GNGTS 2015 - Atti del 34° Convegno Nazionale

GNGTS 2015 S essione 2.3 257 highlight this component, the measurements have been performed in the center of the building floor, in one corner and next to the stairs. The results of ambient vibration surveys are shown in Fig. 3. The frequencies of first three modes are 2, 4 and 5.5 Hz, the corresponding periods are very close to the ones obtained by the Finite Element Model (Tab. 2). It is also possible to identify the direction of the modes: the first and the third one are parallel to the N-S direction of the instrument, corresponding to the major planar dimension, while the second is parallel to the E-W direction (shorter side). The amplification at 4 Hz (Fig. 3 panel c) for the N-S component is much lower than the E-W one. Furthermore, the amplitude of H/H peaks for the test point next to the stairs are lower than the one on the shorter side of the building, that itself is lower than the one on the corner, confirming the identification of the stairs as a stiffness center of the structure. The ensemble of these considerations supports the initial suspicion that the modes are highly coupled and thus the torsional component is relevant for the hangar. Tab. 2 - Comparison between first three modes periods obtained by experimental vibration tests and by Finite Element Model (FEM). Mode Period [s] – Tromino Period [s] – FEM Direction 1 0.52 0.639 N-S 2 0.26 0.284 E-W 3 0.18 0.196 N-S Conclusions. Ambient noise analysis can be successfully applied for the dynamic characterization as for standard buildings as for industrial and/or energy production and distribution facilities. The technique is simple, quick, it does not interfere with the normal operability and, not least, is cheap. It is crucial, in case of an earthquake: if the structure was previously characterized, the comparison between the pre and post event is prompt. Thus, it would be possible to immediately highlight possible major damage and plan the intervention. In this work, noise input technique has been carried out on three different types of structures related to the energy industry: a concrete dam, an oil tank (and a water tank for comparison) and an asymmetric hangar. The measurements on arch dams are quite common, but satisfactory results can be obtained also on dams with complex structural behaviour. As far as tanks are concerned, more tests on tanks of different size should be performed, in order to validate the reliability of these results. Both in the case of the dam and the hangar, it has been seen how the ambient vibration technique can be useful in the detection of planar asymmetry, as in the case of the hangar stiffness asymmetry and consequent torsional component. Likewise, in the case of the dam, the rocking rotational direction was determined, using XNSR Matlab code. Both in the case of the dam and of the tank, the influence of reservoir water level cannot be ignored. Hence, it would be important to repeat the tests with different water level. Lastly, ambient vibration tests should be performed frequently, in order to highlight possible changes in the structural behaviour, necessarily under the same conditions to be sure that the changes are imputable to differences in boundary parameters, allowing prevention activities and assuring a prompt intervention before major damage occurs. References Bukenya P., Moyo P. and Oosthuizen C.; 2012. Comparative study of operational modal analysis techniques using ambient vibration measurements of a concrete dam . Proceedings of ISMA2012-USD2012. Calcina S. V., Eltrudis L., Piroddi L. and Ranieri G.; 2014. Ambient Vibration Tests of an Arch Dam with Different Reservoir Water Levels: Experimental Results and Comparison with Finite Element Modelling. Hindawi Publishing Corporation, The Scientific World Journal, Volume 2014, Article ID 692709, 12 pages – http://dx.doi . org/10.1155/2014/692709