How does fire affect structural steel? Structural steel is one of the most common materials used in building construction, owing to its high strength and ductility. Whilst structural steel is...Continue Reading
How does fire affect structural steel?
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.
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 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.
One of the most cost-effective methods to increase the fire resistance of structural steel is by applying intumescent paint to it. 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.
Generally speaking, the thicker the coating, the higher the fire resistance. Alternatively, cementitious coatings can also be used. 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.