SKEW BainoEs may fairly come under the head of STONE Ii ; for though they are frequently built of brick and other materials, the vast majority of such constructions are either formed of stone, or stone and brick combined.
Oblique, or Skew bridges, have been introduced contem poraneously with railways ; for few bridges of the kind were constructed anterior to that time. We hear, indeed, of one having been thrown over the river Mugone, at Florence, so early as the year 155O; but the true principles on which oblique bridges depended were not previously known or studied, as their application to general purposes was rarely, if ever, absolutely called for, until the plan of rapid locomo tion, which the system of railways gave rise to, made their use constant, and in many eases indispensable.
In the construction of oblique bridges, the main object to be kept in view is to bring the thrust of the arch into such a position as will enable the abutments, or piers, properly to counteract it ; and the most effective way of so doing, and at the same time the most simple, is found to be by making the direction of the courses at right angles to the face of the bridge.
There have been various methods suggested for building these bridges ; but the one most commonly adopted, is to form the arch by regular spiral courses, all parallel to one another, and so arranged, that any section taken at right angles to the axis of the cylindrical segment which constitutes the arch, shall exhibit the coursing joints tending in a radial direction to the centre of the cylinder at the point of section. All the voussoirs are rectangular in plan, both on the extrados and intrados, with the exception of those on the faces of the bridge, because a spiral line, touching the outer edge of both abutments, would form a curve of contrary flexure, falling partly within and partly without the line bounding the arch. In this case, the thee-line will evidently not be parallel to the heading-joints of the vuussoirs, which, therefore, will be of a constantly varying shape and size on the faces of the bridge. Again ; it is important to remark, that in this par
ticular system of construction, the joints of the voussoirs are not, properly speaking, straight lines on the elevation of the arch, but segments, whose chords should all tend to a common centre, which, in every case, must be more or less below the axis of the cylinder, according to the angle of the skew bridge.
One of the best works on the subject of skew bridges, was written by Buck, wherein formulae are given, from which all the dimensions of skew arches of every kind may be arrived at with mathematical accuracy, and the principles of their construction are clearly and correctly explained. Nicholson published two admirable works on this important department of bridge-building—one in 1828, entitled, "A Practical Treatise on Masonry and Stone-cutting ;" and the other in 1S39, called, " A Treatise on the Oblique Areh." To him we are indebted for the first sound exposition of the princi ples on which such constructions are founded ; and from the practical bearing of this, as of all his it was of the greatest use in presenting the subject to men's minds in a correct and intelligible point of view, and setting it forth in its proper light.
The method of forming oblique arches, as above explained and treated of by Buck and Nicholson, supposes the arch to be a segment of a circle on the square section. It is nut, however, necessary that such should be the case ; the form of the arch may be elliptical on the square section, in which case the elevation is a much flatter ellipse, according to the amount of skew • as it is clear that the minor semi-axis is constant, while the major semi-axis will increase with the skew ; and this arch may be again modified by making the joints of the voussoirs perpendicular to the radius of curva ture on the square section, instead of making them tend to the axis of the cylindrical segment.
Many changes of this kind have been recommended and tried, with varying success, by Messrs. 11 art, Adie, Roebuck, and several others.