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.

These coatings, applied by specialist intumescent paint contractors, are the most commonly used structural steel safety system in the United Kingdom. They contain materials which respond to elevated temperatures. The materials dramatically increase in density until exposed to heat. This provides steel with an extra layer of defence. Broadly speaking, there are two types of intumescent coatings:

Thin Film: These coatings are solvent or water-based, and are used in buildings most frequently.

Thick Film: While these coatings can be used for buildings, they appear to be used in industries that need intense temperature operations, such as the oil industry.

Brush, roller, or spray can be applied to intumescent paints. Either on-site or off-site, the application process may take place. On-site application is recommended if aesthetics, as well as short-term costs, are a necessity.

Practical limitations will contribute to intumescent paint becoming less of a practical solution in construction projects. This includes time scales for programmes, environmental conditions, and possible problems with accessibility. Fireproof boards will protect steel columns for up to 240 minutes in these situations, and steel beams for up to 180 minutes.

These boards are made of rigid wood based on minerals and can be mounted to beams, columns, and decking of metal. When the steel structure is being erected, this can be carried out by contractors. Although keeping compliant with industry regulations, this allows for a viable method. It should also be noted that any tarping or ventilation around the construction area is not required for fireproof boards. This will lower increased costs associated with the safety of structural fires.

Firestopping cementitious sprays are a simple and cost-effective structural steel fire safety solution. These sprays can be either mineral-based (a common component being vermiculite) or made using compounds of low-density cement. Firestopping sprays can vary in thickness from 10mm to 70mm and are applied using a spray machine to steel. These sprays provide fire protection for up to 240 minutes, as a rule of thumb.

In cases where aesthetics is not a priority, firestopping sprays are especially useful. For aesthetic purposes, you can apply an additional coating or a primer, but this is not totally important. If you opt for firestopping sprays, it is essential to develop an acrylic copolymer binding film. An additional advantage of fire-stopping sprays is that, between walls and floors, they can enhance sound absorption. Firestopping sprays have a multitude of purposes, suitable not only for steel, but also for products based on wood, fibre, and composite.

Over the last decade, ALERON Fire Protection has worked to develop itself as the UK's market leading firestopping contractors and experts in passive fire protection. We remain passionate about fire protection, implementation of fire safety and fire stopping compliance. ALERON is a team of expert IFC-accredited installers and also maintain accreditations from a number of industry specific professional bodies.

Would you like to find out more about our passive fire protection specialist services? Get in touch today with ALERON team for a free technical advice.