Analysis, design and modeling of unbonded post-tensioned concrete shear walls in seismic areas

In the seismic engineering philosophy it is becoming more and more important to realize structures that not only prevent the collapse but that can be cost-efficient as well, in terms of structural and non-structural repair and in terms of loss of business operation after the earthquake. The self-centering system is a lateral resisting system that could achieve the aforementioned enhancement, proving a restoring force that pulls the structure back to its undisplaced configuration. Particular self-centering systems are the unbonded post-tensioned hybrid concrete walls whose behavior is based on the possibility to have a gap opening at the joint between wall and foundation, combined with the presence of vertical unbonded post-tensioned tendons that provide a clamping force. The damping system is composed by steel bars debonded for a certain length inside the concrete that dissipate energy yielding in tension and compression. For this kind of structures the common code-based design procedures do not give correct results, because they do not take into account the unique behavior of these systems under seismic loads. On the contrary, the Direct Displacement-Based Design method starts from the definition of a target displacement that represents the expected performance of the building and takes into account the typical displacement profile and hysteresis rule of the structure from the very beginning. The strength design carried out with this method for an unbonded post-tensioned concrete wall, must be verified. Analytical models already present in literature used softwares that are not usually present in structural firms and therefore their results are not so easily applicable to real case studies. For this reason, the modeling of this particular structures is developed using a well-known and commercially available finite element software, ETABS 2015. All these considerations are applied to a real case study, the New Long Beach Civic Center, comparing different seismic design procedures and different configurations for the shear wall.