Nonlinear Finite Element Analysis of Temperature Field in Steel Tube Concrete at High Temperature

Literature Overview

Wen Hailin, Yu Zhiwu, and Ding Faxing from Central South University published this study in the Journal of Railway Science and Engineering in 2005 (Volume 2, Issue 5, pages 32-35). Supported by the National Natural Science Foundation (Project 50078007) and Hunan Provincial Natural Science Foundation (Project 03JJY3089), this research addresses a critical gap in fire engineering: the accurate prediction of temperature distribution within steel tube concrete (SRC) members under fire exposure. Understanding the thermal behavior is a prerequisite for assessing the residual load-bearing capacity and deformation characteristics of SRC columns during fire events.

Thermal Mechanisms and Modeling Approach

The study correctly identifies that at elevated temperatures, the thermal properties of materials are not constant but vary significantly with temperature. This temperature-dependent behavior necessitates a nonlinear finite element approach rather than simplified analytical solutions.

Thermal Phenomena Considered

The following thermal phenomena are critical to accurate temperature field prediction:

Free water evaporation: Concrete contains free water that evaporates at approximately 100°C, absorbing latent heat and creating a temperature plateau.
Chemical decomposition: Hydrates and gels in cement paste decompose at temperatures between 200-600°C, absorbing additional heat.
Thermal lag: These endothermic processes create a significant delay in temperature rise at the core of the concrete section compared to the fire-exposed surface.
Steel tube thermal conductivity: Carbon steel maintains relatively high thermal conductivity even at elevated temperatures, facilitating heat transfer to the concrete core.

Finite Element Model Configuration

The authors used ANSYS finite element software with the following modeling assumptions:

Modeling Parameter	Assumption/Value	Justification
Contact condition	Full contact between steel tube and concrete	Conservative assumption ensuring heat transfer
Temperature lag	Modeled by increasing concrete specific heat	Equivalent thermal mass approach
Fire model	Four-side uniform fire exposure	Standard ISO 834 fire curve
Steel thermal properties	Temperature-dependent conductivity and specific heat	Based on Eurocode 3 data
Concrete thermal properties	Temperature-dependent with phase change effects	Captures endothermic reactions

The key modeling innovation is the approach of increasing the concrete specific heat to account for the heat absorption due to water evaporation and chemical decomposition. This is a practical engineering approximation that avoids the complexity of explicit phase-change modeling while capturing the essential thermal lag phenomenon.

Results Interpretation and Engineering Significance

The nonlinear finite element analysis reveals several important thermal behaviors:

Temperature gradient: A significant temperature gradient develops between the outer steel tube surface and the inner concrete core, particularly during the early stages of fire exposure.
Thermal lag magnitude: The temperature lag effect reduces the peak temperature in the concrete core by 100-200°C compared to the fire gas temperature at equivalent time.
Steel tube temperature: The steel tube reaches high temperatures (600-800°C) more rapidly than the concrete core, which has implications for steel strength degradation.
Time-dependent behavior: The rate of temperature increase decreases over time as the thermal gradient reduces and the heat flux diminishes.

Practical Implications for Fire Design

This research provides engineers with the following actionable guidance:

Fire resistance assessment: The temperature field data can be directly input into residual strength models to predict load-bearing capacity at any fire duration.
Protective measures: The thermal lag effect provides a natural fire protection mechanism, but additional insulation may be required for critical structural elements.
Design optimization: The analysis identifies that thicker concrete cores provide better thermal protection, but with diminishing returns beyond certain thicknesses.
Verification approach: The finite element results should be validated against standard fire test data (e.g., EN 1363-1 test configurations) for design code calibration.

Study Insights and Methodological Reflection

The approach of modeling thermal lag through increased specific heat is elegant in its simplicity and effective in practice. However, engineers should be aware of its limitations: this method does not capture the spatial variation of phase change effects within the concrete section, which may be significant for thick sections.

For more refined analysis, explicit phase-change models using the enthalpy method would provide more accurate results, particularly for predicting the temperature distribution during the critical 200-400°C range where most chemical decomposition occurs. The study's assumption of full contact between steel and concrete is reasonable for well-constructed SRC members but may overestimate heat transfer for sections with voids or poor concrete compaction.

The research contributes significantly to the fire engineering knowledge base for SRC structures, providing the thermal analysis foundation necessary for comprehensive fire-resistant design. Engineers should use this methodology as a starting point, supplementing it with experimental validation and appropriate safety margins in the final design.