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Bridge Piers

pier, section and feet

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. Occa'sionally, for economy, piers, particularly pivot-piers, are built hollow—sometimes with and sometimes without cross walls.