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.
FG, HI, are first separate; then in contact; last are joined by a plank laid above them.]
[Illustration: VICTORIA BRIDGE, MONTREAL.
Original tubular form designed by Robert Stephenson.]
[Illustration: VICTORIA BRIDGE, MONTREAL,
[Illustration: CANTILEVER BRIDGE ACROSS THE ST. LAWRENCE, NEAR QUEBEC.
Total length, CF, 3300 feet. Channel span, DE, 1800 feet. Central truss, AB, 675 feet.]
Advantages of the Cantilever, Arch, and Bowstring Designs.
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.