Size effect on axial behavior of stub CFST columns strengthened with square steel tube and sandwiched concrete jackets

The size effect of CFST columns strengthened with square steel tube and sandwiched concrete jackets subjected to axial compression was investigated. Test parameters included specimen size, outer steel ratio and sandwiched concrete grade. The test results showed that the peak nominal stress and ductility index of both CFST and strengthened columns presented a decreasing tendency with the increase of the specimen size. Compared with traditional CFST columns, the size effect on peak stress of retrofitted columns was found to be more significant, while the size effect on ductility index was weaker. The analysis revealed that the level of confinement stress decreased as the size of the confined concrete increased at the peak load. The size effect on the confining stress was much greater than that on the material strength of the sandwiched concrete. Comparison results with four design codes indicated that the existing specifications overestimated the load-bearing capacity of large-size specimens. A formula for calculating the reduction coefficient related to confinement ratio was proposed to consider the size effect on the compressive strength of inner and sandwiched concrete. By introducing the reduction coefficients into formulas of design codes, the trend of overestimating the load-bearing capacity of large-size columns was weakened.