Seismic Response Reduction Systems For Two Series-connected Long-span Cable-stayed Bridges

The cable-stayed bridges are flexible bridges,of which the seismic design is the research hotspot.With the development of the design theory and structural system of cablestayed bridges,it is necessary to conduct special seismic-resistant study on the novel type of cable-stayed bridges and evaluate their seismic performance.This thesis presents a comprehensive study on the seismic response reduction systems for the first seriesconnected long-span cable-stayed bridge termed Honghe Bridge in China.In the thesis,the dynamic characteristics analysis and nonlinear seismic response analysis are carried out,and fluid viscous dampers and C-shape steel dampers are proposed to mitigate the longitudinal and lateral seismic responses of the bridge,respectively.The major findings of this thesis are as follows:(1)According to the design drawings,the finite element model of the Honghe bridge was established using Midas Civil for analyzing the dynamic characteristics.To consider the pile-soil interaction,total four pile-soil interaction models were used in the analysis,termed direct fixation model,equivalent fixation model,six-spring model and m method model.The numerical results show that the series-connected long-span cable-stayed bridge has the characteristics of long natural period and closely-spaced modes.In addition,it is found that the pile-soil interaction will reduce the overall stiffness of the structure,increase the natural vibration period of the structure,and change the high-order vibration modes.The m method model is recommended to simulate the pile-soil interaction for long-span cablestayed bridges at soft soil sites.(2)A comparative study on the seismic performance of four different longitudinal constraint systems,termed semi-floating system,beam-tower fixed system,elastic restraint system and damping restraint system,is conducted through nonlinear time-history analysis.The results show that the damping restraint system is a reasonable longitudinal structural system for the bridge.The velocity exponents and damping coefficients of the fluid viscous dampers of the damping constraint system are optimized via numerical simulation.Finally,the possibility of collision between the east and west Bridges is analyzed(3)Through nonlinear time history analysis,the seismic performance of the bridge with five different lateral constraint systems is compared,which are lateral fixed system,the lateral sliding system,the C-shaped steel bearing system,the lead rubber bearing(LRB)system and the friction pendulum bearing(FPB)system.The results show that the C-shaped steel bearing system is a reasonable lateral structural system for the bridge.The yield strength of the C-shaped steel damper is optimized via parametric study.