Firefighting Lobby Vent to Below Ground Car Park

[Jon Barrett]

Is anyone able to confirm whether the following arrangment has been adopted on any projects they have been involved with?...

The firefighting lobbies, serving the basement levels (2no.) of a car park beneath a shopping centre, are NOT provided with ventilation. This is on the basis that the car park is under negative pressure (from the extract system) so any air movement will be from the firefighting shaft to the car park, rather than smoke passing into the shaft in any large quantities.

Any information would be appreciated.

Thanks,

Jon

[nearlythere]

Must be a hell of an extractor system to keep an underground car park with huge holes in the side to get cars in and out at negative pressure.
Have I picked you up wrong?

[Jon Barrett]

It is a basement car park so is essentially sealed apart from the main car entrance which is arrived at via a covered ramp leading down from street level.

The car park will be under negative pressure by virtue of the fact that air is being removed from the car park at ~30m3/s - the degree of negative pressure has not been determined.

[kurnal]

There is some merit in the idea as the key requirement is that the lobby should be at a negative pressure compared to the firefighing stair. But how can they prove this will be the case in a fire situation without modelling,  and is it right to rely on the car park ventilation system to maintain safe conditions in the fire fighting shaft? My gut tells me probably not, but then it may just be indigestion. I havn't looked it up in 5588 part 5.

[nemodadog]

Yes, on a number of projects that i ahev worked on, the natural vent to lobby is dispensed with on the basis that you have described....mostly within london.

[slubberdegullion]

It is the same principle as dispensing with the lobby on a pressurised staircase - acceptable if done right.  

You should have details of the depressurisation system and it should be demonstrated that the system will overcome local overpressures created by the fire - so if there is a car on fire right next to an exit door smoke will not enter the stairs.  

Wind has a significant effect on horizontal smoke movement within a building - may sound daft for a basement but the staircases do extend up to points where they are exposed to wind - and the system should also be capable of overcoming negative pressures that can occur in staircases due to wind induced venturi effects.  

Generally, if they can demonstrate a depressurisation level of 50 Pa and that with a door from the staircase to the car park open that there will be an airflow towards the carpark through that doorway of around 0.75m/s then there would be little doubt that air will only flow from the staircase to the car park and not vice versa.  And then the lobbies would be unnecessary.

It sounds as though this hasn't been done.  

With depressurisation the fans and ductwork have to have high fire ratings and for sustained periods - ordinary environmental fans with no fire rating are no good.

If impulse fans are used the movement of smoke within the car park should be considered in relation to means of escape

Etc...

Stu

[kurnal]

But if the rating for the fan is in accordance with ADB- 300 degrees for one hour what happens if the fire is hotter or longer duration?  ( I know we are talking test times not the real world ) Who is going to control the fire- the firefighting crews. Without the lobby the fire main outlet will be in the stair. Hose will be passing through the door wedging it partially open. Wheres the safe bridgehead?

[slubberdegullion]

300 degrees is no good.  What if the fire is very close to the fan or duct inlet - we could be looking at 1000 degrees.  Depressurisation can require very specialist, expensive equipment.  And one hour would be no good for fire fighting shafts - two hours required.

The requirement for airflow through the open door you talk about, kurnal, would be 2 m/s for a fire fighting shaft.  This is what is deemed to be necessary to ensure a safe bridgehead within the stairs.

If it's done right, it's better than a ventilated lobby.  It is very probable, though, that the case we are talking about will not be done right.

If they want to do away with the ventilated lobbies, they should pressurise the stairs to BS EN 12101 - 6 really.  They shouldn't rely on some ad hoc, probably environmental, extraction system without the backup of quantitative analysis.

Quick calculation -

assume main vehicular entrance of about 20 sq m - add the open fire fighting door - say 1.6 sq m - add a hefty margin for unanticipated inlet routes - lets be very generous and keep it down to only a few sq m - say 30 sq m overall total inlet area

extraction at 30 cu m/s

30 cu m/s divided by 30 sq m = 1 m/s inlet velocity - far short of the required 2 m/s.

But that's all academic because the fans probably aren't correctly rated for depressurisation anyway!

Stu

[kurnal]

We have probably gone off at a bit of a tangent on this one as re-reading the original post its just the vent thats being dispensed with not the lobby.

[Chris Houston]

ADB- 300 degrees for one hour

That would be the coldest fire I've heard of.

[wee brian]

Yeah - Try and find a fan that works at 1000 degrees then - what would you make from? - ceramic fan blades arent readily available.

The smoke cools as it mixes and moves away from the fire.

[Chris Houston]

Yeah - Try and find a fan that works at 1000 degrees then - what would you make from?

I don't know, fan manufacturing isn't my area of expertise.

[kurnal]

Fans are rated at 300 or 600 degrees - thats normal. The point I was making is that  probably the building involved would have a one hour rated 300 degree fan because that is the recommendation for ventilation of basements in  ADB. If this was the case I would feel uneasy about relaxing ventilation for the fire fighting lobby and relying on such a low spec fan keeping workig for the duration that firefighting is required.
Car parks are historically treated as areas of fairly low risk but recent serious fires- eg the one in Avon have prompted a review of this.

[slubberdegullion]

I would feel uneasy along with you, kurnal.  

This suggestion to dispense with the recommended ventilation is typical of so many proposals today.  

Designers so often push at the boundaries of acceptability, supported only by unverified, untested and unimaginative ideas that, when analysed fully, do not compensate for their deviation from the accepted norm.

Stu