Embedment behaviour of fully threaded bolts in glued laminated timber

As one of the main factors governing the load-carrying capacity of timber connections with dowel-type fasteners, embedment strength is a system property, which is dependent not only upon the material properties but also upon the fastener surface conditions. However, in the current standards for structural timber design, the embedment strength is usually considered to be only related to the fastener diameter and timber density, and in some available formulae, the steel grade of smooth dowels is considered as the important influencing factor for the embedment strength parallel to the grain. The available formulae for determining the embedment strength of dowel-type fasteners in the literature are almost exclusively derived from the embedment tests on smooth dowels. Fully threaded bolts are common dowel-type fasteners in timber structures, but their surface conditions are different from those of smooth dowels. In order to assess the effects of steel grade and surface conditions of fasteners on the embedment strength, embedment tests in glued laminated timber parallel and perpendicular to the grain using fully threaded bolts with two different steel grades were carried out. The test results showed that the steel grade of fully threaded bolts was not a main factor for the embedment strengths. Based on the design formulae given in the standards for structural timber design from both North America and the CEN member states, the formulae for determining the embedment strengths perpendicular to the grain in timber with fully threaded bolts were proposed. In a previous study, it was found that the embedment strength parallel to the grain for fully threaded bolts was significantly greater than that for smooth dowels. Therefore, in this study, the supplementary embedment tests on the specimens with smooth dowels perpendicular to the grain were performed to compare with those with fully threaded bolts. There were no statistically significant differences in the embedment strength perpendicular to the grain with fully threaded bolts and smooth dowels. The effect of surface conditions on the embedment strength perpendicular to the grain is negligible. In addition, there were considerable scatters for the foundation moduli, and the ratio of the foundation modulus to the density showed a decreasing trend with increasing bolt diameter.