THIS IS POLITICS … NOT FIRE SAFETY

Fire safety regulations in the UK are not acts of parliament.

Although we have building regulations and BS codes of practice they are not legally binding. They are flexible and building control officers have the ability to vary them. But they rarely do. Few have the confidence to challenge something written into a British Standard.

And that is fine – so long we know that the guidelines are correct, they are safe and we can rely on them.

If they aren’t … then they shouldn’t be there. But, I’m afraid, they are. Because this is politics, not engineering.

Not surprisingly, therefore, buildings erected on flawed guidelines could be lethal. Such as those high-risers – permitted by the new British Standard 9999 – where, in the event of fire, natural ventilation is allowed to (hopefully) clear smoke from corridors, lobbies and stairwells.

And that alone turns this new set of guidelines into the most contentious modern problem in smoke control.

CONTROVERSIAL

For, according to BS 9999 a blazing 30-metre high office or apartment block is not tall enough to warrant a fan-assisted means of clearing the choking smoke. The stairwell, along with firefighting lobbies and corridors, is apparently perfectly capable of clearing naturally.

Now this murky, controversial area could play dangerous havoc with the lives of the occupants in such a building. Because, on the one hand, building controllers have a right to believe in a British Standard that isn’t going to mislead them. And, on the other, smoke control engineers like Fire Engineering Associates in Trafford Park, Manchester, who I work closely with, can only operate within the same level of a controller’s faith.

PRESSURISATION

Even if those engineers strongly believe that pressurisation systems should be more widely adopted. My colleague Dave Ogden, Technical Director of Fire Engineering Associates argues: “Correctly designed and installed, it is easily the best form of protection.” And his experience with pressurisation systems puts him in a narrow band of smoke control engineers whose contemporaries, more typically, have natural ventilation backgrounds.

Dave has been behind more than 100 FEA pressurisation systems 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.

Although BS 9999 makes tentative steps forward, it far from resolves the dispute over the best method of occupants escaping a blazing building, and firefighters entering it, without being choked to death by hot fire smoke.

Dealing only, at this stage, with fire fighting shafts, section 28 recommends a properly designed and installed pressure differential system in buildings more than 30 metres high and with basements more than 10 metres deep. That’s the good news.

But when it comes to smaller commercial and residential buildings, below 30 metres with basements up to 10 metres deep, 9999 is happy to continue allowing the less reliable natural ventilation as an alternative. And that is where the danger creeps in.

NATURAL VENTILATION

Ventilation, using natural vents and smoke shafts, relies on the buoyancy of smoke and wind generated negative pressure at the outlet. But both these forces are variable and unreliable.

The temperature, and therefore buoyancy, of smoke can be reduced by several factors, such as sprinkler cooling, and negative wind pressure can be very uncertain in built up areas. Adjacent buildings, even parapet walls around the top, can still produce positive pressure footprints at the vent outlets. In many respects, low buildings are more prone to this than their taller neighbours.

When the British Standard Code of Practice CP3 recommended a method of natural ventilation back in 1971 it drew immediate criticism, especially from fire officers. The system was shown to be unreliable and was not adopted in the later BS 5588 Part 1 1990. Instead there was a favoured system of automatic opening vents and corridor smoke doors.

As Dr Howard Morgan pointed out in his article ‘Smoke Clearance’, published in the May 2005 edition of ‘Fire Prevention – Fire Engineering Journal’, this combination has always been difficult to explain in terms of ventilation. And some very comprehensive investigations, by the Building Research Establishment, into the relative performance of external wall vents and natural smoke shafts, found that:

”The condition where external wall ventilation works well, is when the wall ventilators are facing windward, and the wind speed is sufficient to effectively PRESSURISE the fire fighting lobby”.

The debate, and succession of recommendations, has followed one after the other for nearly 40 years. Yet, in spite of all these doubts and uncertainties over natural smoke venting systems BS 9999 recommends their continued use in buildings up to 30 metres high and with the basements no more than 10 metres deep.

FAN POWERED VENTILATION

The area of a natural smoke shaft protecting a firefighting lobby is specified as 3m2 with openings top and bottom. This can and does create space problems. But fans could be used to exhaust the smoke positively, the smoke shaft area reduced to under 0.5m2 and the need for a bottom inlet eliminated.

The Fire Engineering Associates installation at Paddington Central was one of the first examples of fan powered smoke ventilation in the UK. The fan exhaust rate was calculated to provide the same pressure and velocity criteria as would be required in a pressurisation system.

In recent years, concerned by the doubts over natural ventilation, fire safety engineers have been specifying powered ventilation and regulatory authorities have been approving them. However, the powered ventilation option is sadly not, as yet, recommended in BS 9999.

Although natural, and even powered, ventilation allows smoke to enters the escape/entry routes before leaving the building, pressurisation is the only system where the objective of the design is to keep smoke out of the escape routes altogether.

It employs air velocity and pressure differentials, generated by supplying a sufficient volume of ambient air into the escape routes, to produce the specified velocity or pressure.

Design failures in early systems gave pressurisation a bad name but, properly designed and installed, Dave Ogden and I agree that it’s easily the best form of protection.

RELAXED OR CAUTIOUS?

It’s easy to be cynical about BS 9999 – especially when comparing its relaxed position on what I would call dangerous natural ventilation with the very cautious EN12101 Part 6 which insists, for example, on 100 per cent standby fans. According to research the risk of a fan failing at the same time as a fire starting could be as low as 3.7 x 10-8 … so how really necessary are standby fans? Yet EN12101 Part 6 still stubbornly insists.

Those two examples are not the most comfortable or honest bedfellows!

I prefer to stick with the Building Research Establishment’s 2002 report that said alternative designs of smoke shaft were no replacement for pressurisation.

“Pressure differential systems have specific advantages in providing a higher standard of protection in specific buildings, particularly those operating a means of escape strategy based on phased evacuation. They can also provide a greater level of protection to the fire-fighting lobby itself than any of the natural ventilation systems discussed herein”.

That’s good enough for me.

• After originally training in the textile industry Jim Wild – C. Eng F.I Mech. E – spent most of his adult working life with Fläkt Woods, latterly as product manager for the development and application of fans in the fire safety industry. He retired in 2000 and is now an associate fan engineering consultant with Manchester-based smoke control specialists Fire Engineering Associates of The School House, Second Avenue, Trafford Park, Manchester M17 1DZ; Tel: 0161 8727760. www.smoke-control.co.uk

ENDS

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