BBIDGE. 483 BBIDGE. in masonry arch-bridge construction has been the concrete arch with a metal skeleton and arches with hinges in the ring. Concrete, in which a skeleton of metal rods, beams, or netting is im- bedded, is one of the systems of arch construction which has gained prominence in recent years. One of the longest structures of this type so far built is the Topeka (Kansas) Bridge, consisting of five 125-feet spans, two 110-feet .spans, and two 97%-feet spans. Each arch is of concrete, in which are imbedded twelve lattice girders of steel, spaced three feet apart and bent to the curve of the arch. Numerous other spans of this and other systems of concrete metal construction have been built in Europe and . ierica. Con- crete arches without metal skeletons are less commonly used. Perhaps the most notable use of the plain concrete arch so far recorded was in the extension of the West Highland Railroad of Scotland in 1897-98. The longest single span employed was 127 feet, but most of the arches used were of 30 feet and 50 feet span ; the larg- est structure had twenty-one 50-feet semicircular arch spans, and a total length of 1248 feet. Hinged arches of concrete have been built in considerable numbers in Europe. The hinges are sometimes true hinges of metal or stone placed this type, see Encyclopwdia of Civil Engineering (London, 1847), by Edward Cresy, and the vol- umes of Annates des I'onis et Vliausst'cs (Paris, 1831, current) ; Proceedings, Institution of Civil Engineers (London, 1838, current) ; and Engi- neering A'citis. ilETAL Arche.s. The first all-iron bridge ever built was a cagt-iron bridge of 102 feet span crossing the river Severn at Coalliniokdule, in England. This bridge was built in 1770-79. In 17U4 a similar bridge was built near Laasan, in Silesia, Germany, and was the first iron l>ridge constructed ou the Continent of Europe. Both the Coalbrookdale and Laasan bridges were stand- ing in 1!)00, and carrying their tralfic 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 of hinges. At first, following the example of stone arches, hinges were entirely dispensed with, the arch structure being consequently statically in- determinate. Hinges ha<l been suggested early in the Nineteenth Century, but it was not until 1841 that European mathematicians began the Table op Dimensions of Notable Stose-Abch Bhidges IfAUE Place CoMntry Date BuUt No. of Spans Span, feet Rise, feet Luxpmburg Germany 1901 1380 1857-« 1842-3 1S88 1832-3 1833 1888-9 277.65 251 220 213 201.7 200 2L0 190.8 183.7 180 max. 101.08 Italy 87.8 Maryland D. S. A 57.3 59 Lavaur France 90.2 42 Turin Italy 42 52.8 GO Ballochmoyl.... 90 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: Uailway. — 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 1880; ilunderkingen, 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; Eithenstein, Wiirttemberg, span 7.5.44 feet, hinges consisting of lead joints at crown and abutments, built 1893: HohenzoUern, over the Kaiser Wilhelm Canal, Germany, span 98.44 feet, rise 9.84 feet, hinges of granite, bushed with sheet lead, built 1896. These bridges have given excellent ser'ice and have developed no eracks. 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 discu.ssion of the theory of the masonry arch, see A Treatise on Masonry Construction (New York, 1899), by Prof. I. O. Baker, and 4 Treatise on Arches {New York, 1897), by M, A. Howe, and for descriptions of prominent structures of notable series of studies which finally dcvelojied 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 1864 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 ill Continental Europe, although it did not by any means replace the hingeless arch entirely. With this brief sketch of the development of the metal-arch 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 Tbames, London, which was erected in 1819. This ■junc- ture has two side spans of 210 feet and a centre span of 240 feet, with a rise to the arch 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 accompanying table. Probably not more than three cast-iron bridges were built during the twenty-five 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-
