Corbel arch

A corbel arch (or corbeled / corbelled arch) is an arch-like construction method that uses the architectural technique of corbeling to span a space or void in a structure, such as an entranceway in a wall or as the span of a bridge. A corbel vault uses this technique to support the superstructure of a building's roof. A corbel arch is constructed by offsetting successive horizontal courses of stone (or brick) beginning at the springline of the walls (the point at which the walls break off from verticality to form an arc toward the apex at the archway's center) so that they project towards the archway's center from each supporting side, until the courses meet at the apex of the archway (often, the last gap is bridged with a flat stone). For a corbeled vault covering, the technique is extended in three dimensions along the lengths of two opposing walls. Although an improvement in load-bearing efficiency over the post and lintel design, corbeled arches are not entirely self-supporting structures, and the corbeled arch is sometimes termed a false arch for this reason. Different from "true" arches, "false" or corbelled arches are built of horizontally laid stones or bricks, not of wedge-shaped voussoirs converging towards, and being held together by a central keystone. Unlike "true" arches, not all of the structure's tensile stresses caused by the weight of the superstructure are transformed into compressive stresses. Corbel arches and vaults require significantly thickened walls and an abutment of other stone or fill to counteract the effects of gravity, which otherwise would tend to collapse each side of the archway inwards. Some arches use a stepped style, keeping the block faces rectangular, while other form or select them to give the arch smooth edges, usually with a pointed shape. Corbelling is a technique first applied by the ancient Egyptians and Chaldeans. The Newgrange passage tomb, built sometime between 3200 and 2500 BC during the Neolithic period, has an intact corbel arch (vault) supporting the roof of the main chamber.