Contents lists available at ScienceDirect Engineering Structures journal homepage: www.elsevier.com/locate/engstruct Computational modeling of the out-of-plane behavior of unreinforced irregular masonry Micaela Mercuri a , Madura Pathirage a , Amedeo Gregori a , Gianluca Cusatis b, ⁎ a Department of Civil, Building and Environmental Engineering, University of L’Aquila, L’Aquila, Italy b Department of Civil and Environmental Engineering, Northwestern University, Evanston, IL, USA ARTICLE INFO Keywords: Out-of-plane behavior Earthquake Lattice discrete particle model Stone irregular masonry Fracture mechanics Cohesive behavior Non-linear analysis Local collapse mechanisms Global collapse mechanisms ABSTRACT The vulnerability of stone masonry structures to seismic loading constitutes one of the main application areas of research in the field of structural engineering. This paper focuses on the analysis of the out-of-plane response of unreinforced masonry structures. Although successful in many applications, traditional continuum-based ana- lysis, as well as simplified analytical models, fail to a large extent in correctly capturing complex failure me- chanisms occurring for this type of structures. To overcome this issue, this study adopts a discrete model, the so- called Lattice Discrete Particle Model (LDPM), to describe the structural behavior of a variety of stone masonry structures up to their collapse. LDPM simulates the behavior of masonry at the stone level. The interaction between stones that are bounded by weak layers of mortar is governed by specific constitutive equations de- scribing tensile fracturing with strain-softening, cohesive and frictional shearing, and compressive response with strain-hardening. This manuscript aims to validate the proposed model with experimental data available in the literature. Furthermore, overturning walls with and without openings are simulated, the associated local collapse mechanisms are analyzed and compared with the classical kinematic analysis. Finally, more complex mechan- isms are numerically investigated to reproduce the behavior of systems of panels included within the continuity of a facade. The results show that LDPM is able to capture the damage evolution and the fracture propagation that lead to the overall collapse and it can be used confidently as an alternative method to perform the limit analysis of local collapse mechanisms. The proposed numerical approach provides engineers with a powerful modeling tool for the analysis of the behavior of stone masonry structures with a variety of geometrical con- figurations and under very general loading conditions. 1. Introduction Earthquakes are one of the most deadly demonstrations of natural disaster, yet human fatality does not occur directly because of the ground motion. Loss of human lives is often a result of structure failure. Particularly susceptible to collapse during earthquakes are UnReinforced Masonry (URM) structures, which are common throughout North and Latin America, the Himalayan region, Eastern and Western Europe, Indian subcontinent and Asia [1]. Masonry is the material that constitutes most of the cultural and architectural heritage of historical cities. In addition, it is often used for habitats in rural areas of developing countries. Typical URM buildings are composed by several load-bearing ma- sonry walls arranged in orthogonal planes with relatively flexible floor diaphragms [2]. During seismic events, the structure absorbs and dis- sipates the kinetic energy, by relying on its elastic and fracture capacities [3]. The seismic behavior of an URM structure depends on some critical features related to different scales of observation [4,5]. At the macro-scale, factors determining the overall response are the slen- derness of the load-bearing walls [6,7], the effectiveness of the con- nections among walls and horizontal structures to ensure the so-called box behavior [8–11], and the presence of adequate rigid and resistant floor diaphragms [12–18]. Other suitable elements such as ties, string walls or buttresses are also important factors to counteract horizontal thrusts [19]. At the meso-scale and micro-scale, the quality of the used materials [20–23], i.e. mortar and stone aggregate, as well as variations in the quality of the construction work [24,25] are important aspects affecting the global response. Under earthquake excitation, due to multi-directional ground mo- tions, masonry walls are simultaneously subjected to In-Plane (IP) and Out-Of-Plane (OOP) loads. The IP behavior of URM walls has been widely studied among researchers [26–36]. The IP failure is mainly https://doi.org/10.1016/j.engstruct.2020.111181 Received 2 March 2020; Received in revised form 12 July 2020; Accepted 26 July 2020 ⁎ Corresponding author. E-mail address: g-cusatis@northwestern.edu (G. Cusatis). Engineering Structures 223 (2020) 111181 Available online 17 August 2020 0141-0296/ © 2020 Elsevier Ltd. All rights reserved. T