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cLara - smart cities project: ambient vibration survey on strategic buiLdings in matera (itaLy) G. Massolino1,2, D. Sandron1, M.R. Gallipoli3, A. Rebez1, M. Mucciarelli1,4, T.A. Stabile3

1 OGS – National Institute of Oceanography and Experimental Geophysics, Trieste, Italy2 Polytechnic Department of Engineering and Architecture, University of Udine, Italy3 CNR – IMAA , Tito Scalo (PZ), Italy4 Di.S.G.G. – University of Basilicata, Potenza, Italy

Introduction. In October 2015 a survey campaign was performed in Matera (Italy) in the frame of the CLARA Smart City project (Cloud plAtform and smart underground imaging for natural Risk Assessment). Among the objectives of the project there was the aim of non-invasive methods for buildings and infrastructures diagnosis. In this context, 11 building to be tested were identified in Matera, 6 of which had been tested with ambient vibration techniques during a 3 day survey: 94 passive single station measurements have been performed. The main aim was the identification of the main vibrational modes of the structures.

Methods. Tri-axial tromographs TROMINO® (Moho SRL) have been use for ambient vibrations recordings. Data have been analyzed with Grilla software. Outside each building an HVSR (Horizontal to Vertical Spectral Ratio) free-field test has been performed, in order to characterize the seismic response of the foundation soil (Di Giulio et al., 2005; Parolai et al., 2005; Castro et al., 1998). The sampling frequency has been set to 128Hz and the recording time to 30 minutes; triangular smoothing has been set to 10%.

The buildings have been tested with synchronous ambient noise recordings and with different acquisition configurations, to estimate more accurately the main vibrational modes. Testing configuration were usually along a vertical line, with a measure acquired at each floor, and along an horizontal lines, with acquisition at different points of the same floor. The measure at the basement has been taken as a reference for all the measures. Recording time has been set to 12 minutes for each measure and triangular smoothing has been set to 3%. Data have been processed through HVSR and SSR techniques in order to better assess translational and torsional modes (Gallipoli et al., 2004). The map in Fig. 1 report the buildings monitored.

Fig. 1 – Map of the distribution of the identified buildings in Matera to be tested (red symbol) and already tested (green symbol).

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Free field survey. Free field surveys have highlighted a clear high amplitude frequency peak at a 1.5 Hz in all but one the testing sites, showing a significant impedance contrast between calcarenites and shallow sand and clay (Lupo and Gallipoli, 2011). Outside the Province building, instead, the peak was at 5.2 Hz, showing a more superficial layer of clay.

Estimation of building vibrational modes. The buildings tested were:• “Duni” High School. The building has 4 floors. The measures have been performed

along two vertical directions, with an instrument placed at each floor, and one along the horizontal direction at the 3rd floor (not at the top floor because it had different dimension in comparison to the others). In both configurations an instrument at the basement as been used as reference, to use the SSR technique. On the SSR curves, each floor shows two main vibration modes, one at 2.7 Hz for the transverse component and 4.2 Hz for the longitudinal one. The two sides of the building are not symmetric: the right side is stiffer than the left, because of the keystone building of the entrance stairs. This is confirmed by a rocking mode at 2.7 Hz, the peak is present also on the vertical component. Fig. 2 and Fig. 3 report the results for soil and building characterization.

• “Loperfido” High School. Also this building has 4 floors, a rectangular shape with one side much longer than the other. Measures on two vertical directions have been performed in order to characterize both sides of the building, with an instrument at each floor, and one along the horizontal direction at the top floor. An instrument has been kept at the basement to be used as a reference for SSR analysis. The building shows an asymmetric behavior, especially noticeable at on the horizontal measure at the top floor. Mode frequencies were identified at 3.4 Hz and 5.7 Hz on the transversal direction and 4.3 Hz on the longitudinal direction, showing an asymmetrical behavior and a rocking mode at 3.4 Hz, which is evident on the extreme points of the horizontal measure, more flexible.

• Firefighters head office. The building was under construction at the time of the survey. It is a structure designed with antiseismic criteria, regular in plan and in elevation. Also in this case measures have been performed along two vertical directions, one at the centre of the building close to the stairs and one at the left corner. Instruments have been placed at each floor, with the one at the basement used as a reference. At the top floor a measure has

Fig. 2 – a) perspective view of the building and distribution of testing points inside the building and in free-field condition; b) HVSR curve.

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been performed along the horizontal direction. Mode frequencies were very clear at 4.5 Hz on the N-S direction and 6 Hz on the E-W direction.

• City Hall. The structure is more complex than the other buildings analyzed: it has two orthogonal sides linked to a stout central body; the highest side has 6 floors. Also in this case two vertical directions have been tested, one for the main central body and the other for the major side of the building. The measure in the horizontal direction has been performed on the second floor and not at the top floor because the other floors were shorter. Many structural modes have been identified: 1.8 Hz and 2 Hz both on the transversal and longitudinal directions (coupled modes), 2.8 Hz on transversal direction and 4 Hz on the longitudinal one. The 1.8 Hz peak might be imputable on the soil-structure interaction or on a rocking mode.

• Province building. It has a peculiar shape, similar to a cake-slice. Therefore, the measures have been performed along three verticals, one for each corner of the building. Again, one test has been made on the horizontal direction, with an instrument at each corner at the same floor. This is the only building for which SSR analysis could not be performed, since the building is quite stiff and the HVSR function of the ground floor shows the soil amplification contribution, therefore can not be used as a reference. The structural frequencies identified were at 3.7 Hz (longitudinal) and 4.7 Hz (transverse), clearly visible on vertical 1 and 2, while on vertical 3 the frequencies are still visible on the transversal component but on the longitudinal one are higher, 4 and 5 Hz respectively.

• Architecture building – University of Basilicata. The building has a rectangular shape, regular in elevation, with a big internal covered patio. Measurements have been taken along

Fig. 3 – Standard spectral ratio (SSR) curve for each floor, referred to the basement in the longitudinal, transversal and vertical components, along vertical 1 (panel a), vertical 2 (panel b) and horizontal (panel c) arrays.

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only one vertical direction, on a corner of the building, and one on the horizontal direction at the last floor (not accessible to the public). The difficulty to access to other parts of the building made it impossible to place the instruments elsewhere. Only one structural mode was identified, at 6.5 Hz, for both the horizontal components.

For the sake of brevity only the “Duni” High School results are reported. Conclusions. Ambient vibration testing techniques are demonstrated to be a valuable (yet

cheap) help in estimating modal parameters of buildings. Using these techniques the building structural characterization can be very fast (two or three buildings per day), cost effective and informative, since it is possible to detect mode frequencies and double resonance effects. Therefore, ambient vibration techniques represent a cost effective instrument to support decision-making and a precious element for the future of smart cities. Aknowledgements. In the survey logistics and management a valuable contribution was provided by Michele Lupo.

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effects of damage based on observations of horizontal-to-vertical spectral ratio of microtremors. Soil Dynamics and Earthquake Engineering, 24(6):487–495.

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