UCLA

The level of structural damage and the associated economic impact caused by recent earthquakes worldwide have spurred an increased interest in high performance seismic resisting systems that can sustain severe earthquakes with minimal damage. A particularly efficient high performance wall system consists of precast concrete panels vertically post-tensioned to the foundation with unbonded post-tensioning steel. The system relies on the vertical unbonded post-tensioning steel for flexural strength and re-centering, while mild bonded steel bars provide energy dissipation and additional flexural strength. Under lateral loading, the traditional plastic hinge mechanism, associated with structural damage and the potential for large residual deformation, is replaced by a controlled rocking mechanism at the wall-to-foundation interface that allows the system to undergo large nonlinear displacements with minimal damage and minor residual deformations.

This thesis presents experimental results from a dynamic test on a full-scale, four-story precast concrete building that utilized unbonded post-tensioned (UPT) walls in one principal direction of response and bonded post-tensioned concrete frames in the orthogonal direction. The building was subjected to simultaneous three-dimensional shaking using recorded ground motions from the 1995 Kobe earthquake. The excellent performance of the test building in the wall direction of response, exhibiting minimal damage and no residual deformations, confirms that UPT walls are a viable alternative to conventional reinforced concrete (RC) structural walls. In addition to providing experimental evidence of seismic performance of UPT walls incorporated into a building system, the tests provided valuable insight into issues typically not addressed in component-level experimental studies, such as the role of the floor diaphragm, influence of component interactions, and contributions of three-dimensional responses and torsion. As evidenced by the E-Defense test building, these effects need to be considered to obtain realistic estimates of lateral resistance and displacement demands.

The tests also provided a wealth of data against which design methodologies and analytical models for UPT systems can be benchmarked. Based on a detailed assessment of the E-Defense UPT walls in accordance with ACI ITG-5.2 (2009), and the performance of the UPT walls in the tests, design implications were identified and some revisions to ACI ITG-5.2 were suggested. Finally, an analytical model of the building in the wall direction was developed. Experimental results in this direction were used to assess the ability of the model to capture the dynamic responses and interactions of unbonded post-tensioned structural systems. Correlations between analytical and experimental results were satisfactory for a range of global and local responses, and key aspects of the interaction between components such as framing action and beam elongation effects were adequately reflected in the model.