METAL Aacieas. The first all-iron bridge ever built was a cast-iron bridge of 102 feet span crossing the river Severn at Coalbrookdale, in England. This bridge was built in 1776-79. In 1794 a similar bridge was built near Laasan, in Germany. and was the first iron bridge constructed on the Continent of Europe. Both the Coalbrookdale and Laasan bridges were in 1900. and carrying their traffic with perfect safety. As time passed, cast iron was replaced by wrought iron, and the latter in turn by steel in arch-bridge construction. An equally important development in fitting the metal arch to fill the prominent place which it has occupied in bridge construction was the adoption hinges. At first, following the example of stone arches, hinges were entirely dispensed with, the arch structure being consequently statically determinate. Hinges had been suggested early in the Nineteenth Century, but it was not until 1841 that European mathematicians began the at crown and abutments, and sometimes are sim ply lead joints placed at crown and abutments, and often, also, at the quarter points. The fol lowing are the most important hinged concrete arches actually built: Raileeay.—Saxony, span 42.64 feet, rise 9.84 feet, thickness of ring 1.64 feet to 1.96 feet, hinges of sandstone. with con vex surface and concave bearing, built in 1SSO: Slunderkingen, Wiirttemberg, span 164 feet, rise 16.4 feet, arch ring at crown 3.28 feet thick, hinges of steel at crown and abutments, built 1893: Ilithenstein, Wiirttemberg, span 75.44 feet, hinges consisting of lead joints at crown and abutments, built 1893; Hohenzollern, over the Kaiser Wilhelm Canal, Germany, span 98.44 feet, rise 9.84 feet, hinges of granite, hushed with sheet lead, built 1896. These bridges have given excellent serviee and have developed no cracks. The claim made for the hinged arch is, that it is statically determinate, and that since the hinges allow for a certain amount of distortion of the arch ring without rupture, less care and expense are necessary in securing absolutely immovable foundations.
The preceding paragraphs, at best, touch only in the briefest way the development, design, and construction of masonry-arch bridges. For a discussion of the theory of the masonry arch, see A Treatise on Masonry Construction (New York, 1899), by Prof. I. I I. Baker, and A Treatise on Arches (New York, 1897), by Jl. A. Howe, and for descriptions of prominent structures of notable series of studies which filially developed the perfected theory of the hinged arch. In the railway bridge over the Saint-Denis Canal. on the Paris-Creil line, built in 1858, hinges were inserted at the abutments. In ISO-I an arch bridge over the river Wien, in Austria, was built with hinges at both the crown and abut ments. From this time on the development of the hinged metal arch was rapid, particularly in Continental Europe. although it did not by
any means replace the hingeless a reli entirely. With this brief sketch of the development of the metal-areh bridge. attention will be turned to a few representative structures of this type.
As the most notable example of the cast-iron bridge, now seldom built, mention may be made of the Southwark Bridge across the river Thames, London, which was erected in 1819. This strut. ture has two side spans of 210 feet and a centre span id* 240 feet, with a rise to the circle of one-tenth the span in each instance. The arch ribs were composed of cast sections like the stones of a masonry arch, and at best the bridge was little more than a heavy and wasteful imitation of a stone ring. The main dimensions of a number of other cast-iron bridges are given in the aecompanying table. Probably not more than three cast-iron bridges were built during the twenty-tive years preceding 1901, while the number of wrought-iron and steel arches built during that time of sufficient span to be noted in engineering literature number over seventy five, In 1873 the arch bridge over the Slis.sisceipgi at Saint Louis, Mo., was completed with a centre span of 520 feet and two side spans of 50• feet; each span consisted of four parallel east-steel arch ribs, and the bridge carried railway traeks and a highway. The cost of the Saint Louis Bridge was about $5,300,000. No other instance of a bridge of important size being built of cast-steel arches was recorded until 1899, when the Alexander 111. Bridge. across the Seine at Paris, was completed with a single span of 352.6 feet, the arch ribs being made up of wedge-like east-steel sections bolted together. Unlike the Saint Louis Bridge, whieh was built without hinges. the Alexander Ill. arch had hinges at the erown and The longest span arch bridge of wrought steel in the world is the high way and foot bridge built across the gorge of the Niagara River, just below the Falls at Niagara, in 189S-99, with a single span of 840 feet. In 1897, a steel-arch railway bridge was built across the same stream about a mile farther down stream. with it span of 550 feet. Both of these structures were erected without false works to support the metal during erection; each half arch was built out from the shore, member by mem ber, and held back by strong anchors to the shore until the ends met at mid-stream. One is not a single steel-arch span of notable dimen sions.
For the discussion of the theory of the metal arch, see a Treatise on Arches, by Prof. N. A. Howe (New York, 18:17), and fur deneriptions of notable structures of this type, see the vol of Transactions American tio•icly of Ciril Engineers (Ne• York, 1868, current) ; Anna/es des Potts et Chaussees (Paris, 1831, current) ; and the American and European engineering journals.