Mechanical interpretation of infills-to-frame interaction: contributions to the global base shear for strut-based frame models

The presence of infills has substantial effects on the behaviour of frame structural systems, both in the linear and non-linear behaviour. Different effects might arise at global (increase in strength and stiffness, modification of the plastic mechanism) and local level (e.g. triggering of shear failures in the columns). The global effects can be captured by adopting numerical macro-models (e.g. representing the infill panels as equivalent struts), while local effects can be typically better captured by using FEM micro-modelling. Focusing on single-or multiple-strut macro-modelling strategies, in this paper a mechanically-based procedure to decouple the frame and infills contributions to the overturning moment (and hence base shear) capacity for anyvalue of the global displacement is proposed. The method is applicable regardless of the distribution of the infills and of the non-linear Axial load-Axial strain relationship of the equivalent struts. For these reasons, such procedure can be applied to post-process the results of non-linear pushover or time-history analyses of different types of infilled frames (material-wise)as well as be adopted as a simplified analytical approach, such as SLaMA(SimpleLateral Mechanism Analysis), to derive the capacity curve of an infilled frame whilst capturing the proper internal actions. An application of the method on an experimentally-tested one-storey one-bay masonry infilled frame is used to calibrate the adopted numerical model. Furthermore, the method is applied to the pushover analysis of a two-storey one-bay masonry-infilled RC frame, with uniform or “pilotis”infill distributions, modelled by means of a single-strut approach.The results confirm the suitability of the proposed method in decouplingthe frame and infills contributions to the total overturning moment