On Some Common Errors in Iron Bridge Design

27 remedying an entirely imaginary lack of lateral stability. Its radius of gyration is 3-9 inches in the plane of its web, and 1 inch at right angles to that plane. It is 25 feet long and is regarded as fixed in direction at the ends, though whether this fixing is perfectly reliable is not altogether certain. It is braced against lateral bending by a costly system of bars in the plane of the web. Taking the most favourable view, calculation shows that it would give way under a compression of 98,000 lbs., whereas if it were turned the other way, as it easily might have been, so as to be braced in the direction in which it was weakest, its resistance would be more than 200,000 lbs. This is an instructive example of the neglect of condition b, p. 25. Fig. 25 represents a very usual section of compression chord of the earlier types of lattice girder bridges in England and Australia. Here the side plates are so thin and so liable to buckle or wrinkle at the edges that it was decided in the calcula- tions macle by Professor Warren and the writer, and published in the Report of the Royal Commission on Railway Bridges, N.S.W., 1886, to ignore the outer half of their width as contri- buting anything to the compressive resistance of the chord. As an example of improved practice, Fig. *26 is given, representing the upper chord of the very fine steel bridge over the Yarra, on the Gippsland Railway. Of transgression against condition d, I am happily unable to quote a case. Condition, e applies to very numerous cases of compression dia- gonals, where two parallel angle, T or channel bars are braced together. Fig. 27 represents an arrangement that has been adopted on the two most recently erected bridges over the Yarra, and shows that the latest practice is not always the best. The only function that the cross connections perform is to ensure that the two main bars bend the same way, and if they are of themselves inclined to do so no gain of strength ensues as compared with entirely disconnected bars. Fig. 28 shows a vast improvement with the same amount of iron and rivetting, and the dotted lines show how any lateral bending must be of a very different character to that of Fig. 27. In fact the shorter curves and more numerous nodes in the latter case are equivalent to a reduction to the effective length of the column to the distance XY,