Seismic fragility assessment of masonry structural units and masonry aggregates

Aggregate masonry buildings are very common characterized by a set of non-homogeneous structural units; hence, the seismic analysis cannot ignore the unavoidable interactions between adjacent buildings. This work considered a row aggregation of identical structural units but, as first step, a seismic vulnerability assessment methodology with reference to a single isolated structural unit is used. In order to account for some variabilities and uncertainties involved in the problem, the Response Surface statistical method is used, where the expected value of a response parameter (the collapse PGA) is approximated through a polynomial function of a set of chosen variables. The Response Surface model is calibrated through numerical data obtained by non-linear static analyses and used to determine the fragility curves, by applying full Monte Carlo simulations. Identical structural units chosen by the Response Surface generated simulations are aggregated, in order to compare the collapse PGA referred to the isolated structural unit and the one referred to the entire aggregate structure. The results showed significant differences in the value of the collapse PGA considering different directions of the seismic action, due to the geometry of the aggregate structure and the torsion effects deriving from the aggregation of structural units in row. Moreover, comparing the seismic behavior of the isolated structural unit and its seismic behavior when it is in the row aggregate structure, the results showed a decrease of the collapse PGA, if a larger number of structural units is considered.