Characteristics
A dome can be thought of as an arch which has been rotated around its central vertical axis. Thus domes, like arches, have a great deal of structural strength when properly built and can span large open spaces without interior supports. Corbel domes achieve their shape by extending each horizontal layer of stones inward slightly farther than the previous, lower, one until they meet at the top. These are sometimes called false domes. True, or real, domes are formed with increasingly inward-angled layers of voussoirs which have ultimately turned 90 degrees from the base of the dome to the top.
When the base of the dome does not match the plan of the supporting walls beneath it (for example, a circular dome on a square bay), techniques are employed to transition between the two. The simplest technique is to use diagonal lintels across the corners of the walls to create an octagonal base. Another is to use arches called squinchs to span the corners, which can support more weight. The invention of pendentives superseded the squinch technique. Pendentives are triangular sections of a sphere used to transition from the flat surfaces of supporting walls to the round base of a dome.
Domes can be divided into two kinds: simple and compound, depending on the use of pendentives. In the case of the simple dome, the pendentives are part of the same sphere as the dome itself; however, such domes are rare. In the case of the more common compound dome, the pendentives are part of the surface of a larger sphere below that of the dome itself and form a circular base for either the dome or a drum section.
Drums, also called tholobates or tambours, are cylindrical or polygonal walls supporting a dome which may contain windows.
Domes have been constructed from a wide variety of building materials over the centuries: from mud to stone, wood, brick, concrete, metal, glass and plastic.
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