Research On Design Theory Of Partially Cable-supported Bridge

The small span cable-stayed bridge with a single span arrangement about100m is common in city. However, the general design theory of cable-stayed bridge is based on the cable-stayed bridge with economic span. Although it is universal on mechanical calculation, it could not be well applicable to small span cable-stayed bridge on the analysis of structure using efficiency. In recent years, the emergence of partially cable-supported bridge provides a new train of thought for the design of small span cable-stayed bridge. By the design concept of making full use of the girder stiffness of small span cable-stayed bridge, considering the combination mechanical behavior of girder and cable in order to give full play to their respective material performance, partially cable-supported bridge can achieve the unity of the usability and economy. Given there is a few special and systematic research on design theory of partially cable-supported bridge, this paper particularly analyzes and evaluates the characteristic parameters, key problems of design, and long-term performance of this system.Firstly, this paper particularly analyzes the structure and mechanical behavior of small span cable-stayed bridge which designed according to the design concept of conventional cable-stayed bridge based on the Xiangjiang Bridge in Baishan City. According to the analysis results, from improving the utilization ratio of girder stiffness and cable strength, this paper puts forward an optimization design scheme for small span cable-stayed bridge and gets the system characteristic of partially cable-supported bridge. By analyzing dead load share ratio and allowable stress of fatigue strength of cable, this paper proposes that it should take dead load share ratio of cable as the characteristic parameter for defining partially cable-supported bridge. And it points out that dead load share ratio of cable should be taken35%~65%. At the same time, cable safety coefficient should be taken1.67~2.5.And then, this paper researches the key problems in design process of partially cable-supported bridge based on the Sanhao Bridge in Shenyang City. Stiffness expressions of girder, cable, and tower are derived, which provides gist for grasping the whole rigidity of this system from qualitative aspect. Based on the obtained stiffness expressions of girder and cable, the expression of live load share ratio of cable is derived using equivalent girder method. And this paper analyzes the relationship between live load share ratio and structural parameters, evaluates computational accuracy of its expression, which provides reference for grasping live load distribution in structure. Design method of longitudinal prestress in girder is expounded in the paper. It points out the advantage of using bent-up prestressing tendon to provide vertical prestress, and analyzes characters of prestress effect of web, top deck, and bottom slab, which provides lessons for reasonable arrangement of prestress tendon. The finite element model calculation results of simply supported beam, grillage method, and entity unit about cross diaphragm, cable anchored, are compared, and this paper evaluates the applicability of those three kinds of model. Then, the paper analyzes effects of variations in structural parameters upon internal force and displacement of structure.At last, this paper briefly expounds some basic concepts of concrete shrinkage and creep. Through establishing the finite element model considering construction stages, effects of concrete shrinkage and creep upon internal force and displacement of partially cable-supported bridge are analyzed in this paper, and it evaluates the long-term performance of this system. In the end, it studies the influence of concrete loading age to the long-term performance and points out measures to improve its long-term performance.