Experimental and analytical study on the pull-out brittle behavior of hybrid steel-grout-timber shear connectors

In this study, the pull-out brittle behavior of a hybrid steel-grout-timber connection comprising a fully threaded steel rod embedded in a thick epoxy-based grout layer in a cross-laminated timber (CLT) panel is investigated experimentally and analytically. The hybrid steel-grout-CLT connectors were tested under monotonic lateral loading parallel to the grain to understand their ultimate load capacity, stiffness, and failure modes. A total of 45 large-scale specimens were fabricated and tested to evaluate the effect of four key parameters: grout-to-rod diameter ratio, number of connectors, end distance, and row spacing. The results indicated that increasing grout thickness was associated with increased load capacity, consistent with a wider stress-spreading zone reducing stress concentration at the timber bearing surface. Brittle failure of the block tear-out observed in the longitudinal CLT layers was the predominant failure mode. Nevertheless, post-peak residual strength was observed in the specimens, ranging from 30% to 70% of the peak load. The stiffness of the specimens was less sensitive to the grout thickness, while a direct relation was observed between the elastic stiffness of the hybrid connections and the number of connectors. An analytical model was developed based on the shear resistance of timber, considering material property variability and different geometries. The model achieved a mean error of 12.4% when informed by the governing failure mode observed for each specimen, and a mean error of 27.1% when applied conservatively without this prior knowledge, as would be the case in a pre-test design application.