Abutments and Piers

ABUTMENTS AND PIERS Design of Abutments. The supports at each end of a bridge are known as abutments. An abutment serves a number of purposes: it fur nishes a resting-place for the end of the span; it distributes the weight of the bridge over the foundation; and it prevents the bank from run ning out into the stream at the end of the bridge.

It is the object of bridge engineers to plan abutments which will fulfil these requirements and at the same time be as economical as possible to construct; and every man who may be called upon to assist in bridge-building should under stand this object if he expects to do his part of the work intelligently.

The first step in the design of an abutment is to provide a resting-place for the pedestals which carry the bridge-span. These pedestals are made sufficiently large to prevent the crushing of the material of the top of the abutment, or bridge-seat, under the load of the bridge. After the size of the bridge-seat has been determined, the area and depth of the foundation course is calculated, the actual dimensions depending on the nature of the underlying soil, which is known from preliminary investigations. The third and last step is to provide for the embank ment behind the abutment in such a way that it will not encroach on the clear waterway or opening under the span. There have been a n imber of abutment types developed to accom plish this object, among which are: wing-abut ments, Tee-abutments, U-abutments, arch abut ments, and a number of special types.

Wing-Abutments. For the average size of bridge and height of abutment, wing-abutments are the most generally used, and are probably the best. Concrete has replaced stone masonry as the material for constructing such abutments, and answers the purpose excellently. Wing abutments are so called because of the two slop ing walls which extend out from the sides to the full width of the embankment to prevent the earth from running into the stream. These walls are known as wing-walls. The entire abut ment, including the wing-walls, must be designed to retain the earth behind it according to the principles set forth below in the discussion of "Retaining Walls."

Fig. 67 is a plan of a typical railroad abut ment of concrete. A top view, front view, and section are shown, and the technical names of the most important features are marked. The dimensions given are subject to some variation, on account of the differences in the standard em bankments of the various railways. The in structions headed "Rules for Design" (Fig. 67), will enable one to fill out the more important dimensions marked with letters on the drawing. All the blank dimensions shown will have to be figured from these fundamental dimensions, and filled in. When all the distances indicated are given, the form-builder will have sufficient in formation to construct the form work, which should present no especial difficulties after the footings are finished (see Fig. 69). It will be noted that one of the wings of the abutment in Fig. 67 is at a different angle from the other; this angle will have to be decided on for each wing for each abutment, according to the cir cumstances of the case—namely, the direction of the stream and current at the bridge; the con dition of the stream bed, the shores, and the em bankment; the strength of current; the elevation of high water; and the possible shifting of the stream.

Highway abutments of the wing-type are designed and constructed along exactly the same lines as railroad abutments, but they are usually smaller.

When wing abutments exceed a height of about 25 feet, it has been found economical to reduce the mass by introducing thin counter forts, and reinforcing the entire structure to tie it together. Such an abutment is shown in Plate 13 (B) and in Figs. 70 and 71.

Special Abutments. For bridges over streams or valleys with high, steep banks, the plain or even reinforced concrete wing abut ments become uneconomical, on account of the large yardage they require to give them suffi cient weight and stability to prevent overturn ing. Each one of these high abutments is a special problem which must be studied in order to find the cheapest type of structure to fit it; but there are a number of distinctive schemes which may best be illustrated by describing actual examples.