Innovative seismic isolation of masonry infills using cellular materials at the interface with the surrounding RC frames

In this study an innovative concept is proposed to isolate – at low and moderate storey drifts – infill panels from the surrounding reinforced concrete (RC) frame, using thin layers of cellular materials. The concept is verified experimentally through testing of three fully infilled (i.e. at full height) and two partially infilled (i.e. at reduced height) RC frames with different infill-to-frame interface contact conditions under in-plane cyclic loading. The shear strength, hysteretic behavior, damage evolution and stiffness degradation of conventionally infilled RC frames is compared with the respective properties of frames with isolated infills. The experimental results show that fully infilled test specimens exhibit much more severe damage than the isolated ones, leading to the conclusion that the proposed isolation system significantly preserves the integrity of infill panels at moderate storey drifts and increases shear strength and lateral stiffness of the infilled frames at higher deformations. Additional tests on frames with infills at partial height show that cellular material joints at the sides of infills decrease the adverse effects of the infill-frame interaction. Finally, it is demonstrated that mechanical properties, contact conditions and joint thickness of the cellular material influence the overall hysteretic behavior of the specimens. A simple analytical model is developed, combining single-strut elements for the infills with nonlinear springs for the cellular material joints. The model, implemented in OpenSees, is in good agreement with test results. The concept is demonstrated through parametric analyses in full scale RC structures. Overall, it is concluded that the proposed technique has a high potential in reducing infill-frame interactions – hence damage of the infills – up to moderate drifts, whereas full interaction – hence increased capacity – is still in place when drifts are large.