|
Construction 18th August 2001 Back to Home Page back to Bridges 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. 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. This web-site includes very little material about bridge construction, because the subject is so varied and so specialist. As soon as possible, some useful links will be provided, and if possible, some simple explanations as well. Meanwhile, let’s look at the living world, to see how things are done there. Starting at the beginning, an immature form is likely to be vulnerable. A half built medieval castle must have been hard to defend. In modern times, a half built tank or aircraft is vulnerable if bombs can be dropped on the factory. In the animal world, there are many solutions. One is to produce so many offspring that an enormous mortality rate is acceptable. All that matters is that on average, enough survive to produce an equally large next generation. Another way is to nourish and protect young until their probability of survival is sufficiently high. From eagle chicks to tiger cubs to baby crocodiles, even the fiercest animals can be vulnerable when young. Many animals, from earwigs to apes, protect their young. Nevertheless, mortality can be very high once the young leave their parents. In the mammals, this method is taken furthest, by retaining the young inside until they are quite large. Question, why are baby mammals more round and cuddly looking than adults? If the young have already some defence, as in vipers, they can be left alone from the start. But even there, some species retain the eggs inside until they have developed into little versions of the adult. A common strategy to cope with immaturity is to undergo metamorphosis. By adopting very different forms, behaviours and habitats at different stages of growth, animals can inhabit ecological niches that are optimal at each stage. This is most easily seen in many insect orders, though prevalent in other classes which are less commonly seen. Sometimes there is a passive pupal stage, during which the tissues are so radically reorganised that the adult, or imago, is almost unrecognisably different from the larva. The bridge builder, too, has to nurse the embryo 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. Worse still, 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. Tunnels in particular are subject to unforeseen problems and disasters. Tunnelling under the river Thames, river the Severn, and under the Alps, was achieved at cost of life and limb. Above ground, until spans are joined, wind can be a great hazard. Before the Pont du Normandie was completed, there was serious discussion about the use of active stabilisers to keep the long thin cantilevers in place. Large weights would have been moved by rams, in response to amplified and processed signals from accelerometers. A few tall buildings actually use this technique to reduce wind induced swaying. |