Modelling of masonry infilled RC frames subjected to cyclic loads. State of the art review and modelling with OpenSees

Reinforced concrete frames with unreinforced masonry infill walls represent a widely adopted building system. During seismic events, infill walls, usually considered as non-structural elements, may significantly affect the characteristics of the system in terms of in-plane stiffness, strength, and energy dissipation capacity. The assessment of framed structures infilled with unreinforced masonry walls has been investigated since the 1950s. Developing reliable numerical models of infill walls has become an important issue since then. The analytical simplified model based on equivalent diagonal struts is often used to assess the infilled frames response. The nonlinear behavior of the equivalent diagonal strut is usually described by constitutive laws that account for the stiffness, the strength and the hardening or softening behavior of the infill. The present work focuses on the evaluation of various parameters needed to define the monotonic and hysteretic response of infill walls modelled by equivalent struts. In order to select a simple and reliable analytical model that suitable for representing the infill wall response, different strut formulations and hysteretic models have been analysed in detail and used to reproduce several experimental tests available in the literature. The numerical analyses are performed by means of the OpenSees computer program. Three uniaxial material models available in OpenSees are used to assess their capability in reproducing the experimental hysteretic response. Finally, from the comparison among different models and between numerical and experimental results, suggestions are made to properly model the in-plane non-linear response of infills.