Axial Compression Performance of Hollow GFRP-Concrete-Steel Tube Composite Columns

Literature Overview

This paper by Li Wen, Zhang Xuan, and Na Yu, published in 2017 in the Journal of Xuzhou Institute of Technology (Vol. 32, No. 2, pp. 18-25), investigates the axial compression behavior of hollow glass-fiber-reinforced polymer (GFRP) tube-concrete-steel tube composite columns. The research is funded by the National Natural Science Foundation of China (Grant No. 51308028) and the Heilongjiang Provincial Department of Education Science and Technology Research Project (12543023). Two physical specimens were tested under axial compression, and eight additional specimens were analyzed using ANSYS finite element modeling after validating the numerical model against experimental results.

Structural Configuration and Design Rationale

The hollow GFRP-concrete-steel tube composite column represents a hybrid structural system that combines the advantages of multiple materials:

• Steel tube (inner): Provides high compressive strength, confinement to the concrete core, and ductility
• Concrete core: Provides compressive strength and contributes to overall stiffness
• GFRP tube (outer): Provides corrosion resistance, high specific strength, and additional confinement

The hollow configuration between the steel tube and GFRP tube may serve multiple purposes: reducing material weight and cost, providing thermal insulation, creating a pathway for inspection or future strengthening, and potentially reducing the overall density of the composite member.

Experimental Program

Two physical specimens were tested under axial compression. The experimental variables included:

The validation of the ANSYS finite element model against the experimental results is a critical step that ensures the reliability of the parametric study conducted on the additional eight virtual specimens. This experimental-numerical hybrid approach is a standard and accepted methodology in structural engineering research.

Key Technical Findings

Effect of GFRP Tube Wall Thickness

When specimen dimensions are held constant, increasing the GFRP tube wall thickness improves both the ductility and ultimate load-carrying capacity of the composite column. This is attributed to:

• Increased confinement pressure exerted on the concrete core by the thicker GFRP tube
• Greater contribution of the GFRP tube itself to the overall compressive strength
• Enhanced restraint against outward deformation of the steel tube

Size Effect

When specimen dimensions increase, the beneficial effect of increasing GFRP wall thickness on load-carrying capacity diminishes. This observation is consistent with the well-documented size effect in concrete structures, where larger specimens exhibit relatively lower strength per unit area. The confinement effect becomes proportionally less significant as the member size increases, because the confining stress (which is a function of wall thickness) becomes a smaller fraction of the total cross-sectional area.

Stress-Strain Behavior

The stress-strain curves obtained from the finite element analysis reveal the progressive load transfer mechanism in the composite column:

1. Elastic stage: All three components (GFRP, concrete, steel) carry load proportionally to their respective stiffnesses.
2. Cracking stage: Concrete micro-cracking begins, and load transfers to the steel tube and GFRP tube.
3. Confinement stage: The steel tube and GFRP tube provide lateral confinement to the cracked concrete, enhancing its post-peak behavior.
4. Failure stage: Progressive crushing of the concrete core, yielding of the steel tube, and eventual failure of the GFRP tube.

Finite Element Modeling Considerations

The ANSYS modeling approach requires careful attention to several technical aspects:

• Material models: Appropriate constitutive models must be selected for each component—concrete (e.g., Concrete Damaged Plasticity), steel (bilinear or multilinear kinematic hardening), and GFRP (orthotropic elastic or damage-based models)
• Interface modeling: The bond behavior between concrete and steel tube, and between concrete and GFRP tube, must be accurately represented using contact elements or cohesive zone models
• Mesh sensitivity: The finite element mesh must be refined sufficiently to capture stress concentrations and localized failure patterns
• Boundary conditions: The axial compression test setup must be accurately simulated, including any end bearing conditions and friction effects

Comparison with Existing Design Methods

The study implicitly validates or challenges existing design approaches for composite columns. The Chinese standard GB 50017 and various research-based formulas for steel tube concrete columns may need modification to account for the additional GFRP tube contribution. Key considerations include:

• Whether the GFRP tube contribution can be treated as an additional confinement stress in existing SRC formulas
• The appropriate strength reduction factor for the GFRP tube under compressive loading
• The interaction between steel tube confinement and GFRP tube confinement

Critical Reflections and Questions

Several important aspects deserve further investigation:

• Only two physical specimens were tested, which is a relatively small sample size for validating a numerical model and establishing reliable design trends. A larger experimental database would strengthen the conclusions.
• The study focuses exclusively on axial compression. In practice, columns are often subjected to combined axial compression and bending, and the interaction between GFRP tube properties and flexural behavior is unexplored.
• The long-term behavior of GFRP under sustained loading (creep, stress relaxation) is not addressed, which is critical for structural applications.
• The fire resistance of the composite column, particularly the performance of the GFRP tube at elevated temperatures, is a significant concern that is not discussed.
• The hollow space between the steel tube and GFRP tube—its purpose and effect on structural behavior—could benefit from more detailed analysis.

Study Insights and Implications

This research contributes to the growing body of knowledge on hybrid composite columns that combine steel, concrete, and FRP materials. The key insight for practicing engineers is that the GFRP outer tube provides meaningful improvements in ductility and strength, but the benefit is size-dependent—smaller members benefit more from increased GFRP wall thickness than larger ones. This size effect has direct implications for the economic optimization of composite column designs. The experimental-numerical hybrid methodology employed is sound and provides a practical framework for investigating additional design parameters without the cost and time of extensive physical testing. For engineering applications, the findings suggest that GFRP-concrete-steel composite columns are particularly advantageous in environments where corrosion resistance is critical (marine, chemical, or de-icing salt environments), as the GFRP outer tube provides an effective corrosion barrier while maintaining structural integrity.