Page:Collier's New Encyclopedia v. 02.djvu/204

LEFT BRIDGE 174 BBIDGE 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, BTV2 feet high, 18 feet wide, consisting of top and bottom pairs of PARKER TRUSS ^^kN/^^ BALTIMORE TRUSS PENNSYLVANIA TRUSS /1AAA/AA/A WARREN TRUSS 'H"T-/?U5S TYPES OF BRIDGE TRUSSES 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 movinjj; 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. Traversing Bridges. — Movable bridges, sometimes called telescope bridges, capa- ble of being rolled horizontally backward, or in an oblique direction, are occasion- ally employed. The bridge across the Arun, near Arundel, on the South Coast railway, is 144 feet long. It is traversed on wheels, and acts as a sliding canti- lever, the overhanging portion resting on the opposite abutment when in place. Lift Bridges. — A notable example is the one erected over the Surrey canal, which is lifted by the four corners; another over the Royal canal, Dublin. Pontoon Bridges. — Bridges of boats are made of boats laid over with planks, fastened across the stream by means of anchors or stakes. The bridge at Rouen