Nonlinear Behavior of Spirally Stiffened Large-Diameter Thick-Walled Square Steel Tube Concrete Composite Columns Under Eccentric Compression

Overview of the Research Context

The investigation of spirally stiffened large-diameter thick-walled square steel tube concrete (SSTC) composite columns under eccentric compression addresses a critical gap in the structural engineering of composite construction systems. As building heights increase and architectural demands grow more complex, the need for columns with higher load-bearing capacity, improved ductility, and efficient material utilization becomes paramount. The spiral stiffening approach represents an innovative method to enhance the confinement effect on the core concrete while simultaneously improving the local buckling resistance of the thick-walled square steel tube. This study provides valuable insights into the nonlinear mechanical behavior of such members, which is essential for the rational design of composite structures in seismic zones and heavy-load applications.

Core Technical Parameters and Member Configuration

The spirally stiffened SSTC column integrates three primary components: the outer square steel tube, the spiral stiffeners (typically helically wound steel strips or bars), and the core concrete. The spiral stiffeners serve a dual function — they provide lateral confinement to the concrete core and act as local reinforcement against the buckling of the steel tube walls under compressive loading. The key parameters governing the structural performance include the steel tube dimensions (side length, wall thickness), the spiral stiffener geometry (wire diameter, pitch, number of turns), the concrete compressive strength, and the steel grade used for both the tube and stiffeners.

Nonlinear Behavior Under Eccentric Compression

Under eccentric loading, the SSTC column experiences a combination of axial compression and bending moment, which fundamentally alters the stress distribution within the composite cross-section. The nonlinear behavior can be categorized into three distinct stages:

1. Elastic stage (Stage I): The load increases linearly with displacement, and the stress distribution across the cross-section follows the linear elastic assumption. Both the steel tube and core concrete deform compatibly without interface slip.
2. Plastic stage (Stage II): As the load approaches the yield threshold, the steel tube begins to yield on the compression side, while the concrete on the tension side starts to crack. The spiral stiffeners begin to develop tensile stresses as they resist the outward bulging of the steel tube walls.
3. Post-peak stage (Stage III): After reaching the ultimate load, the concrete on the compression side crushes progressively, and the steel tube undergoes significant local buckling. The spiral stiffeners provide a critical restraint against rapid strength degradation, maintaining residual load capacity through the dilation of the confined concrete.

The eccentricity ratio plays a decisive role in determining the failure mode. For small eccentricities (e/b < 0.1), the failure is characterized by concrete crushing on the compression side with limited steel tube yielding. As eccentricity increases beyond e/b = 0.2, the failure transitions to a bending-dominated mode where the steel tube on the tension side yields first, followed by concrete crushing on the compression side. The spiral stiffeners are most effective at intermediate eccentricities, where they prevent premature local buckling while providing confinement to the crushed concrete zone.

Manufacturing and Welding Considerations

From a steel pipe manufacturing and welding perspective, the fabrication of spirally stiffened SSTC columns presents several technical challenges that must be carefully managed:

• Square tube forming: The thick-walled square tubes require high-precision roll-forming or welding and forming processes. The corner radii must be controlled to prevent stress concentrations at the corners, which are critical under eccentric loading.
• Spiral stiffener installation: The helical winding of stiffeners inside or outside the tube requires specialized equipment and precise control of tension and spacing. The stiffeners must be anchored at both ends with reliable welds or mechanical fasteners.
• Welding of stiffener terminations: The connection between the spiral stiffener and the tube wall is a critical detail. Fillet welds or T-butt welds must achieve full fusion to ensure load transfer. Preheating is recommended for high-strength steel grades to prevent hydrogen-induced cracking in the heat-affected zone.
• Concrete placement: The concrete must be placed in controlled lifts to ensure complete encapsulation of the spiral stiffeners without voids. Vibration must be adequate to achieve dense compaction around the stiffeners.

Design Implications and Code Compliance

The research findings have direct implications for the design of SSTC columns in practice. The nonlinear analysis results indicate that the confinement effect of spiral stiffeners can increase the ultimate load capacity by 15–35% compared to unstiffened SSTC columns, with the exact improvement depending on the stiffener geometry and the eccentricity ratio. The ductility enhancement is particularly significant for columns designed for seismic applications, where energy dissipation through plastic deformation is essential.

Current design codes, including GB 50011, GB 51225, and AISC 360, provide provisions for composite columns but do not specifically address spirally stiffened configurations. Engineers must therefore rely on experimental data and nonlinear finite element analysis to establish reliable design procedures. The study recommends incorporating a confinement factor in the design equations that accounts for the spiral stiffener geometry, spacing, and material properties.

Study Insights and Engineering Reflections

This research highlights the potential of spiral stiffening as an effective method to enhance the performance of large-diameter thick-walled square steel tube concrete columns. The key insight is that the spiral stiffeners create a continuous confinement mechanism that is more efficient than discrete ring stiffeners, particularly under eccentric loading where the stress distribution is non-uniform. The nonlinear behavior reveals that the spiral stiffeners not only prevent local buckling but also redistribute stresses within the composite cross-section, leading to a more uniform utilization of materials.

From a manufacturing standpoint, the fabrication of spiral stiffeners requires careful attention to dimensional accuracy and weld quality. Any deviation in the spiral pitch or wire diameter can significantly affect the confinement effectiveness. Furthermore, the weld connections at the stiffener terminations are potential weak points that must be inspected using non-destructive testing methods such as magnetic particle testing or ultrasonic testing. The integration of spiral stiffening into standard fabrication workflows will require the development of dedicated tooling and process specifications, which represents a significant but worthwhile investment for the composite construction industry.