Modeling of masonry of infilled frames, Part I: The plastic concentrator

This paper proposes a model of the behavior of the masonry in infilled frames. The model is based on the theory of plasticity and the concept of an equivalent strut. It is first shown that the equivalent strut model, in its conventional form, introduces artificial effects that do not correspond to the observed behavior. The real infill is a unique element while the conventional model represents it as two independent bars. The conventional strut model is then modified by the inclusion of a new concept: The plastic concentrator. It is assumed that all inelastic effects can be lumped at the concentrator. The idea is that plastic concentrators can be compared with the plastic hinges in the theory of frames. The plastic concentrator links the two bars of the strut model and allows for a transfer of effects between the bars. It is shown that the use of plastic concentrators leads to a more realistic representation of the behavior than the conventional models. The new concept can be used to model in a simplified way the behavior of any infilled frame (i.e. reinforced concrete or steel frame). Finally, it is shown how models with arbitrary force–displacement envelopes can be modified by the inclusion of plastic concentrators.