Occupancy capacity

[Taiter]

Has anyone had experience of calculating occupancy capacity?  There seems to be a couple of different methods - the building standards method and the fire & rescue version.  The latter involves assuming that the largest exit is unavailable.

Can anyone tell me if, when conducting the exercise, I should follow the fire & rescue example?

Any info gratefully received.

[kurnal]

Not quite with you Taiter. It is convention to discount a storey exit (the widest usually) under both the building regs and the fire safety guides.
In some circumstances though it may not be necessary to discount a staircase  when considering the vertical element of the means of escape.

May be able to help further if you could give a little more info on the circumstances you have in mind.

[Taiter]

This is in relation to halls and the like.  The hall in question has 2 x 2 unit width emergency exit doors, at opposite ends of the hall (>45 deg).  The occupancy number for the hall is 430 but using both emergency exits only 400 people could evacuate in 2 1/2 minutes.  However the local fire & rescue service discount one exit therefore occupancy is restricted to only 200 in their calculations.  Do you see where I'm coming from now?  Interested in other opinions.

[PhilB]

If your hall has two exits you should discount the largest. In your case they are the same width so it doesn;t matter which you discount.

The occupancy should then be determined by the number of people that can safely get out the remaining exit in the time available, usually based on 2, 2.5 or 3 minutes depending on construction.

40ish people can pass through one unit of exit width in one minute.

So in your example you have two untis of exit width (1050mm I am assuming).

40 persons per minute per unit would give 200 persons using 2.5 minutes.

[Martin Burford]

Taiter
Whats the Building Standards method of calculating maximum capacities?
Conqueror

[jokar]

The small to medium places of assembly guide gives a table for numbers which we should use and does not involve any calculations.  Dependent on the risk category involved, a 1050 mm exit will allow you to have 160, 200 or 240 people.  See page 64.

[Mike Buckley]

Conqueror look at ADB appendix C which calculates the occupancy of the room by the floor space per person. Hence it would seem to me that the occupancy of the room has been calculated from its area.

The RRO calculates the number of people who can use an escape route based on the width of the route. Here the other side comes in where the widest route is disregarded for the sake of the calculation. Look at section 4 of the guidance.

From the guide for an occupancy of 430 people it would need an escape width from the room of 2300 mm with the widest exit discounted.

The problem would seem to be that the room has a certain capacity based on the area of the room but the available exit width may dictate a different figure.

In short the Building Regs calculate the occupancy on the area whilst the RRO calculate the occupancy on the exit width.

[PhilB]

The small to medium places of assembly guide gives a table for numbers which we should use and does not involve any calculations.  Dependent on the risk category involved, a 1050 mm exit will allow you to have 160, 200 or 240 people.  See page 64.

mmmmmmmmmmmm...would that not be exactly the same as 40 per minute per unit in 2, 2.5 or 3 minutes?

Calculations are not my strongpoint but I was trying to explain the logic rather than just plucking figures from a guide.

[jokar]

PhilB, no argument there just trying to assist the discussion.  The only place that the calculations exist now are in the LDSA Technical Guide for entertainment premises and as such many people are unaware of them.

[Martin Burford]

thanks Jokar and Mike
Conqueror

[kurnal]

Mike I agree but the building regs approved document B needs to be used in its entirety- first determine  capacity by density factor, then calculate capacity of exit routes, both horizontal then vertical phases.
Whichever figure is the lowest is the safe capacity.

 The figures in the tables of approved document B are based on a normal risk building with a target evacuation time of 2.5 minutes. It is ok to interpolate them downwards or upwards for combustible (2mins) or non combustibe buildings (3 mins).

I dont see any  significant differences- apart from the regular massaging or  rounding of the figures with every new edition of the guidance 1000mm seems to be the norm now whereas 1050 used to be the equivalent of 2 units of exit width. Wasn't a single unit of width 21 inches in our day? so 2 units = 42 inches or 1050mm.

[Tom Sutton]

Many assessments of this rate of flow have been made following tests, (particularly those using Paris firemen in 1938 and 1945 and the London underground).

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.

[CivvyFSO]

It seems like there are 2 approaches to occupancy figures.

1) Floor space is governing factor. Work out potential occupancy, THEN make sure exit widths are appropriate, making new exits if necessary. Happens alot with licensed premises. Once the doors are in place this is almost self governing as it is often hard to go over the occupancy figures. (Although some nightclubs and wine bars possibly go quite a bit over.)

2) Exit width is governing factor. Ends up with an occupany limit defined by door widths. Unlikely to be used in licensed premises due to potential for the premises to end up 'rammed' (Sorry for using technical terms) and unsafe.

In Taiter's hall example I would say that its up to the RP to ensure that they have the correct exit widths or limit the occupancy accordingly. Both methods require the loss of the widest door.

I suppose one way of looking at it is:

a) You have a potential capacity of 430, you HAVE to have doors for 430.

b) You only have doors for 200, you should only have 200 people in. +/- for low/high risk etc

[Mike Buckley]

I don't see it as 2 approaches to the figures. I see it as a two stage process. The first is building regs based on floor space. This will set the upper limit on numbers. Then the exit width reduces that number if the exits are not adequate.

Hence if the building regs set an occupancy figure of 400 and the exit width sets a figure of 300 then the capacity of the room should be 300. On the other hand if the building regs figure is 400 and the exit width sets a figure of 600 the capacity of the room is 400.

The method is work out both figures and use the lowest.

[wee brian]

Mike

The floor space factors are only there to give an indication of how many people there might be. Not as a maximum permitted occupancy.

If the exits are adequate theres nothing that would stop you filling the place shoulder to shoulder.

Of course there are other reasons why you wouldnt do this.  Health and Safety, ventilation, number of toilets, risk of fighting in the crouwd etc.

In an exisitng building, you work out the max number of people (for fire safety) by working back from the available exits). If you want more bodies then you need more exits (or something else).  But it will still be necessary to consider the other H & S issues.