A semi-empirical stress-strain model for equivalent strut fiber-section modeling of infilled frames

Infill-frame interaction is commonly modeled by means of equivalent strut macro-models. The main difficulty in using this phenomenological approach is to properly calibrate a force-displacement curve for the struts, as this depends not only on the geometrical and mechanical properties, but more specifically on the different potential damage mechanisms which may occur for the infill-frame system subject to lateral forces. If fiber-section elements are used as diagonal struts, the force-displacement curve is substituted by a stress-strain law. In both cases, the attribution of the inelastic law of the struts, based on a mechanical approaches, is not valid in general, as mechanical approaches are referred to specified hypotheses on the collapse modalities, which are really difficult to predict a priori. Based on this, the paper proposes a new concrete-type stress-strain relationship to be used with fiber-section elements and calibrated using a semi-empirical approach rather than a mechanical one. The proposed stress-strain model depends on four parameters. Experimental data and additional numerical simulations are used to derive semi-empirical correlations associating geometrical and mechanical properties of the frame-infill system with the aforementioned parameters. Analytical correlations of the best fitting equations are provided as tools for direct identification. Validation tests are finally carried out with experimental results different from those used to build the correlations.