Seismic demand of masonry infills in r.c. structures accounting for the in-plane/out-of-plane interaction

The out-of-plane verification of unreinforced masonry infills (MIs) placed at different floor levels of a building is generally carried out through simplified methods, but seismic events in Italy (e.g. L’Aquila, 2009) and worldwide (e.g. Northridge, 1994) have highlighted that code provisions may result in wrong estimations of safety. The types of damage observed for MIs are usually a combination of, or an interaction between, in-plane (IP) and out-of-plane (OOP) mechanisms. Specifically, the IP drift ratio is generally reduced at the upper storeys of buildings, where the OOP drift ratio increases due to an increase of seismic acceleration. Significant OOP damage may also take place at the lower storeys where the highest values of IP drift ratio are attained. The present work is aimed at identifying the effects of the IP and OOP nonlinear interaction of MIs on their seismic behaviour and acceleration demand. A five-element macro-model comprising four diagonal nonlinear beams and one (horizontal) central nonlinear truss for the prediction of the IP and OOP behaviour of MIs, respectively, is first implemented in a C++ computer code for the nonlinear dynamic analysis of r.c. infilled framed structures. The proposed algorithm addresses the issue of nonlinear interaction by modifying stiffness and strength values of the MI in the OOP direction on the basis of simultaneous or prior IP damage and vice versa. Moreover, a lumped plasticity model describes the inelastic behaviour of r.c. frame members, including a 26-flat surface modelling of the axial load-biaxial bending moment elastic domain at the end sections where inelastic deformations are expected. A spatial one-bay multi-storey shear-type model is considered as equivalent to infilled r.c. framed buildings. In particular, the dependence of the results on variation of the following design parameters is considered: i.e. number of storeys; bay length; aspect ratio of MIs, with two leaves of clay hollow bricks, defined as the ratio between the panel length and height; strength level of the r.c. framed structure. Biaxial spectrum-compatible accelerograms are considered at ultimate limit states. A review of the current Italian (NTC18), the American (FEMA356) and European (EC8) code provisions is performed by means of comparison with analyses results.