Study On Longitudinal Reinforcement Of Reinforced Concrete-Filled Steel Tube Column

Reinforced Concrete Filled Steel Tube(R-CFST)has been proven to be a structural system with small volume,high bearing capacity,and good seismic performance.Longitudinal reinforcement is an important component of R-CFST.At present,the research of the influence of longitudinal reinforcement on the mechanical properties of R-CFST is still limited to the macroscopic observation of the overall performance,and the ductility of members,confinement effect,influencing laws of failure mode and toughness have not yet found clear research results.Therefore,in this research work carried out study on longitudinal reinforcement and strength evaluation methods of RCFST,by the means of both analytical and experimental methods(1)Experimental study of longitudinal bar shape: taking the shape of the longitudinal reinforcement(yield strength or yield ratio)as the main parameter,a total of 18 specimens were produced,including 6 reinforced concrete filled steel tube with ribbed longitudinal reinforcement and Reinforced concrete short columns(3 repeated specimens),6reinforced concrete-filled steel tube concrete and reinforced concrete short columns with light round longitudinal bars,3 concrete-filled steel tube shorts(CFST),and 3 plain concrete short columns.Through comprehensive analysis,it is found that the effect of the shape of the longitudinal reinforcement on the mechanical properties of the R-CFST short column is not obvious.Therefore,it is recommended to use the type of longitudinal reinforcement that matches the steel tube.Reinforced cages are arranged in R-CFST short columns,which can make the components have higher bearing capacity,good deformation capacity and energy absorption capacity.(2)Experimental study on longitudinal reinforcement ratio: five groups of R-CFST were prepared using the longitudinal reinforcement ratio as the main parameter.,3concrete-filled steel tube and 3 plain concrete specimens.The experimental results show that the bearing capacity and plastic properties increase with the increase of the reinforcement ratio;with the increase of the reinforcement ratio,the failure changes from shear failure to multi-fold waist drum failure;the reasonable ranges for reinforcement ratio is 1.32% to 3.5%.(3)Numerical simulation study: based on the Saenz concrete constitutive model,a constitutive model of concrete under the multiaxial stress with the confinement index as a parameter was proposed.Based on the above two experiments,36 finite element numerical models with different wall thickness and longitudinal reinforcement ratio were established,and an optimal range which is suitable for the rario of longitudinal reinforcement of R-CFST with any thicknesses of steel tubes was proposed as 1%<ρ<3%.(4)Research on compression bearing capacity: the design methods of CFST(AIJ,EC4,GB 50936-2014,and CECS 28-2012)are modified,and the calculation results were evaluated based on the test results to find the design methods for R-CFST.The results show that the results of the US code formula are conservative,and the revised Japanese,European,and Chinese code formulas are more suitable for evaluating R-CFST compressive bearing capacityThe following conclusions are drawn through the aforementioned research:(1)Effect of the shape of the longitudinal reinforcement on the mechanical properties of the R-CFST short column is not obvious.Therefore,it is recommended to use the type of longitudinal reinforcement that matches the steel tube;(2)with the increase of the reinforcement ratio,the failure changes from shear failure to multi-fold waist drum failure;(3)Optimal range which is suitable for the rario of longitudinal reinforcement of R-CFST with any thicknesses of steel tubes is: 1%<ρ<3%.