Engineering Wonders of the World
Volume I

THE DEVELOPMENT OF THE BRIDGE. 103 fore in modern joists and girders the metal is concentrated as far as possible in flanges at the extreme top and bottom of the section, in order that the resisting power of the fibres may be applied at an advantageous distance from the neutral axis. The carrying power of a beam also varies inversely with the distance between its sup- ports, for the reason that the “ bending moment ”—that is, the leverage of the load— is proportionate to the length. Fig. 2 will (Fig. 2.) help to make this clearer. In case a, the bending moment is equal to half the load acting at a distance of 5 feet from each sup- port (or fulcrum) ; in case b, the leverage is 10 feet, and therefore the bending effect is doubled. The manner in which a load is applied also affects the bending moment. If, for instance, instead of 1 ton concentrated at the centre, as in the above examples, 20 cwt. were dis- tributed equally over the length, as in Fig. 3, affects its carrying capacity. If one end. only be fixed and the other free, forming a “ canti- lever,” as in Fig. 4, the bending action of a (Fig. 4-) weight at the extremity will be four times that of a concentrated load upon a beam of the same length, but supported at both. ends. This is owing to the fact that the weight is not divided between two supports, while the leverage is doubled. On the other hand, if both ends of a beam be firmly secured instead of merely resting upon supports, each end would act as a cantilever for about one-quarter of the span, and so have the effect of shorten- ing the span. The theory of a “ continuous ” beam—that is, a beam having three or more points of support, which divide it into two or more “ spans ”—cannot be stated concisely, because much depends upon the number of spans and the degree and the nature of the loading upon each. Speaking roughly, however, a load upon one span, tends to set up a contrary bending action in. the span, or spans, adjacent to it, as indicated by Fig. 5, and, in consequence, 20 CWT A (Fig. 3.) (Fig. 5.) the leverage would be halved, since the average effect of the load equals a 10-cwt. pull exerted at each, of two points 5 feet from each abutment instead of at the centre. The stress due to a distributed load is consequently only- half that imposed by the same load concen- trated in the centre of the beam. partly neutralizes their respective bending stresses. Each span of a continuous beam may be said to be partly composed of two more or less balanced cantilevers, which, have the effect of making it stronger than a series of short detached beams of the same section. Again, the way in which a beam is supported Besides producing a bending moment, the pressure of the load upon a beam causes, at