Study On Quasi-Static Loading Protocols Considering The Action Characteristics Of Long-Period Ground Motions

Due to abundant low-frequency components of long-period ground motions(LPGMs),long-period structures are susceptible to suffer severe damage,and the corresponding displacement response time-history of long-period structures under LPGMs have significant “large-displacement and long-duration” characteristics.This action characteristics essentially reflect the different loading paths of LPGMs from original ground motions(OGMs)imposed on structures.Hence,the key to study the influence of LPGMs on the long-period structures is to reveal the influence mechanism of the LPGMs action characteristics on the seismic performance of structural components.Based on the engineering background of RC piers in long-span bridges and weak columns in RC frame structures,this paper puts forward the idea of reflecting the large displacement and long-time action characteristics of LPGMs on the structure to quasi-static loading protocols,and aim at develop it.The selection of ground motions,the determination of structural analysis model,the statistics and analysis of cyclic amplitude-cyclic number,the construction and application of quasi-static loading protocols were studied.The main contents are as follows.(1)The selection of the LPGMs and the determination of the structural analysis model.50 representative LPGMs from Chi-Chi earthquake,Wen Chuan earthquake,East Japan earthquake and Tokachi earthquake were selected as input for the nonlinear time-history analyses.15 representative OGMs were also selected for the comparative analyses.Then the RC pier columns in long-span bridges were simplified as SDOF systems,and the high-rise frame structure was simplified as MDOF system.The Bouc-Wen-Baber-Noori(B-W-B-N)model,which considers strength and stiffness deterioration and the pinching effect comprehensively,was adopted to simulate the hysteretic behavior of structural components.The numerical analysis method is a fourth-order Runge-kutta method that meets the requirements of both accuracy,computational efficiency and matching the restorative force model.The solver program is developed by using MATLAB,which provides the ground motion library,analysis model and numerical analysis foundation for the following analysis and research.(2)The quasi-static loading protocols considering the characteristics of LPGMs is established.The elastic-plastic displacement response time history of SDOF and MDOF systems with different parameters satisfying the target ductility under ground motions were calculated.Then based on the improved four-peak-valley rainflow counting algorithm,the cycle amplitude and cycle number of each time-history displacement response are extracted.In addition,the influences of the types of ground motions,the structural natural period,the target ductility,the damping ratio and the strength reduction coefficient on the displacement amplitude and the number of cycles on the total inelastic cycle number and the maximum cumulative displacement ductility were compared and analyzed.The structural natural period and the target ductility have the most significant influence on the cycle number and displacement amplitude distribution of loading protocols under a certain type ground motion.Therefore,according to the regression statistical analysis results of cycle numbers and displacement amplitudes of SDOF and MDOF systems with different periods and target ductility,a double exponential regression prediction model of loading protocols considering the action characteristics of LPGMs was proposed.Based on the proposed double exponential regression prediction model,the quasi-static loading protocols considering the action characteristics of LPGMs were developed.The accuracy and reliability of the proposed double exponential regression prediction model was validated by cycle number,displacement amplitude and cumulative displacement ductility.(3)Based on the completed quasi-static test in Tsinghua university,Open Sees is used for numerical simulation.The reliability of the model is verified by comparing with the test results.Then the influence of the displacement amplitude distribution and the number of cycles on the seismic performance of RC columns is systematically analyzed through the dynamic cyclic test of different loading paths.The seismic performance of RC columns under the traditional loading protocol and the quasi-static test protocol with LPGMs is compared.Finally,by designing RC columns with different axial force ratio,shear-span ratio,strength grade of concrete,reinforcement ratio and stirrup ratio,applying the loading protocol constructed in this paper and the prototype test loading protocol,the impact of loading path on the seismic performance of RC columns with different design parameters was preliminarily analyzed.The results show that the displacement amplitude mainly affects the degradation initial position and degradation speed of RC column,the cycle number reflect the cumulative damage effect,and the pinching effect is mainly related to the energy consumption.The LPGMs has different effects on the RC columns under various parameters.The overall performance is that the degradation degree and path of strength and stiffness is significantly different from the traditional loading protocols.Moreover,due to the accumulation of damage,the steel bars will break prematurely in the later stage,and the hysteresis loop pinching effect will be advanced The final hysteretic energy consumption is about 1.8 times that of the traditional loading protocols.Developed quasi-static loading protocols can be directly applied to the experimental study on the seismic performance of RC piers in long-span bridges and weak columns in RC frame structures under LPGMs,and provide a new method for studying the influence of LPGMs on the seismic performance of structural components.