Notes: 1. α₁, a₂, b₁, b₂—the width of each side, as shown in Fig. (2). 2. t—the thickness of steel tube. 3. f_ys, f_ck—the yield strength of steel and the characteristic compressive strength of concrete prism, calculated by f_ck =0.76 f_cu,k. 4. N_b, ε_b—the load and corresponding average longitudinal strain when the local buckling occurred, it means the local buckling appeared at the post-peak stage when they are marked by “()”, and when local buckling was not found, they are marked by “—”. 5. N_u, ε_u—the experimental maximum load and corresponding average longitudinal strain. 6. ε₈₅—the average longitudinal strain when the load falls to 85% of the maximum load. 7. N_un—the nominal strength of the specimens, N_un= f_ckA_c+f_ysA_s, where A_c is the area of concrete, A_s is the area of steel tube. 8. DI— ductility index, defined as DI=0.75×(ε₈₅/ε₇₅) [¹⁸], where ε₇₅ is the average longitudinal strain when the load reaches 75% of the maximum load in the pre-peak stage.