BRIDGE PIERS.
The thickness of a pier for simply supporting the weight of the superstructure need be but very little at the top, care being taken to secure a sufficient bearing at the foundation. Piers should be thick enough, however, to resist shocks and lateral strains, not only from a passing load, but from floating ice and ice jams; and in rivers where a sandy bottom is liable to deep scouring, so that the bottom may work out much deeper on one side of a pier than on the other, regard should be paid to the lateral pressure thus thrown on the pier. For mere bearing purposes the following widths are ample for first-class masonry—span 50 feet, width 4 feet, span 200 feet, width 7 feet. Theoretically the dimensions at the bottom are determined by the area necessary for stability; but the top dimensions required for the bridge seat, together with the batter, 1 in 12 or 1 in 24, generally make the dimensions of the base sufficient for stability.
The up-stream end of a pier, and to a considerable extent the down-stream end also, should be rounded or pointed to serve as a cutwater to turn the current aside and to prevent the formation of whirls which act upon the bed of the stream around the foundation, and also to form a fender to protect the pier proper from being dam aged by ice, tugs, boats, etc. This rounding or pointing is designated
by the name starling, the best form appears to be a semi-ellipse. The distance to which they should extend from the pier depends upon local circumstances.
A bridge pier may fail in any one of these ways; (1) by sliding on any section on account of the action of the wind against the ex posed part of the pier; (2) by overturning at any section where the moment of the horizontal forces above the section exceeds the moment of the weight of the section; or (3) by crushing at any section under the combined weight of the pier, the bridge and the load. Bridge piers are usually constructed of quarry-faced ashlar backed with rubble or concrete. Occasionally, for economy, piers, particularly pivot-piers, are built hollow—sometimes with and sometimes without cross walls.