A new macroelement model is presented in this paper for the simulation of the in-plane (IP) and out-of-plane (OOP) response of infilled frames subjected to seismic actions. The model consists of two diagonal, one horizontal, and one vertical struts. Each strut is represented by two fiber-section beam-column elements. The model is able to capture the arching action of the wall under an OOP load as well as the interaction between the IP and OOP actions. The proposed modeling approach is sufficiently simple and efficient that it can be used for the static or dynamic analysis of an entire structural system. An experimental validation has been carried out. A further numerical study performed with the macroelement model has shown that wall damage due to IP loads can significantly reduce the OOP resistance of the wall, and this influence depends on the slenderness (height/thickness) of the wall. A more slender wall will suffer a more significant loss of OOP resistance.

The issue of the influence of masonry infills within RC frames structures has been widely investigated in the last decades by several researchers. The large interest addressed to this topic depends on the actual observation that when in presence of seismic events, the response of framed structures is strongly conditioned by the interaction with the infill walls, which however are considered as non-structural elements and not included in the models. The influence of masonry infills role in structural response is so much relevant to affect not only the overall strength and the stiffness but it may also radically change the possible collapse mechanisms of the overall structural complex under the effect of strong ground motions. Infill panels may have a beneficial effect on the structural response, being able in some cases to supply the lack of resistance of structures to lateral actions, or an adverse contribution inducing unexpected and dangerous non-ductile collapse mechanisms. However, the studies carried out on this topic have demonstrated that, independently from the beneficial or adverse contribution of masonry infills on structural response, their presence cannot be neglected in structural modelling both in design and verification phases. The paper provides a large literature review regarding the modelling techniques developed in the last decades, going from refined nonlinear FE micromodel approaches to simplified equivalent single or multiple strut macromodels including also different technical code statements. The reliability of these approaches is discussed highlighting advantages and weakness points. Macromodelling approach is particularly pointed out since it constitutes the most attractive technique to perform complex nonlinear analyses (static and dynamic). A state of the art of the main issues regarding equivalent strut identification (stiffness, constitutive law and cyclic behaviour) across scientific literature is provided describing in detail noteworthy aspects of some approaches.

Equivalent strut macromodels are largely used to model the influence of infill walls in frame structures due to their simplicity and effectiveness from a computational point of view. Despite these advantages, which are fundamental to carrying out seismic simulation of complex structures, equivalent struts are phenomenological models and therefore have to conventionally account for the influence of really large amounts of geometrical and mechanical variables with a relatively simple inelastic response. Mechanical approaches, generally used to evaluate the force–displacement curve of a strut, are based on hypothesizing the damage mechanism that will occur for an infill–frame system subject to lateral forces. This assumption has a great impact on the response of the system and is affected by large uncertainties that also propagate in seismic assessment results. Based on the aforementioned unknowns, this paper proposes a new stress–strain relationship to be used for fiber-section modeling of equivalent struts. The definition of the stress–strain model is based on an empirical approach rather than a mechanical one. The four parameters defining the stress–strain law (peak stress, peak strain, ultimate stress, and ultimate strain) are directly linked to geometrical and mechanical features of an infilled frame through analytical correlation laws, which are determined from an experimental data set enlarged with data from refined finite-element (FE) simulations. The analytical correlations provided in the paper are proposed as a tool for direct modeling of an infilled frame. Validation tests were carried out with experimental results different from those used to build the data set. The extension of the model to the prediction of cyclic behavior is finally proposed.