A Treatise On The Principles And Practice Of Dock Engineering

TESTS. 139 wrought iron breaks suddenly a crystalline fracture is the invariable result. A truer test is a slowly applied breaking weight, which should cause a fibrous fracture. Bad iron is never fibrous. Spécification for Plates and Bars. “ Every plate and bar must be sound, straight, and free from all flaws, and any piece which shows signs of lamination or other defect will be rejected. The edges of all plates are to be planed so that they may bear truly at their joints. All joggles are to be thoroughly well and neatly formed. The butting ends of all ties, angles, and bars are to bear fairly and firmly throughout, and all corners and edges to be neatly finished ofi. Every piece is to be of the full thickness specified, to be tested by gauging, weigh- ing, or otherwise.” Working Strength. —The following table gives the amount of stress generally permissible, in tons, per square inch of sectional area :— TABLE VIII. - Cast Iron. Steel. Wrought Iron. Tension, Compression,* .... Shearing, li 8 2 8 12 6 5 4 4 These figures are based on a factor of safety of 4. The Board of Irade has fixed the limit of stress for bridges of wrought iron at 5 tons per square inch, and of steel bridges at 61 tons. The strength of steel depends on the precise nature of its composition, and the values given above are merely approximate and general. Tests.—Cast iron is usually specified to be tested as follows : A sample bar is cast, 3 feet 6 inches long, 2 inches deep, and 1 inch wide. It is supported on bearings 3 feet apart, and loaded at the centre with a weight variously stated at from 25 to 30 cwts., which it is required to sustain without fracture and without exhibiting a defiection greater than TSK inch. Test bars should, if possible, be cut from the casting, but in any case should be cast under exactly the same conditions. A tensile test is rarely required. Wrought iron is generally required to stand a minimum tensile stress before breaking, the contraction of area at fracture not being less tlian a *These values only apply in the case of short struts. When the length is considér- able, failure is more likely to take place through flexure, and special calculations are necessary for determining the nature and extent of the stress. The problemis ea with in Chapter ix.