Study on octagonal UHPCFDST stub columns subjected to concentric and eccentric compression

This research investigates the behavior of octagonal ultra-high-performance concrete-filled double-skin steel tubular (UHPCFDST) stub columns under concentric and eccentric compression. Twelve specimens were prepared and tested to investigate the influence of loading eccentricity, thickness of outer steel tube, hollow ratio, and steel connecting strips on the response of these octagonal columns. The test results showed that increasing the loading eccentricity reduces the strength of octagonal UHPCFDST stub columns, while improving their moment and ductility. Additionally, increasing the thickness of the outer steel tube enhances the ultimate strength, moment, and ductility of these composite specimens. Conversely, a higher hollow ratio reduces these properties. Furthermore, adding connecting strips to create multi-cell octagonal UHPCFDST columns improves their strength and ductility compared to single-cell counterparts. Finite element (FE) modeling was established and validated against experimental results of UHPCFDST stub columns under concentric and eccentric compression. The FE-predicted results agreed well with the test results. Subsequent parametric studies were conducted to investigate the influence of various parameters on the load-strain curves, ultimate strength, ductility, and moment-load
interaction curves of octagonal UHPCFDST stub columns. The applicability of codified
interaction curves provided in AISC-360-16, EC4, and GB-50936-2014 were validated against experimental and parametric study results. It was shown that EC4 and GB-50936-2014 provided highly conservative predictions, while AISC-360-16 yielded a highly unconservative prediction. Simplified formulas were established to predict the
interaction curves of octagonal UHPCFDST stub columns under eccentric compression. The proposed
interaction curve reliably aligns well with experimental and parametric study results.