Axial Compressive Capacity of Thin Walled Square Concrete Filled Steel Tubular Columns

Literature Overview

The paper by Hao Wenxiu, Cui Boyun, and Xu Xiao (2023), published in Industrial Construction, presents experimental and finite element studies on the axial compressive capacity of thin-walled square concrete-filled steel tubular (CFST) columns. Ten specimens were tested, investigating the effects of slenderness ratio, wall thickness, and concrete strength on failure modes, load-displacement curves, ultimate capacity, and Poisson's ratio. The research was supported by the Hebei Provincial Key R&D Program (Grant 21327209D).

Core Technical Findings

The experimental study reveals several important findings regarding thin-walled square CFST columns:

The failure mode is characterized by crushing of the core concrete with the formation of circumferential buckling bands on the external steel tube surface. The loading process is divided into three stages: elastic, elastic-plastic, and descending.

Interpretation of Technical Points

Local Buckling and Steel Tube Manufacturing Quality

The observation that thin-walled CFST columns fail by local buckling of the steel tube has direct implications for steel pipe manufacturing quality:

1. Wall thickness tolerance: The local buckling load is highly sensitive to wall thickness. A reduction in wall thickness of even 10% can reduce the local buckling load by approximately 20%, based on classical plate buckling theory. Therefore, tight wall thickness tolerances should be specified, preferably within ±0.1 mm or ±5% of nominal, whichever is greater.
2. Surface defects: Surface imperfections such as dents, scratches, or oxide inclusions on the steel tube surface act as initial imperfections that reduce the local buckling load. The steel tube surface should be inspected for defects and any imperfections exceeding 5% of the wall thickness should be repaired or rejected.
3. Flatness of square tubes: For square steel tubes, the flatness of each face is critical. Out-of-flatness imperfections reduce the local buckling capacity. The flatness tolerance should be limited to 1.5 mm per meter of face length, as specified in EN 10219-2 for cold-formed square hollow sections.

Welding of Square Steel Tubes

The fabrication of square steel tubes involves welding of the longitudinal seam, which is a critical quality consideration:

• Longitudinal weld location: The longitudinal weld should be placed on the face of the square tube where the compressive stress is lowest, typically at the corner region. However, for CFST columns under axial compression, the stress distribution is relatively uniform, so the weld location is less critical from a stress perspective but still important from a buckling perspective.
• Weld quality requirements: The longitudinal weld should be a full-penetration weld with complete fusion, as any lack of fusion creates a plane of weakness that can initiate local buckling. The weld should be ground flush with the tube surface to eliminate stress concentration.
• Weld residual stress: The residual stress from the longitudinal weld can interact with the applied compressive stress, potentially accelerating local buckling. Post-weld stress relief, either by heat treatment or mechanical straightening, should be considered for thin-walled tubes.

Concrete Strength and Composite Efficiency

The finding that concrete strength significantly affects the composite efficiency of thin-walled CFST columns, particularly for steel ratios between 0.02 and 0.06, has implications for material selection:

• For thin-walled tubes with low steel ratios, the use of higher strength concrete (C40-C50) provides a more significant improvement in composite efficiency than increasing the wall thickness.
• The concrete should be designed with adequate workability for pumping into the square tube, with a slump of 160-200 mm and an air content of 3-5%.
• The concrete should be consolidated by vibration or pumping pressure to ensure full filling of the steel tube without voids.

Simplified Design Formula

The paper proposes a simplified calculation formula for the axial compressive capacity of thin-walled square CFST columns, referencing GB 50936-2014. The formula uses the regular slenderness ratio as the independent variable for the stability coefficient, which provides a unified approach for different parameter combinations.

Engineering Practice Integration

For steel pipe fabrication and welding of thin-walled square CFST columns, the following practices should be adopted:

1. Material specification: The steel tube material should be specified with a minimum yield strength of 235 MPa (S235) and a maximum carbon equivalent of 0.40% to ensure adequate weldability and ductility. Materials such as S355 with a carbon equivalent below 0.45% are also acceptable with appropriate welding procedures.
2. Welding procedure qualification: The welding procedure for the longitudinal seam should be qualified according to ISO 15614-1, with essential variables including preheat temperature (minimum 50°C for carbon equivalent above 0.40%), interpass temperature (maximum 250°C), and post-weld heat treatment (if specified).
3. Dimensional tolerance control: The following dimensional tolerances should be specified for thin-walled square tubes:

• Side length: ±0.5% of nominal, maximum ±1.5 mm
• Wall thickness: ±0.1 mm or ±5% of nominal
• Out-of-squareness: 2 mm per meter of length
• Out-of-flatness: 1.5 mm per meter of face length
• Twist: 1.5 mm per meter of length

1. Non-destructive testing: All longitudinal welds should be inspected by magnetic particle testing (MT) for surface defects and by ultrasonic testing (UT) for volumetric defects. The acceptance criteria should be based on ISO 5817 Level B for structural welds.
2. Post-weld treatment: For thin-walled tubes, post-weld straightening or stress relief should be performed to minimize residual stresses and geometric imperfections. Mechanical straightening is preferred over thermal treatment to avoid HAZ softening.

Key Questions and Reflections

The study raises the question of whether the current design codes adequately address the behavior of thin-walled CFST columns. The finding that a single equation can represent the stability coefficient for different parameter combinations suggests that the design methodology can be simplified without significant loss of accuracy. However, the applicability of this simplified approach to very thin-walled tubes (steel ratio below 0.02) should be verified through additional testing.

Another consideration is the interaction between the longitudinal weld residual stress and the local buckling behavior. The study does not explicitly address this interaction, but from a steel pipe manufacturing perspective, it is important to recognize that the residual stress pattern from welding can significantly affect the local buckling load. Future research should investigate this interaction and develop welding procedures that minimize the adverse effects of residual stress on buckling capacity.

Study Insights and Implications

This literature provides valuable experimental and analytical insights into the behavior of thin-walled square CFST columns. For steel pipe fabrication and welding engineers, the key takeaways are that the local buckling behavior of thin-walled tubes is highly sensitive to manufacturing quality, particularly wall thickness uniformity, surface condition, and weld quality. The proposed simplified design formula offers a practical tool for design, but its application should be accompanied by strict quality control of the steel tube fabrication and welding processes. The finding that concrete strength significantly improves composite efficiency for thin-walled tubes suggests that material optimization can be achieved through the judicious selection of concrete strength rather than increasing steel tube wall thickness, which has implications for cost and weight optimization in structural design.