On Some Common Errors in Iron Bridge Design

8 There is no doubt whatever that considerable economies are to be effected in the design of columns of future bridges by abandoning old arbitrary methods, and treating the problem scientifically. As a further instance of economical design, Kinzua Viaduct, Xew York State, U.S.A., may be quoted, a section of a wrought iron column of which is shown in Fig. 3. Two of these columns form the support intermediate to a span of 61 and one of 38 feet, carrying a main line of railway 4 ft. 8J in. gauge. Each column is 2/9 feet high and is braced laterally at intervals of 31 feet, r urther comment is needless to show how excessively wasteful bridge columns in Australia have been in cases too numerous to mention. 3. Girders supported in an unfavourable manner.—Under tliis heading come many defective arrangements. The Hist of these is when a girder is supported at the extreme encl, it being possible to support it at a more favourable point. It does not seem to have been generally recognised by engineers that the extremity of a girder is a most unfavourable point of support, giving rise to bending moments and shears of maximum value, and therefore should not be adopted except under the most cogent conditions. If the points of support of a uniformly loaded beam are moved towards the centre, the most surprising diminution both of bending moment and shear takes place, and when the supports are distant from the ends by -207 of the length the maximum bending moment is reduced to -172 and the maxi- mum shear to 'o9 of what it is when the supports are terminal. As in the great majority of beams including rectangular sections of timber and rolled girders of usual proportions, the strength is regulated by the moment and not by the shear, this means that a uniform beam supported at what I propose to call the “efficient points, is very nearly six times as strong as a similar one supported at tlie ends, the load being uniformly distributed. AVith a Jive or variable load, such as a crowd, the advantage is not so great, the stress range being somewhat increased, but even allowing for this in accordance with the Weyrauch vibration formula, the strength of a beam carrying an equal live and dead load—a very usual case—is increased about threefold by its being supported at the efficient points instead of the ends. Thus