Axial Compression Performance of Stirrup-Constrained Thin-Walled Square Concrete-Filled Steel Tube Short Columns

Literature Overview

This paper, published in the Journal of Jilin University (Engineering Science Edition) (2023, Vol. 53, No. 1, pp. 170–177) by Chen Jun and colleagues from Xiangtan University, investigates the axial compression behavior of thin-walled square CFST short columns with internal stirrup confinement. The research was supported by the National Natural Science Foundation of China (Grant No. 12072309), the Hunan Provincial Natural Science Foundation (Grant No. 2019JJ60047), and the Hunan Innovative Province Construction Special Project (Grant No. 2019RS1059). The study examines the effects of concrete strength, stirrup volumetric ratio, and stirrup-to-tube wall connection methods on ultimate load capacity, stiffness, ductility, and lateral deformation coefficient.

Core Technical Findings

The experimental results reveal several important trends:

• Load capacity enhancement: Internal stirrups improve the ultimate load capacity and ductility of thin-walled CFST columns.
• Diminishing returns with concrete strength: The improvement effect decreases as concrete strength increases, suggesting that high-strength concrete provides sufficient confinement on its own.
• Stirrup ratio effect: The improvement effect increases with increasing stirrup volumetric ratio, as expected from confinement theory.
• Connection method irrelevance: Welding the stirrups to the tube wall does not significantly alter the mechanical properties, indicating that the stirrup-tube bond is adequate through friction and mechanical interlock alone.

Technical Interpretation

The use of internal stirrups in thin-walled CFST columns addresses a fundamental limitation: thin-walled steel tubes provide limited lateral confinement to the concrete core, particularly for high-strength concrete where the concrete's lateral expansion demand exceeds the tube's restraint capacity. The stirrups provide additional confinement, effectively creating a multi-layer confinement system.

From a welding and fabrication perspective, the finding that stirrup-to-tube wall welding does not significantly affect mechanical properties is practically important:

1. Simplified fabrication: Stirrups can be placed without welding to the tube wall, reducing fabrication time and cost.
2. Reduced heat-affected zone concerns: Eliminating stirrup welds reduces the number of heat-affected zones in the thin tube wall, which is beneficial for maintaining the tube's local buckling resistance.
3. Quality control simplification: Fewer welds means fewer potential defect locations and reduced inspection requirements.

However, the decision not to weld stirrups to the tube wall should be made with consideration of the following:

• Seismic performance: Under cyclic loading, unwelded stirrups may lose their position, potentially reducing confinement effectiveness.
• Construction vibration: During concrete placement, unwelded stirrups may shift, affecting their confinement geometry.
• Long-term stability: Under sustained loading, the friction-based connection may degrade over time.

Limit State Analysis and Formula Development

The authors derive a formula for the ultimate load capacity of stirrup-confined square CFST short columns using limit strength theory. The formula accounts for:

• The compressive strength of the confined concrete core.
• The contribution of the steel tube to axial load resistance.
• The additional confinement pressure provided by the stirrups.
• The interaction between the tube and stirrup confinement systems.

The formula is validated against experimental data from other researchers, demonstrating its accuracy and applicability.

Process and Quality Control Considerations

For the fabrication of stirrup-confined thin-walled CFST columns, the following quality control measures are recommended:

Engineering Practice Implications

The findings of this study are particularly relevant for applications where thin-walled steel tubes are used to reduce weight and cost, such as:

• High-rise building columns with large diameter-to-thickness ratios.
• Long-span bridge columns where weight is a critical design consideration.
• Retrofit applications where existing thin-walled tubes are to be strengthened.

The formula developed by the authors provides a practical tool for designing stirrup-confined CFST columns, enabling engineers to optimize the stirrup configuration for specific load and ductility requirements. The diminishing returns with increasing concrete strength suggest that stirrup confinement is most beneficial for moderate-strength concrete (C40–C60), while for ultra-high-strength concrete (C80 and above), alternative confinement strategies may be more appropriate.

Key Reflections

This study makes a valuable contribution to the understanding of multi-layer confinement systems in CFST columns. The finding that stirrup-to-tube welding is not critical for mechanical performance is practically significant and could lead to simplified fabrication practices. However, the study focuses on monotonic axial compression, and the behavior under cyclic or seismic loading may differ, particularly regarding the effectiveness of unwelded stirrups. The formula developed should be further validated with full-scale column tests and under combined loading conditions before being adopted into design codes. The research demonstrates the importance of considering internal reinforcement in CFST column design, a topic that deserves further attention in future code revisions.