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generated such that the whole stairway is at higher pressure than the adjoining building spaces. This then affords smoke protection to the stairway.

Mechanical smoke venting

This refers here to the provision of dedicated smoke exhaust by mechanical means other than that included as part of a zoned smoke control (which will generally utilise the air- handling system) or a mechanical dilution scheme.

Mechanical smoke venting, other than for purely dilution intentions, can be designed to serve two possible purposes. Firstly, it may have the purpose of creating tenable conditions inside the fire compartment, corridor etc. In a long corridor this will require the presence of sufficiently buoyant smoke so that a level of stratification is achieved and smoke can be vented at high level (with replacement fresh air at low level). The second purpose may be to depressurise the space in question, e.g. in a corridor this could help protect an adjacent stairwell in a similar fashion to a formal pressure differential scheme.

A special class of mechanical smoke venting is provided by mechanised smoke shafts. Here the function of the fan could be either to provide a fully-mechanical system (in effect a special form of a ducted mechanical smoke extraction system) [Butcher & Parnell, 1998], or as a ‘background’ assistance to overcome adverse wind or building stack effects. The latter form of smoke shaft would be essentially a natural ventilation device with mechanical assistance. The design of mechanical smoke shafts venting directly from the compartment(s) of fire origin were studied in some depth in the early years of smoke control analysis [e.g. Tamura & Shaw, 1973] are probably most appropriate for office spaces where in Canada design guidance is provided [National Research Council Canada, 1995].

Some guidance on the design of mechanised smoke shafts serving common corridors, adjoining the compartment of fire origin, is now available [CIBSE, 2003], where the system is treated as a type of depressurisation scheme. However, the subject is in its early stages of development and further studies (including the current project) should help establish the required mechanical flow rates and make-up air provisions, or alternatively the design pressure differentials if this approach is appropriate..

With all mechanical smoke venting schemes, the provision for make-up fresh air is particularly important [Butcher & Parnell, 1997]. Furthermore, as with a pressure differentials scheme, care is required to ensure adequate pressure relief paths so that hazards due to doors being held tightly shut are avoided. Here the general guidance is that the pressure differential across a closed door should not exceed about 85 Pa.

Mechanical smoke venting from stairwells is not generally considered as this may, in effect, violate a prime objective of a protected stairwell, which is to eliminate or minimise the passage of smoke from adjacent spaces into the stairwell.

As was stressed in the discussion of pressurisation systems above, the need for on- going maintenance and testing of the mechanical components needs to be considered.

© Building Research Establishment Ltd 2005

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