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The load duration class for self-weight is classed as 'permanent', and 'variable' actions come into one of the remaining classes defined in the table, and is determined by duration.

2.2.20.2 Service classes

Because the strength (and creep behaviour) of timber and wood-related products is affected by the moisture content of the material, these properties are dependent on the temperature and relative humidity conditions the materials are subjected to over the design life of the structure. A typical relationship between strength adjustment and moisture content derived from tests is shown for Douglas fir in Figure 2.4. When the moisture content is low, the strength property will be at its maximum and as the moisture content increases the strength is reduced and will reach a minimum value at the fibre saturation point.

To take this effect into account in design, three service classes have been defined in EC5,2.3.1.3, covering the typical environmental conditions that timber structures will function under. These are as follows:

Service class 1 - where the average moisture content in most softwoods will not exceed 12%.

This corresponds to a temperature of 20°C and a relative humidity of the surrounding air only exceeding 65% for a few weeks per year.

Service class 2 - where the average moisture content in most softwoods will not exceed 20%.

This corresponds to a temperature of 20°C and a relative humidity of the surrounding air only exceeding 85% for a few weeks per year.

Service class 3 - where the average moisture content in most softwoods exceeds 20%.

Table 2.4 Values of k*
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