Inventions in the Century. Doolittle William Henry

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СКАЧАТЬ supported by four enormous cables of wire stretching from the Canadian cliff to the opposite United States cliff. The cables pass over the tops of lofty stone towers arising from these cliffs, and each cable consists of no less than 4,000 distinct wires. The roadway hangs from these cables, suspended by 624 vertical rods.

      The engineer of this bridge was John A. Roebling, a native of Prussia, born there in 1806, and who died in New York in 1869. He was educated at the Polytechnic School in Berlin, and emigrated to America at the age of 25. His labors were first as a canal and railway engineer, then he became the inventor and manufacturer of a new form of wire rope, and then turned his attention to the construction of aqueducts and suspension bridges. After the Niagara bridge, above described, he commenced another bridge of greater dimensions over the same river, which was finished within two or three years. His next work was the splendid suspension bridge at Cincinnati, Ohio, which has a clear span of 1057 feet. In 1869, in connection with his son, Washington A. Roebling, he commenced that magnificent suspension bridge to unite the great cities of New York and Brooklyn, and which, by its completion, resulted in the consolidation of those cities as Greater New York. The Roeblings, father and son, were to the engineering of America what George Stephenson and his son Robert were to the locomotive and railway and bridge engineering of Great Britain.

      The Brooklyn bridge, known also as the East River bridge, was formally opened to the public on the 24th of May 1883. Most enormous and unexpected technical difficulties were met and overcome in its construction. Its total length is nearly 6,000 feet. The length of the suspended structure from anchorage to anchorage is 3,454 feet. A statement of the general features of this bridge indicates the nature of the construction of such bridges as a class, and distinguishes them from the comparatively simple forms of past ages. This structure is supported by two enormous towers, having a height of 276 feet above the surface of the water, carrying at their tops the saddles which support the cables, and having a span between them of 1,595 feet. The towers are each pierced by two archways, 31½ feet wide, and 120½ feet high, through which openings passes the floor of the bridge at the height of 118 feet above high water mark. There are four supporting cables, each 16 inches in diameter, and each composed of about 5,000 single wires. The wire is one-eighth size; 278 single wires are grouped into a rope, and 19 ropes bunched to form a cable. The iron saddles at the top of the lofty towers, and on which the cables rest, are made movable to permit its expansion and compression – and they glide through minute distances on iron rollers in saddle plates embedded and anchored in the towers, in response to strains and changes of temperature. The enormous cables pass from the towers shoreward to their anchorages 930 feet away, and which are solid masses of masonry, each 132 x 119 feet at base and top, 89 feet high, and weighing 60,000 tons. The bridge is divided into five avenues: one central one for foot passengers, two outer ones for vehicles, and the others for the street cars. The cost of the bridge was nearly $15,000,000.

      Twenty fatal and many disabling accidents occurred during the construction of the bridge. The great engineer Roebling was the first victim to an accident. He had his foot crushed while laying the foundation of one of the stone piers, and died of lockjaw.

      It was necessary to build up the great piers by the aid of caissons, which are water-tight casings built of timber and metal and sunk to the river bed and sometimes far below it, within which are built the foundations of piers or towers, and into which air is pumped for the workmen. A fire in one of the caissons, which necessitated its flooding by water, and to which the son, Washington Roebling, was exposed, resulted in prostrating him with a peculiar form of caisson disease, which destroyed the nerves of motion without impairing his intellectual faculties. But, although disabled from active work, Mr. Roebling continued to superintend the vast project through the constant mediation of his wife.

       Tubular Bridges.– These are bridges formed by a great tube or hollow beam through the center of which a roadway or railway passes. The name would indicate that the bridge was cylindrical in form, and this was the first idea. But it was concluded after experiment that a rectangular form was the best, as it is more rigid than either a cylindrical or elliptical tube. The adoption of this form was due to Fairbairn, the celebrated English inventor and engineer of iron structures. The Menai tubular railway bridge, adjacent to the suspension bridge of Telford across the same strait, and already described, was the first example of this type of bridge. Robert Stephenson was the engineer of this great structure, aided by the suggestions of Fairbairn and other eminent engineers. This bridge was opened for railway traffic in March, 1850. It was built on three towers and shore abutments. The width of the strait is divided by these towers into four spans – two of 460 feet each, and two of 230 feet. In appearance, the bridge looked like one huge, long, narrow iron box, but it consisted really of four bridges, each made of a pair of rectangular tubes, and through one set of tubes the trains passed in going in one direction, and through the other set in going the opposite direction. These ponderous tubes were composed of wrought-iron plates, from three-eighths to three-fourths of an inch thick, the largest 12 feet in length, riveted together and stiffened by angle irons. They varied in height – the central ones being the highest and those nearest the shore the lowest. The central ones are 30 feet high, and the inner ones about 22 feet. Their width was about 14 feet. They were built upon platforms on the Caernarvon shore, and the great problem was how to lift them and put them in place, especially the central ones, which were 460 feet in length. Each tube weighed 1,800 pounds, and they were to be raised 192 feet. This operation has been described as "the grandest lift ever effected in engineering." It was accomplished by means of powerful hydraulic presses. Another and still grander example of this style of bridge is the Victoria at Montreal, Canada. This also was designed by Robert Stephenson and built under his direction by James Hodges of Montreal. Work was commenced in 1854 and it was completed in December, 1859, and opened for travel in 1860. It consists of 24 piers, 242 feet apart, except the centre one, from which the span is 330 feet. The tube is in sections and quadrangular in form. Every plate and piece of iron was made and punched in England and brought across the Atlantic. In Canada little remained to be done but to put the parts together and in position. This, however, was in itself a Herculean task. The enormous structure was to be placed sixty feet above the swift current of the broad St. Lawrence, and wherein huge masses of ice, each block from three to five feet in thickness, accumulated every winter. The work was accomplished by the erection of a vast rigid stage of timber, on which the tubes were built up plate by plate. When all was completed the great staging was removed, and the mighty tube rested alone and secure upon its massive wedge-faced piers rising from the bedrock of the flood below.

      The Tubular Arch Bridge.– This differs from the tubular bridge proper, in that the former consists of a bridge the body of which is supported by a tubular archway of iron and steel, whereas in the latter the body of the bridge itself is a tube. The tubular arch is also properly classed as a girder bridge because the great tube which covers the span is simply an immense beam or girder, which supports the superstructure on which the floor of the bridge is laid. A fine illustration of this style of bridge is seen in what is known as the aqueduct bridge over Rock Creek at Washington, D. C., in which the arch consists of two cast-iron jointed pipes, supporting a double carriage and a double street car way, and through which pipes all the water for the supply of the City of Washington passes. General M. C. Meigs was the engineer.

      Another far grander illustration of such a structure, in combination with the truss system, is that of the Illinois and St. Louis bridge, across the Mississippi, of which Captain James B. Eads was the engineer. There are three great spans, the central one of which has a length of about 520 feet, and the others a few feet less. Four arches form each span, each arch consisting of an upper and lower curved member or rib, extending from pier to pier, and each member composed of two parallel steel tubes.

      Truss and truss arched bridges.– These, for the most part, are those quite modern forms of iron or wooden bridges in which a supplementary frame work, consisting of iron rods placed obliquely, vertically or diagonally, and cemented together, and with the main horizontal beams either above or below the same, to produce a stiff and rigid structure, calculated to resist strain from all directions.

      Previous to the 19th century, the greatest bridges being constructed mostly of solid СКАЧАТЬ