Zhuojin Pipe Fitting Co., LtdFire Resistance Limit of Square Hollow Sandwich Steel Tube Concrete Columns under Opposite Two-Sided Fire Exposure
Literature Overview
This 2018 paper by Zhang Yuzhuo, Li Xiaoting, and Lü Xuetao, published in Fire Science, investigates the fire resistance limit of square hollow sandwich steel tube concrete (SHS-STC) short columns subjected to opposite two-sided fire exposure. Using ABAQUS finite element analysis validated against experimental data, the study examines the influence of various parameters on fire resistance performance and derives a simplified calculation formula for the fire resistance limit under this non-uniform fire condition.
Research Background and Motivation
Most existing fire resistance research on steel tube concrete columns assumes uniform four-sided fire exposure, which represents an idealized condition. In practical fire scenarios, structural columns are frequently exposed to fire from only one or two sides due to fire compartmentation, adjacent structural elements, or fire origin location. The paper specifically addresses the case of opposite two-sided fire exposure, which creates asymmetric thermal gradients and introduces additional bending moments that significantly alter the structural response compared to uniform fire conditions.
The square hollow sandwich steel tube concrete column is a relatively new structural form that combines a hollow steel tube outer shell with an internal concrete core separated by a sandwich layer. This configuration offers potential advantages in terms of material efficiency and fire resistance, but its behavior under non-uniform thermal loading has not been adequately characterized.
Finite Element Modeling and Validation
The ABAQUS finite element model was developed to simulate the coupled thermal-mechanical response of the column under fire conditions. Key modeling considerations include:
| Modeling Aspect | Approach |
|---|---|
| Thermal analysis | Steady-state and transient heat transfer |
| Material temperature dependence | Steel and concrete properties as functions of temperature |
| Fire exposure | Opposite two-sided ISO 834 standard fire curve |
| Concrete spalling | Material degradation model with temperature threshold |
| Steel tube buckling | Geometric nonlinearity with thermal expansion |
| Boundary conditions | Pinned-pinned for short column simulation |
The model was validated against experimental data from published tests, showing good agreement in terms of temperature distribution, lateral deflection, and ultimate failure load.
Parameter Analysis and Key Findings
The parametric study examined the following variables:
| Parameter | Range | Influence on Fire Resistance |
|---|---|---|
| Load ratio (N/Nu) | 0.2–0.8 | Primary influence: higher ratio, lower fire resistance |
| Tube wall thickness | 4–12 mm | Moderate influence: thicker walls improve performance |
| Concrete strength | C30–C60 | Minor influence: higher strength slightly reduces fire resistance |
| Column length | 1000–3000 mm | Moderate influence: shorter columns have higher fire resistance |
| Sandwich layer thickness | 20–60 mm | Moderate influence: thicker layer improves thermal insulation |
The most significant finding is that the load ratio is the dominant parameter governing fire resistance limit. As the applied axial load increases relative to the room-temperature ultimate capacity, the fire resistance limit decreases substantially. This is attributed to the progressive loss of concrete strength at elevated temperatures, which reduces the composite section's load-bearing capacity until the applied load exceeds the remaining capacity.
Simplified Fire Resistance Formula
Based on the parametric analysis results, the authors regressed a simplified calculation formula for the fire resistance limit:
t_f = f(N/Nu, t_w, f_c, L, t_s)
Where t_f is the fire resistance limit (minutes), N/Nu is the load ratio, t_w is the tube wall thickness, f_c is the concrete compressive strength, L is the column length, and t_s is the sandwich layer thickness.
The formula accounts for the non-uniform thermal gradient effects that create additional bending moments, which are absent in uniform fire exposure cases. The derived formula provides a practical tool for preliminary fire resistance assessment of SHS-STC columns in design practice.
Welding and Fabrication Considerations for Fire-Resistant Steel Tube Concrete Columns
From a steel pipe fabrication standpoint, several considerations are particularly relevant to fire-resistant column design:
- Weld quality and fire resistance: Welded joints in steel tubes are potential weak points under fire exposure. Weld thermal history can alter the microstructure of the heat-affected zone (HAZ), potentially reducing high-temperature strength. Post-weld heat treatment or controlled welding procedures should be specified to minimize HAZ embrittlement.
- Seam location: The position of the longitudinal weld seam relative to the fire-exposed faces should be considered. Welds on fire-exposed surfaces may experience additional thermal stresses from differential expansion between the weld metal and base material.
- Sandwich layer bonding: The interface between the steel tube and the sandwich/concrete layers must maintain integrity at elevated temperatures. Welded connection details between tube segments should provide mechanical interlock that is not solely dependent on adhesive or frictional bonding.
- Fire protection coating compatibility: Any fire-resistant coatings applied to the steel tube must be compatible with the welding procedure. Pre-weld removal and post-weld reapplication of fire coatings should be specified in the fabrication procedure.
Key Questions and Reflections
Several important questions remain open for future research:
- The study focuses on short columns; the fire resistance behavior of slender columns with significant buckling tendencies under non-uniform fire exposure requires further investigation.
- The interaction between fire exposure and seismic loading is not addressed, yet practical structures may experience fire following an earthquake event.
- The long-term degradation of fire protection systems and their effect on the calculated fire resistance limit should be considered in design practice.
- The simplified formula derived from parametric analysis should be validated against additional experimental data from different sources before widespread adoption in design codes.
This research contributes meaningfully to the understanding of fire resistance behavior of steel tube concrete columns under realistic non-uniform fire conditions, providing both analytical insights and a practical calculation tool for engineering design.