Axial Tension Mechanical Properties of Cold-Bent Square Steel Tube Concrete Members

Literature Overview

The study by Gao Huaguo, Zhang Lingxin, Jiao Chenxing, and Wang Qingli, published in Building Structure in 2022, investigates the axial tension behavior of cold-bent square steel tube concrete members. Funded by the Liaoning Xingliao Talent Program (XLYC1902009), Heilongjiang Tayan Action Program, and the China Earthquake Administration Engineering Mechanics Research Institute Basic Scientific Business Fee Special Project (2019A01), this research addresses an under-studied loading condition for composite steel tube concrete members.

The experimental program included five cold-bent square steel tube concrete specimens and one empty cold-bent square steel tube specimen, with concrete strength, section width-to-thickness ratio, and concrete infill presence as parametric variables. Finite element simulation using ABAQUS was employed to complement the experimental data and provide insight into internal load distribution mechanisms.

Core Technical Findings

The research reveals several important findings regarding the axial tension behavior of cold-bent square steel tube concrete members:

1. Fracture strain decreases with increasing width-to-thickness ratio, indicating that thicker sections relative to their width are more brittle under tension
2. Failure locations concentrate at mid-span and at one-third of specimen height from loading ends, with fracture planes oriented horizontally or at 45 degrees to the mid-section
3. Core concrete cracking initiates when longitudinal strain at mid-section reaches approximately 150 με, after which specimen stiffness degrades
4. Within the width-to-thickness ratio range of 15 to 240, concrete infill increases axial tension bearing capacity by 3.03% to 30.75%

Parametric Effects on Axial Tension Performance

Interpretation of Technical Points

The cold-bending process used to form square steel tubes introduces residual stresses and work hardening that significantly affect tensile behavior. Unlike hot-formed tubes, cold-bent tubes have a non-uniform residual stress distribution with compressive stresses on the outer fibers and tensile stresses on the inner fibers at the corners. These residual stresses interact with applied tensile loads, potentially leading to premature crack initiation at the corners where stress concentrations are highest.

The finding that concrete infill increases tensile bearing capacity by 3.03% to 30.75% is particularly interesting. In axial tension, the concrete should theoretically not contribute to load-bearing capacity since concrete has negligible tensile strength. However, the interaction between the steel tube and concrete through bond and friction creates a composite action that enhances the overall tensile capacity. This phenomenon can be explained by the constraint effect: the concrete prevents outward bulging of the steel tube walls under tension, thereby increasing the effective cross-sectional area available for load-bearing.

The 150 με strain threshold for concrete cracking is a critical design parameter. Below this strain level, the composite member behaves elastically with full stiffness. Above this threshold, the concrete begins to crack, and load transfer to the steel tube becomes more pronounced. This transition point is important for serviceability limit state design, as cracking in the concrete core may affect durability and long-term performance.

Load Distribution Mechanism

Integration with Engineering Practice

The axial tension behavior of steel tube concrete members is relevant in several engineering applications:

1. Tie members in trusses: Steel tube concrete members used as tension members in bridge trusses or space structures
2. Prestressed concrete structures: Where steel tubes serve as tendons or reinforcement
3. Seismic isolation systems: Where cyclic tension-compression loading is expected
4. Cable-stayed bridge hangers: Where steel tube concrete members may be used for large-diameter tension elements

From a fabrication and welding perspective, the cold-bending process introduces specific quality considerations:

• Corner quality: The corners of cold-bent tubes are the most critical regions, as they experience the highest work hardening and residual stresses
• Weld seam orientation: If the tube is subsequently welded (e.g., for tube-to-tube connections), the weld should be oriented to avoid placing the weld at the location of maximum stress concentration
• Surface preparation: Cold-bent surfaces may have scale or oxide layers that affect weld quality and should be properly prepared before welding
• Dimensional control: Cold-bending tolerances affect the fit-up of connected members and must be tightly controlled

The width-to-thickness ratio range of 15 to 240 studied in this research covers a wide spectrum of practical tube geometries. The finding that concrete infill provides greater benefit at higher width-to-thickness ratios suggests that the composite action is more beneficial for thinner-walled tubes, which are more susceptible to local buckling under tension.

Key Questions and Reflections

Several aspects of this research merit further consideration:

• How does the cold-bent residual stress distribution affect fatigue performance under cyclic tension loading?
• What is the long-term bond behavior between steel tube and concrete under sustained tensile loads, considering creep and shrinkage effects?
• Can the bearing capacity formula derived from this study be extended to account for combined tension and bending loads?
• How does the cold-bending process affect the microstructure of the steel, and what are the implications for weldability?

The study's focus on cold-bent tubes is timely, as cold-bending is increasingly used for producing square and rectangular steel tubes due to its cost-effectiveness and dimensional accuracy. However, the residual stress and microstructural changes introduced by cold-bending are often neglected in design calculations, and this research helps to address that gap.

Study Insights and Implications

This research provides valuable insight into the axial tension behavior of cold-bent square steel tube concrete members, a loading condition that is often overlooked in the design of composite steel-concrete members. The finding that concrete infill can increase tensile bearing capacity by up to 30.75% challenges the conventional assumption that concrete contributes negligibly to tension capacity. This has direct implications for the economic design of steel tube concrete tension members, where the inclusion of concrete infill may provide unexpected benefits.

For welding engineers and fabrication specialists, the key implication is that cold-bent tube connections require careful consideration of residual stress effects. The non-uniform residual stress distribution in cold-bent tubes can lead to unexpected failure modes if not properly accounted for in connection design. Non-destructive testing protocols for cold-bent tube connections should include specific attention to corner regions and weld seam locations.

The derived bearing capacity formula provides a practical design tool for engineers working with cold-bent square steel tube concrete members in tension. The formula's development through parameter fitting of experimental and finite element data ensures that it captures the essential mechanics of the composite behavior while remaining computationally simple for practical application.