ADB - Floor Space Factors

[StuartH]

I am seeking the forums wider view on the matter of the application of the office floor space factor contained with table C1 of ADB when calculating the occupancy capacity of an existing building.

As you will known ADB confirms a floor space factor of 6m2 per person for areas used as offices.

There is disagreement within my company on the application of the floor space factor contained within Table C1 of ADB. Some are of the view that this figure is absolute (as calculated) even when the actual occupancy is known, others are of the view that the floor space factor becomes irrelevant if the actual occupancy of a given room/floor is known.

As an example:

If a storey has sufficient exit capacity for 110 persons, floor space capacity (based on 6m2 per person) for 80 persons, but a confirmed ACTUAL occupancy (based on desk layout) for a 100 persons; should this storey be limited to 80 persons (ie is the floor space calculation absolute), or limited to 100 persons (based on desk provision) or limited to 110 persons (based on exit provision).

[Davo]

StuartH

Total floor space is therefore 4800 sq m?

To further confuse matters, the H & S (Workplace) Regs 1992 give only a minimum of 3.7 sq m per person for each office.
Square meterige divided by 3.7 gives maximum occupancy. This figure must be rounded down.

Allowing for corridors, toilets etc it appears you may be overcrowded


davo

[deaconj999]

Stu,

For an existing building that has not been refurbished youshould refer to the RRO guide for offices and shops which will have most of the answers for you but maybe not all.

However, if you have an office with 100 persons at their desks and assuming its open plan your calculation would work back to a maximum room size of 600m2. It would be worth taking into consideration the meandering distance around the desks from the furthest point in the room to the nearest exit as this would be your maximum travel distance taking into account the configuration of the desks.

Furthermore, assuming the building is of class B construction you would have an evacuation time of 2.5 minutes. Allowing for 40 persons per minute as an average pass through rate, you would get 100 persons through in 2.5 minutes and on face value you would only need 1 exit. In reality the evacuation time would be a lot quicker again assuming able bodied, familiarity with premises and successful evacuation drills.

But, using ADB as you have done and therefore assuming a major refurbishment or new build ! (which is why you would be referring to ADB) and using the figures above you would only require 1 Exit with a minimum width of 525 mm. Your storey exit is likely to be no less than 762mm anyway and therefore suitable.

All that said, Fire Safety is a dark art and further consideration has ro be given to the fact that with a room or enclosure containing more than 60 persons you are required to have an additional exit.  The travel distance to any of your 2 exits is then classed as travel in more than 1 direction and this also then factors in that within an office, from the furthest point in the office (open plan) you would have to reach the nearest exit witihn 45m.

Because of the 100 persons you can no longer consider a single exit as suitable. If you reduce the occupancy to 80 you would still require a second exit.

Hope this is enough for you to go on, if not add some more info, i.e. dimensions and type of office, cellular with corridors or open plan as I have guessed.

[Tom Sutton]

I think you will find the occupancy factor, is for building when the occupancy is not known. If the occupancy is known then calculate the MOE for that number of persons. A controlling factor is other legislation I believe the 6m2 came from the OSRA 1963 but that may have been superseded by The Workplace (Health, Safety and Welfare) Regulations 1992 which states, but may not apply to offices,

1.  No room in the workplace shall be so overcrowded as to cause risk to the health or safety of persons at work in it.

2.  Without prejudice to the generality of paragraph 1, the number of persons employed at atime in any workroom shall not be such that the amount of cubic space allowed for each is less than 11 cubic metres.

3.  In calculating for the purposes of this Part of this Schedule the amount of cubic space in any room no space more than 4.2 metres from the floor shall be taken into account and, where a room contains a gallery, the gallery shall be treated for the purposes of this Schedule as if it were partitioned off from the remainder of the room and formed a separate room.

So using the average height of room 3m then minimum floor space would be 3.6m2 which I would think is a little tight.

http://www.legislation.gov.uk/uksi/1992/3004/schedule/1/made

I tried to find info on the OSRA 1963 but no luck. There are guides to it if you wish to purchase one.

[kurnal]

Tom is bang on on both counts.

And dont forget the ADB is just  guidance with lots of extra margins built in, but ignoring some  important factors like how fast a fire may develop and how quickly people will respond to a fire. It assumes that everyone will arrive at the exit in a nice orderly manner and pass through it at a nice even 40 persons per minute.  BS9999 does start to address these other factors.

[deaconj999]

Gents,

The Workplace (Health, Safety and Welfare) Regulations 1992 where quoting Schedule 1 Part 1 Space applies to Factories and not offices as Tom rightly mentions may not apply - therefore to accompany the previous post, the following should also be mentioned from the WHSWRegs92.

Room dimensions and space
10.—(1) Every room where persons work shall have sufficient floor area, height and unoccupied space for purposes of health, safety and welfare.

(2) It shall be sufficient compliance with this regulation in a workplace which is not a new workplace, a modification, an extension and which, immediately before this regulation came into force in respect of it, was subject to the provisions of the Factories Act 1961, if the workplace does not contravene the provisions of Part I of Schedule 1.

This space requirment is purely a H&S requirement......

Furthermore, the Offices Shops and Railway Premises act 1963 and the Factories Act 1961 both mention the same point, but for offices and factories respectively and interestingly both use the term overcrowding, albeit the descriptions/definitions/explanations are slightly different to the current H&S reg 92 and to each other.

It is also important to point out that only the Factories act mentions the Fire Authority and that the Fire Precautions Act which brought the fire matters into focus per se wasn't until 1971. Saffron Waldon and all that.

Therefore I don't believe Stu's original post is concerned with H&S matters, but I could be wrong........

[SeaBass]

Tom is correct. The floor space factors are given so that designers can calculate the requirements for means of escape in a building which has yet to be occupied. If one looks at occupancies such as bank trading floors, the space factor per person is usually much less than that given in the guides. (As little as 5.5 sq meters per person)

Many organisations in the City have been focussed on maximising their space factors in recent times and I've undertaken a number of exercises where we have reversed the calcs to see how many people can safely occupy a floor or building.  Interestingly, it is often not the escape capacities that govern maximum occupancy levels, but more mundane issues such as the number of toilets available, and the capacity of the cooling plant.
 
The application of BS 9999 certainly places occupancy levels and escape capaities in a new light and can result in significant reductions in exit width and increase's in travel distance requirements, which boils down to increased occupancy and a reduction in costs for companies.

[StuartH]

Thank you all for your detailed responses. To help clarify a few points, the reason behind this post is that a customer has asked us to carry out occupancy assessments on their existing buildings as part of their Fire Risk Assessment programme. This enables them to ascertain if any premises are current over occupied, or has extra occupancy capacity to move staff around. We are also assessing future works and structural alterations etc holistically to assess its impact on total building occupancy. All parties have agreed to use the information contained within ADB as basis for which to calculate occupancy. Everyone involved seems comfortable with the methodology of calculating exit capacity, and staircases that comply with ADB, but there is some dispute concerning the importance placed on the floor space factors contained within table C1 of ADB when applying it to an existing building with either a known current or proposed occupancy.

Some parties are of the opinion that the floor space factors are absolute, ie if a given room has a floor area of 60m2 and a actual current occupancy of 15 persons, some are saying that this room is over occupied by 5 persons and the occupancy should be reduced to 10 persons as calculated using a floor space factor of 6m2/person. Others are saying that the floor space factor is irrelevant as the actual occupancy is known, and as long as all other means of escape considerations are satisfactory (travel distance, exit capacities, DDA) then the actual occupancy should remain as is / or proposed.

The later is definitely the view being taken by architects and the approved inspectors with new or proposed refurbishments. We are seeing call centres with with occupancy densities as low as 4m2, which some are finding hard to accept.

[Davo]

As a H & S officer of many years, let me confirm that the Workplace Regs do apply to offices, and that the areas 3.7 sq. m. or 11 cu. m. are a carry over from the OSRP.
The OSRP 1963 had the equivalent in 11 sq feet which was also a statutory requirement.. 
Treat DOC B figures as for an empty building and evaluate based on occupancy types, mobility, furniture and layout also affect.


davo

[Tom Sutton]

Davo said the Workplace regs apply to offices but I am not sure if the OSRA is still in force. I know it clearly states in the HSW regs that the relevant section (schedule 1 part1), as stated in my previous posting refers only to factories, but as the OSRA says the very same thing, then the same figures apply.

Check out http://www.statutelaw.gov.uk/content.aspx?activeTextDocId=1167233

Check out contents, schedule 1 at http://www.legislation.gov.uk/uksi/1992/3004/contents/made

[Davo]

TW

Schedule one does indeed refer to factories only, the Regs refer to both.

There are still (very obscure) parts of both the OSRP and Factories Act still not repealed, believe it or not


davo

[wee brian]

If you are trying to establish if a building is over occupied then the ADB floor space factors are irrelevant. As has been said, this is a design tool where occupancy is unknown Para 3.8 spells this out.

For an exisitng building, what you need to know is the capacity of the escape routes. You can use the RRO guides, AD B or BS 9999 etc and work backwards to the max occupancy.

However this only adresses means of escape. You should also consider the provisions for toilets, ventilation etc which have a H&S implication. Sometimes you cant actually fit all the people into the building that the MoE allows, other times the routes are tight and you can over populate a building and get into trouble.

[CivvyFSO]

Stu,

For an existing building that has not been refurbished youshould refer to the RRO guide for offices and shops which will have most of the answers for you but maybe not all.

However, if you have an office with 100 persons at their desks and assuming its open plan your calculation would work back to a maximum room size of 600m2. It would be worth taking into consideration the meandering distance around the desks from the furthest point in the room to the nearest exit as this would be your maximum travel distance taking into account the configuration of the desks.

Furthermore, assuming the building is of class B construction you would have an evacuation time of 2.5 minutes. Allowing for 40 persons per minute as an average pass through rate, you would get 100 persons through in 2.5 minutes and on face value you would only need 1 exit. In reality the evacuation time would be a lot quicker again assuming able bodied, familiarity with premises and successful evacuation drills.

But, using ADB as you have done and therefore assuming a major refurbishment or new build ! (which is why you would be referring to ADB) and using the figures above you would only require 1 Exit with a minimum width of 525 mm. Your storey exit is likely to be no less than 762mm anyway and therefore suitable.

All that said, Fire Safety is a dark art and further consideration has ro be given to the fact that with a room or enclosure containing more than 60 persons you are required to have an additional exit.  The travel distance to any of your 2 exits is then classed as travel in more than 1 direction and this also then factors in that within an office, from the furthest point in the office (open plan) you would have to reach the nearest exit witihn 45m.

Because of the 100 persons you can no longer consider a single exit as suitable. If you reduce the occupancy to 80 you would still require a second exit.

Hope this is enough for you to go on, if not add some more info, i.e. dimensions and type of office, cellular with corridors or open plan as I have guessed.

Fire safety will always be a dark art while people manage to make a relatively simple scenario seem this complicated.

Wee Brian wins the "giving a short and sensible answer" competition.

[deaconj999]

If I'd known a prize was on offer I would have kept it simple........silly me.

When is the next competition open

[Phoenix]

Good God (if there is one), some good answers, some verbose and circumlocutionary diatribes.

It's quite simple.  The table in Appendix C of ADB is there to provide an indication of the number of people who, if there is no specific information on population figures, might be expected to occupy a room/space/building.  If there is specific information available about the occupancy level then that takes precedence.  If not then the Appendix C table gives indicative figures that can be used to assess adequacy of means of escape.

Stu