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Wood Strength and Related Engineering Design Properties

Wood Strength and Related Engineering Design Properties

Properties of Wood at 12% Moisture Content (Average Values from Tests on Clear Pieces 2 x 2 inches in Cross Section per ASTM D143)

Wood is a naturally formed organic material consisting essentially of elongated tubular elements called cells arranged in a parallel manner for the most part. These cells vary in dimensions and wall thickness with position in the tree, age, conditions of growth, and kind of tree. The walls of the cells are formed principally of chain molecules of cellulose, polymerized from glucose residues and oriented as a partly crystalline material. These chains are aggregated in the cell wall at a variable angle, roughly parallel to the axis of the cell. The cells are cemented by an amorphous material called lignin. The complex structure of the gross wood approximates a rhombic system. The direction parallel to the grain and the axis of the stem is longitudinal (L), the two axes across the grain are radial (R) and tangential (T) with respect to the cylinder of the tree stem. This anisotropy and the molecular orientation account for the major differences in physical and mechanical properties with respect to direction which are present in wood.

Standard nomenclature of lumber is based on commercial practice which groups woods of similar technical qualities but separate botanical identities under a single name. For listings of domestic hardwoods and softwoods see ASTM D1165 and the ‘‘Wood Handbook.’’

1 Impact bending height drop in inches for failure with 50-lb hammer
R = radial
T = tangential

Sources:

Tabulated from ‘‘Wood Handbook,’’ Tropical Woods no. 95, and unpublished data from the USDA Forest service, Forest Laboratory.

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