Page:The American Cyclopædia (1879) Volume III.djvu/279

 BRIDGE 273 signed and built byL. Wernwag, and consists of five ribs, each formed of a curved, solid- built beam, connected with an upper single beam by radial pieces, diagonal braces, and in- FIG. 6. SchuylkiU Bridge. clined iron stays. In the Trenton bridge, the roadway bearers are suspended from curved, solid-built beams, by iron bar chains and sus- pension rods ; it consists of five spans, the cen- tral and widest being 200 ft. Burr's plan, which at one time received considerable favor on railroads and aqueducts, particularly in Pennsylvania, consists essentially of open-built beams of straight timber, connected with curv- ed, solid-built beams, termed arch timbers, and which are formed of several thicknesses of scantling, between which the framework of the open-built beam is clamped. Town's plan, commonly known as the lattice truss, consists of two main strings, each formed by two or three parallel beams of two thicknesses, breaking joints with a series of diagonal pieces, crossing each other and inserted between the parallel beams, being connected with the strings and with each other by treenails. As the timbers are of a uniform cross section and length, the construction is simple and economical, though the plan is not well adapted to the resistance of variable strains and jars. Long's truss con- sists in forming the upper and lower strings of three parallel beams, between which are inserted the cross pieces, or posts, which are formed of beams in pairs, placed at regular in- tervals along the strings, and connected with them by wedge blocks; between each series of posts are placed braces and counterbraces, suitably connected by treenails, and in long spans arch braces are also introduced. In Howe's truss, the upper and lower strings are each formed of several thicknesses, breaking joints, while on the upper side of the lower string, and the lower side of the upper, are placed blocks of hard wood or cast iron, insert- ed in notches, and bevelled on each side to form a support for the braces and counterbraces; through the blocks are passed bolts of iron to connect the two strings, and by means of a nut and screw any desired amount of tension can be given to the truss. The preceding combinations are those which are in general use in the United States. One of the most remarkable wooden bridges is that at Havre de Gract), over the Sus- quehanna river ; this is 3,271 ft. long, divided FIG. T. Bridge over the Susquehanna at Havre de Grace. into 12 spans, resting upon granite piers. It is constructed upon Howe's plan, and combines great lightness and strength. There are many works of this character throughout the coun- try, where, owing to the cheapness of timber, they can be built with great economy. The practice of American engineers in this class of structures has been taken as a guide in similar works throughout the world. The modern ten- dency is, however, toward the substitution of iron and steel for timber. Suspension bridges are of very remote origin. Kirchen, in his " Chi- na Illustrated," mentions one which is still to be seen in the province of Yun-nan, and accord- ing to tradition was built by the emperor Ming in A. D. 65 ; it is formed of chains, supporting a roadway of plank resting directly upon them, and is 330 ft. in length. The ancient Peruvians also constructed numerous bridges over the Andes, the principal material being ropes form- ed of the bark of trees ; sometimes a roadway was constructed, and at others the transit was effected by means of a basket supported by the rope, and drawn over alternately from one side to the other. The same plan is used at the present day. Rope suspension bridges have also been used in Europe; they were employed in France, at the siege of Poitiers, to cross the river Clain, and Douglass men- tions their use in Italy in 1742. Iron sus- pension bridges of large span, however, are of modern date. The first of this kind erected in England was in 1819, and was built across the Tweed at Berwick by Capt. Sir Samuel Brown ; it was constructed with chain cables, 12 of which were used in all, six being placed OE