Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 16

Table of Contents

In most cases a bridge crosses a valley or a river in a place which permits the engineer to erect scaffolding to support his trusses until they can be united and become self-sustaining. In some places this course is denied; a river such as the Ohio or the Mississippi may have to be spanned at a point where the waters in a single day may rise forty feet, bearing along trees and timbers with destructive violence. As a rule the difficulty is met by employing cantilever spans which require no scaffolding for their construction. To understand their principle let us suppose that on opposite banks of a creek we roll out to meet each other the joists FG and HI, taking care that the parts over the water shall always be lighter than the parts on land. When the joists at last touch they are secured to each other as a continuous roadway. Or, while they are at a moderate distance apart they may be joined by a third timber laid across the gap from one to the other. In practice the simple principle thus illustrated is developed and varied in many ways, but in every application the one rule is that the trusses as they stretch out from the two sides of a pier shall balance each other, the shore ends being duly weighted down or safely anchored to solid rock. And thus, at length, we come to the wonderful bridge, six miles west of Quebec, whose channel span of 1,800 feet will be the longest ever reared. See illustration, page 29. From the cantilever arms, DA and BE, will be suspended the central truss, AB, of 675 feet. A cantilever span may be much longer than a simple truss because on a pier, as D of this bridge, a part, DA, of the whole span, DE, is balanced either, as in this case, by a shore span, CD, or by a corresponding part of the next span should that span not extend to the shore but pass from one pier to another.

Kentucky river cantilever bridge

The first cantilever bridge in America was designed by C. Shaler Smith for the Cincinnati Southern Railroad, to cross the Kentucky River; it was built in 1876–7.

Arch bridge, Niagara Falls

Spanning the gorge of Niagara, close to the Falls, is an arch bridge of 840 feet in its central span, which, in its construction during 1898, followed the plan originated by James B. Eads in building the St. Louis bridge nearly thirty years before. As scaffolding was out of the question in both cases, each bridge was built out from its piers on the cantilever principle. An arch is sometimes disguised as a modified bowstring, as in the Burr design of 1804, a horizontal tie connecting the extremities of the arched rib and taking its thrust, dispensing with the abutments demanded by an arch. In the chords of such a pattern the strength comes as near to uniformity throughout as practical considerations permit, avoiding the losses of early days when one part of a bridge might be twice as strong as another. The bowstring was adopted for the great span of 542¹⁄₂ feet over the Ohio at Cincinnati built in 1888, and for the span of 546¹⁄₂ feet erected at Louisville in 1893. A bowstring 533 feet long, forming part of the Delaware river bridge of the Pennsylvania Railroad, built in 1896, in Philadelphia, is outlined on page 32. At Bonn, on the Rhine, there was completed in 1904 a bridge whose central span is a bowstring 616¹⁄₄ feet long.

Bowstring Bridge, Pennsylvania R. R., Philadelphia.