On Some Common Errors in Iron Bridge Design

47 ever, is not advisable, as bad workmanship, decay and imperfect jointing may nullify their distributing action. The best thing to do with these weak girders is to introduce longitudinal connections of great stiffness between them. An arrangement in iron or steel similar to what is known as “herring boning” between the joists of a floor was recommended several years ago by the author for Penrith Bridge, N.S.W., see Fig. 42. 24. Improperly designed cross girders.—These occasionally occur, especially in the earlier bridges, and seem to have arisen from an ill-advised attempt to make the cross girders of similar type to the main girders. Badly designed lattice cross girders with vital junctions made by single rivets only, and heavy compressions taken, on thin flat bai’s of considerable length were unfortunately to be found in some early Victorian bridges. Many have now been removed, and their places taken by sub- stantial plate girders. As an example of most improper design, Fig. 43 may be quoted, which, not many years ago, was removed, after a quarter of a century of service, from an important bridge on a busy suburban railway near Melbourne. It will be seen that the tension on. bar AB is 10 tons per sq. inch, the shear on the rivet A, 8 tons per sq. inch, and pressure on the bearing area 25 tons per sq. inch—figures alarmingly high. The marvel is that such girders failed to cause appalling disaster. It is fully illustrated here as a warning to bridge designers, and also as an encouragment; for the fact of these frightfully over-stressed constructions having carried a busy suburban traffic for a number of years without giving way, is a conclusive proof of the extraordinary endurance of the material, which was only- iron, and an evidence of the wide margin of strength against unforeseen contingencies possessed by structures designed in a scientific way under usual limitations as to permissible stress. 25. Unscientific forms of footpath brackets.—Many railway and road bridges are provided with overhung footways outside the main, girders. The brackets supporting these should be rationally designed cantilevers continuous with the cross girders, and consisting of a proper top and bottom chord, and suitable web, either of continuous plate, or triangulated series of bars. Unfortunately, however, an. unhealthy desire for ornament, over- looking the fact that the really beautiful must be based upon 4