Effect of Size on Tension Perpendicular-To-Grain Strength of Douglas-Fir


  • J. D. Barrett


<i>Pseudotsuga menziesii</i>, size effects, tension, Weibull distribution, strength, duration of load, glued-laminated beams, pitched-tapered beams, design of structures


The strength of wood in tension perpendicular-to-grain has been studied by several authors and found to depend on specimen geometry. In this paper, the weakest-link concept has been applied to predict the relationship between specimen volume and load-carrying capacity for Douglas-fir specimens loaded in uniform tension perpendicular-to-grain. The theory allowed the prediction that logarithm of maximum strength should decrease linearly with logarithm of volume. Experimental data taken from the literature were used to evaluate the theoretical model and agreement was found to be high (R2 ≥ 0.85). Average strength of a unit volume is approximately 460 psi, whereas the predicted strength of a 10- X 10- X 20-inch specimen (2000 inches3) is approximately 100 psi. The magnitude of the size effect may depend on the quality of material in the specimens, but certainly any rational development of working stresses for tension perpendicular-to-grain must consider effects of specimen (or structural component) size.


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