Experimental Study on Rectangular Steel Tube High-Strength Concrete Compression-Bending Members

Literature Overview

This 2004 publication in the Journal of Harbin Institute of Technology, authored by Guo Lanhu, Zhang Sumei, and Tian Hua from the School of Civil Engineering at Harbin Institute of Technology, presents an experimental investigation of eight rectangular steel tube high-strength concrete (HSC) eccentrically loaded columns. Funded by the National Natural Science Foundation of China (59808004) and the Heilongjiang Province Outstanding Youth Fund, the study systematically examines the influence of slenderness ratio, eccentricity ratio, and steel ratio on the mechanical performance of these members. The research also compares experimental results with predictions from three major design codes: EC4, LRFD, and AIJ.

Core Technical Content

The experimental program consisted of eight rectangular steel tube high-strength concrete members subjected to uniaxial eccentric compression. The test matrix varied three parameters: slenderness ratio (λ), eccentricity ratio (e/h), and steel ratio (ρ). The high-strength concrete used in the specimens typically exhibits compressive strengths in the range of 60–100 MPa, which introduces unique material behavior compared to conventional concrete-filled steel tube members.

Key experimental findings include:

The critical finding regarding the 8% steel ratio threshold is particularly significant for engineering practice. When the steel ratio falls below this value, local buckling of the steel tube occurs before the member reaches its ultimate load capacity. This means that for slender or lightly reinforced rectangular steel tube HSC members, the design must ensure that the steel ratio exceeds 8% to prevent premature local buckling, or else the design must account for the reduced capacity associated with local buckling.

High-Strength Concrete and Steel Tube Interaction

The interaction between high-strength concrete and the confining steel tube is more complex than in conventional concrete-filled tube members. High-strength concrete exhibits reduced tensile strain capacity and more brittle fracture behavior, which affects the confinement effectiveness of the steel tube. The rectangular geometry further complicates the interaction because the concrete confinement is non-uniform — the flat faces provide effective confinement while the corners experience complex stress states.

From a welding perspective, the rectangular steel tubes used in these members are typically fabricated by HFW welding followed by cold forming. The cold forming process introduces residual stresses that interact with the applied eccentric compression stresses. The weld seam, typically located at the center of one flat face, experiences different stress levels depending on the eccentricity direction. When the eccentricity is perpendicular to the weld seam, the seam is on the tension side or compression side of the section, while when parallel, the seam experiences shear stresses.

The local buckling behavior observed below the 8% steel ratio threshold has direct implications for weld quality requirements. Local buckling initiates at stress concentrations, and any weld defect — such as a lack of fusion, undercut, or excessive weld toe radius — creates a stress concentration that can trigger premature buckling. The welding quality standards for rectangular steel tubes in high-strength concrete applications should therefore be more stringent than for conventional applications.

Code Comparison and Design Implications

The comparison of experimental results with three major design codes reveals important discrepancies:

The finding that LRFD and AIJ provisions are conservative for rectangular steel tube HSC members suggests that these codes were developed primarily based on data from circular steel tube members or conventional concrete-filled tubes. The rectangular geometry and high-strength concrete combination creates a different failure mechanism that the existing codes do not fully capture. Engineers using LRFD or AIJ for such members may be designing for unnecessarily heavy sections, leading to material waste and increased costs.

The EC4 provisions, being closest to the experimental results, provide a more rational basis for design. However, even EC4 may require modification to fully account for the 8% steel ratio threshold effect and the specific buckling behavior of rectangular sections under eccentric compression.

Study Insights and Implications

The most practically valuable insight from this study is the identification of the 8% steel ratio threshold for local buckling prevention. This threshold provides a clear, quantifiable design criterion that engineers can apply directly. For members with steel ratios below 8%, the design must either increase the steel ratio or account for the capacity reduction due to local buckling.

For welding engineers, the study highlights the importance of weld quality in preventing premature local buckling. The weld seam in rectangular steel tubes is a potential initiation site for local buckling, and maintaining high welding quality standards is essential for achieving the predicted structural performance. The use of high-strength concrete further amplifies the consequences of weld defects because the brittle concrete fracture behavior provides less energy absorption capacity.

In conclusion, this experimental study provides essential data for the rational design of rectangular steel tube high-strength concrete compression-bending members, with the 8% steel ratio threshold serving as a critical design parameter and the EC4 code providing the most accurate design basis among the codes examined.