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FRP composites have many advantages as structural materials such as high strength, relatively high elastic modulus, light weight, long-term durability, high dielectric strength, low axial thermal expansion, low maintenance, design flexibility, tailored esthetic appearance, and low tooling and installed costs. On the other hand, their demerits are the rupture without yielding, low elastic modulus of some kinds of the materials, anisotropic properties, low shear and impact resistance, and high material cost that may be gradually improved by the progress of manufacturing technology and widespread use.

Although the use of FRP composites is now becoming popular in structural engineering, carbon fiber-reinforced plastic (CFRP) as a cable material seems the most ideal application. Actually, CFRP stay cables have already been employed on several cable-stayed bridges for pedestrian use. In the far future, CFRP or some other FRPs may be used for the main cables of suspension bridges and greatly increase the limiting span length. The mechanical properties of different cable materials are shown in Table 26.4.

CFRP cables are produced as assemblies of the wires made up of carbon fibers and an epoxy resin matrix. The design considerations for cables made of unidirectional CFRP wires are similar to those for steel cables, with a few exceptions due to the highly anisotropic nature of the material. So far, CFRP stay cables have been made by twisting CFRP wires. Although twisted fiber rope makes coiling possible, resulting in ease in handling, care must be taken on the strength loss of the wires in a bundle and friction between wires. Since the decrease in cable strength is also brought by side pressure and bending, the diameter of a reel and the radius of curvature of the saddle should not be small. As the strength of epoxy resin deteriorates by the effect of ultraviolet radiation, the CFRP cables shall be protected by covering.

The trial design of a super-long-span suspension bridge for which CFRP main cables are used indicates that the bending moment and vertical deflection of the stiffening girder significantly increase due to the light weight of the structure [6], while the dimensions of towers and foundations can be greatly reduced. It is recommended to reduce the sag/span ratio of the main cables.

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