The LWF Blog

Building design & fire safety | Basement ventilation solutions | Part One

July 18, 2013 10:06 am

Fire engineering and fire risk assessment assists designers, architects and building project managers to utilise new ideas and design features by providing innovative fire engineering solutions and guidance. Our article this week is looking at basements and the special attention they require during construction or renovation to avoid fire safety breaches.

A fire in a typical basement, with no windows or vents, builds heat quickly. The smoke has nowhere to dissipate to and so, will utilise the only potential exit available – the staircase to the upper floors. 

This means that not only is a potential escape route filled with smoke and heat exacerbated by the enclosed conditions, but that the fire is likely to need aggressive fire fighting tactics, leading to the fire fighters needing to descend the steps to fight the fire “head on”. This can also mean that they have to pass through the hot gas layer, putting them at considerable risk.

Fire safety requirements for basement situations involve ventilation which would allow the smoke and heat to pass directly into the outside air, reducing the temperature and the amount of smoke the fire produces within the basement.

Which basements require ventilation?

The current Building Regulations in England and Wales require that a smoke control system be provided for any basement storey that has:

  • A floor area of more than 200 square metres, and;
  • A floor more than 3 metres below the adjacent ground level
  • All basement car parks, regardless of the size (to be covered separately in the next blog)

The design requirements in the current guidance are based on Approved Document B (B5) and BS 9999 Section 6, which provide the design criteria for basement smoke ventilation systems.

What types of ventilation are acceptable under the guidance?

Natural ventilation – The provision of natural smoke ventilation can be provided at a high level from the basement floor which achieves a minimum of 2.5% of the floor area of the storey.  So, for example, with a floor area of 200sq.m, the total ventilation area should be a minimum of 5sq.m. The ventilation vents should be evenly distributed around the perimeter of the basement and discharge directly to open air. Smoke outlets should not be placed where they might prevent the use of escape routes, such as staircases.

Separate smoke outlets should be provided from places of particular fire hazard such as: Transformer and switch gear rooms, boiler rooms, storage space for fuel, or other highly flammable substances, and rooms housing a fixed internal combustion engine.

Mechanical Ventilation – To ensure the effectiveness of a mechanical smoke extraction system, it is required that a sprinkler system be installed throughout the basement to control the maximum likely fire size, in accordance with BS EN 12845. (Note: it is not considered necessary in this particular case to install sprinklers for storeys other than the basement, unless there are other indications that necessitate their installation).

Mechanical smoke extraction in a basement is generally achieved by providing ten air changes per hour. Additional design features also apply to ensure a level of redundancy in the system, namely dual smoke rated extract fans (300°C/60 minutes) with twin power supplies. 

Ductwork should be fire/smoke rated, as should any dampers used within it. With powered extract the important aspect is that there must be multiple extraction points in the ceiling to prevent a phenomena commonly termed “plug-holing”, which reduces the effectiveness of the system by extracting fresh air as well as smoke, reducing the buoyancy of the smoke.  Make up/inlet air is also required to replace the air that is being extracted. A powered extract system should come into operation automatically either on activation of the sprinkler system or by an automatic fire detection system conforming to BS 5839-1 (which is at least L3 standard).

When a powered extract system is used, it is recommended that the smoke outlets be positioned similarly to those for a basement natural smoke and heat ventilation, i.e. not in locations where that would prevent the use of escape routes. 

In addition, consideration should also be given to the safety of pedestrians outside the building, prior to the arrival of fire service taking control of an incident. (Note: a powered extract system is activated automatically by the fire alarm system).

Therefore, if mechanical smoke outlets are to be exhausted at ground, they should be positioned at a high level, so that the emissions would not affect any pedestrians or people escaping from the building.

Lawrence Webster Forrest (LWF) are specialist fire engineering and fire risk management consultants who provide a wide range of consultancy services to professionals involved in the design, development and construction of buildings. Call Peter Gyere on (0)20 8668 8663 for more information.

Part two of ‘Basement Ventilation Solutions’ will look at basement car parks.

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