Page:The New International Encyclopædia 1st ed. v. 03.djvu/556

* BRIDGE. 486 BRIDGE. way traffic, the engineers were obliged to devise some new form which should conform to the stipulated conditions. Mr. Stephenson, having decided on the tubular form, proceeded, in con- junction with Mr. Fairbairn. to make an elabo- rate series of experiments on tubes, to detennine the most suitable arrangement of the wrought iron of which they were composed. They found that a rectangular tube, of which the top and bottom were cellular, gave the greatest strength with the least material. The span of the Con- way tube was 400 feet, while the tubular part of "the Britannia Bridge consisted of two spans of 400 feet an<i two of 230 feet each in the clear. The foundation stones of these bridges were laid in 1840 and 184". respectively. In 1854 work was begun on the most important tubular bridge ever built — the Victoria Bridge over the Saint Lawrence River, near Montreal, Canada. The total length of this bridge is 9144 feet, or nearly 1% miles, and it was built in 24 spans of from 242 to 247 feet each, and one of 330 feet. About 9000 tons of iron were used in the tubes. In 1898-99 this bridge was re- placed by a pin-connected truss bridge having 24 spans of 254 feet and one span of 348 feet, and requiring 20,000 tons of steel in its con- .struction. The Victoria Bridge was the last important tubular girder bridge to be built. By the date of its completion, in 1859, the prac- ticability of the suspension system for railway bridges "had been established by the inventions of Roebling, and the application of wrought iron to braced girder bridges had proved the possibility of constructing them in long spans. The braced girder or truss bridge is the most conunon form of bridge now in use. Briefly described, a truss consists, first, of an upper and lower member, each continuous throiighout its length, called the upper and lower chords : and, second, of an open bracing of diagonal members, or diagonal and vertical members combined, which connect these two chords rigidly to- gether, and which are called the web members or tnissing. The web members are connected to the chords by pins or by rivets, the truss being called a pin-connected truss in the first case and a riveted truss in the second case. A truss is continuous when it extends over several supports. Different forms of trusses have dif- ferent outlines and different arrangements of the web members. A truss bridge, in its sim- plest fonn, consists of two trusses placed paral- lel to each other, with their ends resting on piers and .abiitmcnts, and with a floor construct- ed between them. Wlicn the floor is in the plane of the bottom chords, the traflic passes across the bridge between the trusses, aiul the bridge is called a through bridge; when the floor is in the plane of the top chords, and the traflic passes above the trusses, the bridge is called a deck bridge. In a properly designed truss the chords and web memi)crs are subjected only to tensile and compressive strains. The braced girder or truss, as will be inferred from what has preceded, is a modern development of an etlicient and popular bridge type. Its first extensive use was in America, with wood as the structural material. The earliest of these American timber bridges were built by Theodore Burr and Timothy Palmer. The Burr truss consisted of a timber arch rib combined with a stiffening truss, and one of the first bridges in which it was used was at Waterford, N. Y., and had four spans of 1.54 feet, Kil feet, 174 feet, and 180 feet. The Ilarrisburg (Pa.) Bridge, which was also a Burr truss bridgt", had 12 spans of about 210 feet each. The type of bridge built by Timothy Palmer had a cuned bottom chord and a straight top chord, with X-bracing connecting them. A bridge of this type was built over the Schuylkill River, at Philadelphia, Pa., in 1800. which had a centre span of 195 feet and two side spans of 150 feet each. In 1829 Ithiel Towne invented a truss with parallel horizontal chords and a web sys- tem consisting of a close lattice-work of diag- onal members. This was followed by a truss with parallel horizontal chords separated by a web system of vertical jiarts. with X-bracing between them, invented by Col. S. H. Long, United States Kngineers, "in 1850. The first Long truss bridge of which there is any definite record was the Washington Road Bridge, near Baltimore, Jld., with a span of 109 feet. All these early truss bridges were built entirely of wood. In 1840 William Howe invented a truss with parallel hcuizontal chords connected by iron verticals with wooden X-bracing between them. In 1844 Caleb Pratt invented a truss which was a comlunation of iron and wood, and in which the web system consisted of wooden verticals with iron X-bracing between them. The combination truss of Howe came into extensive use for both railway and highway bridges, one of the most notable being the Havre de lirace Bridge, built in 1S50. with 13 250-fot spans and a 170- foot draw span. The Pratt truss diil not lend itself easily and cheaply to combinatiou ((Uislruc- tion. and never attained nuich popularity, as long as wood and iron were used in combination. The first iron bridges in America were built with cast-iron upper chords and struts, anil wrought- iron lower chords and diagonals. Sipiire Whip- ple, Wendell Ballman, and Albert Fink designed combination cast and wrought iron liriilges. and develoi)pd s|iecial truss systems in .America, while al)road Schwedlcr, is'avicr, and Warren performed a similar w(jrk. About 18(>0 bridges built of wrought iron began to be considered, and shortly afterwards the old cast and wrought iron combination structure had practically dis- appeared from use. To attempt to outline the development of the truss during all of the years in which it has been coming to its present state of perfection would involve the reader in techni- calities which hcbmg only in special treatises. It is also im|)ossible to record even the principal truss spans which have been built. In 18S9 Mr. Theodore Cooper estimated that the railways of the United States alone had over 29.000 iron bridge spans. A few of the most notable simple truss bridges of the world are the following: Cincinnati and Covington, double-track railway, over Ohio River, with one span 545 feet, two spans 490 feet: total length, over one mile; total steel contained. 10,180 tons. Henderson, Ky., over Ohio River; total length. 3088 feet; id spans: longest span. i>25 feet. Ohio Con- necting railway. Pittsburg, Pa.; longest span, 523 feet. Norfolk and Western Railroad, over Ohio River, at Credo, W. Va.; two spans 301 feet, two 304 feet, and one 521 feet. Cairo, 111., over Ohio River; two spans 518Vj feet, seven spans 400 feet, three spans of 249 feet; total length, with viaduct, 10,500 feet. Cin-