|The simplest form of arch is the round-headed or semi-circular
design. The form-work off which it is built is easily set out, and thrust
against the abutments is minimised. However, a round-headed arch has large
spandrels, so requires more material than other arch forms. This also means
that the bridge weighs more and will need stronger footings. The curve of the
arch gives limited clearance at the side, so round-headed arches are not
suitable for low bridges.
Both of these bridges are a considerable height above the road, so clearance under the arch is not a problem
Peters Finger Road bridge, on the Romsey line near Salisbury, has quite a low round-headed arch. It bears the scars of regular collisions with road vehicles. A bend in the road makes this bridge particularly vulnerable.
Viaducts often have round-headed arches, because it is helpful to reduce lateral thrust.
Round-headed arches are often found spanning deep cuttings. These two bridges cross the Brighton line between Coulsdon and Merstham. Joliffe Road bridge has a middle arch over the tracks and side arches over the cutting slopes. It also has relieving arches piercing the bridge piers, as a means of reducing the number of bricks used and, therefore, the weight and cost of the structure. Three-arch bridges are quite commonly found crossing cuttings. The huge and impressive single arch bridge at Hooley is less usual and arches of this size are rare. There is a second bridge very like this one at Hooley, but it has been subject to significant modification, with pavements cantilevered out from it, and a pipe bridge is immediately adjacent.
Accommodation bridges, allowing access between fields on either side of the railway, and those for minor lanes, did not require significant headroom, so were often round-headed arches. The arch at Marden is exceptionally small and has given its name to a new street of houses nearby, Barrel Arch Close.
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This page was created 3 January 2010