Discoveries and Inventions of the Nineteenth Century

The great Forth Bridge, now (December, 1889) approaching completion, is the first bridge on the cantilever and central girder principle that has been erected in Great Britain, and it has also the distinction of being by far the widest spanned bridge in all the world. We are told by the engineers of the bridge that the cantilever and girder principle is by no means new, for it has been adopted hundreds of years ago by comparatively rude tribes in the construction of timber bridges, to which it readily lends itself. Such bridges are described as having been erected by the natives of Hindoostan, Canada, Thibet, etc., even at remote periods. The principle of the cantilever and girder construction was well illustrated by Mr. Baker, one of the engineers of the bridge, at a lecture given by him at the Royal Institution, by means of what he termed “a living model,” of which (Fig. 147b) shows the general arrangement. Two men, seated on chairs, extend their arms and hold in their hands sticks, of which the other ends butt against the chairs. The central girder is represented by a shorter stick, suspended at a and b. We have here the representation of two double cantilevers, the ropes at c and d, connected with the weights, representing the anchorages of the landward arms of the cantilevers. When a weight is placed on a b, which was done in the “living model,” by a third man seating himself thereon, a tensile strain comes into action in the ropes and in the men’s arms, while the sticks abutting on the chairs have to resist a compressing force, and the weight of the whole is borne by the legs of the chairs, also under compression. Now let the reader imagine the men’s heads to be 360 feet above the ground, and about a third of a mile apart, while the distance between a and b is 350 feet, and he will have a rough but sufficiently clear idea, not only of the principle upon which the Forth Bridge is constructed, but also of the magnitude of one of its spans. To complete the comparison, Mr. Baker further invited his hearers to suppose that the pull upon each arm 292of the men is equal to 10,000 tons, and that the legs of each chair press on the ground with the weight of more than 100,000 tons. The Forth Bridge spans the estuary at Queensferry nine miles north-west from Edinburgh, and its purpose is to afford uninterrupted railway communication along the eastern side of Scotland. It will, in effect, shorten the railway journey between Edinburgh and Perth, or Aberdeen, by nearly two hours. Queensferry had long been established as a usual place for crossing the Forth, and readers of Scott’s “Antiquary” will remember that the first chapter describes how Monkbarns and Lovel, by some accidental delays to the coach, lost the tide, and had to wait, to sail “with the tide of ebb and the evening breeze,” finding themselves, in the meanwhile, pretty comfortable over a good dinner at the “Hawes Inn.” This inn still stands, its situation being close to the southern end of the great bridge. A design for the erection of a light suspension bridge at the same spot was published at the beginning of the present century, but although the spans were to be equal to those of the present bridge (17,000 feet), the different scale of the projects may be inferred from the total weight of iron to be used being estimated at 200 tons, while 50,000 tons will be required for the structure now approaching completion. In 1873, an Act of Parliament was obtained authorizing the construction of a suspension bridge at Queensferry, to carry the railway over the estuary. The design comprised practically two bridges, each carrying a single line of rails, the bridges being braced together at intervals. The central towers were to have been 600 feet high, or about 100 feet loftier than any other erection then existing in the world. The designer was the late Sir Thomas Bouch, and preparations were made for carrying out the plans by the erection of workshops and the manufacture of bricks for the piers. But the project was knocked on the head by the terrible disaster at the Tay Bridge, in December, 1879, when several of the central piers were overturned by the force of the wind, with swift destruction to a passing train, which was precipitated into the water, and every one of about ninety persons in the train perished. Sir Thomas Bouch having been the designer of the Tay Bridge, public confidence in his plan was shaken to such an extent, that the four railway companies who were promoting the construction of the suspension bridge abandoned the project in favour of a design on the cantilever and central girder system, which was then brought forward by Mr. (now Sir John) Fowler and Mr. Baker. When the Bessemer process had made steel attainable at a cheap rate, these engineers recognized the advantages which cantilever bridges, made of that material, presented for the wide spans required for carrying railways across navigable rivers, and in 1865 they had designed such a bridge, with 1,000 feet spans for a viaduct, across the Severn, near the position of the present tunnel. It was not, however, until 1881 that the designs for the Forth Bridge were published in English and American engineering journals. These designs at once attracted attention, and scarcely a year had elapsed before a railway bridge was built for the Canadian and Pacific Railway, on the same principle, and this has been followed by others since. It is, however, absurd to allege that the engineers took their ideas from America, merely because these smaller undertakings have been completed before the great work that dwarfs them all was open for traffic. The construction of the Forth Bridge on its present design was commenced in January, 1883. Its site at Queensferry is at a point where the estuary narrows, and where, in the very middle of the channel, there is a small 293rocky island, called Inchgarvie, that furnishes a solid foundation for the great central pier. On each side of this island the channels are about one-third of a mile wide, and more than 200 feet deep, and through them the tide rushes with great velocity. The impossibility of building up any intermediate piers, under such circumstances, is sufficiently obvious—the currents must be crossed at one span, if a railway bridge had to be made. The formation of the piers for such a work presented many novel problems, and much of the work had to be commenced in deep water; that is, the ground of rock or hard clay had to be prepared, in some parts, as far as 90 feet below high water. Each pier stands on four caissons, which are great tubes or drums of iron and steel, filled up with concrete. Each weighed, when empty, about 400 tons, but when filled up with concrete, the weight would be about 3,000 tons. The diameter of each is 70 feet, and the deepest one is sunk 89 feet below the water, and it was with no little labour that some of them were put in their places. Each caisson has an outer and an inner tube, is 70 feet in diameter at the base, and 60 feet at the top. Seven feet from the bottom, an air-tight partition formed a chamber in the lower part of the caisson, about 70 feet in diameter, by 7 feet high, and shafts sufficiently large to admit the passage of men and tools led from the top. Air was forced into this chamber, when the caisson had been sunk, expelling the water, and then men descended through the shafts and locks, in which a high pressure of air was also maintained, and excavated the material at the bottom, until the caisson had, by its own weight, sunk to the depth required. The work in this air chamber was carried on by means of electric lights, and ten or twelve weeks were occupied in sinking each caisson. The pressure of the air in the working chamber was sometimes as high as 35 pounds per square inch, or sufficient to maintain the mercurial column in a barometer 72 inches high, instead of the ordinary 29 or 30 inches. It was found that the labour in the compressed air chamber could not be done by our home workmen, as they were quite unaccustomed to the high air pressures required to keep out the water; but arrangements were made for the assistance of a staff of French workmen, inured to the conditions by long working under water in the construction of the docks at Antwerp. PLATE XIII.THE FORTH BRIDGE. The stores, offices and workshops, situated on a slight eminence near the south end of the bridge, are very extensive, occupying, it is said, an area of 50 acres. Here are great furnaces, cranes and machinery for shaping and fitting the steel plates and bars ready for taking their appointed places in the vast structure. An hydraulic crane may, for instance, be seen lifting a ton weight flat steel plate that has been heated to redness in a regenerative gas furnace, and transferring it to an hydraulic press, where it is quickly and quietly bent to the required shape. The plate is then cooled, and, when the edges have been planed, it is placed in position with the adjoining plates, and the rivet holes are drilled by an ingenious machine, specially designed by Mr. Arrol, the contractor, for that purpose. It works upon 8–feet lengths of the tubes, and simultaneously cuts ten rivet holes at different points in the circumference. All the different parts of the structure are temporarily fitted together to ascertain that every piece is properly adjusted. They are then marked according to the position they are to take, and are laid aside until they are wanted. Thus the work at the bridge has proceeded without any awkward hitches arising from ill adjusted sections being brought together. At times, 1,800 tons of finished steel-work has been turned out of these shops in a month, and 294this material, which was supplied by the Steel Company of Scotland, has been found thoroughly trustworthy in every respect. Its strength is one-half greater than that of the best wrought-iron, and the plates have thrice the ductility of iron plates. The steel plates for the great tubes are supplied in lengths of 16 feet, and of different thicknesses, between ⅜ths of an inch and 1¼ inch. Fig. 147c.—Principal Dimensions of the Forth Bridge. The sketch, Fig. 147 c, shows the general dimensions of the bridge proper, or that part of the viaduct which will actually span the estuary. Of the three great piers that support the cantilevers, it will be observed that the central one, which rests on Inchgarvie, is wider than the other two. Each consists mainly of four tubes, 12 feet in diameter, made of plates of steel 1¼ inch in thickness, and these rise to the highest part of the bridge, which is 361 feet above the water, so that the structure is as lofty as St. Paul’s Cathedral. These great tubes are not placed vertically, but incline inwards towards the top, so that while the “straddle legs” of each pair are 120 feet apart at the base, they are only 33 feet apart at the top. These lofty columns are also braced together diagonally by other steel tubes—that is, a tube passes from the foot of every column to each of the other three. At the base of each column, the lowest spanning member springs also (which appears like an arch, but is not so), as a tube of 12 feet diameter. Thus abutting or resting on enormously thick plates of steel that cap the masonry of each pier, are five tubular steel limbs, three of which are 12 feet in diameter, and two are 8 feet, and, besides these five, girder members diverge from nearly the same centre. One of the large tubular members is the first strut that rises obliquely to support the upper structure. From the point where this strut meets the