The response of masonry infills under combined in-plane and out-of-plane loads

Masonry walls are commonly used as infills in steel and reinforced concrete frame structures. Usually, the stiffness and strength of the infill and connections between the infill and frame elements are such that the former affects the overall structural response. The irregular arrangement of infills, which may depend on the original building layout or on the collapse of one or more infills, may cause critical distributions of plastic hinges, high demand of inelastic deformations and reduction of the global dissipation capacity. For these reasons, a substantial number of researches, both analytical and experimental, have been devoted to the capacity of infill walls, especially in the out-of-plane direction. These studies highlighted that the interaction between in-plane and out-of-plane loads plays a fundamental role. Generally, prior in-plane damage produces a reduction of the out-of-plane strength of the infill. In fact, damage caused by in-plane forces, e.g. diagonal cracks in the wall or corner crushing, accelerates the out-of-plane collapse. In addition, observation of damage in the aftermath of earthquakes showed that, for the most part, failure of infills takes place at the lower stories of buildings. This circumstance can be ascribed to the interaction between in-plane and out-of-plane loads; namely, the damage produced by in-plane shear forces, which are larger at the bottom stories, increases the out-of-plane vulnerability of the infills. In this work, the interaction between in-plane and out-of-plane forces is studied by means of numerical non-linear analyses on a single-bay, single-storey reinforced concrete infilled frame. The analyses are performed considering both out-of-plane forces alone and a sequence of in-plane and out-of-plane forces. Different parameters are varied, such as the aspect and slenderness ratios, as well as preliminary in-plane displacement, to assess their influence on the in-plane-out-of-plane interaction.