Strength classes to BS 52682 2002 Grade stresses for permissible stress design code

C14 C16 C18 C22 C24 C27 C30 C35 C40 D30 D35 D40 D50 D60 D70

Strength properties MPa

Bending parallel to the grain 4.1 5.3 5.8 6.8 7.5 10 Stiffness properties MPa

Minimum modulus of 4.6 5.8 6.0 6.5 7.2 8.2 elasticity parallel to the grain

C refers to coniferous softwoods and D refers to deciduous hardwoods.

(BS EN 1995-1-1:2004). Timber to be used in Service Classes 1 and 2 must be adequately protected from the weather when on site.

Service classes of wood in use:

Service Class 1 Timber with a moisture content corresponding to an ambient temperature of 20°C and a relative humidity of the surrounding air only exceeding 65% for a few weeks each year. Average moisture content not exceeding 12% (e.g. internal walls, internal floors except ground floor, warm roofs).

Service Class 2 Timber with a moisture content corresponding to an ambient temperature of 20°C and a relative humidity of the surrounding air only exceeding 85% for a few weeks each year. Average moisture content not exceeding 20% (e.g. ground floors, inner leaf of cavity walls, single leaf walls with external cladding).

Service Class 3 Timber exposed to conditions leading to higher moisture contents than in Service Class 2.

1 1 12 13 9 1 1 12.5 1 6 18 23 8.2 9.0 10.0 6.0 6.5 7.5 12.6 15.6 18.0

Average moisture content 20% and above (e.g. exposed parts of buildings and marine structures).

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