Ben Meek of the Smoke Control Association (SCA) looks at significant upcoming changes to building codes with respect to design for smoke ventilation
The British Standard code of practice BS 9991:2015 – ‘Fire Safety in the design, management and use of residential buildings’ is to be updated, with a new BS 9991:2021 version of the standard in the works. BS 9991:2021 is currently available in ‘draft’ format while comments from the public consultation are reviewed.
Prior to BS 9991:2021, it was common to build tall residential buildings above 18 metres with a single communal escape stairwell. The buildings adopt a ‘defend in place’ evacuation strategy (also known as ‘stay put’), based on the premise that each dwelling is constructed as a ‘fireproof box.’
In a ‘stay put’ building, the occupants of the fire affected apartment are expected to evacuate, while the remaining building occupants should be safe to remain inside their apartments while the fire service tackles the blaze. The National Fire Chiefs Council released a statement in December 2022 noting that designers often misinterpret the definition of ‘defend in place’ by assuming that only the fire affected occupants escape, whereas recent data suggests this is not always the case.
The ‘defend in place’ procedure fails if the apartment fire compartmentation is breached, allowing fire to spread between dwellings. Fires affecting multiple floor levels overwhelm active fire protection systems such as corridor smoke ventilation systems because these systems are designed to handle a single fire on one floor.
If a stairwell becomes smoke logged and it is the only escape route, then the occupants of the higher floor levels find themselves trapped inside the building. In light of this, BS 9991:2022 provides stricter guidance for the use of single stair residential buildings above 18 metres.
Under BS 9991:2022, tall single stair residential buildings above 18 metres are allowed only if certain conditions are met:
- The escape stairwell is separated from the apartment corridor by a dedicated lobby which is not accessed by any dwellings or ancillary accommodation but may contain the lifts. (The stairwell and stair lobby should be pressurised in accordance with BS EN 12101-6:2005);
- The structural elements must be 90 minutes fire rated for buildings up to 30 metres, and 120min above 30 metres
- All load bearing elements must be Class A1
- Stairs must be at least 1.2 metres wide
- The building must have a BS 8629: 2019-compliant alert system
- The single staircase should terminate at ground floor level and should not directly access any stairs to floors below – unless certain provisions listed in Clause 13 of BS 9991:2022 are met.
Stairwell lobby
At present, a typical apartment building has a naturally or mechanically ventilated corridor which directly accesses the communal escape stairwell. This means that the dwelling is separated from the stairwell by no more than two fire doors.
Under these rules, a poorly designed building could have an apartment entrance door directly opposite the stair door, with the two doors only a corridor’s width apart. It’s not difficult to see the challenge this creates for the smoke clearance system to maintain a smoke free stairwell when both doors are open for occupant escape or firefighters.
Including a pressurised stairwell lobby between the apartment and stairwell adds an additional degree of fire separation between the fire zone (dwelling) and the 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 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)