Zhuojin Pipe Fitting Co., LtdExperimental Study on Fire Resistance of Concrete Filled Steel Tube Columns
Literature Overview
This seminal paper by Han Linhai and colleagues (2000), published in the China Civil Engineering Journal, presents the results of fire resistance tests on seven CFST columns under standard temperature-time curves. Supported by the National Natural Science Foundation of China, this research was conducted jointly by Harbin University of Architecture and the Tianjin Fire Research Institute of the Ministry of Public Security. The study provides critical experimental data for fire design of CFST columns in accordance with GB 50045-95 (Code for Fire Protection Design of High-Rise Civil Buildings).
Research Significance
Fire resistance is a fundamental requirement for structural systems in high-rise buildings. CFST columns, while offering superior load-bearing capacity and ductility, present unique fire engineering challenges due to the differential thermal properties of steel and concrete. This study addresses a critical gap in the understanding of how CFST columns behave under fire conditions, providing the experimental basis for practical fire design.
Test Configuration
| Parameter | Specification |
|---|---|
| Number of specimens | 7 |
| Loading condition | Standard ISO 834 fire temperature-time curve |
| Load application | Axial compression at constant load |
| Fire duration | Until failure or 180 minutes |
| Temperature measurement | Thermocouples at steel tube, concrete core, and interface |
| Deformation measurement | Axial displacement, lateral deflection |
| Load level | Varying axial loads to determine fire resistance limits |
Key Experimental Findings
The paper reports several important conclusions:
- Enhanced inherent fire resistance: Due to the synergistic action between the steel tube and concrete core, and the excellent heat absorption capacity of the concrete core, CFST columns exhibit significantly better fire resistance than equivalent unprotected steel columns.
- Temperature gradient development: Significant temperature gradients develop between the outer steel tube surface and the concrete core center during fire exposure. The concrete core acts as a thermal barrier, slowing the temperature rise at the steel tube's inner surface.
- Failure modes: Failure typically initiates at the steel tube's outer surface when the temperature exceeds 550–600°C, leading to local buckling of the heated steel wall. The concrete core continues to carry load even after the steel tube has partially failed.
- Fire resistance limits achieved: With appropriate fire protection coatings applied to the column exterior, the required fire resistance limits specified in GB 50045-95 can be readily achieved.
Fire Resistance Mechanism Analysis
The fire resistance of CFST columns is governed by several interacting mechanisms:
| Mechanism | Description | Effect on Fire Resistance |
|---|---|---|
| Concrete thermal mass | Core concrete absorbs heat, delaying steel temperature rise | Increases fire resistance significantly |
| Thermal expansion differential | Steel and concrete expand at different rates | Can cause debonding at high temperatures |
| Steel strength degradation | Yield strength decreases above 400°C | Reduces confinement and load capacity |
| Concrete spalling | High temperature can cause concrete surface spalling | Exposes steel to higher temperatures |
| Convection and radiation | Heat transfer through the gap between steel and concrete | Affects internal temperature distribution |
Design Implications for Fire Protection
Based on the experimental results, the following design guidelines are recommended:
- Fire protection coating selection: Intumescent coatings with expansion thickness of 20–30 mm can extend fire resistance to 2.0–3.0 hours for typical CFST column dimensions.
- Coating application quality: Uniform coating thickness is critical; thin spots can become failure initiation points.
- Weld joint protection: Welded connections at column joints require additional fire protection as they represent geometric discontinuities.
- Post-fire assessment: Columns exposed to fire require non-destructive testing (UT for wall thickness, MT for surface cracking) before serviceability assessment.
Connection with Steel Pipe Manufacturing
From a steel pipe manufacturing perspective, this research highlights several quality requirements for tubes intended for CFST fire-exposed applications:
- Wall thickness uniformity: Non-uniform wall thickness creates weak points where fire-induced buckling initiates. Ovality should be controlled to less than 1.5% of diameter.
- Material homogeneity: Chemical composition uniformity ensures predictable strength degradation under thermal exposure. Segregation or compositional variation can create unpredictable weak zones.
- Weld quality: For welded tubes (ERW, HFW, LSAW), the weld seam represents a potential weak link under thermal cycling. Full penetration and sound welds are essential.
- Surface condition: The outer surface quality affects fire protection coating adhesion. Surface preparation (cleaning, roughening) must be specified in the steel tube purchase requirements.
Study Insights
This paper remains highly relevant to modern CFST fire engineering practice. The experimental methodology—combining structural engineering analysis with fire science testing—provides a model for future research. The finding that CFST columns inherently possess good fire resistance due to the concrete core's thermal mass is particularly valuable for engineers seeking to minimize fire protection costs while maintaining safety. However, the paper also implicitly warns that relying solely on inherent fire resistance without proper design verification is risky, as failure modes under fire conditions differ fundamentally from those at room temperature.