Structural steel is one of the most common materials used in building construction, owing to its high strength and ductility. Whilst structural steel is noncombustible, its integrity can be...Continue Reading
Structural steel is one of the most common materials used in building construction, owing to its high strength and ductility. Whilst structural steel is noncombustible, its integrity can be compromised at high temperatures. Reinforced concrete, on the other hand, can actually resist the spread of a fire as well as bear higher temperatures.
How does fire affect structural steel?
It can be highly beneficial for construction managers and contractors to better understand the unique way in which structural steel acts and maintains its integrity in the event of a fire. This will hopefully empower them to recommend the most effective and compliant fire protection solution.
Yield temperature of steel
Structural steel can withstand approximately 425°C before it begins to soften. Between 600°C and 650°C, the steel will lose half of its strength, and will pose a risk of failing (depending on the load it bears).
Unsurprisingly, even a house fire will reach very high temperatures of around 600°C or just under. Of course, it depends on what the material being combusted is. A simple candle will often burn at 600°C or more, whilst propane can cause a fire to reach almost 2000°C.
As you can imagine, there are very few large-scale fires that won’t seriously threaten the structural integrity of structural steel. If a simple candle flame can reach a high enough temperature to half the strength factor of steel, the steel is in desperate need of some extra treatment.
Colour of steel at different temperatures
In the event of a fire, the material properties of steel change, and it undergoes a process known as thermal expansion. During this process, its colour changes according to its temperature. It goes from pale yellow (220°C) to a wide range of purple shades (260°C to 285°C), to blue (290°C to 330°C) and eventually to red, yellow and a white sparkle at 1400°C.
After the fire has been extinguished or naturally comes to an end, the steel will endeavour to contract back to its original shape, presuming the deformations are elastic. Otherwise, a permanent set may occur, which is another factor that threatens its integrity, along with its original and residual loads.
Effect of fire on steel structures
Structural steel is one of the first things that will be assessed after a fire. Understanding how long the fire was and what temperature it reached are crucial, because the deformations that we see are extremely unreliable ways to measure its structural damage.
Moreover, the yield temperature of steel is crucial. It’s considered critical (i.e cannot support its load) if the yield stress is reduced to around 60% compared to when at room temperature. The critical temperature will usually be established during construction as part of regulation.
If a fire remains below 700°C and for under 20 minutes, the reduction in both its stiffness and strength will only be temporary. This means that whilst the steel may appear deformed, it will go back to the same properties before the fire, and will not be permanently compromised. Buckling could potentially still occur if the steel is deformed, however.
The melting point of structural steel
The lowest point at which carbon steel will melt is 1130°C, though 0% carbon steel won’t melt until 1492°C. Regardless of the type of steel, it will usually be completely liquid by approximately 1550°C.
How is the fire resistance of steel calculated?
Fire resistance is calculated using three main criteria, all of which are integral to limiting the risk of fire damage to a building. A structural element or product’s ‘fire resistance period’ is dependent on the time (measured in minutes) it takes for any of the below criteria to be compromised.
- Load-bearing capacity – whether or not the steel can support a required load without collapsing
- Integrity – the steel’s ability to prevent the passage of heat, flames and smoke
- Insulation – the steel’s capability to withstand exposure to rising temperature
What are the industry regulations for the fire resistance of steel?
Fire resistance requirements can vary widely, based on the purpose and height of the building. These standards are set by two main pieces of legislation: Fire safety: Approved Document B, which is generally accepted as the main set of regulations for passive fire protection and BS 9999 which are the requirements set by the British Standards Institution for fire safety in the design, management and use of buildings.
Approved Document B provides a range of guidelines on minimum periods of structural fire resistance, primarily for multi-storey non-residential buildings:
- An office building that is up to 30m in height (and does not have a sprinkler system) has a minimum fire resistance period of 90 minutes
- A commercial building (such as a shop) that is up to 19m in above-ground height has a minimum fire resistance period of 60 minutes.
BS 9999 was published in 2008, and offers a more flexible and holistic approach to codes of practice in comparison to Approved Document B. This approach aims to accommodate as many factors as possible, no matter how small, such as the size of fire doors or new fire protection technologies like mist suppression systems. While BS 9999 is not meant to act as an oppositional approach to Approved Document B, it acknowledges that each building has a unique ‘risk profile’ based on occupancy, ventilation and an array of other circumstances.
While the above regulations do not mention steel directly in their specifications for fire resistance, they provide ample context for contractors, managers and site teams to decide on the most effective solution for structural steel fire protection.
How to increase the fire resistance of steel
Given that structural steel is such a common element in construction projects, both old and new, there are multiple ways to increase its fire resistance. These methods will belong to one of two categories: reactive and non-reactive. This is based on whether or not their ability to protect against fire damage is ‘reactive’ to high temperatures. For example, intumescent paint is a reactive solution for protecting steel, as it expands when exposed to extreme heat, whereas cementitious coatings protect steel beams in all conditions.
Spray applied intumescent paint
Intumescent paint for steel is one of the most commonly -used and cost-effective solutions in a passive fire protection project. This special type of paint works by swelling up into a layer of carbonaceous char when exposed to heat, forming a protective layer around steel structures. This layer slows down the transfer of heat to the steel, delaying the time it takes to reach its critical temperature.
As well as spray-applied fireproofing, there is also the option to protect steel using solvent and water-based film coatings. These work in the same way as intumescent paints, in that they expand dramatically when subjected to high temperatures. Thin film intumescent coatings are the industry standard, there is also the option to apply more heavy-duty thick film coatings, but these tend to be reserved for industrial settings such as hydrocarbon plants where extreme temperatures are the norm.
Intumescent board systems
During the construction process, steel erectors can apply intumescent boards to beams, columns and decking. These boards are made of a highly robust, mineral-based wood. Intumescent board systems are a particularly cost-effective solution for increasing the fire resistance of steel, as unlike spray-applied fireproofing there is not a need for additional tarping or increased ventilation. This also means there is less chance of disruption to ongoing construction works.
Cementitious coatings were the default method for protecting steel beams from fire damage until around the 1970s when the market began to diversify to fulfil demands for more lightweight and versatile products.
Unlike intumescent paint, cementitious coatings do not expand when exposed to heat. They work simply by providing a thick, layered barrier to fire, and delaying the transfer of heat to the underlying steel. Cementitious is well-suited to dry environments, where its structural integrity is not threatened by high levels of moisture in the air.
CLM Fireproofing are the UK’s leading experts in passive fire protection. We are on-hand to provide specialist installation and consulting services. Our operatives are fully compliant with the latest industry regulations, so our clients can feel confident that their building is protected from fire. To speak to one of our passive fire protection specialists, contact CLM Fireproofing today.