Axial Compression Performance of Rectangular Multi-Chamber CFST Columns

Research Background and Motivation

Rectangular multi-chamber concrete-filled steel tube (CFST) columns represent an innovative structural system that combines the advantages of rectangular geometry (ease of connection and architectural integration) with the enhanced confinement provided by multiple internal chambers. The division of the cross-section into multiple chambers by internal steel partitions creates a multi-cell configuration that improves the confinement effectiveness, reduces the slenderness of each individual chamber, and enhances the overall load-bearing capacity and ductility under axial compression. This system is particularly suitable for building columns, industrial frames, and bridge piers where rectangular cross-sections are preferred for construction and aesthetic reasons.

Technical Analysis of Confinement Mechanism

The confinement mechanism in a rectangular multi-chamber CFST column differs fundamentally from that in a circular CFST column. In a circular tube, the confinement pressure is uniformly distributed around the perimeter, creating an isotropic triaxial stress state in the concrete core. In a rectangular tube, however, the confinement pressure is non-uniform, with lower confinement at the corners and higher confinement at the mid-sides. The introduction of internal partitions in a multi-chamber configuration helps to regularize this non-uniformity by subdividing the cross-section into smaller rectangular cells, each of which experiences a more uniform confinement pressure.

The confinement effectiveness is quantified by the confining stress (f_l), which is calculated based on the hoop tension in the steel tube and the geometry of the cross-section. For rectangular sections, the confining stress is lower than for circular sections of equivalent cross-sectional area, and this reduction is mitigated by the multi-chamber configuration. The load capacity improvement ratio for multi-chamber CFST columns typically ranges from 1.15 to 1.35, depending on the number of chambers, the partition thickness, and the concrete strength.

Failure Modes and Defect Analysis

The failure modes of rectangular multi-chamber CFST columns under axial compression include:

1. Concrete crushing in the chambers: The concrete in each chamber is crushed at the mid-height, with the failure sequence depending on the confinement effectiveness of each chamber. Chambers with lower confinement (e.g., near the outer walls) fail first, followed by chambers with higher confinement (e.g., near the internal partitions).
2. Local buckling of the outer steel tube: The outer walls of the rectangular tube buckle inward, particularly at the mid-height of the column, reducing the confinement effectiveness and leading to progressive concrete failure.
3. Buckling of the internal partitions: The internal steel partitions may buckle under the lateral pressure from the concrete, leading to a loss of confinement and premature failure. This mode is critical when the partition thickness is insufficient relative to the chamber width.
4. Corner cracking and spalling: The corners of the rectangular section experience the lowest confinement pressure and are prone to early cracking and spalling, which can propagate to the adjacent chambers.

To prevent these failure modes, the following design recommendations are proposed:

• The internal partition thickness should be at least 1.5 times the outer wall thickness to ensure adequate buckling resistance.
• The chamber width-to-thickness ratio should not exceed 40 to prevent excessive local buckling.
• The concrete strength should be limited to 80 MPa for rectangular sections to maintain adequate ductility.
• Corner reinforcement or corner chamfering may be used to improve the confinement at the corners.

Manufacturing and Welding Considerations

The fabrication of rectangular multi-chamber CFST columns involves the welding of internal partitions to the outer steel tube, which requires careful attention to weld quality and heat input control. The welding process is typically performed using submerged arc welding (SAW) or gas metal arc welding (GMAW), depending on the partition thickness and the required weld quality. The weld procedure specification (WPS) must account for the restricted access inside the rectangular tube and the potential for thermal distortion.

Key welding parameters include:

• Preheating temperature: 100-150°C for carbon steel grades above Q345.
• Interpass temperature: 200-250°C to prevent hydrogen-induced cracking.
• Heat input: 1.0-2.5 kJ/mm to control the heat-affected zone (HAZ) width.
• Post-weld heat treatment (PWHT): Required for thick sections (t > 20 mm) to relieve residual stresses.

Post-weld inspection by ultrasonic testing (UT) is mandatory to detect lack of fusion, slag inclusions, and porosity. The acceptance criteria should be based on ASME Section V or EN ISO 17635, with a minimum acceptance level of Level 2 for structural welds.

Study Insights and Implications

The study of rectangular multi-chamber CFST columns demonstrates that the multi-chamber configuration is an effective strategy for improving the confinement effectiveness and load-bearing capacity of rectangular CFST columns. The internal partitions not only subdivide the cross-section into smaller cells but also provide additional restraint against lateral expansion, leading to a more uniform stress distribution and improved ductility. However, the fabrication complexity and cost increase with the number of chambers, and the welding quality of the internal partitions becomes a critical factor in determining the structural performance.

Engineers must carefully evaluate the trade-off between structural performance and fabrication cost when selecting the number of chambers and the partition configuration. For critical applications such as high-rise building columns and bridge piers, the multi-chamber configuration offers a viable solution for achieving high load capacity with controlled ductility. Future research should focus on the long-term durability of multi-chamber CFST columns under cyclic loading, the effect of corrosion on the internal partitions, and the development of simplified design formulas that account for the multi-chamber confinement effect.