17.第 47 題至第 50 題為題組
A suspension bridge is a type of bridge that use overhead cables to support its roadway. The design of a suspension bridge is simple and straightforward, and uses several architectural techniques to distribute the weight of the bridge safely and evenly.
The construction of a suspension bridge starts with its towers, which are usually located on dry land and anchored to bedrock. Once the towers are built, master cables are strung across them and anchored into the bank at both ends of the bridge. Next, suspension cables are strung from the master cables, and then the deck used as roadway is attached to them. Most of the weight of the bridge is transferred by the cables to the anchorage systems. These are embedded in either solid rock or huge 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.
One of the oldest engineering forms, suspension bridges were constructed by primitive peoples using thick vines for cables. A much stronger type was introduced in India around 400 A.D. that used cables of braided bamboo. 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. A single steel wire, only 0.1 inch thick, can support over half a ton of weight without breaking.
Light and strong, a suspension bridge could span distances from 2,000 to 7,000 feet, which is much longer than any other kind of bridge could. Its simple design allows high clearance under the deck, useful when the bridge is built over a major shipping waterway or a very deep gulf. A suspension bridge is also less subjected to collapse than some other bridge types such as those built on support pillars.
【題組】47.Which of the following materials have NOT been used as cables for suspension bridges?