Methods of protecting compartmentation
The Building Regulations require that new buildings must be divided into fire compartments in order that the spread of fire in the building be inhibited.
There are obvious areas of weakness, which include doors, windows and ventilation ductwork passing from one compartment to another.
BS 9999:2008 Clause 184.108.40.206
Fire protection of ventilation ductwork is needed as an integral part of compartmentation and to ensure that means of escape from the building are not prejudiced. There are three basic methods, which are stailed in 220.127.116.11 to 18.104.22.168 the three methods are not mutually exclusive and in most ductwork systems a combination of two, and occasionally all three, will best combat the potential fire dangers.
Method one - Protection using fire dampers - BS 9999:2008 Clause 22.214.171.124
This method does not require the ductwork to provide any degree of fire resistance, since the fire is isolated in the compartment of origin by the automatic actuation of fire dampers within the ductwork system. Fire dampers should therefore be sited in the duct at the point where it penetrates a fire-separating element.
Thermally operated fire dampers should not be used to protect escape routes nor in smoke extract systems
Fire dampers should not be used in extract ductwork for kitchens
Method two - Protection using fire-resisting enclosures - BS 9999:2008 Clause 126.96.36.199
In method 2, a building services duct is provided through which the ventilation ductwork passes and, if the duct is constructed to the highest standard of the fire resistance of the structure that it penetrates, it forms a compartment known as a protected shaft. This method allows a multiplicity of services to be transferred together along a duct traversing a number of compartments and reaching remote parts of a building, without the need for further internal divisions along its length. Fire dampers are then needed only at points where the ventilation ductwork leaves the confines of the protected shaft.
The fire resistance of the ductwork enclosure when tested from either side should be not less than the fire resistance required for the elements of construction in the area through which it passes, unless there are no combustible materials such as insulation between the ductwork and the enclosure and the enclosure facings are constructed of materials of limited combustibility, in which case the fire resistance of the ductwork when tested from either side should be not less than one half of the fire resistance required for elements of construction in the area through which it passes (and in no case less than 30 min).
Method three - Protection using fire-resisting ductwork - BS 9999:2008 Clause 188.8.131.52
In method 3, the ductwork itself itself forms a protected shaft. The fire resistance can be achieved by the ductwork material itself, or through the application of a protective material.
The fire resistance of the ductwork, when tested from either side, should be not less than the fire resistance required for the elements of construction in the area through which it passes. The supporting hangers should be capable of supporting the ductwork for not less than the period of fire resistance of the ductwork.
Fire-resisting ductwork should meet the appropriate fire resistance period for the specific application when tested in accordance with BS 476-24/ISO 6944 or BS EN 1366-1 and classified in accordance with BS EN 13501-3 by a recognised NAMAS/UKAS accredited laboratory.