9999 Risk profiles

[CivvyFSO]

I realise what follows is off theme but I am trying to get to grips with DD 9999 which I hope is close enough to BS 9999 not to make any major difference.

I progressed fairly well until I reached width of doors and I am having difficulty understanding how you calculate the width of a door using the table 13 “Door widths when minimum fire protection measures are provided”. Could any person explain it and show me an example either on or off line?

Take profile A1 and a big square room that can hold 500 people.

3.1mm x 500 people = 1550

You need at least 1550mm of exit width.

But due to the normal discounting rules you will need 2 lots of 1550mm.

[Mr. P]

TW, I understand it as; minimum exit door width for exit point, plus additional width per person over that for the said door width allowance.  ie 1100mm door width allows 183 persons (risk profile B3), then add 6mm per person additional to that.

Note 4 below Figure 13 page 83 basically says this too.

[CivvyFSO]

I realise what follows is off theme but I am trying to get to grips with DD 9999 which I hope is close enough to BS 9999 not to make any major difference.

I progressed fairly well until I reached width of doors and I am having difficulty understanding how you calculate the width of a door using the table 13 “Door widths when minimum fire protection measures are provided”. Could any person explain it and show me an example either on or off line?

Take profile A1 and a big square room that can hold 500 people.

3.1mm x 500 people = 1550

You need at least 1550mm of exit width.

But due to the normal discounting rules you will need 2 lots of 1550mm.

I hindsight my last line is sort of wrong... After discounting the largest exit you need 1550mm of exit width left. And as Mr P points out, this is subject to the normal minimum widths. In this example 3 suitably placed 850 doors cover it, as if you discount one you are left with 1700mm of width left. 1700/3.1 allows 548 people through for that risk profile.

[Ricardo]

I am a little puzzled as to the example given in note 4 page 83, which says 850 divided by 4.4 ( from table 13), is this a typo error? as I cant see any mention in table 13 of 4.4 at all.( or is it me getting it wrong) I see a 4.1 for risk profile B2.

[Mr. P]

Brilliant! Good job Christmas is coming.  Sell ourselves to be pulled as crackers, make a nice little earner by giving them the RA prior to pulling us... we may go spludge, bang or other wise, scream loudly, heres some ear defenders that'll be a nice £2.60 thank-you , cheap at twice the price!!!!

[CivvyFSO]

I am a little puzzled as to the example given in note 4 page 83, which says 850 divided by 4.4 ( from table 13), is this a typo error? as I cant see any mention in table 13 of 4.4 at all.( or is it me getting it wrong) I see a 4.1 for risk profile B2.

You are quite right, it is clearly an error.

850/4.1mm per person gives 207 people per door. Discount one door of the three 850mm doors, you have 2 times exit width appropriate for 207 people giving 414 people.

[Tom Sutton]

Thanks guys, but it looks like I have a problem, in DD9999 B3 indicates 7mm/person not six and there is no notes below Table 13. I do have the 4.4 for B2 Ricardo spoke of but I am afraid it’s a case of saving up my pennies and getting BS 9999 before continuing with this project. 

[wee brian]

The DD is very different from the BS.

[Tom Sutton]

Now I have had time to digest I can now see my calculations were correct despite the erroneous examples shown in DD9999 but what I could not get my head around was the numbers of people a 750mm door could accommodate, seemed so high, I felt I must have made a mistake. For example using previous guides a 750mm door could accommodate between 80 and 120 person depending on the risk category. Using DD9999 a 750mm door could now accommodate between 81 and 241 persons with acceptably an increased number of risk categories.

I now realise that WB but how long do we have to wait for BS 9999 to sort out the mistakes and typos, I do not want to pay twice for the same document?

[kurnal]

BS9999 16.5.6 (e) has the following formula for the calculation of the minimum permissible width of a final exit where flows merge from a staircase and a ground floor storey exit into a ground floor lobby.The formula previously appeared in ADB.

W= [(N/2.5)+(60S)]/80  
where N= the number of persons served by the ground floor storey exit, S = stair width in metres and W=the width of the final exit in metres.

To me this formula appears to undermine the risk based approach of 9999 where we design the means of escape using risk profiles. Surely the 2.5 in the fomula is simply determining the flow rate in persons per minute based on a "Normal risk" evacuation time of 2.5 minutes- ie the 5mm per person derived and extrapolated from the post war building studies. This seems to undermine the general risk based approach- or am I missing something?

Have just been using the document for a new building where I had been using table 13 which I think  should read 4.4mm per person not the 4.1 stated in the table for B2 risk profile?? )  

[Davo]

Prof

On one of the other threads on 9999 I quoted errors in the BS as given to me by the main author

davo

[Phoenix]

Hi kurnal

The formula you quoted can be boiled down to three quarters of the width of the stairs plus 5mm per person for all those coming from the ground floor.

You are right that this is anomalous within the context of 9999 because a different width per person is probably applicable.  It's not perfect, this 9999.  But it's a best guess for the time being (just as are all other fire safety numbers).  You'll find a similar anomaly in the section on shopping mall exit widths, they're still basing mall exits on 5mm per person. 

At least these little quirks are erring on the side of safety.  Also, they are simple (increased accuracy would bring increased complexity).

Another thing to note about the formula you quoted.  Even if only 50 people are joining the flow from the ground floor you still apply the 5mm per person and that doesn't seem to sit right with the fact that you cannot use 5mm per person for less than 220 people.  But the formula does make sense because those few people that are joining the flow from the ground floor are joining a larger flow from the upper storeys which, hopefully, takes the total above 220.

Why do you think B2 in Table 13 should be 4.4? 

Davo, is this one of the errors you spoke of?

Stu

[Mushy]

What happens in a shop when the alarm sounds? Customers ignore the alarm and carry on shopping till they are told what to do. Then they go to the tills and try and pay for what they have in their trolleys and argue about having to go and stand out in the cold before they can finish their shopping. Staff wonder whether to start ther evacuation or not and tend to stand around looking confused till someone on the  non fire rated tannoy tells them what to do.

very true...I was in a supermarket the other day when the fire alarm sounded...it was like the muppets 'pigs in space'...every one looking skywards wondering what the noise was...all those old enough to remember the Woolworths fire in Manchester will know that human behaviour doesn't always do what it says on the evacuation tin

[Davo]

Phoenix

General Interest Jan 28 2009, sorry can't do the technical stuff


davo

[Tom Sutton]

Is this the one you are talking about http://www.kingfell.com/~forum/index.php?topic=4019.msg42354#new