Experimental Investigations Of The Seismic Behavior Of RC Bridge Piers Under Vertical And Horizontal Seismic Coupling

Some macroscopic seismic phenomenon indicated that vertical earthquake effect was more obvious in intensified seismic area.In 1989,A magnitude 6.9 earthquake occurred in Loma Prieta,northern California,US.Concrete piers of Struve Slough Bridge dislocated with bent cap and penetrated bridge deck.The horizontal ground acceleration peak was 0.39g while the vertical ground acceleration peak went to 0.66g.In 2008,a magnitude-8 earthquake hit Sichuan Province where some bridges collapsed including Baihua bridge,Shunhe bridge and Xiaoyudong bridge etc.Specification of Seismic Design for Highway Engineering stated that there was strong vertical earthquake effect near faults and the bridges were substantially affected by the effects of vertical and horizontal earthquake action.According to the seismic phenomenon and specification,it was necessary to study the seismic behavior of RC bridge piers under vertical and horizontal seismic coupling.The axial force of the pier changed violently under the coupling effects,which affected the seismic performance of the pier.By analyzing the elastoplasticity of the north approach of Jiashao bridge,According to record of the datum of displacement and axial load of piers.The load history applied to test was simplified from the record.The maximum drift was 2.8%and the axial compression ratio ranged from 0.05 to 0.25.The positive displacement increased as the axial compression ratio is enlarged and the negative displacement reduced as the axial compression ratio lowered.The relationship between drift and axial compression ratio was linear.The experiments were accomplished by electrohydraulic servo system which was controlled by my program.The RC piers were subjected to various axial load and horizontal displacement.To study the seismic performance of RC bridge piers under variable axial force,cyclic loading test were carried out on six specimens,of which one was under constant axial load and the other five with different longitudinal reinforcement ratio,volumetric percentage of stirrups,and shear span ratio were subjected to variable axial force.Based on the test result,the hysteretic curve,fracture distribution,skeleton line,displacement ductility factor,residual deformation,and energy dissipation capacity of the tested specimens were studied,It indicated that variable axial force has apparent influence on the displacement,residual deformation,and the level of destruction of RC bridge piers.The maximum axial load determinates ultimate state because the rapid development of vertical cracks caused concrete spalling.The formula for residual displacements had been obtained by the regression analysis on the experimental data.The FEM of plastic damage model and fiber model were used to simulate the RC bridge piers and the results agreed well with the experiments results.According to the performance-based seismic design and the codes of highway bridges,suggestion was proposed to consider varying axial load during design.In Chinese bridge codes,damage degrees were classified into no damage(level 1),slight and local damage(level 2),serious damage or collapse(level 3).When the piers subjected to various axial load were valued,followings suggestions were given.(a)To protect the bridge piers under various axial load from damage(level 1)or slight damage(level 2),the minimum axial load was summed to be constant axial load.(b)To protect the bridge piers under various axial load from serious damage or collapse(level 3),the maximum axial load was summed to be constant axial load.Guidelines for Seismic Design of Highway Bridges say that it is hard to present a formula of permissible displacement for double-column pier,for rotation capacity of plastic hinge is uncertain.Based on theoretrical and experiment analysis,a simplified method was proposed.By a contrast of Push-over analysis and the simplified method,the result indicated that the method was safe and the accuracy was acceptable.