Mechanism of Withdrawal Resistance of Screws Embedded Perpendicular to Grain and Evaluation Method of Withdrawal Strength

Abstract

In recent years, the number of wooden structures using screws which are expected to resist withdrawal forces are increasing. It becomes necessary to establish a design method based on a theoretical basis for the withdrawal performance. In this study, FEM (Finite element method) analysis using a 3D model reproducing the threaded part of the screw was conducted, and tensile stress concentration was observed at the tip of the thread under withdrawal force perpendicular to grain. Based on FEM analysis and fracture properties from experiments, an evaluation method of withdrawal strength of the screw embedded perpendicular to grain was proposed using linear fracture mechanics. The calculated values of that method were compared with experimental values of screw withdrawal tests of several woods, such as Japanese cedar, Japanese cypress, Japanese larch and Douglas fir. As a result, calculated values for Japanese cedar corresponded well to experimental ones and the values of Japanese cypress, Douglas fir and Japanese larch were evaluated to be on the safe side.