Enclosure fires

There have been very high demands imposed for a long
time requiring buildings to have structural elements with suffi
cient load bearing capacity to withstand fi re. According to
the available statistics, human fatalities and injuries in a fi re
are very rarely due to the building’s structure collapsing. There
are very good reasons for imposing these high demands, especially
in the case of buildings which cannot be totally evacuated
during a fi re. It is also important for fi refi ghters to be able
to rely on the structure’s bearing capability. The bearing structure
is built differently, according to the type of building. It is
usual to classify buildings in terms of ability to withstand a fi re
for up to several hours.
Other factors affecting a fully developed fi re in a compartment
include:
• the amount and type of combustible material
• the density, shape and arrangement of the material
• the amount of air available
• the size and geometry of the compartment
• the properties of the structure surrounding the
compartment.
The decay period can last a long time and it is very common,
in this instance, for the fi re to return to being fuel controlled.
During this stage, smouldering fi res are very common.
The structural material often loses some of its strength as a
result of the heat’s impact. This can lead to the building collapsing,
either locally or completely.
Materials made from steel are very sensitive to heat and
lose half of their strength at 500 °C. Concrete retains around
75% of its strength at 500 °C. 1 The size of wooden structures
is reduced as a result of combustion and a carbon layer is
formed. Wooden beams become thinner, which makes them
weaker. The smaller the cross-section means the less strength
they have.
Figure 76 (opposite
page). A fully developed
fi re. Part of the
combustion takes place
outside the room.
113