A Macro-Distinct Element Model (M-DEM) for simulating the in-plane cyclic behavior of URM structures

In this work, a new Macro-Distinct Element Model (M-DEM) for the analysis of the in-plane behavior of unreinforced masonry (URM) structures, aimed at combining the efficiency of simplified approaches with the accuracy of discontinuum-based micro-modeling methods, is presented and validated through comparison against a number of both experimental and numerical tests on URM components. In the M-DEM framework, Finite Element (FE) homogenized macro-blocks are connected by discrete spring interfaces, whose layout is determined a priori as a function of the masonry texture. In-plane diagonal and sliding shear failure mechanisms, as well as flexural damage, are accounted for by the discrete spring interfaces. Meanwhile, a new methodology to simulate crushing, which makes use of a strain-softening model originally conceived for modeling concrete failure, is proposed and calibrated against small-scale tests on masonry samples. The strategy is to simulate shear/tension failure in the block interfaces and compression failure within the FE macro-blocks, while discretizing to allow the possibility of simulating out-of-plane failure modes. Using the M-DEM, the observed experimental damage and the hysteretic behavior of various reduced-scale URM specimens, subjected to shear-compression cyclic loading, were satisfactorily reproduced numerically. The capabilities of the M-DEM to predict the influence of the bond pattern on the monotonic behavior laterally-loaded URM piers were also scrutinized through comparison with standard micro-modeling outcomes, focusing on potential differences concerning both accuracy and computational expense. Finally, given the encouraging results obtained, the proposed approach was extended to the simulation of the in-plane cyclic response of a full-scale URM façade. Although the model marginally underestimated the energy dissipation in the first test phases, a good agreement was obtained in terms of peak and residual base shear capacity, initial in-plane stiffness and its progressive deterioration, governing failure mechanisms and final crack pattern, whilst simultaneously keeping computational costs within acceptable limits.