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Tron Bridges

bridge, iron, feet, built, arch, cast and span

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earliest iron bridges were built of cast iron, which in time gave place to wrought iron—a tougher material, and one less liable to be affected injuriously by concussion. In recent years but few large bridges have been constructed of cast iron. Among these may be named the arch bridge of Saint Louis at Paris (built in 186o-1862), -with span of 2 ro.25 feet, the Trent Bridge at Nottingham, with span of roo feet, and certain others. In America, where iron-bridge building has reached its highest development, it is the practice to some extent to employ cast iron for such members as will be subjected to compression in the finished structure, and wrought iron for such as will be subjected to tension. The largest span (239.5 feet) among cast-iron bridges is exhibited by the South wark arch bridge over the Thames at London.

The Firs/ Cast-iron Arch Bridge was built in 1773–r777 by Abraham Darby over the Severn at Coalbrookdale. This bridge spans the river by a single arch of roo feet. Later (1796) there was built over the river Wear, near Sunderland, England, the Wearmouth Bridge, with an arch of open cast-iron panels, haying a span of 236 feet. The most noteworthy struc ture of this class, however, is the cast-iron bridge built about the begin ning of this century by the eng-,ineer Telford over the Spey at Craigellachie (pi. 47, fig. r). In its form and in the distribution of material, this bridge may be regarded as the best example of the use of cast iron in bridge con struction. The success of these bridges gave a decided impetus to the art of building bridges of cast iron in England, and a number of such struc tures—notablv two at Bristol over the New Cut, one over the river Parrot at Bridgewater, and one across the Thames at Staines—were erected during the early years of the present century. In some of these, as in the Wearmouth Bridge, above named, -wrought iron was used in conjunction with cast iron.

Bridges may have the form of either girder, arch, or sus pension bridges. 'Wrought-iron girder bridges exhibit great differences in construction.

the space to be spanned is small, a simple expedient consists in riveting together two rails base to base, forming- a "donble-rail girder " 43, figs.i, 2) whose ends are received in suitable

cast-iron chairs, to which they are firmly clamped. Rolled beams of I-section are also verv serviceable for such simple constructions, in which case the rails either lie directly upon the beams or rest upon interposed wooden cross-ties. With spans up to about 5o feet plate-iron girders may be used with advantage, the metal having-, a thickness of from E1.3 t 0 of an inch and being formed into girders of I-section by riveting (figs. 3, 4, 23). The riyet-bolts are inserted in the rivct-holes and clinched while white-hot. Figure 24 exhibits the appearance of the common form of rivet-bolt employed for this purpose. It consists of a head and shank in one piece. In this form, being- heated white-hot in a forge-fire, it is inserted in its bole and clinched by forming a second conical head upon it throng-1i hand-hammering- \\line it is cooling. The advantage gained by heating- the rivets is not only greater ease of forming the clinching-bead and the lessened liability of shattering the metal of the rivet and plate, but also a firmer union of the plates, since the contraction of the rivet on cool ing draws the joined parts together more firmly than could any mechanical process. Rivet-holes in plates are drilled, not punched, as drilling avoids injury to the surrounding iron and makes a smoother hole. Figures 5 to 7 (pf. 43) are details of a plate-girder bridge, which exhibits the manner in which flat plates placed perpendicularly with respect to one another may be united by the use of angle-irons.

Tubular Bridges, so called, belong in this category (figs. S, 9). These have high side-walls composed of a large number of iron plates riveted together and united transversely at top and bottom by a cellular structure of the same order (fig. 9). The first bridge of this type is believed to have been built at the Bolton Station of the Baltimore and Susquehanna Rail road by James Mulholland. This was a small structure of 55 feet span, and is noted here simply as an interesting item of history. In England and Canada there have been built upon this system truly colossal bridges which for years ranked as the greatest of their class.

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