Abstract: Under the action of earthquake,concrete-filled steel tubular(CFST) columns are often subjected to torsion action or combined action of compression-bending-shear-torsion.The existence of torque will aggravate the damage of CFST columns.In order to study the pure torsional failure and size effect behavior of CFST columns,a numerical model of CFST columns on pure torsional failure was established by using the three-dimensional meso-scale numerical simulation method,considering the meso-heterogeneity of concrete and the contact between steel tube and concrete.The influence of structural size,confinement coefficient and cross-sectional shape on torsional failure of CFST columns with different sizes were analyzed.Finally,the Bažant size effect law was compared and revised,and the formula for the nominal torsion strength in the full size range was proposed.The results show that:(1)Pure torsion failures of plain concrete columns show significant brittle failure characteristics,while pure torsion failures of CFST columns show certain ductile failure characteristics;(2)The pure torsion failure of the CFST column has the size effect,and the size effect of the square columns is stronger than that of the circular columns;(3)Under low constraints(use confinement coefficient to mark),the size effect of the pure torsion failure of the CFST column is more obvious,with the increase of the confinement coefficient,the size effect of the nominal torsion strength is weakened.