拱顶(vault)

房屋建造中用拱作成的天花板或屋顶。源於古埃及和近东的筒形拱顶，实际上是一系列连续的拱。它具有和圆拱同样的外推力，因而必须沿着全长用厚墙作为扶垛，墙上不能开很大的门窗。古罗马的建筑师发现两个相互正交的筒形拱组成一个十字拱顶，如果重复地连续建造，可以覆盖任意长度的矩形面积。由於十字拱顶的推力集中在四个角上，不需要很厚的墙来支承。中世纪发展了一种肋拱顶，即由拱组成骨架，再在其上砌砖石。19世纪时采用铁骨架建造轻质材料的大跨度拱顶（水晶宫）。在现代框架结构中，拱顶已失去其结构意义，钢筋混凝土薄壳顶是一种曲面或其他形式的板，是结构技术上的重大革新。

English version：

vault

In building construction, an arched structure forming a ceiling or roof. The masonry vault exerts the same kind of thrust as the arch, and must be supported along its entire length by heavy walls with limited openings. The basic barrel vault, in effect a continuous series of arches, first appeared in ancient Egypt and the Middle East. Roman architects discovered that two barrel vaults intersecting at right angles (a groin vault) could, when repeated in series, 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. Medieval European builders developed the rib vault, a skeleton of arches or ribs on which the masonry could be laid. The fan vault, popular in the English Perpendicular style, used fan-shaped clusters of tracery-like ribs springing from pendants or columns. The 19th century saw the use of large iron skeletons as frameworks for vaults of lightweight materials (see Crystal Palace). An important modern innovation is the reinforced-concrete shell vault, which, if its length is three or more times its transverse section, behaves as a deep beam and exerts no lateral thrust.