Handbook of Railroad Construction; For the use of American engineers. / Containing the necessary rules, tables, and formulæ for the location, construction, equipment, and management of railroads, as built in the United States.

by George L. Vose

Fig. 122.

The curved wing, in fig. 121, being arched, requires a little less thickness, but at the same time is longer. B B, show the bridge seats. The slope of the wings may be battered with an inclined coping, or off-setted at each course. Wing walls, subjected to special strains or to particular currents of water, require positions and forms accordingly. In skew bridges, as in Chap. V., the wing, at the acute angle, is longer and inclines less from the face of the abutment than that at the obtuse angle. The more the wing departs from the face line and swings round into the slope, the greater the thrust becomes upon it, as the centre of pressure is raised; the thrust becomes a maximum when the wing is inclined from forty-five to seventy degrees from the face of the abutment. The body of an abutment, as well as any other retaining wall, may be much stronger by giving it a trapezoidal instead of a rectangular section, as the resisting leverage is thereby much increased. Abutments may be to advantage buttressed in order to resist special strains, as in case of the arches or braces of wooden bridges.

Fig. 123.

Fig. 124.

257. Railroad abutments except for a double track, require but little breadth on top, except where the truss itself rests. The common T abutment originated by Captain John Childe, and now in very extensive use, seems to fulfil any requirement of a good abutment, see fig. 122, page [242]. B B is the bridge seat, and the mass T T takes the place of wings. The difference of level of the top and of the bridge seat depends upon the difference between the height of the bearing of the lower chord of the bridge, and grade. The line of contact between the earth and the wall is shown by s s′ s″ s‴. The length of the top of the masonry is found thus. Suppose the slope to be one and one half to one, and the whole height thirty feet, the whole horizontal length of slope is then forty-five feet; from this we take the sum of the horizontal distances, s s′ and s′ s″, and suppose these to be, respectively, six and eight feet, we have the whole operation thus:—

30 × 1½ – 6 + 8 = 45 – 14 = 31 feet.

It may be advisable in very high abutments to lighten the masonry by an arched opening as in fig. 123. The walls, also, of the U abutment (see fig. 124), when large, may be pierced with arches to save masonry.

Probably the cheapest mode of bringing a bridge to the embankment is that shown in fig. 125; A being the bridge seat for the main truss, and B that for the trussed girder.