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WHEN a ferro-concrete or a steel bridge is faced with stone and assumes the traditional forms of the architecture of stone, the problem of its design is a very
old one and has already received a large number of satisfactory solutions. It is likely, of course, that in the case of arches, for instance, the proportions of the various parts would tend to become more slender, the piers might be of smaller section and the arch itself might have a very shallow crown, and the haunches may even be lightened of much of their weight and consist in a pattern of upright posts as in example Fig. XXV. shown in the last article and in Figure XXVI. illustrated in the last chapter, but the fact remains that the arch form retains its aesthetic character, which is the source of its perennial appeal to designers in all materials. No future developments in the science of engineering will ever render this form out of date, for even if the arch ceases to have an obvious constructional justification, it will continue to commend itself on account of its simplicity and elegance. That these qualities of the arch are recognised by constructional engineers is abundantly proved by the frequency of the type of bridge illustrated in Figures XXVI., XXVII. and X.XVIII., which so far from exhibiting a spirit of unintelligent conservatism on the
part of their designers, are evidence of an innate asthetic sensibility of which the scientific mind cannot divest itself even if it would. And not only do ferro-concrete bridges composed of slender framework continue to be composed in patterns of which the nether boundary line assumes the form of an arch, but great steel bridges, among which the most notable is the Forth Bridge, exhibit the same characteristic. It is specially noteworthy that in the Forth Bridge beneath the level of the rail-road the profiles of the great cantilevers are curved to form an arch, and we have the authority of Sir Benjamin Baker, who with Sir John Fowler was jointly responsible for the design, for the assertion that the arch form was deliberately chosen because of its beauty. No structural engineer would need to be informed that the lower portion of the cantilever might very well have assumed a thousand other shapes all structurally competent to resist the stresses imposed upon it and that the arch form was certainly not dictated by mechanical considerations. In this matter the engineers of the Forth Bridge acted like the great artists they were, for they deliberately chose the form of the arch for the lower side of the cantilevers, and having by this decision already safeguarded the aesthetic factor in their designs, they then proceeded to determine the dimensions of all the compressional and tensional members required for the purpose,
not only of giving mechanical stability to the bridge, but of maintaining this arch form as a principal element in the pattern of the framework.
A. Trystan Edwards
Figs. 27 and 28 indicate the character of the bending moments created in the transverse
plane by the application of a lateral load such as wind pressure at the upper panel points, in combination with those due to vertical loading, together with the character of the bending or distortion of the members, this latter being much exaggerated.
Professor J. Husband
We have seen already that with a rectangular reinforced concrete beam reinforced on the tension side only, when the bending moment exceeds a certain amount the concrete
reaches its limiting stress before the steel and that we cannot sustain a greater bending moment without over-stressing the concrete.
Ewart S. Andrews