Natural Smoke Vent Design

[David Rooney]

Looking at the FETA guide in a single staircase building it suggests automatic detection in the "access space providing access to the flats" - basically I take this to mean the corridor.

The lobby on each level is simply that ... a lobby and home to a smoke shaft. There are then two corridors off opposing sides of the lobby each entered via a fire door that then leads to the flats.

The corridors are no more the 4m long.

We have been given a design where detectors are installed on the staircase - and the lobby but not in the corridors to the flats (separated from the lobby via a fire door).

So smoke in the corridors won't be detected, neither would it be ventilated - and this doesn't seem to meet the FETA guide?

Also, there is no source vent for fresh air on any floor.

So a detector on level 3 will open the stair vent and shaft vent and the vent to the smoke shaft on that level but will not open any kind of fresh air inlet.... unless you count the stairwell vent but that needs someone (Fire Brigade presumably) to prop open the door from the stair to the lobby?

Is my understanding correct?? 

 

[Jim Scott]

For a standard approach, the lobby and stair would only need venting.  The purpose of the ventilation is to keep the stair clear, not the corridor.

Unfortunately, to be code compliant there would be no need for any make up air in the corridor (unless we go down the road of extended travel distances etc etc), as the stair would provide this to the lobby, being that it is the stair we want to keep clear.  Where a standard BRE shaft approach is used, there is never sufficient replacement air for the system to be effective, for any kind of quick clearance.  However, it appears to be sufficient for keeping the stair clear according to BRE.

I would question the positioning of the smoke detector.  Although I can maybe see some argument to it, the sooner we can get the vents open, the better.

[kurnal]

Good answer Jim.

[David Rooney]

Top man .... thank you, that all makes sense.

But question the position of what detectors - the ones in that aren't in the corridors to the flats?

[kurnal]

Dave the design has been complicated a little by having two 4m corridors serving the flats on each floor. For small single stair buildings ADB recommends that where flats open into a lobby and the maximum travel distance from the furthest flat to the door to the staircase is 4m then the lobby need not be ventilated. In your case they may have  applied an interpretation of the 4m rule applicable to small buildings  in duplicate or  may have implemented the solution for larger buildings with the lobby adjacent to the staircase ventilated. I think probably the latter. There are a number of helpful diagrams in ADB volume 2 and you will see that the solution fits between the provisions for small and larger buildings.

The key to remember is that as Jim says the purpose of the ventilation system is purely to maintain tenable conditions in the staircase for firefighters to access the floor of the fire and set up a an operational bridgehead in clean air.  And then to review the need for evacuation of residents in the clean air. The vent in the lobby is intended to release any smoke and combustion gases that may be driven in by the positive pressure produced by a fire, as a fire  is only likely to be in a flat you might argue that such positive pressure is likely to be contained in the corridor and no help to occupiers - but the whole provision is limited to the  protection of the staircase. The staircase provides inlet air for the vent in the lobby so the vents must open at the same time. Indeed many developers do not install detectors in the staircase and this too can be debatable - but remember is not an alarm system, it is a ventilation system without sounders.

So in summary the ventilation system will perform its limited purpose of protecting the stair without detectors in the flat corridors.

[colin todd]

Mickey, Not being flippant, but for complicated situations like this, where people cant know all the facts, you should not base a critical life safety issue on chatting down the pub.  You should seek advice from a competent person, if necessary by referring back to the specifier.

[David Rooney]

Aye Mr T .....  that is exactly what we have done ..... we have put all the questions back to the consultants involved and asked them to confirm the strategy for our own peace of mind.

But I'm just always interested to hear the logic behind the development of these systems and listening and learning from the fountain of knowledge that exists here... it's all an eye opener for a lowly fire alarm designer. 

It then makes me wonder that although we do our best to show a bit of diligence and question things that don't always appear to make sense on the face of it and actually buy the applicable standards relating to situations like this to help make sense of it, how many firms out there don't question and just "get on with it"?

Thanks Mr K .... I like it when it's put in words of one syllable !! 

Hope all is well ............

[colin todd]

Mickey, I think you need our excellent in-house courses, such as the one on BS 7273-4, which explain the thinking behind the standards.

[David Rooney]

It could be an age thing but I did your most excellent half day training on 7273-4 a couple of months ago but I think this bit about secured doors escaped me ! 

[colin todd]

Oh yes, so you did!  There are 3 possibilities.  You need the refresher course already, you need to re-read the course notes, or you need to ask for your money back cos something was missed out!!

[David Rooney]

But it was a lovely day and I did learn the extraordinary origin of the Knock down Cautionary sign if nothing else !!

[colin todd]

Shhhhhhhh!