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Introduction
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Charring Rate and Depth
Thermal Properties
Mechanical Properties
Light Timber Frame
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Timber

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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Figure 1: Char Layer and Pyrolysis Zone in a Timber Beam (Schaffer 1967)
 
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