The Viaduct Works' Handbook

38 opening of the bridge, being fixed upon suitable machinery, which enables them to be tilted into a horizontal position, and thereby to leave room for the bridge to move over them. The motion to the tilting machinery for the rails is given by a handle fixed on the abutment, and must be turned before the moving gear is set in motion. The whole is worked by two men. The total weight of ironwork in this bridge is 84 tons 19 cwts., and taking the value at £20 per ton, f. o. b. in England, brings the price to £1699. Erecting and Fixing.—The cost of erecting and fixing (in England) wrought-iron girders and bridge tops, upon piers already built, under ordinary circumstances, may be estimated at about £4 per ton. This price will vary with circumstances peculiar to the country, or to the particular locality, height above ground or water, natural obstacles to be overcome, rate of wages, &c., and will be found to vary from £2 to £6 per ton. Quality of Iron.—There are, probably, no structures in iron where the quality of the material employed deserves more careful attention than in bridge tops or girders. Notwithstanding this, com- petition in trade has had a great tendency of late to force down the prices to such a degree as to tempt some manufacturers, regardless of consequences, to use such iron as is totally unfit for the purpose. Iron should not be used of less ultimate tensile strength than 20 tons per square inch of section. As bearing upon this, the following results of testing plates of Blaenavon make, may be found interest- ing, showing it to possess those qualities so well adapted to bridge construction :— Breaking strain per square inch in tons. With the grain. Across the grain. The average result of testing a number of Blae- navon “best best” plates; tested for the Government by Messrs. Laird Brothers, of Birkenhead ...... 26.17 - The Government test is . . .- . 22 18 ■