STATIC FOUNDATION STIFFNESSES BY CONSIDERING REDUCED SOIL SHEAR MODULUS FOR SOIL STRUCTURE INTERACTION ANALYSIS

The most important parameter that will affect the numerical results of the soil-structure interaction problem is the value of the soil shear modulus that will be considered in the calculation of impedance functions. This geotechnical parameter plays an essential role on the dynamic behavior of the site and the structural vibrations under earthquake ground motion. Variation in soil shear strength with shear deformation under cycling loads makes difficult to select an appropriate shear modulus in the computation of the static stiffnesses of the rigid footing for achievement a more realistic dynamic soil-foundation interaction model. The goal of this study is to discuss in detail how to evaluate the effect of the strain depend shear modulus under seismic loads on the impedance functions used in the sub-structure method for idealizing the soil-foundation interaction problem. In the frame of this study, it has been written a MATLAB code for addressing these purposes. The case-study example chosen for the analysis is considered as a 4-story reinforced concrete building structure located in Istanbul consisting of shear walls and moment resisting frames with a total height of 12m from the basement level. The foundation system consists of two different sized strip footings on clayey soil with different plasticity (Herein, PI=13 and 16). In the first phase of this work, the shear modulus reduction factor was not considered in the MATLAB algorithm. The static stiffness and embedment ratio of two rigid rectangular foundations measuring 2m wide by 17m long below the moment frames and 7m wide by 17m long below the shear walls are obtained for translation and rocking vibrational modes. Afterwards, the translational and rotational static foundation stiffnesses by considering reduced shear soil modulus have been calculated through the developed MATLAB code. The embedment effect of the foundation was also considered in these analyses. It can see obviously from the analysis results that the strain induced in soil will depend on the extent of the earthquake demand. It is clearly observed that when the strain range increases, the static stiffness of the foundation medium decreases dramatically. The overall response of the structure can be affected considerably because of the degradation in soil stiffness even for a moderate earthquake. Therefore, it is very important to employ the corrected dynamic shear modulus for earthquake analysis including soil-structure interaction effects.