Seismic Performance Of Resilient Circular Concrete Columns Reinforced By Low-bonded Ultra-high-strength Steel Bars

The traditional seismic design is based on the concept of ductility and requires that concrete structures resist design-strong earthquakes through dissipating seismic energy by plastic deformation within the so-called plastic hinge zones in their components.Although ductile concrete structures can survive without structural collapse under strong earthquakes,they may be severely damaged and left with so large residual deformation after the earthquake that it is difficult to be repaired and had to be demolished.It has become a new requirement for aseismic structures to be able to recover their structural function after being hit by rare or extremely rare earthquakes,and it also has become the key issue to build resilient cities in order to realize prompt recovery of social and economic activities.The resilient structures,which are indispensable to resilient cities,must have at least two characteristics: 1)stable bearing capacity(referred to as drift-hardening behavior in this paper)up to large drift not less than 4%,so as to meet the structural safety requirements under strong and/or very strong earthquakes;2)low residual deformation after being unloaded from large drift(referred to as high re-centering capability in this paper),so as to meet the repairable demand of structures after strong earthquakes.The use of ultra-high-strength(UHS)steel bars with low bond strength as longitudinal reinforcement is an important way to realize the resilient concrete structures.At present,there are few studies on the seismic performance of concrete components longitudinally reinforced by UHS rebars.For this reason,this paper has conducted experiments and numerical analysis on the seismic performance of circular concrete columns with low-bond and UHS steel bars as longitudinal reinforcement.The main research work and results are summarized as follows:(1)Quasi-static tests on seven circular concrete columns were carried out.Four columns were reinforced by UHS PC strands,two columns were reinforced by helical grooved(HG)bars,and the other one was reinforced by normal strength rebars.The effects of longitudinal reinforcement types,shear span ratio,axial load ratio,and the confinement methods within the potential plastic hinge region on the failure characteristics,seismic performance,drift-hardening behavior,and re-centering capability of circular concrete columns were examined.The test results show that the circular columns reinforced by PC strands or HG bars can possess a drift-hardening behavior and significantly improve the re-centering capability of concrete columns deformed to large drifts.For the columns confined by hoops,the damage degree of the plastic hinge zone of the column reinforced by HG bars is lighter than that of the column reinforced by PC strands under high axial compression and large displacement.Therefore,HG bars are more suitable than PC strands to be used as longitudinal reinforcement of resilient concrete columns under high axial compression.Compared with hoops,bolted steel plates(BST)can provide more effective lateral restraint,and further enhance the drift-hardening stiffness and the re-centering capability of the columns under large displacements.The use of BST as the lateral confiner for concrete columns reinforced by PC strands under high axial compression can realize the drift-hardening behavior up to larger drift.In addition,the plane-remain-plane assumption is no longer valid for the longitudinal reinforcement because of the obvious slip between the PC strands or HG bar and surrounding concrete.(2)In order to consider the bonding-slip effect of UHS and HG rebars,based on the finite spring element method(FSM)that combines the lumped plastic hinge method with the strain cumulative effect method,a numerical calculation program for evaluating the lateral force versus displacement relationship of concrete columns reinforced by PC strands or HG bars has been developed.Through numerical analysis,the influence of bond strength on the lateral force versus displacement relationship is carefully studied,and the respective bond strengths of PC strands and HG bars are suggested.The influence of stress-strain hysteretic rule of concrete,unloading stiffness degradation and cracked section effect on residual deformation,energy dissipation capacity and hysteretic loop shape of low-bonded UHS reinforced concrete column,abbreviated as LBHSRC column hereafter,are discussed.The results of numerical analysis show that the cracked section effect is the main factor affecting the hysteretic behavior of LBHSRC columns,and the concrete hysteretic rule and the value of the parameters related to cracked section effect suitable for LBHSRC column are proposed.The analytical results of the proposed method are in good agreement with the test results,implying that it can be used to simulate the hysteresis performance of the LBHSRC columns.(3)Through numerical analysis,the influence of shear span ratio,axial load ratio,longitudinal reinforcement ratio,and other parameters on the skeleton curves of LBHSRC columns is studied,and further combined with the test results,a restoring model suitable for LBHSRC columns is proposed.The calculated results by the proposed model are agree very well with the test results,which indicates that the proposed model can be used for dynamic time history analysis of LBHSRC column.(4)Utilizing the famous Open Sees,the restoring force model of LBHSRC column and Takeda-type restoring force model are developed.The seismic dynamic analysis of LBHSRC column and normal RC column shows that the maximum displacement response of LBHSRC column dose not increase significantly compared with that of RC column,and that LBHSRC column has significant self-centering capability.In addition,the influence of the drift-hardening stiffness,re-centering capability and energy dissipation capacity in the restoring force model on the maximum deformation and residual deformation of the columns is discussed.Furthermore,the seismic dynamic response of the frame structures with LBHSRC columns and with normal RC columns are analyzed by using IDA analysis method.The fragility curves of the two types of frames and the distribution of maximum drifts,residual drifts and component damages of these frames along the story height are compared.The results show that the columns in LBHSRC frame can avoid rapid development of section curvature under strong earthquakes,effectively reduce the risk of column hinge mechanism and weak story,and improve collapse-resistant capacity of the frame structure.