GNGTS 2013 - Atti del 32° Convegno Nazionale

The seismic performance of a building obviously depends on the progression of the fre- quencies along the input time-history, nevertheless the knowledge of its fundamental frequen- cy at low amplitude values and the associated damping are of primary importance to charac- terize 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 the building’s frequencies.Measurements were performed in 50 buildings distinguished according to their construction typology 20 masonry buildings (MA) and 30 reinforced concrete (RC) edifices. As fundamental period for each building we considered the peak with the higher amplitude in both HVNR and SSNR (Fig. 2). Ambient noise was recorded using a three-component velocimeter (Tromino) sampling the signal at a frequency of 128 Hz. In each building, 10 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 10 s were considered, selecting the most stationary part and not including transients associated to very close sources. Fourier spectra were calculated in the frequency band 0.5–15 Hz and smoothed using a triangular average on frequency intervals of ± 5% of the central frequency. In addition to the fundamental period, the damping ratio was computed as well using the nonparametric analysis (NonPaDAn) Matlab routine (Mucciarelli and Gallipoli, 2007). This approach can be summarized as follow: 1) velocity time series are integrated to displacement after time series baseline correction and high-pass filtering; 2) all positive values of the time series, that represent local maxima, are selected and their amplitudes ( Ai for the i th maximum) and times of occurrence ( ti for the i th maximum) are stored in a matrix; 3) for each subsequent Fig. 2 – Examples of SSNR (a) and modal shapes (b) coming from seismic noise data for each floor of two selected buildings in RC (Building n. 5) and MA (Building n. 32). The curves in (a) are plotted using a grey color scale; the light grey curve corresponds to the lower floor measurements while black curve refers to measurements taken on the top floor. 106 GNGTS 2013 S essione 2.1