PSI - Issue 44

Luca Umberto Argiento et al. / Procedia Structural Integrity 44 (2023) 1388–1395 Luca Umberto Argiento et al. / Structural Integrity Procedia 00 (2022) 000 – 000

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1. Introduction The out-of-plane vulnerability of unreinforced masonry buildings under seismic actions is frequently due to the lack of box-type behaviour. Their typical deficiencies concern weak connections, absence of connecting ties, insufficiently rigid floor diaphragms, low strength of materials. In particular, in the framework of the monumental masonry heritage, the churches represent the most widespread building typology that needs to be examined with a special attention in terms of vulnerability assessment. One of the most recurrent out-of-plane failure modes in churches involves the simple rocking of façades with or without parts of sidewalls. In this case, the portions of walls involved in the mechanism can be modelled as rigid blocks and their out-of-plane seismic response can be addressed through kinematic analysis and/or rocking dynamic analysis (Doherty et al. 2002, D’Ayala 2005 , Lagomarsino 2015, AlShawa et al. 2019, Casapulla et al. 2021, Giresini et al. 2019, 2021). Discrete and finite elements can also be used to assess the response of masonry structures under seismic actions (de Felice et al. 2016), but those involve several uncertainties, mainly related to the definition of the constitutive laws of the materials, and are more suitable for masonry walls which do not behave monolithically. Using nonlinear kinematic analysis, capacity curves for the façades of 14 masonry churches hit by the seismic event of 21 st August 2017 in the Ischia Island are obtained in this paper according to the displacement-based approach (Lagomarsino 2015). An advanced macro-block model accounting for frictional resistances (Casapulla and Argiento 2016, 2018) is herein adopted to consider the effect of the interlocking of the façades with the sidewalls. The churches are located in five different villages of the island and have a quite similar seismic hazard risk. The seismic input is represented in terms of elastic acceleration-displacement response spectra (ADRS) derived from analytical functions of the hazard parameters provided by the Commentary to the Italian technical standard for construction, namely CNTC19 (MIT 2019), which is followed to perform the seismic assessment of the selected façades with reference to three limit states. The original contribution of the paper consists in highlighting the role of the geometric parameters of the façade and of the sidewalls and of their interlocking restraints in the seismic assessment of the façade walls with reference to their out-of-plane behaviour at large displacements. The analyses are also prodromal to try to identify ‘ a posteriori ’ homogeneous classes of façades within the examined sample in terms of seismic vulnerability and, in the future, considering a more extended sample, to implement statistical analyses and fragility curves for the identified homogeneous classes. 2. Capacity curves for the rocking façades of 14 masonry churches in the Ischia Island (Italy) In this section, the results of non-linear kinematic analyses carried out to define the capacity curves related to the simple rocking mechanism of the façades of 14 masonry churches are presented. The selected churches were hit by the seismic event of 21 st August 2017 and are located in five different villages of the Ischia Island : Barano d’Ischia, Casamicciola Terme, Forio, Lacco Ameno and Serrara Fontana. Masonry block walls with regular units and staggering are considered (single-leaf walls arranged in a running bond pattern), while the unit dimensions are assumed equal to 150 mm both for length and height of the block (Fig. 1a), as measured on average by in situ surveys. The thickness of the units is always assumed coincident with that of the walls and the friction coefficient is f = 0.6 as suggested by experimental data present in the literature (Vasconcelos and Lourenço 2009). Table 1 reports the dimensions of the façades and the sidewalls (Fig. 1b) for the 14 analysed churches, together with the normalized values of their slendernesses . In particular, λ f = H / s represents the vertical slenderness of the façade, λ s = H / s t represents the vertical slenderness of the sidewalls, λ l = L / s represents the horizontal slenderness of the façade. According to the macro-block model developed by Casapulla and Argiento (2016, 2018) and herein adopted, the façade is assumed rotating around its external bottom edge, while the units along the two vertical crack lines exhibit a rocking-sliding motion with a clear prevalence of sliding. The frictional resistances developed along the vertical crack lines, which separate the façade wall from the sidewalls, exert a restraining action against the rocking motion. The model is used to perform the pushover curves predicting the capacity of the selected façades in terms of horizontal action activating the mechanism and its evolution at large displacements. These curves are obtained