An arch bridge (or a barrel vault) is essentially a two-dimensional structure; the barrel can be cut, hypothetically, into a large number of parallel slices, each of which is identical to its neighbour. Once the structural action of a slice has been understood, then the structural action of the complete bridge or barrel vault is clear.
The simplest form of dome is generated by rotating an arch about its central axis a semicircular arch will generate a hemispherical dome. The generating mathematical curve need not, of course, be a circle a parabolic arch will give rise to a paraboidal dome, and a hen's egg has a more complex profile. However, whatever the profile, there are great differences between the behaviour of a two-dimensional arch and a three-dimensional dome, and these differences are reflected not only in the structural action but also in constructional procedures.
The central span is some 25 metres. The centering rests on voussoirs which project inwards from the arch ring, and also has to be supported from below by scaffolding. The masonry arch cannot carry its own weight until all the voussoirs are in place at this stage, however, because of the method of construction, the weight of the arch is actually carried by the timber centering. It is only when this is removed that the arch will begin to work structurally the problems of de-centering while the full load is being carried are formidable. The gaps shown between the scaffolding...
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