Strength properties of wood
The strength of wood varies between different species, but also within a species, and within a tree. It also depends on the type and direction of the applied force, relative to the wood grain. Wood has very good compression and tension performance in the direction of grain, but is particularly weak against force that pulls the grain apart. These different strengths relate to the way the wood is used in the living tree, since the tree grows wood according to its own needs and circumstances. This is why timber quality varies according to environmental factors, such as climate and rainfall, and site factors such as ground conditions and forest management practices.
In order to build safely and efficiently with timber, engineers need to have information about the level, and variability, of the strength properties. This is achieved by a combination of testing work, to establish the basic information, and a [strength grading process : hyperlink to page] to regulate, standardise, and provide confidence in the strength values to be used in design. In a modern sawmill the grading process is highly automated, reliable and fast.
Most commonly engineers need to know the bending strength (how much load would cause a beam to break) and the stiffness (how much the beam deflects under load), but it is commonly also necessary to have an idea of tension and compression properties (parallel and perpendicular to grain) and shear properties. Other important properties include resistance to splitting, hardness, impact strength and resistance to creep.
These properties must be determined by standardised testing under controlled conditions. Almost all the properties are influenced by the moisture content of the wood, so the testing is done for a reference temperature and humidity – usually corresponding to conditioning to 12% moisture content.
There are factors that affect the properties of sawn timber that are familiar to anyone who has worked with wood. These include the size and position of knots, the closeness, slope and spirality of the grain, the density of the wood, the ratio of earlywood and latewood in the rings, fissures, reaction wood, wane, rot and other damage. These are, however, not the only factors, and there are some that are less apparent, but often more influential. They include, the relative amounts of the main component molecules that make up the cell wall (lignin, cellulose and hemicellulose), the degree of crystallinity of the cellulose, the orientation of those crystals within the cell wall (microfibril angle), and the quantity and nature of extractives.
Timbers are often selected for special strength properties. While most constructional timbers are softwoods that tend to have relatively modest strength (such as pine and spruce) there are species that tend to be stronger (such as larch and Douglas-fir) that can be used in more demanding applications. Very strong tropical hardwoods, such as greenheart, can be used for extreme cases, such as dock works to withstand impact from ships. However, in most cases the most important thing is to know the properties and to design accordingly. Used correctly, species like spruce are perfectly adequate for multi-story construction with light timber frame, and can be used for high-rise construction when used in engineered wood products such as glulam, cross-laminated timber, and laminated veneer lumber.
Wood has very good strength for its weight and was used in both world wars for building aircraft. High quality timber is still highly prized for home-build plane construction and, for similar reasons of good combination of performance and weight, musical instruments.
Edinburgh Napier University has many online resources for those interested in wood engineering.
Contributor: Dr Dan Ridley-Ellis FIMMM