Study On Mechanical Behavior Of Joint Of PVC-FRP Confined Concrete Column And Reinforced Concrete Beam Under Axial Compression

CFRP is a kind of material with excellent performance,which is widely used in structural engineering.The CFRP strips are wrapped at equal distance around the surface of PVC tube,and the steel cage is arranged in the pipe.Finally,the PVC-FRP confined reinforced concrete structure is formed by pouring concrete.Previous studies mainly focused on the aspects of axial compression,eccentric compression,seismic and durability.Based on the research of existing members,the experimental research and the theoretical analysis of the axial compression mechanical behavior of joints of PVC-FRP confined concrete column and reinforced concrete beam were conducted in this paper.Based on the experimental study on axial compression performance of the 10 "strong joint and weak column" specimens(including 1 PVC tube confined concrete strong joint weak column and 9 PVC-FRP confined concrete strong joint weak columns)of group A and the 8 "strong column and weak joint" specimens(PVC-FRP confined concrete strong column weak joints)of group B,five factors including width of the ring beam and the depth of ring beam,reinforcement ratio of ring beam,stirrup ratio of ring beam and hoop spacing of CFRP strips for the specimens of group A were considered,while four factors including width of the ring beam,the depth of ring beam,reinforcement ratio of ring beam and stirrup ratio of ring beam for the specimens of group B were considered.The influence of the above factors on the bearing capacity,deformation,crack and load deformation correlation curve of the specimens was analyzed.In the group A specimens,the failure modes of specimen include: The drum bending deformation of PVC tube confined concrete column,the crushing of the concrete in the column and the buckling of the longitudinal reinforcement,an obvious damage premonition before the destruction,and only a vertical tiny crack appears on the side of the concrete joint.The CFRP strips in the PVC-FRP confined concrete columns were broken,the PVC pipe cracked and the column longitudinal bars buckled,and there were obvious signs of damage before destruction.In group B specimens,the failure mode of the specimen was damaged by the collapse of the node area,with obvious damage omen before the destruction.The results suggested that in terms of the specimens of group A,the slope of load-displacement curve raised with the increase in the width of the ring beam and the reinforcement ratio of ring beam or stirrup ratio of ring beam,as well as the decrease in the spacing of CFRP strips;As the spaces between CFRP strips reduced,the ultimate bearing capacity and deformation of the specimen enlarged and the slope of load-displacement curve raised;while ring beam width,ring beam height,ring beam reinforcement ratio and ring beam stirrup ratio had no influence on the ultimate bearing capacity of the specimen.For group B specimens,with the increase in the width of the ring beam and the reinforcement ratio of ring beam or stirrup ratio of ring beam,as well as the decrease in the ring beam height,the slope of load-displacement curve increased and the bearing capacity of the specimen and deformation evaluated;while the crack resistance of the joints improved in a certain degree along with the increase in the reinforcement ratio of the ring beam.On the basis of experimental research,According to the unified theory of concrete-filled steel tube,and given the static equilibrium condition,the calculation formula of the axial compression bearing capacity of PVC-FRP confined concrete column was derived,and the design formula of bearing capacity of PVC-FRP confined concrete strong joint weak column was demonstrated.On the basis of the local pressure theory and the confined concrete theory,the height influence coefficient of joint was taken into consideration,the calculation formula of the axial compression bearing capacity of the joint zone with multiple ring bars was established.By means of multiple regression analysis,the influence coefficient of ring beam Stirrup ratio was introduced,and the calculation formula of bearing capacity considering the influence of hoop reinforcement was put forward.On the basis of the Mander model,the stress-strain relationship model of the joint zone with multiple ring bars was proposed,and the formula for calculating the cracking load in the joint area is given,and the load-deformation relationship of joint of PVC-FRP confined concrete column and reinforced concrete beam was obtained by combining with the stress-strain relationship model of the PVC-FRP confined concrete column proposed by Yu Feng.Reasonably selecting the strain-stress relationship and element type of PVC tube,CFRP,steel and concrete,setting the analysis step,defining the entity contacts and constraints,creating the loads and boundary conditions,dividing the mesh,establishing the finite element analysis model of joint of PVC-FRP confined concrete column and reinforced concrete beam,and analyzing the influence of concrete strength grade on the curves of load-displacement,the calculation results of model are in good agreement with the experimental results.