Fire compartmentation: why and where

What it is for

Compartmentation is the oldest and strongest tool in fire safety: if fire cannot spread, damage and the number of people at risk stay limited. A compartment is bounded by fire resisting walls and floors and by the fire stopping within them.

The fire resistance requirement for the bounding structures is typically higher than for structures inside the compartment — here the aim is not for the structure to survive but to hold the fire back.

What limits its size

The permitted compartment size is set by the regulation as a function of the risk class and the use. The higher the risk, the smaller the compartments allowed, or the greater the need for additional protection — for example a suppression system, which in return allows a larger compartment.

This has planning consequences: a large open space or a long wing cannot be designed with the compartment boundary appearing only at the end.

Light gauge steel specifics

The bounding structure is a system

A fire resisting wall in this technology is not a “thicker wall” but a verified build-up. The fire resistance requirement has to be verified by testing or calculation just as for any other structure.

The junction is the weak point

A fire resisting wall ends where it meets the floor, the roof or the facade. If a gap remains at these points — under profiled roof sheeting, say, or in the facade cavity — the fire goes round the wall. So the detailing here is not a minor matter.

Penetrations

Every mechanical and electrical penetration is a hole in the fire resisting structure that has to be sealed in a verified way. Fire stopping of penetrations.

Cavities

A light gauge steel wall and floor contain cavities. If these run through the compartment boundary, fire and smoke spread through the cavity. Cavity barriers.

The point

A fire compartment is only worth anything if it closes all the way round. A single unsealed penetration or one continuous cavity renders a well-built fire wall ineffective.