On Some Common Errors in Iron Bridge Design

13 Let us now see how existing structures fail to comply with the above conditions. In connection with main girders the following errors have come under my notice, 5. Insufficient depth.—The extreme top and bottom elements, or chords as they are generally called, of all structures performing the functions of a beam are stressed in inverse proportion to their distance apart. Hence the deeper the girder the less the stress upon and requisite sectional area of these parts. As a matter of pure and bald theory the quantity of material in the web is independent of the depth, so that the most economical girder is one infinitely deep with chords infinitely small and web infinitely thin. Such a result as this is of course valueless as a guide to practice except as showing that as ample a depth as other considerations permit should be chosen. For many years English engineers, following apparently the example of Fairbairn, adopted depths of to of the span- involving very heavy chord sections. The Americans however showed that it was possible to follow theoretical indications much more closely without incurring practical difficulties, and erected many efficient and economical structures with depths of i to the average being about Of late years English practice has been approaching to American, though somewhat hesitatingly. This is of course in cases when the depth is not restricted by such considerations as head way or flood level. As practical illustrations of the defect of insufficient depth, I could refer to, first, the earlier foot passenger bridges over the railways in the vicinity of Melbourne. These are excessively shallow Warren or lattice girders surmounted by gas pipe hand- rails, which in no way add to the strength. A model of one of these at the Prahran Railway Station was made and broken down at the University, and also a model containing the same metal, of decidedly simpler construction and double the depth. The latter was found to be iO per cent, stronger than the former, and further, had the advantage of dispensing with the handrail, being itself deep enough for the purpose of a parapet (see Figs. 8 and 9). The cross girders of the recently erected Tower Bridge London, constitute another example of the same peculiarity, having a depth of only -i- of the span. Consequently the chords are enormously massive, consisting of, in some places, seven layers of