Zhuojin Pipe Fitting Co., LtdFire-Resistant Performance Analysis of Concrete-Filled Steel Tube Planar Frames Under Full Fire Cycle
Literature Overview
The study by Wang Jingxuan and Wang Wenda (2014), published in Vibration and Shock (Vol. 33, No. 11, pp. 124-129), addresses a critical gap in the fire engineering of concrete-filled steel tube (CFST) composite frames. Funded by the National Natural Science Foundation of China (Grant 51268038) and the Gansu Provincial Higher Education Research Project (2013A-032), the research was conducted at the Key Laboratory of Disaster Prevention and Mitigation in Civil Engineering, Gansu Province, Lanzhou University of Technology. The work focuses on the full fire cycle — both the heating and cooling phases — and its combined effect with external mechanical loads on the structural response of CFST column-composite beam planar frames.
Core Technical Approach
The authors established a thermal transfer analysis model and a thermally-mechanically coupled numerical model using ABAQUS, selecting appropriate thermal-mechanical constitutive models for both the steel tube and the infill concrete. The fire exposure follows the ISO 834 standard fire curve, which provides a temperature-time relationship of T = 345 log₁₀(8t + 1) + 20°C, where t is time in minutes. The model was validated against existing experimental data before parametric analysis was conducted.
From a welding and fabrication standpoint, this study is particularly relevant because the fire performance of a CFST frame is directly tied to the integrity of welded connections between steel tubes, composite beams, and any reinforcing elements. The thermal-mechanical coupling analysis implicitly assumes that welded joints maintain their mechanical continuity throughout the fire cycle, which is a non-trivial assumption given that weld metal and heat-affected zone (HAZ) materials exhibit different thermal expansion coefficients and strength degradation curves compared to the base steel.
Key Findings and Interpretation
The parametric study reveals several important conclusions:
| Parameter | Effect on P-Δ Curve | Engineering Significance |
|---|---|---|
| Fire load ratio of column | Significant influence on curve shape | Higher axial load ratios accelerate strength degradation under fire |
| Fireproof coating thickness | Significant influence on curve shape | Coating thickness directly governs steel temperature rise rate |
| Column slenderness ratio | Significant influence on curve shape | Slender columns are more susceptible to thermal buckling |
| Initial stiffness | Reduced after fire exposure | Frame lateral stiffness degrades during heating phase |
| Ultimate horizontal load capacity | Reduced after fire exposure | Overall frame resistance diminishes with increasing temperature |
| Column top axial compression | Decreases during cooling phase | Residual deformation and strength loss persist after fire |
The finding that axial compression at the column top decreases during the cooling phase is particularly noteworthy. This suggests that during the heating phase, thermal expansion of the steel tube induces additional compressive stresses that partially relieve the concrete core, but upon cooling, the steel contracts while the concrete retains some thermal damage, leading to a net reduction in axial load capacity. This behavior has direct implications for the design of welded joints in CFST frames, as the cyclic thermal loading may induce fatigue in weld connections, particularly in the HAZ region where microstructural changes have already reduced ductility.
Connection with Welding and Fabrication Practice
In practical engineering, CFST frames are typically fabricated with full-penetration groove welds or fillet welds at column-beam connections. The ISO 834 fire curve can elevate steel temperatures to 600-800°C in unprotected conditions, at which point carbon steel loses approximately 50-70% of its yield strength. The HAZ of a weld, which may already have a reduced toughness due to grain coarsening during welding, becomes especially vulnerable under thermal cycling. Engineers should consider:
- The use of fire-resistant coatings (intumescent or cementitious) with minimum thicknesses of 10-25 mm to limit steel temperature to below 550°C.
- Post-fire inspection protocols including ultrasonic testing (UT) and magnetic particle testing (MT) of all welded connections to detect thermal cracking or loss of weld integrity.
- Selection of welding consumables with adequate low-temperature toughness, since the cooling phase of a fire creates a rapid temperature differential that can induce brittle fracture in susceptible weld metals.
Study Insights and Implications
This paper provides a systematic numerical framework for evaluating the fire resistance of CFST frames, but it does not explicitly address the behavior of welded joints under thermal cycling. Future research should integrate weld-specific material models into the thermally coupled analysis, accounting for the heterogeneous material properties in the weld metal, HAZ, and base metal. For engineers designing CFST frames in fire-prone regions, the parametric results underscore the importance of fireproof coating specification and column slenderness control as the primary design variables governing post-fire structural integrity. The work serves as a valuable reference for establishing fire design criteria in composite steel-concrete structures, particularly where seismic and fire loads must be considered simultaneously.