Temperature Field Study of Concrete-Filled Steel Tube Composite Columns Under Fire

Literature Overview

This 2015 paper by Xiang Kai and Wang Guohui from the Tianjin Fire Research Institute, Ministry of Public Security, presents experimental and numerical investigation of the temperature field distribution in concrete-filled steel tube (CFST) composite columns under ISO 834 standard fire conditions. Published in the Journal of Natural Disasters (Volume 24, Issue 1, pages 133-140), the study is supported by the "12th Five-Year Plan" National Science and Technology Support Program (2011BAK03B0104) and the National Natural Science Foundation (51408134). The research comprises fire tests on 8 composite column specimens with varying cross-sectional dimensions, section types, and steel tube ratios.

Core Technical Content

The study investigates how fire exposure affects the internal temperature distribution of CFST composite columns, which is fundamental to predicting structural performance during fire events. The temperature field determines the degradation of material properties (steel strength, concrete compressive strength), the development of thermal stresses, and ultimately the failure mode and residual load capacity of the column.

Test Matrix Summary

Parameter	Variations	Purpose
Cross-sectional dimensions	Multiple sizes	Study thermal mass effect
Section type	Circular, square	Compare geometric effects
Steel tube ratio	Varying percentages	Assess protective effect of steel tube
Fire duration	Multiple time points	Establish temperature-time curves
Fire exposure pattern	Full, partial, one-sided	Evaluate asymmetry effects
Slenderness ratio	Various L/D ratios	Study buckling susceptibility
Longitudinal reinforcement ratio	Different percentages	Assess steel reinforcement contribution

Key Experimental Findings

The experimental results reveal several important trends in the temperature field behavior of CFST composite columns:

When the steel tube ratio is constant, larger cross-sectional dimensions result in lower internal temperatures. This is attributed to the greater thermal mass and the increased distance from the fire-exposed surface to the column core.
When cross-sectional dimensions are constant, variations in the steel tube ratio have relatively minor effects on the internal temperature field. This suggests that the concrete core dominates the thermal response once a minimum steel tube thickness is achieved.
When the edge length of a square section equals the diameter of a circular section, the square section column exhibits lower internal temperatures. This is because the circular section has a larger perimeter-to-area ratio, resulting in greater heat input per unit volume.

Finite Element Model Validation

The numerical model was validated against the experimental temperature measurements, showing good agreement between predicted and measured temperature distributions. The model incorporated temperature-dependent material properties for both steel and concrete, thermal contact resistance at the steel-concrete interface, and the phase change of moisture in concrete during heating.

Model Component	Property	Temperature Range
Steel tube	Thermal conductivity	50-200 degrees C
Steel tube	Specific heat	50-800 degrees C
Concrete core	Thermal conductivity	20-600 degrees C
Concrete core	Compressive strength retention	20-800 degrees C
Interface	Thermal contact resistance	Variable

Engineering Practice Implications

For structural engineers designing fire-resistant steel-concrete composite structures, the findings of this study provide quantitative guidance for predicting temperature distributions and, by extension, structural performance during fire exposure. The results indicate that:

Cross-sectional size is the most effective geometric parameter for improving fire resistance, as larger sections inherently possess greater thermal mass.
The steel tube ratio has diminishing returns beyond a certain threshold, suggesting that over-designing the steel tube thickness for fire protection alone may not be economically justified.
Square sections outperform circular sections of equivalent area in terms of internal temperature control, which is a valuable design consideration for composite columns.

Fire Design Recommendations

Design Strategy	Effectiveness	Implementation Difficulty
Increase cross-sectional size	High	Moderate (cost implications)
Apply fire-resistant coating	High	Low
Increase steel tube thickness	Moderate	Moderate
Add internal cooling system	Very high	High
Use fire-resistant concrete admixtures	Moderate	Low
Select square over circular section	Moderate	Low

Study Insights and Outlook

The temperature field is the fundamental input for predicting the structural response of composite columns in fire. Understanding how geometric parameters influence the temperature distribution enables engineers to make informed design decisions that balance structural performance, fire safety, and economic efficiency. The finding that square sections perform better than circular sections of equal area is particularly interesting from a design optimization perspective, as it suggests that the choice of cross-sectional shape should be reconsidered for fire-critical applications. Future research should extend these findings to include coupled thermal-mechanical analysis that captures the interaction between temperature-induced material degradation and structural instability, as well as the effects of fire exposure duration on residual structural capacity after cooling.