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GENERAL REPORT OF THE JUDGES OF GROUP XXVI. 35 pended superstructure will consist of an iron framing, 85 feet in width, suspended from four main cables by wire ropes attached to iron floor-beams placed 7 feet 6 inches apart. The flooring is further to be divided into five spaces by six lines of iron trusses, of which the two centre trusses have a depth of 12 feet, and the others of 8 feet. The outer spaces have a width in the clear between the trusses of 18 feet, and each will accommodate two lines of iron tramways for ordi nary street traffic. The next two spans, 13 feet 2 inches wide, will be provided with iron rails for running two passenger trains to be worked by a wire rope. The bridge is to be supported by four main cables, two outer ones and two near the middle of the flooring. The cables, of which a specimen was in the Exhibition, will be 16 inches in di ameter, composed of galvanized tempered cast-steel wire, No. 6 gauge, having a strength of 160,000 pounds per square inch of section. The cables are to be aided by a system of 104 stays in each quarter, which are together assumed to be capable of upholding the super structure of the main span, the aggregate weight of which, inclusive of cables, will be 5000 tons. Another illustration of bridge-building on a grand scale is the Illi nois and St. Louis Steel Arched Bridge, spanning the Mississippi at St. Louis,—a work remarkable for boldness of design and for origi nality in the mode of construction. This bridge has three main arches of large size. The central arch has a span of 520 feet, with a versed sine of 47 feet; and two arches, one on each side the central arch, have spans of 515 feet. Each of the three arches consists of four ribs about 12 feet deep, the top and bottom of each rib being steel cylinders, 18 inches in diameter, 2 inches thick at the skew-back, and 7/8 of an inch thick at the soffit. These are connected and braced together also by cylindrical tubes. The bridge carries a roadway on the top and a double line of railway beneath. The segments of the ribs as the erection advanced were supported by overhead guys pass ing over temporary towers erected on each pier and abutment, so that all scaffolding, centering, or supports from below were dispensed with. The Raritan Bay Swing or Pivot Bridge, of which a model was ex hibited, is a good illustration of a large bridge of another class. The movable part of the bridge is 472 feet in length, leaving, when the bridge is swung open, clear waterways on each side of 220 feet. The largest iron truss bridge at present in use in the United States seems to be the Newport and Cincinnati Bridge, the channel span of which is 420 feet, but larger bridges of this class are already projected. One proposed to be built over the Hudson River at Poughkeepsie, New York, will have five spans each of 525 feet.
