Design equations for axial capacity and effective flexural stiffness of GFRP-RC columns considering eccentricity and slenderness

This study compiles and analyzes data from 36 experimental programs conducted worldwide between 1999 and 2025 on GFRP-reinforced concrete (GFRP-RC) columns, covering a total of 274 test specimens. A comparison of peak load predictions by different design codes showed that CSA S6:19 provides estimates that align more closely with test results than ACI CODE-440.11–22 and CSA S806:12 (R2021), which neglect the axial contribution of GFRP bars. An empirical equation is proposed to estimate the ratio of experimental to predicted peak load, capturing an exponential decrease with increasing eccentricity and a linear reduction with the slenderness ratio. This equation demonstrates a strong correlation with the experimental data, covering eccentricity ratios up to 100 % and slenderness ratios from 11.5 to 62.3. The study also investigates the effective flexural stiffness using a secant-based approach applied to 147 tests and develops refined stiffness equations through statistical analysis. A Weibull distribution analysis supports modifying the coefficients used in the flexural stiffness expressions in ACI CODE-440.11–22 for improved design accuracy. Furthermore, a refined stiffness formula is proposed, accounting for experimental evidence that GFRP bars exhibit similar elastic modulus in tension and compression, as well as the deeper flexural cracking observed in GFRP-RC columns compared with steel-RC columns. A key limitation of current design codes is their lack of stiffness formulations that incorporate the effects of eccentricity and slenderness. Nonlinear regression of test data yields new empirical expressions that better match observed behavior. Evaluation of the experimental loading paths confirms that the proposed formulations provide closer alignment with actual structural response. • A database of 274 GFRP column axial compression tests is prepared. • An empirical equation is proposed for axial load capacity of GFRP-RC columns after examining code approaches. • Statistical analysis using Weibull distribution to refine flexural stiffness coefficients. • New formulas are compared with and validated against theoretical models and experimental data.