Increasing Nonlinear Behavior at the Soil-foundation Interface to Improve Seismic Performance of the Structure

Abstract

It is well known that the performance of the structure is influenced by nonlinear behavior at the soil-foundation interface. Results of the previous studies have shown that the vertical Safety Factor (FS) of the foundation is not a suitable criterion to estimate the effectiveness level of soil-foundation nonlinearity on the performance of the structure. In order to investigate the subject, three frames were designed and placed on foundations in the current study. The soil was modeled by distributed Winkler elements considering Qz behavior. For assessing the ductility demand of beams and columns and story drifts, the soil-structures systems were subjected to artificial records whose response spectra have been matched with the design spectrum and also real ground motions. It was seen that by decreasing FS from 5 to 2, the ductility demands of the elements and drift of the stories decreased significantly in some cases, while the influence was not noticeable in other cases. It was concluded that soil-structure systems can be divided into two series parts. By introducing an index, it was shown that if the ratio of the moment capacity of the soil-foundation part to the moment capacity of the super-structure is smaller than one, the performance of the super-structure can be improved because it does not receive energy higher than the capacity of the soil-foundation part. Whenever the moment ratio is greater than one, the super-structure does not take noticeable advantages from soil nonlinearity, even if low FS is considered for foundation.

Keywords

• seismic performance of structures
• nonlinear soil-structure interaction
• strip foundation
• ductility
• drift
• safety factor