Investigation of restoration effects on the dynamic characteristics of the Hagia Sophia bell-tower by ambient vibration test

Ambient vibration test is preferred to condition assessment of historical structures due to the fact that this method does not create any negative effect on the measured structures. The method enables to observe the changes in the dynamic characteristics of structures by making measurements for different cases such as damaged, restored, etc. Many historical structures have been restored nowadays because of increasing concern to these structures in the modern societies. The study presents the natural frequencies, mode shapes and modal damping ratios of the bell-tower of Hagia Sophia Church in Trabzon, Turkey, which was restored one year ago by Local Office of Cultural Ministry of Turkey. The masonry bell-tower is about 23.5 m high and dates back to the XIII century. Full-scale ambient vibration tests were performed for pre-restored and restored cases under environmental and randomly generated impact loads. In the measurement, totally nine uni-axial seismic accelerometers were used to collect responses of the tower in the orthogonal directions. The measurement is carried out on the three different levels of the tower by using a reference point. Two commonly used methods which are Enhanced Frequency Domain Decomposition (EFDD) and Stochastic Subspace Identification (SSI) are used to identify the modal parameters of the tower. The first five natural frequencies and their corresponding modal damping ratios are attained by selecting each mode on the power spectral density functions. Also, the bending and torsional modes of the tower are extracted from these spectra. The effects of the restoration are evaluated by comparing the dynamic characteristics of the tower for pre-restored and restored cases.