Effects of the vertical seismic component on seismic performance of an unreinforced masonry structures

The ground motion vertical component is typically ignored in the analysis of ordinary building structures, even though damage observed in recent Emilia (Italy) earthquakes indicate that the vertical accelerations may have influenced the response of older masonry buildings. This manuscript presents a numerical study that investigates the effects of the ground motion vertical component on a reference unreinforced masonry building. These effects are evaluated considering the in-plane wall behaviour due to different input ground motions and different mechanical properties for the masonry walls. Nonlinear dynamic analyses with natural ground motion records selected based on source-to-site distance, fault rupture type, ground category and moment magnitude were carried out. First, the velocity spectrum intensity (VSI) of all records was studied to identify records with strong vertical components. Second, nonlinear history analyses were carried out and selected engineering demand parameters (EDPs: horizontal and vertical displacements, axial and shear forces) were checked. The results show the clear correlation between the vertical VSI and the EDPs and the incidence of the source-to-site distance on the vertical component effects. For records characterized by low source-to-site distances and high moment magnitudes, the vertical component can generate tensile forces in the masonry piers, especially where the gravity loads are small. In these cases, fluctuations in the axial load strongly affect the flexural and shear capacity of the piers. Finally, the analysis of the Eurocode 8 provisions indicates that the absence of clear guidelines on the use of the vertical seismic component can be strongly non-conservative for masonry structures.