The LWF Blog
Fire Safety Engineering for Design – Means of Escape Design – Part 95
August 22, 2022 10:48 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 94, LWF looked at the design codes used for means of escape guidance in the UK and abroad. In part 95, we consider occupant capacity and floor space factors.
When references are made to occupant capacity, it means the maximum number of persons that an area, room, storey or part of a building is designed to hold and for those people to be able to safely evacuate the building in a fire situation.
In an area with fixed seating, designed for people to be seated, such as a concert hall or theatre, the maximum occupancy is easily established from simply totalling the number of seats. In less structured conditions, however, it will be necessary to establish the maximum occupancy using floor space factors.
Floor space factors are calculated using the likely minimum area occupied by each person in m2 and will result in a conservative figure that is likely to overestimate the building population. The population can be calculated by dividing the area in question by the floor space factor.
Codes in England and Wales tend to discount areas such as toilets, stair shafts, voids and fixed elements of structure (but not counters, display units etc. in retail premises) from the overall floor area before calculating the available area and dividing it by the floor space factor.
The NFPA codes (based in the US but used in various parts of the world) tend to utilise the gross floor area without deductions.
Where the client/building owner has definitive figures available to demonstrate maximum occupancies, whether this is via their own trading figures or because they are basing it on the maximum number of covers in a restaurant, these may be used instead of the standard floor space factors.
Guidance on floor space factors in different codes may vary. For instance, in an office environment, commonly a floor space factor of 10 m2 is used, while Approved Document B (ADB) suggests 6 m2 per person is sufficient. In a call centre environment, where people are more densely packed, an exit provision based on 10 m2 per person may be inadequate.
When a building is being designed and constructed, it is important that any reduction in exit capacity based on the client’s current requirements is understood to restrict the future flexibility of use, and that the client understands the condition. Such restrictions must be agreed and recorded in a fire safety management plan.
It should also be noted that the NFPA codes do not permit any relaxation of the specified floor factors.
In part 96 of LWF’s series on fire engineering, we will continue discussing escape designs, occupancies and floor space factors. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.