Kiln Temperature Effect on Tensile Strength of Douglas-Fir and Western Hemlock Lumber

Effect of drying temperatures on tensile strength tension parallel to the grain was studied on 2- by 6-inch Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) and western hemlock (Tsuga heterophylla (Raf.) Sarg.) dimension lumber. The drying temperatures selected were 170?, 200?, and 230?F. The lumber was kiln-dried to 8-10 percent average moisture content and allowed to equalize in the laboratory before the tension test. Two edge-knot classes, small (from 3/4 to 1 inch) and large (from 1-1/4 to 1-1/2 inches), were selected for each species at each temperature. Several physical properties were measured to determine whether one or a combination could be used to predict tensile strength. Increase in temperature caused a decrease in tensile strength for both species. Strength comparisons from 170? to 230?F showed Douglas-fir with small knots reduced 11 percent, with large knots 22 percent; hemlock with small knots reduced 21 percent, with large knots 2 percent. One or a combination of eight physical properties measured to predict tensile strength provided low estimates, with the correlation coefficients ranging from 0.22 to 0.41 in an analysis of covariance. Hemlock was from 3 to 14 percent stronger in tension than Douglas-fir in four of the six combinations of knot sizes and drying temperatures. Douglas-fir was 14 percent stronger than hemlock in one of the combinations and the two species were the same in the remaining combination.

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