Fire Resistance Performance of Square Steel Tube Concrete Columns with Protective Layers

Literature Overview

The study by Han Linhai and Xu Lei from Fuzhou University and Harbin Institute of Technology, published in China Civil Engineering Journal (2000, Vol. 33, No. 6), presents experimental investigation of the fire resistance limit of square steel tube concrete (SSTC) columns with fire protective coatings. Three full-scale specimens were tested under standard fire temperature curves per ISO 834 and GB 9978-88, evaluating fire performance, fire resistance limits, and the adequacy of existing fire protection design methods for steel structures.

Core Technical Analysis

The fire resistance behavior of SSTC columns differs fundamentally from conventional steel columns due to the composite action between the steel tube and concrete core. The key findings include:

The study demonstrates that the concrete core acts as a thermal mass that absorbs heat and protects the steel tube from reaching critical temperatures. The steel tube, in turn, confines the concrete and prevents spalling, creating a synergistic fire resistance mechanism.

Welding Quality and Fire Performance

The welding quality of the steel tube directly affects fire resistance performance through several mechanisms:

1. Weld residual stress: High tensile residual stresses in the weld HAZ can accelerate creep deformation at elevated temperatures. Post-weld stress relief (mechanical or thermal) is recommended for fire-critical applications.
2. Weld integrity under thermal cycling: The differential thermal expansion between weld metal, HAZ, and base metal creates additional thermal stresses during fire exposure. Poorly executed welds with lack of fusion or porosity become preferential failure locations at elevated temperatures.
3. Weld geometry effects: Weld toes and undercut create stress concentrations that become more critical at elevated temperatures when material yield strength decreases significantly (steel yield strength drops to approximately 50% at 600°C and 20% at 800°C).
4. Thermal bridging: Weld seams, particularly longitudinal welds in welded tubes, can create preferential heat transfer paths that affect the temperature distribution within the section.

Fire Protective Coating Design

The study demonstrates that the conventional fire protection coating thickness determination method for steel structures is not suitable for SSTC columns. Key findings for coating design:

The recommended approach is to apply a relatively thin fire protective coating (5-10 mm intumescent coating) to the exterior of the SSTC column, which, combined with the inherent fire resistance of the composite section, achieves the 90-120 minute fire resistance limits required by GB 50045-95 for column structural elements.

Standard Fire Test Methodology

The testing followed standard fire curves that define the temperature-time relationship:

• ISO 834 / GB 9978-88: Standard fire curve reaching approximately 1085°C at 60 minutes, used for structural fire resistance testing
• Temperature measurement: Thermocouples placed at steel tube inner surface, outer surface, and concrete core center to capture temperature gradients
• Load application: Columns tested under axial compression at various load levels to determine the fire resistance limit (time to failure or maximum deflection criterion)

The temperature gradient through the section is critical: the outer steel tube surface reaches higher temperatures than the inner surface due to the thermal mass of the concrete core. This gradient creates thermal stresses that interact with mechanical loading to determine ultimate fire resistance.

Engineering Practice Recommendations

For fire design of SSTC columns in practice:

1. Coating specification: Specify intumescent coatings of 5-10 mm thickness for 90-120 minute fire resistance, recognizing that the concrete core provides significant inherent fire protection.
2. Weld quality: Ensure all welds meet high acceptance criteria (Level A per GB/T 11345) as weld defects become more critical at elevated temperatures when material strength is reduced.
3. Tube material selection: Use fire-resistant steel grades with retained strength at elevated temperatures (such as S355J2 or equivalent) for the steel tube to maximize fire performance.
4. Concrete quality: Use high-quality concrete with adequate durability to prevent spalling under thermal shock. Concrete strength of 30-50 MPa provides optimal fire performance.
5. Design methodology: Do not apply conventional steel structure fire protection design methods directly to SSTC columns; use the composite fire design approach that accounts for the thermal mass and confinement effects of the concrete core.

Summary

This pioneering research establishes that square steel tube concrete columns possess inherently superior fire resistance compared to conventional steel columns, requiring significantly thinner fire protective coatings to achieve code-mandated fire resistance limits. The composite action between the steel tube and concrete core creates a synergistic fire protection mechanism that reduces material costs and construction complexity. Welding engineers should recognize that weld quality becomes more critical in fire-exposed conditions, and all welds in fire-critical SSTC columns should meet the highest acceptance criteria to ensure reliable structural performance under thermal and mechanical loading.