sessione 2.3 - Ogs

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GNGTS 2009 SESSIONE <strong>2.3</strong> INDIVIDUATION OF VUNERABILITY PARAMETERS BY PASSIVE TREMOR MEASUREMENTS F. Barazza, S. Grimaz, P. Malisan Dipartimento di Georisorse e Territorio, Università degli Studi di Udine Observations on the effects of various earthquakes in the world have shown that the presence of synchronization between the natural frequencies of the ground and the structure, and presence of torsional modes can increase the damage during an earthquake. So, even buildings designed according to the latest seismic standard may be called to respond in a more severe way if these effects occur. To investigate this phenomena, the passive tremor measurements on the ground and on buildings would be useful. Combined the measure with a FEM modeling information about the material used to built the structure could be inferred. Obviously, the simple passive tremor measurement not allowed high accuracy in the forecasting of seismic behavior. In fact, due to non-linear effects, the behavior of a structure (or a soil) could be different under an earthquake than a small tremor. The big displacement imposed by a seismic action cause a reduction of the stiffness of the materials and then a correspond reduction of the natural frequencies computed in an elastic hypothesis. Anyway if a synchronization ground-structure in elastic phase exists, then, probably, this condition will persist in a plastic phase too. On contrary, Fig. 1 - Plan of ground floor and first floor of the school of Corno di Rosazzo (UD), and positioning of the measurements point. 430
GNGTS 2009 SESSIONE <strong>2.3</strong> Fig. 2 - SSR on P1 and P2 position. if the natural frequencies of soil and structure are significantly different, then probably they remain different also during the seismic action, and the presence of torsional modes for small tremor, probably lead to the presence of the same modes during the seismic action too. So, the tremor measurement may allows to identify which are the buildings that most likely could suffer more damage due to the phenomena described above. Soil tremor measurement. In order to obtain a first estimation of the first natural frequency of the site the Horizontal to Vertical Spectral Ratio technique (HVSR) has been used. The method, widely used and cited, is really simple to use and have not negligible logistic advantage. On the other hand, the use of HVSR technique for assessing the amplification factor, instead, does not so straightforward, even if a clear peak is an index of presence of a impedence contrast. A case study: primary school in Corno di Rosazzo. A primary school in the Municipality of Corno di Rosazzo (UD, N-E of Italy) has been subject to a site investigation (to individuate the material, the geometry and so on) and a survey of passive tremor measurement. The school is a masonry buildings with two levels. The first Fig. 3 - HVSR for the ground tremor slab is built by concrete and the second one (the roof) is built by wood. Further, the school is connected by a tunnel to another building. Fig. 1 shows the plans of the school, the position of the connection with the other structure and the position of the points of measurement P0, P1 and P2. In order to detect the presence of torsional modes, P1 and P2 are placed close to the centre of the structure and one close to the outside structure respectively, just under the roof. Instead P0 has been placed to the ground in proximity of the structure. The examination of the standard ratio (SSR) between the spectra, P1 on P0 and P2 on P0 (Fig. 2), give useful information about the natural frequencies and eigen-shapes of the school. Fig. 2 shows that the lower natural frequencies of the building in P1 and P2 are about 5 Hz and 7.5 Hz respectively, and that both eigen-shapes are directed belong the y axes. Hence, since the amplitude of the peaks are different, the two frequencies are associated to a torsional modes. The examination of the HVSR computed on P0 (Fig. 3) show a presence of a peak at about 20 Hz. This frequency is far from the natural frequency of the school, so there is not problem of synchronization between ground and structure. A FEM model has been implemented (Figs. 4 and 5 and Tab. 1) in order to: • to verify the macro-behavior of the structure; • to obtain an indirect assessment of mechanical characteristics of the material through a calibration aimed to define the coincidence between natural frequencies derived from measures and modeling. 431
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