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fire highlight another issue that makes
basement and sub-basement fires in
some ways even more dangerous than
cellar and sub-cellar fires—the windows
that can at times allow for ventilation,
hoseline access, and egress also allow
air to enter the fire area right at the base
of the flames, causing a rapid acceleration of fire gases up the interior stairs
as though inside a woodburning stove.
That is called the “chimney effect” and
is worsened by ventilation above the seat
of the fire because firefighters are often
trying to descend through the chimney.
Ventilation and Wind
The San Francisco fire had yet an-
other contributing factor—a slight wind
blowing into the windows at the base of
the flames into the interior of the building.
This is the worst-case scenario. When we
are fighting a fire at street level or above,
we are usually approaching the fire with
our ventilation at or above the source of
heat while we are at or below the escap-
ing gas level, usually in the air intake flow
path. When approaching the belowgrade
fire through an interior staircase, we are
literally descending the exhaust flow
path, right down the chimney.
That in many ways is not new. We have
been going to cellar and basement fires
for more than 150 years, but with our new
understanding of the impact of ventilation of wind-driven fires, we now see
firefighters being caught in the exhaust
flow path when a sudden change occurs
in the fire ventilation profile. The failure
of a wall of floor-to-ceiling high windows
in the basement at a critical time during
this fire, when firefighters were trying
to descend the basement stairs, allowed
an influx of fresh air right at the seat of
the fire. This fresh air at the base of the
flames greatly intensified the fire, which
followed the interior stair to its exhaust
opening on the first floor at the front. In
that regard, if the fire area had been a
cellar with no windows in the fire area,
as opposed to a basement with a full
wall of windows on the upwind side, it
is likely that there would not have been
a sudden intensification of fire. It would
still have been a hot, difficult fire to fight,
( 2) Descending an interior stair into a working
belowgrade fire is one of the most dangerous
activities firefighters can undertake. It is
extremely punishing and time consuming and
should be avoided if other faster, less dangerous
options will achieve the strategic objectives
of protecting life and confining the fire. No
one should be descending ahead of the nozzle
firefighter with a charged hoseline!