Residual Load-Bearing Capacity of Steel Tube Concrete Columns After Fire Exposure

Literature Overview

The paper by Han Linhai, Yang Youfu, and Huo Jingsi, published in Engineering Mechanics (2001, Vol. 18, No. 6, pp. 100-109), presents an experimental investigation of the residual load-bearing capacity of steel tube concrete (CFST) columns after exposure to standard fire temperature-time curves. Six circular-section and six square-section CFST columns were subjected to heating in accordance with ISO 834 and GB 9978-88 standard fire curves, followed by post-fire load testing. The study examines the influence of slenderness ratio and eccentricity ratio on the capacity loss rate, providing essential data for the design of fire-resistant CFST structures and the assessment of fire-damaged structures.

Experimental Configuration and Key Findings

The experimental program was designed to cover a representative range of geometric parameters and loading conditions:

The primary finding is that bare CFST columns experience severe capacity loss after fire exposure, while columns with fire-resistant protective coatings exhibit significantly less degradation. This result is consistent with the well-known phenomenon of steel strength degradation at elevated temperatures, where the yield strength of structural steel can drop by 50% or more at temperatures exceeding 600°C. The concrete core, while gaining some strength at moderate temperatures due to the decomposition of free water and the formation of new hydration products, ultimately loses strength at high temperatures due to the decomposition of calcium hydroxide and the spalling of cover concrete.

Fire-Induced Degradation Mechanisms

The degradation of CFST columns under fire exposure involves several interconnected mechanisms:

1. Steel strength degradation: The yield strength and elastic modulus of structural steel decrease significantly at temperatures above 400°C, following well-established temperature-dependent reduction factors (e.g., Eurocode 3 Part 1-2). The steel tube, which provides both axial load capacity and confinement to the concrete core, becomes progressively weaker as the fire temperature rises.
2. Concrete spalling: The rapid heating of the concrete core can cause explosive spalling, particularly in high-strength concrete with high water-cement ratios or insufficient fiber reinforcement. Spalling removes the concrete cover and exposes the steel tube to direct flame impingement, accelerating the degradation process.
3. Steel tube buckling: The reduction in steel tube stiffness at elevated temperatures can trigger local buckling of the tube walls, particularly in thin-walled sections. The loss of confinement from buckled steel walls further compromises the load-bearing capacity of the concrete core.
4. Thermal gradient effects: Differential heating between the outer surface and the core of the column can induce thermal stresses that contribute to cracking and spalling of the concrete and distortion of the steel tube.

Engineering Practice Implications

The findings of this study have direct implications for the design and assessment of CFST structures in fire-prone environments:

• Fire protection design: The study strongly supports the use of fire-resistant protective coatings or encasements for CFST columns. The choice of protective material—intumescent coatings, gypsum boards, fire-resistant sprays, or concrete encasement—should be based on the required fire resistance rating, the ambient conditions, and the economic considerations of the specific project.
• Post-fire structural assessment: For fire-damaged CFST structures, the residual capacity assessment should consider the measured maximum temperature reached, the duration of exposure, and the presence or absence of spalling. Non-destructive testing methods, including ultrasonic testing (UT) for concrete spalling detection, magnetic particle testing (MT) for steel tube cracks, and hardness testing for steel strength estimation, should be employed in conjunction with visual inspection.
• Repair and rehabilitation: The study provides a basis for determining whether a fire-damaged CFST column can be repaired and reused or must be replaced. Repair strategies may include structural steel jacketing, concrete jacketing, or the installation of additional internal supports. The feasibility of each strategy depends on the extent of damage, the accessibility of the member, and the required residual load capacity.
• Design code provisions: The study contributes to the development of more accurate design provisions for the fire resistance of CFST columns. Engineers should refer to relevant standards such as GB 51248, Eurocode 4 (EN 1994-1-2), and GB 50016 for the design of fire-resistant CFST structures, while being aware that the actual performance may vary depending on the specific material properties and construction details.

Study Insights

This study provides valuable experimental data on the post-fire behavior of CFST columns, filling an important gap in the understanding of fire performance of composite structures. The systematic variation of slenderness ratio and eccentricity ratio provides engineers with a clear picture of how these geometric parameters influence the residual capacity. The finding that eccentricity ratio has a minor effect on capacity loss rate is somewhat counterintuitive, as one might expect that the additional bending demand under eccentric loading would amplify the degradation. However, the dominant role of steel strength loss and concrete spalling appears to overshadow the geometric nonlinearity effects associated with eccentricity. From a practical standpoint, this study underscores the importance of fire protection measures in the design of CFST structures, particularly for bare steel tubes which are most vulnerable to thermal degradation. The data presented here can serve as a reference for the development of more refined fire design models and for the training of structural engineers in post-fire structural assessment and rehabilitation.