On Some Common Errors in Iron Bridge Design

9 it will be seen, that there is an enormous advantage in supporting both main and cross girders at their efficient points, or as near to those points as is possible, and this is true in all cases but most especially so when the dead load is large. Should it be obligatory to support the bfyuA at ope extremity, the position of the other support being^ütM^d, the efficient point is found to be -29 of the length from the other encl, and the maximum bending moment is almost exactly one-third of what it would be were both supports terminal, the load being uniformly distributed. Should it be inconvenient to adopt so large an overhang as above mentioned, considerable advantage may still be obtained with a very moderate amount. For example, a uniform beam uniformly loaded and supported at points one-eight of its length from the ends endures a maximum bending moment of only one-half of what it would if supported at the extremi- ties. As an instance in which an overhanging end might have been advantageously applied certain bridges on the Melbourne and Coburg Railway may be cited. Here, as shown in Fig. 4, the terminal support of the girder is a brick pier imbedded in the embankment, and the girder seat is surrounded with earth in an undesirable way. By moving the pier to the position shown in Fig. 5, the bending moment would be reduced, and the girder seat rendered accessible. A case of unfavourably arranged support is illustrated in Fig. 6, which represents two bridges each crossing six lines of railway in Ya era Park, Melbourne. The girders' are of the lattice type, supported at the ends by a double system of standards or legs. Calculation, shows that the compressions on the diagonals AB and CD are equal. But the tension on AC is equal to the compression on AB as their horizontal resolved parts balance at A. Hence the vertical resolved part of AC balances that of CD at C, and the leg CE carries no weight whatever, except the actual small floor load at C. Now, had the bridge been arranged as at Fig. 7, the main girder would have been shortened by ten per cent., the stresses throughout the remainder would have been reduced considerably, the leg FG would have been saved , and the whole structure