Stiffness contribution and damage index of infills in steel frames considering moderate earthquake-induced damage

This paper examines the contribution of infill wall stiffness to steel frame structures, taking into account moderate earthquake-induced damage and by introducing a damage index based on variations in the out-of-plane dynamic response of infills identified through experimental tests. The research involved an extensive laboratory experiment on a steel-concrete composite mock-up tested before, during, and after infill masonry wall damage. The study initially focuses on dynamic tests performed on infills to determine their out-of-plane dynamic behaviour, aiming to draw conclusions about their post-damage health condition and stability. An improved Matlab routine for the automatic identification of the experimental out-of-plane modal parameters of 2-D elements is developed; the tool is validated on a simple steel plate and then applied to track the experimental modal parameters of the mock-up infills during the progressive induced damage. Successively, the study examines how damage to the infill panels reduces their stiffness and strength. Two exponential equations are derived from the observation of the experimental outcomes. The former equation relates the infill stiffness reduction to a damage index of infill panels expressed in terms of modal frequency variation of their out-of-plane vibration modes. This equation can be used to obtain an estimation of the residual stiffness of infills to be used in the numerical modelling. The latter equation provides an indication about the total stiffness reduction of infilled structures if damage on infills is detected. The proposed strategy, supported by laboratory results, aids practitioners in evaluating the condition and usability of steel infilled structures following earthquakes.