Ultimate Bearing Capacity Analysis of CFRP-Constrained Steel Tube High-Strength Concrete Axial Compression Short Columns

Literature Overview

This paper by Li Song and colleagues from Guangzhou University, published in Engineering Science and Technology in 2022 (Volume 54, Issue 2), presents a theoretical analysis of the ultimate bearing capacity of carbon fiber reinforced polymer (CFRP) constrained concrete-filled steel tube (CFST) short columns under axial compression. The research was funded by the National Natural Science Foundation of China (52078148, 51778158), Guangdong Provincial Water Science and Technology Innovation Key Project (2017-32), and Guangzhou University Graduate Innovation Capability Cultivation Project (2018GDJC-D08).

The study specifically addresses CFRP-constrained steel tube high-strength concrete (HSC) columns, which represent an advanced composite structural system combining the confinement effects of CFRP wrapping with the inherent confinement of the steel tube. This is distinct from conventional CFRP-constrained CFST columns using normal-strength concrete, and the differences in behavior are significant enough to warrant separate analytical treatment.

Theoretical Framework and Limit Equilibrium Method

The analytical approach employed in this study is based on the limit equilibrium method, which provides a rigorous theoretical foundation for predicting the ultimate bearing capacity of composite columns. The method involves stress analysis of each constituent material (high-strength concrete, steel tube, and CFRP) under the confinement conditions developed during axial compression.

The derivation process considers the following key aspects:

1. High-strength concrete stress state: The confinement pressure from both the steel tube and CFRP creates a triaxial stress state in the concrete core. For HSC, which exhibits more brittle behavior than normal-strength concrete, the confinement effect is particularly important for improving ductility and post-peak capacity.
2. Steel tube stress state: The steel tube undergoes both axial compression and hoop tension due to the outward expansion of the concrete core. The interaction between the steel tube and concrete involves radial pressure and shear transfer at the interface.
3. CFRP stress state: The CFRP wrapping provides additional hoop confinement. The stress-strain relationship of CFRP is linear-elastic up to failure, which differs fundamentally from the elastic-plastic behavior of the steel tube.

The theoretical formula derived in this study is validated against experimental test data, confirming the accuracy of the analytical approach. The validation process demonstrates that the limit equilibrium method is applicable to CFRP-constrained steel tube HSC columns, providing engineers with a reliable analytical tool for preliminary design and verification.

Key Technical Findings

The study reveals several important findings that distinguish CFRP-constrained steel tube HSC columns from conventional CFRP-constrained CFST columns using normal-strength concrete.

The finding that CFRP confinement effectiveness is lower in the HSC case is particularly noteworthy. This can be attributed to the higher elastic modulus and more brittle behavior of HSC, which reduces the radial expansion tendency that activates CFRP confinement. In normal-strength concrete CFST columns, the concrete core expands more readily under axial compression, generating higher radial pressures that more effectively engage the CFRP wrapping. In HSC columns, the stiffer concrete core resists radial expansion more effectively, resulting in lower confinement pressures and consequently lower CFRP stress levels at failure.

The finding that thicker steel tubes provide greater advantages for bearing capacity improvement in the HSC case is practically significant. Since the CFRP contribution is reduced in HSC columns, the steel tube becomes the dominant confinement element. Increasing the steel tube thickness directly increases the confinement pressure on the concrete core, leading to more substantial improvements in both strength and ductility.

Comparison with CFRP-Constrained Steel Tube Concrete

The study explicitly analyzes the differences and connections between CFRP-constrained steel tube HSC columns and CFRP-constrained steel tube concrete (normal-strength) columns. This comparative analysis is valuable because it clarifies the applicability boundaries of existing design methods and analytical formulas.

The fundamental difference lies in the stress-strain behavior of the concrete core material. Normal-strength concrete exhibits a more gradual post-peak softening, which allows for sustained radial expansion and progressive activation of the CFRP confinement. High-strength concrete, with its higher compressive strength and steeper post-peak descent, generates less radial expansion for a given level of axial strain, resulting in delayed and reduced CFRP engagement.

The connection between the two systems is that both employ the same basic structural configuration (steel tube + concrete core + CFRP wrapping) and both benefit from the confinement effects of the steel tube and CFRP. The analytical framework developed for normal-strength concrete CFST columns can be adapted for HSC columns by modifying the concrete stress-strain model and confinement pressure calculations to account for the different material behavior.

Engineering Practice Implications

For engineers designing CFRP-constrained steel tube HSC columns, this study provides several practical guidelines:

• CFRP layer design: The number of CFRP layers should be determined based on the specific HSC grade and steel tube thickness, as the confinement effectiveness differs from normal-strength concrete applications. Over-designing the CFRP layers may result in unnecessary material waste without proportional capacity improvement.
• Steel tube thickness selection: Thicker steel tubes should be considered as a primary means of capacity enhancement in HSC columns, as they provide more effective confinement than additional CFRP layers. The cost-effectiveness of increasing steel tube thickness versus adding CFRP layers should be evaluated on a case-by-case basis.
• Analytical verification: The theoretical formulas derived in this study should be used for preliminary design calculations, with verification through finite element analysis and, where possible, experimental validation for critical applications.
• Ductility considerations: The reduced CFRP confinement effectiveness in HSC columns may result in lower post-peak ductility compared to normal-strength concrete columns. Engineers should verify that the ductility requirements of the specific application are satisfied, particularly for seismic design applications.

Summary

This study provides a rigorous theoretical framework for predicting the ultimate bearing capacity of CFRP-constrained steel tube high-strength concrete short columns under axial compression. The limit equilibrium method-based analytical formula, validated against experimental data, offers engineers a reliable design tool. The key finding that CFRP confinement effectiveness is reduced in HSC columns compared to normal-strength concrete columns has important implications for design methodology and material optimization. The demonstrated advantage of thicker steel tubes for capacity improvement in HSC columns provides a practical design strategy that engineers can apply directly. These findings contribute to the ongoing development of advanced composite structural systems that combine the benefits of high-strength concrete, steel tube confinement, and CFRP reinforcement.