Smoke Ventilation Systems

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A basic introduction to smoke control

This document is intended to provide a basic understanding of how smoke control systems can protect multi-storey residential buildings and what the legislative framework is.

Why is smoke control installed in apartment buildings?

o In multi-storey apartment buildings, the main escape route is always via common corridors and/or lobbies to protected stairs.
o Smoke can easily spread from the accommodation, and if a door is simply left open for a short period of time quickly fill a corridor or lobby, making escape difficult for occupants.
o Smoke entering the stairs can also make escape difficult for occupants of higher storeys.
o In taller buildings the fire and rescue services need clear access to stairs and lobbies to form a bridgehead for operations, using a fire fighting shaft which is protected from smoke.

Legislation and standards
The legislative requirements derive from the Building Regulations for England and Wales (2000). Detailed recommendations to meet these requirements are provided in Approved Document B (ADB) 2006. It must be noted that the Building Regulations for Scotland and Northern Ireland vary in some details.

Smoke Control
In designing a building, the incorporation of a smoke control system can significantly improve the life safety protection of the building. Smoke Control systems (sometimes known as fire ventilation or smoke vents) come in many forms and range widely in complexity, but their basic objectives are usually the same.

The objectives of smoke control systems are:-
o To keep the escape routes free from smoke.
o To assist fire fighting operations by creating a smoke free layer.
o To delay and/or prevent flashover, and the subsequent full development of a fire.
o To reduce the thermal damage to the structural components during a fire.
o To reduce smoke and heat damage caused by the fire and associated hot gases.

The key components of smoke and fire ventilation systems are fire dampers; ductwork; smoke curtains, fans etc and these items can form part of either a natural or powered "Smoke and Heat Exhaust Ventilation system" (commonly known as SHEVS).

Smoke Ventilation

Most large buildings contain sufficient air to sustain a substantial fire, so the well known domestic advice to shut all doors in the event of a fire does not apply. The greatest immediate danger to the occupants of a large building in the event of a fire comes from the smoke rather than the heat of the fire. Even a small fire can rapidly fill a large building with smoke to an extent where people escaping the building cannot see to find the escape routes and can be overcome by smoke inhalation.

There are two types of smoke ventilation systems and it is important not to confuse the two.

Smoke Clearance ventilation Systems
o These are installed in the corridors and stairwells of hotels, apartment blocks or in enclosed or underground car parks. Their purpose is to ventilate the common areas which may become completely smoke logged following a fire. They are intended to clear the smoke to facilitate access. They are not specifically intended to offer protection to occupants escaping the building. Escape routes should normally comply with building regulation requirements to facilitate escape before smoke becomes life threatening.

Life Safety smoke ventilation systems
o These are intended to provide a level of protection to people escaping the building. They are typically installed in shopping centres or malls, large warehouses, and public buildings where due to the layout of the buildings the escape routes to places of safety are complex or escape distances are greater than those recommended in the building regulations. Such systems are designed so that the expected quantity of smoke is vented from the area leaving a layer of clean air below. Within this clear layer people are able to pass and exit the building safely.

Fundamentals of design
Smoke ventilation systems do not generally have any influence on the size of a fire in the building and they are therefore often used in conjunction with water sprinkler systems which can often contain the fire to a predetermined size. All smoke ventilation systems must have a provision for air to replace the smoke vented from the building. The inlet air is usually at low level but in the case of large buildings such as a warehouse, the roof may be divided into high level smoke zones by using either fixed or operable smoke curtains; this allows the adjacent zones away from the zone over the fire to be used as the air inlet.

Not all smoke ventilations can be designed around the requirements of Approved Document B, there are many reasons including:-
o An atrium that rises from a basement through several floors
o Escape distances from shops through a covered Mall may exceed recommended safe travel distances
o Large open plan warehouses or supermarkets may exceed the recommendations for compartment sizes
o Underground car parks with little or no inlet air route or perimeter ventilation

The above are just a few of the "challenges" in finding an "engineered solution" to a smoke ventilation issue.

In the above examples, typical engineered solutions may include:-
o Balconies can be protected by automatic smoke curtains and smoke ventilation systems can be designed on a negative pressure basis to reduce the smoke flow from an atrium into the building.
o By providing compartmentation into smoke zones along with smoke ventilation this sometimes will compensate for extended travel distances for escape in shopping centres.
o High level smoke barriers and smoke ventilation can reduce the spread of smoke (along with a sprinkler system) and these can compensate for larger open building areas.
o Covered and basement car parks may be provided with mechanical extract systems using either ductwork or Jet Fan. These systems may create virtual smoke zones, allowing larger open areas.

In a number of instances the performance of any of these systems may be proved by computer modelling (CFD), this is sometimes requested by Building Control to verify the design of the "engineered solution". Unfortunately there are no "catch all solutions" here and each project will end up with a differing solution that is developed by the Smoke Ventilation Company along with the Consulting Engineers and Architects. The approval of fire safety issues, including smoke ventilation in the UK is the responsibility of the local authority building control departments who also consult with the local fire brigade.

Their interest is to establish that the overall building design complies with legal safety requirements as required by building regulations (Approved Document B). Their approval for more complex buildings may take into account fire engineered solutions using fire alarm upgrades, sprinklers, smoke control zones and smoke extract systems. All these items contribute in some way to providing ultimately the "safe means of escape" for the occupants and a "safer" working environment for the fire officers in attendance.

In addition to being satisfied that the overall design is correct they will want to be assured that equipment used for smoke control meets national standards. The British/European Standard EN12101 is for equipment to be used in smoke control systems with part 1 for smoke barriers, Part 2 for natural smoke ventilators, Part 3 for powered smoke ventilators, and Part 6 for Pressure Differential systems. In all cases, independent certification of compliance is generally required for major items of equipment

Once installed the SHEV system should be certified as operating in accordance with the agreed design specification that has the approval of Building Control. This should be done by the installation company and may need a demonstration to the client, building control and possibly by the local fire brigades fire prevention officer. Furthermore, annual testing and recertification is required with the building fire file updated with a current certificate.

Why is Smoke Control and Ventilation used?

It is a mandatory to ensure that Means of Escape routes for occupants of any building during a fire, along with the access routes for the fire fighters, are as clear of smoke as possible and it is also necessary to clear the smoke from a building after a fire. An additional benefit to a well designed Smoke and Heat Exhaust Ventilation system (SHEVs) is that decreasing the spread of smoke in a fire reduces the risk of panic. It should be remembered that a sprinkler system is there to put out the fire, this does little if nothing to get smoke out of the building.

Where is Smoke Control and Ventilation Used?
o Large factories, warehouses and other storage buildings where exit points should be within 45m of any point in the building
o Large shops, stores and covered markets
o Covered shopping centres (or Malls), the landlords domain or common areas has its own extract system, this is not designed to take the smoke load from a large single retail premises.
o Where there is an atrium in a building
o Apartment buildings (especially single stair buildings and those over three floors)
o Where travel distances exceed those in Approved Document B
o Covered or underground car parks where there is no significant ventilation
o Prisons

Smoke and Heat Exhaust Ventilation Systems
Known as "SHEVS" (Smoke and Heat Exhaust Ventilator Systems), this is the generic term for both natural and powered smoke extract systems, and is sometimes referred to as fire or smoke vents. Generally, these systems remove the smoke from the building allowing escape routes to be kept clear of smoke and provide a clear access route for the attending fire officers

Apartment Buildings - Means of Escape
In certain instances, Approved Document B requires smoke ventilation to common corridors and escape stairs in apartment buildings. Smoke control systems are often provided in multi-storey apartment buildings, principally to assist escape in the event of a fire, which need to comply with the recommendations of Approved Document B.

Single stair buildings
In a single stair building, where travel distances exceed 4.5m, by providing ventilation by using an Automatic Opening Ventilator (AOV) the travel distance may be increased to 7.5m. The same can be achieved by providing ventilation to a lobby (known as a protected lobby) that sits between the corridors and the stairwell. Ventilation can be achieved in a number of ways, for a corridor with an outside wall, the addition of actuators to allow windows to open outwards at the top to provide a free area of 1.5m2 (see Approved Doc B diagram C7) will allow the extended travel distance. Alternatively a smoke shaft can be utilised in the corridor, this will require a shaft that terminates above the roof with an opening of 1m2 at each level, again details can be found in Approved Document B on page 30 (section 2.26 smoke control of common escape routes by natural smoke ventilation.)

Multiple stair buildings
Where there are more than one stairwell serving all floors the allowable safe travel distance can be as much as 30m if the corridors between the apartment door and the stairwell are ventilated, again this is achieved by either windows opening to provide 1.5m2 free area or by utilising the smoke shaft as above. If there are dead ends that provide protection to the stairwell by use of fire doors these limit travel distances to 7.5m in the dead-end section and 30m in adjoining corridors.

In most cases, where there is a ventilation requirement, the stairwell is protected by a 1m2 vent at the highest point in the stairwell; this is triggered at the same time as the other vents open. The correct size of vent depends upon its location, this information is found in BS5588-5:2004, (section table 6). It should also be noted that the top of the vent should be at the same height (preferably higher) than the doors that serve the area being ventilated.

Buildings where the uppermost habitable floor is less than 11m high
If the building is under 11m high, has no more than 3 storeys and the staircase does not connect to a covered car park, then only a stairwell ventilator is required, provided that the escape distances are limited to 4.5m in the corridor. If the corridor is also ventilated, then escape distances can be extended up to 7.5m.

All buildings where the uppermost habitable floor is over 11m

In all other apartment buildings over 11m high, the travel distances are limited to 7.5m in one direction and 30m if escape is available in two directions. Any corridor or lobby that opens onto a staircase must also be ventilated. Ventilation can be achieved by an Automatic Opening Vent (AOV) with a minimum free area of 1.5m 2 or if the corridor is landlocked then a shaft system will be required. This can be either natural or mechanical.

Fire fighting shafts in buildings where the uppermost habitable floor is over 18m
In all apartment buildings over 18m high, fire-fighting cores provide smoke free access to the upper floors of a building and allow the fire fighters to attack the fire from a position of relative safety.

There is a requirement for most types of building above 18m to contain a fire-fighting core with at least one fire-fighting lift. This also applies in basements more than 10m deep. Fire fighting cores without a lift are required where there are 2 or more basements each exceeding 900m2. The most relevant standard is BS 5588 -5: 2004: Access and facilities for fire-fighting. Fire fighting shafts in tall or large structures require smoke vents or pressurisation systems to allow clear conditions for fire fighters to enter the building and deal with the situation effectively. The requirement for ventilation of the fire-fighting shaft, the fire fighting lobby or both is established using BS5588-5:2004 and due to the code complexities, information around these requirements is not included in this document.

Fire-fighting cores can include the following elements:
o A protected staircase
o A protected lobby
o A fire-fighting lift (not for shallow basements)

Note that generally for apartment buildings, the protected lobby is not necessary and a normal protected corridor will be sufficient.

Shaft vents (differing types)

Approved Document B
Where natural ventilation is used, ADB recommends that the shaft:
o Is closed at the base
o Has a minimum cross-sectional area of 1.5m2 with a minimum dimension of 0.85m in either direction
o Extends at least 0.5m above the highest structure within 2m
o Extends 2.5m above the ceiling of the highest level served by the shaft
o Should be constructed from non-combustible material and the vents to be equivalent to an E30S fire door
o Is vertical with no more than 4m at an inclined angle (max 30).
o The vent into the shaft, the vent at the top of the shaft and any safety grilles in the shaft should all have a minimum free area of 1.0m2.

Smoke Shaft BS5588 part 5
The standard smoke shaft as recommended in BS 5588 Part 5: 2004 comprises of:-
o a shaft with a minimum cross sectional area of 3m2 terminated with an AOV to provide weather proofing when closed.
o The bottom the shaft is open, or has an automatically opening ventilator or fire damper to the outside with a minimum free area of 1.5m2.
o At each storey an AOV is connected at high level in the lobby into the shaft, providing an air/smoke flow path from the lobby to the shaft.
o Each damper or door has a minimum free area of 1.5m2.

BRE79204 shaft method
Trials at the Building Research Establishment provide a framework for a simpler shaft design; the principles of operation are the similar to BS5588 part 5 however the base of the shaft is closed.

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