It is vital that as construction professionals, we understand the effects of fire on steel structures. A structural fire can exceed temperatures of 800°C or higher, depending on the severity of the...Continue Reading
It is vital that as construction professionals, we understand the effects of fire on 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 fire stopping 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 fire stopping sprays are non-reactive.
This means that they contain fire stopping properties regardless of temperature. In this article, we will outline how all of these methods ensure the compliant protection of structural steel.
What are the regulations for protecting steel structures from fire?
Buildings must be designed so that their stability can be maintained for a reasonable period in the case of a fire. Structural steel is a load-bearing component in most buildings. Therefore, complying with guidelines for structural steel is crucial for maintaining a building’s overall stability.
It’s important to note that regulatory guidelines vary across the UK. Below are the regulations for each region. For more information, please consult our guide to fire safety building regulations.
- England – Approved Document B Fire Safety – Requirement B3: Internal fire spread
- Scotland – Approved Document B Fire Safety – Requirement B3
- Wales – Approved Document B Fire Safety – Requirement B3
- Northern Ireland – Building Regulations (Northern Ireland): Technical Booklet E, 2012.
While some of the finer details may vary across regions, the base principle of maintaining periods of fire protection is clear across all documents. The key takeaway is that the occupancy, height, and function of a building are central to determining its required fire protection period. For instance, Scotland’s Technical Handbook 2 stipulates three distinct fire resistance categories depending on the load-bearing requirement during a fire. These categories are:
- Short (30 minutes)
- Medium (60 minutes)
- Long (120 minutes)
A category is designated to a building based on its height and occupancy as well as its compartment floor area.
Fire protection methods for structural steel
Many structural fire protection strategies include intumescent paint for steel beams 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 paint for steel building regulations 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.
Factors to consider when using intumescent paint
There are a few things to keep in mind when considering intumescent paint. These include:
- Intumescent coatings are often an ideal solution when the aesthetics of a building are important, especially when steel is exposed to the general public.
- The product is applied in the same way as paint, with each layer adding to the overall thickness of the final coat.
- The coating can also be applied to structural wood, which makes it one of the most versatile fireproofing options out there today.
- Intumescent paint can take days to dry, especially if multiple coats are applied. This should be factored into your planning to ensure a timely project delivery.
Should I use water-based or solvent-based intumescent coatings?
Applying water-based intumescent paint for steel beams can be time-consuming due to the number of required coats and the total dry film build needed to provide adequate fire protection. What’s more, water-based intumescent paints also suffer from poor water resistance, which limits their application in indoor spaces and areas that require a waterproof topcoat.
Conversely, solvent-based coatings are generally used on semi-exposed steel due to its ability to withstand various weather conditions. They also offer the added benefit of drying faster and leaving a smoother finish.
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 thinner counterparts, thick film coatings are also often used in projects that require intumescent coatings to have a decorative finish.
Should intumescent coatings be applied on-site or off-site?
Intumescent paint can be applied both on-site as well as off-site. Whether you perform the former or the latter will depend on the wants and needs of the owner of the building, as well as other factors such as how long the paint takes to dry and whether or not the building is still under construction.
Because of access, construction timelines, or logistical considerations, applying intumescent paint off-site may not be the best course of action. In this case, applicators will allow for appropriate time and masking of surrounding areas so there is as little disruption to the construction sequence as possible.
Some of the benefits of on-site applications include:
- No large or complex spray equipment is required
- All steel that needs to be coated is already erected and in-place
- On-site application is not a ‘wet-trade’, meaning that construction activities are not delayed while waiting for access to floors.
The off-site application of intumescent paint can save time in the construction schedule, as it means that trades do not have to spend days moving through floors and applying paint on-site. After the steel has been erected, there is a shorter window in which applicators are required to be on-site to resolve any damages or spray steel connections.
Some off-site application advantages include:
- Reduced number of on-site trades
- Fewer delays in accessing work sites due to continuous bad weather or floor areas being occupied by other ongoing activities
- Improved application productivity, as time is not expended moving equipment, personnel, or products from various floor levels or areas
Flexible blanket systems
Flexible fire blanket systems can be easily applied while also being used on complex details and shapes (in dry environments). Essentially, the blanket is wrapped around a specified area and then fasted in place issuing screws or bolts. Although flexible blanket systems are highly versatile, due to a shortage of manufacturers they can sometimes be difficult to access.
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.
Installing fireproofing boards
Like we mentioned earlier, fireproof boards are generally used in projects where the aesthetic finish of the project is a priority. For this reason, they should be installed with the utmost care, to ensure that their appearance remains pleasing to the eye.
The installation process is as follows:
- Measure the dimensions of the wall where the board will be installed.
- Transfer these measurements to a piece of the fireproof board.
- If the board terminates beyond a wall stud, measure it so that it splits the stud and shares it with any adjacent pieces of drywall or backer board.
- Place the board on top of a sawhorse. Position the saw against the edge of the board so that the blade is at the chalk line. Hold the saw using both hands and push it into the board, cutting along the chalk line.
- Hold the board against the wall and sink screws through it and into the wall studs along the top edge and the topmost corners.
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.
Autoclaved aerated concrete
Autoclaved aerated concrete, or AAC, is made using cement, fine aggregates and an expansion agent. This kind of concrete actually contains 90% air and the material is moulded and cut into precise dimensions during manufacturing. The lightweight material can be used on floors, walls, and roofs and offers excellent thermal insulation. Like all cement-based materials, autoclaved aerated concrete is both fire resistant and durable.
Fire stopping sprays
Fire stopping sprays are a simple and cost-effective steel 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. Fire stopping 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.
Fire stopping 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 fire stopping 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. Fire stopping sprays have a range of applications, being suitable for not only steel but also wood, fabric, and composite-based materials.
CLM Fireproofing is one of the nation’s leading passive fire protection companies, specialising in a range of services including spray applied 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.