Fire safety: smoke control in high-rise residential buildings
19 December 2014
Selecting the most suitable smoke control system for high-rise residential buildings calls for a degree of common sense, says Paul Compton
Why is smoke control needed and what do we want to achieve? Sadly, the answer too often is 'building control sign off with the minimum cost and effort'.
In fact, in high-rise residential buildings the primary objective is to protect stairs against smoke ingress. The key reason for this is because stairs need to be available for escape for a long period where a 'stay-put' policy applies and because they need to be available for firefighters.
Before 2007, the approach was to ventilate dead-ends rather than spaces opening onto stairs (often the same spaces, but not always). The stair ventilator(s) was expected to protect the stair from excess smoke.
Very few people willingly travel more than 4 or 5 storeys by stairs when there is a lift available, especially as stairs are often inhospitable spaces...
Now, the approach taken is to provide ventilation to the stair and to each lobby or corridor opening onto the stair. Smoke control in other corridors is usually provided by doors to limit smoke spread and no smoke control is provided in the apartments themselves.
In the design thinking, it is assumed that there is only one fire, contained within one apartment and that the fire apartment door remains closed except when opened intentionally. While there are rare occasions when these assumptions are not valid, it allows designs that are practical and cost effective.
What is special about a high-rise building? As a starting point, occupants are less likely to be familiar with using the stairs. Very few people willingly travel more than 4 or 5 storeys by stairs when there is a lift available, especially as stairs are often inhospitable spaces, designed for escape rather than circulation. In addition, people are likely to be more reluctant to enter a smoke-filled stair knowing there are 20 storeys to descend.
Building Regulations allow smoke ventilation by automatic opening vents (AOV), natural shafts, mechanical shafts and pressure differential systems. Except for a few special cases, all are treated as equivalent in that they are deemed to satisfy.
BS 9991 does differentiate for tall buildings by requiring a pressure differential system to EN 12101-6 for firefighting stairs in buildings over 30m. This is usually taken to mean a stair-pressurisation system but a depressurisation system would also comply. Some suppliers of mechanical shaft systems might refer to depressurisation to EN 12101-6 but when the standard was written in 1997-98, such systems did not exist and the concept of depressurising referred to the space containing the fire, not an intervening space to the stair.
To some extent, the ventilation system for a tall building is self-selecting:
- stairs and lobbies are usually landlocked so AOV is not suitable, a fortunate obstacle because their performance under wind conditions is very variable
- space is at a premium so space for a natural shaft system can be hard to find.
The premium on space also leads to minimising the number of stairs with extended dead-end travel distances. To compensate, enhanced mechanical shaft systems are usually offered because pressurisation would require air release from every apartment which, apart from being costly, would lead to tough future maintenance problems.
The difficulty that building control bodies face is that the system design and shaft locations are critical and are usually evaluated by computational fluid dynamics. Not only does this call for an expert to generate the design, but also an expert to properly review and approve it.
Designers, therefore, can face a problem if the building is generally designed to BS 9991 because the standard recommends a smoke-control system that is not suitable for extended corridor layouts. Common sense dictates that the most suitable system should be selected.
Whatever system is finally chosen, it is critical that it works well and continues to do so over the years, even as legislation and standards change. For example, while no-one would suggest that all AOV installed to the pre-2006 editions of AD B need to be changed, some will no doubt be too small to the current definition in diagram C7 and some will be in what is now considered to be the wrong location. Maintenance to keep them operating correctly and a sensible risk assessment could allow these 'out of date' AOV to provide suitable protection for many years to come.
Of course, not all systems were perfect when installed. Some issues regularly recur, particularly on older mechanical shaft systems installed when they first became popular around 2003.
Figure 1: Typical roof mounted smoke-shaft extract plant with run and standby fans
Trapped fingers can be an issue because regular tests and false alarm resets are usually carried out remotely. Unless guarding, anti-trapping software is provided on motors or a local manual reset switch, the management system should ensure that vents are overseen during reset.
Lobby vents need to be 30-minute fire-resisting and must fail to their current position and not to the open position to prevent smoke and fire spread between floors and to allow mechanical extract only from 1 storey rather than dissipated performance over several storeys. Electromagnet control does not meet this requirement.
There is nothing wrong with mechanical shaft systems pulling the stair door ajar against a properly set-up adjustable door closer to provide a source of replacement air. But are older systems still properly set up, have any door closers failed and been changed for whatever is easily available and who is responsible for their maintenance?
Running other services through a smoke shaft was only explicitly banned in 2011 by BS 9991. While it has never been good practice, it has always been tempting to use the shaft as an overspill services riser. But what happens if the services collapse under the heat in the riser and partially block it?
Older pressurisation systems also have their issues:
- the building leakage rate is critical to performance and this can change over time. New services openings may not be properly sealed, lobbies may be altered or finishes may be updated
- pressurisation tests are needed regularly to ensure adequate pressurisation is still being achieved
- building control bodies have been known to demand pressurisation right up to the apartment door and there are such systems in use. So accommodation air release, allowing the door open velocities to be achieved, has to be provided from each apartment. How do you control the condition and maintenance of important fire-safety equipment located in private apartments?
In AD B-compliant buildings, the regulations and standards are silent on some architectural decisions that can have a major effect on the system performance. Particularly with natural systems, it is critical that smoke is encouraged to leave the lobby or corridor via the AOV or shaft, not the stairs.
Applying common sense can help here:
- full-height stair doors have no down stand above them to stop a ceiling jet in its tracks so are better avoided
- putting an apartment door opposite a stair door means that the smoke flow is directed straight at the stair door. An offset is good practice
- ideally the location of the extract should pull smoke away from the stair door, not towards it
- the top of ventilators, particularly in natural systems, should be as high as possible, not just level with the top of the door, as the requirement in AD B is often interpreted.
Smoke control is particularly important in high-rise to protect the escape stair. As such it should not just be a final bolt-on solution but considered early, while changes to plans can be accommodated. But just following the letter of AD B and BS 9991 is not necessarily enough. The responsible person needs to know what is installed, how it operates and where it deviates from current good practice.
Paul Compton is Technical Director at Colt International
- Smoke Control Association, Guidance on smoke control to common escape routes in apartment buildings (flats and maisonettes)
- Related competencies include Fire safety
- Image © Colt International
- This feature is taken from the RICS Building control journal (November/December 2014)