| ABSTRACT:Cable stayed bridge with multi-tower has already been built in the middle of last century, but it developed slowly. One of the key factors restricting the development of multi-tower cable stayed bridge is its weakness on rigidity, so many researches are focused on enhancing the integral rigidity of multi-tower cable stayed bridge. Then, the integral rigidity problem of long-span cable-stayed bridge with multi-tower is more prominent. On the basis of predecessors' research results, this paper mainly studies on the integral rigidity of long-span cable-stayed bridge with multi-tower systematically, which is funded by science and technology project of Ministry of Transport "The key technology research of large-span multi-tower cable stayed bridge"(project number:2013315494011). In this paper, the main work includes the following several aspects:(1) An integral rigidity index of multi-tower cable stayed bridge is proposed. A representative value of vertical rigidity is suggested, and the limiting value is discussed as well. It provides a reference standard for the study. On the basis of collecting data widely, statistical analysis on structure parameter of multi-tower cable stayed bridge is conducted, and a optimized range of tower-span ratio is proposed.(2) The isolation structure method is presented in the structural deformation calculation of multi-tower cable-stayed bridges.Based on Matlab software, corresponding program are compiled, and calculation efficiency of deformation of girder under live load is improved when the rigidity parameters need optimizing. According to the difference of loading features between multi-tower cable-stayed bridges and general ones (twin-tower or single-tower cable-stayed bridges), the formulas of cable supporting rigidity coefficient which include tower-beam rigidity are formed and verified. When the formula is derived, the flexibility of girder between middle towers cannot be ignored as the girder in side span. Middle towers are under little constraints transferred by girders. Combining with its resistance to deformation of tower, a parametric study is carried out to analyze the cable support rigidity. Parameters that potentially affect the cable support rigidity are taken into consideration, including tower rigidity, girder rigidity, cable rigidity and angle. Then the conclusion can be used to guide the conceptual design.(3) Based on the design scheme of Qiongzhou Strait Bridge, main span is828m of which is cable-stayed bridge with five-tower, the rigidity problem of large span multi-tower cable stayed bridge is researched. By parameters sensitivity analysis, the rigidity parameters are optimized and an appropriate value range of tower and girder rigidity is presented, as well as cable rigidity and tower-span ratio. After comparative analysis of several stiffening cable design, the static performance optimized design scheme of Qiongzhou Strait Bridge is proposed and mechanical properties checked.(4) The tower-beam rigidity ratio of multi-tower cable-stayed bridge are presented and its mathematical expressions are given. Compared with the rigidity of the cable, rigidity of the tower and rigidity of beam have more obvious influence impact on the on integral rigidity, and their design degrees of freedom are bigger. According to tower-beam rigidity ratio, multi-tower cable-stayed bridges are divided into flexible-tower system and rigid-tower system.(5) A new mechanical structure system of named advanced Morandi structure system (AMSS) is presented, which could solve the problem that integral rigidity of long-span multi-tower flexible-tower system cable-stayed bridge with four or more towers can't be improved by setting stiffening cables. In the AMSS, the vertical bearing is located on a bracket which has a distance from the tower in longitudinal direction in order to meet the requirements of both vertical and rotational constraints between tower and beam. The mechanical behavior of AMSS is approximate to the fixed tower-beam structure system and its performance is that tower and beam are separated. Based on the analysis of its mechanical properties, it can be found that the AMSS has the equivalent function as the fixed tower-beam structure system in improving integral rigidity, and it has the advantage in avoiding large bending moment of the fixed zone between tower and beam. As the bending moment of the end of bracket become larger with the increase of the space between double supporting, the reasonable value of the space between double supporting is studied by the FEM analysis. The rationality of the AMSS and the effectiveness of improving integral rigidity of long-span multi-tower flexible-tower system cable-stayed bridge are verified by the experimental data.(6) Based on dynamic and seismic analysis, the design scheme of Qiongzhou Strait Bridge is further optimized. The rigidity increasing leads to the improving of the structure deformation resistance, but the influence of which to the dynamic characteristics is complex and could not be simply perorated. This paper analyzed the influence of increasing tower rigidity and changing tower beam supporting system to dynamic characteristics of Qiongzhou Strait Bridge design scheme, and the method of obtaining a reasonable rigidity value of multi-tower cable stayed bridge is summarized based on static and dynamic characteristics.(7) The optimizing methods of rigidity of multi-tower cable-stayed bridge by inverse analysis was discussed in this paper. Based on the parameter sensitivity of rigidity, the optimal computation is more targeted by backward deducing the rigidity parameters with a given expected deflection, which in order to avoid repeatedly adjust initial parameters in tentative analysis of finite element analysis. During the parameter optimizing, the FEA model of Qiongzhou Strait Bridge was built in Ansys, and the optimizing model was in Matlab. Through the DOS command, the both programs are successfully connected to transfer from each other, which reduces the workload and enhances the work efficiency... |
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