BROOKLYN SUSPENSION BRIDGE.
Brooklyn Bridge.—The suspension bridge between New York and Brooklyn is the largest bridge of its kind in existence, and, until the construction of the “Forth” bridge, was the longest clear span ever built. Every one is so familiar with this stupendous structure that only a few statements will be made, which may give a better idea of the unprecedented problem which confronted the great engineer, John A. Roebling. When looking at the exceedingly graceful design of the towers, one is apt to forget that a large part of the structure of each tower is hidden from view. The bottom of the foundation of the pier, on the New York side, is 78 feet below mean high tide, and spreads over an area 172 feet long and 102 feet wide. The pressure exerted by the caisson on its base is about 114,000 tons, or 6½ tons per square foot. This great area, 354 feet below the parapet of the towers, is a surface consisting partly of bed-rock and partly of a material so compact that it was found, to be almost impossible to drive an iron bar into it. Down below the mud, below all danger of scour, far below the depth where the dreaded teredo navalis can destroy the timber in the caissons, these piers rest on an immovable foundation, and are an imperishable monument of man’s skill. The floor of the bridge is supported by four cables, each containing 6300 wires. Each wire is supposed to be subjected to a stress of about 570 pounds, and to have an ultimate strength of 3400 pounds. To say that each cable is pulled by a force of 3,591,000 pounds conveys but little real impression to the mind—as little as to say that it would require a pull of over 21,000,000 pounds to break it. And there are four such cables! The main span, including the weight of the cables, weighs about 5000 tons. Some interesting facts concerning the caissons under the piers of this bridge will be given under the heading of “Caissons.”
THE NIAGARA RAILWAY ARCH.
Niagara Railway Arch.—The railway suspension bridge, constructed by Mr. John A. Roebling across the Niagara gorge in 1853–55, was justly considered a monument to the skill of a great engineer, a monument of the world’s progress; and yet so rapid has been the advance in the art of bridge engineering, that this great structure is already a thing of the past, and has now been replaced by another bridge which better fulfills the increased requirements. It was not that Roebling’s bridge was an engineering failure, but that the large increase in the weight and length of trains now requires a much stronger bridge. There were several formidable conditions confronting the engineer who designed the steel arch which has now replaced the suspension bridge. For one thing, a heavy railroad traffic was using the old bridge. The interruption of railroad traffic for even a few day’s is a serious matter. Extend the time to several months, and the consequences are too serious for toleration. And thus it became necessary to so plan and construct the arch that both structures would occupy the same site, not interfere with each other, and not interfere with the running of trains. It is an amazing, almost inconceivable, triumph of constructive skill that this was accomplished so that “not a single train was delayed, and traffic on the highway floor was suspended only for about two hours each day, while the upper floor system was being put in.” The second rigid requirement was the necessity for constructing the arch without any “false works” underneath. Of course it was not practicable to suspend the various members of the arch during construction, from the old bridge, as it was not designed for such a load. Nor would it have been possible to plant false works in the deep and swift current of the Niagara River. And so it became necessary to make each half of the bridge self-supporting, as it hung out over the raging torrent a distance of about 275 feet from the abutments, until the two projecting arms could be joined in the centre. The illustration does not show the independence of the arch from the old bridge. If the old bridge had not been there (as was virtually the case, so far as support given by it is concerned), the independence of those arms reaching out over the river would have been more apparent. Add to all these rigorous conditions the marvelous fact that the erection of this great arch was begun on September 17, 1896, and that the bridge was tested on July 29, 1897 (only 315 days afterward), and we have here one of the greatest triumphs of engineering which could be imagined.
Pecos River Viaduct.—The original location of the Galveston, Harrisburg, and San Antonio Railway included a section of about 25 miles which was very difficult to operate, on account of its very heavy grades and sharp curvature. After some years of study and surveying, a line was found which would save 11.2 miles in distance, 378 feet of rise and fall, and 1933 degrees of curvature, besides being free from land slides which threatened the old line at many points. But the great economic advantages in the expenses of operating could only be obtained at the cost of an almost unprecedented structure,—a viaduct 2180 feet long, which should cross the Pecos River at an elevation of 320 feet 10½ inches above the water surface. There are two bridges in Europe which span very deep gorges by arches, which are higher above the water than this viaduct, but in such cases the depth of gorge is of no engineering importance. There is also a viaduct, for a narrow-gauge railway in Bolivia, 800 feet long and with a height of 336 feet from the rails to the water. But the Pecos viaduct is built to carry standard-gauge railway traffic over a valley nearly half a mile wide, and at such a height that a train moving over it appears diminutive. The stone towers in the illustration appear small, but they are constructed to a height of over 50 feet above the ordinary level of the water, to allow for possible floods. The longest “bents” have a height of 241 feet 0¾ inches. No “false works” were used in erecting the bridge. The “traveler,” shown in the illustration, had an arm 124 feet 6 inches long. After completing the construction on one side of the river (including one half of the “suspended” span immediately over the river), the traveler was taken apart, loaded on cars and transported by rail a distance of nearly 40 miles, in order to reach the other side of the valley. Then the construction was carried on as before, until the two halves of the suspended span met in the centre. The work of erection began November 3, 1891, and on February 20, 1892 (only 108 days later), the two halves of the suspended span were connected. A portion even of this time was lost by inclement weather and unavoidable delays. This light “spider-web” method of construction for crossing very high valleys was originated by American engineers, the first notable instance of it being the construction of the “Kinzua” viaduct, on the N. Y. L. E. & W. R. R., which has a length of 2050 feet and a height of 302 feet above the water—figures which are only slightly less than the above.
THE FIRTH OF FORTH BRIDGE. GENERAL VIEW.
Forth Bridge.—The next type of bridge to be considered has for its example the largest bridge in the world—the “cantilever” crossing the Firth of Forth, in Scotland. The economic design of bridges of this type, on the basis of the mechanical principles involved, is not only an achievement of this century, but of the latter part of the century. Nevertheless, we may find illustrations of the fundamental principle in the stone lintels in an Egyptian temple; in a rough wooden bridge erected by Indians in Canada, near the line of the Canadian Pacific Railroad; and in a bridge erected over two hundred years ago in Thibet, and discovered in 1783 by Lieutenant Davis, of the English embassy to the court of the Teshoo Lama. The principle of these bridges is very graphically shown by a photograph made at the time of the construction of the Forth bridge.