. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . m grade. Concrete pipes, factory made, both plain and with metal rein-forcement, 12 to 48 in diameter, have come into use in recentyears. They are stronger and more dependable than tile andthere is no deterioration. BOX CULVERTS. 222. Wooden box culverts. This form serves the purposeof a cheap temporary construction which allows the use of aballasted roadbed. As in all temporary constructions, the areashould be made considerably larger than the calculated area(§§213-216), not o
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . m grade. Concrete pipes, factory made, both plain and with metal rein-forcement, 12 to 48 in diameter, have come into use in recentyears. They are stronger and more dependable than tile andthere is no deterioration. BOX CULVERTS. 222. Wooden box culverts. This form serves the purposeof a cheap temporary construction which allows the use of aballasted roadbed. As in all temporary constructions, the areashould be made considerably larger than the calculated area(§§213-216), not only for safety but also in order that, if thesmaller area is demonstrated to be sufficiently large, the per-manent construction (probably pipe) may be placed inside with-out disturbing the embankment. All designs agree in usingheavy timbers (12X12, 10X12, or 8X12) for the sidewalls, cross-timbers for the roof, every fifth or sixth timberbeing notched down so as to take up the thrust of the side walls,and planks for the flooring. Fig. 101 shows some of the standarddesigns as used by the C, M. & St, P. Ry. I. —Standard Timber Box Culvert. C., St. P. Ry. (Feb. 1889.) 223. Stone box culverts. In localities where a good qualityof stone is cheap, stone box culverts are the cheapest form ofpermanent construction for culverts of medium capacity, buttheir use is decreasing owing to the frequent difficulty m obtain-ing really suitable stone within a reasonable distance of theculvert. The clear span of the cover-stones varies from 2 to 4feet. The required thickness of the cover-stones is sometimes 1^ §223. CULVERTS AND MINOR BRIDGES. 255 calculated by the theory of transverse strains on the basis ofcertain assumptions of loading—as a function of the height ofthe embankment and the unit strength of the stone used. Sucha method is simply another illustration of a class of calculationswhich look very precise and beautiful, but which are worse thanuseless (because misleading
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