On Some Common Errors in Iron Bridge Design

40 This was undoubtedly the right thing to do, only, in the writer’s opinion, an iron roof would have been more permanent and more in harmony with the monumental character and architectural pretensions of the structure. This question, of temperature stress ought to be looked into in the case of all girders that are partly or wholly exposed to direct solar radiation, and where they are found to exist in any serious degiee, light screens or roofs of sheet iron should be introduced so as to ensure vital and highly stressed parts of the structure being always in the shade. As things are at present, it seems impossible to i'esist the conclusion that structures designed in the usual way for a working stress of 5 or 6 tons per square inch are frequently, owing to the combined effect of secondary and temperature stresses, subjected to actual stresses approaching double their nominal amount. Further, it is recommended that girders exposed to the direct solar radiation, be painted white, in. order to keep them as cool as possible. 21. Insufficient lateral bracing.—While gravity is usually by fai the greatest force acting on bridges, there, are other forces that must not be overlooked operating in non-vertical directions, and which may, and as a matter of history have wrecked structures that gravity was powerless to injure. Of these forces the most important are the horizontal pressure of wind or flood against the structure, the lateral oscillation of badly balanced locomotives, and the tendency of the compression chords of the main girders to bend sideways as long columns. From this point of view, a pair of columns such as shown, in Fig. 2, form a verti- cal cantilever, fixed at the foundation and subjected to horizontal forces from flood and wind. The columns form the chords of the cantilever, and must have sufficient sectional area to endure the consequent compression and tension, as well as the compression due to the load. In this way the bending moment is provided for. I he shear requires a suitably designed web system connect- ing the columns, without which they cannot act together as one efficient cantilever. Now, with regard to this web system, the wildest inconsistency is found in existing bridges. In not a few cases, as for example the Charing Cross Railway Bridge, London, already referred to, it is entirely absent, although the rapid