An arch is a curved structural form that carries loads around an opening, transferring them around the profile of the arch to abutments, jambs, or piers on either side. Arches have been a prominent feature in architecture since the time of the Etruscans who are credited with its invention, although the Romans developed it further and spread its use. The techniques involved in designing and constructing arches have since developed into many other structural forms, including vaults, arcades, and bridges.
Arches are compressive structures, that is, there are no tensile stresses. They are self-supporting, stabilized by the force of gravity acting on their weight to hold them in compression. This makes them very stable and efficient, capable of larger spans and supporting greater loads than horizontal beams.
The downward load of an arch must be transferred to its foundations. The outward thrust exerted by an arch at its base must be restrained, either by its own weight or the weight of supporting walls, by buttressing or foundations, or by an opposing tie between the two sides. The outward thrust increases as the height, or rise, of the arch decreases.
Arch construction
Since many building materials, such as masonry and concrete, can resist compression, arches are commonly constructed using these types of materials. The construction of traditional masonry arches is dependent on the arrangement of the bricks, blocks, or stones over the opening. Wedge-shaped blocks, called voussoirs, are set flank-to-flank with the upper edge being wider than the lower edge. Downward pressure on the arch has the effect of forcing the voussoirs together instead of apart. The voussoir that is positioned in the center of the arch is known as the keystone.
This arrangement means that the arch is self-supporting, but temporary supports from below, usually in the form of timber ‘centers’ (sometimes called ‘centering’ or ‘arch formers’), must be provided until the keystone has been set in place. The interior, lower curve of the arch is known as the intrados. The exterior, upper curve of the arch is known as the extrados. The spring, or springing line, is the point from which the arch starts to rise from its vertical supports.
Types of arch
Most arches are circular, pointed, or parabolic, however, there are a great many variations of these basic forms that have developed during different periods. Ancient Roman architects favored rounded arches, whereas Gothic architects preferred pointed arches and in this respect may have been influenced by Islamic architecture. Some of the most common types of the arch are below.
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