Fire Resistance Limit of Rectangular Steel Tube Concrete Columns Under Adjacent Two-Face Fire

Literature Overview

This paper, published by Lv Xuetao, Zhang Tong, Wang Weiwei, and Zhang Yuzhuo from the School of Architectural Engineering at Liaoning Technical University in 2014, investigates the fire resistance limit of rectangular steel tube concrete (STC) columns subjected to fire on two adjacent faces. The study employs ABAQUS finite element software to construct a thermomechanical analysis model, validates the theoretical results against existing domestic and international experimental data, and conducts parametric analysis on multiple geometric and material factors. The research is supported by the National Youth Natural Science Foundation of China (Grant No. 51208246).

The significance of this work lies in the fact that conventional fire resistance studies predominantly consider uniform four-face fire exposure, whereas in real building configurations, columns located near exterior walls or at structural junctions frequently experience fire on only two adjacent faces. This asymmetric thermal loading scenario creates a more complex temperature gradient distribution and potentially more severe local buckling behavior in the steel tube section.

Core Technical Content and Parametric Analysis

The authors systematically examined the following parameters influencing fire resistance limit:

The primary finding is that within the studied parameter ranges, load ratio, section side length, and slenderness ratio are the dominant factors governing fire resistance performance. The authors derived a theoretical formula for the fire resistance limit, which demonstrated good agreement with experimental data from both Chinese and international sources.

Technical Interpretation of Two-Adjacent-Face Fire Scenario

The two-adjacent-face fire condition produces an asymmetric thermal field within the rectangular steel tube section. The two heated faces experience rapid temperature elevation, while the two unheated faces remain relatively cooler. This creates:

1. Thermal bending effect — The differential thermal expansion between heated and unheated faces induces additional bending moments superimposed on the axial load, accelerating structural degradation.
2. Local buckling vulnerability — The heated faces of the steel tube experience earlier strength degradation due to the rapid decline of steel yield strength at elevated temperatures (approximately 50% reduction at 600°C according to Eurocode 3 Part 1-2).
3. Concrete cover spalling asymmetry — The fire-exposed faces are more susceptible to concrete cover spalling, which further exposes the steel reinforcement and accelerates heat penetration to the steel tube.

The parametric study reveals that increasing the section side length enhances fire resistance because a larger cross-section provides greater thermal mass and delays the onset of critical temperature levels in the steel tube. The load ratio effect is intuitive: lower axial loads relative to the ambient-temperature ultimate capacity allow the column to sustain more thermal degradation before reaching failure. The slenderness ratio effect is particularly important because fire-induced thermal expansion in a restrained column generates additional compressive forces, and slender columns are more susceptible to fire-induced buckling.

Engineering Practice Integration

From a practical standpoint, this research has several implications for structural fire engineering:

• Fire protection design optimization — For columns located at building corners or near exterior walls, where two-adjacent-face fire is the realistic scenario, the required fire protection thickness may differ from that calculated for uniform four-face fire exposure. Engineers should avoid over-designing fire protection in such locations while ensuring adequate coverage on the exposed faces.
• Section geometry selection — The finding that larger section dimensions improve fire resistance suggests that, when fire performance is a design driver, selecting slightly larger rectangular tube sections may be more effective than increasing fire protection thickness alone.
• Load management — The strong influence of load ratio underscores the importance of accurate load assessment in fire design. Over-conservative load assumptions may unnecessarily increase fire protection requirements.
• Slenderness control — Limiting slenderness ratios in fire-critical members through intermediate bracing or section enlargement is an effective strategy, consistent with provisions in GB 50017 and Eurocode 3.

Key Questions and Reflections

Several questions arise from this study that warrant further investigation. First, the study focuses on rectangular sections, but circular steel tube concrete columns are also widely used in practice, and the asymmetric fire scenario for circular sections would produce different stress distributions. Second, the long-term degradation of fire resistance due to repeated thermal cycling (relevant for industrial facilities) is not addressed. Third, the interaction between fire resistance and seismic performance of STC columns under combined loading remains an open challenge, particularly relevant in regions with both seismic and fire hazards.

The theoretical formula derived in this paper provides a useful tool for preliminary design, but engineers should note that its validity is limited to the parameter ranges studied. Extrapolation beyond these ranges requires additional validation through experiments or refined numerical simulations.

Study Insights and Implications

This research contributes valuable quantitative data to the understanding of STC column fire behavior under realistic fire exposure scenarios. The parametric approach and the derived theoretical formula offer practical guidance for engineers designing fire-resistant steel tube concrete structures. The emphasis on load ratio, section geometry, and slenderness ratio as primary design parameters aligns with fundamental structural engineering principles and should be readily incorporated into design practice. Future work should extend these findings to multi-axial loading conditions, consider the effect of fire protection materials on thermal field distribution, and develop simplified design procedures that can be directly applied in engineering practice without recourse to full finite element analysis.