Zhuojin Pipe Fitting Co., LtdTemperature Field Simulation of Steel Tube Concrete Superimposed Columns Under Fire Conditions
Literature Overview and Research Context
The study by Gong Yufa, Qi Xiang, and Zhang Xuan, published in the Journal of Liaoning Technical University (Natural Science Edition) (2017, Vol. 36, No. 12, pp. 1284-1287), investigates the temperature field distribution in steel tube concrete superimposed columns under fire conditions using finite element analysis. The researchers employed ABAQUS software to establish computational models with appropriate thermal parameters for steel and concrete, validated the models against existing experimental data, and conducted parametric studies on the effects of time, fire-exposed surface, and position coefficient on temperature distribution.
This research is directly relevant to the steel pipe and welding engineering community because fire resistance is a critical design consideration for steel structures, and the temperature distribution within composite columns governs the residual mechanical properties of both steel and concrete. For welded steel tube connections in fire-exposed structures, understanding the temperature gradients is essential for predicting post-fire structural integrity and determining the need for repair or replacement. The superimposed column configuration, which combines steel tubes with concrete, introduces additional complexity to the thermal analysis due to the different thermal properties of the constituent materials.
Thermal Analysis Methodology and Model Validation
The researchers selected appropriate thermal parameters for both the steel and concrete materials, which is critical because the thermal conductivity, specific heat, and density of these materials vary significantly with temperature. The steel tube thermal parameters were selected based on standard fire design codes, while the concrete thermal parameters were calibrated to account for the moisture content and aggregate type. The finite element model was validated against experimental data from previous studies, and the agreement between calculated and measured temperatures was found to be satisfactory, confirming the reliability of the model for parametric analysis.
| Thermal Parameter | Steel Tube | Concrete | Temperature Dependence |
|---|---|---|---|
| Thermal conductivity | 50 W/(m·K) at 20°C | 1.5-2.5 W/(m·K) | Decreases with temperature |
| Specific heat | 460 J/(kg·K) at 20°C | 880-1200 J/(kg·K) | Increases with temperature |
| Density | 7850 kg/m³ | 2400 kg/m³ | Relatively constant |
| Emissivity | 0.8-0.9 | 0.8-0.9 | Relatively constant |
| Thermal expansion | 12×10⁻⁶ /°C | 10×10⁻⁶ /°C | Increases with temperature |
The validation process is particularly important because thermal analysis models are sensitive to boundary conditions, material property assumptions, and mesh refinement. The satisfactory agreement between calculated and experimental temperatures provides confidence in the parametric study results and supports the use of the model for design purposes. However, the validation was based on data from other studies rather than the authors' own experiments, which limits the confidence in the model's applicability to the specific superimposed column configuration investigated.
Parametric Study Results
The parametric study examined the effects of three primary variables on the temperature field distribution: fire exposure time, fire-exposed surface configuration, and position coefficient. The fire exposure time directly influences the temperature penetration depth, with longer exposure times resulting in higher temperatures at the column core. The fire-exposed surface configuration determines the thermal boundary conditions and affects the temperature gradient distribution across the column cross-section.
The position coefficient represents the relative position of measurement points within the column cross-section, with values ranging from the fire-exposed surface to the column center. The study found that the temperature distribution was highly non-uniform, with the highest temperatures at the fire-exposed surface and progressively lower temperatures toward the column center. The steel tube, being in direct contact with the fire, experienced the most severe thermal loading, while the concrete core was partially protected by the steel tube and the thermal mass of the concrete itself.
The analysis revealed that the steel tube temperature increased rapidly during the initial fire exposure period, reaching critical temperatures within relatively short times depending on the fire intensity and steel tube wall thickness. The concrete core temperature increased more gradually due to the lower thermal conductivity of concrete and the thermal mass effect. The temperature difference between the steel tube and concrete core created differential thermal expansion, which induced additional stresses in the composite column that must be considered in post-fire assessment.
Engineering Implications for Steel Tube and Welding Practice
The temperature field analysis has direct implications for the fire resistance design of steel tube concrete structures. The steel tube wall thickness is a critical parameter that influences both the fire resistance rating and the temperature distribution within the column. Thicker steel tube walls provide greater thermal mass and delay the temperature rise in the concrete core, but they also increase the thermal gradient across the wall thickness, which can lead to differential thermal stresses.
For welded connections in fire-exposed structures, the temperature distribution near the weld region is particularly important because welds typically have different thermal properties compared to the base metal. The heat-affected zone (HAZ) of welded joints may exhibit different thermal expansion characteristics, leading to additional stress concentrations during fire exposure. The study's temperature field analysis provides a basis for evaluating the thermal loading on welded connections and determining the need for fire protection measures.
The superimposed column configuration, which may involve multiple steel tubes or a combination of steel and concrete elements, introduces additional thermal complexity. The thermal interaction between different elements can lead to non-uniform temperature distributions that are difficult to predict analytically. The finite element analysis approach demonstrated in this study provides a practical tool for evaluating the thermal performance of complex superimposed column configurations.
Key Findings and Design Recommendations
The study identified several key findings that have direct relevance to the fire resistance design of steel tube concrete superimposed columns. The fire exposure time is the most significant factor influencing the maximum temperature achieved, with longer exposure times resulting in more severe thermal damage. The fire-exposed surface configuration affects the temperature gradient distribution and should be considered in the design of fire protection systems. The position coefficient analysis demonstrates the importance of thermal penetration depth in determining the residual structural capacity.
For welding engineers, the key recommendation is to ensure that welded connections in fire-exposed steel tube concrete structures are designed to maintain integrity at elevated temperatures. The temperature field analysis provides the thermal loading data necessary for evaluating weld performance under fire conditions, and the results should inform the selection of welding procedures, filler metals, and post-weld heat treatment requirements. The differential thermal expansion between the steel tube and concrete core should be accounted for in the design of expansion joints and connection details to prevent fire-induced structural damage.
Study Insights and Outlook
This research provides valuable insights into the thermal behavior of steel tube concrete superimposed columns under fire conditions, with practical implications for fire resistance design and post-fire assessment. The finite element analysis approach offers a cost-effective alternative to full-scale fire testing for evaluating the thermal performance of complex column configurations. For the steel pipe and welding engineering community, the study highlights the importance of thermal analysis in the design of fire-exposed structures, particularly regarding the thermal loading on welded connections and the differential thermal expansion effects in composite columns. Future research should focus on developing simplified analytical methods for thermal analysis, conducting full-scale fire tests on superimposed column configurations, and investigating the post-fire residual strength of welded connections to establish reliable repair and replacement criteria.