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Bridges |
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A bridge is basically a beam that is designed to carry a live load such as vehicles, people, |
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and animals across its span. That is, it carries dynamic loads in addition to the dead loads |
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of its own weight and stationary objects. Therefore the stresses imposed on a bridge are |
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more complex than on a structural beam member. Nevertheless the same type of static |
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analysis can be used to see how a bridge reacts to loads in order to shed light on the |
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dynamics of its behavior. In this section you will explore the structure and behavior of |
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beam, truss, cantilever, suspension, arch, and box girder bridges. |
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Beam bridge - typical span 10 to 200 m ( 30 to 600 ft.) |
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Probably the oldest type of bridge used by humans or animals is simply a log fallen across |
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a stream or gully, that is, a simply supported beam. Improvements were made eventually |
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to the log to shape it into a more useable form. Light duty pedestrian or short span road |
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bridges can be constructed from solid, heavy timber beams. Modern highway beam bridges |
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are typically constructed from rows of steel or reinforced concrete I-beam girders which |
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carry the roadbed, or deck. Since the maximum length a beam bridge can efficiently and |
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safely span is limited by the bending moment exerted by a load mid-span, Ι-beam bridges |
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are only suitable for short spans of thirty to six hundred feet. They are used extensively for |
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highway overpasses, elevated walkways, and causeways, which use many short bridge |
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spans joined together in series to cross wide stretches of ground. |
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Plate girder bridge |
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An extreme example of increasing the height of a beam to increase its stiffness is the plate |
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girder bridge design in which the girder is many times taller in cross-section than it is wide. |
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The thinness of the girder makes it susceptible to twisting, or torque, stresses induced by a |
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load, and sideways wind loads. In a deck plate girder bridge, which carries its load on top, |
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Back to Knowhere |
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