Out-of-plane failure analysis of unreinforced masonry using discrete element method

Unreinforced masonry (URM) is often observed in the worldwide built heritage, in the form of either load-bearing components and systems or infill walls in framed structures. Particularly in case of historical buildings where connections between structural elements are poor or even lacking, URM walls frequently experience local out-of-plane (OOP) failure mechanisms under earthquake ground motion. Such failure mechanisms are a source of severe damage to the cultural heritage and huge loss of human life. Research on the OOP structural behaviour of URM walls is therefore required to assess and mitigate disaster risk in several countries. In this paper, the suitability of simplified micro-modelling approaches using the discrete element method (DEM) for numerical simulation of the OOP response of URM walls is investigated. Masonry units are represented as rigid bodies, lumping nonlinearities to zero-thickness interfaces by means of advanced constitutive laws accounting for local failure phenomena such as crushing. Numerical analyses are carried out well into the geometrical and mechanical nonlinear field, simulating fracture processes up to failure and collapse conditions. Numerical models are thus validated against experimental data accounting for varying aspect ratios, boundary conditions and precompression levels of URM walls. Parametric sensitivity analyses are then run to assess model robustness. Finally, the results of the discontinuum-based models utilizing rigid blocks are compared with the output of more computationally demanding models considering the deformation within the masonry units. Computational accuracy is then discussed in terms of damage progression, failure patterns and load-displacement curves.