Discrete-Element Method Simulations of the Response of Soil-Foundation-Structure Systems to Multidirectional Seismic Motion

In this study, a three-dimensional microscale framework utilizing the discrete-element method (DEM) is presented to analyze the seismic response of soil-foundation-structure systems subjected to three-directional base motion. The proposed approach is employed to investigate the response of a single lumped mass on a square spread footing founded on a dry granular deposit. The soil is idealized as a collection of spherical particles using DEM. The spread footing is modeled as a rigid block composed of clumped particles, and its motion is described by the resultant forces and moments acting upon it. The structure is modeled as a column made of clumped particles with a concentrated mass specified for the particle at the top. Analysis is done in a fully coupled scheme in time domain while taking into account the effects of soil nonlinear behavior, possible separation between the foundation base and soil because of rocking, possible sliding of the footing, and dynamic soil-foundation interactions. A technique to idealize several base boundary conditions to mimic rigid and elastic rock as well as an infinite medium is also presented. Microscale energy dissipation in the soil deposit in the free field and in the presence of the structure is quantified. Simulations were conducted to investigate the response of the deposit with and without the structure to various scenarios of multidirectional shaking patterns. Vertical motion amplification in the free field was similar to that of shear wave propagation. However, there was less nonlinearity for vertical motion than there was for horizontal motion. Lateral motion had a small impact on the amplification of the vertical input motion. The inclusion of vertical motion did not influence the amplification of horizontal motion at frequencies far from the resonance frequency of vertical motion.