Research On Seismic Behavior Of Reinforced Concrete Bridge Piers Subjected To Long Duration Earthquake Ground Motions

A strong nonlinear response would occur for reinforced concrete bridge piers under strong earthquakes due to plastic deformation and accumulated damage.Reinforced concrete bridge piers with stiffness and strength degradation characteristics are prone to fatigue damage under long-duration earthquakes,resulting in bridges with minor or moderate damage gradually developing into severe damage under subsequent aftershocks.The piers would be unable to serve or repaired.Therefore,it is of great importance to investigate the seismic damage mechanism of reinforced concrete bridge piers under long-duration earthquake ground motions to accurately obtain the seismic performance of the bridge.The main work and conclusions are summarized as follows:(1)Numerical analysis is an important measure to study the seismic behavior of reinforced concrete bridge piers under long-duration earthquake ground motion.Based on the finite element software OpenSees,a numerical analysis model for cummulative damage analysis of reinforced concrete bridge pier is established.Taking the quasi-static test results of the pier specimens conducted by Kunnath et al and shaking table test results of a full-scale pier specimen conducted by the University of California San Diego(UCSD)as examples,the strength and stiffness degradation of the piers as a result of the cumulative damage under different loading condition was simulated.The results show that the fiber section model with the Mander's concrete material model and the Reinforcing Steel material model for longitudinal bars can accurate simulate the seismic damage accumulation of reinforced concrete bridge piers.(2)In order to study the influence of the duration characteristics of the earthquake ground motion on the nonlinear seismic response of bridge piers,long and short duration earthquake ground motions with similar spectrum and amplitude were obtained and used for seismic analysis of the pier models.A significant duration parameter D_s=5-95%was selected as the duration index of the ground motion.Also,the influence of the spectrum and amplitude of the earthquake ground motion is eliminated based on the spectrum matching method.The results show that the long-duration earthquake ground motion will increase the maximum and residual displacement of the piers.The maximum drift ratio of the RC pier under long-duration earthquake ground motions is about twice of that under short-duration ground motions.The drift ratio of the piers under long-duration earthquake is between 4.5-7.2 times of that under short-duration earthquake ground motion.The damage of the pier under long-duration ground motion is greater than under the spectrum-equivalent short-duration ground motion.The duration of the ground motion affects the seismic performance of the bridge pier and increases the probability of the structure collapse.(3)To study the seismic performance of reinforced concrete bridge pier and its pile foundation under PSSI(Pile-Soil-Superstructure Interaction),a numerical analysis model and its comparison model for the pile-pier considering the PSSI was built.The influence of the sand soil foundation on the nonlinear response of the piers was analyzed,and the mechanism of the residual deformation at the top of the piers was discussed.The analysis results show that the maximum drift ratio of the pier models considering the PSSI under the near-fault ground motion with the PGA of 0.4 g is 1.6-1.9 times of that of the pier models without PSSI.The residual drift ratio of the pier models with PSSI is about 1.1-2.0 times of that of the models without PSSI.The peak displacement and residual displacement at the top of the pier are significantly increased when the pile-soil interaction is considered,that is,the nonlinear response between the pile and soil is an important reason for the bridge pier deformation.The residual displacement of the piers and the pile are inversely proportional to the relative density of the sand foundation.The increase of the relative density of the sand foundation will enhance the soil restraint effect to the pile,and the curvature ductility of the pile will be increased.