A Treatise On The Principles And Practice Of Dock Engineering

140 DOCK ENGINEERING. certain aniount. According to the quality desired the following figures are given :— Ultimate Stress. Contraction. Round or square bars, Flat bars, ..... Angle or tee iron, Plates with grain lengthways, Plates with grain crossways, 23 to 27 tons. 22 to 26 „ 21 to 25 ,, 20 to 24 „ 17 to 22 ,, 20 to 45 per cent. 16 to 40 „ 12 to 30 „ 8 to 20 3 to 12 In addition to this, certain forge tests are required. Thus, 1-inch plates for the Admiralty are to be capable of bending without fracture while hot from 90° to 125° along the grain and from 60° to 90° across the grain, and while cold, 10° to 15° along the grain and 5° across the grain. For 1-inch plates the cold tests are 55° to 70° and 20° to 30° respectively. Steel, according to Admiralty requirements, must have an ultimate tensile strength of between 26 and 30 tons per square inch, combined with an elongation of 20 per cent, in a length of 8 inches. Lloyd’s spécification raises the limits to between 27 and 31 tons with the same elongation. Botli tests apply, indifferently, along or across the grain. As regards temper, strips cut from a plate heated to a low cherry-red and cooled in water at 82° F. must stand bending round a curve of which the diameter is Ij or 3 times the thickness of the plate, according as the authority is Lloyd’s or the Admiralty. Rivets, if of wrought iron, should be capable of being bent double, cold, without sign of fracture. When hot they should stand being hammered down to less than | inch in thickness without cracking at the edge. If of steel they should have an elongation of 25 per cent., with 26 to 28 tons per square inch tensile strength, in test pieces of ten diameters, and should be capable of bending double after the same tempering as that applied to steel plates. Weight of Iron and Steel.—Plates of metal, 12 inches square and 1 inch in thickness, weigh 371, 40, and 40f Ibs. respectively for cast iron, wrought iron, and steel. Corrosion of Iron and Steel.—It is to be regretted that on a point of such vital importance to the dock engineer as the durability of metal structures exposed to atmospheric and aqueous agencies, the evidence is so scanty as to be inconsiderable, so incomplete as to be inconclusive, and so conflicting as to be actually perplexing. This state of things arises from a variety of causes. In the first place, it is only within the last fifty years that iron has begun to usurp the pre-eminence hitherto enjoyed by wood and stone in maritime construction, and steel is an intrusion of still later date. Oonsequently there has hardly yet been sufficient time in which to acquire data for the determination of the actual life of metallic structures