The LWF Blog

Fire Safety Engineering for Design – Means of Escape Design – Part 97

September 5, 2022 11:02 am

LWF’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 96, LWF looked at occupancies, floor space factors, exit widths and the standards for escape design. In part 97, we continue discussing exit widths before considering stair capacities.

While UK regulations for means of escape require that one exit (the one with largest capacity if they differ) is discounted from calculations, NFPA 101, from the United States uses a different approach which means it is not necessary to discount exits or adopt reduced capacities for door openings between 810 and 1050 mm wide.

In NFPA 101, for instance, the main entrance/exit of an assembly building must be designed to accommodate at least half of the full occupancy. In the case of dance halls, night clubs and assembly buildings with festival seating, the main entrance/exit must be able to accommodate two thirds of the total occupancy.

BS 9999 is centred around a risk-based approach which allows for means of escape to be designed on a risk basis. This means that narrow exit widths can be accommodated depending upon the rest of the building design and the fire safety management procedures put into place by the building management. When using BS 9999 in this regard, it is essential it is used in its entirety and that the escape provisions discussed in the standard are not mixed and matched with other standards.

Capacities of Stairways

Simultaneous evacuation applies to very many buildings and so is the form of evacuation most people are familiar with. The alarm is raised and everybody in the building evacuates to an outdoors place of safety at the same time.

When designing means of escape, it may be that the system is designed to work with places of relative safety. A protected stair enclosure can be considered a place of relative safety (due to the fire-resistant construction and compartmentation).

The safe capacity of a stairway is dependent upon the rate at which people can leave the building by the final exit and the number of people who can be accommodated in the enclosure (stacking capacity).

In part 98 of LWF’s series on fire engineering, we will continue to look at stairway capacity and how this can be calculated. 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.