| In modern building structures,RC frame structure is the most common form of building structure,and beam-column joint is an important component of frame structures.The previous earthquake damage indicated that the seismic damage of RC frame structure occurs at the beam-column joint under earthquake action,and the joint is the most important component to maintain the anti-collapse ability of structure under earthquakes.Therefore,the seismic performance of frame beam-column joints is of crucial importance.The main failure modes of beam-column joints can be divided into ductile and brittle failure,and the displacement ductility coefficientμis an important index to evaluate whether ductile failure occurs.In this research,the effects of axial compression ratio（n）,shear compression ratio（v）,reinforcement ratio（ρv）and shear span ratio（λ）on displacement ductility coefficientμunder cyclic loading are discussed in detail.It is concluded that axial compression ratio n is the main factor affecting displacement ductility coefficientμof beam-column joints.ECC is a kind of high toughness composite.Through the bridging and stress transfer of internal fibers,it shows the typical characteristics of strain hardening and steady-state cracking of several fine cracks under tensile loads,and has excellent toughness and remarkable crack control ability,it is a new type of cement-based composite material which can be used to improve the ductility and energy dissipation of structures under earthquakes.Because of the high toughness of ECC,the NC of RC joints can be replaced by ECC to give better ductility.Therefore,this study selected two groups of atypical T shaped and cross shaped RC/R-ECC joint tests,and the FE models are established by using CDPM and reinforcements hysteretic model in ABAQUS.Through the comparison of failure mode,hysteresis curve and displacement ductility coefficient between test and FE,and error analysis,the reliability of FE models are verified.According to the design formula of axial force of beam-column joints,n=0.3 is adopted as the lower limit.And in order to ensure the ductility and collapse resistance of beam-column joints,taking the upper limit of axial compression ratio n=0.9 in the Code for Seismic Design of buildings as the control value,the typical T shaped and cross shaped RC/R-ECC joint models with n=0.3,0.5,0.7,0.9 are designed and established.Based on this,the numerical simulation of seismic behavior of RC and R-ECC beam-column joints under different axial compression ratios are carried out,and the failure mode,ductility,energy dissipation capacity and stiffness recovery are discussed in detail.The results indicate that the seismic behavior of beam-column joints with different cement-based materials,including displacement ductility coefficient（μ）,equivalent damping coefficient(ξeq)and Secant stiffness（Ki）,is strongly dependent on the axial compression ratio n.When the n is over 0.5,the displacement ductility coefficientμand the equivalent damping coefficientξeq were inversely proportional to the n.Under the same n and cyclic loading conditions,the maximum displacement ductility coefficientμof T and cross shaped R-ECC joints was33.7%and 27.9%higher than that of RC joints,respectively.And the brittle failure occurs in two kinds of RC joints when the axial compression ratio n exceeds 0.7,and their ductility decreases seriously,while brittle failure occurs in R-ECC joints when the n exceeds 0.9.Therefore,this study suggests that the n of RC and R-ECC beam-column joints should not exceed 0.7 and 0.9 respectively.Without deleteriously affecting the seismic performance of the structures,the axial compression ratio n of RC/R-ECC joints should be controlled at about 0.5 as far as possible,where the ductility,energy dissipation capacity,stiffness degradation and bearing capacity of the joints performs better than those under the n is 0.3,0.7 and 0.9. |
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