Useful as the barrel vault is, it has several limitations. It exerts a continuous load, necessitating some form of continuous support. Except at its two ends, it is difficult to light the space beneath it. Although it lends itself to unlimited length, increases in width are achieved only with increased thickness of the vault itself and its support. In any case, rergardless of the ratio of length to width, it has only one axis, which contributes to certain desired architectural effects but prohibits others.
The limitations of the barrel vault were overcome by creating a new vaulting type from it-the cross vault. It is formed by interesecting two barrel vaults at right angles and is often called a groin vault, from the groinlike lines along the intersections. The weight of the cross vault is concentrated at the corner, eliminating the need for continuous supports; it has two axes, and the opening of the space in four directions means ample fenestration as well. Its only limitation is its resistance to plans other than the square, but even this can be solved by manipulating the geometry of the vaulting curvature or by extending vaulting arms in the form of supplementary barrel vaults. One other difficulty is a technical one-keeping the groin lines from veering away from precise paths during construction, for visually the success of the cross vault depends on the crispness of the groins. The ultimate solution to this problem is rib vaulting. At this point, it is sufficient to stress the virtues of the cross vault-its economy, openess, and clarity of form-which made it an ever-present aspect of Roman architecture.
The cross vault was employed by the Romans on a great scale and with grandeur in practically all monumental building types. But the grandest type of vault was still another: the dome. It is possible to build a domelike structure in the form of a cross vault-by crossing barrel vaults over an octagonal plan, an eight-sided dome results, often called a cloister vault. This form was common in the medieval period but rare in the Roman era. Romans preferred the true dome, or hemisphere, with its fully rounded perfection. Structurally, the hemisphereic dome may be thought of in two ways: as an arch rotated 360 degrees or as a set of horizontal arched rings culminating in a single "keystone" at the crown. The dome functions both ways, in fact; its vouissoirs are trapezoidal in three dimensions rather than two, as in an arch. Yet, the dome is built up in the later mode- in complete rings; each ring, when finished, forms a self-supporting component of the final dome, a kind of horizontal circular arch.
The arch was one of mankind's greatest discoveries. It enabled architects to span gaps with economy and strength and to enclose vast interiors with assurance. The arch was to architecture what the wheel was to transportation and industry. Just as the wheel reached its heyday in the "machine age" that lasted from the eighteenth to the mid-twentieth century-now being superseded by a second industrial age of electronics, atomic energy, and chemistry-so the masonry arch dominated a lond era that ended a century ago with the widespread introduction of a new architectural technology. Today, it would be difficult for a builder to find a mason anywhere in the Western world capable of building a sizable masonry arch. The art years, has been all but lost. But for centuries, the highest technical skills of the architectural craft were devoted to erecting arches and vaults.
TERMS:
FAN VAULT: In English Gothic architecture, a vault of conoidal form, wherein the ribs have spread out from the sprining point in a way that suggests the appearance of an open fan.
GROIN VAULT: A vault formed when two barrel vaults of identical size intersect at right angles (also known as a cross vault)
CLOISTER VAULT: A dome that rises directly from a square or polygonal base, its curved surface divided into sections by groins or ribs.
CORBELED ARCH: Masonry constructed over a wall opening by a series of courses projecting from each side and stepped progressively further until they meet at midpoint; not a true arch.
CORBEL: A masonry block projecting from a wall to support a superincumbent element. (like a stepping effect- i.e. upside down stairs)
DOME: A curved vault that is erected on a circular base and that is semicircular , pointed or bulbous in section. If raised over a square or polygonal base transitional squinches or pendentives must be inserted at the corners ot the base to transform it into a near circle.
PENDENTIVE: an inverted, concave, triangular piece of masonry serving as the transition from a square support system to the circular base of a dome.
SQUINCH: A small arch, or sometimes a lintel, thrown across the angle of a square or polygon to make them more nearly round and thus able to receive the circular base of a dome or drum.
SPRINGING: The point from which an arch or vault springs or rises from its supports.
DOMICAL VAULT: A rib or groin vault whose center rises domelike above its peripheral levels.
DRUM: The cylindrical or polygonal wall supports.
EXTRADOS: the upper surface of an arch or vault.
INTRADOS: the undersurface (as opposed to the extrados) of an arch (or vault).
KEYSONE: The central vouissor at the top of a completed arch.
