GIRDER BRIDGES.
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 only 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 construction of the bridges to carry that railway across the Menai Strait. It was required by the British Admiralty that these bridges—called the Britannia and the Conway—should lie 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 suitable for heavy and rapid rail 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. Fairhairn. to make an elabo rate series of experiments on tubes, to determine the most suitable arrangement of the wrought iron of which they 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 460 feet and two of 230 feet each in the clear. The foundation stones of these bridges were laid in 1846 and 1S47, respectively. In 1854 work was begun on the most important tubular bridge ever built—the Victoria Bridge over the Saint, Lawrence River. near 3lontreal, Canada. The total length of this bridge is 9144 feet, or nearly 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 1S59, 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 common form of bridge now in use. Briefly described, a truss consists, first, of an upper and lower member, each continuous throughout its length. called the upper and lower chords: and. seeond, 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 trussing. The web members are connected to the chords by pins or by rivets, the truss being called a pin-conneeted truss in the first ease and a riveted truss in the second ease. 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 situ plest form, consists of two trusses placed paral lel to each other. with their ends resting on piers and abutments, and with a floor construct ed between them. When the floor is in the plane of the bottom chords, the traffic passes across the bridge between the trusses, and the bridge is called a through bridge; when the floor is in the Ida 11C of the top chords, and the traffic passes above the trusses, the bridge is called a deck bridge. In a properly designed
truss the chords and web members 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 efficient 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 areh rib combined with a stiffening truss, and one of the first bridges in which it was used was at Waterford, X. V., and had four spans of 154 feet, 161 feet, 174 feet, and 180 feet. The Harrisburg ( Pa.) Bridge, which was also a Burr truss bridge, had 12 spans of about 210 feet each. The type of bridge built by Timothy Palmer had a curved bottom chord and a straight top chord. with Xbraeing CI nmeeting them. A bridge of this type was built over the Schuylkill River, at Philadelphia, Pa.. in 1806. which had a centre span of 195 feet and two side spans of 150 feet each. In 1829 Mile] 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 parts, with Xdiracing between them. invented by Col. S. 11. Long, United States Engineers, in 1850. The first Long truss bridge of whkh there is any definite record was the Washington load Bridge, near Baltimore, Abh, with a span of 109 feet. All these early truss bridges were built entirely of wood. In 1841) William Howe invented a truss with parallel horizontal chords connected by iron verticals with wooden X-braeing between them. In 1544 Caleb Pratt invented a truss which was a combination 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 extewsive use for both railway and highway bridges, one of the most notable being the Ilavre de Craee Bridge, built in 1850. with 13 250-foot spans and a 176 foot draw span. The Pratt truss did not lend itself easily and cheaply to combination construc tion, and never attained much popularity. as long as wood and iron were used in combinathm. The first iron bridges in America were built with cast-iron upper chords and struts, and wrought iron lower chords and diagonals. Squire Whip ple. Wendell 13allman, and Albert Fink designed combination east, and wrought iron bridges, and developed special truss systems in America, while abroad S•hwedler. Xavier. and Warren performed a similar work. About. 1560 bridges built of wrought iron began to be considered, and shortly afterwards the old cast and wrought iron conthination structure had practically dis appeared from use. To attempt to outline the development of the truss (luring all of the years in which it has been coming to its present state of perfection would involve the reader in teehni cantles which belong only in special treatises. It is also impossible to record even the principal truss spans which have been built. In I S9 Thoslorc ('doper estinutted that the railways of the Uffited States alone had over 20,111)1) iron bridge spans. A few of the most notable simple truss bridges of the world are the following: Cineinnati and Covington, double-track railway, over Ohio River, with one span 545 feet, two spans 491) feet; total length, over one mile; t(dal steel contained. 10,180 tons. Henderson, Ky., over Ohio River; total length. :3688 feet; 16 spans; longest span, 525 feet. Ohio Con necting railway. Pittsburg. l'a.; longest. span, 52:3 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 feet, seven spans 401) feet, three spans of 249 feet; total length, with viaduct, 10,560 feet. Gin cinnati, 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 1860, and until 1877 was the longest simple truss span in the world. In England, the Salt-ash Bridge. built in 1859, with two of its 19 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 Mehrtens, only seven have a span of over 300 feet, the longest being the road bridge over the Snide, at Kalbe, with one span of 350 feet.