A sensitivity study of vulnerability parameters for rocking masonry façades of single-nave churches hit by the 2016 Central Italy seismic sequence

The paper is devoted to the large-scale assessment of the vulnerability of masonry church façades to the simple out-of-plane rocking mechanism. The main goal is to derive fragility curves assessing the influence of some geometrical and mechanical parameters and the effect provided by tie rods on the façade vulnerability to this type of mechanisms. According to the displacement-based approach, pushover analysis for each façade is carried out considering both the stabilizing contribution of the interlocking with the sidewalls and the presence of tie rods. The analyses are addressed with reference to a large sample of masonry churches generated starting from the geometrical parameters of the churches hit by the 2016-17 Central Italy seismic sequence using the damage data extracted from the DaDO WebGIS. Geometrical variables, i.e. the length, the height and the thickness of the façade, are treated by means of Monte Carlo simulation to generate a numerical sample of 800 façades. Three limit states are considered, corresponding to the onset of rocking and to moderate and severe motions, and fragility curves are derived for these façades subjected to several ground motions scaled to different values of PGA, using incremental static analysis and multiple stripe analysis. The sensitivity of the fragility curves is investigated with reference to the variation of: i) the friction coefficient influencing the contribution exerted by the sidewalls, ii) the length of masonry units, and iii) the position of the façade mass centre. Moreover, the contribution of strengthening systems made with steel tie rods to mitigate the simple rocking is analysed in terms of vulnerability reduction related to the diameter, position, pre-tension and ultimate strain of the tie rods. Finally, the effect of different seismic inputs on the seismic vulnerability of façades under moderate and severe motions is also examined.