Fire Resistance Performance of Circular Steel Tube Concrete Columns with Internal Steel Sections

Literature Overview

This paper by Han Yi, Wang Jingxuan, and Wang Wenda from Lanzhou University of Technology, published in the Journal of Natural Disasters in 2015, investigates the fire resistance performance of circular steel tube concrete (CSTC) columns with internal steel sections under standard fire conditions. The study is supported by the National Natural Science Foundation of China and addresses the important question of how internal steel reinforcement affects the fire behavior of composite columns. The authors developed a thermo-mechanical coupled numerical model using ABAQUS and validated it against existing experimental data before conducting parametric studies.

Core Technical Findings

The numerical model successfully reproduces the fire behavior of CSTC columns with internal steel sections, as validated by comparison with experimental data. The fire response of the column can be divided into three stages: an initial expansion stage, an axial compression stage, and a failure stage. The fire resistance limit was determined based on the failure characteristics. Parametric analysis revealed that the contact stress between the steel tube and concrete, and between the concrete and internal steel section, decreases with increasing temperature. The fire resistance limit is significantly influenced by the fire load ratio, slenderness ratio, steel tube steel ratio, and fire protection layer thickness.

Technical Interpretation

The thermo-mechanical coupled analysis captures the complex interaction between thermal expansion, thermal degradation of material properties, and structural response under fire loading. The decrease in contact stress between components with increasing temperature is attributed to the differential thermal expansion rates of steel and concrete, as well as the loss of concrete strength and stiffness at elevated temperatures. The three-stage fire response model provides a useful framework for understanding the progressive degradation of structural capacity during a fire event. The identification of key parameters influencing fire resistance limit enables engineers to prioritize design modifications for improved fire performance.

Engineering Practice Implications

For engineers designing fire-resistant composite columns, this study highlights the importance of considering the interaction between all structural components under fire conditions. The internal steel section provides additional load-carrying capacity and can significantly improve fire resistance, but the effectiveness depends on the temperature-dependent behavior of the concrete matrix and the steel-concrete interface. Fire protection layers, such as intumescent coatings or concrete encasement, are critical for maintaining structural integrity during a fire event. Engineers should carefully consider the slenderness ratio and fire load ratio in their designs, as these parameters have a pronounced effect on the fire resistance limit.

Key Questions and Reflections

One important consideration is the long-term durability of the internal steel section under repeated fire exposure or in corrosive environments. The study focuses on standard fire conditions, but real fires can involve more complex thermal profiles with varying temperatures and durations. Additionally, the interaction between the internal steel section and the concrete under fire conditions may involve spalling of the concrete cover, which could expose the steel section to direct flame impingement. The study does not address the effect of fire-induced spalling on the overall structural performance, which is a significant practical concern.

Summary

This study provides a comprehensive numerical investigation of the fire resistance performance of circular steel tube concrete columns with internal steel sections. The validated thermo-mechanical coupled model and the identification of key design parameters offer valuable guidance for engineers seeking to improve the fire performance of composite columns. The findings emphasize the importance of component interaction, fire protection layer design, and geometric parameters in achieving adequate fire resistance limits for steel tube concrete structures.