Experimental assessment of the in-plane/out-of-plane interaction in unreinforced masonry infill walls

During earthquakes, unreinforced masonry (URM) infills are subjected to in-plane (IP) and out-of-plane (OOP) actions. Displacement demands in the IP direction affect the OOP response to seismic accelerations and vice versa: this phenomenon is called IP/OOP interaction. In this study, experimental tests aimed at investigating the IP action effects on the OOP response of thin URM infills are presented. Three URM infills in reinforced concrete frames are first cyclically loaded in-plane up to three different drift levels. Then, on each test specimen, monotonic OOP tests are performed. Tests' results are compared to the pure OOP response of an IP-undamaged reference specimen. For each specimen, the evolution of cracking pattern during the IP and the successive OOP test is presented and discussed. Data concerning the variation of secant stiffness and force at first OOP macro-cracking and at peak load due to the increasing IP damage are presented. Based also on experimental tests presented in the literature, empirical relationships relating the reduction of force and secant stiffness at first macro-cracking and peak load due to the IP damage to the maximum interstorey drift ratio attained during IP tests are proposed. Finally, some considerations concerning the different post-peak behaviour, up to collapse displacement, of IP-undamaged and IP-damaged infills are reported.