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

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-story timber buildings. The benefits of post-tensioning to assemble prefabricated timber elements are rapid erection, simple connections, and high seismic resistance. It has been shown that 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. This article summarizes the results of an investigation into the seismic response of full-scale timber beam-column joints, subjected to extensive experimental study and numerical modelling, performed as part of a larger research program on timber structures at the University of Canterbury, New Zealand. The beams and columns were fabricated from laminated veneer lumber (LVL). The tested joint was designed as part of a moment-resisting frame for a six-story building located in a high-seismic region. The timber members and the re-centering/dissipating elements were optimized to produce the intended dissipating-recentering behavior and commercially available prestressing arrangements were used to verify their applicability in a timber structure. The results of the experiments on both interior and exterior beam-column joints are presented, for both post-tensioned-only and hybrid models. The post-tensioned-only solutions exhibited nonlinear elastic behavior with full re-centering while the hybrid systems provided significantly greater levels of energy dissipation compared with the post-tensioned-only solution. The joints were also tested with a number of additional features such as steel armoring plates at the interface, reinforcement in the form of long screws embedded into the column and combination of the two. The armored the column faces with steel plates exhibited higher stiffness and smaller deformation in compression perpendicular to the grain. The joint region within reinforced column reduce the subsequent drop in prestressing forces in the system. Combination of armoring and reinforcements protect the joint region through minimizing joint shear panel deformation. In general, the tested arrangements exhibited high levels of ductility and negligible residual deformations with no significant damage of the structural elements. The numerical models accurately predict the behavior of different types of beam-column joints tested and can be used to design similar connections. These full-scale tests also demonstrate the practical feasibility of post-tensioned timber frames for multi-story timber buildings.