The LWF Blog
Fire Safety Engineering for Design – Natural Smoke Shafts – Part 208
November 4, 2024 11:21 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 207, LWF talked about natural smoke shafts and their use in residential buildings. In part 208, we discuss natural smoke shafts for firefighting shafts and consider wind overpressures.
BS 9999 Fire Safety in the design, management and use of buildings, code of practice contains guidance on the use of natural smoke shafts in firefighting shafts in UK premises.
In some scenarios, it is necessary that the cross-sectional area of a natural smoke shaft should be at least 3 m2. In 2002, BRE project report 79204 was published and described a closed-base smoke shaft approach. Prior to this, smoke shafts were provided with openings at the top and bottom. The open top and bottom shaft is now rarely seen.
The lobby ventilator should have a geometric free area of at least 1.5 m2, the width and the height of the lobby ventilator should be not less than 1 m. When smoke is detected in the lobby, the ventilator should open.
The adjoining stair should be ventilated with a 1 m2 vent at the head of the stair (direct to outside). This should open at the same time as the lobby vent.
Wind Overpressures
When using natural ventilation for smoke extraction, the positioning is of the utmost importance to avoid wind overpressures. The pressure force from external wind conditions can be significantly greater than the pressures developed by a smoke layer. If such conditions develop at the site of a smoke exhaust opening, it could cause the outlet to act as an inlet and push smoke back down the shaft.
Placement of the ventilation in a negative wind pressure area will have the opposite effect and provide a suction force to assist smoke ventilation.
As well as the effects of localised wind pressure effects, natural ventilation systems are affected by wind pressures around the entirety of the building (global pressures) and this will dictate the smoke flow patterns within the building and the overall effectiveness of the ventilation system design. In a tall building, or tall sections of a building, positive wind pressure can be created on the upstream section of the lower roof area.
The designer must take wind overpressures into account by undertaking wind analysis and to estimate internal pressure. Alterations to positioning may help overcome adverse effects or the installation of wind baffles can help.
In part 209 of LWF’s series on fire engineering we will begin to discuss mechanical 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.