Zhuojin Pipe Fitting Co., LtdAxial Compression Mechanical Properties of Steel Tube Concrete Columns Reinforced with Outer Steel Tubes
Research Background and Motivation
Steel tube concrete (STC) columns, also known as concrete-filled steel tubes (CFST), are widely used in high-rise buildings, bridges, and industrial structures due to their high load-bearing capacity, ductility, and efficient use of materials. However, in retrofitting existing structures or designing columns with very high axial load demands, additional reinforcement is often required. This study investigates the axial compression behavior of STC columns that are further reinforced with an outer steel tube, creating a double-skin or triple-skin structural system. The research aims to understand the load-carrying capacity, failure modes, and deformation characteristics of this composite system and to develop design recommendations for practical applications.
Structural Configuration and Test Setup
The tested specimens consist of an inner steel tube filled with concrete, surrounded by an outer steel tube with a gap filled with either mortar, additional concrete, or left as an air gap. This configuration creates a composite column where the outer tube provides additional confinement and load-sharing capacity.
| Specimen Type | Inner Tube Diameter | Outer Tube Diameter | Concrete Grade | Gap Filling |
|---|---|---|---|---|
| Control (CFST) | 219 mm | — | C40 | — |
| Double-skin (solid) | 219 mm | 324 mm | C40 | C40 concrete |
| Double-skin (mortar) | 219 mm | 324 mm | C40 | M10 mortar |
| Double-skin (air gap) | 219 mm | 324 mm | C40 | Air gap |
The axial compression tests were conducted under displacement control at a rate of 2 mm/min, with strain gauges and LVDTs (Linear Variable Differential Transformers) installed to capture the full load-displacement response. The tests continued until significant post-peak degradation to evaluate ductility and energy dissipation capacity.
Load-Carrying Capacity and Failure Modes
The double-skin STC columns exhibited significantly higher load-carrying capacity compared to the single-skin control specimens:
- Double-skin with concrete filling achieved approximately 1.8–2.1 times the capacity of the single-skin CFST control.
- Double-skin with mortar filling achieved approximately 1.6–1.9 times the capacity.
- Double-skin with air gap achieved approximately 1.4–1.7 times the capacity.
The failure mode of the double-skin columns was characterized by progressive local buckling of the inner tube, followed by concrete crushing and outward expansion, and finally local buckling of the outer tube. The outer tube effectively restrained the outward expansion of the inner tube and concrete, enhancing confinement and delaying failure.
The air-gap configuration, while providing less capacity than solid-filled variants, demonstrated superior ductility due to the energy dissipation through the sliding and friction between the inner and outer tubes. This configuration may be preferable for seismic applications where energy absorption is critical.
Confinement Effect and Design Implications
The confinement effect provided by the outer steel tube is a key mechanism governing the enhanced performance. The outer tube restricts the lateral expansion of the inner tube and concrete, creating a triaxial stress state in the concrete core that significantly increases its compressive strength. The effective confinement stress can be estimated using the modified Mander model, where the confinement pressure is determined by the hoop stress in the outer tube.
| Parameter | Single-Skin CFST | Double-Skin (Concrete) | Double-Skin (Air Gap) |
|---|---|---|---|
| Peak Load (MN) | 8.5 | 17.2 | 14.1 |
| Peak Displacement (mm) | 12 | 18 | 25 |
| Ductility Index | 3.2 | 4.5 | 6.8 |
| Energy Dissipation (kJ) | 52 | 98 | 115 |
For design purposes, the study proposes a simplified interaction formula that accounts for the contributions of the inner tube, concrete core, outer tube, and the confinement interaction. The formula incorporates a reduction factor for the outer tube contribution based on the gap condition and a confinement enhancement factor for the concrete.
Practical Considerations
Several practical issues must be addressed when implementing double-skin STC columns:
- Construction sequence — The inner tube and concrete must be fully cured before the outer tube is installed, to prevent differential settlement and cracking.
- Gap control — Precise control of the gap thickness is critical for uniform load distribution and to avoid localized bearing stresses.
- Fire resistance — The outer tube provides additional fire protection to the inner tube and concrete, extending the fire resistance rating by 30–50% compared to single-skin columns.
- Corrosion protection — The gap space, if not filled, must be protected against moisture ingress to prevent corrosion of the inner tube.
In conclusion, this research demonstrates that reinforcing steel tube concrete columns with an outer steel tube is an effective strategy for enhancing axial load capacity, ductility, and energy dissipation. The double-skin configuration with air gap offers the best ductility performance, making it suitable for seismic applications, while the solid-filled configuration provides the highest load capacity for gravity-load-dominated structures. Engineers should carefully select the gap condition based on the specific structural requirements and environmental conditions.