Triumphs and Wonders of the 19th Century: The True Mirror of a Phenomenal Era / A volume of original, entertaining and instructive historic and descriptive writings, showing the many and marvellous achievements which distinguish an hundred years of material, intellectual, social and moral progress

by James P. Boyd

II. BRIDGES.

Not only is there evidence that bridges of the simplest forms have been used from prehistoric times, but the engineering world has been frequently surprised at the discovery, in semi-barbarous lands where there was evidently no scientific knowledge of bridge construction, of a bridge which, in its mechanical analysis, is a rude example of some one of the more complicated types now in use. But these bridges are always small, and are constructed with an utter disregard of that economy of construction which is one of the great triumphs of modern bridge engineering, being uselessly strong in some parts, considering their weakness in others. At the beginning of this century there was not a wrought-iron or steel bridge in existence. Disregarding stone arches for the present, all other bridges were made of wood—with the exception of a few bridges of cast iron, which were constructed during the latter part of the eighteenth century. But cast-iron is unsuitable for pieces requiring tensile strength; it is also difficult to cast very large pieces with any assurance of uniformity. The best existing examples of cast-iron bridges are, therefore, those of the arch type; but these are very heavy in proportion to their real strength, and would now be much more costly than, as well as inferior to, steel bridges of equal strength. Therefore the great advance in bridge work during this century consists in the development of steel bridge construction, and a brief description will be given of a few bridges which represent the chief types.

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.

PECOS RIVER VIADUCT.

This bridge joins two sections of Scotland which had been previously separated by an arm of the sea, which could only be crossed by a tedious ferry. Even this ferry was frequently tied up by fog or by the strong gales which so often blow up the channel. The prevalence of heavy wind pressure demanded that special attention should be given to this feature, and the most elaborate tests ever made of the effect of wind on a bridge structure formed a part of the preliminary work. The estuary, for a distance of nearly fifty miles, is never less than two miles wide, except at this one place, where it is but little more than one mile wide, with the added advantage of having the island of Inchgarvie nearly in the centre of the channel. The channel on both sides is about two hundred feet deep, which would forbid the location of a pier at any place except on this island, which, being composed of basaltic trap rock, furnished a sufficient foundation at a comparatively slight depth below the surface. To secure the maximum rigidity consistent with economy in weight, the “vertical columns” of the towers were spaced 120 feet apart at the base, but only 33 feet apart at the top. The towers are 330 feet high. As shown in the illustration, the cross-sectional dimensions of the cantilevers diminish rapidly both in width and height, so that although the weight of the steel per running foot at the towers is 23 tons, it becomes only a little over two tons per foot at the centre. The structure is exceptionally rigid.

The picture of any gigantic structure, especially when well proportioned, utterly fails to give an adequate idea of the size of its component parts. It is difficult to realize from the illustration that the four tubular “vertical columns” on each main pier are twelve feet each in diameter at the base—large enough for “a coach and four” to drive into, if they were laid horizontally. Over 50,000 tons of steel were used in the main spans. The total cost of the whole structure was over £3,200,000 ($16,000,000).

Stone Arches.—The nineteenth century has but little to claim as to the development of stone arches. The mechanical theory of their stresses is perhaps better understood now than ever, and the largest masonry arch in existence (the Cabin John arch, having a span of 220 feet, carrying the Washington aqueduct over a creek) is a piece of American work of this century. But it should not be forgotten that more than five hundred years ago there was constructed at Trezzo, Italy, a granite arch of 251 feet span. This arch was unfortunately destroyed in 1427. One of the most remarkable arches in existence was designed and built by an “uneducated” stone-mason at Pont-y-Prydd, Wales, in 1750. A rigorous analysis of its strains—of which the designer probably knew nothing—shows that the “line of resistance” passes almost exactly through the centre of the arch ring. The most highly educated engineer of the present day could do no better. On the other hand, the development of the theory has been shown by the successful construction of an exceedingly bold design for a bridge on the Bourbonnais Railway, in France. The span is 124 feet, and the rise only 6.92 feet. The design was considered so very bold that a model of the arch was first constructed and tested before the design was finally adopted. The extension of the use of stone arches, especially those of very large size, is doubtless prevented by their excessive initial cost over the cost of a steel structure of equal span and strength. Since a stone arch is generally considered more beautiful than a steel bridge, the æsthetical element often demands the construction of stone arches in public parks in situations where a metal structure would be more economical. The great reduction in the cost of steel during the past few years, due to improved processes of manufacture, generally renders the cost of a steel bridge, even with a proper allowance for maintenance, repairs, and renewals, cheaper than a stone arch, unless the span is short.