On Some Common Errors in Iron Bridge Design

16 though this effective remedy could be applied at a mere nominal cost and without any interruption of the traffic, the N.S.W. Railway authorities have persistently refused to yield to the urgent representations of the Royal Commission, of Professor Warren, and of the writer. Should disaster ensure the responsi- bility rests with them. In the later work of the Victorian Railway Department this variation of horizontal shear has been consistently recognised as is evidenced by the varying pitch of the rivets in the girders on the bridge over the Yarra on the Port Melbourne Railway and elsewhere. The considerations applying to the row of rivets con- necting the web and chord angles of these girders also apply to those connecting the chord angles with the chord plates, and a similar variation of pitch is required here. It is to be noted, however, that these latter rivets perform a smaller duty than the former and so may be smaller, or at larger pitch without res4ti> the strength of the girder—Strange to say in Penrith Bridge these rivets, B in Fig. 10, are larger than those at A, though dealing with the shear consequent upon the stress variation in a portion of the chord only, while those at A deal with the whole —a notable anomaly. It is to be added that the stress on the vertical rows of rivets in. a plate web is identical with that in the contiguous portions of the horizontal rows, and that the diameter and pitch should be the same for both. To illustrate this point Fig. 13 has been, prepared, representing, a theoretically rivetted girder with terminal supports and uniformly distributed load. Existing girders ought to be examined and computed to see if there is any weakness as to horizontal and vertical shear, and, if there is, rivets should be cut out a few at a time, holes enlarged and bigger rivets inserted, as recommended by the Royal Commission in the case of Penrith Bridge. 9. Vertical stiffeners absent or wrongly placed.—-Requirement Iff), page 12. These vertical stiffeners are added at intervals along the web of a plate or box girder for one of the following purposes :— 1. To prevent the thin web from being crushed by the local vertical pressure due to the reaction of a support or a concentrated load.