2015-07-21
340
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TEXT 1.
It is one thing to design a bridge. It is another thing to build it. Planning and executing the construction of a bridge is often very complicated, and in fact may be the most ingenious parts of the entire enterprise. An incomplete structure is often subjected to stresses and oscillations that would not arise after completion. The construction work is potentially a grave hindrance to existing traffic and to normal life in the area, especially when large local fabrication works have to be installed.
Even before any actual construction is done, substantial work may be needed in the form of tests. Boreholes will be made to check the condition of the ground, in conjunction with any available geological maps. Records of wind speed and direction will be consulted, and new measurements made if necessary. In the case of a river or sea crossing, records of water levels and velocities will be needed. Models of the bridge or of parts may be tested aerodynamically and hydrodynamically, and of course mechanical tests will be made. Computer simulations will supplement these tests, enabling a great variety of applied forces to be investigated. There may also be investigations into the effects on people and on the natural environment. It may even be necessary to overcome opposition to the construction, from a variety of objectors. There was much opposition, for example, to the construction of the Skye bridge. In older times, ferry operators could be extremely vociferous about a bridge proposal.
The bridge builder has to nurse the structure through difficult stages. Very often, the stresses differ considerably from those of the complete structure, and can be more concentrated. The collapse of several box girder bridges in the 1960s damaged the reputation of this type for a period, until the stresses were better understood. Note the paradox that an apparently simple structure, a set of boxes, can be hard to understand, while a complicated looking truss can be solved, at least in principle, using a set of equations. The variation in stresses during construction may be so severe that jacking must be provided.
Excavation for foundations may have to be taken to great depths, through unsuitable ground, often below water level, before solid rock is reached. Keeping out water and preventing diggings from collapsing can require major feats of engineering in themselves.
The pillars of the towers of big suspension bridges may have to be stabilized by temporary cables until they are completed and joined at the top.
Arches generally need to be supported on falsework until they are complete. Perhaps this is the origin of the word "keystone", the last block without which the structure cannot hold up, though in the finished arch, the keystone is no more important than any other voussoir. So the value of the word keystone is to remind us that until the structure is complete, we have to keep thinking.
Completeness includes completeness of communication. The chain of command and communication must be designed to cope with every foreseeable situation, and it must include rules for dealing with emergencies and unforeseen problems.
