Appraisal of the overburden mass and boundary conditions on the rocking behaviour of the vertical spanning strip wall

Unreinforced masonry structures are particularly vulnerable to seismic events. Specifically, local out-of-plane mechanisms form recurrently, posing a serious threat of collapse. Among them, a Vertical Spanning Strip Wall (VSSW) mechanism occurs when a portion of a wall is solely constrained at its top and base, experiencing out-of-plane bending across its height. During the formation of the mechanism and up to collapse, the VSSW presents a highly nonlinear dynamic behaviour governed by the presence of an overlying wall or diaphragm and a diversity of diaphragm-to-wall connections. The present work simulates the dynamics of a VSSW, accounting for the influence of the overburden mass and a variety of boundary conditions through a two-rigid-body single-degree-of-freedom model. In addition, this study examines the energy losses of the VSSW and provides closed-form expressions to estimate the angular coefficient of restitution either analytically for predictive purposes or experimentally for characterisation campaigns. Finally, a series of illustrative comparative free- and forced-rocking analyses highlight the importance of the overburden mass and boundary conditions on the dynamic stability of the VSSW.