Research On Seismic Performance Of Precast Concrete Column Base Joint With Replaceable Energy Dissipation Connector

Concrete column base joints are usually subjected to a combination of large axial force and bending moment under earthquakes,which are prone to bend failure and the formation of plastic hinges.In this case,there are severe failure phenomena such as concrete spalling and crushing,longitudinal reinforcement buckling under pressure and stirrup fracture.How to reduce the damage to the bottom of the column under seismic action or to focus the damage on some local areas which are easy to repair is a common concern among researchers.In this dissertation,a type of precast concrete column base joint with replaceable energy dissipation connector(REDC-PCCB)is proposed on the basis of the summary of vertical compressive properties,shear force transfer mechanism,the rotation and self-centering effect of the bottom of the column and the connection construction and energy dissipation capability of energy dissipation connectors.It is used at the bottom of rectangular rocking columns of precast concrete structures.The column base is designed as a concrete-filled steel tube structure to form a reinforced part,which is able to bear vertical pressure under earthquakes without obvious damage.The REDCs are installed on the sides of the column base to serve as a high-ductility energy dissipation connector to replace longitudinal reinforcements in the original position,which could participate in the transmission of bending moment at the bottom of the column under normal working conditions.Once a rarely met earthquake happens,the REDCs yield first and dissipate the inputted seismic energy by their axial plastic deformation.Raised steel limit blocks are installed on the sides of the column base to serve as shear connectors,which could limit horizontal slip of the column base effectively.In this dissertation,the theoretical analysis,experimental study and finite element simulation are conducted on the seismic performance of the REDC-PCCB joint.The main works are listed as follow:Firstly,the structural composition,force transmission mechanism,construction technology and advantages of the proposed REDC-PCCB joint are introduced in detail.On the basis of the summary of the existing research results of our research group,the reasonable design requirements of the REDCs are refined.Then the rotational stiffness of the column base joint provided by the REDCs in different phases under horizontal loads and the relationship between the bending moment and rotation at the bottom of the column at each feature point are derived according to the geometric deformation relationship of the REDC-PCCB joint.The shear resistance requirement of the bottom surface is analyzed too.In order to verify the vertical bearing effect of the reinforced part of the column base,several local eccentric bearing tests of the specimens under the specified rocking angle,which could simulate the rocking base of the column,were conducted by a compression testing machine.The results prove that the specimens could meet the requirement of no damage to the bottom of the column under the normal designed axial compression ratio,since only some local areas of the compression side yield while other areas always remain elastic.The ultimate compressive bearing capacities of the specimens are also explored.A refined entity analysis model is established by ABAQUS to compare the numerical results with the test results,further exploring the damage form,strain distribution,distribution laws of vertical pressure and the depth of compression zone of the specimens.Parametric analyses are conducted on factors such as the rocking ratio,the aspect ratio of the specimen and the width-thickness ratio of the steel sleeve on the non-pressure side.On the basis of preliminary tests and analyses,a full-scale REDC-PCCB joint specimen was designed,and then quasi-static tests under five working conditions were carried out to study the effects of relative factors,such as the post-earthquake rehabilitation of the REDCs,the axial compression ratio of the column and the thickness of the REDC core members,on the stress and deformation performance of the specimen.The test results show that even under high axial compression ratio and large drift ratio of the specimen,the concrete column could basically remain elastic.The steel sleeve at the bottom of the column bears most of the vertical pressure without obvious damage,and the local yielding area is only limited to the edge of the bottom compression side.The damage is concentrated in the middle section of the REDC core members on both sides of the column base,and the ductile fatigue fracture occurs after dozens of cycles under a large drift ratio amplitude,thus the good deformability and low cycle fatigue performance of Q235 B steel could be exerted effectively.The shear connectors installed on the sides of the column base effectively limit the tendency of in-plane and out-of-plane slip of the column.The tests obtain full,stable and symmetrical hysteretic curves and bilinear skeleton curves with obvious yield points and no negative stiffness.The hysteretic curves of the specimens before and after rehabilitation basically overlap,and the bearing capacity and stiffness are similar,which could verify the good ductility,energy dissipation capacity and post-earthquake repairable property of the REDC-PCCB joint.A refined entity analysis model of REDC-PCCB joint specimen is established by ABAQUS to conduct monotonic and cyclic loading calculation.The damage form,hysteretic properties,stress and strain distribution and the deformation ability of the REDCs are obtained and compared with the experimental data,which could validate the reliability of the test results and the accuracy of the finite element model.