Smoke clearance system for atria

[Colin Newman]

Hi Guys,

I'm hoping someone can give me some insight into the recommendations in BS 9999 on smoke clearance for atria.  It appears that the recommendations are lifted straight from BS 5588 Part 7, but I've no idea as to what the basis of the recommendations is. According to the Standard, smoke clearance may required to allow for the fire service to clear smoke from the building after the fire has been suppressed.

For atria less than 18m in height the recommendations are ...... "Natural exhaust vents should be provided in the atrium roof. The total area of vents should be not less than 10% of the maximum plan area of the atrium."

For atria of any height ......"A mechanical smoke ventilation system should be provided within the atrium to provide replacement air changes every hour based upon the total volume of the atrium, including the largest floor open to the atrium with an inlet at low level, as follows: a) four air changes per hour in sprinklered buildings where the atrium base has a controlled fire load; b) six air changes per hour in unsprinklered buildings."

Is it just me or do these recommendations appear quite onerous?   

On a recent project, an atrium with a height of 15m and a plan area of 200m² would require a smoke clearance vent of 20m².  Whilst I appreciate that the smoke after the fire is suppressed will have less buoyancy, a 10% vent area or 4-6 air changes per hour seems  particularly high.

Has anyone got any ideas where the recommendations come from? 

[Phoenix]

The parts of 9999 that came from 5588 part 7 have to be quite general in their approach because they offer catch-all solutions. 

If you think this approach is onerous then you are perfectly at liberty to employ alternative means to provide the required level of fire safety.  But you will almost certainly have to use fire engineering techniques to demonstrate that your alternatives are as good as the general catch-all. 

It may be that, for some reason in a particular building, the 9999 guidance is, indeed, onerous.  Well, that is the time to get the fire engineer in to justify the deviation from the standard.  Or, if you think you can justify it yourself, have a go.  You might even get help on this forum.

As for a source, have a look at Approved Document F and all the other publications it references to get a feel for ventilation requirements.

Stu

[CivvyFSO]

6 air changes per hour is not onerous, it is the same as is required in a basement car park to keep CO levels acceptable. 10 air changes are required in basements for clearance.

And it is also to assist firefighting;

From BS9999:2008

B.2.4 Fire-fighting
In order to assist the fire and rescue service in rescue, fire-fighting and
in clearance of smoke after the fire, additional smoke clearance
measures might be necessary.

From BS5588 Pt 7:

29 Facilities for firefighting
29.1 General
The following recommendations are made to assist the fire service during firefighting (see Clause 20).
29.2 Smoke clearance

The main place BS9999 is more onerous is in its definition of atria, bringing more places under the wing of the recommendations.

[Colin Newman]

Thanks for the replies.

Whilst I appreciate that a CFD study may demonstrate that the recommendations for smoke clearance are onerours or otherwise for a specific set of circumstances, I'm really trying to understand the basis for the recommendations.

As you say Civvy, 6 air changes per hour is the rate recommended for maintaining CO at acceptable levels in basement care parks, but where does that figure come from and how does it relate to clearing smoke from atria?  The rate of 10 air changes per hour to remove smoke from basements using a mechanical extract system is cited in AD B as an alternative to natural ventilation which details vents of clear cross-sectional area of 1/40th of the floor area they serve, this is a quarter the size of natural vent recommended for certain atria up to 18m in height.   Again my question is why?

BS 9999:2008 is a little contradictory in its discussion of smoke clearance.  In the definitions it details a smoke clearance system as "smoke control system designed to remove the products of combustion following a fire and used at the discretion of the fire and rescue service to assist fire-fighting operations" implying that the primary purpose is to remove smoke after the fire has been supressed.

In B2.4 it states "In order to assist the fire and rescue service in rescue, fire-fighting and in clearance of smoke after the fire, additional smoke clearance measures might be necessary." indicating that the smoke clearance system assists in fire-fighting.

In B10.1 it then states "The fire and rescue service might need to release smoke and heat from a building after the fire has been suppressed. Ventilation for this purpose is usually obtained by opening windows to provide cross ventilation and smoke clearance. In an atrium building, the spread of smoke to a number of storeys can make it more difficult to open windows on every storey affected by smoke. In such circumstances, a mechanical or natural ventilating system capable of clearing the smoke from the atrium and the affected floor area should be provided. indicating that the smoke clearance system is to be used once fire-fighting operations are complete.

I remain none the wiser and am hoping that someone can point me in the direction of the basis of the recommendations since I sincerely hope it wasn't a wetted finger in the air, albeit probably a committee of wetted fingers in the air!

[Phoenix]

Smoke clearance is exactly what it says.  It is to clear the smoke after the fire is out.  But, in large spaces, it can be very difficult, due to smoke logging, for fire fighters to find the fire, let alone put it out or conduct searches of the building.  So they might want to ventilate the space during the fire fighting operations using the smoke clearance facilities provided.

The figures, frankly, could be plucked out the air.  Equally as important as the area of ventilation are the wind conditions, yet the standard, wisely, makes no requirement on wind conditions at the time of the fire!  Like many figures in ADB and 9999 the figures have an arbitrary nature to them but they are trusted and re-used and up-held because they have been around a long time and because they have stood the test of time over that period.  They're not always going to be right but what would you recommend replacing them with?

As well as smoke clearance there is also smoke control and this is more commonly used during the fire to maintain safe means of escape, to protect the structure and, possibly, to assist fire fighting.  This is more complex and less arbitrary.

Stu

[CivvyFSO]

The rate of 10 air changes per hour to remove smoke from basements using a mechanical extract system is cited in AD B as an alternative to natural ventilation which details vents of clear cross-sectional area of 1/40th of the floor area they serve, this is a quarter the size of natural vent recommended for certain atria up to 18m in height.   Again my question is why?

Because in an atrium you will generally have a much larger volume of air to deal with for starters.

Take a basement with 100m² and a 3m ceiling height. 10 air changes per hour requires 3,000m³ shifting per hour.

Take a 100m² plan area atrium over 6 floors, each with a 3m ceiling height. 6 air changes requires, 10,800m³ shifting per hour.

[wee brian]

In answer to Colin's why;

Smoke clearance is a "nice to have" included in the standard because "consensus" is defined as "absence of sustained objection". Stuff gets into standards just cos people get bored of some old geezer blathering on about something so they put up with some daft advice that hopefully will be ignored by anybody with any sense.

The 10 air changes etc are throwbacks to old (mostly London) standards that are in turn based on the wet finger technique. These were, of course, highly trained fingers not just any old finger.

[Colin Newman]

Oh Brian so cynical and yet so accurate......... probably!