SEISMIC BEHAVIOR OF UNBONDED POST-TENSIONED PRECAST CONCRETE WALLS WITH INTERNAL AND EXTERNAL DAMPERS

In the last few decades, there has been a growing interest in applying unbonded post-tensioned (UPT) precast concrete structures in seismic regions to improve the seismic performance of concrete buildings by reducing the residual damage. Several experimental tests were conducted to investigate the structural performance of UPT walls. In most of the previous tests, UPT precast wall specimens showed a better performance compared to equivalent conventional structural walls. However, it has also been reported that the ultimate behavior of these UPT walls relies heavily on the detailing at their bottom joint, such as mechanical couplers for mild steel reinforcement and confining reinforcing details at their boundary elements. As a result, more research is needed on UPT precast concrete walls to evaluate their post-peak behavior and collapse potential. This paper presents the overview and results of an experimental investigation on the structural performance of UPT precast concrete walls subjected to reversed cyclic loads. In addition, this paper discusses the damage progression and failure mechanism of each specimen. Two half-scaled precast walls were investigated and each specimen consisted of two precast concrete panels joined only by post-tensioned unbonded strands. The bottom joint of each specimen had two types of damper: one specimen featured mild steel reinforcement crossing the bottom joint and the other specimen featured a set of hysteretic dampers attached to the wall surfaces. The hysteretic dampers were also fabricated from mild steel and had a buckling-restraining system. These dampers were set to contribute to the flexural capacity of wall specimens. A prestressing force of about 45% of the nominal yield strength of the strands was introduced to both specimens. In addition to the prestressing, a total axial load of 468.5 kN was applied to each specimen (axial load ratio of about 0.05), before any lateral loading. Fully reversed cyclic displacements were imposed on the wall specimens until a significant strength reduction was observed. Despite that significant axial loads were applied, both specimens sustained large lateral deformation (drifts above 3%) while maintaining their lateral strength, energy dissipation and self-centering capacity. A better performance was observed in the specimen with external dampers, with less residual drifts and less cover concrete spalling; moreover, the specimen with external dampers sustained larger lateral drifts (above 4%) without significant strength degradation. The external dampers were effective in dissipating energy until large drifts, where local buckling at the top end was observed; moreover, these dampers made an addition of about 15% for the equivalent damping ratio.