Axial Compression Performance Test Study of Rectangular Multi-Cavity Steel Tube Concrete Columns

Literature Overview

This research investigates rectangular multi-cavity steel tube concrete (RMCFST) columns, which represent an innovative structural system combining the advantages of rectangular CFST sections with the enhanced confinement provided by internal partitions. The multi-cavity configuration divides the rectangular cross-section into multiple smaller cavities, each filled with concrete, thereby improving the overall confinement efficiency and load-bearing capacity. The study is particularly relevant for applications requiring high axial capacity with improved torsional and biaxial bending resistance, such as building cores, bridge piers, and industrial column structures.

Core Technical Findings

The experimental program reveals that rectangular multi-cavity CFST columns exhibit significantly improved axial compression performance compared to conventional single-cavity rectangular CFST columns. The primary findings include:

• The multi-cavity configuration increases the effective confinement ratio by 40–60%, as the internal partitions reduce the unsupported width of each concrete cavity.
• The failure mode transitions from global buckling in single-cavity columns to local plate buckling followed by concrete crushing in multi-cavity columns.
• The load-bearing capacity per unit cross-sectional area improves by 20–35% with the addition of internal partitions.
• The ductility index increases by 15–25%, attributed to the more uniform stress distribution across the cross-section.

Geometric and Material Parameters

Process and Welding Analysis

The fabrication of rectangular multi-cavity CFST columns involves several critical welding operations that directly influence structural performance. The internal partitions must be welded to the inner surfaces of the outer box section, creating a complex three-dimensional weld network.

Welding process selection:

• Longitudinal partition welds: Gas metal arc welding (GMAW) with flux-cored wire for deposit thickness up to 10 mm, or submerged arc welding (SAW) for thicker sections.
• Transverse welds connecting partitions to outer walls: Manual metal arc welding (SMAW) for accessibility, with E7018 electrodes for Q345-grade steel.
• Circumferential welds at column joints: SAW with automatic or semi-automatic equipment for consistent quality.

Critical quality control measures:

1. Pre-weld inspection of all base metals for surface defects, oxide scale, and moisture contamination.
2. Interpass temperature monitoring maintained between 150–250°C to control microstructural evolution in the heat-affected zone.
3. Post-weld visual inspection combined with magnetic particle testing (MT) for surface-breaking defects and ultrasonic testing (UT) for volumetric defects.
4. Hardness testing in the HAZ to verify that values do not exceed 350 HV, which would indicate excessive hardness and potential brittleness.

The welding sequence should follow a balanced pattern to minimize residual stresses and distortion. A recommended approach is to weld partitions from the center outward, alternating between opposite sides, followed by circumferential welds in a symmetrical pattern.

Engineering Practice Integration

For practical implementation of rectangular multi-cavity CFST columns, several engineering considerations must be addressed:

1. Concrete placement: The internal partitions create multiple small cavities that may be difficult to fill with conventional concrete. Self-compacting concrete with a flow diameter of 600–650 mm is recommended, with superplasticizers to ensure workability without excessive water content.
2. Partition design: The thickness of internal partitions should be at least 0.6 times the outer wall thickness to ensure adequate load transfer and prevent premature local buckling of the partitions themselves.
3. Connection design: The column-to-column and column-to-beam connections must accommodate the enhanced moment capacity of the multi-cavity section, typically requiring extended end plates or stiffened welded connections.
4. Fire protection: The multi-cavity configuration provides inherent fire resistance due to the concrete core, but the partitions may reduce the thermal conductivity of the cross-section. Fire rating calculations should account for the specific geometry.

Key Questions and Reflections

The study raises important questions about the scalability of multi-cavity designs. As the number of cavities increases, the fabrication complexity and welding volume increase substantially, potentially offsetting the structural benefits with higher costs. There is also a question of constructability—how many cavities can be practically fabricated and filled with concrete in a construction site environment?

From a welding metallurgy perspective, the multiple weld seams in a multi-cavity column create numerous heat-affected zones in close proximity. The interaction between adjacent HAZs can lead to microstructural coarsening and reduced toughness in the overlap regions. This phenomenon, known as "heat input accumulation," must be carefully managed through controlled welding sequences and adequate cooling intervals between passes.

Another reflection concerns the inspection accessibility. The internal cavities of a multi-cavity column are largely inaccessible for post-construction inspection. Any defects introduced during fabrication may remain undetected, posing potential risks to structural integrity. This underscores the importance of rigorous pre-assembly quality control and non-destructive testing before concrete placement.

Study Insights and Implications

This research demonstrates that the multi-cavity concept offers a practical and effective means of enhancing the axial compression performance of rectangular CFST columns. The improved confinement efficiency and more uniform stress distribution result in higher load-bearing capacity and ductility without a proportional increase in material usage. For practicing engineers, the key takeaway is that rational partition design—optimizing the number, spacing, and thickness of internal partitions—can yield significant performance improvements within the existing framework of CFST design codes. The work also highlights the critical role of welding quality in determining the ultimate performance of composite structural members, reinforcing the principle that fabrication excellence is inseparable from structural reliability.