Machine Stress Rating (MSR) of Green Douglas-Fir
Keywords:Grading, MOE, MOR, bending strength
A large sample was selected from a production run of unseasoned 2 x 4 Douglas-fir, which had been graded and grade stamped as Standard and Better in accordance with WWPA grading rules. From the large samples, 250 pieces were selected by visual quality rules for machine stress rating and subsequently graded by a qualified lumber grader. The 250 pieces were numbered and rank ordered by MOE and separated into two treatment groups based on the MOE ranking. The even number pieces were placed in an environment of 70 F and 50% relative humidity and allowed to dry.
Both sample groups were tested by a 1/3 point bending load to evaluate the MOE and MOR of the specimens. The average MOEs of air-seasoned and unseasoned samples were equal from a design standpoint. The MOR data were fitted by probability distributions. The exact relationship between the lower percentile values of the air-seasoned and unseasoned MOR was not clear because of a crossing-over of the probability density curves. It was clear from the MOR data analyses that seasoning causes inherently strong material in bending to become yet stronger.
To more accurately assess the impact of drying on lumber bending strength, a reliability type analysis was recommended.
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