Experimental and numerical study of axial behaviour of slender-waist shaped concrete-filled steel tubular stub columns

Due to the transverse expansion speed of the steel in the elastic stage is greater than that of the concrete, the confinement hysteresis effect will occur in the concrete-filled steel tube (CFST) under axial loading case. A novel slender-waist shaped CFST is proposed in this study, with the shape of the outer steel tube smaller in the middle section and larger in both end sections. A series of axial compression tests on newly proposed stub columns are carried out. The test results demonstrate that the core concrete of novel columns is effectively confined by the steel tube. Compare with traditional CFST columns, the slender-waist shaped CFST has a longer elastic stage and its lateral confinement pressure is activated earlier. And the local inward buckling of the outer steel tube is suppressed by the infilled concrete, leading to a very ductile response. It can be found from the failure mode of the slender-waist shaped columns that the middle section is still the weakest. The axial load-bearing capacity can significantly improve by pasting FRP on the surface at the middle position of the column. The numerical modelling approach for the proposed CFST columns is elaborated and validated by comparing with the test data. Finally, based on the limit equilibrium theory, the suggested formula is given in this study to predict the ultimate axial capacity of these new shaped hybrid columns.