Hi kurnal,
Thanks for bringing this up. From the outset I've been disturbed by the anomaly created by interpolation below 1100mm (1050mm for ADB's already cocked up figures). Being a busy little bee though, the path I chose was to gloss over it until it became an issue in an actual building. Well, now you've brought it up, I'll give you my opinion. Another cock up.
There are so many of these in 9999 that one more shouldn't be a shock. In saying that, I must add that I like the principles in 9999 and I think that overall it is a commendable publication (a few errors are inevitable - it was written by humans after all).
What is frightening though is the increased potential that building designers now have to put people at risk.
As well as your A1 building, a nightclub is allowed 3.3mm per person if they have a 7.5m ceiling. That, I can understand a little more because the idea behind 9999 is to move away from the 2.5 minute evacuation time when it is clear that longer than that will be available (for example, because of the high ceiling). But your A1 building might only have a 3m ceiling height. I think the justification would lie in the slow fire growth rate but I agree with you that 6 minutes is a bit long to be waiting at the back of the queue to leave a room with a fire in it and only a 3m ceiling height.
A1 buildings are quite rare but A2 buildings aren't. An A2 building needs (to start, it can go lower!) 3.6mm per person which brings the number for a 800mm door to 222. Now A2 covers rooms like an office or a classroom, the sorts of rooms where we know we want the people out pretty smartish if there's a fire in the room.
A designer will argue for a further 15% reduction to 3.06mm if there's detection in the building. That gives our door a capacity of 261. A building designed to this standard and filled to the safe designated capacity would, in my opinion, not be safe. We could have a room with a typical office type fire loading, with two 800mm doors, a 3m high ceiling and 261 people in the room. Losing one door to the fire (medium growth rate) all 261 people have to leave through the remaining door. I think people who are, to put it in an old colleague's terms, "receiving unambiguous fire cues", will go through a 800mm door quicker than 40 per minute, but to all get out in 2.5 minutes they would have to go through at over 100 per minute. That won't happen.
I'll tell you what would happen, the pressure exerted by those at the back on those who are at or near the door would be sufficient to cause at least one person to trip and fall. After that, many would fall and after that you've lost the only available exit.
Proposals for nightclubs under 9999 often take the 15% reduction offered for beneficial alarm systems when they should not. The alarm provision is often of no benefit to those in the room of origin and so should be applied with reserve.
In general, I'd say the principles of 9999 are good but the guidelines that present those principles need to be read with a judicious eye. And, unfortunately, this frequently does not happen.
Stu
