SUSPENSION BRIDGE'S.
In their crudest form of two parallel cables carrying a floor directly on the cables, suspen sion bridges have doubtless been used since very early times. Such bridges, with the cables com posed of vegetable fibre and creepers. have been found to be used by the aborigines of many countries. It is probable that the Chinese sus pension bridges built during the Sixteenth Cen tury were of this construction, and it is certain that. it was the construction of the few European suspension bridges which were built previous to 'SOO. The first suspension bridge of the modern form, having a platform or a roadway suspended from the cables, is considered to have been erect ed by Joseph Finley, near Greensburg, Pa., in of the handsomest steel-arch structures in the world is the Washington Bridge at New York City, completed in 1889 at a cost of $2,850,000. This bridge has two metal arches of 510 feet span each, flanked at each end by masonry arches of 50 feet span, four at one end and three at the other. The arches of the Washington Bridge and the two Niagara bridges have hinges at the abutments only. Turning to Europe, mention may be made of the :\Itingsten Bridge across the river Wupper on the line of the Solingen-Rem s•heid Railway. This bridge has it centre arch span of 590 feet, without hinges, and steel tres tle approaches at each end. It was completed in 1897. Two notable German steel-arch bridges of recent construction are the Bonn Bridge and Thisseldorf 11ridge over the Rhine at the cities named. The I3onn Bridge has a emaie arch of 614 feet span and two side arches of :307 feet span, and the Diissehlorf Bridge has two spans of 595 feet each. In Frallee the Viaur Viaduct, with a single arch span of 721.0 feet, built. in 1898-99. holds the record for the second longest steel-arch span in the world. In England there 1801. The span was short (only 70 feet), hut it consisted of two chain cables passing over shore towers to inshore anchorages, and had a platform suspended from them. Several other bridges of the same general construction were built by Finley and others in succeeding years up to 1816, when a foot bridge of 408 feet span was built over the Schuylkill River at Phila delphia, whose cables were made of wire. In 1819 the first suspension bridge in England built with a suspended platform was erected over the river Tweed. In 1820 the famous chain suspen sion bridge across the Alenai Strait was com pleted, with a total length of 1710 feet and a suspension span of 580 feet. In Continental Europe suspension-bridge construction was be gun about 1S20, and in 1S34 Continental engi neers established the world's record in long suspension spans by building the Fribourg Bridge in Switzerland, with a span of S70 feet. In 1848 the United States again took the lead through the erection by Charles Ellet of a sus pension span of 1010 feet at Wheeling, W. Va. These early suspension bridges were flexible under unsymmetrical loading, and subject to oscillation from wind-pressures. On account of these defects, they gained but scant favor for roadway struetures, and were totally disregard ed for railways. The possibility of the suspen sion bridge for railway bridges was established by the work of John A. lloebling in building the suspension bridge to carry the Grand Trunk Railway across Niagara Gorge in 1855. In this structure Mr. Roebling placed a stiffening truss on each side of the platform, and by this means gave the structure great rigidity. He also in creased the resistance of the bridge against wind-pressure by inclining the cables toward each other from each tower toward the centre of the bridge. the also used inclined stays from the tops of the towers to the platform, and this is the only feature of Air. Roebling's design which has been discarded in modern practice, for the reason that such stays inevitably produce some uncertainty in the transmission of the load to the eable. The Niagara Suspension Railway
Bridge had a span of 821 feet, a width of 15 feet, and the platform and its wooden stiffening truss was suspended from two 101/4-inch cables. These cables were composed of seven parallel strands of 520 parallel wires each. In 1881 the old wooden stiffening trusses were replaced by steel ones, and in 1887 the old masonry towers were replaced by steel towers. In 1897 the whole structure was replaced by the 550-foot steel arch previously described. Following the Niagara Bridge, Sir. Roebling built the Cincin nati and Covington Suspension Bridge in 1867, with a span of 1057 feet and a total length of 2252 feet, the supporting cables being inches in diameter. This bridge was for high way and street-railway traffic. in 1897-98 it was strengthened by building two new cables di rectly over the old eab]es, and otherwise re modeling the ironwork. In 1872 Mr. Roebling began the construction of his greatest work, the New York and Brooklyn Suspension Bridge, which was carried to completion in 1883 by his son, Washington A. Roebling. This is the long est suspension bridge in the world. but it is surpassed in span by the new East River Bridge, the construction of which was begun in 1897. The main structural features of the New York and Brooklyn Bridge are, briefly, as follows: Length of river span. 15951/4 feet; length of each land span. 930 feet; length of Brooklyn ap proach, 971 feet; length of New York approach, 15621/4 feet: total length of bridge, 5989 feet; width of bridge, S5 feet ; number of cables, 4, each consisting of 6300 parallel (not twisted) steel wires of No. 7 gauge, closely wrapped in a solid cylinder about inches in diameter and having an ultimate strength of 11,200 tons; total cost, $9,000,000, exclusive of land. (See FOUNDATION.) The new East River Bridge crosses the East River at a point about 2 miles above the New York and Brooklyn Bridge, and has the followinr• dimensions: Clear span, 1000 feet ; length between terminals, 7200; total width, 118 feet; clear height for 100 feet each side of the centre, 135 feet; capacity, 4 trolley-car tracks, 2 elevated-railway tracks, one drive, and 2 footwalks; 4 cables, inches in diameter, formed by strands of parallel steel wire 0.203 inch in diameter; height of towers, 235 feet, composed of masonry 20 feet above water, and of steel framework above that point. The follow ing, are the chief points of difference between this bridge and the older Brooklyn Bridge: The use of steel towers instead of masonry; the omission of all inclined stays; the support of the shore spans entirely independent of the cables; and the use of riveted instead of pin connections for the stiffening trusses. (See FOUNDATION.) A form of suspension bridge which has been considerably used is one in which the cables are braced together. The Point Street Bridge, of Pittsburg, Pa., built in 1878, with a span of 800 feet, belongs to this class of structure; and one of the designs for a suspen sion bridge across the North River at New York City has the upper and lower cables on each side connected by a rigid X-braving. As indicating the length of span for which engineers consider suspension bridges available, it is interesting to note that this proposed structure is designed with a clear span of 3100 feet. For the theo retical discussion of suspension bridges, reference may be made to the text-books mentioned at the end of this article; for a concise description of the methods of cable-making on the New York and Brooklyn Bridge, see Cable-Making for S pension Bridges, by Wilhelm Bildenbrand: for description of notable suspension bridges, see the volumes of the engineering periodicals.