Abstract
The aim of this study was to clarify the effects of forest management such as pruning and thinning on the visual grading and bending properties of boxed-heart square timbers of sugi (Cryptomeria japonica D. Don). In this paper, boxed-heart square timbers sawn from sugi logs cut down at control and at managed forests were investigated after drying for visual grading according to the JAS (Japanese Agricultural Standards for Structural Softwood Lumber) and by full-sized bending tests. The results are summarized as follows : 1) The results of visual grading of timbers showed that while the ratios of second grades for control forests increased, for managed forests those of first grades became greater. The difference between the two was affected by the occurrence of knots on the edge of timber surfaces. 2) Results of full-sized bending testing showed MOE (modulus of elasticity) and MOR (modulus of rupture) values of 7.36 kN/mm2 and 40.7 N/mm2 for control forests, and values of 7.52 kN/mm2 and 43.2 N/mm2 for managed forests, respectively. The lower fifth percentile values with 75% confidence level were 29.1 N/mm2 and 32.1 N/mm2 for control and managed forests, respectively. It was recognized that the variations of bending properties for managed forests tended to be smaller. 3) Concerning the relations between visual grading and MOR, the variations of MOR at each visual grade for managed forests tended to be smaller than those for control forests.
