Bridge Piers and Abutments

The pier below high .eater is hollow, with reinforced side walls connecting the towers at the ends. Above high water, the towers are separate and connected by a reinforced arch at the top. Openings are provided through the walls to admit water to the interior spaces.

211. Construction of Piers.—Solid bridge piers are constructed of concrete or of concrete with facing of cut stone. The use of rubble masonry as hacking in such work has given way to concrete on ac count of its less cost and greater case of handling.

Stone masonry facing has the advantage of presenting a pleasing appearance, and offering good resistance to the abrasion of the stream and of floating debris. In constructing a pier by its use forms are unnecessary, which frequently results in lessened cost of con struction, although the cost of the masonry itself is greater than that of concrete. First-class ashlar masonry is required in such work, and the stone must he \veil bonded into the concrete hacking. In im portant work carrying heavy loadings, the facing stones ai c tied to the concrete by the use of steel rods attached to the stretchers at frequent intervals and extending well into the concrete.

When piers are wholly of concrete, it is desirable to place light reinforcement near the surface in the face of the pier to prevent surface cracks, which usually develop in any large exposed surface of concrete. This would require horizontal bars not more than 1 foot apart, and vertical bars every 2 or 3 feet, embedded about 2 inches in the concrete. The top of the coping should he similarly reinforced.

Cylinder piers are most commonly formed by constructing a cylindrical shell of steel and filling it with concrete. Reinforced concrete cylinders are also coming into use, and have the advantage of not requiring painting to prevent rust. A pair of cylinders is gen erally used for a pier and they are connected by bracing near the top or at two points for high piers. This bracing may be of reinforced concrete, or sometimes a steel truss inclosed in concrete.

The masonry of a pier may be supported upon a caisson, or upon hard material or piles in a cofferdam. When the pier rests upon a caisson, a cofferdam is built upon the top of the caisson and the masonry built inside the cofferdam after filling the caisson with con crete. When the pier rests upon piles, the tops of the piles extend upward into and are inclosed by the concrete in the base of the pier.

In such work, it is desirable to place reinforcement in the bottom of the footing of the pier between the piles.

212. Types of Bridge Abutments.

A bridge abutment is a com bination of a pier with a retaining wall; it carries the weight of one end of the bridge with its moving load and retains the bank of earth sustaining the roadway leading to the bridge, the requirements for stability being the same as those for a retaining wall. The weight of the bridge with its live load is brought upon the abutment near the top, and the thrust of the earth filling with that of the load upon the roadway is brought upon the back of the abutment, as shown in Fig. 127. These, with the weight of the pier itself, must give a proper distribution of pressures upon the foundation and safe stresses at any point in the masonry.

The filling supporting the roadway usually has side slopes about 1.5 horizontal to 1 vertical, which must be sustained by walls joined to the abutments. Abutments are divided according to the method used for supporting the side slopes into straight abutments, wing abutments, IT abutments, and T abutments.

Straight abutments are those in which the walls retaining the side slopes are continuations of the abutments in the same lines, as shown in Fig. 127.

Tying abuhtudt are those in which the side .slopes are by wing walls, making an angle, usually about 30° with t tie face of the abutment (see Fig 12S). This type of abutment is selected whore a stream flows past the face of the abutment, as it disturbs the flow of the stream to the least extent and protects the abutment against the stream getting behind it. The wing walls may be shorter and require somewhat less masonry then the walls of straight abutments, when the bottom of the sloping earth is held back to the line of the face of the abutment.

U-Abutments are those in which the walls are turned at right an gles to the abut ment as shown in Fig. 129. The earth slope is then upon the face of the wall. They may be economical when the abut ment is on the edge of a bluff so that the depth of the wall may be reduced by running into the face of the bluff.

Foundations of Bridges and Buildings by Jacoby and Davis, New York, 1914, gives a complete description of the various methods of constructing founda tions, with detailed descriptions of many important constructions.