. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . ion of the load were reversed. The ultimate strength, or maximum capacity for resisting stress, has aratio to the maximum stress due to the working load, which, although less inmetal than in wooden or stone structures, is, nevertheless, made of considerablemagnitude in many cases. It is much greater under moving than under steady dead loads, and varies with the character of the material used. Formachinery it is usually 6 or 8; for structures erected by the civil
. Steel rails; their history, properties, strength and manufacture, with notes on the principles of rolling stock and track design . ion of the load were reversed. The ultimate strength, or maximum capacity for resisting stress, has aratio to the maximum stress due to the working load, which, although less inmetal than in wooden or stone structures, is, nevertheless, made of considerablemagnitude in many cases. It is much greater under moving than under steady dead loads, and varies with the character of the material used. Formachinery it is usually 6 or 8; for structures erected by the civil engineer,from 5 to 6.| In general, parts of structures are so proportioned as to carry their loadswithout risk of exceeding their elastic limits; and in such cases the factor ofsafety should probably always be referred to the elastic limit. * Thermo Electric Measurements of Stress, Transactions American Society of Civil Engineers,Vol. XXVIII, January, 1902, p. 27. t Proceedings International Association for Testing Materials, No. 11, August 4,1909, Article Iron and Steel, Materials of Engineering, Thurston, 1909, p. One Division = in. DeflectionFig. 218. — Effect of Repeated Loadson Beams. (Am. Soc. for TestingMaterials — Moore.) 312 STEEL RAILS The elastic limit is made the basis of estimates by nearly all French engineers,while the ultimate strength is taken by German engineers, using a factor ofsafety of larger magnitude. British and American engineers usually base allcalculations on the ultimate strength, although the former system is extendingin general practice, and the limit of working load is made to fall well within thelimit of elasticity. The general practice at the present time, for railway and highway bridges, isto use a unit strain of about one-half the elastic limit of the material. This factoris considered correct in places where the load assumed is an absolute maximum,as, for instance, where it consists of a definitely determined dead load on
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