On Some Common Errors in Iron Bridge Design

17. Imperfect jointing of tension members.—The form of section and ratio of length to transverse dimension of a tension member are immaterial as regards strength, and so may be ai ranged according to convenience. The joints, however, are the points of vital con'sequence, and malformation of these often leads to serious loss of strength and efficiency. Of these joints there are "two principal classes, rivetted joints and eye-bar and pin joints. The first of these is most usual in European, and the second in American bridge work. In rivetted work, ths incta.1 is used as it conics from th.6 i oiling mill and the attempt to utilize the full section for strength is not made. A rivetted joint, in other words, never claims perfect efficiency, but is always subject to some percentage of loss as compared with ths pieces connected. Io mininiiso this peicen- tage of loss is the object in arranging the joint, and it is possible to reduce this to the proportion of area cut away by one rivet hole. Fig. 32 shows a properly rivetted joint, which should fracture at the line AB, passing through the leading rivet and giving an efficiency less than unity by the fraction represented by the diameter of one rivet hole divided by the width of the bar. It will not fracture through the following pair of rivet holes, because ths tension is too much reduced by ths action, of tlis first rivet to permit it, nor through the third row for a similar reason. All this has been verified by careful experiments at the Melbourne University and elsewhere. In contrast with this, Fig. 33 shows a tension member weakened by no less than three rivet holes in one leading row, and many similar cases might be quoted. There are many more complicated forms of tension joint met with in girder work—to discuss which would occupy too much space here. Suffics it to say that th© same general principles apply as in the simple cases. The leading rivet ought to to placed in the mean fibre of the bar, which in an ordinary rectangular section is the middle of the width and the other rivets arranged symmetrically behind. Sometimes it is convenient to place the leading rivet near one side, and, as far as the writer’s experiments go, the loss of strength clue to such an unsymmetrical arrangement does not seem to be sprious. Still it is a departure from what is obviously the right arrangement, and is therefore not to be encouraged.