Size effect of square steel tube and sandwiched concrete jacketed circular RC columns under axial compression

Abstract This paper aims at investigating the size effect of square steel tube and sandwiched concrete jacketed circular RC columns subjected to axial compression. The experiment part consisted of twenty-seven strengthened columns and three reference specimens, in which the effects of tube size (B) and confinement ratio (ξ) were studied. The results validated the existence of size effect on various indexes including peak nominal stress (σu), peak axial strain (eu), strength index (SI) and ductility index (DI) of specimens. With increasing specimen size, the corresponding σu, eu, SI and DI exhibited a decreasing trend. A finite element (FE) model was developed to further study the mechanical behaviors of the structure, in which the size effect was considered. The FE results demonstrated that the stress of concrete at the peak load point decreased with the increase of specimen size. Moreover, the size effect of confined concrete was more complex than that of the plain concrete. Not only the material strength of concrete but also the confinement effect was influenced by the size effect. The FE analysis also showed that the size effect on large-sized columns in engineering practice was more obvious than that of the tested specimens. It was found that the predictions of the load bearing capacities by codes including GB50936–2014, AISC 360–16, and BS5400were insufficient in terms of accuracy and safety, especially for large-sized specimen. Hence, a size-dependent model was derived, of which the desirable accuracy and credibility was verified with the experimental and the numerical results.