it absorbs moisture. Above the fiber saturation point there is no dimen- 

 sional change with variation in moisture content. The amount of shrinkage 

 and swelling differs in the tangential, radial, and longitudinal dimensions 

 of the piece. Engineering design should consider shrinkage and swelling in 

 the detailing and use of lumber. 



Shrinkage occurs when the moisture content is reduced to a value below 

 the fiber saturation point (for purposes of dimensional change, commonly 

 assumed to be 30 percent of the moisture content at the fiber saturation 

 point) and is proportional to the amount of moisture lost below this point. 

 Swelling occurs when the moisture content is increased until the fiber 

 saturation point is reached, then, the increase ceases. For each 1 percent 

 decrease in moisture content below the fiber saturation point, wood shrinks 

 about one-thirtieth of the total possible shrinkage, and, for each 1 percent 

 increase in moisture content, the piece swells about one-thirtieth of the 

 total possible swelling. The total swelling is equal numerically to the 

 total shrinkage. Shrinking and swelling are expressed as percentages based 

 on the green wood dimensions. Wood shrinks most in a direction tangent to 

 the annual growth rings, and somewhat less in the radial direction, or 

 across these rings. In general, shrinkage is greater in heavier pieces 

 than in lighter pieces of the same species, and greater in hardwoods than 

 in softwoods. 



As a piece of green or wet wood dries, the outer parts are reduced to a 

 moisture content below the fiber saturation point much sooner than are the 

 inner parts. Thus the whole piece may show some shrinkage before the 

 average moisture content reaches the fiber saturation point. Neither the 

 initial nor the final moisture content (M. or M f ) can be greater than 30 

 percent when calculating shrinkage because that is the moisture content at 

 which, when drying, wood starts to shrink or at which, when absorbing 

 moisture, it reaches its maximum dimension. Values for longitudinal 

 shrinkage with a change in moisture content are ordinarily negligible. The 

 total longitudinal shrinkage of normal species from fiber saturation to 

 ovendry condition usually ranges from 0.1 to 0.3 percent of the green wood 

 dimension. Abnormal longitudinal shrinkage may occur in compression wood, 

 wood with steep slope of grain, and exceptionally lightweight wood of any 

 species. 



The cross-laminated construction of plywood gives it relatively good 

 dimensional stability in its plane. The average coefficient of hydroscopic 

 expansion (or contraction) is about 0.000 2 meter per meter (0.000 2 foot 

 per foot) of length or width for each 10 percent change in relative humidity, 

 or 0.2 percent ovendry to complete saturation. 



3. Mechanical Properties of Wood . 



a. Wood as Structural Material . 



Wood is not an isotropic material because strength properties differ 

 along its different axes. It is strongest when loaded in induce stress 

 parallel to grain, either in tension or compression. However, this condition 

 is not always possible and loading perpendicular to grain may be accomplished 

 in a satisfactory manner. 



235 



