| Concrete filled steel tube(CFST)composite structures have the advantages of high load carrying capacity and good ductility.However,the connection between the CFST column and beam is a vital component of a CFST frame structure.Design of connection details is therefore crucial for the safety of the whole structure.Concrete ring beam connection is one of connections linking the CFST columns and reinforced concrete(RC)beams,which has attracted more and more attention recently.The connections have determined force transfer owing to the disconnection of the steel tube in the joint region.However,due to the brittleness of concrete,cracking of concrete is a big problem for the concrete ring beam connections.Also,the axial and shear carrying capacity is inevitably decreased because of the interruption of the steel tube.To solve the problems of cracking and low load bearing capacity of the ring beam connection,a new type of engineered cementitious composite(ECC)encased CFST columns-steel reinforced ECC ring beam connection,which is designed for linking an ECC-encased CFST column and RC beam,is proposed in this study.ECC is a class of high performance cementitious composites with pseudo strain-hardening characteristics and excellent crack control ability.The ultimate tensile strain of this material can reach more than 3% and the crack width of ECC can be controlled below 100?m.Therefore,substitution of ECC with concrete can effectively improve the local compressive bearing capacity and shear bearing capacity of the ring beam connection,and avoid the cracking and durability problems caused by the brittleness of concrete.In this paper,the mechanical properties of ECC ring beam connections are deeply studied from the experimental research,finite element analysis and theoretical analysis.(1)A series of ECC ring beam connections were tested under local compressive loading,and then compared with conventional concrete ring beam connections.According to the test results,substitution of concrete with ECC in the ring beam region effectively decreased the crack width,which improved the normal service life of the ring beam connections.For the unreinforced specimens,suddenly splitting failure occurred for the concrete connection,while ECC connections maintained good integrity.The peak loads of unreinforced ECC specimens were more than 78%higher than those of concrete specimens.For the reinforced specimens,the ductility coefficient and energy dissipation of the ECC ring beam connections were more than 26.1% and 50.7% higher than those of concrete connections,indicating that applying ECC in the ring beam region can increase the ductility and energy dissipation for the connection under local compressive loading.(2)Based on the finite element software ATENA,the local compressive behaviour of ECC ring beam connection was evaluated,and an extensive parametric study of ECC ring beam connections with different design parameters which was not involved in the test was performed.According to the results of finite element analysis,significant improvement of the lateral constraint can be found with the increase of the ring beam width when the ratio of the outside diameter to the inside diameter of the connection was in the range of 0-2.Hence,2 was the recommended maximum for the ratio of the outside diameter to the inside diameter.(3)Based on the tie-arch theoretical model,the calculation methods of the local load bearing capacity for ECC materials were established.Based on the local compressive theory and the constraint theory,two kinds of formulas calculating the local compressive bearing capacity of ECC ring beam connections were proposed.It is found that the two calculation models were in good agreement with the experimental and finite element results.Both calculation methods can be used to predict the local compressive loading for the steel reinforced ECC ring beam connections.(4)The force transfer mechanism of the axial load from the ECC encased CFST column to the ring beam connection region was analyzed by the finite element analysis.It can be found that the vertical force transferred in the joint zone was mainly shared by the longitudinal steel bars.The axial bearing capacity of the joint core region was enhanced by the constraint effect from the circumferential reinforcement.Also,the additional stirrups can increase the axial carrying capacity of the connection by constraining the lateral deformation of ECC in the core area.Based on the finite element analysis,the theoretical models predicting the axial load carrying capacity of the ECC encased CFST columns-steel reinforced ECC ring beam connection were also developed.(5)The force transfer mechanism of the bending moment from the ECC encased CFST column to the ring beam connection region was analyzed by the finite element analysis.According to the results of finite element analysis,the core region of the ring beam connection was under full-section compressive loading state when the eccentricity was small.The influence of the interruption of the steel tube was similar to that under the axial compressive state.When the eccentricity was large,the force transfer mechanism in the compressive zone was similar to that of the axial compression member.However,the strain of the circumferential reinforcement and additional stirrup in the tensile zone was small,and the longitudinal tensile force was shared by longitudinal bars.Based on the finite element analysis,the theoretical models predicting the load carrying capacity of the ECC encased CFST columns-steel reinforced ECC ring beam connection under eccentric loading were also developed.(6)To investigate the seismic behaviours of ECC ring beam connections,seven ring beam connection specimens,including two concrete specimens and five ECC specimens,with different reinforcement ratios,ring beam sizes and axial compression ratios,were tested under reversed cyclic loading.According to the test results,introducing ECC in the ring beam connections exhibited much better mechanical behavior compared with concrete specimens.When failure occurred in the ring beam region,the load bearing capacity of the ECC specimen was 15% higher than that of the concrete connection.Application of ECC in the ring beam avoided the formation of shear cracks in the ring beam region and enabled the failure mode to change to ring beam flexural failure.When failure occurred in the frame beam region,the ductility and cumulative energy dissipation of the ECC connections were 1.4 and 1.9 times larger than those of the specimens with concrete connections.Also,increasing of the circumferential reinforcement ratio and joint size can produce a positive effect on the hysteretic behavior of the ECC ring beam connection.(7)The shear behaviour of ECC encased CFST columns-steel reinforced ECC ring beam connection was analyzed by the finite element analysis.Four kinds of failure modes were found for the ECC encased CFST columns-steel reinforced ECC ring beam connections: joint shear failure mode,ring beam flexural failure mode,frame beam flexural failure mode and an intermediate mode in which failure occurred at the junction of the ring beam and the frame beam.Based on the parametric study results,both increasing the circumferential reinforcement ratio,stirrup ratio,compressive strength of ECC and decreasing the axial compressive ratio can produce a positive effect on the shear bearing capacity and flexural bearing capacity of ring beam connections.(8)The force transfer mechanism of ECC ring beam connection was analyzed.The calculation model of the shear carrying capacity for the ECC ring beam connection was also established.The shear carrying capacity of the connection calculated by the model agreed well with the experimental results,and the model can be used to predict the shear carrying capacity of the ECC ring beam connections.(9)Based on the experimental and analytical research as well as the related design standards,the design methods and suggestions for the ECC encased CFST columns-steel reinforced ECC ring beam connection was proposed in this paper. |
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