| Due to sag effect caused by cable weight, the vertical stiffness of cables will be reduce in cable-stayed bridge. With the increasing of cable length and weight, sagging effect becomes more notable. In the case of2000m-span cable-stayed bridge, length of the longest cable is more than1000m and the stiffness reduction will be significant, leading stiffness to be one of controlling factors in structural design. Being applied and developed in civil engineering, Fiber Reinforced Plastic (FRP) has quite high strength and low weight. The advantages in corrosion and fatigue also help FRP become quite suitable as cable materials.Based on theoretical researches on FRP material, I have studied static and dynamic effect of FRP cables on both single cable and the whole bridge in case of2000m-span cable-stayed bridge. The natural vibration characteristics of single cable and the stiffness of both single cable and entire bridge have been primarily researched. Finally a new method called ’hybrid-cable-stayed Bridge’ has been raised aiming to major bridges with a span of more than2000m. The main ideas and work have been listed below.In Part Ⅰ, static and dynamic characteristics of single cable have been analyzed in case of2000m-span cable-stayed bridge. Based on stiffness formula of catenary cables, the effective stiffness of steel cable and another4kinds of FRP cables have been researched. The effective stiffness curves of these5cables affected by cable length, angular and their horizontal length have been obtained respectively. MATLAB and ANSYS have been used in this part to define the shape of catenary cable and analyze nonlinear dynamical characteristics. By this method, the natural vibration frequency and mode of steel cable and FRP cables have been acquired. Calculation results show that the stiffness of traditional steel cables has reduced more than40～60%, unable to meet the stiffness demand of long cables in2000m-span cable-stayed bridge. In contrast, when cable length exceeds600m, carbon-FRP (CFRP) cable has a highest effective stiffness with lowest natural vibration frequency. Consequently, CFRP is most suitable for cables in major cable-stayed bridges.A steel cable-stayed bridge with a span of2038m has been preliminarily designed in Part Ⅱ. A CFRP cable-stayed bridge with the same span has been acquired using equal-strength method. The comparison of finished state indicates that cable-stayed bridges with CFRP cables could effectively reduce axial compressive forces of main girder and tower. Using CFRP cables could also reduce the cable force and increase the efficiency of cables. Then the vertical displacements of these two bridges have been calculated respectively under the action of concentrated and uniformly distributed loads. Calculation shows that near the center of middle span, CFRP cable-stayed bridge has a quite small vertical displacement more than10%less than that of steel cable-stayed bridge and the stiffness has been obviously improved. But in the range of cables near the tower, cable-stayed bridge with CFRP has a larger displacement, indicating that the vertical stiffness has reduced.In part III, the method of Hybrid-cable-stayed Bridge has been proposed aiming at major bridges with a span of more than2000m. In this part the concept and calculation method of ’long cables’ and ’short cables’ have been defined. Based on preliminary design of a2038m-span hybrid-cable-stayed bridge, the stiffness of this new method has been calculated and compared with traditional ones. It has been shown that hybrid-cable-stayed bridge has stiffness advantage similar to the pure CFRP cable-stayed bridge in the range of long cables and has reserved stiffness characteristics of pure steel cable-stayed bridges in short cables near the tower. Conseqeuntly, it has effectively raised the stiffness for the whole bridge. |
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