. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . reliable to turn the point, it h necessaryto protect the point by some form ofshoe. Several forms in cast iron havebeen used, also a wrought-iron shoe,having four ^straps radiating fromthe apex, the straps being nailed on tothe pile, as shown in Fig. 68 (6). Thecast-iron form shown in Fig. 68 (a)has a base cast around a recess on the top of the base re-FiG. 68. ceives the bottom of the pile and pre- vents a tendency to split the bottom of the pile or to force thes
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools . reliable to turn the point, it h necessaryto protect the point by some form ofshoe. Several forms in cast iron havebeen used, also a wrought-iron shoe,having four ^straps radiating fromthe apex, the straps being nailed on tothe pile, as shown in Fig. 68 (6). Thecast-iron form shown in Fig. 68 (a)has a base cast around a recess on the top of the base re-FiG. 68. ceives the bottom of the pile and pre- vents a tendency to split the bottom of the pile or to force theshoe off laterally. 133. Details of design. No theoretical calculations of thestrength of pile bents need be attempted on account of the ex-treme complication of the theoretical strains, the uncertainty asto the real strength of the timber used, the variability of thatstrength with time, and the insignificance of the economy that .would be possible even if exact sizes could be computed. Thepiles are generally required to be not less than 10 or 12 indiameter at the large end. The P. R. R. requires that they shall. § 134. TRESTLES * 167 be not less than 14 and 7 inches in diameter at butt and smallend respectively, exclusive of bark, which must be removal of the bark is generally required in good durable woods, such as are mentioned in § 129, are bestfor the piles, but the caps are generally made of oak or yellowpine. The caps are generally 14 feet long (for single track)with a cross-section 12Xl2 or 12Xl4. Split capswould consist of two pieces 6Xl2. The sway-braces, neverused for less heights than 6, are made of 3 X12 timber, andare spiked on with f spikes 8 long. The floor system will bethe same as that described later for framed trestles. 134. Cost of pile trestles. The cost, per Hnear foot, of pilingdepends on the method of driving, the scarcity of suitable tim-ber, the price of labor, the length of the piles, and the amountof shifting of the pile-drive
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