Development and practical application of pre-coated parallel wire cable for large cable-supported structures 大規模吊構造用被覆平行線ケーブルの開発と実用化に関する研究

博士論文

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A remarkable progress has been seen in recent years around the world in the construction of cable-stayed bridges, as well as in suspension bridges. As longer cable-stayed bridges have come to be constructed, the multiple-cable system with mono-strand cables has become popular. Stay cables of such long cable-stayed bridges are required to possess excellent mechanical properties, such as a high static and high fatigue strength, and a superior corrosion protective coat. There were few cables in the past which satisfied those requirements, therefore it was necessary to do a study on them. This thesis describes the study on the development of a new-type cable for long cable-stayed bridges, and application subjects for large cable-supported structures. Details of the studies are presented respectively. A slight twist of the wires given to the cable made it possible to coat it with a corrosion protective layer in the manufacturing process and to improve handleability such as installation at the site without damaging the structural performance. It was indicated that the pre- coated parallel wire cable had the same static characteristics (such as the breaking strength and modulus of elasticity) as those of a parallel wire strand, when the twisting angle of a cable was less than 3.5°. It was also shown that the twisting angle had little effect on the time dependent characteristics (such as creep and relaxation) when the twisting angle of a cable was less than 3.5°. Finally, the tensile test was carried out by using full-size cable models and it was confirmed that a pre-coated parallel wire cable with a breaking load of a 3,000 tonf (29,400 kN) class could be put into practical use. A new fatigue resistant socket having a composite-filler consisting of an epoxy resin and a zinc-copper alloy was developed in order to improve the fatigue strength of a socket. It was confirmed that the socket had a higher fatigue strength of about 25kgf/mm^2 (245MPa). It was confirmed that the socket had a higher fatigue strength of the socket when the twisting angle of a cable was less than 3.5°. Experimental results and analysis indicated that the pre-coated parallel wire cable with the sockets had a bending fatigue strength of about 0.6°and the socket using epoxy resin also had an effect on decreasing the secondary stress at the entrance of a socket. A new method in socket design for a large cable, introducing the concept of rigidity for ultimate strength, was examined. As a new protection system against corrosion, an extruded polyethylene coating for a parallel wire cable was developed, which eliminated the need for corrosion protective coating work at the site. A high-density polyethylene was selected as the optimum protective coating material. According to the accelerated weathering test results, it was confirmed that the polyethylene showed no deterioration when 1% carbon black was added. The properties of a polyethylene coated cable were analyzed, and installation characteristics of the cable such as unreeling and clamping were examined using full-size cable models. It was indicated that this protection system was a great advantage in the erection of stay cables of cable-stayed bridges, especially long ones. In addition to fundamental development, the cable that could be applied to any cable-supported structures was also required to have higher performance. One of the application subjects was to make the cable colored and durable. The other was to develop aerodynamic measures against cable vibration to lengthen cable-stayed bridge spans. Details of the investigations with respect to these subjects and the results obtained by the study (a fluoropolymer-coated colored cable and a low drag aerodynamically stable cable with an indented pattern), were presented. A colored cable using a fluoropolymer having outstanding weathering durability was developed. The material properties of the fluoropolymer, especially the weathering durability, were evaluated by using various exposure test methods. It became obvious that the fluoropolymer had excellent weathering durability, which could be estimated to last for more than 100 years if the thickness were 1 mm or more. Structural characteristics of the fluoropolymer-covered cable were examined, and it was indicated that the cable had the same installation properties as those of a polyethylene-coated cable. These characteristics of the fluoropolymer showed that it could be used for various structures and elongated service life and added aesthetic aspects. As a result, a greater freedom of color selection was given to bridge cables that matched the surrounding environment. Finally, it was shown that the colored cable was applied not only to stay cables of long cable-stayed bridges but also to hangers of long suspension bridges. The vibration problems of a stay cable were surveyed and experimental studies on cable vibration and its suppressing method for a long cable-stayed bridge were discussed. After examining the aerodynamic characteristics of a cable with a patterned roughness, using wind tunnel tests, a new aerodynamical cable having both a low drag force and a suppression effect on rain vibration was developed. It was found that it had the same drag coefficient of about 0.6 as that of a cable with a smooth surface in any design wind velocity range. It was also found that a cable with a surface roughness had a vibration suppressing effect when the relative surface roughness had a vibration suppressing effect when the relative surface roughness of 1% of the cable diameter was given discretely on the surface. Suppression effects were analyzed by measurement of the drag coefficient and pressure distribution, indicating that cables with a lumped pattern roughness made it possible to reproduce a supercritical state at a low wind velocity range where rain vibrations occurred. To understand further the aerodynamic characteristics of the surface configuration of the cable, various patterned roughnesses were examined in terms of the Reynolds Numbers. It was found that density and arrangement of lumped roughness were the key factors in controlling the drag characteristics of cables, and that optimization of the drag coefficient-Reynolds number relationship could enable adjusting those two factors. Responses of stay cables by vortex-induced vibration in natural wind were measured at an actual cable-stayed bridge with a main span of 490m. The factors affecting this vibration, such as surface roughness and structural damping, were examined by using a wind tunnel test and analysis. It was shown that the response amplitude was estimated at various damping levels, and that the bending angle of a cable, caused by vortex-induced vibration, to be less than 0.1°, which had little effect on the secondary stress and bending fatigue strength of cables. Cable materials in the future for a super-long cable supported bridge were described. It was shown that the development of cable material was inevitable in order to lengthen a bridge span of a super-long suspension bridge. The present status of cable materials including steel wires and various new materials was surveyed and future problems to overcome were pointed out. As a future prospect, a high strength steel material and a light weight material such as carbon fiber were shown to be possible for use in super-long cable supported bridges. In conclusion, several ideas for the new-type cable discussed here helped in the lengthening cable-stayed bridge spans, and contributed a great deal to the advancement of cable-supported structures.

横浜国立大学, 平成8年6月30日, 博士(工学), 乙第107号