Smoke management measures for protection of common access areas
Each of the principal smoke management methods that can contribute to the smoke management of common access areas is reviewed below. A summary of the method and its appropriate use is given, and reference to relevant research is provided.
This refers to floors, walls, doors and other barriers that have sufficient fire resistance to remain ‘intact’ for a specified duration and provide a level of protection against fire and smoke spread from the location of fire origin. Regions of a building that are enclosed within fire-resisting compartmentation elements may be referred top as protected spaces, e.g. corridors and vertical shafts, the latter for example consisting of a stair and lift shaft and possibly, at each storey, a lobby. Note that the term ‘physical barrier’ here includes, as a subset, the elements of compartmentation as described in, for example, [DETR, 2000].
Physical barriers form an integral part of smoke management for egress form high-rise buildings in all cases. Special provisions that may be found for protected internal stair and lift shafts to open to the outside via a dedicated access level protected corridor [Tamura, 1994] or for there to be at least one protected stair that does not extend to basement storeys [DETR, 2000].
Corridors above specified lengths are often required to be divided into ‘sections’ by fire doors, providing additional smoke containment. In the UK [DETR, 2000], for example, the length of undivided corridor is up to 30 m (depending on number of stairwells and ‘dead end’ arrangements), while in New Zealand [BIA, 2001] distances up to 40 m are allowed. Corridor sub-division is generally not required if the corridor itself is protected by a pressurisation scheme. Furthermore, if sprinkler systems are installed inside the dwellings, then the undivided corridor length requirement may be relaxed, as is the case for example in the USA [NFPA, 1992] and New Zealand [BIA, 2001].
In the current project, it was assumed that each of the 'components' of the common access path identified above is separated by a form of fire barrier, e.g. fire-resisting walls and doors.
This is known also by the names purging and dispersal. By diluting the smoke concentration sufficiently with fresh air, it is possible to create tenable conditions for persons evacuating along a corridor or down a stairwell. The supply of fresh air may be by natural ventilation (generally openings in external walls) or by mechanical means. For openings in external walls, the pressures generated by the buoyancy of the smoke gases will tend to drive smoke out at higher level and entrain fresh in at lower level.
However, a number of problems have been identified with this method. Firstly, the required volume of fresh air is high, making both mechanical and natural ventilation problematic. Secondly, natural supply and exhaust through vents may be subject to
© Building Research Establishment Ltd 2005