Zhuojin Pipe Fitting Co., LtdTemperature Field Analysis and Fire Resistance of Internal Circular Steel Tube Enhanced Square Steel Tube Concrete Eccentric Compression Columns
Overview and Research Significance
The study by Lv Tianqi, Zhao Guofan, and Lin Zhishen (2003) investigates the temperature field distribution and fire resistance limit of eccentrically compressed columns composed of a square steel tube with an internal circular steel tube, filled with concrete. Using finite element and numerical integration methods, the authors analysed the temperature field under fire exposure and computed the fire resistance limit, examining the influence of various parameters on high-temperature performance. The study concluded that the internal circular steel tube significantly enhances the fire resistance of the square steel tube concrete column.
This work is highly relevant to pipe engineers because it addresses a composite structural system where the performance of the steel tubes under elevated temperature governs the overall structural integrity. The thermal behaviour of steel tubes, including the development of thermal gradients, warping, and loss of mechanical properties, directly affects the design of fire protection measures and the selection of appropriate steel grades.
Temperature Field Characteristics and Fire Resistance
Under standard fire exposure (ISO 834 or equivalent), the temperature field within the composite column develops asymmetrically due to the eccentric loading and the geometric complexity of the section. The outer surface of the square tube reaches high temperatures rapidly, while the internal concrete core and the inner circular tube experience delayed and moderated temperature rise due to the thermal mass of the concrete and the insulating effect of the air gap or concrete layer between the two tubes.
| Parameter | Effect on Temperature Distribution | Effect on Fire Resistance Limit |
|---|---|---|
| Concrete cover thickness | Increases thermal lag to inner tube | Extends fire resistance limit |
| Steel tube wall thickness | Increases thermal mass, slows temperature rise | Moderately extends fire resistance |
| Internal tube diameter | Larger diameter increases concrete volume | Extends fire resistance limit |
| Eccentricity ratio (e/h) | Higher eccentricity increases temperature asymmetry | Reduces effective fire resistance |
| Fire exposure duration | Longer duration increases temperature penetration | Governs ultimate fire resistance limit |
The finite element analysis revealed that the temperature gradient across the section creates differential thermal expansion, which induces additional bending moments superimposed on the mechanical eccentricity moment. This thermal-mechanical coupling effect is a critical consideration in fire design, as it can significantly reduce the residual load-bearing capacity of the column.
The internal circular steel tube acts as a secondary structural element that maintains the integrity of the concrete core even when the outer square tube has lost significant strength. This redundancy mechanism is the key to the enhanced fire resistance observed in the study.
Welding and Fabrication Considerations Under Fire Conditions
The fabrication of the internal circular steel tube enhanced square tube column involves multiple welding operations: the longitudinal weld of the square tube, the longitudinal weld of the circular tube (if not seamless), and the connection welds between the two tubes. These welds must be designed and executed with fire performance in mind.
Welding residual stresses, which are typically of the order of yield strength in the HAZ, can interact with thermal stresses during fire exposure to accelerate crack initiation and propagation. The residual stress pattern in the weld, particularly the longitudinal tensile residual stress along the weld seam, can reduce the effective cross-sectional area available for load-bearing at elevated temperatures. Post-weld stress relief through mechanical stretching or thermal treatment can mitigate this effect but may not be feasible for large structural sections.
The connection welds between the internal and external tubes are particularly critical. These welds must resist the differential thermal expansion between the two tubes, which can generate significant shear and tensile forces at the weld interface. The weld design should incorporate adequate throat thickness and penetration to ensure ductile failure rather than brittle weld rupture under fire conditions.
Material selection for fire-exposed steel tubes should consider the temperature-dependent strength retention. Carbon structural steels (e.g., Q235, Q345 per GB/T) retain approximately 50–60% of their ambient temperature yield strength at 600°C. Higher-grade steels (e.g., Q460, Q550) exhibit greater strength loss at elevated temperatures due to their higher initial strength. The choice of steel grade should therefore balance ambient temperature efficiency with fire performance requirements.
Engineering Application and Practical Guidance
The study provides a methodological framework for evaluating the fire resistance of complex composite columns, combining finite element thermal analysis with numerical integration for structural capacity assessment. This approach can be adapted for other composite cross-sections, including multi-tube and multi-cell configurations.
For practical engineering applications, the study suggests that the internal circular steel tube is an effective means of enhancing fire resistance without requiring external fire protection coatings or encasements. This is advantageous for applications where aesthetic or functional constraints limit the use of external protection, such as in industrial facilities or underground structures.
The temperature field analysis also highlights the importance of concrete thermal properties at elevated temperatures. The thermal conductivity and specific heat of concrete decrease with increasing temperature, which accelerates temperature penetration into the core. The use of lightweight aggregate concrete or concrete with added silica fume can improve thermal insulation properties and extend fire resistance limits.
Quality control for fire-exposed composite columns should include verification of weld integrity, concrete density and strength, and tube dimensional accuracy. The concrete density, in particular, affects both the thermal mass and the mechanical performance at elevated temperatures. Low-density concrete, while beneficial for thermal insulation, may have reduced high-temperature strength, creating a trade-off that must be carefully evaluated for each application.
This research, though published over two decades ago, remains relevant as fire safety requirements continue to evolve and composite structural systems become increasingly prevalent. The fundamental principles of thermal-mechanical coupling and redundant structural design demonstrated in this study are directly applicable to contemporary fire engineering practice.