Finite Element Analysis of Temperature Field and Thermal Stress in Rectangular Steel Tube Concrete Columns

Literature Overview

This paper, published in the journal Progress in Water Resources and Hydropower Engineering (Vol. 25, No. 5, 2005, pp. 33–36), was authored by Sun Yuelin, Zhang Liaojun, and Ran Maoge from Hohai University and Guiyang Survey and Design Institute. The study focuses on a rectangular steel tube concrete (SRC) column in a hydroelectric power station powerhouse frame, employing ANSYS finite element software to establish a three-dimensional thermal field model. The authors investigate temperature and stress distributions under both external heating and cooling conditions, examining the fire resistance behavior of rectangular SRC columns and comparing the influence of two different boundary conditions on thermal stress development.

Core Technical Approach

The research adopts a coupled thermo-mechanical finite element methodology using ANSYS, which is widely recognized for its robust capabilities in thermal-stress coupled analysis. The modeling approach involves several critical steps:

Geometric modeling: A three-dimensional finite element model of the rectangular steel tube concrete column was constructed, accounting for the actual dimensions and reinforcement configuration of the powerhouse frame column.
Material property assignment: Temperature-dependent material properties were assigned to both the steel tube and the infilled concrete, including thermal conductivity, specific heat, elastic modulus, and thermal expansion coefficient as functions of temperature.
Boundary condition simulation: Two scenarios were analyzed—external temperature rise (representing fire exposure) and external temperature drop (representing rapid cooling or environmental temperature change).
Thermo-mechanical coupling: The temperature field solution was used as a body load input for the subsequent structural stress analysis, enabling the calculation of thermal stresses throughout the column cross-section.

Key Findings and Technical Insights

The study reveals several important characteristics of thermal behavior in rectangular steel tube concrete columns:

Temperature gradient distribution: Under fire exposure, the steel tube surface reaches high temperatures rapidly due to its high thermal conductivity, while the interior concrete core heats more slowly, creating a significant radial temperature gradient. This gradient is more pronounced in rectangular sections compared to circular ones due to the geometric asymmetry and corner effects.
Thermal stress concentration: The differential thermal expansion between the steel tube (which expands more at elevated temperatures) and the concrete core (which has lower thermal expansion and delayed heating) generates compressive stresses in the steel and tensile stresses in the concrete at the interface. Corner regions of the rectangular section experience higher stress concentrations.
Boundary condition sensitivity: The comparison between two boundary conditions demonstrates that the assumed heat transfer mechanism at the steel-concrete interface significantly affects the computed thermal stress magnitude. A perfect bond assumption yields higher interfacial stresses compared to a debonded or partially bonded condition.

Parameter	Typical Value in Fire Scenario	Effect on Thermal Stress
Steel tube surface temperature	600–800°C at 60 min fire	High compressive stress in steel
Concrete core temperature	200–400°C at 60 min fire	Moderate tensile stress in concrete
Thermal expansion coefficient (steel)	12×10⁻⁶ /°C	Drives differential expansion
Thermal expansion coefficient (concrete)	10–14×10⁻⁶ /°C	Coupled with temperature gradient
Corner stress concentration factor	1.2–1.5× average	Localized stress peaks

Engineering Practice Implications

From a steel pipe manufacturing and structural engineering perspective, this study has several practical implications:

Fire protection design: The findings underscore the need for fire-resistant coatings or concrete encasement on steel tubes in critical structural applications such as hydroelectric power station frames. The thermal stress analysis provides quantitative data for selecting appropriate fire protection thickness.
Material selection: The study implicitly highlights that steel grades with lower thermal expansion coefficients or higher temperature-dependent strength retention would be advantageous for SRC columns in fire-prone environments.
Quality control considerations: For steel tubes used in SRC columns, dimensional accuracy and surface quality are critical, as any geometric irregularities can exacerbate thermal stress concentrations, particularly at corners and weld seams.

Reflections and Critical Assessment

The paper represents an early application of coupled thermo-mechanical finite element analysis to rectangular SRC columns in the Chinese hydroelectric industry. While the methodology is sound, several aspects merit further consideration. The study does not explicitly address the degradation of concrete strength at elevated temperatures, which is a critical factor in fire resistance assessment. Additionally, the long-term thermal cycling effects and their cumulative damage potential are not explored. The boundary condition comparison, while valuable, would benefit from experimental validation through thermal imaging or embedded thermocouple measurements on instrumented specimens.

Study Insights and Concluding Remarks

This literature provides a foundational understanding of thermal stress behavior in rectangular steel tube concrete columns under extreme temperature conditions. The finite element results, while validated against engineering experience, establish a computational framework that can be extended to more complex scenarios including progressive structural failure, spalling of concrete, and coupled mechanical loading during fire events. For engineers involved in the design and quality control of steel tubes for composite structural applications, the key takeaway is that thermal performance must be considered alongside mechanical properties, particularly in critical infrastructure where fire resistance is a safety-critical requirement. The study reinforces the importance of material characterization at elevated temperatures and the value of numerical simulation as a complement to experimental testing in structural engineering design.