Another possible method of failure of a bridge pier arises from forces parallel with the length of the bridge. The stress produced on a bridge by the sudden stoppage of a train thereon, combined with a wind pressure parallel with the length of the bridge, will tend to cause the pier to fail in that direction (sec Fig. 71). Although these forces are never so great as the other external forces, yet the resisting power of the pier in this direction is so very much less than that in the other direction, that the factor of safety against failure is probably less, even if there is no actual danger under any reason able values for these external forces.
233. Abutment Piers. A pier is usually built comparatively thin in the direction of the line of the bridge, because the forces tend ing to produce overturning in that direction are usually very small. When a series of stone arches are placed on piers, the thrusts of the two arches on each side of a pier nearly balance each other, and it is only necessary for the pier to be sufficiently rigid to withstand the effect of an eccentric loading on the arches; but if, by any ac cident or failure, one arch is de stroyed, the thrust on such a pier is unbalanced and the pier will probably be overturned by the unbalanced thrust of the ad joining arch. The failure of that arch would similarly cause the failure of the succeeding pier and arch. On this account a very long series of arches usually includes an abutment pier for every fourth or fifth pier. An abutment pier is one which has sufficient thickness to withstand the thrust of an arch, even though it is not balanced by the thrust of an arch on the other side of the pier. Abutment piers are chiefly for arch bridges; but all piers should have sufficient rigidity in the direction of the line of the bridge so that any possible thrust which may come from the action of a truss of the bridge may be resisted, even if there is no counterbalancing thrust from an adjoining truss.
234. Abutments. The term abutment usually implies not only a support for the bridge, but also what is virtually a retaining wall for the bank behind it. In the case of an arch bridge, the thrust of the arch is invariably so great that there is never any chance that the pressure of the earth behind the abutment will throw the abutment over, and therefore the abutment never needs to be designed as a retaining wall in this case; but when the abutment supports a truss bridge which does not transmit any horizontal thrust through the bridge, the abutment must be designed as a retaining wall. The
conditions of stability for such structures have already been discussed. This principle of the retaining wall is especially applicable if the abutment consists of a perfectly straight wall. There are other forms of abutments which tend to prevent failure as a retaining wall, on account of their design.
235. Abutments with Flaring Wing Walls. These are con structed substantially as shown in Fig. 72. The wing walls make an angle of about 30° to 45° with the face of the abutment, and the height decreases at such a rate that it will just catch the embankment formed behind it, the slopes of the embankment probably being at the rate of 1.5:1. If the bonding of the wing walls, and especially the bonding at the junction of the wing walls with the face of the abutment, are properly done, the wing walls will act virtually as counterforts and will materially assist in resisting the overturning tendency of the earth. The assistance given by these wing walls will be much greater as the angle between the wing walls and the face becomes larger.
236. U-Shaped Abutments. These consist of a head wall and two walls which run back perpendicular to the head wall (see Fig. 73). This form of wall is occasionally used, but the occasions are rare when such a shape is necessary or desirable.
237. T-Shaped Abutments. As the name implies, these consist of a head wall which has a core wall extending perpendicularly back from the center. The core wall serves to tie the head wall and prevent its overturning. Of course such an effect can be produced only by the adoption of great care in the construction of the wall, so that the bonding is very perfect and so that the wall has very considerable tensile strength; otherwise the core wall could not resist the ing tendency of the earth pressure against the rear face of the abutment.