Km

Shear force, Vd

Shear force, Vd

(a) Rolling shear situation

Fig. 4.12. Shear stress with both stress components perpendicular to the grain (rolling shear condition).

(a) Rolling shear situation

Shear force, Vd

Shear force, Vd

(b) Form of deformation

Fig. 4.12. Shear stress with both stress components perpendicular to the grain (rolling shear condition).

where kmod, ksys, km are as described in 4.5.1.1 and /v>k is the characteristic shear strength based on the shear strength across the grain (the rolling shear strength). The rolling shear strength, also referred to as the planar shear strength, is approximately equal to twice the tension strength perpendicular to the grain and values for timber and wood-based structural products are given in Chapter 1.

4.5.3 Bearing (compression perpendicular to the grain)

Although wood is an anisotropic material, when used for structural purposes it is assumed to be orthotropic. This means that it is presumed to have directional properties in three mutually perpendicular (orthogonal) axes. The orthogonal axes are aligned with the grain direction (L), the radial direction (R) and the tangential direction (T) as shown in Figures 4.13a and 4.13b.

The properties of the wood along the direction of the L axis are referred to as properties parallel to the grain, and because of the direction of its cellular structure, the properties associated with this axis are considerably stronger and stiffer than equivalent properties associated with the R and T axes. When loaded in the R or T

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