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PROCEEDINGS OF THE SIXTH INTERNATIONAL CONFERENCE ON ENGINEERING COMPUTATIONAL TECHNOLOGY
Edited by: M. Papadrakakis and B.H.V. Topping
Equivalent Linear Soil Approximation in Soil-Foundation-Structure Interaction
Laboratory of Soil Mechanics, Foundations and Geotechnical Earthquake Engineering, Department of Civil Engineering, Aristotle University of Thessaloniki, Greece
D. Pitilakis, "Equivalent Linear Soil Approximation in Soil-Foundation-Structure Interaction", in M. Papadrakakis, B.H.V. Topping, (Editors), "Proceedings of the Sixth International Conference on Engineering Computational Technology", Civil-Comp Press, Stirlingshire, UK, Paper 171, 2008. doi:10.4203/ccp.89.171
Keywords: soil-foundation-structure interaction, equivalent linear, dynamic, substructure, softening, filtering.
In this study an equivalent linear approach is proposed to account for the effects of the nonlinear soil behaviour of soil-foundation-structure interaction (SFSI). The recently proposed procedure  takes advantage of the features and underlying assumptions of the substructure approximation [2,3] and reveals, with simplicity, the effects of the nonlinear soil deformation, under strong ground shaking, on the dynamic response of the flexibly supported superstructure.
A numerical code was developed incorporating the aforementioned approach. Using the substructure approximation to the soil-structure interaction problem, the equivalent linear behaviour is inherent throughout the whole analysis.The solution of the aforementioned SFSI problem is time efficient and computationally cost effective, compared with the standard finite element methods. Moreover, the numerical code provides the engineer with a simple, yet elaborate, tool to account for the effects of the nonlinear soil behaviour in structural dynamic response. To demonstrate the later, a series of parametric analyses is performed, in order to elucidate the important parameters affecting the vibration of typical structures resting on deformable foundations.
The principal effect of the nonlinear soil behaviour is additional stiffness degradation of the complete soil-foundation-structure system, along with an increase in the energy dissipation. In a linear case of SFSI, the most important parameters affecting the structural response would be the relative stiffness between the soil and the foundation-structure system, the slenderness of the superstructure and the relation between the forcing frequency and the fundamental frequency of the system . On the other hand, in the nonlinear soil case the amplitude and frequency content of the strong ground shaking may alter significantly the structural vibration. The increased system softening reduces further the resonant frequency of the system from the fixed-base conditions. The resonant frequency of the system may be perceived to act as a filter of the incoming seismic wave field, changing the dynamic characteristics of the response. The acceleration response reduces from the fixed-base case, as well as from the linear soil case. The displacement response is generally unaffected by the soil nonlinearity, due to its lower frequency content. The dynamic response of the equivalent linear system is compared for each case with the reference case of the fixed-base structural response.
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