Experimental assessment on the size effects of circular concrete-filled steel tubular columns under axial compression

In this study, eight circular concrete-filled steel tubular (CFST) short columns with diameters (D) of 275–1100 mm and the column diameter-to-steel tube thickness ratios (D/t) of 55 and 69 were tested under axial compression. The bearing capacities of the steel tube and confined concrete were analyzed separately to illustrate the size effect of each component of the composite column. The experimental results show that all test specimens suffered a shear failure. The peak stress, peak strain, ductility index, and strength index decrease with the increasing D, while the composite elastic modules maintain to be relatively constant with the various specimen dimensions. Under the peak load, the vertical stress in the steel tube increases, while the hoop stress decreases with the increasing D. Compared with the specimens with a small D/t ratio, the vertical stress in the steel tube with a larger D/t ratio is lower, while the hoop stress is higher. The confined concrete strength can be divided into the unconfined concrete strength and the increased concrete strength due to lateral confinement, and the reduction of the latter is the main cause of the size effect of the CFST columns. The current design codes such as EC4, AISC, AIJ, and GB50936 overestimate the peak bearing capacity of circular CFST short columns with large diameters, compared to the experimental results. Therefore, a model considering the size effect was established to evaluate the axial bearing capacity of circular CFST short columns.