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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Safety => Topic started by: jayjay on November 07, 2007, 08:16:22 AM
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Final draft is now published copy available on the BAFSA site
See link
http://www.bafsa.org.uk/pdfs/publications/00000050.pdf
Not had a chance to look at it yet.
look forward to members comments.
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Date for full publication is apparently the 9th November, and should be downloadable from:
www.teachernet.gov.uk/fire
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Downloaded the final draft today and had a quick scan through.
Big improvement over the previous version but still a few bits need improvement - one example, open spatial planning (i.e. escape past a void) could do with further clarification as the illustration could be misread; travel C-D not to exceed 18m, but the remaining alternative directions of travel (both apparently within 4.5m) not described.
Will be intersting to see the 'final', 'ratified' publication!
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As I've said on the BB100 thread below, JayJay, the BAFSA version isn't quite complete and contains editorial notes.
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The up-to-date version is now up on the teachernet site
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Has anyone looked at or even better understood the Risk Assessment tool and CBA tool on the teachernet site. Why do we need to reproduce ADB in another document but change some of the figures presented in the original document? For example the occupancy factor for adining room is 0.9 and an assembly hall 0.45.
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And another thing,
Can anyone explain the rationale behind the redistribution of occupants on the fire floor (assuming one exit is unusaable) when calculating stair capacities? I thought the formula used in ADB took this into account; the text following each of the examples in ADB state that that the capacity will be adequate when a storey exit is discounted.
Is BB100 now saying that ADB is wrong?
Oh yes, whilst on the subject of ADB stair calcs - why do we have (n-1)? even discounting a storey exit, the standing capacity of the stair would not change. - Anyone shed any light here?
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The n in the stair formula is relating to the number of floors served. So the standing capacity of the stair will be related to the number of flights but the bottom storey doesnt have a flight so has no standing capacity?
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And another thing,
Can anyone explain the rationale behind the redistribution of occupants on the fire floor (assuming one exit is unusaable) when calculating stair capacities? I thought the formula used in ADB took this into account; the text following each of the examples in ADB state that that the capacity will be adequate when a storey exit is discounted.
Is BB100 now saying that ADB is wrong?
Oh yes, whilst on the subject of ADB stair calcs - why do we have (n-1)? even discounting a storey exit, the standing capacity of the stair would not change. - Anyone shed any light here?
Remember Animal Premises and Stables?
Probably came from the same office.
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Hmmmmm.. Might be right about the guidance coming from the same office!
Kurnal - are we saying that the 'floors served' are in fact the floor count of the building including the ground floor? I didn't think stairs 'served' a ground floor.
I am still confuddled over the calculation change introduced in BB100 though - does anyone know the background to this? Seems as if BB100 is saying that ADB stair sizing gives insufficient capacity based on the fact that one storey exit could be compromised; whereas ADB states that the formula and calculations will provide sufficient capacity without modifying the occupancy spread/split.
Does anyone have anything which documents the evolution of the stair sizing currently in ADB from the original 1950's PWBS?
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For example the occupancy factor for a dining room is 0.9 and an assembly hall 0.45.
I hate to state the obvious, but the people are smaller!!
Now, regarding the (n-1) in the stair calcs, the figures in the table start with an assumption that 40 people will fit in each storey of an 1100mm stair.
They also have a starting assumption that with only one floor served for an 1100mm stair a width of 5mm per person will be sufficient to clear the floor through that stair in 2 and a half minutes (the desired outcome).
They then build on this by assuming that for every 100mm the stair gets wider an extra 5 people will fit per storey.
These three assumptions are the heart of the table and the formula, but they are based on more fundamental assumptions, such as that the height from one storey to the next is 3m (or 10 feet originally). Just look up PWBS for the rest.
This is where the formula comes from:
If
w = width of stairs in metres
p = number of people served by the stair
n = number of floors served by the stair
p is going to be made up of three things:
1. the starting figure given by 5mm per person (e.g. 1100/5 = 220) for one floor served
2. an additional 40 persons for every additional floor served (i.e. add 40 per (n-1))
3. an additional 5 persons for every 100mm extra width on each of the additional floors.
Did you notice the (n-1) up there? It's (n-1) because you add on the additional figures for every floor greater than the original one you started with. For example, three floors served (n=3) you add on (n-1) two lots of 40 for an 1100mm stair.
Mathematically:
(remember w is in metres)
p = 1000w/5 + (n-1)40 + 5(n-1)(w-1.1)/0.1
(that's 5mm per person + 40 per additional floor + 5 per additional 0.1m per additional floor)
This, of course, transposes to:
p = 200w + (n-1)(40 + 50(w-1-1))
which in turn gives:
p = 200w + 50(n-1)(w-0.3) The formula in the book!
Stay awake at the back there!!
Anyway, thats what the n-1 is all about.
It's all very theoretical and hypothetical of course and you can pick it to pieces but it's what we've all been using for 50 years.
Pete, as for your search for the evolution of this since PWBS, good luck!!
Stu
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I hate to state the obvious, but the people are smaller!!
Some of them may be stu. But take a look at the young folk coming out of the everage high school. Bet you wontt be picking any fights!!
I carried out an analysis of a school a few years ago that had been built to ADB by a firm of housebuilders on a PFI scheme in conjunction with an approved inspector. Neither had heard of BB7 or the additional rules for schools that accompanied the ADB 2000 edition. Every lesson change the place just logjammed. Staircases and corridors did not have sufficient capacity. I concluded the reason for this was that every student was also carrying a bag so taking up additional space. Fixed it for fire with a couple of externals for fire (post opening, paid for by the approved inspector) and a one way system for lesson changes.
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That's the problem with designers. They just apply the minimum standards without thinking about day to day use. I suppose in a fire they would (mostly) leave their bags behind.
I had a similar issue with a TA centre. Designed like an office building - not very practical for blokes humping machine guns and mortars about - place got knocked to bits in no time.
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That's the problem with designers. They just apply the minimum standards without thinking about day to day use. I suppose in a fire they would (mostly) leave their bags behind.
They'd probably chuck them on the fire!
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... For example the occupancy factor for adining room is 0.9 and an assembly hall 0.45.
I presume that for an assembly hall, the pupils are standing in their ranks (well, that's what I used to do) and only take up 0.45 of a sq m each.
In a dining hall they are sitting at tables and therefore not so many pupils can be in that room, at least during the lunch break, and some bright spark probably thought they'd just double the number they'd first thought of to allow for the reduced area.
I haven't looked at this document, and wonder if it actually gives advice on the layout of chairs and tables (or seating units combining both) for dining rooms to allow for easy escape in an emergency.
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Under the old BB7, the occupant capacity for dining rooms and gymnasia was 0.9m2 and that for assembly halls was 0.45m2. Presumably they have used BB7 in producing the new guide and listened to those of us who generally lamented it's departure in favour of the 'Constructional Standards' - which were subsequently incorporated in ADB - and then removed.
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Stu,
Now I have the original formula rather than that currently in ADB (with no explanation) I understand where the figures come from and in fact they correspond quite closely to thos in the PWBS when you convert imp to met.
I would still like to know however why BB100 assumes that the stair calcs require adjustment for the discounting of a storey exit when ADB says the exact same formula provides an adequate width (accounting for the loss of a storey exit).
Apologies for sounding like a pain but I am involved in a number of projects where BC and LFRS have differing views on the subject and short of looking to DCLG for a determination I could do with a little background/advice here.
Cheers
Pete
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Pete,
The previous few versions of ADB allowed you more freedom in interpreting them in this respect and it was a logical step for everyone who applied tables 7 or 8 to take into account the loss of a storey exit when there was no requirement to lose a whole staircase (e.g. lobby approach). That tied the vertical assessment in nicely with the horizontal assessment.
But the calculations were always a bit fiddly and people often didn't get them right or forgot to discount a storey exit anyway.
People also argued that the discounting of a single storey exit throughout the whole building didn't have a lot of effect on the final outcome anyway. Ok, by not discounting a storey exit the stair widths would tend to be marginally smaller but it was felt that they would often default up to the same result anyway.
Also, it was recognised that tables 7 and 8 are based on a lot of assumptions that aren't always applicable and yet time has demonstrated that stair widths derived from these tables usually provide satisfactory means of escape. Therefore, the conclusion was that the tables probably err far on the side of safety anyway so there was some leeway to be had.
And how could that leeway be had? Well, lose the fiddly method for discounting a storey exit when no staircase need be lost. In other words, lose a storey exit for assessing exit widths for horizontal means of escape but don't bother to lose the storey exit for sizing the staircases.
My pal Andy Kelly enquired a few times of the authors of ADB how we should reconcile the discrepancy but, to my knowledge, the reply never came. I guess it languished a while on the too difficult pile then one day quietly slipped into the bin.
So staircase calcs are much easier now than they used to be. Except when it comes to schools, it appears. I didn't know that BB100 had resurrected the lose a storey exit method - I bet they didn't mean to!
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Well, just had a look at it and there it is. Put in, apparently, quite deliberately. So, use that method for schools - it errs on the side of safety (which is what we want for the little darlings, isn't it?) - and use the simpler ADB method for all other buildings.....I guess.
The worked example in BB100, if done the ADB method, would have a figure of 315 per stair instead of the 350 and the result would be:
w = (315 + (15x3) - 15)/(150 +(50 x 3)) = 1.15 m a bit smaller than the BB100 figure of 1.267 m
In the circumstances you find yourself in, I wouldn't be surprised if FRS people are still using the old lose-a-SE method while BCOs have more quickly moved on to accept that you no longer have to.
If you compare the methods on some very short buildings, by the way, you can get some very large differences between the results.
Incidentally, BS 5588 part 11 has always had worked examples that do not discount a storey exit for assessing staircase widths and that has always been out of line with the horizontal means of escape. ADB does have a strong tendency to follow the 5588s.
Stu
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Pete could it be when calculating the capacity of a staircase you ignore the ground floor (n-1) as it is only the upper floors who are going to use it?
Another way to look at it is because the staircase is discharging at the base then more people could enter the staircase increasing the numbers. Take a 1000mm width staircase with four storeys, capacity 40 person per storey n-1 = 120. This could be achieved in one minute you still have one and a half minute left which will allow another 60 people to use that staircase = 180, ignore the n-1 and you have a small safety factor?
One group considers the staircase a static component the other a dynamic one?