Behavior of CA-UHPC filled circular steel tube stub columns under axial compression

Ultra-high performance concrete with coarse aggregate filled steel tube (CA-UHPCFST) is a sustainable and high-performance structural form with high carrying capacity. This paper presents experimental and numerical investigations on the axial compressive behavior of circular CA-UHPCFST stub columns. Forty-four specimens were tested to determine the effects of concrete strength, steel fiber content, steel tube thickness, and steel yield strength on the axial compressive behavior. A finite element (FE) model was developed and validated for composite action analysis between steel tube and concrete core. The results show that the failure modes of CA-UHPCFST stub columns depend on confinement coefficient (xi) categorizing as shear failure (xi < 0.9) and bulge failure (xi > 1.4). Correspondingly, as the confinement coefficient grows, the load-shortening curve after the ultimate resistance demonstrates three categories: descending, plateau, or hardening stage. The steel tube exhibits a prominent confinement effect on the CA-UHPC core, with a strength index ranging from 1.066 to 1.225; however, the enhancing effect is not as significant as that of the ordinary concrete-filled steel tubes. The incorporation of steel fiber can improve the ultimate resistance, but has a limited effect on strength index. By comparing the measured ultimate resistances to the predictions, the feasibility of current design codes for the prediction of load carrying capacity is evaluated, and a simplified model is proposed to predict the ultimate resistance of circular CA-UHPCFST stub columns under axial compression.