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Slow dynamics process observed in civil engineering structures to detect structural heterogeneities

Abstract:

Under strong seismic excitation, the resonance frequencies of civil engineering structures rapidly decrease, followed by slow recovery back to their initial values if there is no damage. In this study, we show that as for laboratory trials with rock samples, the properties of the slow recovery characterise the level of heterogeneities, and in this case, the damage rate. First, we validate this concept with laboratory tests applied to continuous beam-like structures in damaged and undamaged states. One recent model is used to fit the observed recoveries, and we show that its parameters (i.e., frequency variation, recovery slope, characteristic times) change with the health of the equivalent structure. In a second step, this concept is applied to two civil engineering structures that experience earthquakes: the first (Factor Building, USA) without observed damage; and the second (Geophysics Institute building, Ecuador) that experienced a fore/ main/ after-shock sequence with apparent damage that was characterised by a permanent drop in resonance frequency. The efficiency of the proposed model is confirmed for monitoring and for the fit of the frequency recovery. We conclude that the recovery process is a clear proxy of the structural state, and that this could be helpful for seismic monitoring of structural health during earthquake sequences.