Length Effect on the Tension Strength between Mechanically Graded High-and Low-Grade Chinese Fir Lumber

Abstract

An experimental study was conducted to evaluate the length effect on the parallel-to-grain tensile strength of Chinese fir (Cunninghamia lanceolata) lumber. In all, 473 pieces of mechanically graded lumber were tested at gauge lengths of 150, 200, 250, and 300 cm. The lumber was sorted into matched groups according to the dynamic Young's modulus as measured using the longitudinal vibration method. The averages of the dynamic Young's modulus of high-grade (H) and low-grade (L) specimens were 11.8 and 8.9 GPa, respectively. Using nonparametric estimates, the estimated length effect parameters of H and L were 0.188 and 0.226, respectively, for the 50th percentile and 0.185 and 0.318, respectively, for the 5th percentile. It was then concluded that the different length effect factors between H and L could be used when using the lumber for practical purposes. The effect of increasing length on the tensile strength was a little larger in H but smaller in L for the 50th percentile compared with the 5th percentile. When two-parameter Weibull distribution functions were fitted to the strength data, the estimated shape parameters of the Weibull distribution by the parametric method were almost identical to the inverse of nonparametric parameters. The influence of defects such as knots on the lower tail of the strength distribution in H may be different than that in L.