Fire-induced changes in modal behavior and seismic implications for historical masonry mosques

Abstract This study investigates the influence of fire on the dynamic behavior of historical masonry mosques and examines the seismic implications of fire-induced changes in modal characteristics. The Hacı Kasım Mosque in Trabzon, featuring stone masonry walls and timber interior elements, was selected as a representative case study. A hybrid analysis framework was employed, beginning with operational modal analysis (OMA) to obtain experimental data, followed by calibration of the finite element (FE) model using these results (with MAC > 0.89 for the first three modes). A two-hour fire scenario conforming to the ISO 834 standard was simulated, and the temperature distribution in critical regions was determined through transient heat transfer analyses in ANSYS. Temperature-dependent reductions in the modulus of elasticity were incorporated into the post-fire model using Eurocode 2 reduction factors, after which modal analyses were conducted. Comparative assessments revealed a 10–15% reduction in natural frequencies, up to a 60% decrease in mass participation ratios in higher modes, and the emergence of localized vibration modes concentrated in fire-affected regions, indicating that fire-induced stiffness degradation not only alters the global dynamic characteristics but also amplifies localized seismic vulnerability. These findings highlight that conventional post-fire restoration practices may be insufficient to ensure the safety of historical structures and emphasize the urgent need for performance-based conservation strategies.