Seismic Collapse Risk Assessment of Reinforced Masonry Walls with Boundary Elements Using the FEMA P695 Methodology

Abstract

Using boundary elements in reinforced masonry (RM) walls allows closed ties to be used and multiple layers of vertical bars to be accommodated, thus providing a confining reinforcement cage. This enhances the overall performance of the RM wall with boundary elements relative to traditional RM walls with rectangular cross sections. This is attributed to the fact that traditional RM walls can typically only accommodate single-leg horizontal reinforcement and a single layer of vertical reinforcement because of practical limitations associated with concrete masonry unit geometrical configuration and construction techniques. Following the FEMA P695 methodology, “Quantification of Building Seismic Performance Factors,” the National Institute of Standards and Technology reported that some low-rise traditional rectangular RM walls (without boundary elements) might experience an excessive risk of collapse under the maximum considered earthquake (MCE). Moreover, although North American codes give seismic modification factors for RM shear walls with rectangular cross sections, no distinctive corresponding values are provided for RM shear walls with boundary elements. To address these issues, this study examines the effect of adopting the seismic response modification factors assigned for traditional RM shear walls on the collapse risk of RM shear walls with boundary elements. In this respect, OpenSees was used to create macro models to simulate the seismic response of 20 RM shear walls with boundary elements, designed with different configurations under different gravity load levels. The modeling approach was experimentally validated and the models were subsequently used to perform nonlinear static pushover analyses and incremental dynamic analyses following the FEMA P695 methodology. The analyses focused on evaluating the wall overstrength, period-based ductility, and seismic collapse margin ratios under the MCE. The results show that RM walls with boundary elements designed considering the ASCE7-10 force reduction factor currently assigned to RM walls with rectangular cross sections experience an enhanced performance that is enough to meet the FEMA P695 acceptance criteria for the expected seismic collapse risk under the MCE.