northern Europe. Each arc of an ogee arch consists of at least two circle segments (for a total of at least four), with the center of an upper circle being outside the extrados. After European appearance in the 13th century on the facade of the St Mark's Basilica, the arch became a fixture of the English Decorated style, French Flamboyant, Venetian, and other Late Gothic styles. Ogee arch is also known as reversed curve arch, occasionally also called an inverted arch. The top of an ogee arch sometimes projects beyond the wall, forming the so called nodding ogee popular in 14th century England (pulpitum in Southwell Minster). Each arc of a four centred arch is made of two circle segments with distinct centers; usually the radius used closer to the springing point is smaller with a more pronounced curvature. Common in Islamic architecture (Persian arch), and, with upper portion flattened almost to straight lines (Tudor arch), in the English Perpendicular Gothic. A keel arch is a variant of four centred arch with haunches almost straight, resembling a section view of a capsized ship. Popular in Islamic architecture, it can be also found in Europe, occasionally with a small ogee element at the top, so it is sometimes considered to be a variation of an ogee arch. Curtain arch (also known as inflexed arch, and, like the keel arch, usually decorative) is not curved. Instead, the arch is flat in profile and can be used under the same circumstances as lintel. However, lintels are subject to bending stress, while the flat arches are true arches, composed of irregular voussoir shapes (the keystone is the only one of the symmetric wedge shape), and that efficiently uses the compressive strength of the masonry in the same manner as a curved arch and thus requires a mass of masonry on both sides to absorb the considerable lateral thrust. Used in the Roman architecture to imitate the Greek lintels, Islamic architecture, European medieval and Renaissance architecture. The flat arch is still being used as a decorative pattern, primarily at the top of window openings.

The corbel (also corbelled) arch, made of two corbels meeting in the middle of the span, is a true arch in a sense of being able to carry a load, but it is false in a structural sense, as its components are subject to bending stress. The typical profile is not curved, but has triangular shape. Invented prior to the semicircular arch, the corbel arch was used already in the Egyptian and Mycenaean architecture in the 3rd and 2nd millennium BC. Like a corbel arch, the triangular arch is not a true arch in a structural sense. Its intrados is formed by two slabs leaning against each other. Brick builders would call triangular any arch with straight inclined sides. The design was common in Anglo Saxon England until the late 11th century (St Mary Goslany). Mayan corbel arches are sometimes called triangular due to their shape.

Few transformations can be applied to arch shapes. If one impost is much higher than another, the arch (frequently pointed) is known as lt=ramping arch, raking arch, or rampant arch (from ). Originally used to support inclined structures, like staircases, in the 13th 14th centuries their appeared as parts of flying buttresses used to counteract the thrust of Gothic ribbed vaults. ), the springing line is located above the imposts (on "stilts"). Known to Islamic architects by the 8th century, the technique was utilized to vertically align the apexes of arches of different dimensions in Romanesque and Gothic architecture. Stilting was useful for semicircular arches, where the ratio of the rise fixed at 1/2 of the span, but was applied to the pointed arches, too. The skew arch (also known as an oblique arch) is used when the arch needs to turn in the horizontal plane, for example, when a bridge crosses the river at an angle different than 90°. A splayed arch is used for the case of unequal spans on the sides of the arch (when, for example, an interior opening in the wall is larger than the exterior one), the intrados of a round splayed arch is not cylindrical, but has a conical shape. A wide arch with its rise less than 1/2 of the span (and thus the geometric circle of at least one segment is below the springing line) is called a surbased arch (sometimes also a depressed arch). A drop arch is either a basket handle arch or a blunt arch.

The practical arch bridges are built either as a fixed arch, a two hinged arch, or a three hinged arch. thus allowing the structure to move freely and compensate for the thermal expansion and contraction that changes in outdoor temperature cause. However, this can result in additional stresses, and therefore the two hinged arch is also statically indeterminate, although not as much as the fixed arch. It is most often used for spans of medial length, such as those of roofs of large buildings. Another advantage of the three hinged arch is that the reaction of the pinned bases is more predictable than the one for the fixed arch, allowing shallow, bearing type foundations in spans of medial length. 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," which further simplifies foundational design. An example of the latter would be the Nippur arch, built before 3800 BC, and dated by H. V. Hilprecht (1859–1925) to even before 4000 BC. Rare exceptions are an arched mudbrick home doorway dated to c. 2000 BC from Tell Taya in Iraq and two Bronze Age arched Canaanite city gates, one at Ashkelon (dated to c. 1850 BC), and one at Tel Dan (dated to c. 1750 BC), both in modern day Israel. An Elamite tomb dated 1500 BC from Haft Teppe contains a parabolic vault which is considered one of the earliest evidences of arches in Iran. The use of true arches in Egypt also originated in the 4th millennium BC (underground barrel vaults at the Dendera cemetery). Standing arches were known since at least the Third Dynasty, but very few examples survived, since the arches were mostly used in non durable secular buildings and made of mud brick voussoirs that were not wedge shaped, but simply held in place by mortar, and thus susceptible to a collapse (the oldest arch still standing is at Ramesseum). Sacred buildings exhibited either lintel design or corbelled arches. Arches were mostly missing in Egypt temples even after the Roman conquest, even though Egyptians thought of the arch as a spiritual shape and used it in the rock cut tombs and portable shrines. Auguste Mariette suggested that this choice was based on a relative fragility of a vault: "what would remain of the tombs and temples of Egyptians today, if they had preferred the vault?" This architectural tradition was continued by the Sasanian Empire (224–651), which built the Taq Kasra at Ctesiphon in the 6th century AD, the largest free standing vault until modern times. An early European example of a voussoir arch appears in the 4th century BC Greek Rhodes Footbridge. Proto true arches can also be found under the stairs of the temple of Apollo at Didyma and the stadium at Olympia. .

Ancient architecture of China (and Japan) used mostly timber framed construction and trabeated system. Arches were little used, although there are few arch bridges known from literature and one artistic depiction in stone carved relief. Since the only surviving artefacts of architecture from the Han dynasty (202 BC – 220 AD) are rammed earth defensive walls and towers, ceramic roof tiles from no longer existent wooden buildings, stone gate towers, and underground brick tombs, the known vaults, domes, and archways were built with the support of the earth and were not free standing. China's oldest surviving stone arch bridge is the Anji Bridge. Still in use, it was built between 595 CE and 605 CE during the Sui dynasty.

Islamic architects adopted the Roman arches, but had quickly shown their resourcefulness: by the 8th century the simple semicircular arch was almost entirely replaced with fancier shapes, few fine examples of the former in the Umayyad architecture notwithstanding (cf. the Great Mosque of Damascus, 706–715 CE). The first pointed arches appear already at the end of the 7th century AD (Al Aqsa Mosque, Palace of Ukhaidhir, cisterns at the White Mosque of Ramle). Their variations spread fast and wide: Mosque of Ibn Tulun in Cairo (876 879 AD), Nizamiyya Madrasa at Khar Gerd (now Iran, 11th century), Kongo Mosque in Diani Beach (Kenya, 16th century). Islamic architecture brought to life a large amount of arch forms: the round horseshoe arch that became a characteristic trait of the Islamic buildings, the keel arch, the cusped arch, and the mixed line arch (where the curved "ogee swell" is interspersed with abrupt bends). The Great Mosque of Cordoba, that can be considered a catalogue of Islamic arches, contains also the arches with almost straight sides, trefoil, interlaced, and joggled. Mosque of Ibn Tulun adds four centred and stilted version of the pointed arch. It is quite likely that the appearance of the pointed arch, an essential element of the Gothic style, in Europe (Monte Cassino, 1066–1071 AD, and the Cluny Abbey five years later) and the ogee arch in Venice ( 1250) is a result of the Islamic influence, possibly through Sicily. Saoud also credits to Islamic architects the spread of the transverse arch. Mixed line arch became popular in the Mudéjar style and subsequently spread around the Spanish speaking world.

The collapse of the Western Roman Empire left the church as the only client of major construction; with all pre Romanesque architectural styles borrowing from Roman construction with its semicircular arch. Due to the decline in the construction quality, the walls were thicker, and the arches thus heavier, than their Roman prototypes. Eventually the architects started to use the depth of the arches for decoration, turning the deep opening into an arch order (or rebated arch, a sequence of progressively smaller concentric arches, each inset with a rebate). Romanesque style started experiments with the pointed arch late in the 11th century (Cluny Abbey). In few decades, the practice spread (Durham Cathedral, Basilica of Saint Denis). Early Gothic utilized the flexibility of the pointed arch by grouping together arches of different spans but with the same height. While the arches used in the mediaeval Europe were borrowed from the Roman and Islamic architecture, the use of pointed arch to form the rib vault was novel and became the defining characteristic of Gothic construction. At about 1400 AD, the city states of Italy, where the pointed arch had never gotten much traction, initiated the revival of the Roman style with its round arches, Renaissance. By the 16th century the new style spread across Europe and, through the influence of empires, to the rest of the world. Arch became a dominant architectural form until the introduction of the new construction materials, like steel and concrete.

The history of arch in India is very long (some arches were apparently found in excavations of Kosambi, 2nd millennium BC. However, the continuous history begins with rock cut arches in the Lomas Rishi cave (3rd century BC). Vaulted roof of an early Harappan burial chamber has been noted at Rakhigarhi. S. R Rao reports vaulted roof of a small chamber in a house from Lothal. Barrel vaults were also used in the Late Harappan Cemetery H culture dated 1900 BC 1300 BC which formed the roof of the metal working furnace, the discovery was made by Vats in 1940 during excavation at Harappa. The use of arches until the Islamic conquest of India in the 12th century AD was sporadic, with ogee arches and barrel vaults in rock cut temples (Karla Caves, from the 1st century BC) and decorative pointed gavaksha arches. By the 5th century AD voussoir vaults were used structurally in the brick construction. Surviving examples include the temple at Bhitargaon (5th century AD) and Mahabodhi temple (7th century AD), the latter has both pointed arches and semicircular arches. These Gupta era arch vault system was later used extensively in Burmese Buddhist temples in Pyu and Bagan in 11th and 12th centuries. With the arrival of Islamic and other Western Asia influence, the arches became prominent in the Indian architecture, although the post and lintel construction was still preferred. A variety of pointed and lobed arches was characteristic for the Indo Islamic architecture, with the monumental example of Buland Darwaza, that has pointed arch decorated with small cusped arches.

Mayan architecture utilized the corbel arches. The other Mesoamerican cultures used only the flat roofs with no arches whatsoever, although some researchers had suggested that both Maya and Aztec architects understood the concept of a true arch.

The 19th century introduction of the wrought iron (and later steel) into construction changed the role of the arch. Due to the high tensile strength of new materials, relatively long lintels became possible, as was demonstrated by the tubular Britannia Bridge (Robert Stephenson, 1846 1850). A fervent proponent of the trabeated system, Alexander "Greek" Thomson, whose preference for lintels was originally based on aesthetic criteria, observed that the spans of this bridge are longer than that of any arch ever built, thus "the simple, unsophisticated stone lintel contains in its structure all the scientific appliances [ ] used in the great tubular bridge. [ ] Stonehenge is more scientifically constructed than York Minster." Use of arches in bridge construction continued (the Britannia Bridge was rebuilt in 1972 as a truss arch bridge), yet the steel frames and reinforced concrete frames mostly replaced the arches as the load bearing elements in buildings.

As a pure compression form, the utility of the arch is due to many building materials, including stone and unreinforced concrete, being strong under compression, but brittle when tensile stress is applied to them.

The voussoirs can be wedge shaped or have a form of a rectangular cuboid, in the latter case the wedge like shape is provided by the mortar. 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. Voussoirs are laid on it until the arch is complete and self supporting. For an arch higher than head height, scaffolding would be required, so it could be combined with the arch support. Arches may fall when the frame is removed if design or construction has been faulty. 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.

The type of arches (or absence of them) is one of the most prominent characteristics of an architectural style. For example, when Heinrich Hübsch, in the 19th century, tried to classify the architectural style, his "primary elements" were roof and supports, with the top level basic types: trabeated (no arches) and arcuated (arch based). His next division for the arcuated styles was based on the use of round and pointed arch shapes.

The steady horizontal push of an arch against the abutments gave rise to a saying "the arch never sleeps", attributed to many sources, from Hindu to Arabs. This adage stresses that the arch carries "a seed of death" for itself and the structure containing it, a statement that can be made upon observation of the Roman ruins. The plot of The Nebuly Coat by J. Meade Falkner, inspired by a collapse of a tower at the Chichester Cathedral plays with the idea while dealing with the slow disintegration of a church building. Saoud explains the proverb by chain like self balancing of the horizontal and vertical forces in the arch and its "universal adaptability".