The LWF Blog
Fire Engineering Design and Risk Assessment – Choice of Smoke Control System – Part 19May 26, 2017 11:28 am
In this Fire Engineering Design and Risk Assessment blog series for Architects and others in the building design business, we have been looking at how the type of building impacts upon the choice of smoke control system. In Part 18, we began looking at the effects of Wind Overpressures and we will continue from that point today.
When attempting to undertake a wind analysis to determine the external pressures at high level vents and low level openings, reference should be made to BS EN 1991-1-4:2005+A1:2010 which deals with wind actions. While there is a commonly accepted criteria of a roof slope 30 degrees or less, such a method is overly simplistic and can only be used as a general guide in cases where the roof profile is simple and the internal pressure is zero or positive. In situations where there are tall buildings close to natural roof vents, a wind analysis should be undertaken.
Ventilators which are to be positioned on the vertical of the building should be casement type, to prevent wind from blowing smoke back into the smoke reservoir. A wind analysis can help to pinpoint areas of the roof which are sheltered from wind and indeed, those which will always produce suction (a vacuum). Where this isn’t fruitful, or sufficient, wind baffles can be used to divert wind interference and in some cases, convert positive pressure into suction.
When considering the low level openings, the aim should be to locate them in areas of positive pressure to assist with venting.
Responsive vents are also a useful possibility. Vents can be opened upon receipt of a signal from a wind direction indicator to manage venting when the wind flows are beneficial.
In the case of buildings which are particularly complex, or those in which the surrounding area provides issues through other tall buildings, a wind pressure study should be undertaken and this can be approached in a few different ways.
A wind specialist can be asked to carry out a desk study, a wind tunnel test can be requested and used in conjunction with structural engineer’s wind tunnel tests or a CFD analysis can be carried out.
While there are various methods to assist and work with the environment in order to use natural ventilation as your chosen smoke control system, there may be some situations which are simply too complex or wind pressures are simply too adverse. In such circumstances, the designer who is addressing smoke control will have to resort to a mechanical exhaust system.
In Part 20 of this series, we will begin to look at Achieving Fire Safety Standards. 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 for over 25 years to produce innovative and exciting building projects. If you would like further information on how LWF and fire strategies could assist you, please contact Peter Gyere on 020 8668 8663.