Out of Plane response of Unreinforced Masonry infills: Comparative analysis of experimental tests for the definition of strategies of macro modelling and fragility prediction

During an earthquake, an interaction between the in-plane and out-of-plane seismic forces occurs and the infilled frames suffer damage in both in-plane and out-of-plane directions simultaneously. Particularly, the out-of-plane collapse of unreinforced masonry infill walls is critical even for new buildings complying with the modern seismic codes, resulting in high casualties and huge economic losses. However, the out-of-plane behaviour of infill walls is yet not fully understood. This study is therefore aimed towards characterizing the out-of-plane seismic capacity of unreinforced masonry infill walls. First of all, available out-of-plane experimental tests performed on unreinforced masonry infill walls are reviewed with a detailed comparison of the experimental results. The influence of parameters like slenderness ratio, aspect ratio, boundary conditions, openings, vertical load, in-plane damage level, the strength of masonry and plaster, and frame stiffness are evaluated, and research gaps are identified. Based on the collected experiments, all available analytical capacity models are checked for their accuracy in the prediction of the out-of-plane capacity of unreinforced masonry infill walls. In doing so, both types of capacity models are evaluated: Type (I) for the estimation of the out-of-plane strength in the in-plane undamaged state; Type-II for the estimation of out-of-plane strength reduction factor for the in-plane damaged state. Afterwards, the best pairs of models from two groups i.e. Type I and Type II, are coupled and checked with the experimented specimens where the reference infill specimen (specimen tested in out-of-plane without prior in-plane damage) is not available. In addition, the influence of orthotropy of the infill masonry in the out-of-plane capacity predicted by the capacity models is analysed. The possibility of using the capacity models in the cases of infill-beam gap and infill with openings is also checked. Different available macro-modelling techniques are investigated and a simple macro-element model which can simulate the behaviour of unreinforced masonry infill walls under in-plane and out-of-plane loads is developed. The model is validated with different sets of experiments. The model takes into account the decrease in out-of-plane capacity due to prior in-plane damage and is capable to capture in-plane/out-of-plane interaction effects of the seismic forces. From the correlation between the experimental and macro-model results, empirical equations are developed that can be used to calculate the stress-strain parameters required for defining the compressive behaviour of the struts. With the provided strategy, the geometrical and mechanical parameters required for the struts can be easily identified for numerical modelling of the infill wall. Using the model, in-plane and out-of-plane responses of the infill wall in lateral loads can be checked. To enrich the information obtained from the experiments regarding the out-of-plane behaviour of infill walls, numerical experimentation is performed by using the developed macro-model covering the range of infill’s geometrical and mechanical properties. From the detailed parametric analysis, the out-of-plane strength of the infill wall is found to be largely influenced by compressive strength, slenderness ratio, aspect ratio, and more importantly by the level of in-plane damage. The decay of strength and stiffness due to prior in-plane damage is also largely governed by the strength and the slenderness ratio of the unreinforced masonry infill. Based on the numerical results, empirical equations are proposed for the evaluation of the infilled frame’s out-of-plane capacity under in-plane damaged or undamaged conditions. The reliability of the proposed equations is proved by comparisons with experimental results. Finally, a procedure for developing the out-of-plane fragility functions is proposed by using the developed macro-model. The fragility is calculated assuming the uncertainty in the geometric and mechanical properties of infill walls instead of the uncertainty in the seismic input. The fragility is defined with respect to the position of the infill wall in a low-rise RC building. Experimental data available in the literature are used for the validation of the output. Overall, the results indicated lower vulnerability in the out-of-plane direction for infill walls without prior in-plane damage and high vulnerability when the infill wall is prior damaged in the in-plane. The proposed procedure can be extended to other types of infill walls depending on the construction technique of the site of interest, obtaining different and specific fragility curves for perming a large-scale risk analysis.