Monitoring the response and the back-radiated energy of a building subjected to ambient vibration and impulsive action: the Falkenhof Tower (Potsdam, Germany)

The response of the soil-structure system near the Falkenhof Tower, Potsdam, Germany, has been monitored during the controlled explosion of a bomb dating to World War II. We installed eight 3-component velocimetric stations within the building and three in the surrounding area. We recorded several hours of seismic noise before and after the explosion, allowing the dynamic characterization of the structure both with ambient noise and forced vibration. We then compared the frequencies values and modal shapes of the structural modes evaluated both by analysing in the frequency domain the transfer functions and in the time-domain the different signals. Moreover, we carried out an interferometric analysis of the recorded signals in order to study the structural behaviour and to characterize the soil-structure interaction. Furthermore, we used normalized Short Time Fourier Transform (STFT) for the continuous monitoring of the structural response, in order to highlight possible frequency variations of the structural mode of vibration, and therefore of the structure's behaviour. To assess structural frequencies and to compare them with those evaluated by transfer functions, we used azimuth-dependent Fourier spectra. We also verified the suitability of the Horizontal-to-Vertical Spectral Ratio (HVSR) for estimating the dynamic characteristics of buildings when only single station seismic noise measurements are available. Regarding the structure-soil interaction, we identified the presence of a wave field back-radiated from the structure within the low amplitude seismic noise signal. In fact, in the free-field seismic noise recording spectra, peaks at frequencies consistent with those of the first two modes of the structure were recognized.