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PIERS OF A BRIDGE, the supports of the arches over the openings, when more than one, not including the supports at the extremities, which are called abutments. See BRIDGE.

M. Belidor observes, that when the height of the piers is about six feet. and the arches are circular, it is sufficient to make their thickness the sixth part of the width of the arch, and two feet more; but when the arches become of a great span, the thickness of the piers may be reduced to the sixth part ; but then the depression of the two feet does not take place at once ; that is, in an arch of about forty-eight feet, three inches are taken oil' for every six feet of increase of the width of the arch. The thickness of the piers supporting elliptic arches is greater than in the thriller proportion: thus, in au arch of seventy-five feet wide, the thickness of the pier, whose height is about six feet, should lie 13.5 when the arch is circular, and fifteen feet when it is elliptical. The

same author makes the abutments one sixth part more than the piers of the largest arch ; and M r. Muller has calculated a table, containing the thickness of the piers of bridges.

Rectangular piers are seldom used except in bridges over small rivers ; in all others they project from a bridge by a triangular prism, which presents an edge to the stream, in order to divide the water more easily, to prevent the ice from sheltering there, as well as vessels from running foul against them. This edge is terminated by the adjacent surfaces at right angles to each other at Westminster Bridge; but those of the Pont-royal, at Paris, make an acute angle of about GO degrees. Engineers, however, in their later constructions, make this angle to terminate by two cylindric surfaces, whose bases are arcs of GO degrees.