While the main focus of the debate on warehouse fires has centred on sprinklers, smoke control is also vital issue, especially for firefighters trying to save a building. Paul Compton unveils the issues.
Hardly a day goes by without one or more fires in warehouses in the UK. Many of these result in the damage or destruction of buildings and contents, financial pressures or worse on the business, and even deaths, as in the Atherstone fire last year. There are around 4000 warehouse fires a year, and between April 2006 and March 2007 there were 11 warehouse fires with insured losses greater than GB pound 250,000.
Much debate about warehouse fires has centred around the role of sprinkler systems and compartmentation, but a topic that is also important is smoke control. A smoke control system can provide occupants with a clear escape path, reduce the likelihood of damage to stock and allow firefighters to enter a building earlier and tackle a fire at source, increasing the chances of saving the building. In some cases, effective smoke control can also be used as a trade-off for other fire safety measures, such as allowing greater travel distances to exits. A well designed system should be able to maintain adequate smoke-free escape conditions at low level, to allow the building to be evacuated with minimum risk of smoke inhalation, injury or death.
In a large single-storey warehouse, a fire can grow much faster than in a more conventional building, due to the large volume of the compartment. Smoke quickly rises into the roof space – it can spread laterally at up to 5m/second (an average person walks at around 1-2m/second and runs at around 7.5m/second). Once the roof space is full, the smoke will start to ‘build down’. The rate at which this occurs varies according to the nature of the combustible contents and the geometry of the building. In an unventilated situation, a building with a volume of 10,000m 3 can become smoke logged in just a few minutes. Although the smoke is largely made up of ‘entrained’ air, it can contain enough toxic substances and asphyxiates to disorientate and disable in seconds and kill within minutes.
The objective of controlling smoke is therefore to limit its spread and provide a means by which smoke and heat can be extracted. A system typically requires three elements:
– high level openings or fans
– barriers to restrict the spread of smoke
– inlet ventilators to provide replacement air
The following needs to be taken into consideration when designing smoke control systems for large, single-storey buildings:
Fire size: The base dimensions of the largest fire which the system is expected to cope with. Calculations may be based on a steady state fire, the largest anticipated fire size in a building, or a time dependent fire based on a fire changing size over a period of time, depending on the contents of the building and the information available.
Heat output: The total heat generated by the fire source – the convective element of the heat output drives the smoke flow.
Ceiling jet: A horizontal flow of hot gasses driven in part by the kinetic energy of the rising fire plume. It typically has a depth of around 10% of the building height.
Clear layer: The objective of most smoke control designs is to maintain a clear layer beneath the smoke for people to escape and firefighters to enter a building. The minimum clear layer for human protection in industrial buildings should be 3m. A much higher clear layer may be selected to limit smoke damage to stock.
Smoke zones: Where the building is large it should be divided into separate zones using high level smoke barriers. Smoke zones should not exceed 2000m 2 if naturally ventilated and 2600m 2 if mechanically ventilated.
Wind effects: If taller structures are situated close to the position of natural ventilators they can divert the wind airflow and cause positive pressures. Instead of these ventilators extracting air they could inlet and cause mixing and deepening of the smoke layer, resulting in smoke logging inside the building.
Air inlet/replacement air: Sufficient replacement air is important for the efficient operation of a smoke control system, to replace air entrained into the smoke plume. For a natural ventilation system opening ventilators, either in walls or in the adjacent non-fire zones in a multi-zone system, can provide the replacement air required. As more inlet air becomes available, the extract ventilation becomes more efficient. Care should be taken to ensure that all inlet air enters the fire zone below the smoke layer, to prevent ‘inlet mixing’ and a deepening of the smoke layer.
High bay storage
The potential for rapid damage in a high bay warehouse can be much greater than normal, due to the rapid growth of fire travelling up the vertical racking. For any fire safety strategy to be effective, it is essential that sprinklers are installed. Most modern high bay warehouses will contain ‘in-rack’ sprinkler systems. A smoke control system can be designed to work with the sprinklers and to remove smoke and so limit damage and assist the fire and rescue service.
Sprinkler systems
Sprinkler systems are designed to control fires but they cannot prevent a building from becoming smoke logged. Therefore a combination of sprinklers and smoke control should provide ideal fire protection. There has been an argument that the removal of heat by smoke vents could delay the operation of sprinkler heads and that by maintaining the oxygen content of a building, the fire might burn more fiercely. Recent research has shown, however, that venting does not significantly delay the operation of sprinklers closest to a fire, but does stop those that are further away from the seat of the fire operating unnecessarily.
Legal requirements
Approved Document B (ADB) gives recommendations on the fire safety requirements for industrial and storage buildings in England & Wales (Scottish and Northern Ireland building regulations apply elsewhere). Where these recommendations cannot be met, trade-offs are allowed and a key one is the limitation on travel distances. In such buildings there is a limit to the travel distance of 25m where travel is possible in one direction, and of 45m where travel is possible in more than one direction.
Where travel distances are to be extended, a smoke control system designed to BS 7346-4 may be accepted to compensate as part of an engineered scheme. Building Control will want to approve the design methodology.
ADB also specifies that the maximum floor area of an unsprinklered single storey building should be no greater than 20,000m 2. A smoke control system might be installed as a compensatory measure as part of an engineered scheme to increase the floor area.
Where local Acts are in force, these require additional fire protection measures to ADB for industrial and storage buildings. Typically, where such buildings exceed 7000m 2 then either a sprinkler or a smoke control system is required for firefighting access. Where such buildings exceed 14,000m 2 then both a sprinkler and a smoke control system is required.
The Regulatory Reform (Fire Safety) Order for England and Wales makes it obligatory in any workplace that the responsible person maintains a current fire risk assessment, and ensures that all relevant fire safety equipment is regularly tested and is maintained in effective working order by competent persons.
With careful design and consideration of all other features of a building, its contents and other fire safety measures, smoke control can be a valuable safety feature of single-storey warehouses and can mean the difference between firefighters saving the building or not. In fire engineered designs, it can provide valuable trade-offs against prescriptive measures.
MORE THAN COSMETIC
The original specification for this 12,077m 2 (130,000ft 2) storage facility for cosmetics company L’Oreal required the sizes of the smoke ventilators to be equal to 2% of the floor area. It also called for restrictions on ventilator sizes, a high local fusible link temperature, and maximum smoke zone of 1600m 2 (thus eight zones would be required).
As part of a value engineering exercise, Colt was asked to review the scheme and put forward a set of alternative proposals for consideration. The building was to be protected by an ESFR (Early Suppression Fast Response) sprinkler system, but such systems usually need smoke ventilators to close upon activation. But in this case, an unventilated fire could quickly lead to smoke logging of most or all of the smoke zones.
So the objective of the scheme was therefore primarily that of activation by firefighters on their arrival in order to clear smoke. Colt proposed that the smoke zones could be increased in size and reduced to three, but with a reduced risk of the building becoming completely smoke logged.
Furthermore, Colt demonstrated that an increase in ventilator size to 7m 2 would not reduce the effectiveness of the scheme, thereby saving the client money. Colt and Alpine, the sprinkler system installer, then ensured that there was adequate clearance between the ventilators and sprinkler heads to ensure that the operation of the ESFR sprinklers would not be compromised.
Colt also worked closely with Hathaway roofing to ensure that the installation of the ventilators did not exceed the 12.0m maximum height restriction imposed by Local Authority Planning. The final system comprised a three smoke zone pneumatic system with 31 ventilators.
While the main focus of the debate on warehouse fires has centred on sprinklers, smoke control is also vital issue, especially for firefighters trying to save a building. Paul Compton unveils the issues.
Hardly a day goes by without one or more fires in warehouses in the UK. Many of these result in the damage or destruction of buildings and contents, financial pressures or worse on the business, and even deaths, as in the Atherstone fire last year. There are around 4000 warehouse fires a year, and between April 2006 and March 2007 there were 11 warehouse fires with insured losses greater than £250,000.
Much debate about warehouse fires has centred around the role of sprinkler systems and compartmentation, but a topic that is also important is smoke control. A smoke control system can provide occupants with a clear escape path, reduce the likelihood of damage to stock and allow firefighters to enter a building earlier and tackle a fire at source, increasing the chances of saving the building. In some cases, effective smoke control can also be used as a trade-off for other fire safety measures, such as allowing greater travel distances to exits. A well designed system should be able to maintain adequate smoke-free escape conditions at low level, to allow the building to be evacuated with minimum risk of smoke inhalation, injury or death.
In a large single-storey warehouse, a fire can grow much faster than in a more conventional building, due to the large volume of the compartment. Smoke quickly rises into the roof space – it can spread laterally at up to 5m/second (an average person walks at around 1-2m/second and runs at around 7.5m/second). Once the roof space is full, the smoke will start to ‘build down’. The rate at which this occurs varies according to the nature of the combustible contents and the geometry of the building. In an unventilated situation, a building with a volume of 10,000m³ can become smoke logged in just a few minutes. Although the smoke is largely made up of ‘entrained’ air, it can contain enough toxic substances and asphyxiates to disorientate and disable in seconds and kill within minutes.
The objective of controlling smoke is therefore to limit its spread and provide a means by which smoke and heat can be extracted. A system typically requires three elements:
• high level openings or fans
• barriers to restrict the spread of smoke
• inlet ventilators to provide replacement air
The following needs to be taken into consideration when designing smoke control systems for large, single-storey buildings:
Fire size: The base dimensions of the largest fire which the system is expected to cope with. Calculations may be based on a steady state fire, the largest anticipated fire size in a building, or a time dependent fire based on a fire changing size over a period of time, depending on the contents of the building and the information available.
Heat output: The total heat generated by the fire source – the convective element of the heat output drives the smoke flow.
Ceiling jet: A horizontal flow of hot gasses driven in part by the kinetic energy of the rising fire plume. It typically has a depth of around 10% of the building height.
Clear layer: The objective of most smoke control designs is to maintain a clear layer beneath the smoke for people to escape and firefighters to enter a building. The minimum clear layer for human protection in industrial buildings should be 3m. A much higher clear layer may be selected to limit smoke damage to stock.
Smoke zones: Where the building is large it should be divided into separate zones using high level smoke barriers. Smoke zones should not exceed 2000m² if naturally ventilated and 2600m² if mechanically ventilated.
Wind effects: If taller structures are situated close to the position of natural ventilators they can divert the wind airflow and cause positive pressures. Instead of these ventilators extracting air they could inlet and cause mixing and deepening of the smoke layer, resulting in smoke logging inside the building.
Air inlet/replacement air: Sufficient replacement air is important for the efficient operation of a smoke control system, to replace air entrained into the smoke plume. For a natural ventilation system opening ventilators, either in walls or in the adjacent non-fire zones in a multi-zone system, can provide the replacement air required. As more inlet air becomes available, the extract ventilation becomes more efficient. Care should be taken to ensure that all inlet air enters the fire zone below the smoke layer, to prevent ‘inlet mixing’ and a deepening of the smoke layer.
High bay storage
The potential for rapid damage in a high bay warehouse can be much greater than normal, due to the rapid growth of fire travelling up the vertical racking. For any fire safety strategy to be effective, it is essential that sprinklers are installed. Most modern high bay warehouses will contain ‘in-rack’ sprinkler systems. A smoke control system can be designed to work with the sprinklers and to remove smoke and so limit damage and assist the fire and rescue service.
Sprinkler systems
Sprinkler systems are designed to control fires but they cannot prevent a building from becoming smoke logged. Therefore a combination of sprinklers and smoke control should provide ideal fire protection. There has been an argument that the removal of heat by smoke vents could delay the operation of sprinkler heads and that by maintaining the oxygen content of a building, the fire might burn more fiercely. Recent research has shown, however, that venting does not significantly delay the operation of sprinklers closest to a fire, but does stop those that are further away from the seat of the fire operating unnecessarily.
Legal requirements
Approved Document B (ADB) gives recommendations on the fire safety requirements for industrial and storage buildings in England & Wales (Scottish and Northern Ireland building regulations apply elsewhere). Where these recommendations cannot be met, trade-offs are allowed and a key one is the limitation on travel distances. In such buildings there is a limit to the travel distance of 25m where travel is possible in one direction, and of 45m where travel is possible in more than one direction.
Where travel distances are to be extended, a smoke control system designed to BS 7346-4 may be accepted to compensate as part of an engineered scheme. Building Control will want to approve the design methodology.
ADB also specifies that the maximum floor area of an unsprinklered single storey building should be no greater than 20,000m². A smoke control system might be installed as a compensatory measure as part of an engineered scheme to increase the floor area.
Where local Acts are in force, these require additional fire protection measures to ADB for industrial and storage buildings. Typically, where such buildings exceed 7000m² then either a sprinkler or a smoke control system is required for firefighting access. Where such buildings exceed 14,000m² then both a sprinkler and a smoke control system is required.
The Regulatory Reform (Fire Safety) Order for England and Wales makes it obligatory in any workplace that the responsible person maintains a current fire risk assessment, and ensures that all relevant fire safety equipment is regularly tested and is maintained in effective working order by competent persons.
With careful design and consideration of all other features of a building, its contents and other fire safety measures, smoke control can be a valuable safety feature of single-storey warehouses and can mean the difference between firefighters saving the building or not. In fire engineered designs, it can provide valuable trade-offs against prescriptive measures.
MORE THAN COSMETIC
The original specification for this 12,077m² (130,000ft²) storage facility for cosmetics company L’Oreal required the sizes of the smoke ventilators to be equal to 2% of the floor area. It also called for restrictions on ventilator sizes, a high local fusible link temperature, and maximum smoke zone of 1600m² (thus eight zones would be required).
As part of a value engineering exercise, Colt was asked to review the scheme and put forward a set of alternative proposals for consideration. The building was to be protected by an ESFR (Early Suppression Fast Response) sprinkler system, but such systems usually need smoke ventilators to close upon activation. But in this case, an unventilated fire could quickly lead to smoke logging of most or all of the smoke zones.
So the objective of the scheme was therefore primarily that of activation by firefighters on their arrival in order to clear smoke. Colt proposed that the smoke zones could be increased in size and reduced to three, but with a reduced risk of the building becoming completely smoke logged.
Furthermore, Colt demonstrated that an increase in ventilator size to 7m² would not reduce the effectiveness of the scheme, thereby saving the client money. Colt and Alpine, the sprinkler system installer, then ensured that there was adequate clearance between the ventilators and sprinkler heads to ensure that the operation of the ESFR sprinklers would not be compromised.
Colt also worked closely with Hathaway roofing to ensure that the installation of the ventilators did not exceed the 12.0m maximum height restriction imposed by Local Authority Planning. The final system comprised a three smoke zone pneumatic system with 31 ventilators.
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[…] example, in a large single-story warehouse, a fire is able to spread much faster than in other buildings. This comes down to the large size of […]