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Use of ambient vibration measurements in Earthquake Protection of Historical Buildings - Case study: Castle Trakošćan, Croatia

Castle Trakošćan, built on the rocky peak of the hill in the late 13th century, is the protected cultural heritage as a historical entity by the Republic of Croatia. Trakošćan is constructed as a highly irregular brick masonry structure with timber or shallow masonry arch, vault or dome floors. It was substantially renewed and upgraded in the late 16th and 19th century. On 16 March 1982, the Tower Castle and a part of the second floor were severely damaged by the M4.5 (VII oEMS) earthquake with the hypocentre below Mt. Ivanščica (approx. 20 km from the Castle). The observed damage was found to be the most significant from an earthquake in the past 50 years, and until year 2020. The Castle was subjected to repair and partial retrofitting until year 2000. The Zagreb M5.5 and Petrinja M6.2 earthquakes, which occurred in March and December 2020, respectively, had strongly shaken the Castle’s structure. The earthquake damage was observed and assessed by a visual inspection accompanied by ambient vibration measurements. As observed, the Castle Trakošćan suffered slight cracking throughout the original and added structure, including the retrofitted parts due to their poor performance. However, the slight cracks that appeared on masonry arches were found to be critically positioned, which can likely lead to their collapse, if their spreading is not prevented. The performed ambient vibration measurements, which were compared to pre- earthquake measurements (from year 2016), revealed the increase in the fundamental period of the castle central tower unit and on the second floor, thus indicating the loss of structural stiffness as a consequence of the earthquake damage. The Castle earthquake performance and protection for the purpose of retrofitting and withstanding the future earthquakes, requires careful investigation by use theoretical analysis and empirical data.

Ambient vibration measurements, Historical buildings, Trakošćan Castle

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