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 Although
timber is classified as combustible material, a properly designed
timber structure has been recognised as performing very well
in fire. Light timber construction is normally protected from
fire by fire resistant cladding materials, while heavy timber
construction has good inherent fire resistance because a char
layer is formed that retards the heat penetration.
When heavy timber members are exposed to a fire, the temperature
of the fire exposed surface of the members is close to fire temperature.
When the outer layer of wood reaches its burning point (about
300°C), the wood ignites and burns rapidly. The burned wood becomes
a layer of char which loses all strength but retains a role as
an insulating layer preventing excessive temperature rise in
the core.
The low conductivity of char will cause a steep thermal gradient
across the char layer. Underneath the char layer, there is a
layer of heated wood with a temperature of above 200°C, which
is known as the pyrolysis zone. This part of wood is undergoing
irreversible chemical decomposition caused solely by a rise in
temperature, accompanied by loss of weight and discolouration.
The inner core wood is slightly temperature affected with some
loss of strength and stiffness properties, mainly due to the
moisture evaporation in the wood. The charring rate is more or
less constant and depends on the density and moisture content
of the wood and heat exposure. Figure 1 shows the typical cross
section of a timber beam subjected to fire (Buchanan 2001; Purkiss
1996).
The fire performance of timber is dependent on the charring
rate and the loss in strength and modulus of elasticity. Strength
and stiffness properties depend on temperature and moisture
content.
The types of timber described include softwoods, hardwoods and
glued laminated timbers (glulam), in the forms of solid timber,
plywood and wood-based panels. Wood-based panels include wood
fibreboard, wood particleboard, medium density fibreboard, oriented
strand boards and cement bonded particleboards.
Due to large variation in the type and quality of timber, a
system of strength classes has been established to group grades
and species with similar strength properties. It gives characteristic
strength and stiffness properties and density values for each
class (EN338: 2003).
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