Zhuojin Pipe Fitting Co., LtdExperimental Study on Residual Bearing Capacity of Concrete-Filled Steel Tube Columns After Fire
Literature Overview
This 2001 paper by Han Linhai, Yang Youfu, and Huo Jingsi, published in the Engineering Mechanics journal, Vol. 18, No. 6, pp. 100–109, presents experimental investigations on the residual bearing capacity of concrete-filled steel tube (CFST) columns after exposure to standard fire curves. The research was conducted at Fuzhou University and Harbin Institute of Technology. The study provides critical data for fire safety assessment and post-fire rehabilitation of CFST structures.
Experimental Program and Fire Conditions
Twelve specimens were tested: six circular section and six square section CFST columns. All specimens were first exposed to fire according to the ISO-834 and GB 9978-88 standard fire curves, which define temperature-time relationships for structural fire exposure:
| Fire Exposure Parameter | Value | Description |
|---|---|---|
| Standard fire curve | ISO-834 | T(t) = 345·log₁₀(8t+1) + 20°C |
| Fire duration | 30–120 minutes | Multiple levels |
| Peak temperature | ~800–1000°C | At 120 min |
| Specimen types | 6 circular, 6 square | Various slenderness ratios |
| Eccentricity ratios | Multiple levels | Including concentric |
The specimens were tested for residual capacity after cooling following fire exposure, with some specimens also tested at elevated temperature for comparison.
Key Findings on Residual Capacity
The experimental results reveal several important patterns:
- Overall capacity loss: Bare CFST columns (without fire protection) experienced severe bearing capacity loss after fire exposure. Columns with protective layers (such as fire-resistant coatings or concrete encasement) showed moderate capacity reduction.
- Slenderness ratio effect: For the same fire exposure conditions, columns with higher slenderness ratios exhibited greater capacity loss rates. This is attributed to the combined effects of material degradation and increased buckling sensitivity at elevated temperatures.
- Eccentricity ratio effect: The load eccentricity ratio had relatively little influence on the post-fire capacity loss rate. This suggests that the material degradation effects dominate over geometric effects in determining residual capacity.
- Section shape effect: Circular and square sections showed different degradation patterns, with circular sections generally performing slightly better due to more uniform temperature distribution and confinement.
Post-Fire Material Degradation
The fire exposure causes progressive degradation of both steel and concrete:
| Temperature Range | Steel Property Change | Concrete Property Change |
|---|---|---|
| 200–400°C | Modulus of elasticity decreases | Thermal cracking initiates |
| 400–600°C | Yield strength reduces 20–50% | Compressive strength reduces 30–60% |
| 600–800°C | Yield strength reduces 50–80% | Compressive strength reduces 70–90% |
| >800°C | Steel loses most strength | Concrete spalling possible |
The steel tube, being exposed directly to fire, experiences more severe degradation than the concrete core, which benefits from thermal insulation by the surrounding steel and the concrete's own low thermal conductivity. However, the steel tube's loss of strength significantly reduces the confinement pressure on the concrete, creating a negative feedback loop that accelerates overall member degradation.
Engineering Practice for Post-Fire Assessment and Rehabilitation
The research findings provide a basis for post-fire structural assessment protocols:
- Visual inspection: Check for steel discoloration (indicating temperature exposure levels), concrete spalling, and visible deformation.
- Non-destructive testing: Ultrasonic testing for concrete strength estimation, magnetic particle testing for steel crack detection, and geometric deformation measurement.
- Load testing: In-situ load testing on representative members to verify residual capacity.
- Rehabilitation options: Based on capacity loss severity, options include:
- No intervention (minor loss, <10%)
- Additional steel jacketing or plating
- Concrete replacement (if spalling occurred)
- Complete replacement (severe degradation)
Study Insights and Reflections
This research is foundational for fire safety engineering of CFST structures. The finding that slenderness ratio is a dominant factor in post-fire capacity loss has important implications for design—slender CFST columns require more fire protection to maintain adequate post-fire performance. The relatively minor effect of eccentricity on capacity loss rate suggests that the material degradation mechanism is more critical than the loading configuration in determining residual strength. From a steel pipe manufacturing perspective, the fire resistance performance of CFST columns is inherently tied to the tube geometry and wall thickness—thicker walls provide better thermal mass and slower heat transfer to the concrete core. Engineers involved in specifying steel tubes for fire-exposed applications should consider wall thickness not only for structural capacity but also for thermal performance. The research underscores the importance of integrating fire safety considerations into the structural design phase rather than treating them as afterthoughts, and highlights the value of composite construction in providing inherent fire resistance through the thermal insulation properties of concrete.