11ME136 Naba Raj Shrestha
Supervisor Assoc. Prof. Masato Saitoh
A simplified and efficient soil structure interaction (SSI) method has been proposed for structures supported by pile foundation system using recently proposed lumped parameter model with gyro-element (GLMP) which shows efficient and accurate representation of frequency dependent impedance function (IFs) of the soil-foundation systems. The original GLPM has been transformed to make the gyro-element compatible with the conventional software framework. The transformed GLPM has been verified with the IFs of a pile group supporting a single degree of freedom system and implemented in multi-story building supported by a pile group. The effects of frequency dependent characteristics have been studied for the multi-story building by comparing the results with conventional Kelvin-Voigt model and rigid foundation system. OpenSees software framework has been used in the modelling of superstructure and foundation system with GLPM. The results show significant effects of the frequency dependent IFs on the response of the foundation.
In general, the impedance functions (IFs) of the soil-foundation system show frequency dependent characteristics and thus frequency domain analysis is compulsory to incorporate them. However, time history analysis is mandatory to integrate the inelastic behavior of materials and structural members which strongly depends on the stress path being integrated stepwise 1). Although varieties of methods are available to consider the frequency dependent IFs in a time history analysis, they are either very complex for practical use having many numbers of elements and degrees of freedom or they do not represent the frequency dependent IFs with sufficient accuracy.
Recently, Saitoh 1) proposed a simple lumped parameter model with gyro-element (GLPMs) which is considered to be a sophisticated model for representing frequency dependent IFs 2). However, the implementation of the GLPMs model in the conventional software in the practical application still remains left.
In this study, a transformation methodology of GLPMs has been proposed and verified by using it in a single degree of freedom (SDOF) structural system supported by pile foundation taken from a literature 2) in the frame work of Open System for Earthquake Engineering Simulation (OpenSees). Transformed GLPMs are then applied to a three dimensional multi-degree-of-freedom system in OpenSees. The effect of frequency dependent IFs on the structural response is studied in terms of the time-history responses and inter-story drift. The input motions are magnified to make the structural behavior in inelastic region.
Fig. 1 The original type II GLPMs model 1) (a); and the transformed GLPMs (b)
2. GLPMS AND ITS TRANSFORMATION
The GLPMs consist of conventional springs and dashpots and newly proposed gyro-elements (Fig 1(a)). Further details of GLPMs can be found in Saitoh 1) .