Long-term behaviour of post-tensioned timber structures

In 2005, the concept of post-tensioned timber joints, known as the ’Pres-Lam’ system, was firstly proposed and developed at the University of Canterbury. The technology relies on unbonded post-tensioning tendons to provide moment capacity to beam-column, wall-foundation or column-foundation connections. Supplemental energy dissipation can be introduced by mild steel bars, or replaceable damping devices, when designing buildings for high seismic risk areas. Nowadays, the research led to the construction of eight post-tensioned timber (Pres-Lam) buildings throughout the world, and others are currently under design. Although the research body is significant, few information is available regarding the performance of the PresLam system over time: because creep occurs within the timber elements, it leads to losses in post-tensioning force over time. This work aims at providing information on the long-term behaviour of such technology. Specifically, the quantification and prediction of the amount of posttensioning loss and creep deformation in post-tensioned timber beams and frames were the objects of this study. Furthermore, the time-dependent seismic performance of post-tensioned timber frames was investigated. Two mains sources of data were analyzed and interpreted: 1) data coming from post-tensioned timber beam specimens, and 2) data monitored on two operative structures: the Trimble Navigation Office (Christchurch, New Zealand) and the House of Natural Resources (Zurich, Switzerland). A numerical approach based on a well know moisture dependent viscoelastic constitutive law was adopted to simulate the behaviour of such structures. Results of the parametric analyses performed with this model, along with analytical equations already present in literature, were combined to propose a design procedure to estimate by hand the post-tensioning trend over time for the most common design cases. Finally, the seismic performance of PresLam frame structures over time was investigated by the development of time-dependent fragility curves