Effects of fastener diameter and strength determination method on orthotropy in wood embedment strength and its unified evaluation across fastener diameter

In current timber structural design codes, embedment strength is generally treated as orthotropic for large fastener diameters, whereas some design provisions use simplified expressions that do not explicitly distinguish between loading parallel and perpendicular to the grain for small fastener diameters. This study examined the validity of this simplified treatment through embedment tests using Japanese cedar, Japanese cypress, and Japanese larch with fastener diameters ranging from 2 mm to 20 mm. Embedment strength was determined using both the offset and maximum-load methods. The offset method showed significant differences between loading parallel and perpendicular to the grain for all diameters. In contrast, the maximum-load method showed that embedment strength perpendicular to the grain was comparable to or higher than that parallel to the grain for small diameters. These results indicate that the apparent disappearance of orthotropy is attributable to the strength determination method rather than the intrinsic embedment behaviour of wood. Furthermore, a natural logarithm-based empirical equation was proposed to estimate the ratio of embedment strength perpendicular to parallel to the grain, and its applicability across the three wood species was demonstrated using the offset method.