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N. Central Idaho
Tunnels and Mines
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There are 5 basic types of bridges: arched, post and beam, suspension, cantilevered, and cable stayed. All bridges rest on foundations that transfer loads from the bridge into the ground. The type of bridge suitable for a particular location depends on length of span, terrain, available materials, and navigation needs.
Arched: An arch is a compression structure. The arch shape channels all the weight from above to the foundations below. The earliest arch bridges were built of stone. Stone arches are built over a temporary wooden form called a centering. Once the last piece of stone, called the keystone, has been set in place the arch proper is finished. But if the centering is removed at this point, the uppermost stones would want to fall, and this in turn would push the sides of the arch outward. To prevent this from happening, more material must be build up against the sides of the arch before the centering is removed. This prevents movement between the stones, and also compresses them together. The more the wedge-shaped stones are compressed, the stronger the arch will be.
Post and Beam: To understand how a beam works, imagine a board placed over two supports, creating a bridge. If you stand on the middle of the board, it will bend. The top surface of the board grows slightly shorter because it is being compressed; the bottom surface is being stretched because it is in tension. If the board (the "beam" of a post and beam bridge) cannot withstand one or both of these forces, it will break. In addition to bending, beams must also be able to withstand twisting. To span a large river, massive beams would have to be used. Early post-and-beam bridges were actually tunnels: two parallel tubes made of wrought iron and supported on high stone piers (the "posts"). Later designs created open sides, usually employing trusses. The open design resisted twisting caused by wind by allowing the wind to pass through. A truss is a collection of interconnected triangles, the sides of which carry the tension and compression forces. Today, steel trusses are among the most common bridges in North America. They are very strong and can span long distances.
Cantilevered: A cantilever is a projecting beam supported or fixed at only one end, such as a shelf bracket or diving board. By increasing the depth of a beam, it is possible to reduce the bending. Since most of this bending happens in the center of the beam, that is where the depth should be greatest. Because this becomes the heaviest part of the structure, the supporting post, or pier is typically in the center of the beam.
Suspension: The main parts of a suspension bridge, other than the roadway, are the towers, the cables and the cable anchorages. The roadway itself actually hangs from the cables. If the cables stopped at the tops of the towers, their own weight in addition to the weight of the roadway and that of the traffic would bend the tops of the towers toward each other. To prevent this from happening, the cables pass over the tops of the towers and are connected to concrete anchorage cast into solid rock. The action of the cables pulling down on both sides of a tower creates a strong vertical force that is carried to the foundations.
Cable stayed: In a cable-stayed bridge, the roadway is supported by a series of cables extending in one or two planes from the tower or towers. Unlike the main cable of a suspension bridge, cable stays are straight and anchor directly into the roadway itself. Each forms the third leg of a triangle. The cable stays are in tension while the road and tower are in compression.