A Treatise On The Principles And Practice Of Dock Engineering

DEAD LOAD. 423 Dead Load.—The weight of the main framework can be calculated in detail from the following data : —The weights of a square foot of cast iron, wrought iron, and steel, 1 inch in thickness, are 37-5, 40, and 40'8 Ibs. respectively. But the process would necessitate a design too detailed for merely tentative purposes, and the calculations would be too lengthy for a preliminary estimate. A sufficiently accurate approximation, for practical purposes, can be obtained by the use of some empirical formula, based on existing examples. Trautwine * gives the following :— For lengths not exceeding 75 feet, the weight in Ibs. per foot run of two trusses or main girders, with lateral bracing for a single track, W = -5 x span in feet + 50 ^ span in feet. For spans between 75 and 250 feet, W = 4-5 x span in feet + 22 ^ span in feet. For double-track bridges, add 80 per cent, to the above values, and for narrow-gauge tracks, take 75 per cent, of the standard (4 feet 8| inches) gauge. The foregoing formulæ do not include any provision for the weight of cross girders, flooring, or rails. The weight per foot run of iron floor systems, comprising a longitudinal stringer under each rail, is given by the same authority, as follows :— Span. Single Track. Double Track. 20 to 100 feet. 200 to 275 Ibs. 550 to 700 Ibs. 100 „ 250 „ 250 „ 350 „ 700 „ 800 „ Exclusive of the main girders of a bridge, the dead load, consisting of iron or timber flooring slightly covered with ballast, the permanent way, cross girders with gusset attachments to main girders, and the horizontal bracing, of a double line of railway carried upon two main girders, may be estimated, according to Sir Benjamin Baker, as follows : — 10 to 100 feet span, .... 14 cwts. 100 „ 150 „ „ .... 15 „ 150 „ 200 „ „ .... 16 „ Where the two lines of railway are supported upon three main girders, the above loads may be reduced by 2 cwts. per foot, and where upon four girders, by 4 cwts. per foot.t “The weight of the cross girders and bracing for a railway bridge, to carry two lines of railway between main girders, may be taken on an aver- age to vary from 6-7 cwts. for a 20-foot span to 9 cwts. for a 275-foot span ; but it will be understood that considerable modification in these weights, both of a plus and minus nature, may be effected by a variation in the depth or arrangement of the cross girders. i * The Civil Engineer's Pocket Book, 17th ed., p. 604. + Baker on “Short Span Railway Bridges.” Î Ibid.