. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . rapid construction^ but renewals and repairs are very difficult, itbeing almost impossible to extract a drift-bolt, which has beendriven its full length, without splitting open the pieces contain-ing it. Notwithstanding this objection they are extensivelyused, especially for temporary work which is not expected tobe used long enough to need repairs. 171. Multiple-story construc-tion. Single-story framed trestlebents are used for heights upto 18 or 20 feet and exception-ally up
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . rapid construction^ but renewals and repairs are very difficult, itbeing almost impossible to extract a drift-bolt, which has beendriven its full length, without splitting open the pieces contain-ing it. Notwithstanding this objection they are extensivelyused, especially for temporary work which is not expected tobe used long enough to need repairs. 171. Multiple-story construc-tion. Single-story framed trestlebents are used for heights upto 18 or 20 feet and exception-ally up to 30 feet. For greaterheights some such construction asis illustrated in a skeleton designin Fig. 75 is used. By using splitsills between each story and sepa-rate vertical and batter posts ineach story, any piece may readilybe removed and renewed if neces-sary. The height of these storiesvaries, in different designs, from15 to 25 and even 30 feet. Insome designs the structure of eachstory is independent of the storiesabove and below. This greatlyfacilitates both the original construction and subsequent Fig. 75. 208 RAILROAD CONSTRUCTION. §172, In other designs the verticals and batter-posts are made con-tinuous through two consecutive stories. The structure issomewhat stiffer, but is much more difficult to repair. Since the bents of any trestle are usually of variable heightand those heights are not always an even multiple of the uniformheight desired for the. stories, it becomes necessary to make the I
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