. Railroad construction. Theory and practice . the normal condition of affairs, as well as thecondition w^hich produces the maximum stress, an inclined axisis evidently preferable from a theoretical standpoint; but what-ever design is adopted, the trestle should evidently be suffi-ciently cross-braced for either a moving or a stationary load,and any proposed design must be studied as to the effect of bothof these conditions. Some of the various methods of securingthe requisite superelevation may be described as follows: (a) Framing the outer posts longer than the inner posts, so that the cap i
. Railroad construction. Theory and practice . the normal condition of affairs, as well as thecondition w^hich produces the maximum stress, an inclined axisis evidently preferable from a theoretical standpoint; but what-ever design is adopted, the trestle should evidently be suffi-ciently cross-braced for either a moving or a stationary load,and any proposed design must be studied as to the effect of bothof these conditions. Some of the various methods of securingthe requisite superelevation may be described as follows: (a) Framing the outer posts longer than the inner posts, so that the cap is inclined at theproper angle; axis of posts verti-cal. (Fig. 82.) The method re-quires more work in framing thetrestle, but simphfies subsequenttrack-laying and maintenance, un-less it should be found that thesuperelevation adopted is unsuit-able, in which case it could be cor-rected by one of the other methodsgiven below. The stringers tendto twist when the train is sta-tionaiy. (b) Notching the cap so that the stringers are at a different. Fig. 82. 147. TRESTLES. 167 Fig. 83. elevation. (Fig. 83.) This weakens the cap and requires thatall ties shall be notched to abevelled surface to fit the string-ers, which also weakens the centrifugal force will tend totwist the stringers and rack thetrestle. (c) Placing wedges underneaththe ties at each stringer. Thesewedges are fastened Avith twobolts. Two or more wedges willbe required for each tie. The ad-ditional number of pieces requiredfor a long curve will be immense, and the work of inspection andkeeping the nuts tight will greatly increase the cost of main-tenance. (d) Placing a wedge under the outer rail at each tie. Thisrequires but one extra piece per tie. There is no need of awedge under the inn^r tie in order to make the rail normal tothe tread. The resulting inward inclination is substantially thatproduced by some forms of rail-chairs or tie-plates. The spikes(a little longer than usual) are driven through the wedge
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