FRP-Constrained Rectangular High-Strength Concrete-Filled Steel Tube Long Column Axial Compression Performance

Literature Overview

Du Yansheng, Gao Dinghui, Chen Zhihua, and Zheng Zihan from Tianjin University present a comprehensive finite element analysis of FRP-confined rectangular high-strength concrete-filled steel tube (CFST) long columns under axial compression. The study builds upon experimental test data to develop a validated nonlinear finite element model, then uses this model to investigate the influence of slenderness ratio on the effectiveness of FRP confinement. The research addresses an important practical challenge: extending the applicability of FRP confinement technology from short columns to slender columns, where buckling effects reduce the effectiveness of confinement and the interaction between steel tube local buckling, concrete crushing, and FRP rupture becomes more complex.

Finite Element Model Validation

The nonlinear finite element analysis was conducted using ABAQUS, incorporating appropriate material constitutive models for the steel tube, concrete core, and FRP wrap. The model was validated against experimental load-displacement curves, with the average deviation of peak load being only 0.5% and a variance of 0.080. The failure modes predicted by the FEA matched the experimental observations, confirming the reliability of the modeling approach including material constitutive relationships, element types, contact definitions, and boundary conditions.

Mechanical Behavior and Failure Mechanism

The study reveals a progressive failure mechanism that unfolds in a specific sequence:

1. Local buckling of the thin-walled steel tube: At relatively low compressive stress levels, the thin-walled steel tube initiates local buckling, forming outward鼓曲 (bulges) on the flat faces of the rectangular section.
2. Plastic strain development in concrete: At the locations where the steel tube has buckled outward, the concrete core develops plastic strains and lateral deformation. The concrete in these regions transitions from a confined state to a partially unconfined state, accelerating damage accumulation.
3. FRP stress concentration at corners: Due to the small fillet radius at the corners of the rectangular section, the FRP wrap experiences stress concentration at these geometric discontinuities. The stress concentration is most severe at the location of maximum steel tube鼓曲 (bulge), where the FRP undergoes the greatest hoop strain.
4. FRP rupture: The FRP ultimately fails at the location of maximum鼓曲, where the combined effects of stress concentration at corners and maximum hoop strain from steel tube鼓曲 cause the FRP to fracture.

FRP Confinement Effectiveness by Concrete Grade

The study quantifies the load-bearing improvement provided by FRP confinement for two concrete strength grades:

The higher improvement for C40 concrete is attributed to the greater relative contribution of confinement pressure to the overall strength of lower-strength concrete. For high-strength concrete (C80), the inherent compressive strength is already high, so the additional confinement pressure provided by FRP represents a smaller incremental improvement. This finding has direct implications for material selection in FRP-confined CFST systems: FRP confinement is most beneficial when combined with normal-strength concrete, while for high-strength concrete applications, the cost-benefit ratio of FRP confinement is less favorable.

Slenderness Ratio Effect on FRP Confinement

A critical finding of this study is the relationship between slenderness ratio and FRP confinement effectiveness. The FRP's ability to enhance load-bearing capacity decreases as the slenderness ratio increases. However, for models with a fillet radius of 20 mm, when the slenderness ratio reaches 31.3, the hoop FRP still provides effective confinement, improving the peak load capacity by 11%.

This result is particularly significant because it establishes a practical upper limit for slenderness ratio beyond which FRP confinement becomes ineffective. For slenderness ratios below 31.3, FRP confinement remains beneficial, but the improvement diminishes progressively as slenderness increases. This trend is explained by the fact that in slender columns, global buckling and elastic instability govern the failure mode, and the FRP's local confinement effect becomes less relevant to the overall column behavior.

Engineering Practice Implications

For steel tube manufacturers and structural designers, this study provides quantitative guidance on the applicability of FRP confinement in CFST systems. The findings suggest that FRP confinement is most cost-effective for: (a) short to moderately slender columns (slenderness ratio below 31.3), (b) columns filled with normal-strength concrete (C40 range), and (c) rectangular sections with adequate fillet radii (20 mm or larger) to reduce FRP stress concentration at corners.

From a steel tube manufacturing perspective, the fillet radius at the corners of rectangular hollow sections becomes a critical design parameter when FRP confinement is planned. Larger fillet radii reduce stress concentration in the FRP wrap and extend the slenderness ratio range over which FRP confinement remains effective. This has implications for the roll-forming or bending processes used to manufacture rectangular hollow sections: the tooling and forming parameters must be optimized to achieve the target fillet radius without introducing excessive residual stresses or dimensional deviations.

Critical Reflections

The study's finite element model, while validated against experimental data, assumes idealized FRP behavior that may not capture all real-world complexities. The FRP wrap's bond to the steel tube surface, the effect of surface roughness on bond strength, and the potential for debonding under cyclic loading are not explicitly modeled. Additionally, the study focuses on axial compression only, and the FRP confinement effectiveness under eccentric compression, bending, or combined loading would require separate investigation. The practical implementation of FRP-confined CFST columns in seismic regions would require companion research on the FRP's performance under cyclic loading, as the brittle fracture behavior of FRP under tension is fundamentally different from its behavior under static loading. Despite these limitations, the study provides a solid quantitative foundation for understanding the interaction between FRP confinement, steel tube local buckling, and concrete crushing in rectangular CFST long columns, and the slenderness ratio threshold of 31.3 for effective FRP confinement represents a practically useful design guideline.