Variation in the Modal Response of Retrofitted Unreinforced Masonry Walls at Different Levels of Damage

This article presents the results of an experimental campaign conducted on a set of four unreinforced masonry walls at full scale. The purpose of this study is to assess, using non-destructive methods, the impact of retrofitting and damage on the modal response of masonry wall systems. Each wall underwent a sequence of increasing cyclic displacements applied by an actuator at the upper end of the specimen. Modal tests based on vibrations were performed both before and after rehabilitation, as well as during the sequence of increasing displacements. It was demonstrated that frequencies can identify progressive damage when the maximum crack is about to occur, as well as the effect of wall retrofitting when mass contribution is considerable. However, the modal assurance criterion indicator (MAC) fails to properly identify a trend of decreasing correlations as progressive damage increases; instead, it is sensitive to detecting maximum crack and instability conditions. Furthermore, it was determined that the coordinated modal assurance criterion indicator (COMAC) does not identify the damage distribution as expected. However, the cumulative COMAC provides a useful tool for quick visualization and interpretation of COMAC behavior. Finally, a novel damage indicator was tested, MACVF, which improves the trend and successfully identifies the most damage-sensitive mode, especially when the maximum level of damage is reached, giving MAC values below 80%. In addition, frequency variations ranged from 70% to 110% when TRM and WWM retrofitting techniques were applied.