Group Effects for Shear Connections with Self-Tapping Screws in CLT

Cross-laminated timber (CLT) panels, when used as shear walls or diaphragms, are commonly connected with multiple ($n$) dowel-type fasteners in a row. For such connections, it is frequently observed that the load-carrying capacity of multiple fasteners is less than the sum of the individual fastener capacities, a phenomenon referred to as the group effect. The research presented in this paper investigated the group effect in self-tapping screw (STS) shear connections between CLT panels. Different joint types (surface splines with STS in shear, and half-lap and butt joints with STS in either shear or withdrawal) were evaluated in a total of 175 quasi-static monotonic and reversed cyclic tests, with the number of STS in one row varied between 2 and 32. The results showed that the group effect for the joint capacity (strength) can be expressed as $n_{\mathrm{eff}}=0.9n$ for all joints under static loading, where $n_{\mathrm{eff}}$ is the effective number of fasteners. In case of cyclic loading, a more pronounced group effect was observed that can be expressed as $n_{\mathrm{eff}}=n^{0.9}$. These reductions are significantly less conservative than the current Canadian design provisions for lag screws. For the reduction in stiffness and ductility, $n_{\mathrm{eff}}=n^{0.8}$ and $n_{\mathrm{eff}}=n^{0.9}$ can be used for all joints under static and cyclic loading, respectively. Finally, the capacity, ductility, and stiffness for joints under cyclic loading where the STS acted in withdrawal were on average 10% lower compared with the static values.