Theoretical Analysis and Experimental Study on Axial Compression Mechanical Properties of Square Steel Tube Concrete

Literature Overview

This paper, published in the China Civil Engineering Journal (Volume 34, Issue 2, 2001, pp. 17-25), presents a comprehensive theoretical and experimental investigation of the axial compression behavior of square steel tube concrete (CFST) members. The authors—Han Linhai and Tao Zhong from Fuzhou University—developed numerical methods to calculate the full load-deformation response curves and validated these through extensive testing. The research was funded by the Fok Ying Tung Education Foundation (Grant No. 0501064) and the results were incorporated into a national military standard.

Core Technical Content

Theoretical Framework

The theoretical analysis is built upon the stress-strain relationships of the constituent materials:

1. Steel stress-strain model: A bilinear or multilinear model that captures the elastic, yielding, and strain-hardening behavior of the steel tube.
2. Concrete stress-strain model: A confined concrete model that accounts for the lateral confinement provided by the steel tube, which increases both the strength and ductility of the core concrete.
3. Interaction model: A coupled model that considers the mutual interaction between the steel tube and the concrete core, including the confinement effect of the steel on the concrete and the support effect of the concrete on the steel.

Numerical Solution Method

The authors employed a numerical solution method to calculate the full load-deformation response curves for square CFST members under axial compression. The method involves:

• Discretization of the cross-section into finite elements.
• Application of the stress-strain relationships at each element.
• Iterative solution to ensure equilibrium and compatibility.
• Tracking of the progressive failure mechanism as the load increases.

Experimental Program

Twenty specimens were tested with varying confinement effect coefficients (ξ), ranging from 1.0 to 7.5644. The confinement effect coefficient is defined as:

ξ = (f_y A_s) / (f_c A_c)

where f_y is the yield strength of steel, A_s is the cross-sectional area of steel, f_c is the compressive strength of concrete, and A_c is the cross-sectional area of concrete.

Validation and Comparison

The theoretical results were validated against the experimental data from the current study as well as published results from other researchers. The agreement was described as satisfactory. The simplified calculation formulas for axial compression strength and modulus were compared with the recommendations of:

• AISC LRFD (1994)
• AIJ (1997)
• EC4 (1992)

Reliability Analysis

A reliability analysis was performed for axially compressed CFST members, considering the variability of material properties and geometric dimensions. This analysis provides a probabilistic basis for design.

Technical Interpretation

The confinement effect coefficient ξ is a fundamental parameter in CFST design because it quantifies the relative contribution of the steel and concrete to the overall member strength. At low values of ξ, the concrete dominates the load-bearing capacity, and the steel provides limited confinement. At high values of ξ, the steel provides substantial confinement, significantly enhancing the concrete's strength and ductility. The full range of ξ tested (1.0 to 7.56) covers both practical design scenarios and extreme cases, providing a comprehensive understanding of the confinement effect.

The numerical solution method developed by the authors is particularly valuable because it captures the full nonlinear behavior of the CFST member, including the progressive yielding of the steel tube, the confinement-induced strength increase of the concrete, and the eventual failure mechanism. This level of detail is essential for understanding the post-peak behavior, which is critical for ductility and energy dissipation in seismic applications.

The comparison with international codes (AISC, AIJ, EC4) is particularly significant because it provides a benchmark for the Chinese design approach. The fact that the simplified formulas derived from the theoretical and experimental work are comparable to those recommended by major international codes validates the research methodology and provides confidence in the results.

Engineering Practice Integration

For engineers designing square CFST columns, the following practical considerations emerge from this research:

• The confinement effect coefficient should be carefully selected to achieve the desired balance between strength and ductility.
• The numerical solution method can be used for detailed design analysis, particularly for non-standard geometries or loading conditions.
• The reliability analysis provides a probabilistic basis for design, which is increasingly important as design codes move toward performance-based approaches.
• The incorporation of the results into a national military standard (GJB 2001) demonstrates the practical value of the research for specialized applications.

Key Questions and Reflections

The study focuses on square CFST members, and the results may not be directly applicable to circular CFST members, which exhibit different confinement characteristics due to the uniform lateral pressure distribution. The paper does not address the behavior of CFST members under combined loading (axial force plus bending), which is more representative of actual structural conditions. Additionally, the long-term effects of creep and shrinkage on the bond and confinement behavior are not considered, which may be important for serviceability assessment.

Study Insights and Implications

This research provides a rigorous theoretical and experimental foundation for the design of square CFST members under axial compression. The development of numerical methods for full load-deformation response calculation, combined with extensive experimental validation, creates a reliable design tool. The comparison with international codes and the reliability analysis add practical value for engineers working in different regulatory environments. The incorporation of the results into a national military standard demonstrates the direct impact of academic research on engineering practice. For the steel pipe and welding industry, this work highlights the importance of understanding the interaction between steel tubes and concrete cores, which has implications for the design of composite structures in various applications including military, civil, and industrial construction.