The Role of Steel Sections on the Use of Intumescents

Did you know that the thickness of any fire protection material such as an intumescent coating depends on a number of factors:

• Type of protection used (thin film vs vermiculite for example);
• Duration of the required fire resistance (FRL);
• Perimetre of the steel section exposed to the flames; and
• The dimensions and shape of the steel beam itself.

In fact, the last two points — both in isolation and in combination — have a significant impact on the first two items. The Hp/A is a significant determinant on how thick the protective coating is, and also, what coating is used.

Through this piece we will look to understand:

• Steel Section Factor and how this affects the sections resistance to fire;
• How steel selection therefore can impact construction and selection of fire rating; and
• The options for structural steel fire coatings.

What is steel section factor?

Steel section factor, which is simplified as Hp/A, is a ratio that indicates the heating rate of a structural steel section in case of the occurrence of a fire. Hp stands for heated perimeter or the portion exposed to fire and A stands for the cross-sectional area of the beam.
The thing is, many people do not realise the importance of steel section factor and the fact it is an essential element in determining the amount of intumescent to be applied to ensure an effective passive fire protection of steel.

The concept of steel section factor is simple: the lower the Hp/A (which is common in larger and thicker steel beams), the slower that specific section heats up when subjected to fire. On the other hand, a section with a higher Hp/A (a lighter and thinner steel member) means that that beam section will heat up faster, hence requiring a much thicker application of intumescent fire protective coatings, or some other suitable fire protection.

Now how is steel section factor calculated?

In reality, there is no ONE specific standard for the calculation of Hp/A because it is dependent on the type of material used for fire protection. However, there are two universally accepted methods for calculations, which are the box protection and profile protection methods.

In a box protection, while the cross-sectional area or A remains as the total  cross  sectional  area  of  the  whole steel member; the Hp is calculated by taking into consideration the smallest possible encasements of the section. The inside dimensions of these encasements, whether square or rectangular, are then added together to get the Hp value.

In the profile protection method, the steel section factor calculation is more complex as you need to take note of the beam’s actual and serial sizes, and sometimes, even the nominal density of steel to calculate the Hp. Actual measurements or reference to a steelwork table is also undertaken to compute for the total cross sectional area.

Irrespective of whether you’re calculating using the box or profile method, there are other factors you should consider: the type of steel section (universal beams, universal columns and joists, structural and rolled tees, angles, channels, or hollow sections) and the number of sides protected (four, three, two, one),  if they are flanged, partially exposed or soft fit.

How thick should the intumescent paint for steel beams be?

Once the steel section factor has been calculated, qualified fire engineers can readily identify the correct and required thickness of the intumescent paint so as to achieve the necessary Fire Resistance Level or FRL. As a reminder, these are specified by the classifications provided by the NCC and the AS 2327.1, AS 4100 and AISC Guidelines for Assessment of Fire Resistance of Structural Steel Members (if it is a steel or composite structure).

The NCC has specified the Minimum thickness (mm) of principal material for FRL, depending on the type of building element, steel columns included.

Of course, FRL (which is dependent on the section factor value), is not the only aspect to be considered in identifying the thickness of a fire protection solution. The following shall also be taken into account as well:

• Steel serial size or the dimension of the steel member
• Number of exposed sides
• The critical temperature or design load, if available
• Potential for damage

Your Options for Structural Steel Fire Coatings

If we remove the option of encasing steel in concrete (concrete encasement) then there are three main types of structural steel fire coatings that you can choose from, which you can combine together or use as a stand-alone. The first one is the vermiculite spray, which is preferred by those who are looking for speedy application and ways to reduce the cost involved. Over multiple applications, a vermiculite spray can achieve FRL requirements up to 240/-/-.
The next passive fire protection solution for steel is the board encasement, which can also achieve FRL requirements up to 180/-/-. It is a perfect option for those who are looking for uniform thickness and a clean finish.
The last one, which is ideal for FRL requirements between 60/-/- to 120/-/-, is intumescent paint such as those produced by Nullifire. For qualified applicators, it is easy to apply, cost effective, highly damage and weather resistant and can be top-coated with compatible coatings allowing you to achieve an architectural finish.
At Permax, we offer two high-quality intumescent variants from Nullifire:

• SC803 – Low VOC, thin-film intumescent perfect for internal use and C1 and C2 environments; meets the 60/-/- FRL requirement
• SC902 – A hybrid, low-VOC, one-coat intumescent perfect for internal and external steelwork; meets the 120/-/- FRL requirement

For more information about passive fire protection, or your requirements for what product to apply, get in touch with one of the team today.