Bridges

The three great cla es of bridge structures are beam or girder, suspension, and arched bridges, the first cla exerting only vertical pre ures on their supports, the second a horizontal pull, and the third a horizontal thrust. The various subdivisions of the first cla are, simple, draw, continuous, and cantilever bridges. These, as well as aqueduct, railroad, stone, tubular, etc., receive notice under their respective headings. The following is a brief historical summary of bridge develop ment: The earliest simple bridges were trestle and pile structures like the pons sublicius at Rome, 620 B.C., and Cæsar's bridge over the Rhine, 55 B.C. The tru system of panels was used by Palladio, an Italian architect, ab. 1570, but was not succe fully developed until the present century. Wooden tru es were mostly employed until 1840; among these may be noted the Burr, the Long, and the Town lattice tru ; they were rarely greater in span than 200 feet, the longest being one of 360 feet, built 1760 in Baden by Grubenmann. Combination tru es, having some of the tensile members made of wrought-iron, were first built by Howe in 1840, and bridges wholly of iron came into use soon after. The pioneer in the U. S. in erecting iron bridges was Whipple, to whom the first exact theory and computations are also due. After 1850 the Pratt tru , the Whip Girder Bridge, Aurora, Ind., 310 ft. span, 18 ft. roadway. ple tru , and many others were rapidly developed, and several spans longer than 500 feet have been erected since 1880, the longest being the highway and railway span of 550 feet over the Ohio River at Cincinnati. Tubular bridges have not been built in U. S. Draw bridges, swinging on a central pivot, have been developed since 1850, the greatest being the Thames River draw at New London, Conn., whose length is 503 feet. Continuous bridges resting on several supports have been mainly used in Europe. Cantilever bridges came into use soon after 1880, the first being that at Niagara, and the largest the Forth Bridge in Scotland, with two spans of 1,700 feet each. Steel has largely replaced wrought-iron since 1890 in all cla es of bridge structures, its strength being materially greater and cost about the same. Suspension bridges in the form of a rope carrying a light platform, were in use at a remote antiquity, but the modern structure, in which the floor is hung from cables, was employed first Suspension Bridge over the Scorff, France. in the U. S. by Finlay in 1805, and its adaptation to railway service was demonstrated by Roebling in 1854 by the construction of the Niagara Bridge. The longest suspension span is the East River Bridge at New York, whose length is 1,595 feet, but plans have been made for one of 3,100 feet over the Hudson. The earliest arched bridges were of stone and timber; cast iron was employed late in the eighteenth century. Steel arches of 510 feet span were built 1889 in the highway bridge over the Harlem River at New York, and one of 515 feet, 1873, in the railroad bridge over the Mi i ippi at St. Louis. The latter, which is the longest arched span, appears to be near the practical limit of length on account of the expense of erection as compared with cantilever and suspension systems. Over 600 miles of timber and iron bridges have been built on IMG:content-0617.png:[merged small][graphic] Washington Bridge, Harlem River, N. Y. 6,520 feet. The longest bridge in the U. S. is that over the Ohio at Cairo, Ill., which has two spans of 518 feet, seven of 400 feet, three of 249 feet, and 38 spans of viaduct approaches, making a total of 10,560 feet, or two miles. The longest in the world is the new Tay Bridge in Scotland, which aggregates 10,780 feet.