Study On The Mechanical Properties Of High Strength Concrete Frame Joints With Centrally Reinforced Columns

With high-speed development of economy and technology nowadays,it is difficult for common multi-storey frame structures to meet people's needs of large-span and high-rise buildings.In order to meet the bearing capacity requirements,the existing of "large-sized beams and columns" can not be avoided,which not only occupies the using space,but also increases the structure weight.It is especially detrimental for anti-seismic capacity.High strength concrete frame joints with centrally reinforced columns is formed by adding additional steel bars and composite stirrups in the middle of the columns,which not only improves the bearing capacity and ductility,but also minimize the size of the members.What's more,it brings about higher economic benefits.As a special form of joints,the theoretical analysis of high strength concrete frame joints with centrally reinforced columns is not perfect,and the current code has not explained the mechanism and calculation.Six specimens of such joints have been tested under low-cycle reciprocating loading.However,due to the insufficient number of specimens and the large number of variables,the research results still needed to be further verified.Moreover,the mechanical mechanism and bearing capacity calculation were not studied.Therefore,according to the test conditions,the material constitutive definition and boundary constraints were determined,and the numerical model corresponding to the specimens was established to simulate the whole process of loading from the axial pressure to failure.The numerical results were compared with the experimental results to verify the accuracy of the modeling methods.By the verified numerical model,the number of models was added to study the effects of concrete strength,axial compression ratio,longitudinal reinforcement ratio and stirrup reinforcement ratio in the core area on joint performance.Six finite element models are established to analyze each influencing factor.The results show that concrete strength has a significant effect on bearing capacity.Compared by the joint made of C50,the bearing capacity of joint of C100 increases by 57.82%.The higher the concrete grade,the more obvious the defects of brittleness and poor deformation,the faster the degradation of strength and stiffness;The ductility and energy dissipation capacity of joints are improved by core reinforcement.The bearing capacity of the model with 25 mm longitudinal bars in core is19.59% higher than that of the model without core reinforcement,and the degradation of strength and stiffness is slower;By increasing stirrup ratio,the restraint of concrete is strengthened,the degradation of strength and stiffness is reduced,and the ductility of the joints is improved.Compared with the model with stirrups of ?6@160,the model with stirrups of ?8@110 has a 20% increase in energy dissipation capacity and a 17.26% increase in bearing capacity;Axial compression ratio is positively correlated with the bearing capacity of joints and negatively correlated with deformation and energy dissipation capacity,but this effect is not significant.The axial compression ratio is doubled and the bearing capacity is only increased by 7.9%.The stress,strain and damage distribution graphs of each loading stage of the finite element models were analyzed to study the mechanism.The results show that the joints in the early stage of loading mainly resist external loads by diagonal struts.Then,the concrete damage in the core area enlarges in the later stage of loading,the joints don't damage quickly because of the restraint of stirrups,which is similar to the truss mechanism.Finally,based on the simulation results of shear capacity of four groups of models,the shear capacity formula of high-strength concrete frame joints with centrally reinforced columns was deduced by Origin.It was examined by the test results and found exact and practical.