Our guide to structural steel fireproofing

It is vital that as construction professionals, we understand how fire affects the behaviour of steel structures. A structural fire can exceed temperatures of 800°C or higher, depending on the severity of the conditions. Temperatures of around 550°C will cause a decline in the load-bearing properties of structural steel. The steel will lose its yield strength and buckle, causing it to bend, twist and ultimately collapse.

Unprotected steel frames can resist fire for approximately 15 minutes. According to industry fireproofing requirements for structural steel, this period of resistance must be increased substantially. The length of this period can vary, based on both the size and purpose of the building. For instance, an office over 30m high with sprinklers must have a minimum fire resistance period of 120 minutes.

Until the 1970s, steel beams were encased in concrete for protection. Since then, there has been a demand for more flexible, lightweight solutions. According to data collected by Tata Steel, the majority of UK steel structures are protected in the following ways:

• 70% use intumescent paints
• 25% use fireproof boards
• 3-5% use firestopping sprays

These fireproofing methods can be broadly categorised in two ways: reactive and non-reactive. Intumescent paints are ‘reactive’ as they gain their fireproofing properties once exposed to extreme temperatures. Conversely, board fireproofing and firestopping sprays are non-reactive. This means that they contain firestopping properties regardless of temperature. In this article, we will outline how all of these methods ensure compliant protection of structural steel.

Intumescent Coatings

Applied by specialist intumescent paint contractors, these coatings are the most widely-used method of structural steel protection in the United Kingdom. They contain materials that react to high temperatures. Once exposed to fire, the materials drastically increase in density. This provides an additional layer of protection to steel. Broadly speaking, intumescent coatings are available in two forms:

• Thin Film: These coatings are solvent or water-based, and are most commonly used in buildings.
• Thick Film: While these coatings can be used for buildings, they tend to be used in industries that require working with extreme temperatures, such as the petroleum industry.

Intumescent paints can be applied by brush, roller, or spray. The application process can take place either on-site or off-site. On-site application is recommended if aesthetics are a priority, as well as short-term costs.

Fireproof Boards

In construction projects, practical constraints can lead to intumescent paint being less of a viable solution. These include project timescales, weather conditions, and potential accessibility issues. In these scenarios, fireproof boards can protect steel columns for up to 240 minutes, and steel beams for up to 180 minutes.

These boards are made of rigid mineral-based wood and can be attached to metal beams, columns, and decking. This can be carried out by contractors whilst the steel framework is being erected. This makes for an efficient solution whilst remaining compliant with industry regulations. It should also be noted that fireproof boards do not require any tarping or ventilation around the construction area. This can reduce additional costs associated with structural fire protection.

Firestopping Sprays

Firestopping sprays are a simple and cost-effective fire protection solution for structural steel. These sprays can be either mineral-based (vermiculite being a common component) or made using low-density cement compounds. Firestopping sprays can range between 10mm and 70mm in thickness and are applied to steel using a spraying machine. As a rule of thumb, these sprays offer fire protection for up to 240 minutes.

Firestopping sprays are particularly beneficial in cases where aesthetics are not a priority. You can add an additional coating or a primer for aesthetic purposes, but this is not wholly necessary. If you opt for firestopping sprays, it is important to apply a binding layer of acrylic copolymer. An additional benefit to fire-stopping sprays is that they can increase sound-absorption between walls and floors. Firestopping sprays have a range of applications, being suitable for not only steel but also wood, fabric, and composite-based materials.