The New Student's Reference Work/Bridge

Bridge, a structure to carry a roadway over a body of water or above the general level of the ground, as across a valley or depression. When the purpose of the structure is to avoid the expense or inconvenience of a permanent embankment the structure is sometimes called a viaduct. If it is to provide an elevated water channel, it is called an aqueduct. The two parts of a bridge are the substructure and the superstructure. The substructure consists of the foundation and of the abutments and piers. The abutments and piers are the parts upon which the superstructure rests. According to the superstructure, bridges are divided into classes as follows: girder, arch, suspension and cantilever bridges. The first three are represented by a plank across a stream, a brick arch and two ropes supporting a suspended platform. The cantilever is a combination of two brackets with a girder between them.

The earliest bridges were made of wood, but masonry arches were made in very early times by both the Romans and Chinese. Cast iron was introduced as a bridge material in the arch bridge across the Severn in 1779. At the present time steel has practically superseded wood for bridge construction.

In a girder bridge there are beams extending from support to support of the substructure. When the beam is not solid, but made up of a network of bars and rods, so that the whole acts like a single beam, it is called a truss. Trusses are of various kinds. The Whipple and Pratt are common trusses in American bridges. There are two long beams or cords connected by diagonals. Early in the century trusses were made of wood, and numbers of such bridges are still in use in the eastern United States. They were later succeeded by combination trusses in which wood and cast iron and wrought iron were used, but all trusses are now more cheaply made of steel.

A peculiar form of the girder bridge was the tubular bridge, which consisted of a rectangular iron tube resting on the piers. The roadbed was through the tube. The Victoria bridge across the St. Lawrence at Montreal is one of the largest and the finest bridges of this kind. It is a railway bridge nearly if miles long, with a central span 330 feet long and twenty-four spans of 242 feet. It cost $7,000,000. Tubular bridges have been superseded by other forms which are cheaper and better. Arch bridges are made of masonry, of iron, or of steel. One of the largest masonry arched bridges is the Trezzo bridge over the river Adda in Italy. It has an arch with 250 feet span. Of recent arched bridges, the Eads bridge across the Mississippi at St. Louis is one of the best. It consists of three steel arches, the central one of 520 feet span and each of the others of 502 feet span. It carries a double line of railway and an upper roadway for carriages and foot passengers. The arch span of this bridge has since been exceeded by the Garabit bridge in France (span 541 feet) and one other; but the St. Louis bridge is still one of the finest and most beautiful arched bridges in the world. It cost over $6,500,000 and took six years in building.

EADS BRIDGE OVER THE MISSISSIPPI RIVER

The first large suspension bridge was the bridge across the Niagara below the Falls. It was built by John A. Roebling in 1852-55, and had a span of 821 feet, at a height of 245 feet above the water. It carried a railroad on the upper platform and a carriage and foot roadway underneath. It was suspended by four cables, each ten inches in diameter and containing 3,640 wires. The largest and finest suspension bridge in the world is the Brooklyn bridge, across the East River at New York. It has a span of 1,600 feet, with a total length of over a mile. The towers are 277 feet high. These towers carry the four suspension cables. Each of these cables is 15½ inches in diameter and composed of 5,282 galvanized iron wires. There is a

BASCULE RAILROAD BRIDGE, TAYLOR STREET, CHICAGO

The old swing bridge is shown in center

distance of 135 feet from high water under the center of the bridge. The bridge has a width of 85 feet, and carries two roadways, two railways and a footway. It was opened in 1883 and cost over $15,000,000. The traffic has increased so that two additional bridges have been built, one to Brooklyn costing $26,000,000 and one to Blackwell's Island costing $17,900,000. It was also proposed to bridge the Hudson River from New York to the New Jersey shore, but the plan has been abandoned, and a tunnel under the Hudson and extending under the East River has been built by the Pennsylvania Railroad.

BASCULE RAILROAD BRIDGE, TAYLOR STREET, CHICAGO

The old swing bridge is shown in center