Pushover analysis of rocking façades in masonry churches: the role of friction and geometry in identifying homogeneous classes of vulnerability

The out-of-plane failure of façades is one of the most typical local mechanisms in masonry churches recognizable in the aftermath of a seismic event. The analysis of such a failure is addressed in this paper with reference to a small sample of existing masonry churches hit by the seismic event of 21st August 2017 in the Ischia Island (Italy). The main aim is to assess the influence of the geometric parameters of the façades and of the interlocked sidewalls on the vulnerability of the façades to out-of-plane mechanisms, using nonlinear static analysis. According to the displacement-based approach, the pushover analysis is carried out considering the geometric nonlinearity, namely by evaluating the static multiplier for varied kinematic configurations, and by means of a macro-block model involving the stabilising contribution of frictional resistances exerted by the sidewalls of the churches. In order to develop seismic assessments, the seismic demand is represented by ADRS (acceleration-displacement response spectra) for different limit states, obtained according to the Italian seismic code. Then, the capacity is compared with the seismic demand in terms of both forces and displacements and the relevant influence of the frictional resistances and of some geometric parameters on the response are highlighted. Finally, the capacity curves are critically discussed in order to identify ‘a posteriori' homogeneous classes of façades within the examined sample in terms of vulnerability and to develop future statistical analyses and fragility curves for the individuated homogeneous classes.