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Stairwell lobby access

By separating the stairwell from common corridor(s) / lobby(ies) that adjoin the dwellings by an additional lobby (which does not adjoin any dwelling) additional protection is afforded to the stair. Such lobbies may be unventilated, naturally ventilated or mechanically ventilated (smoke extraction or air supply / pressurisation).

Pressurised vestibules were discussed above under pressurisation. Unventilated lobbies provide additional stair protection by virtue of smoke containment, and are referenced in various regulations [e.g. DETR, 2000]. Elsewhere, naturally or mechanically ventilated lobby (vestibule) access is sometimes specified in cases where the stair in not pressurised [e.g. NFPA, 2000a]. In the USA [International Code Council, 2002], mechanically ventilated vestibules are required to extend vertically by at least 0.5 m above the top of the entrance door, with extraction from this 'smoke trap' space and replacement fresh air at low level.

Limiting the fire size

While this is not a smoke management method in the conventional sense, this may now form part of the analysis, or 'trade-up'. By limiting the size of fire, or by possibly extinguishing it prior to fire service intervention, it has been argued that the requirement for smoke management in the common access areas is reduced, or even eliminated.

The most widely used mechanism for limiting the size of fire in buildings is currently sprinklers, and these are now recommended or required in multi-storey residential premises in various parts of the world, most notably in North America [International Code Council, 2002] and Australasia [Building Industry Authority (New Zealand), 2001]. In some instances, the requirements for smoke management are relaxed if residential sprinklers are installed. However, it is now generally accepted that smoke management remains an important requirement as smoke is still generated when sprinklers are present, and smoke will most likely pass from the fire compartment to the common access areas when the front door to the dwelling is opened. Furthermore, is has been argued that while sprinklers will reduce the temperature of the fire gases, the amount of smoke generated may not be reduced, and in some cases the smoke may be more hazardous in respect to the levels of carbon monoxide [Mawhinney & Tamura, 1994]. Slowly growing and shielded fires pose particular difficulties for sprinkler systems [Williams et. Al., 2004].

Given lower fire gas temperatures and reduced expansion of the air in the fire compartment, a smoke management system using pressure differentials may be ‘helped’ by the presence of sprinklers. Furthermore, the operation of sprinklers will reduce the probability of windows breaking, which eliminates the effect of adverse building stack pressures that may then be generated, and allows for a relaxation in the design pressure differentials in mechanical smoke control systems.

However, there appears to be no formal engineering method to quantify this effect in the design process, and to possibly allow a reduction in the designed pressure differentials, and further research has been recommended in this area [Klote, 1990]. It can be argued that given the uncertainties due to the presence of other controlling mechanisms such as

© Building Research Establishment Ltd 2005

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