Strengthened infilled RC frames: Continuum and macro modeling in nonlinear finite element analysis

Strengthening and retrofitting masonry infill Reinforced Concrete (RC) frames is crucial due to the large use of this structural typology in earthquake-prone countries. Masonry infills can provide a positive and negative effect on the seismic response. A positive effect is due to the fact that the infills dissipate energy and therefore increase the overall damping. A negative effect may take place if their disposition is irregular or if they fail irregularly, thus the structure behavior becomes irregular. Furthermore, the infills generally stiffen the structure and thus increase the base shear force. It has been shown that by using an adequate infill strengthening technique, performance and safety of the structures and of the infills can be improved. Several models for Un-Reinforced Masonry (URM) infills have been developed in the past, but there is a lack of models available to describe the behavior of strengthened infilled frames. A simplified analytical strut and tie model is proposed based on the results obtained by Finite Element (FE) analyses. The mechanical characteristics of the tension tie and of the compression strut depend on the stiffness and strength of the reinforcement in addition to the geometrical and mechanical parameters of the infilled frame. The proposed model is validated with experimental results of a Carbon Fiber Reinforced Polymer (CFRP) strengthened infilled RC frame. The experimental data are reproduced by means of continuum FE models in order to assess the influence of various parameters on the seismic response and to calibrate the simplified model's parameters. Lateral stiffness and strength obtained with this simplified model show a good agreement with the experimental results and the continuum FE models.