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.
III. CAISSONS.
The use of compressed air to keep back the water that would naturally flow through the soil into a deep excavation is a comparatively recent idea. In 1839 M. Triger, a French engineer, conceived the idea of sinking an iron cylinder through twenty metres of quicksand in the valley of the Loire River, in order to reach a valuable coal deposit which was known to be located beneath the river. A chamber with doors, such as is now called an air-lock, was constructed at the top of the cylinder. To pass into the cylinder the lower door, opening downward, was closed, and when the air in the chamber was at atmospheric pressure, the upper door, also opening downward, was opened. Upon entering the chamber the upper door was shut, and air was pumped in until the pressure equaled the pressure in the cylinder underneath, which was also the pressure necessary to keep back the water from the excavation. The lower door could then be opened and the working chamber entered. To pass out, the reverse process in inverse order was necessary. This was the first pneumatic caisson ever sunk, although such plans had been proposed and even patented in England several years before. The idea was essentially the present plan, but the process has been improved and enlarged. The required pressure is substantially that due to the weight of a column of water as high as the depth of the base of the caisson below the water surface. In the case of the St. Louis bridge, the bottom of the caisson was sunk to 109 feet 8½ inches below the water surface, which required an air pressure of about 47 pounds per square inch in the working chamber. Such a pressure is dangerous to those working in it. The men literally “live fast.” Great exertion is easily made, but is followed by corresponding exhaustion after leaving the caisson. Those having heart disease, or who have been debilitated by previous excesses, are liable to be seriously affected—generally by a form of paralysis which has been specifically named by physicians the “caisson disease.” At the St. Louis bridge, when working at the greatest depths, the men were only worked four hours per day, in two-hour shifts. Facilities were likewise provided to have them bathe, rest, and take hot coffee on coming out of the working chamber. Healthy men, who observed these and similar precautions, were not permanently affected by the work.
FORMAL OPENING OF SUEZ CANAL.
Procession of Ships in Canal, November 16, 1869.