Advanced approaches for the seismic vulnerability assessment at territorial scale of masonry buildings in historical centres

Italian historical centres are generally characterized by unreinforced masonry buildings arranged in aggregate configurations. Past and recent earthquakes have underlined the susceptibility of these buildings to out-of-plane failure mechanisms involving their perimeter façades. Assessing seismic vulnerability of these buildings towards the out-of-plane mechanisms, both at the scale of the individual building and on a larger scale, becomes a necessary step for an effective planning of interventions aimed at reducing their vulnerability. One of the most commonly adopted approaches for assessing the seismic vulnerability of buildings stock at large scale is provided by fragility curves, as reported in many studies available in the current literature. Fragility curves at regional base represent, then, the object of the present research work. In general, the process of assessing seismic vulnerability is strongly related to the knowledge of the main properties characterizing the buildings composing historical centres which are, in most of the cases, arranged in an aggregate configuration, as the result of a discontinuous and unplanned development. As a consequence, the single units composing the aggregate, generally built in different eras and characterized by different construction techniques, materials and structural details, can mutually interact with the adjacent ones through the so-called aggregate effect. The aggregate effect itself could play a non-negligible role on the seismic safety level of the building especially toward the occurrence of the out-of-plane mechanisms. Indeed, depending on the construction history of the building, the connection among adjacent structural units in aggregate configurations could affect the out-of-plane seismic response. Based on the above considerations, the present research work aims, first of all, to introduce the aggregate effect into the seismic analysis of the out-of-plane behavior of the façade walls of buildings in aggregate configuration. To this purpose, the contribution of friction actions, developing at the connections between the façade wall, prone to the failure, and the walls of adjacent buildings, is accounted for in the analysis of the out-of-plane mechanism. Afterwards, a multistep approach to evaluate, in analytical terms, the fragility curves for the most probable out-of-plane mechanisms of the perimeter façades of buildings in historical centres is proposed. The fragility curves are, hence, differentiated according to the typology of mechanism that has the greatest probability of being activated and are, also, obtained for two distinct states of damage, representative of two distinct out-of-plane mechanisms, namely: the formation of the first cracks in correspondence of the activation of the mechanism, that is the Damage Limit State, and the collapse corresponding to the achievement of the Life Safeguard Limit State. The approach includes two important aspects characterizing the out-of-plane mechanisms of such buildings: the previously mentioned aggregate effect, that certainly represents one of the main peculiarities of buildings in historical centres, and, then, the various sources of uncertainty involved in the evaluation of the analytical fragility curves. In particular, the approach takes into account the uncertainty due to the following sources: the randomness of the geometric and mechanical parameters affecting the out-of-plane behavior, the record-to-record variability, the accuracy variability of the analytical model adopted to simulate the behavior of the façade walls and, lastly, the epistemic uncertainties. The proposed approach is, finally, applied to the buildings stock characterizing the historical centre of Sora, a medium size town of Central Italy assumed as case study, whose structural typologies have been identified and analyzed within the research project CARTIS (DPC/ReLUIS 2019–2021 and 2022-2024). The study focuses on investigating the influence of the aggregate effect and the seismic input on the fragility curves. Regarding the first aspect, the obtained results, both in terms of capacity curves and fragility curves, clearly show that the aggregate effect increases the level of the seismic safety of a façade wall in an aggregate configuration towards the out-of-plane mechanisms. Regarding the second aspect, both spectral-shapes derived by natural records and smoothed spectra provided by the Italian and European Codes are adopted as seismic input. However, the first choice, not only takes into consideration spectral-shapes compatible with the seismicity of the site under examination, but it also allows the evaluation of the record-to-record variability. To this end, both spectral shapes derived from the Italian Code and spectra obtained by a selection of natural accelerograms are employed for the construction of the fragility curves of the analyzed buildings. In particular, the obtained results highlight that the proposed approach allows to obtain consistent results also by employing the smoothed spectra derived by the Italian Code (NTC18) provided that an adequate value of the dispersion due to the record-to-record variability, derived beforehand on natural records. In addition, the results show that the range of values of the dispersion due to the record-to-record variability changes in function of both the damage state and specific type of mechanism under examination. It is worth pointing out that the fragility curves proposed in the present research work represent an improvement of the different proposals found so far in the literature for the out-of-plane mechanisms of unreinforced masonry buildings which do not distinguish in relation to the specific typology of local mechanisms and generally refer to a single damage state, corresponding to the achievement of the collapse. The approach proposed herein can, then, represent a useful tool to carry out a complete large-scale vulnerability assessment of a specific geographical area for out-of-plane mechanisms.