Finite Element Analysis of Temperature Field in Steel Tube Concrete Beam-Column Joints Under Fire

Literature Overview

The paper by Jiang Ying and Han Linhai from Tsinghua University, published in Industrial Construction in 2009, addresses a critical but often overlooked aspect of steel tube concrete (STC) structural engineering: the thermal behavior of beam-column joints under fire exposure. Funded by the National Science and Technology Support Plan (2006BAJ03A03), this study develops a comprehensive finite element model that captures both the heating and cooling phases of fire, incorporating the influence of water vapor and other thermodynamic factors on the temperature field distribution within composite joints connecting STC columns to either steel beams or reinforced concrete beams.

Core Technical Content and Methodology

The authors establish material thermal property parameters through careful experimental calibration, which is essential because the thermal conductivity, specific heat capacity, and emissivity of steel tubes, concrete cores, and connection plates all vary significantly with temperature. The finite element model accounts for the coupled thermal-mechanical behavior, where the steel tube acts as both a structural element and a thermal shield for the internal concrete core. During the heating phase, the steel tube rapidly reaches high temperatures, creating a temperature gradient between the outer surface and the concrete core. During the cooling phase, the reverse gradient develops, potentially inducing additional thermal stresses.

The inclusion of water vapor effects is particularly noteworthy. When concrete is heated above 100 °C, free water begins to evaporate, and the resulting steam creates a barrier that temporarily reduces heat penetration into the core. This phenomenon, often called the "steam barrier effect," can be a double-edged sword: it provides short-term thermal protection but may lead to explosive spalling when the steam pressure exceeds the tensile strength of the concrete surface.

Comparison with Experimental Results

The validation of the finite element model against experimental data demonstrates good overall agreement, though discrepancies arise in certain regions. The authors note that the model performs particularly well in predicting the temperature distribution within the steel tube walls and the outer concrete layers. However, the core temperature prediction shows some deviation, likely due to the simplified assumptions regarding the concrete thermal properties at elevated temperatures. This is a common challenge in fire engineering finite element analysis, where the material degradation models are inherently uncertain.

Engineering Practice Implications

For practicing engineers, this study provides several actionable insights. First, the temperature field in STC joints is not uniform, and the assumption of a uniform temperature distribution across the joint cross-section can lead to non-conservative designs. Second, the cooling phase should not be neglected in fire-resistant design, as it can induce tensile thermal stresses that may crack the concrete or deform the steel tube. Third, the connection details between the STC column and the beam significantly influence the temperature distribution, and these details should be explicitly modeled rather than idealized as perfectly rigid connections.

Key Reflections and Study Insights

The most valuable contribution of this paper is its systematic treatment of the coupled thermal behavior in composite joints, which is rarely addressed in existing design codes. The Chinese design code GB 51249-2017 provides simplified methods for fire-resistant design of STC members, but the treatment of joints remains largely empirical. This study fills that gap by providing a validated analytical tool that can be used for detailed fire engineering assessments of STC joints. The approach also highlights the importance of material property data at elevated temperatures, which should be obtained from standard tests such as GB/T 23125 or ISO 22676.

Summary

This study represents a significant advancement in the fire engineering analysis of steel tube concrete structures, providing a validated finite element framework that captures the complex thermal behavior of beam-column joints during both heating and cooling phases. The inclusion of water vapor effects and the careful calibration of material thermal properties demonstrate a rigorous research methodology that yields results of practical engineering value. Engineers involved in the design or assessment of STC structures should consider incorporating these findings into their fire-resistant design practices, particularly for critical joints in high-rise buildings and industrial facilities where fire safety is paramount.