. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . , 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 afoimdation, as in Fig. 76, particularly in soft ground, and alsofor temporary structures. Thesefoundations a
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . , 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 afoimdation, as in Fig. 76, particularly in soft ground, and alsofor temporary structures. Thesefoundations are cheap, quicklyconstructed, and are particularlyvaluable when it is financiallynecessary to open the road fortraffic as soon as possible andwith the least expenditure ofmoney; but there is the disad-vantage of inevitable decaywithin a few years unless the piles are chemically treated, as willbe discussed later. Chemical treatment, however, increases thecost so that such a foundation would often cost more than afoundation of stone. A pile should be driven under each postas shown in Fig. 76. (b) Mud-sills. Fig. 76. , n w v\ .:;-::-.:::51:::::i::^ 1 SILL 1 :-.::-:....::::!;:: Fig. 77. (c) Stone foundations,the most expensive. 77 illustrates the use of mud-sills asbuilt by the Louisville andNashville R. R. Eight blocks12X12X6 are used undereach bent. When the groundis very soft, two additionaltimbers (12 X12 X length ofbent-sill), as shown by thedotted lines, are placed under-neath. The number requiredevidently depends on the na-ture of the foundations are the best and For very high trestles the Norfolk and 172 EAILEOAD CONSTRUCTION. §140. Western R. R. employs foundations as shown in Fig. 78, thewalls being 4 feet thick. When the height of the trestle is 72feet or less (the plans requiring for 72 in height a foundation-wall 39 6 long) the foundation is made continuous. The sill
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