Protected stairwells, sometimes known as protected stairways or protected stairs, are an extremely important part of smoke ventilation systems. This post will help you understand exactly what protected stairwells are and how they’re protected.
It will be beneficial to understand what protected space is in general before looking at protected stairwells specifically. Protected spaces are relevant in all smoke ventilation systems, especially in order to identify escape routes.
BS 9991 is a good place to start when wanting to get a handle on the different types of protected space, and in doing so you begin to understand their purpose. For example, for the definition of a fire-fighting lobby, BS 9991 states:
Protected lobby providing access from a fire-fighting stair to the accommodation area and to any associated fire-fighting lift.”
Fire-fighting shafts are defined as:
Protected enclosure containing a fire-fighting stair, fire-fighting lobbies, a fire main, and, if provided, a fire-fighting lift together with any machinery space.
Fire-fighting stairs are referred to as:
Protected stairway communicating with the accommodation area only through a fire-fighting lobby.
There’s then a need to understand exactly what a protected stair is. Approved Document B defines a protected stairway as:
A stair that leads to a final exit to a place of safety and that is adequately enclosed with fire resisting construction. Included in the definition is any exit passageway between the foot of the stair and the final exit.
To break this definition down further, protected stairwells are designed to provide zones free of products of combustion in the event of a fire. The stairwells are used as a means of escape for occupants in the event of a fire, as once somebody reaches the stairwell they’re considered to be safe from the effects of the fire.
Products of combustion must be kept as far away from these zones as realistically possible, this is through the use of fire resisting structures such as concrete and systems like a smoke ventilation system, be it natural, mechanical or pressure differential.
The enclosure of a protected stairwell should have a fire resistance of no less than 30 minutes when tested in accordance with a variety of British Standards such as 476-21 and 476-22, or their European equivalents.
Where the building has a phased evacuation system or serves a storey more than 18m high, the stair will also need a protected lobby or corridor. Alternatively, a pressurisation system can be used.
Another aspect to note is where an escape route from a ground floor or basement is formed from the exit route from the stairwell, it’s likely that the width of the exit route may need to be increased accordingly. This is calculated using the merging flow formula published in BS 9999.
If you have any questions regarding this topic, please don’t hesitate to contact us and speak to our friendly team who will be happy to talk you through the options available. If you’d like to continue learning about smoke ventilation, you’ll find some helpful resources on our blog and YouTube channel.