Strength properties of wood

The strength of wood increases as its density increases. When evaluating the density of wood, the level of moisture in which its mass and volume were measured must always be known. Most commonly the density of wood is given as dry air density, whereby the mass and volume of the wood are measured with its level of moisture at 15 % (or 12 %). Density is often also given as a dry-fresh density, whereby the mass of the wood is measured dry, and the volume saturation point (about 30 %) at a high level of moisture. The main tree species in Finland are pine, spruce and birch. Pine and spruce are the most common in construction. The density of Finnish pine is 370 – 550 kg/m3, spruce 300 – 470 kg/m3 and birch 590 – 740 kg/m3.

In the growth rings of a tree, there is much less lighter-coloured spring wood than darker summer wood. In a normal pine tree, the share of summer wood is on average 25 % and in spruce about 15 %. In Finnish conifers in terms of wood strength, the ideal gap between growth rings is 1-1.5 mm, under which circumstances the relative share of summer wood in the growth rings is greatest. A small gap between rings does not necessarily mean that the wood is denser and stronger. For example, the annual growth of pines in Lapland is almost exclusively the rarer spring wood, even though the gap between growth rings is very small. Because of this, a pine that has grown in Northern Finland is lower in density and its timber lighter than a pine grown in Central and Southern Finland.

The durability of the heartwood is not dependent on density because, in Finnish pine, spruce and birch, the density of the wood increases as you go from the core to the surface. The durability of the heartwood depends on a high resin content, which increases its resistance to decay and pests. In the main Finnish trees, the density and strength of the wood decreases as you go from the base to the top. In pine, the longitudinal change in density is greater than in spruce. The density of the wood increases with age in species of tree in which the density increases from the core out towards the surface.

The strength of the wood is fundamentally affected by the direction in which it is loaded in relation to the grain. In the direction of the grain, the bending strength is directly proportional to the density of the wood. In uniform, flawless wood, the bending strength is as great as the tensile strength.

Tensile strength in the direction of the grain is usually 10-20 times more than its strength perpendicular to the grain. Tensile strength also depends on the density of the wood: for example, the tensile strength of the spring wood in a pine is only 1/6 of that of summer wood. The compression strength of air-dry wood is about half of the corresponding tensile strength.

The shearing strength of wood is 10-15 % of its tensile strength in the direction of the grain. Shearing strength is weakened by knots and faults and cracks that appear in the wood.

The elasticity and durability of wood increase as its density increases. The modulus of elasticity of wood in the direction of the grain may be up to a hundred times more than the same parameter perpendicular to the grain. In the radial direction, the modulus of elasticity is about twice as great as the same parameter in a tangential direction.

Because the properties of wood may vary greatly depending on different factors, it is advisable to sort it according to its purpose. Sorting can distinguish two main examining criteria, which are the appearance and strength-technical properties of the wood. In wood quality and strength sorting, about 90 % of quality criteria are related to its knots. Strength sorting may be done either visually or mechanically.