# The LWF Blog

## Fire Safety Engineering for Design – Exit Widths for Evacuation – Part 124

March 20, 2023 11:54 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 123, discussed exit widths as the final part of exit routes. In part 124, we begin to look at exit flow which is the flow rate of persons evacuating through a doorway, corridor or stairs, and stairway capacity.

When building occupants have begun the evacuation procedure and are moving towards an exit, the limiting factor in how fast this can be accomplished is usually the width of the doorway openings, corridors and stairs that the people must pass through. An equation to calculate and express the maximum flow rate of persons through such a space is given in CIBSE Guide E, as follows:

For openings and corridors of width 1.1 m and greater:

Where Fp is the number of persons passing through the opening in 1 second (person · s–1) and w is the width of the opening or corridor in metres, after allowing for any obstructions.

If the notional exit time of 2.5 minutes applies (as discussed in part 123 of this series), this equation is equivalent to the method of determining exit widths given in British codes and NFPA guidance – that the capacity of an exit is 1 person per 5 mm of exit width (subject to minimum width criteria).

The flow rate through an exit may be reduced where there is downstream congestion, e.g. if the occupant density significantly exceeds 2 persons per meter-2.

### Stairway Capacity

A protected stairway is generally considered a safe space when it comes to evacuation from a building where there is a fire. The fire protection built into protected stairways means that the compartment is separate from the accommodation surrounding it and it should remain clear of smoke and other products of fire.

It should be borne in mind, however, that in very tall buildings, reaching the protected exit stair and beginning to descend may only be the start of an extended evacuation process for the building occupants. It may take over an hour for all of the occupants to descend the stairway and reach open air outside the building.

The stairway must have sufficient capacity to be able to accommodate the occupants of any floors affected by fire.