# The LWF Blog

## Fire Safety Engineering for Design – Stairway Capacity – Part 126

April 3, 2023 11:19 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 125, LWF began to discuss stairway capacity. In part 126, we continue looking at how the exit capacity of a stairway can be calculated. The approach, guidance and methodology should be consistent with guidance for the occupancy and fire strategy, this blog contains an example of one calculation method.

The number of persons who can be accommodated in a stairway is limited to 3.5 people m-2. An occupancy of the area above this level of people would result in extreme discomfort in those attempting to evacuate. The number of people who may exit the stairway is limited by the width of the final exit and can be calculated as follows:

Where Nin(max) is the maximum number of people able to enter the stair within a specified period, Ws is the width of the stair (m), t is the time available for escape (s), A is the horizontal area of stair and landings per storey (m2) and S is the number of storeys served.

The results obtained by using this calculation will give results similar to those found in Table 7 of Approved Document B Volume 2.  ADB used a similar principle but a simplified calculation procedure.

When wishing to calculate the total number of persons a stairway may accommodate in a specified period, the following equation may be used:

Assuming the following conditions:

• An occupant density (p) in the stair of 2 persons . m-2
• A flow of 2.2 persons .s-1.m-1 of stair width (where the effective stair width We is 0.3 metres narrower than the actual width due to the natural tendency of people to keep a distance from walls and handrails.

It should be noted that while this equation gives the maximum acceptance capacity of the stairway itself, it does not calculate bearing in mind the width of the storey exit onto the stair. If this exit is too narrow, the flow onto the stair may be limited.

In taller buildings, where the stairway will extend to more than 30 metres long, the width must be constrained to 1.4 m wide or it should be at least 1.8 metres wide and contain a central handrail. People prefer to stay within reach of a handrail when making a prolonged descent on stairs and a wide stair with no central handrail would result in a middle-ground which is generally avoided and could become hazardous (as well as limiting the number of persons in the stair).