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Timber trestles can be divided into two general classes, viz.: First, Pile-trestles, or those in which each bent is formed of several piles, a cap, and transverse sway-bracing; and,
Second, Framed Trestles, or those in which each beat is composed of squared timbers framed together and braced.
Owing to the excessive length of piles required for greater heights, pile-trestles should rarely, if ever, exceed thirty feet in height; while framed trestles, if properly designed for rigidity as well as for strength, may be carried up to much greater heights, the economic limit being probably about one hundred feet.
PILE-TRESTLES.
The bents of a pile-trestle should contain at least four piles each. Where the trestle does not exceed ten feet in height, the piles may be driven vertically, and no sway-bracing need be used, provided that the piles have a good penetration in reliable material. For greater heights of trestle than ten feet, the two outer piles of each beat should be given a batter of from two to three inches to the vertical foot, Each bent should also be braced with one or two sets of sway-bracing, each composed of two 3" X 10" yellow-pine diagonals, thoroughly bolted to the piles, wherever they cross them, by 3/4" bolts. Wherever the piles are of irregular sizes, they should be trimmed off so as to make the diagonal bracing fit properly.
The piles for such bents should be so spaced laterally as to
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