(a) Axes related to tree trunk (b) Axes related to cut wood

Fig. 4.13. Principal axes of wood.

(a) Axes related to tree trunk (b) Axes related to cut wood

Fig. 4.13. Principal axes of wood.

directions, the cellular structure is compressed in its weakest directions and although the values of the respective properties in each direction are not the same, the differences are small and for practical design purposes these properties are treated as one group. Properties in the R and T directions are referred to as properties perpendicular to the grain.

The symbols used in EC5 for compressive strength properties parallel and perpendicular to the grain are as follows:

ac>o,k: characteristic compressive strength parallel to the grain. ac,9o,k: characteristic compressive strength perpendicular to the grain.

The subscripts:

c (or t) refers to the type of stress (c - for compressive; t - for tensile). 0 (or 90) refers to the direction of the applied stress relative to the grain direction (0 - parallel to the grain direction; 90 - perpendicular to the grain direction). k (or d) refers to the nature of the stress (k - characteristic strength; d - design strength).

When timber is compressed perpendicular to the grain the wood fibres, which can in principle be likened to a bundle of narrow thin-walled tubes loaded laterally, withstand increased loading as they are squeezed together and as they start to collapse the rate of load increase reduces. This behaviour continues until the fibres are fully squashed and as the wood is strained beyond this stage the sustained load will continue to rise until eventually failure occurs, usually by shearing across the grain. The strain in the wood can exceed 30% and failure may still not arise. To control deformations at the failure condition, BS EN 1193 [3] requires the compression strength of timber perpendicular to the grain to be determined using a 1% strain offset. However, where the bearing width extends over the full width of the member, depending on the position of the bearing area, the bearing length and the member depth, this strain limit can be exceeded and the rules in EC5 allow the bearing strength to increase by up to a factor of 4. At this value, the strain will be of the order of 10% and the effect of the associated increase in deformation at the bearing on the structure must be taken into account in the design. Examples of bearing effects (failures) are illustrated in Figure 4.14. For compression perpendicular to the grain the condition to be met is:

^c,90,d < kc,90fc,90,d (EC5, equation (6.3)) (4.22)

Here ac,90,d is the design compressive stress perpendicular to the grain = Vd/bl, where Vd is the design bearing force, b is the bearing width, and £ is the bearing length. fc,90,d is the design compressive strength perpendicular to the grain and is defined as:

0 0

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