Machine Stress Rating (MSR) of Green Douglas-Fir
Keywords:
Grading, MOE, MOR, bending strengthAbstract
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.
References
ASTM. 1982. Standard methods of testing small clear specimens of timber, D 143. ASTM Book of Standards Part 22, American Society of Testing and Materials, Philadelphia, PA.nGallioan, W. L., D. W. Green, D. S. Gromala, and J. H. Haskell. 1980. Evaluation of lumber properties in the United States and their application to structural research. For. Prod. J.30(10): 45-50.nGreen, D. W. 1980. Adjusting the static strength of lumber for changes in moisture content. Proceedings of the workshop How the Environment Affects Lumber Design: Assessments and Recommendations. U.S. Forest Products Laboratory, Madison, WI. 146 pp.nMadsen, B. 1975. Moisture content-strength relationship for lumber subjected to bending. Structural Research Series Report No. 11, University of British Columbia, Vancouver, B.C. 24 pp.nUnited States Department of Commerce. 1970. American softwood lumber standard. NBS Voluntary Product Standard, PS 20-70. 26 pp.nWilson, F. J. 1981. The effect of moisture content on the flexural strength and stiffness of southern pine dimension lumber. M.S. thesis submitted to Virginia Polytechnic Institute and State University, Blacksburg, VA. 196 pp.nWWPA. 1981. Standard grading rules for western lumber. Western Wood Products Association, Portland, OR.n
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