Skew arch

A skew arch (also known as an oblique arch) is a method of construction that enables an arch bridge to span an obstacle at some angle other than a right angle. This results in the faces of the arch not being perpendicular to its abutments and its plan view being a parallelogram, rather than the rectangle that is the plan view of a regular, or "square" arch.

In the case of a masonry skew arch, the construction requires precise stonecutting, as the cuts do not form right angles, but once the principles were fully understood in the early 19th century, it became considerably easier and cheaper to build a skew arch of brick.

The problem of building skew arch masonry bridges was addressed by a number of early civil engineers and mathematicians, including William Chapman (1787), Benjamin Outram (1798), Peter Nicholson (1828), George Stephenson (1830), Edward Sang (1835), Charles Fox (1836), George W. Buck (1839) and William Froude (c. 1844).

Skew bridges are not a recent invention, having been built on exceptional occasions since Roman times, but they were little understood and rarely used before the advent of the railway. One notable exception is an aqueduct, designed by British engineer Benjamin Outram, constructed in masonry and completed in 1798, which still carries the Ashton Canal at an angle of 45° over Store Street in Manchester. Outram's design is believed to be based on work done on the Kildare Canal in Ireland in 1787, in which William Chapman introduced the segmental oblique arch to the design of Finlay Bridge at Naas, employing an arch barrel based on a circular segment that is smaller than a semicircle and which was repeated by Thomas Storey in 1830 in the bridge carrying the Haggerleases branch of the over the River Gaunless near Cockfield, County Durham with a skew angle of 63° and a skew span of 42 feet (13 m), resulting in a clear span of 18 feet (5.5 m) and a rise of 7 feet (2.1 m). The common method they all used was to clad the timber centring (also known as falsework) with planks, known as "laggings", laid parallel with the abutments and carefully planed and levelled to approximate closely the required curve of the intrados of the arch. The positions of the courses in the vicinity of the crown were first marked out at right angles to the faces using long wooden straight-edges, then the remaining courses were marked out in parallel. The masons then laid the stones, cutting them to shape as required.

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