Time-Dependent Seismic Performance of Post-Tensioned Timber Buildings

Abstract eng:
As a result of the Precast Seismic Structural System (PRESS) program in the 1990s coordinated by the University of California, San Diego, the hybrid connection proved to be an efficient solution for achieving low damage concrete buildings. The combination of unbonded post-tensioning tendons and mild steel bars, allows the accommodation of the seismic demand through controlled rocking between structural members. While tendons provide the system re-centering capability after a seismic event, dissipation is localized to specific replaceable elements. In 2005 the technology was extended to engineered wood materials such as Laminated Veneer Lumber (LVL), glue laminated timber and cross laminated timber. Extensive laboratory tests carried out at the University of Canterbury showed good results in the connection performance, with no residual damage in the remaining structure. However, there is still little information about the long term behavior of post-tensioned timber, related in particular to the amount of possible posttensioning loss due to creep effects arising inside compressed members. The beam-column joint detailing represents in fact a critical part in terms of creep development, since timber column is loaded perpendicular to the grain. A case study building is presented in this paper, where the expected amount of post-tensioning loss varies between 5% and 45% in 50 years depending on the joint detailing solution. Two possible alternatives in the column-foundation connection are proposed, assuming different non-structural components drift resilience. The building seismic response is then analysed for different scenarios of post-tensioning loss up to 90% of the initial value. For this, Acceleration-Displacement Response Spectra (ADRS) and Non Linear Time History Analysis (NLTH) are applied and compared to investigate the structural behaviour considering events with return period equal to 50 years (SLS) and 500 years (ULS). The numerical results are then presented and discussed in terms of drift, accelerations, damping, and residual deformations. The seismic response in terms of re-centering capacity, drift control and dissipation properties is found satisfactory for posttensioning loss up to 50%. In case of greater amount of loss (i.e. 90%) the seismic behaviour is still acceptable providing however a residual drift.

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