Civil Engineering Reference
In-Depth Information
unburnt fuel (unburnt products of pyrolysis) in the smoke
can reignite on reaching its spontaneous combustion
point causing a substantial rise in temperature. This
rise in temperature, if provided with a fresh source of
oxygen (window glazing fails), has the ability to cause a
fl ashover or explosion.
Clearly there is a substantial difference in a fi re
starting in a confi ned area such as a building, in compari-
son to one that starts in the open air and it is this issue
that will need to be addressed when designing buildings
that minimise smoke and fi re spread enabling people to
escape safely.
considerably according to the type of material, e.g. metal
is a much better conductor than brick. It should be
noted that conduction may occur in solids, liquids or
gases; however, in relation to fi res within buildings it is
most prevalent in solids.
The thermal conductivity (the ability to conduct heat)
varies between materials and is a key element in building
design and construction, which will be considered in
Chapter 9.
A fi re in one room can spread to adjacent rooms by
heat being conducted through the fabric of the building
(walls/ceilings, etc.), especially via metal pipes or frames
used in building construction. The heat can then ignite
materials in direct contact with the surface, or radiate
out from the surface. This can raise the temperature
of materials in the adjacent room to their spontaneous
combustion temperature, thus spreading the fi re.
The relative conductivity of building materials is
therefore an important factor in the fi re resisting ability
of a structure or building. This issue is considered within
the Building Regulations Approved Document B.
Convection and the effects of smoke
Although it is important to restrict the spread of fi re
within a building, it is equally important to consider the
speed of spread and effect of the smoke created by the
fi re as it burns.
As a fi re develops it will create large quantities of
smoke which will, usually, spread ahead of the fi re quickly
fi lling a building. The effect of this is to present a toxic
and/or asphyxiant hazard to people within the building.
Smoke also reduces visibility and obscures escape
routes; this linked to people's natural reluctance to walk
into or through smoke can lead to, or increase, panic,
which in turn leads to disorientation reducing the chances
of safe escape.
Figure 7.18
Fire spread through a fi re resisting wall by
conduction along a steel pipe
7.4.3 Radiation
Radiation is the transfer of heat energy as electro-
magnetic waves, which heat solids and liquids (but not
gases) encountered in its path.
Fire radiation paths do not require any contact
between bodies and move independently of any material
in the intervening space. If not absorbed by fi re resist-
ant material the electromagnetic radiation can radiate
through glazing causing fi res to spread and involve a
number of compartments/rooms. As with the example
of a heater or open fi re, fi res can spread to combustible
Figure 7.17
Smoke spread - convection
7.4.2 Conduction
Conduction is the movement of heat through a mater-
ial. The ability of conductors to transfer heat varies
