Abstract Summary
The European historical built heritage largely consists of constructions made of masonry, such as palaces, towers, and churches. Ancient masonry is characterized by a complex mechanical response due to considerable material heterogeneity, a common presence of structural pathologies (either consisting of preexisting cracks or induced by aging and material degradation), as well as the occurrence of severe loading conditions such as those caused by differential soil settlements and seismic events. In this context, the lack of maintenance often plays an important role in aggravating and accelerating the development of structural pathologies. Assessing the structural integrity of historical masonry constructions during their lifetime is often a challenging task, yet of pivotal importance for management authorities who are responsible for (a) preservation of the cultural heritage, (b) life safety, and (c) scheduling of retrofit interventions. In this light, Structural Health Monitoring (SHM) approaches are becoming increasingly popular for the evaluation of the structural integrity of historical masonry constructions during operation. These techniques advocate the use of continuous monitoring of certain static/dynamic response parameters of structures to achieve a prompt identification of initiation/propagation of damage phenomena. Specifically, Operational Modal Analysis (OMA) methods are particularly well-suited for historical constructions given their non-destructive nature, global damage identification capabilities, and minimum intrusiveness upon the monitored assets. By processing their dynamic response under white noise ambient excitations, OMA methods allow characterizing the actual dynamic behavior of historical structures in terms of their modal features, i.e. vibration frequencies, damping ratios, and mode shapes. Despite their numerous benefits, the low sensitivity of OMA techniques to localized damage, as well as the difficulties in establishing robust correlations between changes in the modal features and the residual capacity of the structure, are often indicated in the literature as major limitations. This work is aimed at deepening into these aspects via an experimental program carried out on a full-scale masonry wall system with single opening, which was subjected to progressive damage. During the experimental activity, out-of-plane loading conditions of increasing magnitude were systematically applied to the wall specimen to induce damage initiation and progressive propagation. Ambient Vibration Tests (AVTs) were carried out on the specimen at each step to estimate the correlations between modal features and damage severity. The obtained results show clear variations in modal features consistent with the damage level suffered by the specimen. Interestingly, the variations in the mode shapes were found particularly sensitive to damage even at early stages of development. To further validate the experimental evidence, a Finite Element Model (FEM) of the specimen was developed and experimental results were replicated through non-linear modal analysis based on linear perturbation. The presented results demonstrate that FEM can mimic the damage-induced variations experimentally observed in the modal features of the specimen, providing a sound demonstration of the usefulness of modal features to identify slight-to-moderate damage.
