Lawrence Webster Forrest (LWF), Fire Engineering and Fire Risk Management Consultants
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E-mail: fire@lwf.co.uk

Basement Smoke Ventilation

Basement Fires

Fire in a basement is very different to that above ground for two fundamental reasons.  Firstly, true basements are enclosed, with no windows or means of ventilation, this condition allows for rapid heat build up with nowhere for the heat to dissipate to, other than internal openings within the basement. Within a basement, the most common internal vertical opening into which smoke will want to dissipate is the stairs, which are likely to be required and possibly used for means of escape.  This is obviously an undesirable situation, i.e. excessive heat build up, with the only natural path for dispersal being the escape route.  Secondly, fires below ground are likely to require offensive fire-fighting, whereby the Fire Service need to enter the space and fight the fire internally.  To do so, fire-fighters will need to descend from the ground floor into the basement area, which may require them to pass through the hot gas layer, this can put fire service personnel at considerable risk.

On the basis that increased heat build up is the primary concern with basement fires, it should be no surprise that the critical fire safety requirement placed on a basement is ventilation, i.e. a means of allowing the smoke and heat to pass from the basement area to external, thus reducing the risk associated with the conditions described above.  There are two common methods of reducing the smoke / heat build up within basements, namely natural and mechanical ventilation, although it is noted there are variations thereof.

It is noted that requirements under current guidance can be difficult to achieve and in some cases, due to a lack of design understanding, basement smoke vents have been removed / covered during refurbishment projects, leading to non compliances.

This bulletin discusses the design requirements set out in the current guidance.

Basements Requiring Ventilation

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

·         A floor area of more than 200m2 and;

·         A floor more than 3m below the adjacent ground level.

·         All basement car parks, regardless of the size. 

Design Guidance – Basements other than Car Parks 

In order to achieve compliance with the Building Regulations, the following primary guidance is available: 

Approved Document B (B5) 
BS 9999: Code of practice for fire safety in the design, management and use of building: Section 6 (supersedes BS 5588 Part 5)  

Approved Document B (B5) and BS 9999 Section 6 provide the design criteria for basement smoke ventilation systems (except basement car park smoke ventilation). To comply with the prescriptive guidance there are two options: 

Natural Ventilation:

Provide natural smoke ventilation at high level achieving a minimum of 1/40th (2.5%) of the floor area of the storey. Smoke 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 would prevent the use of escape routes from the building. Separate smoke outlets should be provided from places of special 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:

Mechanical smoke extraction can be utilised as long as the basement is protected by a sprinkler system in accordance with BS 5306-2 or BS EN 12845. To ensure the effectiveness of the mechanical smoke extraction system, it is required that a sprinkler system be installed throughout the basement to control the maximum likely fire size. (Note: it is not considered necessary in this particular case to install sprinklers on the storey’s other than the basement(s) unless they are needed for other reasons). 

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 smoke rated, as should any dampers used within it. With powered extract the important aspect is that there must be multiple extract 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 (at least L3 standard). 

When a powered extract system is used, it is recommended that the smoke outlets be positioned similar to that 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 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 it should be positioned at high level so that they would not affect any pedestrians or people escape from the building. 

Design Guidance – Basements Car Parks 

In order to achieve compliance with the Building Regulations, the following primary guidance is available: 

Approved Document B (B3) – Basement Car Park Smoke Ventilation
BS 7346-7: 2006 Code of practice on functional recommendations and calculation methods for smoke and heat control systems for covered car parks  

Approved Document B (B3) – Basement Car Park Smoke Ventilation 

This Approved Document covers the provision of natural and mechanical ventilation for basement car parks.  

Natural Ventilation:

Naturally ventilated car parks require a minimum 1/40th (2.5%) of the net floor area of the basement car park in openings linked directly to the atmosphere, with at least 50% of the opening being split between two opposing walls, 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.

 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 (other than 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 50% of the rates set out in above and designed so that each part may operate singly or simultaneously;
Each part of the system should have an independent power supply which would operate in the event of failure of the main supply;
Extract point should be arranged so that 50% of the outlets are at high level and 50% at low level; and
The fans should be rated to 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 extract system is used.

 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 a conventional natural ventilation / mechanical ventilation, BS 7346-7 also covers performance-based mechanical ventilation in basement car parks using jet/impulse fans. Where jet fans are employed the design utilises sets of fans to exert thrust on the air within a space to accelerate air, creating flow in a desired pattern, effectively moving smoke and smoke within that space. In large basement car parks, selective operation of the jet fans associated with the affected fire zone will direct the smoke to the appropriate extract location by the most efficient route. The jet fans will induce air from other areas outside the fire zone, moving the smoke within a predetermined path; the objective being to protect unaffected zones, escape routes and fire-fighters access routes from smoke contamination.

The effectiveness of a jet fan system in basement car parks is often demonstrated by using computational fluid dynamics (CFD) model. However, it must be borne in mind that CFD modelling should not be used as the only tool for system design as it is possible that inaccurate information may be used within models, resulting in substandard analyses.

Conclusion

Basement smoke ventilation is fundamental to ensuring that the specific conditions that occur in a basement environment can be appropriately dealt with. The published guidelines for basement smoke ventilation designs have been discussed along with the recommended methods of meeting the performance requirement of the Building Regulations. The optimal solution for a specific building is not always clear and will require careful consideration.  In some instances, the guidance document requirements can not be easily applied to a design, if this is the case a fire engineering solution that meets with the functional objectives of the Building Regulations can be adopted and applied on a bespoke basis.

Should you wish to receive any further information on LWF and the services we provide please contact Peter Gyere, Marketing Director Telephone 020 8668 8663.

 This bulletin was written by Ken Seow MSc Fire Engineer

LWF are fire engineering and fire risk management consultants with over twenty five years experience in the development of fire engineered technology and the application of fire safety standards including fire engineered techniques.

Should you wish to receive any further information on LWF and the services we provide please contact our Marketing Department.

 

 

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