The Structural Engineer > Archive > Volume 4 (1926) > Issues > Issue 11 > The Structural Engineer as Artist Chapter XI Bridges (Continued)
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The Structural Engineer as Artist Chapter XI Bridges (Continued)

THE last chapter of this series was devoted to an analysis of the aesthetic qualities of the arch and arguments were brought forward to show that structural engineers are fully justified in their common practice of utilising the arch form in a manner which has little constructional significance but which indicates their recognition of the artistic qualities possessed by a certain geometrical curve. But just as in ferro-concrete and steel bridges, the arched form is arbitrarily chosen for the sake of the pleasing pattern which it produces other mathematical curves are also commonly employed by engineers, and it remains to be considered whether these also have or have not in their application to architecture and engineering an aesthetic quality which can elevate them to the same classic status as the arch now occupies. There are, however, two separate questions to be here considered. In the first place, there is the quality of the curve itself, and then there is the relation of that curve to other members in the composition of which it forms a part. For instance, the arch form itself can be used in such a manner that its aesthetic virtue has entirely left it. In the last chapter it was pointed out that where the arch assumes the " segmental " form its relation to the jambs of the opening is not nearly such an intimate one as when the arch is semicircular and merges at its extremities into the vertical lines of the jambs, but a far more flagrant discord than this can be produced by the juxtaposition of circular curve and straight line. Such a discord is familiar in the " bow string " type of bridge which is of two principal kinds. The first, represented in Fig. XXXIA, where the sector stops at the level of the roadway while exhibiting an aesthetic crudity inasmuch as there is no formal relationship between the tangent at the extremity of the curve and the line of the roadway is less offensive than Example C, where the curve proceeds below the roadway and thus cuts the latter at a quite arbitrary point in its length. The arbitrariness will be apparent as soon as we compare diagrams XXXc and D, for an examination of these immediately reveals the fact that the curve, as it were, " floats " about and the roadway is in no way inflected to take cognisance of it. The obvious test is this. Take the curve away and there is nothing left on the roadway to indicate that at any particular point in its length an important element of structure was intended to impinge upon it. That is to say, the roadway has not been inflected to take account of the curved truss nor has the truss been inflected to take account of the roadway. One or other of these inflections must necessarily take place if the design is to have an organic quality. Example XXXIE shows this latter kind of inflection, because the lower curtain-like brackets as soon as they reach the level of the roadway do not continue their curvature without interruption as in Examples C and D, for the curve makes