Counter-arch

Historically, the term counter-arch was used in architecture to describe multiple types of arches that provide opposing action:^[1][2]

• 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;^[3]
• 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"^[4] following the works of John Smeaton;
• an arch that is built adjacent to another arch to oppose its forces or help stabilize it.^[5] The counter-arch can be used, for example, when constructing the flying buttress,^[6]
• buttressing arches built between the opposing building facades over narrow streets of old cities;^[7][8]
• in fortification, an arch built on the tops of counterforts behind the bastion walls intended to limit the scope of the potential wall breaching;^[9]
• 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.^[10]

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  The Enneüs Heerma Bridge showing the inverted counter arch in the middle^[11]
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  Drawing of a Neo-Gothic flying buttress for the late 19th-century Votive Church, Vienna
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  Buttressing counter-arches in Bonifacio, Corsica
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  Counter-arched wall of a bastion (the voids are usually filled)
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  Old Westminster Bridge, with open spandrel
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  An old Ouse Bridge, York with middle arch using three rings (two "counter-arches")^[12]