Zhuojin Pipe Fitting Co., LtdFinite Element Analysis of Temperature Field and Temperature Stress in Rectangular Concrete-Filled Steel Tube Columns
Research Background and Engineering Context
The paper by Sun Yuelin, Zhang Liaoqun, and Ran Maoge, published in Progress in Water Conservancy and Hydropower Science and Technology in 2005 (Vol. 25, No. 5, pp. 33-36), presents a finite element analysis of the temperature field and temperature stress in rectangular concrete-filled steel tube (CFST) columns under fire conditions. The study was conducted for a specific hydropower station powerhouse structure, making it directly relevant to the design of fire-resistant structural systems in critical infrastructure facilities.
Finite Element Modeling Approach
The authors utilized the ANSYS general-purpose finite element software to establish a three-dimensional temperature field model of a rectangular CFST column. The model considered both external temperature rise (fire exposure) and temperature drop (cooling phase) scenarios, which represent the two primary phases of a fire event. The boundary conditions were carefully defined to represent realistic fire scenarios, including convective and radiative heat transfer at the exposed surfaces and adiabatic or specified conditions at the unexposed surfaces.
The temperature-dependent material properties of both the steel tube and the concrete core were incorporated into the model. For structural steel, the yield strength decreases significantly above 600 degrees Celsius, and the elastic modulus also degrades. For concrete, the thermal conductivity decreases at high temperatures due to microcracking, and the compressive strength drops substantially above 400 degrees Celsius.
Temperature Field Distribution and Stress Analysis Results
The analysis revealed important characteristics of the temperature field distribution in rectangular CFST columns:
| Location | Temperature Rise Scenario | Temperature Drop Scenario |
|---|---|---|
| Exposed steel tube surface | Reaches fire temperature rapidly | Cools slower than interior |
| Steel-concrete interface | Lagged temperature rise | Thermal gradient develops |
| Concrete core center | Lowest temperature rise | Slowest cooling rate |
| Corner regions of rectangular section | Higher stress concentration | Residual stress patterns differ |
The temperature stress analysis showed that the thermal gradients within the cross-section generate significant bending moments and membrane stresses in the steel tube. The rectangular geometry creates stress concentrations at the corners, which are particularly critical during the cooling phase when differential contraction between the steel tube and concrete core can lead to localized yielding or cracking.
Comparison of Boundary Condition Effects
A key contribution of this research is the comparison of two different boundary condition assumptions and their effect on the predicted temperature stress. The first boundary condition assumed a standard fire curve (ISO 834 or similar) applied to all exposed surfaces. The second boundary condition incorporated more realistic fire scenarios specific to the hydropower station environment, including partial exposure and ventilation effects.
The comparison demonstrated that boundary condition assumptions can significantly affect the predicted temperature stress, with differences of 15-25 percent in peak stress values. This finding underscores the importance of using appropriate fire scenario assumptions in structural fire engineering analyses, rather than relying solely on standard fire curves.
Engineering Practice and Design Recommendations
From a practical standpoint, this research provides several actionable insights for the design of fire-resistant CFST columns in critical infrastructure:
First, the rectangular section geometry requires special attention to corner regions during fire design. The stress concentrations at the corners may necessitate additional local reinforcement or modified section design to prevent premature failure.
Second, the cooling phase of a fire event should not be overlooked in design. The thermal gradients during cooling can generate stresses that are comparable to or even exceed those during the heating phase, particularly in restrained structural systems.
Third, the finite element methodology presented in this paper provides a practical framework for evaluating the fire performance of specific CFST column designs, allowing engineers to optimize section dimensions, steel grade, and concrete mix for fire resistance.
Study Insights and Concluding Remarks
This research demonstrates the power of finite element analysis in understanding the complex thermal-mechanical behavior of CFST columns under fire conditions. The three-dimensional modeling approach captures the non-uniform temperature distribution and stress state that cannot be adequately represented by simplified analytical methods.
The findings have direct implications for the fire design of CFST structures in hydropower stations, nuclear facilities, and other critical infrastructure where fire resistance is a primary design requirement. The methodology is transferable to other structural configurations and can be extended to include coupled thermal-mechanical analysis with progressive failure simulation.
This paper provides a solid technical foundation for the fire design of rectangular CFST columns and contributes valuable insights that should inform future fire design code provisions for concrete-filled steel tube structures.