Improving Structural Lumber Quality in a Sample of Picea Mariana Logs Sawn According to the Knots

Hugo Lemieux, Michel Beaudoin, S. Y. Zhang, François Grondin


This paper examines the effect of knots on the strength recovery of black spruce lumber. A model was developed and used to simulate sawing and grading of boards from knotty logs. Since a log internal defect scanner was unavailable, the internal knot morphology was modeled from external measurements. A standard cant and flitch sawing pattern was used in the simulations and rotated about the log axis. for each 30° of log rotation, the theoretical lumber grades were obtained based on knot sizes and positions within the boards. A best and worst sawing rotation angle based on the potential lumber grade yield was retained for each of 54 logs simulated Half of the logs were sawn into 2 X 4 nominal lumber according to the best rotation angle and the other half according to the worst rotation angle. The resulting pieces of lumber were first visually graded according to the knots and then according to all defects, followed by dynamic MOE testing and finally tested to destruction using a third-point standard bending procedure. The results demonstrate that there was little difference in visual grades between the "best" and "worst" groups and that knots played a minimal role in grade determination of the boards. However, there was significant difference in terms of MOE values, where the group of "best" boards showed an overall 15% increase over the "worst" boards. This result significantly impacts the potential MSR yield of the sample pieces of lumber. Bending tests showed a lurther 25% difference in average MOR between the two groups. These results suggest that there is potential for black spruce to yield higher strength lumber when knots are considered during breakdown. Further refinements should include a model that determines quality in terms of knot position within the board section rather than one that determines quality in terms of potential visual grades.



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