FF-Bottom chord links of flat iron, with heads at each end, bored to receive the pins (Fig. 181c).

G G-Rolled iron floor-beams, suspended to chord pins. HH-Bottom lateral ties, round iron rods with screws. II-Bottom lateral angle block, cast iron.

K-Cast-iron strut shoes, having sockets to receive struts

and drilled holes for chord pins passing through flanges or

ribs below the sockets.

640. Alleghany River Bridge at Pittsburgh, Pa. This is a lattice iron bridge (Fig. 182), and is similar to several

Fig. 182.

other structures which have been made in this country. There is a similar one on the New York Central Railroad, at Schenectady, N. Y., and another near Rome, of the same State.

641. St. Louis and Illinois Bridge. This noted structure might properly be called a steel arch. It is now in course of erection, and is to consist of three spans, the central one of which is 515 feet, and each of the end ones 497 feet. There are eight arches in each span, arranged in sets of two and two; and in each set one arch is directly over the other, and the two are trussed together by link-bars. The arches are composed of steel tubes, which are made of steel staves, as will now be explained.

All the steel in this structure is of the very best quality. The standard fixed for it by the Chief Engineer, Capt. Eads, was so high as to make it almost impossible for our best steel manufacturers to produce it. The coefficient of elasticity was

Fig. 183-Section of a tube, St. Louis and Illinois Bridge, a a is a steel caseing about three-eighths of an inch thick, which is lapped over, and riveted like the plates of a steam-boiler. b, b are steel staves which are forced into the caseing.

A A, Figs. 183 and 184, are cross-rods for connecting the arches together laterally.

BBB are diagonal rods in a vertical, for connecting the upper arch in one set to the lower arch in the adjacent set.

CCC are diagonal rods in the plane of the tubes, for connecting the joint of one set with the joint which is in advance of or back of the corresponding joint in the adjacent set, D is a vertical diagonal rod for trussing the roadway.

EE are trussed vertical posts, the lower ends of which are secured to the arch, and the upper ends support the roadway.

to be between 26,000,000 pounds and 30,000,000 pounds, and it was to sustain a strain of 60,000 pounds, without producing a permanent set.

All the workmanship is of a higher order than is usual in bridge construction. Special machines and tools were made for making the several joints. An error of one thirty-second of an inch might, in most cases, be very troublesome, if not fatal.

The tubes are straight throughout their length, but the ends are planed off in the direction of the radius of the arch, so that the arch is really a polygon having short sides. Several rectangular annular grooves are cut near the ends of each tube; and after the tubes are put in place, their ends abutting against each other, they are joined, and firmly secured by means of a heavy and nicely-fitted iron coupling. In this way the arch is made continuous. A strong steel pin

passes through the coupling and the ends of the tubes, one half of the pin being in each tube. One length of tube is put up at a time, and is connected to all the others, which are properly placed by cross-rods, A A, Figs. 183 and 184, and also diagonal rods C C and B B. The diagonals G G are also secured. These are secured to the pins c c, Fig. 185. The vertical posts EE, which support the railroad, are trussed by means of diagonal bars, as shown in Fig. 184. Each skewback of the arch is secured to the abutments by means of two six-inch steel rods or bolts, which pass through the wroughtiron skew-backs, and several feet into the masonry. This bridge, when completed, will be one of the most remarkable structures of its kind in the world, and can hardly fail to establish many important principles in iron structures.

642. Kuilenberg Bridge. The span of this bridge is about the same as that of the St. Louis and Illinois bridge, as will be seen from the following dimensions. The lower chord of this bridge (Fig. 187) is horizontal, and the upper chord is

Fig. 187-Kuilenberg Bridge. Span between the abutments, 152 meters. Total length, including the parts on the abutments, 156.8 meters (about 515 feet). Length of each bay, 4 meters. Depth of the truss at the centre, 29 meters.

the arc of a circle, the radius of which is 809 feet. It is of the general plan of the Pratt or Whipple systems, only that the upper chord is curved.

VI.

TUBULAR BRIDGES.

643. Tubular Frames of Wrought-iron. Except for the obvious application to steam boilers, sheet iron had not been considered as suitable for structures demanding great strength, from its apparent deficiency in rigidity; and although the principle of gaining strength by a proper distribution of the material, and of giving any desirable rígidity by combinations adapted to the object in view, were at every moment acted upon, from the ever-increasing demands of the art, engineers seem not to have looked upon sheet iron as suited to such