Dynamic identification of the Augusta hybrid base isolated building using data from full scale push and sudden release tests

Abstract

A three-story reinforced concrete building in Augusta (IT), isolated at the base and designed according to the provisions of the latest Italian seismic regulations, was subjected to a series of push and sudden release tests in March 2013. The Augusta isolation system is hybrid, consisting of 16 High Damping Rubber Bearings (HDRB) and 20 Low Friction Sliding Bearings (LFSB). The tests were characterized by a long quasi-static phase, where the building was pushed slowly to the desired displacement amplitude (sliding velocity $\approx 0.1mm/sec$, strain bearing demand $\gamma=0.39-0.78$), and a dynamic phase where the building was left free to oscillate. The duration of the dynamic phase was utmost $1 \%$ the duration of the static phase. The recordings included the displacements at the isolation level and the floor accelerations. A baseline fitting scheme was developed for the removal of the low frequency noise in the records. Application of the adjustment scheme provided reliable estimates of the floor velocities and displacements. The advantage of the proposed signal processing method other than its simplicity, is its ability to account for boundary conditions, for instance initial and residual displacements. Once the signals obtained from all eight tests performed were adjusted, they were used in the identification of the non-linear isolation system and the flexible superstructure (linear in the range of interest). The identification was performed in the time domain using the Covariance Matrix Adaptation Evolution Strategy, a stochastic algorithm for difficult, non linear black-box optimization. The identification of the isolation system provided the mass of the rigid block, the bi-linear properties used in the mechanical representation of the rubber bearings and the sliding coefficient of friction for the Coulomb model used in the modelling of the sliders. The obtained parameters, showed that rubber bearing properties were closer to the corresponding static laboratory properties, therefore implying that after the long quasi-static phase the HDRBs did not have time to recover their dynamic properties. The identified sliding coefficient of friction was in average $1\%$, leading to significant energy dissipation. The identified superstructure properties were the distribution of the floor masses, the modal frequencies, damping ratios and mode shapes. The identified data for the isolation system and the superstructure were input in a synthesized model of the isolated Augusta building, for the dynamic response simulation of the structure. A constrained optimization algorithm was developed ad hoc for the time-step solution of the coupled equations of motion. The obtained simulated response of the Augusta building matched the experimental response, in terms of displacements, velocities and accelerations.