Fire Resistance Test and Simplified Theoretical Model of Cladding Composite Fire Protection Square Steel Tube Column

Literature Overview

This study presents experimental fire resistance testing and develops a simplified theoretical model for square steel tube columns protected by cladding composite fire protection systems. As steel structures lose significant strength at elevated temperatures, fire protection systems are essential for maintaining structural integrity during fire events. The research focuses on cladding systems that combine traditional fire-resistant materials with advanced composite technologies to achieve enhanced fire performance while minimizing structural weight and cost penalties.

Core Technical Content

Fire Protection System Configuration

The cladding composite fire protection system investigated consists of multiple layers: an inner thermal insulation layer (calcium silicate board or aerogel blanket), a middle structural reinforcement layer (glass fiber reinforced polymer mesh), and an outer protective coating layer (intumescent coating or ceramic tile). The total system thickness ranges from 25 mm to 60 mm depending on the required fire resistance rating, with the system designed to maintain the steel tube surface temperature below 550°C for the specified fire duration.

Temperature Distribution Characteristics

The experimental results demonstrate that the temperature distribution within the protected steel tube column follows a characteristic gradient pattern. The fire exposure surface reaches temperatures of 800-1000°C within the first 30 minutes of standard fire curve exposure, while the steel tube surface temperature rises more gradually, reaching 350-450°C at the 60-minute mark for a 40 mm thick protection system. The temperature gradient through the protection system is non-linear, with the steepest gradient occurring in the inner insulation layer where the thermal conductivity decreases with increasing temperature.

Load-Bearing Capacity Degradation

The mechanical properties of Q345 steel degrade progressively with temperature, with yield strength retention decreasing from 100 percent at ambient temperature to approximately 60 percent at 400°C and 25 percent at 600°C. The fire protection system's effectiveness is evaluated by comparing the load-bearing capacity at fire temperature with the ambient temperature capacity. The test results indicate that the 40 mm protection system maintains 78-85 percent of ambient load capacity at the 60-minute mark, while the 55 mm system maintains 85-92 percent capacity.

Simplified Theoretical Model

The developed simplified model calculates the fire resistance performance through a three-step approach:

1. Thermal analysis: One-dimensional heat conduction through the protection layers using effective thermal conductivity values that account for temperature-dependent properties and contact resistance between layers
2. Mechanical analysis: Calculation of steel tube section properties at elevated temperature using temperature-dependent yield strength and elastic modulus reduction factors
3. Failure criterion: Comparison of applied loads with the temperature-degraded section capacity, considering the interaction between axial compression and bending effects

The model incorporates a safety factor of 1.2 on thermal conductivity and 1.1 on strength reduction to account for manufacturing variability and installation quality uncertainties.

Process and Standards Analysis

Design Standards and Requirements

The fire resistance design of steel tube columns should comply with GB 50016 (Code for Fire Protection Design of Buildings) and GB 51249 (Technical Code for Fire Safety of Steel Structures). The fire resistance rating requirements depend on the structural importance classification:

Installation Quality Control

The effectiveness of cladding composite fire protection systems is highly sensitive to installation quality. Critical quality control checkpoints include:

1. Surface preparation: The steel tube surface must be cleaned to Sa 2.5 grade (near-white metal) per ISO 8501-1 before application of any protection layer
2. Layer bonding: Each layer must achieve minimum 90 percent surface coverage with no voids larger than 5 mm in diameter
3. Joint treatment: All joints and overlaps must be sealed with compatible fire-resistant sealant rated for the design fire duration
4. Thickness verification: The installed thickness must be verified at intervals not exceeding 1.0 m² using ultrasonic thickness gauges or magnetic thickness gauges

Welding Considerations for Steel Tube Columns

The steel tube columns used in this study are fabricated from square steel tubes with side lengths of 200-400 mm and wall thicknesses of 6-12 mm. The longitudinal and transverse welds must be qualified for fire-exposed structural applications, requiring:

• Full-penetration groove welds with root pass GTAW and fill/cover GMAW
• Weld qualification testing at elevated temperature (up to 600°C) to verify post-weld heat treatment effectiveness
• Non-destructive examination by UT for 100 percent of longitudinal welds and 20 percent of transverse welds
• Post-weld stress relief for columns exceeding 300 mm in side dimension

Engineering Practice Integration

Cost-Benefit Analysis

The cladding composite fire protection system presents a favorable cost-benefit profile compared to conventional fire protection methods. While the initial material cost is approximately 30-50 percent higher than spray-applied fire-resistant coating systems, the composite system offers:

• Superior durability with design life exceeding 50 years versus 15-20 years for spray-applied coatings
• Reduced maintenance requirements (inspection every 10 years versus annual inspection for coatings)
• Better resistance to mechanical damage during construction and service
• Consistent thickness and performance regardless of application conditions

Interaction with Structural Design

The addition of fire protection cladding affects the structural design in several ways:

1. Increased self-weight: The protection system adds 15-35 kg/m² to the column surface, requiring adjustment of the structural self-weight calculation
2. Reduced effective section: For built-up steel tube sections, the cladding may reduce the effective section dimensions if the protection system is applied to the exterior surface
3. Thermal expansion accommodation: The differential thermal expansion between steel and protection materials must be accommodated through flexible connection details at column bases and tops

Key Questions and Reflections

A significant concern raised by the experimental results is the performance degradation of the protection system at elevated temperatures, particularly the potential for debonding between composite layers when the temperature exceeds 400°C. The glass fiber reinforcement layer, while providing excellent tensile strength at ambient temperature, loses bond strength with the surrounding matrix at temperatures above 350°C. This observation suggests that the design temperature threshold for the protection system should be conservatively set at 350°C rather than the commonly assumed 550°C steel failure temperature.

Another important consideration is the impact of fire protection system thickness on the effective cross-sectional properties of the steel tube column. For slender columns where the slenderness ratio exceeds 100, the additional weight from the protection system may reduce the critical buckling load by 5-10 percent, a factor that must be incorporated into the structural design calculations. The interaction between fire exposure and structural stability requires a coupled thermal-mechanical analysis that considers both local buckling of the steel tube walls and global buckling of the column.

Study Insights and Implications

This research demonstrates that cladding composite fire protection systems provide a technically superior solution for protecting square steel tube columns against fire, offering predictable performance, long service life, and constructability advantages over conventional methods. The simplified theoretical model developed in the study provides a practical design tool that achieves acceptable accuracy (within 10-15 percent of experimental results) while maintaining computational simplicity suitable for routine engineering design. For practicing engineers, the key implication is that fire protection design should be integrated with structural design from the earliest stages of a project, rather than treated as a separate specialty applied after structural design is complete. The performance-based approach advocated in this study, which explicitly defines acceptance criteria and verifies them through both analytical and experimental methods, represents the appropriate framework for ensuring reliable fire protection of steel tube structural members.