Numerical study of the effects of tilting base on seismic out-of-plane response of one-way spanning unreinforced masonry walls

This paper investigates the response of one-way spanning unreinforced masonry (URM) walls first statically tilted in the in-plane (IP) and out-of-plane (OOP) directions and then subjected to seismic OOP loading, a subject largely overlooked in the literature. The study is motivated by this knowledge gap and its particular relevance to Netherlands, where ground settlement often leads to visible tilting in buildings, yet sufficient evidence for if and how such tilting should be explicitly considered in design and assessment does not exist. The numerical modeling approach previously proposed by the authors, validated against complex experimental data, is employed in a comprehensive parametric study to provide preliminary insights into the seismic response of tilted walls. The model represents URM unit-by-unit using nonlinear 3D solid expanded blocks and cohesive-frictional zero-thickness joints. Two wall specimens, one short and one long, are subjected to one level of IP base tilting and two levels of OOP base tilting. Static and dynamic OOP loading is then applied while maintaining the prescribed tilt. Under static OOP loading, three levels of vertical pre-compression are considered, representing conditions in low-rise residential buildings. For dynamic OOP analyses, a multi-step loading sequence with varying levels of overburden is used. The results show negligible sensitivity to IP tilting (even up to 4°). While the specimens exhibit slightly greater sensitivity to OOP tilting, primarily due to reduced vertical confinement and increased uplift, responses remain largely stable even with large OOP drift (up to 22% of wall thickness). Aside from the aforementioned findings, this study, being the first one in the literature studying load-bearing tilted walls, highlights key limitations which may have affected the outcomes and emphasizes the need for further research to better understand the behavior of tilted URM walls.