Finite Element Analysis of Temperature Field in Concrete-Filled Steel Tube Columns with Partial Fireproof Coating Detachment

Literature Overview

This paper by Zhou Wei, Bi Ying, and Zhao Lei (2022), published in Fire Science and Technology, investigates the temperature distribution in CFST columns when the fireproof coating on the column surface undergoes partial detachment. Using ABAQUS finite element analysis, the authors examine how various parameters affect the temperature field and provide insights for fire-resistant design of CFST columns with imperfect coating conditions.

Fire Resistance Design Background

The Role of Fireproof Coating

Fireproof coating is applied to the exterior surface of CFST columns to protect the steel tube from rapid temperature rise during a fire. The coating acts as a thermal barrier, reducing the heat flux to the steel tube and thereby delaying the onset of structural failure. The effectiveness of the coating depends on its thickness, thermal conductivity, and adhesion to the steel surface.

In practice, fireproof coating may suffer from partial detachment due to:

• Mechanical damage during construction or maintenance
• Thermal cycling during repeated heating and cooling
• Inadequate surface preparation before coating application
• Poor adhesion due to environmental factors (moisture, corrosion)

Problem Significance

Partial coating detachment creates a localized zone where the steel tube is directly exposed to fire, leading to a significantly higher temperature in that region compared to the coated areas. This localized heating can cause:

• Local buckling of the steel tube wall
• Premature loss of concrete strength in the adjacent zone
• Reduced overall column capacity due to non-uniform temperature distribution
• Potential progressive failure if the local buckling initiates a chain reaction

Finite Element Modeling and Analysis

Model Configuration

The ABAQUS finite element model incorporates the following features:

Parametric Study Variables

The authors conducted a systematic parametric study varying the following parameters:

1. Section shape factor: Circular vs. square sections with different aspect ratios.
2. Column length: Affecting the heat conduction along the column axis.
3. Coating thickness: 5 mm, 10 mm, 20 mm, and 50 mm.
4. Coating thermal conductivity: 0.04 to 0.12 W/(m·K).
5. Detachment position: At column ends, mid-span, or intermediate locations.
6. Detachment length: 0.1 m, 0.5 m, 1.0 m, and 2.0 m.

Key Results

Temperature Distribution Patterns

The analysis revealed several important patterns in the temperature distribution:

1. The detachment zone temperature rises significantly faster than the coated zones, with the steel tube temperature in the detachment zone reaching 600°C to 800°C within 30 to 60 minutes of fire exposure.
2. The affected zone extends beyond the physical detachment length due to heat conduction along the column axis.
3. The length of the affected zone increases with fire duration, detachment length, and coating thermal conductivity, and decreases with coating thickness and section shape factor.
4. Circular and square sections exhibit similar temperature distribution patterns, with the circular section showing slightly higher temperatures due to its larger surface-to-volume ratio.

Engineering Practice and Quality Control

Fireproof Coating Application Quality

The partial detachment phenomenon underscores the critical importance of fireproof coating application quality. The following quality control measures are recommended:

Inspection and Maintenance

For existing structures with fireproof coatings, regular inspection and maintenance are essential to detect and address coating detachment before it compromises fire resistance:

1. Visual inspection of the coating surface for cracks, blisters, and detachment.
2. Ultrasonic thickness measurement to verify coating thickness at critical locations.
3. Pull-off adhesion testing at representative locations.
4. Documentation of any detected defects and prompt repair.

Repair of Detached Coating

When coating detachment is detected, timely repair is essential. The repair procedure should include:

1. Cleaning of the detached area to remove loose coating and surface contamination.
2. Application of a compatible primer to ensure adhesion.
3. Reapplication of the fireproof coating to the specified thickness.
4. Verification of adhesion and thickness after cure.

Standards and Regulatory Framework

Study Insights and Reflections

This research addresses a practical and often overlooked issue in fire-resistant design: the degradation of fireproof coating over time. The assumption that the coating remains intact throughout the fire event is not always valid, and the consequences of partial detachment can be significant.

From a steel pipe manufacturing and welding perspective, the findings of this study have important implications. The steel tube wall thickness directly affects the temperature rise rate in the detachment zone. Thicker walls provide greater thermal mass and delay the temperature rise, but they also increase the cost and weight of the structure. Engineers should consider the trade-off between wall thickness, coating quality, and fire resistance requirements when selecting the pipe specification.

The welding quality of the steel tube also plays a role in fire resistance. Weld defects such as incomplete fusion or lack of penetration create local weaknesses in the tube wall that may fail at lower temperatures than the base metal. In a fire scenario, these defects can initiate local buckling and compromise the overall structural integrity. Therefore, rigorous welding quality control, including 100% UT inspection of all welds, is essential for fire-critical structural members.

The finite element model developed in this study provides a valuable tool for evaluating the fire resistance of CFST columns with imperfect coatings. However, the model should be validated against full-scale fire tests before being used for design purposes, as the thermal properties of materials at elevated temperatures are uncertain and may vary significantly from the assumed values.

The study also highlights the need for more research on the long-term durability of fireproof coatings, including the effects of environmental exposure, mechanical damage, and thermal cycling on coating adhesion and thickness. Engineers should adopt a proactive approach to coating maintenance and inspection, recognizing that the fire resistance of a structure is only as good as its weakest fire protection component.