Temperature Field Analysis of Steel-Reinforced Square Steel Tube Concrete Columns Under Two-Sided Fire Exposure

Literature Overview

The paper by Zhou Kui, Meng Fanqin, and Zhu Meichun, published in the Journal of Shenzhen University (Science and Engineering) in 2014, addresses a critical yet under-explored scenario in structural fire engineering: the thermal behavior of steel-reinforced square steel tube concrete (SRC-CHS) columns subjected to fire on two adjacent faces rather than the conventional four-sided exposure. This research was supported by the Shanghai Natural Science Foundation (11ZR1426400) and represents a significant advancement in understanding localized fire scenarios that are increasingly relevant in modern building configurations where fire compartmentation may limit flame exposure to specific faces of a column.

The study combines experimental testing under the ISO 834 standard fire curve with finite element analysis using ANSYS, creating a validated computational framework that can be extended to complex fire scenarios beyond laboratory capabilities. From my decades of experience in steel pipe metallurgy and structural integrity assessment, I find this approach particularly commendable because it bridges the gap between controlled laboratory conditions and the chaotic reality of fire events in real structures.

Core Technical Findings

The primary finding is that under two-sided fire exposure, the cross-sectional temperature distribution is significantly lower than under four-sided exposure, resulting in relatively mild material degradation. However, the temperature field exhibits approximate symmetry along the diagonal of the square section, which introduces a critical secondary effect: the development of initial deflection and additional eccentricity in the column. This is a subtle but dangerous phenomenon because it means that even though the absolute temperatures are lower, the asymmetry of heating can produce bending moments that would not exist under uniform heating conditions.

The parametric study reveals that heating duration and section side length are the two most influential parameters on the temperature field distribution. This aligns with fundamental heat transfer principles: longer exposure allows deeper thermal penetration into the section, while larger sections provide more thermal mass but also more area for differential heating.

Key Thermal Parameters and Their Influence

Interpretation of Technical Points

The diagonal symmetry of the temperature field under two-sided fire exposure deserves particular attention from a welding and metallurgical perspective. In steel tube concrete columns, the steel tube is typically fabricated through longitudinal or circumferential welding, and the heat-affected zone (HAZ) already represents a region of altered microstructure and reduced toughness. When subjected to fire, the thermal gradient superimposed on the existing HAZ can create localized stress concentrations that accelerate degradation mechanisms such as low-temperature hydrogen embrittlement (LTHE) and delayed cracking in high-strength steels.

The finding that material damage is "relatively mild" under two-sided fire exposure compared to four-sided exposure must be interpreted with caution. While the absolute temperatures may be lower, the thermal gradients across the section are more severe in localized fire scenarios. From a welding quality control standpoint, this means that the residual stresses locked in during fabrication—particularly from longitudinal submerged arc welding (LSAW) or high-frequency welding (HFW) processes—can interact adversely with fire-induced thermal stresses, potentially leading to premature failure at weld locations even when bulk material temperatures remain below critical thresholds.

The validation of the ANSYS finite element model against experimental results provides confidence in the computational approach, but I would note that the thermal conductivity and specific heat capacity of concrete are highly temperature-dependent and moisture-content-dependent parameters. In fire conditions, moisture evaporation from concrete creates additional complexity that may not be fully captured in standard material models.

Integration with Engineering Practice

In practical engineering applications, this research has direct implications for the design of steel tube concrete columns in buildings where fire compartmentation strategies may limit fire exposure to specific faces. For example, in multi-story office buildings with interior columns, a fire in an adjacent room may expose only two faces of a column. The design engineer must account for:

1. The asymmetric temperature distribution that induces secondary bending moments
2. The potential for thermal buckling in the steel tube due to differential expansion
3. The degradation of concrete cover that may expose the steel tube to direct flame contact

From a fabrication quality perspective, columns designed for fire resistance should receive particular attention to weld quality. Any weld defects—porosity, incomplete fusion, undercut—represent potential initiation sites for crack propagation under thermal cycling. Non-destructive testing (NDT) protocols for fire-exposed structural steel should include ultrasonic testing (UT) and phased array ultrasonic testing (PAUT) of all longitudinal and circumferential welds, particularly in columns where fire exposure is anticipated.

The parametric finding that section side length significantly influences temperature distribution has implications for pipe selection. In the context of steel pipe fabrication, this suggests that larger diameter pipes require more careful consideration of wall thickness ratios to ensure adequate fire resistance without excessive material usage.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation:

• How does the presence of a steel inner core (the "steel-reinforced" component) modify the thermal response compared to a conventional steel tube concrete column without inner reinforcement?
• What is the interaction between fire-induced thermal stresses and residual stresses from the welding and forming processes used to fabricate the square steel tube?
• Can fire-resistant coatings or thermal barrier materials be effectively applied to square steel tube sections to mitigate the asymmetric heating effects identified in this study?

The study's focus on the ISO 834 standard fire curve is appropriate for regulatory compliance, but real fires may follow different temperature-time profiles. Flashover events, for instance, can produce much higher temperatures in shorter durations, potentially overwhelming the thermal protection provided by concrete infill.

Study Insights and Implications

This research contributes meaningfully to the understanding of fire behavior in composite steel tube concrete columns under non-uniform fire exposure. The identification of diagonal temperature symmetry as a source of additional eccentricity is a subtle but important insight that can prevent under-design of columns in fire-exposed scenarios. For engineers involved in the fabrication, welding, and quality control of steel tube concrete columns, the key takeaway is that fire design must consider not only absolute temperature levels but also thermal gradients and their structural consequences.

The validated finite element model provides a valuable tool for parametric studies and design optimization. However, I would recommend extending such models to include explicit weld simulation with accurate representation of HAZ properties, as this would provide more realistic predictions of failure initiation locations under fire conditions. The convergence of computational modeling and experimental validation in this study sets a strong precedent for future research in structural fire engineering of composite steel-concrete members.