On Some Common Errors in Iron Bridge Design

46 A further instance of this fault was to be seen in the eastern extension of the Victoria Street Bridge, Melbourne, erected in 1884, and represented in Fig. 41. This, however, was remedied at the writer’s recommendation. 23. Cross girders too numerous and individually too weak.— This fault is very commonly to be found in the earlier railway bridges of Victoria and New South Wales that have come under the author’s notice. The heaviest local load to be provided for is that of the driving wheel of a locomotive, and amounts to slightly over 8 tons for one type of Victorian locomotive, and between 7 or 8 tons in five other types. In New South Wales the weights are practically the same, for, though the engines are heavier, they are supported on a greater number of wheels. But it must not be forgotten that this load is liable to be largely increased, first by the-downward resolved part of the thrust or pull of the connecting rod, and second by the effect of imperfect balancing, so that 10 tons is the very least amount it would be safe to allow as the actual wheel load. Now, this load may come upon every individual cross girder in succession, and therefore every cross girder should be strong enough to resist it. At the same time there is no advantage in placing them nearer together than the minimum distance between the locomotive driving wheels, which is about 5 feet. Cross girders then, at not less than 5 foot intervals, each strong enough to carry the heaviest loaded pair of engine wheels, constitute reasonable practice, and many good recent structures correspond closely to this arrangement. On some of the earlier Victorian, bridges, however, the interval is only 2 feet 6 inches, and in New South Wales, 3 feet. The stresses in these girders are found by computation to be from 7 to 12 tons per square inch—very alarming figures. Looking at many of these earlier bridges it is difficult to understand why their bottoms have not dropped out long ago. It is, however, to be remembered that some of the worst have never been fully loaded, double line bridges as at Penrith in New South Wales, and Kororoit Creek, Victoria, carrying, hitherto, only single lines of way, while in most cases heavy timber longitudinals or stout continuous decks interposed between the track and the cross girders tend to spread the load and prevent any individual girder receiving its full punishment. Reliance upon these, how-