BRIDGE, a structure consisting of an arch or series of arches supporting a roadway above it, designed to unite two banks of a river or the two sides of an open space. A bridge is generally made of wood, iron, stone, steel, or of brick. The extreme supports of the arches at the two ends are called butments, or abutments; the solid parts between the arches, piers, and the fences on the sides of the road or pathway, parapets. Bridges are now of many kinds, the most usual varieties being the following: Suspension Bridges. — These bridges are such in which the roadway is sus pended from chains, links, cables or ropes, passing over piers or towers, fixed or anchored at their extremities. Another line of evolution had its origin in the principle of suspension. In the typical modern suspension bridge, when the weight of the roadway is known by the stress on the suspending links, the problem of statical equilibrium assumes the simplest form, and the conditions of strength and stability are steadily de termined. But when there is a shifting or rolling load on the roadway, which is heavy in proportion to the weight of the bridge, as, for example, a railway train, the conditions are involved. When the train occupies, say, only one-half of the bridge, the chain is depressed on one side, and is raised on the other side. Thus an undulation is produced in the bridge, which, especially if the train be moving rapidly, may seriously disturb the equilibrium, and even endanger the stability of the bridge. Various com binations have been devised to overcome this difficulty. The simplest and prob ably the best course is to stiffen the roadway, so that the stress of the pass ing load may be distributed over a con siderable length of chain. In this man ner large railway bridges have been constructed, for example, the Roebling bridge (1855) over the Niagara, miles below the falls, having a span of 822 feet, and being 245 feet above the level of the stream.
The Brooklyn suspension bridge, across the East River, between New York and Brooklyn, opened in 1883, is built of steel. It has a central span of 1,5951/2 feet, and two land spans of 930 feet each; making, with approaches, a total length of 5,989 feet, or about one mile and one furlong. The anchorage at each end is a solid cubical structure of stone, measuring 119 feet one way, by 132 feet the other, rising to a height of 90 feet above high water mark, weighing 60,000 tons each. The
towers are 278 feet high. The weight of the whole structure suspended be tween the towers is nearly 7,000 tons. The stress of suspension is borne by four cables of 5,296 steel wires each, 15% inches in diameter. The founda tions of the towers were laid by means of caissons and compressed air, at a level of about 80 feet below high water mark. The Manhattan Bridge (con structed 1901-1911) across the East River between New York and Brooklyn, has a river span of 1,470 feet and a shore span of 725. It is 132 feet wide. The Queensboro bridge (1901 - 1909) over the East River, joining New York and Long Island City, is 7,144 feet long and 90 feet wide.
Cantilever Bridges.—A cantilever is a bracket. It is a structure overhung from a fixed base. The bridge across the river Forth on the North British railway system is one of the largest and most magnificent bridges in the world. The site of the bridge is at Queensferry. At this place, the estuary of the Forth is divided by the island of Inchgarvie into two channels, whose depth, as much as 200 feet, precluded the construction of intermediate piers. Hence, two large spans of 1,700 feet each were adopted. Between these, the central pier is founded on the island midway across, and is known as Inch garvie pier. There are two other main piers, shore piers, known respectively as the Fife pier and the Queensferry vier. Of these three piers respectively three double lattice work cantilevers like scalebeams, 1,360 feet, or a quarter of a mile in length, are poised in line, reach ing toward each other, and connected at their extremities by ordinary girders 350 feet long, by which the two main spans are completed. The bridge con sists of two main spans of 1,700 feet, or nearly one-third of a mile each; two of 675 feet each, being the shore ends of the outer cantilevers; and 15 spans of spans with fixed continuous spans con necting them. The Queensboro bridge is of the cantilever type. The Quebec bridge, 640 feet long, which fell while in course of construction in 1907, and again in 1917, was completed in the latter year. The Spokane bridge over the Willamette river has a draw span of 521 feet, the largest in the world (1920). The Quebec cantilever-truss bridge (1916) has a span of 1,800 feet. Other notable cantilever bridges are those across the Colorado river, at Red Rock, Cal., and across the Mississippi river, at Memphis, Tenn.