On Some Common Errors in Iron Bridge Design

 I 20 11. As an error in the opposite direction to the last redundancy may be next quoted.— This fault is very wide-spread, and has- received a good deal of defence from influential quarters. But such defence has usually been more in the direction of palliation than justification. Redundancy may be defined as a duplicate system of triangulation connecting identical points. In such a case the stresses in the bars become indeterminable by statical calculation, and can be computed only by a much more complex * and less satisfactory method based upon the elastic deformations of the various parts, and tbc result is likely to be vitiated by variation in the coefficient of elasticity, and to a still more serious degree by imperfections of workmanship invisible in the com- pleted structure. To explain more fully, a redundant structure- minus certain of its bars presents a complete system of triangu- lation. Suppose now that the remaining bars are by accident made a little too long or too short, and are forced into their places with violence, a set of stresses o£ possibly great severity is induced throughout the structure which may modify profoundly the result of any calculation. Hence redundancy, while rarely,, if ever, of any real advantage, may lead to most undesirable consequences. As a gigantic example of this defect, the Charing Cross Railway Bridge over the Thames at London, illustrated in. “Humber’s Iron Bridges,” and shown in outline in Fig. 15, may- be noted. Minor instances of this fault are very common, and to forbid redundancy absolutely would mean condemning many otherwise meritorious designs. We may, however, I think, say first, that other things being equal or nearly so, preference should always be given to non-redundant arrangements, and second, that if for sufficient reasons redundant ones be adopted special care should be taken that they are put together in a perfectly unstressed condition. In French practice this fault is very prevalent, and a gigantic example of it is to be seen in the- Eiffel Tower, a structure which, like the Charing Cross Bridge, would undoubtedly be much improved by the removal of numerous costly parts. 12. Curvature of members.—Considering that every part of a properly designed framed structure is a simple strut or tie subject to longitudinal stress only, the necessity of absolute straightness-