  an arch is a structure that spans a space and supports structure and weight below it. 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. ==basic concepts== 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. ===fixed arch vs hinged arch===  the most common true arch configurations are the fixed arch, the two-hinged arch, and the three-hinged arch. :*the fixed arch is most often used in reinforced concrete bridge and tunnel con
Arch_bridges Arches_and_vaults