Experimental Study on Axial Compression Strength of Rectangular Steel Tube Concrete Members

Literature Overview

The landmark study by Han Linhai and Yang Youfu (Fuzhou University and Harbin Institute of Technology, 2001) presents a comprehensive experimental investigation of rectangular steel tube concrete (SRC) members under axial compression. Funded by the Fok Ying Tung Education Foundation (Grant 0501064), the research was published in the China Civil Engineering Journal, Vol. 34, No. 4, pp. 22-31. Twenty-four rectangular SRC short column specimens were tested, varying the confinement effect coefficient from 0.5 to 1.3 and the section aspect ratio from 1.0 to 1.75.

Test Matrix and Design Parameters

The confinement effect coefficient is defined as the ratio of the steel tube cross-sectional area to the concrete core cross-sectional area, adjusted for the effectiveness of confinement. This parameter directly controls the degree of lateral restraint provided by the steel tube to the concrete core.

The section aspect ratio, defined as the ratio of the longer dimension to the shorter dimension of the rectangular cross-section, influences the uniformity of confinement. A square section (aspect ratio 1.0) provides more uniform confinement than a rectangular section with a higher aspect ratio, where the corners experience greater confinement than the flat faces.

Results and Code Comparison

The experimental results demonstrated that both the confinement effect coefficient and the section aspect ratio significantly influence the axial compression mechanical properties and strength capacity of rectangular SRC members. Higher confinement coefficients led to increased strength and ductility, while higher aspect ratios generally reduced the effective confinement efficiency.

All four codes produced conservative predictions compared to the experimental results, but the Chinese standard GJB (2000) provided the closest agreement with the test data. This finding suggests that the GJB design equations, developed with consideration of Chinese material properties and construction practices, are particularly well-suited for rectangular SRC member design.

Engineering Practice Implications

For engineers designing rectangular SRC columns, this study provides critical experimental data that validates and refines the understanding of confinement effectiveness. The results indicate that rectangular SRC members with aspect ratios up to 1.75 can achieve significant strength enhancement through confinement, provided that appropriate confinement coefficients are maintained.

The comparison with international codes reveals that designers using LRFD, AIJ, or EC4 may be inadvertently conservative when designing rectangular SRC members, potentially leading to uneconomic designs. The GJB standard offers a more accurate and economical basis for design, although engineers should verify that the material properties and construction quality meet the assumptions underlying the GJB equations.

Key Questions and Reflections

Several important considerations emerge from this study. First, the test specimens were short columns, meaning that buckling effects were not investigated. In practical applications, slender SRC columns may exhibit different behavior due to the interaction between local confinement effects and global buckling.

Second, the study focuses on monotonic axial loading. Under cyclic loading conditions, such as those experienced during earthquakes, the confinement effectiveness may degrade due to steel tube yielding and concrete crushing. The long-term durability of rectangular SRC members under sustained loading also requires attention, particularly regarding creep and shrinkage effects.

Third, the transition from laboratory-scale specimens to full-scale structural members may introduce additional variability in the confinement effectiveness. Factors such as concrete placement quality, steel tube fabrication tolerances, and connection details can all influence the actual performance relative to the idealized test conditions.

Study Insights and Reference Value

This experimental study provides a robust database for the design and analysis of rectangular steel tube concrete members. The systematic variation of confinement coefficient and aspect ratio enables engineers to establish reliable design correlations and to select optimal cross-sectional configurations for specific structural applications. The code comparison results are particularly valuable for international projects where multiple design standards may be applicable, as they guide the selection of the most appropriate and economical design approach for rectangular SRC members.