Bridges are in general the purest expression of the art of the structural engineer. The core disciplines in their design are the theory of structures, the behaviour and strength of materials and the search for economy in materials and in methods of construction. In order to be able to resolve the tension between appearance and economy that is the source of creativity in the design of bridges, it is essential that the designer should have a deep and intuitive understanding of these disciplines, which leads one to the conclusion that he should be an engineer.
However, bridge owners may well have reasons for building a bridge that are other than purely functional. For instance, it is not uncommon that a bridge is required to be a 'landmark', or a symbol of the regeneration of an area. In these circumstances, it ceases to be principally an engineering artefact, and becomes a cultural artefact, a form of functional sculpture. The structural disciplines cease to be dominant, and in particular, the search for economy and rationality that is at the heart of engineering design, will be downgraded as a constraint. It may well be better for such a project to be led by an architect, who can best make the synthesis that is necessary to meet the client's needs.
For instance, the Hungerford Footbridge across the Thames in London, Figure 1.11, is a cable-stayed structure where the fan of stays only supports about two-thirds of the length of each span. This quite defeats the logic of providing expensive pylons and stays, as it leaves the deck with substantial bending moments, and also gives rise to very uneven loads in the stays, some of which must be virtually unloaded and hence redundant. Although this is a parody of a bridge, using bridge vocabulary out of context, it may be very successful with the public.
A good example of the difference between the architectural and the engineering design of a bridge is given by the Runnymede Bridge that carries the M25 and the A30 across the River Thames near Staines.
The original Runnymede Bridge was designed by Sir Edwin Lutyens, and built posthumously. It appears to be a stone and concrete arch with brick spandrels, Figure 1.12 (a). In fact the spandrels conceal a steel portal structure, and the brick and Portland stone towpath arches conceal massive reinforced concrete abutments, transferring the thrust of the portal to the clay foundation, Figure 1.12 (b) . Consequently, the bridge is in no way expressive of the structural actions, and is not at all economical or rational, principally because the clay foundations are not well suited to carry the thrust of the 55 m span portal subjected to highway loading. However, it is well loved and admired, and is one of only two major bridges designed by Lutyens.
When the M25 came to be built in the 1970s, the author, then working for Arup, was responsible for the design of a parallel bridge, Figure 1.12 (c). This bridge has precisely the same function and span as Lutyens' original, although it works quite differently. For instance, the arch thrust is balanced by the thrust of the rear, raking strut, so only vertical loads are applied to the foundation, Figure 1.12 (d). Although it was more expensive than a simple girder bridge, this was justified by the precedent set by the adjacent structure. The design has been guided by the principles of reconciling economy, rationality and appearance, as described above, and expresses its structural action. This is no guarantee that it will be as well loved as the original.
The other major bridge designed by Lutyens is the Hampton Court Bridge, also across the Thames, Figure 1.13. This is a series of Portland stone arches with brick spandrels, or is it? Does it matter?
Figure 1.12a Runnymede Bridge: original Lutyens design (Photo: Arup)
Figure 1.12b Long section of Lutyens bridge (Source: Thomas Telford Ltd, adapted from D.W.
Cracknell, Proceedings of the Institution of Civil Engineers, Vol 25, July 1963, pp. 325-44)
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