Evaluation Procedures On Post-earthquake Residual Drift And Seismic Capacity For RC Columns

After strong ground motions,the residual drift to some extent is structural important damage observed in reinforced concrete(RC)structures.Post-earthquake residual drift typically demonstrates concrete crushing and reinforcements buckling after structures entered non-linear stage,which has advantages over convenient viewing and quick measurement etc.Residual drift can also be applied to quantatively analyze the residual seismic capacity of damaged structures,which provides evidence to evaluate structural capacity to resist aftershocks or other loads and determine relevent strengthening strategies.Thus,this investigation focuses on RC moment-resisting frame which is widely used and has large amount in China,and concentrates on its post-earthquake residual drift and residual seismic capacity.The topics of this research includes RC column,post-tensioned column and self-centering RC frame with slotted columns.For RC column,using energy dissipation capacity of damaged member as the main index to decide its residual seismic capacity,the relationship of residual seismic capacity and redisual drift is investigated.The damage states of RC columns are classified based on residual drift and residual seismic capacity,then quantitative analysis method for post-earthquake residual drift is proposed.For post-tensioned column which arranged unbonded prestressing tendons along the centroid of column cross-section,the effects of the tendons on residual drift and residual seismic capacity are analyzed.Evaluation index for damage classification using residual drift is determined.For self-centering RC frame with slotted columns,hysteretic performance is investigated,and structural residual drift and damage evaluation are also analyzed.To simulate the column in ground floor of existing RC moment-resisting frames,24 RC column speimens are designed and fabricated.These specimens applied aspect ratio,axial load ratio and volumetric ratio of stirrups in critical zone as the main analytical parameters,and tested under cyclic lateral loads.By analyzing the development of cross-section curvature in critical zone,the equivalent length of plastic hinge zone is determined when the specimens reach 2.0% drift.The drift at specimen yielding is calculated which composes of three parts,flexure drift,shear drift and the drift caused by the slip of longitudinal reinforcements at column end.And the ultimate drift at specimen failure is calculated wh en the longitudinal reinforcements at column bottom end evidently buckled.Thus,the drift capacity of RC column is evaluated based on equivalent length of plastic hinge zone and ductility coefficient,which is the ratio of ultimate drift to yielding drift.The drift capacity of RC column specimen also lays foundation for the proposal of calculation model for residual drift.Based on specimen hysteretic curves which obtained in cyclic lateral loading experiments,the lateral drift and corresponding residual drift at each loading level of each specimen can be achieved.Then normalized these values by the lateral drift and residual drift at specimen failure,respectively.The residual drift at spcimen failure is estimated by discovering the unloading rules fro m ultimate lateral drift.Then the relationship of relative drift and relative residual drift is built that quantitative analysis method for residual drift is proposed.The residual seismic capacity of damaged RC column is described by its energy disspatio n capacity,and the residual seismic capacities corresponded to any relative residual drift are calculated for each RC column specimen.The relationship between relative residual drift and residual seismic capacity is built,that relative residual drift is proposed as the evaluation index for the assessment of residual seismic capacity of damaged RC column.The damage states of RC column are classified by relative residual drift,and the evaluation index in terms of relative residual drift considering structural repairability after damaged is also advised.Another 24 post-tensioned column specimens with unbonded prestressing tendons arranged along the centroid of its cross-section are selected by contrast,which share the same dimensions and same design paramaters with the RC columns.The two types of column specimens are compared in failure modes,drift capacity and energy dissipation capacity in cyclic lateral loading experiments.With unbonded prestressing tendons,the post-tensioned column specimens can significantly postpone the development of structural damage,but severe damage can not be avoided.The drift capacity and energy dissipation capacity show the similar results with the RC columns.After analyzing the residual drift,post-tensioned columns can effectively control the magnitude of residual drift,and the maximum control degree of residual drift can reach 38%.When specimen failed,the average residual seismic capacity of post-tensioned column specimens improved by 10%.The evaluation index in terms of relative residual drift considering structural repairability is increased.To achieve the seismic design purpose in controlling residual drift,diminishing structural damage and quickly restoring structural original function,a new type of RC frame which called self-centering RC frame with slotted column is proposed.Three specimens are fabricated and tested subjected to cyclic lateral loads.According to experimental observations,damage of the column is controlled at the slotted end and no plastic hinge appeared.After some energy dissipating longitudinal reinforcements in the slot fractured,structural energy dissipation capacity shows no weakness,but stiffness degradation aggravates.To predict structural residual drift,the quadra-linear skeleton curve model is proposed and unloading stiffnesses in different periods are investigated.The development rule of residual drift is analyzed,and the maximum control degree of residual drift can reach 58%.The evaluation index to assess residual seismic capacity of the self-centering RC frame with slotted column in terms of relative residual drift is also provided.By compared with RC columns,the results reveal that range of drift that severe damage happened is only accounts for 23% of the whole loading period.The residual seismic capacity at specimen failure is improved by 21%.Comparing with RC columns and post-tensioned columns,the damage evaluation index in terms of relative residual drift considering structural repairability is furthuer improved.