Study On Seismic Behavior Of Reinforced ECC-encased Concrete-filled Steel Tube Composite Columns

Steel-concrete composite columns have gained popularity for use in high-rise building constructions in regions of high earthquake intensity.One is the steel tube-reinforced concrete column(ST-RC column),which has been widely studied and applied due to its good carrying capacity,ductility and durability.However,experimental research and engineering practice show that,the ductility of outer reinforced concrete in STRC columns is far less than that of inner concrete-filled steel tube,leading to incoMPatible deformation between them and the peeling of outer reinforced concrete.Besides,the peak load of inner concrete-filled steel tube lags the outer reinforced concrete,reducing the ultimate load-carrying capacity of ST-RC column.At the same time,concrete is a quasi-brittle material with poor tensile strength and easy to crack.Under horizontal earthquake load,the outer reinforced concrete is easy to crack in tension,leading to rapid degradation of bonding strength with steel bars and debonding failure.The failure can reduce the seismic performance,durability and fire resistance of the structure,which limits the application of ST-RC column in engineering.Engineered cementitious composites(ECC)is a kind of high performance fiber reinforced cementitious composites with ultra-high toughness and excellent energy dissipation.It can be used in the structure to improve the seismic performance of the component.In this dissertation,a new kind of section,named reinforced ECC-encased concrete-filled steel tube composite column(R/ECC-CFST column)is introuduced.It is proposed to substitute the outer concrete with ECC to improve the seismic performance.The specific contents are as follows:(1)Based on the basic design theory of reinforced concrete and concrete-filled steel tube,adopting rational constitutive models and assumptions,the mechanical behavior of R/ECC-CFST column under axial and eccentric loading is studied.The simplified computation model and formula are given,which is in good agreement with the test results.(2)To study the failure mechanism,hysteretic loop,strength,stiffness,ductility,energy dissipation and strain of R/ECC-CFST column with different axial load ratios,stirrup ratios and tube diameters under low cycle loading,11 composite column specimens were designed,including 7 R/ECC-CFST column specimens and 4 ST-RC column contrast specimens.The test results indicate that,all R/ECC-CFST columns have better ductility and energy dissipation while coMPared with ST-RC columns.The hysteretic loops of R/ECC-CFST columns are fuller without obvious pinch phenomena.The more the axial load is,the more the load-carrying capacity is,and the worse the ductility and energy dissipation are.It can improve the load-carrying capacity,ductility and energy dissipation to a certain degree by improving the tube diameter and stirrup ratio.Under the same conditions,the accumulated energy dissipation of ST-RC columns is about 50% of R/ECC-CFST columns with same parameters.(3)A software of ABAQUS was used to simulate the failure process of R/ECC-CFST columns under horizontal low cyclic loading.The calculation results are compared with the test results.Based on the reliable simulation,the failure mechanism of composite columns under low cyclic loading is studied.The influence of different parameters on the seismic performance of composite columns is analyzed.The results show that,increasing the yield strength of longitudinal bars and steel tubes,steel tube ratios and longitudinal reinforcement ratios,the load-carrying capacity and ductility are improved.Increasing the strength of inner concrete and outer ECC could improve the initial stiffness and load-carrying capacity.Improving the axial load ratio,the initial stiffness and load-carrying capacity are improved while the ductility and energy dissipation capacity are reduced.(4)Based on computing theories of restoring force model of reinforced concrete and concrete-filled steel tube,adopting the seismic test results of composite columns,the restoring force model of R/ECC-CFST column is determined as a three-line curve of the stiffness degradation model.Then,the calculation formula and hysteretic rules are given.The characteristic point values calculated by this model agree well with test results.The hysteretic loops described by this model agree well with test results.