An experimental approach to determine pull-out strength of single and multiple axially loaded steel rods bonded in glulam parallel to the grain

A glued-in rods’ connection is generally constituted by a group of steel bars bonded by an adhesive into timber elements. In the past, most of the research focused on single-rod connections, in order to exploit the maximum resistance of the connection without accounting for interaction among bars or splitting failure in the timber member due to close edge distances or spacing between bars. Such interaction problems arise when dealing with multiple rods, thus requiring specific investigation to fully understand the behavior of the connection as a whole and to determine its capacity. In both cases, existing test procedures determine bond strength in specific geometrical configuration. The paper aims to determine the pull-out strength of single and multiple axially loaded steel rods bonded in glulam parallel to the grain differentiating the adhesive failure from the other failure modes. After an initial review of typical applications and existing design procedures, test results on single rod with confined or unconfined test setup on single rod at different embedment depths are presented and discussed, indicating that the confinement has a negligible influence on the pull-out capacity. Subsequently, interaction between bars is investigated by a specific unconfined configuration. The accounted parameters are the embedment depth of the bars, the dimensions of the timber section, and the spacing between bars. Results are discussed and compared with threedimensional numerical simulations. Both experimental and numerical results suggest that the critical value at which the transition from pull-out to timber-related failure is observed depends on the mechanical properties of the timber and on the properties of the adhesive, such that a single value of spacing should not be provided in design standard if the full capacity of the adhesive is to be exploited.