Zhuojin Pipe Fitting Co., LtdTemperature Field Analysis of Square CFST Columns Under Three-Sided Fire Action
Literature Overview
The research by Yang Hua, Zhang Sumei, and Wang Yuyin from Harbin Institute of Technology, published in the Journal of Harbin Institute of Technology (2007, Vol. 39, No. 2, pp. 216-219), investigates the temperature field distribution in square concrete-filled steel tube (CFST) columns subjected to three-sided fire action. This study addresses a critical gap in fire engineering knowledge, as most existing research focuses on four-sided fire conditions, which represent a relatively rare scenario in real building fires.
Three-Sided Fire Scenario and Theoretical Analysis
Three-sided fire action occurs when a structural member is exposed to fire on three faces while the fourth face is shielded by an adjacent wall, partition, or structural element. This scenario is common in real building fires where interior columns are located near walls or partitions. The study develops a theoretical model for predicting the temperature distribution within the cross-section of a square CFST column under three-sided fire exposure.
The theoretical analysis considers the following heat transfer mechanisms:
- Convective heat transfer: Heat transfer from the fire-gas to the steel tube surface through convection.
- Radiative heat transfer: Heat transfer from the fire-gas and hot surfaces to the steel tube surface through radiation.
- Conductive heat transfer: Heat transfer through the steel tube wall and the concrete core.
The theoretical predictions were validated against experimental data from previous tests, demonstrating good agreement between the analytical model and test results. This validation establishes the reliability of the model for parametric studies and design applications.
Parametric Study Results
The study systematically examined the influence of five parameters on the temperature field distribution:
| Parameter | Effect on Temperature Field | Relative Influence |
|---|---|---|
| Fire duration (heating time) | Longer duration leads to higher temperatures throughout the cross-section | High |
| Section side length | Larger sections result in lower core temperatures due to increased thermal mass | High |
| Steel ratio | Higher steel ratio increases heat conduction and may raise core temperatures | Moderate |
| Protection layer type | Different protection materials exhibit varying thermal insulation effectiveness | High |
| Protection layer thickness | Thicker protection layers provide better thermal insulation | High |
Fire Duration Effect
The heating time is identified as one of the primary influence parameters on the temperature field. As the fire duration increases, the temperature throughout the cross-section rises, with the maximum temperature occurring at the fire-exposed surface of the steel tube. The temperature gradient between the fire-exposed surface and the shielded surface increases with time, reflecting the progressive heat penetration through the column cross-section.
Section Side Length Effect
The section side length directly affects the thermal mass of the column and the distance between the fire-exposed surface and the interior regions. Larger sections exhibit lower core temperatures for the same fire duration, as the increased concrete volume provides greater thermal inertia. This parameter is critical in the fire-resistant design of large-diameter CFST columns.
Steel Ratio Effect
The steel ratio, defined as the ratio of steel tube cross-sectional area to the total column cross-sectional area, has a moderate influence on the temperature field. A higher steel ratio increases the heat conduction through the steel tube, potentially raising the temperatures in the concrete core. However, the steel tube also acts as a thermal barrier, limiting the rate of heat transfer to the concrete.
Protection Layer Type and Thickness Effects
The type and thickness of the fire protection layer applied to the steel tube surface are identified as major influence parameters. Different protection materials (such as gypsum boards, vermiculite coatings, and intumescent paints) exhibit varying thermal insulation properties, which directly affect the temperature rise on the steel tube surface. Thicker protection layers provide better thermal insulation, resulting in lower steel tube and concrete temperatures for the same fire duration.
Comparison with Four-Sided Fire Conditions
A significant contribution of this study is the comparison between three-sided and four-sided fire conditions. The analysis reveals that the temperature distribution patterns differ substantially between the two scenarios:
- Three-sided fire: The temperature field is asymmetric, with the maximum temperature occurring at the fire-exposed surface and a significant temperature gradient across the section. The shielded face experiences minimal temperature rise.
- Four-sided fire: The temperature field is symmetric, with the maximum temperature occurring at all four fire-exposed surfaces and a more uniform temperature distribution within the section.
The three-sided fire condition results in a lower average temperature within the cross-section compared to four-sided fire, but the temperature gradient is more severe, which can lead to differential thermal expansion and potential structural issues.
Engineering Practice and Design Implications
The findings from this study have direct implications for the fire-resistant design of CFST columns in real building structures. The identification of fire duration, section side length, protection layer type, and protection layer thickness as the primary influence parameters provides engineers with clear guidance for optimizing fire-resistant design. Key design considerations include:
- Protection layer design: Select appropriate protection materials and thicknesses based on the required fire resistance rating and the expected fire scenario.
- Section design: Optimize the section dimensions to achieve the required structural and fire-resistant performance.
- Fire scenario assessment: Conduct a detailed fire scenario analysis to determine the likely fire exposure conditions for each structural member.
- Temperature monitoring: Implement temperature monitoring systems in critical structural members to verify the fire-resistant performance during actual fire events.
Summary and Reference Value
This study makes a significant contribution to the fire engineering knowledge of CFST columns by providing a validated analytical model for predicting the temperature field under three-sided fire action. The comprehensive parametric analysis identifies the key parameters governing the temperature field distribution and offers practical design guidance for engineers. The comparison between three-sided and four-sided fire conditions highlights the importance of realistic fire scenario modeling in fire-resistant design. For practitioners in the steel pipe and concrete construction industry, this research supports the development of more accurate and realistic fire-resistant design methods for CFST structural systems, ultimately contributing to improved building safety and structural performance in fire events.