Basic Stresses for Wood

Basic stresses for western Canadian softwood species are given and their application to squared timbers of solid wood, joists, planks, beams/ stringers, posts, and timbers is discussed. Average strength properties, as determined by laboratory tests, have been reduced by applying appropriate factors for variability, long-term loading, and safety. By applying a further “grade factor” based on the impairment in strength resulting from the defects permitted in that trade, working or design stresses are secured. Included are a table showing basic fiber stresses for the species covered, illustrations showing the variation in strength of Douglas-fir, and data on the relation of basic stress to duration of maximum load. Stress values given are for extreme fiber in bending or tension parallel to grain, maximum longitudinal shear, compression perpendicular and parallel to grain, and modulus of elasticity in bending.

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