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University of Salento - Progress Report [July-November 2013]

July: San Giacomo Theatre measurements, Corfu

This activity was conducted in collaboration with LP. The experimental measurements were carried out with several difficulties, both for the particularity of the building bot for logistical problems. The building monitored is actually the place where is located the office of the Major of Corfu and the working place of many Municipality employers. The technical difficulties were related to the characteristics this masonry historical construction built in the XIX century. It´s built in a rectangular plant of 25x14 m with tree different levels of wooden floors, the third was built some year after the initial building. This configuration presents a particularity in this structure for a dynamic identification because its shape is extremely stocky. The monitoring system consists of several elements properly connected: in total eighteen accurate accelerometers have been positioned at different levels and in different rooms, according to the orthogonal directions.

In Figure 1a) and 1b) the frontal and lateral view of the building, in Fig. 2a) and 2b) some details of the internal part of the building; in Fig. 2a the main stair between the first floor and the super elevation of the second floor, in Fig. 2b) the long corridor at 1st floor (in wood), and in Fig. 2c) the corridor at 2nd floor (in wood).

The vaults are very high, but by using long stair, 18 accelerometers have been positioned at the superior corners at different levels (in Fig. 3a two accelerometer at a corner the second floor, in Fig. 3b two accelerometers at the corners of the corridor, in figure 3c two couples of accelerometers on the basis and on the high corner at the end of the first flight of the main stair).

The installation procedure and the following measurements have been performed in a period really difficult from the social point of view; a huge strike has been carried out by all the employers of the Municipality and the Major was obliged to close the structure.

Before the closing it was possible just in time to fix 18 accelerometers and to perform two carrots for having information on the materials characteristics (the carrots extraction is depicted in Figs 4a and 4b).

A stratagem has been performed in order to realize the foreseen acquisition of 10 minutes; a window was left opened before leaving the building and a wire connected to the central acquisition unit remote control was passed through the window (Fig.5a). This wire was suspended and passed to the other side of the street (Fig. 5b) and finally connected to a driving pc placed on a table of the bar placed in front of the building (the bar entrance is indicated in Fig. 5c). In this way it was possible to complete the experimental measurement campaign also if the building was officially closed for the strike.

Finally, the working group depicted together with the Major and some architects employed in the Municipality who helped in the operation of experimental monitoring (Figs. 6a and 6b).

September: analysis of the experimental data and comparison with a Finite Element (FE) model

The analysis of the experimental data has performed by the University of Salento together with a comparison with a detailed FE model realized by the LP. For the particularity of the structure that is extremely stocky and that has wooden floors, there is not the safety of a global localization of the identified frequencies.

For this reason, we retain that a preliminary analysis becomes necessary before applying the usual methodologies of model updating; this preliminary analysis, actually in progress, tries to analyze the phase shift of different acquisition points for the identified frequencies by means of digital band pass filter opportunely designed. This preliminary analysis will be useful to analyze the global characteristic of the frequencies identified with the OMA (Operational Modal Analysis) methods, EFDD (Enhanced Frequency Domain Decomposition) that operates in the frequency domain, and the SSI (Stochastic Subspace Identification) technique that operates in the time domain.

The scheme of the points where the accelerometers have been placed (indicated as arrows) is shown in the Artemis model of the building reported in Fig. 7 where it is also introduced the Oxyz reference system used.

The FE model realized in SAP with shell elements is shown in Figure 8; the possibility of updating the model in such a way that it could have the same dynamic behavior of the experimental data is the next challenge of the project for the working group.