cell walls remain saturated until the free water has been evaporated. The 

 point at which evaporation of free water is complete and cell walls begin to 

 lose their moisture is called the fiber saturation point (f sp) . This point 

 occurs between 25 and 30 percent moisture for most species. 



Moisture in wood is expressed as a percentage of the ovendry weight and 

 is determined most accurately by weighing a representative sample, drying 

 it at slightly more than 100° Celsius (212° Fahrenheit), until no further 

 loss of weight takes place, reweighing, and then dividing the difference 

 between the original and final weights by the final (ovendry) weight. 

 Electric moisture meters offer a simpler though less exact method of 

 determining moisture content. With slight seasonal variations, wood in use 

 over a period of time attains an equilibrium moisture content (emc) corres- 

 ponding to the humidity and temperature of the surrounding atmosphere. 

 When exposed to similar atmospheric conditions, different woods will have 

 the same moisture content regardless of their density. 



Moisture content has an important effect upon susceptibility to decay. 

 Most decay fungi require a moisture content above fiber saturation point to 

 develop. In addition, a favorable temperature, an adequate supply of air, 

 and a source of food are essential. Wood that is continuously water-soaked 

 (as when submerged) or continuously dry (with a moisture content of 20 

 percent or less) will not decay. Moisture content variations above the 

 fiber saturation point have no effect upon the volume or strength of wood. 

 As wood dries below the fiber saturation point and begins to lose moisture 

 from the cell walls, shrinkage begins and strength increases. 



c. Directional Properties . Wood is not isotropic because of the 

 orientation of its cells and the manner in which it increases in diameter. 

 It has different mechanical properties with respect to its three principal 

 axes of symmetry: longitudinal (parallel to grain), radial (perpendicular to 

 grain), and tangential (perpendicular to grain) (see Fig. 54). Strength and 

 elastic properties corresponding to these three axes may be used in design. 

 The difference between properties in the radial and tangential directions is 

 seldom of practical importance in most structural designs; for structural 

 purposes it is sufficient to differentiate only between properties parallel 

 and perpendicular to the grain. 



d. Specific Gravity . Solid wood substance is heavier than water, its 

 specific gravity being about 1.5 regardless of the species of wood. Despite 

 this fact, dry wood of most species floats in water because a part of its 

 volume is occupied by air-filled cell cavities. Variation among species in 

 the size of cells and in the thickness of cell walls affects the amount of 

 solid wood substance present and hence, the specific gravity. Thus, 

 specific gravity of wood is a measure of its solid wood substance and an 

 index of its strength properties. Specific gravity values, however, may be 

 somewhat affected by gums, resins, and extractives which contribute little 

 to strength. The relationship of specific gravity to wood strength is 

 recommended in the practice of assigning higher basic stress values to 

 lumber designated as "dense." 



e. Dimensional Stability . 



(1) Effect of Temperature . Wood, like most other solids, expands 

 on heating and contracts on cooling. In most structural designs, the 



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