Grades and Mechanical Properties of Dimension Lumber for Wood Frame Construction Obtained from Large-diameter Sugi ( Cryptomeria japonica ) Logs

In this study, we assessed the grades and mechanical properties of dimension lumber with size classifications of 206, measuring 38-mm thickness and 140-mm width, and 306, measuring 64-mm thickness and 140-mm width, for wood-frame construction, as obtained from 31 large-diameter sugi (Cryptomeria japonica) logs. The lumber, graded as class A in the Japanese Agricultural Standard (JAS), is visually graded dimension lumber primarily used for members requiring high bending performance. Numerous types of lumber have been ranked as select structural grade in terms of both size classifications. However, the ratio of select structural grade is higher in 306 than in 206. The mechanical properties of the lumber in each size classification showed no significant differences. In this study, by conducting bending tests, we investigated the effect of loading configuration, for which an international standardization is planned, with respect to the mechanical properties. Three loading configurations (JAS, an international standard, and a different standard) were employed, and we found that the value of the Young’s modulus in static bending could be adjusted using the adjustment ratios mentioned in the JAS, irrespective of the type of loading configuration used. Our bending test results for different loading configurations showed small differences in bending strength. The 5th percentile lower limits of bending strength in both the 206 and 306 size classifications fell slightly below the specified design strength by the Ministry of Land, Infrastructure, Transport and Tourism. Finally, to facilitate the efficient collection of lumber from large-diameter logs, we compared the amount of lumber of class A, No. 2 grade and better obtained from logs via the estimation and measurement methods and found that the results matched well. As such, we can conclude that the estimation method is beneficial.