Editing Arch bridge (section)

== Simple compression arch bridges ==
=== Advantages of simple materials ===
[[File:MonroeStreetBridgea.jpg|thumb|550px|Falsework [[centering]] in the center arch of [[Monroe Street Bridge (Spokane River)|Monroe Street Bridge]], [[Spokane, Washington]]. 1911.]]
[[File:Vanha kivisilta DSC08273 C.JPG|thumb|The old stone arch bridge over the [[Kerava River]] in [[Kerava]], [[Finland]]]]
Stone, brick and other such materials are strong in [[compression (physical)|compression]] and somewhat so in [[shearing (physics)|shear]], but cannot resist much force in [[tension (mechanics)|tension]]. As a result, masonry arch bridges are designed to be constantly under compression, so far as is possible. Each arch is constructed over a temporary [[falsework]] frame, known as a [[centring]]. In the first compression arch bridges, a [[Keystone (architecture)|keystone]] in the middle of the bridge bore the weight of the rest of the bridge. The more weight that was put onto the bridge, the stronger its structure became. Masonry arch bridges use a quantity of fill material (typically compacted rubble) above the arch in order to increase this dead-weight on the bridge and prevent tension from occurring in the arch ring as loads move across the bridge. Other materials that were used to build this type of bridge were brick and unreinforced concrete. When masonry (cut stone) is used the angles of the faces are cut to minimize shear forces. Where random masonry (uncut and unprepared stones) is used they are mortared together and the mortar is allowed to set before the falsework is removed.

Traditional masonry arches are generally durable, and somewhat resistant to [[settlement (construction)|settlement]] or undermining. However, relative to modern alternatives, such bridges are very heavy, requiring extensive [[foundation (architecture)|foundations]]. They are also expensive to build wherever labor costs are high.

===Construction sequence===
[[File:Limyra Bridge Workflow.gif|thumb|300px|Workflow on the Roman [[Limyra Bridge|Bridge at Limyra]]: the falsework was moved to another opening as soon as the lower arch rib had been completed ]]
[[File:Limyra Bridge Arch.svg|thumb|300px|The Roman segmental arch [[Bridge at Limyra]]]]

* Where the arches are founded in a watercourse bed (on piers or banks) the water is diverted so the gravel can first be excavated and replaced with a good footing (of strong material). From these, the foundation [[pier (architecture)|piers]] are erected/raised to the height of the intended base of the arches, a point known as the '''springing'''.
* Falsework [[centering]] (in British English: arch frame) is fabricated, typically from timbers and boards. Since each arch of a multi-arch bridge will impose a thrust upon its neighbors, it is necessary either that all arches of the bridge be raised at the same time, or that very wide piers be used. The thrust from the end arches is taken into the earth by substantial (vertical) footings at the canyon walls, or by large inclined planes forming in a sense ramps to the bridge, which may also be formed of arches.
* The several arches are (or single arch is) constructed over the centering. Once each basic '''arch barrel''' is constructed, the arches are (or arch is) stabilized with infill masonry above, which may be laid in horizontal [[running bond]] courses (layers). These may form two outer walls, known as the '''spandrels''', which are then infilled with appropriate loose material and rubble.
* The road is paved and [[parapet]] walls protectively confine traffic to the bridge.