Page:The New International Encyclopædia 1st ed. v. 03.djvu/555

* BRIDGE. 485 BRIDGE. uiuier unsymmetrical loading, ami subject to oscillation from wind-pressures. On account of these licfccts, they gained but scant favor for roadway structures, 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. Koehling in Iniihling the suspension bridge to carry the (Irand 'Iriuik Railway across Niagara Gorge in 1855. In this structure .Mr. Roehling placed a stitTening truss on each side of the platform, and by this means gave the structure great rigidity. He al.so 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. He also used inclined stays from the tops of the towers to the platform, and this is the only feature of Mr. 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 cable. 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 lOV-2-incli cables. These caliles 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, Jtr. 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 12 14 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 cables, 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 188.3 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 Brooklvn Bridge are, briefly, as follows: Length of river span, 1595V-! feet: length of each land span. 930 feet; length of Brooklyn ap- proach, 971 feet: length of New York approach, 1562V„ feet: total length of bridge, 5989 feet: width of bridge, 85 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 15'^ inches in diameter and having an ultimate strength of 11,200 tons; total cost, .$9,000,000, exclusive of land. (See FoVNUATlox.) 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 following dimensi(ms: Clear siKin. 1600 feet: length between terminals. 7200; total widtli, 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, 17V-! 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 KouxD.TiON. ) 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-bracing. .s 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 Ctiblc-Malnnf/ for Stix- pensiun Bridi/cs, by ^YilheIm Hildenbrand; for description of notable suspension bridges, see the volumes of the engineering periodicals. GIBDEB BBIDGES. On account of its general applicability and wide utility, the girder bridge is perhaps the type of bridge structure most available to the engi- neer. The three forms of girder bridge which have been used are the plate girder, the box or tubular bridge, and the braced girder or truss. The tubular girder is now never used, and the plate girder is used onl}' for comparatively short spans. Briefly described, a plate girder consists of a top and bottom flange connected by a solid web plate; in cross-sections a plate girder re- sembles a letter I. To form a plate-girder bridge, two or more plate girders are placed parallel, with their ends resting on piers or abutments, and with a platform or floor be- tween them. Plate girders are particularly suitable for spans less than 100 feet, but they are occasionally built in greater lengths up to 150 feet. They are much used in elevated rail- ways and in trestle viaduct construction, and for railway bridges up to 100 feet spans. The tubular girder may be popularly defined as a rectangular tube of metal plates. Usually the sides of the tube consist of single plates, while the top and bottom have a cellular construction consisting of two plates separated by vertical diaphragms. The tubular bridge was the de- velopment of special conditions, and as .soon as these had ceased to exist it disappeared from engineering practice. The possibilities of the tube for carrying a level railway across a large span were brought into general notice by Robert Stephenson, engineer of the Chester and Holy- head Railway, in the const met ion of the bridges to carry that railway across the Mcnai Strait. It was rciiuired by the British Admiralty that these bridges — called the Britannia and the Conway — should be constructed so as not to interfere with navigation, with clear spans of upward of 400 feet. The longest arch spans that had previously been constructed did not exceed 240 feet, and suspension bridges, as built at that date, not being suit;iblc for luMvy and rapid rail-