Flexural rigidity of SFRC columns at the onset of buckling failure: Analytical and numerical study

The performance of slender columns under eccentric loading is influenced by second-order effects, which can significantly reduce their flexural rigidity, stability, and ultimate capacity. While incorporating steel fibers in concrete improves strength and ductility, the impact on the flexural rigidity of slender steel fiber reinforced concrete (SFRC) columns, particularly at ultimate capacity (at the onset of buckling failure), remains underexplored. This study addresses this gap by developing a practical analytical approach to estimate the flexural rigidity of SFRC columns at buckling failure, using sectional analysis and concentric buckling behavior. To validate the proposed approach, finite element models were developed based on experimental study benchmarks. Using the validated FE models and the proposed approach, the flexural rigidity of the benchmarks was estimated and compared under various load eccentricities. The results of the approach showed strong agreement in predicting the flexural rigidity at ultimate capacity. A parametric study was then conducted to assess the influence of fiber volume fraction (ranging from 0% to 2%), slenderness ratio (ranging from 80 to 106.66), reinforcement ratio (ranging from 0.5% to 2%), concrete strength (ranging from 20MPa to 40MPa), and loading eccentricity on the behavior of SFRC columns under ultimate loading conditions. The findings highlight that the proposed analytical approach estimates the flexural rigidity of SFRC columns with a conservative error margin of 5–7%. Steel fibers enhance flexural rigidity and capacity, particularly at lower load eccentricities and fiber volumes around 1.5%, though their effect diminishes with higher eccentricities and fiber content. Increasing concrete strength, longitudinal reinforcement ratio, and slenderness ratio also improve flexural rigidity and capacity, but their influence reduces under bending-dominated conditions. Slender columns experience capacity reductions due to amplified P-Δ effects, which are partially mitigated at higher axial loads. Current codes of practice overestimate flexural rigidity at higher load eccentricities and underestimate it at lower eccentricities, underscoring the need for updated approaches that more accurately account for the effects of load eccentricity, material properties, and slenderness on column behavior.