Swelling and shrinkage of wood

Swelling and shrinkage are changes in the volume of wood that occur as a result of moisture influences. Swelling is an increase in the volume of the wood due to an increase in moisture, whereas shrinkage is a reduction in volume due to a decrease in moisture. The moisture content within the cell walls or the wood fibre changes, which leads to deformations. However, these deformations only occur up to fibre saturation of approx. u = 30% wood moisture. Beyond that, the water is deposited in the intercellular spaces, which has no influence on the swelling and shrinkage behaviour.

Swelling and shrinkage behaviour
in the main directions of the wood

In the three main anatomical directions of the wood, in the fibre direction (=longitudinal), in the wood ray direction (=radial) and in the annual ring direction (=tangential), there are great differences with regard to the swelling and shrinkage behaviour. In general, swelling is most pronounced in the tangential direction, while in the longitudinal direction it is rather negligible. However, swelling and shrinkage vary depending on the type of wood. In general, it increases with increasing density, as the proportion of cell walls (wood fibres) in the total volume increases.

Swelling and shrinkage
in constructions

The material-specific swelling and shrinkage behaviour must be taken into account, especially in the structural design of connections and supports. In combination with other materials such as large-format steel parts, an obstruction of the swelling and shrinkage behaviour over a higher beam area can lead to massive cracks and to cases of damage. For example, the attempt to join large-sized, high bending beams by means of a slotted steel part to make them rigid in bending and shear usually leads to a case of damage.

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