Development and validation of a nonlinear mechanical model for fiber concrete-enhanced steel columns with flexural-shear failure mode

The Fiber Concrete–Enhanced Steel (FCES) column is a composite column consisting only of a steel section and fiber-reinforced concrete, without conventional reinforcement, thereby reducing construction complexity. Currently, ASCE 41 does not provide modeling parameters for FCES columns. In addition, the AIJ guidelines propose a mechanical model for FCES columns based on the assumption of a flexural failure mode. However, this model does not reflect the actual behavior observed in many experimentally tested FCES columns reported in previous studies, where the predominant failure mode is flexural–shear. This study develops a nonlinear mechanical model for FCES columns exhibiting flexural–shear failure. The proposed model is modified from an existing Steel-Reinforced Concrete (SRC) model and is validated using 25 FCES column test datasets. The model is applicable to concrete compressive strengths ranging from 31.2 to 65.3 MPa, axial load ratios of 0.09–0.30, fiber contents of 1–2 %, XH-steel ratios of 5.64–6.65 %, and H-steel ratios of 3–7.01 %. The considered fiber types include Hybrid fiber concrete (HyFC), Steel fiber concrete (SFC), and PVA fiber concrete (PVA-FC). Three fiber concrete material properties—compressive strength, flexural strength, and elastic modulus—are examined. For HyFC, regression formulas are developed to predict compressive and flexural strengths with average errors of 8.4 % and 14.8 %, respectively, while the elastic modulus is estimated using an existing empirical relation due to limited available data. For SFC and PVA-FC, existing predictive formulas are evaluated and the most reliable are adopted. The proposed model defines four key points—cracking, peak, stable post-peak, and ultimate—based on elastic analysis, P–M interaction, and modified deformation relationships. The results demonstrate conservative predictive performance. Nevertheless, further studies are required to extend the applicability of the model to higher axial load ratios, broader steel section ratios, and more reliable elastic modulus models for HyFC.