CHAPTER IV.
THE GREATEST BRIDGE IN THE WORLD.
“Haveyou heard the news? The Tay Bridge is blown down!”
“Yes. A terrible disaster. I should think they would give up their scheme of bridging the Firth of Forth after that.”
“Not they! The scheme may be altered, but bridge it they will. Engineers never give in.”
The comments of these newspaper readers were right. The Tay Bridge, the longest in the world, had been blown down one wild December night in 1879, and girders, towers, and the train which was rushing over it, were suddenly hurled into the surging flood.
At that time a scheme was in hand to bridge the Forth for the North British Railway system, and Sir Thomas Bouch had proposed two suspension bridges hung by steel chains. But ultimately a new design altogether was adopted, the plan being by Sir Benjamin Baker and Sir John Fowler.
It was the new principle—or, rather, a remarkable development of an old principle—for which the bridge-making world was waiting: the principle, namely, of the cantilever.
A cantilever is, in fact, a bracket; and Sir Benjamin Baker has described it as such. It is a strong support, built out from a firm base, and is like a powerful and magnified bracket upholding a shelf.
In the Forth Bridge there are two huge spans, 1700 feet wide, crossed by these cantilevers; bridging channels of some 200 feet deep.
The longest spans on the Tay Bridge were 245 feet; it was over two miles long, and had ninety spans. It was an iron girder bridge, and was opened on the 31st of May, 1878. Not to be beaten, however, after the panic had subsided, another and more stable bridge was constructed, also a girder, but not so high in elevation, and sixty feet further up the river. It was opened in 1887, and is 10,779 feet long, with 85 piers, the navigable channel being under four of the spans, the centre spans being 245 feet wide.
It will be seen at once that the cantilevers at the Forth Bridge cover very much wider spans; and the channel being so deep, the impossibility of building piers will also be obvious. The best place for the bridge was marked by the projection of the Inverkeithing peninsula on the north shore, and also the Inchgarvie rock in the channel itself. The peninsula brought the two shores together, reducing the space to be bridged, and the rock gave firm support for a pier. Still there were the two immense spans of 1700 feet to be crossed, and the engineers decided on the cantilever principle. Thus, though the TayBridge was the longest in the world, the Forth presented by far the greatest spans—viz., the two main spans of 1700 feet each, in addition to which there are two of 675 feet each, and fifteen of 168 feet each.
The total length of this magnificent bridge, which Sir Benjamin Baker rightly claimed was the most wonderful in the world, is somewhat over 1½ miles in length, or 8296 feet, including the piers, while almost a mile is bridged by the huge and superb cantilevers. This is, perhaps, the great marvel. The clear space under the centre is no less than 152 feet at high-water, while the highest portion is 361 feet above the same mark.
And now, how was this great bridge constructed? Workshops were erected at South Queensferry, and the mammoth cantilevers were put up there piece by piece. They were fitted together and then taken plate by plate to the bridge itself. The shops were lit by electricity, and furnished with appliances for bending, cutting, moulding, holing, and planing plates. The workshops were surrounded by quite a maze of railways.
But what of the piers, without which all these preparations would be unavailing? Now the foundations of piers are usually laid by means of cofferdams; that is, piles of timber are driven down through the water into the bed of the river close together, and the interstices filled with clay; or a casing of iron may be used instead. The water in the enclosure thus formed can be pumped out and excavation proceeded with, and the foundations laid. Cofferdams are sometimes made of iron boxes or caissons with interstices fitted with felt, and caissons of this kind about 12½ feet long and 7 feet wide were used in constructing the Victoria Embankment on the Thames.
But with certain of the piers for the Forth Bridge the water was too deep for timber cofferdams, and the usual diving-bell was not sufficiently large. The piers were to be of immense size, no less than 55 feet indiameter, and the diving-bell of ordinary size would not cover that great width.
Huge caissons were therefore made, 70 feet wide, constructed of iron plates and rising in height, according to the depth of water, up to 150 feet. The lower part of the immense caisson or tank was fitted as a water-tight division and filled with compressed air, the object being to resist the pressure of the water. Two shafts communicated with this air-tight division or mining chamber, one for the removal of the earth excavated, and the other for the men to pass up and down. The escape of the air through the shafts was prevented by the use of an air-lock, working on the same principle as a water-lock on rivers or canals. There were two doors in the lock, one communicating with the shaft and the other with the outside air. When the latter was closed and the lock filled with compressed air by opening a valve or tap, the door of the shaft could be opened and the man could descend to his work below.
That work consisted chiefly of excavation in the bed of the river. Drills, hydraulic cutters, and dynamite blasting were all utilised until huge holes, many feet below the river bed, were hollowed out. As the caisson was filled with concrete above the air-tight chamber where the men worked it was exceedingly heavy, and sank by its own weight into the space prepared.
The mining chamber was lit by electricity, and was about seven feet high. The mud of the river bed was mixed with water and blown away by the compressed air which seems to have been about 33 lbs. to the square inch. The caissons were sunk down to rock or boulder clay, and when they had reached the required distance the mining chamber was filled with concrete, and the same material used to the level of the water; the piers were then built up with huge stones placed in cement, the whole forming a magnificent mass of concrete and masonry, carried down in some cases to about 40 feet below the bed of the river.
THE FORTH BRIDGE.
THE FORTH BRIDGE.
The three chief piers consist of groups of four columns of masonry, each gradually tapering from 55 feet in diameter to 49 feet at the top, and about 36 feet high. From these rise the huge cantilevers connected together by girders 350 feet in length.
The centre of these three main piers rests on the island of Inchgarvie; the two others are known as the Fife and the Queensferry piers respectively, and are placed on the side of the deep water channels. In addition to these three main piers are several others, some in shallow water and some on land. The part of the bridge which they carry is an ordinary girder of steel leading to the immense cantilevers. For founding the shallow water piers, cofferdams were used; the caissons with compressed air chambers being for the deep water structures.
They were put together on shore, launched, floated, steered to the desired position, and sunk. One proved cranky and turned over, and was only brought right after much expense and difficulty.
The cantilevers are bolted down to each pier by numbers of huge steel ties, 24 feet in length and 2½ inches in diameter, embedded in the masonry, there being 48 of these bolts or ties to each column. And now as to these cantilevers.
Four huge tubular shafts, two on each side, rise from the group of columns forming each pier, to the height of 350 feet. From these shafts, which slope slightly inward, project the cantilevers, the upper and lower parts being strongly braced together by diagonal ties. In shape the gigantic brackets taper towards a point, the width decreasing as much as from 120 feet at the commencement of the piers to 32 feet at the ends. The wind, it is believed, will be more effectually resisted by this means.
The cantilevers are hung back to back, one to some extent counter-weighing the other. The component parts consist of cylinders of steel or struts for resisting compression—these are the lower parts; and ties oflattice-work made of steel plates for resisting tension,—placed above.
Thus, then, from each of the three chief piers two pairs of gigantic brackets project, each pair placed side by side and braced together, and forming one composite cantilever jutting to the north and one to the south. The rails run on sleepers placed lengthwise and fixed in troughs of steel, so that should a train run off the line the wheels will be caught by these supports.
It is calculated that there are about 45,000 tons of steel in the bridge, and 120,000 cubic yards of masonry in the piers. The contract price was £1,600,000, which works out at about £215 per foot; and the contractors, who were able to obtain an admirable organisation of some 2000 men to carry out the magnificent design, were Messrs. Tancred, Arrol, & Co. Some special tools for use in the work were planned by Sir William Arrol. The bridge was opened by the Prince of Wales on the 4th of March, 1890.
The success of this magnificent structure has assured the wider adoption of the cantilever principle. Long-span bridges, in several cases, have since been built on this design. Its engineers may claim indeed to have widened the scope and possibilities of bridge-building.
Still, when another bridge was wanted over the Thames, at a busy spot, crowded with shipping and near the historic Tower of London, another kind of structure was adopted. What was it?