Design of novel vibrating barriers for the seismic protection of the Messina Cathedral

Heritage buildings are amongst the most vulnerable structures affected by natural hazards due to their inherent historic nature with regards to materials and construction techniques. Although traditional strengthening and vibration control strategies are technically sound, they might face serious challenges such as the compatibility of materials between new and historic construction and the requirements of the structural intervention. For heritage structures, these challenges are a clear barrier to the seismic protection, and the use of such techniques is therefore limited. In this context, a novel non-invasive passive control device called the Vibrating Barrier (ViBa), has been recently proposed. The ViBa is a large-scale oscillating mass-spring-damper unit contained in the ground and tuned to mitigate the motion of surrounding structures under earthquake-induced ground motion, without being directly in contact to them, through a structure-soil-structure interaction mechanism. The effectiveness of the ViBa device to protect various idealized structures and clusters of buildings has been proved in previous contribution co-authored by the first and last author. In this contribution, a real existing structure is investigated: the Messina Cathedral. The Cathedral was originally built during Norman time (XI century), and was destroyed by the Messina earthquake in 1908. The Cathedral was rebuilt shortly after by adopting a mixed masonry-reinforced concrete structure that for the bell tower was realized following the innovative confined masonry typology recommended by a structural code just after the Messina earthquake. Currently, the Cathedral bell tower hosts the largest and most complex mechanical and astronomical clock in the world. The design of the Vibrating Barrier for this real case scenario has required the following steps: i) the structural identification of the structure and the development of a consistent FE model; ii) the definition of a realistic ground motion model due to the high seismicity of the area and iii) the calibration of the ViBa’s unknown mechanical parameters to minimize the dynamic response. Ambient vibration tests have been performed and a permanent monitoring system has been recently installed in the bell tower by the Department of Civil Protection (Seismic Risk Office) allowing the calibration of a reliable FE model including soil-structure interaction effects. A stochastic approach has been used to determine a pertinent ground motion model for determining the probability of exceedance of a selected response parameter consistent with the Response Spectrum at the site. Multiple ViBas have been designed to protect both the bell tower and the Cathedral. A novel Vibrating Barrier has been also designed to control both translational and rocking behavior of the bell tower. Numerical results are presented to show the effectiveness of the ViBa technology to cope with complex real case scenarios offering a novel viable strategy to reduce the seismic risk of existing structures, from future earthquakes, without altering their heritage value.