Fig. 281.—Framed Pier.
If the sills of a bridge are laid directly upon the dry walls of an abutment, or upon a heavy plank, the jar of passing teams soon displaces some of the stones, and brings undue strain upon certain portions of the wall. To avoid this, abutments are best made of cut stones, and laid in cement. A wooden bent for the support of the ends of the bridge may be made as shown in [figure 280]. The whole should be constructed of heavy timber, pinned together. A coat of white lead should cover the interior surface of all joints. The number and position of the posts of the wooden abutment are seen in the engraving. A log should be laid upon the wall at m, to relieve the bridge from the shock of the passing wagons. A center pier should be avoided as much as possible, as it offers serious obstruction in floods, and ice, drift wood and other floating matter become piled against it, seriously imperiling the entire structure.
Fig. 282.—Bridge Supported By Piles.
Fig. 283. —— Railing Of Bridge. —— Fig. 284.
But in cases where the length of the bridge is so great as to require one or more piers, they may be constructed on the plan shown in [figure 281], or in case the bottom is so soft as to render the mudsill insecure, a line of piles supporting a cross-timber, as in [figure 282]. A strong, reliable parapet or railing should always be provided. The want of one may be the cause of fatal accidents to persons and horses. [Figure 283] gives a side view of a good railing, and [figure 284] shows the manner of bracing the posts to the ends of the cross-beams. They should be thus braced at every alternate post of the railing. The floor should be double, as shown in [figure 285], the lower planks laid diagonally, and the upper layer crosswise.
Fig. 285.—Plank Floor Of Bridge.