BRIDGE ABUTMENTS There are four forms of abutments in more or less general use. 1. A plain wall parallel to the current, shown in elevation at Fig. 126, with or without the wings ADF and BEG. The slopes may be finished with an inclined coping, as AD, or offset at intervals, as BE. When abutments were made of stone masonry, the latter was the usual method of finishing; but since abutments are generally built of concrete, the former is the more common. The abutment shown in Fig. 126 is called a straight abut ment or less appropriately an abutment with straight wings. 2. The wings may be swung around into the bank at any angle, as shown (in plan) in Fig. 127. The angle so is usually about 30°. This form is known as the wing abutment or an abutment with splayed wings. 3. When so of Fig. 127 becomes 90°, the result is Fig. 128, which is called the U abutment. 4. If the wings of Fig. 128 are moved to the center of the head-wall the result is Fig. 129, which is known as the T abutment.
The form of the abutment to be adopted for any rarticular case will depend upon the locality,— whether the bridge is over a waterway or over a street or railway. If for the former, the form of abutment depends upon whether the banks are low and flat, or steep and rocky; whether the current is swift or slow; and also upon the relative cost of earthwork and of masonry.
Fig. 126 is the usual form for a street or railway crossing; but is not suitable for a stream crossing, owing to the danger of the water's flowing along immediately behind the wall.
Fig. 127 is the usual form for a stream crossing, particularly where there is a contraction of the waterway at the bridge site, since de flecting the wing walls, on both the up-stream and down-stream side, slightly increases the amount of water that can pass. This advantage can be obtained, to some degree, with the straight abutment (Fig. 126) by thinning the wings on the front and leaving the back of the wings and abutments in one straight line. Not only there is no hydraulic advantage, but there is a positive disadvantage, in in creasing the deflection of the wings beyond, say, 10° or 15°. The more the wing departs from the face line as it swings round into the embankment, the greater its length and also the greater is the thrust upon it. The wings are not usually extended to the toe, B, of the embankment slope, but stop at a height, depending upon the angle of deflection of the wing and the slope of the embankment, such that the earth in flowing around the end of the wall will not get into the channel of the stream. It can be shown mathematically that, if the toe of the earth which flows around the end of the wing is to be kept three or four feet back' from the straight line through the face of the abutment, an angle of 25° to 35° is best for economy of the material in the wing walls. This angle varies slightly with the proportions
adopted for the wing wall and with the details of the masonry. This form of construction is objectionable, since the foot of the slope in front of the wing is liable to be washed away; but this could be rem, edied somewhat by riprapping the slope, or, better, by making the wing longer.
Fig. 126 is one extreme of Fig. 127, and Fig. 128 is the other. As the wing swings back into the embankment the thrust upon it in creases, reaching its maximum at an angle of about 45°; when the wing is thrown farther back the outward thrust decreases, owing to the filling up of the slope in front of the wing. Bringing the wings perpendicular to the face of the abutment, as in Fig. 128 also de creases the lateral pressure of the earth, owing to the intersection of the surfaces of rupture for the two sides, which is equivalent to removing part of the "prism of maximum thrust." If the banks of the stream are steep, the base of the wing walls of Fig. 128 may be stepped to fit the ground, thereby saving masonry. Under these conditions, also the wing abutment, Fig. 127, can be treated in the same way; but the saving is considerably less. When the masonry is stepped off in this way, the angle thus formed becomes the weakest part of the masonry; but, as the masonry has a large excess of strength, there is not much probability of danger from this cause, provided the work is executed with reasonable care.