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An alternative to pressurising the stairwell is to pressurise the vestibules only, assuming than that this will protect both the vestibules and the stairwell. This has been practiced in the Japan and elsewhere in the Far East [e.g. Kujime et. al., 1999], where it has been combined with powered smoke extraction from the access corridors. The idea has also been looked at in North America [Tamura, 1980], and is referenced in the Canadian regulations [National Research Council Canada, 1995].

Wind effects, building stack effects and open doors on the stairwell all need to be considered in the design of a stairwell pressurisation system. Quite a lot of research has been undertaken to show that these can all have a significant influence on the pressure distribution inside the stairwell [e.g. Yuill & Haddad, 1994a & 1994b]. The pressurisation system needs to be sized to cope with the expected variations in stack effect, leakage (e.g. door) paths etc. Appropriate pressure relief control may also be required to ensure that the pressure-differentials across closed doors is not so great that it hinders egress.

It has been shown in some cases to be beneficial to distribute the supply of inlet air at multiple locations (storeys) within the stairwell (multiple injection systems) rather than just at the top or bottom (single injection systems). In particular, for buildings greater than about ten storeys in height, single injection systems may be insufficient [Clark & Buckley, 1995]. The North American recommendation is for a maximum of about eight (sometimes up to 12) storeys for a pressurised stairwell served by a single injection system [Klote & Milke, 2002]. An alternative solution to the provision of multiple injection is to divide the stairwell into sections of ,say, eight storeys each. The latter approach is considered less desirable if the anticipated volume of people using the stairwell in case of emergency evacuation is high, as may be the case when the whole building is to be evacuated. Numerical model analysis is generally required in the design of stairwell pressurisation systems involving multiple injection,

There is a well developed body of research into the required pressure-differentials to prevent smoke entering the stairwell, or other protected space, and guidance is widely available. While It is appropriate specify a pressure-difference across a closed door, when the door is open the pressure differential will be 'lost', and in order to prevent smoke ingress to the 'protected' side an airflow (from to protected to unprotected side), above some 'critical' value, is required (see above section on airflow). This has been the subject of various studies [e.g. Tamura, 1992]. The magnitude of the required airflow will be dependent on the temperature of the smoke gases, and so the 'design fire' conditions is important.

A minimum (critical) velocity of air flow (form the high to low pressure side) to prevent smoke propagation into the protected (high pressure) side, is sometimes specified in design codes [e.g. BSI, 1998]. However, there is concern is some quarters, however, that imposing open door airflow criteria generates conditions such that additional oxygen is supplied to the fire, and can make matters more hazardous. In the USA, for example, particular concern has been expressed, and there the building code [International Code Council, 2002] specifically argues against the design of open door velocities greater than 1 ms-1.

© Building Research Establishment Ltd 2005

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