The tragic fire at Grenfell Tower, in West London, has turned the spotlight on the importance of fire engineering in buildings.
As with all major blazes, there are many contributory factors that transform a nuisance fire into a serious one that leads to large loss of life.
Following the public inquiry, it is likely that important changes will be made in the UK’s fire codes of practice and associated guidance documents, and how they are enforced.
But, until those changes are made, designers must continue to create well-performing high-rise buildings, and – rather than focusing on one issue – they should consider the broader fire life safety aspects of a building to ensure designs are robust.
CIBSE Guide E 2017 addresses the full range of fire life safety issues, from evacuating the building and giving access for firefighters, to controlling the spread of smoke and fire, including new guidance on fire safety of building façades and external wall construction.
The guide also includes more references to international good design practice, including North American codes, standards and guidance that is frequently used in the Middle East and Asia. Because the UK regulations are ‘functional performance standards’, international codes and practices can also be applied here.
Chapter 10 on smoke ventilation has been substantially updated to simplify it and make it relevant to its audience. This article highlights some of the guidance contained in the new chapter and describes the objectives of smoke ventilation systems and addresses system considerations, tenability criteria, and system design and components.
Smoke control in high-rise buildings
Smoke is the major risk to occupants in a high-rise building fire. The toxic products of fire include irritant and narcotic components so, as well as preventing escape routes from being used because of poor visibility, smoke also causes disorientation, incapacity or death.
The tenability design criteria – including temperature, radiation visibility and toxicity – are supplied in detail in Guide E.
Unless controlled, smoke can spread from the source of the fire. Measures by which its movement can be minimised and controlled include passive barriers, fire-resisting construction and ventilation systems, often in combination with other active systems.
At the outset, it should be clear why a smoke control system is required and how it meets the objectives of the overall fire strategy.
For high-rise buildings, there will be a greater requirement because some form of phased evacuation will be in place, meaning people will remain in the building longer than a low-rise building.
For office towers, only the fire floor, and those adjacent to the fire floor, are evacuated in the first phase, followed by others as required.
For residential towers, a stay-in-place policy is currently typical. Another design consideration is that firefighters need to be able to attack the fire and rescue people. The system should be as simple and as reliable as possible, because an overly complex design can lead to failure. This is especially true in a high-rise building with multiple floors, resulting in an increased number of potential points of failure.
The objectives of the smoke control systems in a high-rise may include:
- Minimising the risk of smoke spread to other floors of the building because of phased evacuation
- Maintaining staircases free of smoke
- Maintaining escape corridors free of smoke.
Maintaining a clear layer in the room of origin is usually not an objective as travel distances in high-rises tend to be relatively short. Clear layer systems are more suited to shopping centres and exhibition halls. Guide E details advice on design of clear-layer systems.
Minimising spread of smoke and keeping escape routes clear
Typically, the method for achieving this is to install fire-rated floor construction. The weak links are where the floors are penetrated. This includes stairs, lift shafts and mechanical, electrical and plumbing shafts.
Because stairs contain openable doors and lifts contain non-smoke- rated doors they will leak smoke, so additional measures are required, including another set of doors forming a smoke lobby.
The measures for reducing smoke spread to other floors will also assist in keeping staircases free of smoke. CIBSE Guide E gives detailed advice on the various control methods, including natural smoke shafts, powered ventilation and pressurisation systems.
For buildings such as open-plan office towers, there are no corridors to protect, and people will escape directly to the staircases – the distance to the stairs being limited by code.
However, in residential buildings, the common corridors leading to the stairs – although limited in length – extend travel distances to the stairs, enabling smoke control.
Robust smoke control design
A basic assumption of the codes is that a design is based on a single fire event – one fire at any one time. A robust system should consider the potential that smoke may spread to an adjacent, or multiple, floors.
An overly complex design should be avoided. One with many moving parts and different operational modes reduces reliability and increases the chance of system failure.
Sprinklers play a significant role in any design. If the sprinklers fail to extinguish the fire, they will at least significantly reduce the temperature and volumes of smoke. This is important when designing stair-pressurisation systems.
US codes and standards recognise the role of sprinklers and allow the pressure differences in stairs to be reduced from 25Pa to 12.5Pa, compared with 50Pa for a European system.
Stair pressurisation is commonly used for high-rise buildings overseas, with many international codes requiring stair-pressurisation systems only. In the UK, such systems are often avoided because their design, testing and maintenance are considered onerous.
The design of stair-pressurisation systems using US codes is less imposing, and this may encourage UK designers to use them. Both approaches and calculation methods are described in the new Guide E.