Appearance Best Curves Force Forces Formulas How many Load paths Location Longest Longest Longest Measuring Small beams Stress Stress/strain Strongest Strongest What holds it up Where What should bridges look like? What is the best kind of bridge? What is the correct curve for a suspension bridge? Does a force have a direction? What forces act on a bridge? What formulas are used in bridges? How many bridges exist? What is a load path? Where should bridges be built? What are the longest actual bridge spans? What is the longest possible span? Why are the longest spans all in suspension bridges? How can I measure small deflections of a model? Why are there so many small beams and small cantilevers? What determines the stresses in a structure? Does stress cause strain or vice versa? What is the strongest kind of bridge? What can models do to find the strongest kind of bridge? What makes a bridge hold up? Where should bridges be built?

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 What makes a bridge hold up? This is either easy to answer or difficult to answer, depending on the kind of answer you are seeking. The easy answer is - A     The designers designed it to hold up under the likely conditions        at the site, and they got it right. B     The construction procedure was designed correctly, and the        construction process was adequately supervised and executed.        Construction includes the preparation of all the parts and the        materials from which they are made. C     The maintenance schedule was well planned and correctly        carried out. This all seems obvious, but there have been famous examples in which A, B or C was not done right, resulting in problems, or even collapses. An apparently different answer for A concerns energy. The lowest possible energy state of a bridge occurs when it is lying on the bottom of the river, or of the valley, having collapsed. One job of the designer is to ensure that between this condition and the actual state of the bridge as built, there are energy states so high that they will never be reached under any foreseeable loads. This has to be the case both for the whole bridge and for every individual part that isn't redundant. This can be symbolized by a simple graph, showing the energy of a system versus the force applied. The graph above applies to any linear, elastic structure. But what happens if we apply more force? The difference here is that when a certain critical condition is reached, something breaks or buckles and the structure collapses, and the energy plummets down on one of the vertical lines. This happened to the first Quebec bridge during construction.  In the case of the first Tay bridge, the structure actually got built, but the effect of strong winds had been greatly underestimated, and the construction had not been performed adequately. The red line has been drawn as a possible limit to the load, giving a safety factor.  The structure would be designed so that no foreseeable loading could make the energy go past the red line. The curve is symmetrical in this example. In the case of the Tay bridge, the wind could have blown from either side, with the same disastrous result. For actual live loads, such as traffic, the loading would seem to be always be in the same direction. This is not necessarily true of every individual member. It is even possible that the stress in a member could change from compressive to tensile as a load moved, or a bending or a torsion could change direction. In steady flight, the wings of an aircraft are always pushed upwards, but on the ground, the wings hang down.In time of war, bridges may be among the early structural casualties, the energy needed to overcome the stability being provided by explosives.Another way to look at this is that the forces in a bridge act in such a way as to produce equilibrium at all points. If at any points there is no equilibrium, the structure will deflect until equilibrium is reached. If a structural member reaches its limit before complete equilibrium is reached then the structure is not viable.

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 What is the best kind of bridge? If you need build across a deep valley, a simple crossing could be made by building an embankment. If you do it properly, it will be very strong. This is, of course, seldom practical, as there is usually a river, a road or a railway in the valley, which would have to go through a tunnel in the embankment. There is a also the question of economics, and in a wide V-shaped valley, there may be embankments at the ends, and a viaduct in the middle. The use of embankments might become cheaper if the road had required a cutting nearby, making large quantities of material available. So we probably have to build a bridge. The type of bridge will depend on the length of the longest gap that has to be cleared by a single span, on the type and volume of traffic, on the type of ground, on environmental considerations, on the available budget, and on the relative cost of different designs. An extreme example of having to build a bridge, rather than an embankment is found at Millau, in France, where some of the piers of the bridge are taller than the Tour Eiffel. The relative costs of different designs does not necessarily remain the same under a change of scale. If you scaled the Forth railway bridge down by a factor of ten in length, you would have the same number of parts, but vast numbers of them would be ridiculous complications for such a small structure. If you look at a very short truss, all the members will almost certainly be very simple struts and ties, but if you look at a really large truss bridge, many of the members will structures in their own right, such as trusses, tubes or box girders.Scale is a major factor in the choice of structure. In fact, for spans above 890 metres, there are at present only suspension bridges. Galileo explained the effects of changing scale very well. Compare, for example, aphids, dragonflies and eagles, or insects and elephants.The great variety of bridges tells us that there is more to the art of bridge building than span and load. Technical and aesthetic change, for example, must be taken into account. Travel north from London on the M1 motorway and you travel in time.  Look at the Forth railway bridge and the Quebec bridge. Why has this type of bridge not been repeated on that grand scale? Was it because railways had reached their peak of expansion? Was it because there were no more wide and deep gaps for them to cross? Was it because better and cheaper types of bridges were invented?See also strongest bridges.

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 Why are the longest spans all in suspension bridges? The longest component of a suspension bridge is the set of main cables, which are all in tension, a stable state. In an arch, the main members are all in compression, which means that they are liable to buckling. Hence the thickness of the members and the large amount of bracing that you see in structures such as Sydney Harbour bridge. Suspension bridges do have to include anchorages and towers, which are massive constructions, but they are relatively simple compared with the trussed arch. Worse still are beams, which suffer both tension and compression, unlike the suspension bridge and the arch, which have mainly the one type of stress. In a sense, a beam is like a very shallow tied arch, or a very shallow self-anchored suspension bridge. The cantilever bridge can just about compete with the arch, but the Quebec bridge remains the longest span attempted. With road traffic dominating, the cable-stayed bridge has taken over much of what cantilever bridges used to do. Click here for a more detailed discussion of this topic. Why are there so many small beams and cantilevers? Vast numbers of modern bridges over roads and railways are beams and cantilevers, often plate girder/I-beam or concrete. Why is this? These bridges have the following advantages - The spans can often be built off-site and lifted into place. The designs are relatively simple and the principles are well understood. The piers can be vertical, because there is no horizontal reaction, so the ground area needed is small. Clearance between road and bridge can be maintained high throughout, as compared with the arches which were commonly built in the 19th century.