Zhuojin Pipe Fitting Co., LtdFire Resistance Behavior of Concrete-Filled Steel Tube Composite Frame Structures Under Full Fire Process
Literature Overview
This paper by Wang Jingxuan, Zhang Pengpeng, and Wang Wenda from Lanzhou University of Technology investigates the mechanical performance of a single-bay, single-story composite frame consisting of circular concrete-filled steel tube (CFST) columns and composite beams, subjected to the complete fire process including heating and cooling phases. The study employs ABAQUS finite element analysis under the ISO-834 standard fire curve, examining both thermal transfer and structural response. Funded by the Ministry of Education Key Science and Technology Research Project (210228) and the Gansu Provincial Natural Science Foundation (096RJZA100), the work contributes to the understanding of real fire scenarios in composite steel-concrete structures.
Core Technical Content
The research establishes a numerical model that captures the full thermal-mechanical coupling behavior of CFST composite frames. The thermal analysis phase focuses on temperature field evolution within both the CFST column cross-sections and the composite beam sections, while the mechanical analysis phase evaluates structural performance through four sequential stages: ambient-temperature loading, fire heating, post-fire cooling, and post-fire residual capacity assessment.
Thermal Transfer Modeling
The thermal analysis requires careful selection of thermophysical parameters that vary with temperature. The following parameters are critical for accurate simulation:
| Parameter | Material | Temperature Range | Typical Values |
|---|---|---|---|
| Thermal conductivity | Structural steel | 20–1000 °C | 50–25 W/(m·K) |
| Thermal conductivity | Concrete | 20–1000 °C | 1.7–2.5 W/(m·K) |
| Specific heat | Structural steel | 20–1000 °C | 460–1000 J/(kg·K) |
| Specific heat | Concrete | 20–1000 °C | 880–1200 J/(kg·K) |
| Convective heat transfer coefficient | Fire environment | ISO-834 curve | 25 W/(m²·K) |
| Emissivity | Steel surface | 20–1000 °C | 0.65–0.95 |
| Emissivity | Concrete surface | 20–1000 °C | 0.70–0.90 |
The ISO-834 standard fire curve is defined by the temperature-time relationship T(t) = 345·log₁₀(8t+1) + 20, where t is time in minutes and T is temperature in °C. The heating phase typically reaches peak temperature at approximately 90 minutes, after which the cooling phase begins. The cooling rate significantly affects thermal gradient development within the cross-section, potentially inducing secondary thermal stresses that may exceed those present during heating.
Mechanical Performance Analysis
The mechanical modeling requires appropriate constitutive models for both steel and concrete at elevated temperatures. The steel stress-strain relationship follows a modified bilinear or multilinear model with strength reduction factors per Eurocode 4 or GB 51249. Concrete requires consideration of creep, thermal expansion, and moisture loss effects. The composite beam behavior depends critically on the degree of shear connection between the steel beam and the reinforced concrete slab through shear connectors.
Key modeling decisions include:
- Element types: 3D solid elements (C3D8R) for steel tubes and concrete cores, shell elements for composite beams
- Mesh density: refined at critical zones including column bases, beam-column joints, and mid-span regions
- Boundary conditions: fixed support at column bases, lateral restraint proportional to actual frame connectivity
- Contact definitions: thermal contact resistance between steel tube and concrete core
Engineering Practice Integration
From a practical engineering standpoint, this research addresses a critical gap in fire design methodology. Traditional fire resistance design often assumes that structural capacity is monotonically decreasing with temperature rise, neglecting the beneficial effects of thermal expansion, triaxial confinement, and post-cooling behavior. The finding that composite frames retain relatively high load-bearing capacity after fire exposure is particularly significant for performance-based fire engineering.
The reinforced concrete slab plays a dual role: it absorbs heat during the fire, reducing the temperature rise in the steel beam flange, and it provides lateral restraint that enhances the effective fire resistance of the steel beam. This composite action is quantified through the interaction factor ψ, which ranges from 0 (no restraint) to 1 (full restraint).
Practical Implications for Steel Pipe Selection
For CFST columns used in building structures, the steel pipe grade (typically Q235 or Q345 per GB/T 1591) and wall thickness directly influence fire performance. Thicker-walled tubes provide greater thermal inertia, delaying the temperature rise in the concrete core. The concrete cover thickness and aggregate type also affect the thermal gradient through the wall thickness.
Common design considerations include:
- Steel tube outer diameter to wall thickness ratio (D/t) should generally not exceed 90 for fire design
- Concrete strength grade should be at least C30 to maintain adequate residual strength
- Fire protection measures (intumescent coatings, fire boards) may be required for unprotected steel beam portions
Key Reflections and Technical Insights
The full fire process approach—incorporating both heating and cooling—represents a significant advancement over traditional isothermal fire testing. During the cooling phase, differential thermal contraction between the steel tube and concrete core can generate tensile stresses in the steel tube, potentially leading to cracking in the concrete cover. This phenomenon is not captured in standard fire resistance tests that terminate at peak temperature.
The study's conclusion that composite frames maintain substantial residual capacity after fire exposure has direct implications for post-fire structural assessment. Engineers involved in post-fire structural evaluation can use such analytical models to estimate remaining load capacity and determine whether the structure requires immediate demolition or can be safely loaded for salvage operations.
However, several limitations should be noted: the study focuses on a single-bay, single-story frame, which may not capture progressive collapse mechanisms in multi-story buildings. The thermal contact resistance between the steel tube and concrete core is difficult to calibrate experimentally and represents a significant source of modeling uncertainty. Future work should incorporate experimental validation with full-scale fire tests and extend the analysis to include cyclic loading effects from seismic activity combined with fire exposure.
Study Insights and Implications
This research reinforces the importance of considering composite action in fire design. The synergistic behavior between the steel tube, concrete core, and reinforced concrete slab creates a system that outperforms the sum of its individual components. For steel pipe manufacturers and structural engineers, this translates into practical guidance: CFST columns with appropriate D/t ratios and concrete fill grades provide inherent fire resistance that may reduce or eliminate the need for additional fire protection systems, resulting in significant cost savings over the building lifecycle. The methodology presented provides a foundation for developing more sophisticated performance-based fire design tools that can be integrated into structural analysis software platforms.