Determination of Soil-Structure Interaction Effects for a Model Test Structure Using Parametric System Identification Procedures

Earthquake recordings from a model test structure located at Garner Valley California are used to evaluate inertial soil-structure interaction (SSI) effects. The test structure consists of steel columns and bracings 4.06 m in height supporting a 4m x 4m x 0.4m reinforced concrete roof slab. The structure rests on a 4m x 4m x 0.5m reinforced concrete foundation with no embedment. Surficial soil conditions consist of organic soil overlying silty sand alluvium with an average shear wave velocity in the upper 15 m of 200m/s. Seismic monitoring is performed with uni-axial (horizontal) accelerometers on the roof and foundation as well as vertical sensors at the corners of the foundation. Parametric system identification procedures are implemented to evaluate fundamental-mode vibration frequencies for a fixed- and flexible-base condition, from which SSI effects can be inferred. Data is available for the structure with and without bracing from two earthquakes. The lengthening of the fixed-base period due to SSI is a period lengthening of 12% and 74% for the braced and unbraced structure, respectively. The corresponding foundation damping levels for these two cases are about 1% and 5%. The different levels of period lengthening and foundation damping for the two structural configurations reflect the strong increase of inertial SSI effects with the ratio of structure-to-soil stiffness, which is naturally greater for the fixed- base configuration. The observed levels of SSI are reasonably well predicted by available theoretical models.