The LWF Blog
Fire Safety Engineering for Design – Smoke Ventilation – Part 188
June 17, 2024 10:37 amLWF’s Fire Safety Engineering blog series is written for Architects, building designers and others in the construction industry to highlight and promote discussion on all topics around fire engineering. In part 187, LWF talked about replacement air and looked at extract points. In part 188, we will begin to discuss smoke layer depth and suspended ceilings.
Smoke Layer Depths
A design smoke layer depth is required. It is acknowledged that when smoke rises and reaches a ceiling it will spread laterally and form a ceiling jet, this will have a depth of smoke. Additionally, when the smoke hits walls etc., this is likely to cause the smoke layer to drop and flow under the ceiling jet.
The ceiling jet and smoke layer under must be taken into consideration when designing the smoke ventilation system. The system should be designed with a smoke layer depth of not less than 10% of the ceiling height, typically measured from the base of the fire height (usually the ground floor) to the ceiling. The smoke layer design depth must also take into account the egress routes and maintaining a clear layer above them. To achieve the smoke layer depth requirements the top floor of the atrium is commonly enclosed or a ‘dummy floor’ is added.
NFPA 92 suggests that smoke layer depth should be a minimum of 20% of the floor-to-ceiling height, unless based on an engineering analysis.
Suspended Ceilings
Suspended ceilings which are not perforated should be treated as the top of the smoke layer. If a suspended ceiling is designed in such a way that, should it be exposed to the predicted design temperatures it would not fail, then smoke barriers and channelling screens would not normally need to be continued in the space above a closed suspended ceiling. A suspended ceiling should be fire-rated to withstand 300 degrees of heat exposure.
Partially-open/perforated suspended ceilings, with more than 25% of evenly-distributed geometrical free area do not normally need to be taken into account when considering smoke movement. In such cases, channelling screens and smoke barriers should be continued above the suspended ceilings as far as structural soffit where necessary.
In cases where a partially-open suspended ceiling has less than 25% of its geometrical free area open, it should be treated as a plenum chamber – that being an air compartment above a suspended ceiling.
In part 189 of LWF’s series on fire engineering we will begin to discuss pressure differentials and smoke ventilation. In the meantime, if you have any questions about this blog, or wish to discuss your own project with one of our fire engineers, please contact us.
Lawrence Webster Forrest has been working with their clients since 1986 to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact the LWF office on 0800 410 1130.
While care has been taken to ensure that information contained in LWF’s publications is true and correct at the time of publication, changes in circumstances after the time of publication may impact on the accuracy of this information.