Counter-arch

Historically, the term counter-arch was used in architecture to describe multiple types of arches that provide opposing action:

• an inverted arch used opposite of a regular one. For example, an inverted arch in an open spandrel or in "Moseley bridges", a popular American Civil War-era design by Thomas William Moseley, where the counter-arches were intended as a low-cost alternative to diagonal bracing;
• any relieving arch;
• outer "rings" of compound arches overlaying the one forming the intrados, used in old English bridges since medieval times, are called "counter-arches" following the works of John Smeaton;
• an arch that is built adjacent to another arch to oppose its forces or help stabilize it. The counter-arch can be used, for example, when constructing the flying buttress,
• buttressing arches built between the opposing building facades over narrow streets of old cities;
• in fortification, an arch built on the tops of counterforts behind the bastion walls intended to limit the scope of the potential wall breaching;
• when a pier of the Old Westminster Bridge started sinking during the construction, Charles Labelye was forced to retrofit the bridge with open spandrels using the counter-arches springing off haunches of the two adjacent arches of the bridge thus relieving the pier.

Image:Enneus-heermabrug.jpg|The Enneüs Heerma Bridge showing the inverted counter arch in the middle File:Die Votivkirche in Wien; Denkschrift des Baucomit'es veröffentlicht zur Feier der Einweihung am 24. April 1879 (1879) (14597612677).jpg|Drawing of a Neo-Gothic flying buttress for the late 19th-century Votive Church, Vienna File:Arches in the Old Town - panoramio.jpg|Buttressing counter-arches in Bonifacio, Corsica File:Counter-arched revetment.jpg|Counter-arched wall of a bastion (the voids are usually filled) File:Old Westminster Bridge, open spandrel.png|Old Westminster Bridge, with open spandrel File:Engraving of Old Ouse Bridge.jpeg|An old Ouse Bridge, York with middle arch using three rings (two "counter-arches")