Size effect in circular concrete-filled steel tubes with different diameter-to-thickness ratios under axial compression

Axial compression tests of circular concrete-filled steel tubes with different diameters (219 mm, 426 mm, and 630 mm) and ratios of tube diameter to steel thickness (55 and 88) were conducted to investigate the effect of size on the bearing capacity. The experimental results indicated that the peak nominal stress decreased as the size increased, and the decrease in the nominal stress due to the size effect increased at higher ratios of diameter to thickness. At the peak load moment, an increased specimen diameter corresponded to a decreased hoop stress in the steel tube as well as a decreased concrete strength due to the confinement effect of the steel tube. When the ratio of diameter to thickness increased, the extent of reduction of the hoop stress and the confining effect of the steel tube influenced by the increasing specimen size increased. However, the vertical stress in the steel tube was increased at increased size, and increases in the ratio of diameter to thickness improved the increase degree of the vertical stress of steel tube due to the enlargement of specimen size. Hence, the vertical bearing capacity of the steel tube was affected by both the specimen size and the ratio of diameter to thickness. Based on the size effect law (SEL) proposed by Bazant, and taken the effect of the ratio of diameter to thickness into consideration, a size-dependent formula to evaluate hoop stress in the steel tube was developed. A size-related model considering situations with different ratios of diameter to thickness was established in order to estimate the bearing capacity of large-size circular concrete-filled steel tubes. The model and experimental results showed good agreement.