Static Analysis And Distortional Study On Double Decked & Partially Cable-Stayed Continuous Box-Girder Bridge

Recently, with the advancement of modernized construction method and analysis theory, the box-section structure has developed from thick-wall and narrow-box to thin-wall, wide-box and few diaphragm, even non-diaphragm. And further, the distortion effects of box-girder become more and more predominant. In this thesis a new double decked, separating up and down vehicles, non-diaphragm and partially cable-stayed bridge structure is presented. The research on this type of bridge is rarely dealt with so far. Therefore, it is very important to carry out the static analysis and study of distortion effects for the structure, because of widely applied prospects.Firstly, a practical design of double decked continuous box-girder bridge has been done and then the large and universal finite element software ANSYS has been used to build a spatial model for the structural analysis. As results of analysis, the nodal stress, deflection and the change of cable force under dead load, prestressing force and live load on main sections are gained, and come to the conclusion that: (a) The stress distribution rule of the top slab or the bottom plate under the dead load is similar to that under the prestressing force. The prestress can improve the distribution of internal force of the girder remarkably. When the theoretical mode of lane loading in the new Norm is adopted to calculate the spatial bridge structure, the standard value of concentrated load in lane loading will cause the stress concentration, especially obvious for wide-box and thin-wall box-section structure. The transverse distribution of stress is uneven extraordinarily. In view of the difference from the old Norm, designers should pay much attention to it. (b) The stay cable can reduce the maximum sagging and negative moment effectively, cut the peak value of stress and improve the internal force distribution of whole bridge structure. Its function is equivalent to an elastic support. The stay cable make the vertical rigidity of girder strengthen markedly. The numerical results and model test both indicate that it is potential to increase the bridge span further.(c) The cable force caused by live load accounts for relatively little proportion of the total cable force. That is to say, the stress amplitude of cable is relatively small. The fatigue phenomenon of it is not outstanding, hence the allowable stress can take 0.6σu, which will increase the utilization ratio of stay cable.In addition, some of the main design parameters such as the tower height and the cable force which influence on the whole structure are still discussed. The results indicate that, with the decrease of the tower height, the vertical rigidity of girder will fall and the deflection will increase. The change of initial tensioning cable force is a main influencing factor on the deflection and the distribution of internal force of the girder. Its influence effect is more obvious than that of the change of tower height. So the purpose of reducing the tower height can be achieved by an adjustment of the initial tensioning cable force. In order to verify the reliability of theoretical analysis, the scaled plexiglass model test for the designed bridge structure form has been carried out. The values of stress, strain, deflection and the change of cable force in the bridge under different work conditions have been determined through the test. On these grounds, the finite element model has been checked up and perfected. Furthermore, a new amending method was put forward to the measurement of the distortion angle. By means of further analysis, the static distribution characteristic and distortion effects of every main test section of this bridge structure have been obtained.