EFFECTS OF IN‑ AND OUT‑OF‑PLANE NONLINEAR MODELLING OF MASONRY INFILLS ON THE SEISMIC RESPONSE OF R.C. FRAMED BUILDINGS

The present work is aimed at identifying the effects of in-plane (IP) and out-of-plane (OOP) damage of unreinforced masonry infills (MIs) on the nonlinear seismic response of r.c. framed buildings. To this end, the state secondary school De Gasperi-Battaglia in Norcia (Italy) is considered for the numerical investigation. The school is a reinforced concrete framed structure with a long-shaped rectangular plan, due to the ex-post structural linking of three adjacent blocks originally separated by Gerber saddles, and regular elevation. Composed of a basement and three storeys above the ground level, the structure was built in the early 1960s in compliance with the 1939 Royal Decree Law. The infill typologies consist of: (i) two leaves of hollow clay bricks, for the interior frames; (ii) one leaf with solid bricks and the other leaf similar to the previous MI, for the exterior frames. Partial height infill walls inducing short column effects are placed along the long side of the perimeter, where the low height is attributed to window openings. First a five-element macro-model comprising four diagonal nonlinear beams and one (horizontal) central nonlinear truss for the prediction of the OOP and IP behaviour of MIs, respectively, is 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. Three-dimensional models of the test structure are subjected to biaxial spectrum-compatible accelerograms, assuming: (i) bare structure with non-structural MIs; (ii) infilled structure with structural MIs. Moreover, three assumptions are made for the behaviour of MIs: i.e. elastic both IP and OOP; inelastic IP and elastic OOP; inelastic both IP and OOP. The numerical results confirm the validity of the five-element macro-model in simulating IP and OOP failure mechanisms of MIs in De Gasperi-Battaglia school, and highlight the importance of including nonlinear OOP behaviour.