102 SAFETY & SECURITY The inclusion of a


pressurised stairwell lobby between the apartment and stairwell adds an additional degree of fire separation


escape route. Furthermore, pressurising the stairwell and stair lobby should provide tenable conditions within the stair lobby for the fire service to use it as staging point/fall- back position.


Stair & lobby pressurisation Pressurisation works by using supply fans to pump fresh air into the protected areas in order to direct airflow away from them when doors are opened and thus repel any smoke and hot gas. However architects need to consider the following provisions in the concept design.


The corridor layout must be designed to include the stair lobby, which does not directly access any dwellings or ancillary accommodation. An air supply shaft (riser) is required to serve the stairwell – this could be of the order of 1.0 m2


cross sectional


area. Openings are required in the stair core wall at every third floor level for the air injection points. A second air supply shaft of similar size is required to jointly serve the stair lobby and lift.


Suitable plant space is required at ground floor or roof level for the supply fan sets. If they are at roof level, twin intakes should be provided extending to different roof facades to mitigate contamination of supply air. The fans must have a secondary independent power supply.


Air release vents must be provided in the apartment access corridor – natural facade vents or a smoke shaft riser with AOVs at each level. Finally, if the escape travel distance from the furthest apartment door to the stair lobby door exceeds 15 metres then a dedicated smoke control system is required in the corridor to maintain suitable conditions for occupant escape, which could be two smoke shafts at either end of the corridor. A total of four builders’ work shafts


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could be required for a typical residential tower with a long corridor.


Evacuation lifts


BS 9991:2022 provides clearer and more detailed guidance on evacuation lifts, which facilitate vertical escape for people with disabilities or requiring assistance. While BS 9991:2015 recommended that they be installed if an assessment deems that there are sufficient people requiring assistance to evacuate vertically, BS 9991:2022 says that in all developments where passenger lifts are installed, at least one lift must be an evacuation lift. Also, buildings in excess of 18 metres should be provided with more than one evacuation lift. It also states that evacuation lifts should be located within protected lobbies; with direct access to a stairwell and be served by a smoke control system.


Conclusion To comply with BS 9991:2022, a single stair residential building above must have a pressurised stair and stair lobby. The apartment access corridor must – as a minimum – have air release vents, or a smoke control system if travel distances exceed 15 metres.


The location and sizes of builders’ work shafts, plant space requirements and power supply requirements requires specialist design, therefore a Smoke Control Association member should be engaged at an early design stage in order to assist the design team. The SCA website provides a list of suitable smoke control contractors with mechanical smoke ventilation and pressurisation design capabilities.


Ben Meek is design manager for Group SCS (members of the Smoke Control Association)


ADF FEBRUARY 2023


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