Forced Vibration Test in a Centrifuge Test Examining SSSI Effect

Both numerical simulations and field observations have illustrated the significant effects of structure-soil-structure interaction (SSSI) in dense urban environments during seismic events. A multiyear research program is being conducted to investigate the effects of SSSI in a series of geotechnical centrifuge model tests. In a recent test, two structures that were placed directly adjacent to each other were tested. The goal of this thesis is to focus on the attenuation of radiated waves from a vibrating structure to evaluate its effects on a neighboring structure to aid investigating the effects of SSSI. In addition to excitation by the shaking table on the centrifuge, forced vibration tests with a specially designed electromagnetic shaker were performed on the tested structure to evaluate the effects of one vibrating structure on its neighbors. During the forced vibration test, one structure was excited sinusoidally at different frequencies by a voice coil mass vibrator installed on the top of the structure. Accelerations at a neighboring structure and at various points in the soil were recorded. A signal processing method was developed to extract the amplitude and phase of the recorded accelerations. It employed a technique of convolution of a windowed version of the data with a windowed version of sine and cosine functions. The attenuation of the waves resulting from the forced vibration tests was quantified. A wave attenuation prediction model was developed based on a wave attenuation function that accounted for geometric and material damping. The proposed wave attenuation prediction model also considered the boundary effects of the centrifuge container by employing the method of images. During the forced vibration test, the footings are excited in horizontal, vertical, and rocking modes simultaneously, and each footing responded differently depending on the footing loads. Given the long wavelength of the waves compared to the spacing of the footings and the container, the waves radiated from each footing interfered with each other significantly. Although the system was complex, it appeared that the attenuation of a particularly surface wave from a source can be described by the proposed attenuation model. Using this model, one can estimate the magnitude of the motion underneath a neighboring structure and use that value as an input parameter to determine the response of the structure.