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It has been shown in experiments that mechanical venting of smoke from the fire space helps the performance of stairwell pressurisation schemes [Tamura, 1990]. The location and size of pressure relief vents on the common corridors, or lobbies, connecting to the pressurised stairwell also needs careful consideration.

In summary, while a properly engineered stairwell pressurisation scheme seems to be an effective way to protect these spaces, the extension of the pressurised space beyond the stairwell is less well ‘understood’. Furthermore, a pressurisation system requires commissioning following installation, so that the fan settings can be adjusted for the operating conditions. And, in common with all systems using mechanical air supply or extract, there is the issue of on-going maintenance of the equipment. This is of particular concern in dedicated smoke control systems, i.e. where the mechanical components are not part of the day-to-day air-handling system, since in the absence or routine maintenance testing the systems may become less reliable with time. An Australian study [Zhao, 1998] highlighted the need for regular maintenance, particularly for components not in regular use. Furthermore, smoke dampers were found to be system component most prone to failure.

Finally, note that some mechanical smoke venting schemes (covered below) can be considered as using the pressure-differential approach, where the vented space is depressurised relative to the adjoining 'protected' spaces.

Zoned smoke control

This is in effect a special application of pressure differentials. The basic concept is to exhaust smoke, generally be mechanical means, from the storey containing the fire and to supply fresh air at a number of storeys above and below. By maintaining the fire storey at a negative pressure relative to the storeys above and below, smoke is kept to this storey and does not spread to other storeys. Typically, the ventilation is provided by the building air-handling system, which in ‘fire mode’ is operated with extract only on the fire storey and supply only (100% fresh air) on the adjacent storeys. While smoke extraction may improve conditions on the fire storey, it is not expected to be sufficient to create tenable conditions for long term exposure. Rather, the main purpose is to create a pressure differential with the adjacent storeys [Klote & Milke, 2002].

It is known also by the term sandwich pressurisation [Marchant, 1992], the terminology referencing the fact that the fire storey at negative relative pressure is ‘sandwiched’ between storeys at positive pressure above and below. It has found favour in other parts of the world for the control of smoke in high-rise commercial (office) buildings, in particular where the occupied space is mainly open plan.

However, it is not considered generally suitable for residential buildings. One reason is that the intention is not to maintain tenable conditions (office staff would be expected to evacuate the fire storey), so that persons asleep for example would be placed in danger. Another reason, in the UK at least, is that the provision for extraction within the dwellings is not considered to be generally practical. Other climates may dictate the general inclusion of air-conditioning systems, so that much of the necessary ducting and plant is

© Building Research Establishment Ltd 2005

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