Seismic Collapse Risk Assessment of Posttensioned Controlled Rocking Masonry Walls

Abstract

The use of vertical unbonded posttensioned (PT) bars in masonry walls results in a controlled rocking behavior that can provide a high drift capacity and recenter the wall to its vertical alignment, minimizing residual drifts after a seismic event. However, because posttensioned controlled rocking masonry walls (PT-CRMWs) are a relatively new seismic force resisting system relative to conventional reinforced masonry (RM) walls with bonded reinforcement (i.e., fixed base walls), no distinct seismic response modification factors are yet provided in North American building codes and design standards for PT-CRMWs. In addition, following the FEMA P695 methodology, the National Institute of Standards and Technology (NIST) reported that some conventional low-rise RM walls could experience an excessive risk of collapse under the maximum considered earthquake (MCE). For these reasons, the current study evaluates the collapse risk of PT-CRMWs when designed using the seismic response modification factors currently assigned for special RM walls. In this respect, OpenSees is first used to develop and validate multispring macro models to simulate the seismic response of 20 PT-CRMWs with different configurations and axial load levels. The models are then used to perform nonlinear static and dynamic analyses following the FEMA P695 methodology, which involves evaluating the wall overstrength and seismic collapse margin ratio relative to the MCE. The results demonstrate that low-rise PT-CRMWs, designed with the seismic response modification factors currently assigned by the North American building codes and design standards to special RM walls, can meet the FEMA P695 acceptance criteria for the expected seismic collapse risk under the MCE. However, the peak forces in high rise PT-CRMWs are governed by higher mode effects, which increase the collapse risk due to shear. Finally, the influence of confinement on reducing the collapse risk of PT-CRMW archetypes is evaluated.