CRITERIA FOR IDENTIFYING VULNERABILITY CLASSES AND FRAGILITY CURVES FOR ROCKING CHURCH FAÇADES

The out-of-plane failure of façade walls is one of the most typical local mechanisms in masonry churches recognizable in the aftermath of a seismic event. The purpose of this paper is to assess the influence of some geometrical parameters of the façade on its vulnerability to simple rocking and, therefore, to define reliable criteria for identifying homogeneous vulnerability classes. By adopting the displacement-based approach, pushover analyses are carried out considering the stabilizing contributions of the frictional resistances exerted by the sidewalls. The analyses are addressed with reference to a sample of 14 existing masonry churches hit by the seismic event of 21st August 2017 in the Ischia Island (Italy). Fragility curves for moderate (LS1) and severe (LS2) motions are derived for these façades under 15 ground motions with their site effects (including the occurred seismic input in Ischia) scaled at different values of the peak ground acceleration (PGA), using incremental static analysis (ISA), multiple stripe analysis (MSA) and acceleration-displacement response spectra (ADRS). For the onset of rocking (LS0), depending only on the PGA, fragility curves are developed with reference to the variability of the geometrical parameters of the facades both for the sample of the 14 churches of Ischia and for an extended one (400 façades) generated by means of a Monte Carlo simulation (MCS). Two conditions are considered for the examined façades: a ‘free rocking’ condition and a ‘restraint’ condition related to the interlocking with the sidewalls. All the obtained fragility curves are critically discussed in order to identify 'a posteriori' homogeneous classes of seismic vulnerability for masonry church façades for each limit state.