. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . —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-st
. Railroad construction, theory and practice; a text-book for the use of students in colleges and technical schools . —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) on account of the hopeless imcer-. PLAN Fig. 102.—Standard Single Stone Ctjlvert (3X4). (1S90.) N. & W. tainty as to the true value of certain quantities which must beused in the computations In the first place the true value ofthe unit tensile strength of stone is such an uncertain and variable 256 RAILROAD CONSTRUCTION. §223. quantity that calculations based on any assumed value for it areof small reliability. In the second place the weight of the prismof earth lying directly above the stone, plus an allowance for liveload, is by no means a measure of the load on the stone nor ofthe forces that tend to fracture it. All earthwork will tend to
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