Masonry nonlinear response: modeling and analysis of the effects of damaging mechanisms

Over the last decades, many efforts were devoted to develop efficient and accurate numerical procedures for the assessment of the structural capacity of masonry constructions. The main difficulties in modeling this type of material are due to its heterogeneous nature. Indeed, masonry is composed by blocks, stones or bricks, connected with or without mortar, whose geometry, mechanical properties and arrangement strongly affect the overall response. Among the available modeling strategies, finite element models appear to be suitable tools to describe the evolution of the nonlinear mechanisms developing in the material under typical loading conditions. Within this framework, macromechanical models, which consider masonry as an equivalent homogeneous, isotropic or anisotropic medium, are a fair compromise between accuracy and computational burden. Stemming on the above considerations, this work focuses on the development of constitutive laws involving damage and plasticity inner variables, tailored to the macromechanical analysis of 2D masonry structures. Herein, a new isotropic damage-plastic model, which is an enhanced version of that presented by Addessi et al. (2002), is proposed. This model is able to capture the degrading mechanisms due to propagation of microcracks and accumulation of irreversible strains, as well as the stiffness recovery related to cracks re-closure. Moreover, to account for the variation of the mechanical properties in the different material directions, a novel orthotropic damage model is developed to deal with regular masonry textures. The proposed models are implemented in finite element procedures, where the mesh-dependency problem is efficiently overcome by adopting nonlocal integral formulations. Numerical applications are performed to assess the models capacity of describing the material inelastic behavior and comparisons of numerically and experimentally evaluated responses are also provided for some masonry panels. Finally, the effects of degrading mechanisms on masonry dynamic behavior are investigated. For this purpose a systematic approach is adopted, based on the evaluation of the frequency response curves of masonry walls. The obtained curves show peculiar characteristics due to the irreversible effect of damage, which leads to degradation of the structural mechanical properties and the related variation of the natural frequencies, which in turn significantly influence the dynamic amplification of the response. The numerical results are also confirmed by shaking table tests performed on tuff masonry walls loaded out-of-plane.