Evaluation of infilled frames: an updated in-plane-stiffness macro-model considering the effects of vertical loads

The influence of masonry infills on the in-plane behaviour of RC framed structures is a central topic in the seismic evaluation and retrofitting of existing buildings. Many models in the literature use an equivalent strut member in order to represent the infill but, among the parameters influencing the equivalent strut behaviour, the effect of vertical loads acting on the frames is recognized but not quantified. Nevertheless a vertical load causes a non-negligible variation in the in-plane behaviour of infilled frames by influencing the effective volume of the infill. This results in a change in the stiffness and strength of the system. This paper presents an equivalent diagonal pin-jointed strut model taking into account the stiffening effect of vertical loads on the infill in the initial state. The in-plane stiffness of a range of infilled frames was evaluated using a finite element model of the frame-infill system and the cross-section of the strut equivalent to the infill was obtained for different levels of vertical loading by imposing the equivalence between the frame containing the infill and the frame containing the diagonal strut. In this way a law for identifying the equivalent strut width depending on the geometrical and mechanical characteristics of the infilled frame was generalized to consider the influence of vertical loads for use in the practical applications. The strategy presented, limited to the initial stiffness of infilled frames, is preparatory to the definition of complete non-linear cyclic laws for the equivalent strut

Influence of the Modelling Approach on the Failure Modes of RC Infilled Frames Under Seismic Actions

The influence of the masonry infills on the seismic performances of Reinforced Concrete (RC) frames is generally evaluated in analytical and numerical studies by adopting the equivalent strut model; it is based on experimental observations showing that at the onset of damage, stresses migrate to the diagonal of the panel and are transferred to the surrounding frame through the contact zones at the corners. Above the different equivalent strut models available in literature, single-strut models are generally used to evaluate the global behaviour, while multi-strut approaches are preferred to investigate on local interaction phenomena between panel and frame. In case of existing buildings, with poor transversal reinforcement of the columns, the presence of the infills can lead to pre-emptive brittle failure. The present study is aimed at evaluating the influence of the modelling approach on the evaluation of the seismic performance both in terms of global and local behaviour. Nonlinear dynamic analyses have been performed on an 8-storey infilled RC frame, following the Incremental Dynamic Analysis procedure, in order to evaluate structural performances depending on the model adopted to simulate the infills