Residual out-of-plane capacity of infills damaged by in-plane cyclic loads

During earthquakes, infills are subjected to In-Plane (IP) and Out-Of-Plane (OOP) actions. In the case of strong earthquakes, infills may progressively change their mechanical behavior resulting in a reduction of IP and OOP stiffness and strength. Recent earthquakes have proved that the OOP collapse of infills is a diffused mechanism also for buildings designed to resist seismic events in agreement to the most modern codes. This is potentially a very dangerous event with risk for human health. The strong interaction between IP and OOP behavior of infills traduces in a progressive reduction of the OOP strength. The IP damaging loads may cause a loss of the OOP capacity not predicted during the design process or during the safety assessment. Unfortunately, a very low number of experimental tests is available in the literature that does not allow definitively reliable prediction laws for the degrading of the OOP mechanical properties. Further, only in few cases codes suggest adequate strategies to face this question. For what above, in this paper the reduction of OOP stiffness and strength because of the IP damage is studied by an extended numerical experimental campaign based on FE models to be considered as complementary of the few laboratory experimental tests available in the literature. On the base of the numerical results, a proposal for the prediction of the variation of the mechanical characteristics of infills with respect to OOP actions is presented after an extended discussion on the existing strategies. The nonlinear behavior of infills and the arch effect between infill and the surrounding frame, where effective, have been replicated by the FE models here discussed, trying to simulate the real case of unreinforced masonry (URM) infills.