| Reinforced concrete beam-column joints are the key components affecting the seismic behavior of frame structures.At present,a lot of systematic studies have been reported in the literature on the seismic behaviors and failure modes of RC beam-column connections.However,the major attention of these studies was paid on non-damaged specimens,and less attention has been paid to the durability damage factors of existing frame structures under load and complex external environments.In this paper,based on a number of systematic studies on the durability damage of concrete materials and the seismic performance of reinforced concrete beam-column joints,the seismic behaviors of RC beam-column joints with Initial Damage is studied.Considering the actual engineering environmental conditions of the frame structures,freeze-thaw cycles and concrete carbonization were selected as the initial damage variables.The mechanical properties of the RC beam-column joints with different initial damage factors under cyclic loads were analyzed.Then a seismic restoring force model suitable for the beam-column connections with initial damage is proposed through regression analysis.The main contents of this paper are as follows:(1)By analyzing and comparing the existing constitutive relation of steel bar and concrete materials,as well as the bond-slip constitutive models between the steel bar and concrete,various material constitutive models used in the nonlinear finite element analysis software ABAQUS are selected reasonably.In the numerical analysis process,the uniaxial compressive stress-strain curve equation of concrete considering the effect of freeze-thaw cycles proposed by our research group can be used,which is based on the experimental study.Adopting the nonlinear stress-strain curves,like the carbonization constitutive relation of concrete,the constitutive relation of steel bar,which takes the initial durability damage of the material into consideration,and then a nonlinear finite element simulation method with higher accuracy was applied in this study.(2)Taking the experimental studies of reinforced concrete beams under the freeze-thaw cycle damage and the seismic performance of beam-column joints without concrete damage as controls,the material constitutive model and the finite element method in this paper are used for numerical analysis of the test specimens.The accuracy and applicability of the material constitutive model in this paper can be verified by comparing the results of nonlinear finite element analysis with the experimental results.(3)To compare and analyze the seismic behaviors of reinforced concrete beam-column joints under freeze-thaw damage and concrete carbonation,the external reinforced concrete beam-column joints that meet the requirements of the codes are designed and numerically analyzed with ABAQUS.The results show that the carbonation of concrete improves the maximum bearing capacity by about 15% of the specimens under cyclic load,but the decline in ductility can reach 32%.Besides,we can see that with the increase of freeze-thaw cycles,the peak load and ultimate load of the beam-column joints are reduced,and the ductility and the energy-dissipation capacity also deteriorate to varying degrees.The freeze-thaw damage reduces the seismic performance of the beam-column joints.(4)The characteristics of the load-displacement skeleton curve and the hysteresis curve of specimens are analyzed,and the data of the finite element model are analyzed by the regression analysis method.Then the functional relationship of the characteristic point on the load-displacement skeleton curve between the reinforced concrete beam-column joints considering initial damage and the undamaged specimen can be established.With reference to the tri-fold line restoring force model,the seismic restoring force model of reinforced concrete beam-column joints with initial durability damage was initially created.The stability of the restoring force model can be verified by comparing the results of the finite element analysis with the theoretical results.This can provide guidance for studying the seismic performance of reinforced concrete beam-column joints with initial damage. |
