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Vault (architecture)
From Wikipedia, the free encyclopedia
A Vault (French. voute, Italian. volta,) is an architectural term for an arched form used to provide a space with a ceiling or roof.[1] The parts of a vault exert a thrust that require a counter resistance. When vaults are built underground, the ground gives all the resistance required. However, when the vault is built above ground, various replacements are employed to supply the needed resistance. An example are the thicker walls used in the case of barrel or continuous vaults. Buttresses are used to supply resistance when intersecting vaults are employed.
The simplest kind of vault is the barrel vault (also called a wagon or tunnel vault) which is generally semicircular in shape. The barrel vault is a continuous arch, the length being greater than its diameter. As in building an arch, a temporary support is needed while rings of voussoirs are constructed and the rings placed in position. Until the topmost voussoir, the keystone, is positioned the vault is not self-supporting. Where timber is easily obtained, this temporary support is provided by centering consisting of a framed truss with a semicircular or segmental head, which supports the voussoirs until the ring of the whole arch is completed. With a barrel vault, the temporary support is then shifted on to support the next rings. In earlier times, particularly in Chaldaea and Egypt where timber was scarce, other means of support had to be contrived. Apparently only in Roman times was centering regularly employed.[citation needed] From the Roman Architectural Revolution onwards, vaults along with arches and domes, became a regular and technically demanding architectural form for shaping interior large spaces.
Vault types
Dome
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Amongst the earliest known examples of any form of vaulting is to be found in the neolithic village of Khirokitia on Cyprus. Dating from ca. 6000 BCE, the circular buildings supported beehive shaped corbel domed vaults of unfired mud-bricks and also represent the first evidence for settlements with an upper floor. Similar Beehive tombs, called tholoi, exist in Crete and Northern Iraq. Their construction differs from that at Khirokitia in that most appear partially buried and make provision for a dromos entry.
The inclusion of domes, however, represents a wider sense of the word vault. The distinction between the two is that a vault is essentially an arch which is extruded into the third dimension, whereas a dome is an arch revolved around its vertical axis.
Barrel vault
A barrel vault is the simplest form of a vault and resembles a barrel or tunnel cut lengthwise in half. The effect is that of a structure composed of continuous semicircular or pointed sections.[2] The earliest known examples of barrel vaults were built by the Sumerians, possibly under the ziggurat at Nippur in Babylonia,[3] which was built of fired bricks cemented with clay mortar.[4] The earliest barrel vaults in Egypt are thought to be those in the granaries built by Ramesses II, the ruins of which are behind the Ramesseum, at Thebes.[4][5][6][7] The span was 12 ft (3.6m). and the lower part of the arch was built in horizontal courses, up to about one-third of the height, and the rings above were inclined back at a slight angle, so that the bricks of each ring, laid flatwise, adhered till the ring was completed, no centering of any kind being required; the vault thus formed was elliptic in section, arising from the method of its construction. A similar system of construction was employed for the vault over the great hall at Ctesiphon, where the' material employed was fired bricks or tiles of great dimensions, cemented with mortar; but the span was close upon 83 ft (25m), and the thickness of the vault was nearly 5 ft (1.5m) at the top, there being four rings of brickwork.[4]
It is probable that the great vaults of the Assyrian palaces were constructed in the same way, but with unburnt bricks dried only in the sun: one of the drains discovered by Sir Austen Henry Layard at Nimrud was built in rings sloping backwards. From the fact that each Assyrian monarch on his accession to the throne commenced his reign by the erection of a palace, it is probable that, owing to the ephemeral construction of these great vaults, half a century was the term of their existence. This may also account for the fact that no domed structures exist of the type shown in one of the bas-reliefs from Nimrud; the tradition of their erection, however, would seem to have been handed down to their successors in Mesopotamia, viz. to the Sassanians, who in their palaces in Serbia and Firouzabad built domes of similar form to those shown in the Nimrud sculptures, the chief difference being that, constructed in rubble stone and cemented with mortar, they still exist, though probably abandoned on the Islamic invasion in the 7th century.
In all the instances above quoted in Sumeria and Egypt the bricks, whether burnt or sun-dried, were of the description to which the term "tile" would now be given; the dimensions varied from 18 or 20 in. to 10 in., being generally square and about 4 to 2 in. thick, and they were not shaped as voussoirs, the connecting medium being thicker at the top than at the bottom. The earliest Egyptian examples of regular voussoirs in stone belong to the XXVIth Dynasty (ca. 650 B.C.) in the additions made then to the temple of Medinet Habu, and here it is probable that centering of some kind was provided, as the vaults are built in rings, so that the same centering could be shifted on after the completion of each ring. The earliest example of regularly shaped voussoirs, and of about the same date, is found in the cloaca at Graviscae in Etruria, with a span of about 14 ft., the voussoirs of which are from 5 to 6 ft. long. The cloaca maxima in Rome, built by Lucius Tarquinius Priscus (603 B.C.) to drain the marshy ground between the Palatine and the Capitoline Hills, was according to Commendatore Boni vaulted over in the 1st century B.C., the vault being over 800 ft. long, 10 ft. in span, with three concentric rings of voussoirs.
The enormous Eyvan-e Khosro at Ctesiphon (near present-day Baghdad) was built during the Persian Sasanian period as a throne room. The arch was about 37 meters high, 26 meters across and 50 meters long, entirely without centering and it was the largest vault ever constructed at the time.[8]
Groin vaults
So far, all the vaults mentioned have been barrel vaults, which, when not built underground, required continuous walls of great thickness to resist their thrust; the earliest example of the next variety, the intersecting barrel vault, is said to be over a small hall at Pergamum, in Asia Minor, but its first employment over halls of great dimensions is due to the Romans. When two semicircular barrel vaults of the same diameter cross one another their intersection (a true ellipse) is known as a groin, down which the thrust of the vault is carried to the cross walls; if a series of two or more barrel vaults intersect one another, the weight is carried on to the piers at their intersection and the thrust is transmitted to the outer cross walls; thus in the Roman reservoir at Baiae, known as the Piscina Mirabilis, a series of five aisles with semicircular barrel vaults are intersected by twelve cross aisles, the vaults being carried on 48 piers and thick external walls. The width of these aisles being only about 13 ft. there was no great difficulty in the construction of these vaults, but in the Roman Baths of Caracalla the tepidarium had a span of 80 ft., more than twice that of an English cathedral, so that its construction both from the statical and economical point of view was of the greatest importance.[4][9]