In 1897, a steel arch bridge was completed that was built around the old suspension bridge spanning the Niagara River over the Whirlpool Rapids. The old suspension bridge had been in continuous service since 1855 and had outlived its usefulness. It was decided to build a new one on the same spot, and yet the traffic in the meantime must not be disturbed in the least. It would seem that this was impossible, but the engineers intrusted with the work undertook it with perfect confidence. To any one who has seen the rushing, roaring, foaming waters of unknown depth that race so fast from the spray-veiled falls that they are heaped up in the middle, the mere thought of men handling huge girders of steel above the torrent, and of standing on frail swinging platforms two hundred or more feet above the rapids, causes chills to run down the spine; yet the work was undertaken without the slightest doubt of its successful fulfilment.
It was manifestly impossible to support the new structure from below, and the old bridge was carrying about all it could stand, so it was necessary to build the new arch, without support from underneath, over the foaming water of the Niagara rapids two hundred feet below. Steel towers were built on either side of the gorge, and on them was laid the platform of the bridge from the towers nearest to the water around and under the old structure. The upper works were carried to the solid ground on a level with the rim of the gorge and there securely anchored with steel rods and chains held in masonry. Then from either side the arch was built plate by plate from above, the heavy sheets of steel being handled from a traveller or derrick that was pushed out farther and farther over the stream as fast as the upper platform was completed. The great mass of metal on both sides of the Niagara hung over the stream, and was only held from toppling over by the rods and chains solidly anchored on shore. Gradually the two ends of the uncompleted arch approached each other, the amount of work on each part being exactly equal, until but a small space was left between. The work was so carefully planned and exactly executed that the two completed halves of the arch did not meet, but when all was in readiness the chains on each side, bearing as they did the weight of more than 1,000,000 pounds, were lengthened just enough, and the two ends came together, clasping hands over the great gorge. Soon the tracks were laid, and the new bridge took up the work of the old, and then, piece by piece, the old suspension bridge, the first of its kind, was demolished and taken away.
Over the Niagara gorge also was built one of the first cantilever bridges ever constructed. To uphold it, two towers were built close to the water's edge on either side, and then from the towers to the shores, on a level with the upper plateau, the steel fabric, composed of slender rods and beams braced to stand the great weight it would have to carry, was built on false work and secured to solid anchorages on shore. Then on this, over tracks laid for the purpose, a crane was run (the same process being carried out on both sides of the river simultaneously), and so the span was built over the water 239 feet above the seething stream, the shore ends balancing the outer sections until the two arms met and were joined exactly in the middle. This bridge required but eight months to build, and was finished in 1883. From the car windows hardly any part of the slender structure can be seen, and the train seems to be held over the foaming torrent by some invisible support, yet hundreds of trains have passed over it, the winds of many storms have torn at its members, heat and cold have tried by expansion and contraction to rend it apart, yet the bridge is as strong as ever.
Sometimes bridges are built a span or section at a time and placed on great barges, raised to just their proper height, and floated down to the piers and there secured.
A railroad bridge across the Schuylkill at Philadelphia was judged inadequate for the work it had to do, and it was deemed necessary to replace it with a new one. The towers it rested upon, therefore, were widened, and another, stronger bridge was built alongside, the new one put upon rollers as was the old, and then between trains the old structure was pushed to one side, still resting on the widened piers, and the new bridge was pushed into its place, the whole operation occupying less than three minutes. The new replaced the old between the passing of trains that run at four or five-minute intervals. The Eads Bridge, which crosses the Mississippi at St. Louis, was built on a novel plan. Its deep foundations have already been mentioned. The great "Father of Waters" is notoriously fickle; its channel is continually changing, the current is swift, and the frequent floods fill up and scour out new channels constantly. It was necessary, therefore, in order to span the great stream, to place as few towers as possible and build entirely from above or from the towers themselves. It was a bold idea, and many predicted its failure, but Captain Eads, the great engineer, had the courage of his convictions and carried out his plans successfully. From each tower a steel arch was started on each side, built of steel tubes braced securely; the building on each side of every tower was carried on simultaneously, one side of every arch balancing the weight on the other side. Each section was like a gigantic seesaw, the tower acting as the centre support; the ends, of course, not swinging up and down. Gradually the two sections of every arch approached each other until they met over the turbid water and were permanently connected. With the completion of the three arches, built entirely from the piers supporting them, the great stream was spanned. The Eads Bridge was practically a double series of cantilevers balancing on the towers. Three arches were built, the longest being 520 feet long and the two shorter ones 502 feet each.
Every situation that confronts the bridge builder requires different handling; at one time he may be called upon to construct a bridge alongside of a narrow, rocky cleft over a rushing stream like the Royal Gorge, Colorado, where the track is hung from two great beams stretched across the chasm, or he may be required to design and construct a viaduct like that gossamer structure three hundred and five feet high and nearly a half-mile long across the Kinzua Creek, in Pennsylvania. Problems which have nothing to do with mechanics often try his courage and tax his resources, and many difficulties though apparently trivial, develop into serious troubles. The caste of the different native gangs who worked on the twenty-seven viaducts built in Central Africa is a case in point: each group belonging to the same caste had to be provided with its own quarters, cooking utensils, and camp furniture, and dire were the consequences of a mix-up during one of the frequent moves made by the whole party.
And so the work of a bridge builder, whether it is creating out of a mere jumble of facts and figures a giant structure, the shaping of glowing metal to exact measurements, the delving in the slime under water for firm foundations, or the throwing of webs of steel across yawning chasms or over roaring streams, is never monotonous, is often adventurous, and in many, many instances is a great civilising influence.