Information on Bridges - Ms. Sulik's Teacher page



Information on Bridges

There are 5 main types of bridges, each of which has its strong points and its weak points.

The first type is the Beam Bridge:

Beam bridges are beams that are supported by piers (support columns). These are the simplest type of bridge and were the first bridges ever used when people would lay a board across a stream in order to be able to get across. Beam bridges are commonly made from box girders or concrete. A box girder is a hallow box structure, typically made out of steel and/or concrete and are more resistant to torsion forces then other types of supports used with beam bridges. To build a beam bridge, one would typically use one horizontal beam with two supports, typically at each end. These bridges are not able to span very large distances and in larger distances require support columns throughout the middle of the span. These types of bridges are usually made for people, cars and some light-weight trains to cross, or for any type of transportation over a very short distance.

The Cantilever Bridge:

The Cantilever bridge is characterized by the cantilever structure that gives it its name. A cantilever is a structure that extends horizontally into space and is only supported at one end. For smaller bridges this can be a simple beam but for larger bridges a truss or box girder is needed. When the steel truss cantilever bridge was first used it was a major engineering breakthrough as it can span distances of over 460 m. This type of bridge is also less complicated to build as it can be done with little or no temporary construction known as falsework. The cantilever bridge is classed as a short to medium span bridge designed for pedestrians, automobiles, trucks, and light and heavy rail freight.

The Truss Bridge:

The truss bridge is made of several connecting elements, typically straight, which may be stressed from compression or tension elements or both in response to dynamic loads. The design of a truss bridge is actually quite simple, which is why it was one of the first modern bridges designed and put into use. In most truss bridges, the lower parts are under tension and the upper parts are under compression. Since truss bridges are typically comprised of many small beams they can support a large amount of weight and span great distances. In order to do this though, the trusses take up a lot of space due to their thickness, and this has been reported to be a distraction for drivers. Because the strength of the supports can be supplemented by adding to the number of support elements, engineers can choose more economical materials to construct these bridges, making them very economical. The design allows for smaller individual parts and this is ideal for locations that would not allow large parts to be shipped to or where large cranes and heavy equipment cannot be used during the assembly. These bridges are used for pedestrians, automobiles, trucks, pipelines, light and heavy rail freight. Usually they are used for small to medium spans and can be constructed using timber, steel, iron and concrete.

The Arch Bridge:

An arch bridge is a bridge that has supports, called abutments, at each end and is shaped like an arch. The shape of an arch bridge allows the transfer of some of the weight of the bridge and its loads to the horizontal part of the structure, which is in turn supported by the abutments on each side. A long arch bridge, called a viaduct, is made of a series of arches but today much more economical alternatives exist to the viaduct. These bridges are so strong that some still exist today from as long ago as 1300 BC. Typically, arch bridges are constructed using stone or brick. This makes them more resistant to shearing and compression forces but they are weak to tension forces. These bridges are used for pedestrians, vehicles, light and heavy rail as well as water in aqueducts. Since each arch needs an abutment, arch bridges can only span a short distance but are often set end-to-end to span greater distances.

The Suspension Bridge:

In a suspension bridge the load bearing parts are hung, or suspended from suspension cables. When suspended from a high canyon or river, the simplest suspension bridges have a downward arch and are not able to support modern roads or railways. Advances in technology have allowed for a modern bridge that can carry vehicles and rail. This is done by stopping the downward drop of the bridge by placing towers between the load bearing cables and vertical cables are added above the deck that support the weight of the deck below, which is where the vehicles cross. By adding these additional supports the bridge is made to appear straight or even to curve upward. In order for these cables to successfully support the loads, there must be an anchor at each end of the bridge so that the tension forces may be transferred through the cables into these supports. Mainly, tension and compression forces act on the structures of a suspension bridge. One of the major challenges that engineers face when designing these bridges is that they must be made very stiff in order to prevent the deck vibrating in high winds. Also, since there is limited support under the deck of the bridge, these bridges are less suited for heavy rail transport. They are used however, quite extensively for pedestrians, automobiles, trucks, and light rail and can span medium to long distances with ease.

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