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Wind Driven (Forced Draft) Building Fires
The hazard to firefighters from wind driven forced draught (draft) fires is immense.
It is not a new problem but is an issue that has been associated with several
firefighter fatalities and near misses. In 1992 I included
a chapter in my book (Fog Attack) on the dangers of
'blow-torching' effects caused by wind-driven structure fires and
discussed ways of reducing the hazards. In 1999 I published an article on this website entitled 'Flashover
Pathways' which investigated the effects that 'point to point' air tracks have
on the flashover phenomenon. Some of my earlier articles proposed a phenomenon that became known as 'high
pressure backdraft', where exterior wind effects cause an undesirable
increase in pressure inside a structure. Unlike a 'backdraft', the wind driven 'forced
draft' fire is caused by air being 'forced' in rather than 'sucked' into
the fire compartment. The outcome may however be the same although the tactical
approaches may well be different from a firefighting point of view.
Further
research by scientists at the University of Manchester in the UK have looked
at how air flows into windows affect the rate of heat release inside a fire
compartment and how the external flaming effect may spread the fire. They have
further looked at the wind driven forced draft effect which is created where -
There are additional factors involved that can dramatically
influence the rate of burning, being exterior wind effects or interior air
movements caused by natural influences such as stack effect in tall buildings.
Having been on the wrong end of such a situation on more than one occasion I can
assure you its not an experience you need! The wind driven forced draft fire is also a killer of firefighters
and is most certainly a situation you need to plan for; train for and
equip for.
This is a warning to firefighters of a fire-ground hazard that may be well known,
but is rarely credited with the respect it deserves. In order to
prepare for and deal with wind driven fires in buildings, we must address the
following -
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Training in how our actions may influence internal air dynamics associated with tall buildings
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Training in tactical deployment issues, eg; selecting points
of entry
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Training in risk control measures, eg; confining,
sectorizing or zoning off areas of a building; and the early use of 'back-up' hose-lines
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Defensive nozzle techniques
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How fire protection design and construction may assist
Such fires are renowned for creating havoc on the fire floor and,
although most commonly
encountered in tall buildings, this hazard may also affect your strategic
approaches at ground level on a day where wind is gusting moderate to heavy.
If you look back at fire reports there are countless incidents
where wind speed and direction has played a major part in causing abnormal and
rapid fire development. Such fires may burn with great intensity and create
excessively high temperatures, forcing firefighters to retreat from their
position with great haste. Many others have not always been so lucky.
As an Incident Commander, make sure you account for this hazard
when you position and flow the primary attack hose-line. Never under estimate
the potential effects on the rate of heat release where a second opening is
created (point to point air-track); or where an exterior wind (or interior stack
effect) might initiate an event of rapid fire development that overcomes the
capability of the hose-line in use. If you care about your firefighters you need
to make them aware of these hazards.
'Momentum' and inertia forces may also exist inside a structure
that will directly affect the likelihood and intensity of various rapid fire
phenomena. These could be related to air-flow dynamics caused through -
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Stack Effects
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Forced Drafts
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Temperature Differences
Whilst many scientists have suggested that cool air flowing, or
being drawn, into a structure might have some cooling effect on the hot gases
accumulating inside a fire compartment, we have learned through experimentation
that the opposite effect is more likely! In situations where the outside air is
cold we have observed more frequent events of rapid fire phenomena that occur
with greater intensity, than where the air is warmer. These conditions are
easily replicated in small 'dolls house' flashover demonstrators and even
in larger steel shipping containers to some extent.
This may suggest that the specific gravity, or weight, of air
features in these flashover momentum or inertia forces in some way. As the air
exchange occurs in a compartment fire, with hot gases leaving the compartment
and cold air replacing them, the density of of the outside air may have some
effect on the severity of any subsequent rapid fire phenomena. As colder air
becomes more dense, the air exchange with heated gases in a fire compartment may
be more aggressive.
The relevance of this is that rapid fire phenomena may become
more likely, and indeed more fierce, on a very cold day, especially in sub zero
temperatures and this has been noted in several real fires.
Flashover 'Pathways' HERE
NIST Research HERE
NFPA Research HERE
Paul GRIMWOOD
London Fire Brigade retd
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