Читать книгу Inventors at Work, with Chapters on Discovery - George Iles - Страница 15

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The Burr bridge of 1804, already mentioned, included an arch and was in part sustained by struts projecting from abutments. These features were omitted by William Howe in the bridge which he patented in 1840, and which was, as far as is known, the first successor to a design of Palladio in employing a simple truss for long spans. The Howe truss was built of wood, except its terminal tie-rods, which were of iron; it has been repeated thousands of times throughout the world. In 1844 Thomas W. and Caleb Pratt patented a bridge which in design was the converse of Howe’s. Its diagonals of iron were used in tension, while its vertical struts of timber were in compression; in the Howe pattern the diagonals were in compression, the verticals in tension. This plan, by shortening the struts, diminished the cross-section necessary in a truss. When wrought iron took the place of wood for bridges, the Pratt design became the most popular of all, combining as it did more desirable features than any of its rivals. To-day for long spans the Baltimore truss is much in favor. Its stresses, that is, its resistances to change of form under strain, are readily ascertained; the shortness of its panels means strength; and its diagonals have the inclination which wide and varied experience has shown most desirable. The roadway, it will be observed, is upheld by sub-verticals, that is, by verticals which reach the floor from half the height of a panel.

Diagram of Baltimore truss.

Whipple Bridge.

An important study concerns itself with the intensity and distribution of strains, first in girders, next in trusses, and lastly, in bridges as units, all with intent to ensure the best possible designs throughout. In this field of inquiry the pioneer was Squire Whipple, a maker of mathematical instruments in Utica, N. Y., who published in 1847 his analysis of the strains in a truss bridge due to its own weight and to its moving loads. With the laws of these strains in mind he devised several bridges of great merit, the most noteworthy being reared in 1852 on the Rensselaer & Saratoga Railroad, seven miles north of Troy, which did service until 1883; its sides or web system had ties extended across two panels in double intersection.

In a long truss bridge, which in its entirety may be regarded as a girder of the utmost size, the cross pieces between the main beams of the structure are much less heavy than if continuous plates, of no more strength. The original form of the Victoria Bridge at Montreal was that of a continuous tube of iron, square in section; it has given place to a truss bridge of five times greater capacity which weighs only twice as much. (Illustrations of both on pages 27 and 28.)

Thus to lessen weight in comparison with strength is a matter of great importance in a suspended structure, which must not only bear its own weight, but carry heavy moving loads.

Simple cantilevers.

FG, HI, are first separate; then in contact; last are joined by a plank laid above them.

VICTORIA BRIDGE, MONTREAL.

Original tubular form designed by Robert Stephenson.

VICTORIA BRIDGE, MONTREAL,

Rebuilt with trusses.

CANTILEVER BRIDGE ACROSS THE ST. LAWRENCE, NEAR QUEBEC.

Total length, CF, 3300 feet. Channel span, DE, 1800 feet. Central truss, AB, 675 feet.