Abstract
The purpose of this study was to determine the size effect parameter for the design strength that corresponds to the Japanese Agricultural Standard for lumber. This was performed using an existing database that contains lumber strength properties. Initially, each size effect parameter for the constant span, specimen depth, and span/specimen depth ratio of the E70 grade sugi (Japanese cedar) was obtained using its bending strength, and each size effect parameter was confirmed to agree with the values reported in previous studies. Then, the size effect factor was considered with an objective to reduce the design strength based on the specimen depth. The bending strengths of sugi, akamatsu (Japanese red pine), and Douglas fir were adjusted to the values under the standard loading conditions using an existing adjustment formula. In addition, the 5% lower limit of the bending strength was obtained for each species, grade, and specimen depth. The ratio of the 5% lower limit of bending strength and the design strength corresponding to the Japanese Agricultural Standard for lumber (5% lower limit/design strength) was calculated and compared with the size adjustment factor for the standard specimen depth of 150 mm. Thus, under the standard loading conditions, it was inferred that the value of the size effect parameter for the size adjustment factor was approximately 0.4–0.5.
