On Some Common Errors in Iron Bridge Design

?;V' ■ > 33 section in view of the circumstance that with varying conditions of loading they may be called upon to endure either compression or tension. When two of these angle bars intersect it is a usual practice to cut away one limb of one of them, reducing it to a simple flat bar, which is further reduced by the hole needed to receive the connecting rivet. Thus the remaining or net sectional area becomes about one-third of the gross area, and as it further is subjected to the most injurious kind of stress, that alternating from compression to tension, during the passage of the load, a very serious but easily overlooked weakness ensues. Fortunately the remedy is simple and cheap. It consists in adding a second layer or reinforcing plate to the diagonal at the weak point, extending about a foot on each side of the inter- section, and connected with the unmutilated part of the angle bar with a sufficient number of rivets. 19. Arrangements involving serious secondary stress are not uncommon, especially in English types of girders. The stress upon any bar of a framed structure with rigid joints is of two kinds, primary and secondary. The primary stress is that computed by the ordinary methods of analytical or graphic statics; in other words, by the successive application of the proposition known as the parallogram or triangle of forces. This investigation, provided the structure is not redundant, is simple, and the result certain, admitting no possibility of dispute. All such calculation, however, is based on the assumption that each set of bars meeting at a point is connected by a perfectly frictionless hinge joint. This assumption is by no means strictly true, even in eyebar work, on account of friction, while in structures having the joints made by complicated groups of rivets it is manifestly highly erroneous. What then is the nature and magnitude of the extra, or, as they are now called, secondary stresses, clue to the friction or rigidity of joints’, and how may they be minimised ? Ibis question was first discussed by the Austrian investigator, Manderla, in 1878, but owing to the intricacy of the calculations, and the delicate nature of the experiments needed, we have so far arrived at but approximate determinations of its value. The completest treatment that the writer has met with is that of Professor W. Ritter, o£ the Polytechnic School of Zurich, and was published in 1884. 3a