| Long-span cable-stayed bridges are now widely used in practical projects as flexible system structures with long self-oscillation period and low damping,and the issues related to seismic protection are different from those of conventional bridges,and it is a research priority to ensure their seismic safety if they are located in high intensity seismic zones.At present,the seismic design of cable-stayed bridges usually requires the use of a full-bridge finite element model at the preliminary design stage,and repeated nonlinear calculations are inevitable to obtain the appropriate damper parameters.Therefore,it is necessary to design the damper parameters simply and quickly at the preliminary design stage and have a preliminary understanding of the corresponding seismic response values of the structure,so that the designer can make a quick preliminary judgment on the bridge scheme and even make adjustments to the scheme based on the simplified calculation results to improve the design efficiency.In this thesis,based on a cable-stayed bridge with a main span of 350 m,a longitudinal double-degree-of-freedom simplified model is established,focusing on the methods of determining the parameters required for the simplified model,the simplified calculation methods and influencing factors of the damper parameters and longitudinal seismic response under simple harmonic loads and arbitrary nonperiodic earthquakes,as well as the analysis and evaluation of the overall seismic performance of the cable-stayed bridge based on the fragility theory.In addition,based on the simplified model and the seismic susceptibility analysis method,the optimized design of the viscous damper parameters was further investigated.The following studies were mainly conducted:(1)The nonlinear models of long-span cable-stayed bridges were established using Midas Civil and Open SEES finite element software,and the results of structural dynamic properties and nonlinear seismic response of both were compared,which showed that the structural self-oscillation frequencies and longitudinal seismic response were very similar.The first-order longitudinal drift period of the bridge is5.819 s.(2)Based on the double-mass model,a simplified model of longitudinal double degrees of freedom for a large span cable-stayed bridge is established,and the influence of vertical bending of the main girder is further considered to achieve a reasonable calculation of the relevant parameters of the simplified model.The energy method is used to propose the calculation method of horizontal thrust stiffness of the main girder relative to the tower,and the relevant analytical method is used to propose the principle of determining the equivalent height and the simplified algorithm of horizontal thrust stiffness of the tower,then the required parameters of the simplified model are obtained.The calculation error of the first-order longitudinal drift period based on this simplified model is only 2.03%.(3)Based on the longitudinal double-degree-of-freedom simplified model of the long-span cable-stayed bridge,a simplified calculation method of the damper parameters and the longitudinal response of the structure under the simple harmonic load is derived.Under a simple harmonic load with a peak acceleration of 0.17 g,the calculation errors of the girder displacement and the bending moment at the bottom of the tower can be controlled within 30% when the equivalent damping ratio of the structure does not exceed 0.50.The equivalent damping ratio and load frequency are the key influencing factors for the seismic response of the structure.(4)Based on the fast Fourier transform,a simplified calculation method of the damper parameters and longitudinal seismic response under arbitrary nonperiodic seismic action is proposed,and thus a design method of the damper parameters with the objective of controlling the seismic response is proposed.Under the action of nonperiodic earthquake with peak acceleration of about 0.28 g,the calculation error of the girder displacement,tower mass displacement,the bending moment at the bottom of the tower and damping force can be basically controlled within 30% when the equivalent damping ratio of the structure does not exceed 0.90,which can significantly reduce the requirement of dampers while meeting the requirements of structural seismic design.In addition,the optimal damping parameter is the one that minimizes the bending moment at the bottom of the tower,and the optimal damping coefficients for different velocity indices are obtained based on the simplified algorithm of nonlinear viscous damping parameters of the simplified model.(5)Based on the vulnerability theory,the Open SEES finite element model is used to analyze and evaluate the cable-stayed bridge,focusing on the damage probability of each key member of the cable-stayed bridge under the longitudinal seismic action,thereby evaluating the overall seismic performance of the bridge.The final results show that setting viscous dampers at the towers can significantly reduce the damage probability of the towers and spherical steel bearings,while have little impact on the damage probability of the cables and piers;the cable-stayed bridges in this thesis have good seismic performance,and the elements are not easily damaged under E2 earthquakes,and the towers will probably always be in elastic working condition.(6)The seismic susceptibility analysis of bridge samples with different damper parameters was conducted to further explore the optimal design of the damper parameters.The method of designing the damper parameters with the objective of controlling the bridge failure probability while ensuring the design parameters of each member remain unchanged is proposed;and the method of designing the member parameters with the objective of controlling the bridge failure probability by changing the cross-sectional size and specification of each member while adopting the optimal parameters of the damper. |
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