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...Continue Reading
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.
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 the compliant protection of structural steel.
Many structural fire protection strategies include intumescent paint for steel as a critical solution. Once applied, it expands into a solid, carbonaceous layer when subjected to temperatures of around 250°. This creates a protective coating around the steel to increase its fire resistance rating, which is usually either between 30, 60 and 90 mins. By increasing the period that the steel can withstand high temperatures, intumescent paint extends the window of opportunity for a building to be safely evacuated.
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.
Intumescent film coatings
Intumescent 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 and thick film.
Thin film coatings
Thin film coatings are solvent or water-based and consist of a primer, basecoat and sealer code. They are usually the industry standard as they are suitable for buildings where fire resistance regulations (specified by Approved Document B) are either 30, 60 or 90 mins. In the event of a fire, thin film coatings can expand at a rate of approximately 50:1.
Thick film coatings
As their name suggests, what differentiates thick film and thin film coatings is that thick film coatings are much denser. They also have a lower expansion rate, a ratio of around 5:1. As they are a heavy-duty solution for protecting structural steel, they are usually used in industries that require working with extreme temperatures such as the petroleum industry. It’s also worth mentioning that in comparison to their less dense counterparts, thick film coatings are also often used in projects that require intumescent coatings to have a decorative finish.
Over recent years, technology and service innovation has allowed for more intumescent film coatings to be applied off-site. While this involves more upfront costs, specialists are able to uphold the highest possible quality control and compliance standards, making the installation process as a whole more efficient. Therefore, off-site installation is widely considered to be worth the initial investment.
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.
Fireproof boards are factory manufactured, so installers can rely on them to have a consistent weight and density. Broadly speaking, they will fall into one of two categories: lightweight or heavyweight. Lightweight boards will range between 150-250kg/m³ whereas heavyweight boards can weigh up to 950kg/m³. Aside from their weight, another distinguishing factor is that heavyweight boards are usually used in projects where aesthetic finish is a priority.
Fireproof boards offer a range of benefits for a passive fire protection project. As a ‘dry trade’, they can be installed quickly with minimal impact on other ongoing activities. For instance, in comparison to spray-applied fireproofing it is not necessary to have any tarping or ventilation around the construction areas. This can reduce costs and help streamline overall project delivery.
Prior to the 1970s, cementitious fire protection was the primary way to safeguard structural steel from fire damage. This would involve coating steel beams and other elements in a combination of cement and gypsum. This coating is applied in layers, each one protecting the underlying materials. While cementitious coatings benefit from a high fire resistance rating, over the last few decades they have been superseded by solutions that are lighter and more versatile while offering a similar level of protection.
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.
CLM Fireproofing are one of the nation’s leading passive fire protection companies, specialising in a range of services including structural steel fireproofing. Our experienced team of passive fire protection specialists work with clients to deliver fire protection projects swiftly, efficiently and in full compliance with the latest industry regulations. Contact CLM Fireproofing to find out more.