The largest is the bridge over the Susquehanna at Rockville, Pa., begun in 1901. It has 48 spans of 70 feet each, and is in all 3,820 feet in length. The facings are of masonry, and the body is filled in with concrete. Many other stone bridges were built on other railroads in the period 1900 to 1910. Since then the use of concrete as railroad bridge material has come into great favor. The more recent stone bridges in the United States have been ornamental structures for architectural effects in parks and cities. They are mostly arches of comparatively small dimensions.
About the beginning of the 19th century, metal began to be extensively introduced as a material of construction, and although the masonry arch was superior in beauty and dura bility, the metal bridge gave greater strength in, proportion to the weight of the structure; was capable of being built more quickly and cheaply, and therefore, being better adapted to keep pace with the tremendous activity of modern railway construction, it soon almost completely sup planted the stone structures.
The first structure in which the new material was used exclusively appears to be the bridge across the Severn near the town of Ironbridge, Shropshire, England. It was *built by Abraham Darby, the owner of the ironworks of Coal brookdale, in 1779, and consists of a single arch 100 feet in span, with a rise of 45 feet. The arch is composed of five cast-iron ribs in the form of a segment of a circle. The successful construction of such a structure being thus clearly demonstrated, several others, of holder design, were built during the last quarter of the 18th century, of which the Wearmouth Bridge, over the Wear at Sunderland, completed in 1796, is the most elegant example of the type. As originally constructed, it consisted of a single hingeless arch 236 feet in span, with a rise of 34 feet above the springing lines, which were 65 feet above the level of the river.
From these beginnings the evolution of vari ous types of metal arch structures was rapid. A large percentage of wrought iron was em ployed in the material of construction, affording greater flexibility in the methods of construc tion and design, and the capability of spanning greater intervals with single spans. The most notable of the structures built about this time (the early part of the 19th century) was the Southwark Bridge across the Thames, com pleted in 1819. It consists of three arches, of
which the central arch is 240 feet in span, the largest cast-iron span ever built, and the side arches 210 feet each. They are composed of massive cast-iron arch ribs, which being set without any provisions to counteract the effects of expansion, and being much heavier than is necessary to sustain the loads to which they are subjected, gives a structure which is merely the imitation of one of stone, with the additional fault of a great wastefulness of material. Its construction, however, served a valuable pur pose in the development of metal arch engineer ing. It suggested the principle of hinged arches, which was subsequently taken under considera tion by the mathematicians of Europe in 1841, and developed into a perfect theory, which made thel metal arch a statically determinate structure on absolutely immovable foundations, and one that was provided against distortion and rupture under conditions of varying temperature.
Although the fine cast-iron bridges such as the Westminster and the Blackfriars, built across the Thames during the period covered by 1860 to 1870, consisted of arches with spans as great as 185 feet, the metal hinged-arch bridges built since 1873 have surpassed them greatly, not only in the length of the spans, but in the economy of the material of construction.
An exception to this statement, however, must be noted in the case of the cast-steel Eads Bridge across 'the Mississippi, at Saint Louis, Mo., which was completed in 1874. It consists of three unhinged structures, the centre arch being 520 feet in span. Of the hinged-arch bridges the following are the most notable: The Alexander III Bridge across the Seine at Paris, built in 1899, consisting of a single arch with a span of 353 feet, composed of arch-ribs made up of wedge-shaped cast-steel sections bolted together; the steel-arch railway bridge across the Niagara River, finished in 1896, con sisting of a single arch with a span of 550 feet; and the highway and foot-bridge completed in 1899 across the Niagara Gorge, just below the Falls, consisting of a single steel arch with a span of 840 feet, which made it the largest single span steel-arch bridge in the world' until surpassed by the Hell Gate Bridge.