Moisture & Wood
|Moisture in wood exists in two forms:
As wood dries, the free water in the cell cavities is drawn away first. Once the free water is removed, the bound water is gradually released from the cell walls.
|FIBER SATURATION POINT
Fiber saturation point (FSP): The moisture content at which all of the free water is removed - the cell cavities are empty - but the cell walls are still completely saturated.
This is a key concept in wood design since moisture affects the physical and mechanical properties of wood differently depending on whether the MC% is above or below the FSP.
The FSP varies for different species of wood, but is typically around 30%. Table M-1 lists FSP values for various wood species. The rate of change of physical properties is also dependent on wood species.
Table M-1: Fiber Saturation Point
The following demonstration is based on the properties of Douglas fir.
As the above example demonstrates, wood changes dimensionally as it gains and/or loses moisture at levels below the FSP. It swells as moisture content increases and shrinks as moisture content decreases. This is because below the FSP all moisture is bonded to the cell walls, which act like sponges, swelling when water is added and shrinking as they dry.
The anisotropic nature of wood causes it to shrink at different rates along each of its three principal axes.
The inequality of shrinkage along the three axes causes wood to deform as it dries. The amount and type of deformation of a piece of lumber varies depending on the orientation of the annual rings. The first demonstration showed shrinkage at the micro level, the following demonstration illustrates the effects of shrinkage macro level.
Demo 2: Effects of Shrinkage on Cross Sections
Drying causes not only cross sectional distortion, but may also result in warping along the length of lumber. Various types of warp are due to discontinuities such as knots as well as the annual ring orientations.