f . Wood . 



(1) General . As a construction material, wood is available almost 

 everywhere and at reasonable cost. It is a cellular organic material made up 

 principally of cellulose, which comprises the structural units, and lignin, 

 which cements the structural units together. A tree has distinct zones: 

 bark, sapwood, heartwood, and the pith at the center. There is no consistent 

 difference between the weight and strength properties of heartwood and sap- 

 wood. Because wood is produced by nature under various uncontrolled environ- 

 mental conditions, such as geographical location, precipitation, exposure, 

 and elevation, the product is highly variable. Also, trees are alive, 

 producing wood of different properties at different ages. For a given 

 characteristic or property of wood, such as its bending strength, both the 

 mean value and its variation encountered about the mean should be considered. 



Lumber grading rules are, in effect, specifications of quality. The size 

 and number of knots, slope of grain and other strength reducing character- 

 istics are judged and graded according to uniform standards so that working 

 stresses can be assigned to specified quality. 



Common construction species generally available in the United States are 

 Douglas fir, southern pine, spruce, hemlock, redwood, cedar and other pine 

 species such as lodgepole, ponderosa and white. 



(2) Properties . The major mechanical properties of wood as they 

 affect engineering design are: 



(a) Tension Parallel to Grain . Tension parallel to grain 

 creates a tendency to elongate wood fibers and cause them to slip 

 by each other. Resistance to tension applied strictly parallel to 

 the grain is wood's highest strength property, but if tension is 

 applied at an angle to the grain or the cross section of the piece 



is reduced by knots or holes this strength may be materially reduced. 



(b) Tension Perpendicular to Grain . Tension perpendicular 

 to grain tends to separate the wood fibers along the grain and is 

 the direction in which wood has the least strength. 



(c) Compression Parallel to Grain . Compression parallel 

 to grain creates a tendency to shorten the wood fibers in the 

 lengthwise direction. Resistance of wood to this force is good but 

 is affected by the angle of the load to grain and by the presence 

 of knots and holes. 



(d) Compression Perpendicular to Grain . Compression 

 perpendicular to grain, such as the bearing under the ends of a beam 

 or under a column, tends to compress the wood fibers together. The 

 wood becomes more dense and the action may cause slight displacement 

 at the bearing face. 



(e) Shear Parallel to the Grain . The largest stress 

 usually occurs along the neutral axis of a beam. During the drying 

 of lumber, checks and splits may occur reducing the area in the plane 

 of maximum shear; therefore the shear strength for design is reduced 

 to accommodate this probability. 



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