A Treatise On The Principles And Practice Of Dock Engineering

424 DOCK ENGINEERING. The following tabular values for the weight, in cwts. per foot run, of the main girders and of the entire bridge are condensed from an extensive compilation of data, from existing railway bridges, by Sir Benjamin Baker.* TABLE XXXII. Span in Feet. Plate Gikdeks. Lattich Girders. Two Main Girders with Lower Cross Girders. Two Main Girders with Upper Cross Girders. Four Main Girders under Rails without Cross Girders. Two Main Girders with Lower Cross Girders. Three Main Girders with Lower Cross Girders. Main Girders. Total Iron- work. Main Girders. Total Iron- work. Main Girders. Total Iron- work. Main Girders. Total 11*011- work. Main Girders. Total Iron- work. Cwts. Cwts. Cwts. Cwts. Cwts. Cwts. Cwts. Cwts. Cwts. Cwts. 20 3-4 10'1 3'4 8'2 4-7 62 31 9'8 3-7 8-3 25 3'9 10'6 3'9 8-7 5'5 7'1 3'6 10-3 4-2 8-9 30 4-4 11'1 4'4 9-2 62 7'8 4'1 10'8 4-7 94 35 5'0 11-8 5'0 9-9 7'0 8'6 4'5 11'3 5-3 10'0 40 5'5 12'3 5'5 10-4 7'8 9'5 5-0 11'8 5'8 10'6 45 61 12'9 61 11 0 8'6 10'3 5'4 12'2 6-3 11'1 50 6-6 13'5 66 11 -6 9'3 11'1 5-9 12-8 6-9 11-8 60 7'7 14'7 7'7 12-8 110 12-8 6'8 13-8 7'9 12-8 70 8-8 15'9 8-8 140 12-4 14-3 7'7 14-8 9-0 140 80 9-9 17'2 9-9 15'1 141 16'0 8'6 15-8 10'1 151 90 11 0 18'3 11 0 16-3 15-8 17'8 9'5 16'8 111 16'2 100 121 195 121 17'5 17'7 19'8 10'4 17'8 12'2 17'4 120 14-3 21 9 14-3 19-8 12'2 19'8 140 16-5 24'3 16-5 22-1 14'0 21-8 160 18-7 267 18-7 24-4 15-9 23'9 180 20-9 29'1 20 9 26'8 17'7 25'9 200 231 31-5 23'1 29-1 19-5 27'9 The preceding formulæ and data are only relatively applicable to movable bridges under certain modifications and restrictions. The structure of a swing bridge has necessarily to be more substantial than that of a fixed bridge for the same span, on account of the more exacting nature of its functions and also because it has to be provided with a heavy pivot girder with other fittings. From a comparison of the lengths and weights of a number of existing bridges, the writer has found the total structural weight per foot run (exclusive of counterpoise) to range, generally speaking, from about 1 ton for small spåns to 2 tons for large spans; there being, of course, instances in which these limits are not maintained. For moderate spans, say openings of from 50 to 125 feet, the dead load might fairly be estimated at 30 cwts. per foot run of the extreme length of the bridge, allowing for the accommodation of a double railroad track and a double footway. Live Load.—While in a long bridge the weight of the locomotive and its tender may form a comparatively small proportion of the loaded length *Baker on “ Short Span Railway Bridges.”