Skew Bridges: History
Skew, or oblique arch bridges became an unavoidable necessity with the coming of canals and railways in the Industrial Revolution. Before that, when bridges were only built for roads and pedestrian use, it was usually much easier to align the bridge to cross the waterway normally (i.e. at right angles).
This does not imply that skew bridges only appeared in the late 18th century. Much earlier examples are sometimes encountered, but they were built with the voussoirs laid in courses parallel with the abutments, in the same way as common non-skewed arches. This was not ideal, and limited their use to small arches.
There are main types of construction of skew masonry arches:-
- False skew arch: The coursing joints are parallel to the springing.
- Helicoidal, or English type: The courses - as shown by the joints between the faces of the blocks seen underneath the arches - follow a helical path. This arrangement is far superior to the 'false' type, and is by far the most commonly used arrangement. William Chapman (1749-1832) is credited with its introduction. The arrangement of the masonry would become more apparent with a 'developed' view of the inderside (the soffit) of the arch. A developed view in this context refers to a drawing with the underside 'unrolled' or 'flattened out'. In this view the end faces of the walls would have a sinusoidal form. The courses of stone blocks (or bricks) would be shown lying parallel to each other, set at an angle. This angle is the angle at which the blocks start out (spring) from the abutment. At the highly visible faces of the bridge, where masonry blocks meet the end face of the arch, it is normal to have the cut the faces at an angle, except at the crown of the arch, where the helical courses mean that the ends can be square to their flanks.
- Orthogonal, or French type: Here, the coursing joints are perpendicular to the face of the arch at all elevations. This was a theoretically pure arrangement, but the practical difficulty and cost greatly limited its application. Various workers are credited with proposing such an arrangement. Edward Sang first presented his 'logarithmic' proposals in 1835-6 and published them in 1840 . One source shares the credit between A. J. Adie, M. F. Lefort, and William Froude.
'SKEW BRIDGES. Very little is known, says the Architect, respecting the origin of skew bridges. It has been repeatedly asserted that those built by George Stephenson on the Liverpool and Manchester Railway were the first erections of the kind, but this is incorrect, there being some of earlier date even in Lancashire. A paper in Transactions of the Institution of Civil Engineers, Vol. ?, p. 185, alludes to an oblique arch erected about the year 1530 by Nicolo, called "Il Tribolo,” over the river Mugnone, near Porta Sangallo, at Florence. It appears, however, that the principle upon which such bridges should be constructed was too little understood to render an attempt at constructing them on large scale admissable. Chapman mentions oblique bridges being in use prior to 1787, when he introduced a great improvement in their construction. Down to that time, as far as he was informed, such bridges had always been built in the same way as common square arches, the voussoirs being laid in courses parallel with the abutments. This plan could therefore only be adopted for bridges of very slight obliquity, and even then with considerable risk. About the time mentioned above Chapman was employed as engineer to the Kildare Canal, a branch from the Grand Canal of Ireland to the town of Naas, on which it was desired to avoid directing certain roads which had to crossed. He was therefore led to think for some method of constructing oblique arches upon a sound principle, of which he considered that the leading feature must be that the joints of the voussoirs, whether of brick or stone, should be rectangular with the face of the arch, instead of being parallel with the abutment. One of the first bridges built on this plan, the Finlay Bridge, near Naas, crossed the canal at an angle of only 30deg., the oblique span being 25ft and the height of the arch 5ft 6in. Mr Chapman observes that the lines on which the beds of the voussoirs lie are obviously spiral lines, and to this circumstance may be attributed much of the singular appearance of oblique arches. Finlay Bridge stood well, but the ingenious designer did not think it prudent in any other case to attempt so great a degree of obliquity, although he built several other bridges on the same principle over the Grand Canal of Ireland, and over some wide drains in the East Riding of Yorkshire. He recommends carrying the masonry as equally as possible from each abutment in order to avoid unequal strains the centering.'
1836 'OBLIQUE BRIDGES. TO PETER NICHOLSON, ESQ. —In reference to a lecture stated in the public prints, to have been delivered by Charles Fox, at the Royal Institution a few months ago, on what he termed his "new mode of constructing the Oblique Arch," and stating that, formerly the stones were cut by general rule, but merely fitted to their particular place'," and further, in regard to what has been published by his desire in the Philosophical Magazine" of April last, and, by " his permission" in the London's Architectural Magazine" June last,—l think it an act of justice to you, and I believe, that every person who is acquainted with the valuable instructions which you have frequently, during a long course of years, produced for the advancement of science, will concur in the propriety of setting the public right as to whom the merit due, for a proper and certain rule for the construction of the Oblique Arch. I therefore, deem it right to state, that in your book on "Masonry and Stone Cutting," published in 1828, there is an elaborate illustration for the working the spiral or twist upon the stones, and the explanation is so clear, that Mr. James Hogg, operative mason, residing at Brandling Place, Newcastle, has certified that, in 1834, he built an Oblique Arch entirely from the instructions which are given your book ; and so certain did I feel of the practicability of your rule, that I have adopted it upon the River Coquet, Felton. The chord of the arch being 33 feet, and the angle of obliquity 19°, and which case the stones were cut, or dressed previously to the erection of the centre. Having received approval of the arch as being in accordance with your design, I think there can be no doubt that your claim to the rule for the proper formation of the stones, is prior to that of Mr. Fox; and I have yet to learn that any rule exists by which the Oblique Arch can be so truly built as the one which you have published. I am not aware, although I have endeavoured to learn, and shall be glad, for the sake of the profession which I have the honour to be a member, to discover the contrary, that any of the Oblique Arches which have been erected upon any the public or private works in the North of England, except those above-mentioned, are constructed upon any general principle ; and it is very remarkable, however, much it is to be regretted, that up to the present time, there does not appear nave been any proper drawing prepared, nor the necessary practical instructions given for the certain construction of the Oblique Bridges, which have been built upon a very extensive public work since 1832; but that the contractors were suffered to exercise their own judgment the erection of them, and in one case, (decidedly the most important one of the whole), the entire management was, it appears, left to one of the operative masons in the employ of the contractor for the building the bridge. I have been particular in relating these circumstances, in order to show how little attention has been paid to the proper construction of Oblique Bridges hitherto, which require the greatest care, notwithstanding your valuable work the subject, as well also to suggest the probability that Mr. Fox was aware of the want of a general and proper rule in the cases alluded to, and hence his mistaken notion that " formerly," that is, previously to his lecture few months ago, the stones were cut by no general rule, but merely fitted to their particular place."
I sincerely hope that the enquiry which has been set on foot, with a view to prove that a sure rule for the spiral formation of the stones, has existed for several years, will lead to some advantage, and, I am informed, that already, that is, since the commencement of the enquiry, the executive engineer of a very extensive public railroad, has, very prudently applied competent person for a definite development of your principle, so that in future he may possess a safe guide—thus verifying the adage, better late than never.
It is too clear that, at the present time, there is a sort of mania for Oblique Bridges. That many cases, though not so frequently as they occur, they are indispensable upon railroad lines, but for turnpike roads, where the rate of travelling rarely reaches 11 miles an hour, and considering the absence of lateral friction, now that our roads are longer " Gridironed," and the little risk, comparatively, of departing from the proper course, so great an inconvenience in railroads though strikingly disregarded in various situations, I am firmly of opinion, after an attentive observation of the practical working of the best rule, that it very injudicious to adopt them, except in cases of absolute necessity. The fact too, that your opinion fully coincides with the one represented the Encyclopaedia Britannica, article ' Stone Masonry," viz “That Oblique Bridges should be avoided whenever it is possible," is, I submit, very strong proof of their inferiority. "I am, Sir, very respectfully, Your obedient Servant, HENRY WELCH, Surveyor of the County Bridges of Northumberland. Elswick Villa, Newcastle, Nov. 17, 1836.' 
Sources of Information
-  An Essay on the Construction of Oblique Arches, by Edward Sang M.S.A., The Civil Engineer and Architect's Journal, July 1840, pp.232-6
- J. E. PALLETT (1972) The Contribution of William Froude to the Development of the Oblique Bridge with Mechanically Correct Spiral Tapered Courses, Transactions of the Newcomen Society, 45:1, 205-215, DOI: 10.1179/tns.1972.012
-  DIPLOMA WORK: DISCRETE ELEMENT ANALYSIS OF SINGLE SPAN SKEW STONE MASONRY ARCHES by TAMÁS FORGÁCS, Budapest, 2015
-  'A Practical Treatise on the Construction of Oblique Bridges: With Spiral and Equilibriated Courses' by Francis Bashforth, London, 1855
- Abergavenny Chronicle - Friday 15 September 1899
- Newcastle Journal - Saturday 19 November 1836, with a minor correction added in the edition of Saturday 26 November 1836