Identification of varying modal parameters of a tall building from the full-scale wind-induced responses

The dynamic behaviour of a tall building can change notably during the construction phases, under the varying degree of occupation, loading patterns, wind conditions, and aeroelastic phenomena. In this study, acceleration measurements taken over a period of one year and the corresponding wind conditions for a 150 m tall building in London, UK, are analysed to identify the reasons for variations in the structural modal parameters. Several environmental parameters are investigated, including the magnitude and the relative directions of the wind with respect to the building motion. The identified dominant effects are the response amplitude and the observation time. A finite element model of the tower is employed to support the experimental observations. The amplitude-dependent behaviour is studied for the natural frequencies and damping ratios of the first three vibration modes. The changes in the identified natural frequencies, with the monitoring time and response amplitudes, are attributed to the mass increase under the increasing occupancy and large-amplitude structural softening, respectively. In particular, it is shown that the natural frequencies experience an asymptotic drop of up to 5% with increasing time. The identified trends between the measured total damping and wind speed for different relative wind directions indicate that the amplitude of the lateral building motion, rather than aerodynamic wind characteristics, primarily influence the observed variation in the modal damping. Based only on the operational observations, an updated empirical model is proposed and compared against previous results from the literature. The established empirical model indicates the increase of the damping ratio values from around 0.6 % for the near-zero amplitude conditions to up to 2.1 % for the high amplitude conditions.