Research On Size Effect Of Axially Loaded Circular Concrete-filled Steel Tubular Stub Column

Concrete-filled steel tube(CFST)has been widely used in the construction of highrise buildings and long-span arch bridges as it can meet the demands of modern structures.The section size of CFST is increasing with the development of these giant structures,and then the problem of size effect is attracting more and more attention.Moreover,the CFST columns with large section size are commonly used as the main load-carrying members in some landmarks.Thus,the size effect of CFST should be emphasized.Compared with that of plain concrete,the size effect of CFST columns is more complicated due to the effect of confinement.However,limited research efforts have been put into this area,and the relevant study is not afford to predict the influence of confinement on size effect of concrete core.The purpose of this thesis is to carry out a further study on the size effect behavior of CFST under axial compression.The main work is presented as follows:(1)Fifteen concrete columns were tested to failure to investigate the size effect behavior of concrete,including the failure mode,elastic modulus,peak axial stress and peak axial strain.The predicting accuracy of the typical size effect formulas was studied by being comparing with the statistical test data,and then the size effect law of peak axial stress of concrete was obtained.Based on the analysis of relationship between peak axial stress and peak axial strain for standard and infinite specimens,the size effect law of peak axial strain was determined.The size-dependent stress-strain curve of concrete under axial compression were proposed after considering the size effect of peak axial stress and peak axial strain.Based on the size effect law of concrete,the influences of size effect on reliability index of the structures were studied.(2)Axially loaded tests were conducted on 36 specimens to study the size effect behavior of CFST columns,and the influences of specimen diameter on failure mode,elastic modulus,peak axial stress,peak axial strain and ductility coefficient were analyzed.Based on the test results,the predicting accuracy of the typical codes for CFST with different diameters was studied,and then a size-dependent coefficient was proposed to consider the size effect of concrete core.The size effect of load-carrying capacity of CFST was obtained by introducing the coefficient into the relevant formulas in EC4.Subsequently,the size effect of the statistical specimens in laboratory and engineering was analyzed respectively.(3)A comparison between statistical test data of actively confined concrete and the theoretical formulas was conducted,then the stress-strain curve of actively confined concrete were determined.Based on the model equations of actively confined concrete and passively confined concrete,the influences of loading path on axial strain and axial stress of passively confined concrete were analyzed.The analytical model for passively confined concrete was developed,which can predict not only the behavior of FRPconfined concrete in linear confinement but also the behavior of concrete-filled steel tube in nonlinear confinement.By combining the analytical model for passively confined concrete and the bi-directional constitutive equations of steel,an analytical model for CFST columns under axial compression was proposed.The size effect of actively confined concrete was investigated by analyzing the influence of confinement on size effect of plain concrete,then it was introduced to the analytical model for CFST columns to study the size effect behavior of CFST.The calculation process was realized in MATLAB,and the predicted results were compared with the test results to demonstrate the accuracy of the proposed model.(4)The influences of specimen diameter on steel tube and concrete core were studied based on the proposed size-dependent analytical model,then the size effect behaviors of stress-strain curves of concrete core and CFST were obtained.By analyzing the lateral dilation of steel tube and concrete core,shrinkage effect on confinement strain was investigated,and then the effect on load-carrying capacity of CFST columns having different diameters was studied.Investigation shows that the influence of specimen diameter on stress-strain curve of steel tube is slight,while that on stress-strain curve of concrete core is significant.For CFST specimens with the confinement index in the range of 0<?<1.0,size effect can lead to 6.5%?26.7% and 7.5%?39.3% decrease in peak axial stress and peak axial strain,respectively.The shrinkage of concrete core will delay the occurrence of confinement,and it can lead to 2% and 3% decrease in load-carrying capacity for CFST specimens with diameter of 150 mm and 1500 mm respectively when the autogenous shrinkage strain is within 150??.Thus,the influence of autogenous shrinkage on load-carrying capacity can be neglected.