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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Safety => Topic started by: Underground on October 24, 2007, 04:10:28 PM
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I was looking at the ways of calculating exit widths and occupancy factors - and to no surprise became quite confused. That said, one thing that I would like to focus on is where is it written down as to what a unit of exit width is.
I can calculate units of exit widths, (I came up with 7), but do not know how big an exit width is?
In searching this forum I found the following:-
"The generally accepted rate is 40 persons per minute per unit exit width.
The width of a unit corresponded to the average shoulder width and was determined at 525mm. Two units of 525 (i.e. 1050mm) were required for 2 people to travel through, shoulder-to-shoulder.
However, due to overlapping effects, further units of width required were found to need only another 450mm. Hence, 3 units of 525 + 525 + 450 = l500 mm and 4 units of 525 + 525 + 450 + 450 = 1950mm.
Units greater than 4 units wide were considered to slow flow rates down, and so the maximum width of an exit is 4 units or 1950mm.
It should be noted here that the figure of 40 per minute was an average figure while the actual figures varied between 20 and 170."
This all seems Ok to me, but I cannot find reference to a unit width in the B doc??
Your help and guidance is appreciated.
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Try BS5588 Pt6.
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Part 6 : 1992 Code of practice for places of assembly
I think I am going to have to buy a copy as I cannot find one off hand!
Does it collaborate with the contents of the post above please?
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http://www.communities.gov.uk/documents/fire/pdf/144821
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Many thanks nearlythere - this guide throws up yet more ways of calculating things -
"A width of at least 1,050 mm can accommodate
up to:
• 160 people in higher risk premises;
• 200 people in normal risk premises; or
• 240 people in lower risk premises.
An additional 75mm should be allowed for
each additional 15 persons (or part of 15)"
Can't quite see the references (first post above) though - need to keep searching.
Alternatively, £71 for the BS5588 pt6!
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Post war building studies No.29 - fire grading of buildings refers to various tests that were carried out including some in USA and some by fire-fighters in Paris. From all of these they opted for 21 inches as the average width of a person....525mm...it actually coverts to 533mm. That is the unit of exit width.
Obviously until a door is double that width only one person can fit through at a time. I have the document as a PDF if you have trouble sleeping, contact me and I will e-mail it to you.
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Minimum exit width is 800mm for up to 50 persons, in some guides 60, and for wheelchair use.
900mm for up to 110 persons
1100mm for up to 220 persons
and through extrapolation 100mm for every additional 20 persons.
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The old yellow entertainments guide also referred to the old units of exit width. I have pdf of that if you need it.
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Just to throw a spanner into the works, have you taken into account the number of cows and horses you would get through one unit of exit width in a minute, should it be a stable or other animal premises?
Please, please check out the new guide on the following link: www.communities.gov.uk/documents/fire/pdf/animalpremisesfull
especially pages 71, 75 & 79. It's the funniest thing I have seen in ages, thanks DCLG, you have made my night!
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If you are interested in Post war building studies No.29 try http://www.mace.manchester.ac.uk/project/research/structures/strucfire/DataBase/References/defaultOther.htm
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Similar thing, if you want a copy of BS5588 part 6 or PWBS 29 as pdf files let me know. The calculation is also in the technical guide for entertainment premises, the a5 book issued by the LDSA. Again I have a copy in A4 format if wanted. The issue really is that you do not need to use it anymore as the tables in the guides do that for you in existing buildings and for new builds ADB does the same.
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I don't know why you're bothering with all this. Just use Table 4 from ADB but replace the 60 persons with 100 persons in the first line.
The 60 persons is the limit for the situation where this is the only exit - but this applies no matter how wide the exit route is.
Here is a graphical representation of Table 4 (with the above correction incorporated)
(http://i119.photobucket.com/albums/o128/slubberdegullion2/exitwidths.jpg)
Note that all widths are 50mm less than in other guidance. This is down to a mis-alignment of methodologies for measuring exit doors, and when ADB changed their method they neglected to also change their figures. This anomaly accounts for the blip between 220 people and 221 people. I believe there are political reasons for maintaining the status quo.
It is important to note these figures are portrayed as deterministic facts, whereas they are nothing of the sort. They are about right and, if adhered to, will probably mean that the means of escape within the building provide a reasonably safe environment. They are a "best guess" that has stood the test of time (mostly).
Stu
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Very pretty graph Stu but it only really applies for a building of normal fire risk with a required evacuation time of 2.5 minutes. High risk buidings need wider exits low risk buidings can have narrower exits as you say this is set out to some extent in the new guides and explained in greater depth in BS (or is it still DD) 9999.
On a more serious note I have made a formal complaint to your local fire authority over the overcrowding in your bedroom according to your avatar - I counted 17 895 sheep sharing your room before I fell asleep.
ps is a cattle shed a HIMOO?
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Very pretty graph Stu but it only really applies for a building of normal fire risk with a required evacuation time of 2.5 minutes. High risk buidings need wider exits low risk buidings can have narrower exits as you say this is set out to some extent in the new guides and explained in greater depth in BS (or is it still DD) 9999.
On a more serious note I have made a formal complaint to your local fire authority over the overcrowding in your bedroom according to your avatar - I counted 17 895 sheep sharing your room before I fell asleep.
ps is a cattle shed a HIMOO?
Yeah, good point about the 2.5 mins.
I'm a big fan of the principles set out in DD9999 and advocate the use of those principles wherever I can - the document itself currently leaves a lot to be desired - the BS will hopefully be a lot better and is supposed to be coming out in about March I hear, but let's not hold our breath.
By the way, that 2 minute evac time for high risk, 2.5 for normal and 3 for low has always struck me as odd because (I don't know if anyone still believes this) the high, medium and low were dependent upon the construction of the building! I'd like to know how the construction of the building is deemed to be a more significant factor in the rate of fire growth in the first few minutes than the contents!
Oh and I'm sorry you counted all those sheep, it's actually the same one over and over again. He will drink coffee at bed time!
Stu
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I remember the high/normal/low you are referring to, it was denoted as construction class a,b and c. The low risk being concrete/non combustible construction i.e concrete everything, high being completely combustible construction, i.e. Wood hut. :)
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So you see that most of the guides that some of you like to cling onto are based on the principle that there are only 3 classes of buildings based on primary construction only. i.e. no account is taken of the combustible linings.
Once you determine the class the number and size of exits is based on how many frenchman can fit through a small gap in the time it takes to play God Save the King when your a***e is on fire.
Carry on code huggers & don't forget the latest guide when determining the maximum travel distance for a cow.
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when your a***e is on fire.
Phil,
Jim Slimmon told me that the correct phrase is, "when you are receiving unambiguous fire cues!"
Stu
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Stu, never trust a Scottish fisherman who once worked in London ........Jim once told me that the job was f****d.........and we all know that's just not true!!!
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I think that was a quote about Jim, and that may be true.
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I think that was a quote about Jim, and that may be true.
True indeed......
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Hasn't there always been confusion around between published tables for numbers able to use particular exit widths and the concept of 'units of exit width'? I've tended to refer to whatever publication seemed to be relevant for the particular premises type at the time - but even ADs aren't law and we're now in the risk assessment age - aren't we?
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we're now in the risk assessment age - aren't we?
Yes we are but we still need clues as to how many people can get out of a given exit width in a given time.
There are different figures in different guides but they're all roughly similar....... and that's the point - I've said this before - the figures, wherever you get them from, aren't exact and precise. If a door is 10mm, 20mm or even 50mm narrower than it should be, no one is going to die because of that. If it's 500mm narrower, they might.
No one evacuates in the way that ADB (or the other guides) assume anyway.
So let's just be happy with the "clues" we have, no matter where we get them from. They have stood the test of time with more success than failure (and there have been tragic failures) and are usually conservative.
If you're really interested in the topic read DD9999 first, then move into evacuaton modelling and fire modelling.
Stu
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Agreed, Stu.
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we're now in the risk assessment age - aren't we?
Yes we are but we still need clues as to how many people can get out of a given exit width in a given time.
There are different figures in different guides but they're all roughly similar....... and that's the point - I've said this before - the figures, wherever you get them from, aren't exact and precise. If a door is 10mm, 20mm or even 50mm narrower than it should be, no one is going to die because of that. If it's 500mm narrower, they might.
No one evacuates in the way that ADB (or the other guides) assume anyway.
So let's just be happy with the "clues" we have, no matter where we get them from. They have stood the test of time with more success than failure (and there have been tragic failures) and are usually conservative.
If you're really interested in the topic read DD9999 first, then move into evacuaton modelling and fire modelling.
Stu
History has tragically shown all too often that the width of a doorway is not as important as its ability to open.
When risk assessing escape route capacities does one take into consideration people size eg Weightwatcher meeting.
Who would be brave enough?
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You could disguise it under a "mobility co-efficient" and make it look like some random fire engineering calculation.
i.e. a) Weightwatchers meeting has a mobility co-efficient of 1.2. So work out normal door width required and multiply it by 1.2. b) Gymnast meeting has a mobility coefficient of 0.8. c) Colin Todd has a mobility co-efficient of 1.9
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You could disguise it under a "mobility co-efficient" and make it look like some random fire engineering calculation.
i.e. a) Weightwatchers meeting has a mobility co-efficient of 1.2. So work out normal door width required and multiply it by 1.2. b) Gymnast meeting has a mobility coefficient of 0.8. c) Colin Todd has a mobility co-efficient of 1.9
What about racing snakes like me?
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Hey thats a very good point civvyfso - do the Post War Building Studies have any relevance to the current population with so many of us much larger than our fore fathers.
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You could disguise it under a "mobility co-efficient" and make it look like some random fire engineering calculation.
i.e. a) Weightwatchers meeting has a mobility co-efficient of 1.2. So work out normal door width required and multiply it by 1.2. b) Gymnast meeting has a mobility coefficient of 0.8. c) Colin Todd has a mobility co-efficient of 1.9
What about racing snakes like me?
Clearly you would fall into the gymnasts?
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You could disguise it under a "mobility co-efficient" and make it look like some random fire engineering calculation.
i.e. a) Weightwatchers meeting has a mobility co-efficient of 1.2. So work out normal door width required and multiply it by 1.2. b) Gymnast meeting has a mobility coefficient of 0.8. c) Colin Todd has a mobility co-efficient of 1.9
What about racing snakes like me?
Clearly you would fall into the gymnasts?
With a phased evacuation approach then those with a lesser MC (mobility coefficient) would get to the escape door quicker than those with a higher MC. Hence with the natural order of things by the time the 1.2 MCs got to the door the 0.8 MCs would be away.
Do you think that this is something Darwin missed when writing up his theories? If not, and as I thought about it first, I want ownership of it.
I will call it "The theory that skinny people can run faster than fat people".
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Kurnal,
Why have you four fathers?
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Kurnal the PWS covers more than exit widths but why does it still cast its shadow sixty years later, when more recent research should have cast it to the trash can years ago.
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Would this sign help?
(http://i119.photobucket.com/albums/o128/slubberdegullion2/mobilitycoefficientsign.jpg)
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PWBS actually make a lot of sense when read in context. The story about the 2 and a half minutes national anthem in Edinburgh in 1911 even makes sense when read in context. And the context hasn't changed that much in 50 years. We're still trying to achieve the same goals. People weren't stupid back then.
There are a lot of assumptions made, and reading these makes it clear that the principles derived should only really be applied to "typical" buildings.
Anyway, the findings have largely stood the test of time (I know I've said that before) and are usually conservative.
What else have we got to go on?
Should we re-invent the wheel and call it a "circular locomotion facilitation device?"
Stu