Effect of the location and intensity of damage on the dynamic characterisation of the brick masonry arch using different damage identification indices

This study presents a comprehensive, time-resolved investigation into the effects of localised damage and subsequent restoration on the dynamic behaviour of a semi-circular brick-and-gypsum masonry arch, representative of Persian architectural heritage. Using Operational Modal Analysis (OMA) with Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification (SSI), dynamic responses were evaluated across intact, under three distinct damage scenarios, and following restoration. The findings reveal that both damage location and symmetry significantly influence the arch's dynamic properties. Localised damage led to substantial reductions in natural frequencies, with mode two exhibiting up to 26.6 % reduction in initial damage and 43.5 % in the most severe scenario. Symmetric damage reduced sensitivity in fundamental modes, underscoring the need for multi-modal assessment. Average modal damping ratios increased by up to 107.2 providing more consistent and reliable detection compared to EFDD. Restoration using traditional gypsum mortar significantly improved dynamic characteristics, with natural frequencies recovering by approximately 13.5 % EFDD and 13 % SSI relative to the damaged state. Modal parameters stabilised within 24 h post-restoration; however, certain modes showed incomplete recovery, especially near sensor locations indicating residual stiffness deficits. Damage detection indices of Modal Assurance Criterion (MAC), Normalised Modal Difference (NMD), and Coordinate Modal Assurance Criterion (COMAC) have effectively identified damage, reinforcing their critical role in heritage structural health monitoring. The research highlights the importance of considering restoration as a time-dependent, evolving process, and advocates for integrated and multi-parameter monitoring frameworks. The results offer practical insights for optimising the conservation strategies of historic masonry structures, with recommendations for future work addressing environmental effects, numerical modelling, and advanced restoration materials.