Pull-out strength of axially loaded steel rods bonded in glulam parallel to the grain

Bonded-in steel rods are very efficient in withstanding high forces applied to timber members. Investigations of bonded-in rods started in the late eighties of the last century and several design models were published since. By comparing these approaches on base of an extensive literature review a certain degree of discrepancy and partly even contradiction can be found. The paper describes a test program which aimed to study the influence of timber density and of geometric parameters on the pull-out strength of single axially loaded steel rods. Following the GSA(R)-procedure, which is a well established glued-in rods joint in Switzerland, rods with metric thread were bonded in glulam made of Norway spruce lamellas using an epoxy-type adhesive. The tests showed that the influence of the timber density can be quantified by a power function of p with an exponent of 0.6. The parameters length of the glued zone l and diameter of the hole d(H) can be summarized in the slenderness ratio lambda = l/d(H), which itself is related to the mean shear strength in the anchoring zone by an exponent of approximately -1/3. In order to prevent the specimens from premature splitting, distances between the axis of the rod and the edge of the specimen of 2.3 times the diameter of the rod have been used.