Parabolic or suspension bridges have the greatest span compared with beam and arch bridges. A modern suspension bridge can reach a span of 7,000 feet while a modern beam and arch bridge can reach only around 200 to 1,000 feet. This is because a suspension bridge is the most efficient in dealing with the forces of compression and tension; however, due to the complexity and size of a suspension bridge, it is generally the most expensive to build.
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Basic Design of a Parabolic Bridge
Dutch scientist Christian Huygens believed that two cables running between two towers could support weight that was much greater than its own weight. The cables suspended between the two towers naturally took the shape of a parabola. As opposed to a catenary, the parabolic shape of the cable is created not only by gravity but also by the compression and tension forces acting on the bridge. Today's scientists believe that this is the most efficient design because it requires the least amount of materials to support a heavy load. The modern suspension bridge is composed of a deck, supports, foundations, long wire cables and hangers.
Notable Suspension Bridges
The Akashi-Kaikyo Bridge is the longest suspension bridge in the world. Its span is 6,529 feet, while its towers are 928 feet tall. The second longest suspension bridge is the Xihoumen Bridge connecting the Cezi and Jintang islands in China; its main span measures 5,413 feet long. The third longest is the Great Belt Bridge in Denmark with a span of 5,328 feet.
In the U.S., the Verrazano-Narrows Bridge has the longest span at 4,260 feet. The second longest is the Golden Gate Bridge with 4,200 feet, while the third longest is the Mackinac Bridge with a span of 3,800 feet. The Brooklyn Bridge, which was completed in 1883, was the first major suspension bridge constructed in the U.S.
Latest Research on Suspension Bridges
Research on structural topology optimisation on a suspension bridge's parabolic cable has shown that it is possible to increase its capacity to withstand compressive forces. Researchers at the University of Sheffield in the United Kingdom suggest that replacing the ends of the parabolic cable with the Henchy net truss network can improve its capacity to withstand compression forces, as well as reduce material requirement by 0.3%. Other researchers, however, believe that this may not be feasible for future suspension bridges because the cost of the more complex design will outweigh the little cost-saving with the reduced material requirement.
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