Pres-Lam timber structures are being adopted throughout New Zealand and around the world. This innovative method of timber construction combines the use of large engineered timber members with posttensioning cables/bars. The hybrid version of the Pres-Lam system improves seismic performance through the addition of external or internal steel reinforcing. While the post-tensioning provides re-centering properties, the steel increases energy dissipation from the system as well as increasing moment resistance. The design of these structures is performed to withstand high levels of seismic loading without damage to the structural system. Over time, the post-tensioning force being applied to the structural timber members causes them to reduce in length that has a subsequent impact on the quantity of force being applied. This paper looks at the dynamic characteristics of fundamental period and elastic damping of three recently constructed Pres-Lam buildings, investigating the influence of these losses on the dynamic characteristics. Following this a study of the performance under strong motion is performed. The paper concludes that although the losses in post-tensioning are clear they do not impact on the dynamic characteristics and have only a minor impact on strong motion response.
The benefits of longitudinally post-tensioning in timber beams has been highlighted in several laboratory tests carried out at the University of Canterbury. However, there is still some concern about the long-term behaviour of post-tensioned engineered wood, which relates in particular to the amount of possible post-tensioning loss due to creep effects inside compressed members. For this reason, research has been conducted at the University of Canterbury including both experimental and numerical work on post-tensioned members. In the first part of this paper, the experimental results relative to four years of monitoring of post-tensioned LVL beam specimens with different tendon profiles are presented. In the second part, a coupled finite difference-finite element procedure is proposed to numerically simulate the beams behaviour. Results are then critically discussed, in particular concerning the amount of post-tensioning losses as well as creep deformations on the beams deflection.