Passive fire protection and the importance of limiting temperature for steel structures

All materials become weaker when they get hot, and structural steel is not an exception. Depending on the design of the structure, the steel will have a limiting or critical temperature. What does this mean, and why is it important ? How does it affect the amount of protection needed ?

What is a limiting temperature or critical temperature of a steel structure ?

The strength of steel at high temperature is well understood and documented. It is known that at a temperature above 500°C, hot rolled structural steel loses a significant amount of its ambient temperature load capacity.  

Steel frame structures can be designed so that the maximum stress on the steel members is limited to 60% of their individual maximum capacity at ambient temperatures. This additional 40% redundancy provides the frame with strength in abnormal situations, such as a fire. Dependant on the size, shape and orientation, this additional capacity may equate to a calculated collapse temperature of 550°C, rather than just above ambient. This means that above 550°C, the structural steel member doesn’t have enough strength to support the structure it is maintaining. This is the limiting or critical temperature.  

• If the steel is utilised more than 60%, the limiting temperature reduces.
• If the steel is utilised less than 60%, the limiting temperature increases.

To know more, have a look at this graph:

How does the intumescent coating react to fire ?

From the example above, if 550°C is the limiting temperature before the structure collapses, the graph below demonstrate the typical temperature profiles:

• BS EN 1363-1 heating curve,  
• a steel section protected by an intumescent coating, in green  
• an unprotected steel section, in red.

The intumescent coating doesn’t really start to protect significantly until ~250-270°C, which is commonly about 10 minutes after the fire starts. This is where we can first see a reaction. That’s linked to the chemical reaction of the intumescent coating, converting from a paint film to an insulating char.

Once the char is in place, it begins to insulate at a lower rate, slowing the build-up of heat in the steel core. The slight curve is generally the same shape regardless of the thickness of the intumescent, but the thicker the coating, the more elongated the green curve. This equates to the more time to reach the limiting temperature. The extra time needed divided by the difference to critical core temperature (CCT) is a factor in the protection thickness requires.  

For example:

• For 60 minute protection with a limiting temperature of 350°C:
• To add 50 minutes to protect an additional 80°C of temperature (based on 270°C completed reaction) = 50/80 = 0.625

Another example:  

• For 60 minute protection with a limiting temperature of 750°C:
• Additional 50 minutes to protect an additional 480°C = 50/480 = 0.104

Why is the thickness need lower at a higher temperature ?

This is not an absolute value, but a demonstration that approximately six times more intumescent is needed to protect the same piece of steel for 60 minutes if the limiting temperature is 350°C, compared to if it was 750°C. This is protecting with the same coating.  

On all Nullifire intumescent coatings there are tables to give you the correct assessed protection requirements. The tables don’t always show this factor of 6 as demonstrated above because the performance is not always linear as the example suggests.  

Performance varies for every product, over different substrates, and over different fire resistance periods - which is why performance can’t be modelled. Performance must always be tested and assessed.  

Why is the critical temperature important in steel protection ?

To understand with the critical temperature is important, you should keep in mind this information:

• 350°C could be the failure temperature if the steel is very highly utilised, used to almost the limit of its capability. There is not much extra strength to resist any heating above ambient.  

• 750°C could be the failure temperature when the steel is very under-utilised. There is a lot of extra strength to resist the additional heat increase.  
• The actual design of the building and the principles used dictate the actual limiting or critical temperature of the steel structure. If the steel utilisation is known, it can be calculated. If it unknown, the country building code default limiting or critical temperature must be used.

Only a trained and competent person, such as a structural and/or fire engineer, can determine the limiting or critical temperature of structural steel based on the steel dimensions, length, strength, loading and orientation, etc.  

• Using the protection thicknesses for a limiting temperature of 650°C and higher, without understanding the actual steel utilisation or loading, may significantly under protect the structural steel frame. The fire protection will not be installed to the required level.