Axial Compression Mechanical Properties of Composite Steel-Concrete Short Columns After Fire Exposure and Cooling

Literature Overview and Research Significance

The paper by Xiang Kai, Pan Yanchong, Zhao Bi, and Wang Guohui from the Tianjin Fire Research Institute of the Ministry of Public Security, published in the Journal of Southwest Jiaotong University in 2017 (Vol. 52, No. 6, pp. 1173-1181), investigates the axial compression mechanical properties of composite steel-concrete short columns after fire exposure and subsequent cooling. Funded by the National Natural Science Foundation of China Youth Science Fund (Grant 51408134), this research addresses a critical safety concern in structural engineering: the residual strength and deformation characteristics of steel-concrete composite columns following a fire event. The study combines experimental testing with finite element simulation to provide comprehensive insights into the post-fire behavior of these important structural elements.

From a steel pipe engineering perspective, this research is particularly relevant because steel tubes in composite columns are subjected to severe thermal gradients during a fire, leading to differential thermal expansion between the steel tube and the concrete core. The resulting thermal stresses can cause cracking in the concrete, buckling of the steel tube, and degradation of the bond interface—all of which significantly affect the post-fire load-bearing capacity. Understanding these mechanisms is essential for the design of fire-resistant steel pipe composite structures and for the assessment of damage after a fire event.

Experimental Program and Test Parameters

The authors conducted fire exposure tests on eight composite short columns under the ISO-834 standard fire curve, followed by axial compression tests after cooling. The test matrix considered three key parameters: section shape, steel tube dimensions, and slenderness ratio. This systematic approach allows for the isolation of individual parameter effects on the post-fire performance, which is essential for developing reliable design guidelines.

The fire exposure duration of 180 minutes corresponds to a severe fire scenario that would be encountered in high-rise buildings, industrial facilities, and transportation infrastructure. The ISO-834 curve represents a standard fire temperature-time relationship that reaches approximately 1000°C at 180 minutes, which is sufficient to cause significant degradation of both steel and concrete materials.

Core Findings on Post-Fire Mechanical Properties

The research revealed several important findings regarding the post-fire behavior of composite steel-concrete short columns. The failure mode after fire exposure and cooling was compressive failure, similar to that observed in reinforced concrete short columns at room temperature. This indicates that the composite action between the steel tube and concrete core is preserved to some extent even after severe thermal exposure, although the load-bearing capacity is significantly reduced.

The most striking finding is the magnitude of strength and stiffness degradation. After 180 minutes of fire exposure and subsequent cooling, the ultimate axial compression load-bearing capacity decreased by 27.4% to 38.2%, while the axial stiffness decreased by 61.4% to 69.0%. These degradation rates are substantial and have important implications for the structural safety assessment after a fire event. The stiffness degradation is particularly severe because it reflects the loss of composite action and the development of cracks in the concrete core and at the steel-concrete interface.

The study also examined the effect of the external reinforced concrete layer thickness on the post-fire performance. When the external RC layer thickness was increased by 1.53 times, the reduction in axial load-bearing capacity for circular composite columns decreased by 8.4%, and the reduction in axial stiffness decreased by 1.3%. For square composite columns, the corresponding reductions were 5.8% and 4.1%, respectively. This finding suggests that increasing the protective RC layer can improve the fire resistance of composite columns, but the effect is more pronounced on load-bearing capacity than on stiffness.

Steel Pipe Material Degradation Under Fire Conditions

From a steel pipe metallurgy perspective, the fire exposure causes significant changes in the microstructure and mechanical properties of the steel tube. At temperatures exceeding 400°C, the yield strength of structural steel begins to decrease noticeably, and at temperatures above 600°C, the strength reduction becomes severe. During a 180-minute ISO-834 fire, the steel tube surface can reach temperatures of 800-1000°C, while the inner surface in contact with the concrete core may reach lower temperatures due to the thermal insulation effect of the concrete.

The thermal gradient through the steel tube wall creates differential thermal expansion, which generates hoop stresses and axial stresses in the tube. These thermal stresses can cause local buckling of the steel tube, particularly in regions where the concrete core has cracked and lost its lateral support. The buckling pattern depends on the wall thickness, diameter, and the extent of concrete cracking. In severe cases, the steel tube can experience significant outward bulging, which further reduces the effective confinement of the concrete core.

The welding joints in the steel tube are particularly vulnerable to fire exposure. The heat-affected zone (HAZ) of the weld already has a different microstructure and mechanical properties compared to the base metal, and the additional thermal cycling during a fire can exacerbate this difference. Welds in the steel tube may experience increased residual stresses, microcracking, and reduced toughness after fire exposure. These effects should be considered in the post-fire structural assessment.

Finite Element Simulation and Simplified Calculation Method

The authors employed ABAQUS for nonlinear finite element simulation of the composite short columns under fire exposure and subsequent axial compression. The simulation captured the complex thermal-mechanical coupling effects, including the thermal expansion of the steel tube, the cracking of the concrete core, and the degradation of the bond interface. The comparison between simulation results and experimental data showed good agreement, validating the numerical model.

A simplified calculation method was also proposed for the post-fire axial compression load-bearing capacity and axial stiffness. The average deviation between the simplified calculation results and the simulation results was 4.9% with a standard deviation of 5.3% for the ultimate load-bearing capacity, and 13.8% with a standard deviation of 9.8 for the axial stiffness. These deviations are acceptable for practical engineering design purposes, and the simplified method provides a convenient tool for preliminary design and post-fire structural assessment.

Engineering Practice Implications and Quality Control

For steel pipe fabrication and welding engineers, this research has several important implications. First, the welding quality of the steel tube is critical for the fire resistance of the composite column. Poor weld quality can lead to premature failure at the weld location during a fire, which can compromise the entire composite action. Second, the wall thickness of the steel tube should be selected not only based on the structural load requirements but also considering the fire resistance requirements. Thicker walls provide better thermal mass and can delay the temperature rise at the inner surface, preserving the concrete core for a longer duration.

The quality control procedures for steel tubes used in fire-exposed composite columns should include rigorous inspection of welds, particularly at locations where the steel tube is connected to other structural elements. Non-destructive testing methods such as ultrasonic testing (UT) and radiographic testing (RT) should be employed to detect weld defects that could compromise the fire resistance. The residual stress distribution from welding should be assessed, and post-weld heat treatment may be necessary for critical applications.

Key Questions and Reflections

Several questions remain open from this research. First, the study focuses on the post-fire axial compression behavior, but in practice, composite columns may be subjected to combined loading (axial compression and bending) after a fire event. The interaction between these loading components and the post-fire degradation mechanisms is more complex and requires further investigation. Second, the long-term durability of composite columns after fire exposure and repair is not addressed. In many cases, fire-damaged structures are repaired rather than replaced, and the long-term performance of repaired composite columns under cyclic loading is an important practical concern.

From a standards perspective, the current fire resistance design standards for steel-concrete composite columns often rely on simplified assumptions that may not fully capture the complex thermal-mechanical behavior revealed in this study. The significant stiffness degradation observed (61.4%-69.0%) suggests that the serviceability limit state may be the governing criterion for post-fire structural assessment, rather than the ultimate limit state. This has implications for the design of fire-resistant composite columns and for the development of post-fire repair guidelines.

Study Insights and Implications

The research by Xiang et al. provides valuable experimental and numerical insights into the post-fire behavior of composite steel-concrete short columns. The systematic test matrix, combined with finite element simulation and a simplified calculation method, offers a comprehensive framework for understanding and predicting the post-fire performance of these important structural elements. The significant degradation in both load-bearing capacity and axial stiffness underscores the importance of fire protection measures for steel-concrete composite structures.

For steel pipe engineers, the research highlights the critical role of steel tube wall thickness, weld quality, and the protective RC layer in determining the fire resistance of composite columns. The fabrication and welding of steel tubes for fire-exposed applications require strict quality control and adherence to relevant standards. The simplified calculation method provides a practical tool for preliminary design and post-fire assessment, but engineers should be aware of its limitations and the need for detailed finite element analysis in critical applications.

In summary, this research significantly advances the understanding of post-fire mechanical behavior of composite steel-concrete short columns and provides practical tools for engineers to assess and design fire-resistant composite structures with steel pipe components.