Vault. A vault is a ceiling or roof, traditionally of stone or brick, that depends on the principle of the arch. It may be executed in various geometrical shapes depending, to some extent, on the shape of the area to be covered. The arrangement of the blocks (bond) is determined by whether the vault is to be built with the aid of a temporary wooden support (centering) or without it. Since the timber necessary for centering is not readily available everywhere (it was scarce, for example, in Eastern regions but not in the West), the bond used in the East was different from that used in the West.
The simplest form of vault is the barrel vault, known in Egypt from the architecture of the Old Kingdom. In shape, it is similar to a horizontally placed cylinder sectioned along its axis. It may be executed with or without centering. The longitudinal barrel bond built up of horizontal courses (layers) of blocks laid parallel with the imposts (top of the wall) is very stable but requires centering. The bond of vertical or, preferably, slightly canted (tilted) ring courses, which arch from one wall to the other, does not need centering. It is this method that was used most frequently in Egyptian vault constructions (Clarke, 1912, pp. 26-27).
Because constructing a barrel vault around a corner is possible only with centering, this type of vault was avoided as far as possible in Eastern architecture. If it was unavoidable, however, intermediate arches were introduced, or the vaulting surfaces were set at different heights. In one of the corridors in the big keep of Dayr Anba Hadra, the external corner of the corridor bend was covered with diagonally spanned ring courses, forming a kind of squinch (diagonal arch). On its frontal side leaned rampant ring courses springing from the inner corner of the corridor (Grossmann, 1982, pp. 245-46, pl. 36).
The most important derivations of the barrel vault are the groin vault (or cross vault) and the cloister vault (or domical vault). These forms are created by the intersection of two barrel vaults at right angles. In the groin vault the sections of the generating barrel vault that lie outside the edges of intersection are retained, so that the sharp edges (groins) appear from the underside. In the cloister vault only sections of the barrel vault within the edges of intersection are retained, so the edges appear as valleys. These types of vault can be executed only with the help of centering. Since both types of vault consist of surfaces that are curved only in one plane, the centering is simply made of straight wooden boards. Because these vault types need centering, they must both be considered typically Western construction types. In the East they have been constructed only in special cases. The cloister vault executed in ring courses without centering gained a certain currency in the East. In this construction, squinchlike structures executed in vertical ring courses were built
diagonally over all four corners and joined together in a dovetail over the middle of the walls. In this way, a bellylike curvature was created over the corners instead of the sharp edges of the valleys. This type of vault is best designated a pseudo-cloister vault. (The formerly more common name, squinch vault, does not do justice to the peculiarities of the construction; Grossmann, 1982, pp. 246-50.) Such vaults were first archaeologically documented in Iran; they existed, however, in Egypt as well, from the late Roman period.
The sail vault consists of a truly spherical dome shell whose diameter equals more or less the diagonal of the area to be covered and which, in a sense, is suspended between the supporting vertical side walls. The sail vault may be executed either in horizontal ring courses (most frequently used in wide spans) or in vertical ring courses and dovetail joints over the corners. It does not require centering. In order to achieve a flawless spherical curvature for the dome shell, a rotating template fixed to the center of the curvature is used to indicate the position of each stone or brick (Clarke, 1912, pp. 29-31; Grossmann, 1982, pp. 250-57). Once a ring course has been closed, the construction is self-supporting. In the West, especially during the Roman imperial period, the dome was most frequently executed not of stone or brick but of a different, usually cruder material, a sort of concrete made of rubble and waste materials. This material required centering, but since only straight wooden boards were available to make it, it difficult to achieve a flawless spherical curvature. Consequently, domes were often imperfect in the West.
The dome whose radius of curvature equals half the distance between facing supporting walls can just as easily be executed in a free bond without centering. The dome shell itself is produced, like the sail vault, with the help of a rotating template. Only the center of curvature must be set higher. Nevertheless, difficulties are posed by the corners of the square area that the dome is to cover, since the circular base of the dome does not extend to the corners. In order to solve this problem, two fundamentally different structural designs have been developed. The simpler and older method is to bridge the corners with beams or squinches (ring courses of increasing radius) creating four more points of support, in addition to those at the middle of the walls. Many domed churches in the Nile Valley are built with squinches.
The other solution is the pendentive, a cantilevered construction with a spherically curved surface and a triangular shape. A pendentive requires construction materials with load-carrying
capacity such as stone blocks or Roman concrete, but not the mud brick used in the East. It is, therefore, no coincidence that the pendentive dome was developed and almost exclusively used in Byzantine areas of the East. In Egypt such domes are quite rare and occur only over small areas.
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