Arch Bridges. —Bridges in the shape of arches are often built in places where a more artistic structure than a truss is desired. The High bridge and Wash ington bridge across the Harlem river in New York City are examples of this style of bridge. The High bridge was built to carry the Croton aqueduct across the Harlem river. It consists of 13 168 feet each. The total length of the viaduct, including piers, is 8,296 feet, or a little over Ph miles, of which almost exactly one mile is covered by the great cantilevers. The clear headway under the center of the bridge is 152 feet at highwater, and the highest part of the bridge is 361 feet above the same level. Between the two main girders a double line of railway is carried on an internal viaduct supported by trestles and cross girders. The whole of the metal work of the superstructure is of Siemens steel. The way consists of heavy bridge rails laid on longitudinal sleepers bedded in four steel troughs, into which the wheels will drop in case of derailment, when they will run on the sleepers.
In the piers there are about 120,000 cubic yards of masonry, and in the su perstructure 44,500 tons of steel.
There are several of these bridges in the United States, the first of any size being the Niagara cantilever, built in 1883. Its total length is 910 feet, and it is 295 feet above the surface of the river, with steel towers 130 feet high. The Hudson river bridge at Pough keepsie, built in 1889, has a length of 6,767 feet and is built in five spans; the first, third and fifth being true cantilever 1 2—Vol.
granite arches, the highest one being 116 feet above the river. The bridge, cross ing the river and valley, is 1,460 feet long. The Washington bridge is situated a short distance N. of the High bridge and consists of nine arches, three of granite on the E. side, four of granite on the W., and two central steel spans connecting them. The entire length of the bridge is 2,300 feet, and width, 80 feet; the central spans being each 510 feet long and 135 feet above high water. Another noted arch bridge is located at St. Louis, Mo. It has two spans of 497 feet and one of 515, with a total length, including abutments of 1,700 feet. It is a two story affair with a double rail road track below and a roadway above. The Hell Gate bridge over the East River in New York City is 1,000 feet in length, and is the largest arch bridge yet constructed. It was completed in 1916.
American Quadrangular Girder Bridges.—One of the best examples of American long-span iron-bridge con struction is the bridge across the Ken tucky river on the Cincinnati Southern railroad, noteworthy for its economical design and comparatively light weight. The iron work of the bridge is 1,138 feet in length, and it consists of three spans of 375 feet each. It crosses a limestone
canon at a height of 280 feet above the bed of the stream. The piers are of stone to a height of 60 feet, to clear the highest recorded floods; and they are about 34 feet thick at the flood level. Above the stonework the piers are of iron. The truss or girder is rectangular in section, feet high, 18 feet wide, consisting of top and bottom pairs of booms, forming the corners, united by panels or frames at intervals of 18% feet longitudinally, stiffened and bound with diagonal tie rods. The booms each consist of flat plates placed vertically, riveted together. The piers consist of hollow pillars of plate iron riveted to gether in box form. The diagonal rods are pin connected, that is to say, they are connected to the framework with cylindrical pins, a form of connection much practiced in the United States. The bridge was completed in February, 1877.
Lattice Girder Bridges.—The iron lattice bridge, so called from having sides constructed with cross bars, like lattice work, is the natural outcome of the tubular bridge for long spans, developing equal strength with considerable economy of material and labor. Lattice girders are now almost universally adopted for iron bridges for long spans.
Movable Bridges.—Movable bridges are usually required in the neighborhood of rivers, docks, wharves, canals, and like situations for the passage of ships and boats. They are variously designed and adapted to particular situations, and may be classified as (1) bascules or draw bridges, (2) swing bridges, (3) travers ing bridges, (4) lift bridges, (5) pontoon bridges.
Bascules or Drawbridges.—The bas cule bridge is such as is raised by turn ing, in one piece or in two pieces, round one or two horizontal axes or hinges. For large dimensions it is convenient to construct the bridge in two halves, lift ing from each side, and abutting to gether to the middle. A bridge of this sort has been built across the Thames at London and another for elevated rail road use across the Chicago canal. They are now comparatively common. The largest one of the type was completed over the Chicago river in 1919.
Swing Bridges.—Swing bridges are by far the most commonly employed of movable bridges. The large rivers to be crossed in the United States have de manded swing bridges of great span, with excellent contrivances for minimiz ing friction and insuring steadiness when closed. The swing bridge over the Rari tan, in New Jersey, allows two free passages, each 216 feet wide. The Kan sas City bridge crosses two passages, each 160 feet wide. The total moving weight is 303 tons. The bridge is opened by steam power in about one and a half minutes, or by manual power in two minutes.