Arch
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, 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. Their role in construction was diminished in the middle of the 19th century with introduction of wrought iron : 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 held together by compression. In much of the world introduction of the true arch was a result of European influence. 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, with no precisely defined boundary, is called a crown
- Voussoir. 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
- Impost is block at the base of the arch. The tops of imposts define the springing level. A portion of the arch between the springing level and the crown 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
- Rise
- Clear span
- Abutment The roughly triangular-shaped portion of the wall between the extrados and the horizontal division above is called spandrel.
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, while in the beam a bending moment is present, with compression at the top and tension at the bottom.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. 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, not only the arches themselves got lighter, but the horizontal thrust can be further relieved by a tie connecting the ends of an 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 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 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, 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 can be calculated or modeled, and as long as this curve stays within the confines of the voussoirs, the construction will be stable.
Classifications
There are multiple ways to classify arches:- [|by the geometrical shape] of its intrados ;
- for the arches with rounded intrados, by the number of circle segments forming the arch ;
- [|by the material used] and construction approach. For example, the wedge-shaped voussoirs of a brick arch can be made by cutting the regular bricks or manufactured in the wedge shape ;
- structurally, by the number of hinges between solid components. For example, voussoirs in a stone arch should not move, so these arches usually have no hinges. Permitting some movement in a large structure allows to alleviate stresses, so many bridge spans are built with three hinges since the mid-19th century.
Arrangements
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 blind and decorative arches. Most likely of Islamic origin, the interlaced arcades were popular in Romanesque and Gothic architecture. Rear-arch is the one that frames the internal side of an opening in the external wall.
Structural
Structurally, relieving arches can be used to take off load from some portions of the building. Transverse arches, introduced in Carolingian architecture, are placed across the nave to compartmentalize 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" 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.Shapes
The large variety of arch shapes can mostly be classified into three broad categories: rounded, pointed, and parabolic.Rounded
"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.A segmental arch, with a rounded shape that is less than a semicircle, is very old. Since then it was occasionally used in Greek temples, utilized in Roman residential construction, Islamic architecture, and got popular as window pediments during the Renaissance.
A basket-handle arch consists of segments of three circles with origins at three different centers. Was used in late Gothic and Baroque architecture.
A horseshoe arch has a rounded shape that includes more than a semicircle, is associated with Islamic architecture and was known in areas of Europe with Islamic influence. Occasionally used in Gothics, it briefly enjoyed popularity as the entrance door treatment in the interwar England.
In a Florentine arch, the distance between intrados and extrados grows closer to the apex. It can combine rounded intrados with pointed extrados.
Pointed
A pointed arch consists of two or more circle segments culminating in a point at the top. It originated in the Islamic architecture, arrived in Europe in the second half of the 11th century and later became prominent in the Gothic 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 an 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 and became prominent in England in the late 12th and early 13th centuries. If the centers are closer to another, the result is a wider blunt arch.The intrados of the cusped 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, 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.
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, and, with upper portion flattened almost to straight lines, 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 uses two drooping curves that join at the apex. Utilized as a dressing for windows and doors primarily in Saxony in the Late Gothic and early Renaissance buildings, associated with. When the intrados has multiple concave segments, the arch is also called a draped arch or tented arch. A similar arch that uses a mixture of curved and straight segments or exhibits sharp turns between segments is a mixed-line arch. In Moorish architecture the mixed-line arch evolved into an ornate lambrequin arch, also known as muqarnas arch.