Nonlinear Finite Element Analysis of Temperature Field in CFST Columns Under High Temperature Conditions

Literature Overview

This study by Wen Hailin, Yu Zhiwu, and Ding Faxing from Central South University, published in the Journal of Railway Science and Engineering (2005, Vol. 2, No. 5, pp. 32-35), addresses the thermal behavior of CFST columns exposed to fire conditions. Supported by the National Natural Science Foundation (Grant 50078007) and the Hunan Provincial Natural Science Foundation (Grant 03JJY3089), the research employs nonlinear finite element analysis using ANSYS to determine the temperature distribution within CFST columns under uniform fire exposure.

Thermal Modeling Approach and Key Assumptions

The study recognizes that at elevated temperatures, the thermal properties of both steel and concrete vary significantly with temperature, necessitating a nonlinear thermal analysis approach. The fire exposure model assumes a standard ISO 834 fire curve, and the column is subjected to uniform heating on all four sides. The thermal conductivity and specific heat capacity of both materials are defined as temperature-dependent functions in the finite element model.

A critical aspect of the thermal modeling is the treatment of the temperature lag phenomenon caused by endothermic physical and chemical reactions within the concrete. At temperatures above approximately 100°C, free water in the concrete begins to evaporate, and between 400-600°C, chemically bound water and hydration products decompose. These endothermic reactions absorb significant thermal energy, resulting in a temperature lag that retards heat propagation into the core of the concrete. The study models this effect by artificially increasing the specific heat capacity of the concrete at elevated temperatures, effectively simulating the energy absorption without explicitly modeling the phase change processes.

The assumption of perfect contact between the steel tube and concrete is maintained throughout the analysis, which simplifies the thermal coupling at the interface. In practice, the contact condition may degrade at high temperatures due to differential thermal expansion and potential debonding, which would affect the accuracy of the predicted temperature field.

Results and Engineering Significance

The finite element analysis provides detailed temperature distributions at various time intervals and locations within the CFST column cross-section. The steel tube heats up rapidly due to its high thermal conductivity, while the concrete core heats more slowly due to the combined effects of lower thermal conductivity and the endothermic water loss mechanisms. The temperature gradient across the section is a critical parameter for subsequent structural analysis, as it determines the residual strength and deformation behavior of the column under fire conditions.

The research contributes to the understanding of CFST fire performance by establishing reliable temperature field predictions that serve as boundary conditions for subsequent nonlinear structural analyses. This two-step approach (thermal analysis followed by structural analysis) is a standard methodology in fire engineering and is essential for assessing the fire resistance rating of CFST structural members.

Study Insights and Limitations

The methodology adopted in this study represents a pragmatic engineering approach that balances computational efficiency with physical accuracy. The use of an enhanced specific heat to model endothermic effects is a well-established technique in thermal finite element analysis of concrete structures. However, the assumption of perfect contact between steel and concrete may not hold at temperatures above 300-400°C, where differential thermal expansion and chemical degradation can cause interface debonding. Future research should consider incorporating contact elements with temperature-dependent contact properties to capture the potential loss of composite action at elevated temperatures. The results of this study provide a valuable foundation for developing simplified fire design methods for CFST columns in building and infrastructure applications.