Micromodelling of low-strength URM walls: In-plane shear behaviour and effect of minimal RC band strengthening

Weak unreinforced masonry (URM) buildings exhibit poor in-plane shear behaviour under earthquake loading. Conventional strengthening techniques may be difficult to implement in resource-constrained settings. Therefore, there is a need for practical strengthening solutions and reliable numerical approaches capable of assessing their effectiveness with limited experimental testing. This study investigates the in-plane shear behaviour of weak unreinforced masonry (URM) walls and their strengthened counterparts using a simplified yet physically representative micromodelling framework. Unlike conventional macromodelling, the approach explicitly represents masonry units and mortar joints. Masonry units are modelled using plasticity-based constitutive laws to capture compression-induced crushing, while mortar joints are simulated through cohesive interface models to represent degradation, sliding, and loss of cohesion. Effective stiffness formulations are incorporated to account for mortar thickness, ensuring computational efficiency without compromising realism. The interaction between minimally reinforced-concrete (RC) bands and weak masonry is explicitly considered to evaluate confinement and load redistribution effects. An experimental program of diagonal compression tests was conducted to validate the numerical model, and analytical in-plane shear strength evaluation methods were used for comparison in a simplified manner. Strengthened masonry walls exhibit significantly improved load-carrying capacity, enhanced post-peak behaviour, and increased ductility. The inclusion of minimal RC bands alters the failure mechanism from brittle sliding to a more distributed damage response. The study demonstrates that the proposed micromodelling approach is an effective and practical tool for assessing the adapted strengthening scheme for weak URM structures, where extensive testing is not feasible.