BENDING PROPERTIES AND VISUAL GRADE OF CHINESE FIR PLANTATION DIMENSION LUMBER: EFFECT OF PITH AND GROWTH RING WIDTH

摘要

The dimension lumber of plantation Chinese fir was graded to four different grades as SS, No.l, No.2 and No.3, according to national lumber grades authority (NLGA) standard grading rules. At 15％ moisture content, bending strength and stiffness were tested according to American Society for Testing and Materials D4761-02a. The results showed that for SS and No.2 grades lumber the MOE and MOR values were significantly different between the lumber with and without pith, but contrary to No.3. The mean MOE and MOR decreased as growth ring width (GRW) of each graded lumber increased. MOR and MOE of the SS and No.2 grades dimension lumber were very significantly different between GRW＜6mm and GRW≥6mm group. Therefore, it is suggested that No.2 and better grades lumber should be absence of pith and with the GRW less than 6mm, the mean MOE value of No.2 and better grades lumber was increased approximately 5.0％, and the mean and 5~(th) percentile MOR values of those grades lumber were also increased about 7.0%. In addition, the dimension lumber of plantation Chinese fir with pith and GRW ＞6mm should be graded into No.3 grade, and the mean MOE, MOR and 5~(th) percentile MOR values were not affected.