Thermal Response Characteristics of Steel Tube Components Under Fire Conditions

Literature Overview

The paper by Chen Changkun and Yao Bin (2008), published in Journal of Combustion Science and Technology, investigates the thermal response behavior of steel tube structural components under fire conditions using the ISO 9705 standard fire test system. The study measures heat release rate, temperature field distributions, and steel component temperatures under varying fire source powers and positions. A key finding is that although steel has a relatively high thermal conductivity, the thin-wall nature of structural steel tube components results in internal heat transfer along the longitudinal direction being much smaller than radial heat exchange with the external environment, leading to significant temperature non-uniformity along the component length.

Core Technical Points

The fundamental thermal behavior observed in this study can be understood through the dimensionless Biot number, which characterizes the ratio of internal conductive resistance to external convective resistance. For thin-walled steel tubes with small cross-sectional areas, the Biot number is low, indicating that the component temperature is dominated by the external heat flux rather than internal conduction. This has profound implications for fire protection design and structural fire resistance assessment.

The experimental setup follows ISO 9705, which defines a standard fire test procedure for determining the heat release rate of burning materials. The test system allows precise control of fire source power and position, enabling systematic investigation of thermal response under different scenarios. The measured parameters include:

The finding that longitudinal temperature distribution is primarily governed by local surface gas temperature rather than internal heat conduction is critical for understanding failure modes in steel tube structures under fire. Localized heating can cause thermal buckling, local yielding, and progressive collapse even when the average component temperature remains below the conventional fire resistance threshold of 550 °C.

Process and Standards Analysis

From a welding and pipe manufacturing perspective, the thermal response characteristics studied here have direct relevance to several aspects of steel tube component integrity. First, weld joints in steel tube structures are particularly vulnerable under fire conditions because the HAZ typically has different thermal properties and residual stress states compared to the base metal. The non-uniform temperature distribution identified in this study can exacerbate thermal stresses at weld locations, potentially leading to premature failure.

Relevant standards for structural fire design include EN 1993-1-2 (Eurocode 3: Design of steel structures – Fire design), GB 50016 (Chinese Code for Fire Protection Design of Buildings), and NFPA 251 (Standard Fire Tests for Endurance of Construction Materials and Structural Elements). These standards typically assume uniform temperature exposure, which the present study demonstrates is not realistic for thin-walled steel tube components. The ISO 9705 test protocol used here provides a more realistic fire scenario, and its results should inform the development of component-specific fire design provisions.

Integration with Engineering Practice

For engineers designing steel tube structures in fire-prone environments such as offshore platforms, chemical plants, or high-rise buildings, the findings of this study have several practical implications. First, fire protection systems (intumescent coatings, fireproof boards, water spray systems) should be designed to address localized heating rather than uniform temperature rise. Second, structural analysis under fire conditions should incorporate non-uniform temperature fields rather than assuming isothermal conditions. Third, inspection and maintenance programs for steel tube structures should pay particular attention to areas where thermal non-uniformity is likely to be most severe, such as near fire source locations, around penetrations, and at connection details.

The study also highlights the importance of material selection. Steels with higher thermal conductivity (such as certain austenitic stainless steels) may exhibit more uniform temperature distribution, but their elevated-temperature strength properties must also be considered. Carbon-manganese structural steels (such as Q235, Q345, S355) have adequate thermal conductivity but lose significant strength above 400 °C, making them vulnerable to thermal failure even at moderate fire severities.

Key Questions and Reflections

The study raises important questions about the applicability of current fire design codes to thin-walled steel tube components. Most codes were developed for solid structural sections (beams, columns, plates) where the Biot number is higher and temperature gradients are less pronounced. For steel tubes with D/t ratios exceeding 20, the thermal response may deviate significantly from code assumptions. Engineers should consider conducting fire-specific finite element analyses that incorporate realistic temperature distributions rather than relying solely on code-based fire resistance ratings.

Another important consideration is the interaction between fire exposure and pre-existing defects. Welding defects such as lack of fusion, porosity, or slag inclusions can act as stress concentrators under thermal loading, potentially initiating cracks at temperatures well below the material's elevated-temperature yield strength. Non-destructive testing (NDT) protocols for steel tube structures in fire-prone environments should therefore be more stringent than for structures in normal service conditions.

Study Insights and Implications

This research provides valuable experimental data on the thermal behavior of steel tube components under realistic fire conditions. The key insight for practicing engineers is that thin-walled steel tubes behave thermally very differently from solid sections, and fire design must account for this difference. The longitudinal temperature non-uniformity identified here is a critical factor in fire-induced structural failure that has been largely overlooked in conventional design practice. Future research should focus on developing simplified analytical models for predicting temperature distribution in steel tube components under fire, as well as on validating these models through full-scale fire tests of steel tube structures.