| Reinforced concrete slab column structure is widely used in various public buildings,commercial buildings and underground garage buildings because of its advantages of high utilization rate of building space,simple construction and effective reduction of building height.However,the force of the plate-column joint area of the structure is complicated,which is easy to cause local failure and collapse accident.Domestic and foreign scholars have conducted a large number of studies on the factors affecting the failure of plate-column joints,but most of the studies focus on the influence of longitudinal reinforcement ratio and concrete strength,while few studies consider the impact of impact ratio on the failure pattern of plate-column joints.Moreover,further studies are needed on the failure mechanism of plate-column joints and the boundary of their influencing factors.The bearing capacity calculation formula considering the above three factors is also to be established and improved.In this paper,based on the existing research results at home and abroad,the three-dimensional solid degradation virtual laminar element finite element analysis method was used to design a total of 42 models of slab column nodes without belly reinforcement by taking the impact ratio,longitudinal reinforcement ratio and concrete strength as variables.The nonlinear finite element analysis program VFEAP was used for modeling and simulation analysis.The failure process of the specimen was observed,evaluated and analyzed theoretically by extracting load deflection curve,reinforcement stress curve,section crack analysis diagram and stress contour map.The longitudinal reinforcement characteristic value(?f_y/f_c and impact span ratio,which are synthesized by concrete strength and longitudinal reinforcement ratio,are taken as the theoretical analysis elements,and the three failure forms(bending,bending and punching of plate-column joints and the limits of their influencing factors are studied in depth,and the calculation formulas of the bearing capacity of plate-column joints in various countries are compared and analyzed.A failure model of slab-column joints without belly reinforcement was established considering impact ratio and characteristic value of longitudinal reinforcement,and the formulas for calculating ultimate bearing capacity of slab-column joints including three failure modes of slab-shear,bending and bending were derived.The main research work and conclusions of this paper are as follows:(1 According to the method presented in this paper,the anti-impact failure tests of 5 reinforced concrete slab column joints were simulated.The calculation analysis and test results are in good agreement,indicating that the method presented in this paper can better simulate the mechanical properties and failure modes of concrete slab column joints.(2 A total of 36 plate-column joint models with thrust span ratio of 3～13,concrete strength of C30 and C50,longitudinal reinforcement ratio of 0.86%,1.28%and 1.73%were established and simulated.The results showed that:With the increase of impact span ratio,the ultimate bearing capacity of plate-column joints decreases and ductility increases.The failure mode changes from punching failure to bending failure and then to bending failure.With the increase of the characteristic value of longitudinal reinforcement,the shear capacity of plate-column joints increases in a certain range.When the impact span is small or large,the failure pattern of plate and column joints is not affected by the characteristic value of reinforcement:when the impact span is small,all punching failure occurs.When the impact span is relatively large,all bending failure occurs.(3 Based on the simulation analysis results of 36 models with impact span ratio of 3～13 and 6 models with impact span ratio of 15,the stress change of reinforcement at 45°crossing position of plate surface was extracted from the bending failure and bending failure models,and the plastic strand was determined whether the plate and column joints appeared by combining the stress contour map and the load deflection curve.In this paper,the failure modes of plate and column joint punching,bending and bending are proposed,considering the impact span ratio and the characteristic value of longitudinal reinforcement.(4 Based on the theoretical analysis and the linear regression analysis of samples,the calculation formula of the dip Angle of the impact plane was fitted out.Combined with the above discriminant formula of the plate and column joints,the calculation formula of the ultimate bearing capacity of the plate and column joints covering the impact of the impact ratio and the characteristic value of the longitudinal reinforcement was obtained,which comprehensively considered the impact of the three failure modes of the plate and column joints,namely the punching,bending and bending.The formula in this paper is modified with 95%guarantee rate,and compared with the formula of our specification GB50010-2010,the formula of the United States and Europe,the factors are considered comprehensively,the accuracy is higher,and the safety is better. |
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