Experimental and Numerical Studies on the Behavior of Eccentrically Loaded Square FRP Tube Concrete–Encased Steel Columns

This study investigated the load-bearing capacity, ductility, and confinement effectiveness of square fiber-reinforced polymer (FRP) tube concrete–encased steel columns (FCSCs) under eccentric loading using a combined experimental testing and finite-element model (FEM) validation approach. Seven square FCSCs with different steel ratios (8.3%, 11.3%, 14.3%, and 17.3%) and eccentricities (40 and 80 mm) were tested to obtain the failure mode, confinement mechanism, and relationship between the load-bearing capacity and these variables. The experimental results validated the accuracy of the FEM, which was subsequently used for parametric analysis to further investigate the effects of concrete cover thickness and steel ratio. Additionally, the performance of the FCSCs was compared with that of steel columns (SCs) and FRP tube concrete columns (FCCs) to assess the contribution of the encased steel. The results revealed that the FCSCs exhibited flexural failure under eccentric loading. Both the FRP tube and encased steel contributed significantly to the confinement of the concrete, with the web providing more effective confinement than the flange. Eccentricity reduced confinement provided by both the FRP tube and encased steel. The FCSCs demonstrated superior load-bearing capacity and ductility to the SCs and FCCs. Both reducing concrete cover thickness for a constant steel ratio and increasing steel ratio for a constant concrete cover thickness enhanced load-bearing capacity, with this effect becoming more pronounced at higher eccentricities. Furthermore, under constant concrete cover thickness, the flange area had a greater effect on the load-bearing capacity and ductility of the column compared to the web area when both were increased by the same amount.