The LWF Blog

Alternative Design Solutions – Smoke control inside Atria pt 2

May 3, 2014 2:01 pm

The first part of this series on smoke control within buildings which are designed with an atrium began to look at smoke movement within atria. Part two continues on this theme and examines the reduction of ventilation rates through compartmentation, how smoke moves through the building, touches on flashovers and looks at fire protection systems and their affect on smoke density and temperature.

Consideration of these elements early in the design process can help to provide fire safe solutions to architectural and design issues.

Reducing ventilation rates with compartmentation

The volume of smoke produced in an atrium mainly depends on the height of the ceiling above the fire source and the size of the fire. Additional compartmentation in the atrium can reduce the volume of smoke. Compartmentation at the lower levels of the building should reduce the risk of a fire on the ground floor spreading to the atrium and requiring extraction. 

When applied to an entire level, this reduces the pressure to consider the level as a floor of fire origin and means that only the levels that are open to the atrium must be considered for the design-fire scenario. Where only upper levels are open to the atrium, they are likely to provide a smaller smoke volume than that expected from lower levels.

Alternatively, fire or smoke-rated separations can be provided where high levels open to the atrium. This will allow the smoke reservoir to drop to a lower level, reducing entrainment to plume and providing a reservoir, thereby reducing the extract-rate from the building. This solution will also give upper-level occupants a greater degree of safety, as smoke from lower levels should not penetrate to the upper floors and affect their means of escape.

Smoke movement through the building 

The volume of smoke that a fire produces is largely determined by the smoke’s route from the lower levels to the top of the atrium. Wherever smoke flows through an opening such as a shop window, or if a barrier interrupts the flow, the volume of air entrained into the smoke increases. Features such as architecturally designed ceilings of varying depths, deep beams or downstands transverse to the flow of smoke, or balconies distant from the opening to the atrium, can combine to increase the volume of smoke produced in the building.

Although it is not always feasible to reduce smoke movement through the building, this can be addressed by providing smoke separations at points where this will have a significant effect on smoke production. 

Flashover 

Flashover is considered to occur in a space when the whole of the compartment is involved in the fire. This typically happens when smoke layer temperatures reach around 600 C, or the radiative heat flux from the hot layer to other objects in the room is greater than 15-20 kW/m2.

Flashover is less likely to happen in large spaces, so the risk of this happening should be addressed in spaces adjacent to the atrium and leading to it. A flashover in one of the building’s small spaces will affect smoke flow from the area, as flames will project from the space. This will change the origin of smoke, size of the source and so increase the difficulty of estimating the smoke produced. This means that if the building is believed to contain a flashover risk, a more detailed risk and fire safety solution based analysis is required. 

Fire systems interaction

Activating a sprinkler system will have a significant impact on the size of a fire and the temperature and volume of the smoke produced. All fire systems will need to be taken into account for the smoke control solution. A suppression system will greatly affect fire growth and smoke production. An automatic fire detection system will usually be required to activate such features of the smoke control solution as the opening of automatic vents or release of downstands.

Several factors influence the volume of smoke produced within an atrium, or similar building with a high ceiling, when fires have started at low levels. A fire engineer involved with the design team will always seek to reduce the magnitude of smoke needed for extract from the building. Incorporating the above considerations into the design at an early stage will reduce smoke control requirements.

Next week’s blog will continue looking at smoke control. In the meantime, if you have any queries about this week’s blog, or wish to discuss smoke control in your own building 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.