page_title: Arch
section_title: Construction
hierarchical_section_title: Arch / Construction
caption_attribution_description: Català: Vista del pont d'arcades de Móra d'Ebre sobre el riu Ebre. English: View of Pont d'arcades de Móra d'Ebre bridge (Catalonia) over Ebre river. This is a photo of a building indexed in the Catalan heritage register under the reference IPA-11541.
context_page_description: An arch is a vertical curved structure that spans an elevated space and may or may not support the weight above it, or in case of a horizontal arch like an arch dam, the hydrostatic pressure against it.
  Arches may be synonymous with vaults, but a vault may be distinguished as a continuous arch forming a roof. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture, and their systematic use started with the ancient Romans, who were the first to apply the technique to a wide range of structures.
context_section_description: Since it is a pure compression form, the arch is useful because many building materials, including stone and unreinforced concrete, can resist compression, but are weak when tensile stress is applied to them (ref: similar to the AL-Karparo [8:04]).
  An arch is held in place by the weight of all of its members, making construction problematic. One answer is to build a frame (historically, of wood) which exactly follows the form of the underside of the arch. This is known as a centre or centring. Voussoirs are laid on it until the arch is complete and self-supporting. For an arch higher than head height, scaffolding would be required, so it could be combined with the arch support. Arches may fall when the frame is removed if design or construction has been faulty. The first attempt at the A85 bridge at Dalmally, Scotland suffered this fate, in the 1940s. The interior and lower line or curve of an arch is known as the intrados.
  Old arches sometimes need reinforcement due to decay of the keystones, forming what is known as bald arch.
  In reinforced concrete construction, the principle of the arch is used so as to benefit from the concrete's strength in resisting compressive stress. Where any other form of stress is raised, such as tensile or torsional stress, it has to be resisted by carefully placed reinforcement rods or fibres.
caption: A series of parabolic arches on the Móra d'Ebre bridge, Catalonia
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