Cyclic behaviour of hold-downs using mixed angle self-tapping screws in Douglas-fir CLT

Abstract

Long self-tapping screws are now commonly used in mass timber construction in New Zealand and globally due to their high strength and ease of installation on site. Under axial loads, self-tapping screws are strong and stiff but are not ductile. However they are very ductile under lateral loads similar to other dowel-type fasteners. Previous research has shown the possibility to achieve strong and ductile connections when self-tapping screws are installed with mixed angles so that axially and laterally loaded screws work together to resist the loads. However, previous research was limited to timber-to-timber joints. So far, no experimental testing has been done to investigate the performance of steel-to-timber joints using self-tapping screws with mixed angle installations. These steel to timber joints can be used as hold-down connections in cross laminated timber (CLT) shear walls. In this study, cyclic performance of the mixed angle steel-to-timber connections were experimentally investigated in Douglas-fir CLT. Testing results showed that mixed angle installations can provide high strength and stiffness, as well as high ductility and displacement capacity. The performance of these connections was also compared to similar dowelled hold-down connections with the results showing an increase in initial stiffness as well as the elimination of initial slip due to installation tolerances, confirming the suitability of these connections as a viable alternative for dowelled connections as hold-downs in CLT shear walls.