Zhuojin Pipe Fitting Co., LtdSimplified Temperature Field Calculation Method for Steel Tube Concrete Columns Under Fire
Literature Overview
This paper by Lv Xuetao, Mi Zhenwei, Wang Weiwei, and Ren Qingju from Liaoning Technical University, published in 2016 in the journal Fire Science, presents a simplified calculation method for the temperature field distribution in steel tube concrete (STC) columns subjected to uniform fire exposure. Funded by the National Natural Science Foundation Youth Fund (Grant No. 51208246) and the Liaoning Technical University Top Talent Enhancement Program (20130309), the research addresses the critical need for practical and efficient fire resistance design tools for STC structural members.
Core Technical Content
The authors used ABAQUS finite element software to simulate the temperature field distribution in both circular and square STC columns under 80 different loading conditions. The study systematically analyzes the influence of steel tube wall thickness, cross-sectional dimensions, and fire exposure duration on the temperature distribution within the composite section. Based on the numerical simulation results, simplified analytical formulas were derived for predicting the temperature at critical locations within the steel tube and concrete core.
Simulation Parameters
| Parameter | Range | Number of Cases |
|---|---|---|
| Cross-sectional shape | Circular, square | Both |
| Steel tube thickness | 4 mm to 12 mm | 3 levels |
| Section dimension (D or B) | 200 mm to 600 mm | 4 levels |
| Concrete grade | C30, C40, C50 | 3 levels |
| Fire duration | 30 min to 240 min | 5 levels |
| Total combinations | — | 80 cases |
The simplified formulas were calibrated and verified against both experimental data and numerical simulation results. The validation showed that the calculation accuracy meets engineering design requirements, with deviations typically within acceptable limits for fire resistance design purposes.
Technical Analysis and Engineering Practice Integration
Thermal Property Considerations
From a steel pipe manufacturing perspective, the thermal behavior of the steel tube under fire conditions is governed by several factors that are directly related to material quality and fabrication:
- Steel grade and composition: Carbon steel grades commonly used in STC columns (Q235, Q345, Q390) exhibit different thermal expansion coefficients and thermal conductivity values. Higher alloy content generally reduces thermal conductivity, affecting heat transfer rates.
- Steel tube thickness: Thicker walls provide greater thermal mass and slower heat transfer to the concrete core, effectively acting as a thermal shield. However, thicker walls also experience larger thermal gradients through their thickness, potentially causing thermal stresses that may lead to cracking or delamination.
- Surface condition and coatings: Fire-resistant coatings applied to the exterior of steel tubes significantly alter the heat flux boundary condition. The quality of coating application—uniformity, adhesion, and thickness consistency—directly affects the predicted temperature field.
Temperature Distribution Characteristics
| Location | Temperature Trend with Fire Duration | Critical Concern |
|---|---|---|
| Exterior steel surface | Rapid rise to ~800°C within 60 min | Steel strength loss above 550°C |
| Interior steel surface | Delayed rise, depends on wall thickness | Thermal gradient stress |
| Concrete core center | Slowest rise, buffered by steel and concrete thermal mass | Concrete spalling above 300°C |
| Concrete-steel interface | Moderate rise, affected by bond quality | Interface debonding |
The study reveals that the temperature field in square STC columns is more complex than in circular sections due to corner effects and non-uniform heat transfer paths. The corner regions experience higher heat flux concentrations, leading to localized high temperatures that may trigger concrete spalling at the corners before the mid-span regions reach critical temperatures.
Engineering Practice Implications
For steel pipe manufacturers, the fire performance of STC columns has several practical implications:
- Material selection: The thermal conductivity and specific heat of the steel grade should be considered in fire-critical applications. Q235 grade steel, with its higher thermal conductivity, transfers heat to the concrete core more rapidly than Q345 grade, potentially reducing the effective fire resistance time.
- Wall thickness tolerance: The simplified formulas are sensitive to steel tube wall thickness. Manufacturing tolerances that allow ±10% variation in wall thickness can lead to significant deviations in predicted fire resistance performance. Tighter dimensional control is recommended for fire-critical structural applications.
- Weld quality in multi-segment columns: For columns composed of multiple pipe segments joined by butt welds, the weld zones may have different thermal properties than the base metal. Heat-affected zones (HAZ) with altered microstructure may exhibit different thermal expansion behavior, potentially creating differential stresses under fire exposure.
Key Questions and Reflections
A significant question arises regarding the applicability of the simplified formulas to real-world fire scenarios. The study assumes uniform fire exposure, which represents a standard fire curve (ISO 834 or similar). However, real fires in buildings are often compartment fires with non-uniform temperature distributions, localized heat sources, and variable ventilation conditions. The simplified formulas may need modification for non-uniform fire scenarios, such as pool fires or jet fires in industrial facilities.
Another important reflection concerns the long-term behavior of STC columns after fire exposure. The temperature field calculation provides the basis for assessing structural integrity, but the residual mechanical properties after cooling—particularly the residual strength and stiffness of both steel and concrete—require additional evaluation. Concrete that has been heated above 300°C may experience significant strength loss due to dehydration of hydration products, while steel heated above 550°C loses a substantial portion of its yield strength.
Study Insights and Implications
This research provides a practical tool for fire resistance design of STC columns, bridging the gap between computationally intensive finite element analysis and simple hand calculations. For steel pipe manufacturers, the study highlights the importance of consistent material quality and dimensional accuracy in fire-critical applications. The simplified formulas can be incorporated into structural design software or used as verification tools during the design phase. Engineers should note that while the formulas provide acceptable accuracy for uniform fire scenarios, additional analysis may be required for complex fire exposure conditions. The research also underscores the value of numerical simulation as a design tool, and future work could extend these methods to include non-uniform fire scenarios, progressive collapse assessment, and post-fire residual strength evaluation.