Seismic performance of full-scale post-tensioned timber beam-column joints

Structural frames made of prefabricated laminated timber beams and columns connected by unbonded post-tensioning and additional mild steel reinforcement have recently been proposed for multi-storey timber buildings (referred to as "Pres-Lam®" technology). The benefits of post-tensioning to assemble prefabricated timber elements are rapid erection, simple connections, and high seismic resistance. Prefabricated post-tensioned timber members can be designed to have excellent seismic resistance, with the post-tensioning providing re-centering capacity after major earthquakes, while energy is dissipated through yielding of replaceable mild steel devices. Both post-tensioning and mild steel devices contribute to the stiffness and strength of the overall system. This paper summarizes a large experimental investigation into the seismic response of full-scale timber beam-column joints, performed as part of a larger research programme on timber structures at the University of Canterbury. The beams and columns were fabricated from laminated veneer lumber (LVL). The tested joint was designed for a moment-resisting frame for a six-storey building located in a high-seismic region. The timber members and the re-centering/dissipating elements were optimized to produce the intended behaviour. Commercially available prestressing arrangements were used, with post-tensioning forces up to 1000kN. The mild steel energy dissipaters wee attached externally so that they could be removed and replaced easily after a major earthquake. Both exterior and interior joints were tested. This paper presents the results of the experiments on interior beam-column joints, showing excellent seismic behaviour with very little residual damage. These full-scale tests demonstrate the practical feasibility of post-tensioned timber frames for multi storey timber buildings as well as their excellent behaviour in seismic regions.