Buckling analysis of hybrid composite columns

This study investigates the effect of epoxy-based fiber-reinforced polymer (FRP) jackets on the load-carrying capacity of columns with high buckling risk under seismic conditions using the finite element method. Column models consisting of a C30 concrete core, four S355 steel rebars, and a six-layer FRP jacket were generated in three reinforcement series with 3 mm, 4 mm, and 5 mm bar diameters. A total of 273 configurations were analyzed by varying the fiber orientation between 0° and 90° in 15° increments. This study introduces a novel modeling strategy that enables FRP jackets to be examined in an integrated manner with three-dimensional solid column models, thereby extending beyond the classical surface- and shell-based approaches in the literature. Critical buckling loads were obtained through Static Structural–Eigenvalue Buckling analyses, and all percentage variations were evaluated with respect to unjacketed reference models. Furthermore, Force Reaction analyses performed under a 2 mm displacement-controlled loading on carbon, kevlar, and glass-fiber models demonstrated that FRP jacketing significantly alters the axial stiffness and ductile–brittle tendencies of the columns. Results showed that at 0° fiber orientation, capacity enhancements reached 172.2% for the 3CCCCCC configuration, 160.7% for 4CCCCCC, and 145.8% for 5CCCCCC. Fully carbon configurations (CCCCCC) provided the highest capacity, while carbon-dominated hybrid layups such as ACACAC, CAGCAG, and GCGCGC achieved moderate increases between 60% and 110%. With increasing fiber angle, the capacity decreased, yielding losses of 5–7% in carbon configurations, 12–18% in hybrid ones, and 17–23% in kevlar–glass-based configurations within the 60°–75° range. Increasing the rebar diameter from 3 mm to 5 mm improved the overall stiffness by 8–12%, and the maximum load-carrying capacity was obtained as 926.88 kN in the 5CCCCCC model. Consequently, FRP jackets with high carbon content and fiber orientations between 0° and 30° were identified as the most effective configurations for enhancing buckling resistance, and hybrid FRP systems were shown to provide an efficient strengthening method by improving the stiffness and ductility of columns and reducing buckling-induced collapse risk in seismic regions.