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FIRE SERVICE AND GENERAL FIRE SAFETY TOPICS => Fire Safety => Topic started by: colin todd on December 27, 2015, 07:36:14 PM
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Here is a wee analogy that has bugged me for some time, but can be solved by anyone with a Scottish o grade in statistics. If you can see the relevance to fire safety before the bells toll on new years eve, there will be a bottle of wine.
Imagine that the 1st percentile of the height of males was 5 feet. Further imagine that the 99th percentile was 7 feet. Should we then design doorways in building to be 12 feet in height to be sure they are ok for 100% of people.
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Here is a wee analogy that has bugged me for some time, but can be solved by anyone with a Scottish o grade in statistics. If you can see the relevance to fire safety before the bells toll on new years eve, there will be a bottle of wine.
Imagine that the 1st percentile of the height of males was 5 feet. Further imagine that the 99th percentile was 7 feet. Should we then design doorways in building to be 12 feet in height to be sure they are ok for 100% of people.
Only if the dwarves are riding piggyback on the shoulders of giants.
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I don't think you can use the term dwarfs anymore? it's persons of restricted growth.
Colin, you need to get out more or take a longer Christmas break? ;)
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If the term dwarf is inappropriate then giant must also be so? The jury is out on the term piggyback as its use must depend on context.
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To answer Colins question that would be onerous in the extreme and therefore not cost effective. You need to determine a percentile range that is considered societally acceptable recognising that not all persons needs will be met by the building /means of escape / lighting / fire alarm / signage design then recognise that those who are therefore disadvantaged by the compromise have their needs met as a special case. Personally I like a nice Chianti please.
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Kurnal you are getting warm but only like to the level of 3/10. Try a bit harder.
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But if you only make the door 12 ft without increasing storey height then they will bang their head on the floor above unless you raise this too.
But I guess you are seeking to explore the old chestnut with regard to whether there is life beyond the 99th percentile. Everybody would be smaller than your notional 100 percentile. Riserva will suffice.
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Yes, yes, yes Big Al, but what is the relevance of the statistical point to fire safety design of buildings.
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Why? It never seems to have much before. If one is considering piggy back, then surely the faithful horse must have a shot too? Just resurrect the brief references for animal premises. After all, there but for the grace wee Rab Burns, we may all go and bang our heads on cave walls.
HNY All.
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I have recently returned from a couple of pleasant days in Glasgow. I must say that you should be less concerned with exit height and turn your attention with a degree of urgency to exit width. Apart from Oklahoma City, I don't think I have seen so many fat arses in the one place! No disrespect to Glasgow intended, Belfast is not far behind, if you will forgive the pun. Twelve feet sounds about right for the minimum exit width from all fast food outlets.
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I have recently returned from a couple of pleasant days in Glasgow. I must say that you should be less concerned with exit height and turn your attention with a degree of urgency to exit width. Apart from Oklahoma City, I don't think I have seen so many fat arses in the one place! No disrespect to Glasgow intended, Belfast is not far behind, if you will forgive the pun. Twelve feet sounds about right for the minimum exit width from all fast food outlets.
Glasgow - Where chips are a pizza topping.
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I have recently returned from a couple of pleasant days in Glasgow. I must say that you should be less concerned with exit height and turn your attention with a degree of urgency to exit width. Apart from Oklahoma City, I don't think I have seen so many fat arses in the one place! No disrespect to Glasgow intended, Belfast is not far behind, if you will forgive the pun. Twelve feet sounds about right for the minimum exit width from all fast food outlets.
You obviously don't go to the right places Lyle? There are fat arses everywhere. Its really about not frequenting the same places.
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There is clearly a role for statistical analysis of human size, speed of movement, sensory thresholds etc in the design of fire safety within buildings. Such considerations are already included in fire engineering to some extent. Whilst studies of population characteristics and related percentiles are of direct relevance and use in some aspects of fire safety design, they are of less or indirect relevance in others.
For example it is clear that tenability and toxicity limits in a fire engineered building must always exceed safe limits plus a safety margin for all occupiers- 100% of the population. It would be farcical to design CO levels at a toxic level for a percentage of the population during the evacuation phase. Statistical analysis will identify tenability limits. But levels of toxic substances this is of course interlinked to other factors that contribute to the evacuation time, and this is where an overall suitable safety margin will contribute to an acceptable level of safety.
In respect of means of escape, exit width calculations should be reviewed using statistical analysis taking account of the increased stature of the population since the Post War Building Studies were published. In order to set an appropriate percentile range, studies could be carried out across a range of occupancies. Height of doors is a less important factor for many reasons.
In many other aspects some degree of compromise is appropriate to ensure the provisions are reasonably practical. This includes fire alarm audibility, signage visibility, lighting levels. All these factors can reasonably be addressed in other ways for those who fall outside the percentile threshold.
All of the above needs to be balanced by a judgement on what Society will tolerate.
The hard bits include a detailed analysis of all human factors and a decision as to whether they have a direct bearing on fire safety through the ASET/ RSET approach.
For those that have only an indirect bearing (audibility of alarms, standard of lighting etc) an arbitrary percentile range could be set without danger knowing that the provisions for persons with special needs will provide a reasonable level of safety.
For those that have a direct bearing (escape route width for example) then it might be necessary to widen the percentile range based on intelligence linked to studies of occupancy types.
Finally for some factors (toxicity for example) only 100% plus a suitable safety margin will suffice.
Theres more to this than meets the eye once you start to think about it. Penny for your thoughts Colin?
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Mr P
Surely you are right, as I seem to remember HMG issuing guidance regarding the MoE for horses ???
However, what it failed to mention was taking into account those elderly animals unable to open the doors due to closer tension. Perhaps one should look to a certain company to make a larger version of their well known product.
In regard to percentiles, the answer is 95.
Chairs are made to BS using 95% for height adjustment, weight etc etc.
Central heating in public buildings is designed to account for 95% of all weather conditions
I rest my case (of Retsina)
davo
ps.speaking of MOE, anyone notice how bad a certain large chain signs its premises??????????
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Door heights and other ergonomic things are usually aimed at the 95th centile or thereabouts as Davo says.
a sensible approach (very tall people will have to duck through doors, very short people may find it difficult to reach things - but then the whole access issue gets into all this......).
But with fire safety we're talking about peoples lives and its hard to get people to talk sensibly.
of course, we (on Firenet) all understand that absolute safety is not possible. so we have to adopt what is reasonable.
I remember Howard Morgan (sadly lost to us now) telling me that a worst case scenario was not a sensible design parameter as this would involve a explosion and fire ball engulfing the whole building. What we needed was a "reasonably pessimistic scenario"
For deterministic analysis we need to pick a number - not easy. so we tend to us a comparative approach comparing what we've got with a "code compliant" solution. thus using a magic number made up by "them"......
or people just blag their way through it and try to sound like they know what they are doing.
but you knew all of this, so why the question?
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Big Al, while all that waffle (no disrespect intended Old Timer!) might be true, it is not what I was getting at.
It is also true that, in medicine and biology, the 5th -95th percentile is deemed to be normal, so it is the case that, by definition, 10% of the population are abnormal. (Most of them working in the civil service and my bestest not favourite FRS in the world). All you would have to do is change the definition of normal from 1st-99th percentile and you would reduce at a stroke the number of abnormal people in the world.
The 75dB(A) used in fire alarm systems in rooms in which people sleep is based on the 95th (or it might have been the 99th) percentile of American students woken by this SPL in the 1970s.
But none of this was the point I was getting at, which I cannot reveal to you yet WeeB, as the bells have not yet tolled to end the year.
I will have to give you a clue-think fire engineering (though the more I do think about it these days the more I become disillusioned by the smoke and mirrors).
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I guess I should have known better than to try. I should have remembered from experience that Dottys arms are in the 5th shortest percentile and his pockets are within the deepest 95th percentile.
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I suppose, being abnormally tall just means you have to duck through doors.
But if your slower than the walking pace included in a fire engineers escape sums then your might die.
Saying that, you walk through doors all the time but might never be in a burning building....
This is all very deep, you'll start confusing yourself.
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And if a number of the occupants are overweight they will take up more space extending the time taken for the occupants to pass through exits and staircases leading to an extended evacuation time for everyone.
If they cant see very well they will move more slowly.
If they cant hear well their response time will be affected.
If they have learning disabilities this may affect response.
If they have consumed too much of Dotty's wine....no that's never going to happen ......
One thing leads to another....... and the beat goes on.
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Brian, you are getting warm, but are not quite there yet.
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I get bored easily
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Ok, let me put this point to you, if the 1st percentile for pre-movement time was say 30 seconds, and the 99th percentile were 90 seconds, why on God's earth would any sane fire engineer regard total pre-movement time as 120 seconds and design accordingly.
This would assume that the 99th percentile was the group of people between the 1st percentile and the 99th percentile (assuming you were trying to design for 99% of people), when in fact the 99th percentile already includes the people in the 1st percentile.
Alternatively, it would assume that the 99th-100th percentile was the same as the 0-1st percentile, which clear is unlikely to be the case (as it is unlikely to be a normal distribution) and that you were trying to cater for 100% of people, which is not possible.
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Because most "sane" fire engineers know nothing about statistics.
Most people, including fire engineers, think they understand statistics and can employ them usefully to some degree. They're virtually all wrong. Particularly journalists (and fire engineers).
You clearly have a particular case in mind here. 120 seconds might sound like it possesses a feasible safety margin above the 99th centile value of 90 seconds but rigorous sensitivity analysis of what might reasonably be expected to cause delays would be the only correct way to establish the best safety margin.
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I often get complaints from fire engineers (young ones) that building codes are full of magic numbers made up by committees with no basis in science etc.
I like to explain to them that the numbers they use aren't much better.
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Ashes, it is even worse than a particular case. PD 7974-6 Table C1, says that total pre-movement time equals the time for the 1st percentile to move plus the time for the 99th percentile to move. A number of the worked examples then use the two added together (although some of them use different figures so maybe that is another error).
I agree with your view on people's handling of stats, but I was serious when I said that a Scottish O grade statistics would give you enough understanding of percentiles to appreciate what seems to me to be a schoolboy howler in the PD
I also agree with wee b (who is a cool dude for a civil servant). I rarely admit it, but I was one of a cohort of 4 students who undertook the first ever university education in fire engineering in the UK. The stuff I see young (and sometimes not so young) fire engineers turn out today makes me ashamed. A lot of it is smoke and mirrors designed to do little else than prove that black is white, often to suit a client or a court case. Bring back old fashioned fire prevention officers and building control officers, but it will never happen.
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I accept this is way above my head, but is there good available data to make all these assumptions it looks like C1 doesn't think so, as they say garbage in garbage out?
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Exactly, Tam.
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The stuff I see young (and sometimes not so young) fire engineers turn out today makes me ashamed. A lot of it is smoke and mirrors designed to do little else than prove that black is white, often to suit a client or a court case. Bring back old fashioned fire prevention officers and building control officers, but it will never happen.
Precisely Colin, for once I find myself in complete agreement with you. Have said it several times myself and have completely dissociated myself from several projects when I have smelt a rat. I am not a fire engineer, but always found basic firemans questions enough to confirm my concerns.
However getting the developer and client to agree at the design team meeting to spend more money when the greenhorn fire engineer ( sometimes working for the same group as the AI) actually believes everything his modelling shows him and the AI is happy to sit on the fence and accept the fire engineers report without reading it or understanding it ( it's backed up by their PI insurance isn't it so if the engineering is wrong they will be sued first) .
That's the time to walk away from the client, there are plenty of others out there who expect and appreciate a more diligent approach.
We are where we are as a direct consequence of the privatisation of building control in the early 1990s. Vested financial interests will often take priority over good fire safety design.
I fully agree with your final comment "Bring back old fashioned fire prevention officers and building control officers, but it will never happen" , so here's a New Years resolution for you Colin.
It's clearly time to ditch your avatar message "civilianise enforcement- you know it makes sense"
Happy new year
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Yes, I see what you mean, Colin. It's not right but I can see that there would be some sort of pragmatic applicability of this approximation if the distribution of occupants' individual pre-movement times was symmetrical. But, as you have already pointed out, these times are not symmetrical about a mean and, indeed, Figures C1 and C2 immediately above Table C1 show a number of distributions, all of which are skewed.
Of course, the distributions approach the x-axis asymptotically which means, in practice, that we can never be sure how long it will take to get that last centile moving.
But there are solutions to the problems that can be used to achieve useful predictions and they depend upon the type of premises under investigation. In densely populated spaces where queues can be expected we're only interested in the first few centiles and in sparsely populated spaces we cannot predict the last centiles so we have to impose management regimes to control pre-movement times.
All parts of 7974 tend to over-theorise their respective subject matter, often with errors.
As for the general quality of fire engineering in the commercial world...well...I don't know where to begin...
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Ashes, it a basic fact that where the lowest centile of a distribution can be zero but cannot be less than zero and there is no theoretical limit to the upper centiles, the distribution is bound to be skew wiff. The maximum possible pre-movement time is infinity, as people might never bother to evacuate. I did this myself once, while 'er indoors went and stood dutifully in the rain and ruined her hair do.
As for fire engineers, the reports I love are those than involve thousands of pounds of computer modelling to show that B1 is met in a supermarket with code compliant travel distances, so allegedly proving that sprinklers are not required, without even a notion that sprinklers would be necessary for B3 not B1. I saw this from a large, well-known practice of fire engineers.
And what about using a soot load related to PU foam in a warehouse full of paper. Duh.
Enforcing authorities and BCBs should never trust fire engineers, but alas all you have to say to them is "its a fire engineering solution , guv" and any old tat gets approval.
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Big Al on a point of interpretation of my avatar, civilianise does not mean privatise.