Zhuojin Pipe Fitting Co., LtdMechanical Performance of Square Steel Tube Concrete Composite Columns Under Adjacent Two-Face Fire Exposure
Literature Overview
This paper published in Building Structure (Vol. 55, No. 14, 2025) by Yang Li and Bao Yanhong from Qinghai University presents a numerical investigation of the fire resistance behavior of square steel tube concrete composite columns subjected to fire exposure on two adjacent faces. The research was supported by the Qinghai Provincial Natural Science Foundation (Grant 2024-ZJ-745). ABAQUS finite element analysis was employed to study temperature field distribution, failure modes, internal force redistribution, and stress-strain development, with parametric analysis identifying key factors affecting fire resistance limit.
Core Technical Findings
Temperature Field Distribution
The temperature field in the cross-section exhibits single-axis symmetry under adjacent two-face fire exposure. This asymmetric temperature distribution creates:
- Non-uniform thermal expansion leading to lateral deflection toward the cooler faces
- Differential degradation of steel tube properties on exposed versus unexposed faces
- Asymmetric concrete strength loss creating eccentricity in the effective cross-section
Failure Mode and Internal Force Redistribution
Under progressive heating, the column experiences:
- Initial elastic stage — Uniform heating with thermal stresses developing from restrained expansion
- Thermal gradient stage — Significant temperature differential between exposed and unexposed faces causing lateral bending
- Material degradation stage — Progressive loss of steel strength and concrete compressive strength on heated faces
- Failure stage — Combined axial compression and bending exceeding the reduced capacity of the heated section
Parametric Analysis Results
| Parameter | Effect on Fire Resistance Limit | Sensitivity Level |
|---|---|---|
| Load ratio (N/N₀) | Higher load ratio → shorter fire resistance | High |
| Slenderness ratio (λ) | Higher slenderness → shorter fire resistance | High |
| Column cross-section size | Larger section → longer fire resistance | Moderate |
| External concrete compressive strength | Higher strength → shorter fire resistance (generally) | Moderate |
| Steel ratio (ρ) | Higher steel ratio → longer fire resistance | High |
| Load eccentricity (e) | Higher eccentricity → shorter fire resistance | High |
Simplified Calculation Formula
A simplified formula for predicting the fire resistance limit was developed considering the six most influential parameters. The formula provides good agreement with finite element analysis results, offering a practical tool for fire resistance design of square steel tube concrete composite columns under adjacent two-face fire exposure.
Technical Analysis from Steel Tube and Welding Perspective
Steel Tube Performance Under Fire Conditions
The fire performance of the steel tube component is governed by several key factors:
- Steel strength retention: Q235-Q355 structural steel retains approximately 80% of ambient strength at 400°C, 60% at 550°C, and rapidly degrades above 600°C
- Thermal conductivity: Steel conducts heat rapidly (approximately 50 W/m·K), providing some protection to the internal concrete but also experiencing significant temperature gradients through the wall thickness
- Tube wall thickness: Thicker walls provide greater thermal mass and longer time for heat to penetrate to the inner surface, protecting the concrete core longer
- Weld quality: Welded seams in the square tube experience thermal cycling during fire exposure, with potential for weld degradation depending on base metal composition and weld quality
Welding Considerations for Fire-Exposed Steel Tube Structures
| Welding Factor | Impact on Fire Performance | Recommendation |
|---|---|---|
| Weld metal composition | Different thermal expansion from base metal may cause residual stress redistribution | Use matching filler metals |
| Weld residual stresses | Pre-existing residual stresses may accelerate failure under thermal loading | Consider stress relief welding or post-weld treatment |
| Weld geometry | Fillet welds at tube connections may act as stress concentrators under thermal gradients | Design for adequate weld throat and smooth transitions |
| Heat-affected zone | HAZ may have different thermal properties than base metal | Control heat input to minimize HAZ width |
Concrete Behavior Under Asymmetric Fire Exposure
The concrete core in square steel tube columns experiences complex thermal and mechanical effects under adjacent two-face fire exposure:
- Spalling risk: Rapid heating of the concrete surface may cause explosive spalling, particularly in high-strength concrete (>50 MPa)
- Thermal cracking: Temperature gradients through the concrete section create tensile stresses that may cause cracking
- Strength degradation: Concrete loses approximately 50% of strength at 400°C and becomes essentially ineffective above 500°C
- Creep under sustained load: At elevated temperatures, concrete exhibits significant creep, contributing to additional deformation
Fire Protection Strategies for Steel Tube Concrete Columns
Based on the parametric analysis findings, the following fire protection strategies are recommended:
- Increase steel ratio — Higher steel content provides better fire resistance due to the steel tube's thermal protection of concrete and the steel's relatively better fire performance compared to concrete
- Reduce load ratio — Design for lower utilization at ambient temperature to provide greater margin under fire conditions
- Minimize eccentricity — Symmetric loading reduces the combined bending-compression demand under asymmetric thermal conditions
- Optimize slenderness — Shorter, stockier columns perform better under fire exposure due to reduced buckling sensitivity
Engineering Practice Integration and Study Insights
This research addresses an important practical scenario in fire engineering where columns at building corners or along exterior walls may be exposed to fire from two adjacent directions. The asymmetric fire exposure creates complex thermal-mechanical behavior that differs significantly from the more commonly studied uniform fire exposure conditions.
From a steel tube fabrication and erection perspective, the findings have several practical implications:
- Tube wall thickness selection: The parametric analysis confirms that larger cross-sections and higher steel ratios improve fire resistance. Engineers should consider slightly thicker tube walls in fire-critical locations, recognizing that the additional cost is modest compared to the fire safety benefit.
- Weld quality importance: The integrity of welded connections becomes even more critical under fire conditions, as residual stresses from welding may interact with thermal stresses to accelerate failure. Full-penetration welds with proper NDE coverage are essential at all tube-to-tube and tube-to-plate connections.
- Material selection: The study findings support the use of steel tube concrete composite columns as a fire-resistant structural system, particularly when designed with adequate steel ratio and appropriate load utilization. The simplified calculation formula provides a practical design tool that can be integrated into routine structural design workflows.
The development of simplified calculation formulas validated against finite element analysis represents a valuable contribution to practical fire engineering design. Engineers should note that while these formulas provide good accuracy for preliminary design, detailed finite element analysis remains essential for critical applications and performance-based fire design. The research also highlights the importance of considering realistic fire exposure scenarios in structural design, moving beyond the simplified uniform fire model to address the asymmetric conditions that may occur in real fire events. The parametric study methodology and simplified formula approach provide a framework that can be extended to other composite structural systems and fire exposure scenarios in future research.