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Last edited 08 Oct 2019
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, stabilised 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.
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 centre 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 'centres' (sometimes called 'centreing' 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.
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 favoured 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 arch are described below.
Also known as a semi-circular arch, this is formed in a continuous curve and was developed by the Romans. They were often used side by side in a series to create an arcade. An adaptation is the rampant round arch which has unequal lengths of support on either side.
This is an arch that has a rise that is less than a semi-circle. In a flatter form, segmental arches were commonly used for bridges as larger spans are possible without excessively increasing height. Since the flatter the arch gets the more thrust is delivered sideways to the abutments, there bridges require large abutments at either side.
This was a form of pointed arch that was developed during the Gothic period. It was often used for windows and roof structures in churches and cathedrals. The arch is tall and narrow with a pointed apex.
Also from the Gothic period, equilateral arches were often used for decorative entrances and windows. The two springing points and the crown of the intrados form an equilateral triangle, meaning that each curve has a chord length equal to the span.
Also known as a jack arch, a camber arch is similar to a lintel in that it is flat, or almost flat, in profile, however, the voussoirs use their compressive strength in the same way as a regular arch.
A parabolic arch follows the principle that when there is a uniformly applied load from above, the internal compression that results will follow a parabolic curve. Parabolic arches produce the most thrust at the base, but can span the greatest distance, and so are commonly used in bridge design.
A catenary arch looks very similar to a parabola, but is slightly more 'flat' at the bottom, and rises faster than the parabola. The catenary is the solution to a differential equation that describes a shape that directs the force of its own weight along its own curve, so that, if hanging, it is pulled into that shape, and if standing upright it can support itself. The parabola does not have the same property, but is the solution of other important equations that describe other situations.
 Related articles on Designing Buildings Wiki
- Barrel vault.
- Blind arch.
- Bridge construction.
- Compressive strength.
- Gateway Arch.
- Hyperbolic paraboloid.
- Long span roof.
- Optimal arch bridge.
- Portal frame.
- Shell roof.
- Types of ceiling.
- Types of dome.
- Types of structural load.
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