Quasi-Static Testing of a 1/3 Scale Precast Concrete Bridge Utilising a Post-Tensioned Dissipative Controlled Rocking Pier

Abstract eng:
Low damage design is an alternative design philosophy to conventional capacity design. Its aim is to significantly reduce seismically induced damage to structural members and improve post-earthquake functionality, through replacing member plastic hinging, with replaceable ductile connections. An example of a low damage design structure is one which utilises dissipative controlled rocking (DCR) or the hybrid PRESSS connection. In such a structure, dissipative devices are combined with rocking and unbonded post-tensioning to form a structural system with self-centering behaviour and damage confined to replaceable dissipative devices. For the seismic design of bridges, the need for low damage design is becoming more apparent due to major indirect losses caused by damaged bridges in past major earthquakes. Much experimental research on low damage bridge design has focused on the testing of sub-assemblies, such as, bridge piers. Despite, these experiments being valuable in confirming the performance of these sub-assemblies, they are unable to indicate the real behaviour of these components within a complete bridge system or their effect on other components of the bridge (e.g. displacement compatibility effects). At the University of Canterbury, a 1/3 scale, two span, precast concrete bridge supported by a cantilever, post-tensioned, rocking column was designed and tested as part of a government funded program called Accelerated Bridge Construction and Design (ABCD). In addition to investigating interaction effects between the pier and the rest of the bridge, the behaviour of the same bridge utilising a modified dissipative controlled rocking (MDCR) pier was also investigated. The pier type is said to be modified because it utilises a novel technique of increasing structural redundancy called “hierarchical activation”. The motivation for this being, that, cantilever piers utilise a column sway mechanism whereby all inelastic rotation is concentrated at the base. Therefore, it is vitally important that the base connection is protected from failure in order to prevent collapse. For DCR piers in particular, they rely on the restoring force of the post-tensioning and multiplicity in the number of dissipators to provide structural redundancy under lateral loading. However, once rupture of a few dissipators and yield of the post-tensioning occur, the pier would lose significant stiffness and would be prone to P-Δ effects eventuating in collapse. The technique of “hierarchical activation” involves having a second set of dissipative devices which are only activated after the structure exceeds a certain drift level e.g. ultimate limit state. Hence, for seismic demands larger than the design level, the pier is given an extra layer of structural robustness in the form of increased damping and stiffness which contribute to limiting excessive displacements protecting the post-tensioning from yielding. This paper describes the results of the experimental work undertaken covering 8 tests, each, using different configurations (e.g. dissipation device locations, post-tensioning levels in pier and deck). Significant interaction effects between the pier and deck were observed, in addition to, unexpected deck elongation effects. These observations are shown to have important consequences for the design of cantilever/hammerhead pier bridges in general and bridges using discontinuous decks between spans.

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