SAFETY & SECURITY 101
Better safety by design T
he 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
ADF FEBRUARY 2023
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
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Ben Meek of the Smoke Control Association (SCA) looks at significant upcoming changes to building codes with respect to design for smoke ventilation
The National Fire Chiefs Council released a statement in 2022 noting that designers often misinterpret the definition of ‘defend in place’
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