Page:The New International Encyclopædia 1st ed. v. 03.djvu/559

BBIDGE. 487 BRIDGE. oinnati, Ohio, over Ohio River; five spans, hav- ing total length of 1600 feet; longest span, 515 feet. Kuilenburg Bridge, over the river Lek, Holland ; one span, 515 feet; this bridge was built in 1806, and until 1877 was the longest simple truss span in the world. In England, the Saltash Bridge, built in 1859, with two of its lit spans 455 feet long each, is the longest simple truss bridge. Of 92 remarkable German simple truss bridges whose statistics have been tabulated by Prof. Georg Alehrtens, only seven have a span of over 300 feet, the longest being the road bridge over the Saale, at Kalbe, with one span of 350 feet. Continuous-truss bridges are comparatively lit- tle used, but are far more popular in Europe than in America. A continuous truss, as al- ready stated, differs from a simple truss by ex- tending continuously over three or more sup- ports. Theoretically, a c<mtinuous-truss bridge requires less material than a bridge with the same number of simple-truss spans; but it has a number of practical disadvantages which make engineers loth to use it. Perhaps the most notable continuous-truss bridge in the world is the Ijiichine Bridge across the Saint Lawrence River at Montreal, Canada, built in 1887. It consists of two centre through spans of 408 feet each and two side spans '209 feet, forming a con- tinuous structure over five piers. Cantilever bridges are, strictly speaking, hinged continuous girders. They were not much used until the last quarter of the Nine- teenth Century, when the buiMing of the Ken- tucky Viaduct (1870) and the Niagara Canti- lever (1883) demonstrated the practicability of erecting such bridges without the use of sup- porting false works. Since these dates nu- merous cantilever bridges have been erected in both Europe and America. Briefly defined, a cantilever bridge consists of a continuous-truss span starting from an abutment and extending over and beyond a second support on each side of the stream. The two projecting ends are connected by a truss span suspended from them. The shore ends of the spans are anchored to the abutments. The Niagara Cantilever Bridge, completed in 1883, has a total length between abutments of 910 feet. The projecting arms or cantilevers are each 175 feet, and the truss span which they support is 175 feet long. The Saint John liiver (New Brunswick) Cantilever, built in 1895, has a total length of 813 feet and a main span of 477 feet. The Poughkeepsie Bridge, built in 1889, has a total length of 6767 feet, with five river spans, of which the first, third, and fifth are cantilever spans, and are 548 feet, 546 feet, and 548 feet, respectively. The Red Rock Cantilever .Bridge, built in 1890, Red River, Cal., has a total length of 990 feet and a centre span of 600 feet. The Memphis Bridge, built in 1892 over the Mississippi River, at Memphis, Tcnn., has a truss span of C21 feet and two cantilever spans of 790 feet, which are the longest cantilever spans in the United States. The longest cantilever span in Conti- nental Europe is the 623-foot span of the Danube Bridge, near Czemavoda, in Rumania. Great Britain has the honor of having at present the longest cantilever span in the world, in the ^eat Forth Bridge across the Firth of Forth, in Scotland. This bridge has two cantilever shore arms of 680 feet, and two main cantilever spans of 1710 feet. It was begun in 1883 and was completed in 1890. The Quebec Bridge across the Saint Lawrence River, upon which construction was begun in 1900, has a main can- tilever span of 1800 feet. For theoretical dis- cussions of girder bridges, see the text-books named at the end of this article; and for de- scriptions of many of the prominent structures mentioned, see Transactions American Society of (,'ivil Engineers; T. Cooper, American Railway Bridges; and the volumes of the engineering journals. DBAWBBIDGES. Under the generic term of drawbridges are included all forms of bridges designed to be opened and closed so as to give passage alter- nately to vessels and to the road or railway traffic passing over the bridge. Drawbridges may be divided into the following types: Swing bridges, in which the opening span swings hori- zontally on a turntable or vertical pivot carried by the pier supporting the span ; bascule bridges, which consist of one or two draws hinged at the abutment on horizontal shafts so that they are opened by raising the bridge to a vertical posi- tion, leaving the channel clear; rolling bridges, which have the span mounted on rollers, so that it may be hauled inshore and leave the channel clear; lift bridges, in which the span is placed be- tween two tall towers and so arranged that it is lifted clear of passing vessels, like an ordinary passenger elevator. There are several modifica- tions of each of these types, some of these being patented in part, and for these the special treatises on bridge construction, which are men- tioned later, should be consulted. The most commonly used form of a draw- bridge is the swing bridge. Generally the sup- porting pier, called the pivot pier, is placed under the centre of the span, which is always either a plate girder or a truss span. Sometimes, how- ever, the pivot pier is located nearer to one end of the span than the other, in which case the shorter arm has to be counterweighted to balance the longer arm. When a swing bridge is open, the axis of the span is parallel to the axis of the stream ; there are two open channels on each side of the centre pier and the ends of the s|)an are swnmg clear of any support. When the bridge is closed the ends of the span are supported on piers. The longest swinging span ever con- structed is that for the Interstate Bridge at Omaha, Neb., which has a total length of 520 feet. The Tliames River Drawbridge, New Lon- don, Conn., has a swing span of .503 feel, and the Arthur Kill Drawbridge between Staten Island, X. Y., and the New Jersey mainland, has a swing span of 490'/. feet. The manner of supporting a swing s])an on the pivot pier so that movement is possible is quite simple. A casting at the centre of the ])ier carries a vertical pivot. Around this pivot as a centre is bolted to the pier a circular, track having gear-teeth on the outer circumference. To the bottom of the span is attached a circular drum of the same diameter as the track on the [)icr, and at the centre of this drum is a socket which fits over the pivot on the pier. Between the bottom of the drum and the pier-track is placed a chain of rollers kept in position by radial arms ruiuiing to the centre pivot. The span swings on these rollers, the centre pivot serving only to hold it in posi- tion or true to its centre. Slovemcnt is brought