When iron and steel took the place of stone as the chief bridge-building materials, bridges of many different kinds were designed and built. Sometimes, however, the arch pattern is still used for still bridge, especially when a deep but not very wide gap has to be spanned neatly. A good example is the bridge which crosses the Zambezi River in Africa, just below the famous Victoria Falls. The bridge stands four hundred feet above the dashing and fifty feet long.
Lower down its course, in East Africa, the Zambezi River is crossed by the longest bridge in the world. It is built of steel girders resting upon concrete pillars sunk into the bed of the river. The main girders are strengthened by arch-shaped girders and by a latticework of smaller girders. The bridge is five hundred yards longer than the present Tay Bridge, which is Great Britain’s longest bridge.
We read in the last lesson of the disaster that be feel the first Tay Bridge. The engineer who designed it had planned another mightily bridge to cross the firth of forth, but he was not allowed to carry out his idea. Instead, two other engineers, with the terrible lesson of the Tay Bridge fresh in their minds, designed an entirely different type of bridge.
The forth Bridge is said to be the strongest bridge in the world. It is called a ‘cantilevers bridge’, from the three immense diamond-shaped ‘cantilevers’ which from the main part of it. The great central cantilever rests firmly upon a small rocky island in mid-stream; it is connected by lengths of Girder Bridge with the two outer cantilevers, which are supported by sturdy foundations of granite.
The forth bridge is a mile and half in length. Another cantilever bridge, with a single span six hundred yards long, crosses the St. Lawrence River near Quebec. To keep such large bridges in good condition they must be painted regularly. Over forty men are required to paint the Forth Bridge from end, and the task occupies three years.
Another common type of bridge is the ‘suspension bridge,’ that is, one in which the roadway is suspended from chains, which pass over towers and are fastened securely at either end. As early as 1825, a suspension road-bridge was built across the Menai Strait, and in 1860, the graceful Clifton Bridge was slung across the river Avon at Bristol. From these smaller bridge have sprung some of the amazing suspension bridges of today.
The largest and most wonderful of them all is the one, which spans the entrance to San Francisco harbor in Western America. The bridge is supported by two giant steel towers; over these pass the steel cables, a yard thick, from which the roadway is suspended. The distance between the great towers is over four thousand feet; this is the longest bridge-span in the world.
Within San Francisco Bay is another bridge, which is sometimes claimed to be the world’s longest bridge, for its total length, is over eight miles. It is not, however, a single bridge but a number of bridges strung together; it includes suspension spans, a cantilever span, a girder spans, and even a very long tunnel through an island.
These bridges at San Francisco cost twenty-four million pounds to build. We cannot help thinking that the name for the entrance to San Francisco harbor, ‘The golden Gate,’ is a very fitting one indeed!
Another of the world’s great bridges stands astride the harbor of Sydney, Australia. It is called a ‘bow-string bridge’; a glance at the above picture will explain the name. The roadway of the bridge is supported, by steel hangers, from the great steel arch overhead. Twin towers of stone at either end form the entrance to this wonderful rainbow arch of steel.
One of the most difficult problems of the engineer is that of building a bridge over a busy shipping river, which flows between low banks. If he builds his bridge straight across, it high above the level of banks and river, road and rail traffic may not be able to cross it easily.
The famous Tower Bridge across the Thames at London shows how the difficulty may be overcome. The main roadway of the bridge is made in two halves, which may be raised to allow a large vessel to pass. This type of bridge is called a ‘bascule bridge,’ from the French word ‘bascule’ meaning ‘see-saw.’ The word is a good one, for each half of the bridge is really a giant see-saw worked by powerful but hidden machinery. Problems of the engineer are that of building a bridge over a bridge.
High overhead is a footway, which may be used by the passengers when the lower bridge is raised; but since the passing of a ship causes only about five minutes’ delay, most people prefer to wait until the bridge is lowered again.
Another kind of movable bridge is the ‘swing-bridge,’ in which the middle portion of the bridge may be swung round, like a turnstile, to allow a ship to pass.
The giant ‘swing’ is controlled by the pressing of an electric button!
The great Howrah Bridge at Calcutta, completed in 1946, is a cantilever road bridge-not a railway bridge, as are the forth and Quebec bridges. It carries a wide roadway and two footpaths.