Influence Of Construction Defects On Mechanical Performance Of RC Beam-column Joints

Joint Core is the key component of the reinforced concrete(RC)frame structure,with complex mechanical behavior in this area.As longitudinal reinforcements and stirrups of the beam and the column intersect at the RC joint core,the intensive distribution of reinforcements there may cause casting difficulties and vibrating imperfect situation in construction.In addition,construction processes like formwork and steel bar binding often lead to defections in core joints like stirrup shift,a lack of stirrups and bare longitudinal reinforcements,which causing a decrease in the joint property and the overall mechanical performance of the RC structure.Existing researches on joints,both experiments and numerical simulations,are focus on its seismic performance,seismic damage model and node reinforcement method.And researches on the effects of node construction defects are not sufficient.Therefore,researches about the influence of construction defects on the performance of reinforced concrete frame joints are of great importance.Causes and characteristics of construction defects in beam-column joints in practical construction are summarized,and theories of joint mechanism behavior and its calculation methods are also presented.RC frame beam-column joint numerical models are established using finite element model software ABAQUS,with construction defects like a lack of stirrups and bare longitudinal reinforcements considered in the FEM model.Then uniaxial loads which increasealong the members are applied in the model to simulate the influence of construction defects on the mechanical properties of beam-column joints.The results show that the stress distribution of joints with construction defects changed under the load applied.The stress mainly concentrate in the joint area,at which the steel bar yields,compared with in the end of the beam for the defect-free model.By comparing the force and displacement curves of the defective nodes with the intact nodes under uniaxial loading,the bearing capacity of the defective nodes is inferior to that of the intact nodes.Larger transverse deformation,lower binding capacity of joints core,and worse ductility are shown for the joints with defects compared with the perfect ones.Based on the numerical results of ABAQUS,a nonlinear frame model with joint construction defects considered are established by using SeismoStruct.Then the nonlinear seismic dynamic time-history analysis of the joints is carried out to analyze the influence of joints construction defects on the seismic performance of RC frame.According to the results,the frame with joints defects has a longer natural vibration period and a larger displacement for each floor compared with the structure without joints defects,which indicate a decrease in structural stiffness in the defective frame.In the case of frequently occurred and rare earthquakes,the intact structures show a larger interlaminar shear,then a smaller interstory drift compared with the joint-defective frame,which means a worse seismic capacity for the frame with joint construction defects.The simplified method for the FEM modelling of RC frame structures with beam-column joints construction defects is presented in this paper.In this method,the element stiffness reduction of joints is introduced to simulate the influence of defects on mechanical properties of joints.This simplified method provides guidance for the evaluation of construction defects,and for the selection of the reinforcement scheme.It is of high engineering value,and it lays the foundation for the further study of RC frame structure performance with defective beam-column joints.