On Some Common Errors in Iron Bridge Design

On Some Common Errors in Iron Bridge Design. —-------------- In the Australian Colonies, as in other parts of the world, there is a large and increasing number of iron (in which is included steel) bridges. These bridges are of ages varying up to about fifty years. Many of them were designed at a time when the proper mode of proportioning the various parts was but imperfectly understood, while in some the material and workmanship is by no means up to the modern standards. Unlike wine, bridges do not improve with age—on the contrary, corrosion is always going on, sometimes rapidly, but generally very slowly, but no less surely, and is bound sooner or later to cause a perceptible diminution in strength. It is also thought by many that there is a tendency for the metal in course of time to become hard and brittle and so less able to endure shocks. Thus the bridges are without doubt growing weaker with effluxion of time. Meanwhile the loads they have to endure show a distinct tendency to increase. Steam rollers, traction, engines, and other specially heavy loads, undreamt of at the time our earlier bridges were designed, are now common, while locomotives, with the universal call for more power, become constantly larger and heavier, and powei’ful continuous brakes, unknown when, the earlier bridges were built, introduce longi- tudinal stresses of serious magnitude. From these combined causes it is plain that the margin of safety is steadily diminishing, and it is only a question of time for the point of absolute danger to be reached. Again, there is reason to believe that many parts of the older bridges are excessively and unnecessarily strong while other portions are weak, and that the general arrangement of parts is often far from the most economical.