On Some Common Errors in Iron Bridge Design

19 Bridges defective in these respects should be reinforced by rivetting on the necessary additional parts. Leaving the girders with continuous plate webs we next have to consider those in which the chords are connected by some arrangement of bars forming an open lattice or trellis work of some kind. These have enjoyed a great popularity for bridges of th^ largest size, and also for smaller ones, where the load being • * light it was difficult to design an economical plate girder without using a web undesirably thin for practical conditions. In these as in the plate web girders the fault of insufficient depth and consequently needlessly large chord sections has been very prevalent. At Cremorne on the Melbourne and Brighton Railway there is a lattice girder bridge 140 feet span and 10 feet deep, built about forty years ago. Some ten years since, it was required to build a second bridge at one side of it to carry the Gippsland Railway, and, although there was a desire for the sake of appearance to keep the two structures of uniform depth, the advantage of increasing it was so great that the new bridge was macle 20 feet effective depth for the same span. Apart from this defect the principal faults to be found in. girders of this class are— 10. Incomplete triangulation.—As stated on p. 12 require- ment c, every framed structure should consist of a complete and continuous series of triangles, the triangle being the only polygon whose figure is fully determined when the length of its . sides are fixed. Such a structure is subject to longitudinal • tensions and compressions only, and is free from bending moment f» and shear, and so utilizes most advantageously the material of which it is composed. Now in actual existing structures glaring •departures from this rule are sometimes seen, as in the. case of the shore girders of the old footbridge over the Yarra at the Botanical Gardens, Melbourne (Fig- 14). Here it must be ■obvious to anyone having the slightest knowledge of the subject that a single strut, as shown by the dotted line, would have ■carried the triangulation, to its proper termination, and been far cheaper and in every way better than the complex and costly arrangement of plates, angles and rivets actually employed. As this structure is about to be removed its defects have no further interest. In future structures they should, however, be avoided.