Fire protection

There are four common methods of fire protecting structural steelwork;

Board based systems are used to form rectangular encasements around steel members, such as internal beams and columns.  Paint or other finishes can be applied directly to the boards. The level of fire resistance achieved depends upon the type and the thicknesses of the boards used and upon the method of attachment.


Intumescent coatings may be brushed or sprayed onto steelwork rather like paint. The materials expand when subjected to fire and form an insulating foam. Intumescent coatings can achieve up to 120 minutes fire resistance, and are used mostly on exposed steelwork.

Sprayed fire protection systems are usually based upon cementitious materials and are applied directly onto the surface of steelwork. They are generally low cost, but cannot receive finishes owing to their coarse uneven texture. Sprayed materials tend to be used where steelwork is concealed or where appearance is unimportant. Fire resistance is similar to that of board based materials.

Concrete Filled Structural Hollow Sections

Structural Hollow Sections (SHS) can be fire protected by filling with reinforced concrete. Concrete filled structural hollow sections can achieve 120 minutes fire resistance.


The Slimdek® system has inherent fire resistance as the ASB section is encased in concrete with only the bottom flange exposed to fire. Without fire protection Slimdek® canachieve 60 minutes fire resistance.

Periods of fire resistance in excess of 120 minutes can be achieved if ASB is fire protected.

Multi-storey frames requiring 30-60 minutes can have 40% of the floor beams unprotected by following the recommendations of a special design guide.

Protection thicknesses

The section factor of a particular steel section is its surface area per unit length divided by its volume per unit length (A/V). This parameter defines how quickly a steel section will heat up when subjected to fire. The section factor for a member with box protection is lower than that for a member with profile protection, and hence box protected steelwork heats up more slowly and requires less protection.

Typical spray or board thicknesses for a column in a multi-storey building are as set out in the table below.

Fire resistance(minutes)

Profile Protection(mm)

Box Protection   (mm)













Typical spray or board thicknesses based on 254UC x 89 kg/m column in a multi-storey building.


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