Browsing by Author "Dar, Emrullah."

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    Long-term dynamic response of Hagia Sophia in Istanbul to earthquakes and atmospheric conditions
    (Thesis (M.S.)-Bogazici University. Kandilli Observatory and Earthquake Research Institute, 2015., 2015.) Dar, Emrullah.; Çaktı, Eser.
    The effects of atmospheric factors on structural behavior have gained more importance due to climate change as result of global warming in recent years. In this thesis, the effects of ground motions and atmospheric variations on the modal parameters of Hagia Sophia in Istanbul, which is one of the most prominent structures created in the history and is in the UNESCO world heritage list, are examined. Firstly, the frequency variation of Hagia Sophia due to atmospheric conditions such as temperature, wind speed, humidity and precipitation has been assessed. For this purpose, acceleration records are divided into half-hour segments and the Fourier amplitude spectra of each segment are calculated using short-time Fourier transform. In this way, an annual frequency variation of the structure is achieved by combination of frequency values obtained from these spectra. Afterwards, the frequency variation of Hagia Sophia is compared with the variation of atmospheric conditions and the results are assessed individually. Secondly, the dynamic behavior of the Hagia Sophia during different earthquakes is analyzed. The modal parameters and mode shapes of the structure are determined. The decrease associated with the first two modal frequencies of the structure during each earthquake is calculated. Finally, the effect of maximum acceleration and duration of strong ground motion on modal frequencies of the structure is investigated.
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    Real-time structural health monitoring using statistical methods
    (Thesis (Ph.D.)-Bogazici University.Kandilli Observatory and Earthquake Research Institute, 2023., 2023) Dar, Emrullah.; Çaktı, Eser.
    The detection of structural damage relies on understanding the long-term variation of modal parameters and their relationship to changes in atmospheric conditions. This thesis aims to address this challenge by developing a real-time algorithm for structural health monitoring systems, which are becoming increasingly important. The algorithm uses statistical models developed by analyzing four years of modal frequencies, damping ratios, and mode shapes of Hagia Sophia, a UNESCO World Heritage structure, and their correlation with atmospheric parameters such as temperature, humidity, and wind speed. The algorithm uses four different regression models to predict the modal frequency as a function of the atmospheric conditions and selects the most suitable one. It then compares the predicted and measured frequencies to identify structural anomalies. The algorithm also employs the Modal Assurance Criterion (MAC), Coordinate Modal Assurance Criterion (COMAC), and Enhanced Coordinate Modal Assurance Criterion (ECOMAC) methods to examine the long-term variation of mode shapes. The algorithm is implemented in a user interface software called “AISHM,” which displays the modal parameters and the 3-D animation of the structure in real-time. The software also has the capability to track earthquakes and analyze the structural response in real-time. In summary, this thesis presents a comprehensive approach to real-time structural health monitoring using statistical models and advanced analysis techniques, which can have significant implications for maintaining and preserving historical structures.