DETECTION AND IDENTIFICATION OF SOIL-STRUCTURE INTERACTION IN BUILDINGS FROM VIBRATION RECORDINGS

Soil-structure interaction (SSI) can significantly alter the character istics of recorded motions in buildings. The dominant frequency record ed 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 superst ructure. The presence of SSI can be detected by investigating the caus ality of the building's impulse response. For causal systems, the ampl itudes of the impulse response function at negative times are zero, wh ereas for noncausal systems they are comparable to those at positive t imes. The identification of SSI refers to extracting natural frequenci es of the fixed-base building and the foundation from the recordings o f the foundation and upper stories. The ratio of the Fourier amplitude spectrum of the top-story accelerations to that of the foundation acc elerations permits the identification of the natural frequency of the fixed-base building. Two examples confirm the validity of the suggeste d methods. The first example involves a 10-story simulated building an d its calculated response, whereas the second example involves a 30-st ory real-life building and its recorded response during a large earthq uake.