13 WIND EFFECTS

The designer should consider the effect of wind pressures
on smoke extraction systems.
Information published by the Building Research
Establishment and BSI (CP3, chapter V pt 2) shows for a
building considered in isolation, natural ventilators will
be subject to negative pressure of roof slopes of 30o, or
less, from the horizontal.
Installations of natural ventilators on roofs over 30 from
the horizontal should not be considered without some form
of baffling unless supporting data from wind tunnel tests
and/or computer simulation is available. This baffling can
be external to the unit or part of the ventilator design.
When changing wind directions may cause positive or
negative pressure fluctuation in the building structure,
natural extract ventilators should be installed in
sufficient numbers and positions and controlled via wind
sensor/pressure monitors to ensure that an appropriate
number open at any one time.
If in doubt about pressure distribution on the building
structure, a powered system should be used.
Snow loadings should also be considered when designing/
siting of both powered and natural supply/extract systems.
Any effects caused by the wind which may affect the smoke
ventilation system proposed, must be clearly stated by the
designer.