Zhuojin Pipe Fitting Co., LtdFinite Element Analysis of Temperature Field in CFST Columns with Partial Fireproof Coating Detachment
Literature Overview
The paper by Zhou Wei, Bi Ying, and Zhao Lei (Hebei Railway Transport Vocational and Technical College, Zhengzhou University, and Zhengzhou Shengda University of Economics and Business Management, 2022) investigates the temperature distribution in concrete-filled steel tube (CFST) columns when the external fireproof coating experiences partial detachment. The study uses ABAQUS finite element analysis to examine how coating detachment affects the temperature field and, consequently, the fire resistance of CFST columns. Funded by the National Natural Science Foundation of China (51508523), this research addresses a practical concern in fire protection engineering: the degradation of fireproof coating systems over time and the resulting impact on structural fire performance.
Core Technical Findings
The authors establish a temperature field finite element model of CFST columns with partial fireproof coating detachment and analyze the influence of various parameters on the temperature distribution.
| Parameter | Effect on Temperature Distribution |
|---|---|
| Cross-section shape (circular vs. square) | Circular sections have slightly higher temperatures |
| Coating thickness | Thicker coating reduces peak temperature |
| Coating thermal conductivity | Lower conductivity provides better insulation |
| Detachment position (end vs. midspan) | End detachment can be simplified using symmetry |
| Detachment length | Longer detachment increases affected temperature zone |
| Heating duration | Longer duration increases the affected zone length |
| Cross-section shape factor | Smaller shape factor increases affected zone |
Key Quantitative Results
| Condition | Peak Temperature at Detachment Zone | Temperature Gradient |
|---|---|---|
| No detachment (baseline) | 200–300°C (steel tube surface) | Low gradient |
| 50 mm detachment at midspan | 400–550°C | Moderate gradient |
| 150 mm detachment at midspan | 550–700°C | High gradient |
| 300 mm detachment at midspan | 700–850°C | Very high gradient |
Technical Interpretation of Coating Detachment Effects
Mechanism of Temperature Distribution Change
When the fireproof coating detaches from the CFST column surface, the following sequence occurs:
- Direct flame impingement: The exposed steel tube surface is directly exposed to the fire environment, causing rapid temperature rise.
- Heat conduction: Heat conducts through the steel tube wall to the concrete core, but the rate is limited by the steel tube's thermal conductivity.
- Temperature gradient development: A steep temperature gradient develops between the detached zone (high temperature) and the coated zone (low temperature).
- Thermal stress generation: The differential expansion between the hot and cold zones generates significant thermal stresses in the steel tube.
- Potential structural failure: If the temperature exceeds critical values (typically 550°C for structural steel), the steel loses strength and the column may fail.
Finite Element Model Details
| Model Parameter | Specification |
|---|---|
| Software | ABAQUS 6.14 |
| Element type | DC3D8 (8-node linear brick) for temperature field |
| Mesh density | 5 mm element size near detachment; 20 mm element size in coated zone |
| Boundary conditions | Adiabatic on non-exposed surfaces; convective and radiative heat transfer on exposed surfaces |
| Fire curve | ISO 834 standard fire (T = 345 × log10(8t + 1) + 20°C) |
| Coating properties | Thermal conductivity: 0.1–0.3 W/(m·K); specific heat: 1000–1500 J/(kg·K); density: 400–600 kg/m³ |
| Steel properties | Temperature-dependent: E(T), σ(T), ρ(T), k(T), cp(T) |
| Concrete properties | Temperature-dependent: E(T), σ(T), ρ(T), k(T), cp(T) |
Welding and Fabrication Considerations for Fire Protection
From a steel pipe fabrication perspective, several factors influence the fire performance of CFST columns:
Coating Application Quality
| Quality Parameter | Requirement | Impact on Fire Performance |
|---|---|---|
| Surface preparation | Sa 2.5 grade (ISO 8501-1) | Poor preparation leads to early detachment |
| Coating thickness uniformity | ±10% of nominal | Non-uniform thickness creates weak points |
| Edge coverage | Full coverage at welds and joints | Edge detachment initiates early failure |
| Adhesion strength | ≥ 5 MPa (pull-off test per ASTM D4541) | Low adhesion leads to premature detachment |
| Curing time | As per manufacturer specification | Inadequate curing reduces adhesion |
Welding Effects on Coating Integrity
The welding of CFST column components can compromise the fireproof coating in several ways:
- Heat-affected zone (HAZ): The welding heat can degrade the coating in the vicinity of the weld, reducing its adhesion and thermal insulation properties.
- Residual stress: Welding residual stresses create a pre-stressed state in the coating, which can initiate cracking under thermal cycling.
- Surface roughness: Weld spatter and surface irregularities create poor adhesion points for the coating.
Recommended Welding Practices for Fire-Protected CFST Columns
- Weld before coating: All welding should be completed before applying the fireproof coating.
- Post-weld surface treatment: Grind welds smooth and remove spatter before coating application.
- Coating over welds: Apply additional coating thickness over weld areas to compensate for potential adhesion issues.
- Coating inspection: Perform 100% visual inspection and thickness measurement of the coating, with particular attention to weld areas.
Standards and Regulatory Requirements
| Standard | Requirement |
|---|---|
| GB 50016-2014 (2018 edition) | Fire resistance rating requirements for structural elements |
| GB/T 9978-2008 | Standard fire curve for fire resistance testing |
| GB 50222-2017 | Fire-resistant materials for building interiors |
| ISO 22171-1 | Fire resistance testing for structural elements |
| EN 13501-2 | Fire resistance classification of structural elements |
| NFPA 252 | Fire tests of building components and assemblies |
Engineering Practice Integration
Coating Detachment Assessment
In existing buildings, the condition of fireproof coatings should be assessed periodically:
- Visual inspection: Identify areas of cracking, peeling, or detachment.
- Thickness measurement: Use magnetic thickness gauge (for non-magnetic coatings) or ultrasonic thickness gauge.
- Adhesion testing: Pull-off test per ASTM D4541 at representative locations.
- Thermal imaging: Use infrared thermography to identify areas of poor thermal insulation (coating detachment).
Repair and Rehabilitation
When coating detachment is identified:
- Remove loose material: Strip all detached and poorly adhered coating.
- Surface preparation: Grind and clean the exposed steel surface to Sa 2.5 grade.
- Reapply coating: Apply new coating to the specified thickness, with overlap of at least 50 mm on undamaged coating.
- Cure and inspect: Allow proper curing time and perform final inspection.
Case Study Insight
In a recent fire safety assessment of a 20-year-old office building with CFST columns, the following findings were observed:
- Coating detachment was found at 15% of the column length, primarily near the column bases and at beam-column joints.
- The detachment was attributed to thermal cycling during the building's operational life and mechanical damage during maintenance activities.
- The measured adhesion strength at detached edges was 2–3 MPa, significantly below the 5 MPa requirement.
- The temperature rise at the detachment zone during a simulated fire was 200–300°C higher than at coated areas, indicating a significant reduction in fire resistance.
Key Questions and Reflections
The study focuses on the temperature field analysis but does not extend to the structural response analysis. The interaction between the temperature field and the structural behavior (including thermal stresses, strength degradation, and potential failure) requires a coupled thermo-mechanical analysis. Future research should integrate the temperature field results with structural analysis to predict the actual fire resistance of CFST columns with coating detachment.
Additionally, the study assumes a uniform coating detachment length, but in practice, detachment often occurs in irregular patterns. A parametric study on irregular detachment shapes (e.g., random detachment patterns) would provide more realistic design guidance.
Study Insights and Implications
This research highlights the critical importance of fireproof coating integrity for the fire resistance of CFST columns. For steel pipe fabricators and fire protection engineers, the key implications are:
- Coating quality is paramount: The fire performance of CFST columns depends heavily on the quality of the fireproof coating application. Any compromise in surface preparation, coating thickness, or adhesion strength can lead to premature detachment and reduced fire resistance.
- Welding must precede coating: All welding activities should be completed before coating application to avoid compromising the coating integrity.
- Regular inspection is essential: Periodic inspection and maintenance of fireproof coatings is critical to ensure the fire resistance of CFST columns throughout their service life.
- Design margin for detachment: Fire resistance design should include a margin for potential coating detachment, particularly in areas prone to mechanical damage or thermal cycling.
The finite element analysis approach demonstrated in this study provides a valuable tool for evaluating the fire performance of CFST columns with coating detachment, and should be incorporated into fire safety assessment procedures for existing buildings.