Detection and Identification of Soil-Structure Interaction in Buildings from Vibration Recordings

Soil-structure interaction (SSI) can significantly alter the characteristics of recorded motions in buildings. The dominant frequency recorded in a building subjected to SSI is always smaller than the dominant frequencies of the fixed-base building, and of the foundation when no building is present. A building with SSI is a noncausal system because of the coupling between the motions of the foundation and the superstructure. The presence of SSI can be detected by investigating the causality of the building's impulse response. For causal systems, the amplitudes of the impulse response function at negative times are zero, whereas for noncausal systems they are comparable to those at positive times. The identification of SSI refers to extracting natural frequencies of the fixed-base building and the foundation from the recordings of the foundation and upper stories. The ratio of the Fourier amplitude spectrum of the top-story accelerations to that of the foundation accelerations permits the identification of the natural frequency of the fixed-base building. Two examples confirm the validity of the suggested methods. The first example involves a 10-story simulated building and its calculated response, whereas the second example involves a 30-story real-life building and its recorded response during a large earthquake.