Soil Structure Interaction (SSI) Analysis of Shear Wall Buildings with a Below-Ground Parkade

The soil surrounding and underlying the below grade portion of the structure affects the response of buildings to earthquakes. A common seismic design approach is to neglect the soil and analyze the entire free-standing structure for its response to earthquake ground motion applied at the foundation. Concerns have been expressed whether this simpler approach is acceptable, particularly when the below grade parkade is comparatively tall. This study evaluates the response of shear wall buildings with towers of varying heights, but all with a five-level below-grade parkade. The buildings are assumed to be located in Vancouver and subjected to earthquake loads prescribed by the National Building Code of Canada [6]. Two finite element models, both with soil surrounding the parkade, are analyzed. In the first case the tower and the parkade are supported by foundations placed directly on bedrock, and in the second model the tower and the parkade are supported on a rigid mat foundation resting on a soil deposit overlying the bedrock. A number of ground motions that are compatible with the uniform hazard spectrum for Vancouver are used to assess the response of each building model for a range of subsoil conditions. The results show that soil-structure interaction (SSI) can significantly increase the seismic forces in the structure. This effect is less pronounced for models in which kinematic interaction is present. Overall, the present study shows that for the type of buildings studied SSI magnifies the forces in the structure and needs to be considered in design.