Research On Stiffness And Seismic Resistance Of Bridge Towers Of Thousand-meter Railway Suspension Bridge In High-intensity Mountainous Area

With the development of the economy,more and more railway lines have been built in the southwest region of China.In order to cross the western mountainous areas,the span of railway bridges needs to be increased.Research on long-span railway suspension bridges has gradually increased,and design and construction have also begun to appear.Suspension bridges are flexible structures,but trains have higher requirements for the rigidity of the bridge structure.Therefore,it is necessary to study how to make long-span railway suspension bridges meet the requirements of railway line operation.Taking a railway suspension bridge in the mountainous areas of western China as an example,the effect of different structural design parameters on the stiffness of the bridge is studied.Two methods to improve the stiffness of long-span railway suspension bridges are proposed.The two methods are statically and dynamically calculated,and the influence of different parameters and different pylons on the stiffness is discussed.The main work and conclusions of the thesis are as follows:(1)Based on the summary of national standards on the rigidity limits of railway bridges,this paper proposes the rigidity limit standards applicable to long-span railway suspension bridges based on the operation requirements of Chinese railway lines.(2)Modeling calculation based on BNLAS,to study the influence of different structural parameters on structural stiffness under different load combinations.The results show that reducing the rise-span ratio of the main cable will reduce the vertical rigidity of the structure,while slightly increasing the lateral rigidity,but the effect is not worth the loss compared with the increased cross-section of the main cable.Setting the overhanging span of the stiffening beam will effectively increase part of the vertical and lateral rigidity indexes of the structure,but the overhanging span length is too short will cause the structural support to be unfavorable.Increasing the truss height can comprehensively improve the vertical stiffness index,but it will cause the vertical support to be unfavorable.Increasing the width of the stiffening beam can comprehensively improve the transverse rigidity index of the structure,and the transverse support has little change.(3)Two cable structures,stayed cables and lateral stiffening cables,are set up to analyze whether these two measures can improve the vertical and lateral stiffness of long-span railway suspension bridges.By adjusting the cable setting parameters,the influence of different setting schemes on the efficiency of structural rigidity improvement is analyzed.The results show that the installation of stay cables can increase the vertical rigidity partly equivalent to increasing the height of the stiffening beam,and can reduce the beam end displacement and the beam end moving speed of the stiffening beam.The best effect is when the anchor point of the stay cable on the main cable is located at 1/8 of the mid-span.Increasing the stay cable section will slightly increase the stiffness improvement effect and reduce the stress amplitude of the stay cable during the passage of the train.The provision of transverse cables can bring about an increase in transverse rigidity equivalent to increasing the width of the stiffening beam.When the anchor point of the transverse cable on the stiffening beam is located at 1/4of the mid-span and the angle between the stiffening beam and the stiffening beam is 45°,the effect of increasing the stiffness is the best.(4)Calculate the natural vibration characteristics of the structural schemes of stay cables and transverse cables,as well as the structural schemes of steel bridge towers.Three artificial seismic waves were fitted according to the design response spectrum,and nonlinear time history analysis was performed on the above structural scheme.The results show that the installation of stay cables and transverse cables will improve the overall rigidity of the structure,which is reflected in the increase in the fundamental frequency of the structure and the frequency of the same-mode state.The increase of structural rigidity after the installation of stay cables and transverse cables will reduce the displacement response in the corresponding direction,but the adverse effect on the internal force of the bridge tower is very limited.After replacing the concrete pylons with steel pylons,the longitudinal and lateral dynamic forces of the pylons have been greatly improved.The internal force at the bottom of the tower is reduced by 75% to 90%,and the axial force of the stay cable and lateral cable is also reduced,and the corresponding displacement response is increased.