Bending Strength, Stiffness and Visual Grade of Plantation Chinese Fir Dimension Lumber: Effect of Pith and Ring Width

摘要

@@1. Introduction Chinese fir (Cunninghamia lanceolata (Lamb.) Hook.) is a kind of unique valuable species in China. It grows very fast, upward in shape, with straight and uniform grain, easy to plant with excellent capacity to resist injury and decay. In GB 50005-2003 code for the visual grading lumber method is almost same as the Canadian National Lumber Grades Authorities (NLGA) grading rules. The strength reducing characteristic for visual grading considered in NLGA grading rule, are: knots, slope of grain, decay and wane etc, ring width is prescribed for different grades only to Douglas fir lumber, but pith is permitted in any grade and species. To second-growth' Douglas-fir dimension lumber, a decrease in density, MOR, and MOE with increasing percent juvenile wood was found for all grades. Ring width was very an important index to plantation Chinese fir dimension lumber, and if the lumber included pith, the ratio of juvenile wood was higher than without pith lumber. The relation ship between MOE and ring width was built, the coefficient of determination (R2) was 0.21. The previous studied showed that MOE of mature wood was 20% higher than that of juvenile wood, ring width of mature wood was 7.08mm, and one of juvenile wood was 3.08mm to Chinese fir. Consequently, the main of this paper was to present and discuss the result of an investigation regarding the visual parameter, ring width and pith influence on the mechanical properties of Chinese fir.