Zhuojin Pipe Fitting Co., LtdAnalytical Method for Axial Compression Bearing Capacity of Steel Tube Confined Concrete Considering Temperature Additional Ring Effect
Literature Overview
This study presents an analytical method for determining the axial compression bearing capacity of steel tube confined concrete (STCC) columns, with a particular emphasis on incorporating the temperature-induced additional ring effect. The research addresses a significant gap in existing design codes, where thermal residual stresses and hoop confinement effects under elevated temperatures are often oversimplified or neglected entirely. The authors propose a refined analytical framework that accounts for the interaction between the steel tube's thermal expansion behavior and the resulting additional ring compression imposed on the confined concrete core. This work is particularly relevant to engineers designing composite columns in high-temperature environments such as industrial furnaces, nuclear facilities, and fire-resistance-critical structural systems.
Core Technical Framework
The analytical model is built upon the principle of strain compatibility between the steel tube and the concrete core. Under axial compression at elevated temperatures, the steel tube undergoes thermal expansion that generates an additional circumferential (ring) compressive stress in the tube wall. This additional ring effect enhances the lateral confinement pressure on the concrete, thereby improving the concrete's compressive strength and ductility beyond what is predicted by conventional confinement models.
Key Assumptions and Governing Equations
The model assumes that the steel tube and concrete core deform together without relative slip along the interface, and that the temperature distribution across the cross-section is uniform during the analysis. The additional ring stress is derived from the thermal strain incompatibility between the steel tube's free thermal expansion and the restrained expansion caused by the concrete core.
The effective confinement pressure is expressed as:
- f_c' = f_c0 + k * σ_ring, where f_c0 is the unconfined concrete strength, k is a confinement efficiency factor, and σ_ring is the additional ring stress induced by temperature.
- The ring stress is calculated as σ_ring = E_s α_s ΔT * (D_o / t), where E_s is the steel modulus at temperature T, α_s is the thermal expansion coefficient, ΔT is the temperature rise, D_o is the outer diameter, and t is the tube wall thickness.
Parameters and Typical Values
| Parameter | Symbol | Typical Range | Notes |
|---|---|---|---|
| Steel yield strength at 20°C | f_y | 235–460 MPa | Q235 to Q345 grade |
| Concrete compressive strength | f_c0 | 30–80 MPa | C30 to C80 |
| Temperature rise | ΔT | 0–800°C | Service to fire conditions |
| Tube outer diameter | D_o | 100–600 mm | Structural range |
| Tube wall thickness | t | 4–20 mm | D/t ratio 25–100 |
| Confinement efficiency factor | k | 1.5–3.5 | Depends on D/t and concrete grade |
| Thermal expansion coefficient (steel) | α_s | 1.2×10⁻⁵ /°C | Carbon steel |
Process and Standards Analysis
The analytical method is benchmarked against existing standards including GB 50017 (Chinese steel structure code), Eurocode 4 (EN 1994-1-2), and AISC 341. The study identifies that conventional codes underestimate the bearing capacity of STCC columns at elevated temperatures by 12–28%, primarily because they do not account for the additional ring confinement effect. The proposed method aligns more closely with experimental data obtained from fire tests on STCC columns conducted in accordance with ISO 834.
Comparison with Existing Codes
| Standard | Method | Temperature Range | Additional Ring Effect Considered | Deviation from Test Data |
|---|---|---|---|---|
| GB 50017 | Empirical reduction factor | 20–600°C | No | −15% to −25% |
| Eurocode 4 | Strength reduction + confinement | 20–800°C | Partial | −10% to −20% |
| AISC 341 | Strength reduction only | 20–800°C | No | −18% to −30% |
| Proposed method | Analytical ring effect model | 20–800°C | Yes | −3% to +5% |
Integration with Engineering Practice
From a manufacturing and fabrication standpoint, the findings have direct implications for the selection of steel tube specifications in high-temperature applications. Tubes with higher D/t ratios benefit more from the additional ring effect, as the thermal hoop stress is amplified with increasing diameter-to-thickness ratio. However, excessively large D/t ratios may lead to local buckling instability under combined thermal and mechanical loading. Engineers should therefore optimize the D/t ratio to balance the confinement benefit against the buckling risk.
In terms of welding and fabrication quality, the integrity of the steel tube-concrete interface is critical. Any gaps or voids at the interface will reduce the confinement efficiency factor k. This underscores the importance of proper surface preparation, vibration compaction, and potentially the use of high-flowability concrete mixes to ensure full densification within the tube.
Fabrication Recommendations
- Surface treatment: Shot-blast or sandblast the inner tube surface to improve bond strength.
- Concrete placement: Use pumpable concrete with slump of 180–220 mm for adequate flow within the tube.
- Vibration: Apply external vibration to the tube exterior to ensure compaction of the concrete core.
- Welding joints: For multi-segment columns, ensure full-penetration butt welds with post-weld heat treatment to relieve residual stresses.
Key Questions and Reflections
One critical question raised by this study is whether the uniform temperature assumption holds for large-diameter tubes with thick walls, where significant temperature gradients may develop between the outer surface and the concrete core during fire exposure. This non-uniformity could lead to differential thermal strains that either enhance or counteract the ring confinement effect. Future research should incorporate transient heat transfer analysis to validate the model under realistic fire conditions.
Another point of reflection concerns the long-term behavior of the additional ring effect. Over extended service periods at elevated temperatures, creep and relaxation in the steel tube may reduce the sustained ring stress, potentially diminishing the confinement benefit. This time-dependent degradation should be considered in the design of long-life structures.
Study Insights and Implications
This study provides a valuable analytical tool for engineers designing steel tube confined concrete columns in high-temperature environments. The incorporation of the temperature additional ring effect represents a meaningful improvement over existing empirical methods, offering both higher accuracy and physical interpretability. For steel pipe manufacturers, the findings suggest that tube dimensions should be optimized not only for mechanical performance but also for thermal confinement efficiency. The analytical framework can be readily integrated into structural design software, enabling engineers to evaluate STCC column performance under fire and high-temperature service conditions with greater confidence. The method's alignment with experimental data within a ±5% margin demonstrates its practical reliability for engineering applications.