On Some Common Errors in Iron Bridge Design

45 points, so that in theory, at least, one triangulation carries the whole weight. Mr, J. A. L. Waddell, a leading American Bridge Engineer, to whom photographs and particulars of this structure were sent, expresses himself as follows :— “ For unparalleled ignorance and stupidity the triangular truss illustrated bears off the palm. It is almost incredible that any man with common sense could put in a whole system of triangulation where it is impossible for it to do any work whatever.” While generally agreeing with Mr. Waddell’s drastic condem- nation of the absurdity of this design, I question whether his closing words are strictly correct, for the following reason : If all the load was carried on one triangulation only, the panel points connected with it would deflect more than those connected with the unloaded triangulation, and hen.ce the top and bottom chords instead of bending in an approximately circular curve would bend in a sinous way, having numerous points of contra- flexure. But such bending is impossible without evoking a considerable beam action in the chords, which are several inches in depth. Hence, some part of the load must be transferred to the other triangulation.. How much would be so transferred it is difficult to compute, but certainly not sufficient to cause even an approximate equalization in the stress on. the two triangula- tions. If we assume three-fourths of the total load to come on one and one-fourth on the other triangulation, we shall probably be taking a fairly favourable view. This bridge, undoubtedly, might be largely strengthened by inserting cross girders at all the panel points, and by replacing the thin flat diagonals near midspan of the main girders by proper double acting (strut and tie) sections capable of resisting the action of unsymmetrical loading, and this should be done at once. Another example of a similar fault is be seen in an important bridge at Southampton, England, illustrated in “Engineering,” 4th January, 1884, and criticised by the writer in “Engineering,” 6th June, 1884. Here the cross girders are placed at intervals having no direct relation to the panel points of the main girder. At one point a cross girder comes exactly midway between, panel points, thus bringing an unnecessary and undesirable bending- moment of some magnitude upon the bottom chord.