On Some Common Errors in Iron Bridge Design

44 and European bridges of recent construction. If the headway is insufficient to permit this overhead bracing, then the top member must be kept straight by some stiff construction continuous with the cross girders, and extending from the ends of these upwaid, and it is desirable that the cross girders bs very deep and stiff. This is well arranged in the Toolaraba Railway Bridge, Victoria, a section of which is shown in Fig. 39, and in addition the bottom chords must be kept from bending horizon- tally by a stiff deck or complete horizontal system of triangulation. Very often the former of these requirements is met by projecting the ends of the cross girders some distance outside the main girders, and inserting an inclined strut from the end of the cross girder to the top of the main girder. If this system be used, these struts should be straight, of stiff section, capable of bearing compression as well as tension, and making an angle of about 30 deg. to the vertical. Ihe costly and complex arched connections extending overhead from one main girder to the other at the Footscray Railway Bridge, near Melbourne, and on most of the earlier railway bridges in New South Wales, are of very little value. Existing bridges should be carefully examined, and if found insufficiently braced, have proper triangulated systems added, capable of bearing a wind pressure on the loaded structure of 20 lbs. per square foot, with a safety factor of at least 5, seeing the wind may blow from either side, rendering the question one of vibration strength. Numerous errors occur in connection with the design and arrangement of cross girders in many early road and railway bridges, of these the following deserve notice :__ 22. Cross girders not placed in proper relation to the web system of main girders. —A most glaring instance is to be seen in the bridge carrying the Heidelberg and Eltham Road across the Plenty River, near Melbourne, shown in outline at Fig. 40. Here the main girders are of the X type, and both triangulations are equally strong. The cross girders, howevei’, are so placed as to discharge their load immediately upon the alternate panel