A study of building vibrations induced by weak motions: effects of earthquake excitation, ambient noise and wind speed

We carried out a vibration study experiment on a masonry building in the town of Ariano Irpino, southern Italy, using six-channel stations equipped with three-component velocity-transducers and accelerometers and running in continuous modality from January 2006 to December 2007. The analysis of weak motions from several local earthquakes, together with the 3D numerical modelling of the structure, allowed us to identify the first three vibration modes of the target building. Therefore, we checked the validity of ambient noise data to determine the vibration frequencies of buildings. The analysis tools based on earthquake and ambient noise data were conventional, i.e. spectral ratios between homologous components of stations at high floors in the building with respect to a station installed at the basement, and single-station spectral ratios between horizontal and vertical components. The indications derived from earthquakes and ambient noise result in a satisfactory agreement for frequencies between 1 and 20 Hz when using recordings characterized by low levels of amplitude, both for cultural and meteorological noise. In contrast, when the wind speed increases (above 20 km/h, approximately) seismic noise shows an excess of horizontal vibrations at low frequencies (below 2 Hz). These extra-amplitudes are not related to the seismic input vertically incident to the basement, but are probably due to the lateral action of the wind on the building. In contrast anthropic activities do not affect considerably the trend of spectral ratios in the range of frequencies that include the first modes of vibration of the building, even at high noise level.