Suspension Bridge Of 1883.

Draw Bridges. A draw B. is one that can be moved, either by swinging or lifting, so as to permit ve els to pa the structure. The usual type is that which swings on a turn-table on a central pier, the tru es of the B. generally being deeper over the pier than at the ends. The lower chords of a swing draw B. are subject to stre es of compre ion when open and to tension when closed. The span of a swing draw B. is counted from end to end, as the tru es are continuous over the center pier. While the usual length of span is le than 200 feet, many have been built exceeding 300 feet. One over the Mi i ippi Fig. 2743. THE WASHINGTON BRIDGE OVER HARLEM RIVER. (Fig. 2743) is for highway traffic only, and has two spans of 510 feet each; it was completed in 1889, and its cost, with approaches, was $2,900,000. The Garabit arch in France, built in 1885, has a span of 541 and a rise of 169 feet. The arch at Niagara Falls, completed in 1897 to replace the highway suspension B. , has a span of 840 feet, and a rise of 150 feet, being the largest in the world. One of about the same span was built in 1897-98 to replace the railroad suspension B. one mile north of Niagara Falls. The arch is one of the most graceful forms of B. construction, and hence particularly adapted for use near cities, although its cost is usually somewhat greater than for the cantilever system on account of the greater expense of erection. Suspension Bridges . A suspension B. consists primitively of a rope or cable stretched acro a river. Such were frequently used in ancient times as rope ferries, a basket suspended from the rope being pulled acro the river. In the eighteenth century the idea was extended by laying a platform on two parallel cables or chains, thus forming a suspended foot B. The modern suspension B. , however, consists of a platform hung from parallel cables, which pa over towers, and have their ends secured in anchorages. The first B. on this plan was built near Greens burg, Pa., in 1801, having a span of 70 feet, and during the decade following at least eight others of short spans were erected in Pennsylvania, the longest being one of 306 feet over the Schuylkill at Philadelphia. All these had the cables made of chains or links of wrought iron, but in 1816 wire was first used for cables in a foot- B . of 408 feet span near Philadelphia. A similar development occurred in other countries, and in 1834 the great span of 870 feet was attained in the B. at Freiburg, in Switzerland. These structures were deficient, however, in rigidity, and many of them were blown down. In 1848, at Wheeling, W. Va., a suspension B. of 1,010 feet span was erected, in which 12 cables were used; IMG:content-0870.png:[graphic] IMG:content-0871.png:[graphic] support the roadway, and the tru itself was suspended from the cables. The action of the tru was to distribute the load over the cable and thus prevent local distortion and the accompanying stre es. The Niagara B. also demonstrated the practicability of the suspension system for railroad traffic, owing to the influence of the suspended tru . The B. at Niagara was 810 feet in span between the towers, and had eight cables, each 10% inches in diameter. In 1881 the wooden tru es were replaced by wrought iron ones, and in 1887 the stone towers were likewise replaced. In 1897 the entire structuro was removed and a steel arch erected to replace it. The largest suspension B. in the world is that over the East river at New York (Figs. 2744, 2746), ite IMG:content-0872.png:[blocks in formation] span between the towers being 1,595 feet, with two side gans of 930 feet each. This was completed in 1883, after 13 years' work in construction, both the founding of the piers and the manufacture of the cables being slow and difficult. Each of the four cables contains 5,296 steel wires bound together into a solid cylinder 16 inches in diameter. The bridge carries two tracks for cable cars, two roadways, and a foot-walk; its cost was about $15,000,000. The second East river suspension B. (Fig. 2745), begun in 1897, and to be completed in 1901, has a central span of 1600 feet, and its estimated cost is $7,500,000. The suspension system has now been so developed that it is recognized as applicable to the longest spans and heaviest traf B. Each cantilever arm is secured at its shore end to an abutment or anchorage, and is supported by a tower, the space between the abutment and the tower being called the shore span; beyond the tower the tru projects out over the river. From the opposite shore a similar cantilever arnı rests upon an abutment and projects out over another tower. Between the ends of the two cantilevers there is a space over which stretches a short span whose ends are supported by the two ends of the cantilevers. For instance, in the cantilever B. over the Schuylkill, at Philadelphia, the total length of the structure is 410 feet; this is made up of the two cantilever arms, each 177 feet long, and the short central span of 56 feet; in each cantilever arm the length of the shore span is 98 feet and that of the part projecting beyond the tower is 79 feet. With this arrangement a load placed upon one cantilever arm has no effect upon the other, while the loads upon the central span are transferred in both directions. In building such a structure the shore spans are erected upon false works in the usual way, and the projecting arms of the cantilevers are then built out, panel by panel, beyond the pier, without the nece ity of supports beneath. This method of construction is particularly adapted to deep river gorges, where the erection of simple bridges or of arches would be very difficult and expensive. This Fig . 2748. CANTILEVER RAILROAD BRIDGE AT NIAGARA. plan of erection was first employed in 1875 for a B. of 1,125 feet in length over the Kentucky river, which, however, was a continuous rather than a true cantilever structure. Designs for a cantilever B. over the East river near Black well's Island, at New York, were made as early as 1870, but it was not built. Plans were also prepared for an immense cantilever structure to span the Hudson at New York (Fig. 2749), but these were abandoned for the suspension B. above mentioned. The first structure which involved all the elements of the cantilever system was erected over the Niagara river in 1883 (Fig. 2748); the total length is 910 feet, each shore span being 195 feet, and the span between the towers 470 feet; the height of the roadway above the river is 245 feet. The Niagara cantilever B. was built in eight months, at a cost of $600,000, and it demonstrated the feasibility and economy of the system. In 1889 a B. over the Hudson river at Poughkeepsie was completed, which has five main spans, the first, third and fifth having the cantilever arms, while the alternate spans are connected with the others by continuous tru es; the longest span is 548 feet. In 1890 the B. over the Colorado river at Red Rock, Col., was built, the span between the towers being 660 feet, and the total length 990 feet. Before 1890, indeed, several other cantilevers had been erected, and the system thoroughly established as one of great advantage for spans too great to be covered by a simple tru . In 1892 the great B. over the Mi i ippi at Memphis was finished, which has a cantilever span of 790 feet between towers, as also an adjacent fixed span of 621 feet. This was the largest cantilever B. in the U. S. until the erection of the B. over the Mi i ippi river at New Orleans (Fig. 2750), in 1896-98, which has a total length of 2,274 feet, the shore span being 502 feet, and the span between the towers 1,070 feet; the floor is 85 feet above high water, thus nece itating a long viaduct on each side, so that the total length of steel construction is 10,610 feet, making it the longest in the world. The Forth cantilever bridge in Scotland, finished in 1887, is the greatest ever constructed, there being two shore arms of 685 feet each, and two cantilever spans of 1,700 feet each; the depth of this B. over the towers is 350 feet, and indeed it is probable that in these great spans the limit of economy of the cantilever system is surpa ed. The Montreal cantilever, 1898, has a span of 1,250 feet between the towers, and two shore spans, each 600 feet long. Comparison . With respect to materials of construction, it is now settled that timber can only be used economically for bridges in the case of temporary structures or in localities far removed from manufacturing centers. Cast iron has been entirely discarded on account of its brittlene and unreliability. Wrought iron, though long in serviceable use, has now been largely supplanted by steel, which is somewhat stronger IMG:content-0873.jpg:[graphic] IMG:content-0874.jpg:[graphic] Fig . 2749.-DISCARDED DESIGN FOR CANTILEVER BRIDGE ACROSS THE HUDSON AT NEW YORK. IMG:content-0875.png:[graphic] up to 550 feet in length, but for longer spans they become too expensive on account of the rapid increase in weight. For spans between 500 and 1,000 feet in length either the arch or the cantilever system is to be employed, the latter being usually the cheaper. For the longest spans the stiffened suspension system is the best and most economical. In each particular case the civil engineer is to study the local conditions and so design such a structure that, while safely accommodating the traffic, its cost of construction and maintenance shall be the lowest po ible.