Estimation of Load Multipliers in Overturning Mechanisms with Frictional Resistances: Comparison Among Literature Approaches

The load multiplier of failure mechanisms represents an analytical evaluation of the behavior of a building during an earthquake. Provided that disaggregation phenomena are inhibited, out-of-plane mechanisms are the most hazardous among the possible failure modes in masonry buildings. The overturning can involve a wall either by itself (simple overturning) or by dragging a portion of the orthogonal wall (wedge-shaped overturning). Many literature works explored the geometry and load factor of these mechanisms. Notwithstanding, they led to various interpretations of the same failure mechanisms, especially when friction among the masonry units is considered. In addition, these theoretical approaches assume a monolithic drystone ashlar masonry, but common historical buildings in Italy, with roughly cut stone units randomly arranged on wide joints filled with poor mortar, can barely fit this condition. The paper compares the load multipliers for nine overturning mechanisms, both simple and wedge-shaped, calculated according to five literature approaches, of which three consider the influence of friction. These mechanisms were observed on seven masonry buildings in the historical center of Castelsantangelo sul Nera, hit by the 2016 Central Italy earthquake. The comparison enables to evaluate the contribution of frictional resistances to the loading conditions. Among those explored, two approaches which consider friction obtained similar outcomes, whereas the third one overestimated the multipliers. In general, frictional resistances increased load factors. Furthermore, the analysis on real buildings gave the possibility to compare the analytical outcomes of the expected failure mechanism with the real damage pattern and the real registered seismic action.