What is it?
Fire protection to steel is a casing round a steel beam or column that slows down how fast the steel heats up in a fire. In houses it's usually a box of fire-rated plasterboard (such as Gyproc FireLine, which is pink) fixed round the beam on a light steel frame, then skimmed.
Other ways of doing it include intumescent paint, which swells into an insulating layer when heated, and sprayed coatings, which are more common in commercial buildings.
What does it do?
Steel doesn't burn, but it loses about half its strength by around 550°C. Unprotected, a beam can reach that in a few minutes of a fire and sag, bringing down what it carries. The casing insulates the steel so it stays below its critical temperature long enough for people to get out: in a house, normally 30 minutes.
Where is it used?
Round steel beams and columns that hold up the structure, such as the beam over a knock-through or a wide opening, or a beam carrying a floor.
Types
- Boarded casings: fire board fixed to a framework round the steel, finished with skim or jointing (for example British Gypsum's GypLyner Encase system). The common choice in houses.
- Intumescent paint: a thin coating that foams up in a fire; keeps the steel visible.
- Frameless boards: rigid boards such as Glasroc F FireCase fixed to each other round the steel.
How to specify
The thickness of board needed depends on the fire period and on the steel's section factor: how much of its surface is exposed to the fire compared with how much steel there is to heat up. A light, thin section heats faster than a heavy one, so it needs more protection.
The board maker publishes tables of fire period, section factor and board build-up. For example, one layer of Gyproc FireLine MR 12.5 in British Gypsum's GypLyner Encase system gives 30 minutes up to a section factor of 260. The system has to be built and finished exactly as tested to count.
Other considerations
- Building control inspects the steel before it's boxed in.
- Joints and fixings follow the system's specification; ordinary plasterboard or a different fixing can mean it no longer gives the rated time.
Interacting elements
- Steel Beam: the casing is fixed round it
- Plasterboard: fire board is a type of plasterboard, finished with the same skim