Discoveries and Inventions of the Nineteenth Century

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 upper member, a stay passes downwards with an opposite inclination to the lower member, from its point of junction with which another strut rises, and so on. All the struts, as being subject to compressing force, are made of steel tubes; the straight upper members and the stays are lattice braced girders of rectangular section. The apparent curve of the lower member—for it is really made up of sections of straight tubes—may suggest the notion of an arch; but the reader must remember that the principle of this bridge has no relation to that of the arch. The cantilevers do not unite the long arms they stretch, but each is an independent structure with its own perfect stability, and it will not be clutched on or locked up to its neighbours by the central girders. The weight of one of these 1,700 feet spans is about 16,000 tons, and the heaviest train loads might be two coal trains, weighing together, say 800 tons, or only one-twentieth of the 295dead weight of the structure. But, what would not generally be supposed, the pressure of the wind is an element of much more importance in considering the stability of the bridge than the weight of the rolling load. It is to resist the wind pressure that the lofty columns that are only 33 feet apart at the top across the bridge, plant their bases 120 feet asunder. The estimated lateral pressure of the wind on one of the cantilevers, assuming it as equal to 56 lbs. per square foot, would amount to 2,000 tons. These strains are so fully provided for that the engineers are confident that a hurricane of such a force as would desolate the country would leave the Forth Bridge intact, even if the wind blew in opposite directions on the two arms of the cantilever. To rend asunder the top ties, a pull equivalent to the weight of 45,000 tons would be required, whilst the utmost strain that passing trains could possibly bring upon these ties would be less than 2,000 tons. A striking illustration of the strength of these huge brackets was lately given by Mr. Baker himself, when in a public lecture he assured his audience that half a dozen of our ponderous modern ironclads might be hung from the cantilevers. Everyone knows that a bracket requires to be strongest nearest the base, and the lower steel arms that stretch out 680 feet each diminish in diameter until at the end it has decreased to five feet, and the pairs approach each until, from being 120 feet apart at the base, they are only 33 feet apart at the ends. The central girders will each weigh about 1,000 tons, and only one end of each will be attached to a cantilever, the other ends will simply rest on what are called “rocking columns,” so that there may be freedom of motion to allow play for the changes of position that will be induced by changes of temperature expanding or contracting the huge masses of metal. The reader can hardly have failed to observe that the chief element in the stability of the structure depends upon balancing a great mass of metal on the one side of a pier by an equal mass on the other side. But while each end of the central cantilever bears half the weight of a central girder, the two shoreward cantilevers have this load at their inner ends only. How is their balance maintained? In this way: the shoreward arms are made about 10 feet longer than those that stretch over the water and their extremities are also loaded with about 1,000 tons of iron, built up within the shore piers. The lofty columns of the piers were erected without any external staging, from a temporary platform surrounding the piers and supporting the necessary machinery. The weight of this platform with the machinery on it was about 400 tons, and as the work proceeded it was raised as required by hydraulic machines placed within the vertical columns. As the height of these increased, the men and materials had to be conveyed to the platform by cages moving between guide ropes and worked by steam engines. From this platform were constructed not only the main columns, but the great diagonal tubes, the bracing girders, and the viaduct girder. The cantilevers were also put together without scaffolding. When the first few feet of the lower member had been built out from the base, a movable platform was hung round it, and on this platform were the cranes for putting the plates into position, the furnace for heating the rivets, and