Impact of Fire on Structural Steel
It is important that as building professionals, we understand how fire affects the behaviour of steel structures and its performance at high temperatures. Depending on the severity of the circumstances, a structural fire may reach temperatures of 1000°C or higher. It has been demonstrated, that load-bearing properties of structural steel significantly decrease starting from temperatures of 400°C, and losing almost half of its design strength at 550°C. During a fire event, a steel structure will suffer from material property changes, such as thermal elongation, strength reduction and a reduction in stiffness. The steel would lose the strength of its yield and buckle, causing it to bend, twist and eventually collapse.
For approximately 15 minutes, unprotected steel frames can resist fire, however this period is seen as inadequate to guarantee safe evacuation of most buildings or deployment of fire-fighting actions by a fire brigade. This time of fire resistance must be significantly improved, according to industry fireproofing standards for structural steel. Based on both the size and function of the building, the duration of this time will vary. An office over 30m high with sprinklers, for instance, must have a minimum fire resistance time of at least 120 minutes. Building Regulations Approved Document B provides detailed guidance and specification for the minimum periods of fire resistance (table B.4)
Steel beams were encased in asbestos for fire protection until the 1970s. Asbestos is a hazardous material which is currently banned in most countries around the world. There has been a need for more versatile, lightweight solutions ever since then. The majority of UK steel structures are covered in the following ways, according to data collected by Tata Steel:
· 70% use intumescent paints
· 25% use fireproof boards
· 3-5% use firestopping sprays
It is possible to roughly categorise these fireproofing approaches in two ways: reactive and non-reactive. Intumescent paints are 'reactive' until exposed to high temperatures as they obtain their fireproofing properties. Board fireproofing and firestopping sprays, by comparison, are non-reactive. This implies that, regardless of temperature, they contain firestopping properties. In this article, we will outline how all these techniques ensure that structural steel is covered in a compliant manner.