The response of buildings to earthquake is affected by the soil surrounding the below grade portion of the structure. The soil layers, if any, between the foundation and the underlying rock also affect the response. 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. Soil-structure interaction (SSI) effects are not considered in routine design as it is often presumed that SSI is beneficial and ignoring SSI would lead to a conservative design.

Concerns have been expressed whether the foregoing approach is acceptable, particularly when the below grade parkade is comparatively tall. The present 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 (NBCC 2015).

Two different models are analyzed, (1) the tower and the parkade with soil surrounding the parkade while the foundation of the building rests on bedrock, and (2) the tower and the parkade supported on a rigid mat foundation resting on a soil deposit overlying the bedrock. The latter model also has soil layers surrounding the parkade. In the first case the response is affected only by the inertial effect of SSI, while the latter considers both inertial and kinematic interactions.  Programs SAP2000 and OpenSees are used to create finite element models for analysis. The far ends of the soil layers are attached to absorbing boundaries to represent infinite extent of the layers. A number of ground motions that are compatible with the uniform hazard spectra for Vancouver are used in the analysis. The response of each building model is assessed for a range of subsoil conditions.

The results show that the effect of SSI depends on many different parameters, including shear wave velocity in the soil, and characteristics of the earthquake and of the structure. Soil-structure interaction 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.