Bending strength and stiffness of second-growth Douglas-fir dimension lumber

The second-growth managed resource will have different wood quality characteristics than old growth. This study addresses the problems in converting second-growth coastal Douglas-fir to dimension lumber. Logs from selected stands were sawn into dimension lumber and the 2 by 4 material was sorted by visual grade, log position, and percent juvenile wood. The lumber was kiln-dried and tested in bending. A decrease in density, modulus of rupture (MOR), and modulus of elasticity (MOE) with increasing percent juvenile wood was found for all grades and log positions. The implications of the low properties were assessed by comparing the grade mean MOE and fifth percentile MOR results with the design properties. As the proportion of juvenile wood in the lumber increases from 0 to 100 percent the proportion of grade-log position combinations that do not meet the design requirements also increases. These results do not reflect the design values that could be assigned to random samples of visually graded lumber typically marketed today in British Columbia. They represent, rather, the challenge that must be faced when second-growth stands are increasingly harvested. One approach to reducing the potential degrade is the use of machine stress rating (MSR) of dimension lumber. An analysis of the strength and stiffness values found in this study suggests up to 50 percent of the potential loss in value might be recovered by means of MSR grading.

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