Vertical collapse mechanisms in masonry buildings due to seismic vertical component

The acceleration histories recorded in the epicentral areas during the last earthquake in Central Italy (2016-17) show very intense vertical components. During the post-earthquake damage assessment operations in the epicentral areas, some damage frameworks different from those described in the literature have been observed, with a macro-element collapse mode attributable to the action of the earthquake vertical component. The paper presents some of these buildings characterized by the presence of horizontal cracks, by the displacement of the highest levels, by vertical cracks in the sub-window walls. The masonry walls are undamaged without the typical shear or flexure failure cracks and the lower levels do not show any damage. The crack pattern and the associated collapse mechanisms cannot be included within the classic in-plane and outof- plane mechanisms. A collapse mechanism characterized by the loss of vertical connection of entire structural bodies is therefore hypothesized. The activation of this type of mechanism would seem to exclude the formation of the classic collapse mechanisms on which the seismic capacity checks of the masonry structures provided by the technical codes are based. A simplified linear kinematic analysis able to analyze the described behavior is proposed and illustratively applied to a case study building. Criteria for the identification and verification of the proposed mechanisms are described, with the aim to introduce the method in design codes.