Experimental Testing of a Rocking Cross-Laminated Timber Wall with Pinching-Free Connectors

Abstract

A cross-laminated timber (CLT) wall panel offers an efficient solution to resist seismic loads. Earthquake-induced vibrations can be accommodated and dampened by ductile connections that hold down a rocking wall panel. In conventional hold-downs, irreversible damage from localized crushing of timber and/or withdrawal of slender steel fasteners results in pinching behavior. The impaired stiffness and dissipation are understood to cause larger peak displacements and increase the vulnerability of structures to aftershocks. To overcome this, a pinching-free connector (PFC) was developed previously, and in the present study it was tested as hold-downs for a rocking CLT wall. Quasi-static and pseudodynamic tests demonstrated that the PFCs significantly reduced pinching and provided additional restoring forces from the hold-downs near the rocking toe by responding elastically between 0.6% and 2% drift. Below this drift, the wall recentered under self-weight, because the PFCs are ratcheting tension-only connectors that do not resist recentering. Nevertheless, restoring forces may be possible over a larger range of drifts by positioning the PFCs closer to the edges of the wall. Although minimal restoring forces allow high dissipation in every cycle, the number of cycles is limited by the elongation capacity of the mild steel rods that yield monotonically. Stainless-steel rods therefore are suggested as alternative dissipators with greater ductility. In the ratcheting mechanism (screw threads), inherent backlash of 1.38 mm manifested as slack on reloading or out-of-sync ratcheting and reloading when a pair of hold-downs was used. Consequently, the initial stiffness can decrease by as much as one-half, because one PFC engages only after the other has started to yield. Future research is recommended with roller clutches and circular friction pads to eliminate the backlash and provide large and stable ductility and dissipation. The lightweight CLT wall tested (<200 kg) demonstrates the potential of ratcheting connectors to create efficient seismic-resisting systems that self-center without any posttensioning.