Editing Arch (section)

=== Arch action ===
[[File:Arch action English.png|thumb|Arch (A) action diagram in comparison with a beam (B)]]
A true arch, due to its rise, resolves the vertical loads into horizontal and vertical reactions at the ends, a so called '''arch action'''. The vertical load produces a positive [[bending moment]] in the arch, while the inward-directed horizontal reaction from the spandrel/abutment provides a counterbalancing negative moment. As a result, the bending moment in any segment of the arch is much smaller than in a beam with the equivalent load and span.{{sfn | Au | 1960 | p=169}} The diagram on the right shows the difference between a loaded arch and a beam. Elements of the arch are mostly subject to compression (A), while in the beam a bending moment is present, with compression at the top and tension at the bottom (B).

In the past, when arches were made of masonry pieces, the horizontal forces at the ends of an arch (so called '''thrust'''{{sfn|Calvo-López|2020|p=8}}) caused the need for heavy abutments (cf. [[Roman triumphal arch]]). The other way to counteract the forces, and thus allow thinner supports, was to use the [[counter-arch]]es, as in an [[Arcade (architecture)|arcade]] arrangement, where the horizontal thrust of each arch is counterbalanced by its neighbors, and only the end arches need to [[buttressed]]. With new construction materials (steel, concrete, [[engineered wood]]), not only the arches themselves got lighter, but the horizontal thrust can be further relieved by a [[Tie (engineering)|tie]] connecting the ends of an arch ([[bowstring arch]]).<ref name=britannica>{{Britannica|32510}}</ref>