The LWF Blog

Fire Safety Engineering for Design – Stairway Capacity – Part 125

March 27, 2023 11:36 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 124, LWF began to look at exit flow, which is the flow rate of persons evacuating through a doorway, corridor or stairs, and stairway capacity. In part 125, we continue to discuss stairway capacity.

In a very tall building and in a fire situation, escape from the floor of fire origin onto the escape stairway(s) is necessary for any affected building occupants. The escape stairway is considered a place of safety until those people evacuating can reach a place of safety outside the building. The complete evacuation of those persons to ground level outside the building may take some significant time.

It is important that the necessary number of people who must evacuate can be accommodated by the escape stair in a suitable amount of time. There are certain factors that must be taken into account:

  • The width of the storey level exits
  • The width of the stairway and the final exit width
  • The number of persons who may be accommodated within the stairway (stacking capacity)

The doorway opening onto the stair must be of an appropriate size to accommodate the anticipated number of people at each level of the building.  However, in a multi-level evacuation, the stairway must not be so congested with people from the storeys above any given entrance to the stairway that the occupants of that storey cannot enter the stair.

During an evacuation, people will enter the stairway at different floor levels and some will be leaving through the final exit. The stairway must have sufficient floor space to accommodate those persons who are remaining within the stair enclosure.

An equation can illustrate the maximum number of people who can be accommodated in a stairway at any one time:

Where Nc(max) is the maximum number of people that can be accommodated within a stairway at any one time, p is the maximum occupant density of the stair (person · m–2), A is the horizontal area of the stair and landings per storey (m2) and S is the number of storeys.

In part 126 of LWF’s series on fire engineering, we will continue looking at how the exit capacity of a stairway can be estimated. 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 since 1986 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.



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