Anisotropy

Wood is'anisotropic', i.e. its properties are directionally dependent and vary lengthways, radially and tangential ly, with the grain and across the grain. The structural cells of both coniferous and deciduous wood are aligned lengthways, as are (most of) the channels which transport nutrients during the tree's lifetime. The maximum tensile and compressive load is markedly lower in the radial and tangential directions than in the axial direction (Figure 8.4). The tensile strength parallel to the grain or fibre varies from 30 to 140 N/mm2 according to the type of wood.The compressive strength varies from 20 to 75 N/mm2.These values depend on the type of wood, its individual grain pattern, the number of 'knots' and the moisture content. All stated values are theoretical, in that they are derived from tests on small, physically perfect samples. The standard values for actual use in construction are much lower; and vary from country to country in practice. The marked variance in strength can be due to the slope of grain, knots or splits in the wood. Major differences can be seen between the

SAPWOOD PITH CAMBIUM BARK RADIAL

AXIAL SECTION GROWTH RING LATE WOOD EARLY WOOD TANGENTIAL SECTION

8.4 The treetrunk, structure and terms.

SAPWOOD PITH CAMBIUM BARK RADIAL

AXIAL SECTION GROWTH RING LATE WOOD EARLY WOOD TANGENTIAL SECTION

8.4 The treetrunk, structure and terms.

various types of wood, and even between two samples taken from the same tree. The tensile strength (suts) across the grain is just 5% of that parallel to the grain. This accounts for the propensity of wood to split, and is a major factor to be taken into account when determining the form, number and size of the fasteners to be used.

A significant advantage of wood is that, alongside a good strength-to-weight ratio, it also offers a reasonable degree of thermal insulation. Here too, the values measured parallel to the grain differ from those measured radially or tangentially to the grain.

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