On Some Common Errors in Iron Bridge Design

39 As some of the above remarks may appear condemnatory o£ the American type of girder, of which the llawkesbury Bridge is so magnificent an example, it is well to point out that, owing to the narrowness of the web members relatively to their length, the secondary stress is here comparatively small, not exceeding, according to the writer’s method, 10 per cent., even on the assumption that the joints are perfectly rigid, which, being eye- bar and pin construction, they hardly can be, no matter how tightly fitted and bolted up. 20. Arrangements involving severe temperature stresses.—The existence of heavy stresses due to variations of temperature has been generally recognised in th© case of metallic arches and suspension bridges, but not in the case of ordinary girders. There is, however, reason to believe that in a subtropical country like Australia, and especially in those parts where the air is dry, and so imposes but little resistance to the solar radiation, very serious stresses, amounting possibly to tons per square inch, may be produced'by one of a series of parts that ought to act in unison, being exposed to the direct rays of the sun, while the others are in shade. In this way a difference of temperature of 30 cleg, or 40 deg. may easily be produced, and the consequent difference of expansion, will give rise to temperature stresses of 2 to 3 tons per square inch, which are cumulative upon the ordinary stresses given by statical calculation. That these temperature stresses really exist is abundantly proved by recent experiences at the great IVIoorabool A iaduct on the Geelong and Ballarat Railway. Here there are fom precisely similar continuous girders, each 1300 feet long. The} are completely sheltered from the sun by a broad overhanging deck that surmounts them, with the exception of the lower chord of the girder on the northern side. This chord consists of a number of eyebars placed side by side, and it was observed by the officers of the Railway Department that the outside bai when highly heated by the sun actually buckled under the compression, although at a part where the heaviest tension due to the load existed. The result of this buckling was to increase the stress on the adjoining bars by probably 30 per cent. To remove this serious source of weakness, a wooden roof, the whole length of the viaduct was erected over the chord so affected.