Testing and strength prediction of polygonal UHPCFDST short columns under concentric and eccentric compression

This study investigates the structural behavior of concentrically and eccentrically-loaded ultra-high-performance concrete-filled double skin steel tube (UHPCFDST) short columns with polygon sections. A total of eight specimens, including two concentrically-loaded specimens and six eccentrically-loaded specimens, were prepared and tested. The effects of loading eccentricity (0, 50, 75, and 100 mm) and outer steel tube shape (hexagonal and octagonal) on the behavior of UHPCFDST short columns are investigated. Based on the experimental results, it is found that the failure modes of concentrically-loaded UHPCFDST short columns include outward buckling of outer steel tube, oblique shear, corner and weld fractures, crushing of sandwiched concrete, and inward buckling of inner steel tube, while the eccentrically-loaded specimens experienced outward local buckling of outer steel tube, concrete crushing at the compression side, concrete cracking at tension side, and bending of inner steel tubes. It is also observed that increasing the eccentricity reduces the initial stiffness and ultimate strength of the test specimens. In contrast, it increases the bending capacity and ductility of eccentrically-loaded specimens. Besides, it is demonstrated that specimens with octagonal sections provide higher initial stiffness, ultimate strength, and ultimate moment compared to those of hexagonal sections across all the eccentricities. However, the hexagonal specimens exhibit higher ductility than octagonal specimens. The test results are further employed to validate the applicability of codified N-M interaction curves provided in AISC-360, EC4, and GB-50936 standards. It is revealed that AISC-360 significantly underestimates the ultimate strength of polygonal UHPCFDST columns, while EC4 and GB-50936 show reasonable predictions of ultimate strength with a slight discrepancy.