Soil-foundation-structure interaction and demand spectra in the performance based design

Performance based design is traditionally performed assuming the structure to be clamped on a rigid foundation. The free field soil response is evaluated and simply applied at the base of the structure. Nevertheless, this kind of procedure does not account for the interaction of the soil with the foundation and the structure during a strong ground motion. In this study, the soil-foundation-structure-Interaction (SFSI) is taken into account in the estimation of the demand spectrum at the foundation level. A parametric analysis is conducted in order to elucidate the influence of different parameters that affect the foundation input motion (FIM), and consequently on the estimation of the demand spectra at the foundation level. In a similar manner, the capacity curve of the compliant foundation structure system will be different from the rigid-fixed base system. Depending on the parameters, the FIM is found to differ significantly from the free field ground response. Similarly, the modification of the system's capacity curve, when it is considered compliant, is highlighted through case studies. In the case where the SFSI effects are important for the dynamic response of the system, the demand spectra at the foundation level need to be calculated taking into account this interaction. In these latter cases, the spectral values for the design in the performance based concept may be quite different from the traditionally adopted from a free field analysis.