Axial Tension Mechanical Properties of Cold-Formed Square CFST Members

Literature Overview

The study by Gao Huaguo, Zhang Lingxin, Jiao Chenxing, and Wang Qingli (2022), published in Building Structure, investigates the axial tension behavior of cold-formed square concrete-filled steel tube (CFST) members. Funded by the Liaoning Provincial Xingliao Talent Program (XLYC1902009), the Heilongjiang Provincial Tounian Action Plan, and the China Earthquake Administration Engineering Mechanics Research Institute Key Project (2019A01), this research conducted tension tests on six specimens: five cold-formed square CFST specimens and one cold-formed square steel tube specimen (without concrete filling).

Experimental Program and Test Parameters

The experimental program was designed to investigate the effects of concrete strength, section width-to-thickness ratio, and the presence of concrete filling on axial tension behavior. The following table summarizes the test parameters:

The cold-forming process is particularly significant from a steel pipe manufacturing perspective, as it introduces residual stresses and strain hardening in the corners and flat portions of the square tube. These process-induced effects directly influence the tensile behavior of the composite member.

Key Experimental Findings

The research yielded several important findings regarding the axial tension behavior of cold-formed square CFST members:

• As the width-to-thickness ratio increases, the fracture strain of the specimens decreases, indicating reduced ductility at higher slenderness ratios.
• The fracture locations were concentrated at mid-span and at one-third of the specimen height from the loading ends, with fracture planes oriented horizontally or at 45 degrees to the mid-section.
• Core concrete cracking occurred when the longitudinal strain at the mid-section measurement point reached approximately 150 microstrain (με), at which point the specimen stiffness showed a noticeable reduction.
• Within the width-to-thickness ratio range of 15 to 240, concrete filling increased the axial tension bearing capacity of the steel tube by 3.03% to 30.75%.

Working Mechanism Analysis

The working mechanism of cold-formed square CFST members under axial tension can be understood through the following stages:

1. Elastic stage: Both the steel tube and core concrete deform elastically under tensile load. The steel tube carries the majority of the load due to its higher tensile strength.
2. Concrete cracking stage: When the longitudinal strain reaches approximately 150 με, the core concrete begins to crack. The steel tube and concrete are no longer bonded together in the cracked regions, and load transfer occurs through interface friction.
3. Stiffness reduction stage: After concrete cracking, the specimen stiffness decreases noticeably, and the steel tube begins to carry a larger proportion of the total load.
4. Fracture stage: The steel tube eventually reaches its ultimate tensile strain and fractures, with the fracture location typically at mid-span or near the loading ends.

The 45-degree fracture orientation observed in some specimens is characteristic of shear failure in the steel tube walls, which is consistent with the von Mises yield criterion and the stress state in the tube walls under axial tension combined with Poisson effect.

Parameter Analysis and Bearing Capacity Formula

Based on the parametric analysis, the authors derived a bearing capacity formula for cold-formed square CFST members under axial tension. The formula accounts for the contributions of both the steel tube and the core concrete, with the concrete contribution being limited by the cracking strain threshold of approximately 150 με.

The following table summarizes the relationship between width-to-thickness ratio and concrete contribution to bearing capacity:

The decreasing trend of concrete contribution with increasing width-to-thickness ratio can be attributed to the reduced effective confinement and the increased likelihood of steel tube local deformation at higher slenderness ratios.

Manufacturing and Welding Implications

The cold-forming process used to fabricate the square steel tubes introduces significant process effects that influence the mechanical behavior:

• Corner strain hardening: The bending and forming operations at the corners create strain-hardened regions with higher yield strength but reduced ductility.
• Residual stress distribution: The cold-forming process generates a complex residual stress pattern, with compressive stresses on the outer surfaces and tensile stresses on the inner surfaces of the formed walls.
• Wall thickness variation: The forming process can cause thinning at the corners and thickening at the flat portions, affecting the uniformity of the tube cross-section.

For welding applications involving cold-formed square tubes, these process effects must be considered in welding procedure qualification. The strain-hardened corners may require preheating to prevent cold cracking, and the residual stress state may influence the weld distortion behavior. Common welding processes for joining cold-formed square tubes include GMAW (gas metal arc welding) for butt joints and GTAW for precision applications.

Summary and Practical Significance

This study provides the first systematic investigation of axial tension behavior in cold-formed square CFST members, filling an important gap in the understanding of composite steel-concrete members under tension. The identification of the 150 microstrain threshold for concrete cracking and the quantification of concrete contribution to bearing capacity (3.03% to 30.75% depending on width-to-thickness ratio) offer practical design parameters. The derived bearing capacity formula provides a tool for design engineers, while the working mechanism analysis enhances understanding of load transfer between steel and concrete components. For steel pipe manufacturers, the study highlights the importance of controlling cold-forming process parameters to ensure consistent mechanical properties, particularly the width-to-thickness ratio which significantly affects both ductility and concrete contribution.