Modelling of masonry infills in existing steel moment-resisting frames: Nonlinear force-displacement relationship

The study described and summarised in this paper was aimed at developing a framework for the definition of force-displacement relationships for single-strut models for masonry infill walls within steel moment-resisting frames. The methodology is based on a genetic algorithm optimisation and can be used for the calibration of force-displacement curves based on databases from either experiments or numerical simulation. A case study is also tested to demonstrate the framework in detail. Due to limited available experimental data on the seismic response of existing steel frames with masonry infills, a set of comprehensive finite element micro-models developed in Abaqus are used to generate a database. The optimal values of the parameters to feed a force-displacement relationship of the single-strut model of the masonry infills are obtained for each micro-model by solving optimisation problems with a genetic algorithm. The optimisation problem involves the minimisation of the discrepancies between the global responses from the database and their corresponding single-strut models through least square minimisation. With the optimal values as the input variables, a generalised quadrilinear model of the masonry strut is obtained through regression analysis and is validated against additional micromodels of infilled steel frames.