| In order to reduce the huge internal forces during earthquake,long-span suspension bridges generally adopt the longitudinal unconstrained floating system,and additional dampers are used to suppress excessive displacement of the girder ends.However,due to the structural characteristics of the suspension bridge itself,the stiffening girder will have continuous longitudinal reciprocating movement in the daily operation state.Excessive accumulative travel will lead to performance degradation and fatigue damage of the expansion joints and fluid viscous dampers at the girder ends within a few years.It not only increases maintenance costs,but also becomes a major safety hazard in the event of an earthquake.In view of this problem,this thesis analyzes the longitudinal motion characteristics of stiffening girder of a suspension bridge and the causes of performance degradation of dampers in a short period of time.At the same time,a large scale rotary axial eddy current damper with high energy density is developed,which can not only meet the requirements of anti-fatigue and vibration reduction performance under the condition of daily low-speed reciprocating motion,but also has similar damping performance to the fluid viscous damper under the condition of high-speed seismic vibration,so as to achieve the goal of dual vibration control.The prototypes of eddy current damper were installed on Jiangyin Yangtze River Bridge,and the results show that the vibration suppression effect is as expected.The detailed work and main conclusions are as follows:1.Based on the measured time-history data of longitudinal motion of Jiangyin Yangtze River Bridge,the spectral characteristics,peak-to-peak value-cycle number characteristics and cumulative stroke characteristics of stiffening girder longitudinal motion of suspension bridge were obtained.The results show that the longitudinal movement of the stiffening girder can be divided into three parts.The first part is the stretching or shortening caused by the temperature difference between day and night.The second part is the longitudinal reciprocating motion caused by quasi-static factors whose frequency is much lower than that of the first longitudinal mode.The third part is the longitudinal vibration response whose frequency is equal to or higher than the first longitudinal modal frequency,which corresponds to the longitudinal resonance response of the stiffening girder.The quasi-static longitudinal reciprocating motion of stiffening girder accounts for most of the acumulative stroke and it is studied by using the finite element method.The analysis results show that the reason of the quasi-static longitudinal reciprocating motion of stiffening girder is the longitudinal asymmetric vertical load caused by vehicle running.2.Based on the generation mechanism of quasi-static longitudinal reciprocating motion,a method to identify the equivalent vertical load time history based on quasi-static longitudinal displacement time history was proposed.The time history of external load obtained by this method can be used to conveniently evaluate the suppression effect of vibration reduction measures on the quasi-static longitudinal motion,and avoid the difficulty of simulating the actual random vehicles and the time-consuming dynamic calculation of the full bridge response.3.The low-speed damping characteristics of the eddy current damper were studied,and an approximate linear constitutive relation was obtained.A single-degree-of-freedom simplified analytical model and a finite element model were established respectively.The vibration suppression effects of eddy current dampers and other control methods were analyzed and evaluated.The results show that increasing the damping ratio can effectively suppress the longitudinal vibration resonance of the stiffening girder,but the control effect of quasi-static longitudinal reciprocating motion is not ideal,and the control effect of parallel combination of velocity-dependent damper and friction damper is better.4.The nonlinear damping characteristics of eddy current damper at high speed were studied,and its energy dissipation characteristics were analyzed by analytical method.There is a critical velocity point on the velocity-damping force curve of the eddy current damper,which makes the damping force reach the maximum value.When the velocity exceeds the critical velocity,the damping force begins to decrease.The results show that the energy dissipation of the eddy current damper reaches the maximum when the critical velocity is 0.786 times of the maximum velocity,if the maximum damping force is fixed.If the maximum damping force of the eddy current damper is equal to that of the fluid viscous damper,when the velocity exponent of the fluid viscous damper is greater than 0.2,the energy dissipation of the eddy current damper can always be higher than that of the corresponding fluid viscous damper by choosing an appropriate critical velocity.Based on this principle,the displacement control effects of fluid viscous dampers and eddy current dampers on suspension bridges under harmonic and seismic loads were studied.The analysis results show that the eddy current damper has the same or better control effect than the corresponding fluid viscous damper under the action of harmonic and seismic loads.5.The optimal control strategy of the eddy current damper to the peak seismic displacement response and the accumulative stroke under operation condition was studied.When the maximum damping force of the eddy current damper is selected as the maximum allowable value in engineering practice,and the critical velocity is set to a certain value between zero and the maximum velocity in the uncontrolled state,then the peak seismic displacement response is minimized.The Pareto optimal solution set of two parameters,the maximum damping force and the critical velocity,was obtained by using the multi-objective optimization algorithm,considering the dual vibration control problem of cumulative stroke under the operation condition and the peak displacement response subjected to seismic loading.6.A type of large-scale rotary axial eddy current dampers with high energy density was developed.Their working performance and mechanical properties were studied and tested on a damper test platform.The results show that the developed eddy current damper meets the expected performance requirements,and it has low friction damping and good temperature adaptability.The developed eddy current dampers have been installed on Jiangyin Yangtse River Bridge,and their actual working performance has been tested on site.The test results show that the large-scale rotary axial eddy current damper works normally and their performance meets the expectation. |
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