Example Essay - Bridges
...t be strong so that it doesn't bend under its own weight and the added weight of crossing traffic. When a load pushes down on the beam, the beam's top edge is pushed together (compression) while the bottom edge is stretched (tension). Pre-stressed concrete is an ideal material for beam bridge construction; the concrete withstands the forces of compression well and the steel rods imbedded within resist the forces of tension. Pre-stressed concrete also tends to be one of the least expensive materials in construction. But even the best materials can't compensate for the beam bridge's biggest limitation: its length. The farther apart its supports, the weaker a beam bridge gets. As a result, beam bridges rarely span more than 250 feet. This doesn't mean beam bridges aren't used to cross great distances, it only means that they must be daisy-chained together, creating what's known in the bridge world as a "continuous span." In fact, the world's longest bridge is a continuous span beam bridge with almost 24 miles, the Lake Ponchartrain Causeway. Suspension bridges tend to be the most expensive bridges to build. With aesthetic, light, and strong, suspension bridges can span distances from 2,000 to 7,000 feet, far longer than any other kind of bridge. True to its name, a suspension bridge suspends the roadway from huge main cables, which extend from one end of the bridge to the other. These cables rest on top of high towers and are secured at each end by anchorages. The towers enable the main cables to be draped over long distances. Most of the weight of the bridge is carried by the cables to the anchorages, which are imbedded in either solid rock or massive concrete blocks. Inside the anchorages, the cables are spread over a large area to evenly distribute the load and to prevent the cables from breaking free. Some of the earliest suspension bridge cables were made from twisted grass. In the early nineteenth century, suspension bridges used iron chains for cables. Today, the cables are made of thousands of individual steel wires bound tightly together. Steel, which is very strong under tension, is an ideal material for cables; a single steel wire, only 0.1 inch thick, can support over half a ton without breaking. Since 1998, the Japanese is the $7.6 billion Akashi Kaikyo Bridge, linking the islands of Honshu and Shikoku. The bridge's center section stretches a staggering 6,527 feet. A similar form to suspensions bridges is the Cable-stayed bridges -- both have roadways that hang from cables and both have towers. But the two bridges support the load of the roadway in very different ways. The difference lies in how the cables are connected to the towers. In suspension bridges, the cables ride freely across the tower...