Units of exit widths?

[Underground]

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.

[nearlythere]

Try BS5588 Pt6.

[Underground]

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?

[nearlythere]

http://www.communities.gov.uk/documents/fire/pdf/144821

[Underground]

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!

[PhilB]

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.

[nearlythere]

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.

[kurnal]

The old yellow entertainments guide also referred to the old units of exit width. I have  pdf of that if you need it.

[johno67]

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!

[Tom Sutton]

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

[jokar]

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.

[slubberdegullion]

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)



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

[kurnal]

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?

[slubberdegullion]

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

[CivvyFSO]

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.