A WHIFF OF DISHONESTY

Arguably, the most important problem areas for smoke control engineers are the corridors, lobbies and stairwells of high-rise, multi-room buildings.

And this issue is also the most controversial.

Not because there is any disagreement over whether smoke control is necessary in such buildings, but it’s the method of ensuring that in the event of a fire, occupants can escape and firefighters can enter without either being gassed to death by hot fire smoke.

BS 9999 (2008) contains the latest recommendation in this saga, and makes tentative steps forward. But it does not resolve the debate.

Dealing only at this stage with fire fighting shafts, section 28 recommends a properly designed and installed pressure differential system in buildings more than 30m high and basements more than 10m deep.

But it continues to allow less reliable natural ventilation as an alternative method in smaller commercial and residential buildings. And that’s where the danger lies.

Few smoke control specialists believe as passionately in pressurisation systems as Fire Engineering Associates, in Manchester, who I work closely with. FEA has installed more than 100 over the last decade, often involving several integrated stairwells. These have included Ontario Tower, in London’s Dockland, which has two pressurised systems – a fire fighting shaft covering 30 floors and an escape stair of 21 stories.

VENTILATION

On the other hand, ventilation of corridors and lobbies in high-rise buildings, using natural vents and smoke shafts, relies on two forces – the stack effect and wind-generated negative pressure at the vent or shaft outlet. Both these are variable, in any particular situation, and therefore unreliable.

Smoke buoyancy, a major component of stack effect, is a function of its temperature and can be reduced by several factors, such as sprinkler cooling. And negative wind pressure can be very uncertain in built up areas. Even adjacent buildings, and parapet walls around the top, can produce positive pressure footprints at a vent outlet. And in some way low buildings are more prone to this than their taller neighbours.

NATURAL

In 1971 British Standard Code of Practice CP3 recommended a method of natural ventilation – and drew immediate criticism, especially from fire officers. It recommended automatic openings or 1.5m2 vents to allow wind pressure to clear smoke from a corridor.

This system was shown to be unreliable – according to later research by the now Building Research Establishment – and it was not adopted in the later BS 5588 Part 1 (1990).

Instead, a system of corridor smoke doors and automatically opening vents was recommended – it’s difficult to explain how this works in ventilation.

But in spite of all the doubts and dangers surrounding natural venting BS 9999 recommends their continued use in buildings up to 30m high, and for basements no more than 10m deep.

This does not reflect continuing concerns and doubts about natural ventilation during a period when fire safety engineers have been specifying, and regulatory authorities approving, powered ventilation.

But the powered ventilation option is not, as yet, recommended in BS9999. And it should be.

PRESSURISATION

The common weakness of natural and powered ventilation is that smoke often enters the escape/entry routes before leaving the building.

A pressure differential system, on the other hand, is the only system where the design objective is to keep smoke out of the escape routes.

It employs John Klote’s two principals of smoke control – air velocity and pressure differential. The design involves supplying a sufficient volume (m3/sec) of ambient air into the escape routes to produce the specified velocity or pressure.

Smoke control by pressurisation is not a new idea. The first UK Code of Practice – BS 5588 Part 4, published in 1978 and updated in 1998, effectively became the European standard (EN12101 Part 6). It is this that BS 9999 Section 28 now specifies for the protection of fire fighting shafts.

Based on this standard, the design of the air systems is pure fan application engineering – hence the importance of designers understanding the principals of fan engineering.

Design failures in early systems gave pressurisation a bad name but, properly designed and installed, it is easily the best form of protection.

There is no doubt that EN12101 Part 6 is very cautious and can produce an over-engineered system. It insists, for example, on 100% standby fans. Early research into fan reliability reported that the risk of a fan failing at the same time as a fire starting could be as low as 3.7 x 10-8, and standby fans were unnecessary. Yet EN12101 Part 6 still insists.

Comparing this degree of caution with the doubt and weakness surrounding natural ventilation, one cannot help but detect a whiff of dishonesty.

Fire Engineering Associates and other smoke control engineers can only work within the guidelines laid out in these codes of practice. Some flexibility exists, but authorities will not usually reject schemes based on these guidelines.

We need to be reassured that all these guidelines produce safe results. But at present the natural ventilation of smoke from the corridors and lobbies, of high-rise buildings, does not appear to meet this requirement.

• JIM WILD – a fan engineering consultant specialising in fire smoke control systems – is an associate with Manchester-based smoke control specialists Fire Engineering Associates.

ENDS

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