. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . int just outside / finally^ one-half the width of the roadbed times the sum ofthe slope-stake heights. 94 BAILROAD CONSTRUCTION. 82. If one of the sides is perfectly regular from center to slope-stake, it is easy to show that the rule holds literally point next inside the slope-stake in this case is thecenter; the intermediate terms for that side vanish. The lastterm must always be used. The rule holds good for three-levelsections, in which case t
. Railroad construction. Theory and practice. A textbook for the use of students in colleges and technical schools. By Walter Loring Webb . int just outside / finally^ one-half the width of the roadbed times the sum ofthe slope-stake heights. 94 BAILROAD CONSTRUCTION. 82. If one of the sides is perfectly regular from center to slope-stake, it is easy to show that the rule holds literally point next inside the slope-stake in this case is thecenter; the intermediate terms for that side vanish. The lastterm must always be used. The rule holds good for three-levelsections, in which case there are three terms, which may bereduced to two. Since these two terms are both variable quan-tities for each cross-section, the special method, given in § 78, in which one term (-^J is a constant for all sections, is pref-erable. In the general method, each intermediate breakadds another term. 82. Volume of an irregular prismoid. If there is a break atone cross-section which is not represented at the next, the ridge(or hollow) implied by that break is supposed to vanish atthe next section. In fact, the volume will not be correctly. Fig. 52. represented unless a cross-section is taken at the point wherethe ridge or hollow vanishes or runs out. To obtainthe true prismoidal correction it is necessary to observe on theground the place where a break in an adjacent section, whichis not represented in the section being taken, runs out. Forexample, in Fig. 52, the break on the left of section A. at a g S3. EARTHWORK. 95 distance of y/from the center, is observed to run out in sectionA at a distance of yi from the center. The vohime of theprismoid, computed by the prismoidal fornmkx as in § 70, willinvolve the midsection, to obtain the dimension of which willrequire a hiborious computation. A simpler process is to computethe volume by averaging end areas as in § 81 and apply aprismoidal correction. To do this write out an expression foreach end area similar to that given in Eq. 61.
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