# The LWF Blog

## Fire Safety Engineering for Design – Means of Escape Design – Part 98

September 12, 2022 11:04 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 97, LWF discussed exit widths before beginning to look at stair capacities. In part 98, we will continue on stair capacities and with simultaneous evacuation.

Simultaneous evacuation is where each floor or storey of a building and its occupants receive the fire alarm signal to evacuate the building at the same time. The capacity of a stairway designed for simultaneous evacuation, under British codes, can be established with the following equation:

In this instance, P is the number of people that can be served by the stair, w is the stair width in metres and n is the number of storeys served.

The required width of the stair can be found by adjusting the equation as follows:

NFPA 101 (a U.S. standard, also used widely) employs a simpler approach with the total number of people evacuating the building being multiplied by the required width per person.

In a high rise building, it is common for means of escape to be designed based on a phased evacuation from the premises. This means that the floor of fire origin (and perhaps the floor above and floor below) are evacuated when the fire is discovered and the remaining floors are evacuated as necessary, when it becomes so.

A phased evacuation system requires that adequate compartmentation is in place between levels to protect the floors not immediately evacuated from the effects of fire. This protection will be sufficient for a short amount of time and the evacuation plan should be based conservatively on those figures.

Most British codes require a stair width allowance of 5 mm per person, which is the same width allowance for horizontal travel. However, NFPA 101 increases this allowance to 7.6 mm per person to allow for the slower rate of travel down stairs when compared to horizontal travel and subject to minimum stair widths. NFPA 101 can also increase the stair width allowance per person to 18 mm per person, depending upon the occupancy type of the building.

For phased evacuation to work effectively and safely, it may be necessary for additional fire protection measures to be put into place. This might be an enhanced level of compartmentation, a PA system to guide those evacuating, fire telephones and, almost certainly, an automatic fire detection system.

In part 99 of LWF’s series on fire engineering, we will begin to look at the provision of alternative exits for means of escape. 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.