Embedment strength and foundation modulus at an angle to the grain for fully threaded bolts

As the main parameter, embedment strength is used to assess the load-carrying capacity of timber joints with dowel-type fasteners and foundation modulus is used to assess their stiffness. The embedment properties, i.e. embedment strength and foundation modulus, are system properties, which depend not only upon the material properties but also upon the load-to-grain angle and surface conditions of fasteners. The formulae for the embedment strengths of dowels and bolts in the design standards of timber structures are almost exclusively derived from the embedment tests on smooth dowels, with the surface conditions different from those of alternative fully threaded bolts in timber structures. Moreover, there are hardly any formulae for determining the foundation modulus. In this study, the embedment tests at different angles to the grain were carried out using the fully threaded bolts with different diameters in oversized holes as they would be used in practice. The load-to-grain angle had statistically significant effects on these embedment properties. The empirical formulae for the embedment strength and foundation modulus were derived. The embedment tests on fully threaded bolts and smooth dowels in tight-fitting holes were also performed to explore the effects of bolt hole sizes and surface conditions.