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

Literature Overview

This research by Li Song and colleagues from Guangzhou University, published in Engineering Science and Technology in 2022, investigates the ultimate bearing capacity of short columns composed of high-strength concrete (HSC) confined by both a steel tube and externally wrapped carbon fiber reinforced polymer (CFRP). The study was supported by the National Natural Science Foundation and Guangdong provincial programs. The work employs the limit equilibrium method to derive theoretical formulas and validate them against experimental data.

The motivation for this research lies in the growing demand for high-strength, lightweight structural members. Conventional reinforced concrete columns face limitations in strength and ductility, while steel tubes alone are susceptible to local buckling under compression. The combination of steel tube confinement with CFRP external wrapping offers a multi-level confinement strategy, but the interaction between these confinement layers and the high-strength concrete core requires careful analysis.

Theoretical Framework

The limit equilibrium method provides a rigorous approach to determining ultimate bearing capacity by analyzing the stress state at failure. The authors derive theoretical formulas by performing stress analysis on three components:

1. High-strength concrete core — subjected to triaxial compression from both steel tube and CFRP confinement
2. Steel tube — providing radial confinement and resisting axial compression
3. CFRP jacket — providing additional radial confinement through hoop tension

A key concept introduced is the lateral pressure effect coefficient, which quantifies the efficiency of lateral confinement in enhancing concrete strength. This coefficient accounts for the non-uniform distribution of confinement pressure around the column perimeter and the geometric effects of the steel tube and CFRP layers.

Key Findings and Comparisons

The study reveals important differences between CFRP-confined steel tube high-strength concrete and CFRP-confined steel tube normal-strength concrete:

• CFRP confinement effectiveness is reduced in HSC. This is because high-strength concrete has a lower lateral expansion ratio compared to normal-strength concrete under axial compression. Since CFRP confinement relies on the radial pressure generated by concrete expansion, the lower expansion of HSC results in less effective CFRP engagement.
• Thick-walled steel tubes provide greater bearing capacity improvement. The steel tube is the primary confinement mechanism, and increasing wall thickness directly increases the confinement pressure. This finding suggests that for HSC applications, investment in steel tube wall thickness is more effective than adding CFRP layers.
• Theoretical predictions correlate well with experimental values. The limit equilibrium formulas, when calibrated against test data, provide accurate predictions of ultimate bearing capacity, validating the theoretical framework.

Design Implications

Several design recommendations emerge from this study:

1. Steel tube thickness optimization. For columns using high-strength concrete, increasing the steel tube wall thickness yields more significant bearing capacity gains than adding CFRP layers. Designers should prioritize steel tube thickness in the preliminary design phase.
2. CFRP layer count rationalization. The diminishing returns of additional CFRP layers in HSC applications suggest that over-designing the CFRP wrapping is inefficient. A practical approach would be to determine the minimum CFRP layers needed to prevent steel tube buckling and provide adequate ductility, rather than maximizing CFRP thickness.
3. Confinement efficiency assessment. The lateral pressure effect coefficient should be used to evaluate the effectiveness of the multi-layer confinement system. This coefficient varies with column geometry, material properties, and confinement configuration, and should be determined for each specific design.
4. Ductility considerations. While the study focuses on ultimate bearing capacity, the confinement system also influences ductility. The combination of steel tube and CFRP provides a more gradual failure mode compared to unconfined HSC, which is important for seismic applications.

Key Questions and Reflections

The study raises several questions that merit further consideration. First, the transition from short column behavior to slender column behavior was not addressed. In practice, columns with varying slenderness ratios are common, and the confinement effectiveness may differ significantly for slender members where buckling governs. Second, the study assumes uniform CFRP wrapping, but in practice, defects such as voids, wrinkles, and overlaps can significantly reduce CFRP effectiveness. The sensitivity of the theoretical model to CFRP quality warrants investigation. Third, the long-term behavior under sustained loading, including creep and stress relaxation of CFRP, was not considered, yet these effects may be significant in service.

The limit equilibrium approach used in this study is a classical and rigorous method that provides clear physical insight into the failure mechanism. This is advantageous compared to purely empirical or numerical approaches, as it allows designers to understand the underlying mechanics and make informed design decisions.

Summary

This study provides a rigorous theoretical framework for predicting the ultimate bearing capacity of CFRP-confined steel tube high-strength concrete short columns using the limit equilibrium method. The key finding that CFRP confinement is less effective for high-strength concrete, while thick-walled steel tubes provide superior bearing capacity improvement, has direct implications for design practice. Engineers should prioritize steel tube thickness optimization over excessive CFRP wrapping when designing columns with high-strength concrete cores. The validated theoretical formulas offer a reliable tool for design calculations, complementing experimental data and enabling efficient structural design of confined concrete columns in modern construction applications.