page_title: Skew arch
section_title: Peter Nicholson's helicoidal method in stone
hierarchical_section_title: Skew arch / History / The helicoidal skew arch / Peter Nicholson's helicoidal method in stone
caption_attribution_description: English: Engraved portrait of Peter Nicholson (1765-1844), architect and engineer
context_page_description: A skew 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 Giovanni Barbara, William Chapman, Benjamin Outram, Peter Nicholson, George Stephenson, Edward Sang, Charles Fox, George W. Buck and William Froude.
context_section_description: In his book A Popular and Practical Treatise on Masonry and Stone-cutting (1828), Scottish architect, mathematician, cabinet-maker and engineer Peter Nicholson first set out in clear and understandable terms a workable method for determining the shape and position of the stones required for the construction of a strong skew arch that enabled them to be prepared in advance of the actual construction process.
  Nicholson approached the problem by constructing a development of the intrados of the arch from the plan and elevation drawings, effectively unrolling and flattening the surface, then drawing the courses perpendicular to the faces, adding the header joints perpendicular to the courses, then finally rolling up the development diagram by projecting the detail of the intrados back onto the plan and elevation drawings, a technique also used by others who would later offer alternative solutions to the problem. This method resulted in the courses of stone voussoirs making up the barrel of the skew arch following parallel helical paths between the abutments, giving the view along the barrel an attractive rifled appearance. Although these courses meet the arch faces at right angles at the crown of the arch, the nearer they are to the springing line the greater their deviation from perpendicularity. Thus Nicholson's method is not the perfect solution, but it is a workable one that has one great advantage over more purist alternatives, namely that since the helical courses run parallel to each other, all the voussoir stones can be cut to the same pattern, the only exceptions being the ring stones, or quoins, where the barrel meets the faces of the arch, each of which is unique but has an identical copy in the other face.
  Nicholson never pretended to have invented the skew arch but in his later work The Guide to Railway Masonry, containing a Complete Treatise on the Oblique Arch (1839), he does claim to have invented the method for producing the templates that enabled the accurate cutting of the voussoir stones used in all skew bridges built between the years 1828 and 1836, citing testimonials from the builders of major works, such as the Croft Viaduct at Croft-on-Tees near Darlington. However, by 1836 a young engineer called Charles Fox had improved on Nicholson's helicoidal method and other writers were proposing alternative approaches to the problem.
caption: Peter Nicholson (1765–1844)
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