The LWF Blog
Building design & fire safety | Basement car park solutions | Part twoJuly 26, 2013 11:30 am
LWF’s mission to assist designers, architects and building project managers to provide fire safe design continues with the theme of basement ventilation solutions for basement car parks.
There are two forms of primary guidance which provide the structure to achieve fire safety compliance with current building regulations for basement car park designs and renovations.
The first covers the provision of natural and mechanical ventilation for basement car parks and is: Approved Document B (B3) – Basement Car Park Smoke Ventilation.
Natural ventilation – Naturally ventilated car parks require a minimum 1/40th (2.5%) of the net floor area of the basement car park to be installed in openings linked directly to the open atmosphere. At least 50% of the openings should be split between two opposing walls, as this allows a good flow across the vents and ensures that the system is not undermined by wind pressure acting on one side of the building – as a positive pressure to one face, should assume a negative pressure to the opposing face.
Mechanical ventilation – The minimum rate for a mechanically ventilated car park is 10 air changes per hour under fire conditions. The system should be independent of any other ventilating system (any system providing normal ventilation to the car park).
Other design considerations include:
• The system should be designed to run in two parts, each part capable of extracting half of the prescribed 10 changes per hour under fire conditions.
• Each part of the system should have an independent power supply which would operate in the event of failure of the main supply
• The extract points should be arranged so that 50% of the outlets are at a high level and 50% at a low level
• The fans should be rated sufficiently that they can run at 300°C for a minimum of 60 minutes and the ductwork and fixings should be constructed of materials having a melting point not less than 800°C.
Unlike other basement occupancies, due to the well-defined fire load in basement car parks, a sprinkler system is not required by the guidance, when a powered mechanical ventilation system is used.
The second piece of regulation relating to basement car parks is BS 7346-7: 2006 Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks.
This British Standard gives guidance on providing smoke ventilation from enclosed car parks. It outlines the design requirements for natural and mechanical systems, to ensure smoke clearance, aid fire-fighting and to give protection to the means of escape.
In addition to conventional natural ventilation/mechanical ventilation, BS 7346-7 also covers the use of performance-based mechanical ventilation in basement car parks using jet/impulse fans. This involves a system where sets of jet fans exert thrust to move and flow air in a desired pattern.
In large basement car parks, selective operation of the jet fans associated with the affected fire zone will direct the smoke to the most appropriate extract location by the most efficient route. The jet fans will induct air from other areas that are outside the fire zone, moving the smoke within a predetermined path; the objective being to protect unaffected zones, escape routes and fire-fighter access routes from smoke contamination.
The effectiveness of a jet fan system in basement car parks is often demonstrated by using a computational fluid dynamics (CFD) model. However, it should be noted that CFD modelling cannot be used as the only tool for system design, as it is possible that inaccurate information may be used within models, resulting in sub-standard analyses.
The optimal solution for a specific building is not always clear and will require the careful consideration of the designer or renovator. In some instances, the guidance document requirements cannot be easily applied to a design. On these occasions, a bespoke fire engineering design that meets the requirements of the building regulations is required.
For more information on fire engineering design, and the other services offered by LWF, please contact Peter Gyere, Marketing Director by calling 020 8668 8663.