This paper presents an experimental, numerical, and analytical investigations into the behavior of square concrete filled cold formed stiffened steel tubular stub columns with ring (hoop) stirrups (SC-CFSST) under axial compression. The proposed composite column is fabricated by cold-bending four plates to form four parts of lipped angles, which are assembled by argon welding. Ring stirrups are inserted through the perforations along the longitudinal lips to provide additional confinement. To explore the combined enhancement effects of stirrups and stiffeners, six SC-CFSSTs confined with stirrups and three concrete-filled stiffened steel tubular (CFSST) stub columns without stirrups were tested. Experimental procedures, and results are discussed in detail. A finite element (FE) model was developed to simulate the models of specimens and validated against experimental results. The validated model was employed to conduct a parametric study to better understand the behavior of SC-CFSST columns with different parameters. The results demonstrate that installing ring stirrups significantly enhances confinement effect, improving both ultimate strength and enhances post-peak performance. In addition, international design provisions were utilized to predict the ultimate axial load of CFSSTs and SC-CFSSTs. The results were compared with experimental and numerical results, showing that the DBJ design approach provides the most accurate predictions. A new formula was introduced to estimate the ultimate strength of CFSSTs and SC-CFSSTs, considering combined confinement effects. The formula showed improved accuracy in predicting ultimate strength compared to both FE simulations and existing design predictions.
