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The Tay railway bridge was designed with a badly underestimated resistance to high winds, and there were instances of faulty materials and inadequate responses to detected problems. Several suspension bridges have succumbed to induced oscillation. In the year 2000, the Millennium bridge across the river Thames was closed after only two days of use because of oscillation. One way to reduce the probability of trouble from unknown causes is to make tests on scale models. Computer programs can be used to simulate many structures. In both cases the results will of course be no more accurate than the faithfulness of the simulation. Accidents and problems during construction Some early box-bridge spans fell down during construction, a period during which many structures are more vulnerable than when complete. These bridges were designed as continuous hollow beams, with diaphragms at intervals. This is a strong and light design. But during construction, the growing beams were cantilevers, supported only at one end. Until they were joined to their partners, the parts near the piers had to withstand bending moments that would not occur after completion. In addition to this, there was also the weight of cranes and other equipment, of necessity at the tip of the cantilever. The Milford Haven bridge in west Wales and the West Gate bridge over the river Yarra near Melbourne, Australia, collapsed during construction, with loss of life. The incomplete spans of cable-stayed bridges, or the towers of incomplete suspension bridges are obvious candidates for wind-induced oscillation. Moving and lifting spans or parts of spans are particularly dangerous operations for bridge builders. Large and heavy objects in motion have huge momentum if they move quickly. Lifting them puts them in positions where they are not yet secured in their final places. Wind, tide and river flow are among the factors that can cause trouble at these times. Operations underground and under water are particularly dangerous, and great care is taken to minimise risk to safety. Scouring by rivers is a powerfully destructive force, which can destroy the most solid masonry. Piers from early times have been shaped to reduce resistance to the flow of water, and have often been surrounded by a pavement of stone. Their foundations have to go so far down that scouring cannot remove a dangerous amount of material. Maintenance is a huge task on big structures - "painting the Forth bridge" is in the UK a well-known metaphor. Ideally a structure would be designed so as to require no attention once complete. In practice this is an impossible ideal. At the very least, any structure needs inspection, however well it was built. On any large structure you can see inspection hatches for access to the inside. The collapse of the Tacoma Narrows bridge is well known. Spare a thought for the designer. Books do not record a general outcry against the design, which was presumably accepted as an example of progress in design economy. It is the designer's bad luck that he took his design too far for the current state of knowledge. But other suspension bridges had been damaged or destroyed by oscillations. History is not bunk. Download Some events are outside the control of the designers and constructors. Some earthquakes are so powerful that no structure can withstand them. Ships have been known to collide with bridges, bringing spans crashing into the river. This happened to the Sharpness railway bridge over the river Severn. It has never been replaced. The Sunshine Skyway bridge in Florida was struck by a ship in 1986, bringing down a span. A tragic consequence of building a big bridge at a dramatic location is the attraction to suicidal people. The Clifton bridge is well equipped with cameras and signs about the Samaritans, but it has proved necessary to add curved fences to make climbing over the side very difficult. People sometimes jump off quite small bridges over roads or railways. People have occasionally decided to fly aeroplanes under them. The Tower bridge in London has been the subject of such behaviour. A Vampire was flown under Clifton bridge, but the pilot misjudged the difficult manoeuvre needed to escape from the curving gorge. He didn't make it. Had he flown slower he might have succeeded. This type of manoeuvre is extremely dangerous because it cannot be practised. However well the pilot studies maps and models, he or she cannot practise before the event. Even if nothing apparently goes wrong, without maintenance, most structures will degenerate. Painting the Forth bridge is a well-known metaphor. On the whole, it is rare for a bridge to be left to rot, unless it is disused. Here are pictures that show how plants can seize the smallest opportunity to grow where they are not wanted. Once they have taken root, they can generate more soil, and they can generate pressure that widens the cracks. This allows more water in, increasing the risk of damage by the expansion caused by freezing.
Many a structure that looked so good in the artist's impression, depicting the scene on a sunny spring day, with imaginary trees in leaf, later looks dowdy, as a result of fading or peeling paint, wrinkled or cracked surfaces, dirt, corrosion, creep, and other forms of entropy.
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