Structural Behaviour of Steel-Reinforced Concrete Composite Columns Under Eccentric Axial Loading: Experimental and Analytical Study

Steel-reinforced concrete composite columns, incorporating an encased structural steel section within a reinforced concrete cross-section, offer significantly enhanced strength and ductility relative to conventional reinforced concrete columns and are increasingly considered for high-rise and transfer structure applications in major Nigerian cities. This study investigates the structural behaviour of steel-reinforced concrete composite columns under eccentric axial loading through a combined experimental and analytical programme, motivated by the absence of empirical performance data for composite columns fabricated with Nigerian-grade materials. Eight composite column specimens of 200 mm by 200 mm cross-section, each encasing a 76 x 76 x 6 mm structural steel cruciform section, were fabricated and tested under increasing eccentric axial load at eccentricity-to-depth ratios of 0, 0.1, 0.2, and 0.4. Load-moment interaction responses, lateral deflection profiles, failure modes, and strain distributions across the composite section were recorded. Analytical interaction diagrams were constructed using the Eurocode 4 composite column design method and compared against experimental values. Results indicate that experimental peak loads aligned with Eurocode 4 predictions within a mean error of 7.2 percent for zero and small eccentricity cases but diverged by up to 18 percent at high eccentricity due to premature concrete spalling at the concrete-steel interface under combined bending and compression. Full interaction between the steel section and concrete was confirmed by strain compatibility analysis at load levels below 70 percent of ultimate. Confinement effects from the encased steel cruciform enhanced post-peak ductility, with load retention at 25 mm lateral displacement averaging 62 percent of peak load. The study recommends Eurocode 4 for routine composite column design with a 15 percent capacity reduction factor at high eccentricity pending further calibration. Keywords: composite columns, eccentric loading, steel-reinforced concrete, Eurocode 4, structural behaviour.