GNGTS 2019 - Atti del 38° Convegno Nazionale

676 GNGTS 2019 S essione 3.2 EXPERIMENTAL DYNAMIC CHARACTERIZATION USING AMBIENT VIBRATIONS MEASUREMENTS: THE CASE STUDY OF “S. GIUSEPPE” CHURCH’S BELL TOWER (ACI CASTELLO - CT) S. Imposa 1 , M. Cuomo 2 , S. Grassi 1 , M. Galasso 2 1 Dipartimento di Scienze Biologiche, Geologiche e Ambientali, Università degli Studi di Catania, Catania, Italy 2 Dipartimento di Ingegneria Civile e Architettura, Università degli Studi di Catania, Catania, Italy Introduction. The soil-structure interaction is considered a crucial phenomenon, especially for the evaluation of seismic vulnerability of existing structures. Our study is focused on the “S. Giuseppe” church’s bell tower located in the Aci Castello (CT) municipality (Fig. 1a, b). The technique used in this research is based on the acquisition and analysis of the seismic noise. Two field surveys were performed, using six portable digital seismographs. In each one six data from six points were collected, six of which located inside the bell tower at different heights, and two outside the building, in order to characterize the site. The acquisitions were elaborated using the HVSR (Horizontal to Vertical Spectral Ratio) technique (Nakamura, 1989). To define the experimental dynamic characteristics of the structure, the samplings performed at the different levels of the tower are compared with one of the measurements carried out at external site through the HSSR (Horizontal Standard Spectral Ratio) technique. Both the dynamic characteristics of the structure and the site were defined in terms of fundamental frequencies. Moreover, a numeric model of the tower was created in Midas Fea®, considering the tower isolated from the church. This assumptions allowed to obtain results that satisfactorily compare to the ones obtained using the experimental dynamic characterization. Geometrical-Spatial description and geological context. The S. Giuseppe church’s bell tower, is probably belonging to a pre-existing sixteenth-century church, and suffered many changes over the years. These changes have modified its external configuration respect to the church. The tower is located near the fortress of Aci Castello, it is 15.30 m height and has a 4.80 m x 3.35 m rectangular section. Vertical walls are constituted by an irregular lava stone masonry, with a thickness between 65 and 70 cm. The spaces between the masonries are filled with fragments of volcanic rocks, bricks and smaller stones. From a geological point of view, the area is characterized by the presence of recent alkaline volcanic rocks that cover almost entirely the Pleistocene clay substrate (Monaco et al. , 2010). The clay substrate outcrops widely in the northern portion of the municipal territory. Tholeiitic submarine levels characterized by pillow lavas can be observed along the coast and they form the substratum of Aci Castello fortress. These volcanic products belong to the first emission phase of the evolution of Mount Etna. Data processing and interpretation using HVSR analysis. The site effects were analysed with the HVSR technique (Dal Moro, 2012). This technique is based on the recording of microtremors, related both to natural and anthropic sources, along the three components of the ground motion. The H/V spectral ratio is able to highlight the frequencies at which the ground motion would be amplified during an earthquake; this is due to resonance effects caused by tectonic, stratigraphic and topography characteristics (Imposa et al. , 2018; Grassi et al. , 2019). At each of the two sites outside the bell tower (Fig. 1b) the seismic noise was acquired for 20 minutes with a sampling frequency of 128 Hz. The spectral analysis was carried out subdividing the registration in 60 windows of 20 seconds each. In stationary conditions, the spectral components in the three directions have similar amplitudes, but at the resonance frequency a peak is generated by a local minimum of the vertical component. The fundamental frequency observed in the H\V spectrum related to the measuring point “Est1” is equal to 1.75 Hz, another spectral peak can be observed at higher frequency (11.56 Hz). The resonance frequencies for the measure identified as “Est2” are at 2.16 Hz and 11.09 Hz.