Masonry infilled RC frames: Experimental results and development of predictive techniques for the assessment of seismic response

The issue of the influence of masonry infills within RC frames structures have 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 infills, which however are considered as non-structural elements and not included in the models. The influence of masonry infills in structural response is so much relevant to affect not only the overall strength and the stiffness but it may radically change the possible collapse mechanisms of the structural complex under the effect of strong ground motions. Infills panels may thus 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. As more deeply discussed in this thesis, several modelling approaches have been developed to represent infill-frame interaction, going from refined nonlinear FEM approaches to simplified equivalent strut models. Especially the use of equivalent braced strut approach is pointed out in this work because of its simplicity in and the low computational effort required, which make this technique a predictive tool that is particularly attractive to perform complex nonlinear analyses of large buildings. As base reference of the modelling techniques developed in this thesis a large experimental campaign has been carried out and is presented in Chapter 2. The experimental investigation dealt with the cyclic behaviour of RC frame infilled with different kinds of masonry among the most employed in the worldwide building traditions. The results of this experimental campaign have been fundamental not only to enlarge the experimental knowledge but also for the further processes of calibration and comparison of the proposed models and predictive strategies. The research topics followed in this work regarded 3 fundamental aspects of the infill-frame interaction problem. The first is the calibration of the equivalent strut model, being able to overcome the difficulties in identification of the hysteretic parameters required by the models available in literature. The model makes use of a hysteretic “pivot” law needing few mechanical parameters for the identification and based on geometric rules rather than analytical. The study has shown that despite the simplicity of the model, it is able to provide an adequate accuracy to represent the cyclic hysteretic response of infilled frames. A second topic investigated in this thesis regarded the issue of the local interaction between infills and RC members. The panels are in fact able to attract a large portion of the lateral actions during earthquakes that can be however supported by the frame members if the latter have an adequate transversal reinforcing. The equivalent strut approach is unable to provide information about the additional shear demand arising on beams and column ends, therefore the study was addressed to fill this predictive lack. This was provided by means of a deep parametric study associated with the results of a detailed nonlinear FE analysis. This allowed to define abcorrelation between a geometrical-mechanical parameter identifying the infilled frame system and the shear demand on the beam and column critical end sections. The last topic was developed during a visiting period at University of California - San Diego and regarded the updating of the equivalent strut model able to predict simultaneously the in plane – out of plane response of an infilled frame and the reciprocal damaging in when seismic events occur. The equivalent struts have been modelled by means of fiber elements with distributed plasticity, able to reproduce the arching mechanism developed by the masonry panels confined by the RC frames in presence of out of plane actions. An identification procedure for the definition of an interacting in plane – out of plane model has been developed and validated on experimental basis.