Fire Resistance Performance of Hollow Circular Steel Tube Concrete Columns

Literature Overview

The study by Zha Xiaoxiong, Yu Min, and Liu Yixiang from the Shenzhen Graduate School of Harbin Institute of Technology addresses a critical gap in structural fire engineering: the fire resistance behavior of hollow circular steel tube concrete (HSTC) columns. Published in the Journal of Disaster Prevention and Mitigation Engineering in 2012, this work is significant because while solid steel tube concrete (STC) columns have been extensively investigated for fire resistance, hollow configurations remain under-studied despite their increasing use in practical engineering for weight reduction, material economy, and improved seismic ductility. The research was supported by the 2010 Second Batch Engineering Construction Association Standard Formulation and Revision Plan, indicating its relevance to standard development in China.

Core Technical Findings

The experimental program examined the influence of two key variables on fire resistance duration: the hollow ratio (the ratio of the hollow core diameter to the outer steel tube diameter) and the presence of water filling within the hollow cavity. The principal findings can be summarized as follows:

• Under identical load ratios, the fire resistance duration of HSTC columns first increases and then decreases as the hollow ratio increases, exhibiting a non-monotonic relationship.
• When the hollow ratio is relatively small, water filling in the hollow cavity does not produce a significant improvement in fire resistance duration.
• When the hollow ratio is relatively large, water filling becomes effective in enhancing the fire resistance duration of the member.

This non-monotonic behavior suggests the existence of an optimal hollow ratio that maximizes fire resistance. The mechanism behind this phenomenon relates to the competing effects of thermal insulation provided by the hollow cavity versus the loss of structural confinement and load-bearing cross-section.

Technical Analysis of Fire Resistance Mechanisms

From a materials science and welding engineering perspective, several mechanisms govern the fire resistance of HSTC columns:

1. Thermal insulation effect of the hollow cavity: The air layer within the hollow core acts as a thermal insulator, reducing heat transfer to the inner steel tube surface and the confined concrete. However, this benefit diminishes when the hollow ratio becomes too large because the remaining concrete cross-section becomes insufficient to maintain structural integrity at elevated temperatures.
2. Water vaporization effect: When water is injected into the hollow cavity, the phase change from liquid to vapor absorbs significant latent heat (approximately 2260 kJ/kg at 100°C), creating an endothermic cooling effect that delays the temperature rise of the steel tube and confined concrete. This effect becomes more pronounced at larger hollow ratios where the water volume relative to the concrete volume is greater.
3. Steel tube behavior at elevated temperatures: Carbon structural steel (typically Q235 or Q345) loses strength progressively with temperature. At 550°C, the yield strength drops to approximately 60% of the ambient temperature value. The hollow configuration reduces the steel tube's contribution to load-bearing capacity, making the column more dependent on the concrete core, which itself degrades at temperatures above 600°C.
4. Spalling of confined concrete: The hollow cavity may reduce the lateral confinement pressure on the concrete, potentially increasing the risk of concrete spalling at high temperatures. The presence of water in the cavity can exacerbate this risk through rapid steam generation if the cavity is sealed, creating internal pressures that may cause explosive spalling.

Standards and Design Considerations

The relevant fire design standards include GB 50016 (Code for Fire Protection Design of Buildings), GB 51249 (Technical Standard for Fire Safety of Steel Structures), and EN 1993-1-2 (Design of steel structures - Fire design). For the steel tube material, GB/T 1591 (High-strength low-alloy structural steels) and ASTM A500/A53 are commonly referenced.

Engineering Practice Implications

From a practical engineering standpoint, this research has several important implications:

• Design optimization: Engineers designing HSTC columns for fire resistance should identify the optimal hollow ratio that balances structural efficiency with fire performance. A hollow ratio in the range of 30% to 50% may represent a reasonable design target, though this should be validated through specific fire engineering analysis for each application.
• Water-filled cavity strategy: The practice of filling the hollow cavity with water before a fire event can be an effective passive fire protection measure, particularly for columns with larger hollow ratios. However, the long-term reliability of water retention, the risk of water freezing in cold environments, and the potential for corrosion of the inner steel surface due to prolonged water contact must be carefully evaluated.
• Corrosion concerns: The presence of water inside the hollow cavity creates a closed or semi-closed environment that can accelerate internal corrosion of the steel tube. This is a critical concern from a pipe manufacturing and quality control perspective, as internal corrosion reduces the effective wall thickness and compromises both structural and fire resistance performance.
• Welding and fabrication: The hollow HSTC column typically requires internal welds or sealing arrangements to contain the water. The quality of these welds is paramount, as any leak would defeat the water-filling strategy. Welding procedures should comply with GB/T 985 (Welding procedures for carbon steel and low-alloy steel) and undergo rigorous non-destructive testing (NDT) including ultrasonic testing (UT) and dye penetrant testing (PT).

Key Questions and Reflections

Several questions remain open for further investigation:

• What is the precise mechanism behind the non-monotonic relationship between hollow ratio and fire resistance duration? Is there a transition point where the thermal insulation benefit is overtaken by the structural capacity loss?
• How does the fire resistance performance vary with the type of concrete used (normal strength concrete vs. high-strength concrete vs. fiber-reinforced concrete)?
• What are the long-term effects of water-filled cavities on the structural integrity of HSTC columns under normal service conditions, particularly regarding internal corrosion and moisture-induced degradation?
• Can alternative cavity fillings (such as aerogel, foam, or phase-change materials) provide superior fire protection compared to water?

Study Insights and Implications

This research contributes meaningfully to the understanding of fire behavior in hollow steel tube concrete members. The finding that water filling is only effective at larger hollow ratios is particularly practical: it suggests that the water-filling strategy should be selectively applied based on the specific hollow ratio of the column design. For engineers involved in steel pipe manufacturing, the key takeaway is that the internal surface quality and dimensional accuracy of the steel tube directly influence the effectiveness of cavity-based fire protection strategies. Any internal welds, seams, or surface irregularities can create thermal bridges or stress concentrations that compromise fire resistance. The research also underscores the importance of considering fire engineering requirements during the early stages of structural design, rather than treating fire protection as a retrofit afterthought.