Fire Resistance of Square Steel Tube Confined Steel-Reinforced Concrete Columns Under Two-Sided Fire Exposure

Literature Overview

The research by Zhang Yuzhuo, Liu Yujie, and Lv Xuetao, published in the Journal of Architecture and Civil Engineering in 2020, investigates the fire resistance performance of square steel tube confined steel-reinforced concrete (SRC) columns subjected to two opposite faces exposed to fire. Using ABAQUS finite element analysis calibrated against ISO 834 standard fire curves and validated with existing experimental data, the authors develop simplified calculation formulas for fire resistance limit determination. The study systematically examines the influence of load ratio, effective length, steel reinforcement ratio, cross-sectional dimensions, and concrete strength on fire performance, providing a comprehensive design reference for fire protection engineering of composite columns.

Fire Exposure Conditions and Thermal Analysis

The two-sided fire exposure scenario represents a realistic fire condition for columns located along building edges or in exposed structural positions. Unlike the standard ISO 834 uniform fire exposure assumed in many design codes, two-sided exposure creates asymmetric temperature gradients across the column cross-section, inducing additional thermal bending moments that reduce fire resistance. The finite element model incorporates coupled thermal-mechanical analysis, with the temperature field model providing boundary conditions for the structural analysis model.

Temperature Distribution Characteristics

The temperature field analysis reveals significant thermal gradients across the column section under two-sided fire exposure. The fire-exposed faces experience rapid temperature rise following the ISO 834 curve, while the unexposed faces remain relatively cooler. The steel tube on the fire-exposed side reaches critical temperatures earlier, leading to progressive strength degradation. The concrete core experiences non-uniform temperature distribution, with the fire-exposed region reaching higher temperatures than the opposite side.

Structural Behavior Under Fire Loading

The finite element analysis captures the progressive structural degradation under fire conditions. As temperatures rise, the steel tube and reinforcing steel experience strength and stiffness reduction, while the concrete core undergoes thermal cracking, spalling, and eventual disintegration. The two-sided exposure creates thermal bending that superimposes on the mechanical load effects, creating a combined loading state that accelerates failure.

The load ratio—the ratio of applied axial load to the room-temperature load-bearing capacity—emerges as the most influential parameter. Higher load ratios mean that the column operates closer to its capacity at room temperature, leaving less reserve capacity to accommodate the strength degradation caused by heating. This finding has direct implications for fire protection design, suggesting that columns carrying high service loads require additional fire protection measures regardless of their structural section size.

Steel Tube Manufacturing and Fire Protection Considerations

The square steel tube serves a dual structural role in these composite columns: it provides lateral confinement to the concrete core under normal service conditions, and it acts as a structural element that must maintain integrity under fire exposure. From a manufacturing perspective:

• Wall thickness uniformity: The steel tube wall thickness must be consistent across all four faces to ensure uniform thermal response and structural performance. Variations in wall thickness create localized weak points that can initiate premature failure under fire loading.
• Material grade: The steel grade should provide adequate high-temperature strength retention. Steels with lower carbon content and appropriate alloying elements maintain higher residual strength at elevated temperatures.
• Surface preparation: The exterior surface of the steel tube may receive fire-resistant coatings or be encased in protective layers. The quality of surface preparation directly affects the adhesion and long-term durability of fire protection systems.
• Dimensional accuracy: Square tubes must maintain tight dimensional tolerances to ensure proper fit with connection details and fire protection systems. Deviations in squareness or straightness can compromise both structural and fire protection performance.

Simplified Fire Resistance Calculation Formula

The authors develop a simplified calculation formula for the fire resistance limit of two-sided fire exposed square steel tube confined SRC columns. The formula incorporates the key parameters identified in the parametric study—load ratio, effective length, cross-sectional dimensions, steel reinforcement ratio, and concrete strength—and provides engineering-acceptable accuracy for design purposes. The formula is validated against the finite element results, demonstrating good correlation between calculated and analytical fire resistance values.

The simplified formula enables rapid preliminary assessment of fire resistance during the early stages of structural design, before detailed finite element analysis is undertaken. This is particularly valuable for design engineers who must evaluate multiple design alternatives and optimize structural sections for both serviceability and fire resistance requirements.

Engineering Design Implications

The research findings translate into several practical design recommendations for fire-exposed composite columns:

1. Load ratio management: Design columns with load ratios not exceeding 0.5 to 0.6 to ensure adequate fire resistance reserve capacity under two-sided exposure conditions.
2. Section optimization: Increase cross-sectional dimensions to improve fire resistance, as this provides both additional thermal mass and greater structural reserve capacity.
3. Effective length control: Minimize the effective length of columns through proper lateral restraint at intermediate floors to reduce thermal bending effects.
4. Steel reinforcement optimization: While increasing the steel reinforcement ratio provides limited improvement in fire resistance, adequate reinforcement should be maintained for structural reasons and to prevent premature concrete spalling.
5. Concrete quality: Higher concrete strength provides marginal fire resistance improvement but is primarily justified by structural capacity requirements.

Key Reflections

The two-sided fire exposure scenario is underrepresented in current design codes, which predominantly assume uniform fire exposure conditions. This research fills an important gap by providing design tools specifically for asymmetric fire exposure. The finding that steel reinforcement ratio has limited influence on fire resistance is somewhat counterintuitive and suggests that the fire performance of these composite columns is governed primarily by the thermal behavior of the steel tube and concrete core rather than by the reinforcing steel alone. This insight should inform future code development and design practice for composite columns in fire-exposed structural applications.

Summary and Conclusions

This study provides a comprehensive analytical and computational framework for evaluating the fire resistance of square steel tube confined SRC columns under two-sided fire exposure conditions. The simplified calculation formula offers a practical design tool that captures the essential physics of fire-induced structural degradation with engineering-acceptable accuracy. Steel pipe manufacturers should recognize that their products serve critical roles in fire-resistant composite structures, and that wall thickness uniformity, material high-temperature performance, and dimensional accuracy are essential quality characteristics for this application. The engineering community benefits from having validated analytical tools that enable efficient design optimization for both structural capacity and fire resistance in composite column systems subjected to realistic fire exposure scenarios.