Relative Support Motions on the Out-of-Plane Dynamics of One-Way Spanning Unreinforced Masonry Walls and Insights into Instability Displacement

This study investigates the effects of differential support motions, caused by filtering effects due to the building response to earthquakes, on the one-way bending out-of-plane (OOP) behavior of unreinforced brick masonry walls, a factor often overlooked in seismic assessments. A high-fidelity block–based numerical model is used to simulate walls with varying slenderness ratios, precompression levels, and boundary conditions. Floor motions from shake-table tests on two-story masonry buildings with flexible and rigid attic diaphragms under induced and tectonic seismicity are applied at the top and base boundaries of the models as loading signals. Results show that differential motions between supports significantly influence OOP wall response, reducing the peak acceleration associated with signals to provide collapse while increasing displacement capacity. This effect is most pronounced in walls with lower-slenderness ratios, higher precompression, or constraints against uplift and rotation at the top, as differential loading disrupts vertical arching mechanisms that, otherwise, enhance stability under uniform boundary motions. Using only the base signal, a common simplification in seismic assessments fails to capture these differential effects. The results of the simulations are then examined to investigate the largest OOP displacement at which walls can regain stability under dynamic loading, calculating dynamic stability displacement thresholds and showing an average dynamic-to-static stability ratio of 62%, consistent with the 60% ratio used in safety standards. However, deviations occur in walls with failure mechanisms differing from typical flexural behavior. The findings provide critical insights for more accurate seismic assessment and retrofitting of masonry walls by highlighting the importance of considering differential motions. Additionally, this study offers supporting data for the dynamic stability displacement thresholds currently adopted in seismic assessment guidelines, addressing a gap in experimental data and improving the reliability of safety evaluations for unreinforced masonry structures under seismic loading.