SKEW Bridge. Square arches, or those which stand at right angles to their abutments, and exert their thrust in that direction, have been already treated at considerable length under the article Bridge. Till lately, any other form of arch was rarely to be met with. But there are cases of frequent occurrence, in which various sorts of passages and courses necessarily cross each other obliquely, as well as on different levels;^[1. Indeed to avoid interruptions and accidents, it is always desirable that any crossings with a railway should take place on different levels. The like may be said of canals, which besides, cannot, without some kind of moveable bridges, be crossed closely above by any sort of road, and in fact can only be crossed quite on the same level by some water course; not to say any thing of fording or wading through them. ] and where, on account of the difficulty or impossibility of constructing a square bridge, which should both have sufficient strength, and, at the same time, leave the lower passage quite clear, a different form of arch, placed obliquely to the abutments, and yet exerting its thrust in the direction of the upper passage, is almost the only practicable alternative. Such a structure is called a shew bridge, and is very frequently required since the more general introduction of railways, in which it is of great consequence to preserve as direct or straight a line as possible. Nay, it is not uncommon for a railway to have to traverse districts in which it may neither be allowable nor practicable to use any means of avoiding the obliquity with which it may have to cross canals, roads, &c.; its course being so much controlled by conflicting interests, and by the natural features of the country.

If a common square bridge were to be employed in the circumstances just supposed, namely, in which the passages mutually intersecting obliquely, cannot be so diverted as to cross at right angles, then the arch must either be built stretching in the direction of the upper passage, and of sufficient span to allow the lower passage, with its course unaltered, to pass uninterruptedly under it; or, it must be built stretching at right angles to the lower passage, and having its embankments and abutments so much extended, that the whole structure may both stand securely, and furnish sufficient area above for the upper passage to continue its oblique course over it unaltered. An arch partaking of both these forms might, it is true, be employed; but to such expedients, a proper skew bridge is in general greatly preferable, and is indeed sometimes the only allowable form of construction that will serve the purpose required. As for any alteration in the old courses, they would often be incompatible with the particular or vested rights of proprietors, and especially with the provisions which are generally inserted in the acts of Parliament expressly prohibiting any alteration to be made in the original courses of canals, &c.; and even were there no such restrictions, the diversion of the courses might be attended with great expense, and might occasion much inconvenience to the traffic. The wide or broad arch again, is a most unscientific mode of overcoming the difficulty. Room for it, with the requisite embankments, could not always be had, and it would, besides, great[7:20:376] ly increase the expense, owing to the necessity of constructing a bridge of much larger dimensions than would be required were a proper oblique arch introduced, which, from its exerting its thrust in the most advantageous direction, is stronger than any other of the same magnitude. For, besides endeavouring to preserve a certain degree of equilibrium amongst the several parts of an arch,^[2. The mode of attaining an equilibrium in an oblique arch, is a subject on which no one, so far as we know, bas entered. It seems generally to be taken for granted, that the obliquity makes no difference; but though in this we cannot acquiesce, our limits will not admit of attempting to discuss the question ] it is considered still more necessary to the insuring the greatest strength, especially where the span is considerable, and the upper passage narrow, that the beds of the courses of the stones should be everywhere at right angles, both to the soffit or under surface of the arch, and also to every vertical plane running in the direction of the road or other passage carried over the bridge. There is no difficulty in fulfilling or combining these conditions in a square bridge, but it is very different with the oblique sort.

Writers on the theory of bridges for the most part say nothing of the oblique arch; and the few who do mention it, generally content themselves with slurring it over in such brief, vague, or general terms, as to he nearly unintelligible. Those again who have professedly drawn up, what they reckon practical rules for workmen, have, for the most part, tacitly assumed it as perfectly ascertained and indisputable, that the conditions above mentioned may, with only a little more labour, be equally well attained in the case of the oblique, as in that of the square bridge, by merely making the courses of the stones to form portions of the thread of a square threaded screw, or rather, perhaps, of a thread somewhat of the dovetailed form; the highest part of each thread, or that on the crown of the arch, being at right angles to the direction of the road. Such, in particular, is the doctrine laid down by Mr. Nicholson, and also by Mr. Fox; and it is not a little amusing, that the honour of inventing this supposed valuable theory has been warmly disputed between these gentlemen, or at least their friends.^[3. ’ See Philosophical Magazine for April 1836, January and March 1837, and Reports of British Association for 1838. ] When the soffit is to be a cylindrical surface, the side of the screw-thread may, it is true, be readily formed so as to be everywhere at right angles to the soffit; and, as all the beds of the courses of the stones would have the same curvature, the stones might be conveniently prepared beforehand, which is a great recommendation to the scheme. But unfortunately, the other, and by far the more important condition of the two, namely, to have the beds of the courses everywhere at right angles to the direction of the upper passage, can by no means be accurately fulfilled by the figure of the screw; for no two parallel planes can both of them cut the same thread of the screw at right angles, if they intercept a less portion of it than a revolution, so that a thread which has the proper position or direction at the crown of the arch, can have it at no other point; its direction on the one side of the crown being too much inclined to the course of the under passage, whilst, on the other side, it is too little inclined.

However, although it thus appears that the principle of the screw thread cannot well serve the purpose of the skew bridge, especially when the obliquity is great; yet there can be no question that the beds of the courses of the stones might always be formed with such a curvature as would make them very nearly fulfil both the conditions above specified; but as the several parts of this curved surface, if different from that of a screw, would necessarily be far from being uniform, the stones, owing to their requiring to be of so many different shapes, could not, without immense labour of measurements, drawings, and innumerable patterns, be prepared beforehand; and, therefore, most probably, the easiest way of putting such a scheme in practice, would be to prepare the stones no faster than they are required in building, and then it would be easy to form each stone in such a manner, that two of its opposite sides, or at least the middle parts of these sides, should be as nearly as possible at right angles, both to the soffit, and also to the direction of the passage over the bridge. In this way of working, the soffit need not be restricted to the cylindrical form, because it may suit equally well though of a very different figure. We are aware, that something similar to the method just mentioned has been already acted on, and has also been heartily ridiculed as unscientific and unsystematic; but it will be quite in time to condemn it when once a better shall have been substituted in its stead: for, most assuredly, if closely followed up, it should form an incomparably more perfect structure than that with the screw thread. The like remarks are equally applicable when the material is brick.

Some bridges may be said to consist generally of a platform supported by a set of curved iron ribs, which either abut like a stone arch against the piers, or, if the curvature be small, and especially if there be nothing to abut against, each rib has its extremities connected and kept from spreading, by a straight rod crossing the span underneath. Ribs of this last construction are often used as beams for supporting floors in large buildings; for, being in effect beams of great depth, they only rest or press downwards upon the piers or walls supporting them, and so have no horizontal thrust. Curved iron ribs, whether with or without the rods, are equally applicable to the oblique as to the square arch, since each rib may always be placed in a plane, which is both vertical, and runs in the direction of the upper passage. The horizontal rods, though generally of unnecessarily great strength throughout the most part of their length, are commonly rendered very weak near their extremities, owing to their either being very needlessly pierced with large holes near the ends, or by being very much reduced there, for the purpose of being formed into a screw. Common sense, one would think, could not fail to hint to even the most inexperienced, that to resist a longitudinal pull, there is no use in having one part of the rod stronger than another; because one weak part will render it as likely to fail, as if the whole were equally weak. Neither is it necessary that such rods should be stronger at the middle to sustain their own weight, since they are commonly suspended there by a vertical strap.

It is evident that the opposite horizontal thrusts of an arch, formed of any other materials as well as iron, might, in a similar manner, be made to annihilate or counteract each other by means of horizontal rods beneath, so that a common square arch, having the thrusts obviated in this way, could be used instead of a skew bridge, wherever there is sufficient head-room for the horizontal rods, and likewise space for carrying out a mere wall as far as will square the faces of the arch, and support its weight at the two projecting corners; no additional embankment being required where there is no thrust.

An elegant mode of constructing arches of brick, without requiring the aid of anything like ordinary centring on the great scale, has been introduced by Mr. Brunel, and is equally applicable to the oblique as to the square bridge. The principle of this, as originally adopted, and its sufficiency tested in constructing the shaft of the Thames Tunnel, depends on the cohesive power of Roman cement, coupled with a system of ties, the most eligible material for which Mr. Brunel found to be hoop iron. After the piers have been constructed in the usual manner, a mould, curved according to the figure of the arch, is fixed to, and stretched[7:20:377] between these piers, and upon this a narrow arch or rib of brick is carried over and keyed, using cement, with the occasional insertion of ties. The adhesion of the cement to the brick being greater than the cohesion of the brick itself, enables the rib to be carried to any extent of which the strength of the material will admit. When once a sufficient number of such ribs have been successively constructed, side by side, like so many beams stretched between the piers, they will be in a state to receive and support all the other materials necessary to complete the structure.

The bridge of the Santissima Trinita at Florence, though of ruble construction, affords a magnificent example of the strength of cementitious architecture, and of the durability of the materials; the arches being composed of a mass of irregular stones so strongly united by mortar, as to have the consistence of a single stone. It is evidently a matter of perfect indifference whether such arches were square or oblique. A very ancient bridge, said to have been built by the Romans, over the Danube at Ratisbon, has wonderfully resisted the ravages of time. It consists of gray freestone and thin bricks, firmly united by pozzolano cement.

On several lines of railway, an ingenious mode of constructing arches of laminated ribs of wood or iron has been introduced by Mr. Green, and is alike applicable to the oblique as to the square arch. At the Ouseburn, close to the east side of the town of Newcastle, is one of the great viaducts constructed in this manner by Mr. Green, on the Shields railway. The piers and abutments are of stone, with large projecting buttresses on each side, whilst the arches, which are circular, are of Memel timber. Each arch is one hundred and sixteen feet in span, and consists of three separate and parallel curved ribs, and each rib, which is four feet in depth by twenty-two inches in thickness, is composed of sixteen layers of three-inch planks laminating over each other, and these again, whilst being bent over a curved frame or centre into the form of the required arch, were firmly fixed together by means of oaken pins and iron straps. The three ribs of each arch are connected by diagonal braces, and their ends rest in large iron sockets fixed on the stone piers or abutments. From the upper sides of tl>e ribs a series of very strong struts, braces, and framing, bound with iron straps and bolts, is carried up, filling the spandrils, to support the platform or roadway, which is formed of longitudinal beams; these again are covered with three-inch planks to carry the rails. Hitherto almost all wooden bridges had been constructed of straight timbers, upon the same principles as are used in roofing; and on account of the shrinkage from the frequent joggles and the weight of the structure, the roadway and framing generally became bent or crippled. But the ribs constructed by Mr. Green make a very close approach to solid pieces of timber. Their strength is very little impaired by the joining of the planks, which are from twenty to forty-five feet in length by eleven inches in breadth; particularly since great care has been taken that no two junctions of their ends might occur at the same place. In forming a rib, the first layer of planks consists of two whole deals in breadth, and the next of one whole and two half deals, and so on alternately, until the rib is completed. This construction is supposed to be better able to resist the load which it may have to bear at a given point, than can be done by the same quantity of materials in a different form. The struts which discharge the weight from the spandril beams upon the ribs, stand at right angles to the latter, and divide them into equal parts. The strength, durability, and beauty, can only be exceeded by arches of stone. The whole of the timber was subjected to Kyan’s process, and between every two deals was introduced a layer of brown paper dipped in tar, to exclude moisture. The bridge across the Ousebum has five wooden arches of one hundred and sixteen feet span, and four of stone of forty-three feet span. Each of the wooden arches required three centres, which were made so light as to admit of being shifted from one arch to another without being taken to pieces. The total length of this bridge is nine hundred and fifty feet, and greatest height one hundred and eight feet. At Willington, four miles farther east, on the same railway, is a still longer bridge of the same construction.

Mr. Green has applied the same principle to the construction of iron bridges with laminated ribs of that metal. Wrought iron bars from 1∙5 to 4 inches square, according to the span of the arch, and from fifteen to twenty-five feet in length, grooved on the under, and tongued on the upper side, are laid one over another, and bent over a centre until the rib is formed. The bars are bound together with iron straps at intervals of from four to six feet apart. A considerable saving of expense, and great lightness, as compared with stone or ordinary iron bridges, may thus be attained.