BRACES AND TRUSSES.
bridge with a simple trussFig. 275.—A Simple Form Of Bridge Span.
Fig. 275.—A Simple Form Of Bridge Span.
Fig. 275.—A Simple Form Of Bridge Span.
bridge with truss and support rodsFig. 276.—A Stronger Span.
Fig. 276.—A Stronger Span.
Fig. 276.—A Stronger Span.
The above is the initial strength of the timbers which support the weight of the superstructure, and any load that it may have to sustain. But in bridge building these timbers are reinforced by trusses or braces, which add greatly to the sustaining power of the bridge.
Figure 275shows the simplest form of a self-supporting bridge, which will answer for spans of from ten to fifteen feet in length. The braces,c,c, reach from nearthe end of the sill to about four feet above the center. The truss rod,d, is one inch in diameter for short bridges up to two inches for longer spans; it is provided with an iron washer at the top. The rod passes through the sill, and a cross-sill,e, which passes under the main sills, thus adding firmness to the whole structure. Logs,f,f, are placed against the ends of the sills to keep them in place, and where the wheels will first strike them instead of thefloor plank, thus greatly equalizing the pressure.Figure 276represents a modification of the above. The two truss-rods and braces give the structure greater strength and solidity, adapting it for spans eighteen feet in length. For the latter length, sills should be of good material, ten inches wide and fourteen inches deep, with three middle sills of about the same size.
bridge with a truss as a parapetFig. 277.—A Short Bridge.
Fig. 277.—A Short Bridge.
Fig. 277.—A Short Bridge.
bridge with 3 support rodsFig. 278.—A Bolt Truss.
Fig. 278.—A Bolt Truss.
Fig. 278.—A Bolt Truss.
Figure 277is a more improved style of bridge, the truss serving both to support the structure, and as a parapet. The top railing is of the same width as the sill, about one foot. The lower side may be cut away, giving the bridge a more finished appearance. The railing at the center is six inches thick, and three inches at the ends. The tie,h, is full width and four inches thick. A bridge of this kind will answer for heavy traffic, even if twenty feet in length. The bolt truss, infigure 278, is adapted for a span of twenty-five feet. This makes a bridge of great firmness. Each set of truss-rods support a cross-sill. The road planks are laid crosswise of the bridge. The middle sills are sometimes half an inch lower thanthose along the sides, and should be four or five in number. The ends of the planks fit closely against the inside of the truss sills, thereby keeping the planks securely in place.
bridge with bottom trussFig. 279.—Bridge Braced From Below.
Fig. 279.—Bridge Braced From Below.
Fig. 279.—Bridge Braced From Below.
A common method of bracing is from below as shown infigure 279. This is not usually a good practice, as the braces are liable to be carried away by ice or floods.