Seismic response history analysis including out-of-plane collapse of unreinforced masonry infill walls in RC frame structures

Unreinforced Masonry (URM) walls are commonly used as infill panels for Reinforced Concrete (RC) structures in many countries. In spite of their influence on the seismic behaviour of framed structures, masonry infill walls are often neglected in design practice. Even when they are modelled, usually only their in-plane properties are considered. However, past earthquakes have shown that URM infills can also be susceptible to high levels of damage, including collapse, in the out-of-plane direction. Most current models for the three dimensional response of URM infill walls are micro-models, but macro-models have recently been developed and shown to be much less computationally demanding. However, very few studies have applied these models to the dynamic seismic response history analysis of realistic structures. The purpose of this paper is to use a numerical model to characterize seismically induced out-of-plane failures of URM infill walls in realistic RC framed structures. A URM infill macro-model that captures the interaction between in-plane and out-of-plane response was previously calibrated with experimental results from two types of clay masonry infill walls that are frequently found in Italian buildings, and that are also typical of other Mediterranean countries. These calibrated panels were used as infills in numerical models of two RC framed structures that are representative of the Italian building stock, and both were analysed with a suite of a bidirectional scaled ground motions compatible with Eurocode 8 spectra. The infill walls reach their ultimate displacement capacity by a combination of in-plane and out-of-plane displacements, with the out-of-plane component usually playing the dominant role. The failures occur mainly in the bottom two storeys, where the drift demand is higher. These numerical results are consistent with observations of damage to URM infill walls on similar buildings in recent earthquakes.