Passive Negative Stiffness Damper And Its Control Performance For Vibration Of Stay Cables

With developments of economy and technology,the volume of civil engineering structures gets increasingly large.Costing high,such super large volume civil engineering structures have great economic and social significance.However,these super large civil engineering structures have longer period and lower inherent damping.Under effects of earthquakes,strong winds and other natural disasters,super large volume civil engineering structures are prone to vibrate dramatically,even damage to result in enormous casualties and economic loss.Therefore,it is of crucial importance to study the theories,technologies and devices to control dynamic responses of super large volume civil engineering structures and to improve their ability of disaster prevention and reduction.Conventional design concepts and methods mainly increase structural strength and stiffness,without supplying enough damping.Structural control,which dissipates or absorbs vibration energy by additional control device,can effectively enhance damping level and suppress structural vibrations.Damping force of control devices basically determines its effectiveness.Focusing on low damping of civil engineering structures,this thesis studies passive negative stiffness dampers with different mechanism,control performance and optimal design method,including the following:(1)By connecting with dissipation elements in parallel,passive negative stiffness dampers with different damping force are proposed and modeled theoretically.Based on compressed springs,one passive negative stiffness damper is proposed.The model of damping force is derived theoretically and its characteristics are analyzed.Viscous passive negative stiffness dampers and self-centering negative stiffness dampers are developed,respectively.Mechanical models are established and verified by experiment tests.Effects of negative stiffness force on the ability of self-centering and dissipation are also discussed.(2)Based on parallel and serial mechanism,tunable passive negative stiffness dampers are studied.The linear negative stiffness element is introduced and two basic configurations are proposed,which could tune stiffness only,and tune both stiffness and damping simultaneously,respectively.Equivalent models are deduced and effects of different parameters are studied.Numerical simulations are carried out to reveal the tuning mechanism,including the characteristics of displacement responses and hysteretic loops.(3)Friction tunable passive negative stiffness dampers are investigated.Owing to the serial connection of linear positive stiffness element and linear negative stiffness element,internal displacement is introduced to achieve the tuning of damping force.Experiment tests are performed to investigate the damping force characteristics and tuning mechanism.(4)Numerical simulations and model experiments are conducted to evaluate the control performance of viscous passive negative stiffness dampers for a classical super long member-stay cable.Considering the simplified model of single-mode vibration of a cable,the cable frequency is investigated theoretically by average method and the lower limit of compressed springs is proposed.Numerical examples of cable under sinusoidal excitations and winds are carried out,showing the control performance in both of amplitudes in time history and peak values of frequency response.Single-mode and multi-modes cable vibration control experiments are performed,which verifies the additional modal damping ratio achieved by viscous passive negative stiffness dampers.(5)Optimal matching of the linear negative stiffness and linear viscous damping is studied for suppressing vibrations of stay cables.Considering the case of single model,additional modal damping ratio is derived and the optimal matching of negative stiffness and damping is analyzed.Increasing the degrees of freedoms,the discrete model of stay cable with infinity degrees is established and verified.Additional modal damping ratios are deduced and the optimal matching of negative stiffness and damping is given for different modes.The control performance and mechanism of negative stiffness force are analyzed from the perspective of energy.