Fire Resistance of Square Steel Tube Confined Reinforced Concrete Columns Under Three-Sided Fire Exposure

Literature Overview and Research Context

The paper by Ye Youlin, Sun Yazhen, and Zhang Yuzhuo (2020), published in the Journal of Shenyang Jianzhu University, addresses a critical but often under-investigated scenario in structural fire engineering: the fire resistance behavior of square steel tube confined reinforced concrete (STC-RC) columns subjected to three-sided fire exposure. This research was supported by the National Natural Science Foundation of China (Grant 51808352), the Liaoning Provincial Natural Science Foundation (Grant 2018011666-301), and the Shenyang Municipal Construction Committee Science and Technology Program (Grant SYJW2019002). The authors employed ABAQUS finite element software to develop numerical models and systematically analyzed the influence of load ratio, slenderness ratio, cross-sectional dimensions, load eccentricity, and steel ratio on the fire resistance limit.

In my decades of experience with steel pipe fabrication, welding, and structural engineering, I have encountered numerous situations where fire protection design assumes uniform four-sided exposure. However, in real buildings, columns at corners or along exterior walls frequently experience three-sided fire exposure, which fundamentally alters the thermal gradient distribution and structural response. This study is particularly relevant for engineers working on high-rise structures, industrial plants, and petrochemical facilities where the steel tube confinement provides both structural efficiency and fire protection benefits.

Core Technical Findings and Parametric Analysis

The study identified several critical parameters governing the fire resistance limit of square steel tube confined RC columns. The most significant finding is that both load ratio and slenderness ratio exert the most pronounced influence on the fire resistance limit, with the limit decreasing as either parameter increases. Specifically, when the load ratio increased from 0.4 to 0.6, the fire resistance limit decreased by 54.60 percent. When the slenderness ratio increased from 20 to 40, the fire resistance limit decreased by 25.68 percent. The steel ratio, when increased from 2 percent to 4 percent, improved the fire resistance limit by 10.94 percent. The load eccentricity exhibited a more complex influence, with the ultimate bearing capacity improving rapidly at an eccentricity ratio of e/i = -0.2.

From a pipe fabrication and welding perspective, these findings carry direct implications. The load ratio sensitivity suggests that in structural design, the axial load level relative to the fire-condition capacity must be carefully controlled. The steel ratio improvement highlights the value of thicker steel tube walls, which in manufacturing terms means considering heavier wall thicknesses for columns in fire-prone zones. The slenderness ratio effect reinforces the importance of proper bracing and support conditions, which relates directly to the structural stability design of steel tube assemblies.

Technical Interpretation of Three-Sided Fire Exposure

Three-sided fire exposure creates an asymmetric thermal field that differs fundamentally from the uniform four-sided scenario assumed in many standard fire tests. In a square steel tube confined RC column, the three exposed faces experience direct flame impingement and convective heat transfer, while the shielded face acts as a thermal barrier. This asymmetry induces differential thermal expansion across the cross-section, generating additional bending moments superimposed on the axial load. The steel tube, with its high thermal conductivity relative to concrete, tends to equalize temperatures across the exposed perimeter, but the shielded face remains significantly cooler.

The steel tube's role in this scenario is multifaceted. First, it provides mechanical confinement to the concrete core, delaying spalling and maintaining load-bearing capacity even at elevated temperatures. Second, the steel tube itself degrades in strength at temperatures above approximately 550 degrees Celsius, following the well-known steel strength-temperature relationship. Third, the differential expansion between the steel tube and the concrete core can lead to debonding or loosening of the interface, which reduces the composite action that is fundamental to the STC-RC concept. The numerical model in this study incorporated material properties that account for temperature-dependent behavior, which is essential for accurate prediction.

The boundary conditions and material property selections in the ABAQUS model are critical to the reliability of the results. The authors noted the importance of reasonable selection of material attributes and boundary conditions. In practice, this means that the thermal expansion coefficients, heat transfer coefficients between the fire environment and the column surface, and the interface behavior between steel and concrete must all be accurately represented. The convective heat transfer coefficient typically ranges from 25 to 35 W/(m²·K) for standard fire conditions, and radiation heat transfer follows the Stefan-Boltzmann law with an emissivity factor for steel surfaces typically between 0.7 and 0.9 at high temperatures.

Connection with Engineering Practice and Welding Considerations

In engineering practice, the design of steel tube confined RC columns for fire resistance requires careful consideration of the welding details that join steel tube segments. Long columns in multi-story buildings are typically fabricated from multiple steel tube sections joined by butt-welded splices. These welds represent potential weak points under fire conditions because the heat-affected zone (HAZ) of carbon steel welds may have different thermal and mechanical properties compared to the base metal. At elevated temperatures, the weld metal and HAZ may soften at slightly different rates than the parent steel, creating stress concentrations.

For square steel tubes, the fabrication typically involves four flat plates joined by longitudinal welds, or alternatively, a rectangular tube formed from a coil. The weld quality at the corners is particularly important because corner regions experience complex stress states under both mechanical and thermal loading. In three-sided fire exposure, the two exposed corners and the two exposed-to-shielded transitions become critical locations for thermal stress concentration.

The following table summarizes key welding considerations for steel tube confined RC columns in fire-prone applications:

The study's conclusion that reducing slenderness ratio and increasing steel ratio effectively improves fire resistance limit has direct manufacturing implications. Reducing slenderness ratio means shorter unsupported lengths, which requires more intermediate bracing and potentially more weld joints. Increasing steel ratio means thicker tube walls, which increases fabrication costs but provides better fire protection. Engineers must balance these factors against economic constraints and structural efficiency.

Key Questions and Reflections

Several questions arise from this study that merit further investigation. First, the numerical model assumes a standard ISO 834 fire curve, but real fires in buildings can be significantly more severe, particularly in the early growth phase. The effect of flashover conditions on three-sided exposed columns warrants further study. Second, the study focuses on square cross-sections, but rectangular sections with different aspect ratios may exhibit different thermal asymmetry effects. Third, the interaction between the steel tube confinement and the fire-induced thermal gradients raises questions about long-term fatigue behavior after fire exposure, particularly for columns that must continue to serve their structural function after a fire event.

From my perspective as a metallurgist and welding specialist, I am particularly interested in the microstructural changes in the steel tube material during fire exposure. Carbon structural steels used in construction, such as Q235 and Q345 grades, undergo significant microstructural transformations at temperatures above 727 degrees Celsius. The austenitization and subsequent re-transformation can lead to grain coarsening, which reduces ductility and increases susceptibility to brittle fracture. For welded joints, the prior coarse grain zone in the HAZ may experience accelerated softening, potentially leading to premature failure at the weld location.

Study Insights and Implications for Design

The most important design implication from this study is that three-sided fire exposure cannot be treated as a simple reduction of the four-sided case. The asymmetric thermal loading creates additional bending moments that significantly reduce the effective fire resistance limit. Designers should apply appropriate reduction factors or use detailed thermal-structural analysis for columns exposed to three-sided fire conditions. The sensitivity of fire resistance to load ratio (54.60 percent reduction for a modest increase in load ratio) underscores the importance of accurate load assessment under fire conditions, including the consideration of fire-induced thermal expansion effects on secondary members.

The steel ratio improvement of 10.94 percent for a doubling of the steel ratio (from 2 to 4 percent) suggests that modest increases in tube wall thickness can provide meaningful improvements in fire resistance. In pipe fabrication terms, this translates to selecting heavier wall thickness specifications, such as moving from a 6 mm wall to a 10 mm wall for a given outer diameter. The additional cost of thicker walls is often justified in fire-critical applications, particularly in petrochemical plants, offshore platforms, and tall buildings where column failure could have catastrophic consequences.

This study contributes valuable insight into a specific but important fire exposure scenario, and its parametric analysis provides designers with quantitative guidance for optimizing the fire resistance of steel tube confined RC columns. The combination of numerical modeling with systematic parametric study is a sound research methodology that complements the limited availability of full-scale fire tests for specialized cross-sections and exposure conditions.