Atrium definition

[GB]

I have a 5 storey development of self contained flats that contains an atrium within the common escape. No lobby protection therefore to the flats but there is 2 protected staircases at each end of the common escape. All the flats leading onto the common escape are to be sprinklered which will reduce the volume of any smoke entering the atria.  

We are exploring utilsing permanent opening along the top of the atria. I have been guided by a collegue that over a certain % of rood opening, the common escape will become open air rather than atria. I am unable to refernce this. I have checked ADB & 5588 part 7 with no success.

[kurnal]

Hi GB
If only life were so simple!
I am certain the only way forward is to use a  designed smoke control system to ensure that untenable conditions are not created in the atrium.
BS DD9999 does have some guidance on this and refers to the design and calculation procedures set out in BS7346-4, BRE report 368 and CIBSE guide E.

Were you thinking of a stay put policy  in the event of fire in this case? Would there be any potential for fire loading in the arium?

[GB]

Due to the domestic occupancy we expect a stay put policy. We are looking at a minimum of 300mm gap between the wall and effective roof canopy. Inlet air / louvre at the base is equal to the outlet area with a 66% factor allowed for utilising louvres.

If we make the gap 500mm or 1000mm or 1 mile high - when does it no longer become atria rather a weathershield or canopy?

[AM]

I have a 5 storey development of self contained flats that contains an atrium within the common escape. No lobby protection therefore to the flats but there is 2 protected staircases at each end of the common escape. All the flats leading onto the common escape are to be sprinklered which will reduce the volume of any smoke entering the atria.  

We are exploring utilsing permanent opening along the top of the atria. I have been guided by a collegue that over a certain % of rood opening, the common escape will become open air rather than atria. I am unable to refernce this. I have checked ADB & 5588 part 7 with no success.

I think it comes from the Bs5588 regarding shopping centres. If there is 50% or more open air then it can be classed as an outside space, and the justification used is that an outside space is an outside space, regardless of the building use.

[kurnal]

I am happy to be corrected at any time but I dont think this provision comes from 5588. I wonder if it may have been in the old CP3 chapter IV 1971 in respect of balcony access flats? May be worth a look. But I still think that the approach in BS DD9999 is the way to go.

I can see where you are coming from in so far as sprinklers, compartmentation, and a stay put policy may work together. The problem is in showing that should any stratification occur, where it may occur and whether it may pose a risk to anybody.  

Your proposal  would rely on the stack effect to maintain tenable conditions in the atrium, and all in all I suppose you need to show that the level of safety is at least the same as with the 7.5 m dead end ventilated corridors permitted under the ADB. Whilst as a plus you are suggesting sprinklers to reduce the risk, you will have many more flats on a number of levels  opening into a large space which on one hand may result in air being entrained increasing the volume of the smoke whilst on the other hand the smoke should not be very hot or bouyant.

[John Webb]

It should be remembered that the smoke calculations for atria in connection with shopping malls, offices and other commercial premises are based on a fire size limited by sprinklers on the almost 'worst case'  assumption that the sprinklers limit the spread but don't actually put the fire out. The smoke therefore retains bouyancy and natural venting systems (as opposed to powered extract) will work.
In the building which GB is asking about, the building's fire separation between flats will limit fire spread, so the fire size is going to be potentially less to start with. Domestic sprinklers have a good record of extinguishing fires within the room of origin, so the fire size and duration will be much smaller than the 'design fire' for venting for commercial premises. So the venting system will have a small amount of cool smoke to deal with. I have a gut feeling that the problem will be not so much 'is there enough venting in the atrium?' as 'will the smoke have enough bouyancy to get to the vents?'. How this might affect means of escape I could not predict at all.

[kurnal]

Hi again GB
Just to satisfy my own curiousity I have checked the old CP3 chapter 4 series  documents all the way back to 1958 and the earliest versions of BS5588 (1985) and cannot find any reference to treating ventilated areas as open air as you describe. If ever you do find out more please let us know.