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【正文】 dies must be made to guarantee the stability of the cables and the bridge in the wind. The lighter weight of the bridge, though a disadvantage in a heavy wind, is an advantage during an earthquake. However, should uneven settling of the foundations occur during an earthquake or over time, the cablestayed bridge can suffer damage so care must be taken in planning the foundations. The modern yet simple appearance of the cablestayed bridge makes it an attractive and distinct landmark. The unique properties of cables, and the structure as a whole, make the design of the bridge a very plex task. For longer spans where winds and temperatures must be considered, the calculations are extremely plex and would be virtually impossible without the aid of puters and puter analysis. The fabrication of cable stay bridges is also relatively difficult. The cable routing and attachments for the girders and towers are plex structures requiring precision fabrication. There are no distinct classifications for cablestayed bridges. However, they can distinguished by the number of spans, number of towers, girder type, number of cables, etc. There are many variations in the number and type of towers, as well as the number and arrangement of cables. Typical towers used are single, double, portal, or even Ashaped towers (illustration 2 amp。s longest center span of any bridge at 1,991 meters. A typical suspension bridge (illustration 1) is a continuous girder with one or more towers erected above piers in the middle of the span. The girder itself it usually a truss or box girder though in shorter spans, plate girders are not unmon. At both ends of the bridge large anchors or counter weights are placed to hold the ends of the cables. The main cables are stretched from one anchor over the tops of the tower(s) and attached to the opposite anchor. The cables pass over a special structure known as a saddle (illustration 2.) The saddle allows the cables to slide as loads pull from one side or the other and to smoothly transfer the load from the cables to the tower. From the main cables, smaller cables known as hanger cables or hanger ropes are hung down and attached to the girder. Some suspension bridges do not use anchors, but instead attach the main cables to the ends of the girder. These selfanchoring suspension bridges rely on the weight of the end spans to balance the center span and anchor the cable. Thus, unlike normal bridges which rest on piers and abutments, the girder or roadway is actually hanging suspended from the main cables. The majority of the weight of the bridge and any vehicles on it are suspended from the cables. In turn the cables are held up only by the tower(s), there is an incredible amount of weight that the towers must be able to support. As explained in the cable stayed bridge section, steel cables are extremely strong yet flexible. Like a very strong piece of string, it is good for hanging or pulling something, but it is useless for trying to push something. Long span suspension bridges, though strong under normal traffic loads, are vulnerable to the forces of winds. Special measures are taken to assure that the bridge does not vibrate or sway excessively under heavy winds. The most famous example of an aerodynamically unstable bridge is the Taa Narrows Bridge in Washington state, USA. This page on the Taa Narrows Bridge Disaster at the University of Bristol has some excellent photos and short movies showing why aerodynamic stability is important.
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