An arch is a curved structure that spans a space and may or may not support weight above it.〔"arch, n. 2" ''Oxford English Dictionary'' 2nd ed. 2009.〕 ''Arch'' may be synonymous with ''vault'' but vault may be distinguished as a ''continuous arch''〔"vault, n. 2." ''The Century Dictionary and Cyclopedia'' Dwight Whitney, ed.. vol. 10. New York. 1911. 6707. Print.〕 forming a roof. Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture〔(【引用サイトリンク】url=http://oi.uchicago.edu/OI/MUS/ED/TRC/MESO/architecture.html )〕 and their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.
An arch is a ''pure compression'' form. It can span a large area by resolving forces into compressive stresses and, in turn eliminating tensile stresses. This is sometimes referred to as ''arch action''. As the forces in the arch are carried to the ground, the arch will push outward at the base, called ''thrust''. As the ''rise'', or height of the arch decreases, the outward thrust increases. In order to maintain arch action and prevent the arch from collapsing, the thrust needs to be restrained, either with internal ties, or external bracing, such as abutments.
抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』
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