THE CRUMB OF COMFORT IN A BLAZING BUILDING

Anyone unfortunate enough to find themselves on the top floor of a blazing 30-metre high office or apartment block is entitled to one crumb of comfort.

Well, if the new British Standard 9999 is anything to go by, that crumb of comfort may well be that their escape route is not tall enough to need a fan-assisted means of clearing the choking smoke.

The stairwell is perfectly capable of clearing naturally… apparently. And the same goes for the firefighting lobbies and corridors.

Now this is a murky and controversial area – and one that could play dangerous havoc with the lives of the occupants of such a building.

Controversial? Because smoke control engineers like Fire Engineering Associates in Trafford Park, Manchester, who I work closely with, can only operate within the guidelines laid out in various codes of practice.

There may be flexibility – but authorities will not usually reject schemes that are based on these guidelines. And that’s why we need the reassurance of knowing that all of them produce safe results.

At present the natural ventilation of smoke from the corridors and lobbies of high-rise buildings – as permitted in the new British Standard 9999 – does not appear to meet this requirement.

And that, arguably, has to turn the control of smoke in the corridors, lobbies and stairwells of high-rise, multi-room buildings into the most important modern problem for smoke control engineers.

Few of them argue in favour of pressurisation systems quite so fervently as Fire Engineering Associates. “Correctly designed and installed, it is easily the best form of protection,” says the Technical Director, Dave Ogden whose experience in 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.

NO ARGUMENT

There is no disagreement over whether or not smoke control is necessary. The only argument is over the best method of ensuring that, in the event of a fire, occupants can escape and firefighters can enter the building without being gassed to death by hot fire smoke.

BS 9999-2008 contains the latest recommendations in this now long-running saga. And although it makes tentative steps forward, it far from resolves 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 30 metres high and with basements more than 10 metres deep.

That is the good news.

But when it comes to smaller commercial and residential buildings 9999 is happy to continue allowing the less reliable natural ventilation as an alternative method.

And that is where the danger creeps in.

UNRELIABLE

Ventilation of the corridors and lobbies of high rise buildings, using natural vents and smoke shafts, relies on two forces. One is the buoyancy of the smoke. The other is the wind generated negative pressure at the outlet of the vent or smoke shaft.

Both these forces are variable, in any particular situation, and therefore unreliable.

Smoke buoyancy is a function of its temperature … and this can be reduced by several factors such as sprinkler cooling for example. And negative wind pressure can be very uncertain in built up areas. Putting aside the variability of wind speed and direction, and adjacent buildings, even parapet walls around the top of buildings can still produce positive pressure footprints at the vent outlets. And, in many respects, low buildings are more prone to this than their taller neighbours.

EXTERNAL WALL VENTS

Back in 1971 British Standard Code of Practice CP3 recommended a method of natural ventilation – and drew immediate criticism, especially from fire officers. The recommendations included automatic openings or vents of 1.5m2 free area on adjacent walls with the idea of allowing wind pressure to clear smoke from the corridor.

This system was shown to be unreliable, by some later research from the now Building Research Establishment (BRE), and was not adopted in the later BS5588 Part 1 1990.

Instead, a system of automatic opening vents and corridor smoke doors was recommended.

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 the results of some very comprehensive investigations, by the Building Research Establishment, into the relative performance of external wall vents and natural smoke shafts, were published in their report no. 79204 – 2002.

I quote from their summary:

”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”.

NATURAL SMOKE SHAFTS

In situations where external wall mounted ventilation is not possible, smoke is often ventilated via a natural ventilated smoke shaft. Here openable vents within both the lobbies and the stairwell, and sometimes the corridors, are used to vent the smoke into the smoke shaft.

BRE – Report no. 79204, investigated in some detail the performance of these smoke shafts, and concluded that, even in adverse wind condition, their performance would be at least as good as external wall vents.

The report stressed the importance of the opening at the top of the shaft being situated where it would not be affected by adverse wind conditions. Positive wind pressure can be much greater than the pressure head developed by the smoke.

In spite of all these doubts and uncertainties with 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. The area of a smoke shaft protecting a firefighting lobby is specified as 3m2 with openings top and bottom. This can and does create space problems.

POWERED VENTILATION

A simple solution to overcome the doubt and uncertainties with natural smoke ventilation is to use fans to exhaust the smoke positively. In addition, the area of the smoke shaft can be reduced to under 0.5m2 and the need for a bottom inlet eliminated.

Paddington Central – installed by Fire Engineering Associates – is possibly 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.

Over the past few years, concerned by the doubts over natural ventilation, fire safety engineers have been specifying, and regulatory authorities approving, powered ventilation. However, the powered ventilation option is not, as yet, recommended in BS9999.

It should be.

PRESSURISATION

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

On the other hand, a pressure differential system – commonly known as ‘pressurisation’ is the only system where the objective of the design is to keep smoke out of the escape routes altogether.

It employs John Klote’s two principals of smoke control as its active forces – 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, was published in 1978, updated in 1998 and effectively became the European Standard, EN12101 Part 6 in 2006. BS9999 Section 28 now specifies EN12101 Part 6 Category B 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 that designers understand 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 per cent standby fans. Early research into fan reliability retorted 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 insists.

Comparing this degree of caution with the doubt and weakness of the guidance for natural ventilation, one cannot help but detect a hint of untruthfulness.

To quote that BRE report no. 79204-2002, again, these alternative designs of smoke shaft are:

“… not a replacement for pressure differential systems. 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”.

And that, in my book at least, is game, set and match.

• Jim Wild – a fan engineering consultant specialising in fire smoke control systems – is an associate 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

References

1. BS9999, 2008
Code of practice for fire safety in the design, management and use of buildings.

2. BS – Code of Practice CP3 – Chapter IV
Precautions against fire. Part I, flats and maisonettes (in blocks over two storeys)

3. BS5588 Part I – 1990
British Standards Institution, fire precautions in the design, construction and use of buildings – code of practice for residential buildings.

4. Morgan HP, Marshall MR & Williams C
Smoke dispersal validation building research establishment, client report CR7/87.

5. BS5588 Part 4 – 1978
British Standard Institution fire precautions in the design of buildings, smoke control in protected escape routes using pressurisation.

6. EN12101 Part 6 – 2006
Specification for pressure differential systems.

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