A dome (from Latin: domus) is an architectural element that resembles the hollow upper half of a sphere. The precise definition has been a matter of controversy. There are also a wide variety of forms and specialized terms to describe them. A dome can rest upon a rotunda or drum, and can be supported by columns or piers that transition to the dome through squinches or pendentives. A lantern may cover an oculus and may itself have another dome.

Domes have a long architectural lineage that extends back into prehistory and they have been constructed from mud, snow, stone, wood, brick, concrete, metal, glass, and plastic over the centuries. The symbolism associated with domes includes mortuary, celestial, and governmental traditions that have likewise developed over time.

Domes have been found from early Mesopotamia, which may explain the form’s spread. They are found in Persian, Hellenistic, Roman, and Chinese architecture in the Ancient world, as well as among a number of contemporary indigenous building traditions. Dome structures were popular in Byzantine and medieval Islamic architecture, and there are numerous examples from Western Europe in the Middle Ages. The Renaissance architectural style spread from Italy in the Early modern period. Advancements in mathematics, materials, and production techniques since that time resulted in new dome types. The domes of the modern world can be found over religious buildings, legislative chambers, sports stadiums, and a variety of functional structures.

Etymology
The English word “dome” ultimately derives from the Latin domus (“house”)—which, up through the Renaissance, labeled a revered house, such as a Domus Dei, or “House of God”, regardless of the shape of its roof. This is reflected in the uses of the Italian word duomo, the German/Icelandic/Danish word dom (“cathedral”), and the English word dome as late as 1656, when it meant a “Town-House, Guild-Hall, State-House, and Meeting-House in a city.” The French word dosme came to acquire the meaning of a cupola vault, specifically, by 1660. This French definition gradually became the standard usage of the English dome in the eighteenth century as many of the most impressive Houses of God were built with monumental domes, and in response to the scientific need for more technical terms.

Terminology
A dome is a rounded vault made of either curved segments or a shell of revolution, meaning an arch rotated around its central vertical axis. The terminology used has been a source of controversy, with inconsistency between scholars and even within individual texts, but the term “dome” may be considered a “blanket-word to describe an hemispherical or similar spanning element.” A half-dome or semi-dome is a semi-circular shape often used, especially in apses.

Sometimes called “false” domes, corbel domes achieve their shape by extending each horizontal layer of stones inward slightly farther than the lower one until they meet at the top. A “false” dome may also refer to a wooden dome. “True” domes are said to be those whose structure is in a state of compression, with constituent elements of wedge-shaped voussoirs, the joints of which align with a central point. The validity of this is unclear, as domes built underground with corbelled stone layers are also in compression from the surrounding earth. The Italian use of the term finto, meaning “false”, can be traced back to the 17th century in the use of vaulting made of reed mats and gypsum mortar.

As with arches, the “springing” of a dome is the level from which the dome rises. The top of a dome is the “crown”. The inner side of a dome is called the “intrados” and the outer side is called the “extrados”. The “haunch” is the part of an arch that lies roughly halfway between the base and the top.

The word “cupola” is another word for “dome”, and is usually used for a small dome upon a roof or turret. “Cupola” has also been used to describe the inner side of a dome.

Drums, also called tholobates, are cylindrical or polygonal walls with or without windows that support a dome. A tambour or lantern is the equivalent structure over a dome’s oculus, supporting a cupola.

Characteristics

Internal forces
A masonry dome produces thrusts down and outward. They are thought of in terms of two kinds of forces at right angles from one another. Meridional forces (like the meridians, or lines of longitude, on a globe) are compressive only, and increase towards the base, while hoop forces (like the lines of latitude on a globe) are in compression at the top and tension at the base, with the transition in a hemispherical dome occurring at an angle of 51.8 degrees from the top. The thrusts generated by a dome are directly proportional to the weight of its materials. Grounded hemispherical domes generate significant horizontal thrusts at their haunches.

Unlike voussoir arches, which require support for each element until the keystone is in place, domes are stable during construction as each level is made a complete and self-supporting ring. The upper portion of a masonry dome is always in compression and is supported laterally, so it does not collapse except as a whole unit and a range of deviations from the ideal in this shallow upper cap are equally stable. Because voussoir domes have lateral support, they can be made much thinner than corresponding arches of the same span. For example, a hemispherical dome can be 2.5 times thinner than a semicircular arch, and a dome with the profile of an equilateral arch can be thinner still.

The optimal shape for a masonry dome of equal thickness provides for perfect compression, with none of the tension or bending forces against which masonry is weak. For a particular material, the optimal dome geometry is called the funicular surface, the comparable shape in three dimensions to a catenary curve for a two-dimensional arch. The pointed profiles of many Gothic domes more closely approximate this optimal shape than do hemispheres, which were favored by Roman and Byzantine architects due to the circle being considered the most perfect of forms. Adding a weight to the top of the pointed dome, such as the heavy cupola at the top of Florence Cathedral, changes the optimal shape to perfectly match the actual pointed shape of the dome.

The outward thrusts in the lower portion of a hemispherical masonry dome can be counteracted with the use of chains incorporated around the circumference or with external buttressing, although cracking along the meridians is natural. For small or tall domes with less horizontal thrust, the thickness of the supporting arches or walls can be enough to resist deformation, which is why drums tend to be much thicker than the domes they support.

Zone of transition
When the base of the dome does not match the plan of the supporting walls beneath it (for example, a dome’s circular base over a square bay), techniques are employed to bridge the two. The simplest technique is to use diagonal lintels across the corners of the walls to create an octagonal base. Another is to use arches to span the corners, which can support more weight. A variety of these techniques use what are called “squinches”. A squinch can be a single arch or a set of multiple projecting nested arches placed diagonally over an internal corner. Squinches can take a variety of other forms, as well, including trumpet arches and niche heads, or half-domes.

The invention of pendentives superseded the squinch technique. Pendentives are triangular sections of a sphere, like concave spandrels between arches, and transition from the corners of a square bay to the circular base of a dome. The curvature of the pendentives is that of a sphere with a diameter equal to the diagonal of the square bay. The precise definition of “pendentive” has been a source of contention among academics, including whether or not corbelling is permitted under the definition and whether or not the lower portions of a sail vault should be considered pendentives.

Domes with pendentives can be divided into two kinds: simple and compound. In the case of the simple dome, the pendentives are part of the same sphere as the dome itself; however, such domes are rare. In the case of the more common compound dome, the pendentives are part of the surface of a larger sphere below that of the dome itself and form a circular base for either the dome or a drum section.

Acoustics
Because domes are concave from below, they can reflect sound and create echoes. A dome may have a “whispering gallery” at its base that at certain places transmits distinct sound to other distant places in the gallery. The half-domes over the apses of Byzantine churches helped to project the chants of the clergy. Although this can complement music, it may make speech less intelligible, leading Francesco Giorgi in 1535 to recommend vaulted ceilings for the choir areas of a church, but a flat ceiling filled with as many coffers as possible for where preaching would occur.

Cavities in the form of jars built into the inner surface of a dome may serve to compensate for this interference by diffusing sound in all directions, eliminating echoes while creating a “divine effect in the atmosphere of worship.” This technique was written about by Vitruvius in his Ten Books on Architecture, which describes bronze and earthenware resonators. The material, shape, contents, and placement of these cavity resonators determine the effect they have: reinforcing certain frequencies or absorbing them.

Materials
The earliest domes in the Middle East were built with mud-brick and, eventually, with baked brick and stone. Domes of wood allowed for wide spans due to the relatively light and flexible nature of the material and were the normal method for domed churches by the 7th century, although most domes were built with the other less flexible materials. Wooden domes were protected from the weather by roofing, such as copper or lead sheeting. Domes of cut stone were more expensive and never as large, and timber was used for large spans where brick was unavailable.

Roman concrete used an aggregate of stone with a powerful mortar. The aggregate transitioned over the centuries to pieces of fired clay, then to Roman bricks. By the sixth century, bricks with large amounts of mortar were the principle vaulting materials. Pozzolana appears to have only been used in central Italy. Brick domes were the favored choice for large-space monumental coverings until the Industrial Age, due to their convenience and dependability. Ties and chains of iron or wood could be used to resist stresses.

The new building materials of the 19th century and a better understanding of the forces within structures from the 20th century has opened up new possibilities. Iron and steel beams, steel cables, and pre-stressed concrete have eliminated the need for external buttressing and enabled far thinner domes. Whereas earlier masonry domes may have had a radius to thickness ratio of 50, the ratio for modern domes can be in excess of 800. The lighter weight of these domes has not only permitted far greater spans, but also allowed for the creation of large movable domes over modern sports stadiums.

Experimental rammed earth domes were made as part of work on sustainable architecture at the University of Kassel in 1983.

Symbolism
According to E. Baldwin Smith, from the late Stone Age the dome-shaped tomb was used as a reproduction of the ancestral, god-given shelter made permanent as a venerated home of the dead. The instinctive desire to do this resulted in widespread domical mortuary traditions across the ancient world, from the stupas of India to the tholos tombs of Iberia. By Hellenistic and Roman times, the domical tholos had become the customary cemetery symbol.

Domes and tent-canopies were also associated with the heavens in Ancient Persia and the Hellenistic-Roman world. A dome over a square base reflected the geometric symbolism of those shapes. The circle represented perfection, eternity, and the heavens. The square represented the earth. An octagon was intermediate between the two. The distinct symbolism of the heavenly or cosmic tent stemming from the royal audience tents of Achaemenid and Indian rulers was adopted by Roman rulers in imitation of Alexander the Great, becoming the imperial baldachin. This probably began with Nero, whose “Golden House” also made the dome an essential feature of palace architecture.

The dual sepulchral and heavenly symbolism was adopted by early Christians in both the use of domes in architecture and in the ciborium, a domical canopy like the baldachin used as a ritual covering for relics or the church altar. The celestial symbolism of the dome, however, was the preeminent one by the Christian era. In the early centuries of Islam, domes were closely associated with royalty. A dome built in front of the mihrab of a mosque, for example, was at least initially meant to emphasize the place of a prince during royal ceremonies. Over time such domes became primarily focal points for decoration or the direction of prayer. The use of domes in mausoleums can likewise reflect royal patronage or be seen as representing the honor and prestige that domes symbolized, rather than having any specific funerary meaning. The wide variety of dome forms in medieval Islam reflected dynastic, religious, and social differences as much as practical building considerations.

General types

Beehive dome
Also called a corbelled dome, or false dome, these are different from a ‘true dome’ in that they consist of purely horizontal layers. As the layers get higher, each is slightly cantilevered, or corbeled, toward the center until meeting at the top. A monumental example is the Mycenaean Treasury of Atreus from the late Bronze Age.

Braced dome
A single layer space frame in the form of a dome, a braced dome is a generic term that includes ribbed, Schwedler, three-way grid, lamella or Kiewitt, lattice, and geodesic domes. The different terms reflect different arrangements in the surface members. Braced domes often have a very low weight and are usually used to cover spans of up to 150 meters.

Cloister vault
Called domical vaults (a term sometimes also applied to sail vaults), polygonal domes, coved domes, gored domes, segmental domes (a term sometimes also used for saucer domes), paneled vaults, or pavilion vaults, these are domes that maintain a polygonal shape in their horizontal cross section. The earliest known examples date to the first century BC, such as the Tabularium of Rome from 78 BC. Others include the Baths of Antoninus in Carthage (145–160) and the Palatine Chapel at Aachen (13th – 14th century). The most famous example is the Renaissance octagonal dome of Filippo Brunelleschi over the Florence Cathedral. Thomas Jefferson, the third president of the United States, installed an octagonal dome above the West front of his plantation house, Monticello.

Compound dome
Also called domes on pendentives or pendentive domes (a term also applied to sail vaults), compound domes have pendentives that support a smaller diameter dome immediately above them, as in the Hagia Sophia, or a drum and dome, as in many Renaissance and post-Renaissance domes, with both forms resulting in greater height.

Crossed-arch dome
One of the earliest types of ribbed vault, the first known examples are found in the Great Mosque of Córdoba in the 10th century. Rather than meeting in the center of the dome, the ribs characteristically intersect one another off-center, forming an empty polygonal space in the center. Geometry is a key element of the designs, with the octagon being perhaps the most popular shape used. Whether the arches are structural or purely decorative remains a matter of debate. The type may have an eastern origin, although the issue is also unsettled. Examples are found in Spain, North Africa, Armenia, Iran, France, and Italy.

Geodesic dome
Geodesic domes are the upper portion of geodesic spheres. They are composed of a framework of triangles in a polyhedron pattern. The structures are named for geodesics and are based upon geometric shapes such as icosahedrons, octahedrons or tetrahedrons. Such domes can be created using a limited number of simple elements and joints and efficiently resolve a domes internal forces. Their efficiency is said to increase with size. Although not first invented by Buckminster Fuller, they are associated with him because he designed many geodesic domes and patented them in the United States.

Hemispherical dome
The hemispherical dome is half of a sphere. According to E. Baldwin Smith, it was a shape likely known to the Assyrians, defined by Greek theoretical mathematicians, and standardized by Roman builders.

Onion dome
Bulbous domes bulge out beyond their base diameters, offering a profile greater than a hemisphere. An onion dome is a greater than hemispherical dome with a pointed top in an ogee profile. They are found in the Near East, Middle East, Persia, and India and may not have had a single point of origin. Their appearance in northern Russian architecture predates the Tatar occupation of Russia and so is not easily explained as the result of that influence. They became popular in the second half of the 15th century in the Low Countries of Northern Europe, possibly inspired by the finials of minarets in Egypt and Syria, and developed in the 16th and 17th centuries in the Netherlands before spreading to Germany, becoming a popular element of the baroque architecture of Central Europe. German bulbous domes were also influenced by Russian and Eastern European domes. The examples found in various European architectural styles are typically wooden. Examples include Kazan Church in Kolomenskoye and the Brighton Pavilion by John Nash. In Islamic architecture, they are typically made of masonry, rather than timber, with the thick and heavy bulging portion serving to buttress against the tendency of masonry domes to spread at their bases. The Taj Mahal is a famous example.

Oval dome
An oval dome is a dome of oval shape in plan, profile, or both. The term comes from the Latin ovum, meaning “egg”. The earliest oval domes were used by convenience in corbelled stone huts as rounded but geometrically undefined coverings, and the first examples in Asia Minor date to around 4000 B.C. The geometry was eventually defined using combinations of circular arcs, transitioning at points of tangency. If the Romans created oval domes, it was only in exceptional circumstances. The Roman foundations of the oval plan Church of St. Gereon in Cologne point to a possible example. Domes in the Middle Ages also tended to be circular, though the church of Santo Tomás de las Ollas in Spain has an oval dome over its oval plan. Other examples of medieval oval domes can be found covering rectangular bays in churches. Oval plan churches became a type in the Renaissance and popular in the Baroque style. The dome built for the basilica of Vicoforte by Francesco Gallo was one of the largest and most complex ever made.

Parabolic dome
A parabolic dome is a unique structure in which bending stress due to the uniformly distributed load of its dead load is zero. Hence it was widely used in buildings in ancient times, before the advent of composite structures. However, if a point load is applied on the apex of a parabolic dome, the bending stress becomes infinite. Hence it is found in most ancient structures, the apex of the dome is stiffened or the shape modified to avoid the infinite stress.

Sail dome
Also called sail vaults, handkerchief vaults, domical vaults (a term sometimes also applied to cloister vaults), pendentive domes (a term that has also been applied to compound domes), Bohemian vaults, or Byzantine domes, this type can be thought of as pendentives that, rather than merely touching each other to form a circular base for a drum or compound dome, smoothly continue their curvature to form the dome itself. The dome gives the impression of a square sail pinned down at each corner and billowing upward. These can also be thought of as saucer domes upon pendentives.

Saucer dome
Also called segmental domes (a term sometimes also used for cloister vaults), or calottes, these have profiles of less than half a circle. Because they reduce the portion of the dome in tension, these domes are strong but have increased radial thrust. Many of the largest existing domes are of this shape.

Masonry saucer domes, because they exist entirely in compression, can be built much thinner than other dome shapes without becoming unstable. The trade-off between the proportionately increased horizontal thrust at their abutments and their decreased weight and quantity of materials may make them more economical, but they are more vulnerable to damage from movement in their supports.

Umbrella dome
Also called gadrooned, fluted, organ-piped, pumpkin, melon, ribbed, parachute, scalloped, or lobed domes, these are a type of dome divided at the base into curved segments, which follow the curve of the elevation. “Fluted” may refer specifically to this pattern as an external feature, such as was common in Mamluk Egypt. The “ribs” of a dome are the radial lines of masonry that extend from the crown down to the springing. The central dome of the Hagia Sophia uses the ribbed method, which accommodates a ring of windows between the ribs at the base of the dome. The central dome of St. Peter’s Basilica also uses this method.

Source From Wikipedia