The effects of the vertical ground motion on the rocking response of masonry walls

The seismic performance of ancient masonry structures is often dominated by out-of-plane failure mechanisms. When the masonry exhibits sufficient quality to behave monolithically, the dynamic response can be effectively modelled using Housner’s rigid rocking block approach. However, traditional analyses typically disregard the vertical component of seismic action, which may significantly influence the rocking response and, consequently, the seismic vulnerability of such structures. This study examines the effects of vertical ground acceleration on the out-of-plane rocking behavior of free-standing masonry walls subjected to bidirectional seismic input. The analysis considers three archetype walls of different heights but identical slenderness, subjected to ten natural ground motion records from the 2016 Central Italy earthquake sequence. Through numerical integration of the governing equations of motion, the results reveal that vertical excitation can modify the collapse conditions, vibration frequency, and peak rotation amplitude. While the horizontal component remains the primary driver of collapse, the vertical acceleration can substantially influence the initiation of rocking and the overall dynamic behavior, particularly in low-rise walls or in scenarios characterized by high vertical-to-horizontal peak ground acceleration (V/H) ratios. These findings highlight the need to account for vertical ground motion in the seismic assessment of vulnerable masonry structures, where its omission may lead to unconservative predictions of structural performance