This article was written by Ben Meek for the Smoke Control Association and published in the March 2023 edition of the CIBSE Journal.
The British Standard code of practice BS 9991:2015 – Fire Safety in the design, management, and use of residential buildings is being updated. The new version will be published once all outstanding comments from the consultation period have been addressed.
Up until now, it has been common to build residential buildings above 18m with a single communal escape stairwell. The buildings adopt a ‘defend in place’ evacuation strategy (also known as ‘stay put’), meaning that each dwelling is constructed as a fireproof box.
This ‘defend in place’ procedure fails if the apartment fire compartmentation is breached, allowing the fire to spread between dwellings. Fires that affect multiple floor levels overwhelm active fire-protection systems, such as corridor smoke ventilation systems, because the system is designed to handle a single fire on one floor.
If a stairwell becomes smoke-logged and it is the only escape route, 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 18m.
At present, a typical block of flats has a naturally or mechanically ventilated corridor that directly accesses the communal escape stairwell. This means the fire zone (the dwelling) is separated from the stairwell by no more than two fire doors.
The inclusion of a pressurised stairwell lobby between the apartment and stairwell adds an additional degree of fire separation between the fire zone and the escape route.
Furthermore, by pressurising the stairwell and stair lobby, the fire service should be afforded tenable conditions within the stair lobby to use it as staging point/fall-back position. ‘Pressurisation’ works by using supply fans to pump fresh air into the protected areas to direct airflow away from them when doors are opened and thereby repel any smoke and hot gas.
the proposed changes to BS9991 provide clearer and further-reaching guidance on the provision of evacuation lifts. It also states that evacuation lifts should be located within protected lobbies, which should have direct access to a stairwell and be served by a smoke-control system.
The SCA website provides a list of suitable smoke-control contractors with mechanical smoke ventilation and pressurisation design capabilities. See www.smokecontrol.org.uk.