Zhuojin Pipe Fitting Co., LtdAxial Compression Performance of Multi-Chamber Irregular Steel Tube-Concrete Short Columns
Literature Overview
This study examines the axial compressive behaviour of short columns composed of multi-chamber irregular steel tube sections filled with concrete. Multi-chamber steel tube-concrete (SRC) columns represent an innovative structural system that divides the internal cross-section into multiple independent chambers using internal steel dividers or webs. This configuration offers enhanced confinement efficiency, improved material utilisation, and increased design flexibility compared to conventional single-chamber SRC columns. The irregular geometry of the outer section further expands the design envelope, allowing optimisation of load capacity, stiffness, and serviceability for specific structural applications.
Structural Configuration and Design Parameters
The multi-chamber SRC column consists of an outer irregular steel tube, internal steel dividers creating multiple concrete-filled chambers, and high-strength concrete filling each chamber independently.
| Design Parameter | Typical Range | Design Consideration |
|---|---|---|
| Outer section shape | Rectangular, hexagonal, octagonal, irregular polygon | Load path optimisation |
| Outer section dimensions | 300–800 mm (major axis) | Architectural and structural requirements |
| Outer wall thickness | 8–20 mm | Local buckling resistance |
| Number of chambers | 2–6 | Confinement efficiency vs. constructability |
| Internal divider thickness | 6–14 mm | Chamber separation and composite action |
| Concrete strength | C50–C100 | Material synergy with steel |
| Steel grade (tube and dividers) | Q345–Q690 | Yield strength and ductility balance |
| Concrete volume ratio | 0.35–0.65 | Load sharing between steel and concrete |
The irregular outer section shape is particularly advantageous for columns subjected to biaxial bending or torsional loading, as the cross-sectional geometry can be tailored to the specific load demand. For example, a hexagonal section provides more uniform confinement than a rectangular section of equivalent area, while an irregular polygon can be designed to match the architectural envelope of the structure.
Confinement Mechanism in Multi-Chamber Configuration
The multi-chamber configuration fundamentally enhances the confinement mechanism compared to single-chamber SRC columns. In a single-chamber column, the steel tube confines the concrete core, but the confinement pressure is non-uniform due to the stress distribution across the section. In the multi-chamber configuration, each chamber is independently confined by the outer tube wall and the internal dividers, creating a more uniform confinement pressure distribution.
The confinement pressure in each chamber can be estimated using the modified Mander model:
| Chamber Type | Confinement Pressure Formula | Effective Confinement Factor |
|---|---|---|
| Single chamber (rectangular) | fcl = fc + 3.33 * fcc | k = 1.0 (reference) |
| Double chamber | fcl = fc + 4.20 * fcc | k = 1.28 |
| Triple chamber | fcl = fc + 5.10 * fcc | k = 1.54 |
| Quadruple chamber | fcl = fc + 5.80 * fcc | k = 1.75 |
Where fcc is the lateral confining pressure and k is the effective confinement factor. The multi-chamber configuration increases the effective confinement factor by 28–75% compared to single-chamber columns of equivalent steel area.
Axial Compression Performance
The test results demonstrate significant improvements in axial compression performance:
| Performance Indicator | Single-Chamber SRC | Multi-Chamber SRC | Improvement |
|---|---|---|---|
| Peak load | 1.00 (reference) | 1.20–1.45 | 20–45% |
| Peak strain | 1.00 (reference) | 1.30–1.60 | 30–60% |
| Ultimate strain | 1.00 (reference) | 1.50–2.00 | 50–100% |
| Energy absorption (up to 3% strain) | 1.00 (reference) | 1.60–2.20 | 60–120% |
| Post-peak ductility index | 1.00 (reference) | 1.40–1.80 | 40–80% |
The enhanced ductility of multi-chamber SRC columns is particularly valuable for seismic applications, where the ability to sustain large inelastic deformations without collapse is critical. The multiple chambers distribute the inelastic deformation more uniformly across the cross-section, preventing localised crushing and providing a more stable post-peak response.
Failure Modes and Defect Analysis
The failure modes observed in multi-chamber SRC columns provide important insights into design and fabrication requirements:
- Chamber wall buckling: The internal dividers may buckle locally under high confinement pressure, particularly at the junction with the outer tube. This is mitigated by providing sufficient divider thickness (minimum 8 mm for Q345 steel) and ensuring full-penetration welds at all junctions.
- Concrete crushing in individual chambers: When one chamber fails, the load redistributes to adjacent chambers, which may experience higher stresses than anticipated. The design should account for this load redistribution by ensuring that all chambers have sufficient reserve capacity.
- Interface debonding: The bond between the internal dividers and the concrete is critical for composite action. Shot blasting of the divider surfaces to Sa 2.5 grade and the use of self-compacting concrete with appropriate viscosity are essential for achieving adequate interface shear strength.
- Outer tube local buckling: The outer tube may buckle locally between the internal dividers, particularly if the divider spacing is large. The maximum divider spacing should not exceed 3.0 times the outer tube wall thickness to prevent inter-divider buckling.
Welding and Fabrication Quality Control
The fabrication of multi-chamber SRC columns involves several critical welds and assembly operations:
- Internal divider to outer tube welds: These welds create the chamber separation and must achieve full fusion with minimum residual stress. GTAW (TIG) or SAW processes are recommended, with heat input controlled below 1.5 kJ/mm for steels above Q420. Preheating to 100–150°C is required for wall thicknesses exceeding 10 mm.
- Longitudinal welds in the outer tube: The longitudinal seam weld in the outer tube creates a continuous stress path that can influence the buckling behaviour. Full-penetration welds with smooth transition profiles are essential, and the weld should be ground flush with the tube surface.
- Transverse welds at column ends: The end connections must transfer the full axial load and any bending moments to the structural frame. Full-penetration groove welds with a minimum weld throat thickness of 0.7 times the connected element thickness are required.
Quality inspection requirements include:
- Full UT examination (ISO 17635 Level 2) of all structural welds, including internal dividers
- MT inspection of all weld surfaces for surface-breaking defects
- Dimensional inspection of chamber geometry to ensure uniform concrete placement
- Hydrostatic pressure test of the assembled tube system to verify watertightness before concrete placement
Engineering Practice Applications
Multi-chamber SRC columns are particularly suitable for:
- Super-tall buildings: Where the combination of high axial loads and seismic demands requires enhanced confinement and ductility. The multi-chamber configuration provides superior energy dissipation capacity for seismic design.
- Heavy industrial structures: Where the column must support heavy equipment loads while maintaining sufficient ductility for accidental loading scenarios. The irregular section shape can be tailored to the specific load pattern.
- Bridge piers: Where the column must resist combined axial, bending, and torsional loads from seismic and wind events. The multi-chamber configuration provides enhanced biaxial flexural capacity.
- Nuclear and critical infrastructure: Where the column must withstand extreme loading scenarios without collapse. The enhanced ductility and redundancy of the multi-chamber system provide superior performance under severe conditions.
Key Questions and Reflections
The constructability of multi-chamber SRC columns is a significant practical concern. The internal dividers create obstacles for concrete placement, and ensuring void-free filling of each chamber requires careful control of concrete properties and placement methods. Self-compacting concrete with a slump flow of 280–320 mm and a V-funnel time of 8–14 seconds is recommended, with placement through multiple access points to ensure uniform filling.
The cost-effectiveness of the multi-chamber configuration must also be evaluated. While the strength and ductility improvements are significant, the additional material cost (internal dividers), fabrication complexity (additional welds), and inspection requirements (access to internal welds) increase the overall cost. The technology is most appropriate for critical columns where the performance benefits justify the additional expense.
Study Insights and Outlook
The multi-chamber irregular steel tube-concrete column represents a significant advancement in composite structural engineering. The enhanced confinement efficiency, improved ductility, and design flexibility make it a promising solution for demanding structural applications. Future research should focus on cyclic loading behaviour to evaluate seismic performance, fire resistance performance considering the complex internal geometry, and the development of simplified design equations that can be incorporated into standard structural design codes. The technology also warrants investigation for application in offshore platforms, marine structures, and underground structures where the combination of axial loading, environmental degradation, and seismic demand creates challenging design requirements.