Zhuojin Pipe Fitting Co., LtdFire Resistance of Square Steel Tube Confined Reinforced Concrete Columns Under Two-Adjacent-Face Fire
Literature Overview
This study by Zhang, Xu, and Lv (Industrial Construction, 2019, Vol. 49, No. 2, pp. 148-154), supported by the National Natural Science Foundation of China (Grants 51208246, 51808352), investigates the fire resistance performance of square steel tube confined reinforced concrete (RC) columns subjected to two-adjacent-face fire exposure. The research employs ABAQUS finite element software to model the thermal and mechanical fields, validates the numerical model against experimental data, and derives simplified calculation formulas for fire resistance limits under varying parametric conditions.
Core Technical Findings
The study establishes that the fire resistance limit of square steel tube confined RC columns under two-adjacent-face fire is governed primarily by three parameters: slenderness ratio, load ratio, and load eccentricity. The steel ratio and reinforcement ratio, while important for structural design, exert comparatively minor influence on fire resistance performance. This finding is significant for steel pipe selection in fire-exposed structural applications, as it implies that the geometric proportions and loading conditions dominate over material volumetric ratios in determining fire survival time.
Parametric Influence Analysis
| Parameter | Influence Level | Engineering Implication |
|---|---|---|
| Slenderness ratio | Significant | Higher slenderness reduces fire resistance due to increased thermal bowing and reduced stability margin |
| Load ratio | Significant | Higher axial load accelerates core concrete spalling and steel tube yield under thermal gradients |
| Load eccentricity | Significant | Eccentric loading creates asymmetric thermal-mechanical stress states, reducing fire resistance capacity |
| Steel ratio | Minor | Within typical ranges, increasing steel ratio does not proportionally improve fire resistance |
| Reinforcement ratio | Minor | Internal reinforcement provides limited contribution to overall fire resistance limit |
Steel Pipe Engineering Perspective
From a steel pipe manufacturing and selection standpoint, this research carries several important implications. The use of square hollow sections (SHS) as confining elements in RC columns introduces specific thermal challenges during fire exposure. Two-adjacent-face fire creates an asymmetric temperature gradient across the column cross-section, which generates differential thermal expansion between the heated and unheated faces. This differential expansion induces additional bending stresses in the steel tube, superimposed on the thermal bowing effect caused by the temperature gradient through the wall thickness.
The steel tube material grade and wall thickness become critical design parameters under fire conditions. Carbon structural steel grades such as Q235 and Q355, commonly specified in Chinese standards (GB/T 1591, GB/T 6728), exhibit significant strength degradation at elevated temperatures. The yield strength of Q355 steel, for instance, drops to approximately 50% of its ambient temperature value at 600°C. The confinement effect provided by the steel tube is therefore diminished as fire temperatures rise, and the column's fire resistance is ultimately determined by the combined degradation of both the steel tube and the core concrete.
Thermal Field Characteristics and Steel Tube Behavior
The finite element simulation reveals that under two-adjacent-face fire exposure, the temperature distribution within the column cross-section is highly non-uniform. The heated faces experience rapid temperature escalation following the standard ISO 834 fire curve (T = 345 × log₁₀(8t + 1) + 20°C), while the unheated faces remain relatively cooler. The thermal gradient through the wall thickness of the square steel tube creates a peeling stress that can initiate local buckling at temperatures well below the critical steel yield temperature.
For square steel tubes with typical wall thicknesses of 6-10 mm used in structural confinement applications, the thermal gradient through the wall thickness becomes significant at fire exposure times exceeding 30 minutes. The outer surface of the steel tube reaches temperatures 100-150°C higher than the inner surface at this stage, creating a thermal bending moment that can initiate local instability of the tube walls.
Simplified Fire Resistance Calculation
The study proposes simplified calculation formulas for fire resistance limits that incorporate the identified dominant parameters. These formulas are designed to be practical for engineering use while maintaining acceptable accuracy. The key innovation is the explicit inclusion of load eccentricity and slenderness ratio effects, which are often neglected in conventional fire resistance assessments for composite columns.
Recommended Design Guidelines
- For two-adjacent-face fire scenarios, the design fire resistance target should be determined using the simplified formulas with appropriate safety factors
- Slenderness ratio should be limited to values that maintain adequate fire resistance under the governing fire scenario
- Load eccentricity effects must be explicitly considered in fire design calculations, particularly for columns supporting eccentric beam connections
- Steel tube wall thickness should be selected to ensure adequate thermal resistance and local buckling resistance during fire exposure
Integration with Engineering Practice
In practical structural engineering applications, the findings of this study should inform the selection of steel tube dimensions and material grades for fire-exposed composite columns. The observation that steel ratio and reinforcement ratio have minor influence on fire resistance suggests that optimizing these parameters for fire performance is not cost-effective. Instead, design efforts should focus on controlling slenderness ratios and managing load eccentricities through structural layout optimization.
For welding connections between steel tubes and structural steel elements, the thermal cycling during fire exposure can affect weld integrity. Welded joints in steel tube confinement systems should be designed with appropriate weld procedures to ensure adequate high-temperature performance. Post-weld heat treatment or selection of weld consumables with favorable high-temperature properties should be considered for fire-critical applications.
Study Insights and Implications
The research demonstrates that fire resistance assessment of steel tube confined RC columns requires a holistic approach that integrates thermal analysis, mechanical behavior, and structural stability considerations. The two-adjacent-face fire scenario, while less common than uniform four-face fire in code provisions, represents a realistic hazard in many building configurations where fire originates from one side. The simplified calculation formulas developed in this study provide a practical tool for engineers to evaluate fire resistance performance during the preliminary design phase, enabling early optimization of steel tube dimensions and structural configurations.