On Some Common Errors in Iron Bridge Design

11 Girders of less than 100 feet span are usually supported on planed surfaces of metal which slide one upon the other. In some cases pressures of over a ton. per square inch have been imposed on. such surfaces with success, but usually the pressure is much less. Larger bridges are as a rule supported on rollers and it is desirable that these be of ample diameter and so placed as not to be choked up with dirt or injured by moisture. Rollers 4 inches diameter are allowed | ton. load per inch length by good American authorities, but this pressure is often exceeded in English practice and apparently with impunity. Whether the sliding plates or rollers should be surmounted by a rocking arrangement is a point on which practice varies. In theory the rocker is undoubtedly correct, giving a perfectly definite point of support, and obviating the unequal pressure on the rollers due to the slope of the deflected girder. Many excellent and experienced authorities however omit it. The Great Hawkes- bury Bridge, N.S.W., for example, has no rockers, though carried out in the best possible style in. other respects. On the other hand, the bridge over the Yarra, carrying the Port Melbourne and St. Kilda Railways is carried everywhere on rockers. The fact that in the former case the girders are very deep and therefore stiff, while in the latter they are shallower, may perhaps justify the difference. It is often forgotten by bridge designers that expansion takes place transversely as well as longitudinally, and that roller systems should be arranged accordingly. One point of support being fixed, all the others should be provided with rollers acting in directions radiating from the fixed point. Tall thin columns, such as those at Johnston Street Bridge, previously referred to, do not need expansion arrangements as a rule, the column itself being capable of springing an. inch or two without injury. In dealing with the expansion arrange- ments of existing structures, 1 would suggest that they be kept clean and lubricated and protected from dust and moisture, and that all impediment to free motion be removed. We next have to consider the main girders constituting a large and costly part of the structure. These are of various types, including plate girders, box or tubular girders, closely latticed girders of the type used 30 years ago, and the many modern