The LWF Blog
Alternative Design Solutions – Smoke control inside atriaApril 22, 2014 1:49 pm
When designing a new build with an atrium, the design is usually affected by the prescriptive recommendations for smoke control within atria outlined in BS 9999:2008 Code of practice for fire safety in the design, management and use of buildings. While the code outlines options for enclosure, smoke ventilation, fire alarms and fire suppression systems, a performance-based design can offer alternative solutions to design issues with atria.
Additionally, the performance-based approach can be applied to other large areas within existing buildings such as warehouses, shopping centres or other complex buildings which require smoke control, to reduce smoke ventilation requirements.
Part one of this short series of blog articles looks at the factors which influence the volume of smoke produced within an area, which in turn directly affects the choice of smoke control system. The information given includes general rules and guidelines for design, but as every building’s fire safety requirements are unique, we recommend consultation with an appropriately qualified fire-engineering consultant at an early stage of your project.
Calculating the volume of smoke produced
The volume of smoke produced is generally based on the geometry of the atrium space, the fuel load within the building and the spaces adjacent to the atrium.
Height and size of atrium
A large proportion of the smoke produced comes from air entrained into the smoke/fire plume. As the height of this plume above the fire increases, so does the volume of smoke produced, due to greater volumes of entrainment.
Although the mass of toxic gases produced by a fire does not necessarily increase with increased atrium height, the volume of gas will do so, placing higher demands on the smoke control system. Generally, smoke temperature decreases as the volume of cooler air entrained into the smoke increases. However, as tenability limits are governed by smoke immersion as well as the temperature and concentration of gas products, these limits will be exceeded if smoke threatens occupants on the upper levels.
The height of the smoke plume (to the base of the smoke layer) is proportional to smoke volume, so small increases in plume height will further increase the volume of smoke produced.
As mentioned, the volume of smoke produced in a building depends upon the height of the ceiling above the fire. A reservoir space for smoke, where feasible, will help to reduce the required smoke extraction rate. Allowing the smoke layer to drop reduces the height of the smoke plume, so decreasing the rate of air-entrainment to the smoke layer. The reservoir also provides a space for the smoke to accumulate, giving occupants of the building’s upper levels more time to egress to a place of safety, away from the atrium.
The maximum recommended reservoir size is 2000 m2 where a smoke control system is installed for life safety, or 3000 m2 where the system is installed for property protection. For larger atria, the space can be subdivided with smoke curtains or barriers to achieve this reservoir size. These sizes are based on a natural venting solution; reservoir sizes can be larger for mechanical solutions.
Fuel load and fire size
The fuel load and grouping or layout of the fuel, located either at the base of the atrium or in accommodation areas adjacent to the atrium, is a factor influencing the volume of smoke produced by the fire, due to the influences on the design fire.
Although this may seem obvious, it is not always the case that a higher fuel-load and larger fire require greater smoke extraction systems. Where there is a reasonable fire load in the building, slow-burning material will produce a smaller fire with probably cooler smoke that may not rise to the top of the atrium. But a larger and hotter fire would produce hotter smoke that would be more buoyant and accumulate nearer the ceiling.
The size of the fire should be calculated based on the fuel load and layout of the building, including the heat release-rate, the perimeter and the area of the fire. Where there are obvious groups of combustible materials in a space, these can be used for the area and perimeter (such as exhibition stalls). But design guidance from standards and research papers are used if there is no limit on the layout of goods.
The fire’s location in a building – the distance of a fire in a compartment from the opening to the atrium for example, will affect the volume of smoke produced in the atrium space. Fires in a corner or against a wall also produce less smoke due to lower entrainment, but the smoke will be hotter.
The location of the design fire is based on engineering judgement and the layout of the building. Different scenarios should be modelled to identify the worst case.
Next week’s blog will continue looking at smoke management within atria. In the meantime, if you have any queries about this article, or wish to receive advice on a specific project, please contact Peter Gyere at Lawrence Webster Forrest Limited on 020 8668 8663.
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.