Experimental Investigation On Self-reset Performance And Seismic Performance Of Damaged By Earthquake Concrete Columns

Most of the damaged concret columns after earthquake did not achieve complete damage with a certain bearing capacity.Especially,the unbonded prestressed concrete frame column / pier column has a certain self-resetting capability,the smaller residual deformation and the higher repair value.If the plastic hing area which has been caused after earthquake can be repaired and the enlarged cross section with both new and old materials can work together sffectively,the bearing capability and deformation capability of the strengthened seismic damage columns can be improved,with meeting the seismic design saftety requirements and reducing the economic losses effectively after earthquake.In this paper,the unbonded prestressed concrete rectangular section columns subjected to cyclic loading were used as seismic damage test columns which were strengthened by the method of increasing the positive and negative bending moment sides.And the pseudo-static test was carried out to study their seismic performance and to put forward some suggestions for seismic design.In this paper,according to different design parameters,including two types of shear span ratio(3.8and 3.0),four types of design value of axial load ratio(0.2,0.3,0.4 and 0.5),three types of volume stirrup ratio(0.6%,0.95% and 1.3%),a class of unboned prestressing force level(0.7fptk),24 sismic damage unbonded prestressed concrete test columns strengthened with enlarged section method were designed and finished the pseudo-static loading tests.The whole testing performance of concrete test columns was recorded: flexural cracking,tensile yield of longitudinal reinforcements,vertical compressive cracking,concret cover spalling,buckling of original longitudinal reinforcements and failure.Seismic performance of specimens were investigated in hysteretic curves,envelope curves,displacement ductility,resistance decrease,stiffness degradation,energy dissipation and equivalent viscous damping,and the effects of test parameters on seismic performance were also analyzed.Meanwhile,to consider the possible effects of hysteresis cycles on seismic performance of specimens,eight loading cycles were applied at 2% drift ratio,3% drift ratio and 4% drift ratio,other force control conditions were applying one loading cycle and displacement control conditions were applying three loading cycles.The experiments result shows that the influence of cyclic loading cycles on the resistance decrease and stiffness degradation of test columns is significant.The resistance attenuation rate and stiffness degradation amplitude of the second cycle are the maximum,and they are both gradually decreasing from the third to eighth cycle.When the loading control displacement is small,the stiffness of each cycle is obviously degraded;when the loading control displacement is large,the stiffness of each cycle is degraded slowly.In order to quantify the residual deformation of the test columns,the residual deformation rate and the residual deformation ratio are defined.The numerical values of cracking,yielding,peak capacity,2% drift ratio,3% drift ratio,3.5% drift ratio,4% drift ratio and damage states are obtained.The influence of three design paramters and the number of loading cycles on the residual deformation of test columns is analyzed and summarized,and compare the differences between the strengthened test columns and original test columns.Meanwhile,the 2% drift ratio is as an elastoplastic limit state,and the residual deformation ratio fitting formula is put forward for two kinds of shear span ratio test columns.Finally,by defining general provisions,materials,section selection and limit drift,some design recommendations are proposed for the design of unbonded prestressed concret columns strengthened by enlarging the cross-section.