Fire Resistance Performance of Hollow Steel Pipe Concrete Columns Experimental Study

Literature Overview

This paper by Zha Xiaoxiong, Yu Min, and Liu Yixiang from Harbin Institute of Technology, Shenzhen Graduate School, published in the Journal of Disaster Prevention and Mitigation Engineering in 2012 (Vol. 32, No. 1, pp. 66-70), addresses a significant gap in the fire engineering literature. While extensive experimental research exists on solid steel pipe concrete (SRC) columns under fire conditions, hollow steel pipe concrete (HSC) columns remain largely unexplored. The authors conducted fire resistance tests on circular hollow steel pipe concrete columns, considering different hollow ratios and investigating the effect of water injection into the hollow cavity on fire resistance performance.

Core Technical Findings

The key experimental results reveal a non-monotonic relationship between hollow ratio and fire resistance duration under identical load ratios. The fire resistance time first increases and then decreases as the hollow ratio increases. This behavior can be attributed to competing thermal mechanisms: at moderate hollow ratios, the air layer inside the hollow cavity acts as a thermal insulator, slowing heat transfer to the concrete core and delaying the degradation of concrete strength. However, when the hollow ratio becomes too large, the reduced concrete cross-sectional area and increased steel-to-concrete surface ratio accelerate structural failure.

A particularly noteworthy finding concerns the water injection strategy. When the hollow ratio is small, injecting water into the hollow cavity provides only marginal improvement in fire resistance time. However, when the hollow ratio is large, water injection significantly enhances fire performance. This is because the latent heat of vaporization of water absorbs substantial thermal energy, creating an effective cooling mechanism that compensates for the reduced structural mass. The phase change from liquid to vapor acts as a thermal sink, delaying the temperature rise in the critical load-bearing sections.

Engineering Practice Implications

From a steel pipe manufacturing and welding quality perspective, the fire resistance behavior of these columns is intimately connected to the quality of the longitudinal welds in the steel pipes. The heat-affected zone (HAZ) of the pipe weld, particularly in ERW or HFW welded pipes, may exhibit different thermal degradation characteristics compared to the base metal during fire exposure. Engineers must ensure that the welding process parameters, including welding current, voltage, and travel speed, produce a HAZ with minimal microstructural coarsening, as this directly affects the residual strength at elevated temperatures.

Furthermore, the choice of steel grade for the hollow steel pipe is critical. Carbon structural steel grades such as Q235 and Q345 exhibit different strength retention curves at elevated temperatures. According to EN 1993-1-2 and GB/T 19813, the reduction factor for yield strength of structural steel begins to decrease significantly above 400°C and drops to approximately 50% at 600°C. The steel pipe's contribution to the composite column's load-bearing capacity diminishes rapidly in these temperature ranges, making the concrete core's thermal protection increasingly important.

Key Questions and Reflections

The experimental scope, while valuable, remains limited in several respects. The study does not address the long-term durability of water injection systems or the potential for water leakage under thermal cycling. In practical applications, the integrity of the water injection seal must be guaranteed throughout the column's service life, which introduces additional quality control requirements for the steel pipe fabrication and welding joints. Any porosity or lack of fusion defects in the longitudinal weld could compromise the seal under thermal stress.

Additionally, the interaction between the steel pipe and concrete during fire exposure involves complex bond-slip behavior. The thermal expansion mismatch between steel and concrete, exacerbated by the hollow geometry, may lead to localized debonding at critical elevations. This phenomenon is not adequately captured in the current experimental framework but deserves attention in future research.

Study Insights and Reference Value

This study provides essential baseline data for the fire design of hollow steel pipe concrete columns, a structural form increasingly adopted in modern construction for its weight efficiency and architectural flexibility. The findings directly inform the selection of steel pipe specifications, wall thickness, and hollow ratio for fire-critical applications. For pipe manufacturing engineers, the study underscores the importance of weld quality and steel grade selection as these factors directly influence the fire performance of the composite structure. The water injection strategy offers a practical engineering solution for enhancing fire resistance in large hollow ratio columns, though its implementation requires careful attention to sealing integrity and maintenance protocols.