GNGTS 2016 - Atti del 35° Convegno Nazionale

GNGTS 2016 S essione 2.2 369 the vibration energy during an earthquake. Such energy causes a structure to have the highest amplitude of response at its fundamental frequency, which depends on the structure’s mass and stiffness. Therefore, the damping level, as well as the knowledge of the fundamental period (T) of the building are particularly important for estimating the seismic base shear force F in designing earthquake resistant structures. Following the Eurocode8 (2003), F can be expressed as: F = Sa (T; ζ • m • λ where Sa ( T; ζ) is the ordinate of the target spectrum at period T and damping ζ, m is the total mass of the building above the foundation or above the top of a rigid basement and λ is a correction factor. The Sa ( T; ζ) is evaluated considering the local geological features, which influence the site response in term of amplification of the ground motion, as the seismic input travels from the bedrock to the overlying soil deposits. The seismic performance of a building obviously depends on the progression of the frequencies along the input time-history, nevertheless the knowledge of its fundamental frequency at low amplitude values and the associated damping are of primary importance to characterize the initial seismic behaviour of a structure. These parameters can usually be obtained either through numerical modelling or experimental monitoring of the edifice using different input motions. In the present study, the horizontal to vertical noise ratio (HVNR) and the standard noise spectral ratio (SSNR) techniques were used to identify site and the building fundamental frequencies respectively.Measurements areplanned tobeperformed in70buildings distinguished according to their construction typology in masonry buildings (MA) and reinforced concrete (RC) edifices. Ambient noise was recorded using a three-component velocimeter (Tromino) sampling the signal at a frequency of 128 Hz. In each building, 20 minutes length of ambient noise were recorded both at the top and at the ground floor.According to the guidelines suggested by the European project Site Effects assessment using AMbient Excitations (SESAME 2004), time windows of 20 s were considered, selecting the most stationary part and not including transients associated to very close sources. Fourier spectra were calculated in the frequency Fig. 1 – Location of Palazzo Boscarino and free field noise measurement sites (a); View of the building (b); H/V spectral ratios obtained at the free field recording sites #1, #2, #3.