Understanding Steel’s Vulnerability to Heat
Despite its reputation for durability, steel can become vulnerable when exposed to elevated temperatures such as the intense heat of fires.
At around 550°C, structural steel reduces in strength, retaining only about 60% of its room-temperature strength, rather than half as commonly misunderstood. This critical temperature threshold, known as the limiting temperature, varies depending on the specific grade of steel and the load it carries.
Steel also undergoes thermal expansion during heating, which can generate additional stresses in restrained structural members, further compromising the building’s stability. This combination of strength loss and thermal stress creates the need for comprehensive fire protection strategies in steel-framed buildings.
Passive Fire Protection Methods
Passive fire protection (PFP) is the first line of defence against thermal effects on steel structures. These systems work by providing thermal insulation that slows the rate of temperature rise in steel members during a fire.
Spray-applied fire-resistant materials (SFRM), commonly known as fireproofing sprays, form a thick, insulating layer around steel beams and columns. These cementitious or mineral-based coatings provide varying degrees of fire resistance depending on the thickness applied and the specific product used. The application process involves spraying these materials onto clean steel surfaces to achieve the required thickness for the desired fire rating.
Active Fire Protection Systems
Active fire protection systems complement passive measures by attempting to control or suppress the fire itself, thereby reducing the thermal load on steel structures and providing additional safety margins.
Water-based sprinkler systems are the most common active protection method, designed to activate automatically when predetermined temperature thresholds are exceeded. These systems can reduce fire temperatures around steel members and may even extinguish fires in their early stages, protecting both the steel structure and the building’s occupants. The cooling effect of water can help maintain steel temperatures below critical thresholds.
Deluge systems, which flood entire areas with water simultaneously, are used in high-risk industrial applications where rapid fire spread is a concern. These systems provide immediate and comprehensive water coverage but require careful design to ensure adequate water supply and drainage.
Testing Standards and Performance Evaluation
The effectiveness of fireproofing systems is rigorously evaluated through standardised testing procedures that ensure consistent performance across different products and applications. Fire resistance testing in the UK historically followed BS 476 standards.
Now, however, European harmonised standards such as BS EN 1363 and related standards are more commonly used. These tests work by subjecting protected steel specimens to controlled fire conditions using standardised time-temperature curves designed to simulate real fire scenarios.
