PSI - Issue 78

Beatrice Travasoni et al. / Procedia Structural Integrity 78 (2026) 1111–1118

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1. Introduction Unreinforced masonry (URM) buildings represent a large share of the existing building stock in seismic-prone regions, where their seismic performance is strongly influenced by local mechanisms such as out-of-plane collapse of façade walls. This phenomenon is particularly critical in the presence of flexible floor diaphragms, typical of traditional construction, and nonlinear connection systems whose behaviour often governs the onset and evolution of damage. The seismic interaction between vertical walls, horizontal diaphragms, and their connections has been widely investigated in recent years. Experimental campaigns on full-scale masonry prototypes, such as the one tested at the University of Pavia in the 1990s (Calvi and Magenes; 1994, Magenes et al.; 1995), have provided reference case studies for the numerical benchmarking of URM systems under seismic loads. More recently, investigations on timber diaphragms with varying reinforcement levels (Baldessari; 2010) have demonstrated the role of in-plane stiffness in affecting global seismic behaviour. These findings have been complemented by the analytical formulations proposed by Guerrini et al. (2021), which allow for the estimation of the orthotropic stiffness of existing timber floors, both unreinforced and retrofitted. In parallel, Mirra (2021) conducted experimental testing on wall-to-diaphragm connections, highlighting the importance of their nonlinear tensile response, particularly in traditional timber floor systems. Complementary to this, Tselios et al. (2023) focused on the tensile behaviour of mechanical anchors embedded in masonry, providing key data for modelling failure mechanisms on the wall side of the connection. Despite this, many simplified assessment approaches still rely on idealised hypotheses, assuming rigid diaphragms or perfectly effective connections. These assumptions fail to capture the combined nature of out-of-plane mechanisms, which often evolve from an initial overturning motion to a vertical bending response as connections degrade or rupture. This study focuses on the out-of-plane response of the façade wall in a two-storey URM benchmark building. A nonlinear finite element model is developed to investigate the effect of different diaphragm configurations and connection systems, with the aim of identifying the mechanical parameters that govern the transition between overturning and bending mechanisms under seismic actions. 2. Methodology and benchmark case study The adopted methodology is based on the calibration of a finite element model (FEM), starting from the definition of nonlinear material properties representative of the masonry under investigation. To this aim, a benchmark structure made available within the research framework of the Italian Network of Seismic Laboratories (ReLUIS) was selected. The case study, referred to as BS4, consists of a two-storey unreinforced masonry (URM) building with rigid floor diaphragms. The structural layout is inspired by a full-scale prototype tested under static cyclic loading at the University of Pavia during the 1990s by Calvi and Magenes (1994) and Magenes et al. (1995). The building features a rectangular plan measuring 6.0 × 4.4 m², with four perimeter walls of 0.25 m thickness and an overall height of 6.44 m. In the present study, only the symmetrical plan configuration, identified as BS4_P1, is considered. This layout is characterized by a single typology of longitudinal walls, symmetrically arranged with respect to the plan axes, and by two blind transverse walls without openings (Fig. 1). The mechanical properties adopted for the masonry materials are reported in Table 1, and represent typical values for unreinforced clay brick masonry as found in Italian historical construction.

Table 1. Mechanical properties of materials.

f m = 6.2 MPa f tm = 0.04 MPa

τ 0 = 0.163 MPa f v0 = 0.23 MPa

E = 1800 MPa f tb = 1.22 MPa

G = 600 MPa μ = 0.58 rad

w = 17.5 kN/m 3

Masonry

l b / h b = 4 f m masonry compressive strength; τ 0 masonry shear strength; E masonry elastic modulus; G masonry shear modulus; w masonry specific weight; f tm average tensile strength of mortar; f v 0 initial shear strength between adjacent units; f tb tensile strength of masonry units; μ friction coefficient for sliding between adjacent units; l b brick length; h b brick height