Study on axial compression behavior of CFST stub columns with spherical-cap gaps strengthened by outer steel tubes

Outer steel tube strengthening technology is effective in improving the bearing capacity and ductility of concrete-filled steel tube (CFST) members. However, its effectiveness and mechanism for CFST members with spherical-cap gaps (CFST-SG) remain unclear. To address this, the axial compression behavior of CFST-SG stub columns strengthened with outer steel tubes (TCFST-SG) is systematically investigated. After the reliability of the finite element modeling method is verified against experimental data, 109 refined finite element models are established, with comprehensive consideration given to the coupling effects of gaps and strengthening measures. Parameter analysis demonstrates that improvements in ultimate bearing capacity (N_u) and strength index (SI) increase significantly with increases in sandwich material thickness (t_c), the diameter-to-thickness ratio of the inner steel tube (D_i/t_i), the yield strength of the outer steel tube (f_syo), and the strength of the sandwich material (f_cus). Conversely, these improvements gradually diminish with increases in unstrengthened length (L), gap ratio (χ), the diameter-to-thickness ratio of the outer steel tube (D_o/t_o), the yield strength of the inner steel tube (f_syi), and the strength of the core concrete (f_cui). Specifically, as χ increases from 2.2% to 8.8%, the improvement in N_u decreases from 38% to 26%, respectively, while the improvement in initial stiffness (K) exceeds 9.72%. Mechanistic analysis indicates that this strengthening enhances the interfacial contact between the inner steel tube and core concrete, suppresses lateral deformation, and consequently improves overall bearing capacity and stability. Based on the superposition theory and the constraint regionalization model, an axial compression bearing capacity formula for TCFST-SG stub columns is derived, showing good agreement with simulation results. Finally, an efficient construction method is provided. These findings provide a theoretical basis and technical guidance for the engineering application of outer steel tube strengthening technology.