EFFECT OF SOIL CHARACTERISTICS ON THE ROCKING OF FOUDATIONS

Rocking of shallow foundations is a consequence of soil-structure interaction, in which the base of the foundations subjected to strong vibration may separate from the underneath soil by rotating about the longitudinal axis. In this rotational mode, plastic hinge spreads in the underlying soil instead of the main elements of the structure such as columns, therefore prevents a catastrophic collapse of the structures during strong vibrations, however, resulting in residual deformation in the soil and permanent displacement of the foundation. Rocking of shallow foundation is an intricate phenomenon and its results including displacement time history of the foundation, amount of mobilized moment and dissipated energy, are dependent on several factors such as specifications of the underlying ground, properties of the foundation and the superstructure and characteristics of inducing vibration. In this investigation, the effects of soil behavior including nonlinearity, plasticity and the soil mechanical propertiessuch as stiffness or shear modulus and frictional angle, on the response of rocking of shallow foundations of high-rise buildings subjected to dynamic loading, is studied by simulation of the problem using ABAQUS software. Four scenarios namely linear, nonlinear (elastic and elastoplastic) were considered for modeling of the soil. For efficiency modeling and applying nonlinearity behavior in the soil, subroutine USDFLD coded in FORTRAN was deployed . The soil-structure interaction is modeled completely in which the whole structure and the affected soil are modeled altogether. The structure in this study is modeled as a rectangular block which is integrated with its foundation. The earthquake propagates in the soil layer from the bedrock. The bedrock and the lateral boundaries of the soil were considered to be five times of the foundation width away from the boundary of the foundation to avoid reflecting seismic waves inside the model. Predominant frequency of the imposed dynamic loading is considered to be 1/758 Hz and the duration of this loading is considered to be 8/75 seconds. The studied soil is sand with a maximum shear modulus, obtaining from Equation 1. This equation is proposed by Seed and Idriss (1970).

Keywords:

Rocking , soil structure interaction , ABAQUS , nonlinear elastic-perfect plastic