On Some Common Errors in Iron Bridge Design

15 the Victorian Railways this has been clone to a most remarkable extent. At Kororoit Creek, on the Geelong Railway, there is a double line bridge of 80 feet span, the chords of the girders of which consist of two 3 j x 3j x % angles and three 24 x f plates continuous throughout, and representing an enormous waste of metal towards the ends o£ the girders. As a pleasing contrast to this the new steel bridges over the Inkermann and Balaclava Roads on the Brighton Railway, near Melbourne, may be noted. Heie the plates are arranged just as the stress requires and for the last few feet of each end are omitted, the angle bars forming the whole chord. Figs. 11 and 12. 8. Insufficient connection between chords and web.— The stress in the chord of a plate web girder is given with approximate accuracy by dividing the bending moment by the depth. If this stress be determined at two points say one foot apart, different values will be obtained, the difference between which will be the horizontal or longitudinal shear for that foot in length tending to separate the chord and web. This has to be resisted by the rivets. As the bending moment varies rapidly near the supports, find slowly near midspan, this shear will vary correspondingly. Hence, for equal strength throughout, the pitch of the connecting rivets should vary from the centre to the ends, being very large at midspan, and becoming smaller and smaller as the supports aie appi cached. To carry this out exactly as calculation requires would involve too much complicated measurement for practical conditions, hence one or at most two variations of pitch must suffice, portions of the work being a little over-rivetted to secure uniformity. The great Penrith Railway Bridge, N.S.W., shows a notable neglect of this requirement. The rivetting at the point A in Fig. 10 being small and uniformly pitched throughout, the rivets are seriously over-stressed for a distance of 40 feet on each side of the piers, and for 10 feet at each end of the bridge (which is continuous over thre© equal spans). This grave defect was discovered by Professor Warren of Sydney University and verified by the writer. It is fully dealt with in the Report of the Royal Commission on Railway Bridges of N.S.W., 1886, and the proper remedy, viz., the replacement of the f inch rivets by 1 inch rivets for the distances mentioned, pointed out. But 2