Other Noteworthy Bridges are the Rhine Bridge at Coblenz 05/. 43, figs. zo-27), completed in 1864, with three spans of 317.3 feet width; the Intel de Ville Bridge in Paris, with a span of 262.5 feet; the drawbridge at Marburg, with three spans of 172.3 feet; the Theiss I3ridge at Szegedin (1858), with eight spans of I3S.S feet; the railway bridge over the canal St. Denis (1858), with a span of r4S.3 feet; the street bridge over the canal St. Denis at Villette (1867), with a span of 147.7 feet; the Ruhr Bridge at with three channel spans of rIS.5 feet, etc. What is believed to have been the first wrought-iron arch bridge was built over the Cron at St. Denis (r8o8).
Susfiension Bridges have as their prototypes the crude rope bridges known to the natives of Hindustan and South America and to the Chinese from time immemorial. These were formed of a couple of ropes or twisted vines (lianas) thrown across a river or precipice, with a roadway of bamboo or the like lashed to them; other ropes stretched across the space at suit able height above those first named formed hand-rails. Figure 3 05/. 38) exhibits one of these primitive bridges in the Himalaya Mountains. It is made of two parallel canes, resting- at their ends on posts, and from these are suspended loops at intervals in which the floor of cane is laid. This particular structure spans the Runject near Darjiling, and is 24o feet long.
In the suspension bridges proper GO/. 41, figs. I; 2 ; 48; figs. the roadway is not placed directly upon the cables or chains, but is suspended from these by means of suspension-rods and stays. The form and inclina tion of the roadway, therefore, are quite independent of the cables or chains, and in recent bridges of this type it is customary to give the road way an inclination upward from both ends toward the middle of the span.
Classes of Suspension Bridges: bridges may be of two kinds—chain bridg-es and wire-rope or cable bridges. In those of the first class (pl. 4r,fig. 3) the chains are formed of rods or bars of forged iron or steel, of which a number—as many as twelve—placed side by side, form a link of the chain. These rods have at their extremities eyes through which bolts or pins are passed, fastening them to one another, and with which the suspension-rods supporting the roadway are likewise secured. Frequently two chains (as in the Figure) are used, one over the other.
The earliest wrought-iron chain bridges of which there is record were those built by Finlay in the United States. The supporting structure in these was a chain formed of 7-foot links of square bar-iron, from witch the roadway (of wood) was hung by iron snspension-rods. The towers were generally of wood. The first of these chain bridges was the road bridge with a span of 7o feet built in 1796-97 over Jacob's Creek on the highway between Uniontown and Greensburg, Pennsylvania. The general appear drice of these bridges is shown on Plate 48 (fig. 1).
In the wire-rope or cable bridges (pl. 41, figs. r, 2; pi. 4S, figs. 2, 3), instead of the above-described iron or steel links, many strands of iron or steel wire are laid up to form a cable, which is finally firmly bound round with wire. One of the first (if not the first) cable suspension bridges in the United States was that built in 1842 by Charles Ellett, Jr., over the Schuylkill River at Fairmount, Philadelphia. It was in its time quite a noted structure. It had ten cables, five on each side, and the span from centre to centre of towers was 358 feet. The clear width between side rails was 25 feet. This bridge remained in service for about thirty years, when it was replaced by the present double-deck bridge of iron.
Anchorages and chains or cables may be anchored directly to a rock anchorage, or, as is almost universally done, may be extended backward over piers or towers and firmly secured to the solid rock or beneath a mass of masonry by means of " anchor-plates " (151. 41, figs. pl. 42, fig. 9). The anchorage must, of course, be amply strong to resist the pull of the cables. That the slight movement of the cables proceeding- from changes of loading and of temperature may not exert an injurious bending strain upon the towers, it is necessary that the bearings on which the cables rest shall be such as will allow the cables a certain freedom of motion. Figure 4a, b 41) and Figure io („6/. 42) exhibit some of the numerous artifices employed for this purpose. In these devices the cables rest npon movable saddles with a gentle curvature to avoid a sharp bend, and the saddles, in turn, rest upon a series of horizontal steel rollers which are free to move in response to the action of the cables. The under-bearing is a flat plate of iron or steel fastened by bolts to the top of the pier.