VAULT

in building construction, a structural member consisting of an arrangement of arches, usually forming a ceiling or roof. The basic barrel form, which appeared first in ancient Egypt and the Middle East, is in effect a continuous series of arches deep enough to cover a three-dimensional space. It exerts the same kind of thrust as the circular arch and must be buttressed along its entire length by heavy walls with limited openings. Roman architects discovered that two barrel vaults that intersected at right angles formed a groin vault which, when repeated in series, could span rectangular areas of unlimited length. Because the groin vault's thrusts are concentrated at the four corners, its supporting walls need not be massive and require buttressing only where they support the vault. The groin vault, however, requires great precision in stone cutting, an art that declined in the West with the fall of Rome. Vaulting was continued and improved in the Byzantine Empire and in the Islamic world. Medieval European builders developed a modification, the rib vault, a skeleton of arches or ribs on which the masonry could be laid. The medieval mason used pointed arches; unlike round arches, these could be raised as high over a short span as over a long one. To cover rectangular areas, the mason used two intersecting vaults of different widths but of the same height. Using 19th-century materials, builders could construct large iron skeletons as frameworks for vaults of lightweight materialsfor example, the glass-vaulted Crystal Palace of the 1851 Exhibition in London. Because the new materials eliminated weight and thrust problems, the simple barrel vault returned to favour for such structures as railroad terminals and exhibition halls. In many modern frame systems the vault has lost its functional significance and become a thin skin laid over a series of arches. The reinforced-concrete shell vault, a bent or molded slab, is one of the most important innovations. The steel-reinforced shell exerts no lateral thrust and may be supported as if it were a beam.