Study For Adaptive Variable Curvature And Friction Pendulum Bearing Isolation System

The traditional friction pendulum bearing(FPB)has insufficient displacement capability and a constant isolation period,which does not meet the seismic performance requirement under a wide range of earthquakes.The multi-spherical friction pendulum bearings were developed to improve the traditional FPB.However,the configuration of the multi-spherical friction pendulum bearings is typically complex,which complicates the design and construction and reduces the controllability of the isolation system.The variable friction pendulum bearing(VFPB)and the variable curvature and friction-friction pendulum bearing(VCF-FPB)are proposed to upgrade traditional FPB without a complex configuration.Therefore,there is the more practical significance for the research and application of VFPB and VCF-FPB.A series of laboratory experiments have been conducted on polytetrafluoroethylene(PTFE),modified polytetrafluoroethylene(M-PTFE),modified ultrahigh molecular weight polyethylene(M-UHMWPE)and modified polyformaldehyde(M-POM)-stainless steel interfaces to determine the effect of sliding velocity,bearing pressure,type of test,the surface finish of stainless steel,temperature on the friction coefficient.The friction coefficients of PTFE,M-PTFE,M-UHMWPE increase rapidly with sliding velocity,up to a certain value of velocity beyond which it remains almost constant.The friction coefficient reduces with increasing pressure.The friction coefficient of the M-POM-stainless steel interface remains almost constant with increasing sliding velocity and decreases with increasing pressure.Compared with the other three types of friction material,sliding velocity and bearing pressure have a smaller effect on the friction coefficient of M-POM.The surface finish of stainless steel has a greater effect on the friction coefficient of M-UHMWPE than M-POM.The effect of temperature on the friction coefficient of M-UHMWPE and M-POM is obvious.The friction coefficient decreases with increasing temperature.After several times of cyclic displacements,four types of friction materials appear adhesive wear.PTFE wear is the most serious,followed by M-PTFE,then the M-UHMWPE,and the wear degree of M-POM is minimum.The VFPB is proposed in this paper,the configuration of the VFPB is similar to the traditional FPB,the sliding surface of the VFPB arranges a series of concentric rings.The mechanical characteristic of the VFPB is investigated by theoretical analyses and experimental tests.The VFPB provides stiffness and damping that can be changed with increasing displacement amplitude as desired by properly designing its sliding bands.The number,width,and friction coefficient should be considered in arranging sliding bands.The effective stiffness of the VFPB increases with the maximum friction coefficient,while the effective damping ratio states decrease.Moreover,the effective stiffness of the VFPB decreases as the number of bands increases,while the effective damping ratio states increase.The test hysteretic loops under different vertical loads like dog bone shape,small in the middle,big at both ends,which conforms to the shape of the theoretical hysteretic loops.The hysteretic loop becomes fatter as the vertical load increases for the same peak velocity,which implies increasing energy dissipation capacity.The friction coefficient of the M-POM-stainless steel interface is obtained by parameter identification based on hysteretic curves from tests,which basically remains constant with sliding velocity and decreases with the vertical load.The friction coefficient increases from the inner band to the outermost band,which matches as expected.The friction coefficient of the inner band varies between 0.061 to 0.096 under different vertical loads.The friction coefficient of the middle band varies between 0.075 to 0.108 under different vertical loads.The friction coefficient of the outermost band varies between 0.098 to 0.132 under different vertical loads.An analytic model is proposed for the VFPB,whose accuracy is verified through comparison with experimental data.The tests with sinusoidal motion,triangular motion and actual ground motion,respectively,are conducted to verify the analytic model.The effective stiffness and effective damping ratio are calculated based on the hysteretic curves from the analytic model,experimental test and theoretical analysis.The effective stiffness and effective damping ratio obtained from the three methods agree within 5% and 8%,respectively,under sinusoidal motion displacement-controlled loading with different sliding velocities.The effective stiffness and effective damping ratio obtained from the three methods agree within 8%and 7%,respectively,under triangular motion displacement-controlled loading with different sliding velocities.The full-scale VFPB was subjected to a displacement history corresponding to a realistic ground motion.The proposed analytic model accurately reproduces the experimental result.The two hysteretic curves show that the differences in terms of the maximum displacement and maximum force between the two methods are within 3% and 9%,respectively.To verify the basic nonlinear accuracy of the dynamic response-history analysis procedures,a nonlinear single-degree-of-freedom system was analyzed.The results obtained by using the analytic model and solving dynamic equations agree within 8%.In general,the analytic model can accurately reproduce the real behavior of VFPB.Combined with the concept of variable friction and variable curvature,a novel VFC-FPB is proposed.The force-displacement relationship,residual deformation,shear behavior parameter,and variation rules of shear behavior are derived by theoretical analysis.The finite element simulation is conducted to verify the results from theoretical analysis.The agreement between the results obtained from theoretical analysis and finite element simulation validates the accuracy of the force-displacement relationship,effective stiffness,effective damping ratio,and energy dissipation capacity of VCF-FPB from theoretical analysis.VCF-FPB has two different post-yield stiffness.The width of the sliding band Ⅰ,d1,affects the transition point from first post-yield stiffness to second post-yield stiffness.The second post-yield stiffness decreases as the radius of the sliding band Ⅱ,R2,increases.The effective stiffness and energy dissipation capacity linearly vary with the friction coefficient of the sliding band Ⅱ,μ2.The effective damping ratio varies as an inverse proportional function with μ2.R2 does not affect the energy dissipation capacity of VCF-FPB.Compared with traditional FPB,the effective stiffness of the VCF-FPB increases by 15%~160%.The increment of effective stiffness increases as μ2increases,and decreases as R2 increases.Compared with traditional FPB,the energy dissipation capacity of VCF-FPB increases by 1%~230%.The increment of energy dissipation capacity increases as μ2,and remains constant with R2.Just for the particular combination of R2 and μ2,the effective damping ratio of VCF-FPB is 0%~80% greater than that of FPB.The increment of the effective damping ratio increases as R2 and μ2 increase.Generally,compared with VFPB,the effective stiffness of VCF-FPB increases,the maximum increment is 82.3%.The increment of effective stiffness decreases as R2 and μ2 increase.Compared with VFPB,the effective damping ratio decreases.The decrement of the effective damping ratio decreases as R2 and μ2increase.The VCF-FPB with μ2>0.13 exhibits greater effective stiffness and effective damping ratio.The analysis model and dynamic equation of the VCF-FPB isolation system are developed.The nonlinear time-history analysis is conducted based on the Matlab platform.The effect of the VCF-FPB isolator parameter on the seismic response of the VCF-FPB isolation system is studied.The displacement response of the VCF-FPB isolation system increases with R2,and decreases with μ2.Compared with the FPB isolation system,the displacement response of the VCF-FPB isolation system decreases.The decrement of displacement response increases as R2 decreases and μ2 increases.The acceleration response of VCF-FPB decreases as R2 increases,and increases as μ2 increases.Compared with the traditional FPB isolation system,the acceleration response of the VCF-FPB isolation system increases.The increment of acceleration response increases as R2 and μ2 increase.The increment of acceleration response is smaller than the decrement of displacement response.Displacement and acceleration response are the most sensitive to d1,and the least sensitive to μ2.A design method of isolated building using friction pendulum system based on the Standard for seismic isolation design of the building is proposed.Taking into account the difference in the damping characteristics of the isolation layer and superstructure when calculating the seismic force of the isolated structure,this method leads to a more reasonable design.Nonlinear time-history analyses are performed for the reinforced concrete frame and modular steel frame isolated structures with FPBs or VFPBs to study the seismic performance of the base-isolated building.For the reinforced concrete frame isolated structures,the FPB and VCF-FPB are effective to improve the seismic performance of the structure.Compared with isolated structure with FPB,isolated structure with VFPB experiences a lower isolation layer displacement and exhibits smaller seismic responses.Compared with FPB,using VFPB can reduce the damaged degree of the superstructure by approximately 10%,for the rare earthquake scenario,and by approximately 15%,for the very rare earthquake scenario.For the near-fault earthquakes,the ground motion intensity indices have considerable effects on the seismic responses of the buildings isolated using VFPBs or FPBs.The velocity-related indices have the greatest impact on the roof displacement and isolator deformation,whereas the roof acceleration is mostly influenced by the acceleration-related indices.Displacement-related indices have the second-greatest impact on roof displacement and isolator deformation.The effect of the ground motion intensity indices on seismic responses of the isolated structure using FPB is more remarkable than using VFPB.Under near-fault earthquakes,the proposed VFPB can simultaneously reduce the isolator displacement by 30% and the responses of the superstructure by 5% compared with the conventional FPB.For the modular steel frame isolated structures,employing VFPB and FPB,both reduce the seismic response of the superstructure.Compared with FPB,using VFPB can reduce the isolation layer displacement by approximately20%.The seismic responses of the superstructure of isolated structures with VFPB increase slightly,however,the isolator displacement decreases significantly.The proposed VFPB can achieve more remarkable performance as the intensity of ground motion increases,which exhibits adaptive characteristics.