The LWF Blog
Fire Safety Engineering for Design – Means of Escape Design – Part 96
August 30, 2022 10:42 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 95, LWF began to consider occupant capacity and floor space factors. In part 96, we continue from that point looking at occupancies, floor space factors, exit widths and the standards for escape design.
Where an escape design includes the use of refuges, the idea behind it is that some people may need to be accommodated in a protected refuge area prior to being fully evacuated from the building. When the capacity of such a refuge is being considered, it is acceptable to base it on 2 persons per m2 for maximum occupant density. Fairly obviously, a refuge is a short-term fire protection measure.
Once the occupant capacity of a building has been established, the width of exits should be calculated. The recommendations vary depending upon which fire code is used. BS 9999 allows the clear exit width per person to be as low as 2.44 mm per person. NFPA 101 states it can be as high as 7.6 mm per person. The figures quoted are subject to overall minimum width constraints.
As any exit can be blocked by the fire necessitating evacuation, British codes tend to discount the largest exit from the calculations.
If a building has three equally sized points of egress, and they need to serve a capacity of 500 persons, the required width overall would be (500 x 5 mm) = 2500 mm. If one exit is discounted completely, the required minimum clear width of each of the two remaining doors would be 2500/2 = 1250 mm clear width per exit.
The third exit, which was discounted during the calculations should also be at least the same calculated dimension as the other two exits.
The example does not mean to infer that all exits from a building must be the same size, merely that they must all be at least a certain size to accommodate safe evacuation from the premises.
ADB Volume 2 assumes that an exit less than 1050 mm wide will have a proportionately lower capacity than a larger exit and the exit capacities given below are widely adopted:
Maximum no. of occupants Minimum clear width of exit / mm
60 750
110 850
220 1050
In part 97 of LWF’s series on fire engineering, we will continue looking at exit widths before considering stair capacities. 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.