To investigate the bearing behavior of symmetrical full-scale different strength concrete filled double skin steel tube (CFDST) stub columns, 19 full-scale specimens were designed, considering the slenderness ratio (λ); the compression strength of core concrete and sandwich concrete (f_cki, f_cko), the thickness of the inner and outer steel tubes (t_i, t_o); the diameter of inner and outer steel tubes (D_i, D_o); and the tensile strength of the inner and outer steel tubes (f_yki, f_yko) as the main parameters. Nonlinear constitutive models for concrete considering constraint effect were adopted, and a finite element (FE) model was established using ABAQUS software. By comparing the results between simulations and experiments, the rationality of the modeling method was verified. Based on the FE model, the parameter analysis for CFDST columns were conducted, and the force mechanism, stress distribution, and deformation process were analyzed. The results showed that the axial compression bearing capacity ($N_{\mathrm{s}}^{\mathrm{u}}$) increased significantly with an increase in f_cki, t_o, D_o, and f_cko, while $N_{\mathrm{s}}^{\mathrm{u}}$ decreased gradually with an increase in λ. Finally, according to the calculated results of the specimens, the calculation formula for $N_{\mathrm{s}}^{\mathrm{u}}$ of full-scale composite columns was statistically regressed using 1stOpt software and showed a good agreement with the FE.

In order to investigate the mechanical behavior of special-shaped reinforced concrete composite columns encased with GFRP core columns (EGCSSCs) subjected to axial load, twenty-seven full-scale EGCSSCs were designed with varying parameters: axial compressive strength of core concrete (f_cc), axial compressive strength of peripheral concrete (f_co), thickness of GFRP tube (t_gfrp), ratio of longitudinal reinforcement (ρ_v), stirrup ratio (ρ_s) and GFRP ratio in the cross-section (α). The three-dimensional finite element refined models of EGCSSCs were established by ABAQUS finite element software, and the response of EGCSSCs under axial load was studied based on the verification of finite element modeling. The influence of different parameters on the ultimate axial compressive strength (N_us), initial stiffness (K), and ductility index (µ) of EGCSSCs was obtained, and the typical failure mode of EGCSSCs was clearly described. The results showed that the main failure mode of the EGCSSCs subjected to axial load was bulging outward at the middle of the EGCSSCs, showed yielding of the longitudinal steel bars, and was crushing both ends of the peripheral concrete. When the column was damaged, the peripheral concrete reached peak stress earlier than the core concrete. All specimens exhibited excellent load-carrying capacity and good ductility. Moreover, with the existence of GFRP core columns, the N_us and µ of the columns were increased by 11.61% and 140.86%. In addition, K increased with the increase in f_cc, f_co, t_gfrp and α, and the largest increments were 23.99%, 50.54%, 21.77%, and 34.19%, respectively. µ decreased with the increase in f_cc and f_co, which decreased by 14.05% and 40.28%, respectively. By using statistical regression and introducing the constraint effect coefficients and the reduction coefficient, the calculation formula for the axial compression-bearing capacity of EGCSSCs was derived, which could lay a foundation for the popularization and application of this kind of composite column in practical engineering.

In order to acquire the hysteretic behavior of the asymmetrical composite joints with concrete-filled steel tube (CFST) columns and unequal high steel beams, 36 full-scale composite joints were designed, and the CFST hoop coefficient (ξ), axial compression ratio (n₀), concrete cube compressive strength (f_cuk), steel tube strength (f_yk), beam, and column section size were taken as the main control parameters. Based on nonlinear constitutive models of concrete and the double broken-line stress-hardening constitutive model of steel, and by introducing the symmetric contact element and multi-point constraint (MPC), reduced-scale composite joints were simulated by ABAQUS software. By comparing with the test curves, the rationality of the modeling method was verified. The influence of various parameters on the seismic performance of the full-scale asymmetrical composite joints was investigated. The results show that with the increasing of f_cuk, the peak load (P_max) and ductility of the specimens gradually increased. With the increasing of n₀, the P_max of the specimens gradually increases firstly and then gradually decreases after reaching a peak point. The composite joints have good energy dissipation capacity and the characteristic of stiffness degradation. The oblique struts force mechanism in the full-scale asymmetrical composite joint domain is proposed. By introducing influence coefficients (${\xi }_1$ and ${\xi }_2$), the expression of shear bearing capacity of composite joints is obtained by statistical regression, which can provide theoretical support for the seismic design of asymmetrical composite joints.