. Railroad construction. Theory and practice . Fig. 75, ments a minimum. The higher the trestle the greater the costof each bent, and the greater the span that would be all trestles have bents of variable height, but the advan-tage of employing uniform standard sizes is so great that many § 139, TRESTLES. 161. Wi4 roads use the same span and sizes of timber not only for thepanels of any given trestle, but also for all trestles regardless ofheight. The spans generally used vary from 10 to 16 feet. TheNorfolk and Western R. R. uses a span of 12 6 for all single-story trestles,
. Railroad construction. Theory and practice . Fig. 75, ments a minimum. The higher the trestle the greater the costof each bent, and the greater the span that would be all trestles have bents of variable height, but the advan-tage of employing uniform standard sizes is so great that many § 139, TRESTLES. 161. Wi4 roads use the same span and sizes of timber not only for thepanels of any given trestle, but also for all trestles regardless ofheight. The spans generally used vary from 10 to 16 feet. TheNorfolk and Western R. R. uses a span of 12 6 for all single-story trestles, and a span of 25 for all multiple-story stringers are the same in both cases, but when the span is25 feet, knee-braces are run from the sill of the first story belowto near the middle of each set of stringers. These knee-bracesare connected at the top by a straining-beam on which thestringers rest, thus supporting the stringer in the center and vir-tually reducing the span about one-half. 139. Foundations, (a) Piles. Piles are frequently used as afoundation, as in Fig. 76, particularly in soft ground, and alsofor temporar}^ structures. Thesefoundations are cheap, quicklyconstructed, and are particularlyvaluable when it is financiallynecessary to open the road fortraffic as soon as possible andwith the least
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