Biaxial eccentric behavior of improved multi-cell T-shaped concrete-filled steel tubes

The special-shaped concrete-filled steel tube (CFST) structure has attracted considerable scholarly interest in recent years. However, limited attention has been given to research on multi-cell T-shaped CFST (MT-CFST) column, resulting in a lack of uniformity in calculation methods. This paper presents an improved MT-CFST column composed of three rectangular steel tubes, a steel plate, and concrete. This design ensures that welds avoid cold bending zones in steel tubes, thereby enhancing quality of welds. In practical engineering applications, columns often experience biaxial eccentric loads. A biaxial eccentric compression test was conducted on fifteen MT-CFST specimens. This study investigated the influence of eccentricity, eccentric angle, and web height on performance of MT-CFST column. The experimental results revealed a similar failure mode among specimens: prior to peak load, minor bending deformation was observed. For specimens T56.6–60-45° and T40–90-180°, the smaller width-to-thickness ratio of steel plates on compressed side prevented local buckling. For other specimens, slight local buckling on compressed side near mid-span section was observed. After peak load, local buckling progressively intensified, primarily due to substantial bending deformation. The failure of specimens was attributed to local buckling and overall bending deformation. For eccentricity greater than 20 mm, the bearing capacity and elastic-plastic stiffness of specimens exhibited an increasing trend followed by a decreasing trend with the increase in eccentric angle at the same eccentricity, with the peaks occurring at around 90°. Specimens with the compressed side located at flange demonstrated higher bearing capacity and elastic-plastic stiffness. Additionally, FE models have been established to simulate the bidirectional eccentric behavior. A simplified method for calculating flexural capacity was developed, grounded in stress analysis of section at ultimate conditions. In accordance with ANSI/AISC 360, a method was established for predicting uniaxial eccentric capacity. By conducting a parametric analysis of M_x/M_ux,n-M_y/M_uy,n curves, a simplified method for determining M_x/M_ux,n-M_y/M_uy,n curves was proposed, with calculation results in good agreement with FE results.