On Some Common Errors in Iron Bridge Design

41 tidal current of the Thames and the prevalence of heavy barges drifting therewith would appear to call for more than, usual provision against lateral shocks. In others, as for example the railway bridge over the Murray, on the main, line from Melbourne to Adelaide, it consists of very light round rods. On the Vic- torian Railways a fairly massive T iron bracing is usually employed, while on the older New South Wales Railway Bridges a still more massive and complex arrangement is used. The N.S.W. Roads and Bridges Branch has adopted a practice different from all the proceeding, and connects the two columns by a continuous web of sheet iron, lightened by being pierced with large oval openings. This arrangement, it is contended, gives ample strength, and is not so likely to be injured by floating logs as the others. It however involves the use of a large amount of metal and comparatively complex workmanship. Now, various and inconsistent as is the practice of Engineers in this respect, the principles of design are few and simple, and are identical with those applying to the design of framed girders. A non- redundant system of triangles, free from eccentricity, with good joints and bars massive enough to be safe from accidental blows, is all that is required. The bracing of the Johnston Street Bridge, as shown in Fig. 2, is open to serious criticism. It is in the first place redundant. The horizontal pieces are costly and complex rivetted girders, while the diagonals are angle irons of unduly light section, so light, in fact, that they can be sprung some inches by the pressure of the hand. Had the horizontal connections been left out altogether, and a fourth of the money so saved expended in making the diagonals twice as massive as at present, the structure would have been strengthened and cheapened at the same time, and the calculation of stress on its various parts made easy and certain. In this and many other cases, the bracing terminates above the summer level of the stream, leaving the bottom part, when the shear is greatest, unbraced. This course is usually excused on the ground of the difficulty of making attachments under water, and if the lower part of the columns is much more massive than the upper and well supported by the firm ground around the excuse may be accepted. It is, however, far more scientific and satisfactory to carry the bracing clown to the bottom, and this has been done in two of the most recent bridges over the Yarra at Melbourne.