On Some Common Errors in Iron Bridge Design

14 plates rivetted together, an undesirable arrangement, as it is very difficult to effect satisfactory rivetting through so many super- posed layers of metal, and, further, there is some doubt as to whether the outer layers really do their fair share in resisting the bending moments. Had the Tower Bridge cross girders been supported at the efficient points, which He at the edge of the footpath, the reduction in material and weight required would have been enormous, and it is not clear that there is any insuperable objection, to this being clone. As examples of excessive shallowness and most unfavour- able mode of support these girders are very notable. This defect is obviously incurable in existing structures. 6. Unfavourable disposition of material for enduring bending moment.—This is an infraction of requirement a on page 12, and is occasionally seen in the older type of box girder. A notable example is the Railway Bridge at Penrith, N.S.W., the section of which is shown, in Fig. 10. Here there are four chords instead of two, and those nearer the neutral axis represent a most unfavourable disposition of material. The intermediate chords are 10 feet apart and the extreme ones 13. Consequently, first, the stress on the intermediates is only of that on the extremes, and as it acts at only of the distance from the neutral axis, the value of every square inch of metal is only or n°t much more than, half of what it would have been if placed at the extreme distance. In this way the bridge is loaded with a vast quantity of metal which perforins only about 60 per cent, of the duty it should. Further, this arrangement involves the existence of cells 18 inches square, and nearly 600 feet long, which are most objectionable from the point of view of inspection, painting and repair. Fortunately, this form of girder is now quite obsolete, and the error is not likely to be repeated. 7. Uniformity of chord section throughout the length.—This uniformity is justifiable on grounds of simplicity in rolled beams and the smaller varieties of built girders when a pair of angle bars of convenient size suffices for the maximum chord section. But when one or more plates have to be added, to carry these throughout the whole length regardless of variations of chord stress is absurd. In some of the earlier plate girder bridges on