Forces Acting on Bridges
Many different forces act on bridges. No bridge is completely permanent. These forces will, one way or another, break any bridge. It is an engineer's job to try and delay this destruction for as long as possible by building the strongest bridge possible. Knowledge of the forces acting on bridges is crucial in this endeavor.
Compression is a force pushing inward. An example of compression is two people leaning on one another with their hands out, so that their hands are compressed together. Another example would be a soda can squashed underneath a car as the car rolls over it.
Tension is a force pulling outward. To explain it in terms of the previous example of compression, an example would be two people linking hands and leaning backwards, so that their arms are taut. Another example is a rope in a game of tug-o-war.
Torsion involves two forces. When forces at opposite ends of a bridge rotate the bridge in different directions, torsion is acting on the bridge. An example is a dish towel being wrung out. In a bridge, however, a much more rigid structure is needed, so torsional effects are far more severe than those from a wrung dish towel. The torsion in the center of a bridge is zero, while torsion is greatest on the edge of the object it is acting on.
Shear is a force that pushes ends of an object in different directions. For example, when a person sits on a bench, shear is present. The legs of the bench push the seat up, while the middle is pushed down by the person sitting on it.
Tension is a force pulling outward. To explain it in terms of the previous example of compression, an example would be two people linking hands and leaning backwards, so that their arms are taut. Another example is a rope in a game of tug-o-war.
Torsion involves two forces. When forces at opposite ends of a bridge rotate the bridge in different directions, torsion is acting on the bridge. An example is a dish towel being wrung out. In a bridge, however, a much more rigid structure is needed, so torsional effects are far more severe than those from a wrung dish towel. The torsion in the center of a bridge is zero, while torsion is greatest on the edge of the object it is acting on.
Shear is a force that pushes ends of an object in different directions. For example, when a person sits on a bench, shear is present. The legs of the bench push the seat up, while the middle is pushed down by the person sitting on it.
http://images.books24x7.com/bookimages/id_9027/fig12-1.jpg
Dead load is the weight of bridge structure. This is the bridge itself. In suspension bridges, this is a major force that must be accounted for.
Live load is the weight of traffic on bridge. This could be people, cars, trains, trucks, or anything else that is not attached to or part of the bridge. Engineers need to be aware of the maximum possible weight that this could be, but, more importantly, what the maximum probable live load could be. On a small bridge, there are decent chances of semi trucks covering the entire bridge. However, on a long, large bridge, it is unlikely that there will ever be bumper-to-bumper semi trucks for the entire length of the bridge. So, most bridges are designed only to be capable of carrying a realistic live load, not the absolute maximum.
Live load is the weight of traffic on bridge. This could be people, cars, trains, trucks, or anything else that is not attached to or part of the bridge. Engineers need to be aware of the maximum possible weight that this could be, but, more importantly, what the maximum probable live load could be. On a small bridge, there are decent chances of semi trucks covering the entire bridge. However, on a long, large bridge, it is unlikely that there will ever be bumper-to-bumper semi trucks for the entire length of the bridge. So, most bridges are designed only to be capable of carrying a realistic live load, not the absolute maximum.
http://www.pbs.org/wgbh/buildingbig/lab/loads.html
In this picture, the elephants are the live load, while the gray and white lines (the bridge) are the dead load.
In this picture, the elephants are the live load, while the gray and white lines (the bridge) are the dead load.
Wind is the main cause of many of the forces that act on bridge. Wind can push a bridge horizontally, vertically, or in any other way. Cross bracing, an X-shaped structure that pushes on the joints of beams to prevent them from wiggling, can help keep a structure stable in wind. One of the most interesting effects of wind on bridges is resonance. Information on resonance is located under the "Forces Acting on Bridges" tab, and then the "Resonance" tab.
Temperature has an impact on bridges, especially arch bridges. Expansion and contraction can cause a bridge to swell or contract. Thermal expansion can be combated with finger joints or roller joints, and hinges in arch bridges. These joints allow parts of a bridge to move a bit, so that cracks do not form.
Earthquakes are natural disasters that severely affect bridges. They have the least effect on bridges with the least dead weight. Bridges can be strengthened against earthquakes with shear walls, or cross bracing under the bridge.
Settlement load affects the area under the bridge. As soil settles under bridge foundations, the bridge sinks into the soil. Settlement load is lessened with piles.
Earthquakes are natural disasters that severely affect bridges. They have the least effect on bridges with the least dead weight. Bridges can be strengthened against earthquakes with shear walls, or cross bracing under the bridge.
Settlement load affects the area under the bridge. As soil settles under bridge foundations, the bridge sinks into the soil. Settlement load is lessened with piles.