On Some Common Errors in Iron Bridge Design

42 The lateral bracing of the girders of existing bridges is just as various and inconsistent as that of the piers—in some cases being entirely absent and in others overdone at unnecessary expense. The older type of tubular bridges, such as the Footscray Railway Bridge, Victoria, and those at Menangle and Penrith, N.S.W., though presenting enormous surfaces to wind pressure, are invariably absolutely devoid of lateral bracing. They resist the wind simply by the resistance to bending of the main and cross girders in their weakest direction, and must be subject to extra stresses of serious amount in consequence. On the other hand, one not unfrequently sees small open lattice girders offering but insignificant surface to the wind, braced in the most costly and elaborate manner. Now, both these extremes must be wrong,, and a discussion of what is really needed will be of advantage. A little consideration will show, as has been abundantly verified by experiments on models at the University, that a girder may be wrecked by lateral movement in one or more of the following ways :— («) It may fall over on its side, turning on its lower chord as an axis. This may be caused by wind or other lateral force acting on the top chord and web. The tendency will be greatest with deep and narrow girders, and becomes insignificant in the case of those that are shallow and broad. If the traffic is- carried on the top of the girders this tendency may be effectively met by inserting a diagonal of sufficient section between the bottom of one girder and the top of the other at each point of support. This, with the cross girders, will keep all secure. Should, however, the traffic pass between the girders, as is often the- case where headway is limited, such a diagonal is inadmissible. We must, therefore, make the base of the girder broad enough to secure stability, and carry a pillar up, starting the full width of this- broad base, but tapering, if desired, toward the top, as indicated in Fig. 12, or, as an alternative, cross girders may be made continuous with stiff web members, so that the verticality of the plane of the latter is secured by the i’esistance to bending of the